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B‐mode ultrasonography of bull testes was first described by Pechman and Eilts [8] in 1987. A thorough visual and manual examination of the scrotum and its contents should always precede the ultrasound examination. The scrotum and testes are then examined by ultrasound in both a sagittal and transverse plane, with the probe applied to the cranial, lateral, or caudal surface of the testis depending on the examiner's preference. A complete examination includes the spermatic cord, the entirety of the testis parenchyma, the epididymis, and the scrotal wall. The body of the epididymis and the ductus deferens are difficult to detect unless they are grossly abnormal.

Normal ultrasonic appearance of the testis is shown in Figure 10.1. The testis parenchyma appears as a homogene ous tissue with a stippled medium echogenicity that surrounds the centrally located and more hyperechoic (whiter) mediastinum testis. The mediastinum testis is normally less than 5–6 mm wide. The parenchyma consists primarily of convoluted seminiferous tubules that contain the sustentacular (Sertoli) and germinal cells but also includes the adjacent interstitial (Leydig) cells along with associated vascular, neural, and stromal tissues. The seminiferous tubules are connected by the straight tubules to the rete testis that comprises a series of interconnected channels within the mediastinum testis. Sperm pass dorsally through the rete testis to the efferent ducts that in turn penetrate the capsule of the testis before uniting as the epididymal duct.

Figure 10.1 Caudal ultrasonographic views of the left testis of a yearling dairy bull. Caudal is at the top and cranial at the bottom of each ultrasound image. On the left is a sagittal view, with the mediastinum testis (between arrows in each image) appearing as a more hyperechoic linear structure running through the approximate center of the less echoic testis parenchyma. On the right side of the image is a transverse view of the same testis, with the mediastinum in cross‐section, again in the middle of the less echoic parenchyma. The scrotal wall is visible adjacent to the probe on the top (caudal) and on the bottom (cranial) of the testis in each image. The bright white line at the bottom of each image is a gas artifact that marks the far side of the scrotal skin.

Changes in the ultrasonic appearance of the testis parenchyma can reflect normal events around puberty as well as pathology. An increase in echogenicity (increased pixel intensity, or a whiter parenchyma) has been reported to occur in association with sexual development around the time of puberty [9, 10], but ultrasonography was no better at predicting puberty than was measurement of scrotal circumference [10]. While pixel intensity has been correlated with attainment of sexual maturity as assessed by semen quality [11], it seems to be a better predictor of future semen quality than the present status of the bull [12]. Subsequent studies in mature bulls have failed to correlate pixel intensity with semen quality at BBSE [13] or with histologic measures of spermatogenesis [14].

In any event, objective assessment of these subtle changes in pixel intensity is not possible by real‐time visual interpretation of an ultrasound image. To accurately assess pixel intensity, images must be obtained and digitally preserved using rigorously standardized methods. The digital files are then sampled and evaluated using computer software that can objectively assess the gray scale of each pixel in the sampled area, providing both an average intensity and measure of variation. Given the limited clinical utility and the great effort required, Brito et al. [10] concluded that “though testicular ultrasonogram pixel intensity might be useful for research purposes, clinical application of this technology in the present form for bull breeding soundness evaluation is not justifiable.”

The majority of the discrete anomalous ultrasonographic changes detected in the testis are more hyperechoic than normal tissue. These may appear as small foci scattered throughout the testis (Figure 10.2) or they may be more localized (Figure 10.3). Larger hyperechoic lesions are often seen to radiate from the rete to the periphery of the testis, suggesting the complete involvement of one or more seminiferous tubules (Figure 10.4). The histologic basis for the increase in echogenicity is usually fibrosis [15] but the echogenicity itself does not confirm histology or etiology. Where a specific histologic diagnosis is provided in the accompanying ultrasound images, it is based on histopathologic assessment of excised tissues.

Figure 10.2 Sagittal view of the left testis of a mature bull with small hyperechoic foci distributed throughout the testis parenchyma.

Figure 10.3 Sagittal view of a testis with small hyperechoic foci (large arrow) surrounding the mediastinum (small arrow) suggestive of lesions in the straight ducts that drain the seminiferous tubules into the rete testis.

