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IMAGING OF PENIS.
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Anatomy and Physiology The penis is composed of three cylindrical bodies of endothelium-lined cavernous spaces: the paired dorsolateral corpora cavernosa, and the single, ventral, and midline corpus spongiosum. The variable MR signal intensity of these structures is dependent on the rate of blood flow within the cavernous spaces that constitute the corporal bodies. In general, the three corpora are of intermediate T1- weighted and high T2- weighted signal intensity. The corpora cavernosa are isointense relative to one another, as they are connected via fenestrations in their septum and therefore have similar flow.
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The corpus spongiosum is a separate space and may normally have flow and signal intensity characteristics different from those of the cavernosa. The posterior portions of the corpora cavernosa are known as the crura, which flare laterally to attach to the ischiopubic rami. The corpus spongiosum arises within the perineum from the bulbous spongiosum and extends anteriorly to form the glan penis. On axial T2-weighted images, the deep arteries of the corpora cavernosa are depicted as small, round areas of hypointensity (due to faster flow)within the medial, dorsal portions of the cavernosa. A layer of fibrous tissue, the tunica albuginea, surrounds both the corpora cavernosa and the corpus spongiosum, but the layer is thicker around the cavernosa
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A second fibrous layer, the Buck fascia, surround the corpora cavernosa and separates them from the corpus spongiosum. Composed of mature fibrous tissue, both the tunica albuginea and Buck fascia are imaged as low T1-weighted and lowT2-weighted signal-intensity bands that surround the corporal bodies. External to the Buck fascia is a loose layer of subcutaneous connective tissue, which appears hyperintense relative to the fascia at T2 weighting The deep dorsal vein of the penis lies in the midline within this layer
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The posterior urethra consists of the prostatic and membranous portions. The latter traverses the urogenital diaphragm. The anterior urethra, which begins at the urogenital diaphragm, consists of the bulbous and pendulous portions of the urethra, both of which are surrounded by the corpus spongiosum. If not dilated or distended, the anterior urethra is difficult to see at MR imaging. On heavily T2-weighted images, the collapsed muscular walls of the urethra are revealed as a band of relative hypointensity compared with the bright cavernous spaces of the corpus spongiosum.
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Vascular Anatomy The arterial supply to the penis originates from the right and left internal pudendal arteries,which, in turn, arise from the anterior division of the internal iliac arteries . Each internal pudendal artery gives rise to the perineal and common penile arteries. The branches of the common penile artery are variable but classically consist of three: the bulbourethral artery, the dorsal artery of the penis, and the cavernosal artery.
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The bulbourethral artery supplies the penile bulb, the bulbourethral (Cowper) glands, and the posterior aspect of the corpus spongiosum. It gives rise to the urethral artery, which parallels the course of the urethra within the corpus spongiosum. The paired deep dorsal arteries lie external to the tunica albuginea within the hypodermal connective tissue and course lateral to the deep dorsal vein. They supply the penile skin and the glans penis.
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The cavernosal arteries are easily visible on axial MR images of the penis within the dorsal, medial third of the corpora cavernosa. They give rise to multiple helicine arteries, which supply the sinusoids of the corpora cavernosa. After the administration of gadopentetate dimeglumine, enhancement of the cavernosa proceeds centrifugally, extending from the region surrounding the cavernosal arteries to the periphery of the corporal bodies and from the proximal part of the cavernosa to the distal
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The cavernosa are drained through the emissary veins of the wall of the corpora. Distally these veins drain into the unpaired deep dorsal vein of the penis. Proximally, they drain into the cavernous veins, which join with the bulbar vein to form the internal pudendal veins.
