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Due to its excellent soft tissue contrast and direct multiplanar capabilities, magnetic resonance imaging (MRI) is well suited for evaluation of the prostate gland. Currently, most prostate magnetic resonance (MR) examinations are performed for staging of biopsy-proven prostate malignancies. However, prostate MRI can also be used to assess response to tumor therapy, to evaluate Mullerian abnormalities, and to diagnose complications of prostatitis. Applications of MRI in management of prostate disease will likely evolve with improvements in MR spectroscopic techniques, increased availability of 3 T scanners, and increased availability of MR scanners designed to permit MR-guided interventions.

MR TECHNIQUE Endorectal MR technique

The endorectal coil is an intracavitary surface coil that provides high spatial resolution images with high signal-to-noise ratio (SNR) due to its ability to acquire thin slices at small fields of view (1-7). Disposable endorectal coils (Medrad, Pittsburgh, Pennysylvania) are commercially available for use on 1.0 and 1.5 T MRI scanners and are combined with a pelvic, phased-array coil for improved imaging of the anterior and superior aspects of the prostate gland as well for the remainder of the pelvis (Fig. 1) (5,7-11). The coil is placed in the rectum and an attached balloon is inflated with 60 to 100 cc of air to prevent movement of the coil. Antiperistaltic medications, such as glucagon (1 mg intramuscular; Glucagen, Bedford Laboratories, Bedford, Ohio), scopolamine butylbromide (20 mg intramuscular or intravenous; Buscopan,

Endorectal Coil Medrad Pittsburgh

Figure 1 Endorectal coil. A disposable endorectal coil such as the one shown here (with permission by MedRad, Pittsburgh, Pennysylvania) provides high spatial resolution images of the prostate due to its ability to acquire thin slices at a small field-of-view.

Boehringer Ingelheim, New Zealand) or dicyclomine hydrochloride (10-20 mg oral; Bentyl; Aventis Pharmaceutricals, Bedford, New Jersey), are routinely administered to reduce motion artifacts and blurring that result from bowel peristalsis or the passage of gas into or out of the rectum during the MRI examination (11-13).

Imaging Protocol

Although diagnostic studies can be obtained on 0.5 to 1.0 T scanners, endorectal imaging of the prostate is usually performed on a 1.5 T field strength system to ensure good image quality and staging accuracy (14-19). A large field-of-view sagittal localizer sequence is used to ensure correct coil placement within the rectum and to prescribe subsequent sequences. Standard imaging includes a T1-weighted spinecho sequence obtained in the axial plane and T2-weighted fast spin-echo sequences obtained in the axial, coronal, and sagittal planes. Endorectal coil images are obtained from the top of the seminal vesicles through the apex of the prostate gland. Sagittal and coronal planes are helpful in assessing the relationship of the prostate gland to the seminal vesicles and bladder base and in detecting tumors in the prostatic base and apex (11). Finally, larger field-of-view axial images are obtained through the entire pelvis using a phased-array pelvic surface coil to detect lymph node enlargement and osseous metastatic disease.

Endorectal coil imaging employs a small field-of-view (10-14 cm), thin slices (3-4 mm skip 0-1 mm), and high matrix (256 x 256 or 256 x 192). Although fat suppression is known to improve tissue contrast by increasing the dynamic range for signal intensity throughout the body, it has not been shown to improve the diagnosis of prostatic neoplasm or extracapsular extension (20). Therefore, fat suppression is generally not used for prostatic imaging because it decreases SNR, which limits visualization of anatomic detail, such as the prostatic capsule, and reduces contrast between low signal intensity extraprostatic tumor and high signal intensity periprostatic fat and definition of periprostatic anatomic planes and intra-prostatic tumor (20-24).

Role of Intravenous Gadolinium

Intravenous contrast is not routinely used for endorectal prostate MRI because of conflicting results in the literature. Several studies have suggested that dynamic, contrast-enhanced T1-weighted imaging during the early phase of enhancement improves the depiction of tumor margins, extracapsular tumor, and seminal vesicle or neurovascular bundle invasion (25,26). Gadolinium has also been shown to differentiate between benign and malignant low T2-signal intensity foci in the peripheral gland of the prostate gland (27-29). However, other investigators have failed to demonstrate any improvement in staging accuracy and tumor localization with dynamic contrast administration when compared to unenhanced T2-weighted imaging, and these researchers have concluded that the routine use of contrast is not warranted (19,27,30-33).

