Magnetic Resonance Angiography: Clinical Techniques

Published on 13/02/2015 by admin

Filed under Cardiovascular

Last modified 22/04/2025

Print this page

This article have been viewed 1337 times

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.

FIGURE 82-3 Navigator-gated coronary three-dimensional SSFP MR angiography. A-C, MPR reconstructions (A, oblique axial; B, oblique coronal; C, oblique sagittal) from the same data set illustrating an aberrant origin of the right coronary artery (arrow). This vessel may be confidently visualized throughout much of its length, passing proximally between the ascending aorta (A) and pulmonary trunk, following an interarterial course. Motion artifacts were minimized by the use of respiratory and ECG gating.

Indications

Physicians have seemed reluctant to integrate three-dimensional SSFP MR angiography into routine clinical practice; this is a reflection of the long imaging durations required for its successful implementation. In many cases, this technique is used because of necessity rather than desire, with a typical scenario involving a patient with contraindications to CT angiography or conventional catheter angiography or both, in whom confirmation of vascular patency is required. Nonetheless, the potential value of this approach has gradually achieved acknowledgment in recent times with its successful application to aortic, renal, carotid, and peripheral arterial imaging. The current environment of heightened sensitivity toward the administration of intravenous gadolinium-chelate contrast agents, discussed in greater detail subsequently, may also serve to broaden the spectrum and availability of this versatile technique.

Contraindications

After general contraindications to MRI have been excluded, all patients may be considered candidates for three-dimensional SSFP MR angiography. This technique may play a significant role in the future with increasing acceptance of this technique into routine imaging practice.

Pitfalls and Solutions

SSFP techniques are sensitive to off-resonance artifacts, manifesting as dark bands traversing the images acquired. These potentially detrimental regions may prove particularly difficult to avoid when imaging at 3.0 T. Preventive methods have been described, including small volume frequency scouting to select the optimal frequency at which the bands are eradicated.¹⁶

Whole heart coronary MR angiography using three-dimensional SSFP may represent a source of considerable frustration owing to its occasional production of poor-quality studies despite its apparently adequate implementation. In many cases, these suboptimal studies stem from poor delineation of the luminal and mural margins, with resultant blurring of the images obtained. Because successful three-dimensional SSFP demands careful coordination of numerous key components, knowledge of each is necessary so that the offending factor may be addressed.¹⁷

Heart Rate and Rhythm

ECG gated studies involve data acquisition during a predefined interval of the cardiac cycle. Erratic cardiac rhythms serve to deprive each cycle of this portion of the R–R interval, prolonging data acquisition times. Tachycardia also has a detrimental effect on three-dimensional SSFP imaging, increasing the degree of cardiac motion experienced during a single image acquisition window.

Respiratory Rhythm

Ideal conditions for respiratory-navigated MR angiography studies involve a patient with a slow, consistent respiratory pattern, such that its predictability may be employed in selecting a window during which data acquisition occurs. In the absence of such conditions, rapid or varying respiratory rates and depths prolong study times and predispose toward degradation of image quality.

Respiratory Acceptance Window

High-quality free-breathing SSFP MR angiography requires that k-space filling occurs during a specific phase of respiration. This requirement has the effect of ensuring that the structures within the imaging FOV maintain a near-exact position throughout data acquisition. Definition of an acceptance window (the volume within which the diaphragm must be located for data collection to occur) for respiratory navigation represents a compromise between image quality and duration of the examination. When acceptance windows are narrow, k-space filling occurs for a diminutive proportion of each respiratory cycle, and study times are prolonged, although less respiratory motion artifact results. Conversely, loosening of acceptance window constraints expedites study durations through an increase in the respiratory phase during which data collection occurs, although at the expense of compromised image quality.

Navigator Band Location

Triggering of k-space filling depends on identification of a soft tissue interface within a predefined acceptance window. Commonly employed interfaces include interfaces between the lung and liver or mediastinum. Adequate selection of navigator band location is central to the successful implementation of this technique. Sharply defined interfaces allow confident automated detection of the tissue margin, whereas ill-defined margins (e.g., between liver and atelectatic lung) make identification of the interface difficult.

Patient Motion

A perpetual source of image degradation during MRI, patient motion is of particular relevance to three-dimensional SSFP MR angiography because of the relatively long imaging durations involved. The long duration has the effect of introducing motion artifact and degradation of image quality. Although shortening of the duration of the examination (by widening the respiratory navigation acceptance window or removal of ECG gating) may reduce the probability of significant patient motion, these adjustments also have the effect of compromising image integrity.

