© 2007 Schattauer GmbH, Stuttgart
Blood Coagulation, Fibrinolysis and Cellular Haemostasis
Incidence and distribution of lower extremity deep venous thrombosis at indirect computed tomography venography in patients suspected of pulmonary embolism Alain Nchimi*, Benoît Ghaye, Charlemagne T. Noukoua, Robert F. Dondelinger Department of Medical Imaging, University Hospital of Liège, Liege, Belgium
Summary Indirect computed tomography (CT) venography reportedly provides high accuracy for detection of venous thrombosis in patients suspected of pulmonary embolism (PE). Nevertheless, the extent of the scanning range for lower limb and abdominal veins remains to be determined.It was the objective of this study to investigate the distribution of venous thrombosis in order to identify the most appropriate extent of scanning range when using CT venography.We reviewed 1,408 combined CT pulmonary angiographies (CTPA) and indirect CT venographies of the lower limbs, performed in patients suspected of PE. Percentage of venous thromboembolism (VTE), which includes PE and/or venous thrombosis was calculated. Location and the upper end
of clots were recorded in 37 venous segments per patient from calf to diaphragm. PE, venous thrombosis and VTE, were found respectively in 272 (19.3%), 259 (18.4%) and 329 (23.4%) patients.Addition of CT venography to CTPA increased depiction of VTE in 17.3%.The upper end of venous thrombosis was located below the knee in 48%, between knee and inguinal ligament in 36% of the patients, and above the inguinal ligament in 15%.Ninety-six patients had thrombosis in a single vein,of which none occured above the iliac crests in a patient without PE at CTPA. In conclusion, when added to CTPA, optimal scanning of CT venography should extent from calves to the iliac crests in patients suspected of VTE.
Keywords Pulmonary embolism, computed tomography (CT), venography, angiography, veins, thrombosis, thromboembolism
Thromb Haemost 2007; 97: 566–572
Introduction Deep venous thrombosis is a serious clinical condition that may be complicated by pulmonary embolism (PE), resulting in a continuous disease process termed venous thromboembolism (VTE). VTE is estimated to be associated with 300,000–900,000 hospitalizations and results in 50,000–150,000 deaths each year in the USA (1). Early diagnosis and treatment of VTE significantly improve survival, but diagnosis of both deep venous thrombosis and PE, based on clinical findings alone is difficult (2, 3). In PE, the major risk of death results from a recurrent embolic episode, which arises in more than 90% of patients from deep venous thrombosis located in the lower limb or the pelvis (4). Therefore, imaging of lower limb veins has been advocated in diagnostic algorithms for patients suspected of PE when imaging of pulmonary arteries, i.e. ventilation-perfusion lung scan
or computed tomography (CT) pulmonary angiography (CTPA), do not allow for a definite diagnosis of PE (5–7). Indirect CT venography, combined with CTPA, was introduced to allow a one-stop-shop diagnostic examination for patients suspected of VTE (7), providing results similar to lower limb ultrasound (US) for the diagnosis of deep venous thrombosis in the femoro-popliteal veins (8–14). The reported incremental value of adding indirect CT venography to CTPA has been reported in a broad range of 8–27% of patients with negative CTPA (10, 13–18). Optimal scanning coverage for CT venography remains a matter of debate, fueled by the fact that venous thrombosis located in below-knee veins has variable clinical significance across the literature (19). In the published series, the scanning range of indirect CT venography is variable, with the lowest level empirically located at the ankle, upper-calf, knee or pelvis (9, 10,
Correspondence to: Alain Nchimi, MD Department of Medical Imaging University Hospital of Liege, B 35 B – 4000 Liege, Belgium Tel.: +3243667249, Fax: +3243667772 E-mail:
[email protected]
*Current address: Dept of Medical Imaging, Clinique Saint Joseph, Rue de Hesbaye, 52, B-4000 Liège, Belgium. Received January 13, 2006 Accepted after resubmission February 9, 2007 Prepublished online March 7, 2007 doi:10.1160/TH06–01–0021
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13, 16, 17, 20). Despite the fact that the knowledge of distribution of venous clots at CT venography may have important implications on acquisition of CT venography (21), data currently available are limited (22). Aim of this study was first to assess the incremental value of adding CT venography to CTPA in patients with clinical suspicion of VTE; second to define the optimal acquisition range by reviewing lower limb clot distribution at CT venography in a large series of patients.
