Pages From 2003, Vol.41, Issues 1, Body MR Imaging

MR angiography of the abdominal aorta

and peripheral vessels

Vincent B. Ho, MDa,*, William R. Corse, DOb

aDepartment of Radiology, Uniformed Services University of the Health Sciences, 4301 Jones Bridge Road, Bethesda,

MD 20814, USAbMR Imaging, Doylestown Hospital, 595 West State Street, Doylestown, PA 18901, USA

MR angiography (MRA), in particular gadolinium

(Gd)-enhanced three-dimensional MRA, is well

suited for the evaluation of patients with suspected

or known disease of the abdominal aorta and pe-

ripheral vessels [13]. In most of these patients, the

underlying vascular disease is degenerative and asso-

ciated with atherosclerosis. Atherosclerosis and its

related sequelae are leading causes for morbidity and

mortality in the Western world. Renal insufficiency is

prevalent in this population because of underlying

diabetes mellitus or renal artery disease. The ability

of MRA to provide reliable arterial depiction without

the need for the use of nephrotoxic contrast agents

(eg, iodinated contrast media) is one of the more

compelling arguments for the use of MRA in this

patient population.

Time-of-flight and phase-contrast MRA

The potential of MR imaging to illustrate arterial

structures noninvasively using time-of-flight (TOF)

and phase-contrast (PC) MRA has been known for

almost two decades [46]. These techniques rely on

the properties of moving protons (flowing blood) and

can be performed as either a two- or a three-dimen-

sional acquisition. TOF MRA relies on flow-related

enhancement or in-flow effect caused by the entry

of unsaturated protons into the imaging slice (two-

dimensional) or volume (three-dimensional). Arterial-

to-background image contrast is improved by the

application of repetitive radiofrequency pulses to

suppress the signal from stationary background tis-

sue. Arterial in-flow (and arterial signal) is highest if

blood flow is brisk; the imaging slice (or volume in

the case of three-dimensional TOF) is thin; and

imaging is performed perpendicular to the direction

of flow. Vertically oriented vessels, such as the aorta

and peripheral arteries, are best imaged using axial

TOF scanning. Unfortunately, the acquisition of

images perpendicular to the length of the vessel is

inefficient and can result in long acquisition times.

Axial two-dimensional TOF MRA of the peripheral

vessels (from the aortic bifurcation to the ankle) can

often require 2 or more hours to accomplish. Long

imaging times increase the likelihood of motion

artifacts not only from physiologic motion (eg, res-

piration and peristalsis) but also from bulk patient

movement. This is further complicated by the fact

that viewing of the vessels is best performed in their

long axis. The use of thicker slices or partitions to

shorten acquisition time has the untoward effect of

not only increased saturation concerns but also

decreasing the spatial resolution of the vessels on

the subsequent longitudinal image reformation used

for image interpretation.

The other traditional MRA technique is PC, which

relies on the phase shifts that protons experience

0338-3890/03/$ see front matter D 2003, Elsevier Science (USA). All rights reserved.

PII: S0338 -3890 (02 )00062 -3

The opinions or assertions contained herein are the private

views of the authors and not to be construed as official or

reflecting the views of the Uniformed Services University of

the Health Sciences or the Department of Defense.

* Corresponding author.

E-mail address: [email protected] or [email protected]

(V.B. Ho).

Radiol Clin N Am 41 (2003) 115144

when they move along a gradient field. In practice,

PC MRA generally requires longer scan times and is

technically more challenging to perform than TOF

MRA and not widely used. For targeted imaging of

smaller vascular regions, however, such as the renal

arteries, PC MRA has been found to be useful,

especially as an adjunct to TOF MRA [79]. For a

PC MRA, the operator must set an appropriate flow

direction and velocity encoding, which in cases with

complex flow or geometries may be difficult to

determine a priori.

Because TOF and PC imaging rely on flow, both

techniques are prone to artifacts related to disruptions

or variations in blood flow (eg, pulsatile flow and

turbulent flow) [10,11]. Turbulent flow is particularly

problematic because it results in intravoxel phase

dispersion and ultimately in vascular signal loss. This

is common about stenoses and regions with complex

geometries, which unfortunately are typically the

regions of clinical interest. Intravoxel dephasing is a

known limitation of TOF and PC MRA, which results

in the overestimation of the severity of a stenosis or

even in the erroneous appearance of a stenosis or

occlusion. These pitfalls and the other previously

mentioned issues related to TOF and PC imaging

have contributed to the general lack of enthusiasm for

their routine clinical use for imaging of the aorta and

peripheral vessels.

Gd-enhanced three-dimensional MRA

In the early 1990s, a new MRA technique

for aortography called Gd-enhanced three-dimen-

sional MRA was introduced by Prince et al [12].

Unlike TOF and PC MRA, this method does not

rely on blood flow but rather on the T1 shortening

effects of circulating Gd-chelate contrast media. The

technique is quick, fairly easy to perform, and provides

high-resolution three-dimensional arterial image sets.

Artifacts related to spin saturation, slow flow, or

turbulent flow encountered with TOF are minimal

for Gd-enhanced three-dimensional MRA. As a result,

Gd-enhanced three-dimensional MRA can be tailored

for most efficient high spatial resolution imaging

(ie, parallel to the length of the vessel).

On Gd-enhanced three-dimensional MRA, arterial

signal relies on the concentration of Gd within

the lumen of the vessel during image acquisition.

The resultant images are essentially lumingrams

Fig. 1. A 77-year-old man with hypertension. On standard coronal maximum intensity projection (MIP) (A) from a Gd-enhanced

three-dimensional MRA, the proximal renal arteries are noted to be normal and patent. On oblique coronal MIP (B) and axial

subvolume MIP (C), however, the occluded left upper pole segmental renal artery (large arrow) and the moderate stenosis (small

arrow) of the left lower pole segmental renal artery are better visualized.

V.B. Ho, W.R. Corse / Radiol Clin N Am 41 (2003) 115144116

Fig. 2. A 49-year-old man with rectal cancer. Gd-enhanced three-dimensional MRA of the aortoiliac vessels was performed as a

preprocedural road map for intra-arterial chemotherapy planning. The arterial anatomy is well seen on volume-rendered pro-

jection of the three-dimensional data set. (A) Coronal maximum intensity projection (MIP). (B) Sagittal MIP.

V.B. Ho, W.R. Corse / Radiol Clin N Am 41 (2003) 115144 117

similar to that of conventional x-ray angiography.

Gd-enhanced three-dimensional MRA provides

data that can be formatted in angiographic projections

identical to those of x-ray angiography (views famil-

iar to referring surgeons) without the risks related

to catheterization, nephrotoxicity, and radiation expo-

sure inherent to x-ray angiography. Gd-enhanced

three-dimensional MRA has the additional benefit

of providing volumetric (three-dimensional) angio-

graphic data sets, which can be used for improved

projectional viewing. Arteries are often tortuous

or obscured by overlapping structures (eg, large

aneurysm) on conventional x-ray angiography. Gd-

enhanced three-dimensional MRA can provide not

only the standard angiographic projections but also

the opportunity to view arteries in nonstandard obliq-

uities using maximum intensity projection (MIP).

Overlying structures can be removed easily using

subvolume MIP (Fig. 1) or multiplanar reformation

(MPR). The three-dimensional data can also be

processed for advanced viewing using volume ren-

dering, shaded surface display, and virtual intra-

arterial endoscopy [13,14]. Direct volume rendering

(Figs. 24) often can provide a different perspective

and may be helpful not only for image interpretation

but also for preoperative surgical discussions and

conferences. Standard MIP and MPR, however,

are all that are typically needed for routine interpreta-

tion [13].

