Ultrasound Physics & Terminology

This module includes the following:

• Basic physics terms
• Basic principles of ultrasound
• Ultrasound terminology and terms
• Common artifacts seen
• Doppler principles
• Terms for labeling and scan orientation

BASIC PHYSIC TERMS

- Ultrasound: Utilizes sound waves of very high frequency (2MHz or
greater). It is propagated via waves of compression and
rarefaction, and requires a medium (tissue) for travel.
The higher the frequency, the less depth penetration,
however the resolution is improved.

- Resolution: Is the parameter of an ultrasound imaging system that
characterizes its ability to detect closely spaced
interfaces and displays the echoes from those interfaces
as distinct and separate objects. The better the
resolution, the greater the clarity of an ultrasound
image.

- Axial Resolution: Is the minimum required reflector separation along the
direction of propagation required to produce separate
reflections. Good axial resolution is achieved with short
spatial pulse lengths. Short spatial pulse lengths are a
result of higher frequency and higher damped
transducers. Therefore the higher the frequency the
better the resolution.

- Lateral Resolution: Is the minimum reflector separation perpendicular to
the direction of propagation required to produce
separate reflections. Good lateral resolution is achieved
with narrow acoustic beams. A narrow acoustic beam is
the result of a long near zone and a small angle of
divergence in the far zone.

- Transducers: Convert one form of energy to another. Ultrasound
transducers convert electric energy into ultrasound
energy and vice versa. Transducers operate on
piezoelectricity meaning that some Materials (ceramics,
quartz) produce a voltage when deformed by an applied
pressure, and reversely results in a production of
pressure when these materials are deformed by an
applied voltage.

- Pulsed Transducers: Consists of one transducer element which functions as
both the source and receiving transducers.

- Mechanical Probes: Allows the sweeping of the ultrasound beam through
the tissues rapidly and repeatedly. This is accomplished
by oscillating a transducer. The oscillating component is
immersed in a coupling liquid within the transducer
assembly. In our case the coupling fluid is deionized
water. It is important that the fluid is bubble free, so
that your image is not compromised. Check the
water level in the transducer assembly before
scanning and if you see air bubbles, make sure
you fill it with the deionized water.

- Attenuation: A decrease in amplitude and intensity, as sound travels
through a medium. Attenuation occurs with absorption
(conversion of sound to heat), reflection (portion of
sound returned from the boundary of a medium, and
scattering (diffusion or redirection of sound in several
directions when encountering a particle suspension or a
rough surface). These different forms of attenuation are
responsible for artifacts that may be in your
image. Some of these artifacts are useful and some are
not. Some artifacts are produced by improper
transducer location or machine settings.

BASIC PRINCIPLES OF ULTRASOUND

Sound Waves

Audible sound waves lie within the range of 20 to 20,000 Hz. Clinical
ultrasound systems use transducers of between 2 and 17 MHz. The
Vevo VisualSonics ultrasound system uses frequencies of between 20
to 55 MHz. Sound waves do not exist in a vacuum, and propagation in gases is
poor because the molecules are too widely spaced which is why lung
does not image well with ultrasound.

A gel couplant is used between the skin of the subject and the
transducer face otherwise the sound would not be transmitted across
the air-filled gap.

The strength of the returning echo is directly related to the angle at
which the beam strikes the acoustic interface. The more nearly
perpendicular the beam is the stronger the returning echo; smooth
interfaces at right angles are known as specular reflectors. This is best
seen in the walls of a large blood vessel such as the aorta or the
carotid artery.

Transducers

The choice of which transducer should be used depends on the depth
of the structure being imaged. The higher the frequency of the e
transducer crystal, the less penetration it has but the better the
resolution. So if more penetration is required you need to use a lower
frequency transducer with the sacrifice of some resolution.

The shape of the beam is varied and is different for each transducer
frequency. There is a fixed focused region of the ultrasound beam
which is indicated on the system with a small triangle to the right of
the image. This indicates the focal zone of that transducer and is
where the best resolution can be achieved with that particular
transducer. Effort should be taken to position the object of interest in
the subject to within that focused area to obtain the best detail. This
can be achieved with the use of more or less ultrasound gel and
moving the transducer closer to or farther away from the subject.