Patients and methods Inclusion and exclusion criteria Study was approved by the institutional review board, and, because of its retrospective nature, patients’ written informed consent was not required. From 1998 to 2004, 1,673 consecutive patients presenting with clinical suspicion of PE underwent combined CTPA and indirect CT venography of the lower limb and abdominopelvic veins. Two hundred sixty-five (15.8%) were excluded from analysis for the following reasons: venous thrombosis documented prior to combined CTPA-CT venography (134 patients); follow-up examinations of PE after treatment (104 patients); and examinations unavailable for review (27 patients). As a result, the final study population consisted of 1,408 patients (mean age ± SD: 61.3 years ± 17.2; age range: 19–100 years; 692 male and 716 female). All patients had given oral informed consent prior to combined CTPA-CT venography examinations. CT acquisition The first 1,344 exams were performed with a single-detector scanner (PQ 5000, Philips, Eindhoven, The Netherlands), and the last 329 with a multiple-detector scanner (Sensation 16, Siemens Medical Solutions, Erlangen, Germany). Pulmonary arteries were scanned with helical acquisition, using 0.75 to 2-mm collimation, 0.7- to 1-mm reconstruction increment, 100–125 mAs and 120–130 kVp, starting 20–25 seconds (sec) after the beginning of intravenous injection of 140 ml of 30% iodinated contrast material (Xenetix® 300, Guerbet, Aulnaysous-Bois, France), at a flow rate of 3 ml/sec. CT venography was obtained 210 sec after the start of injection, extending from ankle to diaphragm, using either sequential acquisition of 5-mm thick slices at 20-mm intervals, 100–125 mAs and 130 kVp (single-detector scanner), or helical acquisition of 1.5-mm collimation, 5-mm thick reconstructed slices, 5-mm reconstruction increment, 100 mAs and 120 kVp (multiple detector scanner). CT venography was started at 240–300 sec in patients suspected of low cardiac output. When the acquisition showed an inhomogeneous luminal enhancement, a second scanning was repeated at the corresponding venous level 60–120 sec later. CT interpretation All 1,408 examinations were evaluated by two readers (a consultant chest radiologist with more than five years of experience in interpretation of combined CTPA and CT venography, and a resident radiologist in charge) reaching a consensus. Interpretation was formulated according to a predefined protocol in terms of positive, negative or indeterminate results for PE and venous thrombosis:
– (i) The presence of pulmonary arterial clot was assessed from main pulmonary artery down to at least the subsegmental level. CT signs of acute PE were interpreted along guidelines published in the literature (23). Indeterminate results were examinations negative for PE in which one or several pulmonary arteries were not assessable beyond the lobar level, mainly due to insufficient intravascular enhancement or motion artifacts. – (ii) Distribution of acute venous thrombus was assessed in 43 venous segments, including posterior and anterior tibial veins, peroneal veins, sural veins, gastrocnemian veins, tibio-peroneal trunks, popliteal veins, lesser and greater saphenous veins, common, deep and superficial femoral veins, circumflex veins, common, internal and external iliac veins, spermatic or ovarian veins, renal veins and inferior vena cava (IVC). CT signs of acute venous thrombosis were interpreted along guidelines published in the literature (24). Indeterminate results were examinations negative for venous thrombosis in which one or several venous segments were not assessable, due to insufficient intravascular enhancement, beam hardening or motion artifacts. Data analysis Incidence of VTE, PE and venous thrombosis were calculated. The incremental value of adding indirect CT venography to CTPA for the diagnosis of VTE was defined as the percentage of patients with venous thrombosis and without PE reported to the total number of patients with VTE. The incidence, location and distribution of acute venous clots were assessed in relation with results for PE at CTPA. A venous segment was defined as a vein with its duplicate or triplicates, if any. Thrombosis in a duplicated or triplicated vein was considered a single segment thrombosis. Location of the upper end of the thrombus relative to three pre-defined anatomical landmarks (namely below knee, between knee and inguinal ligament, and above inguinal ligament) were assessed. Venous thrombosis located between the popliteal and the common femoral veins were classified as lying between the inguinal ligament and the knee. Statistical analysis The software used for statistical analysis was Systat 9.0 (Systat software Inc., Richmond, CA, USA). 95% confidence intervals (CI) for means and proportions were calculated using the modified Wald method. Independent proportions were compared with z-statistics. The Woolf method was used to calculate odds ratios (OR) and their CI. A p-value less than 0.05 was considered to express a statistically significant difference.