Figure 10.4 Sagittal view of a testis with multiple linear radiating hyperechoic lesions, two of which are highlighted by the smaller arrows. A normal mediastinum testis is shown (large arrow).

Tumors are generally more hyperechoic than the normal testis parenchyma (Figure 10.5), but mixed or hypoechoic echogenicity is also possible [4]. Mineralized lesions are extremely hyperechoic and are usually accompanied by intense or complete shadowing causing a loss of image distal to the lesion (Figure 10.6).

Figure 10.5 Sagittal view of the right ventral testis of an aged beef bull. Histologically, the hyperechoic lesion (arrow) was a Sertoli cell tumor.

Figure 10.6 Sagittal views of the left and right testes of a yearling dairy bull. The left testis is normal, while the hyperechoic areas of the right testis cast prominent dark shadows in the distal tissue and represent areas of mineralization.

Barth et al. [15] have proposed a scoring system for fibrotic lesions in the bull testis based on the size and frequency of hyperechoic lesions detected by ultrasonography in a single transmediastinal plane. They found that the occurrence of fibrotic lesions in young bulls aged between 3 and 20 months was not associated with decreased semen quality. This is consistent with their histologic findings, where seminiferous tubules within the area of fibrosis were generally affected, but immediately adjacent germinal tissue was quite often completely normal. However, bulls that experienced a dramatic increase in fibrosis score at a second examination (≥4 points, 4–13 months later) were more likely to have poor semen quality than bulls that did not experience this degree of change [15]. While bulls in this report with up to 50% of their parenchyma affected were able to produce mostly normal sperm, it is reasonable to suspect that they would have reduced production capacity.

The cause of fibrotic lesions in the testis is unknown. Possible etiologies include infectious or inflammatory conditions, developmental defects of the seminiferous tubules or their connecting ducts, autoimmunity, degenerative conditions, or aging. Ram testes inoculated with Trueperella pyogenes (formerly Arcanobacterium pyogenes) eventually developed hyperechoic lesions that consisted of fibrotic tissue [16]. A severe outbreak of bovine respiratory syncytial virus (BRSV) in a group of bulls was associated with an increased prevalence of fibrotic lesions in one report, though a cause‐and‐effect relationship could not be confirmed [15]. On the other hand, scrotal insulation resulting in a dramatic decrease in semen quality did not cause fibrosis or any other ultrasonically detectable change in the testis within four months [17] or six months [12] after the insulation event. A progressive fibrosis that is more marked in the ventral testis has been described as a histologic feature of aging in the bull [18].

Distinct hypoechoic lesions in the testis parenchyma are seen less commonly (Figure 10.7). Intratesticular inoculation of T. pyogenes in rams caused an initial hypoechoic change associated with edema, neutrophilic infiltration, and degenerative changes in the seminiferous tubules [16]. Tumors may present as hypoechoic areas in some cases. Since an increase in echogenicity is seen at puberty in association with proliferation of the spermatogenic tissue, it seems reasonable to speculate that areas affected by a degenerative process would be more hypoechoic by contrast.

Figure 10.7 Transverse view of the testis containing two discrete hypoechoic wedge‐shaped areas (between arrows) of unknown etiology or significance.

A hypoechoic distension of the rete testis was first reported in Ayrshire cattle [19]. Most of the bulls in this report had bilateral distension and were azoospermic by 16 months of age, suggesting a block to sperm outflow. A genetic predisposition to the condition was suspected. A similar unilateral lesion was reported in a mature Holstein bull in conjunction with agenesis of the ipsilateral seminal vesicle, epididymal body and tail, as well as low semen volume [20]. Figure 10.8 shows an ultrasonogram from a yearling Holstein bull with a history of oligospermia and low semen volume. Note the enlarged hypoechoic appearance of the mediastinum. The contralateral testis and mediastinum appeared normal (not shown). Surgical removal of the affected testis confirmed a diagnosis of agenesis of the body of the epididymis and fluid distension of the rete testis. The distension can also affect the efferent ducts. In our experience, extensive and uniform hypoechoic distension of the rete testis as seen in Figure 10.8 is pathognomonic for sperm outflow obstruction, most often congenital. In unilateral cases, the presenting complaint is usually low sperm numbers and semen volume in an otherwise normal young bull. Congenital cases may be associated with aplasia affecting the epididymis, deferent duct, ampulla, or seminal vesicle.

