Quality Assurance and Maintenance Issues

Why is quality assurance so important?

• the weak links in the digital imaging chain are difference than that of film/screen

• unlike film/screen systems that break down suddenly, problems with the digital imaging chain usually occur subtly and over long periods of time

What are the three main types of acceptance testing carried out?

• visual check

• phantom image test

• image erasure test

What components of the digital imaging system require routine evaluation?

• monitor

• technologist workstation

• PACS workstation

• photostimulable phosphor plate (PSP)

• photostimulable phosphor plate reading device

• flat panel receptor

Daily tests include?

• visual inspection

• plate reader inspection

• erasure of suspect PSP plates

• laser printer QC

• monitor QC

Monthly tests include?

• erase all PSP plates

• phantom image test

• calibrate monitor

Quarterly tests include?

• cleaning and inspecting all PSP plates

• phantom image test

• determine repeat rate and causes

What all does the phantom image test consist of?

• resolution

• contrast

• noise

• laser jitter

• exposure indicator

Annual tests include?

• repeat acceptance tests

• track changes

• review service reports

• review patient exposure records

• suggest improvements

What is done for ongoing quality control testing of PSP image receptors?

All readers must be calibrated, in the same way, to accurately determine the exposure indicator calibration

Dark noise evaluation

• noise that is present on an image receptor before it has been exposed

• excessive dark noise degrades images

If there is failure to demonstrate a uniformly bright image what does it mean?

There may be a problem with the PSP plate or the processor

What % variation is allowed in pixel value for image brightness uniformity?

10%

What does the linearity and slope test assess?

Pixel values

What does the linearity and slope test involve?

• pixel values are plotted and then compared to the level of exposure

• this will determine whether or not they demonstrate a linear relationship of increasing pixel density with increasing exposure levels

• this is done using a PACS or other device to plot the exposure level and relative change in pixel density on an X and Y graph

How do you test laser beam function?

• radiograph a metal ruler

• process it though the PSP reader to determine if its edges, of the displayed image, are straight and continuous

• if it isn’t tracking straight across the plate you will see poor image quality caused by laser jitter

How do you test for high contrast resolution?

• use a resolution grid that measures line pairs per millimeter to assess the high contrast resolution capability of PSP systems

• looking at the ability to see small lines, or spatial resolution, at a high contrast level

What happens if PSP plates are not thoroughly erased?

Noise is added to the next image

How do you test for plate erasure thoroughness?

• a plate exposed with a lead block in the center, is exposed

• after processing the image, it should show complete uniformity in its display

• indicates that the erasure was effective

Noise and low contrast resolution can be assessed by evaluating exposure of a device called the?

Leeds TO.12 Low Contrast Resolution Phantom

How do you test for distortion?

• using a wire mesh screen

• the uniformity of grid spacing is measured to determine whether the capture process has distorted the image

When does anti-aliasing occur?

When the laser sampling frequency of a PSP plate and the anti-scatter grid frequency match

If the laser scan runs parallel to the grid lines, what occurs?

Moire artifact

What does the positioning and collimation errors test look for?

Determines uniformity of an image when combining different positions to create and image

Daily monitor test

Performed using the Society of Motion Picture and Television Engineers (SMPTE) test pattern or the TG18QC test pattern

What is an example of a visual inspection done for plate maintenance?

Looking for damage (ex: cracked or broken hinges)

How often should PSP plates be cleaned?

• every three months

• or when an artifact is seen on a plate

What do you do with a cracked or broken PSP plate?

Dispose of it properly

How often should PSP plates be erased of they have not been used?

Every 48 hours

Why should you erase PSP plates even if they have not been used?

Will help reduce fog and background radiation to its lowest possible levels

Where can information on designing a comprehensive quality control program for your facility be found?

AAPM Report No. 93

What are repeat radiographs also known as?

Reject, retake, or double-dose studies