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1 Department of Diagnostic Radiology, University of Kentucky, 800 Rose St., HX
318, Lexington, KY 40536.
2 Department of Internal Medicine, University of Kentucky, Lexington, KY
40536.
3 Joyner Sports Medicine Institute, 601 Perimeter Dr., Ste. 110, Lexington, KY
40517.
4 Present address: Department of Biomechanics, McKinly Lab, Newark, DE
19701.
5 Division of Pulmonary and Critical Care Medicine, University of Kentucky,
Lexington, KY 40536.
6 Division of Hematology and Oncology, University of Kentucky, Lexington, KY
40536.
Received October 30, 2003;
accepted after revision January 18, 2004.
Partially supported by an unrestricted grant from Amersham Health,
Princeton, NJ.
Abstract
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SUBJECTS AND METHODS. We prospectively enrolled 200 adult outpatients. Patients were randomized to receive either the low-osmolar (hyperosmolar to blood) nonionic contrast medium, iohexol, or the nonionic isosmolar contrast medium, iodixanol. Images were obtained before contrast administration and 180 sec after contrast administration through the pelvis at the level of the external iliac vessels. Opacification of the external iliac vessels was assessed both objectively and subjectively.
RESULTS. The arterial and venous densities before contrast
administration were approximately 45 H for both groups. On delayed images
obtained after contrast administration, the mean venous density was 95.2 H for
iohexol and 101.4 H for iodixanol. Changes in venous density due to
administration of iohexol and iodixanol were 49.8 and 56.1 H, respectively.
This 12.5% difference was highly significant (p = 0.002). Sixty-six
percent of the images in the iodixanol group were rated either 4 (good) or 5
(excellent), whereas only 36% of the iohexol group achieved a similar rating
on our subjective rating scale. This difference was statistically significant
(
2 = 16.4, p < 0.001, df = 1).
CONCLUSION. Our study shows that isosmolar contrast material provides significant improvement in delayed opacification of the external iliac vessels in comparison with conventional low-osmolar contrast medium (hyperosmolar to blood).
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Early reports of deep venous thrombosis on CT scans were considered incidental findings [1]. Subsequently, protocols were devised specifically to identify deep venous thrombosis on CT venography [2]. With CT venography, 2- to 3-min delayed imaging is performed from the lower abdomen to the knees. Studies have shown that its sensitivity for the detection of deep venous thrombosis is comparable with that of sonography. In a study of 71 patients evaluated with both CT venography (combined with pulmonary CT angiography) and lower extremity venous sonography, Loud et al. [3] found 100% sensitivity and specificity for CT venography [3]. In a similar study of 70 patients, Garg et al. [4] found the sensitivity and specificity of CT venography to be 100% and 97%, respectively. CT venography has the advantage of allowing visualization of the deep femoral veins, iliac veins, and the inferior vena cava [3, 5]. An additional theoretic advantage to CT venography is the capability to reveal thrombus in duplicated or partially duplicated deep venous systems. Quinlan et al. [6] recently reported the frequency of these duplications and suggested that they may be a source of false-negative findings on venous Doppler sonography examinations.
Unfortunately, in our experience venous opacification is often less than ideal and the density difference between clotted and opacified vessels can be subtle [7]. At our institution and in previously reported series, nonionic low-osmolar contrast material has been used for these examinations [2–4, 7, 8]. Nonionic low-osmolar contrast agents are, in fact, at least two times the osmolarity of blood. The term "low-osmolar" is a historical reference to traditional ionic contrast agents, which are five to six times the osmolarity of blood. Dilution of contrast material, caused by the influx of water into the intravascular space (as the blood seeks isosmolarity), may prevent optimal venous opacification on delayed images [9].
Nonionic contrast agents that are isosmolar to blood are now commercially available and provide comparable opacification of vessels and solid organs on abdominal CT [9]. They have been proven to be at least as safe as conventional nonionic contrast media and may have less severe nephrotoxic effects [10–13]. For CT venography, isosmolar contrast material offers a potential advantage compared with conventional contrast agents because the dilutional effects seen with low-osmolar contrast material are eliminated.
