Clinical Applications Multistation MRA

With the development of moving or stepped table techniques, multi-station MRA has become a clinical staple for the evaluation of the peripheral runoff vessels [12-17]. The stepping table technique, akin to that used for peripheral x-ray angiography, consists of the progressive movement of the imaging field of view in conjunction with the arterial transit of a Gd-chelate contrast bolus down the lower extremities. This technique also called bolus chasing has become a clinical standard for peripheral MRA and is now achievable on any new MR scanner, using scanner-specific software [12-14] and hardware or a third-party movable tabletop that mounts on the scanner bed [15, 16]. Bolus chase peripheral MRA is typically performed using three (or possibly more) overlapping contiguous 3D MRA acquisitions. The initial 3D MRA is either centered about the lower abdominal aorta (the aortoiliac segment) or the most distal part of the imaging volume (feet) with each subsequent 3D MRA prescribed in overlapping fashion for distal or proximal coverage. Improved distal anatomic coverage can be provided by the performance of a pre-contrast 2D TOF MRA of the most distal arterial segments (e.g. calves and feet) prior to the bolus chase.

Early iterations of this technique required the initiation of imaging to begin with arrival of contrast into the abdominal aorta (i.e. initial or first station) to continue with the rapid acquisition of subsequent distal 3D MRAs. Arterial phase timing was performed using any of the previously mentioned timing techniques (e.g. test bolus scan, MR fluoroscopic trigger, MR SMARTPREP). Since arterial phase timing was only performed for 3D MRA of the initial station, the arteries of the more distal stations, particularly the infrapopliteal arteries, are often unreliably visualized.Wang et al [6] reported obtaining diagnostic quality images of the in-frapopliteal arteries in as few as 43% of cases as opposed to diagnostic quality images of the proximal two stations in 100% and 96% of cases, respectively. This is due in large part to the wide variation in bolus arrival times to the lower limbs, which hampers proper synchronization of imaging with arterial enhancement in the distal run-off vessels. Differences in contrast arrival times are more significant distally, especially in patients with underlying vascular disease. Prince et al [18] reported arrival times to the distal tibial artery (at the ankle) that ranged from 18 to 64 sec and disparate flow (greater than 5 sec difference in contrast arrival times) between right and left lower legs in about a quarter of their 87 patients with peripheral vascular disease. These variations in contrast arrival understandably result in inconsistent arterial illustration of the infrapopliteal arteries using a single injection bolus chase method alone.

Time resolved imaging has been found to be particularly helpful for ensuring proper visualization of the infrapopliteal arteries as an adjunct to bolus chase MRA. One technique called the "hybrid technique" [19] integrates time-resolved imaging into a peripheral MRA exam (Fig. 7). This technique, developed at Northwestern University, is predicated on a basic 3D gradient echo pulse sequence (TR 3.5 msec / TE 1.2 msec / flip angle 25 degrees) and dedicated peripheral vascular coil utilization. In this hybrid technique, two separate bolus-timing scans are performed (one in the pelvis and the other in the calves). The pelvis and calf bolus timing scans utilize a relatively high (inplane) spatial resolution 2D gradient echo scheme to acquire images in the axial plane every second for 40 seconds in the pelvis at the aortic bifurcation and for 80 seconds in the calves at the level of the posterior tibial and peroneal artery bifurcation. The timing sequence uses an image subtraction algorithm describe by Finn et al [19] where the first 3 images serve as a subtraction mask for all subsequent frames and the images are displayed on-line in near real time as the images are acquired. Each timing run is performed with a 2 mL bolus of Gd-chelate contrast media injected at 2 mL/sec followed by a 20 mL saline flush injected also at 2 mL/sec. Following the two bolus-timing scans, a pre-contrast mask 3D MRA acquisition is obtained in the distal station, which includes the calves and feet. Onset of distal station 3D MRA acquisition is based on the estimated arrival time of contrast media demonstrated on the initial 2D calf bolus timing scan. The actual calves/feet 3D MRA is performed with a 20 mL Gd-chelate contrast bolus injected at 2 mL/sec with two consecutive acquisitions obtained with no delay between acquisitions. Usually, each 3D MRA of the calves/feet are 22 seconds ±2 seconds in duration. If contrast arrival in each calf was demonstrated to be discrepant (i.e. asymmetric) by more than 10 seconds on the 2D calf bolus timing scan, then a third 3D MRA acquisition is performed at this station. The calves/feet 3D MRA slab is typically 80 mm thick with partition thickness of 1 to 1.2 mm each.

