Download Lecture 15 QA programs Fri - gnssn

RTC on RADIATION PROTECTION OF PATIENTS
FOR RADIOGRAPHERS
Accra, Ghana, July 2011
QA Programmes in Diagnostic Radiology
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International Atomic Energy Agency
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• QUALITY ASSURANCE
• An overall system which deals with quality in all
its aspects, qualitative and quantitative
• QUALITY CONTROL
• The quantitative aspects of a quality assurance
programme
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Quality assurance programs (I)
• Radiology imaging equipment should produce
images that meet the needs of the radiologist or
other interpreters without involving
unnecessary irradiation of the patient.
• Quality assurance actions contribute to the
production of diagnostic images of a consistent
quality by reducing the variations in performance
of the imaging equipment.
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• The quality control aspects of a quality
assurance program are, however, not
necessarily related to the quality (information
content) of the image.
• They may (and often do) relate to the
radiation dose to the patient
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Quality assurance programs (II)
• It has been increasingly recognized that
quality assurance programs directed at
equipment and operator performance can
be of great value in improving the
diagnostic information content, reducing
radiation exposure, reducing medical
costs, and improving departmental
management.
• Quality assurance programs thus
contribute to the provision of high quality
health care.
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Quality assurance programs (III)
• Several studies have indicated that
many diagnostic radiological facilities
produce poor quality images and give
unnecessary radiation exposure.
• Poor equipment performance makes
a significant contribution to the high
prevalence of poor image quality.
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Effect of poor quality images
 A poor quality image has three negative effects:
 If the image is not of adequate quality, practitioners
may not have all the possible diagnostic information
that could have been made available to them, and
this may lead to an incorrect diagnosis.
 If the quality of the radiograph is so poor that it
cannot be used, then the patient shall be exposed
again, causing an increase in the cost of diagnosis.
 Unnecessary radiation exposure also occurs in the
production of inadequate quality radiographs.
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Standards of acceptable image quality
• Prior to the initiation of a quality control program,
standards of acceptable image quality should be
established.
• Ideally these standards should be objective, for
example “acceptability limits for parameters that
characterize image quality”, but they may be
subjective for example “the opinions of
professional personnel” in cases where adequate
objective standards cannot be defined.
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Retake analysis
• The analysis of rejected images is a basic
component of the quality assurance program
• Those images judged to be of inadequate quality
are categorized according to cause of reject, which
may be related to the competence of the technical
personnel, to equipment problems or specific
difficulties associated with the examination, or
some combination of these elements
• Examples of the main causes of retake:
• Exposure faults (particularly important in mobile
radiographic equipment)
• Bad positioning
• Equipment malfunction
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How to start ? (I)
• Look for past experience in the
existing literature.
• Taking into account the personnel
and material available.
• Define priorities if it is not
possible to develop the full
program.
• Look for the usefulness of the
actions to be done.
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How to start ? (II)
• With the “basic” quality items (image
quality and patient dose).
• Use criteria to decide if the results of
the controls are good enough (eg.
comparison with guidance levels) or if
it is necessary to propose corrective
actions.
• Leave the more difficult items for a
second step!
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Basic advice !
• Any action (quality control,
corrective action, etc) should be
reported and documented, and:
• Should be performed within a
reasonable time.
• The reports should be understood
and known by radiologists and
radiographers.
• The cost of the proposed corrective
actions should be taken into
account (useless actions should be
avoided).
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Test objects for objective image
quality evaluation
Test for QC
of monitors
and laser
printers
Test for QC
of geometry
in
fluoroscopy
Test for QC
of
radiography
Test for QC in
mammography
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Clinical images and quality criteria for image
quality evaluation (I)
For a chest examination (P/A) projection:
• Performed at full inspiration (as
assessed by the position of the ribs
above the diaphragm - either 6
anteriorly or 10 posteriorly) and with
suspended respiration.
• Symmetrical reproduction of the thorax
as shown by central position of the
spinous process between the medial
ends of the clavicles.
• Medial border of the scapulae outside
the lung fields.
