CT Angiography and MR Angiography of the Peripheral and Visceral Vasculature

1

What are the clinical indications for computed tomography angiography (CTA) and magnetic resonance angiography (MRA) of the peripheral and visceral arteries?

CTA and MRA are noninvasive methods of assessing the arteries and veins, which have all but replaced catheter angiography. The clinical indications for these examinations can be thought of in the following broad areas:

• Ischemia.

• Aneurysm/dissection.

• Compression syndromes.

• Traumatic injury.

2

What is the most important principle for performing contrast-enhanced CTA and MRA?

The most important principle in performing contrast-enhanced vascular imaging is the timing of the image acquisition. The images must be acquired when the opacification of the vessel (artery or vein) of interest is as high as possible. This requires careful synchronization of image acquisition with the injection of contrast material.

3

How is the image acquisition timed following contrast administration in a CTA?

The image acquisition can be timed in one of three ways:

• Empirical delay.

• Bolus tracking.

• Test bolus.

When using an empirical delay, the images are acquired after a preset delay from the time of contrast administration. This is the method generally used for imaging of the veins. The time delay depends on the vein of interest. To assess the infrarenal inferior vena cava (IVC) and the iliac veins, a delay of 140 seconds from the time of contrast injection is used. To assess the portal veins and the renal veins, the delay is typically 70 to 90 seconds. For assessment of the central veins of the chest and upper extremities, a delay of 90 seconds is used.

Imaging of the arteries requires more nuance because the window for optimal enhancement of the arteries is short. This is most often achieved by tracking the bolus of injected contrast material. A cursor is placed in a region of interest (ROI), which is the aortic segment most proximal to the artery of interest. For example, if the artery of interest is the celiac artery, then the ROI can be the descending thoracic aorta or abdominal aorta. After a specified time delay, known as the monitoring delay, low dose single slice images are obtained through the ROI every 1 to 2 seconds, while simultaneously tracking the attenuation of the aorta. When its attenuation exceeds a predetermined threshold, typically 130 HU, the scan commences after another time delay known as the scan delay.

When using a test bolus, a small volume of contrast material, typically 20 ml, is injected at the same rate as the main injection. An ROI is placed in the aorta and low dose single slice images are obtained. The attenuation of the artery is then measured on each image at different points in time, and the time to peak maximal enhancement (PME) is calculated. The image acquisition for the diagnostic scan is then timed by using the time to PME plus a few extra seconds because a larger contrast volume is administered.

Note that these techniques of image acquisition timing can also be utilized with MRA.

4

When should a test bolus be used?

There may be local preferences, but in general, a test bolus is used in the following circumstances:

• The aorta is diseased by atherosclerosis, or there is aortic dissection. In both situations, the ROI may inadvertently be placed in a segment of the aorta which does not opacify following contrast administration as much as it should.

• The patient has a depressed ejection fraction (typically a left ventricular ejection fraction of less than 20%).

• The volume of administered contrast material is reduced because of renal impairment.

5

Describe a typical computed tomography (CT) protocol for assessing the arteries.

Unenhanced precontrast images are often, but not always, obtained. Some indications for obtaining unenhanced precontrast images include: suspected hemorrhage, penetrating trauma, evaluation of the aorta following stent or surgical repair, and suspected acute aortic syndrome.

Arterial phase images are obtained after the injection of 100 ml of iodinated contrast material mixed with saline at a rate of 4 to 5 ml per second, using bolus tracking, with a monitoring delay of 12 seconds and a scan delay of 6 seconds.

Venous phase images are not routinely obtained. Some indications for venous phase imaging include: suspected mesenteric ischemia, suspected hemorrhage, evaluation of an indwelling stent graft to detect an endoleak, and suspected venous thrombus.

6

Is the quality of evaluation of the arteries of the lower extremities better with a wide-detector multidetector row CT (MDCT) scanner?

Asked differently, is a 256-detector MDCT scanner better than a 64-detector MDCT scanner to assess the lower extremity arteries? Not necessarily. The acquisition time of wide-detector MDCT scanners is shorter and could lead to outrunning of the contrast bolus. That is, the scan could reach the lower extremities before the contrast material has arrived in this location.

7

How can we mitigate outrunning of the contrast bolus when using a wide-detector MDCT scanner?

