Technique

Linear-array transducer with frequency of 5-10 MHz (typically 7.5 MHz) is employed for laparoscopic ultrasonography. Laparoscopic ultrasonography probe consists of transducers mounted on or near the tip of a slender shaft. The shaft is usually 10 mm in diameter and at least 15-20 cm in length, and is introduced into the peritoneal or retroperitoneal cavity by way of a 10 mm port.

The depth of ultrasound penetration with 7.5 MHz transducers is approximately 6-8 cm, which is commonly adequate for laparoscopic surgery, because laparoscopic ultrasonography scanning is performed directly on the surface of the target organ or lesion (contact scanning). Laparoscopic ultrasonography can detect small lesions such as stones as small as 1 mm, cysts as small as 2 mm, and tumors as small as 3 mm with reliable accuracy (3).

In practice, during contact scanning, three basic probe maneuver techniques are used: lateral (sliding), rotational (angulation), and withdrawal (advancement) techniques.

There is sufficient moisture naturally to allow good acoustic contrast, although sterile saline or gel may be used as acoustic coupling medium, if necessary. A strictly rigid system often loses acoustic coupling because of its inability to maintain direct contact with the organ surface. Major limitations of laparoscopic ultrasonography are the facts that the probe entry is limited through the established laparoscopic port, and also that freedom of rigid-shaft movement is restricted. Because the rigid shaft is pivoting, the freedom of the scanning direction is limited, such that it is often difficult to maintain the probe in standard transverse and longitudinal orientations. An alternative, although suboptimal, approach to contact scanning using a nonflexible system is to fill the surgical cavity with fluid and scan through the fluid as an acoustic medium (stand-off scanning technique). The use of a flexible probe toward multiple directions is a more versatile approach and reduces the number of laparoscopic port sites and scanning time. Movement of the flexible portion of the probe is controlled by the operator using an external lever mechanism similar to that of a flexible endoscope. The ability to flex or extend the probe is of critical importance in maintaining contact with organs having curved surfaces such as the kidney.

In practice, during contact scanning, three basic probe maneuver techniques are used: lateral (sliding), rotational (angulation), and withdrawal (advancement) techniques (4).

In lateral movement technique, the probe shaft slides horizontally in contact with the surface of the target while the probe-to-surface geometry is maintained. In rotational technique, the probe shaft in the port is rotated (clockwise or counterclockwise). In withdrawal technique, the transducer surface is maintained in the same direction while the probe shaft is manipulated longitudinally using an advancement-withdrawal maneuver. Occasionally, combinations of these two or three scanning maneuvers are performed simultaneously. Using a combination of these techniques, the complete laparoscopic ultrasonography examination of, for example, the kidney takes less than 10 minutes. Although the learning curve for laparoscopic ultrasonography has been speculated to be between 20 and 50 procedures (5), the author believes that with the acquisition of basic ultrasound principles and a good knowledge of surgical anatomy, laparoscopic ultrasonography can be a straightforward procedure for many urologists. Working with an experienced partner or collaboration with the radiologist may accelerate the learning curve for urologists interested in laparoscopic ultrasonography. Laparoscopic ultrasonography can be performed at any time during laparoscopic surgery, most frequently, early during the course of surgery. Because laparoscopic ultra-sonography can provide information regarding adequacy of tumor extirpation, it is recommended to keep the laparoscopic ultrasonography equipment available and the probe sterile until the end of the procedure so that laparoscopic ultrasonography scanning can be repeated whenever indicated. Picture-in-picture capability displays the laparoscopic ultrasonography image on the same screen as the laparoscopic image; the laparoscopic ultrasonography echo image is usually shown in the corner of the screen. This capability has eliminated the need of a separate screen for the ultrasound image, reducing surgeon strain and minimizing operating room equipment (2).

Doppler ultrasound facilities allow identification of vessel anatomy and characterization of blood supply of and within the lesion. Spectral-wave-form analysis has capabilities to distinguish between arterial and venous flow, and to measure velocity as well as resistive index.

In color Doppler mode, it is useful to determine flow direction and velocity. The advantage of power Doppler function is that blood flow sensitivity is increased by a factor of three to five times in comparison with conventional color Doppler function. Power Doppler is much less angle dependent.

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