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Original Research |
1 Diagnostic Imaging Center, Saiseikai Kumamoto Hospital, 5-3-1 Chikami,
Kumamoto-shi, Kumamoto 861-4193, Japan.
2 Diagnostic Radiology, Graduate School of Medical Sciences, Kumamoto
University, Kumamoto 860-0811, Japan.
3 Diagnostic Image Analysis, Graduate School of Medical Sciences, Kumamoto
University, Kumamoto 860-0811, Japan.
4 CT Systems Division, Toshiba Medical Systems, Tokyo 113-8456, Japan.
Received September 14, 2004;
accepted after revision January 27, 2005.
This study was presented at the ARRS 2005 annual meeting (New Orleans) and
received the Certificate of Merit.
Abstract
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SUBJECTS AND METHODS. Forty-five patients were divided into two groups on the basis of the CM (300 mg I/mL) administration protocol: group 1 (23 patients) received 100 mL of CM at 3.0 mL/sec; and group 2 (22 patients), 50 mL of CM at 3.0 mL/sec followed by a 20-mL saline flush at 3.0 mL/sec. All patients underwent 16-MDCT angiography of the entire aortoiliac region. Seven regions of interest (ROIs) were drawn from the ascending aorta (ROI 1) to the external iliac artery (ROI 7). Quantitative analysis was performed by calculating the mean aortoiliac attenuation and the mean difference between the maximum and minimum attenuation values. Vascular enhancement of the renal arteries was visually assessed using 2D and 3D postprocessing techniques.
RESULTS. The mean aortoiliac attenuation in group 1 was 314.3 ± 45.9 H and that in group 2 was 306.1 ± 35.0 H. The difference was not statistically significant. Adequate mean aortoiliac attenuation was achieved in 95.7% (22/23) and 95.5% (21/22) of patients in groups 1 and 2, respectively. The difference was not statistically significant. The mean difference between the maximum and minimum attenuation values was significantly smaller in group 1 (41.3 ± 16.8 H) than in group 2 (57.2 ± 25.3 H). The renal arteries were assessable in all patients in both groups.
CONCLUSION. This protocol of 50 mL of CM with a saline flush provides attenuation comparable to that obtained with the 100 mL of CM in aortoiliac 16-MDCT angiography.
Keywords: aneurysm • aorta • aortography • contrast media • MDCT angiography
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To obtain satisfactory CT angiography images, aortic enhancement (e.g., > 200 H) should be maintained for a certain period depending on the helical CT scanning time. Using 16-MDCT, the scanning time for examining the entire aorta has been reduced to approximately 10-15 sec [6, 14-16]. When a protocol of 50 mL of CM injected at a rate of 3 mL/sec followed by a saline flush is used, the duration of arterial enhancement of more than 200 H can be maintained for more than 10 sec on the basis of a simulating model [17] and our preliminary experience; the aortoiliac region can be covered with 16-MDCT within this time duration. Thus, we hypothesized that the higher efficiency of 16-MDCT should make it possible to reduce the volume of CM to 50 mL.
The objective of the present study was to evaluate whether a low-dose CM protocol with a saline flush might provide sufficient contrast enhancement of the aortoiliac region on 16-MDCT angiography.
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After enrollment, 50 patients were randomized into two groups of 25 patients each using a random number table for examination with one of two CM injection protocols: according to the first protocol (hereafter referred to as the 100-mL protocol), group 1 patients received 100 mL of CM administered at 3 mL/sec; and for the second protocol (hereafter referred to as the 50-mL protocol), group 2 patients received 50 mL of CM administered at 3 mL/sec followed by a 20-mL saline flush at 3 mL/sec. In both protocols, patients received iomeprol at 300 mg I/mL (Iomeron, Eisai). In general, most patients in Japan weigh approximately 60 kg or less [18]. We chose a fixed injection rate of 3 mL/sec for this study because we believe that it not only provides adequate aortic enhancement but also prolongs contrast enhancement.
The age and body weight of each patient were recorded. Five patients were later dropped from the study population. Three of these five patients were excluded because the scan delay was determined by monitoring a region of interest (ROI) in the aortic arch rather than an ROI in the descending aorta. In the other two patients, image quality was insufficient because of extravasation of CM (n = 1) or an allergic reaction (n = 1). Thus, a total of 45 patients (33 men and 12 women; mean age, 71.4 years; age range, 55-85 years) were included in the study, with 23 patients assigned to group 1 and 22 patients assigned to group 2. The ages (mean ± SD) of the patients in groups 1 and 2 were 72.0 ± 7.2 years and 71.0 ± 7.6 years, respectively. The body weights of the patients in groups 1 and 2 were 56.9 ± 9.8 kg and 57.3 ± 9.9 kg, respectively. Age and body weight were not significantly different between the two groups (p = 0.29 and 0.90, respectively). The diagnoses of the patients in both study groups are shown in Table 1.
