Thorax 1996;51:23-28 23 Original articles Thorax: first published as 10.1136/thx.51.1.23 on 1 January 1996. Downloaded from Role of spiral volumetric computed tomographic scanning in the assessment of patients with clinical suspicion of pulmonary embolism and an abnormal ventilation/perfusion lung scan

Astrid B van Rossum, Frank E E Treurniet, Gerard J Kieft, Sierd J Smith, Relinde Schepers-Bok

Abstract monary in the diagnostic Background - A study was carried out strategy for pulmonary embolism. to evaluate the potential place of spiral (Thorax 1996;51:23-28) volumetric computed tomography (SVCT) in the diagnostic strategy for pul- Keywords: pulmonary embolism, pulmonary arteries, monary embolism. computed tomography, lung scintigraphy. Methods - In a prospective study 249 patients with clinical suspicion of pul- monary embolism were evaluated with Pulmonary embolism is a major complication various imaging techniques. In all patients of venous thrombosis with an incidence in a ventilation/perfusion (V/Q) scan was per- the West of 2-3 per 1000 inhabitants and a formed. Seventy seven patients with an mortality rate, if untreated, of 30%.' Adequate abnormal V/Q scan underwent SVCT. Pul- anticoagulant therapy reduces the mortality monary angiography was then performed rate to 8%,' so a diagnostic test for pulmonary in all 42 patients with a non-diagnostic embolism has to be sensitive and specific be- V/Q scan and in three patients with a high cause treatment has a high complication rate.23 emboli on Symptoms and signs of pulmonary embolism probability V/Q scan without http://thorax.bmj.com/ the SVCT scan. Patients with an abnormal are non-specific and laboratory tests do not perfusion scan also underwent ultra- provide an accurate diagnosis, so diagnosis re- sonography of the legs for the detection of lies heavily on imaging techniques. Chest radio- deep thrombosis. graphic and perfusion lung scan abnormalities - hundred and two are non-specific, although a normal perfusion Results One seventy lung scan essentially rules out pulmonary patients (69%) had a normal V/Q scan. embolism.45 If the perfusion lung scan is ab- Forty two patients (17%) had a non-diag- normal, combination with a ventilation lung nostic V/Q scan, and in five of these scan raises the specificity. A high probability on October 1, 2021 by guest. Protected copyright. patients pulmonary emboli were found V/Q scan gives an 86-92% chance of pul- both by SVCT and pulmonary angio- monary embolism being present and warrants graphy. In one patient, although SVCT anticoagulant therapy.6-9 Unfortunately, up to showed no emboli, the angiogram was 60% of the patients suspected of pulmonary positive for pulmonary embolism. In one embolism have an equivocal so-called non- of the 42 patients the SVCT scan showed diagnostic V/Q scan6-81015 and in these patients an embolus which was not confirmed by Department of it is necessary to visualise the pulmonary Diagnostic pulmonary angiography. The other 35 arteries to rule out the presence of emboli. A B van Rossum patients showed no sign of emboli. Thirty Pulmonary angiography is the reference stand- F EE Treumiet five patients (14%) had a high probability G J Kieft ard for detecting clots, but is invasive. The R Schepers-Bok V/Q scan, and in 32 patients emboli were challenge is to find a less invasive test to identify seen on SVCT images. Two patients had the true positive and negative patients amongst Department of both a negative SVCT scan and a negative Internal Medicine those with a non-diagnostic V/Q scan. S J Smith pulmonary angiogram. In one who had an Computed tomographic (CT) scanning with inconclusive SVCT scan pulmonary angio- intravenous contrast medium is relatively non- Leyenburg Hospital, graphy was positive. The sensitivity for invasive and the of electron beam Leyweg 275, development The Hague, 2545 CH, pulmonary embolism was 95% and the CT scanning and spiral volumetric CT (SVCT) The Netherlands specificity 97%; the positive and negative scanning makes it possible to depict contrast predicted values of SVCT were 97% and Correspondence to: enhanced pulmonary arteries down to seg- Dr A B van Rossum. 97%, respectively. mental and subsegmental levels using one Received 22 March 1995 Conclusions - SVCT is a relatively non- intravenous bolus of contrast material in Returned to authors invasive test for embolism 1-3 5 June 1995 pulmonary approximately 30 seconds." SVCT allows Revised version received which is both sensitive and specific and continuous CT scanning oforgan volumes dur- 1 September 1995 Accepted for publication which may serve as an alternative to vent- ing a single breath hold by advancing the patient 4 September 1995 ilation scintigraphy and possibly to pul- through the x ray beam. 24 van Rossum, Treurniet, Kieft, Smith, Schepers-Bok

