The basic principle behind CE-MRA is imaging the arterial first pass of paramagnetic contrast agent in the vessels after intravenous injection (12). The injected contrast agent produces a strong shortening of the T₁-relaxation time of blood. This results in high signal intensity of the arteries on T₁-weighted, spoiled gradient-echo images. The delay between arterial and venous enhancement provides a time window for pure arterial imaging. This time window depends on the rate of contrast agent injection, but also on the anatomic region being imaged. In the upper limbs, venous return is much faster than in the lower limbs, requiring a much shorter measurement time. Since the technique depends on imaging of the first pass of contrast agent, timing the start of image acquisition after injection is essential. Various techniques have been developed to calculate this so-called scan delay, as will be explained later.

The quality of the MRA images with regard to selective arterial visualization, resolution, and volume of interest depends on both sequence parameters used and the contrast
medium bolus geometry. Arterial enhancement depends on individual physiologic parameters, such as cardiac output and blood volume, but also on contrast agent application parameters, which can be manipulated (flow rate, dose, and saline flush volume). The T₁-shortening of blood depends on the intravascular concentration of the contrast agent, which, in turn, depends on the rate of injection. In general, the higher the injection rate, the higher the concentration will be, although rates exceeding 5 mL/s do not result in further increase in signal intensity. In fact, intravascular signal may become lower at rates higher than 6 mL/s (13). On the other hand, a higher infusion rate results in faster venous return and a shorter bolus length, requiring a shorter measurement time. Always a compromise between imaging time, resolution and arterial/venous time window must be sought. Since, for examination of the lower limbs, the entire vascular tree from renal arteries to the ankles has to be imaged with high resolution (especially in the lower legs), a long arteriovenous time interval is necessary. Therefore, in the lower extremities injection rates of 0.5 to 1.5 mL/s are used. After injection of contrast agent, the venous system should be flushed with saline to push the contrast medium column from the arm veins into the circulation.

A heavily T₁-weighted, spoiled gradient-echo sequence [short repetition time/echo time (TR/TE); flip angle, 25 to 50 degrees] is used in which the achievable field of view, matrix size, scan volume, and number of partitions are determined by the arterial/venous time interval. Subtraction of pre- and postcontrast images is performed to visualize only the arteries for greater ease of image interpretation and to reduce artifacts (Fig. 35.1) (14). The obtained volumetric dataset containing high intensity voxels corresponding to arteries can be postprocessed using the maximum intensity projection (MIP) algorithm. For improved diagnostic performance, review of the individual source images or reformatting images in the transverse plane is required.

In CE-MRA, adequate timing of the start of data acquisition, with arrival of the contrast agent in the vessels of interest, is essential. Arterial enhancement must coincide with acquisition of central k-lines, and for improved arterial/venous differentiation, the central k-lines have to be sampled before venous return. Essentially, three methods have been developed to ensure proper timing.

Adequate timing can be performed by measuring the circulation time after intravenous injection of a small test bolus (1 to 2 mL) of contrast agent, using a rapid dynamic imaging sequence. With this dynamic series, the arrival time of the test bolus after injection in the region of interest (ROI) can be imaged and, therefore, the scan delay can be calculated. To avoid pooling of the test bolus in veins, the tubing has to be flushed with saline. Both the test bolus and the final infusion should be injected at the same rate. This timing method is fairly robust and easy to perform. A disadvantage is that the method requires additional administration of contrast agent. It lengthens the total procedure time with 2 to 3 minutes. Also, in case of irregular heart action, the circulation time calculated from the test bolus may sometimes differ from that during injection of the final bolus.

Fast imaging sequences can be used to visualize the influx of contrast into the ROI. MRA data acquisition is then started automatically or manually [MR Smart-Prep (15), bolus track (16)]. A disadvantage of this method is the delay of 2 to 4 seconds that occurs between visualizing the arrival of the contrast bolus and the actual start of acquisition. Also, time for breathing instructions, if required, may be too short.

With ongoing improvements in hardware and gradient technology, the time required to image a large volume of interest with high resolution has greatly reduced. With a 2D slab, the entire anteroposterior diameter of the legs can be covered in 2 to 3 seconds (17). Therefore, by performing a dynamic volumetric series after intravenous injection of the contrast agent, the arterial phase will always be included in one of the datasets, thus obviating the need for bolus timing (18).

