Peripheral MR angiography (PMRA), especially using bolus chase techniques, is steadily replacing conventional x-ray angiography as the primary diagnostic method for evaluation of patients with peripheral vascular disease. Bolus chase contrast-enhanced PMRA can reliably and accurately illustrate the peripheral vasculature from the abdominal aorta through the trifurcation vessels in one to two minutes.1-5 In addition to the inherent clinical benefits of noninvasiveness and avoidance of ionizing radiation and iodinated contrast agents, MRA incurs lower cost relative to conventional catheter angiography.
Most vendors offer specialized software and/or equipment for bolus chase CE PMRA, which reliably provides accurate diagnostic-quality peripheral angiograms if used properly. While subtle differences involve the individual MR scanner and its commercial product offerings, the basic concept for bolus chase PMRA is fairly universal. Common practical considerations may not be specifically addressed by vendor product inserts or instructions, and experience is often the best source for tips to optimize this technique.
Bolus chase CE PMRA requires the ability to perform multiple 3D MRAs rapidly, move the patient accurately and quickly, and administer the contrast bolus at a constant rate. Bolus chasing by MR is akin to early bolus chase imaging used for cut film x-ray runoff studies, which used a stepping table to follow the contrast column during its distal progression. With bolus chase MRA, overlapping 3D acquisitions are performed during the arterial transit of a gadolinium chelate contrast bolus down the peripheral vessels.
Early attempts at bolus chasing were manual, and operators simply unlatched the scan table and slid the patient between rapid 3D MRA acquisitions. They used a measuring stick to achieve accuracy of patient positioning.1,2 Most vendors now offer dedicated bolus chase pulse sequences that integrate 3D imaging with table positioning. A separate movable platform that sits on the scanner table can also be used for speedy patient translation between scans, but this requires an operator to be physically present in the scanner room to manually reposition the patient. The separate movable table platform may, however, be retrofitted for use on different scanners within the same clinic, and it enables bolus chasing on older scanners for which dedicated bolus chase pulse sequences may not exist.
With all techniques, accurate table repositioning is critical, as image mask subtraction of pre- and postcontrast 3D image sets is often necessary for proper visualization of the small distal runoff vessels. Accurate positioning also ensures proper overlap between imaging fields-of-view (or stations) and complete anatomic coverage.
MRA imaging protocol. Localizers are typically performed for each station using bright blood gradient-echo pulse sequences. Although simple sagittal localizers can be used, blood inflow is often poor and arterial structures suboptimally depicted. If possible, it is often preferable to localize from axially acquired gradient-echo images (e.g., axial 2D time-of-flight MRA). Axial imaging provides improved arterial depiction, as vascular inflow (i.e., TOF effect) is optimized. With axial 2D TOF images, localization of individual 3D MRA volumes for bolus chasing should be performed on sagittal maximum intensity projections (MIP) so that oblique coronal 3D volumes can be prescribed. Oblique coronal prescription of the 3D MRA volumes optimizes spatial resolution and minimizes scan time for each station.
For bolus chase MRA, a standard fast 3D spoiled gradient-echo pulse sequence is used. A multistation pass is performed prior to and in conjunction with the bolus administration, with precontrast 3D MRA providing the mask for subsequent image subtractions. The number of partitions, partition thickness, field-of-view, and matrix sizes should be selected to ensure optimum arterial depiction of the individual station. Generally, spatial resolution demands are higher for the infrapopliteal arteries than for the abdominal aorta. For imaging the aorto-iliac segment and the superficial femoral arteries, relatively larger voxel sizes (2 to 2.5 mm) are often adequate, with smaller sizes (1.5 mm or smaller) preferable for the infrapopliteal vessels. The table shows sample pulse sequence imaging parameters for performing timing bolus and bolus chase PMRA that offer general guidelines for the users to modify to fit preferences and equipment. Actual parameters used may vary considerably and will be vendor-specific.
One critical consideration is imaging time for each station. Higher spatial resolution requires longer acquisition times, which in turn delay time of imaging for all subsequent stations. In general, imaging of each of the initial two stations (station 1: abdomen/pelvis, and station 2: thigh) should be no longer than 20 to 25 seconds. The time required for the third and usually final station is typically 30 to 45 seconds, as high spatial resolution is often necessary.
Taking advantage of k-space phase ordering can be useful. As with traditional contrast-enhanced MRA, arterial signal is dependent primarily on the timing for central k-space data acquisition, preferably during peak arterial gadolinium enhancement. Partial Fourier imaging (i.e., 0.5 NEX) with station-specific phase ordering (sequential for proximal and reverse sequential for the remaining two stations) diminishes bolus duration requirements and can optimize arterial depiction.9 Imaging time and/or spatial resolution can be improved by using a rectangular field-of-view and/or parallel imaging.
