The prostate is a heterogeneous, oval-shaped organ that surrounds the proximal urethra. In the adult, the normal gland measures approximately 3.8 cm (cephalocaudal) by 3 cm (anteroposterior) by 4 cm (transverse).1 It normally weighs about 20 g, but it can be slightly larger in men older than 40 years. The prostate is composed of glandular and fibro-muscular tissue and is located in the retroperitoneum between the floor of the urinary bladder and the urogenital diaphragm. The base of the prostate, its superior margin, abuts the inferior aspect of the urinary bladder. The gland is bounded anteriorly by prostatic fat and fascia, laterally by the obturator internus and levator ani muscles, and posteriorly by areolar tissue and Denonvilliers’ fascia, which separates it from the rectum.
The seminal vesicles are two sac-like lateral structures that outpouch from the vas deferens and are situated on the posterior-superior aspect of the prostate between the bladder and the rectum. The seminal vesicles join the vas deferens to form the ejaculatory ducts, which then enter the base of the prostate to join the urethra at the verumontanum. The verumontanum is a midpoint region between the prostatic base and apex and surrounds the urethra. The size and fluid content of the seminal vesicles are variable.
The prostatic urethra courses through the substance of the gland and is divided into a proximal and a distal segment. The proximal segment extends from the neck of the bladder to the base of the verumontanum; the distal segment begins at this point and extends to the apex of the gland.
Blood supply to the prostate is from the internal iliac arteries, which eventually give rise to urethral and capsular arteries. Venous return is via the prostatic plexus, which drains into the internal iliac vein.2 The prostate produces seminal fluid, which is essential to the function of the spermatozoa.
NORMAL SECTIONAL ANATOMY
The earlier anatomic descriptions of the prostate divided the gland into five major lobes: anterior, posterior, media, and two lateral. More recent histological studies, however, have divided the prostate into three glandular zones: the transitional, central, and peripheral zones. There is also a nonglandular region called the anterior fibromuscular stroma2 (Fig. 6–1 A, B).
FIGURE 6–1. (A) An axial view of normal prostate anatomy: CZ is the central zone, FS is the fibromuscular stroma, TZ is the transition zone, and PZ is the peripheral zone. (B) Sagittal view of the normal prostate anatomy: SV is the seminal vesicle, ED is the ejaculatory duct, DU is the distal urethra, PC is the prostatic capsule, VD is the vas deferens, VM is the verumontanum, PU is the proximal urethra, CZ is the central zone, FS is the fibromuscular stroma, TZ is the transition zone, PS is the periurethral stroma, and PZ is the peripheral zone. (Modified with permission from Dakin R., et al. Transrectal ultrasound of the prostate: Technique and sonographic findings. JDMS. 1989; 5(1):1-15.)
The transitional zone represents about 5% of the glandular prostate and is located in the central region on both sides of the proximal urethra.2 The ducts of the transitional zone run parallel to the urethra and end in the proximal urethra at the level of the verumontanum.
The central zone constitutes approximately 25% of the prostatic glandular tissue and is located at the base of the gland.2 It is wedge-like in shape, is oriented horizontally, and surrounds the ejaculatory ducts throughout their course. The zone narrows to an apex at the verumontanum. Ducts of the vas deferens and seminal vesicles come together to form the ejaculatory ducts, which pass through the central zone and join the urethra at the verumontanum.
The peripheral zone constitutes about 70% of the glandular tissue.2 This zone consists of the posterior, lateral, and apical parts of the prostate and also extends anteriorly. The ducts of the peripheral zone enter the urethra at, and distal to, the verumontanum.
Anterior Fibromuscular Stroma
The anterior fibromuscular stroma is a thick nonglandular sheath of tissue that covers the entire anterior surface of the prostate. This tissue is composed of smooth muscle and fibrous tissue.
NORMAL SONOGRAPHIC ANATOMY
Sonographically, the prostate is a homogeneous gland with low-level echoes. The periurethral glandular tissue that surrounds the proximal urethra is homogeneous and isoechoic. The central zone is normally more echogenic than the peripheral zone because it has a greater amount of corpora amylacea (calcified deposits) in the central zone. The fibromuscular capsule, located anteriorly, is smooth, hyperechoic, and sharply defined.
In sonography, the terms outer and inner gland are sometimes used to distinguish between the above zones. The outer gland consists of the peripheral and central zones, whereas the inner gland consists of the transitional zone, the inner anterior fibromuscular stroma, and the internal urethral sphincter. The surgical capsule separates the inner gland from the peripheral zone.