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Data Acquisition
All CT scans were obtained using a 16-MDCT system (Aquilion 16 CFX Edition,
Toshiba Medical Systems) with a 0.5-sec gantry rotation speed, a tube voltage
of 120 kV, and a tube current of 300 mA. All examinations were performed using
a collimation of 16 x 2.0 mm, a helical pitch of 0.94, and a table speed
of 30 mm per rotation. The CM was injected via a 20-gauge IV catheter placed
in an antecubital vein for all examinations. The iodinated CM was administered
using a double-head power injector (Dual Shot-Type C, Nemoto-kyorindo).
Synchronization between the flow of CM and CT acquisition was achieved using a
real-time bolus-tracking system (Sure Start, Toshiba Medical Systems).
The anatomic level for monitoring was set in the descending aorta at T7 on the scout MDCT image. We chose the T7 level because it is easy to monitor all of the ascending aorta, descending aorta, and pulmonary artery at that level. The trigger threshold was set at 150 H for the aortic ROI. The distance from the T7 level to the actual starting position was approximately 20 cm. The use of a 16-MDCT scanner made it possible to announce the breath-hold and move the imaging table to the starting position simultaneously in 4 sec. Four seconds after the trigger, CT was started. Data were acquired during a single breath-hold in the head-to-foot direction. CT scans were obtained from the level of the sternal end of the clavicle to the groin. In each patient, the scan delay and scanning time were recorded.
The helical data were reconstructed in the axial plane as 2.0-mm sections at 1.0-mm intervals before storage and were then transferred to a workstation (M900, Zio), where the reconstructed axial helical sections were reformatted in the coronal plane at 1.0-mm intervals with a slice thickness of 2.0 mm, and volume-rendered images were generated for visual evaluation. The reformatted images included the entire thickness of the aorta, the renal arteries, and the kidneys.
Quantitative Analysis
Images were displayed on a computer monitor with a 1,028 x 1,024
matrix (TWS-5100, Toshiba Medical Systems). The author who interpreted the
images was blinded to the protocol used to obtain the images. The arterial
attenuation values (in Hounsfield units) were measured at the center of
circular ROIs placed in the arteries at five levels as follows: The first
level was the ascending aorta (ROI 1); second, the aortic arch (ROI 2); third,
the descending aorta at T7 (ROI 3) and T11 (ROI 4); fourth, the abdominal
aorta at L2 (ROI 5) and bifurcation levels (ROI 6); and, fifth, the most
distal portion of the external iliac arteries (ROI 7).
The sizes of the ROIs in the aorta and the ROIs in the external iliac arteries were kept constant in each patient. An attempt was made to select an ROI area of approximately 100 mm2—that is, not so small as to be affected by pixel variability and not so large as to approach the edges of the vessel. Attenuation values in the left and right external iliac and renal arteries were averaged. To compare the magnitude of the contrast column, we calculated the mean aortoiliac attenuation of each scan as the average of the mean attenuation values from ROI 1 to ROI 7. To compare the uniformity of the contrast column, we calculated the difference between the maximum and minimum attenuation values along the z-axis for each patient.
Awai et al. [19] adopted an attenuation value of 250 H as an index of adequate attenuation of the aorta. On the basis of their findings, we also selected an enhancement value of 250 H to indicate adequate aortic attenuation.
Visual Assessment
Two diagnostic radiologists (with nine and 15 years of experience) visually
evaluated the axial, reformatted coronal, and volume-rendered images in terms
of vascular enhancement. Image findings were classified as follows: 1, origins
of the renal arteries could not be identified; 2, bilateral renal arteries
could be identified; and 3, bilateral renal arteries and their first branches
could be identified. Final visual evaluation results were based on consensus
between the two observers.
Statistical Analysis
All data, including patient age and weight, scan delay, scanning time,
aortoiliac attenuation, and the difference between the maximum and minimum
attenuation values, were reported as mean ± SD. The two-tailed
Student's t test was used to investigate the intergroup differences
in patient age and weight, scan delay, scanning time, aortoiliac attenuation,
and the difference between the maximum and minimum attenuation values along
the z-axis. The Fisher's exact test was used to investigate the
intergroup difference in the proportion of adequate mean aortoiliac
attenuation (250 H). The Mann-Whitney U test was used to verify
visual evaluation results. Interobserver variability was also assessed using
kappa statistics. Probability values of less than 0.05 were considered
statistically significant.
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Aortoiliac Enhancement
The mean attenuation of the aortoiliac region with the 100-mL protocol was
314.3 ± 45.9 H, and the mean attenuation with the 50-mL protocol was
306.1 ± 35.0 H (Table
3). The aortoiliac enhancement profile with the 100-mL protocol
showed a more constant level, whereas the profile with the 50-mL protocol
showed a decrease in the attenuation values at the aortic bifurcation and in
the external iliac artery (Fig.
