Contrast Enhanced MRA of the Peripheral Vasculature

Three different approaches may be used, as follows:

1. Single-station MRA of the peripheral arteries [18,19,30]

This approach evaluates a single field-of-view only, and relies on prediction of the site of pathology by clinical or imaging (e.g. duplex) criteria. However, anatomical prediction of the location of occlusive lesion is difficult and multiple lesions occur at discrete sites, many of which lie outside a single-field-of-view in the majority of patients. Although of proven efficacy, this approach is mostly used for evaluating parts of the peripheral vasculature that have been "missed" by arteriography, especially the pedal arteries in patients in whom distal bypass grafting is an option. And, the approach plays an essential role in imaging patients with aortic or bilateral iliac occlusion where catheter arteriography is not possible. See Chapter VI.i on aorto-iliac arteries as an example of "single-station" CE-MRA.

2. Multi-station, multi-injection MRA of the peripheral arteries [31]

This approach employs a single-station MRA at each of two or three consecutive locations, with a separate injection for each location. Although proven in clinical practice, this technique is now rarely employed since refinements in moving-table approach have been made (Table 3).

3. Moving Table Contrast-Enhanced MRA [27,28, 32-35]

By moving the patient rapidly through the isocenter of the magnet and acquiring 3D data-sets at three coronally-oriented overlapping FOV's, it is possible to image the arteries of the entire lower half of the body during a single contrast injection (Fig. 2-7). Initially, this approach employed manual movement of the table-top (detached from the drive mechanism) by the operator and "fixed" imaging parameters (scan volume, resolution, etc.) for all locations. Although successful, the requirement for higher resolution of the infra-popliteal vessels (due to their small size) and requirement to minimize acquisition time for the first two locations (to reduce venous enhancement within the legs) has fuelled advances in technology as follows:

• Automated table movement (a "floating" table)

• Resolution optimized individually for each location [36]

• Imaging volume optimized individually for each location [36]

Table 4 summarizes the advantages and disadvantages of moving-tabel CE-MRA.

The main challenge of MT CE-MRA is to eliminate or reduce to an acceptable level the degree of venous-enhancement in the 3rd location, which may degrade images of the infra-popliteal arteries.

Moving Table Peripheral Mra

Fig. 3a, b. a Images from a time-resolved single thick slice MR fluoroscopic detection sequence (1 image/second) demonstrating arrival of contrast within the aorto-iliac arteries over 4 consecutive images. b There is occlusion of the right common and right superficial femoral arteries with reconstitution of flow into diffusely diseased popliteal artery. There is a good anterior tibial artery, a diffusely diseased but patent peroneal artery and occlusion of the posterior tibial artery. On the left side, there is occlusion of the common and external iliac arteries, and also of the superficial femoral artery throughout its length. The popliteal reconstitutes for a short distance, before occluding at the level of the joint with further reconstitution of popliteal flow more distally. There is a good left anterior tibial artery with a stenosis above the ankle joint, a diffusely diseased but patent peroneal artery, and occlusion of the left posterior tibial artery

Peroneal Artery Occlusion

Fig. 4a, b. Frontal (a) and lateral (b) MIPs demonstrate occlusion of the right superficial femoral and popliteal arteries, with a single patent infrapopliteal artery (the anterior tibial which reconstitutes via collaterals). On the left side, note diffuse aneurysmal disease of the distal SFA and popliteal arteries. The posterior tibial artery is patent and enlarged, the peroneal artery is occluded and the left anterior tibial artery is of small caliber

Fig. 4a, b. Frontal (a) and lateral (b) MIPs demonstrate occlusion of the right superficial femoral and popliteal arteries, with a single patent infrapopliteal artery (the anterior tibial which reconstitutes via collaterals). On the left side, note diffuse aneurysmal disease of the distal SFA and popliteal arteries. The posterior tibial artery is patent and enlarged, the peroneal artery is occluded and the left anterior tibial artery is of small caliber

