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Using Dual-Detector Helical CT Angiography to Detect Deep Venous Thrombosis in Patients with Suspicion of Pulmonary Embolism

Diagnostic Value and Additional Findings

¹ Department of Radiology, Cliniques Universitaires St-Luc, av. Hippocrate 10, B-1200 Brussels, Belgium.
² Department of Internal Medicine Unit, Cliniques Universitaires St-Luc, B-1200 Brussels, Belgium.

Received August 2, 2000; accepted after revision September 20, 2000.

 
Address correspondence to E. E. Coche.

Abstract

Top Abstract Introduction Subjects and Methods Results Discussion References

 
OBJECTIVE. The purpose of this study was to assess the value of dual-slice helical CT angiography in detecting deep venous thrombosis in patients in whom acute pulmonary embolism was suspected and to describe the additional extrathoracic findings.

SUBJECTS AND METHODS. Sixty-five consecutive patients were examined for suspected pulmonary embolism using helical CT of the chest (2.7-mm collimation; table speed, 7.5 mm/sec; 100-140 mL of contrast medium injected at a rate of 3 mL/sec) followed by CT of the lower limbs (6.5-mm collimation; table speed, 10 mm/sec) without any additional contrast medium injection. Sequential scanning of the abdomen was performed using 10-mm collimation and an interval of 40 mm. Color Doppler sonography of the lower limbs was done within 24 hr of CT by two radiologists who were unaware of CT findings. Results of CT venography were compared with those of Doppler sonography and with phlebography or repeated focalized sonography in cases of discrepancy.

RESULTS. Twenty-two patients had pulmonary embolism revealed on chest CT. Sixteen patients had a deep venous thrombosis. Thirteen patients with pulmonary embolism had a deep venous thrombosis. Three patients with deep venous thrombosis had no pulmonary embolism. Sensitivity and specificity for diagnosing deep venous thrombosis with CT was 93% and 97%, respectively ( = 0.88). Additional extrathoracic findings were observed in four patients.

CONCLUSION. Combined CT venography with dual-slice scanning is an accurate method to diagnose deep venous thrombosis that may reveal additional imaging findings in some patients with possible pulmonary embolism.

Introduction

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Pulmonary embolism is a major cause of morbidity and mortality. Clinical diagnosis remains difficult because of the absence of specific symptoms. The clinical management of venous thromboembolism has shown interesting developments in recent years, and enhanced helical CT has greatly contributed to the evolution in the noninvasive imaging of pulmonary embolism. In most situations, there is an association between pulmonary embolism and the presence of venous thrombosis in the lower limbs. A single examination that accurately reveals both pulmonary emboli and deep venous thrombosis (DVT) would be desirable. Recently, a new technique for imaging the veins of the legs, pelvis, and abdomen using the venous opacification that follows the rapid infusion of contrast medium of CT pulmonary angiography (CT) was described by Loud et al. [1]. To our knowledge, only one series has reported the diagnostic accuracy of this technique to detect DVT [2]. The purpose of our prospective study was to test the value of dualdetector helical CT in diagnosing DVT in patients suspected of acute pulmonary embolism and to describe the additional findings of clinical relevance.

Subjects and Methods

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Between December 1998 and June 1999, 79 consecutive patients were referred to our CT unit for suspicion of acute pulmonary embolism. Informed consent was obtained according to the protocol approved by the local ethics committee. Of these 79 patients, 14 were excluded from the study: five because Doppler sonography examinations were delayed due to the unavailability of staff radiologists, five patients had bronchospasm or malaise after chest CT and CT venography was not performed in time, and four patients were excluded for technical reasons (temporary hardware failure). The remaining 65 patients comprised 40 women and 25 men who were 24-89 years old. The patients had been referred for CT from the departments of surgery (n = 12), internal medicine (n = 43), emergency (n = 7), obstetrics (n = 1), psychiatry (n = 1), and the intensive care unit (n = 1). Among the 65 patients, five had a proven malignancy (two hematologic malignancies, three solid tumors). One patient had a cardiac myxoma, which is known to cause coagulation abnormalities. Not all patients were assessed systematically by lung scintigraphy. The medical records were reviewed for each patient, and any subsequent therapy was documented.

Combined CT Venography and Pulmonary Angiography
All CT scans were performed with a CT Twin Flash RTS (Elscint, Haifa, Israel) with a double-detector array, which scans two sections simultaneously. Before CT pulmonary angiography was started, a scanogram of the body, including the chest and the ankles, was obtained using 120 kVp and 30 mAs. One hundred to 140 mL of iopromide (Ultravist-370; Schering, Berlin, Germany) was injected through an antecubital vein using a rate of 3 mL/sec. A single breath-hold helical scan of the chest was performed with a delay of 12-20 sec, depending on the patient's cardiovascular status, from the aortic arch to the diaphragm by using 2.7-mm collimatin, 1 sec per rotation, 7.5 mm/sec table speed, 120 kVp, and 165 mAs per section. Axial sections were reconstructed with a 2-mm increment by using a 180° linear interpolation algorithm and a 512 x 512 matrix.

