New tools could further enhance the diagnostic performance of MDCT in the diagnosis of PE. Earlier studies have evaluated the feasibility and performance of CAD tools for the automated detection of segmental and subsegmental pulmonary emboli. Schoepf et al65 have tested 1 such CAD tool (ImageChecker CT, R2 Technology, Inc) in 23 patients with PE and 13 patients without PE. Data were collected with a 16-slice CT. The performance of the CAD tool for the detection of emboli in the segmental and subsegmental pulmonary arterial tree was assessed. A consensus reading of 2 experienced radiologists revealed 130 segmental pulmonary emboli and 107 subsegmental pulmonary emboli in the 23 patients with PE. All 23 patients with PE were correctly identified as having PE by the CAD system. In vessel-by-vessel analysis, the sensitivity of the CAD algorithm was 92% (119/130) for the detection of segmental pulmonary emboli and 90% (92/107) for subsegmental pulmonary emboli. The overall specificity, positive predictive value, and negative predictive value of the algorithm were 89.9%, 63.2%, and 97.7%, respectively. The average false-positive rate of the CAD algorithm was 4.8 (range, 1 to 9) false-positive detection marks per case. The investigators concluded that CAD of segmental and subsegmental pulmonary emboli based on 1-mm multidetector-row CT studies is feasible. Application of CAD tools may improve the diagnostic accuracy and decrease the interpretation time of CTA for the detection of pulmonary emboli in the peripheral arterial tree and further enhance the acceptance of this test as the first-line diagnostic modality for suspected PE.
In the evaluation of lung function, ventilation assessment is an important component. Presently, ventilation imaging is mainly realized using nuclear medicine methods33 or MRI with the inhalation of polarized noble gases, that is, 3Helium or 129Xenon,66 or gadolinium chelate aerosols.67 A drawback of these methods is the rather limited morphologic information in scintigraphy or SPECT, as well as restricted spatial resolution and lack of information on the pulmonary microstructure in both MRI and nuclear medicine imaging. Whenever high-resolution morphologic information on structural changes of the lung parenchyma is needed, MDCT is the first-line imaging modality that can provide this important morphologic information, applying the so-called high-resolution protocols. In combination with DECT, comprehensive imaging for morphology, angiography, perfusion, and ventilation could become feasible in PE patients. The inert gas xenon has x-ray absorption characteristics that resemble those of iodine and can therefore serve as an inhalative contrast agent for CT ventilation imaging.68,69 Although there have been trials in the past decades to use stable xenon gas for CT ventilation imaging, this method has so far not been used in clinical care. These earlier approaches were based on sequential chest scans, implying an increased patient dose and potential misregistrations because of varying levels of inspiration.70 With dual-source scanners, DECT now has the potential to map xenon distribution patterns by directly visualizing the inhaled xenon gas. Chae et al69 performed xenon-enhanced DECT in 12 patients at an inspiratory xenon concentration of 30%, and showed the technical feasibility of DECT ventilation imaging. Using DECT with inhalation of xenon and intravenous iodine administration, a comprehensive assessment of pulmonary morphology and function, including both ventilation and perfusion imaging, becomes possible with CT. Future studies will deal with the use of DE ventilation imaging in combination with DE perfusion mapping in patients with pulmonary functional impairment, for example, after PE, or in cases of worsening of the gas exchange during intensive care treatment, to evaluate the feasibility of a comprehensive diagnostic evaluation including ventilation, perfusion, morphology, and structure of the parenchyma. However, to differentiate perfusion from ventilation information, 2 DECT scans would have to be performed. The first scan with inhaled xenon, and the second scan after intravenous administration of iodinated contrast material, will have to be performed to map the parenchymal distribution of iodine and to correlate changes in ventilation and perfusion with structural or vascular abnormalities. As inhaled xenon can have anesthetic properties at higher concentrations,25,26 future studies will also have to deal with the optimal inspiratory concentration of the inhaled xenon.
As outlined above, MDCT, and especially DSCT, will remain the most powerful and clinically most important tool in the evaluation of PE patients. Routine MDCT pulmonary angiography offers an accurate and quick depiction of the pulmonary clots, and this method is available nearly everywhere. In addition, MDCT can provide crucial information on the hemodynamic stability and prognosis of a patient suffering from acute PE. New acquisition strategies, such as dual-energy techniques (perfusion imaging, ventilation imaging, direct thrombus imaging) and very fast and low-dose acquisition protocols such as the second-generation DSCT high-pitch chest pain protocol, will further strengthen this position.
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