The goal of this prospective randomized study was to determine whether isosmolar contrast material offers an advantage for delayed venous opacification.
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A random-number generator was used to randomize patients to receive either the nonionic low-osmolar contrast material iohexol (Omnipaque, GE Healthcare) or the nonionic isosmolar contrast material iodixanol (Visipaque, GE Healthcare). Study investigators were blinded to the randomization.
CT scans were obtained using a Somatom Sensation 10 scanner (Siemens). The contrast dose for each patient was calculated according to the patient's weight to provide a standard dose of 430 mg of iodine per kilogram of body weight. This dose corresponds to 1.43 mL/kg of iohexol (300 mg I/mL) and 1.59 mL/kg of iodixanol (270 mg I/mL). Dose was calculated on the basis of our routine use of 100 mL of iohexol for chest and abdominopelvic CT and an assumption of 70-kg body weight for the average-sized adult patient.
Before administration of the contrast agent and regardless of the examination ordered, an initial 6-mm-thick slice was obtained through the pelvis at the level of the lower urinary bladder, approximately between the acetabular roof and the top of the pubic symphysis. Routine scans were then obtained as requested; however, the protocol was modified to obtain a second 6-mm-thick slice through the pelvis at the same level, precisely 180 sec after contrast administration.
Patients were excluded from the study if informed consent could not be obtained, if patient weight required a larger dose of contrast material than our power injector would permit (125 kg for iohexol, 139 kg for iodixanol), or if the patient had a history of deep venous thrombosis or pulmonary embolus.
The complete examination was sent to a PACS (picture archiving and communication system) workstation for routine interpretation. An electronic copy of the image obtained before contrast administration and the electronic copy of image obtained 180 sec after contrast administration through the pelvis were sent to a separate workstation for use in the study. Patient identifiers were electronically removed from these images, and study numbers were assigned to the images on the basis of the randomization chart. The reviewers, who were unaware of the type of contrast material used, then analyzed the data both quantitatively and qualitatively. Round regions of interest were drawn in the right external iliac vein and artery on both pre- and postcontrast images, and the density in Hounsfield units was calculated. The regions of interest were drawn so that the circle incorporated the center two thirds of the vessel diameter. On the basis of their subjective assessment of venous opacification, three investigators assigned a numeric rating by consensus using a 1- to 5-point scale (very poor, poor, fair, good, and excellent, respectively).
One-tailed independent Student's t tests were used to compare the changes in arterial density and venous density between iohexol- and iodixanol-enhanced images (SigmaStat [version 3.0], Statistical Package for the Social Sciences). The familywise alpha rate was set at 0.05. However, Bonferroni's adjustment for multiple comparisons decreased the alpha value to 0.025 for each test. The data for each variable were analyzed for equal variance and for the normality of distribution before testing. In cases in which the assumptions regarding the appropriate use of Student's t tests were not met, the Mann-Whitney test was used to corroborate the findings of the t test. Analysis of the subjective scores for each contrast material was performed using a chi-square test with Yate's correction for continuity and an alpha value of 0.05.
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The precontrast arterial and venous densities were approximately 45 H for both groups (Table 1). The mean delayed postcontrast venous density was 95.24 H for iohexol and 101.43 H for iodixanol. Changes in venous density due to administration of iohexol and iodixanol were 49.78 and 56.01 H, respectively. This 12.5% difference was highly significant (p = 0.002). A similar difference was noted for change in arterial density (66.93 H for iohexol and 72.54 H for iodixanol).
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The ratings for subjective assessment of venous opacification are
summarized in Table 2. In
general, iodixanol received higher ratings than iohexol. Because of the small
number of ratings in the 1 (very poor) and 5 (excellent) categories for both
iohexol and iodixanol, the subjective data were combined into two groups:
ratings of 1, 2, or 3 and ratings of 4 or 5, as shown in
Table 3. Sixty-six percent of
the images in the iodixanol group were rated either 4 or 5, whereas only 36%
of the images in the iohexol group achieved a similar rating. This difference
was statistically significant (2 = 16.4, p <
0.001, df = 1).