Two to three minutes after the beginning of contrast medium injection, axial CT images of the lower limbs were acquired without further contrast material injection from the mid calf to the pelvis using 6.5-mm collimation, 1 sec per rotation, 20 mm/sec table speed, 120 kVp, and 75 mAs. Axial images were reconstructed with a 5-mm increment using a 180° interpolation algorithm and a matrix of 768 x 768. The patients' lower legs were slightly elevated to avoid compression of calf veins and the feet were taped together to decrease motion. Tourniquets were placed above the ankles and thighs to avoid preferential opacification of the superficial veins just before the acquisition and were removed just before the abdominal scanning.

Finally, axial sequential images were obtained from the pelvis to the retrohepatic inferior vena cava using 10-mm slice thickness at 40-mm intervals, 120 kVp, and 150 mAs. CT studies were assessed prospectively by one radiologist. After initial examination, thoracic and lower leg images were assessed by two independent observers who were unaware of the sonographic findings on a workstation (Omnipro O2; Elscint). Subsequently, consensus was achieved for final interpretation in all cases.

The diagnostic criterion for acute pulmonary embolism was a low attenuation area that completely or partially filled the lumen of the opacified vessel [3]. The criteria for the diagnosis of DVT with CT venography was a filling defect in an opacified vein [4] or a nonopacified venous segment if the vein distal and proximal to the non-opacified segment was opacified (Figs. 1A,1B,1C and 2A,2B,2C).

View larger version (76K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1A. —52-year-old man with mild dyspnea and previous cardiac failure. Enhanced dual-slice CT scan (2.7-mm collimation) shows large hypodensity located in left lower lobe pulmonary artery (arrow) consistent with acute pulmonary embolism.

View larger version (110K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1B. —52-year-old man with mild dyspnea and previous cardiac failure. Helical CT scan (10-mm collimation) obtained at level of lower abdomen shows hypodensity (arrow) within inferior vena cava caused by thrombus.

View larger version (123K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 1C. —52-year-old man with mild dyspnea and previous cardiac failure. Sequential CT scan (10-mm collimation) obtained at level of bladder reveals extensive thrombosis (arrow) of left external iliac vein.

View larger version (106K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2A. —70-year-old woman presenting with acute dyspnea several days after neurosurgery procedure. Helical CT scan of chest (2.7-mm collimation) reveals numerous segmental (arrows) and subsegmental hypodensities within arteries consistent with acute pulmonary emboli.

View larger version (90K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2B. —70-year-old woman presenting with acute dyspnea several days after neurosurgery procedure. Lower leg helical CT scan (6.5-mm collimation) obtained 2 min after contrast medium injection shows small hypodensity (arrow) in peroneal vein of left calf consistent with deep venous thrombosis. Color Doppler sonography did not reveal any abnormality in this area.

View larger version (46K): [in this window] [in a new window] [as a PowerPoint slide]   Fig. 2C. —70-year-old woman presenting with acute dyspnea several days after neurosurgery procedure. Phlebography of left lower limb shows deep venous thrombosis (arrows) in left peroneal vein.

Color Doppler Sonography with Compression Examination
All patients were examined with color Doppler sonography within 24 hr of the helical CT examination with a 7-4- or 5-10-MHz flat transducer (HDI 3000; Advanced Technology Laboratories, Bothell, WA) for lower limb assessment, and a 3-5-MHz curved transducer for pelvic and abdominal examinations. In all patients, sonographic examinations included compression and flow-accentuation maneuvers. Color Doppler mapping was used to locate artery and veins and to view the response to compression of the lower calf. Flow sensitivity was set to the maximum. The patient was seated during examination of the calf veins and was in the supine position for the femoral, iliac, and inferior vena cava examinations. Veins were examined in the transverse plane. The diagnostic criterion for DVT was loss of venous compressibility during compression with the sonographic probe [5]. The site of the thrombosis, including the proximal and distal extents of the thrombus, was carefully documented. Venous involvement was also assessed in the iliac veins and the inferior vena cava.

All examinations were performed by two independent trained radiologists who were not aware of helical CT findings. Discrepancies were resolved by consensus. If findings of CT venography and sonography were concordant no further investigation was performed and final diagnosis relied on color Doppler sonographic findings. If discrepancy occurred, two methods were used as reference: phlebography or repeated focalized sonography.