For imaging of the leg vessels, a large anatomic region (approximately 100 cm) has to be covered. Since the longitudinal field of view of MR systems is limited to 45 to 50 cm, complete imaging of the lower extremity requires imaging of several regions (or “stations”). This means repositioning of the patient to the isocenter of the magnet. Two methods for imaging of the entire legs have been developed: So-called multistation/multi-injection imaging and the bolus chase method. In both techniques one has to ensure sufficient overlap between the separate stations. No studies have been performed comparing the two methods directly; however, because of its short examination time and more efficient use of contrast agent, the bolus-chase technique is the current standard of practice.

With this technique, the three regions of the legs (aortoiliac, femoropopliteal, and infrapopliteal vessels) are imaged sequentially with two or three separate bolus injections of contrast medium (19). Imaging of each station involves acquisition of a mask dataset, which is subtracted from the subsequent images obtained after contrast injection. Contrast agent can be injected at moderately high rates of 1 to 1.5 mL/s, ensuring high signal intensity in the vessels. The advantage of this technique is that it can be applied on any clinical scanner without use of special hardware, and imaging parameters can be tailored to every single station. The multistation/multi-injection technique, however, is not very time efficient (30 to 45 min/patient); a total contrast agent dose of 0.2 to 0.3 mmol/kg/patient is required, and the contrast-to-noise ratio of the subtracted images becomes less at the second and third stations as a result of circulating gadolinium (Fig. 35.2) (20). Using larger dosages in the subsequent stages can partly circumvent this decrease in signal intensity. Surface coils can be used for imaging of the different stations. Huber et al. (21) achieved 100% sensitivity and 94% specificity for identifying 80 hemodynamically significant stenoses and 39 occlusions in 24 patients.

In this technique, mask images of all three stations of the legs are made first (16,22,23 and 24). Then, during slow constant infusion of contrast agent, the flow of contrast through the legs is chased using a floating table. This technique requires application of special hardware to move the table to predefined positions. This can be done using homemade table-stopping devices or special MR-table hardware. The advantage of the moving-table technique is that it is very rapid (coverage of the entire legs in less than 4 minutes), requiring a relatively low dose of contrast agent (approximately 30 cc).

Initially, a disadvantage of the stepping-table technique was that the three stations were imaged sequentially while the scan preparation had been optimized for only a single station. Later on, the method has become more flexible (16), and even dedicated surface coils can be used (25,26).

Although with the bolus-chase technique, imaging times of the various segments are short, early venous enhancement may sometimes cause difficulties in evaluation of the arteries of the lower leg (Fig. 35.3). This venous overlay may be especially prominent in patients with diabetes, venous ulcers, or cellulitis. Enhancement of the veins in the third station, the lower legs, can be minimized by using an optimized imaging protocol: Use of a biphasic injection protocol (e.g., 1 mL/s during 10 seconds and the rest with 0.5 mL/s), use of “centric” k-space acquisition, in which the contrastdetermining k-lines are acquired first, and the use of parallel imaging. With the use of parallel imaging techniques such as simultaneous acquisition of spatial harmonics (SMASH) (27) or sensitivity encoding (SENSE) (28) techniques, combination of coils can be used to compensate for omitted gradient steps. The increase in imaging speed can be used to reduce acquisition times or to increase resolution of the scan (29). Binkert et al. (30) used a dual-bolus technique, in which they started with a dedicated calf MRA and—after a 15-minute interval—proceeded with the moving-table MRA. This led to better visualization of the infrapopliteal arteries with less venous overlay and therefore a significantly increased diagnostic accuracy, compared to moving-table MRA alone. Also, venous compression by means of a tourniquet, either
at the midfemoral (31) or at infragenual level (32,33), can be used to slow down the flow in the vessels of the legs.

To encourage acceptance of the technique by referring clinicians, one has to provide them with images to which they are accustomed and which appear similar to conventional angiography. The number of images should be limited as a practical concern for vascular surgeons. The images should be large to be viewed from a distance, displaying standard angiographic views. Also, bony landmarks should be provided for better orientation, similar to images of conventional angiography (Fig. 35.4).