The most widely used EAUS system is the B-K Medical scanner (Pro-Focus 2202, B-K Medical A/S, Mileparken 34, DK-2730 Herlev, Denmark), with a
hand-held rotating endoprobe type 1850, which gives a 360° axial view of the rectal wall (Fig. 4) . The radial probe has a 24-cm metal shaft with a rotating transducer at its tip. This 8539 transducer has a frequency range from 5 MHz to 10 MHz with a focal length of 2-5 cm, and a 90° scanning plane and is rotated at four to six cycles per second to achieve a radial scan of the anus and surrounding structures. The end of the probe is covered with a cone made of sonolucent polymethyl pentene plastic and 1.7 cm in outer diameter, which is filled with degassed water to maintain acoustic coupling between the transducer and the tissue (Fig. 5). It is important to eliminate all bubbles within the hard anal cone, given that these may produce artifact and limit overall utility of the study. The outer walls of this cone are parallel so that
the probe may be moved within the anal canal without causing any anatomical distortion. In patients with a stenotic anus, a smaller endoprobe (7 MHz, type 6005, focal range: 0.5-3 cm) and cone (diameter 0.9 cm) can be used.
Technology progress allows the three-dimensional (3-D) reconstruction of two-dimensional (2-D) images . It is not necessary to use new ultrasound probes but to connect the ultrasound apparatus to a computer equipped with a software (BK3Di). Three-dimensional reconstruction is based on a high number of parallel transaxial images acquired using a special colorectal pullback mover (UA0552) with the B-K Medical ultrasound probe type 1850. The colorectal pullback mover is a computer-controlled, motor-driven device that can be operated at different levels of resolution. For endoanal application, the usual setting is 0.2-0.3 mm between adjacent transaxial images. Scanning the anal canal with these settings over a pullback distance of 35 mm will typically yield 175 parallel images. Data from a series of closely spaced, 2-D images is combined to create a 3D volume displayed as a cube. The 3-D image does not remain fixed; rather, it can be freely rotated, rendered, tilted and sliced to allow the operator to infinitely vary the different section parameters and visualise the lesion at different angles and to get the most information out of the data. After data is acquired, it is immediately possible to select coronal anterior-posterior or posterior-anterior as well as sagittal right-left views. The multiview function allows up to six different and specialised views at once with real-time reconstruction. If one wants to see the internal structure, a volume representation may be chosen. In this, one allows the ray to pass through the data, and contributions from different depths are added together in some way and used to construct the image pixel on the screen (volume rendering). "Volume render mode" is a special feature that successfully can be applied to high-resolution 3D data volumes. Imaging processing includes maximum intensity, minimum intensity and summed voxel projections, combined with positional or intensity weighting. This technique changes the depth information of 3-D data volume so information inside the cube is reconstructed to some extent.
Extensive anorectal examinations, however, require moving the transducer head. Probe movement can cause artifacts and change anatomical presentation. The new B-K Medical 2050 anorectal transducer is designed so that no moving parts come into contact with human tissue (Fig. 6). The transducer's 360° rotating head, the proximal-distal actuation mechanism and the electronic mover are fully enclosed within the housing of the slim probe. Both 3-D data-set acquisition and high-precision position-
ing of the scan head over a longitudinal distance of 60 mm are accomplished at the touch of a button, allowing information gathering without having to move the probe's position. The 2050's double crystal covers a frequency range from 6 MHz to 16 MHz.
In females, transvaginal endosonography (TVS) can be performed in case of anal pain or stenosis or to obtain additional information using the same radial probes. Moreover, for sagittal and conventional transverse imaging of the pelvic floor, including color Doppler, a biplane, high-frequency transducer with a long linear and transverse array can be used (B-K Medical 8658 probe) (Fig. 7). Both arrays are placed at 90° to each other and at 90° to the longitudinal axis. The transducer can be placed resting on the posterior vaginal wall. With the patient lying on her back on a table or in a gynecological chair, the anterior vaginal wall will softly contact the surface of the ultrasound transducer without disturbing the functional anatomy.
Transperineal ultrasound (TPUS) represents another method by which to detect perianal inflammatory disease, which can be performed using regular convex and linear high-resolution ultrasound probes . A convex 3.5 -MHz probe provides a general overview over perianal anatomy, identifying bladder, rectum, prostate gland and urethra or vagina and uterus. Further investigation is performed with a 10-MHz linear probe for a much more detailed imaging of perianal inflammatory disease.
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