ece 501 introduction to bme ece 501 dr. hang. part vi medical imaging diagnostic ultrasound imaging...
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ECE 501 Introduction to BME
ECE 501 Dr. Hang
Part VI Medical Imaging Diagnostic Ultrasound Imaging
ECE 501 Dr. Hang
ECE 501 Dr. Hang
• Physics of Ultrasound
•Ultrasound Imaging System
Introduction
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What is Sound? • Sound is a wave, i.e. traveling variations of pressure. • It involves mechanical motion in the medium through which it travels • Pressure variations cause particles of the medium to vibrate due to increase and decrease of density.
Physics of Ultrasound
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What is Ultrasound?
• Pressure waves with frequencies higher than 20 KHz, higher than audible range of human beings.
• Harmless unless very high intensities are used.
Physics of Ultrasound
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Pulsed Ultrasound
• Emitted from transducer
•
Physics of Ultrasound
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Resolution of Typical Ultrasound Scanner
• Center frequency f0: 2-10 MHz
• Speed of sound in tissue C: 1540m/s
• Wavelength λ = C/f0
Physics of Ultrasound
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How to Get Positional Information?
• Range equation:
Physics of Ultrasound
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Wave Propagation
• Absorption: main factor that contributes to attenuation • Reflection & Scattering: essential to create an image but contribute little to attenuation. •Refraction
Physics of Ultrasound
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Acoustic Impendence
Physics of Ultrasound
CZ
Density of medium
C Speed of sound in the medium
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Reflection
Physics of Ultrasound
12
12RFZZ
ZZ
Reflection Factor:
12
22TF
ZZ
Z
Transmission Factor:
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Oblique Incidence
Physics of Ultrasound
ti
ti
ZZ
ZZ
coscos
coscosRF
12
12
ti
i
ZZ
Z
coscos
cos2TF
12
2
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Scattering
Physics of Ultrasound
Specular scattering
Diffractive scattering
Diffusive scattering
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Attenuation• Scattering 20% • Absorption (conversion of wave energy into heat) 80%• Linear function of frequency: high frequencies are more attenuated than low frequencies
Physics of Ultrasound
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Doppler Effect
Physics of Ultrasound
c
vf cos2f 0
D
fD: Doppler shift frequency
f0: transducer frequency
v: blood moving velocity
c: sound speed
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Transducer• An array of Piezoelectric crystal/ceramic • Deforms when a voltage is applied and transmits acoustic waves
• Electrical charge is created by the mechanical of deformation
Ultrasound Imaging System
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Transducer
Ultrasound Imaging System
Appropriately delayed pulses are transmitted from an array of piezoelectric elements to achieve steering and focusing at the point of interest.
The echoes returning are likewise delayed before they are summed together to form a strong echo signal from the region of interest.
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Transducer
Ultrasound Imaging System
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System Diagram
Ultrasound Imaging System
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TGC (Time Gain Compensation):amplifies the returned signal in
proportion to depthor the time required for the signal to return
Ultrasound Imaging System
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Demodulator:• Removes the carrier frequency• Hilbert transform (for our project)
Ultrasound Imaging System
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Echo processing:• Magnitude: Envelope detection• LOG Compress: reduce the range from 12 bit to 8 bit
Ultrasound Imaging System
)()()( 22 tQtItBa
CtBAtB ab )]([log)( 2
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Scan conversion: Interpolation
Ultrasound Imaging System
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Spectral Doppler: Short Time Fourier Transform (spectrogram)
Ultrasound Imaging System
c
vf cos2f 0
D