Results Incidence of VTE and incremental value of CT venography Mean percentages of VTE, PE and venous thrombosis were 329/1408 (23.4%), 272/1408 (19.3%), and 259/1408 (18.4%), respectively (Table 1). Two hundred two out of 272 (74.3%) patients with PE had concomitant venous thrombosis. The incremental value of adding indirect CT venography to CTPA was 57
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Nchimi et al. Distribution of venous thrombosis in patients suspected of PE
(96.0%) deep veins, 84/1,072 (7.8%) veins located above the inguinal ligament, 292/1,072 (27.3%) veins located between the inguinal ligament and the knee, and 696/1,072 (64.9%) veins located below knee. The peroneal, posterior tibial, sural, popliteal and tibio-peroneal veins were the most commonly involved with 284/1,072 (26.5%), 126/1,072 (11.7%), 121/1,072 (11.3%), 118/1,072 (11.0%) and 115/1,072 (10.7%) venous thrombosis, respectively. At each paired location, there was no difference in the percentages of DVT from one side to the other (p >0.05).
Table 1: Incidence of venous thromboembolism figures. CI = confidence interval; CT = computed tomography; PE = pulmonary embolism; VTE = venous thromboembolism. N
Percentage
95% CI
VTE
329
23.4
21.2–25.7
Venous thrombosis and PE
202
14.3
12.5–16.1
PE without venous thrombosis
70
5.0
3.9–6.2
Venous thrombosis without PE
57
4.0
3.0–5.1
Head of thrombus below knee
44
3.1
2.3–4.2
Head of thrombus above knee
13
0.9
0.5–1.6
Inconclusive CT pulmonary angiography
146
10.4
8.8–12.1
Inconclusive indirect CT venography 179
12.7
11.0–14.6
Inconclusive examinations for VTE
15.0
13.2–17.0
211
out of 329 (17.3%) (95%; CI: 13.4–21.9% ) patients with VTE, in whom venous thrombosis without PE was diagnosed. Distribution of venous thrombosis Distribution of venous clots according to epidemiological and clinical risk factors in the 259 patients with acute DVT are summarized in Table 2. The segmental anatomic distribution of venous clot in these patients is presented in Table 3 and Figure 1. Venous thrombosis was located exclusively on the right side in 82/259 (31.7%), exclusively on the left side in 99/259 (38.2%) and bilaterally in 78/259 (30.1%) patients. A total of 1,117 veins showed acute thrombosis, with a mean number of 0.79 ± 4.45 (median: 8.88; range: 0–26) thrombosed veins per patient. Nine hundred forty-seven out of 1,117 (84.7%) thrombosed veins were found in patients having PE at CTPA. Venous thrombosis involved 45/1117 (4.0%) superficial veins and 1,072/1,117
Subsets of patients with n (%) acute DVT (total=259) Age > 50 years
210 (81.1%)
Single segmental location of venous thrombosis Ninety-six out of 259 (37.0%) patients with venous thrombosis had involvement of a single segmental vein. Seven out of 96 (7.3%) single segmental thromboses were located above the inguinal ligament, 17/96 (17.7%) between inguinal ligament and knee, 69/96 (71.9%) below knee, including 3/96 (3.1%) located in a superficial vein. Distribution of single segmental venous thrombosis, with respect to diagnosis of PE, is listed in Table 4. The percentage of single segmental thrombosis was lower in patients with PE compared to patients without PE for common iliac, circumflex, deep femoral, sural, posterior tibial and greater saphenous veins, and higher for the other veins.
