Core Curriculum, The: Ultrasound
1st Edition

15
Prostate and Seminal Vesicle Ultrasound
The initial enthusiasm for transrectal US (TRUS) imaging of the prostate to screen for and stage prostate cancer has dimmed considerably. Currently, in most practices, TRUS is primarily used to guide prostate biopsy and for a few specific indications, such as evaluation of male infertility and diagnosis and drainage of prostatic abscesses [1,2]. TRUS is an excellent technique for examining the seminal vesicles for diseases associated with infertility, perineal and ejaculatory pain, and hematospermia [3]. Transabdominal US imaging of the prostate through a urine-filled bladder has limited utility, but can be used to estimate prostate size.
Imaging Technique
Using a transabdominal approach, the prostate is imaged through the fluid-distended bladder (Fig. 15.1). A 3.5-4.0-MHz sector transducer is positioned on the lower abdomen just above the symphysis pubis with the sound beam directed caudally. The prostate is examined in transverse and longitudinal planes. Prostate size is estimated and abnormalities, such as calcifications, can be demonstrated.
The transrectal approach allows the use of high-frequency 5-10-MHz transducers that provide the most detailed US examination. If transrectal biopsy is planned, the patient is routinely premedicated with antibiotics. No other patient preparation, such as an enema, is usually needed. The patient is placed in left lateral decubitus position with both knees flexed toward his chest and his head comfortably positioned on a pillow. Digital rectal examination is performed to judge the size of the prostate and to detect any palpable abnormalities. The endorectal transducer is coated heavily with US gel and is sheathed in a condom, which is also generously coated with US gel. The probe is gently placed into the rectum with the tip initially directed toward the sacrum to follow the curve of the rectum. Most endorectal transducers are end-fire sector format. Side-fire linear array transducers
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provide excellent images, but cannot be used to direct transrectal biopsy. The prostate, seminal vesicles, and periprostatic tissues are examined in transverse and sagittal planes. Color Doppler may be used to detect the vascular changes associated with prostate cancer and inflammatory conditions [4]. Images are routinely viewed inverted with the transducer
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position at the bottom of the image (Fig. 15.2). The patient’s right side is depicted on the left side of the image.
Figure 15.1 Transabdominal US of Prostate. Midline sagittal view through the urine-distended bladder (b) shows the enlarged prostate (P) at the bladder base. The urethral orifice (arrow) appears as a V-shaped notch in the prostate.
Figure 15.2 Transrectal US of Prostate. A. Transverse image of the prostate is viewed inverted with the intrarectal transducer (T) positioned at the bottom of the image. This orientation corresponds to images of the prostate as seen on CT or MR with the patient in supine position. The patient’s right side (R) is displayed on the left side of the image. This scan through the mid-prostate shows the normal peripheral zone (p) to be well demarcated from and more echogenic than the hypertrophied inner gland (I). Arrowheads mark the surgical capsule. The wall of the rectum (arrow) is seen adjacent to the transducer. Cursors (+) measure the transverse dimension of the gland. L, patient’s left side. B. Transverse image obtained closer to the base of the prostate in another patient shows excellent demarcation of the peripheral zone (p) from the inner gland. Compare (A) and (B) to observe that the peripheral zone is thinner near the base and thicker near the apex. The prostates of both men show the inner gland changes of BPH.
Figure 15.3 Prostate Anatomy. Drawing demonstrates prostatic anatomy in sagittal and transverse planes. (Adapted with permission from Brant WE, Helms CA. Fundamentals of Diagnostic Radiology. Lippincott-Williams & Wilkins, Baltimore, 1999.)
