Topics covered in this article include:
- Measurement Summary
- Available Tests
- Detailed Discussion of Measurements
- Example File
The Doselab kV 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.
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:
- Normalized MTF Plot
- MTF Critical Frequencies
- MTF Plot Data
- Contrast, Noise, and Uniformity
- Position Offset from Center
- Center Pixel
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 a DICOM image. It is necessary to use the file naming convention or to select "Planar kV Imaging (Doselab kV) in the Type field in the upload widget to identify the phantom. If using the filename to identify the image, use "dlkv" somewhere in the file name. Below is an example of selecting the image type in the upload widget:
The test requires at least one image and it must be a DICOM file. The files must contain "dlkv" 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. 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.
|Monthly Imaging QA [TG142 Table VI]
|Planar kV Imaging (Doselab kV)
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 at isocenter, centered in the field.
- Ensure that the phantom edge is at least 60 pixels from the edge of the image. While we may be able to process the image with a smaller gap, such as 30 pixels, it may not work reliably.
- 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. If all of the test objects are seen, including the 4 numbered circles, we may be able to process a partial phantom image but it is not ideal.
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.
Detailed Discussion of Measurements
Contrast, Noise, and Contrast to Noise Ratio (CNR) are calculated as shown below:
Area refers to the areas marked in the picture below:
Below is an example of the report generated:
Below is an example file for use in testing: