Ultrasonography of the Testes

Ultrasonography of the Testes

Charles Love

Texas A&M University College of Veterinary Medicine, College Station, TX, USA

Ultrasonographic evaluation of the scrotal contents in the stallion includes the spermatic cord (the spermatic artery, ductus deferens, and spermatic venous network (pampiniform plexus), cremaster muscle, nerves, lym- phatics), the epididymis (head, body and tail), the testis, vaginal cavity, and scrotum (skin, dermis). The clinician should be able to identify position, location, and the normal echoic pattern of these structures. The examination of the scrotal contents, in addition to determining normalcy, should also include measurement of the length, width, and height of each testis. These measures are then used to calculate testis volume and determine the efficiency of sperm production.

Stallion Position and Location

The clinician is required to perform the examination in the vicinity of the flank area of the stallion. For obvious reasons this is a risky position for the clinician. If semen collection is performed in conjunction with the evalu- ation, stallions tend to be more tractable following semen collection. Regardless, it is recommended to sedate the stallion to facilitate a thorough and complete evaluation. The ultrasound evaluation of the scrotal contents should be considered a stand-alone primary procedure and therefore sufficient time and patience should be allotted to allow for a thorough examination of the scrotal contents as well as an accurate measure- ment of the testes dimensions. An inadequate exam- ination can result in erroneous testis measurements leading to an incorrect clinical interpretation.

The stallion can be evaluated from the left flank, either confined in a stock or free-standing in the stall following sedation.

Probe Type

The location of the scrotum is in a relatively restricted area between the hind legs of the stallion, which can limit manipulation and probe placement. The linear array probe commonly used for per rectum examination is satisfactory, but its size (i.e. length) can limit examination of discrete structures such as the epididymis. Sector probes can also be used, particularly the “finger” type probe, that is small (surface area ~2.5 cm) and allows easier access to specific areas of interest. The T-type linear probe allows ease of handling and placement in the scrotal area. See Figure 13.1.

Figure 13.1 Ultrasound probes used for scrotal content evaluation. Left: standard linear array; middle: “finger” sector scanner; right: T-type linear array.


Ultrasound evaluation should be preceded by a thor- ough manual evaluation of the scrotum and its con- tents to detect any specific areas that require scrutiny. Ultrasound gel or a similar lubricant can be applied to the probe. Since the scrotum in the stallion has very little hair there is no need to clip the scrotum. However, since sweat glands are present, artifactual changes due to lather and bubble formation require gel removal and gel reapplication to maximize image resolution.

Testis Measures

Measurement of the testis dimensions (width, height, and length) can be performed first.


The height is measured by placing the probe ventrally and directing the beam dorsally so that the central vein is approximately two thirds of the distance from the surface and the spermatic artery can be visualized dor- sally. There is no need grasp or manipulate the testis for this measure (Figures 13.2, 13.3).

Figure 13.2 (A) Cranial–caudal view of the left testis. (B) Lateral view of the left testis. The height of the testis is measured by placing the ultrasound probe ventrally with the beam directed dorsally. (C) The line denotes the orientation of the arrow in B. Notice the hyperechoic line at the tip of the arrow identifying the tunica albuginea on the dorsal surface of the testis. Below the arrow (dorsal) is the caudal edge of the spermatic cord.

Figure 13.3 (A) Central vein in cross-section and (B) longitudinally.


The length is usually measured from the caudal aspect of the scrotum in the vicinity of the tail of the epididymis, directing the beam cranially. The ultrasound probe should be rotated slightly in a horizontal direction so that the maximum length is visualized. The maximum distance can be determined by visualizing the hyperechoic cranial edge of the tunica albuginea of the testis. Similar to the height measurement, the length should be measured in situ (Figure 13.4).

Figure 13.4 (A) Lateral view of the left testis. The solid black line shows the length measure that should extend from the cranial to the caudal edge of the testis, making sure to not include the cauda epididymis. The ultrasound probe is located caudally on the scrotum. Notice the long axis of the testis is not located horizontally, but rather is angled in a dorsocranial direction. (B) Ultrasound picture of the long axis of the testis. The white line corresponds to the black line in A. Notice the hyperechoic line at the tip of the arrow denoting the cranial edge of the testis.


The left testis is measured by placing the probe on the left lateral surface of the scrotum and directing the beam horizontally and medially (Figure 13.5). At the same time the right testis should be pushed dorsally so that the shape of measured testis is not distorted. For the measurement of the right testis, the left testis is pushed dorsally to allow measurement. Grabbing of the scrotal neck, similar to the technique used to measure ruminant testes, should be avoided for several reasons. First, it is easier to perform the examination using only one hand to manipulate the probe on the testis, while the testis hangs freely. Even if the testis is in an inguinal position, the testis can be accurately measured without the need to manually draw the testis into the scrotum. Second, it tends to cause strong contraction of the cremaster muscle, which may elicit a similarly strong response from the stallion (i.e. kick).

Figure 13.5 (A) Cranial–caudal view of the left testis. The width of the testis is measured by placing the ultrasound probe laterally at the widest point of the testis with the beam directed medially. (B) Line denotes the orientation of the beam in A. Notice the hyperechoic line at the end of the arrow identifying the tunica albuginea on the medial side of the testis. (C) The location of the probe on the lateral surface of the testis at the widest point of the testis.

Qualitative Evaluation of the Scrotal Contents

The echotexture of the scrotal contents can be evalu- ated following testes measurement. A routine should be followed to ensure a thorough evaluation is com- pleted. The routine followed by the author starts at the neck of the scrotum and visualizes the spermatic cord above the testis.

