Science Behind Sonography

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  Science  Behind Sonography.


P.K. Ghatak, M.D.

No.33.

Ultrasound is a portion of sound that is beyond human ears can hear. The human hearing range is 20 to 20,000 Hz, and the intensity is 2 to 120 decibels (dB). A decibel is named after Alexander Graham Bell. 

What is a Sonogram:

The sound waves are generated by vibrating crystals. The sound waves, after encountering a barrier, reflect back some or nearly all sound waves. These reflected waves are collected and a computer generates pictures in great detail. The reflected sound waves are often called an Echo, and the images generated are called an Echogram.

The Echo has a special quality, among the reflected sound waves, which must reach the human ears after some delays for it to be registered as a separate sound. This delay is called Impedance and the impedance is variable depending on the tissue, air, and fluid; this characteristic allows sonography to construct an image.

An Italian scientist, Spallanzani,  discovered that bats use ultrasound to locate insects and this process is called Echolocation.

The science behind the conversion of electricity to sound.

When asymmetrical crystals, eg, Quartz or Polycrystalline Ceramics, are subjected to physical stress, they are polarized and generate electricity which is called Piezoelectric.

In a reversed situation, when these crystals are placed in an electric field, they begin to vibrate and generate sound waves. By changing the crystals, frequency and amplitude of the electrical field, sound waves of various frequencies and strengths can be generated.

Components of the Ultrasound (US) instrument:

The basic components are the ultrasound Generator, also called the Transducer or Probe and the Receiver. The receiver is also incorporated into the transducer. The other parts are the monitor and controls. The transducer contains an assortment of crystals, and it comes in various shapes and sizes to fit the part of the body to be examined, eg, the neck is a small saddle-shaped gadget and a large, gently curved transducer is used in the abdomen and pelvis examination.

In addition, there are specially designed probes for special locations, to name a few - transthoracic esophageal ultrasound for examination of the left atrial chamber of the heart, and transrectal for examination of the prostate gland.

Ultrasound images:

US imaging is used for every organ system of the entire body. A US study is also useful for the evaluation of the functional status of an organ, the detection of motion and the direction of blood flow. And it is very useful in following the development of the fetus in utero.


A more detailed discussion can be found in  Medical matters: Archive

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