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Department of Radiology, University Hospitals of Cleveland, 11100 Euclid Ave., Cleveland, OH 44106.
Received May 14, 2004;
accepted after revision August 16, 2004.
Address correspondence to J. D. Prologo
(jdprologo{at}hotmail.com).
Abstract
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SUBJECTS AND METHODS. Two cohorts of patients undergoing CT for suspected PE with either single-detector CT (3-mm collimation and pitch of 1.7) or MDCT (2-mm collimation and pitch of 1) scanners were prospectively observed and compared using predefined criteria for evidence of subsequent thromboembolic disease during the 6 months after the acquisition of their initial scan.
RESULTS. Ninety-eight patients were scanned using a single-detector
CT scanner. Of these, none had evidence of subsequent PE or deep venous
thrombosis (DVT), and six (6.1%) died of unrelated causes. Of the 100 patients
scanned using an MDCT scanner, one (1.0%) had a subsequent nonfatal PE 2
months after the initial scanning, one (1.0%) had DVT 1 month after the
initial scanning, and eight (8.0%) died of unrelated causes. No significant
difference was found in either the probability of subsequent thromboembolic
events (
2 = 0.3183, degrees of freedom [df] = 1,
p = 1) or frequency of unrelated deaths (2 = 0.2655,
df = 1, p = 0.7829) between patients scanned using
single-detector CT or MDCT protocols.
CONCLUSION. Our results show that patients with suspected acute PE and negative CT results have acceptable clinical outcomes in the absence of anticoagulation treatment up to 6 months after acquisition of their initial scan. Furthermore, we found that the increased visualization of smaller, more peripheral arteries afforded by multislice technology did not affect clinical outcome.
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The development of helical CT technology provided a rapid, noninvasive way to evaluate the pulmonary vasculature and led to widespread adjustment of the radiologic contribution to evaluation of suspected PE [5-9]. Early work with single-detector scanners showed similar sensitivity and specificity of CT pulmonary angiography (CTPA) with regard to PE detection when compared with nuclear ventilation-perfusion scanning and conventional invasive pulmonary angiography, particularly in the proximal arteries [10-13] (Fig. 1). The development of MDCT resulted in faster scans with improved contrast enhancement, thinner collimation, and subsequent improvement in visualization of the peripheral arterial tree [14-18]. The ability of CTPA to detect concurrent or mimicking disease further expands its diagnostic utility, and recent studies have shown acceptable clinical outcomes for untreated patients after a CTPA examination with negatively interpreted results [19-25].
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These attributes, plus its efficiency and increasing availability, have led to a widespread increase in the use of CTPA as a first-line imaging technique in patients with suspected cardiothoracic disease [26]. To our knowledge, no study has analyzed the effect of improved ability to interrogate distal vasculature afforded by MDCT on clinical outcome. The purpose of this study is to compare the clinical outcomes of patients in whom PE was ruled out with single-detector CT versus MDCT.
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All patients underwent scanning of the entire chest in a cephalocaudal direction during a single breath-hold after the administration of IV contrast material (iohexol, Omnipaque 300, Nycomed Imaging). For single-detector CT examinations, 140 mL of contrast material was injected at a fixed rate of 2 mL/sec, with an average image acquisition time of 35 sec (3-mm collimation, pitch of 1.7, 120 kV, 250 mA). For examinations performed with MDCT, 90 mL of contrast material was injected at a rate of 4 mL/sec, with an average image acquisition time of 16 sec (2-mm collimation, pitch of 1, 120 kV, 250 mA). Saline flushes were not used in either group.
Patients with negatively interpreted examination results were systematically followed up for evidence of subsequent thromboembolic disease (PE or deep venous thrombosis [DVT]) up to 6 months after acquisition of their initial scan. Specifically, radiographic reports and discharge summaries were evaluated for positive results on other diagnostic studies or the institution of anticoagulation treatment. Also, for those patients who could not be definitively identified as having a clear thromboembolic or life-ending event via report surveillance, a telephone survey was used with the goal of identifying the aforementioned clinical parameters at 3 and 6 months. The survey incorporated a standard questionnaire written using lay terms that included items regarding cause and location of death if the patient died, potential subsequent hospitalizations (including discharge diagnoses, dates, and locations), new diagnoses (e.g., "blood clots" or "thrombus"), and new medications (e.g.,"blood thinners"). The incidence of subsequent thromboembolic events in the two groups was compared using chi-square analysis.
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Of the 98 patients with negative examination results from the
single-detector CT scanner, none had evidence of subsequent PE or DVT, and six
(6.1%) died of unrelated causes. Of the 100 patients with negative results on
MDCT examination, one (1.0%) had a subsequent nonfatal PE 2 months after the
initial examination, one (1.0%) had DVT 1 month after the examination, and
eight (8.0%) died of unrelated causes. There was no significant difference in
either the probability of subsequent thromboembolic events (2
= 0.3183, degrees of freedom [df] = 1, p = 1) or frequency
of unrelated deaths (2 = 0.2655, df = 1, p =
0.7829) between patients scanned using single-detector CT or MDCT
protocols.
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An increased radiation dose is inherently delivered to the patient with MDCT because the width of the X-ray beam exceeds that of the detectors, a well-known dose inefficiency related to the maintenance of uniform section sensitivity profiles with multiple detector rows [27, 29]. Additional increases in patient radiation dose may be experienced when improved signal-to-noise ratios are sought with higher tube currents [29]. No such adjustments were made for this study.
Our results show that patients with suspected acute PE and negative CTPA findings have acceptable clinical outcomes in the absence of treatment or anticoagulation therapy for as long as 6 months. These results are consistent with recent reports regarding outcome after negative results on CTPA [22-25] and are comparable to reports of recurrent thromboembolic disease after negative results on conventional pulmonary angiography [30-32]. In addition, there was no advantage to MDCT with regard to clinical outcome. That is, the increased visualization of subsegmental arteries afforded by MDCT technology did not affect the negative predictive value of our CTPA examination.
It is likely that small distal emboli were missed in the group who underwent CTPA with single-detector CT technique. Oser et al. [33] showed that 30% of pulmonary emboli diagnosed with conventional pulmonary angiography would have been missed with CTPA techniques limited to segmental and larger artery evaluation. The similar clinical outcomes of patients examined with single-detector CT and MDCT protocols in this study suggest that the presence of a subsegmental clot not visualized on single-detector CT technique may not have significant clinical impact. This is an important question because the use of CTPA continues to increase coincident with technologic advances that afford better visualization of smaller clots [14-16, 26].
The extremes of clot burden on the pulmonary vasculature are related to clinical presentation and course. Smaller clots are known to have less dramatic clinical manifestations and lower mortality and morbidity rates [34, 35]. Large clots resulting in right heart strain or hypotension have worse clinical outcomes than smaller clots without these associations [36-38], and it has recently been suggested that indexes of pulmonary embolic obstruction as quantified with CT are predictive of patient survival [39].
The risks associated with anticoagulation are well documented, and protocols involving the long-term use of anticoagulation therapy are currently being re-examined [40]. For example, a recent study found a 10 times higher risk of hemorrhagic stroke in patients receiving oral anticoagulant therapy when compared with the general population [41]. In the future, as the rate of CTPA-detected subsegmental PE increases, clinicians will need to consider the risk-benefit ratio of anticoagulation in patients with previously undetectable small clots in the peripheral pulmonary vasculature.
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