MSCT evaluates upper abdominal vessels

November 5, 2004

Vascular imaging has benefited enormously from the increased speed and enhanced image acquisition capabilities of both single-slice spiral CT and multislice CT.1-4 The ability of 16-slice scanners to acquire images 40 times faster than conventional spiral scanners has revolutionized vascular imaging.

Faster scanning means a reduction in breath-holding, shorter examination times, and fewer motion artifacts, all of which enable examination of the entire thoracic and abdominal aorta in a single scan with high spatial resolution. Acquisition of large volumes with thinner slices generates nearly isotropic data that can be reconstructed in any imaging plane to yield the same resolution as the original axial images. Radiologists are not only adapting imaging protocols to patients' specific diagnostic needs but are also seeking new strategies with MSCT to reduce patient radiation exposure.

Use of 16-slice CT has many advantages for evaluating the abdominal vascular system in normal and pathological conditions. At the University Tor Vergata, we generally use a low-dose acquisition for precontrast scanning on our MSCT system (Light Speed 16, GE Healthcare). Such precontrast scans are advisable when bleeding is suspected, as they can evaluate wall calcifications and intramural hematomas or dissections. We use a 2.5-mm slice collimation for abdominal aorta or stent-graft imaging and 1.25-mm slice collimation for the study of visceral arteries, with a table feed of 13.75 mm/sec for both. Optimal anatomic coverage should range from 1 cm above the diaphragm to the iliac artery bifurcation.

Obtaining valid arterial enhancement is crucial in contrast-enhanced imaging. Optimal synchronization between data acquisition and peak vascular enhancement improves image quality. Because the fast speed of 16-slice CT scanners substantially reduces the time window for this synchronization, we use our scanner's "smart prep" function to calculate timing. This automated bolus triggering technique detects arrival of contrast in the artery of interest. The abdominal aorta at the diaphragmatic level is designated a region of interest. Contrast injection starts simultaneously with a low-dose dynamic monitoring scan. MSCT acquisition begins automatically once intensity measurements at the ROI rise above 180 HU.

Acquisition volume and table feed are identical to those used in the precontrast scan. We use slice collimations of 2.5 mm for the aorta and 1.25 mm for visceral arteries, with incremental reconstructions of 1.25 mm and 0.625 mm, respectively.

The switch to a 16-slice CT scanner with its faster acquisition has enabled us to reduce the volume of contrast used. Injection duration should match acquisition time. High-concentration contrast media (370 to 400 mg/mL iodine) requires injection flow rates between 3 and 4 mL/sec. We usually inject 100 to 120 mL of 370 mg/mL iodine contrast with a flow rate of 4 mL/sec. Flushing the vein with 60 mL saline solution following contrast bolus injection is always advisable.5,6

A delayed scan is preferable for following up a stent-graft or studying a possible aortic dissection. Acquisition begins 60 to 80 seconds after the end of the contrast phase. The presence of leakage in the aneurysmatic sac during evaluation of stent-grafts can be better demonstrated in the delayed scan. True and false lumens that usually present with a slow washout can be differentiated easily.

Many real-time viewing and reformatting options exist for analyzing vascular 16-slice CT studies. We can acquire large volumes of body imaging data in a short time with near-isotropic spatial resolution, allowing us to create high-quality 2D and 3D images on a dedicated workstation (Advantage 4.2, GE Healthcare).

Axial source images remain the basis for diagnostic decision making. Curved planar reformatting, 2D multiplanar reformatting (MPR), and advantage vessel analysis (AVA) are particularly useful in evaluating the vessel diameter and measuring the degree of arterial stenosis. The arterial lumen and thrombus can be displayed simultaneously within the aneurysm. Three-D postprocessing options such as maximum intensity projection (MIP) and volume rendering, which provide angiography-like images, offer excellent demonstration of vascular anatomy. We always choose to reconstruct different projection angles.

Identification of all possible artifacts is an important part of image interpretation. Short acquisition times of 10 to 15 seconds reduce the likelihood of respiratory artifacts and make it feasible to perform the examination in uncooperative patients.

The presence of calcifications and metallic endovascular stents with higher attenuation than opacified vascular lumen can cause blurring artifacts, which may lead to overestimation of the stenosis. Better visualization is obtained with wide window settings. Image noise in 3D reconstructions can cause irregularities of object surfaces or even obscure underlying pathology, and it may produce a veil-like shadowing of structures deeper inside the displayed volume. Intravascular lesions such as mural thrombi or soft plaques sometimes cannot be visualized directly using 3D volume rendering or MIP when a thicker slab is used.7

CLINICAL BENEFITS

The main advantages of using 16-slice CT to assess abdominal vasculature are improved spatial resolution and increased coverage. MSCT scanners can examine a range of vascular pathologies involving the aorta and abdominal vessels.8-10

A 16-slice scanner can cover the entire abdominal aorta within a single breath-hold and still yield isotropic resolution. Aortic aneurysms are more commonly found in the infrarenal portion of the aorta associated with a high grade of atherosclerosis. At our university and in many radiology departments, CT angiography is established as the first-line modality for diagnosis of patients with aneurysm dilatations. CTA enables evaluation of the aneurysm's extension and real diameter as well as vessel calcifications and thrombotic wall apposition (Figure 1). Accurate postprocessing makes it possible to judge the involvement of abdominal vessels such as the celiac trunk, upper mesenteric artery, and renal arteries.

