Ultrasound Study of the Urethra

Fig. 18.1

Anatomy of the male urethra

The posterior urethra is composed of two anatomical segments: prostatic and membranous urethra. The prostatic urethra is almost entirely covered by the prostate gland (Fig. 18.2). The lumen of the posterior urethra is not enveloped by corpus spongiosum but covered by prostate and levator ani muscle. The membranous portion commences about 1 cm distally from the prostatic apex and lies within the urogenital diaphragm and encircled by the external urethral sphincter (levator ani muscles located laterally). Urethral stenosis is the term preferred to describe the narrowing of the urethral lumen that is not surrounded by corpus spongiosum, specifically the membranous and prostatic urethra [1].

Fig. 18.2

The male posterior urethra covered by prostatic apex in axial (a) and longitudinal (b) view using end-fire transrectal probe

The anterior urethra is composed of three anatomical segments: bulbar, penile urethra, and fossa navicularis/urethral meatus. The lumen of the anterior urethra is surrounded by the corpus spongiosum until the external meatus. The corpus spongiosum is covered by the deep fascia of the penis (Buck). The first segment of the anterior urethra is the bulbar urethra that is linked to membranous segment through the urogenital diaphragm. The bulbar urethra is fixed to the perineal membrane that covers the inferior part of the urogenital diaphragm. The corpus spongiosum of the bulbar segment is thick and elastic posterolaterally while rigid and thin superior-medially. During micturition, the intraluminal pressure stretches posterolaterally the expandable and elastic walls of the corpus spongiosum, and the lumen of the bulbar urethra increases significantly if compared to the penile urethra. The bulbar urethra is covered posterolaterally by the bulbospongiosus muscle and deep perineal (Gallaudet) and superficial perineal fascia (Colles) (Fig. 18.3). Ultrasound description of these fascial layers is feasible; however, it has limited clinical relevance. The penile urethra is fixed to the cavernous bodies of the penis and lies in the inferior part of the penile shaft. The penile urethra is fixed and covered by two fascial layers composed of the deep fascia of the penis (Buck) fused with deep perineal fascia and the superficial fascia of the penis. The junction between the bulbar and penile urethra is anatomically located just proximally to the penoscrotal angle. The third distal segment of the anterior urethra is the distal part of the penile urethra which enlarges into the navicular fossa completely covered by corpus spongiosum of the gland and then ends at the urethral meatus. Urethral stricture is the preferred term for narrowing of a segment of the urethra which is surrounded by corpus spongiosum, i.e., the urethral meatus to the bulbar urethra [1]. The severity of a urethral stricture is related to the amount of damage to the corpus spongiosum, the investing vascular layer of the urethra, resulting in a progressive process termed spongiofibrosis (Fig. 18.3b) [2, 3].

Fig. 18.3

(a) Normal bulbar urethra ultrasound. White bar, CS corpus spongiosum; yellow bars, BSM bulbospongiosus muscle, DPF deep perineal fascia, SF superficial fascia; white arrows, BF buck fascia, urethral epithelium (arrowhead). (b) Urethral stricture with spongiofibrosis

The ultrasound examination is not the up-front method to investigate the urethra, first, because distension is always required, and second, because ultrasound urethrogram is not able to study the lumen of the posterior segment which can be achieved only in few cases. Therefore, ultrasound is not a global study of the urethra, and retrograde contrast urethrography is the standard [4–6]. Nevertheless, ultrasound is the first diagnostic tool in the study of paraurethral soft tissues or perineal masses, because traditional contrast urethrography does not evaluate extraluminal lesion. Magnetic resonance is the gold standard in the imaging of the perineal soft tissues. Magnetic resonance has the potential to provide significant additional information for the staging of urethral and penile cancers [7].

18.2 Ultrasound of the Male Posterior Urethra

Ultrasound of the posterior urethra, either transperineal scanning with a 3.5–5-MHz curved array probe or a transrectal probe, can be used. Distension of the posterior urethra is only well achieved by asking the man to void. However, some men may be unable to void because of inhibition. Overall posterior ultrasound urethrogram is much less reliable than micturating or descending contrast urethrography. Indications to posterior urethral ultrasound are vesicourethral anastomosis after radical prostatectomy, posterior urethra and bladder neck after surgery for benign prostatic hyperplasia, study of transurethral catheter placement or displacement, anatomy of Cowper’s gland diseases, and anatomy after pelvic trauma (Tables 18.1 and 18.2).

