Time-resolved MRA revolutionizes imaging

Conventional static MR angiography techniques create high-spatial-resolution structural studies but fail to image physiological information inherent in the delivery of blood or contrast. MR scanner gradient enhancements now enable repetitive data capture over time in the attempt to depict vascular dynamics and physiology in a method similar to that routinely used with conventional catheter x-ray angiography.

The clinical availability of temporally enhanced time-resolved acquisition techniques such as TRICKS (time- resolved imaging of contrast kinetics) revolutionizes the way that MRA is performed in the clinical setting, providing dynamic information along with high-resolution, extended anatomical coverage.

Recent advances that employ parallel imaging to increase temporal resolution extend the clinical applicability of these techniques. A time-resolved approach improves the quality and consistency of contrast-enhanced MRA exams by reducing their reliance on operator performance and precise acquisition timing.

TECHNIQUE CONSIDERATIONS

With improvement in MR scanner gradient performance, echo times and repetition times decrease, allowing greater anatomical coverage or higher spatial resolution imaging in a given physiologically acceptable scan time. Physiologically limiting considerations include arterial to venous transit time of the imaging target and, when applicable, patient breath-holding capability.

Such novel phase-encoding schemes as elliptic centric phase encoding enhance efficiency in capturing fundamental central views that determine contrast resolution. These encoding schemes allow extended scan time for high resolution/extended coverage acquisitions without contamination by the venous signal.

Conversion of an MRA acquisition from single-phase high-resolution to time-resolved generally involves serial scan repetition as the contrast agent is delivered to the target object or organ. Protocol modifications typically include reduction in spatial resolution and anatomical coverage (fewer and/or thicker slices) in the interest of faster cycling of the image capture. The trade-off in quality inherent in this brute force approach with conventional imaging techniques is sufficiently severe to limit applicability in clinical practice.

Modern time-resolved techniques reduce these negative trade-off aspects by creatively altering the way in which image k-space is captured. With TRICKS and its variants, the center of k-space is sampled more often than the periphery. This produces multiple physiologic snapshots per each full pass through k-space and typically delivers a fourfold increase in temporal resolution without a reduction in the signal-to-noise ratio. The addition of parallel imaging to a time-resolved protocol increases by twofold or more the temporal resolution, although with the typical reduction in SNR.

As an example, a 3D scan that would require 16 seconds to acquire with conventional techniques would take as little as two seconds to acquire in a serial fashion with parallel imaging-enhanced time-resolved scanning, yielding eight more time points for imaging per unit time (Figure 1).

FUNDAMENTAL BENEFITS

The benefits of a time-resolved protocol for MRA include close to perfect reliability in the capture of the ideal arterial phase of contrast passage. Multiple snapshots reduce the possibility of being too early or too late with timing of a single-phase high-resolution study. Poorly timed studies are less anatomically accurate and may suffer from venous interference. A time-resolved approach also eliminates the fuss and cost of a timing run or bolus tracking for scan initiation.

A fundamental benefit of time-resolved MRA is dynamic depiction of flow physiology. Information such as reduced flow due to stenosis, vascular occlusion, and collateral flow can be missed or obscured with a single-phase high-resolution approach even if optimally performed. With asymmetrical flow due to proximal occlusive disease, single-phase high-resolution MRA studies cannot consistently deliver the ideal information on bilateral studies such as lower extremity runoff.

Single-phase high-resolution studies are also insufficient for high-flow vascular lesions such as fistulae, for which depiction of the details of arteriovenous transit can be critical to treatment planning.

CLINICAL APPLICATIONS

Time-resolved MRA has shown potential in several applications.

• Aortoiliac. Aortic MRA studies are generally well performed with single-phase high-resolution protocols, as dynamic information is not critical. Some institutions, particularly those with less experience with contrast-enhanced MRA, may employ time-resolved techniques to improve consistency of scan performance.Time-resolved techniques are preferred in disease states in which delineation of altered arteriovenous transit is important. Studies intended to detect endoleaks in patients treated with endovascular stenting for aortic aneurysm may be best performed with time-resolved MRA. Sites of leaks and vascular supply are well rendered with dynamic imaging (Figure 2).
• Peripheral vascular. Stepping table runoff studies involve acquiring a single-phase 3D image at the aortic station and chasing the bolus distally with two or more additional acquisitions. This approach trades off spatial resolution and coverage at each station in the endeavor to capture static, pure arterial information at each more distal station as the contrast passes. Unfortunately, no clinically available stepping table/bolus chase scanning technique can keep pace with the contrast bolus and accomplish scanning before venous transit while delivering adequate image quality. Even state-of-the-art conventional stepping table/bolus chase single-phase scanning results in an unacceptable incidence of venous interference at lower stations and lower than desired spatial resolution and coverage throughout the study.
  Serial multistation time-resolved acquisitions offer several advantages over bolus chase studies. The time-resolved approach generally involves acquiring the distal stations first, followed by more proximal stations (Figure 3). Calf or tibioperoneal arterial studies are obtained first with the time-resolved technique, providing studies free of venous interference. The thigh station is acquired next.