Image Interpretation

Postprocessing

The volumetric nature of three-dimensional SSFP MR angiography with near-isotropic voxel dimensions permits data reconstruction in any desired plane without incurring penalties with regard to image distortion (Fig. 82-4). Comprehensive multiplanar vascular evaluation is typically feasible on the basis of a single acquisition. Coronary artery assessment generally involves curved multiplanar reconstruction (MPR) processing, allowing visualization of the arterial lumen throughout its length, despite its tortuous course. Short-axis reconstructions (perpendicular to the vascular lumen) at sites of suspected pathology may also be derived from this data set. Renal artery evaluations generally involve coronal oblique and axial reconstructions.

FIGURE 82-4 Thoracic three-dimensional SSFP MR angiography. Acquisition of an isotropic three-dimensional data set allows reconstruction in any desired plane without loss of in-plane resolution. A and B, Coronal (A) and sagittal oblique (B) images show the thoracic aorta without motion artifact. C, There is a metastatic lesion (asterisk) in the sternum, its effect on the right ventricle being readily appreciable in this sagittal image. D, Volume-rendered three-dimensional reconstruction.

Reporting

As explained previously, free-breathing, ECG gated three-dimensional SSFP MR angiography exploits the T2/T1 signal ratio of blood for signal generation. In contrast to the other angiographic techniques discussed, SSFP MR angiography provides concomitant arterial and venous luminal depiction. This wealth of information should be evaluated carefully for the presence of incidental or subclinical vascular abnormalities, within the arterial and the venous territories.

TIME RESOLVED MAGNETIC RESONANCE ANGIOGRAPHY

Time resolved MR angiography is a dynamic approach to contrast-enhanced angiography that involves rapid sequential imaging of an anatomic volume during the luminal transit of a contrast bolus (Fig. 82-5). Although most commonly implemented as a two-dimensional technique for the purpose of “bolus timing” in preparation for subsequent high-resolution contrast-enhanced MR angiography, more recent three-dimensional applications of TR MR angiography have shown considerable promise with regard to functional vascular imaging and evaluation of visceral perfusion.

FIGURE 82-5 Bilateral internal carotid arterial bulb occlusions. Sequential time resolved MR angiography frames and full-thickness MIP from contrast-enhanced MR angiography (bottom right two frames, coronal and axial MIPs) examination fail to show opacification of the internal carotid arteries from their origins. The vertebral arteries, collateral pathway, and external carotid circulation are prominent, including ophthalmic branches.

Regardless of the approach used, time resolved MR angiography involves the use of a T1-weighted GRE sequence with sufficiently short TR to allow repeated imaging at temporal resolutions of 1 to 2 seconds per frame. Achievement of such ultrashort temporal resolutions demands that compromises are made, however, most commonly with respect to spatial resolution. One approach is to allow concessions in the slice-select direction, as described by Finn and colleagues.¹⁸ Although this method precludes off-axis reconstruction of the volumetric data obtained, this is rarely of sufficient impact to limit the diagnostic utility of the studies performed. Alternatively, through-plane resolution comparable to that of the in-plane direction may be employed, although at the expense of less frequent data refreshment.¹⁹

Enthusiasm regarding the potential value of time resolved MR angiography in clinical practice stems from its ability to permit starting imaging before the arrival of the contrast bolus and continuation of imaging throughout the arterial and venous phases of luminal opacification. As a result, TR MR angiography is free from the bolus-timing constraints of contrast-enhanced MR angiography and the ever-present threat of venous contamination occasionally experienced during contrast-enhanced MR angiography.

Indications

Widespread appreciation of the potential utility of time resolved MR angiography has resulted in its successful application across a wide range of disciplines (Figs. 82-6 and 82-7). Our experience with this technique has been considerable and similarly encouraging. Table 82-1 summarizes proven indications for the use of time resolved MR angiography, including suggested applications that are based on our personal experience to date.

FIGURE 82-6 Subclavian steal. A, Selected sequential frames from three-dimensional time resolved MR angiography shows proximal occlusion of the left subclavian artery with collateral circulation via retrograde flow in the left vertebral artery (arrow). B, This pathway is eloquently depicted by three-dimensional contrast-enhanced MR angiography (arrow). C-E, Cine axial phase contrast imaging at the level of the mid-common carotid arteries confirms the presence of retrograde left vertebral arterial flow (arrow in C-E; C, magnitude image; D, speed or angiographic image (all flow bright); E, phase map image (ascending flow is bright and descending flow is black). On phase map images (E), retrograde flow within the left vertebral artery is confirmed by its black signal. On the phase map image, flow is bright in the right vertebral and carotid arteries. F, On flow analysis, there is negative flow or descending flow (arrow) measured within the left vertebral artery compared with the positive flow or ascending flow within the carotid arteries (a) and the right vertebral artery (b).

