Ultrasound uses high frequency sound waves to produce images of the inside of the body. There are different modes of ultrasound including A-mode which displays amplitude over time, B-mode which produces two-dimensional images, and M-mode which depicts motion over time. Modern ultrasound systems can produce real-time 2D and 3D images using piezoelectric crystals and array transducers. Doppler ultrasound measures the frequency shift of reflected sound to analyze blood flow velocity.

1. Introduction to Biomedical Imaging
Alejandro Frangi, PhD
Computational Imaging Lab
Department of Information & Communication Technology
Pompeu Fabra University
www.cilab.upf.edu

2. Ultrasound Imaging
Introduction to Biomedical Imaging

3. Ultrasound Imaging
Basic principles. Comparison to X-rays
Ultrasound > 20kHz
Medical/Diagnostic Ultrasound 1-15 MHz
Sound spreads in all directions
Ultrasound can be formed into a narrow beam (it is more “light-like”)
Periodic motion yields pressure waves
Speed of sound vs. speed of light
Ultrasound requires a medium to propagate (no sound in vacuum)
X-rays can be scattered. US can be reflected, refracted and focused
Introduction to Biomedical Imaging

4. Ultrasound Imaging
Ultrasound pulse/continuous wave modes
Pulsed Wave Mode
Short DC pulse is applied to the crystal producing its instantaneous
expansion an tissue compression
Due to elastic tissue properties: high pressure wave front (compression)
travels through the body at speed ν followed by a low wave front
(decompression or rarefaction)
Multiple frequencies are present in the signal and echo (bandwidth)
Continuous Wave Mode
AC voltage applied to crystal
Periodic pattern of compression and rarefaction will travel across the
body with speed ν and wavelength λ
The frequency is given by the AC voltage applied to the transducer
crystal: ν = λ f
Introduction to Biomedical Imaging

5. Ultrasound Imaging
Ultrasound pulse/wave
generation:
transmission
Ultrasound signal
detection
Introduction to Biomedical Imaging

6. Ultrasound Imaging
Ultrasound transducer frequency vs. resolution
Introduction to Biomedical Imaging

7. Ultrasound Imaging
Piezoelectric crystals, beam forming and transducers
Introduction to Biomedical Imaging

8. Ultrasound Imaging
Piezoelectric crystals, beam forming and transducers
Introduction to Biomedical Imaging

9. Ultrasound Imaging
Piezoelectric crystals, beam forming and transducers
Abdominal
Obstetrics
Intraoperative
vascular
(superficial)
Gynecology
Obstetrics
Introduction to Biomedical Imaging

10. Ultrasound Imaging
Ultrasound system overview
Time-gain
compensation
Introduction to Biomedical Imaging

11. Ultrasound Imaging
A-mode Ultrasound (A = amplitude)
Now obsolete in medical imaging. Wave spikes are represented when a single
beam passes through objects of different consistency and hardness. The distance
between these spikes (for example A and B ) can be measured accurately by
dividing the speed of sound in tissue (1540 m/sec) by half the sound travel time.
depth
Introduction to Biomedical Imaging

12. Ultrasound Imaging
M-mode Ultrasound (M = motion)
A single beam in an ultrasound scan can be used to produce an M-mode picture
where movement of a structure such as a heart valve can be depicted in a wave-
like manner. Because of its high sampling frequency (up to 1000 pulses per
second) This is useful in assessing rates and motion and is still used extensively
in cardiac and fetal cardiac imaging.
depth
Mitral valve
LV (parasternal
view
time
Introduction to Biomedical Imaging

13. Ultrasound Imaging
B-mode Ultrasound (B = brightness)
Same as A-mode, but one dimensional
graphical display with brightness
corresponding to amplitude of reflected
sound
2D real-time ultrasound
Most modern ultrasound devices are real-
time 2D imaging systems. Multiple
crystals (linear, curved or phased-array)
or moving crystal
Sequential B-mode pulses sweeping
across a plane to display the image in
either a linear or ‘sector’ format
Displayed as real time imaging with up to
100 images per second.
Introduction to Biomedical Imaging

14. Ultrasound Imaging
Summary A-, B-, M-mode ultrasound
Introduction to Biomedical Imaging

15. Ultrasound Imaging
Ultrasound 2D piezoelectric arrays
Allows real-time volumetric scans
Arbitrary multiplanar reslicing is possible restrospectively
Real time volume rendering
Brest biopsy
Introduction to Biomedical Imaging

16. Ultrasound Imaging
Examples of 3D US
Allows real-time volumetric scans
Arbitrary multiplanar reslicing is possible restrospectively
Real time volume rendering
Miltral valve 3D Color Doppler
Introduction to Biomedical Imaging

17. Ultrasound Imaging
Doppler Ultrasound
Measures the Doppler frequency
shift between the transducer and
the red blood cells
Higher frequency = blood toward
transducer
Lower frequency = blood away
from transducer
Introduction to Biomedical Imaging

18. Ultrasound Imaging
Doppler Ultrasound
In practice non zero Doppler angle
Introduction to Biomedical Imaging

19. Ultrasound Imaging
Doppler Ultrasound
Continuous wave (CW)
Continuous sinusoidal wave,
hence no depth information
Introduction to Biomedical Imaging

20. Ultrasound Imaging
Doppler Ultrasound
Continuous wave (CW)
Continuous sinusoidal wave,
hence no depth information
Pulsed Wave (PW)
Pulsed waves along one scan line
at constant pulse repetition
frequency
Only information of one spatial
position
Sample each reflected pulse at a
fixed time (range gate)
Introduction to Biomedical Imaging

21. Ultrasound Imaging
Doppler Ultrasound
Continuous wave (CW)
Continuous sinusoidal wave, hence
no depth information
Pulsed Wave (PW)
Pulsed waves along one scan line at
constant pulse repetition frequency
Only information of one spatial
position
Sample each reflected pulse at a
fixed time (range gate)
Color Flow (CF)
Information of the whole image
(Doppler equivalent of B-mode)
Velocity is encoded as a color
Introduction to Biomedical Imaging

22. Ultrasound Imaging
Doppler Ultrasound modes
Pulse wave Doppler of mitral
annulus using time-velocity integral
(TVI) for analysis of diastolic
function from an apical 4 chamber
view.
Color Doppler imaging of the heart
CW Doppler representing moderate
to severe mitral regurgitation from
an apical 4 chamber view.
Introduction to Biomedical Imaging

23. Ultrasound Imaging
Modern ultrasound systems
GE Vivid7 GE Voluson 750
Introduction to Biomedical Imaging