Quality Assurance in
Diagnostic Medical
Sonography
HHHoldorf
QUALITY ASSURANCE

Quality Assurance
-The “routine” periodic evaluation of the US
system to guarantee optimal image quality
Requirements
1. Assessment of system components
2. Repairs
3. Preventive maintenance
4. Record keeping

Done for medical and legal matters
QUALITY ASSURANCE
Goals
-proper equipment operation
-detect gradual changes
-minimize downtime
-reduce number of repeat scans
QUALITY ASSURANCE
Methods
-test under known conditions
-constant instrument settings

*Results based on Transducer Type
Receiver Gain
Power
Position of transducer
Depth
*Make sure that when testing equip. that the derived
conclusion is system variation and not change in
procedure
-use phantom with measurable characteristics

QUALITY ASSURANCE
 Performed
by the Sonographer
-It is the duty of the sonographer to perform
quality assurance
-Although the manufacturer’s service engineer
may be involved, the ultimate responsibility
for QA always rests with the sonographer
-Various aspects of all instrumentation should
be routinely inspected to ensure consistency
of its performance
-It is important to validate the reliability of
the images produced and the measurements
made with each transducer on each US
system.
QUALITY ASSURANCE
 AIUM
100mm Test Object
-fluid filled tank containing strategically located
stainless steel pins or plastic strings
-speed of sound in the AIUM object is identical
to that of soft tissue
-Evaluates
accuracy
and
performance
characteristics of a system.
-BUT, the AIUM test object does not have the
attenuation properties of soft tissue, so a
grayscale cannot be evaluated (BISTABLE)
AIUM 100mm Test object
QUALITY ASSURANCE

1.
2.
3.
4.
Placing the transducer on the top, sides and on the
oblique side of the object provides a variety of
orientations between the sound beam and the pins.
Axial resolution= evaluated when the pins in the
test object are parallel to the sound beam’s main
axis
Lateral resolution= evaluated when the pins in the
test object are perpendicular to the sound beam
Electronic caliper accuracy= evaluated by
comparing the distances between reflections on the
display with the actual distances in the test object
Dead zone= evaluated by scanning the pins located
at the top of the test object, very close to the
transducer.
QUALITY ASSURANCE
Tissue Equivalent Phantom
-have ultrasonic features similar to soft tissue.
-more suitable to evaluate the characteristics of
modern systems (grayscale, multi-focus, phased
array transducers)
-strategically located pins, structures that mimic
cysts & solid masses embedded in the phantom
-similar to soft tissue
1. Speed of sound
2. Attenuation
3. Scattering characteristics
4. Echogenicity

Tissue Equivalent Phantom
Tissue Equivalent Phantom
QUALITY ASSURANCE
Doppler Phantom
-flow phantoms are the devices of choice
for evaluating Doppler systems
-used to assess the accuracy of pulsed,
continuous wave, power and color flow
systems.
1. vibrating string
2. moving belt phantoms
3. flow phantom

Doppler Phantom
QUALITY ASSURANCE

Modern phantoms include a circulation pump which propels a fluid through
vessels embedded in a tissue equivalent phantom

Used most often a suspension mimicking blood.
(problems: air bubbles and changing consistency over time)
Tests for effective:
1.
Penetration of Doppler beam
2.
Ability to discriminate between different flow directions
3.
Accuracy of measured flow speed
QUALITY ASSURANCE

Slice Thickness Phantom
-evaluation of slice thickness is also called
elevational thickness
-imaging plane is thicker than either the beam
width or the SPL.
-phantom’s medium mimics soft tissue.
-thicker slices diminish spatial resolution and
reduce the ability to visualize small, low contrast
lesions
-When the US beam is overly thick, cystic structures
may appear filled in “ARTIFACTS”
Slice Thickness Phantom
QUALITY ASSURANCE
Performance Measures
Sensitivity is the ability of a system to display low level
echoes with a tissue equivalent phantom.
1.
Minimum sensitivity (image deeper)
 Pick a deep pin, with the TGC set flat, increasing the
gain from its minimum value to the point when an echo
is displayed on the CRT determines the minimum
sensitivity.
 The same rod in the test object should always be
imaged for continuity.
 Assesses the detection of low level echoes in the far
field

QUALITY ASSURANCE
2.
Normal sensitivity

Settings are those which all the pins, solid masses,
and cystic structures in the test phantom are
accurately displayed.

