Spiral CT optimization for measurement of bronchial lumen diameter using an experimental model

The aim of the study was to determine optimal parameters for demonstrating sublobar bronchi on spiral CT. Measurements were obtained from five paralle l polyethylene tubes embedded in foam matrix with similar radiographic char acteristics to segmental and subsegmental bronchi and to lung parenchyma, r espectively. Collimation widths of 1.5, 2, 3 and 4 mm were used, with a pit ch of 1 or 1.5 and a reconstruction interval of 1 mm or 2 mm. Various slice planes were used. Images acquired orthogonally were viewed normally. Image s acquired in planes oblique or parallel to the long axes of the tubes were reformatted into a plane orthogonal to the long axes of the tubes to be co mparable with the directly acquired orthogonal images. Tube diameters were measured at lung window settings (L, -400; W, 1300) and compared with known true inner and outer tube diameters. Measurements from images acquired ort hogonal to the tube long axes were accurate regardless of slice thickness. Images acquired obliquely or parallel only produced accurate measurements a t the lowest slice thickness (1.5 mm). Pitch and reconstruction interval ha d no effect on measurement error in any scan plane. It is concluded that a slice thickness of 1.5 mm or less, with a pitch of 1.5, should be used when acquiring images at angles other than orthogonal to the long axes of exper imental tubes equivalent to the segmental and subsegmental bronchi. It is s uggested that similar parameters should be used in vivo and that the examin ation should be targeted to the area of the bronchial tree in question to r educe patient dose and length of breath-hold.