Wide-detector MDCT scanners can be slowed by slowing the gantry rotation time, using narrower detector coverage where offered, and by reducing the pitch. Additionally, the threshold attenuation for triggering the scan during bolus tracking can be increased, and the scan delay can also be increased. These measures will ensure that the scan starts later with less chance of outrunning the contrast bolus. Finally, it is prudent to acquire, immediately, a second set of images through the most peripheral part of the peripheral circulation (i.e., through the anatomic region the contrast takes the longest amount of time to reach). For assessment of the arteries of the lower legs, this means a second image acquisition from the knees to the toes.

8

What is the most important consideration when imaging the arteries of the upper extremities?

The most important consideration is laterality. It is important to inject contrast material on the side opposite the side of pathology. Otherwise, concentrated contrast material passing through the upper extremity veins will lead to streak artifact (on CT) or susceptibility artifact (on MRI) that may obscure the adjacent arteries of interest.

9

How is MRA image acquisition timed?

The arterial phase of MRA can be timed by visualization of the contrast material entering the arterial tree of interest. The alternative is to obtain multiple acquisitions (multiphasic imaging) through the anatomic region of interest after contrast administration. Because MRA does not involve ionizing radiation, there is no penalty in terms of radiation exposure with multiphasic imaging.

Multiphasic imaging is typically used to assess the temporal filling pattern of vascular lesions. This is also utilized when the arteries of interest are confined to an extremity, such as in the hand or calf.

10

Describe a typical MRA protocol.

The specific protocol will vary depending on the particular artery or vein of interest, and the clinical indication. However, imaging sequences that are usually obtained include:

• Multiplanar heavily T2-weighted images: These are useful to assess the nonvascular structures in the anatomic region of interest and to detect fluid collections.

• Unenhanced precontrast T1-weighted images: These are useful to detect subacute hemorrhage, which has high T1-weighted signal intensity, and to serve as a baseline of comparison with contrast-enhanced images to assess for presence of enhancement.

• 3D T1-weighted contrast-enhanced MRA images acquired during the arterial and venous phases of enhancement: These are useful to assess the arteries and, to a lesser extent, the veins in high detail.

• 2D axial delayed phase contrast-enhanced T1-weighted images: These are useful to assess the veins.

11

What are the options if the patient cannot receive intravenous gadolinium-based or iodinated contrast material?

This depends on the clinical question. Unenhanced CT can still be useful to detect ruptured aortic pathology, intramural hematoma, and stigmata of mesenteric ischemia in the bowel. However, it cannot generally detect an aortic dissection, and it cannot assess vessel patency, vessel wall abnormalities, or vascular graft dehiscence.

On magnetic resonance imaging (MRI), vascular assessment without intravenous contrast material is still possible using one or more of the following image sequences:

• Time of flight (TOF) images.

• Phase contrast (PC) images.

• Arterial spin labeled (ASL) images.

• Balanced steady state free precession images.

12

What must you tell the MRI technologists when performing a TOF MRA?

There are three important points to communicate. First, you must decide if you want to use a saturation band and, if so, the direction of the saturation band. A saturation band removes signal from a vessel depending on the direction of its blood flow. For example, if you want the lower extremity arteries not to have signal, apply a superior saturation band to nullify the craniocaudal arterial flow of blood. Conversely, if it is signal in the veins that you want to remove, then apply an inferior saturation band to nullify the caudocranial venous flow of blood. Generally, when assessing the arteries, the signal in the veins is removed, and vice versa. However, you may elect not to use a saturation band.

Second, it is important to specify the plane of the sequence. The image acquisition must be perpendicular to the long axis of the vessel of interest, or as close to the perpendicular as possible. For example, to assess the arteries of the calf, the axial plane is used. To assess the renal arteries, the sagittal plane is used.

Third, decide whether the TOF image acquisition should be gated to the cardiac or respiratory cycle. For assessment of the arteries in the calf and foot, gating may help to improve image quality.

13

What is the advantage of TOF MRA?

This approach can determine the directionality of blood flow, which is useful to distinguish between antegrade and retrograde flow.

14

What are the disadvantages of TOF MRA?

The sequence is limited for assessment of vessels that have a tortuous course, such as the splenic artery, and is also limited for assessment of vessels that do not run perpendicular to the plane of section. It is not as good for assessing veins, because blood flow in veins is slower. Furthermore, it does not distinguish high-grade stenosis from moderate-grade stenosis as well as contrast-enhanced MRA.

