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.

CE-MRA can also visualize stenoses within the vertebral artery or within the basilar artery. Reversal of blood flow in the vertebral artery with subclavian steal phenomenon is recognized using CE-MRA in combination with through-plane velocity mapping CMR (after handgrip exercise) at the level of the vertebral arteries. Confirmation is with 2D time-of-flight MRA acquired first with an inferior and then a superior saturation band to confirm flow reversal.

Carotid MRA reports concentrate on:

Pulmonary vessels

Pulmonary arteries

CMR is well suited for the evaluation of PA anatomy and this is widely used in patients with ACHD where evaluation of the size, patency and anatomical relations of the PAs is combined with RV assessment and PV status. It is also being increasingly used in the assessment of patients with pulmonary hypertension (Figure 8.3). CT pulmonary angiography remains the imaging modality of choice for diagnosis of acute pulmonary embolism since MRA has longer acquisition, and thus breath-hold times, and lower spatial resolution leading to smaller peripheral emboli potentially going undetected.

Figure 8.3 CMR assessment in a 59-year-old woman with pulmonary hypertension secondary to chronic thromboembolic disease. (a) MIP reconstruction of PA CE-MRA highlighting proximal obstruction of the right upper and middle lobe arteries (solid white arrow). There was also evidence of multiple distal stenoses predominantly in the lower lobe branches. (b) Concurrent serial SSFP cine cardiac assessment revealed a dilated RV with reduced EF (34%) and moderate TR (black arrow). (c) An incidental liver cyst seen on the pilot transaxial FSE images (dashed white arrow).

Pulmonary veins

The use of CMR in the diagnosis and differentiation of the various forms of partial and total APVD has already been discussed. Additionally, it has a role in the evaluation of the pulmonary veins and LA pre-radiofrequency ablation for cardiac arrhythmias. Subsequent assessment of pulmonary vein stenosis requires CE-MRA combined with velocity mapping (Figure 8.4).

Figure 8.4 MIP reconstruction of CE-MRA data from a 47-year-old man showing moderate stenosis (50%) of the right upper anterior pulmonary vein following radiofrequency ablation (white arrow).

The pulmonary veins should also be imaged using CE-MRA in cases of cardiac malignancy involving the LA in order to exclude extension.

MRA reports of the pulmonary vessels include:

1. Description of the anatomy;

2. Concurrent cardiac assessment, such as the RV in ACHD and pulmonary hypertension; and

3. Measurements of pulmonary vein anatomy and orifice diameters in patients referred for radiofrequency ablation.

Renal and mesenteric arteries

CE-MRA is a good method for assessment of the renal arteries, especially in cases of renovascular hypertension due to atherosclerosis and, to a lesser extent, suspected fibromuscular dysplasia.

CMR screening protocols for secondary causes of hypertension conclude with angiography and are performed during an hour-long study in the following manner:

• Cardiac assessment and later calculation of LV volumes, function and mass;

• Exclusion of coarctation of the aorta;

• Evaluation of the adrenal glands; and

• Renal CE-MRA (Figure 8.5).

Figure 8.5 Part of a CMR hypertension screen from a 55-year-old man.

(a) SSD reconstruction of the renal CE-MRA data showing no evidence of renal artery stenosis. A left accessory renal artery is indicated (white arrow). (b) Prior serial SSFP GE cine cardiac assessment was later analysed to reveal normal LV size, volume and function, with mild LVH. (c) Oblique sagittal SSFP GE cine image showing no evidence of coarctation of the aorta (two-headed white arrow).

High-resolution T1W SE images of the adrenal glands were normal and are not illustrated in this figure.

CE-MRA provides reliable visualization of the major renal arteries and accessory renal arteries along their entire length. However, spatial resolution remains lower than with conventional X-ray angiography, so branch vessels and small accessory vessels are less well depicted. This is of relevance, for example, in some cases of fibromuscular dysplasia where the main renal arteries may not be involved but smaller side branches are affected. Fibromuscular dysplasia is a much less common cause of renal artery stenosis than atherosclerosis. It principally affects young women who present with hypertension and biochemical abnormalities, but rarely renal impairment. Lesions typically affect the main renal artery, are unilateral, beaded in appearance, and may have multiple stenoses. By comparison, atheromatous renal artery stenosis has a predilection for an older, male population with evidence of atheroma elsewhere, treatment-resistant hypertension, and associated renal dysfunction. Importantly, gadolinium contrast agent is not nephrotoxic. The commonest site for atheromatous renal artery stenosis is at the ostium of the renal artery and multiple lesions affecting different-sized vessels may co-exist. Grading of stenosis severity is assisted by post-processing MPR reconstruction and velocity mapping CMR.