Spermatic Cord

The primary structure visualized is the tortuous sper- matic artery as it winds to the testis (Figure 13.6). Pathology of the spermatic cord includes spermatic cord torsion, which commonly occurs 1–3 cm dorsal to the testis (Figures 13.7, 13.8, 13.9). Varicocele (Figure 13.10), dilation of the pampiniform plexus, while uncommon, may also be detected here. In addition, generalized edema resulting from inflammation, such as orchitis or epididymitis, may present as hyperechoic inconsistencies in the spermatic cord (Figure 13.11).

Figure 13.6 (A) Cross-section of the spermatic cord showing the spermatic artery and (B) confirming blood flow with Doppler ultrasound.

Figure 13.7 Spermatic cord torsion. (A) Spermatic cord dorsal to the testis in a case of spermatic cord torsion. Notice loss of the “Swiss cheese” appearance of the spermatic artery and the increase in the hyperechoic appearance of the cord due to loss of circulation, blood stagnation, and edema. (B) Testis associated with the cord lesion. Notice the hyperechoic areas, probably a result of venous congestion.

Figure 13.8 (A) Ultrasonographic appearance of a spermatic cord following torsion of the spermatic cord compared to (B) normal contralateral spermatic cord. Notice the lack of circulation in A compared to the hypoechoic areas of circulation in the normal cord.

Figure 13.9 (A) Affected spermatic cord in Figure 13.8 1 day later, after the torsion has self-corrected. (B) The spermatic artery is patent (Doppler flow), but there is residual edema in the center of the cord.

Figure 13.10 Distended venous supply (varicocele) dorsal to the testis.

Figure 13.11 Generalized edema resulting from inflammation such as orchitis or epididymitis may present as hyperechoic inconsistencies in the spermatic cord.


Following the examination of the spermatic cord the ultrasound probe is passed laterally on the testis to examine the parenchyma, which should be homogene- ous in echotexture, devoid of hyper- or hypoechoic foci, commonly associated with neoplasia (Figures 13.12, 13.13) or benign structures (Figure 13.14). In cross-section the testis shape may be round to oblong.

Figure 13.12 (A,B) Heterogeneous mass within the testis parenchyma that was diagnosed as a Sertoli cell tumor.

Figure 13.13 Heterogeneous mass within the testis paren- chyma that was diagnosed as a seminoma. Notice the promi- nent vascular channels at the periphery and within the body of the mass.

Figure 13.14 Discrete hypoechoic lesion beneath the tunica albuginea of the testis. These lesions tend to be benign, but should be monitored for growth and vascularity.


The epididymis includes head, body, and tail regions that are located craniolaterally, dorsolaterally and caudally respectively, on the horizontally oriented testis. The epididymal tail (Figure 13.15) should be evaluated for location and size. Spermatic cord rotation of 180° occurs where the epididymal tail is located dorsocranially. In addition, the size and ultrasonographic appearance of the tail can vary from small, in the case of hypoplasia, to prominent, when the tail is distended with sperm or due to inflammation. A tail distended by sperm only, may be common in stallions at sexual rest or in cases of sperm accumulation when sperm accumulates from the ampulla all the way back to the ductus deferens and epididymis (Figures 13.16, 13.17).

Figure 13.15 Epididymal tail. Notice the prominent lumen in cross-section.

Vaginal Cavity

The vaginal cavity is a potential space that communi- cates with the peritoneal cavity through the inguinal canal and therefore, peritoneal contents such as fluid and intestines have the potential to pass through the canal and occupy the vaginal cavity. In addition, circulatory (artery, vein, lymphatic) compromise to the spermatic cord can result in free fluid accumulation (hydrocele). This fluid tends to accumulate around the epididymal tail (Figure 13.18).

Figure 13.18 (A,B,C,D) Fluid (hydrocele) in the vaginal cavity around the epididymal tail.

Measurement and Interpretation of Testes Volume

Measurement of testes size is an important part of the stallion breeding soundness evaluation since testes size is associated with sperm production. The number of sperm produced by the stallion may impact fertility and, thus, the number of mares that a stallion can breed. Historically, testes size was determined using the linear measure, total scrotal width, in which cali- pers were used to measure the combined width of the testes, the mediastinum testes, and the scrotum. This technique, however, does not determine the three- dimensional shape (i.e. volume) of each testis. While the relationship of length, width, and height tends to be proportional, there are instances where they are not, and the determination of each dimension will more accurately determine volume.

Subsequently, ultrasonography has been introduced as a technique to measure and evaluate the testes. This technique has the advantage of being specific and accurate since the clinician can visualize the testis parenchyma and identify landmarks that assure measurement of the length, width, and height.

Measurements (length, width, and height) from each testis are performed as described in the preceding section. The measures from each testis are then inserted in a formula that approximates the volume of an ellipsoid [1]:

Figure 13.16 (A) Dilated (~0.5 cm) ductus deferens associated with (B) distended (~3.0 cm) cauda epididymal tail. (C) Contralateral epididymal tail that measures 1.7 cm.

Figure 13.17 Comparison of the size and distension level of the (A) left (1.6 ×2.1 cm) and (B) right epididymal tails (2.5 × 4.0 cm) from the same stallion. Lack of a prominent lumen (A) may result from recent ejaculation or indicate reduced sperm production from that testis. A prominent lumen is usually due to sperm. It may be associated with plugged ampullae (accumulation) or may occur in a stallion at sexual rest.

Testis volume in cm3

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Nov 6, 2022 | Posted by in EQUINE MEDICINE | Comments Off on Ultrasonography of the Testes

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