BRAINCAKEImpedance Matching and Coupling Gel
At an air-skin boundary about 99.9% of the ultrasound intensity is reflected, because the acoustic impedance of air is tiny compared with that of skin. If there were even a thin layer of air between the transducer and the skin, almost no ultrasound would enter the body and there would be no useful echoes.
The solution is impedance matching (acoustic matching): making sure the ultrasound passes between materials of similar acoustic impedance, so that very little is reflected at each boundary and almost all of it is transmitted.
A coupling gel is spread on the skin before the transducer is placed on it. The gel:
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Key terms in this lesson
- acoustic impedance
- The product of the density of a medium and the speed of sound in it, which determines how much ultrasound is reflected at a boundary.
More in Medical Imaging
- PET Scanning
- Ultrasound
- The Piezoelectric Effect
- Ultrasound Transducers
- A-scans and B-scans
- Acoustic Impedance
- Reflection at Boundaries
- Doppler Imaging
All 20 lessons in Medical Imaging · All OCR A-level Physics topics