Magnetic Resonance Angiography: Clinical Techniques

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CHAPTER 82 Magnetic Resonance Angiography

Clinical Techniques

Since its inception slightly more than 2 decades ago, MR angiography has become a preferred noninvasive imaging technique for a wide range of clinical indications. Concomitant technical developments in CT angiography have challenged the potential dominance of MR angiography, most notably during the recent era of multidetector CT. MR angiography permits comprehensive multiplanar endoluminal and vascular mural evaluation with exquisite soft tissue contrast, however, in the absence of ionizing radiation exposure or the requirement for iodine-based contrast medium administration. This chapter reviews the spectrum of unenhanced and gadolinium contrast-enhanced MR angiography techniques, highlighting the current and potential future roles of each in contemporary medical imaging.

DESCRIPTION OF TECHNICAL REQUIREMENTS

Consistent achievement of high-quality diagnostic MR angiography examinations depends on a synergy between appropriate MRI hardware and software and technologist-patient communication. Compromise in any of these components is certain to have a detrimental effect on image quality. Although detailed consideration of the wide range of currently available technical components is beyond the scope of this chapter, many key considerations do exist, each of which is briefly considered.

Gradient Coils

More recent developments in MRI hardware design have facilitated further improvements in gradient coil performance.2 High-performance gradient coils enable optimization of vascular SNR. These SNR improvements incur a penalty, however, in the form of energy deposition and increases in specific absorption rate. In practical terms, limitations in specific absorption rate often necessitate a compromise in attainable slice coverage for a particular repetition time (TR).

Phased-Array Surface Coils and Parallel Imaging Techniques

Comprising multiple integrated receiver coils, phased-array coils combine the advantages of high SNR achieved by smaller coils with the benefits of improved volume coverage, previously afforded only by large coil elements. Parallel imaging techniques (e.g., sensitivity encoding, or SENSE),3 whereby incomplete k-space sampling is tolerated by coil sensitivity profile calculation of the missing data, allow for significant improvements in temporal resolution, spatial resolution, or volume coverage. Parallel imaging depends on phased-array surface coils for its application. Parallel imaging improvements are attained at the expense, however, of reduced SNR. Such SNR loss may be offset, and even reversed, by imaging at higher field strengths (e.g., 3.0 T), allowing the benefit of ever-increasing acceleration factors to be realized without compromise in field of view (FOV) or spatial resolution.4

TIME OF FLIGHT MAGNETIC RESONANCE ANGIOGRAPHY

Repetitive successive radiofrequency pulses, if applied at a magnitude and rate sufficient to prevent interval T1 recovery, results in saturation of signal from tissue within the imaged volume.5 Time of flight (TOF) MR angiography exploits this saturation effect, providing untainted visualization of the signal produced by unsaturated entry of blood (i.e., through-plane blood flow) without the requirement for contrast agent administration. Unidirectional flow may be imaged through the use of presaturation pulses (also known as saturation bands) to eliminate signal from spins traveling in the opposite direction, with the effect of providing pure angiographic or venographic depiction, as desired. These attributes have made TOF MR angiography the most established MR angiography technique currently available, particularly with regard to the carotid, vertebral, and intracranial vascular territories.

Numerous potential implementations of this technique are available. Each varies in its degree of suitability, depending on the clinical indication. Two-dimensional TOF MR angiography involves the excitation of a single anatomic section and has proven useful for the evaluation of anatomic regions where respiratory or cardiac motion precludes useful volumetric evaluation (e.g., chest or abdomen). Multiple breath-holds and sequential, independent two-dimensional TOF MR angiography acquisitions may be used in this instance to provide diagnostic quality examinations, even in dyspneic patients (Fig. 82-1). Three-dimensional TOF MR angiography is preferred for intracranial evaluation in particular, permitting detailed volumetric data acquisition at submillimeter voxel resolution and the potential for subsequent postprocessing (Fig. 82-2). Multiple overlapping thin slab acquisition (MOTSA) represents a compromise in two-dimensional and three-dimensional techniques, integrating the advantages of three-dimensional imaging with the relatively fewer limitations of the two-dimensional approach. MOTSA combines multiple, relatively thin three-dimensional slabs to provide clinically useful volume coverage.6

Pitfalls and Solutions

Despite its popularity and widespread implementation, TOF MR angiography may be extremely challenging to implement and interpret because of its numerous potential pitfalls.8

PHASE CONTRAST MAGNETIC RESONANCE ANGIOGRAPHY

Phase contrast MR angiography is an unenhanced approach to imaging that employs bipolar phase-encoding gradient pairs to encode flow velocity in the gradient direction. Stationary background tissue accumulates a net phase shift of zero. Moving spins experience a net phase shift that produces signal and the image contrast necessary to distinguish between moving and stationary tissue (i.e., angiography).10 Phase contrast MR angiography requires the operator selection of a velocity encoding (VENC) in cm/s, which is responsible for determination of the flow sensitivity of the acquisition. Because assignment of phase shift is limited to a range of −180 degrees to +180 degrees, the VENC represents a flow velocity that would cause a maximal phase shift of 180 degrees. For optimal sensitivity, this VENC should be selected to correspond with or slightly exceed the highest velocity present within the vessel in question. For intracranial applications, a VENC of 70 to 80 cm/s is often sufficient for arterial imaging, whereas a factor of 20 to 30 cm/s should be applied for venous imaging.11 If the flow velocity exceeds the chosen VENC, aliasing results with the effect of apparent flow reversal.

In addition to providing a visual representation of flowing blood, phase contrast MR angiography allows quantitative evaluation of flow velocity, typically acquired in cine mode (i.e., cine phase contrast). This evaluation reflects the direct relationship between the phase shift experienced by flowing spins and their velocity. This technique is being increasingly recognized regarding its potential utility throughout the vascular system, particularly in regard to estimation of pressure gradients or flow quantification.

THREE-DIMENSIONAL STEADY-STATE FREE PRECESSION MAGNETIC RESONANCE ANGIOGRAPHY

SSFP is a low flip angle gradient-recalled-echo (GRE) technique that induces a persistent level of tissue magnetization by means of a TR that is significantly shorter than the T2 of tissue. As a result, this approach permits bright blood vascular imaging, the signal from which is a reflection of the inherent T2/T1 ratio of blood, while precluding gadolinium-chelate contrast agent administration.13 Owing to a very short TR and large flip angle, two-dimensional SSFP techniques allow rapid subsecond image acquisition that does not require respiratory suspension, even when imaging the chest. These attributes have resulted in the adoption of SSFP as a cornerstone imaging technique in many aspects of cardiac imaging, including two-dimensional single-shot multiplanar morphologic and ECG gated cine functional myocardial assessment.

Many three-dimensional implementations of SSFP have been successfully evaluated for the purpose of vascular imaging, most notably with regard to the coronary and renal arteries.14,15 In exploiting the intrinsic T2/T1 signal of blood, three-dimensional SSFP MR angiography allows large FOV vascular coverage, while avoiding the data acquisition constraints because of the contrast bolus imposed during contrast-enhanced MR angiography. Combining three-dimensional SSFP MR angiography with navigator gating allows free-breathing nonenhanced chest and abdominal vascular depiction. Further addition of ECG gating has allowed the realization of free-breathing coronary MR angiography, although at the expense of often prolonged acquisition times (≥10 minutes) (Fig. 82-3). The potential of parallel imaging techniques to aid in reduction of these acquisition times has been evaluated, providing encouraging results to date. Implementation of this data-sharing technique does incur penalties with regard to SNR, however, with the effect of image degradation that may be poorly tolerated.

Contraindications

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