Postprocessing Techniques

Postprocessing techniques (multiplanar reformation, maximum intensity projection, and virtual angioscopy) are not used routinely in the diagnosis of PE. When transverse sections provide a confident analysis of all the pulmonary arteries, 2D and 3D reformations provide no additional information. Sometimes an in-plane or oblique arterial branch or an area of linear perivascular soft tissue may generate an area of low density that may be confused with PE in the axial plane. This artifact is due to partial volume effect between lung parenchyma or perivascular lymphatic tissue and the contrast enhanced pulmo nary artery. The vessels that most often run in the axial plane are the right middle lobe and lingular arteries, the anterior segmental arteries of the upper lobes, and the superior segmental arteries of the lower lobes [25,26]. In these cases, the vessels are displayed better by multiplanar reformations (MPRs) that use data obtained from a contiguous helical data set (Figs. 7 and 8). The images may be reformatted in any anatomical plane (coronal, sagittal, oblique, or curvilinear view). Virtual endoscopy provides a direct view of the intraluminal clot. Its clinical utility has yet to be determined.

Another potential application of MPRs is the evaluation of chronic thromboembolic disease. The creation of MPRs through the longitudinal axis of obliquely oriented vessels can overcome some of the difficulties encountered with axial images in the identification of focal arterial stenoses and webs or in separating eccentric, adherent thrombus from adjacent lymphatic tissue [27].

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