Sfa Occlusive Disease

Fig. 5. Male patient with a previous history of aorto bi-femoral and right femoro-popliteal grafting. Note the typical appearance of the two limbs of the aorto bi-femoral graft. There is a 4cm aneurysm of the native right internal iliac artery. There is also an anastomotic pseudoaneurysm of the right common femoral artery in the right groin. The right femoro-popliteal graft is patent. The right anterior and posterior tibial arteries appear normal. On the left side, there is a tight stenosis in the distal superficial femoral artery just proximal to the point where the superficial femoral artery occludes within the adductor canal. Note the large collateral which runs medially and which conveys blood to the posterior tibial artery which appears normal throughout its length. Normal left peroneal artery, the anterior tibial artery is occluded

Fig. 5. Male patient with a previous history of aorto bi-femoral and right femoro-popliteal grafting. Note the typical appearance of the two limbs of the aorto bi-femoral graft. There is a 4cm aneurysm of the native right internal iliac artery. There is also an anastomotic pseudoaneurysm of the right common femoral artery in the right groin. The right femoro-popliteal graft is patent. The right anterior and posterior tibial arteries appear normal. On the left side, there is a tight stenosis in the distal superficial femoral artery just proximal to the point where the superficial femoral artery occludes within the adductor canal. Note the large collateral which runs medially and which conveys blood to the posterior tibial artery which appears normal throughout its length. Normal left peroneal artery, the anterior tibial artery is occluded

Axillofemoral Bypass

Fig. 6. 78 year old patient with Leriche syndrome in whom an ax-illo-femoral bypass graft has been constructed. There is occlusion of the aorta just below the level of the renal arteries and also occlusion of the iliac arteries bilaterally. Note the bifurcated right axillo-femoral graft which anastomoses with the profunda femoris arteries on both sides. Note occlusion of the superficial femoral artery on both sides, with reconstitution of flow into the popliteal artery on each side via well marked collaterals. Despite the severe proximal disease, three run-off arteries are identified on both sides

Fig. 6. 78 year old patient with Leriche syndrome in whom an ax-illo-femoral bypass graft has been constructed. There is occlusion of the aorta just below the level of the renal arteries and also occlusion of the iliac arteries bilaterally. Note the bifurcated right axillo-femoral graft which anastomoses with the profunda femoris arteries on both sides. Note occlusion of the superficial femoral artery on both sides, with reconstitution of flow into the popliteal artery on each side via well marked collaterals. Despite the severe proximal disease, three run-off arteries are identified on both sides

Left Femoral Popliteal BypassMra With Aorta Run Off

Fig. 7a, b. a Note fusiform aneurysmal disease of the infra-renal abdominal aorta, left common and left external iliac arteries. On the left side, there appears to be occlusion of the poplitreal artery, however, the appearance is somewhat unusual in that the signal intensity within the artery gradually drops of towards the "occlusion". A repeat single-station MRA of the thigh arteries alone 24 hours later (b) demonstrates a patent left popliteal artery, emphasizing the fact that the "pseudo-occlusion" was due to slow flow within the left SFA. Note occlusion of the right common and external iliac arteries and distal right superficial femoral arteries

Fig. 7a, b. a Note fusiform aneurysmal disease of the infra-renal abdominal aorta, left common and left external iliac arteries. On the left side, there appears to be occlusion of the poplitreal artery, however, the appearance is somewhat unusual in that the signal intensity within the artery gradually drops of towards the "occlusion". A repeat single-station MRA of the thigh arteries alone 24 hours later (b) demonstrates a patent left popliteal artery, emphasizing the fact that the "pseudo-occlusion" was due to slow flow within the left SFA. Note occlusion of the right common and external iliac arteries and distal right superficial femoral arteries

Table 4. Advantages and disadvantages of moving table MRA of the peripheral arteries

Advantages

It addresses the issue of extended anatomic coverage in the most time-efficient manner It makes the most efficient use of a single bolus

Disadvantages

It requires additional software and hardware

In some instances results in less venous enhancement than other multi-station approaches

Although this approach offer a practical solution to the limitation of reduced spatial coverage, it is limited by the fact that, even using fast gradients, acquisition of high-resolution 3D MRA data at three consecutive locations takes substantially longer than the transit time from aorta to leg arteries, thus increasing the likelihood of venous enhancement within the legs which undoubtedly impairs image quality.

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