Gadolinium-enhanced three-dimensional MRA

has consistently been shown to be accurate and

preferable to traditional noncontrast MRA techniques

for evaluation of not only the aorta but also the

lower extremity arteries [13]. In many institutions,

Gd-enhanced three-dimensional MRA is steadily

emerging as the preferred method for evaluation

of the abdominal aorta, and the peripheral run-off

vessels. The growing popularity of Gd-enhanced

three-dimensional MRA has been facilitated by MR

scanner and equipment manufacturers who have

designed new pulse sequences, interactive timing

algorithms, improved user-interfaces, and coil prod-

ucts specifically for the performance of Gd-enhanced

three-dimensional MRA. In addition, a large variety

of vendor and third-party products are now available

for soft-copy interpretation and viewing of the three-

dimensional data sets. In the ensuing sections, the

theory and technical considerations associated with

Gd-enhanced three-dimensional MRA are discussed

followed by a brief clinical discussion of interpre-

tative issues for several common clinical indications

for MRA of the abdominal aorta, its branches, and

peripheral vessels.

Principle of Gd-enhanced three-dimensional MRA

Arterial depiction is optimized by proper timing of

data acquisition, especially the low spatial frequency

Fig. 3. An 82-year-old woman suspected of having

mesenteric ischemia. Coronal volume-rendered projection

of the Gd-enhanced three-dimensional MRA demonstrates

normal orientation of the celiac artery and superior mesenteric

artery. Multiplanar reformation (not shown) noted both

vessels to have a normal caliber.

Fig. 4. A 69-year-old woman with a celiac artery aneurysm.

The volume-rendered projection of a Gd-enhanced three-

dimensional MRA clearly illustrates an aneurysm of the

celiac artery (arrow).


k-space data (center of k-space) for the period of peak

arterial enhancement (ie, peak concentration of Gd)

[1517]. Late data acquisition can result in substan-

tial venous contamination of the image sets and can

hinder image interpretation. Premature image acquisi-

tion can result in insufficient arterial Gd and poor

vascular depiction. The acquisition of central k-space

data during the period of preferential arterial enhance-

ment (arterial phase) is generally preferable. Gd-

enhanced three-dimensional MRA was originally

described using a slow venous infusion 40 to 60 mL

Gd-chelate contrast media during a long (3 to

4 minutes) three-dimensional spoiled gradient echo

acquisition, one typically used for three-dimen-

sional TOF MRA, leading to its early description as

Gd-enhanced three-dimensional TOF MRA. The

slow contrast infusion (0.3 mL/second) prolonged

the arterial phase of enhancement and delayed sig-

nificant venous enhancement, extending the time

window for preferential arterial enhancement (Fig. 5).

Improvements in gradient strength and pulse

sequence design have yielded faster data acquisition

speeds (eg, 20 to 30 seconds per three-dimensional

acquisition, and recently with sensitivity encoding

[18] 10 to 20 seconds per three-dimensional acquisi-

tion). This has enabled the performance of breathhold

image acquisition, which minimizes respiratory

motion artifacts and significantly improves arterial

visualization of abdominal aortic branch vessels

[1922]. Faster imaging, however, has necessitated

more accurate timing of data acquisition.

Timing

There are several methods for achieving proper

timing of a Gd-enhanced three-dimensional MRA

[15,19,20,2326]. The simplest is using a fixed

timing delay (eg, 15 seconds). This can often be

unreliable, however, because circulatory times are

highly variable, especially if the patient has a poor

ejection fraction or large capacious aortic aneurysm.

The arrival of a contrast bolus in the abdominal aorta

can take from 10 to 60 seconds [19]. The preferred

methods are the use of a timing bolus injection

[19,20], a triggering algorithm [23,24,26], or a fast

multiphase technique [25]. The timing bolus strategy

entails the administration of a small 1- to 2-mL test

bolus at the same rate as the actual bolus (eg, 2 mL/

Fig. 5. Diagram of vascular signal intensity as it relates to bolus injection rate. Fast bolus injection results in higher arterial

contrast media concentrations and higher signal intensity. With faster injection rates, however, the duration of preferential arterial

enhancement is diminished because of earlier and more significant venous enhancement than seen with slower injection rates.

Slow injection rates prolong the arterial phase; however, the maximum arterial concentration of contrast media is lower. Very

slow injection rates may result in insufficient signal for adequate arterial visualization. (From Ho VB, Choyke PL, Foo TKF, et al.

Automated bolus chase peripheral MR angiography: initial practical experiences and future directions of this work-in-progress.

J Magn Reson Imaging 1999;10:37688; with permission.)


second for a Gd-enhanced three-dimensional MRA of

the aorta) and imaging the target vessel at a high

enough temporal rate (eg, one to two frames per

second). For this technique it is critical that sufficient

flush (eg, 30 mL) is used to ensure that the bolus is

within the central vasculature and that the contrast

injections of the test bolus and actual bolus are

similar. Use of an MR imaging-compatible injector

(eg, Spectris, Medrad, Indianola, PA; and Optistar,

Mallinckrodt, St. Louis, MO) minimizes variations in

the delivery of contrast boluses. If monitoring is

performed in a plane perpendicular to the vessel

(eg, axial timing scan for the abdominal aorta),

superior and inferior saturation bands should be used

to minimize the in-flow signal and ensure vascular

signal is attributable only to the contrast bolus arrival.

The merit of this method is that it can be universally

performed on all current scanner platforms.

There also are several vendor-specific real-time

triggering options for Gd-enhanced three-dimensional

MRA. In one technique called automated bolus

detection algorithm (SmartPrep, GE Medical Sys-

tems, Waukesha, WI [23]), a monitoring volume is

placed in addition to the three-dimensional MRA

volume. The algorithm is able to detect the bolus

arrival into the monitoring volume and automatically

initiates the MRA data acquisition once the operator-

defined thresholds are exceeded. There is a change in

scanner noise between the monitoring phase and the

MRA data acquisition that provides an audible cue

for the coordination of the patients breathholding.

Another real-time triggering algorithm uses MR

fluoroscopy (BolusTrak, Philips Medical Systems,

Best, The Netherlands; Care Bolus, Siemens Medical

Systems, Iselin, NJ). This technique provides a MR

fluoroscopic image of the target vasculature and

Fig. 6. Proper alignment of preferential arterial-phase enhancement for a variety of k-space schemes used for Gd-enhanced three-

dimensional MRA. The critical issue for all the schemes is for the central k-space data (ie, low spatial frequency data) to be

acquired during the plateau phase of arterial enhancement. In the conventional sequential k-space scheme, the central k-space

data are acquired during the middle of the data acquisition period. In both the conventional centric and elliptical centric

acquisition schemes, the central k-space data are obtained at the beginning of imaging. Note that with conventional centric

acquisitions, k-space is only centric in ky and that the high spatial frequency encodings in kz are also acquired during each linear

pass and the central k-space encodings in ky and kz are gathered more efficiently (ie, acquired more quickly) in the elliptical

centric acquisition scheme. Partial Fourier imaging with reverse sequential acquisition ordering can also provide a compact

acquisition of low spatial frequency data during the beginning of image acquisition. Note that low spatial frequency data are best

obtained during the plateau period of arterial enhancement. Acquisition of central k-space data prematurely during the rapid rise

in arterial signal (open arrow) can result in significant ringing artifacts (see Fig. 7). (Adapted from Ho VB, Foo TKF, Czum JM,

et al. Contrast-enhanced magnetic resonance angiography: technical considerations for optimized clinical implementation. Top

Magn Reson Imaging 2001;12:28399; with permission.)


enables the operator to trigger the MRA data acquisi-

tion manually on contrast bolus arrival [26].

