Recently, two-dimensional high-resolution computed tomography (2D-HRCT) and volumetric high-resolution computed tomography (VHRCT) have allowed great progress to be made in estimating small regions of hypoattenuation in the lung. VHRCT allows minimum-intensity projection (MINIP) imaging, which seems to be much more suitable than conventional 2D-HRCT for detecting subtle hypoattenuating regions, although the reliability of MINIP imaging data has not been conclusively shown. Three-dimensional computed tomography (3D-CT) images reconstructed from helical computed tomography (CT) acquisition over the whole lung field have been increasingly used to determine the absolute volume occupied by emphysematous changes. However, depending on the threshold CT values used for image reconstruction, whole-lung 3D-CT images may be distorted by considerable differences between the x-ray beam thickness and the effective section thickness, resulting in erroneous quantitation of emphysematous lesions by helical 3D-CT.
Abbreviations:2D-HRCT two-dimensional high-resolution computed tomography, 3D-CT three-dimensional computed tomography, BMT bone marrow transplantation, BO bronchiolitis obliterans, BOOP bronchiolitis obliterans organizing pneumonia, CT computed tomography, MINIP minimum-intensity projection, MIP maximum-intensity projection, RV residual volume, TLC total lung capacity, VHRCT volumetric high-resolution computed tomography
Computed tomography (CT), including conventional two-dimensional high-resolution computed tomography (2D-HRCT) and the volumetric high-resolution computed tomography (VHRCT) newly developed for thin slabs of restricted lung regions, seems to enable direct analysis of events occurring at the acinar level. This is because current CT scanners can provide scan data with a spatial resolution of less than 0.5 mm-less than the diameter of a single acinus. Furthermore, HRCT permits the acinar distribution of emphysematous changes to be determined in different lung fields. Use of VHRCT reconstructed with sliding thin-slab minimum intensity projection (MINIP) imaging (a technique originally proposed by Napel et al. in 1993) has been gradually increasing as a way to detect small areas with emphysematous changes. Although MINIP imaging has been used to estimate focal hypoattenuating areas caused by bronchiolar obstruction or mild emphysematous destruction with greater sensitivity than conventional 2D-HRCT provides, the reliability of MINIP images has never been convincingly shown. In addition to 2D-HRCT and VHRCT images, three-dimensional computed tomography (3D-CT) images obtained from helical CT whole-lung scans have been actively used to assess lung volume and extent of emphysema. However, whole-lung 3D-CT images are associated with the serious drawback of image distortion along the longitudinal axis of the body, which causes significant disparity in CT-estimated lung volumes.