Evaluation of prostate size: a comparison of ultrasound and magnetic resonance imaging

To determine the relative accuracy of ultrasonography (US) and magnetic resonance imaging (MRI) in the evaluation of prostate volume, we compared US and MR images with surgical findings in 15 patients. Transabdominal US was excellent for determining prostate size in patients with small to moderate enlargement. When compared with surgical specimens, the difference between the weight of the gland as predicted by US and the actual weight was 14% (SD +/- 12). With the transabdominal approach, the length was often inaccurately imaged, but the addition of transrectal scans in the sagittal projection improved results: with combined transabdominal and transrectal US, the average difference in weight was 8% (SD +/- 7). The MRI more accurately predicted prostatic volume (average difference, 6% (SD +/- 6), but the difference between the latter 2 is not significant. In 5 additional patients who had undergone transurethral resection of the prostate, residual prostatic tissue was evaluated by MRI and US. In addition to demonstrating prostate size, transrectal sagittal US showed the relationship among the bladder neck, prostatic urethra, and remaining prostatic tissue. Real-time US also allowed evaluation of dynamic sphincteric contractions, which is a finding not available today with MRI. Tissue differentiation was attempted with both MRI and US. Neither imaging modality could differentiate benign from malignant disease.