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Technical Developments |
1 From the Department of Radiology, Abdominal Imaging, New York University Medical Center, 560 First Ave, Suite HW 207, New York, NY 10016. Received December 11, 2000; revision requested January 26, 2001; revision received February 19; accepted March 8. Address correspondence to M.M. (e-mail: michael.macari@med.nyu.edu).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMaterials and Methods Results DiscussionREFERENCES |
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Index terms: Aorta, CT, 94.12916 • Computed tomography (CT), angiography, 94.12916, 98.12916
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMaterials and Methods Results DiscussionREFERENCES |
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To obtain high-quality CT angiograms, a timing acquisition may be required to ensure that data will be obtained during peak aortic enhancement (2,3). This strategy requires time for two injections of contrast material plus time to evaluate the transverse images (4). In a busy practice, this may not be feasible. An alternative approach is to use a bolus-tracking technique.
Findings in a previous study (4) with a single–detector row helical CT system demonstrated that use of a timing acquisition did not offer advantages over use of an empiric delay in evaluating the abdominal aorta. With use of a single–detector row CT system, only the abdominal aorta, not the pelvic runoff vessels, can be optimally evaluated with thin-section CT. Others (1) have reported excellent CT angiography results with 5-mm-thick sections with use of a dual–detector row system without a timing acquisition of the abdominal aorta and iliac arteries.
The z-axis coverage (40 cm) with thin sections (1.0–2.5 mm) can be obtained in a single breath hold with a multi–detector row CT system (2). However, the temporal window for contrast material administration during CT angiography is short. A timing acquisition may be even more important with multi–detector row CT than with conventional helical CT (5).
The purpose of this study was to evaluate the adequacy of CT angiography of the aorta and iliac and common femoral arteries with a four-channel multi–detector row CT system with thin sections and an empiric delay for administration of contrast material in patients without a history of major cardiac disease.
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Materials and Methods |
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TOPABSTRACTINTRODUCTIONMaterials and Methods Results DiscussionREFERENCES |
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Forty-eight patients underwent evaluation before placement of an endovascular graft, and 22 underwent evaluation at follow-up after placement. The procedure was explained to all patients, and written informed consent was obtained. CT angiography was performed as part of the routine clinical evaluation of patients before and after placement of an aortoiliac endovascular graft.
Scanning Technique
CT angiography was performed with a multi–detector row CT system (Volume Zoom; Siemens Medical Systems, Forcheim, Germany). After acquisition of a localizing scout image in all patients, CT angiography was performed from the superior aspect of the T12 vertebra to the caudal aspect of the symphysis pubis. All studies were performed with the following parameters: 4 x 1-mm section thickness, 120 kV, 0.5-second gantry rotation, pitch of 5–7, 120–160 mAs, 30-second breath hold, 10–14 mm/sec table speed, 1.25-mm-thick sections, 0.75-mm overlap, and 298–400 sections acquired.
Contrast Material Administration
CT angiography studies were performed after administration of 150 mL of nonionic intravenous contrast material (Iopramide [300 mg of iodine per milliliter]; Berlex Laboratories, Wayne, NJ). The contrast material was injected through a 20-gauge catheter inserted into an antecubital vein, at a rate of 4 mL/sec with a power injector (Envision; Medrad, Pittsburgh, Pa). In all patients, data acquisition began 25 seconds after initiation of the bolus injection. We chose 25 seconds as the scanning delay in this study on the basis of our prior experience with a single–detector row scanner for CT angiography and findings in a previous study (4) with a 20-second delay at CT angiography with single–detector row scanners. With use of an empiric delay of 20 seconds in that study, proximal abdominal aortic enhancement to 200 HU was not achieved in 35% of patients. Contrast material extravasation did not occur in our study.
Data Interpretation
Imaging data were evaluated quantitatively and qualitatively with an off-line workstation (Vitrea 2; Vital Images, Plymouth, Minn).
Quantitative Assessment
The adequacy of the 1.25-mm-thick transverse CT angiograms was determined by measuring the attenuation in the vascular lumen at five different levels including the aorta: level 1, the origin of the celiac axis; level 2, the origin of the left renal artery; level 3, 4 cm below the origin of the left renal artery; level 4, the origin of the common iliac arteries; and level 5, the origin of the common femoral arteries. In each case, a circular region-of-interest cursor was placed directly in the center of the vascular lumen by a single radiologist (M.M.). The region-of-interest cursor was made as large as possible while wall calcification or metallic artifact related to the stent was avoided. To evaluate attenuation of the common iliac and common femoral arteries, the single vessel (right or left) that enabled placement of the largest region-of-interest cursor was chosen for the analysis. A mean attenuation value of 200 HU was considered the low threshold defining adequate intraluminal opacification.
