Download Ultrasound Notes - El Camino College

SLIDE 7
They used a through-transmission technique with two transducers placed on
either side of the head, and producing what they called "ventriculograms", or
echo images of the ventricles of the brain. Pulses of 1/10th second were
produced at 1.2 MHz. Coupling was obtained by immersing the upper part of the
patient's head and both transducers in a water bath and the variations in the
amount of ultrasonic power passing between the transducers was recorded
photographically on heat-sensitive paper as light spots (not on a cathode-ray
screen). It was an earliest attempt at the concept of 'scanning' a human organ.
Although their apparatus appeared elaborate with the transducers mounted on
poles and railings, the images produced were very rudimentary 2-dimensional
rows of mosaic light intensity points. They had also reasoned that if imaging
the ventricles was possible, then the technique was also feasible for detecting
brain tumors and low-intensity ultrasonic waves could be used to visualize the
interior of the human body.
SLIDE 12
Discovered by the Curie brothers in 1880 when they realized certain crystals
cycha s quartz undergo mechanical deformation a potential difference develops
across the tow surface crystals.
A piezoelectric crystal has an alternating current applied across it. The
piezoelectric crystal grows and shrinks depending on the voltage run through it.
Running an alternating current through it causes it to vibrate at a high speed and
to produce an ultrasound. This conversion of electrical energy to mechanical
energy is known as the piezoelectric effect
To produce an ultrasound, a piezoelectric crystal has an alternating current
applied across it. The piezoelectric crystal grows and shrinks depending on the
voltage run through it. Running an alternating current through it causes it to
vibrate at a high speed and to produce an ultrasound. This conversion of
electrical energy to mechanical energy is known as the piezoelectric effect. The
sound then bounces back off the object under investigation. The sound hits the
piezoelectric crystal and then has the reverse effect - causing the mechanical
energy produced from the sound vibrating the crystal to be converted into
electrical energy. By measuring the time between when the sound was sent and
received, the amplitude of the sound and the pitch of the sound, a computer can
produce images, calculate depths and calculate speeds.
Whenever a sound wave moving in air hits a solid surface, it reflects off it. This
reflected sound is called an echo. The same applies to a sound wave moving
through water and hitting an obstacle. If we know the speed of sound in the air or
water, we can calculate the distance to the obstacle. To do this we must measure
the time taken for a pulse of sound to travel to the object and back again:
The distance to the object and back is given by
distance=speed x time
As this is the total distance that the sound has traveled to the object and back,
we must divide by 2 to find the one-way distance.
This use of echoes is the basis of sonar (sound navigation and ranging). The
pulse of sound that is used should be short, and high frequencies are usually
used, as they travel further without being absorbed. Sounds with a frequency
above 20 kiloHertz (20 kHz) are called ultrasonic (beyond the range of human
hearing). The sounds used for sonar are well into the ultrasonic range, with
frequencies of 1 - 20 megaHertz (MHz).
In solving problems on sonar, remember that the speed of sound itself varies
from one material to another. The speed also depends on temperature, pressure
and other factors. Typical speeds are approximately 330 m/s in air, 1500 m/s in
water and 5000 m/s in a metal.
The piezoelectric effect also works in reverse. If the crystal is squeezed or
stretched, an electric field is produced across it. So if ultrasound hits the crystal
from outside, it will cause the crystal to vibrate in and out, and this will produce
an alternating electric field. The resulting electrical signal can be amplified and
processed in a number of ways (see questions on A-scan and B-scan). So a
second crystal can be used to detect any returning ultrasound which has been
reflected from an obstacle. Normally the transmitting and receiving crystals are
built into the same hand-held unit, which is a called an ultrasonic transducer
(generally, a transducer is any device to convert energy from one form to
another, usually to or from electrical energy.)
the transducer incorporates a piezoelectric element, which converts electrical
signals into mechanical vibrations (transmit mode) and mechanical vibrations into
electrical signals (receive mode
SLIDE 13
By measuring the time between when the sound was sent and received, the
amplitude of the sound and the pitch of the sound, a computer can produce
images, calculate depths and calculate speeds.
