Cardiovascular Magnetic Resonance Imaging at 3.0 Tesla

Cardiovascular Magnetic ResonanceImaging at 3.0 Tesla

M. Gutberlet1, P. Freyhardt1, B. Spors1, K. Schwinge1, M. Grothoff1, R. Noeske2, T. Niendorf3, R. Felix1

1Department of Diagnostic Radiology and Nuclear Medicine, Charite, Campus Virchow Klinikum, University Medicine Berlin, Berlin,Germany2GE Healthcare Technologies, Berlin, Germany3Applied Science Laboratory (ASL), GE Healthcare Technologies, Boston, MA, USA

Correspondence to:Matthias Gutberlet, MDDepartment of Diagnostic Radiology and Nuclear Medicine, Charite, Campus Virchow Klinikum, University Medicine Berlin, AugustenburgerPlatz 1, 13353 Berlin, GermanyTel: +49 30 450557001; Fax: +49 30 450557901; E-mail: [email protected]

Key words: Cardiovascular MRI, 3 Tesla, 3.0 Tesla, parallel imaging, tagging, viability.

Abstract

Cardiovascular MR imaging often requires high temporal

and spatial resolution, especially in order to acquire data

about cardiac function. Furthermore, the current results at

1.5 T for coronary artery imaging or plaque imaging are

still not satisfying even with the use of the latest technology.

Therefore, cardiac imaging inherently demands high sig-

nal-to-noise (SNR) and contrast-to-noise ratios (CNR) and

hence may benefit from higher magnetic field strengths.

However, higher magnetic field strengths do not inevitably

improve the image quality for all cardiac imaging techni-

ques as compared with their 1.5 T counterparts. At higher

magnetic field strengths one has to cope with increased

field inhomogeneities, longer T1, shorter T2* relaxation

times and radiofrequency power deposition constraints,

which require further methodological developments. Initial

studies using 3.0 T whole-body scanners for cardiac ima-

ging revealed that optimized steady-state free precession or

spin-echo sequences meet the expected SNR increase at

3.0 T but showed different results for CNR. These results

are especially encouraging for cardiac tissue characteriza-

tion at 3 T together with the evolving parallel imaging

techniques. This review focuses on the feasibility of cardiac

MR imaging at high magnetic field strengths. The pros

and cons of cardiac imaging at 3.0 T vs. 1.5 T are

examined and technical solutions are discussed.

Introduction

Limitations in cardiac MR imaging are very often caused

by temporal and/or spatial resolution constraints. There-

fore, a high signal-to-noise (SNR) and/or contrast-to-noise

ratio (CNR) is mandatory (1). One of the main determi-

nants of SNR is the static magnetic field strength (B0) so

that whole-body MRI at 3.0 T may help to overcome

some of these limitations. Higher static magnetic field

strengths (B0) give higher SNR; but the radiofrequency

(RF) field (B1) of both transmit and receive coils is much

more dependent on the sample, i.e. patient (1). In addition,

RF power limitations due to specific absorption rate (SAR)

constraints have to be taken into account (2) for human

studies. Increased susceptibility related B0 field inhomo-

geneities may result in additional artefacts. There are only

few cardiovascular MR (CVMR) studies in which the SNR

has been compared at different field strengths (1, 3) while

the number of high-field 3.0 T whole-body MR systems is

constantly increasing. The majority of studies have been

reported for research platforms (1, 4), but initial studies

recently described the use of 3.0 T in cardiac imaging

using commercially available systems, together with com-

mercial hardware and software (5, 6). All studies demon-

strated a significant SNR increase (3, 4, 6–10) but also

reported some image quality problems associated with B1field inhomogeneities in different regions of the heart (3, 5).

A pronounced sensitivity to susceptibility artefacts, which

might result in a SNR decrease and an image quality

degradation were also reported for 3.0 T CVMR studies

(3, 5). The aim of this review is to give a short overview of

the potential benefits, current results and remaining tech-

nical challenges for cardiovascular MR imaging at 3.0

Tesla.