• Reproduction of the whole rib cage
above the diaphragm.
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Clinical images and quality criteria for image
quality evaluation (II)
EUR 16260. CEC 1996.
For a chest examination (cont’d):
• Visually sharp reproduction of the
vascular pattern in the whole lung,
particularly the peripheral vessels
• Visually sharp reproduction of :
a) the trachea and proximal bronchi,
b) the borders of the heart and aorta,
c) the diaphragm and lateral costophrenic angles
• Visualization of the retrocardiac lung
and the mediastinum
• Visualization of the spine through the
heart shadow
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Patient dosimetry
Dose indicators:
• Entrance dose for simple
examinations.
• Dose area product and total number
of images and fluoroscopy time for
complex procedures.
• For some complex interventional
procedures, maximum skin dose.
• For CT scanner, CTDI and the
number of slices (also Dose Length
Product DLP).
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Repeat Analysis
• Keep a record of repeated x-rays, and
understand WHY a repeat was necessary
• Use in continuing education of radiographers
• Especially important in digital imaging,
where repeats can easily by “hidden” or not
recorded
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Routine QC Testing
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Why QC?
• In general we want best possible image quality for
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least necessary radiation dose
Baseline testing of new equipment
Monitor equipment performance at regular
intervals, and to know when corrective action is
necessary
Check compliance with any regulatory
requirements
Proactive QC rather than reactive ad hoc testing
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Protocols and Guidelines
• Protocols
• need to be able to repeat the
measurement
• perform the same test the same way each
time
• need to be able to compare your results
with others
• sometimes specified in national or
international (eg. IEC) Standards
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Protocols and Guidelines
• Guidelines
• regulatory bodies may want to specify not
only how a test should be performed, but
what range of results is acceptable
• usually obtained from national or
international standards
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What is included?
• General
• Radiation safety – shielding, signage, protective clothing
• Equipment design features
• Operation indicators, exposure switch, control of multiple
tubes, filtration, markings
• Performance testing (excluding mammography)
• kVp, timer, HVL, linearity, AEC, leakage, collimation,
fluoroscopy parameters (resolution, dose rate, image
quality, collimation)
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Beam Half Value Layer (HVL)
• Possibly the most important test
• Checks whether there is sufficient filtration in
the x-ray beam to remove damaging low
energy radiation
• Need not only a radiation detector, but also
high purity (1100 grade) aluminium - most Al
has high levels of high atomic number
impurities eg. Cu
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Unfiltered X-Ray Spectrum (100 kVp)
Radiation
Intensity
Characteristic radiation
(related to target material)
Low energy
radiation,
damaging
to tissue
Bremsstahlung
radiation
kVp
Bremsstrahlung radiation
0
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Energy (keV)
Filtered (~3mm Al) Spectrum (100 kVp)
Radiation
Intensity
characteristic
radiation
Dose
saved
Bremsstrahlung
radiation
0
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100
Energy (keV)
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kVp Accuracy
• kVp should be +- 5% of set value
• Should be measured at acceptance of x-ray
unit, or
• After a tube change, generator maintenance
• As well as regularly
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Timer Accuracy
• Unlikely to be a problem in recent x-ray units
• Measure exposure time at commonly used
time settings
• Calculate error between set and actual time
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Linearity
• Checks that the radiation output per mAs
remains constant as the mA is varied
• Checks so-called kVp and mA
“compensation”, where the extra loads on a
HV generator at high mA are compensated
for – kVp must not fall
• If radiographers make manual exposures,
they should be confident of the result of an
exposure adjustment (less important when
AEC used)
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AEC
• AEC should routinely be used, and is fitted
to table and chest Bucky systems
• Usually 3 detectors, and operator can
chooses whatever combination is desired
Spine
Lungs
Remember that the centre chamber sensitivity is adjusted to
account for the increased density of the spine!
