On completion of this chapter, you should be able to:
Identify the normal anatomy of the scrotum
Explain the vascular supply to the scrotal contents
Describe patient positioning, scanning protocol, and technical considerations for an ultrasound examination of the scrotum
Discuss the role of color and spectral Doppler in scrotal imaging
Describe the ultrasound characteristics of scrotal pathology
Ultrasound is the imaging modality of choice for evaluating the scrotum. High-frequency ultrasound imaging, combined with color and spectral Doppler, quickly and reliably provides valuable information in the assessment of scrotal pain or mass. In particular, color Doppler has a central role in the evaluation of suspected testicular torsion because it can demonstrate absence of flow in the affected testis. Color Doppler also plays a key role in the evaluation of testicular infection by demonstrating hyperemic flow on the affected side. Ultrasound imaging accurately differentiates intratesticular from extratesticular masses and cystic from solid masses. Advances in the development of ultrasound equipment have provided improved spatial and contrast resolution, reduced speckle artifact, and increased sensitivity to the display of scrotal perfusion. The steady progress in ultrasound image quality has enhanced our ability to clearly define the scrotal anatomy and to more accurately depict and differentiate abnormalities. This chapter covers the pertinent anatomy of the scrotum and its contents, including the vascular supply. The ultrasound scanning protocol is discussed along with tips on scanning techniques and potential pitfalls. A review of the disease processes affecting the scrotum is provided, including a description of sonographic findings.
Anatomy of the scrotum
The testes are symmetric, oval-shaped glands residing in the scrotum. In adults, the testis measures approximately 3 to 5 cm in length, 2 to 4 cm in width, and approximately 3 cm in height. Each testis is divided into more than 250 to 400 conical lobules containing the seminiferous tubules. These tubules converge at the apex of each lobule and anastomose to form the rete testis in the mediastinum. The rete testis drains into the head of the epididymis through the efferent ductules ( Figure 23-1 ). Sonographically, the testes appear as smooth, medium-gray structures with a fine echo texture.
The epididymis is a 6- to 7-cm tubular structure beginning superiorly and then coursing posterolateral to the testis. It is divided into head, body, and tail. The head is the largest part of the epididymis, measuring 6 to 15 mm in width. It is located superior to the upper pole of the testis ( Figure 23-2 ). It contains 10 to 15 efferent ductules from the rete testis, which converge to form a single duct in the body and tail. This duct is known as the ductus epididymis. It becomes the vas deferens and continues in the spermatic cord. The body of the epididymis is much smaller than the head. It is difficult to see with ultrasound on normal individuals. It follows the posterolateral aspect of the testis from the upper to the lower pole. The tail of the epididymis is slightly larger and is positioned posterior to the lower pole of the testis. The appendix of the epididymis is a small protuberance from the head of the epididymis. Postmortem studies have shown the appendix epididymis in 34% of testes unilaterally and 12% of testes bilaterally. The normal epididymis usually appears as isoechoic or hypoechoic compared with the testis, although the echo texture is coarser.
At the upper pole of the testis, the appendix testis is attached. It is located between the testis and the epididymis. Postmortem studies have shown the appendix testis to be present in 92% of testes unilaterally and 69% bilaterally ( Figure 23-3 ).
The testis is completely covered by a dense, fibrous tissue termed the tunica albuginea. The posterior aspect of the tunica albuginea reflects into the testis to form a vertical septum known as the mediastinum testis. Multiple septa (septa testis) are formed from the tunica albuginea at the mediastinum. They course through the testis and separate it into lobules. The mediastinum supports the vessels and ducts coursing within the testis. The mediastinum is often seen on ultrasound as a bright hyperechoic line coursing craniocaudad within the testis ( Figure 23-4 ). The tunica vaginalis lines the inner walls of the scrotum, covering each testis and epididymis. It consists of two layers: parietal and visceral. The parietal layer is the inner lining of the scrotal wall. The visceral layer surrounds the testis and epididymis. A small bare area is posterior. At this site, the testicle is against the scrotal wall, preventing torsion. Blood vessels, lymphatics, nerves, and spermatic ducts travel through the area (see Figure 23-1 ). The space between the layers of the tunica vaginalis is where hydroceles form. It is normal to see a small amount of fluid in this space.
