***TASK*** the task is to summarize an input biomedical literature in six sentences 
    ***INPUT*** the input is a biomedical literature 
    ***OUTPUT*** the output is the summary of an input biomedical literature in six sentences 
    ***DOCUMENTATION*** 
    ***EXAMPLES*** 
    Input: We performed a prospect i ve , blinded study to assess and compare the values of preoperative contrast venography and magnetic resonance venography in the detection of deep venous thrombosis in the thigh and pelvis of forty-five consecutive patients who had a displaced acetabular fracture . The magnetic resonance venography and contrast venography were performed an average of seven days ( range , one to twenty-nine days ) after the injury . Twenty-four asymptomatic thrombi were identified with magnetic resonance venography in fifteen ( 33 percent ) of the patients . Four of the thrombi were in the superficial femoral vein , nine were in the common femoral vein , one was in the external iliac vein , seven were in the internal iliac vein , and three were in the common iliac vein . Ten ( 42 percent ) of the twenty-four thrombi were confirmed with contrast venography ; nine of them were located in the thigh . The remaining fourteen thrombi ( 58 percent ) that had been noted on magnetic resonance venography could not be seen with contrast venography because they were located either in the deep pelvic veins or in the uninjured extremity . The thrombi in the internal iliac vein were identified only with magnetic resonance venography . Twelve of the fifteen patients who had thrombi had a filter placed in the inferior vena cava preoperatively . In eight of these patients , the filter was placed because of the findings of magnetic resonance venography alone . Magnetic resonance venography result ed in a change in the therapeutic management of ten ( 22 per cent ) of the forty-five patients . There were no pulmonary emboli . We concluded that magnetic resonance venography is superior to contrast venography for the preoperative evaluation of proximal deep venous thrombosis in patients who have an acetabular fracture .  Magnetic resonance venography is non-invasive , does not require the use of contrast medium , images the proximal aspects of both lower extremities simultaneously , and , most importantly , allows for the identification of deep venous thrombosis in the pelvis Magnetic resonance venography is a recently developed , noninvasive means of visualizing the proximal veins of the lower extremity and pelvis . Magnetic resonance venography is compared with st and ard contrast venography in the diagnosis of proximal deep vein thrombosis after total joint arthroplasty . Two hundred seven extremities were evaluated in a blinded study 5 to 7 days after surgery . St and ard contrast venography identified 11 proximal deep vein thromboses . Initial interpretations of the magnetic resonance venographies by staff radiologists identified 5 of the proximal vein thromboses ( sensitivity 45 % ) . Two patients with negative st and ard contrast venographies were identified as positive ( specificity 99 % ) . A retrospective review of all magnetic resonance venographies by a dedicated magnetic resonance angiographer identified 10 of 11 deep vein thromboses seen on st and ard contrast venography ( sensitivity 91 % ) . Both false negatives were identified as positives . St and ard contrast venography remains the gold st and ard for identifying proximal vein thromboses . Emerging magnetic resonance imaging techniques have created a potential alternative modality by which to identify deep vein thrombosis . The present study suggests that st and ard contrast venography continues to be the most accurate modality currently available . Although magnetic resonance venography seems to be accurate , its interpretation requires experience . As costs diminish and experience increases , magnetic resonance venography will have increased importance in the clinical recognition of deep vein thrombosis This is a prospect i ve comparative study of magnetic resonance imaging ( MRI ) of the deep veins versus contrast venography in consecutive patients treated for various injuries to their lower extremities , showing no clinical symptoms of deep vein thrombosis . The majority of examinations referred to in this study were performed according to the following methodology : First , the patient was subjected to MRI . Subsequently , within a 24-h interval , he/she was subjected to contrast venography . The acquired results were compared in a blinded manner . The diagnostic indices for MRI were calculated on the assumption that the results of contrast venography were sure to give an accurate indication of either presence or absence of thrombosis . Thirty-six patients were included in the study , of which 27 ( 15 males ) completed it . The overall incidence of distal deep venous thrombosis ( DVT ) was 22 % ( 6/27 ) . One patient showed extension of a crural thrombus into the popliteal vein .  