CT Angiography

The advent of helical and now multichannel CT has allowed CT to become a powerful tool for imaging the vascular system. The technique involves a thinly collimated helical scan through the renal vascular pedicle during a venous injection of iodinated contrast media. With each scanner generation, resolution has improved. Now we can image isotropically and reformat in any plane while maintaining image quality. Improved speeds now allow scanning of the aorta from diaphragm to iliac bifurcation within a single breath-hold. However, the imaging of renal arteries sets a difficult challenge for CT, because they run parallel or slightly oblique to the scanning plane and accessory vessels can have submillimeter dimensions. It has several advantages over conventional angiography. It is relatively noninvasive in that it does not require an arterial injection and so has consequent decreases in physician time, cost, in-patient hospital stay, patient discomfort, and complications. With improved technique it may also allow for a lower patient dose (67). CT angiography (CTA) may be used to plan the optimum projection for subsequent angiography and/or intervention (64).

Technique

For evaluation of the renal arteries, bowel contrast is not necessary; however, if this is desired, negative contrast with water should be used in preference to iodinated positive contrast. Precontrast, thick-section, low-dose scans should be obtained to localize the origins of the renal arteries and define the scanning volume. A scan from above the superior mesenteric artery to the aortic bifurcation will miss much less than 1% of accessory renal arteries (68). The thinnest slice collimation that allows adequate volume coverage within a single breath-hold should be selected. With single slice scanners, the volume to be scanned must be balanced against spatial resolution along the patient's (z) axis. This is obviously much less of a dilemma with multichannel scanners. Phantom studies have suggested that a maximum of 2 mm collimation is

Figure 7 (Figure on facing page) (A) LAO 20 view from the flush aortogram of a hypertensive patient. There are one right and two left renal arteries. A significant nonostial stenosis was present in the lower pole, accessory left renal artery (arrow). (B) This was resistant to conventional angioplasty but responded well to treatment with a 4 mm cutting balloon, followed by repeat dilatation with a 5 mm conventional balloon. The posttreatment appearance is shown with the guidewire still in the lower pole artery. (C) Pretreatment carbon dioxide angiogram in the same patient. Note that although the main renal arteries and large accessory artery are reasonably well opacified, the intrarenal vessels are not seen as a result of bolus fragmentation. Abbreviation: LAO, left anterior oblique.

Figure 8 (A) Flush aortogram in a young hypertensive patient demonstrating minor aneurysmal beading (arrow) of the right main renal artery but no definite stenosis. The left renal artery was normal. (B) Carbon dioxide angiogram in the same patient. The aneurysmal segments are visible but less obvious. On selective catheterization and pressure measurement, there was a 30 mmHg gradient in the main artery. This was abolished following angioplasty. The appearances are consistent with FMD. Abbreviation: FMD, fibromuscular dysplasia.

Figure 8 (A) Flush aortogram in a young hypertensive patient demonstrating minor aneurysmal beading (arrow) of the right main renal artery but no definite stenosis. The left renal artery was normal. (B) Carbon dioxide angiogram in the same patient. The aneurysmal segments are visible but less obvious. On selective catheterization and pressure measurement, there was a 30 mmHg gradient in the main artery. This was abolished following angioplasty. The appearances are consistent with FMD. Abbreviation: FMD, fibromuscular dysplasia.

required to accurately demonstrate RAS (69,70). With increasing pitch, the slice sensitivity profile is broadened, resulting in a larger effective slice thickness. However, if pitch is doubled, the volume scanned doubles, but the effective slice thickness only increases by 30% (68,71). Therefore, scans should be performed with high pitch and narrow collimation. Multichannel CT allows for much higher pitch factors. The patient should be coached in the breath-hold technique prior to the scan as respiratory artifacts can mimic a stenosis or aneurysm of the renal artery. Respiratory movement has less effect on the blood vessels caudal to the kidneys due to their being relatively fixed within the retroperitoneum. The data obtained with helical CT is converted into

Imaging of the Renal Arteries: Current Status Table 7 Multichannel CT of the Renal Articles

• No bowel preparation required

• Precontrast thick section localizer scan

• Scan from superior mesenteric artery to aortic bifurcation

• Dynamic bolus tracked injection of contrast

• Thinly collimated scan, for example, a slice thickness of 0.75 mm and table feed of 18 giving a pitch of 1.5

Abbreviation: CT, computed tomography.

axial slices by means of a linear interpolation algorithm. Currently, 180° interpolation is used because it decreases z-axis blur (62), allowing lower effective slice thickness. Contrast administration should maintain an enhancement plateau throughout the entire scanning period. This is best achieved by using bolus-triggering software. A region of interest is set in the aorta and sequential scans are performed at this level. When enhancement in the region reaches a predetermined threshold (usually 50-100 Hounsfield units), the scan is automatically triggered. With single slice scanners, 120 to 150 mL of contrast injected at 3 to 5mL/sec is sufficient (68). As multichannel scanning is faster, the plateau phase can be shorter, and so less contrast is required. In addition, if the contrast bolus is chased with a 40 mL flush of saline, the plateau can be prolonged by up to eight seconds (68). Table 7 outlines an example of a scanning protocol for a 16-channel multislice CT.

Data Presentation

The volume of data gathered by helical scanning can be presented in a variety of ways, including traditional axial images, multiplanar reformats (MPRs) and curved reformats, maximum intensity projections (MIPs), and surface-shaded display (SSD). Axial images remain the mainstay of diagnosis and allow identification of nonvascular renal disease. They should be reviewed interactively on a workstation in a cine fashion, because the renal arteries may be tortuous and course through several images (72,73). Multiplanar reformatting is most useful for providing coronal images that allow appreciation of a cranio-caudally orientated stenosis that may not be obvious on axial imaging. Curved reformats are produced by tracing the course of the artery on source images followed by software straightening. They can accurately display the width of the vessel and help with grading of stenosis but are time consuming. MIP images first require editing of the data to remove overlying bony structures. Newer workstations may do this automatically. MIPs produce an angiogram-like image in which the relative attenuation values of structures are maintained, but their 3-D relationship is lost (Fig. 9). They are very sensitive for vascular calcification; unfortunately, overlying renal veins may obscure the artery. SSDs provide an attractive way of presenting findings to referring clinicians but are of limited diagnostic accuracy. In SSD, adjacent voxels above a predetermined threshold attenuation are modeled into a 3-D object (Fig. 10A,B). Apparent vessel size is very dependent on the chosen threshold for 3-D reconstruction. If the level is too low, artifacts from overlying enhancing structures will be present, too high and small vessels not seen, or stenosis overgraded (72). In single threshold reconstructions, contrast and calcified plaque cannot be separated, leading to underestimation of lumen narrowing. Both MIP and SSD perform poorly in demonstrating accessory arteries (72). Volume-rendering uses opacity curves instead of a single threshold

10 Ways To Fight Off Cancer

10 Ways To Fight Off Cancer

Learning About 10 Ways Fight Off Cancer Can Have Amazing Benefits For Your Life The Best Tips On How To Keep This Killer At Bay Discovering that you or a loved one has cancer can be utterly terrifying. All the same, once you comprehend the causes of cancer and learn how to reverse those causes, you or your loved one may have more than a fighting chance of beating out cancer.

Get My Free Ebook


Post a comment