Scan types and image orientation
Image orientation is essential for clear communication and interpretation of ultrasound images. When scanning in the longitudinal plane, the patient’s head direction is oriented to the left of the monitor. Scans in the transverse plane are oriented so that the left side of the monitor image corresponds to the right side of the patient.
The main difference between laparoscopic and open ultrasound probes is the orientation of the probe. In fact, in contrast with the typical transverse orientation of open intraoperative ultrasound (IOUS), the transducer configuration of LUS is longitudinal. Full liver exploration often requires to change the probe trocar position. Therefore, when starting a new scan, it is necessary to make sure that the transducer and the monitor orientation are aligned; a button is available to change the orientation of the image.
Sliding is the most commonly used movement during laparoscopic ultrasound of the liver. Once the first acoustic window has been identified and the transducer positioned, the probe slide across the Glissonean surface while the probe-to-surface geometry is maintained. This movement provides a series of parallel images to the original scan plane. The sliding can be done with the probe in longitudinal or transverse orientation.
The transducer is rotated clockwise or counterclockwise while the central part remains fixed over the starting point. This allows to scan through longitudinal, oblique and transverse planes of the structure of interest, generating a three-dimensional image. This manoeuvre is only possible with 4-ways flexible probes.
Angulation technique: rocking e tilting
The probe’s head is in a stationary position while the shaft of the probe is moved to different angles. When rocking, the transducer moves parallel to the scanning plane; during tilting, it moves perpendicular to it. These movements allow the scanning of extensive areas and to generate a three-dimensional reconstruction of the image to be studied.
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