Lower Limb Phlebography or Focalized Sonographic Examination
Ascending phlebography of the lower limbs was performed within 12 hr in two patients with discordant results between color Doppler sonography and lower extremity venous helical CT. Conventional venography was performed with a tourniquet around each ankle and thigh. Images were obtained with a conventional fluoroscopy unit (CGR; General Electric Medical Systems, Milwaukee, WI). Fifty to seventy milliliters of iopromide (Ultravist-370) was injected through a 21-gauge butterfly needle inserted in a dorsal vein of the foot. For images of the iliac veins and the inferior vena cava, the patient was placed in the supine position and both legs were raised to optimize opacification of the pelvic veins. Images were interpreted by an independent vascular radiologist who was unaware of the other imaging modality results.

In one patient, phlebography was not performed because of the inability to catheterize the venous system. Two other patients refused the examination. For these three patients a repeated focalized sonographic examination was performed with a third radiologist present. In this situation, the final diagnosis was reached by consensus of the three observers. Sensitivity, specificity, and positive and negative predictive value of CT for the detection of DVT was calculated.

Results

Top Abstract Introduction Subjects and Methods Results Discussion References

 
Twenty-two (34%) of 65 patients had a pulmonary embolism diagnosed with chest CT findings. Sixteen (25%) of 65 patients had a DVT. Thirteen (59%) of 22 patients with pulmonary embolism had DVT and three (19%) of 16 with DVT had no pulmonary embolism. Clinical signs or symptoms of DVT were present in only three patients with DVT. Sensitivity and specificity of helical CT for revealing DVT was 93% and 97%, respectively. Positive and negative predictive values were 93% and 97%, respectively. Interobserver agreement ( = 0.88) for diagnosing DVT was excellent. Sensitivity and specificity for color Doppler sonography for diagnosing DVT were 87% and 97%, respectively, and positive and negative predictive values were 93% and 96%, respectively. With regard to femoropopliteal veins, three patients had DVT of the femoral veins alone, four had DVT of the femoral veins and popliteal veins, three had DVT of the femoral veins and the popliteal veins, and one had DVT of the femoral and fibular veins. The sensitivity for DVT at or above the knee was 100% for helical CT. For helical CT, the level of misinterpretation was located below the knee and for sonography at the level of the popliteal or infrapopliteal areas (Table 1). Additional findings revealed by helical CT (Table 2) were ascites and pelvic mass in one patient, a subcapsular hepatic and retrocaval mass in one patient, hepatic nodules consistent with metastases in a third patient, and a left renal mass in a fourth patient.

Discussion

Top Abstract Introduction Subjects and Methods Results Discussion References

 
In the present study, twenty-two patients had pulmonary embolism revealed by chest CT. Sixteen patients had a DVT and 13 (59%) of 22 patients with pulmonary embolism had DVT revealed on lower leg helical CT and sonography. In patients with confirmed pulmonary embolism, the prevalence of detectable residual DVT is largely unknown. The few studies in which such data can be found provide discrepant results, with the prevalence of DVT ranging from 13-93% [6,7,8,9]. A recent study of 228 consecutive patients with an angiography-proven acute pulmonary embolism and bilateral lower limb venography reported a high prevalence (82%) of residual DVT [10]. Our study found a lower prevalence, with 13 (59%) of 22 patients with pulmonary embolism and DVT depicted by combined venography and helical CT. No systematic angiography or scintigraphy was performed in our patients to confirm the presence of pulmonary embolism. It has been shown recently that pulmonary angiography is an inaccurate standard to diagnose peripheral pulmonary embolism and that single-detector helical CT was able to reveal peripheral pulmonary emboli with the same accuracy [11].

Color Doppler sonography with compression was used as the reference method to reveal DVT in our prospective study. In cases of discrepancy between sonography and helical CT, a repeated focalized sonography or phlebography was performed. Compression sonography has proved to be the diagnostic method of choice in routine clinical practice for detection of clots in the extremities but uncertainty exists concerning the need to evaluate the calf veins similarly [5]. Color-flow imaging was performed as a complement to compression sonography in the present study. Evaluation of the calf veins was done with color-flow imaging to help identify the location of the deep calf veins. Recently, Cornuz et al. [12] examined 977 consecutive patients for possible DVT using color-flow sonography and compression. They concluded that a sonographic examination that includes a careful evaluation of the calf veins in patients without risk factors for DVT can be used to exclude the presence of clinically important DVT. This method is certainly an imperfect standard and is largely dependent on the observer's experience, but it is the most widely accepted reference method in clinical practice.

