At annual repeat CT screening, the finding of an isolated cystic airspace with increased wall thickness should raise the suspicion of lung cancer.

191. The observers examined each hemithorax image for these radiographic features. Also, each observer predicted the presence or absence of pleural fluid on the basis of the combination of signs present in each case, and evaluated the effusions as small, moderate, or large. Disagreements between observers were resolved by another review of the radiographs and mutual consensus. In most cases, disagreements resulted from quantification differences. The observers did not know the results of previous radioDownloaded from www.ajronline.org by 54.245.13.81 on 05/12/18 from IP address 54.245.13.81.

Address reprint requests to W. J. Dodds.

A study comparing precontrast and survey postcontrast dynamic computed tomographic (CT) scanning was performed on 60 patients who had suspected hepatic metastases. An incremental dynamic technique was used during and following a 50-g iodine load administered over two minutes. The survey postcontrast dynamic technique was superior in both sensitivity and contrast differentiation and yielded no known false-negative examinations. No postprocedure renal dysfunction was observed. High-dose contrast-material delivery in conjunction with incremental dynamic CT scanning appears to be the most suitable technique for performing postcontrast hepatic CT examinations.

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Periportal halos are defined as circumferential zones of decreased attenuation identified around the peripheral or subsegmental portal venous branches on contrast-enhanced computed tomography (CT). These halos probably represent fluid or dilated lymphatics in the loose areolar zone around the portal triad structures. While this CT finding is nonspecific, it is abnormal and should prompt close scrutiny of the liver in search of an underlying etiology. Periportal halos which may be due to blood are commonly seen in patients with liver trauma. Periportal edema may cause this sign in patients with congestive heart failure and secondary liver congesion, hepatitis, or enlarged lymph nodes and tumors in the porta hepatis which obstruct lymphatic drainage. This CT sign has also been observed in liver transplants (probably secondary to disruption and engorgement of lymphatic channels) and in recipients of bone marrow transplants who might develop liver edema from microvenous occlusive disease. While the precise pathophysiologic basis of periportal tracking has not been proven, it represents a potentially important CT sign of occult liver disease.

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