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DOI: 10.1148/radiol.2462070319
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(Radiology 2008;246:605-611.)
© RSNA, 2008

Thoracic Imaging

Venous Thromboembolism: Indirect CT Venography during CT Pulmonary Angiography—Should the Pelvis Be Imaged?1

Sanjeeva P. Kalva, MD, Jyothi P. Jagannathan, MD, Peter F. Hahn, MD, PhD, and Stephan T. Wicky, MD

1 From the Department of Radiology, Massachusetts General Hospital and Harvard Medical School, GRB-290, 55 Fruit St, Boston, MA 02114. From the 2006 RSNA Annual Meeting. Received February 15, 2007; revision requested April 24; revision received June 6; final version accepted August 4. Address correspondence to S.P.K. (e-mail: skalva{at}partners.org).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
ADVANCES IN KNOWLEDGE
IMPLICATION FOR PATIENT CARE...
 References
 
Purpose: To retrospectively determine the relative contribution of pelvic and lower-extremity indirect computed tomographic (CT) venography to the diagnosis of venous thromboembolism (VTE) in patients undergoing CT for pulmonary embolism (PE).

Materials and Methods: This HIPAA-compliant study was approved by the institutional review board, and informed consent was waived. The records of 2074 consecutive patients (890 men, 1184 women; mean age, 59 years; age range, 15–97 years) suspected of having PE who underwent combined CT pulmonary angiography and CT venography between May 2005 and March 2006 were reviewed. CT venograms from the iliac crests to the popliteal fossae were reviewed for presence and location of thrombi. Radiology reports were reviewed for CT pulmonary angiographic results. Thrombus detection rates with and without pelvic CT venography were compared by using the {chi}2 test. Separate effective radiation doses for CT venography of pelvis and lower extremities were calculated.

Results: On CT images of the 2074 patients, VTE was detected in 283 (13.6%) patients; PE, in 237 (11.4%); and deep vein thrombosis (DVT), in 121 (5.8%). Forty-six patients had DVT but no PE. Addition of CT venography to CT pulmonary angiography increased the detection of VTE by 19.4% (46 of 237). Isolated pelvic DVT was seen in two (0.1%) patients. There was no difference in the detection of VTE whether or not the pelvis was included (P = .92). Effective radiation dose for CT venography was 5.2 mSv ± 0.5 (standard deviation) for the pelvis and 0.6 mSv ± 0.2 for the lower extremities.

Conclusion: CT venography of the pelvis during CT pulmonary angiography does not significantly improve the detection of VTE. CT venography may be limited to the lower extremities, thus reducing radiation dose.

© RSNA, 2008


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
RESULTS
DISCUSSION
ADVANCES IN KNOWLEDGE
IMPLICATION FOR PATIENT CARE...
 References
 
Venous thromboembolism (VTE) is an important cause of morbidity and mortality (1). The clinical diagnosis of pulmonary embolism (PE) is difficult because of its varied manifestation (2). Clinical probability scores and laboratory tests are used to assess the probability of PE (3). Imaging studies often provide the definitive diagnosis (4,5).

With the advent of multidetector scanners, computed tomographic (CT) pulmonary angiography is now routinely performed in patients suspected of having PE (6). Indirect CT venography of the pelvis and lower extremities has been added to CT pulmonary angiography to increase the rate of VTE detection (7). Multiple studies (8–10) have shown an increase of 14%–38% in VTE detection with the addition of CT venography. The Prospective Investigation of Pulmonary Embolism Diagnosis II (PIOPED II) study (11) confirmed that the overall sensitivity of CT in the diagnosis of VTE improves from 83% to 90% if CT venography is added to CT pulmonary angiography.