Qualitative Assessment
Postprocessed, volume-rendered, coronal, and sagittal maximum intensity projection data sets were evaluated by two radiologists (M.M., G.M.I.) in consensus. The aorta and iliac vessels were divided into two segments. The proximal segment included the aorta from the celiac axis to the aortic bifurcation. The distal segment included the proximal common iliac arteries to the common femoral arteries.
The radiologists assigned a grade for the image quality of the segments, without knowledge of the quantitative information, by using a four-point scale: 1, suboptimal (diagnostic information not obtained); 2, adequate (all clinically relevant diagnostic information obtained with poor differentiation of arterial vasculature from background tissues that required use of source data); 3, adequate (all clinically relevant diagnostic information obtained with good differentiation of arterial vasculature from background tissues that did not require use of source data); 4, excellent (all clinically relevant diagnostic information obtained with excellent differentiation of arterial vasculature from background tissues).
This qualitative analysis was applied to the length of the aortic neck; the relationship of the renal arteries to the aneurysm, including the presence of accessory renal arteries; the location of the aneurysm; and the presence of iliac and femoral stenoses. These anatomic relationships were the clinically relevant information that was used to establish the qualitative assessment.
In addition, the presence of renal vein contamination and whether it interfered with data display were graded on a three-point scale: 1, renal vein not visualized; 2, renal vein partially opacified but does not interfere with data interpretation; and 3, renal vein opacified and interferes with data interpretation.
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Results |
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TOPABSTRACTINTRODUCTIONMaterials and Methods Results DiscussionREFERENCES |
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In five patients, eight mean attenuation values ranged from 150 to 200 HU. In three of these five patients, a single mean attenuation value was less than 200 HU. This measurement was obtained at level 1 (199 and 185 HU) in two of the five patients; at level 5 (197 HU) in one patient; at levels 2 (188 HU) and 3 (189 HU) in one patient; and at levels 1 (167 HU), 2 (197 HU), and 5 (164 HU) in one patient.
In the majority of patients, attenuation varied throughout the course of the aorta and runoff vessels. In 48 patients, attenuation values increased steadily from proximal to distal areas (Fig 6). In nine patients, attenuation increased linearly to the level of the common femoral artery, where there was a decrease in attenuation. In eight patients, attenuation decreased 4 cm below the renal arteries (Fig 7). In five patients, attenuation values were within 10 HU of each other at all five levels (Fig 8).
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Regarding renal vein opacification in the 70 patients, a grade of 2 was given to 13 vessels, and a grade of 1 was given to 57. Renal vein opacification did not interfere with three-dimensional data display in any case.
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Discussion |
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TOPABSTRACTINTRODUCTIONMaterials and Methods Results DiscussionREFERENCES |
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The correct scanning delay can be determined in a number of ways. A timing acquisition can be performed during which a small amount of contrast material (15–20 mL) is injected at the same rate that will be used at CT angiography; simultaneously, low-dose serial transverse CT scans can be obtained at the location where scanning is to be initiated (4). The peak enhancement can then be calculated by reviewing the transverse images with a region-of-interest cursor placed in the center of the aorta. This strategy requires the time for two injections of contrast material plus the time to evaluate the transverse images. In a busy practice, this may not be acceptable. An alternative approach is to perform a bolus-tracking technique, in which scanning begins when a preassigned attenuation level is reached, typically 100–150 HU for aortic evaluation (6). This option is available with some commercially available CT scanners. Although these techniques potentially offer better control of the scanning delay, there are limitations to their use, including availability, cost, time, and potential for computer delay or malfunction.
In our study, which was performed without a timing acquisition, vascular opacification of at least 200 HU was achieved in 342 (98%) of 350 cases, and every measurement was greater than 200 HU in 65 (93%) of 70 cases. Moreover, the measurement was never below 150 HU in cases with a mean attenuation of less than 200 HU.
Image quality was compromised appreciably in only one patient, who weighed 147 kg. In this patient, three of the five measurements were less than 200 HU. Since patient weight has been demonstrated to be inversely correlated with aortic enhancement (7), it is not surprising that we obtained attenuation measurements of 167, 197, and 164 HU in this patient. Volume-rendered and maximum intensity projection images in this patient were evaluated as having suboptimal quality, with a grade of 2 for both proximal and distal segments. In the remaining five cases with low attenuation measurements in our study, mean attenuation was 199, 185, 197, 185, and 189 HU. These measurements were less than 200 HU, but they did not adversely affect image quality (Fig 6).
