qc in a digital world john aldrich phd fccpm department of radiology vancouver coastal health...
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QC in a Digital World
John Aldrich PhD FCCPMDepartment of RadiologyVancouver Coastal Health
University of British Columbia
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Digital Imaging
Any sufficiently advanced technology is indistinguishable from magic… Arthur C Clarke 1961
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Overview
New paradigms Standards Image acquisition systems
Radiography DR, CR Fluoroscopy, Angiography DF CT US
PACS
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New Paradigm
In electronic imaging the functional parts of conventional radiology have been separated:
Image Capture Image Storage Image Display
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Imaging QC Principles
Proactive QC rather than Reactive QC
Test tool/phantom Standard imaging
parameters/conditions Scheduled testing (Daily/Weekly)
Defined and objective acceptance/rejection criteria
Patient replaces the phantom
Non-standard imaging parameters/conditions
Frequent testing (every patient)
Ill-defined and subjective acceptance/rejection criteria
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Quality Control (QC)
Baseline value determination
Clinical use period
Next constancy testing
Data evaluation
Within the
established criteria
Remedy
First constancy testing
FAIL
PASS
Acceptance testing New equipment Conformance to
manufacture’s specs/criteria Routine performance
evaluations Specific tests performed at
regular intervals Consistency checks
Evaluate malfunctioning or out-of-spec equipment
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Digital System QC
FilmDeveloped
AndFixed
Detector ReadingViewedDisplay
DigitalProcessing
StoredPACS
QC of the digital systems is an additional requirement – in addition to the usual x-ray performance tests
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Health Canada - Quality Control
Safety Code 20A (1981-2000)
Recommended safety procedures for the installation, use and control of x-ray equipment. Mainly concerned with the x-ray output parameters of the equipment
Only film processor QC defined
Safety Code XX (due 2008)
Recommended safety procedures for the installation, use and control of x-ray equipment. Mainly concerned with the x-ray output parameters of the equipment
25% of the Code is concerned with QC of the digital imaging detector systems
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Digital X-ray Systems
Direct Radiography DR Formation of image without a secondary read-out device
Computed Radiography CR Use of storage phosphor plate usually in a cassette-based
system Digital Fluoroscopy/Angiography DF
Image intensifier/video system replaced by digital plate. Computed Tomography CT Ultrasound US
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DR, CR and DF – Extra QC
Routine QC interval will depend on system – not less than annually
Dose Calibration Spatial Resolution Low Contrast Uniformity Artifacts Spatial Linearity
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Dose Calibration
Each system should be calibrated according to the manufacturers protocol, as they are all slightly different
General set-up Arrange for defined dose at surface of cassette at
80 kVp Expose and read image Record Exposure Index
The image can also be used to check for uniformity, linearity and artifacts
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Image Quality
All CR and some DR/DF manufacturers have special Image Quality phantoms and automatic software to analyze image quality
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Resolution and Contrast
Any high contrast resolution phantom can be used to provide comparative information
Low contrast resolution is one of the most difficult parameters to measure
There are several phantoms and measurement is subjective, so consistent technique is essential
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Digital Radiography QC
Many DR systems require more frequent calibration of the uniformity eg every month Flat field measurement (uniform copper plate)
Uniformity correctionNoiseArtifacts
Contrast-detail and resolution phantom
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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 take an image
using the same plate and no patient Make sure there is a QC program to detect wear
before you detect it clinically
Hammerstrom et al J Digital Imaging 2006 19:226
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Observations
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Observations
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Observations
Sharp particulates embedded in the felt lining under a plastic clip etched phosphor surface to create density on radiograph
Not enough pressure beside plastic clip to cause 2nd wear mark to effect radiograph
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Observations
Yellowing of phosphor
Virox
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Observations
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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Consistency Checks
Weekly/daily Simple phantom to test reproducibility To use if there seems to be a problem
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Vancouver Phantom
This phantom we have developed for routine constancy QC of digital systems Field collimation Standard operating
conditions Resolution Contrast
Low contrast circles
High contrast mesh
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Orthopaedic Measurements
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QC in CT - Daily
In-air calibration of scanner every 24 hours Adjusts sensitivity of all detectors Important to do this – build into schedule.
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QC Frequency
Function Test Weekly Annually Mechanical Scan plane congruence X Couch movement X Collimated beam width X Image Quality Noise and uniformity X X CT number X X Spatial resolution X Imaged slice thickness X Image linearity X Dose CTDI100 in air X
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QC Phantoms
ACR CT Accreditation Phantom (RMI) Alignment, noise, uniformity, CT
number, resolution, MTF, low contrast, image slice width
Scanner QC phantoms GE: noise, uniformity, resolution, MTF,
low contrast Siemens: noise, uniformity, MTF
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Ultrasound QC - Phantom
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US Probe Test Report
Cracked/Dead Elements
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Ultrasound QC – Clinical
6 dead elements – right image Slight shadowing in the middle of the image Discernable loss of signal amplitude
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Optimization of Displays
Clean the surface of the display With the display OFF look at
reflections on the surface of the display such as lamps, windows, white coats and name tags. Reduce these artifacts as much as possible
Display the SMPTE test pattern Ensure you can see the 5% and
95% grey scales
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Radiology Workstation Contrast
Aldrich JE et al. J Digital Imaging 2005;18:287-295
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Darkened Room Lighting
Bright Room Lighting
AAPM
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Calibration of Displays
Software generates grayscale levels Photometer measures the luminance output
at each level and adjusts video card output to obtain a perceptually linear gradation between grayscale levels
Calibrates display to DICOM standard
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Primary PACS Displays
Primary reporting workstations should be used in custom-built reporting areas with low reflecting surfaces, ergonomically-designed chairs, recessed pot lighting with dimmer controls and climate control.
Our primary reporting stations are calibrated for luminance and contrast ratio every three months.
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Secondary PACS Displays
In contrast, the secondary displays are used under a range of conditions, often with the possibility of distracting reflections and high ambient lighting.
The secondary displays are checked normally only on installation
Calibration factors can often be changed by the user.
Location: Operating Rooms Emergency Rooms 3D Processing workstations (Offices, wards, home)
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The Imaging Chain
Detector ReadingViewedDisplay
DigitalProcessing
StoredPACS
Image are used to follow disease processes so it important that the whole digital chain is linear
Linearity should be checked after changes to software/hardware in any component
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The Future
The only perfect science is hindsight