MRI did not detect any of the thrombi . This lack of result was ascribed to failure to fully demonstrate all segments of the crural veins . However , MRI did show three proximal thrombi in the superficial femoral vein , which were not shown by the venograms . Thus , both the sensitivity and specificity of MRI were 0 % , so MRI proved to be of no value in the diagnosis of asymptomatic deep venous thrombosis in this study RATIONALE AND OBJECTIVES The authors performed this study to compare magnetic resonance ( MR ) venography and conventional venography in the diagnosis of deep venous thrombosis  ( DVT ) in the calf after sonography . MATERIAL S AND METHODS Sonography was performed in 595 patients who were suspected of having lower-extremity DVT . Patients with positive above-knee duplex sonograms , allergy to iodinated contrast material , renal insufficiency , or cardiac pacemakers and patients who were obese were excluded . The remaining 73 patients were asked to undergo MR venography and conventional venography . All studies were to be performed within 48 hours of the clinical diagnosis and according to st and ard clinical practice . Images were interpreted by radiologists who were blinded to the results of other modalities . Two separate analyses were performed : one in which conventional venography was used as the st and ard of reference , and one in which the presence of at least two positive studies for thrombus was considered diagnostic . RESULTS Although 36 patients agreed to participate in the study , only 14 underwent MR venography and conventional venography within 48 hours of the clinical diagnosis . With use of any two positive studies for confirmation , acute DVT was diagnosed in three patients . Conventional venography depicted two of the three cases , whereas sonography and MR venography each depicted all three . The findings were concordant in only five of the 14 patients . CONCLUSION Moderate discrepancy among modalities was demonstrated . This suggests radiologists should undertake comparisons among these three modalities for the detection of calf DVT . In patients with a high clinical suspicion , a second modality may be useful if the initial study is negative The aims of this study were to optimize image quality for indirect CT venography ( sequential versus spiral ) , and to evaluate different image reconstruction parameters for patients with suspected deep venous thrombosis ( DVT ) . Fifty-one patients ( 26/25 with/without DVT ) were prospect ively evaluated for pulmonary embolism ( PE ) with st and ard multidetector-row computed tomography ( MDCT ) protocol s. Retrospective image reconstruction was done with different slice thicknesses and reconstruction increments in sequential and spiral modes . All reconstructions were read for depiction of DVT and to evaluate best reconstruction parameters in comparison with the thinnest reconstruction ( “ gold st and ard ” ) . Image noise and venous enhancement were measured as objective criteria for image quality . Subjective image quality was rated on a four-point scale . Effective dose was estimated for all reconstructions . In sequential 10/50 reconstruction DVT was completely detected in 13/26 cases , partially in 10/26 cases and was not detected at all in 3/26 cases , and 15/26 , 9/26 and 2/26 cases for the 10/20 reconstruction , respectively . DVT was completely detected in all spiral reconstructions . Image noise ranged between 14.8 - 29.1 HU . Median image quality was 2 . Estimated effective dose ranged between 2.3 mSv and 11.8 mSv . Gaps in sequential protocol s may lead to false negative results . Therefore , spiral scanning protocol s for complete depiction of DVT are m and atory Despite considerable recent advances in diagnostic techniques for lower-limb deep venous thrombosis ( DVT ) , current methods have disadvantages . Ultrasonography , the most accurate noninvasive test , is widely available and cheap . As such , it has largely replaced venography as the test of first choice for symptomatic DVT . In a recent meta- analysis , the sensitivity of  ultrasonography  was 89 % overall for symptomatic DVT and 97 % for above-knee thrombosis ( 1 ) . Large outcome studies have shown that patients may be safely left untreated after a negative result on ultrasonography  if they have a low clinical risk score , a low d-dimer level , or a negative result on repeated ultrasonography at 1 week ( 2 - 4 ) . However , these strategies may be complex and still require 3 % to 34 % of out patients and most in patients to undergo repeated ultrasonography at 1 week ( 2 - 4 ) . In practice , retesting after 1 week is inconvenient , and physicians often rely on a single test or request immediate venography ( 5 ) . Other problems with ultrasonography include poor sensitivity for asymptomatic disease , difficulties in diagnosing DVT recurrence , and limited visualization in the pelvis ( 1 , 6 , 7 ) . Impedance plethysmography is also commonly used ; however , it has a lower diagnostic accuracy than ultrasonography and has similar weaknesses in the setting of recurrent thrombosis , asymptomatic DVT , and DVT below the knee or in the pelvis ( 1 , 4 , 6 ) . Computed tomography and magnetic resonance imaging techniques can visualize DVT above the knee and in the pelvis but in general are unsuccessful below the knee ( 8 - 10 ) . The ability of these techniques to diagnose DVT recurrence and asymptomatic disease has not been tested .  