The seminal vesicles are visualized as symmetrically paired structures that are slightly less echoic than the prostate. The vas deferens can be depicted as tubular hypoechoic structures joining the seminal vesicles medially. On transverse imaging, they are round or oval and are located between the seminal vesicles. The ejaculatory duct, when empty, can be seen as a hyperechoic line joining the urethra. The empty urethra is identified by its echogenic walls coursing through the prostate. When filled with fluid, the urethra is recognized more easily. The surgical capsule is usually seen as a hypoechoic line but can also be echogenic due to calcification.
On longitudinal sections, the anterior space between the prostate and the seminal vesicles (prostate–seminal vesicle angle) is variable but is the same bilaterally. Similarly, the posterior space between the prostate and the seminal vesicle (or nipple) is symmetrical on both sides.1
INDICATIONS FOR SONOGRAPHY
Patients can be referred for endorectal prostate sonography for various reasons such as the following.1, 2
• An abnormal digital rectal examination, as indicated by a palpable prostatic nodule or prostate with an asymmetrical size or shape
• Biopsy guidance of sonographically detected abnormal areas
• Clinical evidence of prostate cancer such as an elevated level of prostatic-specific antigen or radio-graphically detected bone metastasis
• Guide treatments for prostate cancer such as radiotherapy and cryotherapy
• Monitoring of a patient’s response to therapy
• Inflammation leading to the formation of a prostatic abscess
• Infertility caused by the absence of the seminal vesicles or a bilateral obstruction of the ejaculatory ducts
• Difficulties in voiding caused by an obstruction of the prostatic urethra
• Calculation of prostatic volume prior to surgery
EQUIPMENT AND EXAMINATION TECHNIQUES
Technical innovations have led to the availability of several types of endorectal imaging systems. The original systems were radial (axial) scanners that produced transverse-oriented slices of the prostate. Later, linear array scanners that imaged the gland in longitudinal sections were introduced. Today, biplanar endorectal probes that can produce both longitudinal and transverse sections of the gland are available, thus eliminating the need for two separate probes. The frequency of endorectal probes ranges from 5 to 8 MHz. A guide can be attached directly to the probe allows one to biopsy suspicious prostatic lesions safely and accurately. Fig. 6–2 demonstrates the placement of the endorectal transducer and anatomy of the prostate.
FIGURE 6–2. Drawing demonstrating the placement of the endorectal transducer and anatomy of the prostate.
Preparation of the patient for endorectal sonography begins with a self-administered enema before the examination. This not only eliminates fecal material from the rectum that might adversely affect the quality of the image, but also reduces the risk of contamination of the prostate. If biopsy is to be performed, prophylactic antibiotics must be given before the procedure and continued for 24–48 hours afterward.3
The patient is generally examined in the left lateral decubitus position. The lithotomy position is sometimes used when other urological procedures are also being performed. The probe is previously sterilized and covered with a condom before insertion. A digital rectal examination is performed to exclude any obstructing lesions or rectal fissures and to correlate the exam with any palpable abnormalities. Axial scanning begins at the level of the seminal vesicles. The probe is then gradually withdrawn to image the gland sequentially down to the level of the apex.
Sagittal imaging begins in the midline and shows the gland from base to apex with portions of the seminal vesicles. The probe is then rotated clockwise and counterclockwise to demonstrate the right and left sides of the gland.
On color Doppler examination, moderate vascularity from the capsular and the urethral arteries and their branches can be visualized.
In the United States 45,000 men die from prostate cancer each year. Men of African American descent and those with a family history of prostate cancer are at higher risk.2 Although the etiology of prostatic cancer remains unclear, the factors implicated in its causation include age, genetic or racial makeup, hormonal influences and diet.
Screening tests for prostate cancer include annual digital rectal exam and prostate-specific antigen (PSA) blood test. Screening is recommended starting at age 50 years, or at age 40 years in men with a positive family history of prostate cancer. Normal PSA is less than 4 ng/mL. Elevated levels can be seen in patients with cancer, benign prostatic hyperplasia, prostatitis, and following procedures such as cystoscopy, prostate biopsy, and Foley catheter insertion. Artificially reduced levels of PSA are seen in patients taking medication such as Proscar (finasteride), which is used for treatment of benign prostatic hyperplasia.2
Anatomic studies have determined that 70% of prostate cancers originate de novo in the peripheral zone, 20% originate in the transitional zone, and 10% originate in the central zone.1 Clinical symptoms include back pain and an obstruction of urinary outflow that may mimic benign prostatic hyperplasia.