Data from 16-slice CT also aid perioperative surveillance of the aneurysm and surgical planning. Curved or AVA reconstructions can be used to gain measurements of the aneurysmal sac diameters and the caliber and angle of the iliac arteries. These reconstructions also help in selecting the best endoprosthesis to exclude the dilatation. Images acquired for follow-up of surgical or endovascular procedures show the correct position and integrity of the prosthesis or stents. Any leakage due to residual perfusion of the aneurysmatic sac from a lumbar or hypogastric artery or caused by damage to the device mesh can be demonstrated (Figure 2). Sac regression can also be monitored.

Images of an aortic dissection acquired with 16-slice CT allow evaluation of true and false channels, the presence and extension of the intimal flap, primary entry and possible distal reentry, and the thrombotic process of the false lumen. MPR reconstructions and AVA protocols may help determine whether surgery or endovascular procedure would be the correct therapeutic workup (Figure 3).11

Sixteen-slice CT can also prove useful for evaluating visceral arteries. The modality is regarded as a reference technique for noninvasive imaging of renal arteries. It can assess stenosis and occlusions with high accuracy: 90% sensitivity and 98% specificity compared with digital subtraction angiography (DSA).11 It depicts delayed parenchymal enhancement and decreased size of the involved kidney well. The images can also be used to grade stenoses and poststenotic dilatations. Two-D and 3D reconstructions allow assessment of the aneurysmatic dilatation, suspected dissection, and thrombosis or trauma of renal arteries.

Additionally, we can evaluate the suitability of potential renal donors, demonstrating their entire vascular map with all possible anatomical variations (Figure 4). We also use this technology to evaluate other splanchnic vessels such as the celiac trunk and its branches and the mesenteric arteries in aneurysmatic dilatation, dissection, and vasculitis.

We can map abdominal vasculature before and after embolization of liver lesions. Abdominal mesenteric arteries can be evaluated in cases of abdominal angina and in patients presenting with acute or chronic ischemia. MSCT images of acutely ill patients may reveal embolic occlusions of cardiac origin (30% to 50%), thromboses due to an atherosclerotic process (15% to 30%), nonocclusive vasospastic mesenteric ischemia (20% to 30%), acute mesenteric thromboses (5% to 10%), or spontaneous dissection.11 (These figures indicate the causes of abdominal ischemia.) Scans of patients with chronic abdominal angina will show any significant hemodynamic stenoses involving the superior or inferior mesenteric arteries (Figure 5).

Sixteen-slice CT can help in identifying stenoses to the anastomoses of hepatic arteries or portal veins in liver transplant patients and in assessing the encasement of superior mesenteric arteries and vein in patients with pancreatic carcinoma.12-16

ACCURACY AND EFFICACY

We investigated the utility of angiograms obtained on our 16-slice CT scanner between April and November 2003, using DSA as a gold standard. Axial MSCT angiograms were compared with DSA results in all 96 patients. Our study included 22 cases of abdominal aortic aneurysm, 14 aortic dissections, 10 stent-grafts, six renal artery aneurysms, 12 renal stenoses, 10 celiac trunk aneurysms, nine celiac trunk stenoses, 16 splenic artery aneurysms, and 12 superior mesenteric artery aneurysms.

We found that the MSCT angiograms provided additional clinical information not seen on DSA. Sixteen-slice CT also proved more accurate than DSA in measuring the real size of aneurysmatic dilatations and in demonstrating the presence of wall calcifications, mural thrombi inside the aneurysmal sac, and bleeding within self-contained ruptures.

Two-D and 3D postprocessed images depict the aneurysmal neck well. Sixteen-slice CT can also establish intimal flap extension into the aortic branches in cases of chronic or acute aortic dissection. It is our modality of choice in the follow-up of endovascular repair, due to its high accuracy in detecting endoleaks. A delayed scan is necessary to detect slow-flowing leaks that may otherwise be missed during the arterial phase. MSCT angiography shows renal artery stenosis clearly17 and is also useful in evaluation of renal artery aneurysms and dissections.

The move from single-slice spiral to multislice CT scanning is changing radiologists' role in the evaluation of vascular structures. Emergence of faster workstations with increasingly sophisticated reconstruction algorithms, more user-friendly interfaces, and smarter investigative tools will further enhance this technology.

PROF. SQUILLACI is a professor of radiology, DR. SPERANDIO is a fellow, and PROF. SIMONETTI is department head, all in the department of diagnostic imaging and interventional radiology at the University Tor Vergata in Rome.

References

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