The vas deferens is a continuation of the ductus epididymis. It is thicker and less convoluted. The vas deferens dilates at the terminal portion near the seminal vesicles. This portion is termed the ampulla of the deferens. The vas deferens joins the duct of the seminal vesicles to form the ejaculatory duct, which, in turn, empties into the urethra. The junction of the ejaculatory ducts with the urethra is termed the verumontanum. The urethra courses from the bladder to the end of the penis. In men, the urethra transports both urine and semen outside the body.
The vas deferens, testicular arteries, venous pampiniform plexus, lymphatics, autonomic nerves, and fiber of the cremaster form the spermatic cord. The cord extends from the scrotum through the inguinal canal and internal inguinal rings to the pelvis. The spermatic cord suspends the testis in the scrotum.
Right and left testicular arteries arise from the abdominal aorta just below the level of the renal arteries. They are the primary source of blood flow to the testis. The testicular arteries descend in the retroperitoneum and enter the spermatic cord in the deep inguinal ring. Then they course along the posterior surface of each testis and pierce the tunica albuginea, forming the capsular arteries, which branch over the surface of the testis. With high-frequency ultrasound imaging, the capsular artery is sometimes seen as a hypoechoic linear structure on the surface of the testis. Color Doppler can be used to confirm its identity ( Figure 23-5 ). The capsular arteries give rise to centripetal arteries, which course from the testicular surface toward the mediastinum along the septa. Before reaching the mediastinum, they curve backward, forming the recurrent rami (centrifugal arteries) ( Figure 23-6 ). These centrifugal arteries branch farther into arterioles and capillaries. With sensitive color Doppler settings, the recurrent rami may be seen giving a candy cane appearance ( Figure 23-7 ).
In approximately one half of normal testes, a transmediastinal (or transtesticular) artery is visualized coursing through the mediastinum toward the testicular capsule. A large vein is often identified adjacent to the artery ( Figure 23-8 ). On color Doppler, the transmediastinal artery will have a different color than the centripetal arteries because its flow is directed away from the mediastinum and toward the capsule. On reaching the testicular surface opposite the mediastinum, the transmediastinal artery courses along the capsule as capsular arteries. Spectral Doppler waveforms obtained from the capsular, centripetal, or transmediastinal arteries show a low-resistance waveform pattern in normal individuals ( Figure 23-9 ). Box 23-1 diagrams arterial branching in the testicles.
↓ Capsular artery
↓ Centripetal artery
↓ Recurrent rami
The cremasteric artery and deferential artery accompany the testicular artery within the spermatic cord to supply the extratesticular structures. They also have anastomoses with the testicular artery and may provide some flow to the testis. The cremasteric artery branches from the inferior epigastric artery (a branch of the external iliac artery). It provides flow to the cremasteric muscle and peritesticular tissue. The deferential artery arises from the vesicle artery (a branch of the internal iliac artery). It mainly supplies the epididymis and vas deferens. The scrotal wall is also supplied by branches of the pudendal artery.
Venous drainage of the scrotum occurs through the veins of the pampiniform plexus. The pampiniform plexus exits from the mediastinum testis and courses in the spermatic cord. It converges into three sets of anastomotic veins: testicular, deferential, and cremasteric. The right testicular vein drains into the inferior vena cava, and the left testicular vein joins the left renal vein. The deferential vein drains into the pelvic veins, and the cremasteric vein drains into tributaries of the epigastric and deep pudendal veins.
