The recent advances in ultrasound technology combined with a better understanding of biopsy, aspiration and drainage techniques, have produced unparalleled growth in the field of interventional ultrasound. There has been a number of new developments in interventional ultrasound, which will be reviewed but before that, one must have a firm understanding of why ultrasound is often the ideal modality in guiding interventional procedures.
The three different methods for guidance of interventional procedures are indirect needle guidance, the freehand technique and use of a needle guidance system.
Indirect Method is used for aspiration or drainage of large fluid collections. It is a blind technique because the needle or catheter is not inserted using real-time guidance.
This method allows direct needle visualization because the needle is placed with ultrasound guidance. The needle may be either adjacent to or remote from the transducer, parallel or, less frequently, perpendicular to the scan plan.
Needle Guidance Systems
Most available needle guidance systems are attached to the transducer. These guidance systems are designed with grooves and slots to allow passage of different needles; some of these are large enough to accompany small catheters. The angle, direction, or depth of the needle can be continuously monitored with real-time guidance. The guidance system holds the needle firmly along a predetermined course, which is displayed as a line or caliper on a video monitor or the ultrasound unit.
The technique for aspiration of fluid depends on the amount of fluid to be aspirated and the consistency of the fluid. For instance, a diagnostic thoracocentesis can be performed by placing the needle into the collection, removing the inner stylet, attaching a syringe directly to the needle and aspirating the fluid.
Percutaneous drainage techniques are usually guided by ultrasound, CT, fluoroscopy or a combination of these. Whenever possible, drainage procedures must be done under ultrasound guidance. If a target area can be visualized under this technique, usually the drainage under the same technique can be done. The advantages of ultrasound are that it can be portable and that the initial needle puncture can be visualized in its entirety. Ultrasound can guide trocar catheter placement for procedures such as percutaneous cholecystostomy and abscess drainage also.
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