What is the normal anatomy and imaging appearance of the testicles on US CT MRI?
The testicles are composed of densely packed seminiferous tubules that each converge posteriorly into large ducts and drain into the rete testis at the testicular hilum (also known as the mediastinum testis). Testicles are normally 2 to 3 cm wide and 3 to 5 cm long. In the rete testis, 15 to 20 efferent ductules converge to form the epididymal head superiorly and then converge into a single convoluted tubule in the epididymal body and tail inferiorly. The tubules emerge at an acute angle from the epididymal tail to form the vas deferens.
On US, the testicles have a homogeneous echotexture, are surrounded by a thin curvilinear echogenic tunica albuginea, and have symmetric blood flow on Doppler US. A linear band of echogenicity, representing the mediastinum testis, and an anechoic or hypoechoic focus containing small cystic or tubular structures, representing the rete testis, may be seen posteriorly at the testicular hilum. On CT, they have homogeneous soft tissue attenuation and enhancement, although CT is not used for primary evaluation of testicular disorders given the exposure to ionizing radiation and suboptimal soft tissue contrast resolution. On MRI, the testicles have homogeneous intermediate T1-weighted signal intensity and high T2-weighted signal intensity relative to skeletal muscle, are surrounded by a smooth thin curvilinear low signal intensity tunica albuginea, and homogeneously enhance. Thin low signal intensity septa radiating toward the mediastinum testis are often seen within the testicles on T2-weighted images. The mediastinum testis appears as a linear band of low signal intensity at the posteriorly located testicular hilum, whereas the rete testis has high T2-weighted signal intensity due to fluid within the seminiferous tubules.
The epididymides are posterolateral to the testicles. On US, they are isoechoic/slightly hyperechoic relative to the testicles and have symmetric blood flow on Doppler US. On CT, they have soft tissue attenuation. On MRI, the epididymides have low-intermediate T1-weighted and low T2-weighted signal intensity relative to the testicles.
The testicular arteries arise from the abdominal aorta near the level of the renal vessels and extend to the scrotum to supply the testicles. The right gonadal vein drains into the inferior vena cava, whereas the left gonadal vein drains into the left renal vein. Normal scrotal skin thickness varies between 2 to 8 mm.