Timing of parenchymal enhancement on dual-phase dynamic helical CT of the liver: how long does the hepatic arterial phase predominate?

OBJECTIVE: Dual-phase dynamic helical CT is now being used to detect and characterize benign and malignant hypervascular lesions in the liver. The purpose of this study is to define the timing and degree of parenchymal enhancement of normal liver during the hepatic arterial phase. SUBJECTS AND METHODS: This prospective study included 102 patients with known or suspected hypervascular hepatic lesions who underwent dual-phase helical CT. After unenhanced CT scanning, we injected iopamidol (Isovue 300; Bracco Diagnostics, Princeton, NJ) at 3 ml/sec for 120 ml, then at 2 ml/sec for 55-60 ml. Scan delay for the hepatic arterial phase was 25 sec and for the portal venous phase was 76 sec. Section thickness was 7 mm and pitch was 1:1. Operator-defined regions of interest were obtained from all three phases. RESULTS: Mean unenhanced attenuation of the liver was 51 +/- 12 H. The liver revealed progressive enhancement during the hepatic arterial phase as follows: an increase of 10 H occurred at a mean time of 33 +/- 4 sec, 20 H at 39 +/- 6 sec, 30 H at 44 +/- 8 sec, 40 H at 46 +/- 6 sec, and 50 H at 48 +/- 5 sec. At 20 H and 30 H of enhancement, we found a statistically significant difference (p < .01) for the mean times of men and women. Mean peak enhancement during the portal venous phase was 89 +/- 23 H. CONCLUSIONS: Because the hepatic arterial contribution to liver perfusion is approximately 30%, parenchymal enhancement greater than approximately 30% of peak might indicate portal venous predominance. In our study, this percentage corresponded to an increase of approximately 30 H. Therefore, detection of hypervascular lesions in the hepatic arterial phase may be compromised when imaging lasts longer than approximately 44 sec after the initiation of contrast material injection because 44 sec was the mean time for 30 H of enhancement in our series. However, variability between patients was marked, particularly between men and women. Furthermore, the data suggests that the hepatic arterial phase may be relatively brief and that it may be difficult to image properly using current helical CT technology.

Intra-anal intussusception: diagnosis by posteroanterior stress proctography.

Intra-anal intussusception was diagnosed in eight of 39 patients on evacuation proctography. Posteroanterior views revealed prolapse of the infolded rectum into the anal canal on staining in seven of eight patients, associated with splaying open of the anal canal and sudden distal movement of the fold during prolapse. Similar changes were seen in four of 31 patients in whom intussusception had not been diagnosed on lateral evacuation proctography. The pattern of the collapsed rectum was assessed for fold length, thickness, and angulation in relation to the midline of the rectum. Infoldings that prolapsed were closer to the anorectal junction on stress (mean 14.6: 42.4 mm, p < 0.0001) showed greater change in height between rest and strain (28.8: 14.6 mm, p < 0.05) and became more acutely angled during straining (41.9: 5.3 degrees, p < 0.01). Intra-anal intussusception may be missed in 33% (four of 12 patients) on routine evacuation proctography. Posteroanterior stress proctography is a simple supplementary examination to validate intussusception.