The most widely used ERUS system is the Bruel and Kjaer scanner (Hawk 2102 EXL, 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. 1) . 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 to 10 MHz with a focal length of 2-5 cm and a 90° scanning plane and is rotated at 4-6 cycles/s to get a radial scan of the rectum and surrounding structures. The end of the probe is covered with either a latex balloon or a plastic cap that is filled with degassed water to maintain acoustic coupling between the transducer and the tissue (Fig. 2). It is important to eliminate all bubbles within the hard anal cap or the balloon, given that these may produce artefacts and limit the overall utility of the study. The latex balloon is chosen when imaging rectal cancers.
The rectum can be of varying diameters and therefore the volume of water in the balloon may have to be adjusted intermittently. If the anus is being evaluated, a water-filled hard plastic cone, made of a sonolucent polymethyl pentene plastic and 1.7 cm in outer diameter, is used in place of the latex balloon. The outer walls of this cone are parallel, so that the probe may be moved within the anal canal without causing any anatomical distortion.
Technology progress allows the three-dimensional (3D) reconstruction of two-dimensional (2D) images . It is not necessary to use new ultrasound probes, but to connect the ultrasound apparatus to a computer equipped with software (L3Di-2000/2100). 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 col-orectal pullback mover is a computer-controlled, motor-driven device that can be operated at different
Fig. 2a, b. Contact methods: a plastic cone and b latex balloon levels of resolution. For the endo-anal 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. The data from a series of closely spaced 2D images is combined to create a 3D volume displayed as a cube (Fig. 3). The 3D image does not remain fixed, but rather 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 are 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 to be x x
seen at once with real-time reconstruction. Extensive anorectal examinations require moving the transducer head. Probe movement can cause artefacts 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. 4). 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 3D data set acquisition and high precision positioning of the scan head over a longitudinal distance of 60 mm are accomplished at the touch of a button, allowing the information to be obtained without having to move the probe's position. The 2050's double crystal covers a frequency range from 6 to 16 MHz. With a shaft length of 270
mm, the probe is long enough to thoroughly cover the entire rectum plus the sigmoideum.
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