Spread through sexual contact, urethritis from Neisseria gonorrhoeae and Chlamydia trachomatis is favoured by promiscuity and low socioeconomic status. Alternatively, urethral infection increasingly results from intermittent or permanent catheterisation or from urologic instrumentation. Symptoms include mucopurulent discharge, alguria, dysuria and urethral pruritus [1, 7].
Acute urethritis is generally diagnosed on the basis of clinical and laboratory findings, but imaging may be required to exclude complications. In the past, conventional CM-enhanced radiographic studies were the primary modalities for imaging of the male urethra, particularly for assessment of traumatic injuries, strictures, and abscesses draining into the urethra. However, retrograde urethrography and voiding cystourethrography were unable to assess the periurethral structures [17, 60, 61].
Ultrasound of the penis may demonstrate a periurethral abscess, but is generally cumbersome because of inflammatory swelling and tenderness of the penile and perineal structures. The use of MRI can effectively visualise abnormalities of the periurethral structures . In our experience, acute urethritis appears as diffuse thickening of the urethra and periurethral tissues, with intermediate to high signal intensity on T2-weighted images and intense contrast enhancement (Fig. 14) [5, 62].
Perineal abscesses from lower urinary tract infection
Urethritis may be further complicated by a periurethral abscess through infection of Littré's glands. Since the penile tunica albuginea prevents the dorsal spread of infection, abscesses tends to track ventrally along the corpus spongiosum. When Buck's fascia is perforated, the process leads to fasciitis and gangrenous necrosis of the subcutaneous tissue .
MRI visualises perineal and penile abscesses as fluid- or pus-filled cavities with enhancing periphery, in communication with the urethra (Fig. 15), and may clearly reveal involvement of the corpora cavernosa and fibrous tunicae. A urethral diverticulum, most commonly located in the distal urethra, may mimic the appearance of an abscess. Treatment requires antibiotics, suprapubic urinary drainage and surgical evacuation [5, 24, 62, 63].
Funiculitis and epididymitis
Colour Doppler ultrasound (CD-US) remains the primary modality for investigating abnormalities of the testis and epididymis, particularly to differentiate torsion from inflammatory conditions [4, 8, 9].
Acute epididymitis is almost always unilateral and has a bimodal distribution, with the majority of cases occurring between 16–30 and 51–70 years of age. Whereas in the former age group, infection is commonly caused by Chlamydia trachomatis or Neisseria gonorrhoeae and is associated with sexual activity, in individuals at an advanced age or immunosuppressed individuals, it results from C-UTI by common aerobic urinary pathogens such as E. coli [1, 64].
Clinical manifestations of epididymo-orchitis include scrotal pain, sometimes radiating to the groin or lower abdomen, fever and other symptoms of UTI, variable scrotal swelling and tenderness at physical examination. Laboratory tests, urethral swab and cultures help to direct antibiotic therapy, which cures infection, relieves symptoms and prevents complications and transmission [1, 64].
When CD-US is not the initial examination, careful scrutiny of CT studies may reveal spermatic cord asymmetry with variable thickening and increased enhancement of vessels on the affected side: this rather subtle finding reflects infectious hyperemia and is strongly associated with ipsilateral infectious funiculitis, epididymitis and/or orchitis; hypervascularity of the epididymis may also be observed at CT (Figs. 16 and 17). Alternatively, spermatic vascular engorgement may reflect the presence of a testicular tumour [10, 65].
Similarly, MRI may depict an enlarged epididymis with increased or heterogeneous T2 signal intensity and engorged vessels [5, 22, 24]. The diagnosis of epididymitis is easily confirmed by CD-US, with the usual features including segmental or global enlargement, inhomogeneously hypoechoic and hypervascularised compared to the testis, commonly associated with thickening of scrotal tunicae or hydrocele (Figs. 16, 17 and 18) [8, 9, 61].
Orchitis and scrotal abscesses
Contiguous infectious involvement of the testis (orchitis) is rather uncommon compared to epididymitis, and generally has similar or more severe clinical and laboratory manifestations [1, 64].
CD-US hypervascularity is the key sonographic finding which allows differentiation of acute orchitis from torsion and infarction [8, 9].
At CT, the normal testes are symmetric and hypoattenuating, and are poorly differentiated by thickened scrotal tunicae and hydrocele. On cross-sectional imaging studies, acute orchitis is suggested by asymmetry with enlargement and increased contrast enhancement of the affected testis compared to the contralateral structure; signs of funiculitis and epididymitis are generally associated (Figs. 17 and 18). In normal conditions, at MRI the testes appear homogeneous, T1-isointense to muscle and T2-hyperintense, and the albuginea and mediastinum testis are identifiable as low-signal bands. Testicular inflammation is better demonstrated by MRI as decreased T1 and increased T2 signal intensity compared with the normal testis, with either intense homogeneous enhancement or the characteristic “tiger skin” post-contrast pattern corresponding to preserved septa. Focal or diffuse orchitis may be challenging to differentiate from testicular tumours, which generally show a mass effect and solid-type CT attenuation and MRI signal features, and are not associated with clinical and biochemical signs of infection [5, 22–24].
Untreated epididymo-orchitis may be further complicated by testicular necrosis and/or development of a scrotal abscess or pyocele, which requires surgical treatment [1, 64]. Abscesses may be sonographically appreciated as an ill-defined lesion with low echogenicity and absent internal flow signals [4, 8, 9]. CT and MRI features of pyocele include complex, heterogeneous fluid collections, surrounded by an enhancing periphery (Fig. 18) or by hyperaemic inflamed surrounding parenchyma [22–24].
Differential diagnosis of perineal and genital infections
Fournier’s gangrene (FG) is a rare, life-threatening necrotizing polymicrobial infection of the perineal, perianal and genital structure, commonly occurring in diabetics, with a striking male predominance (male-to-female ratio, 10:1). FG may further complicate a lower UTI, or originate from different infections such as colorectal infections, anal fistulas or pressure ulceration. Clinical presentation includes local pain, swollen oedematous or gangrenous overlying skin, sometimes with appreciable crepitus, and progressive hyperthermia. FG represents a surgical emergency: aggressive surgical debridement and broad-spectrum antibiotics are required to prevent a fatal outcome [66, 67].
CT is by far the best modality for imaging of FG; it quickly and comprehensively visualises the extent of FG involvement. CT features include subcutaneous fat stranding at the involved areas, fascial thickening, superficial or deep fluid and air-attenuation collections. Subcutaneous emphysema produced by anaerobic bacteria is the hallmark of FG and is present in approximately 90 % of cases (Fig. 19). Furthermore, CT can define the starting point of the infectious process, thereby allowing differentiation of complicated perineal infections from urinary versus an alternative source, particularly cryptogenic perianal sepsis (Fig. 20) [66–68].
Another uncommon differential diagnosis of perineal and scrotal infections is hidradenitis suppurativa (HS), an inflammatory disease of the skin and subcutaneous tissues with unclear pathogenesis and chronic progressive course. Mostly encountered in males and black people with poor hygiene, HS presents with tender subcutaneous nodules, which progress to form painful, deep dermal abscesses, sinus tracts, and eventually ulceration and fibrosis. Recurrence is common despite surgical excision [69, 70].
In our experience, MRI may accurately describe the affected regions, and confirm HS over orchiepididymitis and scrotal abscess by demonstrating that tissue inflammation and abscesses are confined to the superficial planes, with a characteristic symmetric distribution and lacking communication with pelvic organs (Fig. 21) [55, 71–73].