Topics covered in this article include:
- Measurement Summary
- Available Tests
- Image Acquisition Suggestions
- Detailed Discussion of Measurements
- Example File
The QCkV-1 is a common phantom designed to be used weekly and/or monthly to provide an ongoing check of imaging performance, particularly those aspects which are most liable to deterioration. After an initial grey-scale check, image quality is measured simply by counting the number of details detected and the number of bar patterns resolved in the image. An ongoing record of these numbers will reveal any trend towards deterioration in imaging performance.
Task Group 142 (TG-142) of the American Association of Physicists in Medicine (AAPM) recommends that Planar kV Imaging should be checked during monthly Quality Assurance (QA) in Table VI: Imaging.
The analysis provides the following results:
- Spatial Resolution
- Uniformity and Noise
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.
The test requires two images and must be DICOM files. If more than two images are uploaded at once, each set must have matching Series Instance UIDs (tag 0020,000E). In some cases, it may be necessary to assist the automatic image recognition with manual naming conventions. If your images are not recognized automatically for the following types please try adding "qckv" somewhere in the file name. Alternatively, limited capabilities to manually identify planar images using DICOM tag values have been added to the image processing system. This is an extension of the existing naming convention system. The PatientID, StudyID, and SeriesDescription DICOM tags are checked and if the text "qckv" (case insensitive) is found the image set will be processed as a Planar test kV image. For more details see Manual Identification of RT Planar Images and Individual Catphan Slices through DICOM tags.
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.
The following tables show the tests to select in the template builder corresponding to the supported analyses.
|Planar kV Imaging||Monthly Imaging QA [TG142 Table VI]||Planar kV Imaging (QCkV-1)|
Image Acquisition Suggestions
Below are general guidelines on image acquisition. The exact method will depend on the site equipment and configuration.
- Motion enable the kV arms and gantry, then move them into position.
- Remove any the filters (e.g. half fan) from the kV source.
- Place the phantom on the kV detector, ensuring the phantom is right side up with the area marked 1 at the top of the image.
- Ensure the phantom is reasonably centered in the field of view at a 45% angle on the kV detector. Some C-series Varian LINACs have a set of red lasers on the kV source that may be used to align the phantom. TrueBeams do not have this feature but the green wall lasers may be used to extrapolate the positioning.
- Adjust the collimation so that the full phantom is exposed and the entire image outside the phantom is open field. While image processing is robust, excessive artifacts in the image and around the edges can affect the ability to process the image.
- In the imaging software, select the kV acquisition parameters. The parameters below are a good starting point but may need to be adjusted for the site's environment:
- SID = 150 cm
- 50 kVp
- 10 mAs (250 mA, 50 ms)
- In the imaging software, capture the images and export for use.
- Repeat the above steps to acquire a second image.
Detailed Discussion of Measurements
Below is an example of the report generated from the planar kV imaging test:
The Uniformity and Noise value is determined as follows:
- The analysis finds the six areas shown in yellow in the image above on both images.
- The corresponding areas are subtracted between the two images.
- The standard deviation is calculated for each area difference (so 6 numbers at this point)
- The median of the 6 standard deviations is calculated.
- This median value is divided by the square root of 2 and is reported as the uniformity and noise measurement.
Below is an example file for use in testing: