Three-dimensional spiral CT angiography of the abdomen

Three-Dimensional Spiral CT Angiography of the Abdomen

Charles P. Semba, Geoffrey D. Rubin, and Michael D. Dake

Spiral CT angiography is a technical innovation in vascular imaging that can produce spectacular three-dimensional reconstructions of the abdominal vessels and organs using modified CT scanning techniques. Rapid volume data acquisition allows contrast material to be imaged in the arterial or venous phase. CT reconstruction in cross-sections avoids superimposition of overlying structures, The combination of these features allows spiral CT angiography to produce extraordinary images of the abdominal vasculature and organs. This review outlines fundamental techniques in spiral CT angiography and summarizes our initial clinical experience at Stanford University Medical Center, Copyright �9 1994by W.B. Saunders Company

T HE KEY FEATURE of spiral scanning that allows for angiographic reconstruc-

tions is rapid acquisition of imaging data. Spiral CT slip-ring technology provides continuous 360 ~ rotation of the x-ray gantry for up to 40 seconds. 1 Because the table continuously moves the patient through the gantry during scanning, the imaging data is collected along a continuous spiral path instead of by conventional stationary transaxial slices. The simultaneous table movement and continuous gantry rotation (gantry rotation period is 1 second) without an inter- scan delay allow data to be collected very rapidly. When using intravenously administered iodinated contrast, images of the abdomen can be obtained when the contrast is captured entirely during the arterial or venous phase of circulation. A typical high-resolution (< 3-ram collimation) spiral examination of the abdominal aorta can be performed in 30 seconds covering 9 to 18 cm of volume during each 30-second scan. A conventional CT scan may take several minutes to cover the same volume and therefore cannot precisely image during the arterial contrast phase. Spiral CT imaging exclu- sively in the arterial or venous phase of contrast injection thus provides the cornerstone of three- dimensional (3D) angiographic reconstructions.

METHODS

3D spiral CT angiography is being investi- gated actively at Stanford University Medical Center using both Siemens Somatom Plus-S (Siemens Medical Instrumentation, Iselin, N J) and General Electric HiSpeed Advantage (Gen- eral Electric Medical Systems, Milwaukee, WI) scanners. Since December 1991, more than 75 patients with a wide variety of vascular problems have been evaluated using spiral CT angiography and compared with conventional invasive angiography.2, 3

For imaging the abdominal aorta and splanchnic circulation, a routine scout projection is performed and 5 to 10 conventional transaxial images are obtained in the area of interest using a slice thickness of 10 mm. Based on these initial views, a 9- to 24-cm cylindrical scan volume is selected with the key region of interest placed in the center of the scan volume.

An 18- or 20-gauge intravenous 3-cm-long angiography catheter is then introduced into the antecubital vein. A test injection is performed to properly time the injection bolus to the region of interest. Fourteen scans are performed sequentially at 2-second intervals at the most cephalad region to be examined without table movement. Twenty mL of nonionic contrast (Isovue 300; Squibb, New Brunswick, N J) is injected using an automated power injector (Medrad, Pittsburgh, PA) at 4 to 5 mL per second using an 8-second scan delay. The patient is hyperventilated and instructed to breath hold for the entire dynamic 14-image sequence for a total scan time of 28 seconds. Image analysis is then performed to determine the time in which maximum contrast density is reached at the most cephalad portion of the scan volume. For example, if the maximum density within the top portion of the aorta is reached within 18 seconds, the scanning delay is programmed for approximately 16 seconds. Typi- cal scanning delay for the abdominal aorta is 14 to 25 seconds. 4

From the Departments of Cardiovascular/Interventional Radiology and Thoracic Radiology, Stanford University Medi- cal Center, Stanford, CA.

Address reprint requests" to Charles P. Semba, MD, Depart- ment of Cardiovascular/Interventional Radiology, Stanford Universi O, Medical Center, Stanford, CA 94305.

Copyright �9 1994by W.B. Saunders Company 0887-2171/94/1502-000655. 00/0

Seminarsin Ultrasound, CT, andMRI, Vo115, No 2 (April), 1994: pp 133-138 133

134 SEMBA, RUBIN, AND DAKE

After determining the appropriate scan delay, the formal diagnostic scan is performed using a single 30-second breath hold. Spiral data is collected using 30 continuous 360 ~ gantry rotations with a collimation of 3 mm and a table speed of 3 to 6 mm per second. All studies are performed at 120 kV and from 210 to 280 mA. Approximately 120 to 150 mL of iodinated contrast material is injected at a rate of 4 to 5 mL/s for the examination.

With the Siemens Somatom Plus-S, the ac- quired raw data is then processed into axial sections and subsequently into 3D renderings on the scanner. The General Electric HiSpeed Advantage interpolates the raw data into axial sections on the scanner and then transfers that data off-line to a work station (Vox-Tools; Sparc-10; Sun Microsystems, Mountain View, CA) for 3D reconstructions. In a typical 9-cm scan volume, 60 axial reconstructions are created using a 3-mm slice thickness spaced at 2.0-mm increments with a 1.0 mm overlap. The overlapping provides smoother 3D rendering and improved resolution resulting from decreased partial volume artifact. The entire scanning time is 30 seconds, and completed reconstructions can be obtained in less than 1 hour.

There are three main reconstruction formats: (1) conventional transaxial image, (2) shaded- surface display (SSD), and (3) maximum- intensity projection (MIP). 3 The transaxial images are identical to conventional scanners except that the contrast bolus is captured exclu- sively during the arterial or venous phase of circulation: For our spiral CT examinations, interpretation frequently requires both SSD and MIP formats because each rendering technique provides unique and complementary information (Fig 1). SSD's shaded-surface rendering of the vessel appears similar to a vessel filled with wax to create a solid cast. Because it renders only the surface of the vessel, there is no information regarding the "inside," such as complex focal atherosclerotic plaque anatomy or intimal dissection flaps. Additionally, there is no information about the relative density of the vessels, associated calcified plaque, and sur- rounding structures. For example, the aorta appears as equally dense or bright as the neigh- boring bone or solid organ. SSD requires conver- sion of the reconstructed data into a binary data

Fig 1. SSD (A) and MIP (B) images of a normal abdominal aorta and splanchnic vessels. The celiac trunk, superior mesenteric artery, renal arteries, and inferior mesenteric artery are identified readily. SSD, shaded surface display; MIP, maximum- intensity projection.

set, which occurs by selecting a density threshold (typically between 100 and 200 HU) such that any pixel above the threshold is included in the reconstruction and pixels below the threshold are not. Although SSD provides beautiful surface renderings of the vasculature and organs, it does not provide information regarding relative CT attenuation. Therefore, contrast- filled vessels, contrast-enhanced parenchyma, and calcified plaque all appear to have the same "density" or brightness when viewed in SSD projections if their attenuation is greater than the threshold selected.

3D SPIRAL CT ANGIOGRAPHY OF THE ABDOMEN 135

MIPs allow the contrast-filled vessels to be evaluated relative to the densities of adjacent structures. Each pixel in an MIP represents the highest Hounsfield unit (HU) encountered by an imaginary ray as it passes through a volume of interest. Because contrast material has higher attenuation than all structures except bone, the skeletal structures must be deleted to prevent them from obscuring the vasculature. MIP over- comes the problems of SSD in identifying the vascular anatomy within solid organs such as the kidney and liver. Because spiral CT obtains volumetric data, both MIP and SSD 3D reconstructions of the aorta and abdominal organs can create projections from any viewing angle with a single contrast bolus. Therefore, spiral CT can render views of the abdominal vasculature that are impossible to obtain using conventional angiographic techniques and projections.

CLINICAL EXPERIENCE

Most of our initial work in evaluating the utility of spiral CT angiography has been performed in the abdomen and the carotid/ cerebral circulation, z-6 We have examined patients with a variety of abdominal aorta abnormalities including aortic aneurysms, aortic dissections, renal artery disease, aorto-iliac atherosclerotic occlusive disease, and vascular grafts and stents. For aortic aneurysms, 3D spiral CT reliably depicts second-to-fourth- order aortic branches. The surgical neck and length of the aneurysm are appreciated easily, and the extent of disease involvement relative to the branch vessels is discerned clearly. Spiral CT angiography provides enough anatomic information for planning preoperative or endovascular intervention. The branch vessels including the celiac trunk and superior and inferior mesenteric arteries are appreciated easily, and calcified atherosclerotic plaque is identified readily using MIP reconstructions.

With aortic dissections, the extent of the intimal dissection flap and lumen sizes (true versus false) can be evaluated thoroughly (Fig 2). The degree of vessel compromise by the dissection plane can be seen without difficulty as well as the origin of branch vessels from the true or false lumen. We have found spiral CT to be extraordinarily helpful in viewing the true and false lumen in a clear format that is impos-

sine with conventional angiography because views of the aorta can be obtained in any projection without obstruction from the verte- bral bodies. Spiral CT evaluation is much faster than MRI but the field of view is limited to approximately 9 to 24 cm at 3-mm collimation. With 5-mm collimation and a 40-second expo- sure, 20 to 40 cm can be imaged by varying table speeds between 5 and 10 mm/s. Use of 5-mm collimation represents a tradeoffbetween higher resolution evaluation of aortic branches and increased length of coverage.

Spiral CT angiography of the renal arteries appears promising. 6 Despite technical advances in medical diagnosis and treatment, identifica- tion of renovascular hypertension remains difficult. The current gold standard is use of invasive renal angiography for locating renal artery stenosis. Whereas noninvasive tests such as nuclear scintigraphy can provide physiological clues, there are no practical noninvasive methods to evaluate renal arteries. Ultrasound is limited because it is highly operator dependent, can evaluate only a small portion of the renal artery, and cannot discern small accessory renal branches. Magnetic resonance angiography is relatively time consuming and is distorted by motion artifact, extensive calcified plaque, and metallic surgical clips. In our initial experience, spiral CT angiography provided excellent depiction of the renal anatomy and was reliable in detecting significant lesions using MIP analysis (Fig 3). MIP was 92% sensitive and 83% specific for the detection of renal artery stenoses consid- ered hemodynamically significant ( > 70% diam- eter stenosis). 6 Depiction of ostial lesions may be even better with spiral CT angiography than with conventional angiography. As scanning techniques are refined, spiral CT may provide an easier and less invasive alternative for screen- ing renal artery stenoses.

Imaging of vascular grafts and stents with spiral CT angiography offers detailed information that is useful in graft or stent surveillance and diagnostic problem solving. For example, a patient who has undergone surgical vascular bypass graft placement can have the graft evaluated less invasively using spiral CT angiography. Metallic stainless steel stents are mesh-like cylinders used inside arteries and veins to provide internal support, thus keeping the vessels


Fig 2. SSD images (A and B) of a complex abdominal aortic dissection showing two false lumens (arrows) and the posterior true lumen (open arrow). Note that the anterior false lumen supplies the superior mesenteric artery and right renal artery (A) and the smaller false lumen supplies the left renal artery IB). Angiogram of the small false lumen shows filling of the left renal artery (C) with early contrast opacification of the true lumen (open arrow). The angiogram of the posterior true lumen (D) shows opacification of the intercostal arteries and partial fill ing of the smaller false lumen (arrows) but no filling of the superior mesenteric artery. SSD, surface-shaded display.

open. Spiral CT angiography can assess the integrity of the stent lumen quickly (Fig 4). Magnetic resonance angiography may be difficult to interpret because of the magnetic suscep-

tibility artifact created by the stent. Doppler ultrasound is difficult because of overlying bowel artifact or lung. Current investigation includes imaging transjugular intrahepatic portosystemic

3D SPIRAL CT ANGIOGRAPHY OF THE ABDOMEN 137

Fig 3. SSD image (A) of a high grade left renal artery stenosis, which is implied by absence of the proximal renal artery (arrow) and opacification of the distal artery and atrophic renal parenchyma. The corresponding aortogram (B) shows a short segment 99% proximal left renal artery stenosis (arrows). SSD, shaded-surface display.

shunts (TIPS)--stents inserted percutaneously to treat portal hypertension and gastroesopha- geal bleeding. Other applications include evaluation of percutaneously placed endovascular stent/grafts to treat thoracic and abdominal aortic aneurysms. Spiral CT angiography is now a routine examination in our evaluation of percutaneously inserted aortic stent/graft endo- prostheses.

LIMITATIONS

Limitations of spiral CT angiography include the need for intravenous iodinated contrast media. Patients with severe contrast allergies, renal failure, or other contraindications to intravenous contrast cannot undergo either conven-

tional angiography or spiral CT angiography. Also, the cranio-caudal coverage of spiral CT is limited to 9 to 24 cm. In most applications, this coverage is sufficient to evaluate a problem such as an abdominal aortic aneurysm. However, the larger extent of coverage must be balanced with the resultant decreased resolution of small branch vessels. Sequential scans obtained with two contrast injections can be linked to form a single contiguous 3D angiogram encompassing up to 80 cm of coverage. In comparison with Doppler ultrasound and flow-sensitive magnetic resonance angiography, spiral CT angiography lacks the ability to quantify volume and velocity of blood flow within a given vessel. Spiral CT angiography cannot match the spatial and con-