Anatomy
The prostate is shaped like a rounded, inverted pyramid and sits on the urogenital diaphragm, just behind the symphysis pubis (Fig. 15.3). The broader base of the prostate supports the base of the bladder (base to base). The tapered apex rests on the muscular urogenital diaphragm. The seminal vesicles lie in the posterior groove between the bladder and the prostate. The seminal vesicles are convoluted tubes coiled to form a lobulated sac (Fig. 15.4). The terminal ampullary portion of the vas deferens passes medial to and joins the seminal vesicle to form the ejaculatory duct (see Fig. 15.6). The ejaculatory ducts course from superolateral to inferomedial in the upper half of the prostate and empty into the urethra at the verumontanum. The urethra runs through anterior prostate in an arching course from the bladder neck to the prostate apex. In cross-section, the urethra is horseshoe-shaped with a posterior ridge that forms the verumontanum. In the mid-portion of the verumontanum is a 6-mm, blind-ending sac–the utricle. The utricle is the müllerian remnant that forms the uterus and vagina in the female. Prostatic ducts empty into the length of the intraprostatic urethra. The prostate is enveloped by a thin but tough fibrous capsule and is surrounded by fat and a prominent plexus of periprostatic veins.
The prostate gland is divided into three anatomic zones and an anterior non-glandular area (Fig. 15.3). The zones of the prostate can be likened to a catcher’s mitt holding a softball. The “mitt” is posterior and represents the peripheral zone (PZ). The PZ at the base of the prostate is thin, like the finger portion of the catcher’s mitt. The PZ thickens toward the apex like the bottom portion of the catcher’s mitt. The PZ encompasses only the posterolateral rectal surface of the gland, and does not extend circumferentially around the gland. The PZ contains 70% of the glandular tissue. The softball represents the “inner gland,” which consists of the periurethral transitional zone (TZ) and the pyramidal-shaped central zone (CZ). The TZ constitutes only 5% of the total glandular tissue in young men, but is the site of benign prostatic hypertrophy (BPH) and enlarges substantially in most older men. The CZ is largest at the prostate base and tapers toward the apex, constituting 25% of
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the total glandular tissue. With BPH, the enlarging TZ compresses the CZ and thins the PZ. The anterior surface of the prostate consists only of fibrous and muscular tissue without acini. It is termed the anterior fibromuscular stroma and is not a site of disease.
Figure 15.4 Normal Seminal Vesicles. Transverse image shows both seminal vesicles (s) with the normal “bow-tie” appearance. Slight asymmetry in size is a normal variant. b, bladder.
Figure 15.5 Normal Seminal Vesicle Angle. Lateral sagittal image shows the normal fat-filled acute angle (arrow) between the hypoechoic seminal vesicle (s) and the more echogenic base of the prostate (P). The bladder (b) is partially filled with urine.
Most cancer (70%) occurs in the PZ. Approximately 10% arise in the CZ and 20% in the TZ. Transurethral resection of the prostate consists of resection of the periurethral TZ tissue above the verumontanum, leaving a cavity that is continuous with the bladder lumen. Cancer in the TZ is often discovered incidentally in the tissue resected by transurethral resection of the prostate.
TRUS demonstrates the seminal vesicles as oval hypoechoic structures 3-5 cm long and 1-2 cm diameter. In axial plane, the two seminal vesicles have a bow-tie appearance (Fig. 15.4). Mild asymmetry in size is common. Marked asymmetry suggests tumor involvement in patients with prostate cancer. An acute angle between the posterior seminal vesicles and the base of the prostate is invested with echogenic fat. Loss of this acute angle suggests tumor invasion into the seminal vesicles (Fig. 15.5). The thickened ampullary portion of the vas deferens can often be seen separately from the seminal vesicles (Fig. 15.6).
In young men, the PZ is isoechoic with the TZ and CZ. With aging and hypertrophy of the TZ, the inner gland becomes enlarged and heterogeneous. The PZ then appears relatively hyperechoic and distinct. The tissue boundary between the PZ and the inner gland is termed the surgical capsule (Fig. 15.2). The urethra is identified in the midline of the prostate by looking for its V-shaped orifice at the bladder base (Fig. 15.7).
