Magnetic Resonance Angiography: Clinical Techniques

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CHAPTER 82 Magnetic Resonance Angiography

Clinical Techniques

Stefan G. Ruehm, Derek G. Lohan

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.

Field Strength

It has been established that signal-to-noise ratio (SNR) increases in an approximately linear fashion with magnetic field strength, which provides the opportunity for immensely superior vascular depiction with imaging at 3.0 T compared with 1.5 T. High field strength imaging is associated with a realm of potential challenges, however, which are relatively less significant at 1.5 T, including specific absorption rate considerations, T2* and dielectric resonance effects, and a greater incidence of clinically appreciable peripheral nerve stimulation.¹ Use of a 3.0 T system demands familiarity with methods of avoiding and addressing these challenges such that compromised patient safety or image quality is not acceptable. Although higher field strength has advantages for MR angiography, it is not essential, and high-quality diagnostic examinations are routinely produced on 1.5 T MRI systems.

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

ECG Gating

Cardiac gating is typically unnecessary for most body MR angiography applications. During contrast-enhanced MR angiography, implementation of ECG gating would markedly prolong the already relatively protracted data acquisition times currently achievable (approximately 20 seconds), beyond the restrictions allowed by first-pass of the contrast bolus. In certain situations, availability of cardiac gating is desirable, however; these include vascular flow quantification using phase contrast flow-sensitive techniques and three-dimensional steady-state free precession (SSFP) coronary and thoracic aortic MR angiography, both of which are considered in greater detail later.

Careful Patient Preparation

High-quality MR angiography depends on a combination of adequate MRI scanner technology, experienced staff capable of exploiting this technology and modifying the imaging protocol when necessary, and an informed patient capable of adhering to the operator instructions. The impact of this last prerequisite on image quality cannot be overstated. Careful patient preparation results in improved patient cooperation with breath-hold instructions and a reduction of motion artifacts. This level of patient cooperation can be achieved only by complete and detailed explanation of the MRI experience to the patient in a comforting environment before the examination, while addressing the patient’s concerns in a considerate manner.

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

FIGURE 82-1 An 88-year-old man with aortoiliac atherosclerosis and aortoiliac grafts. A-C, Comparative two-dimensional TOF MR angiography (A), arterial phase TR MR angiography frame (B), and full-thickness three-dimensional contrast-enhanced MR angiography (C) coronal images illustrate the typical image quality of each technique when imaging this anatomic region. Performed properly, three-dimensional contrast-enhanced MR angiography provides the highest spatial resolution image.

FIGURE 82-2 Periophthalmic intracranial aneurysm. A-D, Full-thickness MIP (A) and volume-rendered reconstructions (B) from TOF MR angiography examination, and axial thin-section MIP (C) and coronal oblique full-thickness volume rendered (D) reconstructions from contrast-enhanced MR angiography provide comparable information regarding aneurysm morphology and location (arrow in B and C).

Indications

Before the widespread introduction of contrast-enhanced MR angiography techniques for comprehensive large-volume anatomic vascular coverage, TOF MR angiography represented the cornerstone of MR angiography throughout the body. Contrast-enhanced MR angiography, however, required revision of many diagnostic algorithms in favor of this faster and typically higher quality method. Nonetheless, TOF MR angiography remains the technique of choice for intracranial vascular depiction, a reflection of its superb spatial and contrast resolution and its patient acceptability.⁷ Advances in MRI hardware, including the introduction of dedicated head and neck coils and resultant implementation of parallel imaging techniques, have enhanced the value of this approach in clinical practice further.

TOF MR angiography also remains a common means of extracranial carotid arterial evaluation. Artifact relating to patient motion or swallowing often produces less adequate results compared with intracranial imaging. The superb quality of vascular depiction obtained on supra-aortic contrast-enhanced MR angiography studies suggests that TOF MR angiography may soon be superseded by this contrast-enhanced technique for routine extracranial carotid MR evaluation. In the presence of contraindications to contrast administration, TOF MR angiography may also be valuable in head and neck venous imaging, particularly imaging of the intracranial venous sinuses.

Contraindications

No contraindications particular to TOF MR angiography exist, such that it may be safely performed in any patient undergoing an MRI examination. This attribute has undoubtedly contributed to the popularity of this technique and helped maintain its position as a significant part of the diagnostic algorithm.

Pitfalls and Solutions

Image Interpretation

Postprocessing

A TOF MR angiography data set is volumetric and comprises voxels, acquired in slices (two-dimensional TOF) or in the form of digital sections known as partitions (three-dimensional TOF). Selection of isotropic voxel dimensions minimizes the image distortions seen on multiplanar off-axis reconstruction. TOF MR angiography data are most often presented in the form of a full-thickness maximum intensity projection (MIP), rotated through 360 degrees in anteroposterior (somersault) and transverse planes. As a result, small aneurysms or luminal surface irregularities (e.g., intimal flap or ulceration) may not be as well appreciated or may be missed. It is recommended that the partition “source data” are also reviewed so that subtle abnormalities are not concealed by overlapping vessels in areas of complex vascularity.

Reporting

Confident, accurate reporting of TOF MR angiography examinations necessitates that the potential pitfalls referred to earlier and their imaging manifestations are recognized and correctly interpreted. Because there is a tendency for each of these unwanted effects to result in intraluminal signal loss, either focal or widespread, failing to recognize them may result in considerable overestimation in the degree of vascular steno-occlusive disease and prompt unnecessary invasive investigation.

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.

Indications

Before the widespread availability of contrast-enhanced and increasingly impressive TOF techniques, phase contrast MR angiography was relatively successful in the evaluation of various vascular territories. In recent times, this approach has been relegated in importance to that of a “last resort,” should the other angiographic techniques discussed in this chapter be unsuccessful or contraindicated. Phase contrast MR angiography has regained some of its former popularity more recently because of its flow quantification capabilities. Our experience suggests that phase contrast flow quantification is a valuable, versatile tool in the noninvasive evaluation of flow characteristics within almost any vascular bed.¹² Although this technique has not yet been incorporated into widespread clinical practice, its future potential remains encouraging.

Contraindications

Absolute contraindications to phase contrast MR angiography are those of MRI in general; any patient in whom MRI is deemed safe may potentially undergo this angiographic technique. The relatively long data acquisition times involved (often >20 minutes) may render phase contrast MR angiography as a relative contraindication in patients with unstable or rapidly declining clinical condition.

Pitfalls and Solutions

The limitations of phase contrast MR angiography are similar to those of TOF MR angiography, including in-plane saturation, velocity aliasing, voxel dephasing, and long acquisition times. The flip angle may be increased in the presence of intravoxel dephasing or decreased if spin saturation is experienced. VENC may also be increased or decreased to prevent aliasing and poor image contrast. Administration of the contrast agent gadolinium-chelate improves vascular depiction on phase contrast MR angiography and may be considered for further augmentation of luminal vascular image contrast. Nonetheless, despite the presence of potential solutions to many phase contrast MR angiography pitfalls, this technique remains inferior to alternative angiographic approaches, and has been excluded from routine clinical practice for the purpose of vascular depiction.

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.