Patient positioning and scanning protocol
High-resolution ultrasound imaging is the primary screening modality for most testicular pathology. Applications include inflammatory processes of the testes and epididymis, tumors, trauma, torsion, hydrocele, varicocele, hernias, spermatoceles, and undescended testes.
Patient preparation: none.
Transducer selection: 8- to 12-MHz linear array.
Patient position: supine (Valsalva maneuver or upright position to check for varicocele).
Images and observations should include the following:
Long testicle (include medial, mid, and lateral).
Anteroposterior (AP) and long measurements of above anatomy.
Transverse scan of each testis (upper, mid, lower).
Transverse scan of head of epididymis.
Include AP and transverse measurements of the middle pole of the testicles.
If possible, a split-screen image should be obtained to compare the echogenicity of each testis.
Images of the extratesticular area should be obtained to determine the presence of hydrocele, hernia, or other conditions.
Doppler flow analysis of the scrotal area:
When indicated, color and pulsed Doppler analysis of intratesticular flow with resistance measurements should be obtained.
The scanning instrument should be optimized for slow flow detection (e.g., decrease pulse repetition frequency/scale, lower filters, increase gain or power).
Ultrasound examination of the scrotum is performed with the patient in the supine position. The penis is positioned on the abdomen and covered with a towel. The patient is asked to place his legs close together to provide support for the scrotum. Alternatively, a rolled towel placed between the thighs can support the scrotum. It is often unnecessary to place a towel for support if the legs are positioned close together. This may be more comfortable for the patient in pain.
A generous amount of warmed gel is applied to the scrotum to ensure adequate probe contact and eliminate air between the probe and the skin surface. Rarely, a stand-off pad may be necessary to improve imaging of very superficial structures such as a tunica albuginea cyst. However, with the use of high-frequency probes (10 to 14 MHz), this is usually not necessary. Instead of a stand-off pad, an extra-thick mound of gel may be adequate to improve near-field imaging.
Before beginning the scrotal ultrasound, it is necessary to determine clinical findings. Was this patient referred because of a palpable mass, scrotal pain, swollen scrotum, or other reason? It is important to ask the patient to describe his symptoms, including history, location, and duration of pain. Can he feel a mass? If so, ask the patient to find the lump. Then place the probe exactly over this location to examine the site. Did the patient experience trauma? When did the trauma occur? Ask him to describe what happened. Has he had a vasectomy? When? Not only is this information helpful in guiding the examination, but it is important to the interpreting physician and gives confidence to the patient regarding the quality of the ultrasound study. Box 23-2 lists important tips when performing an ultrasound examination of the scrotum.
Explain procedure and preparation to patient, and then allow the patient to get ready in private.
Be sure to take an image of right and left testicles together for comparison in both gray-scale and color Doppler.
Perform Valsalva maneuver when a varicocele is suspected.
Sensitize color Doppler for slow flow when evaluating torsion.
Torsion is a surgical emergency; perform the examination in a timely manner.
Scrotal ultrasound is always a bilateral examination, with the asymptomatic side used as a comparison for the symptomatic side. To begin, it is best to perform a brief survey scan to determine what abnormalities, if any, are present. Each testis is scanned from superior to inferior and is carefully examined to determine whether abnormal findings are present. The size, echogenicity, and structure of each testis are evaluated. The testicular parenchyma should be uniform with equal echogenicity between sides. Think of these questions as you scan: Is the parenchyma homogeneous or heterogeneous? Is there a mass? If so, is it cystic or solid? Is it intratesticular or extratesticular? Is one testis much larger than the other? Which side is swollen, or is one side shrunken? All testes should appear similar in size and shape. Is the epididymis normal? Is the skin thickened? Turn on color Doppler to assess the flow. Is there an absence of flow in the testis, or is it hyperemic? How does the color Doppler compare between sides? Testes should show about the same amount of flow when the same color Doppler setup is used. Check the flow in each epididymis. Again, compare between sides. They should be similar. After the survey scan, images are obtained that demonstrate the findings.