Estimates of prostate weight can be made by digital rectal examination. Measurement of prostate volume by US is easily related to weight because 1 cm3 equals 1 gm. Prostate
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volume is calculated by the formula (length × width × height × 0.52 = volume). In young men the normal gland is 20 gm. In older men, the prostate is considered enlarged when it exceeds 40 gm.
Figure 15.6 Normal Ampullary Portion of the Vas Deferens. Angled coronal image of the left seminal vesicle (s) shows the ampullary portion of the vas deferens (d). The junction of the vas deferens and seminal vesicle is the commencement of the ejaculatory duct (e).
Figure 15.7 Normal Prostatic Urethra. Sagittal midline image of the prostate using an endorectal linear array transducer shows the prostatic urethra (arrowheads) and the urethral orifice (arrow) at the base of the bladder (b). r, wall of the rectum; s, seminal vesicle.
Prostate Cancer
Prostate cancer is the most commonly diagnosed cancer in men. American men have approximately a 10% lifetime risk of having the disease and a 3% lifetime risk of dying from the disease. The incidence of prostate cancer is higher in African-American men and in men with a positive family history. TRUS was initially promoted as an effective imaging method to screen men for prostate cancer. However, its proven sensitivity in the range of 50-70% and specificity in the range of 40-60% makes it inadequate as a screening method [5]. Current American Cancer Society Guidelines for screening recommend yearly prostate-specific antigen (PSA) testing and digital rectal examination for all men older than age 50, and for men older than age 45 who are African-American or have family history of prostate cancer [6]. TRUS-guided biopsy is often recommended for PSA values >4 ng/mL and for palpable abnormalities [6,7].
Normal, hyperplastic, and neoplastic prostatic epithelium produce PSA. Cancers raise serum PSA 10 times as high as an equal volume of BPH tissue. PSA is organ-specific but not disease-specific. In addition to cancer, BPH, prostatitis, prostate infarction, biopsy, and surgery increase serum PSA. Serum PSA values of 0-4 ng/mL are considered normal, although up to 25% of men with prostate cancer will have PSA values in this range [6]. Values of 4-10 ng/mL are considered borderline and carry approximately a 20% risk of cancer. Values exceeding 10 ng/mL have a 67% risk of cancer. In attempts to increase the specificity of PSA, PSA density and PSA velocity calculations are used [8]. PSA density is determined by the ratio of serum PSA to prostate volume with values >0.12 interpreted as at risk for cancer. PSA velocity refers to the rate of increase of serum PSA over time. A 20% increase in serum PSA over baseline in 1 year or an absolute increase of 0.75 ng/mL in 1 year have been used as indications for biopsy.
TRUS-guided biopsy is performed using 18-gauge, spring-loaded, automated biopsy needles [9]. Because the biopsy is performed transrectally, patients are premedicated with antibiotics. Any lesions considered suspicious by digital rectal or US examination are biopsied, and 6-10 random biopsies are taken in an organized pattern to sample all areas of the gland (Fig. 15.8). Potential complications of the biopsy include hematuria, hematospermia, and infection.
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Prostate cancer spreads by direct invasion through its capsule into the periprostatic tissues and seminal vesicles, by lymphatics to regional lymph nodes, and by venous invasion to the spine. Further hematogenous spread may extend to any organ. Findings of prostate cancer include
Figure 15.8 TRUS-Guided Prostate Biopsy. Sagittal image of the left lateral prostate shows the needle (arrow) in the left base region during a random pattern biopsy of the prostate.
  • Discrete nodule in the PZ.
    • - Hypoechoic nodules (70%) (Fig. 15.9) are not specific for cancer. See Box 15.1 for differential diagnosis.
    • - Hyperechoic/heterogeneous nodule (30%).
  • Infiltrative zone in prostate glandular tissue.
    • - Hypoechoic ill-defined mass is common (Fig. 15.10).
