Scrotal (or transscrotal) ultrasound is a medical ultrasound examination of the scrotum. It is used in the evaluation of testicular pain, and can help identify solid masses.
Indications Although the development of new imaging modalities such as computerized tomography and magnetic resonance imaging have opened a new era for medical imaging, high-resolution sonography remains as the initial imaging modality of choice for evaluation of scrotal disease. Many of the disease processes, such as testicular torsion, epididymo-orchitis, and intratesticular tumor, produce the common symptom of pain at presentation, and differentiation of these conditions and disorders is important for determining the appropriate treatment. High-resolution ultrasound aids in improved characterization of some intrascrotal lesions and suggests more specific diagnoses, resulting in more appropriate treatments and the avoidance of unnecessary operation.
Imaging technique For any scrotal examination, thorough palpation of the scrotal contents and history taking should precede the sonographic examination. Patients are usually examined in the supine position with a towel draped over their thighs to support the scrotum. Warm gel should always be used because cold gel can elicit a cremasteric response resulting in thickening of the scrotal wall; hence a thorough examination is difficult to be performed. A high resolution, near-focused, linear array transducer with a frequency of 7.5 MHz or greater is often used because it provides increased resolutions of the scrotal contents. Images of both scrotum and bilateral inguinal regions are obtained in both transverse and longitudinal planes. Color Doppler and pulsed Doppler examination are subsequently performed, optimized to display low-flow velocities, to demonstrate blood flow in the testes and surrounding scrotal structures. In evaluation of acute scrotum, the asymptomatic side should be scanned first to ensure that the flow parameters are set appropriately. A transverse image including all or a portion of both testicles in the field of view is obtained to allow side-to-side comparison of their sizes, echogenicity, and vascularity. Additional views may also be obtained with the patient performing Valsalva maneuver. Online calculators have been introduced to estimate testicular volume based on sonographic measurements.
Anatomy
The normal adult testis is an ovoid structure measuring 3 cm in anterior-posterior dimension, 2–4 cm in width, and 3–5 cm in length. The weight of each testis normally ranges from 12.5 to 19 g. Both the sizes and weights of the testes normally decrease with age. At ultrasound, the normal testis has a homogeneous, medium-level, granular echotexture. The testicle is surrounded by a dense white fibrous capsule, the tunica albuginea, which is often not visualized in the absence of intrascrotal fluid. However, the tunica is often seen as an echogenic structure where it invaginates into the testis to form the mediastinum testis. In the testis, the seminiferous tubules converge to form the rete testes, which is located in the mediastinum testis. The rete testis connects to the epididymal head via the efferent ductules. The epididymis is located posterolateral to the testis and measures 6–7 cm in length. At sonography, the epididymis is normally iso- or slightly hyperechoic to the normal testis and its echo texture may be coarser. The head is the largest and most easily identified portion of the epididymis. It is located superolateral to the upper pole of the testicle and is often seen on paramedian views of the testis. The normal epididymal body and tail are smaller and more variable in position.
The testis obtains its blood supply from the deferential, cremasteric and testicular arteries. The right and left testicular arteries, branches of the abdominal aorta, arise just distal to the renal arteries, provide the primary vascular supply to the testes. They course through the inguinal canal with the spermatic cord to the posterior superior aspect of the testis. Upon reaching the testis, the testicular artery divides into branches, which penetrate the tunica albuginea and arborize over the surface of the testis in a layer known as tunica vasculosa. Centripetal branches arising from the capsular arteries carry blood toward the mediastinum, where they divide to form the recurrent rami that carry blood away from the mediastinum into the testis. The deferential artery, a branch of the superior vesicle artery and the cremasteric artery, a branch of the inferior epigastric artery, supply the epididymis, vas deferens, and peritesticular tissue.
Four testicular appendages have been described: the appendix testis, the appendix epididymis, the vas aberrans, and the paradidymis. They are all remnants of embryonic ducts. Among them, the appendix testis and the appendix epididymis are usually seen at scrotal US. The appendix testis is a Müllerian duct remnant and consists of fibrous tissue and blood vessels within an envelope of columnar epithelium. The appendix testis is attached to the upper pole of the testis and found in the groove between the testis and the epididymis. The appendix epididymis is attached to the head of the epididymis. The spermatic cord, which begins at the deep inguinal ring and descends vertically into the scrotum consists of vas deferens, testicular artery, cremasteric artery, deferential artery, pampiniform plexuses, genitofemoral nerve, and lymphatic vessel.
Intratesticular tumors One of the primary indications for scrotal sonography is to evaluate for the presence of intratesticular tumor in the setting of scrotal enlargement or a palpable abnormality at physical examination. It is well known that the presence of a solitary intratesticular solid mass is highly suspicious for malignancy. Conversely, the vast majority of extratesticular lesions are benign.
Germ cell tumors Primary intratesticular malignancy can be divided into germ cell tumors and non–germ cell tumors. Germ cell tumors are further categorized as either seminomas or nonseminomatous tumors. Other malignant testicular tumors include those of gonadal stromal origin, lymphoma, leukemia, and metastases.