Representative images are obtained in at least two planes—transverse and sagittal—with additional imaging planes scanned as needed to demonstrate the findings. In transverse, images are taken that show the superior, mid, and inferior portions of each testis. The width of the testis is measured in the midtransverse view. A transverse view of the head of the epididymis is included. Superior to the epididymal head, an image is obtained to demonstrate the area of the spermatic cord. In the sagittal plane, images are taken to show the medial, mid, and lateral portions. A long axis measurement of testicular length is obtained in the midsagittal image. Again, additional images may be taken to demonstrate abnormal areas. An image is obtained of the epididymal head superior to the testicle. The body and tail of the epididymis can be demonstrated coursing posteriorly on each side. Scrotal skin thickness is evaluated and compared from side to side. At least one image is taken to show both testes at the same time, so the interpreting physician can compare size and echogenicity ( Figure 23-10 ). Additional views may be taken in patients with suspected varicocele. These include upright positioning and the Valsalva maneuver. Color and spectral Doppler are used in all examinations, with representative images taken to demonstrate both arterial and venous flow in each testis. Table 23-1 lists the scanning protocols for scrotal ultrasound.
|Transverse Image||Sagittal Image|
|Spermatic cord area||Spermatic cord area|
|Epididymal head||Epididymal head with superior testis|
|Superior testis||Long axis midtestis with measurement|
|Mid testis with measurement||Medial long axis|
|Inferior testis||Lateral long axis|
|Transverse view showing both testes||Color Doppler of epididymal head|
|Color Doppler of mid testis|
|Spectral Doppler of artery|
|Spectral Doppler of vein|
High-frequency linear-array transducers are preferred for scrotal imaging because they provide the best spatial resolution. However, the field of view is limited with linear arrays. Occasionally, a larger field of view is required to measure anatomy or display anatomic relationships. Ultrasound systems provide numerous methods to meet this need, including virtual convex imaging, panoramic imaging, stitching images together, and using a curved-array transducer.
Real-time imaging of the scrotum is performed with a high-frequency linear-array probe of at least 7.5 MHz. Because high-frequency transducers have better spatial and contrast resolution compared with lower-frequency transducers, they are preferred for scrotal imaging. Probes with frequencies of 10 to 15 MHz are usually best. Because there is a tradeoff between frequency and penetration, the highest frequency providing adequate penetration should be used. In patients with considerable wall edema and thickening, frequencies as low as 5 to 7.5 MHz may be necessary to adequately penetrate the testis.
Many ultrasound systems have a trapezoid or virtual convex feature that can be selected with the linear-array probe. This is very helpful for measuring the long axis of the testis, or when an abnormal area cannot be entirely imaged with the standard linear format ( Figure 23-11 , A ). It is best to use this feature selectively instead of routinely, because steering the beam to create the wider format has a negative impact on image quality; steering widens the distance between scan lines and degrades lateral resolution.
In cases of large hydroceles, hematomas, or swelling, an even larger field of view may be required. In these cases, a panoramic tool may be useful. This tool allows the image to build as the probe is moved over the skin surface. A very long image can be obtained that shows anatomic relationships ( Figure 23-11 , B ). Images may also be stitched together in a combined mode. The first image is obtained in one window; then the probe is moved, and another image is obtained by attempting to match the boundaries of the first image ( Figure 23-11 , C ). Another way to obtain a larger field of view is to use a 5- to 7.5-MHz curved-array transducer for a portion of the examination to demonstrate the entire scrotal contents. Again, this should be done selectively to obtain the necessary images and should be followed by a return to the high-frequency linear-array probe for further evaluation of each testis ( Figure 23-11 , D ).