Fig 4, Conventional MIP image of a left renal artery stainless steel Palmaz stent (A) and curved planar MIP reformations (B), which allow visualization within the stent. The MIP images can show the precise position of the stent. Poststent placement angiogram (C) shows a widely patent left renal stent (arrows). MIP, maximum-intensity projection.


trast resolution of film-screen or digital-subtrac- tion angiography, but in almost all applications, it provides the crucial anatomic information to solve clinical problems. Finally, upgrading to spiral CT requires a new scanner equipped with a slip-ring gantry. An existing non-slip-ring CT scanner cannot be modified to scan in spiral mode. The most recent spiral CT scanner costs nearly $1 million.

ADVANTAGES

The main advantage of spiral CT angiography is its ability to construct high-quality 3D renderings of the abdominal vasculature without the need for invasive angiography. A typical aortic angiogram requires puncture of the femoral artery, advancement of a catheter into the aorta, contrast injections, and a minimum of 1 hour in the angiography suite, followed by 6 hours of postprocedure monitoring before the patient can leave the hospital. With spiral CT angiography, the patient requires only a periph- eral intravenous injection, and the scan takes 30 seconds to perform. Thus, an aortic study takes approximately 30 minutes for the entire session. 3D reconstructions are available for review approximately 1 hour after the scan. Spiral CT angiography can provide views of the abdominal vasculature that cannot be obtained using conventional angiographic techniques. In conventional angiography, the operator must angle the tube and inject contrast for each different projection. Because of volumetric data acquisition,

spiral CT produces images of the aorta in an infinite number of projection angles. If the images are filmed sequentially at different projection angles, a video loop can be created and the entire vasculature can be viewed dynami- cally.

CONCLUSION

Although conventional angiography probably will remain the gold standard, spiral CT angiography is less invasive, less expensive, easier to perform, and quicker than conventional angiography. The reconstructed images provide excellent anatomic detail that easily enables accurate diagnosis and preoperat ive planning. Spiral CT angiography reduces the risk of invasive diagnosis and is not dependent on the catheterization skills of the operator. Scans are a simpler and faster alternative to magnetic resonance angiography. Spiral CT can be used in nonangio- graphic applications to obtain fast scans of the abdomen and thorax; such uses allow a higher throughput of patients and overall increased examination efficiency. 7-9 Our initial experience has shown that spiral CT angiography is safe, reliable, and efficacious. We are now beginning prospective randomized studies to evaluate spiral CT angiography versus conventional angiography in operative or interventional preproce- dure planning and assessment of renal artery disease. With the rapid explosion of clinical testing and refinement in techniques, spiral CT angiography no doubt will become a routine diagnostic tool.

REFERENCES

1. Kalender WA, Seissler W, Klotz E, et al: Spiral volumetric CT with single breath-hold technique, continuous transport, and continuous scanner rotation. Radiology 176:181-183, 1990

2. Rubin GD, Dake MD, Napel SA, et al: Three- dimensional spiral CT angiography of the abdomen: Initial clinical experience. Radiology 186:147-152, 1993

3. Marks MP, Napel S, Jordan JE, et al: Diagnosis of carotid artery disease: Preliminary experience with maximum-intensity-projection spiral CT angiography. A JR Am J Roentgenol 160:1267-1271, 1993

4. Rubin GD, Walker P J, Dake MD, et al: 3-D spiral CT angiography: An alternate imaging modality for the abdominal aorta and its branches. J Vasc Surg 18:656-665, 1993

5. Napel SA, Marks MP, Rubin GD, et al: CT angiography with spiral CT and maximum intensity projection. Radiology 185:607-610, 1992

6. Rubin GD, Dake MD, Napel S, et al: 3-D spiral CT angiography of the renal arteries. Radiology 1993 (in press)

7. Remy-Jardin M, Remy J, Wattinee L, et al: Central pulmonary thromboembolism: Diagnosis with spiral volumetric CT with the single-breath-hold technique--Comparison with pulmonary angiography. Radiology 185:381-387, 1992

8. Costello P, Dupuy DE, Ecker CP, et al: Spiral CT of the thorax with reduced volume contrast material: A com- parative study. Radiology 183:663-666, 1992

9. Dupuy DE, Costello P, Ecker CP: Spiral CT of the pancreas. Radiology 183:815-818, 1992

Three-dimensional spiral CT angiography of the abdomen

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