Principles of Imaging in Vascular Disease

Time-of-Flight Angiography

TOF angiography is performed using a flow-compensated gradient refocused sequence. Data are typically acquired as a stack of two-dimensional slices or as a single three-dimensional volume. In this technique, stationary tissues in the slice or volume of interest are saturated from repeated radiofrequency pulses and have low signal intensity. Blood flowing into the imaging has not been subjected to these radiofrequency pulses and is therefore fully magnetized (unsaturated). When it flows into the volume, it is bright compared with the stationary background tissues. This two-dimensional inflow technique works well in vessel segments running perpendicular to the imaging plane such as relatively normal arteries and veins, including the carotid arteries, the cerebral vasculature, the inferior vena cava, and iliac veins. Selective saturation pulses can be applied, allowing selective saturation of blood entering the imaging volume, such that either the arterial or the venous signal can be suppressed. With this technique, separate images of the arterial and venous system can be acquired.

A limitation of this technique is in-plane saturation, which results in signal dropout when the long axis of the vessel coincides with the scan plane or in the presence of slowly flowing blood in tortuous arteries. Another limitation is turbulence-induced signal loss in and distal to a stenosis. In addition, the relatively long echo times required for gradient moment nulling make TOF imaging sensitive to susceptibility artifacts from bowel gas, implanted metallic objects (e.g., clips), or other air-tissue interfaces. These artifacts are a primary reason for the inaccuracy of TOF MRA. In addition, imaging times are lengthy because vessels need to be imaged perpendicular to the long axis of the vessel and a stack of slices must be acquired. Long acquisition times can lead to artifacts caused by slice misregistration or patient motion. Three-dimensional TOF MRA is used when a small area is being imaged and vessels run in different orientations such as the carotid bifurcation or Circle of Willis. Nearly isotropic volumes may be obtained allowing vessel reformation in any direction. If offers higher spatial resolution and higher signal to noise. Disadvantages include insensitivity to slow flow and saturation effects that limit slab thickness. Imaging times are longer than two-dimensional–TOF techniques and unlike two-dimensional–TOF techniques where motion artifact affect single slices, in three-dimensional–TOF, motion artifact affects all slices.

Phase-Contrast Angiography

Phase-contrast angiography uses velocity-induced phase shifts, which occur as blood flows through a magnetic field in the presence of flow-encoding gradients. Two images created with opposite bipolar flow-encoding gradients are subtracted from each other; in the phase difference image, the residual phase is proportional to velocity. Stationary tissues do not undergo a velocity-induced phase shift in either image; they are suppressed and therefore subtract completely. The flow-encoding gradients can be in any direction or in multiple directions, depending on the selected flow sensitivity. Phase-contrast angiography can be performed as a two- or three-dimensional gradient refocused sequence and is improved after contrast administration. It has been used in the evaluation of the renal arteries, carotid arteries, and portal veins. It is also used to measure flow velocity. When used with cardiac gating, a time-resolved velocity profile can be generated, providing quantitative measurement of the flow rate.

There are limitations to the technique. The amplitude of the preselected bipolar gradient determines the degree of velocity encoding (VENC). Phase-contrast sequences encode only a specified range of velocities. In occlusive disease, turbulence causes a wide spectrum of rapidly changing velocities, which produce intravoxel phase dispersion and signal loss. Artifactual signal loss at sites of vessel stenosis is common. The sequence is also susceptible to measurement degradation from cardiac, respiratory, and translational motion. Background subtraction can be problematic. Currently, because faster MR angiographic techniques are available, phase contrast techniques are used primarily for the measurement of flow velocity.

Three-Dimensional Contrast-Enhanced Magnetic Resonance Angiography

Three-dimensional TOF gadolinium-enhanced MRA is the most widely used and useful of the MRA techniques. It does not rely on motion of blood to create flow signal. A contrast agent, typically gadolinium chelate, is given to shorten the T1 (spin-lattice) relaxation time of blood so that it is significantly shorter than that of surrounding tissues. Blood is imaged directly using a T1-weighted sequence. Blood is bright relative to background tissues during the arterial phase of three-dimensional contrast-enhanced MRA, because contrast transiently reduces the arterial blood T1 to less than that of the brightest background tissue (fat). This technique reduces the sensitivity to turbulence, and in-plane saturation effects are eliminated. With this technique, a number of slices are oriented in the plane of the target vessels, permitting rapid imaging of a large field of view covering a large region of the vascular system. Contrast-enhanced MRA is fast and affords high-quality breath-hold and non–breath-hold angiograms. Dynamic contrast-enhanced MRA exploits the transient shortening in blood T1 after the intravenous (IV) administration of a contrast agent using a fast, three-dimensional spoiled gradient echo sequence. This is a first-pass technique, because the contrast agents currently used are extracellular agents and the gadolinium chelate rapidly leaks into the extravascular space. However, repeated slice volumes over the region of interest can be acquired during passage of the contrast allowing images in both the arterial and venous phases. Typical repetition times for contrast-enhanced MRA are less than 5 ms, with echo times of 1 to 2 ms and total scan times of 10 to 30 seconds. The sequences that use T1-weighted gradient echo, T1 fast field echo, or fast low-angle shot have high spatial resolution and a high signal-to-noise ratio. The images can be acquired in a breath-hold fashion and reformatted in any plane. Because intravascular signal is dependent on T1 relaxation rather than inflow or phase accumulation, in-plane saturation and signal loss owing to turbulence are not significant.