There is, however, consensus in the literature with respect to special circumstances that may warrant the use of intravenous contrast. For example, equivocal cases of seminal vesicle invasion can benefit from gadolinium administration because enhancement of the lumen of the seminal vesicle (which ordinarily does not enhance) has been observed with tumor invasion and is occasionally superior to T2-weighted imaging (26,30,31). Gadolinium-enhanced MRI is also useful in determining the location and extent of necrosis in the prostate caused by such minimally invasive treatments as cryosurgery and high-intensity focused ultrasound (34). Cryosurgery results in destruction of the internal architecture of the gland with loss of normal zonal differentiation. Cryonecrotic tissue is avascular and demonstrates absent enhancement, which can be used as a sign of successful treatment (35).

Three Tesla Scanners

Endorectal coils are generally used for prostate imaging because a strong signal can be received from the posterior aspect of the prostate where 70% of cancers occur (Fig. 1) (36). However, at the apex of the prostate gland, the peripheral gland wraps all the way around the urethra and extends anterior to it, a location where there is only weak signal from the coil (36). Additional pitfalls of an endorectal coil include signal hyperintensity immediately surrounding the coil (near-field artifact) and structural deformation of the peripheral zone of the prostate gland, which makes image interpretation difficult (37). Also, use of an endorectal coil is not possible in patients who have had an abdominoperineal resection and is contraindicated in patients with active inflammatory bowel disease. Additionally, it is not well tolerated by patients with severe hemorrhoidal disease or radiation-induced proctitis.

Imaging the prostate using a 3 T scanner and an external phased-array coil are being investigated as methods of improving SNR, thereby providing standard anatomic information and spectroscopy data without the use of an endorectal coil

(36). Preliminary studies have shown that the zonal anatomy of the prostate is clearly visualized on a 3-T system and that the SNR is improved relative to a 1.5 T scanner

(37). Limitations of higher field strength scanners include decreased penetration of the radio-frequency pulse and increased energy deposition into the body (37).

NORMAL PROSTATE GLAND Anatomy

The prostate gland is an accessory exocrine gland of the male reproductive system, composed of glandular and fibromuscular tissue. The ejaculatory ducts course through the prostate gland to enter the prostatic urethra at the verumontanum, allowing prostatic secretions to liquefy semen.

From a radiologic standpoint, the prostate is divided into two important components: the central gland and the peripheral gland. The central gland is further divided into the transitional and central zones (38). The central zone surrounds the proximal urethra and ejaculatory ducts and encloses the transitional zone and peri-urethral glands. It is shaped like a funnel with its widest portion comprising the majority of the base of the prostate (39). In young men, the central gland is predominantly composed of the central zone, whereas, in older men with benign prosta-tic hypertrophy (BPH), the central gland is made largely of transitional zone. Ultimately, the central zone is no longer visible on MRI because it is compressed by the transitional zone, which assumes a progressively greater proportion of the prostatic volume (due to BPH) as the prostate gland ages (38,39).

The peripheral gland surrounds the distal prostatic urethra and is the major glandular component of the prostate, comprising the majority of the prostatic apex. It also extends superiorly along the posterolateral aspect of the prostate to surround the central zone (like a waffle cone around a scoop of ice cream) (38,39). Prostate carcinoma and prostatitis are more likely to occur in the peripheral gland (39).

The lymphatic drainage of the prostate gland includes the obturator, internal iliac, external iliac, common iliac, and presacral lymph node chains. Prostatic veins form a periprostatic venous plexus around the lateral and anterior aspects of the prostate, receive blood from the prostate and deep dorsal vein of the penis, and ultimately drain into the internal iliac veins (40). The anterior aspect of the plexus is known as the venous plexus of Santorini. Prominence of the anterior and posterior periprostatic veins near the apex has been associated with greater intraoperative blood loss during radical prostatectomy (41).

MR Appearance of the Normal Prostate

The prostate gland is of homogeneous, intermediate signal intensity on T1-weighted images, with poor differentiation of the zonal anatomy of the gland on this sequence (11,42,43). On T2-weighted sequences, zonal anatomy is better delineated because mucin-producing glands in the peripheral gland result in high signal intensity with respect to muscle (Fig. 2A and B) (11,42,43). A collagenous network can occasionally be seen as curvilinear low signal intensity structures against the background of normal high signal intensity peripheral gland (44). In patients with prostate cancer,

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