Topics covered in this article include:
- Introduction
- Measurement Summary
- Instructions
- Available Tests
- Detailed Discussion of Measurements
- Example File
Introduction
This phantom measures aspects of image quality: contrast scale, high contrast spatial resolution, low contrast detectability, noise and uniformity.
The QA phantom contains three sections, each corresponding to a single scan plane: Section 1: Resolution block S0mm scan location, Section 2: Contrast membrane S40mm scan location, and Section 3: Water bath S60mm scan location.
Measurement Summary
The analysis provides the following results:
- Contrast Scale
- Contrast
- Contrast Detail
- Uniformity
- Bar Pattern Standard Deviation
- Bar Pattern Standard Deviation
- Bar Pattern Standard Deviation Plot
-
Scaling
-
Scaling Profile Plot
The measurements are completely automated, requiring the user only to drag and drop the image set into the web-based software interface. A detailed report is created.
Instructions
The test requires at least one image and it must be a DICOM file. The files must contain "ct_ge" somewhere in the file name or that "GE CT QA Phantom" is selected in the Type drop down as shown below:
When imaging QA tests are added to templates an upload control will appear in the scheduled QA's data entry screen allowing the user to upload images for automated analysis.
To add files to the upload queue simply drag them from a Windows Explorer folder to the drag and drop folder and release them. Alternatively, by clicking on the Add Files button to the lower right of the control a windows file selection dialog will open and files can be selected for upload. Under either method, multiple files may be selected for upload at once.
If the automatically upload checkbox is checked (the default) then file uploading will start immediately as files are added.
If the automatically upload button is turned to off the file upload process must be started manually clicking the Start upload button on the lower right of the control. To clear the upload queue click the Clear button.
Once file series have been uploaded they will be displayed below the upload control. To remove a series from the queue click the Cancel button beside the series. To start processing click the Start Processing button. A description for the image series can be added at this point. Click the Edit button next to the series. Type a description for the series into the text box that appears below Description and either click Save or press the enter key. The description can also be edited after the images have been processed. Descriptions will appear in the report with the analysis of the series.
While files are being processed users may perform other tasks such as data entry.
Available Tests
The following tables show the tests to select in the template builder corresponding to the supported analyses.
Analysis | Template Section | Subsection |
GE CT Quality Assurance Phantom |
Contrast |
|
---|---|---|
Uniformity and Noise |
|
|
Resolution |
|
|
GE CT Scaling |
|
Image Acquisition Suggestions
Reference the GE Technical Reference Manual for phantom placement and scan acquisition parameters. The tolerance suggestions in the measurement discussion match the tolerances in the GE technical Reference Manual.
Detailed Discussion of Measurements
Contrast Scale
The Contrast Scale is determined by subtracting the mean HU value of a water region from the mean HU value of a plastic region. The expected value is 120 HU ± 12 HU. The regions selected are shown below:
Contrast
Contrast is determined by taking the difference between the plastic area and water area.
Contrast Top = Area A Mean - Area B Mean
Contrast Bottom = Area C Mean - Area D Mean
Contrast Detail
The following definitions and calculations are used in the contrast detail analysis:
background = mean pixel value of the area around each hole
meanHole= mean pixel value in the detected hole
contrast = (meanHole- background) / (meanHole+ background) * 100;
sdHole standard deviation of the hole pixels
sdBg: standard deviation of the background pixels
CNR = contrast/noise;
constantContrast = contrast * holeDiameter
constantCNR = CNR * holeDiameter
Uniformity and Noise
Uniformity is determined by the mean pixel value of the given 15 mm ROI. The uniformity difference between the center ROI and the average of the edge ROIs should be 0 ± 3.
Noise is the standard deviation of the mean pixel value of the central 25 mm ROI. Noise should equal 3.2 ± 0.3,
Resolution
An ROI is fitted to each bar pattern and the standard deviation across the region is reported for each bar pattern. Only the largest 5 bar patterns are used.
Scaling
The phantom diameter is determined by measuring the width of the the vertical and horizontal profiles that cross the center of the phantom as shown by the red line in the images below:
Sample Report
Below is an example of the report:
Example File
Below is an example image set for use in testing:
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