The final timing method is simply to perform

multiple fast MRA acquisitions in succession [25].

This tact, also known as multiphase or time-resolved

imaging, assumes that at least one of the MRA

acquisitions is performed properly during the arterial

phase of the bolus. The typical compromise for high

temporal resolution (5 to 8 seconds per three-dimen-

sional acquisition) is lower spatial resolution of any

individual acquisition. This method may have little

use in patients with a poor breathholding capacity

because the limitations in breathholding may pre-

clude the acquisition of sufficient data sets during a

single breathhold to ensure arterial-phase imaging.

Furthermore, respiratory motion during these acqui-

sitions significantly degrades what are already lower

spatial resolution data sets.

Pulse sequence

Gadolinium-enhanced MRA is traditionally per-

formed with a T1-weighted fast three-dimensional

spoiled gradient echo pulse sequence using the short-

est possible repetition time (TR) and echo time (TE)

to ensure the fastest possible imaging speed. The use

of a three-dimensional acquisition provides high

spatial resolution and improves background suppres-

sion. Radiofrequency spoiling and the use of a higher

flip angle improve the T1 weighting of the acquisition

and the arterial signal following contrast administra-

tion. The imaging parameters should be tailored to

afford the highest possible spatial resolution for the

allotted time period, which for a breathhold acquisi-

tion is generally 20 to 30 seconds. Prescription of a

volume with a matrix of 256 224 to 256, partitionthickness of 1.5 to 2.5 mm, and 40 to 60 partitions is

usually sufficient for imaging the abdominal aorta

during a 20- to 30-second breathhold Gd-enhanced

three-dimensional MRA of the abdominal aorta.

Knowledge of the k-space trajectory of the three-

dimensional pulse sequence is also critical for proper

timing [15]. Historically, Gd-enhanced three-dimen-

sional MRA had only been implemented using a

traditional sequential k-space scheme in which the

k-space is filled linearly in a sequential fashion from

top to bottom with the low spatial frequency data

(center of k-space) being acquired during the middle

of the imaging period. Because the central k-space

data are responsible for most image contrast, its

acquisition should be timed for peak arterial enhance-

ment, and preferably before significant venous

enhancement occurs (Fig. 6). With the development

of real-time triggering methods, however, a variety of

Fig. 7. Ringing artifact on breathhold renal Gd-enhanced three-dimensional MRA in a 36-year-old man with left renal artery

stenosis. This artifact is recognized by the presence of bright and dark lines ([A] coronal maximum intensity projection, [B]

coronal source image) that parallel the edge of the enhancing abdominal aorta (small arrows) and results from the premature

acquisition of low spatial frequency data during leading edge of the contrast bolus when arterial signal is rapidly rising. This

artifact is more common with centric acquisition ordering, which acquires central k-space data early. Ringing artifact can be

avoided by timing for the low spatial frequency to be obtained during the plateau phase of the arterial enhancement (see Fig. 6).

Note that despite the artifacts, the patients left renal artery stenosis (large arrow) was well delineated. (Adapted from Ho VB,

Foo TKF, Czum JM, et al. Contrast-enhanced magnetic resonance angiography: technical considerations for optimized clinical

implementation. Top Magn Reson Imaging 2001;12:28399; with permission.)


alternate k-space schemes were designed. In these

alternate k-space schemes, the central k-space data

are acquired during the beginning of the imaging

period, which improves the ability to synchronize the

bolus arrival with the critical central k-space data

acquisition. This allows monitoring of contrast arrival

within the target vessel itself versus proximal or

upstream. Two such alternate k-space schemes are

centric-phase ordering [23] and elliptical centric-

phase ordering (see Fig. 6) [26]. Partial Fourier

imaging (eg, 0.5 NEX) is another alternate method

for selective k-space sampling. As with traditional

sequential phase-ordered acquisitions, partial Fourier

imaging uses a linear k-space sampling scheme but

only a little over a half of k-space is acquired. As

such, the center of k-space can be prescribed to be

acquired either at the end (conventional sequential) or

at the beginning (reverse sequential) of the acquisi-

tion. The disadvantage of partial Fourier imaging is a

decrease in signal-to-noise, but with Gd-enhanced

three-dimensional MRA there is typically sufficient

arterial signal if timing is proper. With these alternate

Fig. 8. A 72-year-old man with a 9-cm abdominal aortic aneurysm. The extent of this large aortic aneurysm is displayed on the

arterial-phase Gd-enhanced three-dimensional MRA ([A] coronal maximum intensity projection [MIP], [B] sagittal MIP) and

delayed-phase Gd-enhanced three-dimensional MRA ([C] coronal MIP). Note the improvement in visualization of the infrarenal

aorta on the later delayed-phase acquisition (C) compared with the arterial-phase images (A,B). This is secondary to the slow

aortic flow within the large abdominal aortic aneurysm. Performing two acquisitions (arterial phase and delayed phase) after the

administration of contrast agent is prudent because it is hard to know a priori whether the patient has slow blood flow.

Furthermore, the delayed-phase images can often provide diagnostic quality angiographic images should there be inadequate

patient breathholding or motion during the arterial-phase acquisition. An axial image (D) from a late delayed-phase axial two-

dimensional spoiled gradient echo acquisition (ie, traditional axial two-dimensional time-of-flight MRA) taken through the

abdominal aorta delineates the circumferential mural thrombus (T) within the aneurysm and provides a better assessment of

actual aortic wall-to-wall diameter (arrows). (Adapted from Ho VB, Prince MR, Dong Q. Magnetic resonance imaging of the

aorta and branch vessels. Coron Artery Dis 1999;10:1419; with permission.)


k-space sampling schemes, it is particularly important

not to acquire central k-space views during the rapid

rise in arterial Gd (see Fig. 6) because this can result

in a ringing artifact [16] seen as alternating black and

white lines about the edges of arteries (Fig. 7).

Bolus delivery

Faster pulse sequences have enabled the achieve-

ment of high-quality Gd-enhanced three-dimen-

sional MRA more efficiently using smaller doses

Fig. 9. A 66-year-old man with a small infrarenal abdominal aortic aneurysm (AAA). On the coronal maximum intensity

projection (MIP) (A), a small AAA can be seen (arrow) well below the renal arteries, which are noted to be solitary for each

kidney and to have a normal caliber. On sagittal subvolume MIP (B), the ventral origins of the celiac artery (thick arrow) and the

superior mesenteric artery (thin arrow) are noted to also have a normal caliber. The contour and shape of the lumen of the small

infrarenal AAA (arrow) is particularly well seen on a volume-rendered projection (C).


of Gd-chelate contrast media [24,27,28]. With their

shorter bolus length requirements, faster imaging

provides the opportunity to deliver the contrast

media at a higher injection rate, which can provide

sufficiently high arterial Gd concentrations using

much lower doses (see Fig. 5). In general, contrast

injections are best delivered by a right antecubital

vein using a 22-guage or larger angiocatheter. The

reliability of contrast administration can be

improved by the use of an MR-compatible injector

and is particularly important for those who choose a

test bolus for timing. In general, contrast media

injections should always be accompanied by a

sufficiently large saline flush to ensure that the

contrast bolus is pushed out of the tubing set and

peripheral venous system and well into the central

circulation. Recently, Boos et al [29] demonstrated a

clear advantage for the use of a 30-mL or larger

saline flush for Gd-enhanced three-dimensional

MRA in that it significantly prolonged the duration

of arterial enhancement.