Published data related to mesenteric CE-MRA is limited. Anecdotally, results appear comparable to renal angiography. Most stenoses in the coeliac trunk and superior mesenteric artery occur within the proximal segment where the diameter of the vessel is the greatest. The inferior mesenteric artery is often less well visualized since it is smaller and consequently stenoses at this site are often overestimated by CMR.

Renal MRA reports include:

Aorta

The thoracic aorta has already been discussed in Chapter 6. Optimal visualization of the aorta is via a combination of SE and GE techniques to ensure accurate evaluation of the aortic wall and peri-aortic tissues, and lumen, respectively. With inflammatory conditions, such as aortitis, contrast agents are used to further highlight aortic wall abnormalities. CE-MRA readily depicts location, extent and exact diameter of aortic aneurysms with measurements being made on the original images (Figure 8.6). In cases of aortic dissection where there is continuation to the abdominal aorta it is important to know whether side branches originate from the true or false lumen. CT is often the modality of choice for investigating acute pathology of the aorta, for reasons of both safety and availability, while CMR is an ideal method for patient follow-up following surgical or medical treatment.

Figure 8.6 CMR study from a 62-year-old man referred prior to elective CABG for further investigation of chronic renal failure.

(a) MIP reconstruction of CE-MRA data showing a diffusely atherosclerotic abdominal aorta with a large (6.4 ×7.0 ×8.7cm³) infra-renal aneurysm (solid white arrow). The aneurysmal segment starts 4.0 cm after the origin of the renal arteries and has a proximal neck measuring 2.5 cm in diameter. The common iliac arteries arise from the distal aspect of the dilatation. Multilayered mural thrombus was noted within the aneurysm on SE imaging (images not shown). The right and left renal arteries had only mild stenoses (dashed white arrow) and the kidneys are of comparable size. (b) Part of the prior SSFP GE cine assessment demonstrating thinning of the mid-anterior wall on the two-chamber view (solid black arrow). (c) This segment was associated with near transmural late enhancement indicative of MI and non-viable myocardium (dashed black arrow).

MRA reports on the abdominal aorta include:

Peripheral arteries

CE-MRA is the best CMR technique for evaluating arterial disease in the upper and lower limbs. Visualization of the subclavian and brachial arteries requires a dedicated body coil, otherwise the technique is similar to that of thoracic aorta MRA. In order to avoid venous overlap the contrast should be injected in the contralateral arm. Imaging of the small arteries of the hand requires high-resolution scanning, dedicated surface coils, and precise timing of the start of imaging. An additional use of CE-MRA is in the diagnosis of thoracic outlet syndrome where images are acquired with the arms in abduction and in neutral position (Figure 8.7). High-resolution targeted T1W SE sequences are required to demonstrate the cause of compression in this condition.

Figure 8.7 MIP reconstruction of CE-MRA data from a 30-year-old woman with thoracic outlet syndrome.

Images were acquired with the right arm in elevation and abduction (symptomatic position) and show a stenosis in the proximal subclavian artery (white arrow). CE-MRA with the arm in neutral position did not reveal any abnormalities (not shown).

For imaging of the lower limbs, the most commonly used technique is the bolus chase technique. With this, the first step is the production of mask images of all three stations of the leg: aortoiliac region, femoral region and lower legs. Then, during constant injection of contrast, the contrast bolus is followed throughout the entire leg using a moving table. Imaging of the tibial vessels may be compromised by venous contamination. Steno-occlusive disease in the distal aorta is readily seen as in Leriche’s syndrome and CE-MRA is also useful in the surveillance of extra-anatomical bypass grafts (Figure 8.8).

Figure 8.8 Lower limb CE-MRA.

(a) Diagrammatic representation of normal lower limb arterial anatomy. (b) MIP reconstruction of CE-MRA data from a 70-year-old man presenting with intermittent claudication on a background of a left femoropopliteal bypass graft operation. The graft is shown to be patent (between white arrows).