15

Is CTA or contrast-enhanced MRA better?

It depends on local preferences and expertise. CTA is quicker than MRA, and therefore is generally preferred for evaluation of potentially unstable or claustrophobic patients, but it exposes patients to ionizing radiation. CTA is superior when there are indwelling metallic stents in the artery. Stents, particularly stainless steel stents, lead to blooming artifact on MRA. CTA is also useful to assess the burden of atherosclerotic calcification, which is useful, for example, for preoperative planning of abdominal aortic aneurysm (AAA). Paradoxically, MRA is better for evaluating heavily calcified arteries that are also small, such as arteries of the calf. This is because it is difficult to distinguish between heavy calcification and iodinated contrast material on CTA, particularly when an artery is heavily calcified.

The spatial resolution of CTA can be as small as 0.6 mm, whereas MRA is typically on the order of 1 mm. MRA is more amenable to postprocessing because it is easier to subtract background structures. Both CTA and MRA can offer a snapshot of organ perfusion, but MRA offers more dynamic information without radiation exposure.

MRA allows image acquisition in any plane, whereas CTA provides only for image acquisition in the axial plane. However, multiplanar reconstructions and reformations can subsequently be created in any plane of interest.

16

When performing a contrast-enhanced MRA, what must you communicate to the technologists?

The plane of section in which you want to reconstruct 2D images from the acquired 3D MRA images. The technologists may call this “the plane of injection.” Put simply, this is the plane that allows for full coverage of the artery of interest while using the thinnest depth of the 3D volume of image acquisition to minimize acquisition time. For the renal arteries, it is either the coronal or axial plane, but not the sagittal plane. For the celiac arterial tree, it is the axial plane. For the superior mesenteric artery (SMA), it is the sagittal plane. For the lower extremity arteries, it is the coronal plane.

17

Is CTA or MRA preferred for assessment of the renal arteries?

Both work well. If MRA is used, then a tight field of view (FOV) should be prescribed (i.e., a tight “plane of injection”) around the renal arteries and abdominal aorta to derive a spatial resolution comparable to CTA. CTA is preferred to assess the patency of an indwelling renal artery stent.

18

Is CTA or MRA preferred for assessment of the iliac arteries for percutaneous access?

CTA is preferred, although MRA may also be used. CTA gives a better indication of the amount of atherosclerotic calcification in the arteries, which affects management.

19

Is CTA or MRA preferred for assessment of the calf and foot vessels?

Both have strengths and weaknesses. CTA ensures reasonable temporal uniformity of arterial opacification, so long as the contrast bolus is not outrun. Multistation MRA may be so delayed as to lead to venous opacification in the calves, making it difficult to distinguish between artery and vein. However, MRA is preferred when heavily calcified arteries are present.

20

Is CTA or MRA preferred for assessment of arteriovenous malformations?

MRA is preferred because multiphasic imaging is feasible and provides information about the vascular abnormality. This helps to distinguish between high and low flow vascular lesions. Also, MRI gives better information about the surrounding soft tissues secondary to superior soft tissue contrast.

21

Is CTA or MRA preferred for assessment of mesenteric ischemia?

Both assess the mesenteric arteries and veins well. However, CT may better depict some stigmata of acute bowel ischemia/infarction such as pneumatosis, portal venous gas, and pneumoperitoneum. Furthermore, CTA is much faster to perform compared to MRA, which is an advantage for the evaluation of potentially unstable patients.

22

Is CTA or MRA preferred for assessment of arterial trauma following penetrating trauma?

CTA is preferred. Metallic shrapnel causes susceptibility artifact on MRA, which can obscure the arteries. Active arterial extravasation of contrast material is well depicted on CTA as extraluminal contrast material which does not conform to a known vascular structure. Again, CTA is much faster to perform compared to MRA, which is an advantage for evaluation of potentially unstable patients.

23

Is CTA or MRA preferred for assessment of endoleaks following stent graft repair of an aneurysm?

CTA is preferred because the stent graft causes susceptibility artifact on MRA. Embolization coils, when present, also cause susceptibility artifacts. These can obscure portions of the aneurysm, limiting evaluation.