Physics of Ultrasound

1. BASIC PHYSICS & SETTING OF
EUS SYSTEM
MR.PRIYARANJAN DAS (OLYMPUS)

2. Basic Properties Of Ultrasound Waves
• Ultrasound- a form of
mechanical energy,
• Propagation occurs in the
form of vibrations in a
medium.i.e displacement &
oscillations of atomic
particles.
• A sinusoidal wave.
• Medical applications-1to
50MHz but in EUS (5MHz –
30MHz)

3. Basic Properties Of Ultrasound Waves
• Velocity=wavelengthx frequency
• Velocity depends on physical
properties of the medium-density &
compressibility.
• Acoustic waves of different
frequencies propagate with the same
velocity in a single medium

4. Properties Of Medium
• Density
• Compressibility
• Bulk Modulus
• As Density increases,
compressibility decreases &
bulk modulus increases

5. What happens to ultrasound in
tissues
• Short pulses of US are
transmitted into the tissue &
reflected tissues are received.
Undergoes:
Reflection.
Refraction
Scattering
Absorption
• Acoustic Impedance: resistance
to sound propagation.
• Impedance=density x velocity

6. SCATTERING
• Scattering: occurs when US wave
interacts with small components in
tissue that are smaller than the
wavelength than incident waves and
different impedance values.
• Eg: individual cells, fat, collagen
• Responsible for different echotextures
of organs.
• Fat: high scattering; appears brighter,
due to multiple reflections

7. THE EUS MACHINE & EQUIPMENT
• Transducer:A device which
converts electrical energy into
mechanical energy and sends as
ultrsound pulses into tissue
• The back received signals are
picked up by the receiver built
in the transducer and sent to
processor which generates real
time images.

8. Linear Array Transducers
• Multiple single element transducers
can be combined to form an array
which gives focussing at different
depths,
• they can be fired individually or in
groups.
• Allows electronic focussing at
different depths

9. Processor Characteristics
• Depth of interface(D)=
VxT/2
• Amplification of output can
be adjusted by
1. GAIN-increasing the
overall gain which
increases the amplitude
of all received echoes but
at the expense of
resolution.
2. TGC: allows selective
amplifictaion of weaker
echoes from deeper
structures

10. IMAGING PRINCIPLES
RESOLUTION
• SPATIAL (X-axis -Axial, Y-axis - Lateral & Azimuthal Angle – Z-axis which is
Manufacturer dependent and fixed)
• TEMPORAL (Depends on FR, FW, Depth) – Function of time.
• CONTRAST
8-Bit Processor(256 shades of grey viz. as Dynamic Range.
CT has 10-Bit processing so gives higher resolution of B-Mode Imaging.

11. IMAGING PRINCIPLES
• RESOLUTION:
• Axial: smallest seperation
distance between two
objects that can be detected
along the beam.
• Determined by spatial pulse
length(SPL)-the length of
time the ultrasound pulse
occupies in space
• SPL =velocity/frequency x n.
• The limit of axial resolution is
SPL/2.

12. High Vs Low Frequency

13. IMAGING PRINCIPLES
• RESOLUTION:
• Lateral:
• Ability to discriminate
between two objects in a
perpendicular plane to that
of beam
• Depends on focusing of the
transducer;highly focused
transducer has narrow beam
width

14. Scanning
• A Mode Scan: Amplitude
Mode gives radiofrequency
wave patterns.
• Rarely used clinically.
• Forms the basis for other
modes.
• B Mode:-Brightness mode.
• Created by processing a
series of A mode signals-
resulting in a compound B
mode image.

15. DOPPLER
• Is used to identify objects that are
in motion relative to transducer.
• An object in motion will reflect a
wave that is of different
frequency; this is called doppler
shift.;this depends on velocity of
moving object.
• Greatest shifts detected when
transducer is aligned to flow
direction(Cos0 or Cos180)

16. Doppler
• Continuous doppler is used for recording high
velocities
• Pulse doppler is used for depth acuity
• Color doppler is like pulse doppler.
• Blood flow towards the transducer-Red.
• Flow away from the transducer-Blue.
• High velocities-turblant flow-green hue-mosaic
pattern
• Power doppler: determines the strength of doppler
signal discards info on velocity or direction.
• H-Flow : most sensitive for detecting blood flow
(Minute blood vessels visualization possible near
pancreatic region for better ROI targeting to collect
FNB/FNAC through EUS needle)

17. Imaging Artifacts
• Reverberation: Single transmitted
pulse undergoes multiple reflections
from a strong reflector,received by
transducer-reflected back.
• Identified by equal spacing between
hyperechoic bands

18. Reverberation Artifact

19. Acoustic Shadowing
• Large impedance encounter
• Majority is reflected and no
echo is detected beyond the
interface.
• Useful in detecting
calcifications.

20. Acoustic Shadowing
• Also seen due to refraction at a
boundary between tissue with
different velocities.-tumor or
cyst.
• Because of bending of the beam
some amount of tissue is not
visible-

21. Reflection-Mirror Imaging
• Occurs when imaging
near an airwater
interface such as
lumen partially filled
with water.
• Becuz of significant
impedance mismatch
the transmitted pulse
is reflected multiple
times.
• can be avoided by
removing air

22. Through Transmission
• Enhancement of a structure
beyond a fluid filled structure
such as cyst.
• Due to less attenuation of the
wave as it passes through the
cyst.
• Seen with cyst or blood vessel.

23. Tangential Scanning
• To accurately measure the
thickness of a strucure the
transducer should be
perpendicular to the structure.
• Tangential scanning results in over
estimation of thickness.
• Importance lies in staging of GI
cancers
• On radial EUS this can be
identified.
• Tip maneuvering should be done

24. Side Lobe Artifacts
• Off axis secondary projections of US
beam.
• They are of low intensities but still can
produce weak echoes but usually are
obscured by on axis high intensity beam.
• But while viewing anechoic structures
these atrifacts become manifest.
• Disappears with repositioning the
transducer.
Harmonics utility is helpful to eliminate
the side lobe artifacts or reverberation
by increasing SNR which results in
increasing resolution of B-Mode imaging
drastically.