SPIRAL CT OF THE LUNGS - OPTIMAL TECHNIQUE AND RESOLUTION COMPARED WITH CONVENTIONAL CT

OBJECTIVE. The purpose of this study was to compare the resolution of spiral and conventional CT for imaging the lungs and to optimize spira l CT techniques for imaging lung parenchyma. MATERIALS AND METHODS. A line-pair phantom, a dried lung specimen, and three human subjects wer e scanned. Spiral CT scans obtained on a Somatom PLUS-S CT scanner wer e compared with conventional CT scans. In addition, the influence of d ifferent collimation (1, 3, 5, and 8 mm), pitch (1 and 2), display ker nel (UltraHi, Hi, Standard, and Soft), and interpolation algorithms (3 60 degrees and 180 degrees linear interpolation) on spiral CT scans wa s evaluated. Scans obtained by using different techniques were compare d for edge sharpness, contrast resolution, overall visibility of vesse ls, and noise. RESULTS. With the line bars parallel to the z-axis, a s patial resolution of 7.7 Ip/cm was obtained at all four levels of coll imation used for both spiral and conventional CT scanning. Resolution in the z-axis (i.e., direction of table movement) on spiral CT scans d ecreased markedly when alignment of the line-pair phantom bars was cha nged to 45 degrees. Resolution along the z-axis decreased with increas ing collimation, and this effect was more marked on spiral CT scans th an on conventional CT scans. The relative decrease in resolution on sp iral CT scans was visible on scans of human subjects only at narrow co llimation (1 and 3 mm). Optimal visualization of structures in the lun g parenchyma was obtained by using 3-mm collimation and the UltraHi di splay kernel. The choice of table speed (pitch 1 vs 2) and interpolati on algorithm (360 degrees vs 180 degrees linear interpolation) did not significantly affect the image quality of scans of human subjects. CO NCLUSION. CT scans of the lung obtained by using spiral and convention al modes at wide collimation (5 and 8 mm) show no differences in resol ution, although, at narrow collimation (1 and 3 mm), curved structures such as walls of bullae and fissures are somewhat indistinct. On spir al CT scans of the lungs, narrow collimation and the UltraHi display k ernel provide the best image resolution and have more effect on image quality than does choice of table speed or linear interpolation algori thm.