MAGNETIC RESONANCE ANGIOGRAPHY

AND ITS ARTIFACTS

MR angiography It is a type of magnetic resonance imaging(MRI) scan that uses a magnetic field and pulses of radio wave energy to provide pictures of blood vessels inside the body.

Advantages of MR angiography: compared with catheter angiography, MRA is less invasive, less expensive, and faster to perform. For conventional angiography, catheter is inserted though the patient groin and threaded up into the artery in the brain.mra does not require this catheter. As a result, it eliminates related complications such as possible damge to an artery.

Disadvantages of MR angiography: it does not depict small vessels or extremely slow blood flow as well as conventional angiography dose.

MR ANGIOGRAPHY IMAGES

CONTRAST ENHANCED MR ANGIOGRAPHY It is a technique involving 3D spoiled gradient-echo(GE) sequences, with administration of Gd-based contrast. It can be utilised to assess vascular structures of almost any part of the body.

It key features are as follows: T1- weighted spoiled gradient-echo(flip angle 25 degree- 50 degree allows T1- weighting)

central k-space acquisition corresponding to arterial phase of the study maximizes preferential visualization arteries

use of Gd- based contrast to shorten T1-interval of the blood which appears bright as a result

Black blood imaging In this techniques blood appears as black as this are spin-echo based sequences techniques. However proton in the flowing blood usually do not receive either 90 degree or 180 degree pulse. Hence signal is not produced and flowing blood appear as dark. Slow flow and clots can produced bright signal because they received both 90 degree or 180 degree pulse.

Blood flow- black image Slow flow and clot- bright image

BLACK BLOOD IMAGING

Bright blood imaging In this type of imaging blood appear bright most of this are gradient echo sequences. In this GRE sequences excitation pulse is slice selected but the rephrasing which is done by gradient rather than 180 degree pulse is not limited to the slice of interest and is applied to whole imaging volume therefore a flowing proton will receive an excitation pulse is rephrased regardless of its slice position.

Blood flow- bright image Slow flow and clot- black image

BRIGHT BLOOD IMAGING

Non contrast enhanced MR angiography

It is performed in several ways including: 1.Time of flight angiography(TOF- MRA) 2. Phase contrast angiography(PC- MRA) Generally, these techniques are time- consuming as compared with contrast enhanced MR angiography.

Non- contrast Contrast image image

Time of flight angiography(TOF) It is an MRI technique to visualize flow within vessels, without the need administer contrast. It is based on the phenomenon of flow- related enhancement of spins entering into imaging slice. As a result of being unsaturated, these spins give more signal that surrounding stationary spins.

TOF can be of three types : 1. 2D time of flight 2. 3D time of flight 3. MOTSA(Multiple overlapping thin slab (acquistion)

TIME OF FLIGHT IMAGES

2D time of flight With 2-D TOF, multiple thin imaging slices are acquired with a flow compensated gradient- echo sequences. These images can be combined by using a technique of reconstruction such as maximum intensity projection(MIP), to obtain a 3-D image of the vessels analogous to conventional angiography.

3D time of flight With 3-D TOF, a volume of images is obtained simultaneously by phase- encoding in the slice-select direction. An angiographic appearance can be generated using MIP, as is done with 2-D TOF.

2D- TOF IMAGE 3D- TOF IMAGE

MOTSA(multiple overlapping thin slab acquisition)

Combined advantages of 2D(large cover area) and (3D high resolution). Imaging volume is divided. these slabs are combined to single volume data.

MOTSA(MULTIPLE OVERLAPPING THIN SLAB ACQUISITION) MRA IMAGE

Phase contrast imaging(PC-MRA) It is an MRI technique that can be used to visualize moving fluid. It is typically used for MR venography as a non IV-contrast requiring technique.

Spin that are moving in the same direction as a magnetic field gradient develop a phase shift that is proportional to the velocity of the spins. This is the basis of phase-contrast angiography.In the simplest phase- contrast pulse sequences, bipolar gradients are used to encode the velocity of the spins.

Stationary spins undergo no net change in phase after the two gradients applied. Moving spins will expierence a different magnitude of the second gradient compared to the first, because of its different spatial position. This result in net phase shift.

PHASED CONTRAST MRA IMAGES

ECG- Gated FSE MRA

In the ECG gated fast spin echo techniques images of the vessels are acquired during diastole and systole.

On the diastole image both artery and vein are bright while on the systole image the artery is dark due to slow flow .

The systole images are subtracted from diastole images giving the bright blood arteriography. The vein and background are subtracted.

Advantages of this technique involves:1. relatively short time2. sensitivity to slow flow3. ability to acquire in coronal plane parallel to the vessels.4.It may not be suitable for patients with arthythmia(asthamic patient)5.it is mainly used for peripheral arteries and aorta.

Examples: FBI(fresh blood imaging, Toshiba) NATIVE SPACE and NATIVE HASTE(SIEMENS) TRANCE(PHILIPS) and flow prep.

SSFP- Based MRA Balanced SSFP sequences are gradient echo steady state sequences with very high SNR and motion insensitivity due to balanced gradients in all three directions.

These sequences are called True FISP(siemens),FIESTA(GE) and balanced TFE(Philips). The contrast is determined by T2/T1 ration giving the blood very high signal without dependence or inflow. Veins other fluid and fat are also bright on this sequences.

Hence some form of subtraction is required arterial spin labelling(ASL) or inversion recovery pulses are used for this purpose.Example: SLIP(Spatial labelling inversion pulse, Toshiba) NATIVE True FISP(Siemens) IFIR(Inflow inversion recovery, GE)

SSFP- BASED MRA IMAGES

Contrast Enhanced MRA(CEMRA) This sequences used for CEMRA is usually a modified T1 weighted spoiled gradient refocused GRE sequences.

Approximate T1 times of blood, muscle and enhanced blood are 1200ms, 600ms and 100msrespectively.

A dose of approximately 0.2m mol/kg of gadolinium is required to make T1 of blood shorter than that of fat and muscle, so that it will appear brighter than fat.

The most important aspect in CEMRA is timing of peak arterial enhancement such that it will fill center of the K-space that is responsible for contrast in the image.

The time to fill the center of the K-space is usually the initial 2-3 seconds of the typical 15-20 seconds run. It is important to match these 2-3 seconds with peak arterial enhancement to get good angiogram.

Keyhole imaging can be applied to CEMRA to improve the temporal resolution.

Examples- TRICKS (GE)-time resolved MRA technique TWIST(SIEMENS)

ANGIOGRAPHY ARTIFACTS1.METAL ARIFACT2. BLOOMING ARTIFACT

METAL ARTIFACT It is intended to reduce the size and intensity of susceptibility artifacts resulting from magnetic field distortion.

A variety of techniques are used foe reducing metal artifacts at MRI, both for addressing artifacts due to the presence of metal in the image plane(in- plane artifacts) and for artifacts due to metal in an adjacent plane(through- plane artifacts).

METAL ARTIFACTS IMAGE

BLOOMING ARTIFACTS Blooming artifacts is a susceptibility artifact encountered on some MRI sequences in the presence of paramagnetic substance that affect the local magnetic milieux. Most affected sequences are T2*, and in many instances they are designed to exploit this phenomenon to make certain pathologies more conspicuous. As such although it is artifact, it is used deliberately to improve detection of certain small lesion, much as the T1 shortening effect of low concentration gadolinium are used to detect contrast enhancement.

The term “blooming” refers to the fact that lesions appear larger than they actually are.

BLOOMING ARTIFACT IMAGE

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