Potential benefits of 3.0 T in cardiac imaging

Cardiac MRI has been proven to be the gold standard for

the depiction of cardiac anatomy under several circum-

stances and also for the evaluation of cardiac volumes and

mass, especially for the right ventricle or in pathological

geometries of the left ventricle (11, 12). 18F-FDG PET is

considered to be the gold standard for myocardial viability

assessment from a nuclear medicine point of view. With

the widespread use of myocardial viability assessment using

the ‘delayed contrast enhanced’ techniques (13) where

3/2004 n IMAGING DECISIONS

several studies suggested advantages over nuclear medicine

techniques (14, 15), MRI becomes more and more the gold

standard for viability assessment. Several encouraging re-

sults were reported for the analysis of myocardial perfusion

using MRI (16). This approach is not been considered as

being clinical routine and hence has to prove its reliability

in larger clinical trials, including direct comparison with

established nuclear medicine techniques like technetium

SPECT or PET (17). Clinical routine coronary MR angi-

ography remains technically challenging, which renders

CT a superior competitor, which is mainly due to the

improvements in the spatial resolution and temporal

resolution of multidetector computed tomography

(MDCT) (18). The even more challenging attempt of MR

coronary plaque imaging remains very difficult with the

current 1.5 T systems. All these applications demand faster

imaging, if possible almost ‘real-time’, and a higher spatial

resolution. It has been demonstrated that high magnetic

field strengths improve the baseline SNR in cardiac ima-

ging (3, 6) as compared with the conventional 1.5 T ap-

proach. This merit of high-field imaging can be translated

into an enhancement in the spatial and temporal resolu-

tion. It has been also predicted that high-field strengths

promise to reduce the noise amplification in parallel

imaging (Fig. 1a and b). The need for reducing the RF

power deposition at high magnetic fields accentuates the

complementary advantage of parallel imaging (19).

Thus high-field strengths together with tailored imaging

strategies offer the potential to overcome physiological

motion and SNR constraints of current cardiovascular MR

imaging approaches (3, 6, 19).

Problems and potential solutions

Cardiac imaging at 3.0 T is different from imaging at

1.5 T due to SAR limitations, increased susceptibility

artefacts (Fig. 2), differences in tissue relaxation (Fig. 6),

and RF homogeneity issues (Fig. 7) (20).

SAR limitations

State of the art routine cardiac imaging procedures for the

assessment of ventricular function andwall motion are based

on steady-state free precession (SSFP) imaging techniques (3,

19, 21). These sequences offer a high intrinsic CNR between

myocardial muscle (Figs 1 and 2) and blood within the

ventricles at a high SNR, but need a short TR, which is

achieved by using themaximumamplitude of theRF fieldB1

c d

a b

j Fig. 1. SSFP 4 chamber view (4 cv) at 3.0 T without (a)/with (b) the use of parallel imaging (ASSET) – acceleration factorR ¼ 2. The acquisition time (a) was 14 s and (b) 7 s. The following imaging parameters were used for the SSFP (FIESTA)sequence: TR ¼ 3.4 ms (minimum), TE ¼ 1.5 ms, slice thickness ¼ 8 mm, field of view ¼ 350 mm, PFOV ¼ 1.0,matrix ¼ 2242, flip angle ¼ 30�, views per segment ¼ 12, retrospective gating, 50 phases per RR interval, TA ¼ 16heartbeats. (c, d) Short axis fast spin-echo ‘black blood’ images which demonstrate the high image quality obtained at 3.0 T.A double inversion recovery sequence (a) and a triple inversion recovery sequence (b) were used together with an eightchannel cardiac phased array coil.

2 4 n C A R D I A C I M A G I N G A T 3 . 0 T

IMAGING DECISIONS n 3/2004

(19), which keep the excitation pulse duration as short as

possible. Schar et al. (19) showed SAR constraints at 3.0 T.

The SAR limit of 4 W/kg (2) has been already exceeded at

lowB1 field strengths. In consequence the repetition timeTR

was increased. This approach can result in suboptimal image

quality at 3.0 T due to the increased sensitivity for B0 in-

homogeneities and off-resonance effects (3, 19) (Fig. 2A).

