Ultrasound physics

Advanced, Basic Sciences

Sound waves are mechanical waves that travel in a longitudinal direction with alternating areas of compression and rarefaction transmitted through solid, liquid, or gaseous medium. Normal human ears can detect sound waves between 20 Hz and 20,000 Hz. Ultrasound waves refer to those with frequencies above 20,000 Hz. Clinical medicine utilizes ultrasound to generate real time images of internal structures.

Ultrasound utilizes the piezoelectric effect (generation of mechanical force in response to electric field application or vice versa) to generate an ultrasound wave when electrical current is applied. The transmitted energy interacts with different tissue structures along its path resulting in attenuation, or weakening of the signal. Attenuation occurs via reflection, scattering, absorption, or rarefaction. Reflection occurs at the interface between two different media, which in clinical medicine is usually air, fat, bone, and/or tissues. These media have different acoustic impedance , or properties that prevent transmission of sound waves. Acoustic impedance is defined as density multiplied by acoustic velocity. The greater the acoustic impedance, the stronger the wave transmitted back to the transducer is, which is responsible for the ultrasound image. 

Frequency, which is a measure of cycles per sound, is inversely proportional to wavelength. In terms of medical ultrasound, higher frequencies (6-13 MHz) result in greater resolution but less depth whereas lower frequencies (2-5 MHz) result in less resolution but greater depth.

There are several different modes of ultrasound: A-mode, B-mode, M-mode, and Doppler color flow. A-mode is when there is a single pulse transmitted from the ultrasound and provides one-dimensional information regarding the depth of the image scanned. B-mode, also known as brightness mode, is the most commonly used mode nowadays, and provides a two-dimensional image as a cross section. M-mode scans along a single line or axis over time and provides information regarding motion, which is commonly used in cardiac imaging. Doppler mode utilizes the Doppler effect , which refers to the change in frequency due to relative motion between a sound source and receiver. In Doppler mode, a color image is superimposed onto a B-mode image and is primarily utilized to detect the flow of blood through blood vessels. Conventionally, the color codes can be remembered by the acronym BART: blue away, red toward. 

Resolution of the ultrasound image is categorized as spatial resolution or temporal resolution . Spatial resolution is then further subdivided into either axial or longitudinal resolution, which is the ability to distinguish two different points in separate space. The higher the resolution, the smaller the distance that can be distinguished. Axial resolution is defined by the below equation, which shows that increasing the wavelength or the number of cycles in the pulse should increase axial resolution. Lateral resolution may be improved by reducing beam width. Temporal resolution is the ability to detect movement of an object with time, which is particularly important in M-mode. Improvements in frame rate will increase temporal resolution at the expense of image quality. 

Axial resolution=  ((number of cycles in the pulse)x(λ))⁄2


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Fan Ye, MD