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Little data are available regarding how to determine the dose of contrast material for CT venography. Garg et al. [4] found no significant difference in the percentage of studies graded as good when comparing 100- versus 150-mL doses of low-osmolar contrast material [4]. Our protocol approximated the doses administered in other studies and was carefully designed to standardize the amount of iodine per kilogram of body weight in each group [3, 4, 8, 14, 16]. Patients did not wear compressive stockings for imaging, although one published series showed significant improvement in delayed venous density using this technique [16].
Our data indicate that the use of an isosmolar contrast agent has advantages in delayed vascular opacification over traditionally used low-osmolar agents. The mean precontrast venous densities were the same in each group, which rules out the possibility that differences in precontrast venous density—due to varying hematocrit, for example—might explain the differences in postcontrast densities. The mean venous density in our patients who received low-osmolar contrast material was virtually identical to that reported by Bruce et al. [8] who also used low-osmolar contrast material. The mean postcontrast venous density of 101.4 H in the isosmolar group represented a 12.5% increase in enhancement compared with that of the low-osmolar group.
Our subjective analysis of delayed venous opacification also showed a higher number of cases with good or excellent ratings for delayed venous opacification in the group with isosmolar contrast material. Our findings differ from the results of Cham et al. [14] who rated delayed venous density to be good or excellent in 77% of patients who had been given low-osmolar contrast material. Only 36% of our hyperosmolar contrast group received a 4 (good) or 5 (excellent) rating. However, multiple differences in study design between our study and that of Cham et al. exist, including a 5- versus 4-point grading scale and a 180- versus 120-sec scanning delay, respectively. Cham et al. also subjectively determined venous density to be equal to that of arterial density in 44% of patients. That finding conflicts with our findings and those of Yankelevitz et al. [15], which indicate that arterial density is considerably greater than venous density during the first several minutes after contrast administration.
The improvement in delayed venous opacification with isosmolar contrast material is a reflection of the delayed vascular opacification in general. A similar improvement in delayed arterial enhancement with isosmolar contrast material was also shown in our study.
A potential pitfall in the design of our study is that data were collected at a single time point after contrast administration. A theoretic possibility does exist that the difference in delayed venous opacification could be greater or less at varying time points. However, our choice of a 180-sec delay was based on previous time–density curves, and our decision to choose a single time point was made to limit radiation exposure [15, 17].
Our study does not prove that the use of isosmolar contrast material increases the sensitivity of CT venography for the detection of deep venous thrombosis. In fact, patients suspected of deep venous thrombosis were excluded from our study. A prospective crossover study in which all patients being evaluated on CT venography are examined with either contrast agent on successive days would be required to prove an advantage of isosmolar contrast material for the detection of deep venous thrombosis. This type of study would be difficult to justify because of the added radiation and contrast exposure required.
Our study also does not compare the two types of contrast agents for use in pulmonary CT angiography. Again, patients being evaluated for possible pulmonary embolism were specifically excluded from our study. However, two previously published studies showed an increase in first-pass pulmonary artery opacification with isosmolar compared with low-osmolar contrast material [17]. Our anecdotal experience and that of others support those results, but whether any improved pulmonary arterial enhancement would translate into an increased sensitivity for the detection of pulmonary emboli is not proven [18].
The increased cost of isosmolar contrast material varies among institutions and buying groups. At our institution, a 100-mL bottle of nonionic isosmolar iodixanol (Visipaque) costs approximately $20 more than a similar quantity of nonionic low-osmolar iohexol (Omnipaque). This difference is certainly a factor to consider in our current health care environment in efforts at cost-containment.
In summary, our study shows that isosmolar contrast material provides significant improvement in delayed opacification of the external iliac vessels in comparison with a conventional hyperosmolar agent. Because we believe that use of isosmolar contrast material will make venous thrombi more conspicuous and will potentially improve our sensitivity to detect them, we now—despite the increased cost— use isosmolar contrast material for all our patients who are being evaluated for possible thromboembolic disease on combined pulmonary CT angiography and CT venography.
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