Although higher intraluminal enhancement may be advantageous (50), the minimum adequate goal for contrast opacification of the arterial system is 150 to 200 Hounsfield Units (HU), with decreased measurement variability above 200 HU (51,52). With this goal in mind, one can adjust the dose, timing, and rate and length of administration of contrast material (CM) to achieve adequate opacification with the minimum dose. To reduce or eliminate venous contamination, the duration of the scan should be short and timed to coincide with the arterial phase of contrast opacification. Approximately 7 seconds are required for recirculation of the intravenous (IV) contrast to reach the arterial compartment in a patient with a normal cardiac output. After that, the arterial attenuation begins to increase. With a uniphasic injector, the attenuation quickly peaks and then fades, resulting in a humplike time-attenuation curve. The ideal time-attenuation curve would be a flat line, with uniform contrast enhancement within a vessel for the duration of the scan. With the use of multiphasic contrast injectors, the time-attenuation curve approaches the ideal plateau, and the issue of timing a scan to coincide with the contrast bolus peak becomes less of an issue (53).

It is important to time the scan precisely in order to minimize the volume of contrast injected and to scan before extensive venous contamination occurs. The three widely used methods are fixed-delay scanning, timing-bolus technique, and bolus tracking (49). Fixed-delay scanning simply refers to starting the scan after a standard interval from the beginning of the contrast injection. Timing bolus, on the other hand, refers to the use of a separate timing run using a small bolus of contrast and measuring the time of injection to peak enhancement in the region of interest. Bolus tracking combines the timing run and the acquisition in a single scan and is performed by monitoring the time-attenuation curve within a particular vessel after the injection of the CM, with the scan initiated after reaching a particular threshold attenuation.

The enhancement dynamics of contrast administration have been reviewed elsewhere (53), but in short, bolus tracking offers several advantages in the setting of carotid steno-occlusive disease. First, in atherosclerosis, the cardiac output of the patient may not be normal, altering the enhancement dynamics in any individual. Bolus tracking individualizes the contrast timing for each patient. Second, because of individual variability in contrast enhancement, most fixed-delay protocols use longer contrast injection times, increasing the total dose of contrast. This is particularly undesirable, as patients with atherosclerotic disease of the carotids often have impaired renal function. During a scan, the delay between the point at which the attenuation reaches the set threshold and the acquisition of images can range from 4 to 10 seconds, increasing the dose of contrast used and the risk of venous contamination. As a practical step, when the minimization of contrast dose is critical, one can initiate scanning before the time-attenuation curve reaches this threshold by monitoring the curve and initiating the scan when the slope of the attenuation curve abruptly increases, thus decreasing the delay between the start of the scan and the acquisition of the initial images.

For CTA of the carotid arteries, MDCT scanning can cover a field of view from the aortic arch to the skull base in as little as 9 seconds. Because the goal of CTA is not parenchymal enhancement but adequate opacification of the vessel lumen, the dose of contrast can be much less than the often-used 30–45 gm of iodine. For a 10-second acquisition performed with a contrast concentration of 350 mg iodine/mL CM, one can use 90 cc injected at 5.0 mL per second, for a total injection time of 18 seconds (53).

The dose can be further reduced by the use of a saline chaser, taking advantage in the recirculation delay to ‘push’ the contrast through to the arterial system using saline, rather than the tail end of the contrast bolus, which would not reach the arterial system during the scan acquisition. Others (54) have increased the length of the contrast plateau 8 seconds on average with a saline chaser. In this scenario, the total contrast volume would be reduced to 50 cc, for a total iodine dose of 17.7 g, yet it would still maintain the iodine flux of 1.8 g per second for the duration of the scan. Faster scanners can potentially reduce the dose further, though timing becomes more critical, and the potential contrast dose reduction can be offset by the risk of acquiring a nondiagnostic scan. Rather, faster scanners can be used to achieve greater longitudinal resolution for a given scan time or to include the intracranial circulation within the same scan time.

From a practical standpoint, injection rates of 5 to 6 cc per second mandate the placement of a large bore (at least 20 gauge) IV catheter. Ideally, a lower extremity vein should
be used, as upper extremity IV injection will result in streak artifact at the level of the brachiocephalic veins; however, this is impractical. The antecubital fossa is the preferred site of IV access.

Several basic steps reduce the amount of artifact in CTA of the cervical vessels. While motion artifact is diminished with the use of rapid scanning technologies, breath holding will reduce motion, especially at the level of the aortic arch. Similarly, instructions to the patient to refrain from swallowing during the scan will reduce artifact. Cardiac pulsation is not a significant source of motion artifact in the cervical vasculature.

Streak artifact in CTA of the neck comes from several sources. While it should go without saying, the arms should be placed at the patient’s side for the scan. For combined scans of the body and neck, we advocate split-bolus scanning with repositioning of the arms rather than scanning the entire neck and body in one setting. Streak artifact from the shoulders can be minimized with patient positioning or the use of a shoulder girdle, if necessary.