After the completion of the single station, multiphase 3D MRA of the calves/feet, the table is moved

Fig. 7a, b. Hybrid 3D peripheral MRA. Patient with peripheral occlusive disease as shown by hybrid peripheral MRA. Coronal MIPs of the two injection MRA can be stitched together to provide an extended FOV (a) view of the run-off vessels. The initial dedicated high spatial resolution, arterial-phase 3D CE MRA of the infrapopliteal arteries (b) provides excellent visualization of this patient with bilateral distal run off disease. Visualization of the proximal vessels is provided by the subsequent bolus chase 3D MRA performed from the abdominal aorta through the knee during the second contrast injection and mask 3D MRA acquisitions of the lower abdomen/pelvis and thigh stations are obtained. A second contrast infusion is performed with two-station moving table 3D MRA of the lower abdomen/pelvis (i.e. aortoiliac region) and thighs (i.e. superficial femoral/popliteal arteries). The start of the lower abdomen/pelvic acquisition is derived from the pelvic bolus timing scan. The thigh acquisition immediately follows that for the lower abdomen/pelvis with interval table translation in a manner similar to any other bolus chase technique. The general bolus profile utilized varies slightly based on patient weight but is as follows. The bolus chase through the lower abdomen/pelvis and thigh is performed using a biphasic contrast injection scheme (the first 20 mL at 2 mL/sec followed by the remaining 16 mL at 0.8 mL/sec, which yields a total pelvis and thigh contrast infusion time of 30 sec) for a total of 36 mL Gd-chelate contrast dose. The Gd-chelate contrast media injection is followed by a 20 mL saline flush injected at 2 mL/sec. The parameters (field of view, matrix size, number of partitions etc.) at each station are optimized to each patient body habitus to permit the most rapid scanning possible without clinically compromising through-plane resolution. Average scanning times for the lower abdomen/pelvis are 14-18 seconds for an 80 to 90 mm volume with partition thickness of 2.0-2.5mm each. If imaging times for the lower ab domen/pelvis are too long, one risks venous contamination in the thigh station that follows. Average scan time for the thigh station is approximately 1418 seconds as well but with a thinner volume and partition thickness (44-60mm and 1.5-1.7mm respectively). For any patients in whom 60 mL of Gd-chelate contrast media dose exceeds 0.3 mmol/kg, the contrast dose should be reduced appropriately so that the initial calves/feet station is performed with 15 mL of contrast media and the lower abdomen/pelvis-thigh contrast infusion is reduced to the remaining amount of a 0.3 mmol/kg cumulative dose. For the lower abdomen/pelvis-thigh contrast infusion, the rate should continue to be biphasic (e.g. initial 15 mL injected at 1.5 mL/sec followed by the remaining contrast dose at 0.5-0.6 mL/sec). In the situation of reduced dose, if contrast is to be infused at 1.5 mL/sec rather than 2 mL/sec, then the initial bolus timing scans (i.e. pelvis and calf 2D bolus timing scans) should be accordingly adjusted to rates of 1.5 mL/second as well.

Wang et al [6] and Khilnani et al [7] have suggested a similar type of integration of time resolved imaging, but using a 2D projectional MR DSA method for the initial bolus timing scan. In their case, 6-10 mL of Gd-chelate contrast media are injected for each projectional 2D MR DSA for the infrapopliteal vessels first. This is followed by moving table 3D MRA of the proximal stations.

Moving Table Mra

Fig. 8a, b. 3D TRICKS of the calf. Selected images (a) from a 3D TRICKS of the calf demonstrates fairly symmetric filling to the tibial and peroneal arterial segments despite an occlusion of the right popliteal artery. Note that the spatial resolution is sufficient to also see the many small collaterals about the right popliteal artery [Images courtesy of Dr. V. Laurent, CHU Nancy, Nancy, France]

Fig. 8a, b. 3D TRICKS of the calf. Selected images (a) from a 3D TRICKS of the calf demonstrates fairly symmetric filling to the tibial and peroneal arterial segments despite an occlusion of the right popliteal artery. Note that the spatial resolution is sufficient to also see the many small collaterals about the right popliteal artery [Images courtesy of Dr. V. Laurent, CHU Nancy, Nancy, France]

The initial distal 2D MR DSA is thus used to approximate circulatory times for proper "bolus sharing" and planning of the multi-station bolus chase 3D MRA.

Hany et al [21] and Swan et al [22] have successfully implemented 3D TRICKS for peripheral MRA. In their original description, separate injections are used for each of the typical three stations, with cumulative contrast doses of 0.3 mmol/kg for each patient. TRICKS imaging of the infrapopliteal arteries (Fig. 8), appears to be particularly good and can be selectively used as described above as an initial lower station 3D MRA exam to be followed by a multi-station bolus chase 3D CE MRA exam for the proximal stations, thereby limiting the injections to two for a three station study.

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