One solution to keep TR values as short as at 1.5 T is the

reduction of the flip angle (Fig. 2), which has already been

described by Schar et al. (19) with a reduction from 54� at1.5 T to 42� at 3.0 T, by Hinton et al. from 60� at 1.5 T to

36� at 3.0 T (6) and by Gutberlet et al. (3) from 45� to 30�(Fig. 2) depending on the different scanners used for com-

parison. Fortunately, it could be shown that the optimal

CNR of cardiac muscle and blood within the ventricles at

3.0 T is achieved also at lower flip angles (19) as compared

with 1.5 T. Another possibility to keep TR values as short as

at 1.5 T without exceeding SAR limits is been offered by the

use of parallel imaging techniques, which reduces the

number of applied RF pulses (3). The application of variable

flip angle and hyperechoes is another technical approach to

offset SNR limitations (22). Busse demonstrated a 75%

reduction in the RF power deposition using fast spin echo

sequences operated at high-field strength together with a

modulated angle refocusing train method to reduce power

and prolong relaxation, and a smooth transition between the

high and low flip angle regimes (23), without impairing the

image quality.

Susceptibility artefacts and RF homogeneity issues

Shorter T �2 values which can be attributed to larger static

magnetic field (B0) inhomogeneities within the left ventricle

(LV), which are mainly caused by the heart–lung interface

(24) and mostly occur close to the posterior coronary vein

(Fig. 2) of the LV (3, 4), make fast imaging of the heart

more challenging (4, 19, 25) at 3.0 T. The most common

artefacts are off-resonance artefacts, which manifest itself

by ‘dark band’ coherence patterns. These artefacts (Fig. 2)

can be minimized by reducing the repetition time (TR), or

by improving the B0 homogeneity using sophisticated

shimming algorithms (3, 19). B1 field inhomogeneities in

spin-echo sequences at 3.0 T tend to deteriorate image

quality depending on patients shape, especially at the right

ventricle (3). These ‘shading artefacts’ occur at the free wall

of the right ventricle, close to the diaphragm (Fig. 7). These

artefacts are already known from 1.5 T (5, 26), but seem to

be pronounced at 3.0 T due to higher B1 field inhomo-

geneities. This renders the application of spin-echo-based

imaging techniques in some cases unsuitable for the

assessment of the free wall of the right ventricle (Fig. 7).

This is in alignment with the observation that the SNR

obtained for the LV (especially the septum) can show lower

values as compared with other regions (5). The application

of adiabatic RF-pulses holds the promise to overcome

B1-inhomogeneity related image artefacts obtained for

spin-echo-based cardiac imaging at 3 T (3).

Differences in tissue relaxation

While SSFP imaging sequences need a reduction in the flip

angle at 3.0 T due to SAR limitations, ‘black blood’ ima-

ging techniques require an increase in the preparation time

(TI) to null signal contributions from blood. The increase

in TI can be as large as 30%, due to longer T1 relaxation

times published by Noeske et al. (3, 4). This finding has

a b

j Fig. 2. Two chamber, short-axis view obtained with 2D SSFP (FIESTA) at 3.0 T. The volunteer data show a susceptibilityartefact at the inferiolateral wall (white arrow) of the left ventricle (LV). Due to a TR prolongation (a) the artefact is morepronounce for higher flip angles [(a) flip angle ¼ 30�; (b) flip angle ¼ 45�].

C A R D I A C I M A G I N G A T 3 . 0 T n 2 5


implications not only for native imaging but also for con-

trast-enhanced techniques like ‘delayed enhancement’,

which has been used for the assessment of myocardial

viability. A higher CNR has been obtained for contrast-

enhanced cerebral MRI due to lengthening in T1. In

analogy to this observation it is to be expected that car-

diovascular ‘delayed enhancement’ studies might require

approximately half of the dose of Gd-DTPA at 3.0 T to

achieve the same image quality derived at 1.5 T (Fig. 6).

Furthermore, the change in the contrast sensitivity might

also result in different inversion times to ‘null’ the myo-

cardium at high-field strengths.

Current studies

Cardiac anatomy

Several cardiac imaging studies consistently demonstrated

an SNR increase at 3.0 T versus 1.5 T (1, 3, 4, 6, 19). The

SNR increase has a large variability ranging from 20% (6)

to 160% (3) depending on the RF coil configuration (6) and

imaging sequence type (3, 19, 20) used. This affords image

quality improvements and facilitates a higher spatial and

temporal resolution (6, 19, 20), which may improve the

depiction of smaller anatomical structures like valves or

trabeculae of the right ventricular musculature (6).

