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Department of Radiology Aga Khan University Hospital Karachi,
Pakistan
Medical College Aga Khan University Hospital Karachi,
Pakistan
We read with interest the article titled "Helical CT for the Evaluation of Acute Pulmonary Embolism" by Patel and Kazerooni [1]. While finding the recommendation of the combined usage of CT pulmonary angiography (CTPA) and CT venography as a "one-stop-shopping" test for ruling out pulmonary embolism as most interesting and indeed justifiable, we believe there is room for some comments.
Recent trials have indeed established the role of spiral CT as a rapid, cost-effective, widely available, and noninvasive technique to safely rule out acute pulmonary embolism. The advent of MDCT, with its ability to visualize smaller subsegmental emboli that may be missed with single-slice technology, and increasing familiarity with the technique may ascertain its place globally as the primary screening tool for the detection of emboli to the central and peripheral pulmonary vasculature.
In comparison with V/Q scintigraphy, the authors quote greater accuracy rates of CTPA in detection of acute pulmonary embolism as a reasonable justification for a substitution in the conventional diagnostic algorithm. However, in most of the comparative studies to date, conventional perfusion imaging has been evaluated versus modernday tomographic scanners, yielding somewhat inconsistent and prejudiced results. An examination with V/Q lung scans in SPECT technique using an ultrafine aerosol [2] showed excellent results with diagnostic accuracy comparable to MDCT, with SPECT showing a superior sensitivity. Similarly, the implementation of digital subtraction technique in catheter pulmonary angiography has led to increased diagnostic yield. Usage of imaging advancements in these conventional screening techniques may tilt the balance against CT, in terms of the scientific advance promised by the latter. These concerns need to be addressed in order to prevent deterrence of the universal acceptance of CTPA as the primary screening tool in acute pulmonary embolism.
Helical CT, however, as the authors mention, holds several other advantages over contemporary techniques, including identification of other pathologies of the thorax and excluding differential diagnoses that may mimic pulmonary embolism in clinical presentation. Right ventricular failure is known as the most significant determinant of mortality in acute pulmonary embolism patients. CT is useful for risk stratification via accurate identification of right ventricular dysfunction [3] and, uninfluenced by increasing patient age [4] and preexisting pulmonary pathology or pulmonary embolism (unlike V/Q scanning), it therefore may be the most important predictor of patient outcome among existing investigations.
When patient safety is argued in diagnostic radiology, debate often centers on adequate radiation exposure. Even though the advent of MDCT may result in incurring of a higher effective dose, as the authors mention, it is pertinent to observe that this is compensated by the noteworthy reduction in the number of nondiagnostic and total number of investigations found to be required per patient [5].
While PIOPED II [6] may indeed establish helical CT and venous-phase CT as first-line studies in patient evaluation, the need is for global availability and familiarity with the above-mentioned techniques in order to counter the fatality rate of one of the most common causes of cardiovascular death, acute pulmonary embolism.
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