QC in a Digital World

John Aldrich PhD FCCPMDepartment of RadiologyVancouver 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

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

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)

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

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

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

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

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

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

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 Flat field measurement (uniform copper plate)

Uniformity correctionNoiseArtifacts

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 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

Consistency Checks

Weekly/daily 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 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 Adjusts sensitivity of all detectors Important to do this – build into schedule.

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

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

Ultrasound QC - Phantom

US Probe Test Report

Cracked/Dead Elements

Ultrasound QC – Clinical

6 dead elements – right image 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

Aldrich JE et al. J Digital Imaging 2005;18:287-295

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Darkened Room Lighting

Bright Room Lighting

AAPM

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.

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)

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

The Future

The only perfect science is hindsight