MRA of the abdominal aorta

An MRA of the abdominal aorta is primarily

performed for the assessment of aortic aneurysm;

aortic dissection; aortic occlusion; or a suspected

Fig. 10. A 75-year-old woman with hypertension and an infrarenal AAA. On coronal maximum intensity projection (MIP) (A) a

long, fusiform infrarenal AAA that extends to the aortic bifurcation. High-grade stenoses are also noted at the origins of both

common iliac arteries. On an oblique coronal subvolume MIP (B), a high-grade stenosis of the left renal artery is noted (arrow).

A high-grade stenosis (arrow) was also noted in the right renal artery at its origin; however, this was best seen on an oblique axial

subvolume MIP (C) from below.


branch vessel stenosis (especially renal artery sten-

osis). As discussed in the ensuing sections, breath-

hold Gd-enhanced three-dimensional MRA can

adequately answer the clinical questions related to

these aortic diseases. Gd-enhanced MRA of the

abdominal aorta is best performed in the coronal or

oblique coronal three-dimensional prescription. On

occasion, a sagittal acquisition may be preferable for

imaging arterial branches that originate ventrally,

such as the superior and inferior mesenteric arteries,

especially if the patients breathholding ability is

limited and the volume needs to be minimized

because of imaging time considerations. The superior

mesenteric artery, however, can often be well visual-

ized on a sagittal reconstruction of a coronal three-

dimensional MRA data set if the vessel is included

within the field of view. The examination, however,

should always include an axial T1-weighted fast spin

echo and an axial T2-weighted fast spin echo, pref-

erably performed before the MRA. These initial

Fig. 11. A 78-year-old man with chronic type B aortic dissection. On oblique sagittal maximum intensity projection (A), the

patient is noted to have a very tortuous thoracic aorta with only a hint of the dissection, which begins in the distal arch beyond the

subclavian artery (not shown). On oblique sagittal multiplanar reformation (MPR) (B), however, spiral extension of the intimal

tear into the abdominal aorta is clearly seen. Oblique axial MPRs at the levels of the celiac artery (C), superior mesenteric artery

(D), and renal arteries (E) demonstrate that all four arteries originate from the true lumen (T ) and not the false channel (F ).


sequences enable the evaluation of the aortic diameter

and wall and screening for any unexpected visceral

pathology (eg, pelvic kidney).

In general, before the breathhold Gd-enhanced

three-dimensional MRA, an unenhanced breathhold

three-dimensional MRA using the same imaging

parameters should be performed as a trial run. This

not only ensures that the three-dimensional volume is

appropriately placed and artifacts, such as phase

wrap, do not overlap critical regions but also provides

a preparatory experience for the patient (and the

operator) in which he or she can familiarize them-

selves with the breathholding requirements.


should always include at least two postcontrast

acquisitions (arterial phase and delayed phase). The

additional time is trivial (additional breathhold) and

the second acquisition (delayed-phase MRA) often

can provide additional information. If blood flow is

slow as in a large aneurysm (Fig. 8), the initial arterial-

phase MRA may be completed before the attainment

of sufficient Gd concentrations have been achieved

within the distal aorta. Similar concerns arise in an

aortic dissection where flow within the false channel

is often slow. Imaging multiple phases has additional

benefits for renal artery evaluations because it also

enables the detection of delays or changes in paren-

chymal enhancement [30]. An additional benefit for

delayed-phase imaging is not only to provide a

secondary set of images but an improved depiction

or mural thrombus. The routine performance of an

axial two-dimensional fast spoiled gradient echo pulse

sequence (ie, a conventional axial two-dimensional

TOF MRA) after the coronal breathhold acquisitions

can be a helpful option to consider for the evaluation

of thrombus (see Fig. 8).

A contrast dose of 20 to 30 mL of Gd-chelate

injected at 2 mL/second [21] usually provides suf-

ficient illustration of the abdominal aorta on Gd-

enhanced three-dimensional MRA. Although a single

dose (20 mL [27]) of a Gd-chelate contrast agent has

been shown to be adequate for renal artery imaging, a

Fig. 12. A 60-year-old woman with chronic type B aortic dissection but worsening vague abdominal pain. Sagittal maximum

intensity projection (MIP) (A) from a sagittal Gd-enhanced three-dimensional MRA demonstrates a narrowing of the celiac

artery (arrow) at its origin. On an oblique axial subvolume MIP (B), the extension of the dissection into the proximal celiac

artery is better visualized. Note that the true lumen is bright but the false channel was thrombosed and only apparent by its mass

effect on the true lumen (arrows) on Gd-enhanced three-dimensional MRA.


larger dose of 30 mL (or 0.2 mmol/kg) is recom-

mended in patients with a large abdominal aortic

aneurysm (AAA), an aortic dissection, or aortic

occlusion because this ensures a sufficiently high

arterial Gd concentration for adequate visualization

of the arterial structures.

Clinical considerations

Abdominal aortic aneurysm

Aneurysms are defined as enlargement of the

arterial diameter by 50% or more from its normal

caliber, which for the abdominal aorta is generally

greater than or equal to 3 cm [31]. AAAs are common

especially in men above the age of 55 and in women

above the age of 70 [32]. The urgency of this

diagnosis relates to its risk of rupture, which is fatal

in most cases (81% to 94% [33]). AAAs are fre-

quently asymptomatic, however, until they rupture.

Large AAAs (greater than 5 cm) have a 25% to

41% likelihood of rupture within 5 years [34,35] and

generally are repaired surgically. The risk of rupture

of small AAAs (aortic diameter less than 4 cm), on

the other hand, is low (0% to 2% [34,35]). Because

the 30-day operative mortality risk for elective AAA

repair is 5% to 6% [36], AAAs with diameters less

than 4 cm are typically followed with periodic

surveillance to check for interval expansion, which

is typically 0.2 to 0.4 cm per year [34,35]. Of course,

rapid expansion of an AAA (eg, greater than 1 cm per

year), widening of the pulse pressure, or the mani-

festation of symptoms (eg, abdominal tenderness or

pain) favors early surgical intervention. Ideally, sur-

gical repair is elective because emergent repair carries

roughly a 10-fold increase in operative morbidity

(30-day operative mortality of 40% to 50% [36]).

The elective repair of AAA with diameters between

4 and 5 cm, however, continues to be debated be-

cause rupture rates are 3% to 12% after 5 years [34].

Improvements in aortic stent grafts has enabled

successful endovascular repair of some aneurysms,

especially small infrarenal AAA. Although preproce-

dural assessments can be obtained using MRA,

magnetic susceptibility artifacts can be significant

for certain stent materials and CT generally has been

the modality of choice for imaging prestenting and

poststenting patients with AAA. There are a growing

number of stent grafts, however, which are made of

materials, such as nitinol or polytetrafluoroethylene,

that seem to have minimal artifacts on Gd-enhanced

three-dimensional MRA [37,38].

Degenerative aneurysms of the aorta are com-

monly associated with atherosclerosis but recent

observations suggest a multifactorial causation and

a categorization of most AAAs as nonspecific

[31,34]. AAA is typically fusiform but may on

occasion be saccular. A saccular aneurysm should

lead one also to entertain an infectious etiology (ie,

mycotic aneurysm) [39]. Aneurysms are typically

infrarenal (84%) but can also involve the entire

abdominal aorta (4%) or be limited to the pararenal

region (12%) [40]. Aneurysms of the proximal

Fig. 13. A 56-year-old man with Leriches syndrome who presented with hypertension and buttock claudication. Preoperative

Gd-enhanced three-dimensional MRA ([A] coronal maximum intensity projection [MIP]) demonstrates the characteristic

occlusion of the distal abdominal aorta below the renal arteries. A high-grade stenosis was also noted in the proximal left renal

artery (arrow). The postoperative Gd-enhanced three-dimensional MRA ([B] coronal MIP) demonstrates the aortobifemoral graft

that included revascularization of the left renal artery at the proximal anastomosis. (Courtesy of Qian Dong, MD, and Martin

Prince, MD, PhD, Ann Arbor, MI.)


abdominal aorta carry a higher operative morbidity

and are particularly challenging because aortic recon-

struction requires proper incorporation of branch

vessel ostia (ie, renal arteries, celiac arteries, or

superior mesenteric artery).