Any residual contrast will be hidden by the background subtraction inherent in acquisition techniques such as TRICKS. The aortic station would be last, and this would benefit from the lack of a need to "step" with greater coverage and resolution. Alternatively, a two-station stepping table/single-phase run could follow the calf time-resolved study.

Imaging over time allows direct depiction of flow dynamics such as slow filling, collateral, and occluded flow. Since there is no need to step to the next station, the individual time-resolved snapshots are typically higher in spatial resolution and coverage than the individual stepping table/single-phase scans obtained on bolus chase studies.

• Carotid-vertebrobasilar. Single-phase high-resolution imaging is very well suited to MRA of the neck if performed with elliptical centric phase encoding, which allows extended scan times (up to about one minute) without incurring venous contamination, if appropriately timed. Many facilities will still choose to employ time-resolved/multiphase techniques to make study performance more consistent and avoid the fuss and cost of a timing run or enhancement-triggered scanning. The eightfold acceleration of parallel imaging-enhanced time-resolved studies, delivering temporal resolution of about two seconds per 3D run, is of particular benefit in neurovascular studies because of the inherently short arteriovenous transit time of the brain. Dynamic time-resolved imaging can be useful in depicting altered flow dynamics, as in subclavian steal syndrome and flow-limiting stenosis.
• Brain. Non-contrast-enhanced time-of-flight studies are generally sufficient for imaging the cerebral vasculature. Contrast-enhanced MRA can be of additional value for the evaluation of large aneurysms. Time-resolved approaches improve the consistency of these MRA studies and can yield information about aneurysm flow dynamics.

Time-resolved techniques are superior to static single-phase approaches for high-flow lesions such as vascular malformations and arteriovenous fistulae. Time-resolved imaging offers information about arteriovenous transit and dynamics.

PHASE ENCODING SCHEMES

The one- to two-second temporal resolution provided by currently available time-resolved techniques is less than ideal for these applications. The eventual clinical availability of more effi¬cient phase encoding schemes, such as undersampled projection recon¬structions (e.g., VIPR), will undoubtedly improve temporal resolution significantly, so that it approaches that of conventional catheter angiographic techniques.

Time-resolved acquisition techniques such as TRICKS revolutionize the way that MRA is performed in the clinical setting, providing dynamic information along with high-resolution, extended anatomical coverage. Time-resolved techniques also improve the quality and consistency of contrast-enhanced MRA examinations by reducing their reliance on operator performance and precise acquisition timing. Time-resolved techniques are commonly employed in clinical practice today and should become increasingly popular with future enhancements in scanner hardware and software.

DR. TANENBAUM is section chief of CT, MRI, and neuroradiology at New Jersey Neuroscience Institute and JFK Medical Center in Edison, U.S.

Bibliography
Korosec FR, Turski PA, Carroll TJ, et al. Contrast-enhanced MR angiography of the carotid bifurcation. J Magn Reson Imaging 1999;10:317-325.
Mell M, Tefera G, Thornton F, et al. Clinical utility of time-resolved imaging of contrast kinetics (TRICKS) magnetic resonance angiography for infrageniculate arterial occlusive disease. J Vasc Surg 2007 Mar;45(3):543-458; discussion 548. Epub 2007 Jan 16.
Mistretta CA, Grist TM. X-ray digital subtraction angiography to magnetic resonance-digital subtraction angiography using three-dimensional TRICKS: Historical perspective and computer simulations. Invest Radiol 1998;33(9):496-505.
Unlu E, Temizoz O, Albayram S, et al. Contrast-enhanced MR 3D angiography in the assessment of brain AVMs. Eur J Radiol 2006;60(3):367-378.