Prostate and Seminal Vesicles: Ultrasound Anatomy and Scanning Methods

Fig. 20.1

Traditional convex probe with 3.5 MHz frequency

The quality of the images is related to the number, dimensions, and structure of the crystals and to the scan converter.

The TR probes show high frequencies which range from 4 MHz up to 12 MHz. The arrangement of the transducers defines the probe: monoplanar, biplanar, and end fire [1].

1.

The monoplanar probes are linear and slim; they show high spatial resolution, but the exam of the prostate is possible only in a single plan and, consequently, with several diagnostic limitations. Currently, the most common use is in case of anal stenosis (Fig. 20.2).

Fig. 20.2

The shape of monoplanar probes is linear and slim; even if the exam of the prostate is possible only in a single plan, this kind of probe shows high spatial resolution
2.

The biplanar probes have two groups of transducers, which form a 90° corner; the most important disadvantage is the big dimension of the probe itself, due to two groups of electric connections. Moreover, the employment of the biplanar probes is more difficult because the scan in the two orthogonal plans is not often simultaneous; additionally, the transducers are located in two different points of the probe; thus, there is no spatial correspondence between the anatomical entities observed in the sagittal and transversal plan (Fig. 20.3a–c).

Fig. 20.3

(a) The biplanar probes have two groups of transducers, located in two different points of the probe, which form a 90° corner and allow almost simultaneously transverse and sagittal visualization; the presence of two distinct groups of transducers implicates the big dimension of the probe itself. It is important to note that there is no spatial correspondence between the anatomical entities observed in the sagittal and transversal plan. (b) Patient in semi-lithotomy position: prostate visualized according to sagittal plan. (c) Patient in semi-lithotomy position: prostate visualized according to transversal plan
3.

The end–fire probes show a fanlike arrangement of the transducers on the distal convexity of the probe itself (Fig. 20.4). The radial disposition of the transducers is the cause of the most important limit of this kind of probe: it offers a trapezoidal vision, with imaging distortion and a reduced spatial resolution in the zones which are distant from the field of view. Additionally, the areas which are located proximally to the apex of the probe show evident reverberation phenomena, with a reduced spatial resolution. In routine practice, the spatial resolution is good if proximal to the probe, whereas it progressively reduces distally, with a final consistent reduction of the resolution power. The exam of the prostate and SV is obtained manually rotating the probe; the employment of different adaptors allows different interventional procedures, including the biopsy with TR approach.

Fig. 20.4

Fanlike arrangement of the transducers on the distal convexity of the end-fire probe. This kind of probe offers a trapezoidal vision, with imaging distortion and a reduced spatial resolution in the zones which are distant from the field of view, i.e., prostate apex

The images from different probes are not correspondent to each other: the sagittal scans from linear probe are different from those from end-fire probe because of both the different fields of view and the compression of the probe on the posterior part of the prostate.

The images in transversal scan with biplanar probe have to be considered as axial, whereas the images from end-fire probe are coronal oblique. According to these considerations, the US anatomy of the apex is better defined with linear probes, whereas the base is well documented with the end-fire ones.

20.2.2 Transabdominal Approach

The TA approach uses the acoustic window of the bladder, which has to be mildly filled; in case of empty bladder, the margins of the glands and seminal vesicles are hidden by the echoes from the adjacent bowel loops (Fig. 20.5). The multifrequency convex probe has to be inclined in cranio-caudal direction, in order to avoid the acoustic shadow of the pubis (Figs. 20.6 and 20.7).

Fig. 20.5

Transabdominal approach with mildly filled bladder. The measurement of the three diameters of the prostate is easily feasible

Fig. 20.6

Difficulties of visualization of the prostate with transabdominal approach caused by the acoustic shadow of the pubis and the excessively filled bladder

Fig. 20.7

Incorrect inclination of the convex probe in craniocaudal direction: the acoustic shadow of the pubis does not allow the visualization of the whole gland in longitudinal scan

The actual machines allow to choose the most appropriate frequency according to:

The physical characteristics of the patient;
The level of fatty tissue in the anterior abdominal wall;
The thickness of abdominal muscles.

The TA approach evaluates the volume of the prostate and its relationships with the adjacent organs; nevertheless, the exam of the internal anatomy of the prostate is not possible. The third lobe, the intraprostatic cysts, or calcifications are easily recognizable. The SV are not always well defined, and they usually appear like two pear-shaped anechoic entities, with transversal orientation, cranially located respecting to the gland in the transversal plan and up to the prostate and behind the bladder in the longitudinal scans.

The TP approach is rarely used and it allows images similar to those of TA approach. This technique is applied to obese patients or those submitted to radical cystectomy or with small bladder due to chronic inflammation or radiotherapy. This approach is also useful in performing prostate biopsy in men submitted to operations because of rectal cancer or those with anal stenosis.

The patient is supine with open legs, the probe is positioned on the perineum, and it is rotated in two orthogonal plans [2, 3].

20.2.3 Transrectal Approach

An enema 2 h before the exam is advisable. It is not necessary to have a full bladder, even if moving the bowel loops, especially the sigma-rectum, facilitates the definition of the prostatic base and SV.

The TR approach was firstly used by Watanabe and coll. in 1967, but the method diffused only after the production of manual probes with the initial frequency of 5 MHz and successively higher.

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