Ultrasound Physics

Gain

Increases or decreases the overall brightness of the image by amplifying returning echoes.

TGC (Time Gain Compensation)

Adjusts brightness at specific depths to compensate for attenuation of sound waves.

Depth

Sets how deep the system listens for echoes, affecting the amount of tissue displayed.

Sector Width

Adjusts the angular width of the scan field, influencing the number of scan lines and frame rate.

Power (Acoustic Output Power)

Adjusts energy of transmitted ultrasound pulse, affecting signal strength and patient exposure.

Dynamic Range

Number of grey shades used to display echoes, controlled via logarithmic compression.

Harmonics

Uses echoes at twice the transmitted frequency to improve lateral resolution and reduce clutter.

Compound Imaging

Combines multiple scan angles into one image, reducing speckle noise and improving border definition.

Zoom

Magnifies a region of interest, either as post-processing or rescanning with higher line density.

Doppler Gain

Adjusts brightness of flow signals by amplifying returning Doppler-shifted echoes.

Angle Correction Cursor

Aligns the Doppler beam with blood flow to ensure accurate velocity measurements.

Scale

Sets the velocity range displayed, adjusting for Nyquist limit to avoid aliasing.

Pulse Repetition Frequency (PRF)

Number of pulses sent per second, determining maximum measurable velocity before aliasing.

Acoustic Impedance (Z)

The resistance of a medium to sound wave propagation, affecting how much sound is reflected.

Acoustic Impedance Mismatch

Difference in impedance between two tissues, influencing reflection and transmission.

Speed of Sound

The rate at which sound travels through a medium, affecting distance calculations in imaging.

Stiffness

Resistance of a medium to deformation, influencing sound speed.

Density

Mass per unit volume of a tissue, impacting acoustic impedance and sound speed.

Ultrasound Frequency

Number of sound wave cycles per second, affecting resolution and penetration depth.

Wavelength (λ)

Distance between two consecutive wave peaks, related to frequency and resolution.

Spatial Pulse Length (SPL)

Length of an ultrasound pulse in space, affecting axial resolution.

Attenuation

Gradual weakening of the ultrasound signal as it travels through tissue.

Absorption

Conversion of sound energy into heat, a major cause of attenuation.

Reflection

Echoes returning to the transducer at tissue boundaries, essential for image formation.

Refraction

Bending of sound waves at an interface due to change in speed.

Scattering

Redirection of sound in many directions by small structures.

Specular Reflection

Reflection off a smooth surface, producing strong echoes.

Diffuse Reflection

Echoes from irregular surfaces that return from many angles.

Speckle

Granular noise due to interference of scattered echoes.

B-mode (Brightness mode)

Standard 2D imaging where each pixel’s brightness reflects echo amplitude.

M-mode (Motion mode)

Displays motion over time along a single scan line.

Pulse-Wave Doppler

Measures velocity of blood at a specific depth using pulsed ultrasound.

Colour Flow Doppler

Displays direction and velocity of flow in a region using colour overlay.

Spatial Resolution

Ability to distinguish two structures that are close together.

Axial Resolution

Ability to distinguish structures along the beam’s path.

Lateral Resolution

Ability to distinguish structures side by side.

Echolocation

Process of emitting sound pulses and analysing returning echoes to locate structures.

MI (Mechanical Index)

Indicates likelihood of cavitation due to peak rarefaction pressure and frequency.

TI (Thermal Index)

Predicts temperature rise due to absorption during ultrasound.

TIS (Thermal Index – Soft Tissue)

Reflects heating in soft tissue during early pregnancy or superficial tissue imaging.

TIB (Thermal Index – Bone)

Reflects heating of bone when fetal bone is present after 10 weeks gestation.

TIC (Thermal Index – Cranial)

Reflects heating near the skull for transcranial imaging.