PRINCIPLES OF ULTRASONOGRAPHY

1. PRINCIPLES OF ULTRASONOGRAPHY
Jerome A

2. CATEGORIES OF SOUND
•Infrasound (subsonic) below 20Hz
•Audible sound 20-20,000Hz
•Ultrasound above 20,000Hz
•Non-diagnostic medical applications <1MHz
•Medical diagnostic ultrasound >1MHz

3. WHAT IS ULTRASOUND?
• Ultrasound or ultrasonography is a medical
imaging technique that uses high frequency
sound waves and their echoes.
• Known as a ‘pulse echo technique’
• The technique is similar to the echolocation
used by bats, whales and dolphins, as well as
SONAR used by submarines etc.

4. ULTRASOUND PHYSICS
• Characterized by sound waves of high
frequency
– Higher than the range of human hearing
• Sound waves are measured in Hertz (Hz)
– Diagnostic U/S = 1-20 MHz
• Sound waves are produced by a transducer

5. • The ultrasound machine transmits high-frequency
(1 to 12 megahertz) sound pulses into the body
using a probe.
• The sound waves travel into the body and hit a
boundary between tissues (e.g. between fluid and
soft tissue, soft tissue and bone).
• Some of the sound waves reflect back to the
probe, while some travel on further until they
reach another boundary and then reflect back to
the probe.
• The reflected waves are detected by the probe
and relayed to the machine.

6. • Transducer (Probe)
–Piezoelectric crystal
• Emit sound after electric charge applied
• Sound reflected from patient
• Returning echo is converted to electric
signal  grayscale image on monitor
• Echo may be reflected, transmitted or
refracted
• Transmit 1% and receive 99% of the time

7. • The machine calculates the distance from the
probe to the tissue or organ (boundaries) using
the speed of sound in tissue (1540 m/s) and the
time of the each echo's return (usually on the
order of millionths of a second).
• The machine displays the distances and
intensities of the echoes on the screen, forming
a two dimensional image.

8. FREQUENCY AND RESOLUTION
• As frequency increases, resolution improves
• As frequency increases, depth of
penetration decreases
– Use higher frequency transducers to image
more superficial structures Penetration
Frequency

9. COMPONENTS

10. TRANSDUCERS/PROBES
• Sector scanner
– Fan-shaped beam
– Small surface required for contact
– Cardiac imaging
• Linear scanner
– Rectanglular beam
– Large contact area required
• Curvi-linear scanner
– Smaller scan head
– Wider field of view

11. MONITOR AND COMPUTER
• Converts signal to an image/ archive
• Tools for image manipulation
– Gain – amplification of returning echoes
– Time gain compensation (curve)
– Freeze
– Depth
– Focal zone

12. • For diagnostic ultrasound, sound wave frequencies of between 2 and 10
MHz (1 MHz =1 million sound waves per second) are most commonly used.
• High frequency sound waves provide greater detail, whereas lower
frequency provides greater tissue penetration.
• With low frequency transducer, a larger area is viewed, but with less
Details.With a high frequency transducer, a smaller area is viewed but with
more detail. The lower frequency transducers (<3.5 MHz) are suited for
viewing larger structures at a greater distance from the transducer.
• The higher frequency transducers (>5.0 MHz) are intended for detailed
study of superficial structures close to the transducer (eg. evaluating the
ovaries, uterus etc).
• Most transducer currently available nowadays produces single as well as
multiple frequency sound waves.

13. MODES OF DISPLAY

14. • Ultrasonography (A, B and M mode, 3D
and 4D imaging)
• Doppler flow measurement, including
Duplex and Triplex methods (Duplex,
Colour Doppler, Triplex, Power Doppler)
• Tissue Doppler imaging
• Ultrasound densitometry

15. • A mode
– Spikes – where precise length and depth
measurements are needed – ophtho
• B mode (brightness) – used most often
– 2 D reconstruction of the image slice
• M mode – motion mode
– Moving 1D image – cardiac mainly

16. 16
A-MODE – ONE-DIMENSIONAL
Distances between reflecting interfaces and the
probe are shown.
Reflections from individual interfaces (boundaries of
media with different acoustic impedances) are
represented by vertical deflections of base line, i.e. the
echoes.
Echo amplitude is proportional to the intensity of
reflected waves (Amplitude modulation)
Distance between echoes shown on the screen is
approx. proportional to real distance between tissue
interfaces.
Today used mainly in ophthalmology.

17. 17
A tomogram is depicted.
Brightness of points on the screen represents intensity of
reflected US waves (Brightness modulation).
Static B-scan: a cross-section image of examined area in the
plane given by the beam axis and direction of manual
movement of the probe on body surface. The method was
used in 50‘ and 60‘ of 20th century
B-MODE – TWO-DIMENSIONAL

18. ULTRASOUND TERMINOLOGY
• Anechoic
– No returning echoes= black (acellular fluid)
• Echogenic
– Regarding fluid--some shade of grey d/t returning echoes
• Relative terms
– Comparison to normal echogenicity of the same organ or
other structure
– Hypoechoic, isoechoic, hyperechoic
• Solid structures – acoustic shadow (caused by
absorption and reflection of US)
• Air bubbles and other strongly reflecting interfaces
cause repeating reflections.

19. PATIENT POSITIONING AND PREPARATION
• Dorsal recumbency
• Lateral recumbency
• Standing
• Clip hair
• Apply ultrasound gel

20. 20
BIOLOGICAL EFFECTS
 Possible bioeffects: inactivation of enzymes, altered
cell morphology, internal haemorrhage, free radical
formation.
 Mechanisms of bioeffects:
– Mechanical effects
– Elevated tissue temperatures (absorption of ultrasound and
therefore increase in temperature high in lungs, less in
bone, least in soft tissue)
 All bioeffects are deterministic with a threshold
(cavitation) or without it (heating).

21. PITFALLS OF ULTRASOUND
• Ultrasound cannot penetrate air or bone
– Size of organs is largely subjective
– Unable to evaluate extra-abdominal structures
– Cost
– User dependent results

22. Thank you