ACCURATE MEASUREMENT OF SURFACE-AREAS OF ANATOMICAL STRUCTURES BY COMPUTER-ASSISTED TRIANGULATION OF COMPUTED-TOMOGRAPHY IMAGES

There is a need for accurate surface area measurement of internal irre gular anatomical structures in order to define light dosimetry in adju nctive intraoperative photodynamic therapy (AIOPDT). No satisfactory p reoperative method exists of measuring this parameter. We have investi gated whether computer-assisted triangulation of serial sections gener ated by computed tomography (CT) scanning can give an accurate assessm ent of the surface area of the walls of the true pelvis after anterior resection and before colorectal anastomosis. We have shown that the t echnique of paper density tessellation is an acceptable method of meas uring the surface areas of phantom objects, with a maximum error of 0. 5%, and is used as the gold standard. Computer-assisted triangulation of CT images of standard geometric objects and accurately-constructed pelvic phantoms gives a surface area assessment with a maximum error o f 2.5% compared with the gold standard. The CT images of 20 patients' pelves have been analysed by computer-assisted triangulation and this shows that the surface area of the walls varies from 143 cm2 to 392 cm 2. Simple step-like analysis of images and approximation to geometric shapes with subsequent calculation give unacceptably high errors. The surface area of an internal, rigid, irregular surface area for illumin ation in AIOPDT can be accurately measured preoperatively by computer- assisted triangulation of CT images.