1. ECE 501 Introduction to BME
Dr. Hang

2. Part VI Medical Imaging Diagnostic Ultrasound Imaging
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Dr. Hang

3. Introduction Ultrasound Imaging System Physics of Ultrasound Dr. Hang
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Dr. Hang

4. Physics of Ultrasound 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.
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5. Physics of Ultrasound 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.
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6. Physics of Ultrasound Pulsed Ultrasound Emitted from transducer
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7. Physics of Ultrasound Resolution of Typical Ultrasound Scanner
Center frequency f0: 2-10 MHz
Speed of sound in tissue C: 1540m/s
Wavelength λ = C/f0
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8. Physics of Ultrasound How to Get Positional Information?
Range equation:
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9. Physics of Ultrasound Wave Propagation
Absorption: main factor that contributes to attenuation
Reflection & Scattering: essential to create an image but contribute little to
attenuation.
Refraction
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10. Physics of Ultrasound Acoustic Impendence Density of medium
Speed of sound in the medium
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11. Physics of Ultrasound Reflection Reflection Factor:
Transmission Factor:
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12. Physics of Ultrasound
Oblique Incidence
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13. Physics of Ultrasound Scattering Specular scattering
Diffractive scattering
Diffusive scattering
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14. Physics of Ultrasound Attenuation Scattering 20%
Absorption (conversion of wave energy into heat) 80%
Linear function of frequency: high frequencies are more attenuated
than low frequencies
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15. Physics of Ultrasound Doppler Effect fD: Doppler shift frequency
f0: transducer frequency
v: blood moving velocity
c: sound speed
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16. Ultrasound Imaging System
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
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17. Ultrasound Imaging System
Transducer
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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18. Ultrasound Imaging System
Transducer
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19. Ultrasound Imaging System
System Diagram
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20. Ultrasound Imaging System
TGC (Time Gain Compensation):
amplifies the returned signal in proportion to depth
or the time required for the signal to return
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21. Ultrasound Imaging System
Demodulator:
Removes the carrier frequency
Hilbert transform (for our project)
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22. Ultrasound Imaging System
Echo processing:
Magnitude: Envelope detection
LOG Compress: reduce the range from 12 bit to 8 bit
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23. Ultrasound Imaging System
Scan conversion: Interpolation
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24. Ultrasound Imaging System
Spectral Doppler: Short Time Fourier Transform (spectrogram)
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