Conventional T1-weighted spin echo images have

been found to be excellent for the delineation of

aortic dimensions [41,42]. Preoperative planning,

however, also requires not only the assessment of

aneurysm size (width, depth, and length) but also its

proximal and distal extent and its relationship to the

visceral branch vessels. In addition, evidence for

rupture, complications of the aortic wall, supernumer-

ary renal arteries, obstructive disease of renal, celiac,

or mesenteric vessels, or an anomaly, such as a

horseshoe kidney, can significantly alter the surgical

plan [43]. Gd-enhanced three-dimensional MRA

(Figs. 9, 10) can illustrate these features accurately

and reliably and has been shown to be sufficient for

preoperative planning for AAA interventions

[22,4447]. For example, Gd-enhanced three-dimen-

sional MRA has been shown to predict correctly the

proximal anastomotic site for AAA repair in 95% of

patients, which was comparable with that of conven-

Fig. 14. A 50-year-old woman with hypertension. On Gd-enhanced three-dimensional MRA ([A] oblique coronal subvolume

maximum intensity projection (MIP), [B] axial subvolume MIP, [C] coronal volume-rendered projection, [D] coronal transparent

volume-rendered projection), the string of beads appearance (arrows) characteristic for fibromuscular dysplasia is noted in the

right renal artery, which looks comparable with that of conventional x-ray angiography (E). Note that the beaded appearance was

well seen in the right renal artery on Gd-enhanced three-dimensional MRA, especially on the volume-rendered projections (C,D).

(F) Mild fibromuscular dysplasia was also noted on conventional x-ray angiography in the proximal left renal artery (arrow).

This was suggested on Gd-enhanced three-dimensional MRA (G) but less clearly seen, most probably secondary to the inherent

lower spatial resolution of MRA.


tional x-ray angiography (97%) [46]. MIP and MPR

are particularly helpful for the identification of ste-

notic branch vessels, which are preferably revascu-

larized at the time of the AAA repair. Gd-enhanced

three-dimensional MRA has been shown to be accu-

rate for the detection of significant occlusive celiac,

renal, mesenteric, or iliac arterial disease (94% sen-

sitivity and 98% specificity [47]).

Aortic dissection

Dissections much more frequently arise in the

thoracic aorta but can often extend inferiorly into the

abdominal aorta to involve the renal, celiac, mes-

enteric, and iliac arteries [48]. The principle concern

with dissections is the involvement of visceral

branch vessels, which can result in their obstruction.

On Gd-enhanced three-dimensional MRA, the true

and false channels and entry and exit intimal tears

can be well illustrated (Figs. 11, 12) [4951]. MPR

of the three-dimensional data sets enables the selec-

tive viewing of individual aortic branch vessels and

the identification of their blood supply (ie, from true

versus false channel). The extension of an intimal

tear into the abdominal aorta typically spirals pos-

terior laterally about the arch with the false channel

coursing to the left of the aorta potentially to involve

the left renal artery and possibly the celiac and

superior mesenteric arteries. Delayed-phase imaging

is recommended (ie, at least two postcontrast MRA

acquisitions) because flow within the false channel

may be slow and not adequately fill with contrast

media during the initial acquisition.

Aortic occlusion (Leriches syndrome)

Occlusion of the abdominal aorta is uncommon

but worth mentioning because MRA can be very

useful in this condition [43,52]. Abdominal aortic

occlusion may occur as a result of a variety of

Fig. 14 (continued )


causes, in the acute setting most commonly as a

result of embolism. Chronic occlusion most com-

monly results from thrombosis superimposed on

severe atherosclerotic involvement of the distal

abdominal aorta and common iliac arteries. Chronic

occlusion can produce Leriches syndrome (named

after Leriche who was the first to describe aortic

occlusion in 1940 [52]). Leriches syndrome refers to

the clinical syndrome that results from occlusion of

the infrarenal aorta. Patients typically have buttock

and thigh claudication, erectile impotence, atrophy of

the thigh musculature, and diminished femoral

pulses. Invariably patients with chronic occlusion

develop a rich collateral circulation. Arterial access

is limited in these individuals and Gd-enhanced

three-dimensional MRA (Fig. 13) can often be suf-

ficient for the primary assessment of the occlusion or

the assessment of graft repairs.

Branch vessels

Renal artery stenosis. Although all major abdom-

inal aortic branches can be well evaluated with

MRA, the renal arteries merit special discussion

because renal artery stenosis is particularly common

among patients with conditions affecting the aorta

and peripheral vessels. Up to 22% of patients with

infrarenal AAA (see Fig. 10) [53] and 45% of patient

with peripheral vascular disease [54] also have renal

artery stenosis. Renal artery stenosis frequently man-

ifests clinically as systemic hypertension (70% [55]),

which can often be reversed with renal revisualiza-

tion by balloon angioplasty, stenting, or vascular

surgery. Patients with renal artery stenosis can often

progress to end-stage renal disease if left untreated.

Atherosclerotic renal artery stenosis has a 35%

cumulative incidence of disease progression at

3 years and 51% at 5 years [56]. Renal artery

stenosis and disease progression are particularly

prevalent in diabetic patients, which comprise

roughly 50% of patients with renal artery stenosis

[55,56].

The preoperative identification of renal artery

stenosis is important and may augment or change

the surgical plan. Concomitant renal revascularization

during surgery for an AAA or aortoiliac occlusive

disease has been shown to result in significant

improvement or reversal of hypertension in most

patients [57,58].

The critical technical issue for achieving diag-

nostic renal MRA is spatial resolution, which ideally

is less than 1.5 mm in any single dimension [59].

The diagnosis that is especially challenging by Gd-

enhanced three-dimensional MRA is that of fibro-

muscular dysplasia because the changes may be

subtle (Fig. 14). Like atherosclerotic renal artery

stenosis, fibromuscular dysplasia can result in revers-

ible systemic hypertension. Patients with fibromus-

cular dysplasia, however, are typically young

women; whereas, atherosclerosis tends to occur in

older men [60]. Fibromuscular dysplasia typically

has a string of beads appearance, which may be

subtle on Gd-enhanced three-dimensional MRA

(see Fig. 14).


has been shown to be very accurate (sensitivity 91%

to 100%, specificity 89% to 100% [30,6166])

for the detection of greater than 50% diameter

stenoses of the main renal artery. The supplementa-

tion of Gd-enhanced three-dimensional MRA with a

postcontrast PC three-dimensional MRA can pro-

vide ancillary and often complementary informa-

tion, which improves the specificity of MRA for

the detection of renal artery stenosis [64,67,68]. On

PC MRA, spin dephasing invariably is present in

hemodynamically significant renal artery stenosis.

PC MRA relies on blood flow and the phase shifts

that it experiences while moving across a gradient

field. Because of the significant time requirements,

this technique was never popular for routine clin-

ical applications. Like the previously discussed

flow-based technique of TOF, PC MRA is also

prone to flow-related artifacts, such as intravoxel

dephasing about regions of arterial narrowing. After

contrast administration, however, arterial signal on

PC MRA is especially high [69]. Because the

technique is still sensitive to turbulent flow, intra-

voxel dephasing is still present on postcontrast

imaging and can be used to confirm the presence

of a hemodynamically significant stenosis (Fig. 15).

