Medical Equipment Section2

1. Cairo University
Faculty of Engineering
Systems and Biomedical Engineering Department
Fall Semester, 2017
SBE 405 Medical Instrumentation IV: Ultrasound Imaging–Section 02
Ghaidaa Eldeeb
Continue Interaction with body:
2.5 Interference of waves
2.6 Diffraction of waves
 If the aperture, the width of the source is smaller than the
wavelength, the wave spreads out as it travels (diverges),
an effect known as diffraction.
 If the width of the source is much greater than the
wavelength of the wave, the waves are relatively flat
(plane) rather than curved and lie parallel to the surface of
the source.
 Such waves travel in a direction perpendicular to the
surface of the source with relative little sideways spread,
i.e. in the form of a parallel-sided beam.
Constructive interference Destructive interference
Waves are in phase and peaks of waves are
coincide
Waves are in anti-phase and peaks of one
wave coincide with the trough of other
Resulting wave amplitude greater than
that of both the individual ones
Resulting pressure is zero , effects of waves
cancel each other

2.  Each of the small sources generates a sound wave of the same frequency and amplitude and all are in
phase.
 The curved waves propagate outwards and the parts of the curve which are parallel to the surface of
the source align to form plane waves. The other, nonparallel parts of the curved waves tend to
interfere destructively and cancel out.
3- Plan Disk:
 Beam shape
 Focusing
o The surface of the disc source can be considered
to be made up of many small elements, each of
which emits a spherical wave.
o The pressure amplitude at each point in the beam
is determined by the sum of the spherical waves
from all of the elements.
o The different path lengths, from the various
elements to the summing point, mean that each
of the spherical waves has a different phase when it arrives.
Near Field Far Field
Non uniform beam Beam diverge
Constant diameter Side lobes
Length=a^2/⅄ a:radius of source =D/2 ϴ=sin-1(0.61 ⅄/a)
Maximum path diff= ⅄/2 Path diff < ⅄/2

3.  if a ≈ ⅄ near field is short and beam diverge rapidly ( theta ϴ large)
 a > ⅄ near field is long and little diverge
 The optimum beam shape is achieved when the aperture is 20 to 30 wavelengths in diameter
 If freq. inc. near dec , diverge dec.
 Side lobes are weaker than the main lobe but can give rise to significant echoes if they are incident
on a strongly reflecting target adjacent to the main lobe, resulting in acoustic noise in the image
 Manufacturers normally design their transducers to minimize side lobes. Tis can be done by
applying stronger excitation to the centre of the transducer than at the edges, a technique known as
apodization.
 Apodization reduces the amplitude of side lobes but leads to an increase in the width of the main
lobe
4- Focusing
 Solution of near field
 Beamwidth dec.……resolution is
high
 W=F⅄/a where W: width of beam at
focus
 Focusing is strong when F<a^2/⅄
 Focusing methods:
o using a curved source
o Acoustic lens to a plane source.
 Lateral resolution is proportional to W
and for circular transducer: Lateral Resolution ≈ 2.44 Fλ/D, where D
is the aperture diameter
 Axial resolution = spatial pulse length/2 =TC/2
5- The ultrasound pulse and spectrum
 To produce a distinct echo which corresponds to a
particular interface, ultrasound must be transmitted in the
form of a short burst or pulse.
 To allow echoes from closely spaced interfaces to be
resolved separately, the pulse must be short
 A pulsed wave can be described as being constructed from
a range of frequencies centered on the nominal frequency
 A short pulse gives precise time resolution and
hence distance resolution and its echoes contain
information at a wide range of frequencies. The
information contained in an echo from a long pulse is concentrated near the nominal frequency and
gives a stronger signal at that frequency. However, a long pulse results in poor distance resolution.

4. 6- Non-linear propagation & harmonic imaging
 At high pressure amplitudes (>1 MPa), this simple
picture breaks down and non-linear propagation effects
become noticeable
 In the high-pressure (compression) parts of the wave,
the medium becomes compressed, resulting in an
increase in its stiffness and hence an increase in the
speed of sound in addition to that the particle motion is
in the direction of propagation, resulting in a slight
increase c, whereas in the low pressure (rarefaction)
parts of the wave motion is in the opposite direction and
c is slightly reduced
 The rapid changes in pressure in the compression part of
the wave appear in the pulse spectrum as high frequency
components, these are multiples of the original or
fundamental frequency f0 known as harmonics. A
frequency of 2f0 is known as the second harmonic, 3f0
as the third harmonic and so on. The figure shows that
the original pulse spectrum is effectively repeated at
these harmonic frequencies.
 Non-linear propagation results in some of the energy in
the pulse being transferred from the fundamental
frequency f0 to its harmonics. As the pulse
travels further into the medium, the high-
frequency components are attenuated more
rapidly than the low-frequency components and
the pulse shape becomes more rounded again as
the overall amplitude is reduced
 Second harmonic imaging
o 1- reduces noise and side lobe artifacts
o 2- Improve depth penetration.
7- Acoustic pressure and intensities

5. Isptp – the spatial peak temporal peak intensity is the maximum value in the pulse at the point in the beam where
it is highest.
Isppa – the spatial peak pulse average intensity is the average value over the pulse duration at the point in the
beam where it is highest.
Ispta – the spatial peak temporal average intensity is temporal average intensity at the point in the beam where it
is highest.
Isata – the spatial average temporal average intensity is the temporal average intensity averaged over the beam
area (usually -6 dB area)
8- Resolution:
 Refers to the ability to distinguish between objects
 Spatial Resolution: The ability to distinguish between objects located at different positions in
space.
o Axial Resolution: The ability to distinguish between echoes originating from two
reflectors lying one behind the other along the axis of the ultrasound beam. It is
sometimes referred to as Depth Resolution
 Factors affecting Axial Resolution:
- Beam Wavelength (ʎ)
- Spatial Pulse length (SPL)
- Beam Frequency (f)
o Lateral Resolution: The ability to distinguish between two reflectors suited side by side
in a direction perpendicular to that of the ultrasound beam.
 Factors affecting Lateral Resolution:
- Beam width
- Beam frequency
- Scan line density

6. Examples:
7-A plane disc transducer, with a diameter of 1.5 cm, is driven at 3 MHz to produce a continuous-wave beam in
tissue with a speed of sound of 1500 m s-1.
(a) Calculate the near-field length of the beam and its angle of divergence in the far field.
(b) Estimate the beam width at the focus if a lens is added with a focal length of 6 cm.
8. Explain how focusing of an ultrasound beam can be achieved and how its effects depend on the dimensions of
the transducer and the ultrasound wavelength.
9. Explain the origin of acoustic noise in B-mode images and how it can be reduced by the use of harmonic
imaging.
10. The peak value of acoustic pressure measured in an ultrasound beam in water is 1 MPa. What is the
corresponding instantaneous intensity in W m-2? Assume the speed of sound in water is 1500 m s-1 and its
density is 1000 kg m-3.