SLIDE 15
Ultrasound is produced and detected using an ultrasound transducer. Ultrasound
transducers are capable of sending an ultrasound and then the same transducer
can detect the sound and convert it to an electrical signal to be diagnosed.
To produce an ultrasound, a piezoelectric crystal has an alternating current
applied across it. The piezoelectric crystal grows and shrinks depending on the
voltage run through it. Running an alternating current through it causes it to
vibrate at a high speed and to produce an ultrasound. This conversion of
electrical energy to mechanical energy is known as the piezoelectric effect. The
sound then bounces back off the object under investigation. The sound hits the
piezoelectric crystal and then has the reverse effect - causing the mechanical
energy produced from the sound vibrating the crystal to be converted into
electrical energy. By measuring the time between when the sound was sent and
received, the amplitude of the sound and the pitch of the sound, a computer can
produce images, calculate depths and calculate speeds.
SLIDE 16
Advantages compared with other techniques 1. Ultrasound examinations
are non-invasive i.e. they do not require the body to be opened up, or anything
to be inserted into the body. This is a major advantage compared to fibre-optic
endoscopy, for example,
which may involve much more patient discomfort as the probe is inserted. 2.
Ultrasound methods are relatively inexpensive, quick and convenient,
compared to techniques such as
X-rays or MRI scans. The equipment can be made portable, and the images
can be stored electronically.
3. No harmful effects have been detected, at the intensity levels used for
examinations and imaging. This
contrasts with methods based on X-rays or on radioactive isotopes, which
have known risks associated
with them, and ultrasound methods are preferred whenever possible. This is
particularly relevant to
examination of expectant mothers.
4. Ultrasound is particularly suited to imaging soft tissues such as the eye,
heart and other internal organs,
and examining blood vessels.
Disadvantages of ultrasound compared with other techniques 1. The major
disadvantage is that the resolution of images is often limited. This is being
overcome as time
passes, but there are still many situations where X-rays produce a much
higher resolution. 2. Ultrasound is reflected very strongly on passing from
tissue to gas, or vice versa. This means that
ultrasound cannot be used for examinations of areas of the body containing
gas, such as the lung and the
digestive system.
3. Ultrasound also does not pass well through bone, so that the method is of
limited use in diagnosing
fractures. It is possible to obtain quite good ultrasound scans of the brain, but
much greater detail
is obtained by an MRI scan.
SLIDE 17
Disadvantages of ultrasound compared with other techniques 1. The major
disadvantage is that the resolution of images is often limited. This is being
overcome as time
passes, but there are still many situations where X-rays produce a much
higher resolution. 2. Ultrasound is reflected very strongly on passing from
tissue to gas, or vice versa. This means that
ultrasound cannot be used for examinations of areas of the body containing
gas, such as the lung and the
digestive system.
3. Ultrasound also does not pass well through bone, so that the method is of
limited use in diagnosing
fractures. It is possible to obtain quite good ultrasound scans of the brain, but
much greater detail
is obtained by an MRI scan.
SLIDE 20
Color Doppler ultrasound of normal femoral vein at the level of the bifurcation of
the deep and superficial femoral veins.