Greenman et al. (5) and others (3) also found a decrease in

image quality at specific regions of the heart depending on

patients shape, i.e. at the septum (5) or the free wall of the

right ventricle (3) when using spin-echo imaging tech-

niques. This effect can be attributed to B1 field inhomo-

geneities at 3.0 T (Fig. 7).

Cardiac function

Various studies suggested that functional imaging tech-

niques, like SSFP sequences (3, 6, 7, 19) or myocardial

tagging (8) (Fig. 3) benefit from the application of higher

magnetic field strengths due to the SNR increase, which

in return allows higher temporal and spatial resolution.

The SNR increase also fosters the use of higher accel-

eration factors in parallel imaging (Fig. 1) as compared

with 1.5 T. As the SNR of a parallel imaging study is

always reduced as compared with an unaccelerated study

obtained using the same coil array and identical imaging

parameter. The scaling of SNR maybe expressed as

follows (27):

SNRaccelerated ¼ SNRunaccelerated=gffiffiffiffi

Rp

j Fig. 3. Short axis view cardiac images obtained with a tagging sequence at 1.5 T (a, c) and 3.0 T (b, d) at end-systole (a,b) and end-diastole (c, d) using the same volunteer. At 1.5 T the tags are hardly visible at end-diastole (c) while the tags arevery well preserved at 3.0 T (d).



where R represents the acceleration factor chosen and g the

so-called geometry factor, which is among other factors

depending on the coil design used. The SNR increase

obtained for SSFP sequences at 3.0 T overcompensates the

signal loss due to the use of parallel imaging with an

acceleration factor of R ¼ 2 (19).

Myocardial perfusion, viability contrast enhancement

To date there are no studies available concerning the use of

3.0 T magnetic field strengths to assess myocardial perfu-

sion, viability or other post-contrast studies. Brain studies

at 3.0 T revealed (22) that the prolongation of T1 relaxa-

tion times halves the amount of contrast agent, which is

needed to achieve the same CNR and image quality at

3.0 T as compared with 1.5 T (Fig. 6). Furthermore, for

delayed enhancement examinations different inversion

times have to be used to ‘null’ the myocardial signal.

Myocardial perfusion deficits can also be detected without

the need of contrast agent applications. This approach

exploits the blood oxygenation level-dependent (BOLD)

contrast, which has been successfully applied in the heart

and for the assessment of the endothelial function at 1.5 T

(28, 29). BOLD imaging at 3.0 T keeps the promise to

benefit from the higher SNR and the increased sensitivity

for microscopic susceptibility gradients.

Coronary artery imaging

The first preliminary report, on in vivo coronary artery MR

imaging at 3.0 T was published in 2002 by Stuber et al.

(10). For this human study free-breathing 3D Navigator

techniques where applied. Although this study was not

j Fig. 5. Coronary artery images obtained at 3.0 T using an eight-channel-phased array surface coil together with a 3D-fatsat SSFP (FIESTA) MR-sequence (30) showing the RCA (a), the left main (LM) and its separation into LAD and CX (bluebox), its magnification (b) and the resulting 3D-MIP (c) in a volunteer. Images (d, e) illustrate the RCA at its origin (d) and atthe magnification of the area of the ‘crux cordis’ (e) as well as the resulting 3D-MIP (f) reconstruction of the same volunteer.

j Fig. 4. Right coronary artery image acquired with a 2Dspiral imaging technique at 3.0 T displaying the origin andthe proximal segment of the RCA. The image also demon-strates distal branches at the ‘crux cordis’ (white arrow). Ao,ascending aorta; RA, right atrium; LA, left atrium.



aiming at a direct comparison between 1.5 and 3.0 T

higher mean SNR and a slightly higher visible vessel length

has been demonstrated at 3.0 T. A recently published

study by Nayak et al. used a ‘real-time’ approach to image

coronaries at 3.0 T (20), including encouraging results with

regard to the observed SNR and CNR improvements

(Fig. 4). Robust breath-held contrast-enhanced magnetic

resonance angiography (MRA) at 3.0 T can be completed

in two to three breath-holds covering the main branches of

the coronary arterial systems. Niendorf et al. (30) used fat-

suppressed ECG gated 3D-SSFP (FIESTA) sequences at

3.0 T combined with an eight-channel cardiac-phased

array prototype coil.

