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Doppler Ultrasound of the Penis
Acknowledgements
Acknowledgements are made to Reginald Bruskewitz, MD; Mike Ledwidge, RDMS, RVT; Jeanne Johnson and Mercedes Kirk.
Introduction
Common indications for penile imaging are for evaluation of erectile dysfunction, trauma, priapism, penile carcinoma and Peyronie’s disease. Because it is a superficial soft tissue structure, the penis is ideally suited for imaging with ultrasound. The addition of colour and spectral Doppler allows the examiner to delineate vascular anatomy, display dynamic variations in blood flow, measure arterial velocity and infer adequacy of venous drainage. The advantages of Doppler over other imaging modalities are its ease-of-use, patient acceptance, versatility, reproducibility, ready availability, and relatively low cost. By combining imaging with the pharmacologic induction of erection, both anatomic and physiologic abnormalities can be assessed during the flaccid and erect states.
The evaluation of erectile dysfunction (ED) has been the dominant application for penile Doppler, especially in the older population since the prevalence and severity of the disease increases with advancing age. While impotence may be the result of psychogenic, neurogenic or hormonal factors, vascular disease is the most common cause of ED. With the introduction of phosphodiesterase inhibitors [specifically Sildenafil citrate (Viagra©), Verdenafil HCl (Levitra©), and Tadalafil (Cialis©)], the frequency of Doppler studies for ED has significantly decreased. Most centres now prescribe a trial of the phosphodiesterase inhibitors as the initial diagnostic/therapeutic test of ED, with only non-responders being sent on for imaging.
Penile Anatomy and Physiology
The penis contains three longitudinal, cylindrical erectile bodies. Two corpora cavernosa are located in the dorsal two-thirds of the penile shaft, and a single corpora spongiosum is located in the ventral one-third of the shaft. The corpora cavernosa are enclosed by the tunica albuginea, a tough, non-distensible fascial layer. The septum that divides the corpora cavernosa contains fenestrations that create multiple connecting anastomotic channels between the sinusoidal spaces, allowing for free communication across the midline. The dorsal arteries, veins and nerves are situated centrally along the penile dorsum, superficial to the tunica albuginea and deep to Buck’s fascia. The urethra is contained within the corpus spongiosium.
On ultrasound, the corpora cavernosa are of uniform hypoechoic echotexture. The tunica can be seen as an echogenic envelope around the corpora. The echogenic walls of the cavernosal arteries can be seen centrally within the corpora. The corpus spongiosum is of higher echogenicity ( Fig. 12-1 ).
ARTERIAL ANATOMY
The internal pudendal artery and its branches are the primary source of arterial supply to the penis. The first three branches are the superficial perineal artery, the bulbar artery and a small urethral artery. The perineal artery is a large and constant branch that, in 80% of cases, has an internal and external branch. The bulbar artery, which supplies the proximal penile shaft, is usually easily identified during angiography because it is associated with a bulbar parenchymal blush in the early arterial phase. The urethral artery, which is of small diameter, arises anterior to the bulbar artery. After these branches, the internal pudendal artery continues as the common penile artery. It then divides into left and right penile arteries, which enter the base of the penis and branch into a dorsal artery and a cavernosal artery. The dorsal artery extends along the dorsal aspect of the penile shaft towards the glans and terminates at the level of the arterial corona of the glans; it supplies blood primarily to the skin, subcutaneous tissues and glans. Collateral vessels from the dorsal artery often communicate with the cavernosal artery. The cavernosal, or deep penile artery, enters the tunica albuginea proximally and extends the length of the corpus cavernosum. The cavernosal arteries and their helicine branches are the primary source of blood flow to the erectile tissue of the penis. Just as the cavernosal artery supplies blood to the corpus cavernosum, the spongiosal artery supplies the corpus spongiosum ( Fig. 12-2 ).
VENOUS ANATOMY
Venous drainage of the penile erectile tissue (i.e. the sinusoidal spaces) primarily occurs through emissary (efferent) veins which drain the corpus cavernosum, penetrate the tunica albuginea and empty into the circumflex veins; these then drain into the deep dorsal venous system of the penis. The emissary veins may also drain directly into the deep dorsal vein. The superficial dorsal vein drains the distal portion of the corpora cavernosa, as well as the skin and glans. The deep and superficial dorsal veins can be routinely visualised by colour Doppler imaging in the midline of the penile shaft ( Figs 12-3 and 12-4 ). The most proximal portions of the corpora cavernosa are drained by the cavernosal veins directly into the periprostatic venous plexus.
ERECTILE PHYSIOLOGY
When the penis is flaccid, its smooth muscle is in a tonic state, the cavernous sinusoids are collapsed, and the cavernous venules are open. The emissary veins drain the sinusoidal spaces and blood circulates into the dorsal veins. During this state, there is relatively high resistance to blood flow into the penis. Erection starts when an autonomic neurogenic impulse relaxes the cavernosal arterioles and sinusoidal spaces. As erection occurs, there is a marked increase in the volume of arterial inflow into the penis as the cavernous arteries dilate. This is accompanied by relaxation of the smooth muscle of the corpora cavernosa with expansion and elongation of the cavernous sinusoids as they fill with blood. Compression of the cavernous venules between the dilated cavernous sinusoids and the unyielding peripheral tunica albuginea decreases venous outflow. This veno-occlusive mechanism (which depends on neurological stimuli, a sufficient supply of arterial blood, and normal function of the tunica albuginea) maintains sinusoidal distension and rigid erection.
Five stages of erectile physiology have been defined: latent, tumescent, full erection, rigid erection and detumescent. During the latent phase, the diameters of the cavernosal arteries are at their greatest and there is maximum inflow of blood with minimal resistance. During tumescence, the sinusoidal cavities of the corpora cavernosa distend with blood. With full erection, blood flow decreases, as do the diameters of the cavernosal arteries. With a rigid erection, blood inflow (and outflow) ceases and the diameters of the cavernosal arteries are at their narrowest. Detumescence occurs when the trabeculae and arteries contract in response to a release of norepinephrine. During the five stages of erection, different arterial diameters and waveform patterns are normally present on Doppler examination.
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