a coupling agent is required to achieve good contact between the probe and skin. Any air or gas between the probe and the tissue to be examined will interfere with the acquisition of an image. The use of ultrasonic coupling gel as a coupling agent has the advantage of long duration, since it does not evaporate. Additionally, it is approved by most manufacturers for contact with the probe surface. Alcohol (70% isopropyl) also makes a good coupling agent, with the advantage that it does not have to be cleaned from the probe or patient after the examination is completed. A disadvantage of alcohol is that it evaporates and often needs to be reapplied to complete the examination. Additionally, not all probe surfaces are approved for contact with alcohol or other solvents. Placing a protective cover containing a small amount of coupling gel over the probe will prevent alcohol from contacting the probe surface. A disposable examination glove works well for this purpose.
For transrectal examination of the pelvic organs, the probe is covered with an examination sleeve containing just enough coupling gel to provide good contact between the probe and sleeve. Most probes designed for transrectal use in horses and cattle will fit snugly within a finger of a glove or sleeve. The feces are evacuated from the rectum and a manual examination is conducted before the covered probe is inserted. No coupling agent is necessary to maintain contact between the sleeve and rectal mucosa beyond the lubricant normally used to facilitate insertion of the probe and arm. If a protective cover is not used, the probe should be cleaned and disinfected between examinations following the recommendations of the probe manufacturer.
While diagnostic ultrasonography is generally regarded as safe for examination of reproductive tissues and fetuses [4], few published data exist in the veterinary literature. Coulter and Bailey [7] exposed yearling beef bulls to ultrasound (three minutes for each testis, one time, using a 5‐MHz linear transducer) and found no discernible effects on sperm numbers, morphology, or motility during a 70‐day period after the examination. They concluded that diagnostic ultrasonography should be safe for examination of the bull scrotum and testes. A reasonable approach is to use the minimum power and contact time necessary to complete your examination.
Examination of the Scrotum
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.