Figure 10.8 Sagittal view of the left testis of a mature Holstein bull that presented with a complaint of low semen volume and oligospermia. Note the hypoechoic distension of the rete testis (dark linear area between arrows). The distension was secondary to a complete outflow obstruction resulting from the absence of the epididymal body on this side.

The head of the epididymis is best seen in a sagittal view and is found on the dorsocranial aspect of the testis. It is usually less echoic than the parenchyma of the testis. The tail of the epididymis can be readily located by palpation and is found on the ventral aspect of the testis. Acquired lesions of the epididymides can lead to outflow obstruction. A cystic distension of the head of the epididymis is shown in Figure 10.9. It is worth noting that cysts of the appendix epididymis (proximal mesonephric duct remnant) are also found in this area and even those as large as a few centimeters in diameter usually do not interfere with normal epididymal function [6]. The epididymal tail (Figure 10.10) can be difficult to image with a transrectal probe and for fine detail one should consider using a higher‐frequency probe with a smaller footprint. Congenital, traumatic, and inflammatory lesions can be seen here, with sperm stasis and granuloma formation as possible sequelae. Alterations of size and echogenicity are characteristic of disease in this tissue. Multiple small sperm granulomas are visible as focal hyperechoic lesions in the tail of the epididymis (Figure 10.11).

Figure 10.9 Sagittal view of the testis. Note the hypoechoic dilations (arrows) in or near the head of the epididymis. Normal testis parenchyma is seen to the right of the cysts.

Figure 10.10 Sagittal view of the ventral testis (large arrow) and tail of the epididymis (small arrow) of a normal bull.

Figure 10.11 Transverse view of the ventral aspect of the right testis of an aged beef bull. The tail of the epididymis is seen between the arrows. The small white foci seen in the epididymal tissue between the arrows are sperm granulomas.

Ultrasonography can provide more specific information about the extent, location, and resolution of scrotal pathology. Examples include scrotal hydrocele (Figure 10.12), inflammation, sperm granuloma, spermatocele, fibrosis, mineralization, herniation, tumors, abscesses, trauma, hemorrhage, outflow obstruction, and lesions of the scrotal wall and vasculature. The echogenic pattern seen is variable and often quite heterogeneous. In cases of testicular trauma (Figure 10.12), ultrasound can confirm the actual degree of damage to the testis and provide a more accurate prognosis for recovery. Ultrasound can be used in cases of scrotal hernia to confirm the presence of herniated bowel as well its viability as assessed by the presence or absence of peristalsis.

Figure 10.12 Sagittal view of the dorsal testis. The small arrow points to a traumatic lesion of the testis capsule and visceral vaginal tunic. The hypoechoic fluid surrounding the testicle is a persistent scrotal hydrocele (large arrow).

The final element of the scrotal ultrasound evaluation is an assessment of the scrotal neck and spermatic cord (Figure 10.13). While herniation, trauma, hemorrhage, and scrotal hydrocele can affect this region, they are usually more evident in the scrotum proper. The vas deferens cannot be reliably imaged in normal bulls. The primary ultrasonic feature of this region is the vascular cone consisting of the prominent tortuous testicular artery overlain by a fine network of very small veins called the pampiniform plexus. Brito et al. [21] reported an increase in vascular cone diameter up to 13.5 months of age in beef bulls and increased vascular cone diameter was positively correlated with an increased percentage of normal sperm.

Figure 10.13 Sagittal view of the vascular cone (between small arrows) just dorsal to the testis (large arrow). The lumen of the testicular artery is represented by the black irregular areas in the vascular cone. The network of veins forming the pampiniform plexus is not usually visible due to their small size.