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MRI OF PENIS MR imaging of the penis is facilitated by appropriate positioning of the patient. With the patient supine, a folded towel is placed between the patient’s legs inferior to the perineum, to elevate the scrotum and penis. The penis is then dorsiflexed against the lower abdomen in the midline and taped in position to reduce motion of the organ during the examination . A 3-inch or 5-inch surface coil is employed to maximize signal- to-noise ratio at small fields of view
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If imaging of the penis in the erect state is required, prostaglandin E1 (alprostadil, 10 g) is injected into one of the corpora. After injection, the patient is asked to massage the injection site and to manually stimulate the penis until it is erect. Intra-cavernosal injection of prostaglandinE1 is contraindicated in men who have conditions that predispose them to priapism, such as sickle cell anemia, sickle cell trait, multiple myeloma, leukemia, cavernosal thrombosis, and tumors that are known to invade the cavernosa. Use of prostaglandinE1 is also contraindicated in men with penile prostheses. Imaging protocols are customized to address the clinical question. In general, though, we acquire axial (short-axis) spin-echo T1-weighted and axial, sagittal, and fat-saturated coronal fast spin-echo T2-weighted images.
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If gadolinium is administered, we employ either a two- or three-dimensional fat-saturated T1-weighted gradient-echo pulse sequence before, during, and after the dynamic administration of intravenous gadolinium chelate. If the entire pelvis also needs evaluation, such as in the case of penile malignancy, we obtain body coil axial T1-weighted spin-echo pelvic images to look for inguinal or obturator lymphadenopathy. Inpatients who undergo evaluation for arteriogenic impotence, pelvic MR angiography is performed with a three-dimensional T1-weighted gradient echo pulse sequence and an approximately 36-cmfield of view to obtain images from the distal aorta to the distal pudendal arteries.
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Penile Malignancies Squamous Cell Carcinoma of the Penis:- Most primary penile malignancies are squamous cell carcinomas. most often during the 6th and 7th decades of life. most commonly diagnosed malignancies of men in Asia and Africa. Uncircumcised men are more often affected, probably because of the chronic irritative effect of smegma. At MR imaging, squamous cell carcinomas are usually hypointense relative to the corpora on both T1- and T2-weighted images. At contrast enhanced imaging, these lesions do increase in signal intensity but less so than the normal corporal bodies
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MR imaging may be performed for staging purposes. In the commonly used Jackson staging system, stage I lesions are confined to the glans or prepuce, stage II lesions involve the penile shaft, stage III extend to inguinal nodes, stage IV involve deep pelvic nodes or distant metastases. Both computed tomography (CT) and MR imaging depict pelvic lymphadenopathy, but MR imaging is superior for evaluation of the primary lesion.
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Imaging of Lymph Nodes Approximately 30%–60% of patients with squamous cell carcinoma have palpable inguinal lymph nodes at the time of initial diagnosis . In about one-half of these patients, the enlarged nodes harbor metastatic disease; in the other one-half, the palpable lymph nodes are reactive. The capacity of CT and MR imaging to help detect lymph node metastases is limited because the diagnosis is based on lymph node size. As a result, occult metastases in normal-sized lymph nodes will go undetected, whereas enlarged nodes secondary to infection or inflammation will be labeled as malignant.
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The recently introduced technique of lymphotrophic nanoparticle–enhanced MR imaging allows the characterization of lymph nodes in patients with various cancers. MR imaging is performed with ferumoxtran-10, which consists of ultra small super paramagnetic iron oxide particles. Normal lymph nodes contain macrophages, which engulf the iron oxide nanoparticles. Malignant lymph nodes lack the phagocytic cells needed to take up the nanoparticles. Therefore, non metastatic lymph nodes show homogeneous uptake of ferumoxtran-10 and appear dark due to shortening of T2 and T2*, whereas malignant lymph nodes do not take up the contrast and appear bright
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Imaging of Distant Metastases.—Distant metastasis is uncommon in patients who present with penile cancer (3% of cases) . Generally, distant metastases occur late in the course of the disease and are associated with a dismal prognosis. The most common sites of metastases are the lung, liver, and retroperitoneum.
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Anterior Urethral Carcinoma:-Carcinomas of the male urethra occur most often in the bulbous and membranous portions of the urethra, followed by the fossa navicularis. The anatomy of the urothelium changes along the course of the male urethra, with 1]transitional cells lining the prostatic and membranous segments, 2]stratified and pseudo stratified columnar epithelium in the bulbous and pendulous parts, and 3]squamous cells in the fossa navicularis and urethral meatus.