    • - Cancers are isoechoic and difficult to detect by US in up to 25% of all cancers. Some lesions show mass effect, contour abnormality, asymmetry of gland size or shape, or loss of PZ differentiation from the central gland. However, most isoechoic tumors are detected only by random US-guided biopsy.
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  • Cystic lesions are very rare. Nodular soft tissue tumor is seen within a cystic mass [10].
  • Focal abnormal vascularity on color Doppler US may identify cancers in the absence of gray scale US abnormalities. Focal increased vascularity or abnormal tangled vessels are signs of malignancy [11].
Figure 15.9 Hypoechoic Nodule in Peripheral Zone. Sagittal image shows an ill-defined hypoechoic nodule (arrow) in the peripheral zone of the prostate. Biopsy confirmed adenocarcinoma.
Benign Prostatic Hypertrophy
Benign hyperplasia of prostatic adenomatous tissue begins as early as age 40 and continues throughout life. By age 80, 90% of men have BPH. Starting at age 50, prostate weight doubles approximately every 10 years. Symptoms relate more to the pattern of hypertrophy than to the absolute volume of enlargement. So-called “median lobe hypertrophy” of periurethral prostatic tissues at the bladder base is primarily responsible for symptoms of hesitancy, decreased force and caliber of urine stream, dribbling, frequency, nocturia, and incomplete bladder emptying. Tissue hypertrophy occurs in the periurethral TZ and usually compresses the CZ and thins the PZ, so that the majority of the gland appears involved. The term inner gland is often used to describe the hypertrophied TZ and indistinguishable CZ. US diagnosis is based on the following findings [12]:
  • Enlarged prostate >40 gm. The inner gland is hypoechoic compared to the PZ (Fig. 15.2). The CZ is compressed and is often not discernible.
  • Inhomogeneous inner gland. The echogenicity of the inner gland is heterogeneous and the echotexture is coarse (Fig. 15.11).
  • Multiple nodules. Most nodules are hyperechoic; some are hypoechoic or isoechoic.
  • Calcifications. Prostatic calculi form primarily along the surgical capsule, the boundary between the hypertrophied tissue and the PZ (Fig. 15.12).
  • Cystic changes. Cystic degeneration of hyperplastic nodules and retention cysts are common (Fig. 15.11).
Figure 15.10 Infiltrative Prostate Cancer. Sagittal image shows an ill-defined hypoechoic mass (arrows) involving both the peripheral zone and the inner gland in the region of the prostate apex. Biopsy revealed poorly differentiated adenocarcinoma.
Figure 15.11 Benign Prostatic Hypertrophy–Cystic Changes. Transverse image through the mid-prostate shows enlargement, inhomogeneity, and cystic changes (arrows) characteristic of benign prostatic hypertrophy. The peripheral zone (p) is normal and slightly echogenic compared with the hypertrophied central gland.
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Inflammation
Acute Prostatitis/Abscess
Acute bacterial infection of the prostate is a clinical diagnosis with no indication for imaging. However, if symptoms fail to abate with antibiotic therapy, then abscess is suspected. TRUS is indicated to diagnose abscess and provide guidance for aspiration and drainage [13]. Prostate abscess is most common in diabetic and immunocompromised patients and in patients with indwelling urinary catheters. Causative bacteria are most commonly Escherichia coli, Staphylococcus, and gram-negative colon bacteria. Findings are
  • Fluid collection with ill-defined borders, irregular walls, and occasionally septations (Fig. 15.13). Fluid may be homogeneous hypoechoic or inhomogeneous with debris. Abscesses are found primarily in the inner gland, although the PZ is commonly involved in the inflammatory process.
  • Fluid collection is compressible and deforms with transducer pressure.
  • Hypoechoic halo may surround the fluid collection.
  • Gas in the abscess produces ring-down and comet tail artifacts.
