Manglos S H, Capone R B, Bassano D A
SUNY Health Science Center, Syracuse 13210.
Med Phys. 1992 Mar-Apr;19(2):491-500. doi: 10.1118/1.596838.
Cone beam transmission CT (CB-CT) improves SPECT imaging by providing high-quality attenuation maps for attenuation compensation and for correlated SPECT and CT imaging. The present work measures the detection nonuniformity for CB-CT implemented with a gamma camera, and applies nonuniformity corrections to make CB-CT more uniform and accurate. Two cone beam collimators were investigated, as well as the uncollimated cone beam geometry, using both uniformity images and CB-CT reconstructions of a uniform circular cylinder. Uniformity images were acquired as a function of point source position relative to the nominal focal point. The uniformity images for both collimators were highly nonuniform, with some regions differing by more than 15% from the average image counts per pixel, indicating that the holes do not focus to the same point. The most uniform images were obtained with the point source located at or near the nominal focal point. Radiographs estimated the misfocusing of the holes to be about 0.6 degrees in some regions. There were no indications that the hole size was nonuniform. The CB-CT reconstructions of data acquired with collimator showed no obvious signs of image artifact from the detection nonuniformities. However, low-noise simulated data with well-localized detection defects produced readily-apparent circular artifacts. The nonuniformity correction was accurate and easy to apply, and should be used whenever quantitative accuracy is required. The uniformity images acquired without collimator lacked the collimator-produced nonuniformities, but had decreased counts near the detector edge. The decrease was predictable, using simple geometric considerations. Uniform cylinder reconstructions of "without collimator" data showed a corresponding decrease in center density relative to the edge (edge-to-center ratio = 1.25), which was improved by the nonuniformity correction (ratio = 0.21). Accurate CB-CT without collimator will require further correction for photon scatter.
锥束透射计算机断层扫描（CB-CT）通过提供高质量的衰减图用于衰减补偿以及用于关联单光子发射计算机断层扫描（SPECT）和CT成像，从而改善了SPECT成像。本研究测量了用伽马相机实现的CB-CT的检测不均匀性，并应用不均匀性校正以使CB-CT更加均匀和准确。使用均匀圆柱体的均匀性图像和CB-CT重建，研究了两种锥束准直器以及非准直锥束几何结构。均匀性图像是作为点源相对于标称焦点的位置的函数获取的。两种准直器的均匀性图像都非常不均匀，有些区域与每个像素的平均图像计数相差超过15%，这表明孔没有聚焦到同一点。当点源位于标称焦点处或附近时，获得的图像最均匀。射线照片估计某些区域的孔的失焦约为0.6度。没有迹象表明孔的尺寸不均匀。用准直器采集的数据的CB-CT重建没有显示出检测不均匀性导致的明显图像伪影迹象。然而，具有良好定位检测缺陷的低噪声模拟数据产生了容易出现的圆形伪影。不均匀性校正准确且易于应用，并且每当需要定量准确性时都应使用。未使用准直器采集的均匀性图像没有准直器产生的不均匀性，但在探测器边缘附近计数减少。使用简单的几何考虑，这种减少是可预测的。“未使用准直器”数据的均匀圆柱体重建显示中心密度相对于边缘相应降低（边缘与中心比 = 1.25），通过不均匀性校正得到改善（比率 = 0.21）。无准直器的准确CB-CT将需要对光子散射进行进一步校正。