In most commercial bolus chase packages, several MRAs are performed serially as quickly as possible after activation or triggering of the imaging sequence. The table moves between MRA acquisitions through a series of operator-defined locations. The inherent speed of the table results in a mandatory delay of three to four seconds between scanning stations. Some older scanners may have slower table motors that are insufficient for proper bolus chase PMRA, and a third-party manual table platform may be necessary. To ensure adequate coverage, a 3 to 5-cm overlap between each 3D MRA should be prescribed. Of course, some tall individuals may require a larger FOV and/or decreased overlap.
Renal artery evaluation needs additional consideration. Because the renal arteries are typically evaluated during a conventional x-ray runoff study, many referring physicians, particularly vascular surgeons, desire this evaluation on PMRA. The increased incidence of renal artery stenosis in patients with peripheral vascular disease further supports this tendency.10 The difficulty with renal artery imaging during a bolus chase exam comes primarily from its spatial resolution requirements, with preferred voxel dimensions of less than 2 mm. For existing bolus chase techniques, high-spatial-resolution renal artery imaging continues to be a challenge. It deleteriously prolongs the initial abdominal station 3D MRA, which can result in serious venous contamination concerns for distal stations. The solution may be the implementation of parallel imaging for station 1, and this approach is under investigation.
INFRAPOPLITEAL ARTERIES
Despite improvements in imaging software and hardware, the depiction of the smaller infrapopliteal arteries of the calves and feet can often be suboptimal on a standard multistation bolus chase CE PMRA exam.2,3,9 Venous contamination may occur secondary to fast circulatory times, or arterial signal may be poor because of inadequate synchronization with arterial enhancement.
One method to overcome these concerns is to perform a traditional axial or oblique axial 2D TOF MRA of the infrapopliteal arteries prior to performing the bolus chase CE PMRA.11 This lengthens the overall exam period but will ensure reliable depiction of these arteries. Another option is to perform a two-injection or hybrid PMRA exam in which a dedicated CE 3D MRA of the calf and foot station is done using a separate contrast bolus injection.12 Using this scheme, the lower leg/foot station is imaged first, as venous contamination on the second injection is typically most problematic for interpretation of the infrapopliteal arteries. By imaging the calf and foot station first, residual background signal in the venous and urinary systems, while present, usually poses less of a problem for interpretation of the suprapopliteal arterial segments. The dedicated distal station CE MRA can be performed using a standard arterial phase 3D MRA6 or a time-resolved MRA technique.12,13
Imaging of the more distal runoff vessels may not always be necessary. Their visualization is most important in patients with limb-threatening ischemia in whom bypass surgery is contemplated. Most patients with peripheral vascular disease do not present with limb-threatening ischemia, however, but with intermittent or exertional claudication in which treatment is often medical or the lesion proximal.10 The anatomic coverage of a three-station bolus chase MRA is typically sufficient for the clinical evaluation. But the expectations of referring physicians and the severity of disease in the referred patient should be considered to ensure that the PMRA technique being implemented supplies sufficient information.
CONTRAST MEDIA INJECTION OPTIONS
The use of extracellular gadolinium chelate contrast media for MRA continues to be an off-label use in the U.S. and many countries, but it has repeatedly been shown to be accurate and reliable2,11,14-17 and has become a clinical standard at many institutions. The ability to illustrate the vessels without the use of nephrotoxic iodinated contrast agents, arterial catheter placement, or exposure to x-rays is a definite benefit, especially in patients with suspected peripheral vascular disease. Koelemay et al and Nelemans et al have, in separate scientific reviews of the published literature, demonstrated the clinical superiority of Gd-enhanced MRA compared with noncontrast techniques, notably 2D TOF MRA, for the diagnosis of peripheral vascular disease, citing speed and accuracy as key factors.4,5
Early bolus chase MRA methods took two to four minutes and employed slow contrast media injection rates of 0.3 to 0.7 mL/sec.1,2 The slow rates ensured sustained preferential arterial enhancement with minimal venous enhancement. Current bolus chase MRA methods are facilitated by faster pulse sequences that have sped up the bolus chase to one to two minutes and enabled the use of faster injection rates of 1 to 1.5 mL/sec. This increases the achievable arterial gadolinium concentration and the resultant arterial signal.7,9,18,19 The faster infusion rates accelerate the venous phase of the bolus, however, and afford a narrower window for preferential arterial imaging.10,20
To accommodate the individual needs of each station, some investigators have advocated a biphasic injection scheme, with an initial faster rate to achieve relatively higher gadolinium concentration during imaging of the aorto-iliac arteries and a slower terminal rate to ensure adequate preferential arterial enhancement during the imaging of the more distal stations. Two typical fixed-dose injection schemes are 40 mL Gd-chelate contrast, with the initial 20 mL injected at 1.5 mL/sec and remaining 20 mL at 0.5 mL/sec, resulting in an injection duration of approximately 53 seconds; or 30 mL Gd-chelate contrast, with an initial 15 mL injected at 1.2 mL/sec and remaining 15 mL at 0.6 mL/sec, for an injection duration of approximately 38 seconds.
