The TRUS of the prostate is usually executed with the patient on the left side. The administration of an enema two hours before the exam is mandatory in order to avoid artifacts due to the presence of feces. The fasting is not necessary. The bladder has to be filled with 250–300 cc in order to better define the base of the prostate and the seminal vesicles.

The probes are transrectal, with high frequencies (≥7.5 MHz); this is due to the superficial position of the prostate respecting to the plan of the probe (internal wall of the rectum). Currently, the commercially available probes are:

The prostate has to be studied with two orthogonal scans, transversal and longitudinal, together with the evaluation of the volume of the whole prostate and the adenoma.

This clinical data plays a fundamental role in a therapeutic and surgical context [1–8] (Fig. 23.4).

Like in suprapubic sonography, BPH can usually be visualized as a low-echo area in transrectal sonography and can easily be distinguished from the peripheral gland with higher echo density. But it is also true that many adenomas are echo equal to the rest of the gland. It is still possible to detect an echo-equal adenoma, if a so-called tangential artifact occurs on the periphery of the adenoma.

This physical phenomenon occurs if the sound wave hits the peripheral surface at an angle. The ultrasound cone is thus split and an acoustic shadow generated. In the adenoma, there are frequently hyperechoic echos that occur more rarely in the peripheral gland. These can be residues of chronically recurrent focal prostatitis. They can be found in many cases where anamnestically no prostatitis has been diagnosed, so it must be assumed that many of these infections proceed subclinically. This would match the frequent histological findings after transurethral resection in patients, the majority of which had shown neither clinical nor anamnestic indications of prostatitis. In autopsies such pathological changes are found in over 50 %. In addition, corpora amylacea may show up as soundproof echos with a consecutive acoustic shadow. They are most likely to be found at the borderline between an adenoma and the peripheral gland. As their position is ventral to the peripheral gland, they do not detriment its feasibility in transrectal sonography (in contrast to suprapubic sonography). Small cystic alterations in adenomas are also not rare although still fairly infrequent, like the soundproof echos in the peripheral gland. Usually, these alterations are distended glandular vessels and in some cases also residues after minor tissutal necrosis. Utriculus cysts are found in the middle line dorsal from the urethra and can sometimes grow up to several cubic centimeters in volume.

It is not normally possible to differentiate anatomically between the central and peripheral zone sonographically, despite its crude and irregular vessel structures. The normal peripheral and central zone presents a homogeneous area in the central zone of the dorsal gland. In contrast to these latter zones, the ventrally located transition zone seems rather low in echo density.

TRUS is a very reliable instrument for the evaluation of BPH and to calculate the BPH. The diagnostic accuracy of the TRUS is very high, with a risk of overestimating the real volume and weight of the gland which ranges only 4–10 % [9–15]. Roehrborn et al. have shown that there was a distinct underestimation of prostate size by DRE when compared with TRUS measurement. The underestimation of prostate volume increased with increasing TRUS volume, particularly if the volume was greater than 30 mL. The average underestimation was between 9 and 12 % for prostate volumes 30–39 mL and between 17 and 27 % for prostate volumes 40–49 mL.

Ahmad et al. have shown that DRE had positive predictive value of 94 % in identifying the prostate above 30 cc. Hence, when considering treatment with 5-ARIs, DRE may be sufficient to identify suitable patients for 5-ARI therapy. However, for prostate volumes between 25 and 30 cc and above 80 cc, TRUS may be required [16].

Additionally, TRUS allows the identification of the medium lobe, its dimensions, and its relationship with the bladder floor [1].

Moreover, the measurement of the transition zone is a fundamental moment during TRUS examination. Different authors have shown that the transition zone volume represents an important parameter predictive of the severity of these symptoms [17, 18] (Fig. 23.5).

The TZ volume strongly correlates with the urodynamic parameters, especially when the transition zone index (TZI) is >0.5 [20]. The TZI is the result of ratio between the volume of the transition zone and the volume of the whole prostate. Additionally, the TZI correlates with the severity of the emptying symptoms [18, 19], and it is very accurate in predicting the risk of acute urinary retention [20] (Figs. 23.6 and 23.7).