The performance of a Gd-enhanced three-dimen-

sional MRA first provides a road map for the

appropriate prescription of the phase-contrast

MRA over a more limited anatomic region and a

more time-efficient PC acquisition.

Renal transplant evaluation. Without the concerns

of nephrotoxicity associated with CT and conven-

tional x-ray angiography, Gd-enhanced three-dimen-

sional MRA can be a good method for the

postoperative assessment of renal transplant recipi-

ents (Fig. 16) [70,71]. Dual-phase Gd-enhanced

three-dimensional MRA easily can assess the

patency of the vascular anastomoses of the trans-

planted kidney.


can also be used to screen potential renal donors

and has been found to be comparable with CT


angiography [72,73]. The critical issue for the pre-

operative evaluation of potential donors is to deter-

mine the most suitable kidney for expedient and safe

removal [74,75]. Imaging is performed to identify the

number of renal arteries, the presence of early branch-

ing arteries, unsuspected renovascular disease, or any

parenchymal disease (eg, renal cell carcinoma) that

may influence the choice of kidney. On dual-phase

Gd-enhanced three-dimensional MRA, renovascular

anatomy and anomalies (eg, renal ectopia and retro-

aortic or circumaortic renal vein) readily can be

identified. Supernumerary renal arteries (Fig. 17)

are particularly common (27% of kidneys [75]) and

although not a contraindication for renal donation

Fig. 15. A 71-year-old man with a history of hypertension and diabetes mellitus. Gd-enhanced three-dimensional MRA ([A]

coronal maximum intensity projection) shows severe renal artery stenosis bilaterally (arrows). On phase-contrast three-

dimensional MRA (B), signal loss distal to the renal artery stenoses (arrows) is seen. This suggests that both arterial narrowings

are hemodynamically significant. Bilateral high-grade stenoses (75% on right and 80% on left) are noted on conventional x-ray

angiography (C). (Adapted from Hood MN, Ho VB, Corse WR. Three-dimensional phase contrast MR angiography: a useful

clinical adjunct to gadolinium-enhanced three-dimensional renal MRA? Mil Med 2002;167:3439; with permission.)


may affect the choice of kidneys for transplant or the

surgical approach.

MRA of the peripheral vessels

The primary indication of peripheral angiography

is for the evaluation of patients with suspected or

known peripheral arterial occlusive disease (PVOD).

Once again, patients typically have atherosclerosis.

Atherosclerotic PVOD is common and its prevalence

increases with age, affecting 20% of the population

over the age of 75 years, and is twice as common in

men [76,77]. Patients typically present with intermit-

tent claudication in calf, thigh, or buttocks, which is

exacerbated with exercise or ambulation. Claudica-

tion is indicative of a diminished arterial flow reserve

and an inability to augment blood flow for the

increased metabolic demands of exercise. Symptoms

are typically self-limiting but can significantly impact

an individuals quality of life. It is this latter effect on

Fig. 16. A 48-year-old man with autosomal-dominant polycystic kidney disease. On Gd-enhanced three-dimensional MRA ([A]

coronal maximum intensity projection [MIP], [B] oblique coronal subvolume MIP, [C] coronal multiplanar reformation [MPR])

a normal and patent arterial anastomosis (arrow) of the transplant kidney (t) with the external right iliac artery (a) is noted. On

MPR (C), overlapping signal from the right external iliac artery could be removed, enabling improved visualization of the

anastomosis (arrow).


quality of life that may bear significantly on the

decision to treat patients with intermittent claudica-

tion more aggressively [7880].

In more severe cases of PVOD, ischemia may be

limb threatening and therapeutic intervention (eg,

balloon angioplasty, stenting, bypass graft placement,

or amputation) is typically required. These patients

typically complain of claudication at rest or develop

nonhealing ulcers or even gangrene in the affected

leg. The therapeutic option depends on the location of

the disease, degree of stenosis, and length of the

lesion. Focal stenosis or occlusion (length less than

5 cm) of iliac artery, for example, responds well to

balloon angioplasty or stenting [8184]. Long iliac

artery stenoses or occlusions, however, have lower

long-term patency success rates and often require a

surgical bypass procedure. The decision to perform a

bypass procedure is always considered carefully

because failure of the bypass graft may result in a

higher level of amputation than initially required

[85,86].

Multistation Gd-enhanced three-dimensional MRA

(bolus-chase MRA)

Arteriography of patients with PVOD has been

particularly challenging because the length of the

vascular anatomy that must be illustrated (from at

least the aortic bifurcation to the level of the ankle or

distal trifurcation vessels) is extensive (eg, greater

than 1 m). This is required because lesions are

typically multiple and tandem lesions are common

(70%) [79]. Surgical planning requires comprehen-

sive evaluation of the entire arterial territory. Repair

of a popliteal artery stenosis, for example, may have

little effect if the patient has an ipsilateral iliac

occlusion and generally in-flow disease (aortoiliac)

is treated first. Arteriography is also necessary to

illustrate potential donor and recipient sites for poten-

tial bypass.

Using individual fields of view of roughly 40 to

50 cm, peripheral MRA typically requires the

imaging of three or more overlapping locations or

stations. Using two-dimensional TOF technique, pe-

ripheral MRA has been successful at detecting arterial

stenoses greater than 50% (eg, sensitivity 85% to

92%, specificity 81% to 88% [87,88]). Aside from its

obvious clinical benefits versus x-ray angiography,

two-dimensional TOF MRA has also been shown

to demonstrate suitable infrapopliteal bypass recipi-

ents not visible on conventional x-ray angiography

(so-called occult run-off vessels), which are critical

for bypass graft planning [87,89,90]. The lengthy

time requirements (often greater than 2 hours) and

numerous pitfalls, however, have limited the accept-

ance of two-dimensional TOF imaging for routine

clinical peripheral MRA. Gd-enhanced three-dimen-

sional MRA is much faster than two-dimensional

Fig. 17. Arterial-phase renal Gd-enhanced three-dimensional MRA using an automated bolus detection scheme that was

prescribed to monitor signal in a 3 3 3cm volume within the mid-abdominal aorta at the level of the renal artery origins.The breathheld renal Gd-enhanced three-dimensional MRA ([A] coronal maximum intensity projection [MIP], [B] oblique

subvolume MIP) in this 46-year-old male renal donor demonstrates supernumerary renal arteries (two right and three left renal

arteries). (Adapted from Ho VB, Foo TKF, Czum JM, et al. Contrast-enhanced magnetic resonance angiography: technical

considerations for optimized clinical implementation. Top Magn Reson Imaging 2001;12:28399; with permission.)


TOF MRA and has been found to afford improved

diagnostic performance for imaging the peripheral

vessels [1,2].

The most recent development of Gd-enhanced

three-dimensional MRA has been the development

of a technique called bolus-chase MRA [91102].

Bolus chasing has been used for many years in

conventional x-ray angiography. The basic con-

cept is to synchronize imaging with the arterial transit

of a single contrast bolus. In MR imaging, this

can be achieved by aligning table translation

with the arterial phase of an intravenously adminis-

tered Gd-chelate contrast bolus (Fig. 18). Typically, a

40-mL or 0.2-mmol/kg dose of Gd-chelate contrast

media is administered at a slow rate (0.3 to 0.8 mL/

second) [93,94,97]. The rate of contrast infusion

should be adjusted so that the length of the contrast

bolus duration matches roughly the time required to

acquire the critical k-space data for the three over-

lapping stations (Fig. 19). For example, if imaging

requires 100 seconds (30 seconds per station with

5 seconds between stations), a 40-mL dose injected at

0.4 mL/second results in a 100-second bolus dura-

tion. Recently, a biphasic injection rate (Fig. 20) has

also been shown to be effective [101].

