Applied Radiology

MR angiography can be performed without contrast, and such techniques often provide excellent information and may, in many instances, produce images of adequate quality. It is important, however, to be aware of the limitations of non-contrast techniques. In Figure 1A, a two-dimensional time-of-flight (TOF) sequence gives a false appearance of only moderate disease of the carotid bifurcation. But on the three-dimensional high-dose gadolinium-enhanced MRA (Figure 2B), it is clear that bifurcation anatomy is more complex. A maximum intensity projection that zooms in on the bifurcation shows that, although the degree of luminal narrowing is only moderate, there is a large ulceration in the atherosclerotic vessel wall that may be contributing to the patient's symptoms (Figure 2C). This abnormality is not visualized by flow-based, non-contrast MRA techniques.

Another shortcoming of flow-based MR techniques, such as 2D TOF, is the tendency for the signal to drop out in sites of very severe stenosis as a result of jet flow--so-called spin dephasing. ^6,7 As shown in Figure 2, 3D gadolinium-enhanced MRA, which is not flow-based, more accurately depicts the true lumen and enables more precise appreciation of anatomic details.

Artifact associated with metal objects in the body represents a challenge for MR imaging. The 3D gradient echo sequence we use for high-dose gadolinium-enhanced MRA employs a very short echo time that helps to eliminate this artifact. Nevertheless, some metal produces such a severe artifact that it cannot be eliminated. The stainless steel Palmaz stent is an example. In the case of platinum stents, the metal stent configuration creates a Faraday cage phenomenon that attenuates the penetration of radiofrequency (RF) energy both into and out of the stent. By increasing the flip angle to 75°, however, it is possible to deliver sufficient RF penetration into the stent to enable visualization of the vessel lumen (Figure 3). ⁸

Another important advantage of 3D contrast-enhanced MRA is the elimination of in-plane saturation. This is critically important in patients with alteration of the normal vascular anatomy. ⁹ In Figure 4, the patient has an occlusion of the left subclavian artery, and flow to the left subclavian is provided by a carotid-to-left subclavian artery graft. Three-dimensional contrast-enhanced MRA readily depicts the unpredictable course and direction of such vessels and, in this case, reveals a complex plaque at the carotid bifurcation.

Patient safety in the MR scanner is an issue that must be considered. In general, MR is not suited to imaging high-risk patients. For example, in the evaluation of dissection, MR would typically be reserved for evaluating Type 3, stable dissections. It is, however, capable of imaging high-risk patients, and may represent the best approach even in an emergency if the risk of subjecting a particular patient to radiation or iodinated contrast is sufficiently high.

In summary, gadolinium-enhanced MRA is able to visualize the origins of the great vessels; it is fast and, therefore, produces less motion artifact than non-contrast techniques; it is associated with less spin dephasing; it can demonstrate plaque features and luminal anatomy in enough detail to reveal ulcerations; it is not hampered by in-plane saturation; it enables the evaluation of small vessels, such as the vertebral arteries; and postprocessing is very simple. Moreover, if gadolinium is already being used for brain MR, there is no extra cost associated with obtaining 3D gadolinium-enhanced MRA at the same time.

Contrast Administration

Several factors must be considered when selecting the dose of gadolinium contrast. ¹⁰ First, it is important to realize that the bolus of contrast evolves as it travels through the body. Initially, at the point of injection, it is very compact. It spreads out as it passes through the pulmonary circulation, and becomes even less concentrated after it passes through the systemic capillary bed.

The selection of gadolinium dose, therefore, must take into account the distance between the injection site and the area of the body that will be imaged, as well as how much gadolinium will be extracted along the way. As might be expected, the higher the dose of contrast, the greater the visualization of vascular detail. This is demonstrated in Figure 5. At a gadolinium dose of 0.1 mmol/kg, it is not possible to see the venous anatomy in the liver, but as the dose is increased to 0.2 mmol/kg and 0.3 mmol/kg, the portal venous anatomy can be appreciated.

The injection method itself is also important to consider. ¹¹ Manual injection has several advantages in MRA, because so many factors must be coordinated perfectly. The delivery of the bolus, the breathholding of the patient, and the initiation of the acquisition all have to occur simultaneously. In my experience, it is easier to coordinate each of these factors while standing next to the patient and performing the injection by hand.

In addition, hand injection necessitates consideration of the viscosity of the contrast agent, which in turn influences decisions on the size of angiocatheter, length and caliber of intravenous tubing, and type of gadolinium contrast to use.

In subclavian venography, for example, two operators are required in order to simultaneously inject the right and left antecubital veins with dilute gadolinium contrast media (20 mL gadolinium in 250 mL normal saline) in order to image the subclavian veins and superior vena cava (Figure 6).

Optimal contrast administration requires an understanding of the basic physics of k-space and how it influences the features of an image. In particular, the center of k-space dominates the contrast features of the image, whereas the periphery of k-space dominates the details. ¹² It is important, therefore, to time the delivery of the contrast bolus so that arterial contrast is at its maximum during the central portion of k-space, to achieve the maximum contrast effect. ¹³

A perfectly timed injection of contrast may provide excellent quality with a relatively low dose of contrast media. But with a small contrast dose, small bolus timing errors can ruin a study. Increasing the contrast dose allows for a greater margin of error in bolus timing. In essence, contrast dose can make up for variations in the skill of the operator. The most talented operators may be able to use a lower contrast dose. On the other hand, if the operator is less skilled or performs MRA infrequently, a larger contrast dose may be warranted in order to demonstrate more angiographic features.