did not perform a pulmonary angiogram if the SVCT was also positive for pulmonary embolism in view of the very high reported positive predicted value of a high probability scan.6`9 Pulmonary angiography was only per- Thorax: first published as 10.1136/thx.51.1.23 on 1 January 1996. Downloaded from formed to establish a final diagnosis in those patients who had a high probability scan but whose SVCT scan was negative. If the V/Q scan was neither normal nor high probability it was classified as non-diagnostic8 101415 and in these patients we performed both SVCT and pulmonary angiography. These two ex- aminations were always performed within 24 hours of each other (fig 1). In addition, ultrasonography of the lower

Figure 1 Flow chart of the study design. SVCT= spiral volumetric computed limb was performed on the same day to detect tomographic scanning; PA=pulmonary angiography; PE=pulmonary embolism. deep vein thrombosis in all patients with an abnormal perfusion scintigram. The purpose of this prospective study was TECHNIQUES to confirm the potential role of SVCT in the V/Q scan detection of pulmonary emboli, and to define Technetium-99m labelled macroaggregated al- its value in the diagnostic strategy for pul- bumin (0 03 mCi/kg) was used for perfusion monary embolism. lung scanning. Six views were obtained: an- terior, posterior, right and left lateral, and right Methods and left posterior oblique. Ventilation was sim- PATIENTS ultaneously performed with krypton-8 1m From August 1993 to January 1995 a total of (40 ,uGy). These six views were obtained in 99 348 consecutive patients had a V/Q scan for patients but in 40 only four views could be clinical suspicion of pulmonary embolism obtained. The quality of the V/Q scans was based on history, physical examination, laborat- determined by the condition of the patients. ory findings, and chest radiographic or electro- Some had ventilation difficulties, others were cardiographic abnormalities, and were eligible bedridden and therefore were hard to position for inclusion in the study. Exclusion criteria correctly. V/Q scans were classified according were the use of anticoagulants for more than to a clinical classification suggested by Moser 48 hours, contraindications for pulmonary

and Hull.8 101415 http://thorax.bmj.com/ angiography - for example, pulmonary hyper- V/Q scans were reported as normal when no tension or severe cardiac dysfunction as in- perfusion defects were present, as high prob- dicated by the referring physician - and contra- ability when one or more large areas of mis- indications to the use of intravenous contrast match were present, areas with no perfusion material such as hypersensitivity, severe con- and normal ventilation. All other scans were gestive heart failure, and pregnancy. Fifty two classified as non-diagnostic - that is, they were patients were using anticoagulants for more not conclusive. V/Q scans were interpreted than 48 hours, 25 patients refused to enter the together with chest radiographic appearances. study, 20 patients had severe cardiac dys- on October 1, 2021 by guest. Protected copyright. function, one had a history of a severe allergic reaction to intravenous contrast medium, and CT scan one proved to have an at SVCT scans were performed using a Somatom SVCT and subsequently went for thoracic sur- Plus S scanner (Siemens, Erlangen, Germany). gery, so no pulmonary angiography could be If possible, scanning was performed during a performed. Thus, 249 patients entered the single breath hold, but inability to hold res- study after informed consent was obtained. piration was not considered a criterion for ex- clusion. If patients were short of breath they DIAGNOSTIC STRATEGY were asked to breathe gently. SVCT scans were If the V/Q scan was normal, investigations for obtained using 210mA and 120kV. A 3600 emboli were stopped and the patients were linear interpolation was used for the re- recorded not to have emboli. Patients with construction of a single image. Scanning time an abnormal V/Q scan underwent SVCT. If was 32 seconds with a 5 mm/s table feed re- patients had a high probability V/Q scan, we sulting in a scanning volume of 16 cm. Five mm slices were reconstructed with a 4 mm increment. In one patient the scan was in- Table 1 Spiral volumetric CTfindings versus V/Q scan findings conclusive and an additional target scan with V/Q scan Spiral volumetric CT 2 mm slices of a specific area was made. As the scanning volume was restricted to 16 cm, the No pulmonary Pulmonary Inconclusive Total embolism embolism SVCT range started at the top of the aortic arch so that the scan would cover all segmental Non-high probability 35 6 1 42 High probability 2 32 1 35 pulmonary arteries. One hundred ml of a low osmolarity 30% iodinated non-ionic contrast Total 37 38 2 77 agent (Iopamiro 300, Bracco, Italy) was ad- CT = computed tomography; V/Q=ventilation/perfusion ministered via an antecubital vein using an SVCT scanning in pulmonary embolism 25