The additional radiation dose (3–8 mSv, depending on CT parameters) associated with CT venography prompted some investigators (12) not to routinely recommend CT venography during CT pulmonary angiography. Further, the field of view for CT venography has not been standardized. A review (13) found that many studies involve imaging from the iliac crests to the popliteal fossae, while others recommend imaging the entire abdomen and lower extremities. Although CT venography has been shown to be useful for the diagnosis of VTE, the relative contribution from imaging the pelvis during CT venography is not known. The high radiation dose during CT venography is predominantly due to irradiation of the pelvis. Thus, the purpose of our study was to retrospectively determine the relative contributions of pelvic and lower-extremity CT venography to the diagnosis of VTE in patients undergoing CT for PE.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
ADVANCES IN KNOWLEDGE
IMPLICATION FOR PATIENT CARE...
 References
 
Institutional review board approval was obtained from the Partners Human Research Committee for this Health Insurance Portability and Accountability Act–compliant single-center retrospective study. Informed consent was waived, and identifying information was removed from the images.

Patients
The study group consisted of 2074 consecutive patients (mean age, 58.9 years; median age, 60 years [range, 15–97 years]) who underwent combined CT pulmonary angiography and CT venography for possible PE between May 2005 and March 2006. Imaging studies were requested on the basis of moderate to high clinical probability of PE. There were 890 (42.9%) men (mean age, 58.6 years; median age, 59 years) and 1184 (57.1%) women (mean age, 59.1 years; median age, 60 years). Subjects were either inpatients (n = 1703, 82.1%) of the hospital or outpatients (n = 371, 17.9%) being treated in the emergency room or other settings.

CT Imaging
At our hospital, patients suspected of having PE undergo a standard diagnostic protocol that includes CT pulmonary angiography and CT venography. The protocol has been in place since 2002 and is based on initial relevant publications (14–18). Nonionic iodinated contrast material (135 mL of Isovue 300; Bracco Diagnostics, Princeton, NJ) was injected at a rate of 4 mL/sec through an 18-guage catheter in the antecubital vein. No saline flush was administered. Helical CT was performed by using a 16-section multidetector CT scanner (GE LightSpeed 16; GE Healthcare, Milwaukee, Wis). The scan was initiated after a fixed delay determined by the formula: Tdel = Tinj – (Tacq/2), where Tdel is the time delay (approximately 27 seconds), Tinj is the total contrast agent injection duration, and Tacq is the acquisition time.

For CT pulmonary angiography, images were obtained in a caudocranial direction from the diaphragm to the aortic arch, with the following scan parameters: detector width, 1.25 mm; 120 kVp in normal-sized patients and 140 kVp in large (>112.5 kg) patients; 380 mA; rotation time, 0.5 second; table speed, 13.75 mm per rotation. Patients were instructed to suspend respiration for the duration of the CT pulmonary angiographic examination (typically 10–12 seconds). Images were reconstructed at 1.25 mm section thickness, with no overlap or intersection gap.

At 180 seconds after administration of the contrast material, CT venography was performed from the iliac crests to the popliteal fossae in a caudocranial direction, with the following scan parameters: 120 kVp; 180 mAs; detector width, 2.5 mm; rotation time, 1 second; table speed, 27.5 mm per rotation. Images were reconstructed at 7.5-mm section thickness without intersection gap.

Image Interpretation and Report Review
CT venographic images were interpreted by two radiologists (S.P.K., with fellowship training in vascular imaging, 10 years experience in general radiology, and 5 years experience in vascular imaging; J.P.J., with fellowship training in body imaging, 7 years experience in radiology, and 3 years experience in vascular imaging) who were blinded to the results of previous venous imaging and concurrent CT pulmonary angiography.

Transverse CT venographic images were reviewed at a picture archiving and communication system workstation (Impax, version 4.5; Agfa, Mortsel, Belgium) with appropriate window settings based on venous enhancement. Multiplanar three-dimensional reformations were not used for interpretation. The interpretation was performed in consensus, and the presence and location (femoral, popliteal, or iliac vein or inferior vena cava) of thrombi were recorded. A deep vein thrombosis (DVT) was defined as a low-attenuating partial or complete intraluminal filling defect surrounded by a high-attenuating ring of enhanced blood that was seen on at least two consecutive transverse images (14).

