concise ultrasound notes

1. US HAND BOOK (1)
Dr. Kamal Sayed / MSc US AAU
Technology/freq & resol/atten/

2. •
Ultrasound Technology
•
Ultrasound wave is produced when an electric current is
applied to an array of piezoelectric crystals.
•
This causes distortion of the crystals, makes them vibrate and
produce this acoustic mechanical wave which is transmitted
into the body (The target reflector).
•
Mechanical sound waves are reflected back into the probe &
PZT convert them into electric signal again which are analyzed
by the US system into an image on the display screen
•
•
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3. •
The ultrasound waves are produced in pulses.
•
Each pulse is 2-3 cycles of the same frequency.
•
The pulse length is the distance each pulse travels.
•
The pulse repetition frequency is the rate at which the
transducer emits the pulses.
•
The pulses have to be spaced.
•
This allows enough time between pulses so the beam has
enough time to reach the target and return to the transducer
before the next pulse is generated.

4. •
Ultrasound image is produced when the pulse wave
that is generated travels through the body, reflects
off the tissue interface (echo) and returns to the
transducer.
•
When the wave is transmitted back to the transducer
its amplitude is represented by its brightness or
echogenicity.
•
It is represented as a dot.
•

5. •
The final image is produced by the combination of
these dots.
•
Strong reflections produce bright dots (hyperechoic
images) e.g. bone.
•
weaker reflections produce grey dots (hypoechoic
images) e.g. solid organs.
•
No reflection produces anechoic images, e.g. blood
vessels

6. •
Frequency and Resolution
•
Ultrasound frequency is above 20,000 Hz or 20 KHz.
•
Medical ultrasound is in the range of 3 -15 MHz.
•
Average speed of sound through most soft human
tissues is 1,540 meters per second.
•
This can be calculated multiplying the wavelength
with frequency.
•

7. •
The best resolution is obtained at the focus.
•
Diagnostic US TXRs often have better axial resolution than
lateral resolution, although the two may be comparable in the
focal region of strongly focused.
•
Elevational (azimuthal) resolution represents the extent to
which an US system is able to resolve objects within an axis
perpendicular to the plane formed by the axial and lateral
dimensions.

8. •
The higher frequency wavelength will have shorter
wavelength whereas lower frequency wavelength
will have longer wavelength.
•
The wavelength for the 2.5 MHz is 0.77 mm whereas
that for 15 MHz is 0.1 mm

9. •
SPATIAL RESOLUTION is the ability of the US system to detect
and display structures that are close together.
•
Since an US image displays depth into the patient and width
across a section of anatomy it is therefore reasonable to
consider two types of spatial resolution – Axial & Lateral.
•
In terms of digital images, spatial resolution refers to the
number of pixels utilized in construction of the image.
•
Images having higher spatial resolution are composed with a
greater number of pixels than those of lower spatial
resolution.
•

10. •
Lateral resolution is the image generated when the two
structures lying side by side are perpendicular to the beam.
This is directly related to the width of the US beam.
•
Narrower the beam better is the resolution.
•
The width of the beam is inversely related to the frequency.
Higher the frequency narrower is the beam.
•
If the beam is wide the echoes from the two adjacent
structures will overlap and the image will appear as one.
•
Lateral resolution is roughly four times worse than
axial resolution in ultrasound.

11. •
Modes in US
•
a mode in US is a process that uses the reflection of high-
frequency sound waves to make an image of structures deep
within the body.
•
US modes are :
•
A-mode: A-mode is the simplest type of ultrasound. ...
•
B-mode: In B-mode ultrasound, a linear array of transducers
simultaneously scans a plane through the body that can be
viewed as a two-dimensional image on screen.
•
M-mode: M stands for motion.
•
Mode doppler

12. •
D – mode
•
Definition. - Constant depth mode. –
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A gate is used to only receive echoes from a specific depth. -
Echoes received from a specific depth is imaged in the plane
and is at a CONSTANT depth from the transducer and
perpendicular to the beam. - A cross-sectional image is
created.

14. Modes of US

16. •
A-mode : is the simplest type of US. A single transducer scans
a line through the body with the echoes plotted on screen as
a function of depth.
•
Therapeutic US aimed at a specific tumor or calculus is also A-
mode, to allow for pinpoint accurate focus of the destructive
wave energy.
•
A-mode scans result in a waveform with spikes or peaks at
the interface of two different tissues (e.g., where
subcutaneous fat and muscle meet).
•
Both A-mode and B-mode ultrasound have been used to
measure subcutaneous fat thickness.

21. •
B-Mode is a two-dimensional ultrasound image display
composed of bright dots representing the ultrasound echoes.
The brightness of each dot is determined by the amplitude of
the returned echo signal.
•
B-mode or 2D mode:
•
In B-mode (brightness mode) US, a linear array of transducers
simultaneously scans a plane through the body that can be
viewed as a two-dimensional image on screen.
•
More commonly known as 2D mode now.