Table 18.1

Filling defects in the urethra

Intraluminal – calculi, foreign bodies, bezoar or hair ball, blood clot

Congenital anatomical – posterior urethral valve, ureterocele

Benign lesion congenital or acquired urethral polyp

Benign inflammatory lesions – urethritis cystica

Malignant lesions – solid tissue or papillary/soft tissue

Table 18.2

Classification of posterior urethral trauma based on site of injury [16]

Grade	Clinical findings
I	Posterior urethra stretched, but integrity is maintained
II	Tear of the membranous urethra, above the urogenital diaphragm
III	Partial or complete tear of both anterior and posterior urethra, with disruption of the urogenital diaphragm
IV	Bladder injury extending into the urethra (IVa) injury of the bladder base, with periurethral extravasation

18.3 Ultrasound Urethrogram of the Male Anterior Urethra

The primary imaging modality for demonstrating the male anterior urethra is fluoroscopic contrast urethrography performed either as a retrograde study via catheter insertion into the distal urethra. The main advantages of the technique are that it has a high sensitivity for the detection of urethral strictures and that the images obtained are easy to interpret for the non-radiologist. However, there are a number of disadvantages, including the fact that it may not be possible to catheterize the distal urethra, particularly in patients with external meatus narrowing or previous surgery. Also, the technique is necessarily invasive, and interpretation may be hampered by the presence of air bubbles, which may obscure pathology or even provide a false-positive study. In addition, as the balloon of the catheter is inflated in the distal urethra, pathology in this area will not be identified.

Ultrasound urethrogram is a suitable technique for visualizing the male anterior urethra, but it cannot replace contrast urethrography. Ultrasound could be strongly considered as follow-up option in the man with known anterior urethral stricture [8, 9].

However, early studies identified not only the ability of ultrasound to demonstrate the exact length of strictures but also the added ability to define the periurethral tissues, as opposed to contrast urethrography, which only demonstrates the lumen. In particular, the presence and degree of periurethral fibrosis can be shown with a view to guiding surgery [10]. The initial experiences with ultrasound evaluation of the urethra were described separately in the late 1980s by McAninch et al. [5] and Merkle and Wagner [11].

A linear probe for superficial tissue, such as 7.5–12 MHz, with an array length of 4 cm or more, is used with direct skin contact along the ventral surface of the penis. The most used method is to distend the urethra using either a Knutson’s clamp or a Foley catheter with the balloon distended in the navicular fossa. The urethra is distended using saline. We use a large syringe with saline 60 cc and manual compression on the gland to distend the urethra during ultrasound. Contrast media, unlike saline, may obscure intraurethral bodies/calculi. The anterior distended urethra is scanned in the transverse and longitudinal planes: description of smoothness of the urethral mucosal lining and narrowings of the lumen are noted; measure in length is carefully reported (Figs. 18.3 and 18.4). Strictures and subtle wall irregularities of the anterior urethra are analyzed after distension of the urethra with saline, using a flat array ultrasound probe.

Fig. 18.4

Ultrasound urethrogram showing a stricture in the penile anterior urethra, the abnormal mucosa is more extensive than estimated by just measurement of the length of the stricture

The ability to visualize the stricture as well as the corresponding surrounding wall thickness change is an advantage of this technique over contrast urethrography. Whether this additional information is of clinical value in the selection of appropriate stricture therapy has been not yet supported by evidence. The length of stenosis is necessary to plan surgery urethroplasty rather than optical urethrotomy. The superiority of ultrasound over contrast urethrography is not yet proven. Moreover, some of the disadvantages associated with retrograde urethrography also apply to ascending urethral ultrasonography. The technique remains invasive, with insertion of a catheter distally, and requires two operators (one to instill fluid to distend the urethra and ensure no displacement of the catheter and the second to perform the ultrasound). Furthermore, despite trimming of the distal catheter beyond the balloon to allow visualization of the penile urethra, pathology in the fossa navicularis cannot be identified owing to the presence of the balloon. The procedure also requires a sterile technique. An alternative method is the ascending technique that has been superseded by a descending approach [12]. The patient attends with a full bladder and voids into a receptacle. Urethral distension is achieved with the urine stream, which is interrupted by the patient gently clamping the penis between thumb and forefinger during voiding, approximately 2 cm proximal to the tip after retraction of the foreskin. If needed in selected patients, views of the navicular fossa are obtained while actively voiding [7]. However, some men may be unable to void because of inhibition, and we do not suggest the self-distension.

18.4 Cowper’s Gland (CG)

The CGs are two periurethral glands and are accessory sex glands that help to lubricate the semen. They are located symmetrically close to the urogenital diaphragm, and their ducts open in the bulbar urethra as separate units or singly (Fig. 18.5). The commonest anomaly is the Cowper’s duct cyst or diverticulum, which is sometimes termed a syringocele. It is a retention cyst, is believed to be a congenital abnormality, and usually presents in childhood but in the occasional case may be first encountered in the adult male. It is postulated that the adult Cowper’s duct cyst is the result of postinflammatory stricturing of the duct, either the result of infection or instrumentation. The underlying fault is the obstruction of the duct of the Cowper’s gland, which results in cystic dilatation. It may be an incidental finding, or it may present with urinary infections, postmicturition dribbling, perineal pain, or lower urinary tract symptoms.