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Foundations of Medical Ultrasonic Imaging Dr. Kazuhiko HAMAMOTO Dept. of Information Media Tech. School of IT & Elec. Tokai University.

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Presentation on theme: "Foundations of Medical Ultrasonic Imaging Dr. Kazuhiko HAMAMOTO Dept. of Information Media Tech. School of IT & Elec. Tokai University."— Presentation transcript:

1 Foundations of Medical Ultrasonic Imaging Dr. Kazuhiko HAMAMOTO Dept. of Information Media Tech. School of IT & Elec. Tokai University

2 Today’s Topics What is Ultrasound ? What is Ultrasound ? Physics of ultrasound Physics of ultrasound Ultrasonic echo imaging Ultrasonic echo imaging  Focusing technique  A-mode signal and B-mode image  Features of echo image Future works Future works

3 What is Ultrasound? The upper limit of hearing : about 20[kHz] The upper limit of hearing : about 20[kHz] Over the upper limit = Ultrasound ? Over the upper limit = Ultrasound ?  This definition is not clear.. “Ultrasound is a sound whose utility value is not to hear” “Ultrasound is a sound whose utility value is not to hear” Application to diagnosis Application to diagnosis  Frequency : 2MHz – 10MHz

4 Physics of ultrasound - propagation - Velocity of propagation Velocity of propagation  About 1540[m/s] in human body  Each tissue has its own velocity.  Ultrasonic diagnostic equipment assumes that sound velocity is constant in the body.  This assumption causes artifacts in echo image Wavelength Wavelength  About 0.437[mm] in the body (3.5MHz)

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

6 Physics of ultrasound - propagation - Spherical wave Spherical wave  Point sound source  Diffuse sound field Point source

7 Physics of ultrasound - propagation - Practical condition –ultrasonic element- Practical condition –ultrasonic element-  Finite element size (about 0.3mm)  Not plane wave, not spherical wave D Near field (Fresnel zone) Far field (Fraunhofer zone) Plane waveSpherical wave L: wavelength = 0.437mm D:diameter = 0.3mm Fresnel zone = 0.052mm

8 Physics of ultrasound - characteristics- Reflection and transmission Reflection and transmission  Acoustic impedance : Z=ρc  ρ: density, c : sound velocity Z1Z1 Z2Z2 θi θr θt Incident wave Reflected wave Transmitted wave Snell’s law For sound pressure R p : reflectance, T p : transmittance For sound intensity R I : reflectance, T I : transmittance

9 Physics of ultrasound - characteristics- Refraction (snell’s law) Refraction (snell’s law)  c : sound velocity c1 c2 c1 > c2 c2 c1 < c2 c1 This phenomenon causes artifacts in medical echo image.

10 Physics of ultrasound - characteristics- Attenuation Attenuation  Diffusion attenuation [dB/m]  Inverse proportion to distance from source  Absorption attenuation [dB/m/MHz]  Frequency dependent attenuation  Reflected wave from deep region has lower center frequency and longer wavelength than incident wave. Attenuation causes low resolution of echo image. Attenuation causes low resolution of echo image.

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

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

13 Ultrasonic echo imaging - focusing technique- Acoustic Lens Acoustic Lens Ultrasonic element Acoustic lens sound velocity : c1 Human body Sound velocity : c2 c1 < c2 Focal point wavefront Weak point : a fixed focus

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

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

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

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

18 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

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

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

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

22 Ultrasonic echo imaging - grouping - Element array linearconvexlinearannular Control of beam direction Switched array method Phased array method mechanical scanlinearOffset sectorsector Probe formlinearconvexsector Region of image thyroid, breast Abdominal region heart

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

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


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