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AEC parameters
• Reproducibility
• Variation between chambers
• Minimum response time
• Exposure termination limit (backup timer)
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Resolution and the Focal Spot
Penumbra
More blur
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Appearance of image
Less blur
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Light Field/X-Ray Field Alignment
• Radiography equipment uses a light field to
show the radiographer where the x-ray field
will (hopefully) be
• The light field can come out of alignment
and must be checked
• Alignment should be with ± 1% of FFD
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GOOD Light Field/X-Ray Field
Alignment
Anode end marker
Coin shadow
X-ray Field
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POOR Light Field/X-Ray Field
Alignment
Anode end marker
Alignment error
Coin shadow
X-ray Field
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X-Ray tube housing leakage
• Tube housing has 2mm+ lead to prevent
excess leakage
• Can be damaged at tube change
• Limit is 1 mGy.hr-1 @ 1m from tube focus,
using maximum continuous rated tube
factors (kVp and mA)
• Measure using kVp, and exposure which will
not damage tube!
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Fluoroscopy QC
• Fluoroscopy equipment must have
normal tests for kVp, field alignment, and
HVL
• Image quality is tested with a “phantom”
• Also need to check maximum and typical
radiation dose rates to the patient
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Control Charts
• An essential tool for detecting changes in
performance
• A plot of a parameter over time, with
permissible limits
• Easy to see when a parameter is likely to
become unacceptable, before it actually
does so
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DR, CR and DF – Extra QC
• Routine QC interval will depend on system
– not less than annually
• Extra tests needed
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Dose Calibration
Low Contrast (contrast to noise ratio)
Uniformity
Artifacts
Spatial Linearity
• AEC will most likely need to be reset after
change from film
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Patient Dose in Digital Imaging
• Because of the very wide dynamic range of
digital detectors, DR/CR can reduce
radiation exposure
• DO NOT simply use the same exposure
parameters as for film/screen
• Because higher dose gives less image
noise, “exposure creep” is a real problem
• “A digital image without a little noise is a bad
image” – Joel Gray
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Special Requirements for CR QC
• In film screen systems the film is changed for
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every image
With CR the IP is read up to 10,000 times
Almost all plates suffer from wear artifacts
If you are suspicious about an artifact in a patient
image, take another exposure using the same
plate and no patient
Make sure there is a QC program to detect wear
before you see it clinically
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CR Plate Problems – Clinical Image
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CR Plate Problems (Fuji IP)
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Yellowing (oxidising)
of phosphor halides
Wear of plate
phosphor edge
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CR Plate Problems
Dust
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Scratches
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CR QC Recommendations
• Quality Control (QC) - perform monthly
• Inspection – cassette and IP
• Visual
• Radiographic
• CR Cassette cleaning
• CR IP cleaning
• Benefits
• Fewer image artifacts and repeated exposures
• Increased life cycle of cassettes, IPs, and readers
• Compliance with vendor warranties
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Calibration of Displays
• Software generates grey scale levels
• Photometer measures the luminance output
at each level and adjusts video card output
to obtain a perceptually linear gradation
between grey scale levels
• Calibrates display to DICOM standard grey
scale display function (GSDF)
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Film Processing
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Film Processing QC
• Why ?
• Sometimes the most crucial part of imaging
• “Most photo labs. have better processor QC
than X-Ray departments”
• How ?
• sensitometry (measurement of the film
response)
• densitometry (measurement of the film density)
• darkroom fog, film/screen contact, chemical
tests
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Film Processor QC
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Not this!!!
