Foundations of Medical Ultrasonic Imaging Physics of

Foundations of Medical Ultrasonic Imaging

Physics of ultrasound n Ultrasonic echo imaging n Focusing technique u A-mode signal and B-mode image u Features of echo image u n Future works

Ultrasound and Acoustic Impedance Density, Speed of Sound, Acoustic Impedance, Air 1. 3 330 0. 0004 Water 1000 1500 1. 5 Blood 1060 1570 1. 66 Fat 925 1450 1. 34 Soft tissue (average value) 1060 1540 1. 63 Muscle 1075 1590 1. 71 Bone (average for adult) 1600 4000 6. 4 Material

Physics of ultrasound - propagation n Velocity of propagation About 1540[m/s] in human body u Each tissue has its own velocity. u Ultrasonic diagnostic equipment assumes that sound velocity is constant in the body. u t n This assumption causes artifacts in echo image Wavelength u About 0. 437[mm] in the body (3. 5 MHz)

Physics of ultrasound - propagation n Plane wave Line sound source, infinite length u No diffusion attenuation u Sound source

Physics of ultrasound - propagation n Spherical wave Point sound source u Diffuse sound field u Point source

Physics of ultrasound - propagation n Practical condition –ultrasonic element. Finite element size (about 0. 3 mm) u Not plane wave, not spherical wave u D Plane wave Near field (Fresnel zone) Spherical wave Far field (Fraunhofer zone) L: wavelength = 0. 437 mm D: diameter = 0. 3 mm Fresnel zone = 0. 052 mm

Physics of ultrasound - characteristicsn For sound intensity RI : reflectance, TI : transmittance Reflection and transmission u Acoustic impedance : Z=ρc u r: density, c : sound velocity Snell’s law Incident wave qr qi Z 1 Z 2 Reflected wave Transmitted wave qt For sound pressure Rp : reflectance, Tp : transmittance

Physics of ultrasound - characteristicsn Refraction (snell’s law) u c : sound velocity c 1 c 2 c 1 > c 2 c 1 < c 2 This phenomenon causes artifacts in medical echo image.

Physics of ultrasound - characteristicsn Attenuation u Diffusion attenuation [d. B/m] t u Inverse proportion to distance from source Absorption attenuation [d. B/m/MHz] Frequency dependent attenuation t Reflected wave from deep region has lower center frequency and longer wavelength than incident wave. t n Attenuation causes low resolution of echo image.

Ultrasonic echo imaging - basic principlen n Same principle as echo among the hills. Estimate the distance from the sound reflection and the sound velocity. Ultrasonic Probe Reflected Signal Ultrasonic Beam Human Body

Ultrasonic echo imaging - focusing technique. Ultrasonic beam is needed for imaging. n Ultrasonic wave is widely spread in human body! n It propagates as spherical wave, not beam! n How to form ultrasonic beam ? u Acoustic lens u Electronic focus n

Ultrasonic echo imaging - focusing techniquen Acoustic Lens Acoustic lens sound velocity : c 1 < c 2 Ultrasonic element Focal point wavefront Human body Sound velocity : c 2 Weak point : a fixed focus

Ultrasonic echo imaging - focusing techniquen Electronic focus (transmission) Array of ultrasonic Element Delay circuit Focal point Desired focal length by control of delay circuit

Ultrasonic echo imaging - focusing techniquen Electronic focus (receiving) delay Array of ultrasonic Element + Point scatterer High S/N The same principle as radar

Ultrasonic echo imaging - focusing techniquen Electronic focus (beam profile) u Use of several elements element acoustic lens longitudinal direction = progression of pulse focus of scan direction = Electronic focus = beam width = resolution of scan direction focus of slice direction = acoustic lens

Ultrasonic echo imaging - A-mode signal- RF signal Envelope detection A-mode signal Pulse length = resolution alongitudinal direction Frequency dependent attenuation

Ultrasonic echo imaging - A-mode signal to B-mode image A-mode signal Log Amp : control of dynamic range STC ( Sensitive Time Control) : compensation of attenuation Amplitude to Brightness

Ultrasonic echo imaging - scanning techniques n n Control of beam direction : switched array Scanning : linear Thyroid image

Ultrasonic echo imaging - scanning techniques n. Control of beam direction : phased array n. Scanning : sector Heart image

Ultrasonic echo imaging - scanning techniques n. Control of beam direction : switched array n. Scanning : offset sector Liver image

Ultrasonic echo imaging - grouping Element array Control of beam direction linear convex Switched array method linear annular Phased array method mechanical scan linear Offset sector Probe form linear convex sector Region of image thyroid, breast Abdominal region heart

Ultrasonic echo imaging - features n Resolution u u u n n n Direction of pulse propagation : pulse width : 1 -2 mm Direction of scanning : beam width : 2 -3 mm Low resolution and low S/N in deep region Ability of imaging of soft tissue Imaging in real time Doppler image Not quantitative image Artifacts due to wave properties

Ultrasonic echo imaging - future works n Quantitative imaging (tissue characterization) u u u n High resolution u n Harmonic imaging (use of harmonic component) 3 D image reconstruction u u n Sound velocity Attenuation coefficient Elasticity imaging 3 D data acquisition system High speed volume rendering Computer assisted diagnosis u Feature extraction (boundary, texture, character etc. )