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Editorial |
1 From the Mallinckrodt Institute of Radiology, 510 S Kingshighway Blvd, St Louis, MO 63110. Received February 21, 2000; accepted March 1. Address correspondence to the author (e-mail: woodardp@mir.wustl.edu).
Index terms: Computed tomography (CT), helical, 60.12115, 944.12915 • Editorials • Embolism, pulmonary, 60.72, 944.77 • Lung, perfusion, 60.12171 • Lung, ventilation, 60.12176 • Pulmonary arteries, CT, 944.12912, 944.12915, 944.12916
Choosing which "road" to follow from the myriad possibilities of imaging examinations for the detection of pulmonary thromboembolic disease can be confusing for clinicians and radiologists alike. Our clinical colleagues want to know not only which examination to order, but also when to order it and what to do with the results. This is especially true with regard to helical computed tomography (CT). Most clinicians are intrigued by the possibility of an examination that enables the diagnosis of pulmonary embolism (PE) and is both noninvasive and more definitive than ventilation-perfusion (V-P) scintigraphy. However, it is stated in the literature (1), and most clinicians are aware, that isolated subsegmental emboli may be missed at helical CT. Some experts suggest that small peripheral emboli are not clinically important (2). Others paint cautionary tales of how a peripheral embolus may have a substantially negative effect on a patient with poor cardiopulmonary reserve (3,4). That subsegmental emboli might be missed at helical CT, combined with the controversy regarding their clinical importance, leaves many of our clinical colleagues concerned. Can helical CT scans that are negative for PE be trusted? Although researchers have published numerous studies on the accuracy of helical CT for the diagnosis of PE, patient outcome has been assessed in only a few studies (5,6).
In this issue of Radiology, Goodman et al (7) present a prospective study in which the outcomes of 198 patients with PE-negative helical CT scans were assessed and the results were compared with those in a similar cadre of 350 patients with negative or low-probability V-P scans. Goodman et al followed up all patients for 3 months: Each patient or his or her family was called at 1 and 3 months, and subsequent imaging studies, clinical records, autopsy reports, and death certificates were assessed.
The results showed subsequent PE in two (1.0%) of the 198 patients in the CT group and in five (3.1%) of the 162 patients in the low-probability V-P scan group. There was no subsequent PE in the patients (n = 188) with V-P scans that were interpreted to be normal. These results are quite helpful to those of us who provide advice to our clinical colleagues. First, with these findings, we are assured that patients with PE-negative helical CT scans do well without anticoagulation therapy. This should be especially true when these patients also have a lower extremity deep venous study that is negative for thrombus. Second, the results provide us with some guidelines as to which types of patients to refer for each modality. For instance, because patients with a normal V-P scan are unlikely to have subsequent PE, those patients who potentially could have a normal V-P scan should be referred for scintigraphy. Likewise, those patients who certainly would have an indeterminate V-P scan are the most likely to derive diagnostic benefit from contrast material–enhanced helical CT. The diagnostic algorithm supported by the results of Goodman et al (7) is a two-armed approach with different pathways based on the presence or absence of lung disease.
Like Goodman et al, we at the Mallinckrodt Institute of Radiology recommend that outpatients who have a normal chest radiograph and no history of lung disease undergo V-P scintigraphy as the first imaging examination in the diagnostic work-up for PE. This is because V-P scintigraphy, although not highly specific for the detection of PE, is highly sensitive for the detection of perfusion deficits. By limiting the patients who undergo V-P scanning to those without demonstrable lung disease at chest radiography, we reduce the number of indeterminate studies and select a group of patients whose V-P scans are likely to show normal or high-probability results. If the V-P scan results are normal or high probability for PE, then we suggest no further testing. For patients whose V-P scans are read as low probability or indeterminate for PE, we suggest further work-up, beginning with a lower extremity Doppler ultrasonographic (US) examination.
For patients with known or radiographically demonstrable pulmonary disease or for inpatients (because of the high likelihood of a confounding pulmonary pathologic entity), we recommend that the first examination for the possible diagnosis of PE be contrast-enhanced helical CT. If the study is positive, then we suggest no further testing. If the helical CT scan is negative for PE, then we suggest lower extremity Doppler US. We recently have begun performing a helical CT examination that includes assessment of the pulmonary arteries and the pelvic and lower extremity deep venous system. CT has the advantage over lower extremity US in that it can enable assessment of veins above the inguinal ligaments and thus provide added information about the absence or presence of pelvic vein thrombus (8,9). Like Goodman et al (7), at my institution we believe that the clinician can safely withhold anticoagulation therapy if the helical CT scan is negative for PE and a lower extremity examination—either Doppler US or CT—reveals no deep venous thrombus.
As noted in the Goodman et al article, their study, which was performed during a 2-year period, was begun before the availability of multidetector helical CT systems. These systems can reduce collimation from the 3.00 mm obtained with the single-detector helical scanner to 1.00–1.25 mm while maintaining or improving the scanning speed. This, along with improved software for better cine viewing and multiplanar image reconstruction, portends improved subsegmental vessel and PE detectability. Indeed, single-detector helical systems will eventually become outdated.
That said, Goodman et al (7), in their article, have provided information that lays a foundation for the future. They have provided data that yield some guidance regarding how helical CT can be used in the diagnosis of PE and, most important, provided an answer to the often asked question, "If the CT scan is negative for PE, then where do we go from here?"
Footnotes
See also the article by Goodman et al (pp 535–542 ) in this issue.
Abbreviations: PE = pulmonary embolism V-P = ventilation-perfusion
References
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