Discussion The incremental value of indirect CT venography Our study showed that the mean incremental value (17.3%) (95%; CI: 13.4–21.9%) of adding CT venography to CTPA is within values (8–27%) reported in the literature (10, 13–17, 25). We found a high percentage of venous thrombosis (74%) (95% CI; 68.7–79.1%) on CT venography in patients having PE at CTPA. Despite being higher than the mean value of 36% (95% CI; 22–52%) reported in a meta-analysis by van Rossum et al. (26), our results are in agreement with recent studies reporting percentage of venous thrombosis in a range of 66 to 83%, when using ascending venography (27, 28), US (29) or indirect CT ve-
n (%) within the subgroup Above-inguinal ligament
Between inguinal ligament and knee
Below-knee
21 (10.0%)
68 (32.4%)
121 (57.6%)
Outpatients
201 (77.6%)
16 (8.0%)
66 (32.8%)
119 (59.2%)
Female gender
132 (50.9%)
23 (17.4%)
40 (30.3%)
69 (52.3%)
Immobilisation
129 (49.8%)
9 (7.0%)
15 (11.6%)
105 (81.4%)
81 (31.3%)
2 (2.5%)
18 (22.2%)
61 (75.3%)
Cancer
59 (22.8%)
25 (42.4%)
19 (32.2%)
15 (25.4%)
Recent surgery
60 (23.2%)
2 (3.3%)
12 (20.0%)
46 (76.7%)
Heart failure
Previous VTE
55 (21.2%)
2 (3.6%)
29 (52.7%)
24 (43.6%)
COPD
54 (20.8%)
4 (7.4%)
29 (53.7%)
21/54
Trauma
32 (12.3%)
4 (12.5%)
12 (37.5%)
16 (50.0%)
Procoagulant disorder
16 (6.2%)
6 (37.5%)
3 (18.7%)
7 (43.7%)
Estrogen use (women)
12 (4.6%)
4 (33.3%)
5 (41.7%)
3 (25.0%)
568
Table 2 : Epidemiological and clinical subsets related to acute deep venous thrombosis risk factors in 259 patients, with distribution. Between parentheses are the percentages. COPD = Chronic obstructive pulmonary disease; VTE = venous thromboembolism; DVT = deep venous thrombosis.
Nchimi et al. Distribution of venous thrombosis in patients suspected of PE Table 3: Odds ratio (OR) for pulmonary embolism with distribution of venous thrombosis in a per segment basis. CI = confidence interval; CT = computed tomography; IVC = inferior vena cava; PE = pulmonary embolism. All patients (n = 259) n (%)
95% CI
Patients with PE (n = 198)
Patients without PE (n = 61)
n (%)
n (%)
IVC
12 (4.6)
2.4–8.0
11 (5.5)
Renal
1 (0.4)
0.01–2.1
1 (0.5)
Spermatic / Ovarian Common iliac
1 (0.4)
0.01–2.1
1 (0.5)
19 (7.3)
4.5–11.2
16 (8.1)
Mean OR (95% CI)
1 (1.6)
3.53 (0.45–27.91)
0 (0)
0.93 (0.04–23.12)
0 (0)
0.93 (0.04–23.12)
3 (4.9)
1.7 (0.48–6.04)
External iliac
19 (7.3)
4.5–11.2
15 (7.6)
4 (6.5)
1.17 (0.37–3,67)
Internal iliac
17 (6.5)
3.9–10.5
14 (7.1)
3 (4.9)
1.47 (0.41–5.29)
Circumflex
1 (0.4)
0.01–2.1
56 (21.6)
16.8–27.1
Superficial femoral
75 (29.0)
23.5–34.9
68 (34.3)
7 (11.5)
4.04 (1.74–9.36)
Deep femoral
29 (11.2)
7.6–15.7
24 (12.1)
5 (8.2)
1.54 (0.56–4.23)
102 (39.4)
33.4–45.6
98 (49.5)
3 (4.9)
18.95 (5.74–62.51)
Common femoral
Popliteal
0 (0) 50 (25.2)
1 (1.6)
0.1 (0–2.49)
6 (9.8)
3.1 (1.26–7.64)
Gastrocnemian
33 (12.7)
8.9–17.4
18 (9.1)
15 (24.6)
0.31 (0.15–0.66)
Tibio-peroneal trunk
98 (37.8)
31.9–44.1
93 (47.0)
5 (8.2)
9.92 (3.81–25.82)
41.3–53.8
105 (53.0)
18 (29.5)
2.7 (1.46–5.00)
47 (23.7)
9 (14.7)
1.8 (0.83–3.93)
Peroneal Posterior tibial Anterior tibial
123 (47.5) 58 (22.4)
17.5–28
4 (1.5)
0.4–3.9
4 (2.0)
Sural
75 (29.0)
23.5–34.9
56 (28.3)
0 (0) 19 (31.1)
2.85 (0.15–53.69) 0.87 (0.47–1.62)
Greater saphenous
21 (8.1)
5.1–12.1
19 (9.6)
3 (4.9)
2.05 (0.59–7.18)
Lesser saphenous
5 (1.9)
0.6–4.5
4 (2.0)
1 (1.6)
1.24 (0.14–11.31)
Other superficial
14 (5.4)
3.0–8.9
5 (2.5)
9 (14.7)
0.15 (0.05–0.47)
nography (9). Since we excluded patients with documented venous thrombosis prior to CTPA to avoid selection bias, our results confirm that patients suspected of VTE require lower limb vein investigation if the assessment of pulmonary arteries fails to show PE. Distribution of venous thrombosis at indirect CT venography Distribution of venous thrombosis has been studied with US and ascending venography (21, 22, 30–35) which are considered as the current reference standards in the workup of DVT. However, both techniques have difficulties in demonstrating thrombosis located in calf, deep femoral, pelvic and abdominal veins, as well as located in anatomical variants. Unsurprisingly, in most of the studies comparing indirect CT venography to the US or ascending venography, the inter-technique agreement was low in these areas, with venous filling defects at indirect CT venography considered as “false-positive” findings (24, 36). Whether these disagreements resulted from pitfalls of the standard examinations rather than inaccuracy of indirect CT venography is of concern from a clinical point of view. The relevance of our study –which is, to the best of our knowledge, the first investigating distribution of venous thrombosis with indirect CT venography on a venous segment basis from the ankle to the diaphragm– lies in the potential challenge of indirect CT venography to the current