  • Color flow imaging shows periabscess hypervascularity.
  • US-guided aspiration is recommended to obtain fluid for culture and for treatment by drainage. Appropriate antibiotic therapy based upon bacterial culture and sensitivity is also needed.
Figure 15.12 Benign Prostatic Hypertrophy–Calcifications. An angled transverse scan through the left base region of the prostate demonstrates calcifications, associated with benign prostatic hypertrophy, along the surgical capsule (small arrows). The peripheral zone (p) is normal. Note the normal sharp demarcation of the prostate (curved arrow) from the periprostatic fat.
Figure 15.13 Prostatic Abscess. Transverse image through the mid-prostate shows heterogeneous low density in the inner gland (I) and in the peripheral zone (p) with an irregularly shaped and poorly defined fluid collection (a) that proved to be an abscess. Transrectal US-guided needle placement allowed complete aspiration with culture of the purulent material. Subsequent antibiotic therapy appropriate for the cultured Escherichia coli organisms resulted in complete resolution.
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Chronic Prostatitis
Chronic inflammation of the prostate may be caused by incomplete treatment of bacterial prostatitis. Chlamydia and Mycoplasma may also cause chronic prostatitis. Commonly no organism is identified. Persistent symptoms include urgency, frequency, nocturia, and perineal pain.
  • The prostate shows diffuse nodularity and inhomogeneous parenchyma.
  • Prostatic calculi, small and in clusters or large and coarse, are common and characteristic. Calculi form in corpora amylacea, a proteinaceous material found in prostate glandular tissue. They are usually found near the urethra in prostatitis (Fig. 15.14), in distinction with the calcifications of BPH that form near the surgical capsule. Calculi may become infected and serve as a reservoir for relapsing bacterial infection.
  • Thickening and irregularity of the prostatic capsule is common.
  • Periprostatic veins may be dilated.
Figure 15.14 Chronic Prostatitis. Coarse calcification (arrow) in the periurethral area is characteristic of chronic prostatitis. This gland is somewhat heterogeneous but of normal size (~30 gm). Changes of chronic prostatitis are often superimposed on changes of benign hypertrophy. b, bladder.
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Cystic Lesions
Retention Cysts
Obstruction of prostatic ductules results in cystic dilatation of glandular acini [14]. These cysts contain clear fluid and no sperm. Symptoms are rare.
  • Smooth-walled unilocular cyst 1-2 cm in diameter.
  • Anechoic fluid.
  • Usually occur away from the midline.
Cystic Degeneration of BPH Nodules
Cystic changes in hyperplastic nodules are common and may be caused by hemorrhage and necrosis.
  • Cysts are small (<1cm) and within a nodule (Fig. 15.12).
  • May contain calculi or echogenic fluid caused by hemorrhage or necrosis.
Müllerian Duct Cyst/Utricle Cyst
Müllerian duct cysts arise from the müllerian remnant in the utricle. Utricle cysts are enlargements of the utricle. Many authors use these terms interchangeably while others believe they are two separate entities [14]. Both extend from the midline verumontanum. Symptoms depend upon size of the cyst. Most utricle cysts are small (<1 cm), whereas müllerian duct cysts are frequently large and extend well beyond the prostate.
  • Midline cyst with thin walls and anechoic fluid.
  • Utricle cysts commonly tubular and <1 cm in length.
  • Utricle cysts fill with urine and may empty with voiding.
  • Large müllerian and utricle cysts may extend well beyond the prostate and present as cystic pelvic masses.
Ejaculatory Duct Cyst
Obstruction of the ejaculatory ducts causes cystic dilatation of the duct and may be a cause of infertility, ejaculatory pain, and hematospermia [15].
Figure 15.15 Seminal Vesicle Cysts. Angled transverse image of the right seminal vesicle shows three small cysts (arrows). This finding is strongly associated with obstruction of the seminal vesicles or ejaculatory duct, which is a cause of infertility. b, bladder.
