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Physical principles of ultrasound

Dr Mirjan - Misko Nadrljanski et al.

Medical Ultrasound is based on the use of high frequency sound to aid in the diagnosis and treatment of patients. The ultrasound frequencies range from 2 to approximately 15 MHz.

The ultrasound image (sonographic / echographic) is based on mechanical oscillations of the crystal excited by electrical pulses (piezoelectric effect). Numerous crystals are assembled to form the transducer. A transducer converts one type of energy into another. Based on the pulse-echo principle occurring with ultrasound piezoelectric crystals, ultrasound transducers convert electricity into sound, i.e., pulse and sound into electricity, i.e., echo.

The ultrasound waves (pulse of sound) are setn from the transducer and propagate through different tissues and then return reflected as the echoes to the transducer. Those returned echoes are converted back into electrical impulses by the transducer crystals and are further processed in order to form the ultrasound image presented on the screen.

The ultrasound transducers contain a range of ultrasound frequencies, termed bandwidth. Medical ultrasound transducers contain more than one operating frequency: 2.5 – 3.5 MHz for general abdominal imaging and for ex. 5.0 – 7.5 MHz for superficial imaging.

The waves are reflected at the surfaces between the tissues of different acoustic density, the reflection being proportionate to the difference in acoustic density.

If the difference in acoustic density is increased, the intensity of the reflected sound is increased and the intensity of the transmitted sound is proportionately decreased.

If the difference in acoustic density is very different, the sound is then completely reflected resulting in total acoustic shadowing. Acoustic shadowing is present behind bone structures, calculi (stones in kidneys, gallbladder etc.) and air (intestinal gas). (See Fig. 1 – Calculosis V. felleae – Gallbladder stone with apparent ultrasound phenomena: acoustic shadow and attenuation ).

Echoes are not produced if there is no difference in acoustic density. Homogenous fluids like blood, bile, urine, contents of the cysts, ascites and pleural effusion are seen as echo-free structures.