Short breath-hold periods were achieved by turning

the higher SNR/CNR at 3.0 T into shorter acquisition

times due to the application of parallel imaging without

impairing the image quality (Fig. 5). It has been also

reported that the SNR improvement afforded by a 3.0 T

field strength coupled with the enhanced CNR between

the blood pool and the myocardium may provide

benefits for clinical coronary MR angiography (30). As

larger acceleration factors are explored with high-chan-

nel MR systems, the benefits of high-field strengths for

coronary artery imaging will become even more pro-

nounced through an increase in the volume coverage,

which may permit the visualization of the entire

Gd-DTPAGd-DTPA0.2 mmol / kg / BW0.1 mmol / kg / BWa b

3.0 T 1.5 T

j Fig. 6. Delayed contrast-enhancement images of the same patient with transmural septal infarction obtained at 3.0 T (a)vs. 1.5 T (b). The delineation of the scar tissue is possible at a high quality for both field strengths. Due to a prolongation of T1

relaxation at 3.0 T, half the amount of Gd-DTPA (Magnevist�; Schering AG, Berlin, Germany) is necessary to achieve thesame image quality at 3.0 T.

j Fig. 7. Images derived from a triple inversion recovery MR sequence using a transverse orientation in a volunteer at 1.5 T(top row) and 3.0 T (bottom row). The following parameters were applied: TR ¼ 2 RR intervals, TE ¼ 42.0 ms, slicethickness ¼ 8 mm, field of view (fov) ¼ (350 · 350)mm2, PFOV ¼ 0.75, TI ¼ 150 ms, matrix ¼ 2562, ETL ¼ 32, receiverbandwidth ¼ ±62.5 kHz, TA ¼ 16 heartbeats. The images show a significantly higher SNR at the myocardium of the leftventricle at 3.0 T as compared with 1.5 T. Higher B1 field inhomogeneity (RF profile) in 3.0 T images on the other hand,causing ‘shading’ artefacts at the free wall of the right ventricle, can also be observed. The signal intensity at the septum islower than at the lateral wall of the LV (3, 5).



coronary tree within a single breath-hold acquisition.

The access to higher accelerations would also (i) further

reduce RF deposition constraints at high-field strengths

and (ii) would serve to enhance the immunity to

physiological motion by using shorter acquisition win-

dows. This offers the potential to integrate breath-held

coronary MRA into clinical cardiac examinations for the

non-invasive detection of coronary artery anomalies or

luminal stenoses in the proximal parts of the coronary

arteries at high magnetic field strengths.

Conclusion

Clinical cardiac MR imaging is very well established at

1.5 T and has been shown to be feasible in volunteers at

3.0 T using commercial whole-body scanners (1, 3, 6–8,

10, 19, 20, 31). The higher magnetic field strength requires

methodological developments and imaging protocol

adjustments due to SAR limits (3, 6, 19), T1 relaxation time

prolongation, T2* shortening as compared with 1.5 T

applications. All applied imaging techniques revealed a

significant SNR increase. In the majority of cases a CNR

increase was also obtained, which primarily improved the

image quality at 3.0 T as compared with 1.5 T. Both

effects allowed a higher spatial and temporal resolution,

which is especially beneficial in conjunction with the use of

parallel imaging. However, increasing susceptibility arte-

facts in SSFP sequences as well as artefacts due to B1-field

inhomogeneities depending on patients shape (3, 5) were

observed, which may also result in a degradation in the

image quality derived from cardiac imaging at 3.0 T.

The first group of artefacts can be compensated by

advanced shimming (3, 19), while it is more challenging to

address the B1-field inhomogeneities (3). First reports on

coronary artery imaging in humans at 3.0 T hold the

promise to integrate coronary magnetic resonance angi-

ography (CMRA) into clinical cardiac examinations for the

non-invasive detection of coronary artery disease (10, 20,

30). Comprehensive studies dealing with the use of cardiac

MRI for the detection of perfusion deficits and the

assessment of myocardial viability have not been published

yet. It is to be expected that these approaches will also

profit from the higher field strength. In summary, further

volunteer and patient studies are required to prove the

potential clinical advantages and extra diagnostic value of

cardiac MRI at 3.0 T over current 1.5 T applications.

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Cardiovascular Magnetic Resonance Imaging at 3.0 Tesla