26. •
M-mode is defined as time motion display of the ultrasound
wave along a chosen ultrasound line. It provides a
monodimensional view of the heart. ... The advantage of
the M-mode is its very high sampling rate, which results in a
high time resolution so that even very rapid motions can be
recorded, displayed, and measured.
•
M-mode, or motion mode, is used clinically for the
assessment of valve motion, chamber sizes, aortic root size,
wall thickness, and ventricular function
•

29. {M- mode} tracing across the aortic sinus and the left atrium ([LT upper)
tracing across the left ventricle (LT lower)
tissue dopp M mode (RT upper) Color Doppler M-mode (RT lower)

30. •
Highly dense tissues such as bone or kidney stones
readily reflect echoes and, therefore, appear bright white on
an ultrasound. Air, such as in the bowel, also readily reflects
echoes. ... Remember, ultrasound does not detect tissue
density

31. •
The physics of US
•
The production of ultrasound waves is based on the so-called
'pulse-echo-principle'.
•
The source of the ultrasound wave is the piezoelectric crystal,
which is placed in the transducer.
•
This crystal has the ability to transform an electrical current
into mechanical pressure waves (ultrasound waves) and vice
versa.
•

32. •
dynamic range (DR)
•
in medical US imaging, DR is defined as the difference
between the maximum and minimum values of the displayed
signal to display and it is one of the most essential parameters
that determine its image quality.
•
Its effect on image is by changing the gray scale mapping.

33. •
image too dark, first, increase receiver gain
image too bright, first, reduce output power.
•

34. Young's Modulus
a mechanical property that measures the tensile stiffness of
a solid material.
•
The Young's Modulus of a material is a fundamental property
of every material that cannot be changed. It is dependent
upon temperature and pressure however.
•
The Young's Modulus (or Elastic Modulus) is in essence
the stiffness of a material. In other words, it is how easily it
is bended or stretched.
•
ss

35. •
To be more exact, the physics and numerical values are
worked out like this:
•
Young's Modulus = Stress / Strain
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where:
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Stress = force / cross sectional area
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Strain = change in length / original length
•

36. •
Acoustic impedance (Z) is a physical property of tissue.
•
It describes how much resistance an ultrasound beam
encounters as it passes through a tissue.
•
Acoustic impedance depends on:
•
the density of the tissue (d, in kg/m3)
•
the speed of the sound wave (c, in m/s)
•
and they are related by:
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Z = d x c (units in Rayels in kg/sq meter)

37. •
Sequencing (to produce scan lines) is the process through
which a sequence of ultrasound pulses are transmitted into
the tissue and the RF data are collected.
•
Line density adjusts the number of scan lines in
your ultrasound image. A higher level provides better
resolution in the image (more scan lines), but reduces the
frame rate. Use this to get the best possible image with the
most acceptable frame rate.

38. •
This Doppler-shifted ultrasonic image of a partially occluded
artery uses color to indicate velocity. The highest velocities
are in red, while the lowest are blue. The blood must move
faster through the constriction to carry the same flow
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(next slide).
•

40. •
Low frequency noises, below the frequency that human ears
can usually detect, are used by elephants to communicate
over long distances. The infrasound frequencies are good for
long distance communication because they travel well
through objects instead of being reflected
•
. People use this frequency range for monitoring earthquakes
and volcanoes, charting rock and petroleum formations below
the earth, and also in ballistocardiography and
seismocardiography to study the mechanics of the heart.
Infrasound is characterized by an ability to get around
obstacles with little dissipation.

41. •
Higher frequencies are absorbed more rapidly in the air. This
effect reduces as the frequency reduces. Hence, infrasound
travels further, but does weaken as it spreads out, just like
any other wave
•
. There is no agreement about the biological activity of
infrasound. Reported effects include those on the inner
ear, vertigo, imbalance, etc.; intolerable sensations,
incapacitation, disorientation, nausea, vomiting, and bowel
spasm; and resonances in inner organs, such as the heart.
•

42. •
7 hz
•
The most dangerous frequency is at the median alpha-rhythm
frequencies of the brain: 7 hz. This is also the resonant
frequency of the body's organs
•
That's due to the refraction of light: the way rays of light bend
when they move from a medium like air to a medium like
water. ... Because sound moves faster in warm air than colder
air, the wave bends away from the warm air and back toward
the ground. That's why sound is able to travel farther in chilly
weather.

43. •
The molecules in the medium, as they are forced to vibrate back and forth,
generate heat. Consequently, a sound wave can only propagate through a
limited distance. In general, low frequency waves travel further than high
frequency waves because there is less energy transferred to the medium.
•
What is Infrasound? ... For example, some animals, such as whales,
elephants and giraffes communicate using infrasound over long distances.
Avalanches, volcanoes, earthquakes, ocean waves, water falls and meteors
generate infrasonic waves.
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44. •
vacuum
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Sound cannot travel through a vacuum.
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A vacuum is an area without any air, like space.
•
So sound cannot travel through space because there is no
matter for the vibrations to work in.

45. •
Why sound is louder at night ?
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There is a phenomenon called refraction that affects the
direction of sound propagation.
•
During the day, the sound bends away from the ground;
during the night, it bends towards the ground.
•
Hence at night you have additional "sound" reaching you,
making it louder.

46. •
During attenuation the ultrasound wave stays
on the same path and is not deflected.