1). However, there was no statistically significant difference in
mean aortoiliac attenuation between the two protocols (p = 0.50). The
mean difference between the maximum and minimum attenuation values along the
z-axis was significantly smaller with the 100-mL protocol than with
the 50-mL protocol (41.3 ± 16.8 H and 57.2 ± 25.3 H,
respectively, p = 0.02) (Table
3).
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Figures 2 and 3 are scattergrams that show the relationships between body weight and the mean aortoiliac attenuation values for each protocol. The Pearson's correlation coefficients and the corresponding p values were R = -0.284 and -0.692 and p = 0.19 and 0.0002 with the 100- and 50-mL protocols, respectively. Aortic attenuation of 250 H or more, however, was kept during CT with the 50-mL protocol in patients weighing between 37 and 75 kg. Adequate mean aortoiliac attenuation (250 H) was achieved in 22 patients (95.7%) and 21 patients (95.5%) in groups 1 and 2, respectively. The intergroup difference in the proportion of adequate mean aortoiliac attenuation was not significant (p = 0.74).
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= 0.64) in
groups 1 and 2.
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TopAbstractIntroductionSubjects and MethodsResultsDiscussionReferences |
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In the present study, the 50-mL protocol with a saline flush was found to provide good attenuation and uniformity throughout the imaging volume that was comparable to that obtained with the 100-mL protocol. Aortic branches could be visually assessed using either protocol. There was a tendency toward a negative correlation between the mean aortoiliac enhancement value and patient weight in both protocols. The correlation was significant with the 50-mL protocol, but it was not significant with the 100-mL protocol. However, adequate aortoiliac attenuation (250 H) was achieved in all patients but one with either protocol. We believe that body weight had a greater influence on aortic enhancement with the 50-mL protocol. Our study population included no patients weighing more than 75 kg. When the 50-mL protocol is used in patients weighing more than 75 kg, reassessment of the CM dose tailored to patient weight is required.
A saline bolus may help to reduce the volume of CM required for consistent and uniform enhancement. Haage et al. [10] reported that the degree of reduction in CM providing similar enhancement with or without the use of a saline flush was in the range of 20-40%. Furthermore, a saline flush helps to maintain a more compact bolus of injected CM [11]. The benefit of a saline flush may be more apparent in MDCT scanners with 32 or more detector rows. In addition, a saline flush is useful in reducing beam-hardening artifacts arising from the superior vena cava [4]. In the present study, standard CM (300 mg I/mL) was used, and no beam-hardening artifacts interfering with visual evaluation were observed with either protocol. However, Becker et al. [22] reported that when high-density CM (400 mg I/mL) at high flow rates (2.5 or 3.5 mL/sec) is used, beam-hardening artifacts interfering with visual evaluation may arise.
There are several limitations in the present study. First, the ranges and mean values of body weight in the patients examined in this study are smaller than those of people in North America and Europe. Our low-dose protocol using 50 mL of CM injected at a rate of 3 mL/sec may correspond to a CM protocol using 70-80 mL of CM volume injected at a rate of 4 mL/sec in heavier patients. Whether our results are applicable to populations of heavier patients must be verified in future studies. Second, there were no patients with large aneurysms measuring 7 cm or more in our study population. It is possible that good uniformity may not be maintained in patients with large aneurysms. Studies involving larger numbers of subjects must be conducted. Third, aortic attenuation in the delayed phase was not assessed in this study. In three cases of aortic intramural hematoma in group 2, CT images in the delayed phase were obtained. In these three cases, differentiation between enhanced aortic lumen and intramural hematoma was possible in the delayed phase with the low-dose protocol. Visual assessment of the aorta in the delayed phase may be possible with a low-dose protocol, but additional studies are needed to verify this assumption.
There is a possible problem in assessing the delayed phase images with a low-dose protocol. It may be difficult to differentiate a coexisting tumor bordering on the aorta and the intramural thrombus of aortic aneurysm because of low contrast in the parenchymal organs or tumors in the delayed phase using the low-dose protocol. Fourth, the volume of the saline flush was as small as 20 mL. This is due mainly to the packaging of this product. The product package that is one size larger than a 20-mL ampule is a 100-mL bottle. In the clinical setting, a 20-mL saline flush is considered to be desirable from the viewpoint of cost, and we therefore used this volume. Adequate aortoiliac attenuation was achieved with a saline flush of 20 mL in the present study, but a saline flush of 40 mL might be preferable.
In conclusion, 50 mL of CM with a 20-mL saline flush provides attenuation and uniformity of the contrast column comparable to that obtained with 100 mL of CM in aortoiliac 16-MDCT angiography, resulting in cost savings. Moreover, a low-dose protocol may potentially reduce the number of cases of contrast nephropathy.
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and
dotted line). Aortoiliac enhancement profile with 100-mL protocol
shows more constant level, but there was no significant difference. ROI =
region of interest.