Most modern ultrasound scanners offer additional features that enhance the quality of the ultrasound image. These features include, but are not limited to, compound imaging, harmonics, extended field-of-view imaging, virtual convex, speckle reduction algorithms, and use of multiple focal zones ( Table 23-2 ). All of these controls may be adjusted to improve image quality.
|Feature||What Is It?||Advantage||Disadvantage|
|Harmonics||Selective reception of penetration (uses frequencies generated within tissue)||Improved contrast resolution |
Improved visibility of low-level echoes
Reduction of artifacts
|Less harmonic penetration (uses higher frequency)|
|Compound imaging||Uses multiple-angled firings to create one image||Improved border definition |
Less angle dependence
|Slowed frame rate |
Loss of some beneficial artifacts (i.e., shadowing, refraction, and enhancement)
|Speckle reduction algorithms||Sophisticated algorithms applied to the image to reduce speckle (salt and pepper appearance of ultrasound image)||Improved contrast resolution |
Improved conspicuity of masses
|Extended field of view imaging||Image builds up as probe is moved across anatomy||Improved ability to show anatomic relationships of structures too large to fit in linear-array format||May be difficult to perform on uncooperative patient or over sharply curving interface|
|Trapezoid or virtual convex imaging||Steering of linear-array probe to create sector format||Larger field of view with linear-array probes||Reduced lateral resolution|
|Multizone focus||Use of multiple focal zones to create an extended area of focus on one image||Improved lateral resolution||Slowed frame rate|
Color and spectral Doppler play an important role in scrotal ultrasound. The typical color/spectral Doppler frequencies used for scrotal ultrasound are between 4 and 8 MHz. The upper frequency range is used to improve sensitivity to slow flow. This is important in evaluation of testicular torsion or tumor vascularity. Penetration is decreased with higher frequencies, so it is important to make sure that the color penetrates to the depth of interest. Color and spectral Doppler findings on the symptomatic side are always compared with the asymptomatic side.
Power Doppler is often used as a way to quickly get to a sensitive setting that will demonstrate slow flow. Power Doppler shows the amplitude or power of the moving signal, whereas color Doppler shows the frequency shift. Power Doppler does not demonstrate flow direction or aliasing, and to some offers a more straightforward display of blood flow. Presets for power Doppler are often set at a lower pulse repetition frequency (PRF) than color Doppler because aliasing is not an issue, so pushing the power Doppler button may show more flow with fewer adjustments to the controls. This often provides a quick way to get to a more sensitive flow setting. Persistence is usually much greater with power Doppler, requiring a steady hand and slower movement of the probe. To further enhance power Doppler, the same parameters are adjusted as for color Doppler.
Familiarity with color Doppler controls is very important when performing scrotal ultrasound. The sonographer may need to adjust some of the following color Doppler parameters throughout the study to enhance the visibility of scrotal perfusion ( Table 23-3 ):
Gain— The color gain control is used to amplify the reflected color Doppler signal. Whenever the expected amount of color is not visible in the image, the color gain should be increased until noise is present. Once color noise is visible, the gain can be decreased until it just disappears. At this point, the color gain setting is optimized.
Scale/pulse repetition frequency (PRF)— The PRF is the number of pulses transmitted in 1 second. This important color parameter affects the sensitivity of the system in displaying slow flow. It also sets the point at which color aliasing occurs (Nyquist limit). The control has different names depending on the ultrasound equipment being used. It is variably named scale, PRF, or flow rate. The PRF is reduced to improve sensitivity to slow flow. This is critical when ruling out testicular torsion. If the PRF is set too high, slow flow may not be visible. When the PRF is set too low, excessive color aliasing occurs, which makes it impossible to determine flow direction or to assess flow quality. Neither of these factors is significant in scrotal ultrasound, so it is common to use low PRF settings. However, flash artifact from patient motion is more apparent with very low PRFs and may make scanning difficult. It is recommended to adjust the PRF so that the asymptomatic testicular flow is well demonstrated without excessive flash artifact. Then compare the same settings on the contralateral side ( Figure 23-12 ).