Expanding Coverage

MRA offers the exciting opportunity to expand beyond a scanner's usually limited field of view to image extensive regions of anatomy, simply by moving the table during contrast injection. ^14-16 For example, after performing an initial image in the pelvis, we can pull the table and acquire another image of the thigh, then pull the table again and image the calf. It is also possible to deliver several small injections of gadolinium contrast as the scan moves from the feet to the knees, then finish with a bolus chase. The ability to scan large regions of the body is especially exciting because of the systemic nature of atherosclerotic disease.

Bolus-chase MRA has been extrapolated to cover arteries of the entire body, beginning with the aortic arch and carotid arteries, and moving sequentially down the body to image arteries of the chest, abdomen, pelvis, thigh, calf, and feet. ¹⁷ The patient in Figure 7 was unable to undergo conventional angiography, because access through the iliac and common femoral arteries was not possible. But with MRA, only a peripheral intravenous line in the arm was needed before imaging the arteries of the entire body with a single bolus injection.

Conclusion

Contrast-enhanced MRA offers important advantages over other forms of angiography. It poses no risk of ionizing radiation or nephrotoxicity. Source data can be reformatted for any view. It can depict anatomy and physiology in great detail, and it is highly accurate. As researchers continue to work on refining and advancing the techniques of MR angiography, its future looks bright. *

Discussion

TG: Thank you, Dr. Prince. Let me ask a few specifics about how you administer the contrast, and what doses you use. You mentioned that you give this by hand injection, and that viscosity is an issue. Does that effect your decision to use different agents?

MP: Yes, if you need a small caliber angiocatheter, then the higher viscosity agents are going to be more difficult to inject, especially by hand. Or especially if you are going to be imaging a baby with a very small caliber angiocatheter, then the viscosity becomes very important. So, we would prefer agents that have the lowest viscosity.

Another important detail is that viscosity is related to temperature. So if you find that the resistance to injection is too great, you can warm up the gadolinium to body temperature. And that will drop its viscosity by approximately a factor of two. Then that reduces the force required to inject the contrast by hand.

DAVID ROBERTS, MD: We routinely give high-dose gadolinium for peripheral bolus-chase MRA applications. We find that the injection protocol is critically important for these studies. If we inject too quickly, certainly anything above 1 mL/sec, we notice significant superficial femoral venous (SFV) enhancement and tibial venous enhancement, which can really degrade the studies. For that reason, we have found that we really can't get by with hand injection, because many of the fellows and residents, despite training and stressing this point, simply inject too quickly. Therefore, we found power injectors fairly mandatory for these studies. What's your take on this?

MP: We've had the same experience. When we are performing a multistation examination with one bolus and are trying to share that bolus between multiple stations, if we time it for the first station, our timing for the second and third station may not be accurate, depending on whether the patient has fast or slow flow. So we have begun doing a test bolus at the calf. Then that gives us a better sense of the timing for all three stations. If a patient has very fast flow, if it is <20 seconds to the calf, for example, then we won't even attempt to do a bolus-chase exam, because we know that we just cannot move the table fast enough to keep up with really fast flow.

DR: How do you address the problem of asymmetry, left to right, in a patient?

MP: Our pattern has been to try to figure out which leg is the clinically significant leg, which is the one that's on track for being operated on first or is providing the most difficulty for the patient. Then we time for that leg, and hope for the best on the other leg.

HR: I have a question about a potential pitfall with great vessel and carotid imaging. Most of these patients, of course, are going to be injected in one arm or the other and as that tight bolus goes past the common carotid artery, especially with a left-sided injection, we sometimes get a false drop-out in signal over a short segment related to the venous gadolinium passing there. How do you approach that? Do you routinely inject the right arm? Do you sometimes inject a femoral vein?

MP: That's an excellent point. It brings up one of the advantages of the hand injection. We always try to inject the right arm. As we are injecting, we want to get the bolus in and flushed out of the vein before we start collecting the central k-space data. So we use a fluoroscopic triggering technique for our carotids, If we are injecting and also tracking the pace of the passage of the contrast through the lungs and as it reaches the great vessels, if we see that it is going quickly and we are not going to get all the contrast in and flushed, I may actually terminate the gadolinium and immediately switch to flush earlier, just to get it flushed out of the subclavian vein and innominate vein.

If you have to inject the left arm, you may not be able to escape some venous artifact, superimposing it over the great vessel origins. Part of the reason for that is that in a lot of older patients, the aorta is unfolded and very hard and it pinches the brachycephalic vein between the aorta and the sternum. It causes the gadolinium to hang up there, and despite with your best efforts to flush it through, you may not be able to clear that.

LARRY KRAMER, MD: Martin, you showed in one graph that as you increase your contrast dose, that you see more and more detail. Is there a point at which as you increase dose, susceptibility takes over and you actually decrease signal and detail?

MP: Yes, that's an excellent question. I think it's more related to injection rate. There's going to be an injection rate at which the concentration of gadolinium in the blood becomes so high that you begin to have T2* effects, which actually cause the signal to drop. In my experience, with hand injecting, it's hard to inject fast enough to reach that point. But if you are performing venography and injecting that vein, you have to dilute the gadolinium to avoid that T2* effect. So that is a very important point.

LK: For venography, how much do you dilute the gadolinium?

MP: I typically take a bottle of 15 or 20 mL of gadolinium and inject that into a 250 mL bag of normal saline. Then I'll load up four 60-mL syringes. That gives me a very large volume of diluted contrast to work with, so I can slam that diluted gadolinium in, in such a way that it will tend to distend the veins. Yet it will not be excessively concentrated. I'll inject a full 60 mL before beginning the 3D acquisition. Then I will continue to inject another 60, then that would be two syringes on each arm, so that I can get bilateral information.