FIGURE 82-7 Iatrogenic type B aortic dissection in a 10-year-old boy. A, Disparity between true and false luminal flow is seen on time resolved MR angiography frames (these frames are of diagnostic quality despite respiratory motion). B-D, The intimal flap is clearly visible on direct coronal (B) and sagittal oblique (C) three-dimensional contrast-enhanced MR angiography MIP and volume-rendered (D) reconstructions, obtained with suspended respiration. E, This dissection was subsequently confirmed at conventional catheter angiography.

TABLE 82-1 Indications for the Use of Time Resolved Magnetic Resonance Angiography

Discipline	Application
Neurovascular	Cerebral and spinal vascular malformations
	Spinal arteriovenous shunts
	Subclavian steal syndrome
	Dural arteriovenous fistulas
	Carotid bifurcation imaging
	Postoperative assessment of extracranial to intracranial arterial bypass
	Intracranial venous sinus evaluation
	Assessment of bilateral symmetric cerebral parenchymal perfusion in the presence of internal carotid or vertebrobasilar steno-occlusive disease
Cardiac	Work-up of complex pediatric and adult congenital heart disease
	Evaluation of patency of intracardiac and extracardiac surgical shunts (e.g., Fontan, Glenn shunt)
	Detection of right-to-left intracardiac shunts
Pulmonary	Assessment of pulmonary perfusion
	Differentiation between idiopathic and thromboembolic pulmonary hypertension
	Evaluation of pulmonary arteriovenous malformations
	Primary screening of patients with suspected pulmonary embolism
	Pulmonary venous ostial evaluation for stenosis
	Pulmonary venous assessment for anomalous drainage
Chest/abdomen	Assessment of differential flow in true and false lumens after aortic dissection
	Evaluation for endoleaks after endograft repair of abdominal aortic aneurysm
	Preoperative evaluation of hepatic arterial and portal venous anatomy in potential liver donors
	Determination of adequacy of renal parenchymal perfusion in the presence of renal artery stenosis
Extremity	Evaluation of the hemodynamic significance of infrageniculate arterial occlusive disease
	Assessment of soft tissue enhancement in the presence of cellulitis
	Determination of degree of arteriovenous shunting in peripheral arteriovenous malformations
Venous	Confirmation of venous patency
	Determination of dominant venous collaterals in the presence of venous occlusion
General (incidental detection)	Hypervascular thyroid nodules
	Unknown arterial abnormalities (e.g., splenic/renal artery aneurysms)

Contraindications

A significant proportion of current interest in time resolved MR angiography has been generated as a result of its relative lack of specific contraindications: it may be performed in any patient in whom contrast-enhanced MR angiography is planned. When general contraindications to MRI (e.g., severe claustrophobia, presence of certain implanted metallic devices such as cardiac pacemaker leads) and previous severe reactions to gadolinium-chelate contrast agents have been excluded, time resolved MR angiography can be considered in all patients.

The potential role of time resolved MR angiography in clinical practice has been readdressed in recent times as a result of the emergence of a potential link between administration of gadolinium-chelate contrast agents and development of the potentially fatal condition nephrogenic systemic fibrosis in patients with severe, end-stage renal failure.²⁰ Although some authors have questioned the justification for contrast-enhanced MR angiography and time resolved MR angiography in these patients, others have focused on the potential of time resolved MR angiography to replace its high-resolution three-dimensional counterpart in certain instances. Such preclusion of the requirement for contrast-enhanced MR angiography is wholly desirable, a reflection of the ability of time resolved MR angiography to provide highly diagnostic studies with the injection of only 0.01 to 0.02 mmol of gadolinium-chelate contrast agent per kilogram of patient body weight compared with 0.1 to 0.2 mmol/kg for contrast-enhanced MR angiography.²¹