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Film Processor QC
• Most important QC features :
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proper film storage
cassette and screen care
processor chemical care
sensitometry
artifacts
processor cleanliness
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Film Processor QC - Film Storage
• Film should be stored in cool, dry conditions
< 26° C, 30-60% relative humidity
• Too low humidity allows static discharge
• Storage period must not be too long
• Stack film boxes vertically to avoid pressure
on films (causes pressure marks)
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Film Processor QC - Cassette and
Screen Care
• Clean screens regularly to avoid dust
shadows and scratches
• Use manufacturer’s recommended cleaning
solutions
• An ultraviolet light (“Black Light”) can show
up dust
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Film Processor QC - Processor
Chemical Care
• Chemicals (developer and fixer) degrade
with time and use
• Developer in particular will oxidize (go
brown) and cause poor, dirty films
• Fixer will change pH and lose emulsion
hardener
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Film Processor QC - Processor
Chemical Care
• Chemicals must be replaced or replenished
(continual automatic replacement) regularly
• Use manufacturer’s recommendations
• Check the developer temperature daily processing is very sensitive to temperature
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Sensitometry
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Sensitometer and densitometer required
Essential to keep the process under control
To be performed daily
Main parameters investigated:
• base + fog
• speed
• contrast
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Sensitometry (1)
• Use a sensitometer to expose a film to light
and insert the exposed side into the
processor first
• Before measuring the optical densities of the
step-wedge, a visual comparison can be
made with a reference strip to rule out a
procedure fault, like exposure with a different
colour of light or exposure of the base
instead of the emulsion side
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Sensitometry (2)
• From the characteristic curve (the graph of
measured optical density against the
exposure by light) the values of base and
fog, maximum density, speed and mean
gradient can be derived.
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Densitometer
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Sensitometric strip
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A method of exposing film by means of a
sensitometer and assessing the response
of film to exposure and development
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Characteristic curve of a radiographic
film
Optical
Density (OD)
D1
Saturation
Visually invaluable
range of densities
 = (D2 - D1) / (log E2 - log E1)
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D22
The  of a film : the gradient
of the straight line portion
Normal range of the characteristic curve
of exposures
Base
+ fog
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E11
E
E22
Log Exposure (mR)
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Film sensitometry parameters
• Base + fog: The optical density of a film due to its
base density plus any action of the developer on
the radiographically unexposed emulsion
• Sensitivity (speed): The reciprocal of the exposure
value needed to achieve a film net optical density
of 1.0
• Gamma (contrast): The gradient of the straight
line portion of the characteristic curve
• Latitude: Steepness of a characteristic curve,
determining the range of exposures that can be
transformed into a visually invaluable range of
optical densities
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Sensitometry
Limiting value : base + fog:  0.20 OD
contrast:
Mean Grad: 2.8 - 3.2
speed:
reference to
baseline value 10%
Frequency :
Daily
Equipment :
Sensitometer and
densitometer
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Film Processor QC - Artifacts
• Anything on the film which is not related to
the x-ray image
• Examples :
• dust marks, static discharge
• fixer stains (poor washing)
• film storage problems
• processor problems (roller marks, scratches)
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Film Processor QC - Processor
Cleanliness
• All processors will eventually get dirty
• Strip down and thoroughly clean processor
at least every 6 months
• Daily cleaning of entrance trays
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Darkroom light leakage (I)
• Remain in the darkroom for a minimum of
five minutes with all the lights, including the
safelights, turned off
• Ensure that adjacent rooms are fully
illuminated
• Inspect all those areas likely to be a source
of light leakage
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Darkroom light leakage (II)
• To measure the extra fog as a result of any
light leakage or other light sources, a preexposed film of about 1.2 OD is needed
• Always measure the optical density
differences in a line perpendicular to the
tube axis to avoid influence of the heel effect
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Darkroom light leakage (III)
• Open the cassette with pre-exposed film and
position the film (emulsion up) on the
(appropriate part of the) workbench
• Cover half the film and expose for four
minutes.
• Position the cover also perpendicular to the
heel effect to avoid the influence of this
inhomogeneity in the measurements
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Darkroom safelight (I)
• Perform a visual check that all safelights are
in good working order (filters not cracked)
• To measure the extra fog as a result of the
safelights, repeat the procedure for light
leakage but with the safelights on
• Make sure that the safelights were on for
more than 5 minutes to avoid start-up effects
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QC Summary
• QC is meant to help take good
radiographs
• The best radiographer in the world will
still take bad x-rays if the equipment is
not working properly
• More importantly, the patient will get
higher and unnecessary radiation
doses, as will the staff
• You will also waste money on x-ray film
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