Presentation on theme: "Foundations of Medical Ultrasonic Imaging"— Presentation transcript:
1 Foundations of Medical Ultrasonic Imaging Dr. Kazuhiko HAMAMOTODept. of Information Media Tech.School of IT & Elec.Tokai University
2 Today’s Topics What is Ultrasound ? Physics of ultrasound Ultrasonic echo imagingFocusing techniqueA-mode signal and B-mode imageFeatures of echo imageFuture works
3 What is Ultrasound? The upper limit of hearing : about 20[kHz] Over the upper limit = Ultrasound ?This definition is not clear..“Ultrasound is a sound whose utility value is not to hear”Application to diagnosisFrequency : 2MHz – 10MHz
4 Physics of ultrasound - propagation - Velocity of propagationAbout 1540[m/s] in human bodyEach tissue has its own velocity.Ultrasonic diagnostic equipment assumes that sound velocity is constant in the body.This assumption causes artifacts in echo imageWavelengthAbout 0.437[mm] in the body (3.5MHz)
8 Physics of ultrasound - characteristics- Reflection and transmissionAcoustic impedance : Z=ρcρ: density, c : sound velocityFor sound pressureRp : reflectance, Tp : transmittanceFor sound intensityRI : reflectance, TI : transmittanceSnell’s lawIncidentwaveθiθrReflected waveZ1Z2Transmitted waveθt
9 Physics of ultrasound - characteristics- Refraction (snell’s law)c : sound velocityc1c1c2c2c1 > c2c1 < c2This phenomenon causes artifacts in medical echo image.
10 Physics of ultrasound - characteristics- AttenuationDiffusion attenuation [dB/m]Inverse proportion to distance from sourceAbsorption attenuation [dB/m/MHz]Frequency dependent attenuationReflected wave from deep region has lower center frequency and longer wavelength than incident wave.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 ProbeReflectedSignalUltrasonicBeamHumanBody
12 Ultrasonic echo imaging - focusing technique- Ultrasonic beam is needed for imaging.Ultrasonic wave is widely spread in human body!It propagates as spherical wave, not beam!How to form ultrasonic beam ?Acoustic lensElectronic focus
14 Ultrasonic echo imaging - focusing technique- Electronic focus (transmission)Array of ultrasonic ElementDelay circuitFocal pointDesired focal lengthby control of delay circuit
15 Ultrasonic echo imaging - focusing technique- Electronic focus (receiving)Array of ultrasonic Elementdelay+Point scattererHigh S/NThe same principle as radar
16 Ultrasonic echo imaging - focusing technique- Electronic focus (beam profile)Use of several elementselementacoustic lenslongitudinal direction= progression of pulsefocus of slice direction= acoustic lensfocus of scan direction= Electronic focus= beam width= resolution of scan direction
17 Ultrasonic echo imaging - A-mode signal- Frequency dependent attenuationRF signalEnvelope detectionA-mode signalPulse length= resolution along longitudinal direction
18 Ultrasonic echo imaging - A-mode signal to B-mode image - Log Amp : control of dynamic rangeSTC ( Sensitive Time Control) :compensation of attenuationAmplitude to Brightness
19 Ultrasonic echo imaging - scanning techniques - Control of beam direction : switched arrayScanning : linearThyroid image
20 Ultrasonic echo imaging - scanning techniques - Control of beam direction : phased arrayScanning : sectorHeart image
21 Ultrasonic echo imaging - scanning techniques - Control of beam direction : switched arrayScanning : offset sectorLiver image
23 Ultrasonic echo imaging - features - ResolutionDirection of pulse propagation : pulse width : 1-2mmDirection of scanning : beam width : 2-3mmLow resolution and low S/N in deep regionAbility of imaging of soft tissueImaging in real timeDoppler imageNot quantitative imageArtifacts due to wave properties
24 Ultrasonic echo imaging - future works - Quantitative imaging (tissue characterization)Sound velocityAttenuation coefficientElasticity imagingHigh resolutionHarmonic imaging (use of harmonic component)3D image reconstruction3D data acquisition systemHigh speed volume renderingComputer assisted diagnosisFeature extraction (boundary, texture, character etc.)
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