standards. In our series, venous thrombosis extended above the inguinal ligament in 15% of the patients, with internal iliac vein thrombosis occurring in 6.5% of patients with venous thrombosis and accounting for 4.2% of patients with single segmental venous thrombosis. Furthermore, this is an important issue considering that a thrombus in a pelvic location could likely be large and result in massive pulmonary emboli. On the other hand, intra-abdominal veins, such as the inferior vena cava, renal and ovarian veins, respectively, accounted for less than 5% of the thrombosed veins and never occurred as a single segmental vein thrombosis. In order to limit radiation exposure, the upper limit of the scanning should therefore be the level of the iliac crests, particularly if there is no venous thrombosis evidenced at a more lower level. One of the most debated issues in the literature is investigation of veins located below the knee, i.e. the percentage of calf vein thrombosis. In our series, in nearly half of patients with venous thrombosis (48%) the upper end of the thrombus was located between the ankle and the popliteal vein, including commonly the peroneal, posterior tibial and sural veins, which are known to be the source of deep venous thrombosis due to slow flow and larger number of valves in these veins compared to the other calf veins (30–32, 34). This high percentage of below-knee thrombosis suggests that the caudal scanning limit for CT venography should include the whole calves. Other published
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Figure 1 : Histogram of distribution of venous thrombosis in 256 patients. White columns = patients without pulmonary embolism (PE); grey columns = patients with PE.
series, in which imaging of the veins located below and above the knee was included, also found such a high but broad range of percentage (40–70%) of caudally-located vein thrombosis (30–33). Specific etiologic factors may explain this broad range. Similar to literature, our results show that conditions such as malignancy, estrogen use and procoagulant disorders are mainly associated with proximal thrombosis, whereas thrombosis related to recent surgery, trauma or immobilization tends to be more caudally located (31). Similarly, the presence or absence of clinical symptoms of venous thrombosis may also be a determinant factor, since proximal thrombosis is more commonly symptomatic than distal thrombosis (30, 33). Our series is likely to represent a realistic distribution of venous clots in patients presenting with symptoms of PE, as we have excluded patients who presented with PE as a complication of a previously documented venous thrombosis. Clinical implications of the scanning range Thirty seven percent of patients in our study had thrombosis involving a single venous segment. This rate is in accordance with the study of Kerr et al., who reported an incidence of 40% of
single segmental venous thrombosis when using ascending venography (32). In our study, 76% of single segment thrombi were located below knee. In the study by Rose et al. this rate was even higher (96%) (30). This is of clinical importance since, in our series, excluding scanning of the calves would have resulted in overlooking 125/259 (48%) of patients with venous thrombosis and, more importantly, 38/57 (67%) of patients with venous thrombosis but without PE. We speculate that the use of limited US scanning (i.e. confined to femoral and popliteal veins) as proposed for clinical routine in patients suspected of PE (35, 37) would have resulted in overlooking 81/96 (84%) patients with a single segmental venous thrombosis. Elias et al. (38) recently found a higher sensitivity for complete US scanning compared to limited US. In how far these patients require anticoagulation is another matter of debate. However, there is growing evidence that, if left untreated, patients with calf venous thrombosis should undergo at least one or more follow-up US examination(s) to exclude possible cephalic extension that may occur in 20–30% (39, 40). Patients with calf venous thrombosis also have a 20–30% risk of subsequent PE, which may be clinically relevant in patients with limited cardio-pulmonary reserve (41).