ULTRASOUND TERMINOLOGY AND TERMS

- Anechoic: A structure that does not produce any internal echoes

- A-Mode/Amplitude modulation: A single dimension display consisting of a horizontal
baseline. This baseline represents time and or distance with upward (vertical) deflections (spikes depicting the acoustic interfaces)

- Attenuation: The ultrasound beam undergoes a progressive
weakening as it penetrates the body due to absorption, scattering and beam spread. The amount of weakening is dependent on frequency, tissue density, and the number and types of interfaces

- B-Mode/Brightness modulation: A two-dimensional display of ultrasound. The A-mode
spikes are electronically converted into dots and displayed at the correct depth from the transducer.

- Complex: Refers to a mass that has both fluid-filed and solid
areas within it

- Cystic: This term is used to describe any fluid-filled
structure, for example, the urinary bladder

- Acoustic Enhancement: Sound is not weakened (attenuated) as it passes
through a fluid-filled structure and therefore the
structure behind appears to have more echoes than
the same tissue beside it

- Frequency: The number of complete cycles per second (Hertz)

- Gain: Refers to the amount of amplification of the returning
echoes

- Gel Couplant: A trans-sonic material which eliminates the air
interface between the transducer and the animal’s
skin

- Homogenous: Of uniform appearance and texture

- Hypoechoic: A relative term used to describe an area that has
decreased brightness of its echoes relative to an
adjacent structure

- Hyperechoic: Also a relative term used to describe a structure
which has increased brightness of its echoes relative
to an adjacent structure

- Interface: Strong echoes that delineate the boundary of organs,
caused by the difference between the acoustic
impedance of the two adjacent structures; an
interface that is usually more pronounced when the
transducer is perpendicular to it

- M-Mode: is the motion mode displaying moving structures
along a single line in the ultrasound beam

- Noise: An artifact that is usually due to the gain control
being too high

- Reverberation: An artifact that results from a strong echo returning
from a large acoustic interface to the transducer. This echo returns to the tissues again, causing additional echoes parallel and equidistant to the first echo.

- Shadowing: Failure of the sound beam to pass through an object,
e.g. a bone does not allow any sound to pass through
it and there is only shadowing seen behind it

- Time-Gain Compensation: Compensation for attenuation is accomplished by
amplifying echoes in the near field slightly and
progressively increasing amplification as echoes
return from greater depths

- Transducer: A device which houses the element for transmitting
and receiving ultrasound waves. Also referred to as a
probe or Scanhead

- Velocity (of sound): Is the speed at which a sound wave is traveling. In
soft tissue at 37 degrees C. sound travels at 1540
m/second

COMMON ARTIFACTS SEEN

- Reverberation: Multiple reflections commonly seen in the bladder or
heart

- Shadowing: Created by strong reflectors, or attenuating
structures, i.e. bone, gas, calcifications and air

- Enhancement: Increase in reflection amplitude from reflectors that
lie behind a weakly attenuating structure, i.e. cysts,
solid masses

- Speckle: The granular appearance of images and spectral
displays that is caused by the interference of echoes
from the distribution of scatterers in tissue.

Within transducers, there is a FOCUS which concentrates the sound
beam into a smaller beam area than would exist otherwise. This area
of focus is where you will obtain your best images. You will find the
focus on the monitor (arrow), on the vertical millimeter scale. So when
positioning your anatomy make sure it is in the region of the
focus, so that you obtain your best images.

Time Gain Compensation (TGC)

Equalizes differences in received reflection amplitudes because of the reflector depth. Reflectors with equal reflector coefficients will not result in equal amplitude reflections arriving at the transducer if their travel distances are different. TGC allow you to adjust the amplitude to compensate for the path length differences. The longer the path length the higher the amplitude. The TGC is located on the right upper hand corner of the monitor, and is displayed graphically.

B-MODE (brightness mode)

The mode that is used for the display of echoes that return to the transducer. There is a change in spt brightness for each echo that is received by the transducer. The returning echoes are displayed on a television monitor as shades of gray. Typically the brighter gray shades represent echoes with greater intensity levels. This mode allows you to scan.

M-MODE (motion mode)

Is a graphic B-mode pattern that is a single line time display that represents the motion of structures along the ultrasound beam, 1000fps. This mode allows you to trace motion i.e. heart wall motion, vessel wall motion.

