Transcript QC in a Digital World - Diagnostic Accreditation Program

QC in a Digital World
John Aldrich PhD FCCPM
Department of Radiology
Vancouver Coastal Health
University of British Columbia
Digital Imaging
Any sufficiently advanced
technology is indistinguishable
from magic… Arthur C Clarke 1961
Overview
 New paradigms
 Standards
 Image acquisition systems
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Radiography DR, CR
Fluoroscopy, Angiography DF
CT
US
 PACS
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
Quality Control (QC)
 Acceptance testing
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First constancy testing
New equipment
Conformance to
manufacture’s specs/criteria
Baseline value determination
Clinical use period
 Routine performance
evaluations
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Data evaluation
Specific tests performed at
regular intervals
 Consistency checks
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Next constancy testing
Evaluate malfunctioning or
out-of-spec equipment
Within the
PASS
established criteria
FAIL
Remedy
Digital System QC
Film
Developed
And
Fixed
Detector
Reading
Digital
Processing
Stored
PACS
Viewed
Display
QC of the digital systems is an additional requirement
– in addition to the usual x-ray performance tests
Health Canada - Quality Control
 Safety Code 20A (19812000)
 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
Digital X-ray Systems
 Direct Radiography DR
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Formation of image without a secondary read-out device
 Computed Radiography CR
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Use of storage phosphor plate usually in a cassette-based
system
 Digital Fluoroscopy/Angiography DF
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Image intensifier/video system replaced by digital plate.
 Computed Tomography CT
 Ultrasound US
DR, CR and DF – Extra QC
 Routine QC interval will depend on system
– not less than annually
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Dose Calibration
Spatial Resolution
Low Contrast
Uniformity
Artifacts
Spatial Linearity
Dose Calibration
 Each system should be calibrated
according to the manufacturers protocol,
as they are all slightly different
 General set-up
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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
Image Quality
 All CR and some DR/DF manufacturers have
special Image Quality phantoms and
automatic software to analyze image quality
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
Digital Radiography QC
 Many DR systems require more frequent
calibration of the uniformity eg every month
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Flat field measurement (uniform copper plate)
Uniformity correction
 Noise
 Artifacts
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Contrast-detail and resolution phantom
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
Observations
Observations
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
Observations
 Yellowing of
phosphor
 Virox
Observations
Dust
Scratches
CR QC Recommendations
 Quality Control (QC) - perform monthly
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Inspection – cassette and IP
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Visual
Radiographic
CR Cassette cleaning
CR IP cleaning
Benefits
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Fewer image artifacts and repeated exposures
Increased life cycle of cassettes, IPs, and readers
Compliance with vendor warranties
Consistency Checks
 Weekly/daily
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Simple phantom to test reproducibility
To use if there seems to be a problem
Vancouver Phantom
 This phantom we have
developed for routine
constancy QC of
digital systems
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Field collimation
Standard operating
conditions
Resolution
Contrast
Low contrast
circles
High contrast
mesh
Orthopaedic Measurements
QC in CT - Daily
 In-air calibration of scanner every 24 hours
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Adjusts sensitivity of all detectors
Important to do this – build into schedule.
QC Frequency
Function
Mechanical
Test
Scan plane congruence
Couch movement
Collimated beam width
Weekly
Annually
X
X
X
Image Quality
Noise and uniformity
CT number
Spatial resolution
Imaged slice thickness
Image linearity
X
X
X
X
X
X
X
Dose
CTDI100 in air
X
QC Phantoms
 ACR CT Accreditation Phantom
(RMI)
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Alignment, noise, uniformity, CT
number, resolution, MTF, low contrast,
image slice width
 Scanner QC phantoms
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GE: noise, uniformity, resolution, MTF,
low contrast
Siemens: noise, uniformity, MTF
Ultrasound QC - Phantom
US Probe Test Report
Cracked/Dead
Elements
Ultrasound QC – Clinical
 6 dead elements – right image
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Slight shadowing in the middle of the image
Discernable loss of signal amplitude
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
Radiology Workstation Contrast
350.00
Darkened Room
Lighting
300.00
Bright Room Lighting
250.00
Contrast Ratio
AAPM
200.00
150.00
100.00
50.00
0.00
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Unit
Aldrich JE et al. J Digital Imaging 2005;18:287-295
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
181
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.
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:
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Operating Rooms
Emergency Rooms
3D Processing workstations
(Offices, wards, home)
The Imaging Chain
Detector
Reading
Digital
Processing
Stored
PACS
Viewed
Display
 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
The Future
The only perfect science
is hindsight