Artifacts from dental amalgam can be reduced with either neck extension or angling of the gantry to throw off the artifact from the vessels (52,55). With thin-collimation volumetric scanning, reformatted true axial images can be created despite oblique scan planes. This technique can also be used in cases of suboptimal patient positioning.

Streak artifact from dense IV contrast at the level of the subclavian and brachiocephalic veins can be managed by using a lower extremity vein when possible, the use of a saline chaser, and injection of the arm contralateral to suspected pathology (56,57).

Generally, the neck should be scanned in the cranial to caudal direction, which can minimize streak artifact at the brachiocephalic vein, at the risk of venous contamination or breathing motion near the arch. Scanning in the caudal to cranial direction can be performed if a multiphasic contrast injector is not available in order to follow the contrast bolus and avoid excessive variability in the degree of luminal enhancement. Further, with caudocranial scanning, any difficulty with maintaining breath hold should not affect imaging.

Another source of artifact in CTA is the variability in the appearance of a contrast-enhanced vessel with different window and level settings (57,58,59). This variability is not solely limited to CTA; however, practically it is less of a factor with MRA and DSA. Liu et al. (59) made over 25,000 measurements of vessel caliber in phantoms, using both transverse and maximum-intensity projection (MIP) images. The optimal window and level settings for measurement of vessels caliber were found to vary with the degree of luminal opacification and whether transverse or MIP images were examined. It is important to be consistent with whichever display settings are used.

Longitudinal resolution, or CT section thickness, is determined by the collimation of the scanner. Helical CT scanners can acquire angiographic images of the neck at 1- to 2-mm slice thickness in 15 to 25 seconds. MDCT scanners can obtain much thinner and essentially isotropic data with much shorter scan times. Higher resolution imaging can provide information for plaque characterization and more accurate assessment of stenosis of the carotid and the origin of the vertebral arteries (Fig. 12-5). With high-resolution imaging, one can potentially visualize dissection flaps, or mural irregularities in the setting of vasculitis. For most purposes, 1- to 2-mm axial reconstructed images provide sufficient diagnostic detail. For the creation of source images for 3D postprocessing, one should use the thinnest possible reconstruction that the collimator settings allow, with 50% overlapping reconstruction, to reduce noise and stair-step artifact (49,57).

CT protocols should be designed based on different clinical priorities. For a given scanner, the section thickness will be inversely proportional to the scan length for a given pitch and longitudinal field of view. Routinely, one may use the thinnest available collimation for the scanner and adjust the pitch (generally between 1 and 2), scan time, mA, and contrast volume appropriately (54). The extracranial circulation, from the aortic arch to the skull base, can be scanned at very thin collimation in a reasonable amount of time, without an excessive contrast dose. If the intracranial circulation is to be included, one can increase the section
thickness for the cervical scan to include the head within the larger scanned area without lengthening the scan time. Some representative scan protocols for different CT scanners can be found online at www.ctisus.com. For the patient in whom contrast dose is an issue, the scan time can be adjusted to minimize total contrast dose, with the collimation adjusted proportionately.

Time-of-flight (TOF) imaging, which is based on differences in the excitation history between flowing protons in blood and the stationary protons in background tissues, can be performed as a two-dimensional (2D) technique, or as a 3D volumetric acquisition. Two-dimensional imaging is rapid but subject to stair-step artifacts due to motion between repeated acquisitions of adjacent slices to cover the area of interest (Fig. 12-6).

TOF imaging with 3D technique offers the ability to achieve a smaller voxel size, resulting in higher resolution, as compared with 2D TOF. However, the smaller voxels result in lower signal-to-noise ratio (SNR), and the use of a larger imaging volume predisposes the technique to signal loss from saturation effects. In addition, 3D imaging offers a shorter allowable echo time (TE), which can minimize signal losses from phase dispersion.

Phase contrast (PC) imaging uses two gradients of equal strength but opposite polarity (a bipolar gradient) to induce a phase change in excited protons. In stationary tissue, the bipolar gradient has no net effect, but in moving blood, the protons acquire a net phase change proportional to the velocity of the blood. PC can thus measure flow velocity as well as directionality. Technical limitations and issues related to scan time limit the clinical application of this technique for cervical arterial disease. In this capacity, PC techniques are largely focused on providing scout information for planning other MRA acquisitions and for showing flow direction, as when subclavian steal physiology is suspected.

Contrast-enhanced magnetic resonance angiography (CE MRA) refers to the injection of an intravascular contrast agent, typically a chelate of gadolinium, to shorten the T1 of blood and thus provide contrast against surrounding tissue. As with CTA, several techniques can be used to time the administration of contrast and to achieve arterial selectively: A fixed delay time, a timing run, an acquisition triggered off detecting the arrival of contrast, and temporally resolved strategies can all be employed.