PW MODE (pulsed-wave mode)

Frequency change of reflected sound waves as a result of reflection motion relative to the transducer used to detect the velocity and direction of blood flow. This reflection shift can be displayed graphically, as well as audibly. During Doppler operation the reflected
sound has the same frequency as the transmitted sound if the blood is
stationary ( we know that blood is not stationary because it moves) therefore if
the blood is moving away from the transducer a lower frequency is
detected (negative shift) the spectrum appears below the baseline. If
the blood is moving toward the transducer a higher frequency (positive
shift) is detected and the spectral displays above the baseline

Doppler shift

Is dependent on the insonating frequency, the velocity of moving
blood and the angle between the sound beam and direction of the
moving blood. If the sound beam is perpendicular to the direction of
blood flow, there will be no doppler shift therefore there would be no
display of flow in the vessel. The angle of the sound beam should be
less than 60 degrees at ALL times.

Sample Volume

Is the gate length which chooses the doppler shifts that will be used to produce audible sounds or spectral display. The larger the sample volume the more Doppler frequencies detected.

Aliasing

Is the production of false doppler shift and blood velocity information
when the Doppler shift exceeds a threshold. It appears as if the spectral display is cut off and wraps around and reappears in the opposite region of the display.

Spectral Broadening

The widening of the doppler shift spectrum. Meaning the increase of the range of doppler shift frequencies present, owing to a broader range of flow speeds encountered by the sound beam.

Color Doppler

Doppler echoes are usually displayed with gray scale brightness
corresponding to their intensities. In color doppler echoes are
displayed with colors corresponding to the direction of flow that their
positive or negative doppler shifts represent (toward or away from the
transducer). The brightness of the color represents the intensity of the
echoes, and sometimes other colors are added to indicate the extent
of spectral broadening.

Power Doppler

Depicts the amplitude or power of doppler signals rather than the
frequency shift. This allows detection of a larger range of doppler shifts
and therefore better visualization of the smaller vessels, but at the
expense of directional and velocity information.

DOPPLER PRINCIPLES

Doppler

Displays the change in frequency of a wave resulting in the motion of
the wave source or reflector. In ultrasound the reflector is the moving
red blood cell. The Doppler shift is dependent on the insonating
frequency (transducer frequency), the velocity of the moving red blood
cells, and the angle of the sound beam and direction of the moving red
blood cells. The following is the Doppler equation:

Df = (2f v cos q)/c
Df = Doppler shift frequency (difference between
transmitted and received)
f = Transmitted frequency
v = Velocity of the blood
c = Speed of sound
q = Angle of the sound beam and direction of
moving blood cells

The equation can be rearranged to detect the velocity of the blood flow
with the following equation:

v = (Df c)/(2 f cos q)

Two important facts can be observed from this equation. First, since
the cosine of 90 degrees is zero, if the ultrasound beam is
perpendicular to the direction of the blood flow a Doppler shift and
potentially incorrect impression of the blood flow velocities. Second,
the cosine function is steeper above 60 degrees and therefore errors
are magnified with insonating angles above this. Therefore, careful
consideration should be taken to obtain an angle of less than 60
degrees to the direction of the blood flow to obtain reliable and
accurate results in quantifying the velocity in a certain blood vessel.

Power Doppler

Depicts amplitude or power of the Doppler signal rather than the
frequency shift. Therefore, there is less angle dependence and a
visualization of smaller vessels with a Doppler shift, however, with the
sacrifice of velocity and directional information.

Pulsed Wave Doppler

With the use of a sample gate or volume gives a graphical display of all the velocities within the area sampled. The amplitude of the signal is proportional to the number of blood cells and is indicated as a shade of gray.

TERMS FOR LABELING AND SCAN ORIENTATION

- Coronal: The long axis of a scan performed from the
subject’s side where the slice divides the anterior
from the posterior or the dorsal from the ventral in
the long axis

- Transverse: A cross-sectional view

- Sagittal(Longitudinal): The long axis plane

- Superior, Cranial, Cephalad, Rostral: Interchangeable terms indicating the direction towards the head

- Inferior or Caudal: Indicating the direction towards the feet

- Anterior or Ventral: A structure lying towards the front of the subject

- Posterior or Dorsal: A structure ling towards the back of the subject

- Medial: Towards the midline

- Lateral: Away from the midline

- Proximal: Towards the origin

- Distal: Away from the origin


spotted at healthsci.tufts.edu