Lower Genitourinary

Scrotum

Sonography is the primary method used to image the scrotum. Patients undergoing sonography of the scrotum are usually examined in the supine position. A towel can be draped between the thighs to help support the scrotum. Warm gel should always be used because cold gel can elicit a cremasteric response and make it very difficult to perform a thorough examination.

The anatomy relevant to the scrotum is shown in Fig. 6-1 . The normal testicles appear as homogeneous ovoid organs that are symmetric bilaterally. The normal testis measures 4 to 5 cm in length, 2 to 3 cm in width, and 2 to 2.5 cm in depth. The normal testicular volume using the formula for an ellipsoid (length × width × depth × 0.53) is 15 to 20 mL. Testicular size decreases with age. The seminiferous tubules converge to form the rete testis, which is located at the testicular mediastinum. The mediastinum itself appears as a peripherally located, elongated hyperechoic structure ( Fig. 6-2A and B ). The rete testis connects to the epididymal head through the efferent ductules. The head of the epididymis is semilunar with rounded edges and its echogenicity is similar to that of the testis (see Fig. 6-2C ). It rests directly on the upper pole of the testis and should be seen in almost all men. It continues inferiorly into the body and tail of the epididymis, which are hypoechoic to the testis (see Fig. 6-2D and E ). The body and tail of the epididymis are smaller and more difficult to identify than the head, but with practice they can usually be seen along the anterolateral or posterior aspect of the testis. The body of the epididymis tends to move with the mediastinum of the testis. The vas deferens can also be seen (with practice) in the spermatic cord as a straight, noncompressible, hypoechoic tube measuring approximately 2 mm in thickness that becomes progressively more tort­uous as it enters the scrotum and connects to the epididymal tail. The tiny lumen, which measures approximately 0.3 mm, can usually be seen with very-high-resolution probes ( e-Fig. 6-1 ).

The vascular supply of the testis is shown in Fig. 6-3 . Unlike other organs, the major arteries of the testis are located peripherally and are called capsular arteries . These arteries supply blood to the testicular parenchyma by branches called centripetal arteries . The centripetal arteries enter the testis and travel toward the mediastinum. As they approach the mediastinum, they branch into recurrent rami that curve away from the mediastinum. In approximately 50% of testes, one or more major branches of the testicular artery enter the testis through the mediastinum. These transmediastinal arteries are often large enough to be seen by gray-scale sonography and are often accompanied by a transmediastinal vein. As with other solid parenchymal organs, the testicular arterial waveforms have a low-resistance pattern. The veins of the testis drain through the mediastinum as well as through the capsule of the testis. They are more difficult to visualize than the arteries, but can be seen in the majority of normal testes. Blood flow may or may not be seen in the normal epididymis, but it should not be as vascular as the testis. Table 6-1 presents a summary of the characteristics of normal testes.

One of the major roles of sonography is the evaluation of scrotal masses, and the most important determination is whether the mass is inside or outside the testis. Although the vast majority of extratesticular masses are benign, intratesticular masses are more likely to be malignant. In addition to the location, it is also important to determine whether the mass is cystic or solid, whether it has detectable inter­nal vascularity on color Doppler, and whether it is palpable ( Box 6-1 ).