To ensure that the bolus duration and injection

rates are standardized, another tact is to dilute a

0.2-mmol/kg dose to a fixed volume (eg, 45 mL)

and inject it at a fixed rate (eg, 1 mL/second) [94].

The use of a 0.2-mmol/kg dose ensures the ability

to perform another Gd-enhanced three-dimensional

MRA using 0.1 mmol/kg should a segment require

additional investigation. The actual technique varies

with the imaging capabilities of the scanner that is

being used [95]. Some scanners, for example, are

capable of partial Fourier imaging (ie, 0.5 excita-

tion or NEX) which allows for the foreshortening

of the bolus duration requirements (see Fig. 19).

Table translation can be performed manually by

simply unhooking the scanner table and sliding the

patient out of the bore [9193,97]; by using a

specially designed coil on platform apparatus (eg,

SKIP, Magnetic Momments, Bloomfield, MI [101];

or AngioSURF, MR Innovation, Essen, Germany

[99]); or using software that integrates imaging with

automated table motion (eg, MoBI-Track, Philips

Medical Systems, Best, The Netherlands [96]; and

SmartStep, General Electric Medical Systems, Wau-

kesha, WI [100]). The selection of table translation

method primarily depends on the individual MR

scanner because options vary between vendors. Each

vendor has similar variation in timing method options

(MR fluoroscopic trigger, automated bolus detection,

and so forth) and pulse sequence choice (partial

Fourier versus full Fourier, sequential versus centric

phase ordering, and so forth). Operators are advised

to familiarize themselves with the specific options

available with their scanner.

Although specific table motion technique and

timing methods may differ, the basic bolus-chase

MRA procedure remains fairly similar. All techniques

require a multistation localizer and matching precon-

trast coronal (or oblique coronal) three-dimensional

acquisitions at each location, typically using the same

table motion procedure as for the subsequent bolus-

chase MRA. As with single-station Gd-enhanced

three-dimensional MRA, the precontrast multistation

three-dimensional MRA serves to ensure appropriate

Fig. 18. Schematic of multistation peripheral bolus chase three-dimensional MRA. Imaging of the peripheral vasculature requires

the imaging of three contiguous anatomic regions: the aortoiliac segment (station 1); the femoropopliteal segment (station 2); and

the tibioperoneal or trifurcation segment (station 3). (From Ho VB, Choyke PL, Foo TKF, et al. Automated bolus chase

peripheral MR angiography: initial practical experiences and future directions of this work-in-progress. J Magn Reson Imaging

1999;10:37688; with permission.)


anatomic coverage and familiarize the patient with

the procedure. For bolus-chase MRA, however, the

precontrast images have an additional benefit in that

they provide a mask for image subtraction, which can

significantly improve arterial visualization in the

peripheral vasculature, especially in the calf

[94,103,104]. It is advisable to take efforts to min-

imize bulk patient movement between the precontrast

and postcontrast imaging by minimizing not only the

time between acquisitions but also securing the

patients lower extremities whenever possible.

Bolus-chase MRA has been shown to depict

reliably and accurately peripheral arterial stenoses

greater than 50% (eg, sensitivity 81% to 95%; spec-

ificity 91% to 98% [93,97,98]). This technique can be

helpful not only for the initial screening of patients

with suspected PVOD (Figs. 2022) but also for the

postoperative surveillance of graft patency (Fig. 23)

[91]. A known pitfall for the use of Gd-enhanced

three-dimensional MRA for postoperative evaluations

is its diminished ability to access stent graft patency

in patients who have undergone endovascular repair

with a stent that contains stainless steel (eg, Palmaz

stent) or cobalt-based alloy (eg, Wallstent). The

magnetic susceptibility from these stents results

in significant signal loss and precludes proper visu-

alization of the stent lumen even on Gd-enhanced

three-dimensional MRA [37]. Stents that are made

of nitinol wire (eg, Cragg stent, Cragg Endo ProSys-

tem 1, and Passager stent) and polytetrafluoroethy-

lene (eg, Hemobahn stent), however, have been

shown to have minimal artifacts on Gd-enhanced

three-dimensional MRA [37,38]. In patients who

have undergone endovascular therapy using nitinol-

or polytetrafluoroethylene-based stent grafts, Gd-en-

hanced three-dimensional MRA can be a suitable

modality for the assessment of graft patency.

Current bolus-chase MRA methods are reliable

for imaging the peripheral vessels through the level

of the trifurcation. Visualization of the distal run-off

vessels, however, is often variable [15,94,102]. This

results from a combination of issues. Timing with

Fig. 19. Schematic of arterial-phase imaging of the contrast bolus at stations 1 through 3. The relative timing of the data

acquisition of the half-Fourier three-dimensional gradient echo sequences is diagrammed as A (station 1), B (station 2), and C

(station 3) with the center lines of k-space for each marked by diagonal lines. Note that the use of sequential view ordering for

station 1 and reverse sequential view ordering for station 3 results in a shortened duration required for central k-space coverage

during the arterial phase (ie, shortened critical arterial imaging period). (From Ho VB, Choyke PL, Foo TKF, et al. Automated

bolus chase peripheral MR angiography: initial practical experiences and future directions of this work-in-progress. J Magn

Reson Imaging 1999;10:37688; with permission.)


current techniques is only provided for the initial

station and timing for arterial-phase imaging of the

terminal station (third or fourth station) is under-

standably variable. In addition, imaging of the ter-

minal station begins only after completion of imaging

for the proximal stations. These concerns are am-

plified in patients with PVOD because they often

have slow flow or significant intervening aneurismal

or occlusive disease in which the transit time for

contrast media can be highly variable and often

asymmetric. In patients with limb-threatening ische-

mia, the identification of recipient run-off vessels is

critical for successful bypass grafting. Visualization

of distal run-off vessels can be ensured by the

preliminary performance of a traditional two-dimen-

sional TOF below the knee (especially foot). Most

patients with PVOD present with milder disease and

intermittent claudication, however, and therapy is

typically conservative (eg, smoking cessation and

regular exercise) and noninvasive [77]. In these

individuals, a three-station bolus-chase MRA typ-

ically can provide sufficient diagnostic information

for patient management.

Future directions

In addition to the use of higher field strength MR

scanners (eg, 3 T) and high performance gradients,

there are a variety of new techniques that may

significantly improve the speed of MRA data acquisi-

tion. New parallel imaging techniques, such as simul-

taneous acquisition of spatial harmonics [105] and

sensitivity encoding [18], use the spatial-encoding

properties of multiple phased-array coil elements to

reduce the number of requisite spatial-encoding

views. These can result in a significant reduction

(twofold or threefold) in scan time but at the cost of

signal-to-noise (approximately equal to the square

root of the scan time reduction factor). This is

especially promising for MRA of the abdominal aorta

Fig. 20. A 60-year-old man with atherosclerosis. Coronal

maximum intensity projection from a three-station Gd-en-

hanced bolus chase three-dimensional MRA using a biphasic

injection provided sufficient Gd for good visualization of the

abdominal aorta and iliac arteries (station 1); the femoropo-

pliteal arteries (station 2); and the trifurcation vessels (station

3). The contrast was injected intravenously at 0.6 mL/second

for initial 20 mL and at 0.4 mL/second for the remaining

20 mL. This was followed by a saline flush at 0.4 mL/sec-

ond. This provided a sufficiently long bolus to match the

100 seconds required for data acquisition (30 seconds for

each of three stations plus 5 seconds for each of the two table

movements between stations). Although, a single slow injec-

tion rate of 0.5 mL/second provides near equivalent bolus

duration, the slightly faster initial rate of injection provides a

higher concentration of Gd for improved visualization of the

abdominal aorta. The decrease in the injection rate for the

later half of the bolus ensures sufficient arterial Gd con-

centrations during the imaging of the distal trifurcation ves-

sels. On this examination, fusiform dilatation of the distal

abdominal aorta and common iliac arteries and a moderate

stenosis in the distal right external iliac artery were noted.


