Introduction
The EPID installed at the LINAC can be used for patient positioning, pretreatment patient QA and Dose monitoring. The tests of the EPID Plugin are based on the recommendations of AAPM TG 142[1] and DIN 6847-6[2].
Imaging QA is generally performed by capturing an image through a specially designed phantom. The EPID QA Plugin is designed to analyze MV and kV phantoms for flat panel imager QA.
Table 5.1. The EPID QA Plugin test types and test parameters
Test result type | Test result parameter [unit] |
Scaling | scaling discrepancy [mm] |
Spatial resolution | spatial resolution [lp/mm] |
contrast | contrast [%] and CNR |
Uniformity and Noise | Minimum uniformity [%] |
The software provides Individual tests for each test type and a combined test for all test result parameters. Supported file formats: DICOM RT Image and tiff image.
EPID Test Setup and ROIs
The nominal setup for EPID measurements is illustrated below. The phantom is placed at isocenter (SAD) on the patient couch. The imaging device, EPID, has a fixed distance to the source (SID, Source to Imager Distance). Values for SID and SAD are extracted from the image files via the DICOM tags RTImage SID (3002| 0026) and Radiation Machine SAD (3002|0022), respectively. If necessary, these values can be changed by the user. The length scales of the image are projected to isocenter using the projection factor:
proj_fac = SID/SAD
Therefore, SID entered for calculation must be the same as the one used in measurements.
Illustration of principle setup of an EPID measurement with a phantom placed on the table.
The figure below shows an image of a supported phantom, the QC-3 phantom from Standard Imaging. The length scale in mm is projected to isocenter and the displayed size corresponds to the actual dimensions of the phantom. The defined ROIs for the different tests are indicated and numbered on the image. The user can define ROIs for the different tests. The pre-defined ROIs are provided for the supported phantoms.
Image of the QC-3 phantom from Standard Imaging
Workflows for EPID QA Plugin Tests
Note: For detailed information on the myQA - Imaging QA – TestSetup/TestRun EPID window, see Section 5.2, myQA Machines Reference Guide.
Setting up Baselines in Test Setup
Pre-conditions
The phantom is aligned to the center of the image. The MV or kV image has been acquired.
Note: The baselines and settings defined in Test Setup will be used for every execution in Test Run. However, they cannot be changed in Test Run.
Procedure
- Click Edit in the Test Setup ribbon to enter the Edit mode.
- Select the desired machine in the Machines & Templates panel.
- In the Protocol Tree panel, click New in the ribbon to create a Task.
- With the task is selected, click New in the ribbon to create tests.
- In the New Test Definition dialog (see next page), enter the required name of the test and optionally a description.
- Select Category (1) (Planar MV (EPID), for information display only).
- Select Result Type (2) (e.g., EPID (MV)/Planar (kV): Combined or Contest). Implementation will be aumatically selected according to Result Type.
- Define Energy (3): select the MV round button and enter the value of the MV.
When Undefined is selected, the exact value of energy cannot be entered.
Note: Only tests with the same defined energy or those with Undefined selected can be shared. Migrated tests from a previous version will have an undefined energy by default. A manual adjustment in the Test Setup to match the actually used energy type and value are advisedNew Test Definition dialog
- Click OK (4). The test is listed in the Protocol Tree and the Test Definition panel is displayed.
Define the test baseline
Test Setup workspace – Test definition panel
- In the Test Definition panel, click Open Module (5). The myQA – Imaging QA – TestSetup window
opens. - In the myQA – Imaging QA – TestSetup window,
- Machine: Displays the machine selected in the beginning of this procedure.
- SID and SAD: Values are extracted from the imported file.
- Phantom model: a predefined phantom can be loaded from the dropdown table.
- Phantom image(s): For importing the reference image. It will be displayed in the Images view.
- There are 3 modes to import the DOCOM data:
Import file: click it, browse, and select the *dcm file and then click Open.
Import Listener and Import Query: See Section 4.1.1, myQA Machines Reference GuidemyQA - Imaging QA - Test Setup CBCT window – the Equipment setup and Images sections
Inspect the predefined ROIs on the reference image. If necessary, modify them.