A CT venographic study was considered indeterminate if there was poor venous contrast enhancement or if major artifacts from bones or metallic prostheses limited the evaluation. Poor contrast enhancement was defined as attenuation of the common femoral vein that was less than that of adjacent muscle and the common femoral artery (14). Findings of chronic DVT (calcification in the wall of the vein, diffuse narrowing of the vein) were not considered a positive finding as this diagnosis would not alter patient care.

During CT venography the deep veins were assessed from the inferior vena cava to the popliteal veins. For the purpose of our study, bone landmarks were used to define the anatomic boundaries of the pelvis and the thighs, which are easily recognized by CT technologists. The pelvis was defined as the region from the iliac crests to the superior margin of the acetabulum. Anatomically, the femoral vein continues as the external iliac vein medial to the midpoint of the inguinal ligament. This point roughly corresponds to the superior margin of the acetabulum. The lower extremities were defined as the regions from the superior margin of the acetabulum to the neck of the fibula, which correspond to the lower extent of the popliteal fossa.

The radiology reports of the CT pulmonary angiographic studies were reviewed by one author (S.P.K.). The presence and location (main, lobar, segmental, and subsegmental) of PE were recorded. CT pulmonary angiographic studies of patients (n = 6) who had isolated pelvic vein thrombosis with no lower-extremity DVT detected at CT venography were reviewed by two authors (S.P.K., J.P.J.) for the presence of PE, and the results, if different from original reports, were recorded.

Effective Radiation Dose
Effective radiation dose was calculated from imaging studies performed during March 2006 for 200 consecutive study patients. The dose was calculated separately for the pelvis and the lower extremities, as anatomically defined earlier. The calculations were performed by using the ImPACT CT Patient Dosimetry Calculator (version 0.99x; ImPACT, Medical Devices Agency, London, England). The input values for the calculator included the scanner type, peak voltage, tube current–time product, beam width, pitch, body part being scanned, and the extent of the scan. Radiation dose calculations were limited to 200 consecutive patients due to little variability (standard deviation < 10% of the mean) of the calculated effective radiation dose.

Calculations and Statistical Analysis
The individual frequencies of VTE, PE, and DVT were calculated. Additionally, the frequencies of DVT in patients with PE and of PE in patients with DVT were calculated. The value of imaging the pelvis during CT venography was assessed by calculating the frequency of pelvic vein thrombus in the absence of PE.

The incremental increase in VTE detection when CT venography is added to CT pulmonary angiography was calculated by using the following formula: [(VTEboth – VTECTPA)/VTECTPA]·100, where VTEboth is the number of patients with VTE at CT pulmonary angiography and/or CT venography and VTECTPA is the number of patients with VTE at CT pulmonary angiography alone. Similarly, the incremental increase in VTE detection with the addition of CT venography of the lower extremities alone (no pelvis) to the diagnosis of VTE was calculated.

When calculating the percentage of positive studies, the denominator used was the total number of patients in the study. Indeterminate studies were included with negative studies. A study was considered positive if DVT was seen in any evaluable segment, irrespective of findings in other segments.

Statistical analysis was performed to determine if there existed any significant difference in the overall detection of VTE if CT venography of the pelvis was excluded. The {chi}2 test was used to compare the detection of VTE between (a) CT pulmonary angiography with CT venography of both the pelvis and lower extremities and (b) CT pulmonary angiography with CT venography of the lower extremities alone. Commercially available software (SAS; SAS Institute, Cary, NC) was used. A P value less than .05 was considered to indicate a significant difference.

