The accuracy of magnetic resonance imaging and ultrasonography compared with surgical findings in the localization of the undescended testis.

Objective: To determine the accuracy of ultrasonography (US) vs magnetic resonance imaging (MRI) in the detection of undescended nonpalpable testis.

Design And Setting: A clinical, radiological, and surgical study was undertaken in boys with undescended nonpalpable testes at Pavia and Milan (Italy) University Hospitals.

Participants: Seventeen patients with undescended nonpalpable testes aged 10 months to 14.5 years who were examined between 1989 and 1993. A total of 22 nonpalpable testes were examined using US and MRI techniques. Twenty-one testes were explored surgically.

Results: The US located 13 (62%) of 21 testes, 12 of which were found at surgery while one (5%) proved to be false positive. Eight (38%) of 21 evaluations were negative; four (19%) were true-negative results. Of the 12 gonads correctly localized with US, two (17%) were located near the internal inguinal ring while 10 (83%) were within the inguinal canal. The MRI examination located 11 (52%) of 21 gonads, all found at surgery. Of the 10 negative results (48%), five (24%) were false negatives and five agenetic testes (24%) were not found surgically. Of the 11 testes correctly localized using MRI, four were located in the abdomen or just proximal to the internal inguinal ring and seven were in the inguinal canal. Five of the six high testes were correctly localized using US and MRI (two using US and four using MRI).

Conclusions: Neither US nor MRI is currently sensitive enough to stand alone as a screening modality for a nonpalpable testis since the two techniques used separately gave a useful result in 16 (76%) of 21 testes. They appear to be complementary since their combined specificity compared with surgical findings was 95%. Since the localization of an undescended testis is important for planning surgical strategy, US should be used for screening evaluation, and if it is not successful, it should be followed (before any invasive diagnostic procedure) by MRI, which appears to be more reliable in the localization of undescended, high, and "functioning" testes.