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Squamous cell carcinomas, Followed by transitional cell carcinomas and adenocarcinomas, are the most common anterior urethral Carcinomas. At MR imaging, urethral carcinomas are usually hypointense relative to the normal cavernosa on both T1- and T2-weighted images
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Penile Sarcoma:- Primary sarcomas can also occur in the penis. These include epithelioid sarcoma, Kaposi sarcoma, leiomyosarcoma, and rhabdomyosarcoma. All these lesions are rare and together represent less than 5% of all penile malignancies. Epithelioid sarcoma isointense relative to the corpora on T1- weighted images, was hypointense relative to the corpora on T2-weighted images, and enhanced less than the normal corpora after administration of contrast material.
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Leiomyosarcomas of the penis may arise from either the smooth muscle of the glans or the corpora cavernosa . Those that arise in the latter tend to metastasize early because of their proximity to the vascular structures of the penis. Rhabdomyosarcoma is the most common malignancy of the lower genitourinary tract in the first 2 decades of life. At MR imaging, rhabdomyosarcoma generally is isointense relative to skeletal muscle on T1-weighted images, is hyperintense relative to muscle on T2- weighted images, and enhances heterogeneously after administration of contrast material.
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Metastases to the Penis Metastases to the penis are rare and may manifest with malignant priapism . In approximately 70% of cases, penile metastases arise from other primary malignancies of portions of the genitourinary tract such as the prostate or urinary bladder . Metastases from the colon, stomach, esophagus, and pancreas have been reported less often Metastatic spread of malignancy to the penis represents an advanced stage of disease, and prognosis is generally poor.
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Benign Masses:- MR imaging can be used to evaluate a number of benign entities that manifest as palpable penile masses, the nature of which may not be apparent at clinical evaluation. Cowper Duct Syringocele Cowper duct syringocele represents cystic dilatation of the main duct of the bulbourethral (Cowper) glands. At MR imaging, Cowper duct syringocele appears as a midline oval structure at the penile base adjacent to the ventral aspect of the proximal bulbous urethra and is of high T2 signal intensity.
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Periurethral Abscess Uncomplicated cases of lower genitourinary tract infection or inflammation do not require imaging. Cases in which abscess is suspected may benefit from imaging evaluation, and the inherently high soft-tissue contrast afforded by MR imaging makes it an excellent modality for this purpose MR imaging can show the relationship of the inflammatory mass to the corporal bodies and urethra.
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Partial Cavernosal Thrombosis:- At MR imaging, the affected cavernosal segment is distended with thrombotic blood and may compress the contralateral corpus cavernosum. The signal intensity of the affected segment is dependent on the age of the thrombus. Generally, it is hyperintense relative to the normal cavernosum on T1- weighted images and hypointense on T2- weighted images.
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Peyronie’s disease Peyronie’s disease accounts for 0.3-0.7% of all urologic disorders. It occurs most often in the fourth to sixth decades of life, and occasionally in men less than 20 years old . Patients usually have increasingly painful erection and deviation of the penis. The main clinical findings are hard, well-demarcated plaques in the region of the tunica albuginea enclosing the corpora cavernosa and of the septum penis.The cause of the disease is still not fully explained, although some evidence suggests that repeated microtrauma, hormonal dysfunction, metabolic disorders (diabetes, gout), and a general fibroplastic disposition may play a role in the pathogenesis.
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Peyronie Disease Chronic inflammation that leads to fibrosis and focal thickening of the tunica albuginea is termed Peyronie disease. The indurated area, which maybe painful, results in varying degrees of penile deformity with erection. If severe, this deformity may interfere with sexual intercourse. Peyronie plaques may or may not calcify. If calcified, the plaque may be visible at radiography, CT, and US. At MR imaging, Peyronie disease is seen as focal thickening of the tunica albuginea and is best seen on T2-weighted images. After intravenous administration of gadolinium, enhancement of the plaque has been shown to correlate with the presence of active inflammation.
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