A low threshold of 200 HU for mean attenuation was chosen as a legitimate test of adequate aortic enhancement in this study. In previous studies (4,8), which evaluated the adequacy of opacification of the abdominal aorta with a single–detector row helical CT scanner, an attenuation value of 160 HU was used to define adequate vascular opacification. With 160 HU as a low threshold of enhancement, no benefit was reported from a timing acquisition before CT angiography in patients without cardiac disease (4). In that study, CT angiography was performed from the celiac axis to the aortic bifurcation.
Twenty-five seconds was chosen as the scanning delay in this study, primarily on the basis of our prior experience with a single–detector row scanner for CT angiography and findings in a previous study (4) in which a scanning delay of 20 seconds was used at CT angiography with single–detector row scanners. In that study (4), 26 patients underwent evaluation without a timing acquisition; mean attenuation values were less than 200 HU in nine patients and were less than 160 HU in the proximal aspect of the aorta in three. On the basis of these data, we elected to delay the beginning of data acquisition until 25 seconds to improve proximal attenuation. The entire acquisition was tailored to last 30 seconds for complete z-axis coverage. With this technique, mean attenuation from proximal to distal areas increased in a majority (48 [68%] of 70) of patients and decreased distally in only nine (13%).
Longer z-axis coverage is needed at CT angiography because of the increasing availability of bifurcated endovascular stents with distal ends that are deployed in the iliac arteries. In addition to determination of aneurysm size, the presence of accessory renal arteries, and the angulation and tortuosity of the aneurysm and aneurysm neck, the presence of iliac and femoral stenoses also needs to be determined (1,9). Traditionally, a combination of both conventional and CT angiography was believed necessary to provide all these data (9). CT angiography with multi–detector row technology can provide high-quality CT data sets that may answer all of the clinical questions, potentially obviating preoperative invasive studies (1,2,10).
There are two requirements for CT angiography to answer all the clinical questions. First, the vascular lumen must be adequately opacified with iodinated contrast material (1,2,4). Second, thin-section CT scans, which improve z-axis resolution, need to be obtained in such a way that the data can be optimally postprocessed (6,11,12). In our study, we demonstrated that these requirements—vascular opacification and thin sections—for optimal three-dimensional qualitative assessment can be achieved with multi–detector row CT without a timing acquisition.
There are several limitations to our study. First, we did not formally evaluate cardiac status; instead, our assessment of heart disease was based on the patient history. Many of our patients may have had some degree of atherosclerotic coronary artery disease, and some patients were being treated with cardiac medications, including beta and calcium channel blockers. Initially, patients with a history of congestive heart failure or previous myocardial infarction were excluded from our study. During the 4 months of this study, six (8%) of the original 76 patients were excluded for cardiac indications. The apparently low frequency of heart disease in this population may be partially attributable to selection bias inherent in the study of only outpatients. When a patient with known cardiac disease is seen in a practice with a high frequency of cardiac disease, a timing acquisition must be performed before CT angiography to ensure optimal vascular opacification.
Second, we did not use a saline flush, which has been advocated by others (6) to increase the circulation of the contrast material through the vasculature. As our results indicate, however, this was not necessary with our protocol, since adequate quantitative enhancement was obtained at almost all levels.
Third, our CT scanner allowed use of 1-mm-thick sections with 0.5-second gantry rotation speed. This enabled maximal z-axis resolution over a greater acquisition range within the time that most patients can hold their breath. Therefore, our results may not be generalizable to scanners that operate with different specifications.
Fourth, a small amount of opacification of the renal veins was present occasionally during data acquisition with our CT angiography protocol. As demonstrated in the qualitative assessment of three-dimensional data displays, however, this small amount of opacification did not interfere with the quality of volume-rendered or maximum intensity projection images in any case.
Fifth, 150 mL of iodinated contrast material was used to ensure optimal vascular enhancement throughout our study. Similar opacification might have been obtained with smaller amounts of contrast material if bolus-tracking techniques were used or a small amount of contrast material were injected as a test bolus. The aim of our study, however, was to design a rapid, reliable technique that would ensure optimal opacification.
In conclusion, we demonstrated that adequate quantitative and qualitative enhancement of the aorta and iliac and common femoral arteries can be obtained in patients without cardiac disease by performing thin-section multi–detector row CT angiography with an empiric scanning delay of 25 seconds.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMaterials and Methods Results DiscussionREFERENCES |
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