automatic injector. We used a biphasic contrast A *'';"Stinjection!,'.';with a flowF,.:.t>.i.Ctjof3ml/s during.. thej9 first 15 seconds, then a flow of 2 ml/s up to 100 ml, with a 15 second scan delay. The entire ex- Thorax: first published as 10.1136/thx.51.1.23 on 1 January 1996. Downloaded from MN, .L ..... Lamination required 10-15 minutes. All images were viewed using lung (window width 1500 HU; window centre 500 HU) and me- ~~diastinal (window width 400 HU; window centre 40 HU) settings. of the ... ~~~~~~~~~Theopacification pulmonary arteries was analysed using mediastinal window set- P SIT E tings. Opacification was graded as good when it was adequate for detecting filling defects but the density was less than 200 HU, and as excellent when it demonstrated a high degree ...... of vascular opacification (>200HU). SVCT . : i| .!'..scanso were classified as negative or positive for pulmonary embolism. The diagnosis of pul- g!ii0* monary embolism was based solely on the pres- ence of one or more defects in the 0 0 2SS filling pulmonary arteries. Indirect signs such as a B wedge-shaped pleurally-based parenchymal shadow corresponding to a pulmonary infarct _ were best evaluated at lung window settings and were only used as a support ofthe diagnosis of pulmonary embolism. Ancillary findings such as pneumonia were scored separately. With our SVCT protocol, pulmonary arteries had a good enhancement with an average of 200HU in the main pulmonary artery. The vascular opacification was excellent in 51 *_T;i; :-'''!:;.t .g,'x''.°fi holdpatients (>200 HU) and good in 27 patients (<200 HU). The quality of the images was ...... L.P.. reduced in one obese patient, in two patients http://thorax.bmj.com/

a be adequate for interpretation (97%). esu,; Angiography Using the Seldinger technique a 5 French a l _ ~~Grollman (Cordis, Roden, The Neth- l - l ~~~~~~~~~~~~erlands)was inserted into the femoral vein. 1 I ~~~Pulmonary arterial pressure was measured in on October 1, 2021 by guest. Protected copyright. _l-l _ ~~~~~~~~~~~~thepulmonary trunk, as a pulmonary arterial ...... suret~~~~~~~~~~~to.anof >50 mmHg wasa ontraic _g - A_ _ ~~~~~~~~~~non-ioniccontrast agent (Iopamiro 300) was

Figure 2 An 84 year old woman with sudden onset of dyspnoea and chest pain. (A) and (B) The VIQ scan showing matching defects in the right posterobasal and lateral middle lobe segment, and in the left anterobasal segment. (C) The pulmonary angiogram Figure 3 A 24 year old woman with a non-diagnostic of the right lung showing intraluminalfilling defects in the anterobasal segmental artery VIQ scan which showed a matching defect in the right (curved arrow) and in the lateral segmental artery (straight arrow), which could not be lower lobe. SVCT scan showing normal enhancing right seen on SVCT scanning. lower lobe arteries (white arrows). 26 van Rossum, Treurniet, Kieft, Smith, Schepers-Bok