Pitfalls and Solutions

Time resolved MR angiography represents a balance between in-plane spatial resolution, through-plane spatial resolution, FOV, and rate of temporal data refreshment. Because the equilibrium between these factors is often quite tenuous, improvement in one parameter often incurs penalties with respect to one or more of the others. Many of the pitfalls of time resolved MR angiography occur as a result of this complex relationship, such as exclusion of a vital vascular structure owing to FOV constraints, inability to produce suitable off-axis MIPs as a result of limited through-plane resolution, or failure of detection of a transient vascular phenomenon because of suboptimal temporal resolution. Avoidance of such adversities involves careful preemptive tailoring of the imaging plane, three-axis spatial resolution, and temporal resolution to suit the clinical indication. In many cases, technical restrictions prevent avoidance of all of these pitfalls during a single time resolved MR angiography acquisition. As a result, a low threshold should be maintained for consideration of multiple time resolved MR angiography acquisitions using complementary parameters so that a comprehensive morphologic and dynamic evaluation is obtained.

Image Interpretation

Postprocessing

Generally, time resolved MR angiography is postprocessed inline on the MRI console, involving subtraction of the initial unenhanced data set from all subsequent data sets and presentation of the subtracted temporal volumes in a full-thickness MIP format for immediate review. Certain situations may require that off-axis reconstruction of one or more of the volumetric data sets be obtained, the diagnostic quality of which largely depends on the through-plane resolution and relative magnitudes of in-plane and through-plane resolutions. In cases in which these postprocessed images are inadequate for diagnostic interpretation, repeat time resolved MR angiography at a different projection or imaging parameters or both should be considered in view of the relatively small contrast dose involved.

Reporting

A single time resolved MR angiography acquisition, often of duration of no longer than 20 seconds, provides a considerable amount of morphologic and dynamic information that should be alluded to in the ensuing report. Arterial luminal patency, directional flow, parenchymal perfusion, venous drainage, and a wide variety of site-specific considerations, including degree of arteriovenous shunting within vascular malformations and differential perfusion with regard to symmetric viscera, must be carefully evaluated to detect subtle derangements. Because other MR angiography techniques provide little if any dynamic vascular information, particular reference should be made to this parameter when considering any time resolved MR angiography examination.

THREE-DIMENSIONAL CONTRAST-ENHANCED MAGNETIC RESONANCE ANGIOGRAPHY

High-resolution three-dimensional contrast-enhanced MR angiography has become one of the most powerful diagnostic tools in diagnostic imaging. This is due in large part to the introduction of high field MRI systems with rapid high-performance gradient coils and the development of phased array coils and, subsequently, parallel imaging techniques.²² These advances have conspired to make large FOV high-resolution three-dimensional contrast-enhanced MR angiography during comfortable breath-hold times and in the absence of venous contamination a clinical reality.

Contrast-enhanced MR angiography depends on the ability of intravenously administered gadolinium-chelate contrast agents to shorten the T1 time of the target vascular bed. Properly timed, the T1 signal from the target vascular territory far exceeds that of adjacent nonenhanced extravascular background soft tissue, allowing confident detailed evaluation of luminal patency or mural irregularity. Using a three-dimensional T1-weighted spoiled GRE sequence similar to that described for time resolved MR angiography, the presence of an ultrashort TR results in saturation of signal from background soft tissues, enhancing the conspicuity of contrast-enhanced blood. The illustration of vessels may be augmented further by acquisition of an unenhanced data set before administration of contrast agent for use in subsequent “mask” subtraction.

Indications

The efficacy of three-dimensional contrast-enhanced MR angiography has been proven for almost every vascular territory, with the result that it has now replaced conventional diagnostic angiography as the imaging modality of choice for various clinical applications. The most common indication for contrast-enhanced MR angiography is the investigation of suspected atherosclerotic steno-occlusive disease.²³ This approach allows comprehensive, large-volume, detailed evaluation for the presence of mural irregularity, stenosis, dissection, or aneurysmal dilation, and permits accurate assessment of their effect on the adjacent vessel lumen. The most common territories routinely evaluated using this technique are extracranial carotid, supra-aortic, thoracic and abdominal aortic, renal, mesenteric, and peripheral extremity vessels (Figs. 82-8, 82-9, and 82-10).

FIGURE 82-8 Fibromuscular dysplasia in a young woman with hypertension. A and B, Arterial phase time resolved MR angiography reconstruction (A) and coronal oblique contrast-enhanced MR angiography MIP (B) show characteristic renal arterial “beading” of the right renal artery (arrow) of fibromuscular dysplasia. C, This was confirmed on conventional x-ray angiography and treated using percutaneous balloon angioplasty, with satisfactory clinical improvement. D, A good therapeutic result is also noted on postoperative contrast-enhanced MR angiography.

