IAEAInternational Atomic Energy Agency

RTC on RADIATION PROTECTION OF PATIENTS FOR RADIOGRAPHERSAccra, Ghana, July 2011

QA Programmes in Diagnostic Radiology

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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

influoroscopy

Test for QC of

radiography

Test for QC in mammography

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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.

Clinical images and quality criteria for image quality evaluation (I)

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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 costo-

phrenic angles• Visualization of the retrocardiac lung

and the mediastinum• Visualization of the spine through the

heart shadow

EUR 16260. CEC 1996.

Clinical images and quality criteria for image quality evaluation (II)

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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 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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0 20 40 60 80 100 120

Energy (keV)

kVp

RadiationIntensity

Unfiltered X-Ray Spectrum (100 kVp)

Bremsstahlungradiation

Characteristic radiation (related to target material)Low energy

radiation,damaging to tissue

Bremsstrahlung radiation

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0 20 40 60 80 100 120

Energy (keV)

characteristicradiation

RadiationIntensity

Filtered (~3mm Al) Spectrum (100 kVp)

Bremsstrahlungradiation

Dose saved

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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

Appearance of imageMore blur 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

Coin shadows

X-ray Field

Anode end marker

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POOR Light Field/X-Ray Field Alignment

Coin shadows

X-ray Field

Anode end marker

Alignment error

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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• 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 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

Hammerstrom et al J Digital Imaging 2006 19:226

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CR Plate Problems – Clinical Image

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CR Plate Problems (Fuji IP)

• Yellowing (oxidising) of phosphor halides

• Wear of plate phosphor edge

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CR Plate Problems

Dust 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 :• 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

• 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

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

A method of exposing film by means of asensitometer and assessing the response

of film to exposure and development

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Characteristic curve of a radiographic film

Optical Density (OD)

Visually invaluablerange of densities

Base + fog

Saturation

Log Exposure (mR)

Normal range of exposures

The of a film : the gradient of the straight line portion of the characteristic curve

D1

D2

E1 E2

= (D2 - D1) / (log E2 - log E1)

E1 E2

D2

E1 E2

D1

D2

E1 E2

Saturation

= (D2 - D1) / (log E2 - log E1)

Saturation

= (D2 - D1) / (log E2 - log E1)

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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 - 3.2speed: 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 pre-exposed 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