Output power, TGC, and amplification are adjusted
to establish normal sensitivity

Found at higher gain than the minimum sensitivity

All subsequent quality assurance and performance
measurements are made at the normal sensitivity.
(because we have to see all the structures to make
measurements)
QUALITY ASSURANCE
3. Maximum sensitivity (image shallower)

Evaluated with the output power and amplification of
the system set to the maximum practical levels

With these settings, a tissue equivalent phantom is
imaged, and the depth of tissue-like texture is
determined.

Maximum visualization depth is used to assess
sensitivity, and should not differ from one routine
evaluation to the next

***Sensitivity is also assessed when the sonographer
adjusts the system controls to change echo brightness
from barely visible to full brightness
QUALITY ASSURANCE

Dead Zone
-results from the time that it takes for the system to switch from the transmit to the
receive mode
-region close to the transducer where images are inaccurate
-extends from the transducer to the shallowest depth from which meaningful
reflections appear
-information within the dead zone is unreliable and may not be used in the
diagnostic setting
-the dead zone is assessed with the most shallow series of pins in a test object
Dead Zone
QUALITY ASSURANCE

Dead Zone
-an acoustic standoff, or “gel pack” positioned between the transducer and the
patient allows accurate imaging of important superficial structures (50 cc bag
of IV fluid)
-an increasingly deeper dead zone may indicate a cracked crystal, detached
backing material, or a longer pulse duration
QUALITY ASSURANCE

Registration Accuracy
-ability of the system to place reflections in proper positions while imaging from
different orientations

Range Accuracy (vertical depth calibration)
-system’s accuracy in placing reflectors at correct depths located parallel to the
sound beam
-if differences appear the error may be caused by
1.
System malfunction
2.
The speed of sound in the phantom is different than 1,540 m/s
QUALITY ASSURANCE

Horizontal calibration
-system’s ability to place echoes in their correct position perpendicular to the sound
beam

Focal zone (surrounds focus)
-focus of phased array transducers, must be carefully evaluated
-lateral resolution is excellent in the focal zone
-systems with dynamic receive focusing should produce narrow reflections over a
wide range of depths
QUALITY ASSURANCE

Axial Resolution
-smallest distance at which two pins parallel to the sound beam are displayed as
two distinct echoes.
-evaluated by scanning a set of closely spaced pins within the phantom
-uses pins parallel to the sound beam

Lateral Resolution
-minimum distance that two side-by-side rods are displayed as two distinct
images
-look to see if pins are perpendicular to sound beam
QUALITY ASSURANCE
 Compensation
Operation or Uniformity
-Using a tissue equivalent phantom and scanning
from the top, the echoes are displayed with
TIME GAIN COMPENSATION (TGC) off and then
with the TGC on.
-With the TGC off, the echoes should be
displayed with reduced amplitude as depth
increases
-With the TGC on, all of the echoes should have
the same amplitude, regardless of depth
QUALITY ASSURANCE
Mock Cysts and Tumors
-Using the tissue equivalent phantom to check
dimensions of cysts
-Also, note texture and fill-in
 Display and Grayscale Dynamic Range
-Adjusting the system’s output power and
amplification should produce changes in the
grayscale display.
-Important to compare the relationship between the
image on the system’s screen with the output of all
other display devices, such as remote viewing
stations

BIOEFFECTS

GOLD STANDARD: a perfect technique that we deem 100% accurate to which
our ultrasound results are compared.

