Improving the evaluation of renal transplant collecting system dilatation by computerized ultrasound image digitization.

43% of the renal allografts transplanted at our centre demonstrate pelvicalyceal dilatation on ultrasonography. However, only 6% of grafts are ultimately shown to be obstructed by antegrade pyelography. We have attempted to improve the specificity of ultrasonography by a quantitative analysis of 394 serial ultrasound studies from 150 renal transplant recipients. Representative images in the longitudinal and transverse axes were selected by a single observer for each of the studies and the outline of the kidney and its pelvicalyceal system was traced out. The images were then digitized using a Summagraphics tablet so that the length, width and area of the kidney and its pelvicalyceal system were calculated for each image. A step-wise discriminant analysis was performed which showed that the ratio of the area of the kidney to its pelvicalyceal area in both axes discriminated between normal, dilated but non-obstructed, and obstructed grafts. The median ratio of kidney to collecting system area in the obstructed grafts was 5.2 in the longitudinal and 4.4 in the transverse scan planes (vs median ratios of 32.8 and 22.0 for non-dilated grafts). Using canonical discriminant function, an index was derived which combines these ratios for the longitudinal and transverse images of the same kidney and this has further improved the diagnostic accuracy (median index of 2.45 in obstructed group vs -0.85 in non-dilated kidneys). This method improves upon the radiologist's report and identifies those grafts which require intervention.