Main differences between Ultrasound and X-rays
Diagnostic Ultrasound
X-rays
(radiology)
wave type
longitudinal
waves
electromagnetic waves
transmission
requirements
elastic medium
No medium
generation
stressing the medium
accelerating
charges
velocity
depends on the medium
through which it
propagates
It is relatively constant:
299,792.456.2 m/s
similar waves
seismic, acoustic
radio, light
mechanical
electric
Velocity of sound in some Biological Materials
Velocity of sound in some Biological Materials
Material
Velocity of Sound
(m/s)
Impedance (Rayl x 10
-6)
Air
330
0.0004
Fat
1450
1.38
Water
1480
1.48
Average Human Soft
Tissue
1540
1.63
Brain
1540
NA
Liver
1550
1.65
Kidney
1560
1.62
Blood
1570
1.61
Muscle
1580
1.7
Lens of eye
1620
NA
Skull Bone
4080
7.8
Main differences between Ultrasound and X-rays
Diagnostic Ultrasound
X-rays
(radiology)
wave type
longitudinal
waves
mechanical electromagnetic
waves
transmission
requirements
elastic medium
No medium
generation
stressing the medium
accelerating
charges
velocity
depends on the medium
through which it propagates
similar waves
acoustic
electric
radio, light
Velocity of sound in some Biological Materials
Velocity of sound in some Biological Materials
Material
Velocity of Sound
(m/s)
Impedance (Rayl x 10
-6)
Air
330
0.0004
Fat
1450
1.38
Water
1480
1.48
Average Human Soft
Tissue
1540
1.63
Brain
1540
NA
Liver
1550
1.65
Kidney
1560
1.62
Blood
1570
1.61
Muscle
1580
1.7
Lens of eye
1620
NA
Skull Bone
4080
7.8
Let’s begin by breaking the meaning of Ultrasound.
Can anyone break down the word Ultrasonography?
Answer: Ultra stands for ultra high frequency. Sonic means sound. Graphy is a
permanent record.
So now put it all together. What is the process of Sonar?
Answer: It is a technique of sending sound waves thru water and observing the
returning echoes to identify submerged objects.
Frequencies of sound that are heard by the human ear are called audible sound.
What is the difference in the sound waves in ultrasound and audible sound
frequencies?
Answer: Sound waves have a higher frequency than audible sound waves.
In ~ 1947 patients with suspected ventricular disease, tumors or other intracranial
lesions were sent to get an US. Top part of the patients head was immersed in
water and two transducers were placed on opposite sides of the head. The
images were recorded on paper (photographic) as rudimentary 2-dimensional
rows of light spots.
Present day patients are not submerged in water and the transducer are much
smaller as are the machines. The water has been replaced by gel.
Piezoelectric effect: Defined as “pressure electric”
Curie brothers discovered when AC is applied to certain crystals they expand
and contract, causing the crystals to vibrate, in response to the electrical field.
The electric energy (AC) is converted to mechanical energy in the form of an
ultrasound wave. The sound bounces off the anatomy and is returned to the
crystals and they start to vibrate again. This is when the mechanical energy is
converted back into electrical energy. The transmitting and receiving crystals are
built into the same handheld device.
What is this handheld device called?
Answer: transducer
Once US waves are produced they are directed into the body. They travel thru
the body until striking a tissue barrier that reflects the sound waves back to the
transducer: they are called echoes.
Acoustic impedance: sound impedance
Impedance delays or prevents the progress of the acoustic waves from being
transmitted thru a medium.
Ultrasound has properties very similar to light:
1) Focused with a smaller transducer
2) Refracted: US signal is deflected from a straight path and the angle of the
deflection is away from the transducer
3) Reflected: US signal is reflected back toward the transducer
4) Scatter: tissue boundaries are less than the wavelength of US
wavelengths. This is the most common cause of scattering. Blood is the
most common anatomy that causes scatter. This is because blood has a
wavelength 20 times less than US wavelength.
What is the velocity of sound?
Answer: Sound is vibration that travels thru a medium (anatomy) as a wave.
Velocity describes how far the wave will travel in a given amount of time.
The way in which the US responds when it comes into contact with anatomy is
dependent on the density of the anatomy.
US travels faster or has an increased velocity when traveling through water in
comparison to air.
If we know US travels faster in water what can we deduce when we are trying to
determine whether US travels faster thru fat or bone? Explain?
Answer: The higher the atomic number of the material the faster US passes thru
a given medium.
The sound waves returned are picked up by the computer to make images. The
intensity and pitch images produces images. The computer uses this data to
calculate depth and speed.
There are many types of transducers with varying frequencies and shapes
depending on the body part to be imaged. First lets do a review on the
realationship between wavelength and frequency.
How are frequency and wavelength related in x-ray?
Answer: Frequency increases wavelength decreases.