Ultrasound
Deniz Nevşehirli
• Ultrasound is a medical
imaging technique that
uses high frequency sound
waves and their
reflections.
• A basic ultrasound machine consists of the
following parts:
– Transducer Probe - the part that sends and receives the
sound pulses.
– Central Processing Unit (CPU) - computer that does
all of the calculations and contains the electrical power
supplies for itself and the transducer probe.
– Transducer Pulse Controls - changes the amplitude,
frequency and duration of the pulses emitted from the
transducer probe.
– Monitor - displays the image from the ultrasound data
processed by the CPU.
How it works?
•
•
•
The machine transmits high-frequency sound
pulses ranging from 1 to 5 Megahertz into the
body using a probe.
As the sound waves travel into the body they
encounter a boundary between tissues (e.g. soft
tissue and bone).
During the transition between two materials with
different physical properties, some of the waves
get reflected back to the probe, while some get
refracted and travel further until they reach
another boundary and get reflected.
Behavior of an ultrasound beam
within tissue
•In part A we see the exponential attenuation of ultrasound
beam intensity within an homogenous material
•In Part B we see reflection and refraction occurring at interface
between two materials with different physical properties.
How it works?
•
•
•
The probe detects the reflected waves and
transfers to the machine.
The distance from the probe to the tissue or
boundaries is calculated by the machine using
the speed of sound in tissue (1,540 m/s) and the
time elapsed until the detection each echo
(normally on the order millions per second).
The machine displays the distances and
intensities of the echoes on the screen.
An Example of an Ultrasound Image
Major Uses of Ultrasound
• Obstetrics and Gynecology
• Urology
• Cardiology
– To observe structures or functions of the
hearth to identify abnormalities.
– To measure blood flow through the heart
and major blood vessels.
• Lungs filled with air and ribs limits the application.
A-Mode Applications
• Excellent resolution for short
penetration distances using high
frequencies. (5-15 MHz)
• 1 dimensional images are obtained.
• Small size transducer is an advantage.
B-Mode Applications
• Light intensity versus time is used to generate
images.
– Stronger reflections cause brighter line.
• Mechanical Scanning
– Transducer is moved and placed on different locations
on the patient. From each position an image is obtained
and combined to form the display.
• Electronic Scanning
– Linear array of transducers are used. (64 or more)
– Faster operation. Several hundreds of images per
second.
C-Mode Applications
• Through transmission imaging is used.
• Separate transducers are used.
• By moving transducers, 2d images are
obtained.
• Two tissue properties are derived:
– Total attenuation is calculated using absorption,
scattering and reflection losses.
– Index of refraction along the path using the
time delay which is inversely proportional to
the velocity.
M-Mode Applications
• Quantitative and qualitative analysis of
heart valve motion are obtained.
• Transducer is stationary. Positions of heart
valve leaflets are displayed at varying times
using echoes.
• Vertical deflection is scanned at a rate of 23 cardiac cycles to form a single display.
And the vertical axis is given in units of
mm/sec.
Doppler Ultrasound
• Doppler ultrasound is based upon the Doppler Effect.
– When the object reflecting the ultrasound waves is moving, it
changes the frequency of the echoes.
• If the object is moving toward the probe, this will result in an
increase in the frequency.
• If the object is moving away from the probe, this will result in a
decrease in the frequency.
– The change in frequency depends on how fast the object is
moving. Doppler ultrasound measures the change in frequency of
the echoes to calculate how fast an object is moving.
• Reflected by moving red blood cells, doppler ultrasound
has been used mostly to measure the rate of blood flow
through the heart and major arteries.
Doppler Ultrasound
• Pulsed Wave Doppler Imaging
– The time interval between transmission and reception is used to calculate
distance. So it is depth selective.
– Gives precise information about the location of target area and the flow.
– Minimum range problem.
– Aliasing problem
• Continuous Wave Doppler Imaging
– Receives information about all moving reflectors along the path of the
beam.
– Is not depth selective because there is no basis for the measurement of the
time delay.
– Maximum velocity is calculated.
• Color Doppler Imaging
– Pulsed Doppler Imaging is used to obtain a static image of blood flow
velocity waveforms.
– Different flow directions and rates are coded with different colors.
Future of Ultrasound
• Biomedical engineers at Duke University's Pratt
School of Engineering have created a new threedimensional ultrasound cardiac imaging probe.
• Inserted inside the esophagus, the probe creates a
picture of the whole heart in the time it takes for
current ultrasound technology to image a single
heart cross section.
• Transesophageal echocardiography, (TEE) is one
form of ultrasound cardiac imaging. In this
technique a probe is inserted down the patient's
throat and behind the heart to capture ultrasound
heart images.
Future of Ultrasound
• Current TEE systems can quickly generate only two-dimensional
cross-sectional images. This limitation makes it impractical for use in
guiding therapeutic treatment devices such as ablation probes. 2-D
probe must repeatedly repositioned during treatments so, instead,
fluoroscopy (X-ray) is used.
• But the use of X-ray imaging results in radiation exposure for patients
and requires lead-shielding for clinicians. In addition, such procedures
take up to seven hours to complete.
• Since 3-D imaging requires significantly more sensors than 2-D
imaging. The new Duke 3-D probe has the size of normal TE probes
but contains an array of 504 individual ultrasound sensors. (~8 times
the usual number: 64)
• The probe generates ultrasound at 5 million vibrations per second.
Having 504 sensors, it provides great sensitivity and a sharp image.
And because the image is large enough to map the whole volume of
the heart, fewer images need to be taken, reducing the required time.
Dangers of Ultrasound
• Since ultrasound is energy, the question is what is this
energy doing to the tissues.
• There are two major possibilities of problem with
ultrasound:
– Tissues or water absorb the ultrasound energy and that increases
local temperature.
– Solubility of gases decrease with increasing temperature.
Dissolved gases can form bubbles due to local heat caused by
ultrasound.
• Although, there are no records of bad effects of ultrasound
in studies in either humans or animals, ultrasound should
still be used only when necessary.
References
• Lectures on Ultrasound by Prof. Yekta Ülgen, Institute of Biomedical
Engineering BOĞAZİÇİ UNIVERSITY
• http://www.online-medical-dictionary.org
• http://electronics.howstuffworks.com/ultrasound.htm
• http://ocw.mit.edu/OcwWeb/index.htm
• http://www.cardiovascularultrasound.com/articles/browse.asp
• http://www.physorg.com/news4277.html
• http://www.mgdinc.com/pdfs/Comparison%20of%20Continuous%20
Wave%20Doppler%20vs%20Pulse%20Doppler%20Profiling%20Tech
nology.pdf
• http://www.centrus.com.br/DiplomaFMF/SeriesFMF/doppler/capitulos
-html/chapter_01.htm