Sonographically, prostate cancer varies in echogenicity. However, the most common appearance is a hypoechoic nodule on the peripheral zone. Hyperechoic cancers can rarely present as focal areas of calcification. Isoechoic cancers are difficult to detect, although secondary signs such as capsular bulging and asymmetry of the gland may aid in the diagnosis. Tumor invading the entire gland may have an inhomogeneous appearance.2
Invasion of the tumor into the seminal vesicles can be seen as solid material within this normally fluid-filled structure. Invasion may make the size, shape, and echogenicity of the seminal vesicles asymmetrical in appearance.
Obliteration of the nipple or the prostate-seminal vesicle angle is another diagnostic criterion for invasion by the tumor.1 However, because the nipple is not imaged consistently, the criterion is of limited usefulness. Doppler ultrasound has not proven to be useful in the diagnosis of prostate cancer. Staging of prostatic cancer with ultrasound is also feasible but is limited by problems of resolution.
Benign Prostatic Hyperplasia and Hypertrophy
Benign prostatic hyperplasia affects 80–90% of adult men.1 Its etiology is believed to be related to hormonal factors. The clinical symptoms of the disease may include decreased flow of urine, difficulty in initiating and terminating urination, nocturia, and urinary retention. Benign prostatic hyperplasia originates in the transitional zone and in periurethral glandular tissue.
The sonographic characteristics of hyperplasia nodules are variable. They can be hypoechoic, hyperechoic, or of mixed echogenicity. Enlargement of the central gland by benign prostatic hyperplasia causes lateral displacement of the peripheral zone. The prostatic calculi that are often encountered with benign prostatic hyperplasia are believed to be the result of stasis of prostatic secretions. Corpora amylacea are seen as echogenic foci similar to prostatic calculi. Benign prostatic hyperplasia causes the number of cells in the prostate to increase, whereas benign prostatic hypertrophy refers to an increase in the size of existing cells. Hyperplasia and hypertrophy often develop concurrently and result in the enlargement of the prostate gland. Transrectal ultrasound is not usually indicated in patients with benign prostatic hyperplasia unless prostate cancer is a clinical concern.
Prostatitis and Prostatic Abscess
Inflammation of the prostate can be the result of acute or chronic bacterial infections or of unknown nonbacterial factors. Clinical symptoms of prostatitis may include fever, pelvic and low back pain, urinary frequency and urgency, and dysuria. Although prostatitis usually involves the peripheral zone in its initial stages, it can originate in any area of the gland.
In acute prostatitis, the main sonographic finding is a hypoechoic gland with anechoic areas that may mimic carcinoma. Increased blood flow may be seen on color Doppler. In chronic prostatitis, sonographic findings may include focal masses of variable echogenicity, ejaculatory duct calcifications, thickening or irregularity of the prostatic capsule, dilatation of the periprostatic veins, and distention of the seminal vesicles.2
A prostatic abscess may develop secondarily to prostatitis. Endorectal sonography may show hypoechoic areas corresponding to liquefaction within the abscess. Sonography can be used to guide aspiration of an abscess if necessary.
Prostatic Utricle Cysts
Prostatic utricle cysts occur as a result of dilatation of the prostatic utricle. On sonography, they are small, anechoic structures located in the midline. They can, however, become large and measure several centimeters in size.
Ejaculatory Duct Cysts
Ejaculatory duct cysts occur secondarily to obstruction or a diverticular of the duct. They contain spermatozoa and are associated with infertility. On sonography, they are seen as anechoic masses within the ejaculatory ducts.
Seminal Vesicle Cysts
Seminal vesicle cysts result from an anomaly of the Wolffian duct. Large, solitary cysts may be associated with renal agenesis. They can also be associated with infertility when they obstruct the seminal vesicle.
Patients with azoospermia (no sperm in the ejaculate) can be examined to exclude ejaculatory duct obstruction. This is diagnosed when the seminal vesicle measures more than 1.5 cm in anteroposterior diameter; presence of a dilated ejaculatory duct and a midline cyst. Additional ultrasound findings in infertility may include the following: bilateral absence of the vas deferens; bilateral occlusion of the vas deferens, seminal vesicles, and ejaculatory ducts by calcification or fibrosis; and obstructing cyst of the seminal vesicle, ejaculatory ducts, or prostate.
1. Rifkin M. Ultrasound of the Prostate. New York: Raven Press; 1988.
2. Toi A, Bree R. The prostate. In: Rumack C, Wilson S, Charboneau W, et al., eds. Diagnostic Ultrasound. 3rd ed, Vol. 1. St. Louis: Mosby; 2005.
3. Reiter R, Dekernion J. Epidemiology, etiology and prevention of prostate cancer. In: Walsh P, Ritik A, et al., eds. Campbell’s Urology. 8th ed. Philadelphia: WB Saunders; 2001.