Fig. 21. A 65-year-old man with right lower extremity

claudication. Coronal maximum intensity projection from a

three-station Gd-enhanced bolus chase three-dimensional

MRA demonstrates bilateral narrowing of the common iliac

arteries, which is much worse and high-grade in the right

common iliac artery (arrow). The remaining arterial seg-

ments were patent.

Fig. 22. A 53-year-old man with bilateral occlusion of the

proximal superficial femoral arteries. Coronal maximum

intensity projection from a three-station, Gd-enhanced bolus

chase three-dimensional MRA demonstrates not only the

occlusions but also the reconstitution of superficial femoral

artery at the mid-thigh level (arrows). Note the numerous

collateral vessels in the thigh about the regions of occlusion.


where imaging speed can be used to acquire a larger

number of partitions for improved anatomic coverage

or more importantly higher spatial resolution. Alter-

natively, the speed can be used to achieve improved

temporal resolution and an increase in the number of

phases during a breathhold.

There also has been the introduction of hybrid

bolus-chase schemes. Maki et al [106] have demon-

strated the feasibility of an interleaved image acquisi-

tion in which a two-dimensional MRA is performed at

the thigh station (station 2) between two three-dimen-

sional MRAs (abdomen-pelvis, station 1; and calf,

station 3). This expedites imaging of the calf and can

minimize the concerns related to venous contamina-

tion. Another approach was described by Foo et al

[107] in which the three-dimensional acquisitions

were segmented such that only the central portions

of k-space (low spatial frequency data) are acquired

during an initial pass during the arterial phase of the

contrast bolus and remaining k-space data are

acquired later during a second pass during the delayed

phase. This technique called segmented volume

acquisition (shoot and scoot) enables more efficient

data acquisition because the time-critical portions of

k-space (ie, the center of k-space) are preferentially

acquired during the arterial phase of the bolus and

provides high spatial resolution data sets (Fig. 24).

Another innovation is time-resolved two-dimen-

sional [102] and three-dimensional (eg, TRICKS

[108]) digital subtraction angiography. These tech-

niques are somewhat similar to the multiphase three-

dimensional MRA scheme previously discussed

under timing methods but require specialized off-line

computer equipment or software, which are not yet

commercially available. Like multiphase three-

dimensional MRA, MR digital subtraction angiogra-

phy may have limited use for imaging regions where

breathhold acquisitions are desired (ie, aortoiliac

region) but for imaging of the thigh, calf, and feet,

these techniques may be very helpful.

There have also been technical improvements

that relate to the pulse sequence. Until recently,

Gd-enhanced MRA has typically been performed

using a T1-weighted spoiled fast gradient echo pulse

sequence. Foo et al [109] recently reported success

using a steady-state free precession (TrueFISP, Sie-

mens Medical System; FIESTA, General Electric

Medical Systems; balanced FFE, Philips Medical

Systems) for Gd-enhanced MRA. On steady-state

free precession, vascular signal is a function of the

ratio of tissue T2 to T1 relaxation times. This effect

can provide additional vascular signal contributions

that may improve luminal visualization despite low

Gd concentrations (Fig. 25).

Fig. 23. Adult patient with atherosclerosis and a history of

abdominal aortic aneurysm (AAA) repair. The Gd-enhanced

bolus chase three-dimensional MRA demonstrates a residual

AAA in the proximal abdominal aorta but the aortobifem-

oral graft in the distal abdominal aorta is intact. Mild

irregularity is noted in the distal superficial femoral arteries

bilaterally consistent with mild to moderate disease.


The final developments have been in the contrast

agents [110]. There are a variety of new contrast agents

that have unique binding to large molecules like

albumin (eg, MS-325, Epix Medical, Cambridge,

MA) or inherently large structure (eg, NC100150,

Amersham Health, Buckinghamshire, United King-

dom) and are retained within the blood pool for

a prolonged period of time, whereas conventional

extracellular Gd-chelate contrast agents leak out

of the vessels within 2 to 3 minutes of venous

injection. Blood pool agents like conventional extrac-

ellular Gd-chelates rely on their T1-shortening effects

for improved signal on contrast-enhanced MRA. The

prolonged window of arterial signal improvement

affords a large temporal window for high spatial

resolution scanning. The main limitation of this tech-

nique is the significant venous enhancement that is

typically present after the initial 1 to 2 minutes. Given

the systemic nature of atherosclerosis, however, the

use of blood pool agents may be beneficial for whole-

body screening. A hybrid contrast agent called

gadobenate dimeglumine (MultiHance, Bracco Diag-

nostics, Milan, Italy) has been approved for use in

Europe and has some protein binding, which has been

shown to improve arterial signal-to-noise significantly

when compared with traditional Gd-chelate contrast at

a comparable Gd dose of 0.1 mmol/kg [111]. Recently,

Ruehm et al [99] demonstrated the feasibility of

performing a five-station bolus-chase MRA using a

0.3-mmol/kg dose of gadobenate dimeglumine.

Any of the aforementioned improvements may

significantly expand the current role and diagnostic

accuracy of aortic and peripheral MRA. Specifically,

they may improve the reliability of infrapopliteal

imaging and even renal imaging during a bolus-

chase MRA. A high percentage of patients with

peripheral vascular disease have renal artery steno-

sis, yet most of the current bolus-chase techniques

fail to produce diagnostic-quality images of the renal

arteries reliably. Although this may be secondary to

the height of the patient (insufficient superior ana-

tomic coverage of overlapping stations), more com-

monly time considerations often result in the use of

Fig. 24. Segmented volume bolus chase acquisition (shoot

and scoot). In this acquisition scheme, the low spatial fre-

quency data for each of the three stations are acquired during

the arterial phase of the bolus. Imaging was initiated by an

automated bolus detection algorithm that monitored the

contrast bolus arrival in the mid-abdominal aorta, which

represented the center of the proximal station. After

acquiring the critical central k-space data, the algorithm

returns the table to the proximal stations so that remaining

high-spatial frequency data can be acquired to complete

data acquisition for each station. By segmenting k-space

data acquisition into two separate passes, this technique

shortens the time requirements for the duration of the arterial

enhancement and minimizes the time delay before

imaging of the terminal station. This in turn enables the

use of faster injection rates for improved arterial visual-

ization of the infrapopliteal arteries. (Adapted from Ho VB,

Foo TKF, Czum JM, et al. Contrast-enhanced magnetic

resonance angiography: technical considerations for opti-

mized clinical implementation. Top Magn Reson Imaging



thicker partitions (eg, 2.5 to 3 mm thick partitions)

in the aortoiliac station such that renal artery assess-

ment is suboptimal.

Summary

Contrast-enhanced MRA can be an accurate and

reliable method for the arterial evaluation of the

abdominal aorta and peripheral vessels. This tech-

nique can be adapted for a variety of anatomic

regions. The basic issues relate to proper synchro-

nization of imaging with peak arterial enhancement

and to optimization of voxel dimensions for adequate

depiction of the arterial structures.

Acknowledgments

The authors thank Michael Schweikert, RT(R),

(MR), lead MR technologist at Doylestown Hospital,

and Maureen N. Hood, BSN, RN, RT(MR), of the

Uniformed Services University of the Health Sci-

ences for their invaluable assistance in the preparation

of this manuscript.

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