Obstructive shadow of ribs imaged on the axial transverse tomogram of the chest.

In order to take an axial transverse tomogram having few obstructive shadows, the phantom and human experiment were perfomed and studied. Thorax cage phantom composed of ribs made of lead wire was laid on the table of axial transverse tomograph of horizontal type and the axial transverse tomograms were taken with the central X-ray of caudalocranial or craniacaudal direction with the range of rotation of 180°, rotating from the right, front and left side of the body. Obstructive shadow arose mostly when projected caudalocranial direction of cent ral X-ray, while fairly few when taken with craniocaudal direction. With exposure of full rotation of 360°, the obstructive shadow was more fa int than the above two. This was confirmed also with the human experiment. When the axial transverse tomogram of the chest is taken, the obstructive shadows of the rib are imaged at times on the lung field and desturb correct interpretation. In order to take tomograms having few such shadows the studies of the optimal range of rotation of the X-ray tube around the human body and the angle of the central X-ray to the ribs at the tomography were made with phantom and human body experiments. 1. EXPERIMENTS WITH A PHANTOM For this experiment the presentaions of the problem are as follows: I) is there any difference of the appearance of obstructive shadows between the tomography conducted with the range of rotation of 180° and that of 360°? 2) does the angle of inclination of the ribs to the horizon influence on the grades of appearance of these shadows? An axial transverse tomograph of the erect type (Shimadzu) 1 >2> was used for the experiments, with the phantom to be radiographed placed on a rotating table in the erect position. The distance of X-ray tube, phantom and film were so arranged as to produce an X-ray image 1.27 times the actual size. The fr'J Ji'J llB Received for publication February 13, 1968.