Contrast-enhanced MRA

The basic pulse sequence for ultrafast CE-MRA is a 3D GE acquisition which is usually designed to acquire data encoding for contrast first—centric-ordering. The use of gadolinium contrast agent shortens the T1 value of blood so that it appears bright irrespective of flow patterns or velocities (Figure 8.1).

Figure 8.1 Carotid CE-MRA.

(a) MIP reconstruction of the entire CE-MRA data. The left common carotid artery (solid white arrow) is seen to have a stenosis following its bifurcation (white square). (b) Enlarged and rotated view of the initial MIP highlights a severe, discrete stenosis at the left internal carotid artery (dashed white arrow).

Correct timing of scan acquisition is of paramount importance for optimal imaging of the passage of contrast through the blood vessel of interest. This is usually achieved by application of ‘bolus-track’ techniques, i.e. a real-time 2D fluoroscopic sequence visualizing the arrival and passage of contrast in the great vessels, which subsequently trigger the high-resolution 3D CE-MRA sequence. The time delay in switching between these two sequences must also be taken into account. Alternative timing methods are the use of a test bolus in order to calculate bolus arrival time, or obtaining a series of ultrashort acquisitions so that at least one acquisition coincides with bolus arrival time.

Time-of-flight (2D or 3D)

This is an older GE technique in which flow signal is enhanced by inflow effects. Stationary tissue within a region of interest is saturated by rapid application of RF pulses. This results in fresh blood flowing into this plane being non-saturated and therefore appearing bright. Unwanted venous signal can be eliminated by application of specific saturation bands. Time-of-flight methods are sensitive to flow velocity; for example, slow flow near the vessel wall appears less bright with this technique than faster flow in the middle of the vessel. Importantly, the 2D time-of-flight sequence has the disadvantage of overestimating vessel stenosis due to a phenomenon known as phase dispersion. This effect is less pronounced using 3D time-of-flight imaging.

Maximal intensity projection

In this ray tracing post-processing algorithm, the desired viewing plane is specified first. Then the maximum intensity encountered in parallel rays along this viewing plane is assigned to the displayed pixel. MIP reconstructions are applicable to both time-of-flight and CE-MRA data and have the tendency to overestimate the degree of vessel stenosis (Figure 8.2b).

Volume rendering technique and shaded surface display

This is a ray casting algorithm which selects visible voxels by tracing rays from an instantaneous viewing position. The surfaces are identified by a threshold technique and the resulting SSD provides 3D appreciation of the vessel. Resulting images can also overestimate the degree of vessel stenosis (Figure 8.2c).

Curved multiplanar reconstruction

Curved multiplanar reconstruction (MPR) can be used to obtain images in views other than that of the native acquisition (Figure 9.3). These work particularly well on 3D data sets with isotropic voxels in which resolution is identical in any obtained plane.

Carotid arteries

The most commonly used sequences for carotid MRA are time-of-flight and CE-MRA (Figure 8.1). For imaging of the aortic arch and cervical arteries, CE-MRA is the only option. Both 3D TOF and CE-MRA are as accurate as conventional X-ray angiography in the measurement of internal carotid artery stenosis, and because of the small but significant risk of stroke with the invasive technique (0.5–1.0%), CMR is recommended as the optimal method of evaluating carotid artery disease. Data interpretation requires careful evaluation of the raw data to avoid overestimation of stenosis severity. MIP projections will aggravate signal loss and cause vessels to appear narrower because the algorithm selects brightest intensities both within the vessel and in the background. Such overestimation with MIPs is more of a problem in 3D time-of-flight than with CE-MRA, in which background intensities are suppressed.

MRA is excellent for the diagnosis of carotid dissection. Dissection typically consists of haemorrhage in the media, sometimes extending into the adventitia, and the intimal flap is not always apparent. Angiography identifies a smooth or irregular narrowing and high-resolution SE images with fat saturation prepulse can help to identify the false lumen.