THE OPTIMIZATION OF HELICAL THORACIC CT

Purpose: Our purpose was to determine the optimal helical thoracic CT scanning protocol. Method: Three adult Suffolk sheep under general ane sthesia were repeatedly scanned by a variety of variable thickness hel ical and conventional plus thin section high resolution (lung gold sta ndard) CT sequences, reconstructed for mediastinal (standard interpola tor and algorithm) and lung parenchymal (extrasharp interpolator, bone algorithm) detail. The images were evaluated in a random order by fiv e separate blinded, experienced imagers utilizing a predetermined grad ing scale. Results: At equivalent slice thicknesses, the mediastinal i mages showed no statistically significant differences between conventi onal and helical CT using pitches of 1.0, 1.5, and 2.0. However, the 5 -mm-thick sections, regardless of technique, performed better than did either the 2- or the 10-mm-thick section images, For the lung interst itium, there was an obvious and marked advantage to reconstructing the lung images separately from the mediastinal images with edge-enhancin g algorithms and interpolators. With l-mm-high mA thin section, high r esolution lung CT as the gold standard, 2 mm conventional and helical pitch 1.0, 1.5, and 2.0 images were all graded equivalent. Of the 5 mm images, the helical pitches of 1.0 and 1.5 were graded equivalent to the gold standard. All of the 10 mm lung sections using both conventio nal and helical CT were graded statistically worse than the gold stand ard (p < 0.05). Conclusion: The use of helical CT with a 5 mm beam col limation and a pitch of 1.0 or 1.5 reconstructed twice to maximize bot h the mediastinal and the lung parenchymal detail provides the optimal way to routinely evaluate the chest.