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Ultrasonography

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Over the last 20 years or so, ultrasonography has also made its way into the field of regional anaesthesia and attracts considerable interest there. The frequency of its clinical use is growing exponentially - as is also the number of publications on this topic. This growth has been made possible primarily by technical improvements in the area of ultrasonic devices, which today can provide a very detailed image of superficial structures. This is accomplished by means of high-frequency, low-distortion probes.

 

Physical Foundations

The starting point for the development of ultrasonic devices which could be used in medicine was the discovery of the so-called piezoelectric effect by Jacques and Pierre Curie at the end of the 19th century. Put very simply, the generation of ultrasonic waves is based on electrical stimulation of certain piezoelectric crystals to the point that they oscillate interiorly and emit ultrasonic waves (the „reverse piezoelectric“ effect). These waves pass through the body and are subject in the process to diverse physical phenomena such as refraction, absorption, dispersion and reflection. Only the reflected waves, i.e. those which return to the probe, can be used for imaging. They lead to mechanically conditioned voltage changes within the piezo crystals which are then ultimately converted into an image by means of electronic calculation.

 

The Ultrasonic Devices

sonositeAs mentioned at the beginning, the impulse for frequent use of ultrasound to create regional blocks came first from the introduction of so-called high-frequency probes. These deliver ultrasound waves in a range between ca. 8 and 20 MHz and can thus show superficially positioned structures in great detail. The trade-off for this advantage, in comparison to probes with a lower frequency (< 5 MHz), is a relatively low penetration depth of only a few centimetres. Fortunately, most of the relevant neural structures are close to the surface of the body. Industrially, a successful effort has been made to adapt modern devices to requirements in the perioperative environment. This has resulted in a tendency to quick-starting, easily operated, compact and thus mobile devices. In the process, image quality no longer suffers due to size limitations and adjustment parameter restrictions.