Fixed delays are limited by the wide range of circulation times among patients, causing marked differences in the time to peak enhancement and the most vulnerability to venous contamination. Time-resolved imaging (e.g., time-resolved imaging of contrast kinetics [TRICKS]) increases the likelihood of obtaining an arterial phase image but may still benefit from a timing run. Generally, for all these techniques, a noncontrast image is obtained to use as a mask for subtraction.

Black blood imaging can provide complementary information to TOF and CE MRA. This technique is particularly useful for assessing atherosclerotic plaque (Fig. 12-7). A
common approach to black blood imaging is to use a double-inversion pulse to null signal from flowing blood (33). As such, it is less susceptible to dephasing effects and can help to confirm lumen patency when this effect is present.

Slower blood protons may become saturated and lead to a signal loss that is caused by the acquisition and not necessarily the vessel morphology. This effect is important for clinical assessment in that it can cause near-occlusive stenosis to appear as complete occlusion.

A similar phenomenon is noted in areas where blood recirculates, crossing the excitation slice repeatedly, as in the carotid bulb. This effect also can be seen from recirculation of blood flow within an ulcer crater. As well, this is the basis for signal dropout from in-plane blood flow: When blood is flowing parallel to the imaging plane rather than perpendicular to it, the blood will be within the imaging slice for a longer period of time, resulting in loss of signal.

Such saturation effects can be minimized by making the imaging slice thinner (decreasing the Δz) or to increase the TR to prolong the time between excitation pulses, minimizing the number of pulses experienced by the protons when traversing the slice. The ability to obtain a thin Δz (and increase the sensitivity to demonstrating slow flow) relative to 3D techniques represents an important advantage of 2D imaging. However, a Δz reduction will require more slices to cover a similar anatomic area of interest and has the potential to increase sensitivity to motion and, thus, stair-step artifacts. Prolonging the TR is problematic, as this will decrease the degree to which the signal from background stationary tissue is suppressed.

MOTSA (multiple overlapping thin-slab acquisition) is a 3D technique that uses a relatively thin-slab thickness to minimize signal loss from saturation while maintaining the advantages of 3D imaging. The saturation effect is less pronounced than with standard 3D imaging, though more pronounced than with the thinner slices used for 2D imaging. TONE (titled optimized nonsaturating excitation), also referred to as ramped excitation, is another technique used to reduce signal loss from saturation, which uses a variable flip angle to reduce the saturating effect of the excitation pulse as protons enter the image volume.

The use of gadolinium contrast in CE MRA offers a very effective means of reducing proton magnetization saturation by shortening the T1 of the blood pool. This allows for the use of thick-slab acquisitions without the consequence of saturation. Therefore, a very large field of view (FOV) can be acquired parallel to the vessels of interest, such as a coronal acquisition to cover the aortic arch to the intracranial extent of the cervical arteries without signal loss from in-plane flow (Fig. 12-8). An important advantage of this expanded coverage is that it enables assessment of tandem lesions. It also nullifies the consequent drop in signal when acquiring smaller voxels with 3D imaging.

Phase dispersion, also referred to as dephasing artifact, is another source of signal loss in MRA. One solution to phase dispersion artifact is to decrease the size of the voxel
so that each voxel contains a smaller range of velocities of flowing blood protons. The smaller distribution of velocities will result in less intravoxel signal loss from dephasing. Furthermore, the phase change is also dependent on the TE, so another approach to decreasing the dephasing artifact is to decrease this parameter. TOF imaging using a 3D technique offers smaller voxels than are generally achievable with 2D techniques. That reduces signal loss from dephasing effects with further benefits from the shorter TE values also achievable relative to 2D imaging. Administering gadolinium compensates for the signal loss in 3D imaging from the saturation effects described earlier while compensating for signal losses from dephasing.

Flow compensation gradients, also known as gradient moment nulling (GMN), are refocusing gradients that can be applied to correct for spin dephasing in flowing blood. Phase dispersion from protons flowing at higher orders of velocity can be effectively refocused to recover lost signal by application of more complex gradients, though this is at the expense of longer echo times, which begins to counteract the reduction of dephasing.

It is clear that CE MRA can minimize signal loss from saturation by reducing the T1 relaxation of the entire blood pool, and it can be acquired using a 3D sequence so that dephasing effects are reduced but the accompanying reduction in signal due to the smaller voxel size and thicker Δz of 3D imaging is no longer problematic. One might suspect that a TOF MRA sequence is not necessary if a CE MRA is acquired; however, a TOF sequence can complement a contrast study with several added benefits.

The CE MRA is generally acquired as a coronal slab so that a large area can be imaged in order to evaluate for tandem lesions, but the large FOV in the z-direction minimizes the resolution in this plane (Fig. 12-9). A TOF MRA that focuses on the carotid bifurcation allows for a more detailed depiction of stenosis if present because of the smaller FOV in this direction. Furthermore, CE MRA has been shown to overestimate stenosis relative to TOF techniques (60) (Fig. 12-10). Finally, an inadequate CE MRA study is difficult to repeat because of venous contamination and gadolinium dose limitations, so acquiring a TOF MRA prior to contrast administration can serve as a fail-safe measure.