Seminoma
Approximately 95% of malignant testicular tumors are germ cell tumors, of which seminoma is the most common. It accounts for 35%–50% of all germ cell tumors. Seminomas occur in a slightly older age group when compared with other nonseminomatous tumor, with a peak incidence in the fourth and fifth decades. They are less aggressive than other testicular tumors and usually confined within the tunica albuginea at presentation. Seminomas are associated with the best prognosis of the germ cell tumors because of their high sensitivity to radiation and chemotherapy.
Seminoma is the most common tumor type in cryptorchid testes. The risk of developing a seminoma is increased in patients with cryptorchidism, even after orchiopexy. There is an increased incidence of malignancy developing in the contralateral testis too, hence sonography is sometimes used to screen for an occult tumor in the remaining testis. On US images, seminomas are generally uniformly hypoechoic, larger tumors may be more heterogeneous [Fig. 3]. Seminomas are usually confined by the tunica albuginea and rarely extend to peritesticular structures. Lymphatic spread to retroperitoneal lymph nodes and hematogenous metastases to lung, brain, or both are evident in about 25% of patients at the time of presentation.
Nonseminomatous germ cell tumors Nonseminomatous germ cell tumors most often affect men in their third decades of life. Histologically, the presence of any nonseminomatous cell types in a testicular germ cell tumor classifies it as a nonseminomatous tumor, even if most of the tumor cells belong to seminoma. These subtypes include yolk sac tumor, embryonal cell carcinoma, teratocarcinoma, teratoma, and choriocarcinoma. Clinically nonsemionatous tumors usually present as mixed germ cell tumors with various cell types and in different proportions.
Embryonal cell carcinoma
Embryonal cell carcinomas, a more aggressive tumor than seminoma usually occurs in men in their 30s. Although it is the second most common testicular tumor after seminoma, pure embryonal cell carcinoma is rare and constitutes only about 3 percent of the nonseminomatous germ cell tumors. Most of the cases occur in combination with other cell types. At ultrasound, embryonal cell carcinomas are predominantly hypoechoic lesions with ill-defined margins and an inhomogeneous echotexture. Echogenic foci due to hemorrhage, calcification, or fibrosis are commonly seen. Twenty percent of embryonal cell carcinomas have cystic components. The tumor may invade into the tunica albuginea resulting in contour distortion of the testis [Fig. 4].
Yolk sac tumor Yolk sac tumors also known as endodermal sinus tumors account for 80% of childhood testicular tumors, with most cases occurring before the age of 2 years. Alpha-fetoprotein is normally elevated in greater than 90% of patients with yolk sac tumor (Woodward et al., 2002, as cited in Ulbright et al., 1999). In its pure form, yolk sac tumor is rare in adults; however yolk sac elements are frequently seen in tumors with mixed histologic features in adults and thus indicate poor prognosis. The US appearance of yolk sac tumor is usually nonspecific and consists of inhomogeneous mass that may contain echogenic foci secondary to hemorrhage. Choriocarcinoma --- Choriocarcinoma is a highly malignant testicular tumor that usually develops in the 2nd and 3rd decades of life. Pure choriocarcinomas are rare and represent only less than 1 percent of all testicular tumors. Choriocarcinomas are composed of both cytotrophoblasts and syncytiotrophoblasts, with the latter responsible for the clinical elevation of human chorionic gonadotrophic hormone levels. As microscopic vascular invasion is common in choriocarcinoma, hematogeneous metastasis, especially to the lungs is common. Many choriocarcinomas show extensive hemorrhagic necrosis in the central portion of the tumor; this appears as mixed cystic and solid components at ultrasound.
Teratoma Although teratoma is the second most common testicular tumor in children, it affects all age groups. Mature teratoma in children is often benign, but teratoma in adults, regardless of age, should be considered malignant. Teratomas are composed of all three germ cell layers, i.e. endoderm, mesoderm and ectoderm. At ultrasound, teratomas generally form well-circumscribed complex masses. Echogenic foci representing calcification, cartilage, immature bone and fibrosis are commonly seen [Fig. 5]. Cysts are also a common feature and depending on the contents of the cysts i.e. serous, mucoid or keratinous fluid, it may present as anechoic or complex structure [Fig. 6].
Non-germ cell tumours Sex cord-stromal tumours Sex cord-stromal (gonadal stromal) tumors of the testis, account for 4 per cent of all testicular tumors. The most common are Leydig and Sertoli cell tumors. Although the majority of these tumors are benign, these tumors can produce hormonal changes, for example, Leydig cell tumor in a child may produce isosexual virilization. In adult, it may have no endocrine manifestation or gynecomastia, and decrease in libido may result from production of estrogens. These tumors are typically small and are usually discovered incidentally. They do not have any specific ultrasound appearance but appear as well-defined hypoechoic lesions. These tumors are usually removed because they cannot be distinguished from malignant germ cell tumors.
Leydig cell tumors are the most common type of sex cord–stromal tumor of the testis, accounting for 1%–3% of all testicular tumors. They can be seen in any age group, they are generally small solid masses, but they may show cystic areas, hemorrhage, or necrosis. Their sonographic appearance is