The Incidence of Subsegmental Pulmonary Emboli in Multi-Detector Row and Single-Detector Row CT

Introduction The anatomic distribution of pulmonary embolism (PE) in central, segmental and subsegmental arteries is understudied and an often-debated issue is the possible limitation of computed tomography (CT) to accurately detect peripheral emboli. Multi-detector row CT (MDCT) is thought to increase the detection rate of sub-segmental emboli compared to single-detector row CT (SDCT). We evaluated the prevalence and anatomic distribution of PE in consecutive patients with proven PE diagnosed by MDCT or SDCT. Methods Data were obtained from a diagnostic outcome study of patients suspected of PE. An algorithm consisting of sequential application of a clinical decision rule, a quantitative Ddimer test and singleor multi-detector row CT diagnosed PE. The location of PE was classified into three groups (central, segmental and sub-segmental PE) with emphasis on the largest pulmonary arterial branch involved. Results A total of 3306 consecutive patients were included in the diagnostic study, of whom 674 (20%) were diagnosed with PE. Data regarding the localisation of PE were missing in 41 patients. Localisation of PE in MDCT was central in 160 patients (29%, 95%CI: 25-33), segmental in 293 patients (53%, 95%CI: 49-57) and sub-segmental in 98 patients (18%, 95%CI: 15-21). In patients diagnosed with SDCT, PE was central in 31 patients (38%, 95%CI: 27-49), segmental in 39 patients (48%, 95%CI: 36-59) and sub-segmental in 12 patients (15%, 95%CI: 8-24). The percentage of detected PE did not differ significantly between MDCT and SDCT (31% vs. 32%, p=0.65), neither the percentage of subsegmental PE (18% vs. 15%, p=0.48) detected by MDCT or SDCT. Conclusions The percentage of detected subsegmental PE did not differ between MDCT and SDCT. There seems to be no danger of over-diagnosis of small subsegmental PE using multidetector row systems. Nijkeuter_V4.indd 66 02-05-2007 15:09:37 67 The Incidence of Subsegmental Pulmonary Emboli in Multi-Detector Row and Single-Detector Row CT C ha p te r 5 Introduction Over the past years the role of contrast material-enhanced computed tomography (CT) in the diagnosis of pulmonary emboli (PE) at the level of segmental and central pulmonary arteries is well established; hence this method has become the first line diagnostic test for the evaluation of PE in many institutions1-3. The reliability of CT in the detection of smaller emboli in subsegmental arteries has been the subject of debate however4;5. Within the past years, multi-detector row CT (MDCT) has been introduced. The most prominent feature of MDCT is its high-speed acquisition, enabling quick coverage of large volumes, and improved spatial resolution. This should theoretically result in the visualisation of more than 90% of subsegmental arteries6 . Importantly, the need for diagnosing isolated subsegmental pulmonary embolism, i.e. PE limited to subsegmental arteries, is still uncertain7. This is reflected by the discrepancy between the overall sensitivity for PE for single-detector row CT (SDCT) and the outcome studies in which patient management was based on a normal SDCT. Thus, in two large prospective studies the sensitivity of SDCT for detecting all PE has been found to be around 70%4;8 while the same research groups observed a low 1-2% three months thrombo-embolic risk in patients left untreated based on a single normal SDCT and normal lower limb compression ultrasound3;9. It has been argued that diagnosing more subsegmental PE with multi-detector row CT may therefore lead to overtreatment of small pulmonary emboli without apparent clinical need with associated risk of bleeding. Contra wise, there is also expert opinion based consensus that the presence of peripheral emboli may be an important indicator of concurrent deep vein thrombosis and thus potentially heralds more severe embolic events; this would then underscore the need to accurately diagnose subsegmental emboli10-12. However, the need for anticoagulant treatment in the presence of an isolated subsegmental pulmonary embolus has never been studied. In the PIOPED study, the proportion of PE limited to the subsegmental arteries using pulmonary angiography was 6% (95%CI: 4-9%)13. In three other studies that used pulmonary angiography, of which one was a retrospective study10, isolated subsegmental PE was observed in 10-36 % of patients10;14;15. Establishing the true prevalence of subsegmental PE is complicated by the 45-66% reported inter-observer variability for detecting emboli at the subsegmental level in pulmonary angiography16;17. Using singledetector row CT, the prevalence of subsegmental PE was 22% (29/130)18 in one study. In two other studies, using multi-detector row CT, the prevalence was 7% (14/187)2 and 15% (8/54)19. These data underline the uncertainty of establishing the prevalence of subsegmental PE and indicate that it is currently not established whether multidetector row CT is more accurate in diagnosing subsegmental PE than single-detector row CT. We analysed the distribution of central, segmental and subsegmental PE in a large cohort of consecutive patients diagnosed with PE by multior single-detector row CT as part of a large management study in patients presenting with clinically suspected PE1. Nijkeuter_V4.indd 67 02-05-2007 15:09:38