During the first-pass arterial phase, imaging is done before vascular contrast equilibration. Using a first-pass technique, steady-state background signal is nearly eliminated. Once the contrast is injected and dynamic imaging is done, however, a second bolus injection of contrast material must be given to repeat the process over, for example, a second region of interest. When the second dose is given, residual soft tissue enhancement can obscure vascular detail. Subtraction techniques, using a precontrast mask, are typically used to remove background tissues. When the second or third dose of contrast is used, subtraction is mandatory. Timing is important in contrast-enhanced MRA. Unlike computed tomography (CT), MRA does not map spatial data linearly over time. With three-dimensional MRI, all the three-dimensional Fourier or K-space data (the information from which the image is constructed) are collected before individual slices are reconstructed. K-space maps spatial frequencies rather than spatial data. Consequently, K-space data do not correspond to image space directly. Different portions of K-space determine the features of the image. The center of K-space, which records low spatial frequencies, affects contrast, whereas the periphery of K-space records high spatial frequencies, which contribute to the fine details, such as edges.

Intravascular T1 signal intensity is determined by the gadolinium concentration at the time the center of K-space is collected. The timing is synchronized so that the midportion of the bolus arrives at the desired site as the center of K-space is being collected. Perfect timing produces maximal arterial signal with minimal venous signal. If central K-space data are acquired too early, while arterial gadolinium is increasing rapidly, ringing or banding artifacts may be generated. Acquiring central K-space data too late leads to reduced arterial signal intensity and enhancement of venous structures. Several methods have been used to obtain proper bolus timing, including simple estimates of the travel time of the bolus from the site of injection to the region of interest. For example, in a healthy patient, the travel time from the antecubital vein to the aorta is approximately 15 seconds; in a patient with cardiac disease or an aneurysm, it is 25 to 35 seconds. To estimate contrast travel time more precisely, a test bolus can be used. In this technique, 1 to 2 mL of contrast followed by a 10- to 15-mL saline flush is injected at the same rate as the planned injection. Multiple single-slice fast gradient echo images of the appropriate vascular regions are then obtained as rapidly as possible—typically, every 1 to 2 seconds for a given period—and the time to peak enhancement (contrast travel time) is determined in the region of interest. A limitation of this technique is the setup time; also, the redistribution of the test bolus to the interstitial space may add to the background signal. Other timing techniques used include MR fluoroscopy. In this technique, two two-dimensional sagittal gradient refocused images are obtained rapidly (<1 second per image) throughout the region of interest. Images are generated in near real time and updated at a rate greater than one image every second. The bolus time is watched, and when it arrives, the operator switches over to the three-dimensional MRA sequence. This technique may be helpful in cases of asymmetrical flow because of asymmetrical stenoses.

Another technique is a temporally resolved method in which multiple three-dimensional data sets are rapidly acquired (over 2 to 8 seconds) without any predetermined timing; injection and scanning are begun simultaneously. The operator then selects the desired image set. Other techniques used to facilitate MRA involve simply scanning faster using parallel imaging techniques and using alternative K-space acquisition techniques.

Respiratory motion causes image blurring, ghosting, and signal loss. In three-dimensional imaging, blurring occurs in the direction of motion, whereas ghosting is more pronounced in the phase-encoding direction. Before the availability of fast imaging systems, MRA techniques were too long to permit breath holding. Breath holding results in improved images in abdominal and thoracic MRA and facilitates the visualization of small vessels, such as the renal arteries. It does not appear to be as critical in the evaluation of the carotids. Most ambulatory patients can hold their breath 20 to 30 seconds, and imaging is usually done on inspiration.

Another MRA technique that is now widely used is a time-resolved three-dimensional MRA technique with very high subsecond temporal resolution and submillimeter spatial resolution, which allows capture of multiple arterial, mixed, and venous phase images during the passage of contrast agent through the vascular anatomy (e.g., TWIST sequence). No test bolus or bolus timing is required with this technique. Imaging is so fast that venous contamination is eliminated. The technique is helpful for monitoring flow through vascular beds. It is helpful in determining flow patterns through cerebral and peripheral malformations and in delineation of shunt characteristics.

There is also renewed interest in non–contrast-enhanced MRA techniques using inflow inversion recovery techniques, and these sequences are an area of research interest, particularly for assessment of the renal arteries.

Clinical Applications

Extracranial Carotid Arteries

The carotid arteries can be well seen with contrast-enhanced MRA ( Fig. 15.1 ). The short circulation time of the carotid circulation (4 to 6 seconds) makes timing critical; otherwise, venous overlap of the images results. Both long and short imaging times have been used to overcome this problem.

High-resolution imaging with short echo time is recommended. A major problem that impedes accurate stenosis assessment is the occurrence of high-speed turbulent jets at the site of stenosis. Specialized coils are recommended. The origins of the vessels from the aorta should be included in the imaging plane. Numerous studies have shown a high degree of correlation between stenosis measurement with contrast-enhanced MRA and DSA. MRI has also been used to evaluate atherosclerotic plaque in the carotid arteries.

Thoracic Aorta

Contrast-enhanced MRA is used routinely to evaluate the thoracic aorta. It is the standard of practice for following the status of dissections and monitoring aneurysm enlargement ( Fig. 15.2 ). Imaging is usually performed in the oblique sagittal plane or coronal plane. In aortic dissection, the origins of the brachiocephalic vessels should be included. A phased array body coil is recommended when the aneurysm is confined to the thoracic aorta. The ascending, descending, and abdominal aortas are evaluated. A full evaluation of the aorta should include multiplanar cardiac-gated black blood imaging of the aortic wall to detect intramural hematoma.