injected at a rate of 20-30 ml/s up to a total of ASSESSMENT OF SCANS 40-60 ml in two seconds. Exposure rates were The V/Q and SVCT scans were interpreted four per second for four seconds, followed by separately by two radiologists. The V/Q scan one per second for 4-6 seconds. Radiographic reader was blinded to the SVCT results but parameters were in the range of 120-130 kV had access to chest radiographs. The SVCT Thorax: first published as 10.1136/thx.51.1.23 on 1 January 1996. Downloaded from and 1-5-3 mA. Anteroposterior, right posterior reader was only aware of the fact that the V/Q and left posterior oblique projections were ob- scan was not normal. The pulmonary angio- tained. The criterion for the diagnosis of pul- gram was our standard of reference. To monary embolism was the identification of a guarantee an optimal final diagnosis it was filling defect in, or an abrupt cutoff of, a pul- interpreted, together with the SVCT and V/Q monary artery. scan findings, by the same two radiologists in consensus. Ultrasound Ultrasound was performed using a 7 5 MHz linear transducer (Toshiba 270 HE). The ven- Results ous system of both lower limbs was examined The V/Q scans were normal in 172 of the 249 from the inguinal canal to the mid calf. The patients (69%), non-diagnostic in 42 (17%), diagnosis of deep venous thrombosis was made and high probability in 35 (14%) (table 1). when the vein was incompletely compressible and/or an intraluminal thrombus was dem- onstrated. Ultrasonography was performed in Non-diagnostic VIQ scan and SVCTfindings 75 of the 77 patients with an abnormal per- The non-diagnostic group consisted of 42 fusion lung scan. patients, 35 ofwhom had no signs ofpulmonary

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Figure 4 A 45 year old woman with right sided chest pain in whom the VIQ scan was non-diagnostic. (A) and (B) There is a partial mismatch on the posterior view in the basal segments of the left lung. LPO view shows a more matching defect in the basal segments. On the right are several peripheral small partly matching defects. (C) SVCT scan showing an intraluminal filling defect in the anterobasal segmental artery of the left lung (white arrow) and some pleural effusion (open arrow). (D) Pulmonary angiogram which confirmed the SVCTfindings (black arrow). SVCT scanning in pulmonary embolism 27 A confirmed by pulmonary angiography (fig 4). In one patient the angiogram was negative for pulmonary embolism (table 2). Thus, six of the 42 patients with a non- diagnostic V/Q scan proved to have pulmonary Thorax: first published as 10.1136/thx.51.1.23 on 1 January 1996. Downloaded from embolism and the SVCT scan was falsely neg- ative in one of these.

High probability VIQ scan and SVCTfindings The group of high probability V/Q scans con- sisted of 35 patients (14%). In 32 cases the SVCT scans showed thromboembolism (fig 5). As for indirect signs of pulmonary embolism, consolidation interpreted as "pulmonary in- farction" was seen in 10 cases and pleural effusion in eight. Several patients also showed other abnormalities such as emphysema (2) and two patients had a bronchial carcinoma. In two ofthe 35 patients with a high probability V/Q scan the SVCT scan showed no pulmonary emboli. In one of these the SVCT scan showed emphysema in the non-perfused areas, and in the other it was normal. Pulmonary angio- graphy was negative in both cases. Finally, one of the 35 patients with a high probability V/Q scan had an inconclusive SVCT scan and the pulmonary angiogram was positive. Figure 5 A 44 year old In our population sensitivity and specificity man with high probability of SVCT for pulmonary embolism were 95% VIQ scan. (A) The perfusion scan showing and 97%, respectively. The positive and neg- several segmental perfusion ative predicted value of SVCT were both 97%. defects, ventilation scan Due to the study design the sensitivity of the was normal (not shown). (B) SVCT scan showing V/Q scan was 100% by definition, and speci- large central emboli in both ficity was 82%. In patients with an abnormal right and left pulmonary perfusion scan the positive predictive value of

arteries (white arrows). http://thorax.bmj.com/ SVCT was 97% and ofV/Q scanning was 52%. In the group of patients with a non-diagnostic embolism on SVCT. In 34 of these 35 patients V/Q scan the positive predicted values ofSVCT pulmonary angiography confirmed the SVCT and V/Q scans were 83% and 17%, respectively. findings with normal enhancing pulmonary ar- teries. One patient with a negative SVCT scan had a positive angiogram (fig 2). Ultrasonography Twenty four patients with a negative SVCT Ultrasonography revealed deep vein throm- on October 1, 2021 by guest. Protected copyright. scan for pulmonary embolism showed other bosis in 12 patients, all of whom had a high abnormalities - ranging from emphysema (6), probability V/Q scan and pulmonary embolism pneumonia (9) (fig 3), pleural effusion (5), on the SVCT scan. Thus, in this study 12 out empyema (1), lung fibrosis (1), lymph- of 39 patients with pulmonary embolism had adenopathy (1) to diaphragmatic hernia (1) - deep vein thrombosis (31%). which could explain the V/_Q scan defects. One of the 42 non-diagnostic patients had an inconclusive SVCT scan which was difficult Discussion to interpret because of moderate vascular If a patient is suspected of having a pulmonary opacification. There was no clear depiction of embolism there are several strategies to follow. the right lower lobe arteries, making it im- One possibility is to treat all patients with possible to rule out thromboembolism in this anticoagulant drugs but, as pulmonary emboli area. However, there was consolidation at the are only found in a minority ofthose suspected, same location as the matching defects on the many patients would receive unnecessary and V/Q scan. The pulmonary angiogram showed potentially harmful treatment.23 no pulmonary embolism. Six SVCT scans from It is therefore imperative to establish or ex- the non-diagnostic group showed pulmonary clude the presence of pulmonary embolism (or emboli. In all but one case the diagnosis was deep vein thrombosis) in each patient suspected