Since the majority of sound energy produced by the transducer remains in the
body, it is important to know output of the ultrasound system.
BIOEFFECTS

Hydrophone
-small needle with a piezoelectric crystal at its end
- a wire connects the PZT crystal to an oscilloscope and the needle is placed in the
ultrasound beam
-displayed are acoustic signals received by the crystal
-since the hydrophone is small, the acoustic pressure is measured at specific
locations within the sound beam
-can quantitate amplitude, period, pulse duration and pulse repetition period
Hydrophone
BIOEFFECTS
 Radiation
Force
-transducer’s sound beam creates a very
small, but measureable force on any target
that it strikes.
-if the target is a balance or a float, the
measured force relates to the power in the
beam
-when the sound beam is entirely absorbed
or reflected by the target, the target acts
as an extremely sensitive miniature postal
scale
-measures SATA or SPTA intensity
BIOEFFECTS
 Acoustic-Optics
-Based on the interaction of sound and
light
-A shadowing system, called Schlieren,
uses this principle to allow us to view the
shape of a sound beam in a medium
-Quantitation of the PRP, amplitude,
period, and pulse duration
BIOEFFECTS
Three devices measure the output of ultrasound
transducers by absorption, the conversion of
sound energy into heat:
1. Calorimeter
-measures the total power of the entire sound
beam through the process of absorption
-the sound beam is directed into the calorimeter
where the sound energy is converted into heat
(absorption)
-the sound beam’s total power is calculated by
measuring the temperature rise and the time of
heating

BIOEFFECTS

Thermocouple
-tiny electronic thermometer
-a dab of absorbing material is placed on the thermocouple
-the thermocouple is inserted into the sound beam and the temperature is measured
by absorbing material
-the temperature rise is related to the power of the sound beam at the particular
location where the device is located
BIOEFFECTS
 Liquid
Crystals
-certain crystals change color based on
their temperature
-tank full of crystals
-When a sound beam strikes a crystal, the
sound energy is absorbed
-The change in temperature causes a
change in their color, providing insight into
the shape and strength of the sound beam
BIOEFFECTS

RISK-BENEFIT RELATIONSHIP
1.
Benefits must outweigh the risks of the exam
2.
Obstetrics and fetal medicine is a common application of diagnostic sonography.
There is no confirmation of harm resulting from its use
3.
Extremely high ultrasound intensities damage biologic tissue
4.
Low intensity ultrasound has no known Bioeffects
5.
Under controlled circumstances, Bioeffects are beneficial
BIOEFFECTS

AIUM’S AND FDA’S Bioeffects intensity limit:
SPTA (Spatial Peak/Temporal Average)
 Highest
output intensities with pulsed
Doppler
 Lowest output intensities with gray
scale imaging
 100 mW/cm2 unfocused
 1,000mW/cm2 focused
BIOEFFECTS

Focused beams are less likely to cause temperature elevation in tissues

Unfocused beams are more likely to cause temperature elevation in tissues
**this occurs because a narrow beam heats only a small region of tissue and the heat is
rapidly transferred to and dissipated by adjacent tissues that were not heated by
the US beam.
BIOEFFECTS
Dosimetry
-Science of identifying and measuring the
characteristics of an ultrasound beam that
are relevant to its potential for producing
biological effects.
 Bioeffects may be conducted in two broad
areas:
1. In Vivo- living
2. In Vitro- nonliving

BIOEFFECTS

1. In Vivo
-performed within the living body of a plant or an animal.
e.g. Studying the effects of US exposure on the lung tissue
of a laboratory rat.
***Difficult to study in vivo because of attenuation***

2. In Vitro
-performed outside the living body in an artificial
environment.
e.g. A computer model estimating the temperature
elevation of tissues during exposure to US.
BIOEFFECTS

AIUM Statement on In Vitro Bioeffects
1.
In vitro Bioeffects research is important
2.
In vitro Bioeffects are real even though
they may not apply to the clinical setting
3.
In vitro Bioeffects which claim direct
clinical significance should be viewed with
caution (without in vivo validation)
BIOEFFECTS

1.
STUDY TECHNIQUES:
Mechanistic Approach
- begins as a proposal that a specific
mechanism has the potential to produce
Bioeffects
-based on that proposal, a theoretical
analysis is performed to estimate the scope
of the Bioeffects at various exposure levels.
-searches for a cause and effect
relationship
BIOEFFECTS

