Residency · Residency · Diagnostic Radiology
Scrotal Ultrasound: Testicular Torsion and Mass Evaluation
Introduction
Scrotal ultrasound is the primary imaging modality for evaluating acute scrotal pain, palpable masses, and infertility. It provides exquisite anatomic detail with high-frequency linear transducers (12-18 MHz) and is the definitive study for distinguishing surgical emergencies such as testicular torsion from nonsurgical conditions. Accurate characterization of testicular masses is critical, as virtually all solid intratesticular lesions are malignant until proven otherwise.
Normal Scrotal Anatomy
The testis is ovoid with homogeneous, medium-level echogenicity, measuring approximately 3-5 cm in length. The mediastinum testis is an echogenic linear structure along the posterior aspect representing the fibrous septum. The epididymis head (globus major) is isoechoic to slightly hyperechoic to the testis, while the body and tail are thinner and extend posterolaterally. The tunica albuginea is a thin echogenic capsule surrounding the testis. The appendix testis and appendix epididymis are small pedunculated structures at the superior pole that represent potential sites of torsion.
Testicular Torsion
Pathophysiology
Intravaginal torsion is the most common type and involves twisting of the spermatic cord within the tunica vaginalis, typically due to a "bell-clapper" deformity. Extravaginal torsion occurs in neonates, where the entire testis and tunica vaginalis twist. Testicular salvage rates decline rapidly: greater than 90% if detorsed within 6 hours and less than 10% after 24 hours.
Ultrasound Findings
The most important finding is absent or markedly decreased intratesticular blood flow on color and power Doppler compared to the contralateral side. The torsion knot sign describes a whirlpool appearance of the twisted spermatic cord on grayscale and color Doppler at the external inguinal ring or within the scrotal sac. The testis may be enlarged, hypoechoic (edema), or heterogeneous (hemorrhagic infarction in late cases). A reactive hydrocele and scrotal wall thickening are common. Absent venous flow may precede absent arterial flow in partial or early torsion.
Pitfalls
Partial torsion may show reduced but not absent flow; the waveform morphology (elevated RI) should be compared with the contralateral testis. Spontaneous detorsion may result in normal or hyperemic flow at the time of imaging. In prepubertal boys, normal intratesticular flow may be difficult to detect; always compare with the asymptomatic side and optimize Doppler settings.
Epididymo-orchitis
Epididymo-orchitis is the most common cause of acute scrotum in adults. The epididymis appears enlarged, hypoechoic, and markedly hyperemic on color Doppler. Orchitis manifests as testicular enlargement and hyperemia when infection extends to the testis. A reactive hydrocele and scrotal wall thickening are present. An abscess appears as a focal hypoechoic or complex fluid collection with peripheral hyperemia and may require drainage. | Feature | Testicular Torsion | Epididymo-orchitis |
| Blood flow on Doppler | Decreased or absent | Increased (hyperemia) | |
|---|---|---|---|
| Testicular echogenicity | May be hypoechoic or heterogeneous | Enlarged, hypoechoic if orchitis | |
| Epididymis | Normal or enlarged | Enlarged, hypoechoic, hyperemic | |
| Torsion knot sign | Present (whirlpool of twisted cord) | Absent | |
| Management | Urgent surgical exploration | Antibiotics, conservative |
The key distinction from torsion is that flow is increased in epididymo-orchitis and decreased or absent in torsion.
Appendage Torsion
Torsion of the appendix testis (most common) or appendix epididymis presents as an enlarged, hyperechoic or hypoechoic round structure adjacent to the superior testicular pole with absent internal flow. Surrounding hyperemia of the epididymal head is typical. The condition is self-limited and managed conservatively.
Testicular Masses
Approach
| Location | Malignancy Risk | Common Diagnoses |
|---|---|---|
| Intratesticular solid | ~95% malignant | Seminoma, NSGCT, lymphoma |
| Extratesticular | Almost always benign | Lipoma, spermatocele, epididymal cyst |
Intratesticular solid masses are malignant until proven otherwise, with an approximately 95% malignancy rate. Extratesticular masses are almost always benign (lipoma, spermatocele, epididymal cyst).
Germ Cell Tumors (95% of testicular malignancies)
Seminoma is the most common testicular malignancy, appearing as a homogeneously hypoechoic, well-defined mass with hypervascularity on Doppler and a peak incidence at age 30-40. Non-seminomatous germ cell tumors (NSGCT) show mixed echogenicity and heterogeneous composition, often with calcification, cystic areas, or hemorrhage. Subtypes include embryonal carcinoma, yolk sac tumor, choriocarcinoma, teratoma, and mixed tumors.
Stromal Tumors
Leydig cell tumors are small, well-defined hypoechoic masses that may cause hormonal effects (gynecomastia). Sertoli cell tumors are rare with variable echogenicity.
Benign Intratesticular Lesions (Important Mimics)
Epidermoid cysts are well-defined with a laminated "onion-ring" appearance and no internal vascularity on Doppler. Tubular ectasia of the rete testis shows cystic dilatation near the mediastinum testis and is associated with spermatoceles. Segmental testicular infarction presents as a wedge-shaped avascular area and may mimic a tumor.
Varicocele
A varicocele consists of dilated veins of the pampiniform plexus (greater than 3 mm diameter) posterior and superior to the testis. The veins increase in caliber and show retrograde flow with the Valsalva maneuver. Grading is by size: Grade I (subclinical), Grade II (palpable with Valsalva), and Grade III (visible). Varicoceles are predominantly left-sided (95%); an isolated right varicocele should prompt evaluation for a retroperitoneal mass (renal cell carcinoma compressing the gonadal vein).
Key Clinical Pearls
In suspected torsion, never delay surgical exploration for imaging if clinical suspicion is high; ultrasound sensitivity is approximately 86-100%, but false negatives occur. Always compare both testes side-by-side to assess relative flow and echogenicity. A small (less than 1.5 cm) hypoechoic intratesticular lesion in a patient with prior contralateral tumor or leukemia/lymphoma should raise concern for metastasis or lymphoma. Microlithiasis (5 or more calcifications per transducer field) is associated with an increased risk of germ cell tumors, and clinical surveillance and self-examination are recommended. Any solid intratesticular mass in a young male warrants urgent urology referral and serum tumor markers (AFP, beta-hCG, LDH).
References
- Dogra VS, Gottlieb RH, Oka M, et al. Sonography of the Scrotum. Radiology. 2003;227(1):18-36.
- Hertzberg BS, Middleton WD. Ultrasound: The Requisites. 3rd ed. Elsevier; 2016.
- Williamson RCN. Torsion of the Testis and Allied Conditions. Br J Surg. 1976;63(6):465-476.
- Woodward PJ, Sohaey R, O'Donoghue MJ, et al. From the Archives of the AFIP: Tumors and Tumorlike Lesions of the Testis: Radiologic-Pathologic Correlation. Radiographics. 2002;22(1):189-216.