The gonads initially develop near the kidneys in the retroperitoneum. The formation of the gonad is dependent on three primordia: (1) primordial germ cells, (2) genital ridge, and (3) coelomic epithelium. The testes are suspended by the mesorchium, a double peritoneal fold. The upper fold transmits the spermatic vessels, and the lower fold forms the Hunter gubernaculum. The downward journey through the retroperitoneum commences at approximately the third month of gestation. During the seventh month the testes are found normally at the level of the anterior superior iliac spine. The peritoneum enters the inguinal canal ahead of the testes, but extends down the gubernaculum only part way. The testes and gubernaculum subsequently extend into the inguinal canal. The testes begin to enter the internal ring as the gubernaculum emerges from the external ring. As the gubernaculum reaches the bottom of the scrotal sac, it begins to shorten until its lower two-thirds have disappeared completely. At about the end of the seventh month, the testes pass through the inguinal canal. Although descent through the canal is accomplished in a few days, it takes four additional weeks for the testes to pass from the external ring to the bottom of the scrotum. Descent may be complete or may still be incomplete at birth. After the testes emerge through the external ring, the external ring contracts [3] (Fig. 26.2).

When diagnosing disease processes that involve the scrotum and contents, ultrasound remains the primary modality of evaluation. Ultrasound has the advantages of general availability, portability, high resolution, functional information (blood flow), and relatively low cost. Limitations of the technique are its high dependency on the quality of the equipment and the expertise of the operator [4]. MR imaging is an excellent adjunct or problem solving tool for further evaluation of equivocal findings on ultrasound, complex clinical situations, or in patients where there is discordance between ultrasound findings and clinical presentation [5].

The examination should be performed in a warm room to avoid a cremasteric reflex (more pronounced in children), which pulls the testes upward, possibly outside the scrotal sac. The patient should be supine and the scrotum elevated by placing a towel under the scrotal sac. A second towel may be used to lay the penis on the anterior aspect of the lower abdominal wall. High-frequency linear transducers in the range of 8–15 MHz with color Doppler capability are necessary. For color Doppler studies, the highest possible power output, lowest scale and wall filter, and optimal gain should be used. The presumed normal testis should be imaged first to optimize scanning parameters. Then, without changing the parameters, the contralateral side should be imaged. This applies to both grayscale and color Doppler techniques. Side-by-side grayscale and color images of both testes should be obtained for comparison. For evaluation of varicocele or inguinal hernia, the patient is also scanned during a Valsalva maneuver and/or while standing. If a tumor of the testis is found and no previous abdominal imaging has been performed, the periaortic and renal hilar regions should be searched for possible lymphadenopathy [4].

MR imaging of the scrotum is performed with the patient in the supine position. The scrotum should be elevated either with a foam block or towels placed between the thighs. A 3 or 5 in. coil is placed on the area of interest so thin section, high-resolution, small field-of-view images can be obtained. Additionally, depending on the clinical question, a body coil can be used to acquire larger field-of-view images of the pelvis and/or abdomen (generally to the level of the renal hilum). Three orthogonal planes of T2-weighted images and axial T1-weighted images should always be obtained [6, 7]. One plane of the T2-weighted images should have fat suppression to increase dynamic range, increasing sensitivity for subtle differences in signal intensity [6]. If gadolinium is being administered, 2D or 3D fat-suppressed T1-weighted gradient-recalled echo images should be obtained in one plane precontrast and in at least two planes post contrast. As in other body imaging applications, the precontrast fat-suppressed T1-weighted gradient-recalled echo images are the most sensitive for detecting hemorrhage secondary to the increased dynamic range [6].

The echogenicity of the testis is of intermediate level, which is often compared to that of the thyroid gland and is thought to result from the presence of seminiferous tubules [4]. The mediastinum testis is demonstrated on ultrasound images as a linear echogenic structure [8] (Fig. 26.3). The testis is surrounded by a fibrous capsule, the tunica albuginea, which cannot be seen on ultrasound. Along the length of the testis, the epididymis lies at the posterolateral aspect. Usually only the head of the epididymis is visualized in the normal adult on ultrasound. The echogenicity of the epididymis is similar, or minimally increased, with respect to the testis (Fig. 26.4). Its blood flow is slightly less than that of the testis. Two embryologic remnants measuring a few millimeters each are occasionally visualized when the testis is surrounded by fluid, one at the upper pole of the testis and the other at the head of the epididymis. These are called appendix testis and appendix epididymidis, respectively [4, 9] (Fig. 26.5). The spermatic cord extends superiorly from the testis through the inguinal canal. Pulsed Doppler tracing can recognize the internal spermatic artery by its low-resistance pattern, as opposed to the high-resistance pattern found in the cremasteric and deferential arteries. The spermatic cord contains the vas deferens, a single tubular structure connecting with the seminal vesicles at the base of the bladder to form the ejaculatory ducts that carry the sperm to the urethra. The vas deferens connects with the epididymis at the tail [4].

The scrotal wall usually appears as a low T2-weighted signal intensity structure [5, 10]. Normal testes are ovoid structures that show hyperintense T2-weighted signal and isointense T1-weighted signal when compared to skeletal muscle (Fig. 26.6). T2-weighted images provide excellent tissue contrast between the testes and other scrotal structures and are therefore the pillar of scrotal MR imaging [5–7, 10–12]. As in other areas of body imaging, T1-weighted images are useful for tissue characterization, particularly in the depiction of hemorrhage or fat [6, 10, 11]. The testes demonstrate strong, homogenous contrast enhancement following the administration of intravenous gadolinium [5, 10]. The mediastinum testis appears as a hypointense band on T2-weighted images relative to the testicular parenchyma [6, 7, 10, 12] (Fig. 26.6). The tunica albuginea, which is approximately 1 mm thick, shows low T1-weighted and T2-weighted signal intensity, secondary to its fibrous nature [5–7, 10, 11]. The epididymis has similar T1-weighted signal intensity and lower T2-weighted signal intensity than the testis [5–7, 10, 11]. The vas deferens has lowT2-weighted signal intensity in the wall and high T2-weighted signal intensity within the lumen [6].

Scrotal pathologies can be divided into the following classifications (Table 26.1).

When approaching scrotal masses, the two most important considerations are whether the lesion is intratesticular or extratesticular and whether the lesion is solid or cystic (Table 26.2).

While the dichotomous division of intratesticular scrotal masses into solid and cystic lesions is useful for the purposes of developing a differential diagnosis, it may also be helpful to approach intratesticular masses as single and unilateral or multiple and bilateral (Table 26.3).