My post today is on the physics behind uses of ultrasound because, when touching upon this topic in a lesson, it got me wondering what the process is that allows things such as imaging of the inside of your body to be done to such a high level.
Ultrasound is a sound wave with frequencies higher than what the human can hear from 20kHz up to several gigahertz. The process of using ultrasound for imaging of the body starts with an ultrasound transducer or otherwise known as an ultrasound probe, which will send sound waves into the patient’s body and bounce of certain tissues and bones and then finally return to the transducer. These echoes will produce a black and white image of the organs.
Ultrasound transducers consist of piezoelectric crystals which emit the sound waves. Piezoelectric crystals generate a small voltage, commonly known as piezoelectricity when subject to vibration or mechanically deformed. The material used in ultrasound transducers that make the piezoelectric crystals can change the electrical signals received into mechanical vibrations and vice versa which enables the reflected waves to create an image on the ultrasound screen. When the electrical current is applied to a piezoelectric crystal, it causes the crystal to expand and contract causing a pressure wave, and these are the same pressure waves that propagate in the body and get reflected to create the image of the body.
To be more specific about the wave propagation, when the sound waves reach a boundary between two different media in the body such as tissue and bone, some of the waves reflect back to the source with the reflected angle being same as the transmitted angle. Structures such as bone are strong reflectors which mean most of the incoming waves are reflected and a shadow of the sound waves will be produced behind the structure. Irregular surface structures cause ultrasound waves to be scattered in all directions, forming speckles in the image and giving it a grainy appearance. The ultrasound wave must be coupled with gel to travel through the air as it is easily reflected by it. Gel ensures proper contact between the transducer and patient and allows sound to travel through tissues.
Ultrasound waves can be controlled in how they are emitted from the ultrasound transducers. By changing the frequency of ultrasound waves, the penetration and resolution of the images will vary. The higher the frequency the lower the penetration of the waves, but the better the resolution of the image. For lower frequencies, the resolution of the image will be reduced but the waves will penetrate a much greater distance.
Reference:
https://www.explainthatstuff.com/piezoelectricity.html
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3214508/
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