- If desired, the Images view can be displayed in full window by clicking the icon and the user can zoom the image furthermore.
- The ROIs on the test image on the right can be resized and moved by dragging-and-dropping.
- Use the tools on the Tools panel to change the brightness and contrast of the images if necessary. Changes of the brightness and contrast have no influence on the calculation of the test results.
- The image can be displayed in several options of color palettes by selecting one in the Color Palette dropdown menu.
- The ROIs can be edited using the Edit button in the Regions of Interest panel.
Images view in full view
- After checking the ROIs, click the Process button in the Baseline values section. The Expected and
tolerance (Warn and Fail) values will be filled. Edit the tolerances if necessary. They can be edited by changing the values in the corresponding boxes. - Click OK, the reference image and configuration files are listed in the Test Definition panel.
The current configuration files will be deleted by clicking Reset configuration or Open Module.
- Repeat the above steps until all baselines for other EPID QA tests are created.
- Click Set Active in the ribbon if you want to perform the tests.
- Click Save in the ribbon.
Note: In this procedure, you can also create a test template first, and then copy the template into
the machine protocol.
Executing EPID Tests in Test Run
Pre-conditions
The protocol is set to active in Test Setup.
Procedure
- In the Test Run workspace, select the corresponding protocol and task in the Agenda panel
- In the content area, select a test.
Note the test results of the four EPID tests can be shared with each other. Checkmark the Share Results box. By starting one test, all of four EPID tests will be performed.Test Run workspace – an EPID task is selected (before tests are executed)
For a combined test, the Share Results box does not appear since it is obsolete. - Click Start. The myQA – Imaging QA – TestRun EPID window opens.
In the Equipment setup section, parameters (machine, SAD, SID, phantom model, MTF and scaling discrepancy distance), the reference image, expected values and tolerances are loaded from the baseline setup and displayed; none of them can be modified. - In the Phantom image(s) combo box, import the test image file. It will be displayed in the Images view.
There are 3 modes to import the DOCOM data:Import file: click it, browse, and select the *dcm file and then click Open.
Import Listener and Import Query: See Section 4.1.1, myQA Machines Reference Guide.myQA – Imagine QA – TestRun EPID window
Inspect the predefined ROIs on the test image. If necessary, modify them.- If desired, the Image view can be displayed in full window by clicking the icon and the user can zoom the image furthermore.
- The ROIs on the test image on the right can be resized and moved by drag-and-dropping
- Use the tools on the Tools panel to change the brightness and contrast of the images if necessary; ROIs can be edited using the Edit button in the Regions of Interest panel.
myQA - Imaging QA - TestRun EPID window
- Click Process; the test parameters of the test image are determined and compared to the specified
tolerances. The result of the test (pass, fail, warning) will be set accordingly.Test results (Example) - Baseline values and MTF chart sections. In this example, more EPID tests
are defined and Share results is enabled, all the tests were run automatically and the results were displayed. - Click Ok to close the myQA – Imagine QA – TestRun EPID window.
Test Run workspace – an EPID task is selected (after tests are executed)
- Click Finish in the ribbon and then finish in the Finish Task dialog. Afterwards, the test results will
appear in myQA Cockpit and a report can be created in the Test Repository page. Test Repository page with executed test list
References
[1] F. Klein, "Task Group 142 report: Quality assurance of medical accelerators," Med. Phys., vol. 36, p. 4197, 2009.
[2] DIN, “Elektronische Bildempfänger (EPID) – Konstanzprüfung“, DIN 6847-6, 2012.
[3] DIN, "Leistungsmerkmale zur Bildgebung von Röntgeneinrichtungen für die Computertomographie," DIN EN 61223-2-6, 2008.
[4] D. Dance, "Diagnostic Radiology Physics," IAEA, 2014.
[5] H. de las Heras-Gala, "Quality Control in Cone Beam Computed Tomography EFOMP-ESTRO-IAEA
Protocol," http://dx.medra.org/10.19285/CBCTEFOMP.V1.0.2017.06, 2017.