Table 2 VIQ scan findings compared with spiral volumetric computed tomography (SVCT) and pulmonary angzography (PA) (n = 45) V/Q scan SVCT-IPA- SVCT+IPA+ SVCT-IPA+ SVCT+IPA- SVCT?IPA- SVCT?IPA+ Non-high probability 34 5 1 1 1 High probability 2 - - - - 1 28 van Rossum, Treurniet, Kieft, Smith, Schepers-Bok

of having the condition. A perfusion lung scan ning and angiography.'2 Later they reported a is a sensitive screening test; if the perfusion sensitivity of 65% and a specificity of 97%.13 scan is normal there is no clinically relevant Remy-Jardin, using SVCT, found a sensitivity pulmonary embolism present and no further of 100% and a specificity of 96%." Our study, investigation is necessary.4 In most series about which correlates V/Q scanning with SVCT Thorax: first published as 10.1136/thx.51.1.23 on 1 January 1996. Downloaded from 30-40% of patients had a normal perfusion scanning and pulmonary angiography, confirms scan.679 In our series 172 of 249 patients had these findings with a high sensitivity and speci- a normal perfusion scan (69%), suggesting that ficity for pulmonary embolism (95% and 97%, our clinicians consider pulmonary embolism respectively ). The positive and negative pre- too frequently in their differential diagnoses. dicted values were 97%. We encountered only An abnormal perfusion lung scan can be one false positive and two false negative SVCT caused by several conditions, some of which scans. It is to be expected that more false mimic the clinical signs and symptoms of pul- negative CT scans will be encountered in monary embolism. If a patient has an abnormal patients with small peripheral emboli. A large perfusion lung scan, additional tests are multicentre clinical study comparing the needed. One possibility is to try to detect the different diagnostic strategies is necessary, and thrombosis in the pelvis or legs and ultra- will show whether false negative SVCT scans sonography is a very specific test for this.'5 If in patients with small peripheral emboli are a thrombosis is found, anticoagulant therapy clinically relevant. is warranted and no additional test is needed. In the population studied, SVCT scanning In our study additional tests would have been could have replaced ventilation scanning in unnecessary in 12 out of 77 patients (16%). patients with an abnormal perfusion scan. The Other authors have reported a similar limited sensitivity of SVCT scanning for pulmonary value for ultrasonography in the diagnostic as- embolism was almost as high as that of V/Q sessment of patients suspected of having pul- scanning (95% versus 100%), specificity was 80%). The main benefit of monary embolism.'617 If ultrasonography is higher (97% versus negative for deep vein thrombosis, the diag- SVCT scanning compared with V/Q scanning value in patients nostic strategy has to be directed to the lungs is the high positive predicted with a non-diagnostic V/Q scan. SVCT scan- themselves. ning could also replace pulmonary angiography The usual next step is a ventilation-perfusion as the diagnostic test in patients with non- scan. If scan shows one or more large lung this diagnostic V/Q scans. segmental mismatches, there is a greater than 90% chance of pulmonary embolism being 1 Dalen JE, AlpertJS. Natural history ofpulmonary embolism. present, but only 10-20% of the patients sus- Prog Cardiovasc Dis 1975;259-70. 2 Robin ED. Overdiagnosis and overtreatment of pulmonary pected of pulmonary embolism will have such embolism: the emperor may have no clothes. Ann Intern a high probability scan.i9 In our series 35 Med 1977;87:775-81. 