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 ? Physics of ultrasound
Ultrasonic echo imaging
Focusing technique
A-mode signal and B-mode image
Features of echo image
Future 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 diagnosis
Frequency : 2MHz – 10MHz

4. Physics of ultrasound - 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
About 0.437[mm] in the body (3.5MHz)

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

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

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

8. Physics of ultrasound - characteristics-
Reflection and transmission
Acoustic impedance : Z=ρc
ρ: density, c : sound velocity
For sound pressure
Rp : reflectance, Tp : transmittance
For sound intensity
RI : reflectance, TI : transmittance
Snell’s law
Incident
wave θi θr
Reflected wave Z1 Z2
Transmitted wave θt

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

10. Physics of ultrasound - characteristics-
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.

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
Reflected
Signal
Ultrasonic
Beam
Human
Body

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 lens
Electronic focus

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

14. Ultrasonic echo imaging - focusing technique-
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)
Array of ultrasonic Element
delay +
Point scatterer
High S/N
The same principle as radar

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

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 -
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
Scanning : linear
Thyroid image

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

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

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

23. Ultrasonic echo imaging - features -
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
Imaging in real time
Doppler image
Not quantitative image
Artifacts due to wave properties

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