The effective radiation dose reduction when the pelvis was excluded from CT venographic dose calculations was determined. The effective radiation dose during CT venography of both the pelvis and lower extremities was compared with that during CT venography of the lower extremities alone by using the paired Student t test, and a P value less than .05 was considered to indicate a significant difference.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
ADVANCES IN KNOWLEDGE
IMPLICATION FOR PATIENT CARE...
 References
 
PE and DVT
Of 2074 patients, 283 (13.6%) had VTE detected at CT venography and/or CT pulmonary angiography. Two hundred thirty-seven patients (11.4%) had PE, and 75 of these also had DVT (Table 1). Thus, the frequency of DVT among patients with PE was 31.6%. One hundred twenty-one patients (5.8%) had DVT, and 75 of these also had PE (Table 2). Thus, the frequency of PE among patients with DVT was 62.0%.


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  		Table 1. Presence of PE and DVT at CT Venography and CT Pulmonary Angiography

 

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  		Table 2. DVT Location and Association with PE

 
CT Venography and CT Pulmonary Angiography
With the addition of CT venography to CT pulmonary angiography, DVT in 46 additional patients was detected. Thus, the frequency of isolated DVT was 2.2%. This amounts to an incremental increase of 19.4% (46/237) in the detection of VTE. Among the 46 patients with DVT but no PE, the distribution of thrombi was as follows: 36 patients had femoral-popliteal vein thrombosis; eight, femoral-popliteal and pelvic vein thrombosis; and two, isolated pelvic vein thrombosis.

Sixty-five (3.1%) of the 2074 patients had indeterminate CT venographic studies. Among these, the pelvic veins alone were uninterpretable in one patient, pelvic and lower-extremity veins were uninterpretable in 13 patients, and lower-extremity veins alone were uninterpretable in 51 patients.

Pelvic DVT
Pelvic DVT was found in 22 (1.1%) patients (Table 2). Of these, 16 had associated DVT in the lower extremities. The other six had DVT confined to the pelvis, and four of the six had PE on CT pulmonary angiographic images. Thus, the overall frequency of isolated pelvic DVT was two of 2074 (0.1%). CT venography of the pelvis contributed to VTE detection in just two of 2074 patients. Of the two patients who had isolated pelvic vein thrombosis, one had a left gonadal vein thrombus following abdominal hysterectomy and bilateral salpingo-oopherectomy. The other patient had an acute isolated right common iliac vein thrombus with signs of chronic DVT in the lower extremities.

Area of CT Venography
If CT venography had been restricted to the lower extremities, VTE would have been detected in 281 (13.5%) patients instead of 283. Without pelvic images, the increased yield from adding CT venography to CT pulmonary angiography would be 18.6% (44 of 237) instead of 19.4% (46 of 237). There was no significant difference in the overall detection of VTE, whether or not CT venography included the pelvis (P = .92).

Effective Radiation Dose
The effective radiation dose for CT venography was 5.2 mSv (standard deviation, 0.5; standard error of the mean, 0.0368) for the pelvis and 0.6 mSv (standard deviation, 0.2; standard error of the mean, 0.0141) for the lower extremities. A significant reduction in the effective radiation dose could be achieved if CT venography was limited to the lower extremities (P < .001). Ninety percent (5.2 of 5.8) of the effective radiation dose during CT venography is due to irradiation of the pelvis. Thus, an almost 10-fold reduction in radiation dose can be achieved by excluding the pelvis and limiting CT venography to the lower extremities.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
ADVANCES IN KNOWLEDGE
IMPLICATION FOR PATIENT CARE...
 References
 
Approximately 90% of pulmonary emboli originate in the deep veins of the lower extremities (19). The frequency of DVT among patients suspected of having PE varies from 10% to 18% (20,21). The frequency of reported DVT in patients with proved PE varies from 29% to 43% (21,22). In one series (23), 63 patients had DVT on conventional venograms among 125 patients who had abnormal ventilation-perfusion scan findings. The frequency of DVT among patients with no imaging evidence of PE varies from 2% to 5% (24,25). In our series, the frequency of DVT in patients with and those without CT evidence of PE was 31.6% (75 of 237) and 2.5% (46 of 1837), respectively. Recurrent PE following negative pulmonary angiographic findings has been associated with a history of DVT. In one prospective series (26), PE after negative pulmonary angiography was associated with a history of prior DVT in 83% of cases.

