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Commentary |
1 Both authors: Department of Radiology, Johns Hopkins University School of Medicine, 601 North Caroline St., Rm. 3254, Baltimore, MD 21287.
Received March 27, 2000;
accepted after revision March 27, 2000.
This article is a commentary on the preceding article by Garg et al.
Introduction
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Several years after the introduction of helical CT, articles began appearing in the literature touting the potential advantages of a dedicated CT study for the evaluation of pulmonary embolism. Advocates noted the short study times, good patient tolerance, and ability to directly visualize a thrombus in a pulmonary artery without relying on secondary signs of pulmonary embolism such as decreased perfusion on a ventilation—perfusion scan [2,3,4]. The initial literature was mixed in its acceptance of this application of helical CT. However, with each new advance in scanners, new articles appeared showing "better than ever" results for the detection of pulmonary embolism [2,3,4]. Once scanners could perform subsecond acquisition and routinely use 2- to 3-mm collimation, articles began to suggest that the accuracy of CT had come close to that of classic catheter-based pulmonary angiography.
At this point, a common strategy began to develop that included chest CT followed by Doppler sonography to rule out deep venous thrombosis in the extremities. Negative findings for both studies seemed to be an ideal way of excluding clinically significant pulmonary embolism; however, obtaining two separate studies is time-consuming and expensive. Over the past two decades, radiologists have striven for speed and efficiency in a single comprehensive examination, whether it be for staging tumors or for detecting pulmonary embolism. In this issue of the AJR, Garg et al. [5] address this goal from two perspectives. They look at the success of combining CT pulmonary angiography and CT venography and directly compare CT venography with sonography for detection of femoropopliteal vein thrombosis.
First, it is important to review the technical aspects of the examination. CT pulmonary angiography was performed with a well-accepted protocol of 3-mm collimation and pitch of 2 after the administration of 100-150 mL of iopramide or iothalamate meglumine contrast material injected at a rate of 4 mL/ sec. Scans of the leg veins were obtained 3 min after the start of the injection using 10-mm-thick sections at 20-mm intervals from the level of the renal veins to the knee. Sonography of the lower extremities was then performed and compared with CT venography using criteria outlined in the Materials and Methods section of the manuscript. The authors found that 97% of the CT venography studies could be classified as good or satisfactory. CT venography was better and more efficacious than sonography in 36% of patients, equivalent in 37%, and not as good in 27% of patients. In the cases in which CT venography was not as good, certain predictive elements such as the presence of orthopedic hardware or insufficient scan delays were to blame. Patient comfort was definitely an advantage of CT venography; it did not require the direct compression used with sonography. Results from visualization of the inferior vena cava or iliac veins were not included in the final results analysis.
The authors' results are an important extension of the drive for CT to be both a primary imaging technique for processes that involve the vascular system such as pulmonary embolism and a study that leads to more accurate diagnoses to enhance patient care. The use of CT as a single examination for evaluation of both the chest and the lower venous system by appropriate scanning techniques provides a potentially ideal imaging scenario by eliminating the need for a second examination (sonography). The time to complete and interpret the CT scan of the extremities is obviously far less than the time to obtain the sonogram, and, probably, the examination is performed at a lower cost.
In the past, one of the limitations of many of the helical scanners was the limited scanning range. In our institution even our 1-year-old 0.75-sec single-detector helical scanners could not cover the patient distance needed for a combined CT pulmonary angiography and CT venography without moving the patient between the two portions of the study. Although this might not be difficult with a mobile patient, it is extremely difficult with an emergency patient with shortness of breath or an intensive care unit patient receiving intubation.
However, technology continues to solve many of these potential problems. The introduction of multidetector CT provides the ability to scan long distances without having to move or reposition the patient. We are all now familiar with the images of a CT angiogram covering the distance from the great vessels of the aortic arch to the runoff vessels of the lower extremity in a single helical acquisition. Although for the combined CT pulmonary angiography and CT venography a single long acquisition is not needed, the ability to scan the entire patient without moving the patient makes the technology ideal for this application.
Before it becomes the accepted standard of care, several questions will still need to be addressed. Garg et al. [5] waited 3 min from the start of injection to the start of CT venography. Their study errors seem to be due in great part to starting too early, which resulted in flow artifacts. In fact, the authors now wait 4 min in many patients in whom slower flow is suspected. Another question relates to the scan protocols. Is the ideal protocol 10-mm-thick sections at 20-mm intervals, or are thinner sections at closer intervals really needed? Should scans be obtained from the knees to the renal veins (which is what Garg et al. obtained) or from the mid pelvis downward? Should the calf veins also be imaged? Although timewise this makes little difference, it could become an issue in terms of radiation dose, especially in the younger patient. Garg et al. note that many of these questions will need to be addressed to define the optimal technique.
Several recent articles published in the AJR address similar issues. Shah et al. [6] examined 52 patients who underwent both CT and lower extremity venous sonography and found agreement in 94% of cases. Of the three cases with discordant results, two patients had findings of deep venous thrombosis on sonography and negative findings on CT, and one had positive findings on CT and negative findings on sonography. In that study, scans were obtained from the level of the iliac crest to at least the level of the inferior aspect of the symphysis pubis.
Yankelevitz et al. [7] tried to determine the ideal time for performing CT venography after the completion of CT pulmonary angiography and found that near-peak enhancement could be obtained in most patients 2 min after completion of the CT pulmonary angiography.
Most recently Loud et al. [8] reviewed 71 consecutive patients who underwent CT pulmonary angiography by CT venography. In this series the CT venography protocol consisted of 5-mm-thick sections obtained at 5-cm intervals from the level of the diaphragm to the upper calves after a 3.5-min delay after the CT pulmonary angiography. CT and sonography correlated exactly in the femoropopliteal deep venous system. CT had an added advantage in patients who had thrombi in the iliac veins and the inferior vena cava.
Although all the current literature points to the combined CT pulmonary angiography and CT venography as the ideal single study solution, larger clinical series will need to be published before specific protocols can be set. The diffusion of multidetector CT into the community provides this opportunity. We congratulate Garg et al. [5] for focusing on such an important topic in radiology and look forward to future optimizations of this technique that may combine all steps of evaluation of suspected pulmonary embolism into a single study with high sensitivity and specificity and yet lower or reduced cost. Once again it appears that the pursuit of technologic advances in radiology will benefit our most important resource, our patients.
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