MR Anatomy

Both the corpora cavernosa and the corpus spongiosum are of intermediate to high signal on Tl-weighted sequences, and high signal on T2 (Figs. 4.1-4.5). The corpus spongiosum is of similar signal to the glans, and may be higher or lower than the corpora cavernosa on T2-weighted sequences.3 Variable layering effects are a normal appearance in the tumescent corpus cavernosum (Fig. 4.2b). Contrast between high signal in the corpora and the fascial layers of the penis is higher on T2 than Tl-weighted sequences.4

Both the corpus spongiosum and the corpora cavernosa are surrounded by a fibrous sheath: the tunica albugĂ­nea, with inner (circular) and outer (longitudinal) layers,5 but is a single smooth low signal structure on both Tland T2-weighted sequences. Outside the tunica albuginea is a tough, enveloping layer of deep fascia, often termed Buck's fascia, which fuses proximally with the deep fascia of the urogenital region. Like the tunica albuginea, it is of low signal on T1 and T2-weighted sequences, and most authors assert that the two layers appear fused on MR and cannot be readily distinguished.6,7 However, connective tissue and fat between the tunica albuginea and Buck's fascia in the midline posteriorly contains the low signal deep dorsal vessels (with the vein and sometimes the arteries seen on axial scans) and often allows the two structures to be differentiated for the dorsal part of their circumference5; laterally they are usually apposed but can sometimes be differentiated on both Tl and T2-weighted sequences, where Buck's fascia is of slightly higher signal than the tunica albuginea8 (Fig. 4.2 a, c). The tunica albuginea is thin-

Fig. 4.2 Axial T2-weighted (a, b) and Tl-weighted (c) images through the penis in different patients after intracavernosal alprostadil. Note the differing conspicuity of the Buck's fascia and the dorsal vessels: clearly visible in patients (a, c) but not (b). Black arrowheads mark the tunica albuginea, and white arrowheads Buck's fascia. The thick white arrow shows the superficial dorsal vein in (a), and the thinner white arrows the deep dorsal vessels. The cavernosal arteries are marked by black arrows. The urethra, lying in the middle of the corpus spongiosum, is marked by an asterisk. Note the layering of signal within the corpora cavernosa in (b), a normal finding

Fig. 4.3 T2-weighted sagittal section close to the midline (a) after intracavernosal alprostadil and (b) without tumescence. Black arrows mark the tunica albuginea, large white arrows the corpus spongiosum, small white arrows the urethra within it, and black arrows the bulbocavernosus muscle. The white arrow head marks the entry of the urethra into the roof of the bulb. An asterisk marks the glans. The 'corrugated' appearance of the corpus cavernosum in (a) is because of the midline intercavernosal septum. Note the considerably thicker tunica albuginea in the detumescent state, and the lower signal in the corpus cavernosum; the glans is not in the midline sagittal plane

Penis Cancer

ner over the corpus spongiosum than cavernosum, and in the glans it is hard to distinguish, fusing completely with the subepithelial connective tissue toward the tip.9 The superficial dorsal vein may be seen outside Buck's fascia in the midline. After intravenous contrast, enhancement in the corpora cavernosa radiates axially from the cavernosal arteries, and from proximal to distal.10 Outside Buck's fascia lie the superficial vessels in a loose, superficial fascial layer (also called Dartos fascia), b

Fig. 4.4 T2-weighted coronal section through the base (a) and shaft (b) of the penis after intracaver-nosal alprostadil. In (a) a white arrowhead marks the ischiocavernosus muscle, and a black arrowhead the bulbocavernosus. A white arrow shows the urethra entering the bulb. Inferior pubic rami are marked by asterisks. In (b), a black arrowhead marks the tunica albuginea and a white arrowhead Buck's fascia. The glans is well seen (thick white arrows)

Fig. 4.4 T2-weighted coronal section through the base (a) and shaft (b) of the penis after intracaver-nosal alprostadil. In (a) a white arrowhead marks the ischiocavernosus muscle, and a black arrowhead the bulbocavernosus. A white arrow shows the urethra entering the bulb. Inferior pubic rami are marked by asterisks. In (b), a black arrowhead marks the tunica albuginea and a white arrowhead Buck's fascia. The glans is well seen (thick white arrows)

continuous with Colles' fascia of the perineum and containing a few thin dartos muscle fibers. Hematoma or urine arising deep to an intact Buck's fascia is confined to the penis. In contrast, blood or hematoma lying in the superficial fascia may extend to the scrotum and anterior abdominal wall.11

The most proximal part of the corpus spongiosum is the bulb, surrounded by the low signal bulbospongiosus muscle (Fig. 4.4). Its roof is pierced by the urethra, which then runs centrally within the corpus spongiosum and is of intermediate to low signal on T1 and T2-weighted sequences. The most proximal part of the corpora cavernosa are the crura, attached to the ischium and with their medial parts covered by the low signal ischiocavernosus.3

The superficial and deep inguinal nodes are well seen on MRI (Fig. 4.5), as are the pelvic nodes. The fascia lata separates the superficial from deep nodes, with the deep lying medial to the femoral vein, and Cloquet's node often the most conspicuous.12 Superficial nodes may be divided into five subgroups, defined according to their relations to a central group at the confluence of greater saphenous and femoral veins13 (Fig. 4.5a). The conspicuity and accuracy of measurement of lymph nodes depends very much on the techniques used for both CT and MRI,14 but MR with a slice thickness of 3 or 4 mm has the potential to accurately delineate nodes well under 8 mm in short axis diameter, 1 4 with a very low coefficient of variability between observers (0.05 in one series15). Ultrasound using a high resolution linear probe will be at least as accurate. Discriminating features of benign and malignant nodes will be discussed later in the chapter.

Fig. 4.5 (a) Thick-slab T1 spin echo coronal image of the superficial inguinal nodes. The horizontal group is shown by the white arrows: the most medial (asterisk) is defined as the 'sentinel' node by Senthil Kumar et al.85 and would also fit the definition by Cabanas.90 Black arrowheads mark the superficial epigastric vein; note that on the left the sentinel node lies medial to the superficial epigastric vein, but on the right is anterior to it. White arrowheads mark the long saphenous vein and the black arrow the junction between the superficial epigastric and long saphenous veins. (b) Axial T2-weighted image of the groin in a patient scanned because of a penile prosthesis. Note the horizontal chain of the superficial nodes (small white arrows, each with a fatty hilum). They lie superficial to the fascia lata (white arrowheads). The saphenous vein (black arrow) passes through the cribriform fascia (lying in the oval defect of the fascia lata) to join the femoral vein (v). The femoral artery is marked a, the femoral nerve n, and the spermatic cord s. The larger white arrow shows a deep inguinal node medial to the femoral vein

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