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Nchimi et al. Distribution of venous thrombosis in patients suspected of PE Table 4: Distribution of venous thrombosis in 96 patients with single segmental venous thrombosis. CI = confidence interval; CT = computed tomography; IVC = inferior vena cava; NA = not applicable.
All patients (n = 96) n (%)
95% CI
Total above inguinal ligament
7 (7.3)
IVC
0 (0)
Renal Spermatic / Ovarian
3.0–14.5
Patients with PE (n = 51)
Patients without PE (n = 45)
n (%)
n (%)
5 (9.8)
2 (4.4)
NA
0 (0)
0 (0)
0 (0)
NA
0 (0)
0 (0)
0 (0)
NA
0 (0)
0 (0)
Common iliac
2 (2.1)
0.3–7.3
1 (2.0)
1 (2.2)
External iliac
1 (1.0)
0.03–5.7
1 (2.0)
0 (0)
Internal iliac Total between inguinal ligament and knee Circumflex
4 (4.2)
1.2–10.3
3 (5.9)
1 (2.2)
17 (17.7)
10.7–26.8
13 (25.5)
4 (8.9)
1 (1.0)
0.03–5.7
0 (0)
1 (2.2)
Common femoral
4 (4.2)
1.2–10.3
4 (7.8)
0 (0)
Superficial femoral
7 (7.3)
3.0–14.5
5 (9.8)
2 (4.4)
Deep femoral
1 (1.0)
0.03–5.7
0 (0)
1 (2.2)
Popliteal
4 (4.2)
1.2–10.3
4 (7.8)
0 (0)
Total below-knee
69 (71.9)
61.8–80.6
31 (60.8)
38 (84.4)
Gastrocnemian
11 (11.4)
5.9–19.6
1 (2.0)
10 (22.2)
3 (3.1)
0.7–8.9
2 (3.9)
1 (2.2)
26 (27.1)
18.5–37.1
16 (31.4)
10 (22.2)
6 (6.2)
2.3–13.1
3 (5.9)
3 (6.7)
Tibio-peroneal trunk Peroneal Posterior tibial Anterior tibial Sural
0 (0) 23
Greater saphenous
2 (2.1)
Lesser saphenous
0 (0)
Other superficial
1 (1.0)
Thus, in a clinical perspective, any attempt to limit scanning range of venous segments to the femoro-popliteal level, i.e. below the pelvis and above the calves, would result in a significant reduction of diagnostic sensitivity for VTE. Limitations The retrospective nature of our study may undoubtedly lead to unintentional selection bias. The study design also prevents intra- and inter-observer variation analysis, a priori clinical probability of VTE or three-month follow-up assessment in the patient group. Evaluating the data of a multidetector scanner together with sequential scanning protocols is another speculative limitation on this study, since differences between both technologies may lead to differences in sensitivity of CT venography. However, with the sequential scanning parameters we used (i.e. 5 mm thickness and 20 mm interslice gap), the risk of missing DVT is extremely low (12). On the other hand, in a separate study evaluating differences between a single detector and a multidetector scanner, we found no significant difference in the percentage of DVT (42).
NA
0 (0)
15.8–33.8
9 (17.6)
14 (31.1)
0.3–7.3
1 (2.0)
1 (2.2)
0 (0)
0 (0)
1 (2.0)
0 (0)
NA 0.03–5.7
0 (0)
Radiation dose Radiation dose has been a growing concern over the last decade, and recent studies reported median cumulative effective doses of 8.3–9.3 mSv and median effective gonadal doses in the range of 3.4–4.4 mSv related to CT venography (43–45). Nevertheless, VTE is a disease that concerns mainly elderly patients, as confirmed by the mean age over to 60 years in our and most other study populations. Furthermore, as a rule, we do not perform CT venography in patients below 40 years of age, unless a previous US of the lower limb has been inconclusive. Dose modulation systems, minimizing the dose required without compromise in image quality, are currently developed by all manufacturers and could reduce the dose by 35 to 60% (46). Conclusion The addition of indirect CT venography to CTPA is useful in patients suspected of PE. In our study, it resulted in a 17% increase in VTE diagnosis. Scanning range should extent from calf to iliac crests in patients presenting with suspicion of VTE.
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Nchimi et al. Distribution of venous thrombosis in patients suspected of PE
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