Conventional venography has long been used for evaluating DVT and is still considered valuable in evaluating calf vein DVT. This requires intravenous access in a foot vein and administration of contrast material with fluoroscopic guidance. The invasive nature, adverse reactions to contrast material, and requirement of trained personnel make this test cumbersome. Further, conventional venography is technically inadequate in 5%–10% of cases (27–29), and interobserver variability is reportedly 10% (30).

Sonography has replaced conventional venography for the evaluation of DVT. The femoral and popliteal veins can be evaluated with compression sonography. Sonography has proved to be highly sensitive (89%–100%) and specific (99%–100%) for DVT when compared with conventional venography of the thigh (31–33). However, evaluation of the calf veins is limited. The importance of evaluating the calf veins has been debated. Kakkar et al (34) examined 132 consecutive postoperative patients to determine the natural history of DVT. Forty patients had DVT, and four patients with DVT developed PE. Patients who had DVT confined to the calf veins had no complications. Other studies (35,36) have demonstrated that sonography of the calf veins is unnecessary. Although sonography has high sensitivity for the diagnosis of symptomatic DVT, it is less accurate with regard to detecting DVT among asymptomatic individuals (37).

Direct CT venography, CT imaging of the lower-extremity veins following contrast material administration in foot veins, has been reported (38). However, it has the same risks as conventional venography. CT venography following a single upper-extremity venous injection of contrast material for CT pulmonary angiography was first reported by Loud et al (15). It was called indirect CT venography and has been adopted for the detection of lower-extremity DVT during CT pulmonary angiography. The addition of CT venography to CT pulmonary angiography requires only 3 more minutes to perform, potentially obviating a separate lower-extremity examination that could delay provision of results (15,16). CT venography has been shown to have sensitivity and specificity comparable to those of lower-extremity sonography for the detection of DVT (17,18). In addition, CT venography has been found to be useful in detecting isolated abdominal or pelvic vein thrombus (15). The addition of CT venography to CT pulmonary angiography increases the detection of VTE by 14%–38% (8–10,14–18). Dodd (39) reviewed CT venographic studies for an evidence-based practice series and suggested that CT venography was useful in patients with a high pretest probability of VTE and was less useful in patients with a low pretest probability of VTE.

Protocols for CT venography have not been standardized. Many authors (13) recommend imaging the pelvis and the lower extremities to the popliteal fossae. Addition of CT venography to CT pulmonary angiography is associated with additional radiation dose. The effective radiation dose during CT venography ranges from 2.3 to 11.8 mSv (40,41). Most of the absorbed dose during CT venography results from radiation exposure to the pelvis and lower abdomen. Rademaker et al (42) have shown that the addition of CT venography that includes the pelvis to CT pulmonary angiography increases the gonadal radiation dose 500- to 2000-fold compared with the dose from CT pulmonary angiography alone. Measured gonadal radiation doses ranged from 2.1 to 10.7 mSv (41,42). Owing to the high radiation dose associated with CT venography (42–44), some authors have questioned its clinical utility during CT pulmonary angiography (11,12). The risks of pelvic irradiation have been well discussed (42). The calculated risk of radiation-related death from leukemia induced by an effective dose of 2.5 mSv during CT venography is on the order of 1:8000, and the genetic risk in the reproductive age group is around 1:15 000 (42).

Radiation exposure during CT venography can be reduced in several ways. The use of a 5-cm scan interval would decrease the dose by 80% but has a 40% chance of missing a thrombus measuring less than 5 cm (8). Automated tube current modulation can potentially decrease the effective radiation dose. Another approach is to decrease the field of view. In our study, inclusion of the pelvis contributed 90% of radiation dose associated with CT venography (5.2 mSv of the total 5.8 mSv). Limiting CT venography to the lower extremities would reduce the radiation dose to the patient without significantly altering the overall detection frequency of CT in detecting VTE. Exclusion of the pelvis during CT venography is associated with substantial reduction of gonadal radiation in women.