FIGURE 82-9 Vascular mapping using time resolved MR angiography and contrast-enhanced MR angiography. A, Multiple uterine leiomyomas seen on postcontrast fat-saturated T1-weighted GRE imaging. B and C, Isolated arterial phase TR MR angiography (B) and full-thickness contrast-enhanced MR angiography MIP (C), performed as a prelude to interventional therapy, allow high-quality depiction of the internal iliac arteries and branch vessels in the absence of venous contamination. D and E, Conventional catheter angiography, obtained at the time of fibroid embolization, are provided for comparison.

FIGURE 82-10 Traumatic arteriocavernosal fistula. A-G, Opacification of the right corpus cavernosum (arrows in D, E, and G) is seen on late arterial phase time resolved MR angiography images (A-F) and first-pass contrast-enhanced MR angiography (G). H, Disparity between right (arrow) and left cavernosal opacification is seen on postcontrast two-dimensional coronal fat-saturated T1-weighted imaging.

Pulmonary vascular imaging has also benefited significantly from developments in contrast-enhanced MR angiography; this technique allows depiction of branch vessels of calibers of 1 to 2 mm. Contrast-enhanced MR angiography has now become a valuable tool in the evaluation of suspected pulmonary hypertension and determination of its potential etiology, such as chronic thromboembolic disease. The efficacy of this technique in diagnosis of acute pulmonary embolism has also been shown, and this may be of particular value in patients in whom iodinated contrast media are undesirable or contraindicated.²⁴

The reproducibility of contrast-enhanced MR angiography has also led to its widespread adoption in the follow-up of patients who have undergone open surgical intervention for atherosclerotic occlusive disease, most often in the form of bypass grafts, especially extra-anatomic bypass grafts. Although the presence of anastomotic hemostatic clips occasionally may preclude visualization of such conduits throughout their length owing to metallic susceptibility artifact, this is rarely of such severity as to limit diagnostic interpretation.²⁵ Three-dimensional contrast-enhanced MR angiography has also proven to be of high diagnostic utility in the diagnosis of acute aortic events, such as dissection or intramural hematoma, vascular inflammatory conditions (e.g., Takayasu and giant cell arteritis), evaluation of coronary arterial course and patency, and pediatric vascular abnormalities, including aortic coarctation (Figs. 82-11 and 82-12) and truncus arteriosus (Fig. 82-13), among many other conditions (Fig. 82-14).

FIGURE 82-11 Aortic coarctation in a 2-year-old child. A and B, The coarctation of the aorta and the abundant intercostal, parascapular and internal mammary collateral vessels (arrows on B) are well illustrated on contrast-enhanced MR angiography sagittal oblique MIPs.

FIGURE 82-12 Newborn girl with preductal interruption of the aortic arch. A and B, On contrast-enhanced MR angiography (A, oblique sagittal MIP; B, opposite oblique sagittal volume rendered), hypoplasia of the aortic arch distal to the left subclavian origin is seen, followed by a short segment of occlusion (arrow on A). The distal thoracic aorta is perfused by a large patent ductus arteriosus.

FIGURE 82-13 Newborn with a variant of truncus arteriosus type 1. On contrast-enhanced MR angiography, the pulmonary trunk (arrow) arises directly from the innominate artery, subsequently branching into normal-sized pulmonary arteries.

FIGURE 82-14 Total anomalous pulmonary venous return in a 2-month-old girl. A-D, Coronal contrast-enhanced MR angiography MIP reconstructions (A and B) and volume rendered reconstructions (C and D) illustrate confluence of the pulmonary veins bilaterally to form a horizontal vein with a vertical component (arrow) on the left side that subsequently drains to the coronary sinus. Volume rendered reconstructions are often valuable in preoperative assessment.

Contraindications

As considered previously with regard to time resolved MR angiography, after general contraindications to MRI and previous reactions to gadolinium-chelate contrast agents have been excluded, contraindications specific to contrast-enhanced MR angiography relate to more recently developed concerns regarding the potential induction of nephrogenic systemic fibrosis. Contrast-enhanced MR angiography does differ from time resolved MR angiography, however, because the dose of contrast agent required is considerably larger. The larger dose is needed because of the longer acquisition times involved, demanding protracted bolus infusions at rates sufficient to reduce adequately the T1 of blood in which the contrast agent resides at a level sufficient for diagnostic interpretation. Although little is known at present regarding the relationship between gadolinium-chelate contrast agents and nephrogenic systemic fibrosis, current U.S. Food and Drug Administration recommendations are that these agents should be avoided, if possible, in high-risk individuals. In this situation, dose reduction using TR MR angiography or obviation of the need for contrast administration by use of one of the alternative MR angiography techniques described earlier may be considered, depending on the clinical indication.