WEIGHT-BASED DOSING
An alternate injection scheme uses a weight-based dose (e.g., 0.2 mmol/kg). In order to ensure consistent bolus duration, the dose can be diluted to a fixed volume (e.g., 0.2 mmol/kg diluted to 45 mL) and injected at a fixed rate (e.g., 1 mL/sec).9 Weight-based dosing provides an opportunity to perform an additional Gd-enhanced MRA using 0.1 mmol/kg dose, if desired. As a rule, the bolus duration should match the time required to acquire the center one third of k-space for all imaged stations (Figure 3). Three-station bolus chase MRA imaging and intervening table translation, for example, may require 60 seconds to complete, with the requisite central k-space views taking only 45 seconds if preferential k-space ordering is chosen. Furthermore, the duration of a bolus injection does not necessarily equal the actual arterial enhancement phase of the bolus. The bolus will broaden over time, and actual in vivo arterial enhancement is approximately twice as long21 as the actual injection duration.
Bolus chase MRA is improved by the use of a large saline flush. Boos et al have demonstrated that a saline flush of at least 30 mL beneficially prolonged the arterial phase of a bolus and delayed its venous phase.21 The saline flush should be infused at the same rate as the gadolinium chelate contrast media bolus in order to maintain a constant rate of bolus delivery. In a biphasic injection scheme, the terminal rate of the contrast bolus injection would be the rate used for the saline flush. Contrast and flush administration can be greatly facilitated by the use of an MR-compatible power injector, which provides more consistent bolus administration and can easily implement biphasic injection schemes.
ARTERIAL PHASE TIMING
Synchronization of imaging for the arterial phase of the bolus injection is critical for successful arterial illustration on bolus chase PMRA, as with other types of CE MRA. The bolus chase is typically timed to begin with contrast arrival in the abdominal aorta, and all subsequent imaging is performed serially as quickly as possible. Performance of a test bolus scan can help estimate arterial contrast arrival time.22 Investigators have shown improved success using two test bolus scans: one at the abdominal aorta and a second at the popliteal or trifurcation vessels.23,24 Obtaining two spatially separate test bolus scans allows optimized imaging of the arteries tailored to individual patient runoff dynamics. It has the disadvantage, however, of increasing background, especially venous, contamination prior to the actual bolus chase and prolonging the preparation time.
Arterial phase timing for bolus chase MRA can also be achieved using special real-time triggering algorithms. Commercially available bolus chase pulse sequences have been integrated with a real-time triggering scheme such as an automatic bolus detection scheme or an MR fluoroscopic trigger. The automatic bolus detection scheme is completely automated: The pulse sequence monitors an operator-defined volume of interest for contrast arrival and automatically initiates MRA imaging upon detection of the bolus arrival. The MR fluoroscopic trigger provides high-temporal-resolution 2D images, which the operator uses to visually detect arrival of the contrast bolus and manually initiate the MRA imaging.3 Maki et al23 have described the use of an MR fluoroscopic trigger for a multistation bolus timing scan. In this application, instead of initiation of a 3D MRA, the table is moved to image the lower leg, where a typical timing scan using a fast 2D gradient-echo pulse sequence is performed. This multistation single-bolus technique provides timing data that enable the operator to adjust the scan parameters for each station to better coincide with the arterial phase of the bolus.
POSTPROCESSING
Bolus chase 3D MRA presents an interpretive challenge not only because of the number of images but also the time required for image postprocessing. Use of an independent computer console equipped with specialized software for MIP (Figure 4) and multiplanar reconstruction facilitates image interpretation considerably. Volume-rendered software also can enhance image interpretation and display. In addition to actual image interpretation, postprocessed views should be archived for review by referring physicians. Vascular surgeons often prefer hard-copy films, as they enable easy review at preoperative conferences and in the operating room.
In most cases, filming should resemble that used for traditional x-ray angiography, such as large MIP views (e.g., "1 on 1" or "2 on 1") of each station on standard 14 x 17-inch films supplemented with smaller format (e.g., "4 on 1") MIP views of pertinent anatomic regions. Some surgeons prefer reversing the gray scale to show black arteries, as it provides arterial illustration similar to that of conventional digital x-ray subtraction angiography. These and other details should be discussed with the referring clinicians so that the information and representation of the information can be of greatest assistance to them.
References
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MS. HOOD is an assistant professor of radiology, and DR. HO is an associate professor of radiology, both at the Uniformed Services University of the Health Sciences in Bethesda, MD. MR. SCHWEIKERT is clinical manager of MRI, and DR. CORSE is director of MRI, both at Doylestown Hospital in Doylestown, PA.
