J. David Godwin

Problems and pitfalls in the evaluation of thoracic aortic dissection by computed tomography

Several conditions can lead to cither false positive or false negative diagnoses of aortic dissection by computed tomography (CT) with intravenous administration of contrast medium. Insufficient contrast enhancement may cause intimal flaps to be missed, leading to a false negative diagnosis. False positive diagnoses result when extraaortic structures (e.g. mediastinal veins. pericardium, thickened pleura, and lung) are mistaken for false channels in the aorta. Superimposition of structures in thick CT slices may cause intimal calcifications to appear displaced. Streak artifacts across the descending aorta can resemble double aortic channels or intimal flaps. Fusiform aneurysms with thrombus are often hard to distinguish from thrombosed dissections by CT as well as by aortography.

Contrast-related flow phenomena mimicking pathology on thoracic computed tomography

Flow phenomena occurring after the bolus injection of contrast material can simulate the presence of pathology on thoracic computed tomography. In a review of 50 dynamic scans performed after contrast medium injection, apparent filling defects in the superior vena cava were seen in 46. In four cases, retrograde filling of the azygos or hemiazygos veins occurred. In two cases, layering of contrast material within the descending aorta simulated dissection. In many cases, the dynamic series of scans helped in diagnosing these appearances as flow related.

CT of obstructive lobar collapse.

The varied computed tomographic (CT) appearance of obstructive lobar collapse is described in 25 cases. CT is helpful in understanding the morphology and mechanisms of lobar collapse and in diagnosing atypical cases. The final shape of the collapsed lobe is dependent on the size and location of the obstructing tumor and on the amount of retained lung fluid. Focal bulging of the fissure (S-sign) is the most helpful sign in identifying the obstructing tumor. Differential enhancement could not separate the tumor from collapsed lung in six of eight (75%) of the cases studied with intravenous contrast, a finding at variance with previous reports. The limitations of CT in evaluating mediastinal or pleural invasion in the presence of lobar collapse are discussed.

CT of localized lucent lung lesions.

OCALIZED lucent lung lesions depicted by the standard chest radiograph are relatively rare, except for those occurring in patients with generalized chronic obstructive pulmonary disease. The standard chest radiograph provides an inexpensive, sensitive means for determining the presence of most clinically significant focal, thinwalled, sharply demarcated areas of avascularity. Some of the limitations of the chest radiograph in this regard are: (a) it permits only a broad differential diagnosis; (b) it sometimes does not define the full extent of disease; (c) it sometimes does not allow localization to the precise anatomic compartment; and (d) it sometimes fails to demonstrate a small accompanying lesion adjacent to or remote from the lesion under investigation. Even with these radiographic limitations, other available data including clinical history, pulmonary function studies, and laboratory results significantly enhance the radiologist’s diagnostic accuracy. For those few but potentially life-threatening localized lucent lesions, more aggressive and risky procedures must be employed for diagnostic or therapeutic purposes, such as biopsy, chest tube insertion, or even thoracotomy. With the advent of CT, considerable improvement in the overall diagnostic accuracy of these localized lucent lung lesions has been obtained. By defining more precisely the location, margins, thickness, contents, and overall configuration, CT significantly narrows the differential diagnosis. If the CT characteristics of the lesion are nondiagnostic, interventional procedures such as bronchoscopy or percutaneous aspiration biopsy can more easily be obtained by CT guidance. The CT cross-sectional display and geometric resolution sometimes more clearly define previously recognized standard radiographic features of complicated versus uncomplicated and benign versus malignant lucent lung lesions. In addition, CT uniquely distinguishes pleural from subpleural lucent lesions. Probably the most important role CT serves is in demonstrating occult opaque or lucent lesions in proximity to the original lucency or in the contralateral lung. The presence or absence of abnormal pulmonary vascularity, adenopathy, calcification, or small pleural

Residual cavities of lung metastases following chemotherapy

At the sites of previously documented lung metastases from bladder carcinoma, computed tomography identified lung cavities that were virtually imperceptible by plain radiographs. Such thin-walled cavities, which developed following chemotherapy, have not been previously described in patients with bladder carcinoma.

CT in congenitally-corrected transposition of the great vessels

Congenitally-corrected transposition of the great vessels (CTGV) may be detected de novo in adulthood and the plain radiographic findings may be ambiguous or they may be mimicked by a mediastinal mass. CT readily shows the malposition of the aorta and pulmonary artery, and may also show associated congenital heart lesions. The following cases demonstrate the CT findings in CTGV and the distinction of CTGV from conditions resembling it on radiographs.

Computed tomographic evaluation of lesions in the thoracic apex

Fifty patients with clinically suspected lesions of the chest apex underwent computed tomography (CT) and plain film (PF) examinations in an attempt to define the underlying etiology as benign or malignant. CT was slightly more sensitive in diagnosing malignancy (90%) compared with PF (77%), with a higher predictive value of a negative test (85% vs. 70%), although the differences were not statistically significant. The specificity and predictive value of a positive test were similar for CT and PF. CTs advantage was better elucidation of the soft tissues of the chest apex. CT provided additional information beyond PF in 17/20 patients with benign conditions and in 29/30 patients with cancer. In 21 of these 29 cancer patients, CT findings significantly altered subsequent management.