Venography  is the reference st and ard diagnostic test , but it has in large part been replaced by noninvasive tests . In clinical practice , it is the most reliable test for the diagnosis of asymptomatic thrombosis and thrombosis isolated within the calf or pelvis . However , imaging in the pelvis is inadequate in up to 24 % of normal studies , and the proximal extent of thrombosis is frequently not delineated in patients with above-knee DVT ( 11 ) . Underfilling of vessels and vessels overlying one another also create problems with venography below the knee . Studies have shown that interobserver variability for venography is high ( 10 % to 16 % ) , especially below the knee ( = 0.46 to 0.73 below the knee and 0.46 to 0.84 above the knee ) ( 12 , 13 ) . In addition , a high proportion of studies are nondiagnostic for possible DVT recurrence ( 1 , 6 ) . A noninvasive test is needed that accurately diagnoses above-knee DVT and thrombus below the knee , in the pelvis , and in asymptomatic limbs . Unlike most imaging techniques , which identify thrombus as filling defects , magnetic resonance direct thrombus imaging ( MRDTI ) visualizes thrombus against a suppressed background ( 14 ) . In an unblinded comparison with venography , we previously showed that MRDTI precisely visualizes acute deep venous thrombus ( 14 , 15 ) . In the current study , we sought to assess prospect ively whether MRDTI is a reliable diagnostic test for suspected acute symptomatic DVT . Methods The ethics committee at our institution granted approval for the study , and all participants gave written informed consent . With the exceptions of pregnant women , patients with known contrast allergy , and those with renal failure , all patients with DVT suspected on the basis of lower limb symptoms are investigated by using venography at our institution . Participants were recruited after routine venography was done between May 1998 and September 1999 . During this time , 338 consecutive patients underwent routine contrast venography .  Consecutive patients with positive venograms were selected , along with one quarter of those with negative venograms , according to a predetermined r and om sequence . This protocol was chosen to equalize the numbers of positive and negative cases and was based on a 6-month audit of venograms in our institution that found that 22 % of venograms were positive . Clinical diagnostic criteria were not used , and the decision to request investigation for suspected DVT had been made by the attending clinician ; however , patients who did not have leg symptoms were not recruited . Other exclusion criteria were failed or inconclusive venography , failed or inconclusive MRDTI , contraindications to MRI , and claustrophobia ( Figure 1 ) . Individual venous segments that were nondiagnostic at venography were also excluded from analysis . Figure 1 . Outline of the study . Magnetic resonance direct thrombus imaging was performed on all patients recruited within 48 hours of venography . The scans were interpreted by an experienced radiologist ( review er A ) and by a nonradiologist ( review er B ) trained to read MRDTI scans . For venograms and MRDTI scans , the review ers noted the presence or absence of DVT ; the diagnostic classification of DVT , divided into isolated calf DVT , femoropopliteal DVT , and ileofemoral DVT ; and the presence of thrombus in the calf , femoropopliteal , and iliac venous segments . Venograms were obtained and initially reported by the radiologists on duty . This initial report was used to make recruitment decisions ; if the results were discordant with those of MRDTI , ultrasonography was also performed . However , ultrasonography was not used in the calculations of the accuracy of MRDTI . After completion of the study , venograms were interpreted by an independent radiologist , and these results were used as the gold st and ard against which MRDTI was compared . Results of MRDTI and venography were reported without 
    Output:
    
MRI has equivalent sensitivity and specificity to ultrasound for diagnosis of DVT , but has been evaluated in many fewer studies , using a variety of different techniques