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  • Cystic mass along the expected course of the ejaculatory duct.
  • Small cysts are confined within the CZ.
  • Large cysts may extend to the seminal vesicles [16].
  • Cyst fluid contains spermatozoa.
Seminal Vesicle Cyst
Congenital cysts of the seminal vesicles occur in association with unilateral renal agenesis, atrophic kidney, and ectopic ureter draining into the seminal vesicles [16]. Acquired cysts are found with conditions that obstruct the seminal vesicles such as infection, chronic inflammation, fibrosis, and seminal vesicle stones. Cysts also occur in association with autosomal dominant polycystic disease. Cysts of the seminal vesicles have a high association with obstruction of the vas deferens and infertility [17].
  • Cystic dilatation of the seminal vesicles (Fig. 15.15).
  • Dominant cyst of the seminal vesicles as large as 6-7 cm.
  • Look for stones and inflammation in the seminal vesicles, obstruction of the vas deferens or ejaculatory duct, ectopic ureter, and renal anomalies.
References
1. Melchior SW, Brawer MK. Role of transrectal ultrasound and prostate biopsy. J Clin Ultrasound 1996;24:463-471.
2. Kuligowska E, Fenlon HM. Transrectal US in male infertility: spectrum of findings and role in patient care. Radiology 1998;207:173-181.
3. Littrup PJ, Lee F, McLeary RD, et al. Transrectal US of the seminal vesicles and ejaculatory ducts: clinical correlation. Radiology 1988;168:625-628.
4. Neumaier CE, Martinoli C, Derchi LE, et al. Normal prostate gland: examination with color Doppler US. Radiology 1995;196:453-457.
5. Smith JA, Jr. Transrectal ultrasonography for the detection and staging of carcinoma of the prostate. J Clin Ultrasound 1996;24:455-461.
6. von Eschenbach A, Ho R, Murphy GP, et al. American Cancer Society guidelines for the early detection of prostate cancer: update 1997. CA Cancer J Clin 1997;47:261-264.
7. Olson MC, Posniak HV, Fisher SG, et al. Directed and random biopsies of the prostate: indications based on combined results of transrectal sonography and prostate-specific antigen density determinations. AJR Am J Roentgenol 1994;163:1407-1411.
8. Brawer MK. How to use prostate-specific antigen in the early detection or screening for prostatic carcinoma. CA Cancer J Clin 1995;45:148-164.
9. Bostwick DG. Evaluating prostate needle biopsy: therapeutic and prognostic importance. CA Cancer J Clin 1997;47:297-319.
10. Agha A, Bane BL, Culkin DJ. Cystic carcinoma of the prostate. J Ultrasound Med 1996;15:75-77.
11. Lavoipierre AM, Snow RM, Frydenberg M, et al. Prostatic cancer: role of color Doppler imaging in transrectal sonography. AJR Am J Roentgenol 1998;171:205-211.
12. Chong CL, Butler EB, Price HM, et al. Anatomy and pathology of the prostate: an overview of the prostate gland. The Radiologist 1997;4:257-276.
13. Barozzi L, Pavlica P, Menchi I, et al. Prostatic abscess: diagnosis and treatment. AJR Am J Roentgenol 1998;170:753-757.
14. Nghiem HT, Kellman GM, Sandberg SA, et al. Cystic lesions of the prostate. Radiographics 1990;10:635-650.
15. Meacham RB, Townsend RR, Drose JA. Ejaculatory duct obstruction: diagnosis and treatment with transrectal sonography. AJR Am J Roentgenol 1995;165:1463-1466.
16. King BF, Hattery RR, Lieber MM, et al. Congenital cystic disease of the seminal vesicle. Radiology 1991;178:207-211.
17. Asch MR, Toi A. Seminal vesicles–imaging and intervention using transrectal ultrasound. J Ultrasound Med 1991;10:19-23.