EMPERICAL APPROACH
-based on acquisition and review of information from patients or animals exposed to
US
-studying charts of patients who have been exposed to US
-seeks a relationship between the exposure to US and the effects of that exposure
-searches for a relationship between an exposure and a response
***strongest conclusions when conclusions to both approaches agree.
BIOEFFECTS

1.
MECHANISMS OF BIOEFFECTS
Thermal=proposes that Bioeffects result from
tissue temperature elevation
-as sound travels in the body, energy is
converted into heat.
-bone is an absorber. There fore temperature
elevation at a tissue-bone interface is more
likely.
-temperature elevation in fetal soft tissue is
considered of potentially greater harm than
adults. Thus, fetal soft tissues adjacent to
bone are of great concern
BIOEFFECTS
 Body
core temperature is regulated at 37
degrees C. Life processes do not function
normally at other temperatures.
 Any exam that causes an elevation in
temperature of less than 2 degrees C may
be used without reservation
 Any exam that causes a temperature
elevation to greater than 41 degrees C is
considered potentially harmful to a fetus.
BIOEFFECTS

Thermal index (TI):useful predictor of max. temperature increase under
most clinically relevant conditions.
3 forms:
1.
TIS=assumes that sound is traveling in soft tissue
2.
TIB=assumes that bone is at or near the focus of the sound beam
3.
TIC=assumes that the cranial bone is in the sound beam’s near field
BIOEFFECTS

1.
2.
3.
4.
Empirical findings:
Serious tissue damage occurs from
prolonged elevation of body temperature
Tissue heating is related to the output
characteristics of the transducer and the
properties of the tissue
A 2 degree to 4 degree in testicular
temperature can cause infertility
A combination of temp and exposure
time determine the likelihood of harmful
Bioeffects
BIOEFFECTS
5. No confirmed Bioeffects have been
reported for temp elevations of up to 2
degrees above normal for exposures of
less than 50 hrs.
6. Max. heating is related to the beam’s
SPTA intensity
7. Fetal tissues appear less tolerant of
tissue heating than adults.
8. A greater amount of acoustic energy is
absorbed by bone than by soft tissue.
BIOEFFECTS
 Mechanistic findings:
1. Theoretical models appear to correlate
with experimental data even though:
-the ultrasound beam is quite complex
-diagnostic equipment is diverse
-tissue characteristics are different
***strong argument***
BIOEFFECTS
2. Cavitation=interaction of sound waves with
microscopic, stabilized, gas bubbles in the
tissues.
-Also known as gaseous nuclei= NOT CONTRAST
AGENTS. NATURAL GASEOUS NUCLEI THAT
OCCURS NATURALLY IN THE BODY.
Excitation takes on the form of shrinking and
expanding of the bubble. Potential of near
total energy absorption where the nuclei exist
may lead to thermal injury
MI relates to cavitation
BIOEFFECTS
1. Stable Cavitation
-occurs at lower MI levels
-the gaseous nuclei tend to oscillate or expand and contract.
-bubbles that are a few millimeters in diameter might double in size-bubble does
not burst
-the bubbles intercept and absorb much of the acoustic energy
-EFFECTS: the fluids surrounding the cells undergo micro streaming and the cells
are exposed to shear stresses
BIOEFFECTS
2. Transient Cavitation (inertial or normal)
-at higher MI levels, transient cavitation occurs
-bubble bursting
-EFFECTS: produces highly localized, violent
effects, including:
1. Colossal temperatures
2. Shock waves
-The destructive effects of transient cavitation are
not considered clinically important since they
are highly localizes and affect few cells.
-The pressure threshold for transient is only 10%
higher than that required for stable cavitation
Conclusion
The American College of Radiology (ACR) strongly
recommends that the training for “appropriately trained
sonographers or service engineers” conducting routine QC
be provided by a qualified medical physicist.
If unable to acquire training by a qualified medical
physicist, training can be achieved through the ultrasound
equipment manufacturer or through an appropriate course.