Reports (43,44) show that isolated pelvic DVT is uncommon, composing 1%–4% of all cases. Cogo et al (45) reviewed 542 conventional venograms for distribution of DVT in symptomatic individuals. Of these, 189 had positive findings for DVT and none had isolated pelvic DVT. In PIOPED II (11), 105 patients had positive results at CT venography, with thrombi in the inferior vena cava or pelvic veins alone in three patients (3%), in the thigh veins alone in 89 patients (85%), and in both in 13 patients (12%). Cham et al (14) reported a 1% prevalence of pelvic vein thrombosis seen at CT venography among 541 patients suspected of having PE. In that study, all patients with pelvic DVT had DVT affecting other veins in the lower extremities. Only one patient had isolated DVT in the inferior vena cava. In another study (46) of 61 intensive care unit patients suspected of having PE, none had isolated pelvic vein thrombosis. In our study, isolated pelvic vein thrombosis was seen in six of 2074 patients. Of these six patients, four also had PE. Thus, the overall frequency of isolated pelvic vein thrombosis without associated PE was 0.1% (two of 2074). Recent recommendations by PIOPED II investigators (47) suggest scanning only the femoral and popliteal veins during CT venography in patients with a low clinical probability of PE. Similarly, in women of childbearing age, these investigators recommend scanning below the level of the acetabulum if CT venography is contemplated.

CT venography of the pelvis may be useful in certain clinical circumstances, such as postoperative PE, after pelvic surgery, or in the presence of pelvic mass wherein DVT may be isolated to the iliac or gonadal veins. In our study, an isolated gonadal vein thrombus was noted in one patient who had undergone pelvic surgery.

There are limitations in our study. We did not review the CT pulmonary angiographic studies but rather recorded the reported findings. Reports had been rendered by board-certified subspecialty-trained radiologists with clinical information about the patients. However, any retrospective disagreement about CT pulmonary angiography would likely have little effect on the findings of our study, as we focused on the utility of adding CT venography of the pelvis to the diagnosis of VTE. Another limitation was that we did not study the contribution of CT venography of the pelvis to other invasive procedures (such as inferior vena cava filter placement) or to clinical care (finding an incidental abscess or pelvic mass). Many interventional radiologists would place a filter in the inferior vena cava whether or not pelvic imaging was available. A vena cavogram is performed during filter placement to look for inferior vena cava thrombus and structural anomalies of the inferior vena cava.

In conclusion, CT venography of the pelvis during CT pulmonary angiography does not significantly add to the overall detection of VTE. CT venography may be limited to the lower extremities, excluding the pelvis, thus reducing the effective radiation dose to the patient.


   ADVANCES IN KNOWLEDGE
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
ADVANCES IN KNOWLEDGE
IMPLICATION FOR PATIENT CARE...
 References
 


   IMPLICATION FOR PATIENT CARE
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
ADVANCES IN KNOWLEDGE
IMPLICATION FOR PATIENT CARE...
References
 


   FOOTNOTES
 

Abbreviations: DVT = deep vein thrombosis • PE = pulmonary embolism • PIOPED II = Prospective Investigation of Pulmonary Embolism Diagnosis II • VTE = venous thromboembolism

Author contributions: Guarantors of integrity of entire study, all authors; study concepts/study design or data acquisition or data analysis/interpretation, all authors; manuscript drafting or manuscript revision for important intellectual content, all authors; approval of final version of submitted manuscript, all authors; literature research, all authors; clinical studies, all authors; statistical analysis, all authors; and manuscript editing, all authors

Authors stated no financial relationship to disclose.


   References
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
DISCUSSION
ADVANCES IN KNOWLEDGE
IMPLICATION FOR PATIENT CARE...
 References
 

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