In our series we found two false-negative and one false-positive result (Table 1) for color Doppler sonography with compression. The abnormalities in these misdiagnosed cases were at the infrapopliteal areas, which is comparable with the results of the literature [13, 14]. The sensitivity and specificity were 87% and 97%, respectively, for color Doppler sonography. Phlebographies were performed in two of the five patients in whom CT venography and Doppler sonography results were discordant. In one patient phlebography findings were concordant with those of the CT venography, confirming a thrombosis of the peroneal vein misdiagnosed with Doppler sonography (Fig. 2A,2B,2C). In the other patient, venography findings ruled out posterior tibial and peroneal vein thrombosis that had been misdiagnosed with dual-slice helical CT. In three patients phlebography was not performed because of technical problems or because the patient refused. Repeated focalized sonography was performed on the suspicious venous segment with confirmation of DVT in a popliteal vein, which was not seen on the initial sonography, and confirmation of DVT in a peroneal vein not seen on helical CT. In addition, the repeated focalized sonography also ruled out a posterior tibial vein thrombosis misdiagnosed on color Doppler sonography.

The results of our prospective study show the excellent performance of dual-slice helical CT venography when combined with chest helical CT in revealing DVT. Our study revealed three DVT among 43 patients (5%) without CT-proven pulmonary embolism. These patients had no signs or symptoms of DVT and combined CT venography provided pertinent data to start the patients on anticoagulation therapy. The sensitivity and specificity of dual-slice CT venography in revealing DVT were 93% and 97%, respectively. This technique had 100% accuracy in the femoropopliteal system. One false-negative and one false-positive diagnosis in our series were probably a result of lack of enhancement in the infrapopliteal veins (Table 1). These data are similar to those reported by other researchers who studied patients with suspicion of pulmonary embolism and used collimation as thin as 5 mm for chest and 10 mm for lower extremities. Loud et al. [2] used single-slice helical CT in 71 consecutive patients with combined CT venography and pulmonary angiography and showed 100% sensitivity and specificity. A comparison of the findings of CT venography with color Doppler sonography for detection of deep venous thrombosis in our study with those of the study of Loud et al. suggests that CT reveals approximately the same number of clots as sonography and that the error rates are similar.

Cost-effectiveness should be taken into consideration when deciding a diagnostic strategy for the assessment of pulmonary embolism. In a recent review of cost-effective diagnostic algorithms in pulmonary embolism, Van Erkel et al. [15] reported that most diagnostic strategies for DVT rely on helical CT. The most cost-effective strategy consisted of performing sonographic examinations of leg veins to reveal DVT followed by helical CT of the pulmonary vessels in cases in which no DVT was detected. No cost-effectiveness analysis was performed in our study concerning this point; therefore, we do not know whether combined CT venography performed systematically in all patients with suspected pulmonary embolism is more or less expensive than sonography and chest CT. Nevertheless, in terms of medical efficacy, it seems interesting to perform combined chest CT with venography in patients with difficulties to be assessed by sonography (e.g., patients with a leg cast, edema of lower limbs, or limited mobility or in centers where trained radiologists are not available for performing sonographic examination). Imaging of the abdomen using CT is useful to assess the inferior vena cava and iliac veins, which are sometimes difficult to assess with color Doppler sonography. Furthermore, these images may give incidental findings that may change the patient's treatment. In our study, dual helical combined CT venography provided additional extrathoracic findings for four patients (Table 2). For two patients, these findings had an impact on therapy with local radiotherapy and surgery. In one patient, extrathoracic findings did not change the therapy because the abdominal diagnosis was known. No further assessment was performed for the other patient because of the patient's age. Additional extrathoracic findings, which may affect patient management, have also been reported by Katz et al. [16]. In their series of 300 patients, they reported three cases consistent with a renal cell carcinoma, subcapsular fluid collection, and psoas hematoma.

Our technique differed from the technique described by Loud et al. [1, 2] mainly by the mode of image acquisition in the lower limbs. Dual CT detectors allowed a maximum helix length of 1550 mm. We were able to image the lower limbs in one shot using spiral acquisition, which allowed us to assess all segments of the venous system without any gap. The delay we used to assess the lower limb veins was 2-3 min after the beginning of contrast medium injection. Yankelevitz et al. [17] found a highly variable time to peak for venous enhancement in the 20 patients studied by combined CT pulmonary angiography and CT venography. Eighty-five percent of patients were within 90% of their peak value at 120 sec, and 95% of patients were within 75% of peak enhancement at that time. We placed tourniquets to allow preferential opacification of the deep venous system as described with the phlebographic technique. We obtained a good to excellent opacification in most patients. To reduce irradiation, we used low dose consistent with 75 mAs and 120 kVp.

We can conclude from this study that combined dual helical Ct of the chest, abdomen, and lower limbs is an accurate technique to reveal associated DVT in patients examined for acute venous thromboembolism. In rare cases, this technique may depict DVT requiring anticoagulation therapy in patients without any obvious pulmonary embolism on chest CT, or additional extrathoracic findings requiring an appropriate treatment or complementary imaging techniques. Further work considering cost-effectiveness analysis must be performed to consider the real value of this technique in the diagnostic algorithm for pulmonary embolism.

Acknowledgments
 
We thank Anne Smith for her assistance with manuscript preparation.

References

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