Pitfalls and Solutions

Successful high-resolution contrast-enhanced MR angiography may be a challenging feat even with a cooperative subject, unless one possesses knowledge of the numerous grounds for potential error during this technique. Important prerequisites for diagnostic contrast-enhanced MR angiography include the following:

1 Timing data acquisition such that it corresponds with the arrival of the contrast bolus within the vessels in question. Specifically, filling of the center of k-space should be synchronized with peak luminal enhancement of the target vascular territory.

2 Maintenance of a compact contrast bolus with the effect of optimizing the T1 difference between enhanced blood and adjacent soft tissues.

3 Complete data acquisition during a period sufficient for avoidance of venous contamination.

4 Optimization of signal reception after the contrast medium is within the vascular territory of interest.

5 Prevention of patient motion artifact, in particular artifact related to respiratory suspension when imaging the chest and abdomen.

Compromise in any of these prerequisites may result in suboptimal contrast-enhanced MR angiography. Premature central k-space filling relative to the peak of the contrast bolus may result in intraluminal “pseudofilling defects” owing to inappropriate filling of the low spatial frequency data in this region of k-space. Failure of coordination of the contrast bolus and data acquisition such that the arterial phase has passed and venous contamination is present may also obscure vascular detail, which is a particular disadvantage when imaging the lower extremities. Dispersion of the contrast bolus or poor signal reception may result in inadequate luminal enhancement, making identification of subtle vascular abnormalities impossible. Motion artifact also may have a significantly detrimental effect on image quality, owing to the production of blurring of vessel margins.²⁶

Preemptive avoidance of many of these potential adversities is often readily achievable, however. Numerous methods have been developed to aid in bolus timing, including use of a test bolus, “fluoroscopic” or automated bolus detection of contrast arrival, incorporation of a fixed delay between contrast injection and start of data acquisition, and repetitive scanning so that different phases of opacification are obtained.²⁷ Although some methods (e.g., test bolus, real-time “fluoroscopy,” and automated bolus detection algorithm) may be expected to produce more consistent results than others (e.g., fixed delay and multiphase imaging), familiarity with more than one technique is recommended so that timing issues in uncharacteristic cases may be addressed. Optimization of intravascular signal should involve a combination of MRI-compatible power injector and appropriate surface receiver coils, such that image contrast between the lumen and extravascular soft tissues is maximal. Motion artifact may be difficult, or occasionally impossible, to address—particularly in dyspneic patients because of the relatively long breath-hold times required for chest and abdominal contrast-enhanced MR angiography (approximately 20 seconds). Nonetheless, preprocedural coaching of patient respiratory suspension and attention to patient fears should represent a routine part of any contrast-enhanced MR angiography examination.

Image Interpretation

Postprocessing

Three-dimensional contrast-enhanced MR angiography provides a complete volumetric data set amenable to a wide variety of image post-processing.²⁸ MPRs are of significant practical value in the assessment of vascular patency, particularly in regions where overlapping vascular structures with tortuous courses are present (e.g., chest MR angiography). This holds particularly true when isotropic or near-isotropic voxels have been acquired because MPR allows reconstruction at any conceivable projection without the risk of image distortion or elongation. In certain instances, curved MPRs (whereby the axis of the reconstruction deviates from a single plane) may also be considerably valuable, most commonly in the determination of changes in caliber of long, tortuous vessels, such as the coronary arteries and thoracic aorta.

MIP reconstructions increase the conspicuity of the contrast-enhanced lumen by mapping the maximum signal intensity value at each location within a desired plane. Thin-section MIPs are popular in the evaluation of vascular patency, although they are known to result in overestimation of the significance of luminal stenoses, when present. Specific rotational full-thickness MIPs are routinely used at many institutions because they provide a quick means for demonstration of overall luminal integrity and vascular patency in a series of fixed projections.

Volume rendered techniques are among the most versatile for reconstructing three-dimensional contrast-enhanced MR angiography data. Volume rendering makes use of the information within each voxel to provide a two-dimensional projection of the three-dimensional data set, without compromise in through-plane attributes, and retains depth perception, a distinct advantage over MIP that provides a two-dimensional projection without depth information.