Principles of Contrast enhanced and non contrast enhanced MRA will be reviewed, as well as their clinical application.
· The principles of contrast enhanced MRA and advances with regards to spatial and temporal resolution will be reviewed
· Non-contrast MRA techniques that are available in clinical practice will be reviewed, including flow-dependent techniques, phase-dependent techniques and those dependent upon the physical MRI properties of blood
· Clinical application of contrast and non-contrast MRA techniques will be discussed, based on the vascular bed of interest
1. To understand the principles of contrast enhanced MRA
2. To review non-contrast MRA techniques
3. To review clinical application of these techniques and when/where they can or should be used
MRA is a widely-used technique, that carries no risk of ionizing radiation, and allows volumetric, multiplanar assessment. In contrast to CTA, it allows assessment of luminal stenosis even in the presence of calcified plaque, and can be used in severe renal insufficiency. Ideally, bright arterial (or venous, for MRV) signal is desirable, with suppression of unwanted background tissue.
Contrast-enhanced MRA is widely used in clinical practice, and has been used for neurovascular and body applications, allowing anatomic but also haemodynamic assessment if desired. This presentation will briefly discuss the principles of contrast enhanced MRA, and discuss recent advances in temporal and spatial resolution, such as methods to accelerate acquisition and minimize artifacts due to motion. Time permitting, ferumoxytol enhanced MRA (off-label use) will also be discussed.
There are several non-contrast MRA techniques available for clinical use, with some developed in response to concerns about gadolinium chelates in patients with renal failure. Reports of cerebral gadolinium deposition, although its long-term implications are unknown, have also engendered increased enthusiasm for techniques that do not require contrast.
Non-contrast MRA techniques can be divided based on how arterial signal and background suppression is obtained:
A. Flow-related Techniques
· Time of Flight MRA (TOF MRA): A well-established and clinically validated technique, where the basic premise is that background tissue is suppressed, enabling contrast with arterial spins flowing into the imaging field of view. It may be applied as a 2D or 3D technique, and selective saturation pulses may also be applied to suppress undesirable inflowing (e.g. venous) spins. As TOF MRA depends on arterial inflow to visualize the arteries, it is most effective for vascular beds where there is fast or continuous inflow, and smaller volumes, since the imaging plane should ideally be perpendicular to the main direction of vessel flow. It is therefore most effective in neurovascular imaging for both MRA and MRV.
· Inflow/outflow Techniques: These are flow dependent techniques that do not require subtraction and are acquired in a fashion that differs from standard time of flight MRA. One such volumetric technique that is commercially available is inflow inversion recovery MRA, where a selective inversion prepulse is applied to suppress background tissue, and during the inversion time (TI), fresh arterial blood enters the region of interest prior to a relatively rapid readout. As the technique remains dependent on arterial inflow, it is most effective for arterial beds with relatively rapid flow, e.g. the renal arteries. It is particularly effective for vascular beds with cardiac or respiratory motion, where techniques requiring image subtraction are problematic.
A robust 2-dimensional flow-dependent technique is Quiescent Interval Single Shot MRA (QISS MRA), an ECG-gated technique that employs selective saturation pulses to suppress background tissue and inflowing venous blood, and depends on fresh arterial blood to enter each slice during a “quiescent interval” in the cardiac cycle, timed to include systole, prior to a diastolic phase readout. Due to relative speed and ease of implementation, and sensitivity to relatively slow arterial flow (superior to traditional TOF MRA), the technique can be best applied to large coverage lower extremity MRA. Another example of a flow dependent technique is velocity sensitive angiography, which selectively suppresses background or slowly moving spins with a velocity selective inversion pulse.
B. Phase Dependent Techniques
· Phase contrast MRA (PC MRA): PC MRA depends on the phase of moving spins to provide vessel contrast, with stationary spins effectively experiencing no overall phase shift, resulting in excellent background suppression. It is a technique that can also be applied to relatively slowly flowing vessels and hence can be applied to MR venography as well as MRA. Due to the need for multiple flow-encoding gradients, it is traditionally a relatively slow technique, again best applied to neurovascular imaging, although scan times can be markedly reduced with the use of undersampling and non-Cartesian readouts. However, it can provide functional information in addition to morphologic information, and 4 dimensional PC imaging that includes a temporal element is an emerging technique with exciting potential clinical application to aortopathy and other vascular disease. 4D PC MRA can also be performed following administration of a blood pool agent, e.g. ferumoxytol, for improved SNR.
· Subtraction Arterial Spin Labeling (ASL) Techniques: With this technique, two sets of images are acquired, with differences between magnetization of arterial spins between the sets, such that subtraction of one set from the other leads to cancellation of background tissue. Either a balanced steady state free precession (bSSFP) or fast spin echo (FSE) readout can be employed, with choice dependent on the region of interest. Different magnetization or labeling of arterial spins can be achieved either by depending on flow-related dephasing from application of gradients, or by applying radiofrequency pulses. These techniques achieve superb background suppression, but are more susceptible to motion artifact given relatively long scan time required for 2 successive acquisitions, and hence are best applied to relatively motionless vascular beds, e.g. the peripheral arteries.
C. Flow-Independent Techniques
The intrinsic properties of blood are used to achieve bright arterial signal with these techniques, and contrast with other (background) tissue, with no reliance upon blood flow. The most commonly used of these techniques is magnetization-prepared bSSFP MRA, where chemically selective fat suppression and T2 preparation are used to minimize signal from fat or tissues with relatively short T2 relaxation times. The technique is effective even in relatively slow flow vascular beds or where there is complex multidirectional anatomy, where flow dependent techniques may be challenging. This technique has particularly been applied to coronary MRA, particularly in younger patients where ionizing-radiation associated with coronary CTA is undesirable. Alternatively, bSSFP MRA (or spoiled gradient echo readout at 3T) can also be performed following contrast administration, if no contra-indications are present, with blood T1 shortening enabling increased SNR and CNR.
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