3 Levine MN, Raskob GE, Hirsh J. Hemorrhagic com- http://thorax.bmj.com/ patients had a high probability scan, and 32 of plications of long-term anticoagulant therapy. Chest 1989; scan. 95:s26-35. these had a positive SVCT We felt that 4 Hull RD, Raskob G, Hirsh J. Clinical validity of a normal it was unnecessary to perform a pulmonary perfusion lung scan in patients with suspected pulmonary embolism. Chest 1990;97:23-6. angiogram in these patients, as not only is the 5 Kipper MS, Moser KM, Kortman KE, Ashburn WL. Long V/Q scan specific in such patients, but the first term follow up of patients with suspected pulmonary embolism and a normal lung scan. Chest 1982;82:411-5. studies with SVCT scanning also showed high 6 PIOPED investigators. Value of the ventilation/perfusion specificity."'-" Two patients with a high prob- scan in acute pulmonary embolism: results of the pro- spective investigation of pulmonary embolism diagnosis. ability V/Q scan had a negative SVCT scan J7AMA 1990;263:2753-9. and the pulmonary angiogram also showed no 7 Biello DR, Mattar A, McKnigt RC, Siegel BA. Ventilation- on October 1, 2021 by guest. Protected copyright. perfusion studies in suspected pulmonary embolism. AJ7R emboli. Therefore, in our population SVCT 1979;133:1033-7. scanning was a good alternative to the vent- 8 Hull RD, Hirsh J, Carter CJ, Raskob GE, Gill GJ, Jay RM, et al. Diagnostic value of ventilation-perfusion scanning ilation scan in the high probability group. in patients with suspected pulmonary embolism. Chest The main issue in the diagnosis ofpulmonary 1985;6:819-28. 9 Hull RD, Raskob GE, Carter CJ, et al. Pulmonary embolism embolism is how to manage patients with a in outpatients with pleuritic chest pain. Arch Intern Med scan. to the lit- 1988;148:838-44. non-diagnostic V/Q According 10 Hull RD, Hirsh J, Carter C, Jay RM, Dodd PE, Ockkford erature, 40-60% of patients suspected of pul- PA, et al. Pulmonary angiography, ventilation lung scan- ning and for clinical suspected pulmonary monary embolism will have such a V/Q scan, embolism with abnormal perfusion scan. Ann Intern Med and the frequency of pulmonary embolism in 1983;98:891-9. 18 '0 11 Remy-Jardin M, Remy J, Wattine L, Giraud F. Central these patients varies from 10% to 40%.2 It pulmonary thromboembolism diagnosis with spiral vo- is therefore necessary to identify or exclude lumetric CT with the single breath hold technique: com- parison with pulmonary angiography. Radiology 1992;185: the presence of blood clots in the pulmonary 381-7. arteries of those subjects. To date, pulmonary 12 Teigen CL, Maus TP, Sheedy PF, Johnson CM, Stanson AW, Welch TJ. Pulmonary embolism: diagnosis with elec- angiography remains the standard test for de- tron beam CT. Radiology 1993;188:839-45. picting pulmonary emboli, but there is often 13 Teigen CL, Maus TP, Sheedy PF, Stanson AW, Johnson CM, Breen JF, et al. Pulmonary embolism: diagnosis with reluctance to perform this invasive procedure. contrast enhanced electron-beam CT and comparison Fast CT scanning with bolus intravenous with pulmonary angiography. Radiology 1995;194:313-9. 14 Hull RD, Raskob GE. Low probability lung scan findings; contrast media is a new, less invasive technique a need for change. Ann Intern Med 1991;114:142-3. arteries. It 15 Moser KM. Venous thromboembolism. Am Rev Respir Dis for studying the pulmonary provides 1990;141:235-49. a straightforward diagnostic criterion - namely, 16 Cronan J. Venous thromboembolic disease: the role of US. Radiology 1993;186:619-30. the presence ofintraluminal filling defects. First 17 Quinn RJ, Nour R, Butler P, Glenn DW, Travers PL, studies with SVCT and electron beam CT Wellings G, et al. Pulmonary embolism in patients with intermediate probability lung scans: diagnosis with Dop- scanning were promising. 1'1-1 Teigen et al found pler venous US and d-dimer measurement. Radiology 85% agreement between findings at CT scan- 1994;190:509-1 1.