Topics covered in this article include:

• Introduction
• Measurement Summary
• Instructions
• Available Tests
• Image Acquisition Suggestions
• Detailed Discussion of Measurements
• Example File

Introduction

This test is used to determine scaling discrepancies on the MV imager by comparing a known distance between BB's in the FC-2 phantom against the measured distance in a DICOM image.

Task Group 142 (TG-142) of the American Association of Physicists in Medicine (AAPM) recommends that MV scaling should be checked during monthly Quality Assurance (QA) in Table VI. Imaging.

Measurement Summary

The analysis provides the following results:

• MV Imager Scaling Discrepancy
• BB Location Detail 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

MV scaling images must be DICOM files. In some cases, it may be necessary to assist the automatic image recognition with manual naming conventions. If the images are not recognized automatically please try adding radlight somewhere in the file name or selecting the "Radiation Light Field Coincidence (FC-2)" in the Type drop down in the upload widget. 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 radlight (case insensitive) is found the image set will be processed accordingly. 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.

Available Tests

Image Acquisition Suggestions

Below are the recommended steps to acquire the radiation/light field coincidence images:

1. Move the couch to 100 cm SSD.
2. Set the jaws or MLC so that the field size is at least 15x15cm.
3. Add the center marker plate if desired and ensure the central BB is within 2.5 cm of the image center.
4. Acquire the image.

Optionally you may want to capture another image with the current setup using the kV imager. For more details, please see the kV Scaling (FC-2) help article.

Detailed Discussion of Measurements

The scaling discrepancy is reported as a percentage and calculated as follows:

1. The first step is to find the pixel size and account for any magnification due to the placement of the imager by dividing the SAD by the SID. We refer to this as the pxISO as dividing the SAD by SID gives the pixel size projected to the isocenter. pxISO = pixel size (SAD / SID). The following is pulled from the DICOM tags:
   • • pixel size in mm is pulled from the 3002,0011 Image Plane Pixel Spacing tag
     • SAD is pulled from the3002,0022 Radiation Machine SAD tag
     • SID is pulled from the 3002,0026 RT Image SID tag
2. Find the BB pixel locations and calculate the length in pixels between the BB locations along each side and multiplying by pxISO to produce the length of the sides in millimeters.
3. Calculate the average side distance, referred to as measuredSideMM, by taking the mean of the four sides. 
4. Determine the physical BB spacing, referred to as bbDistance. For a 15x15 cm field the BB spacing is 130 mm and for a 10x10 cm field the BB spacing is 80 mm.
5. To calculate the percentage difference between the physical BB distance and the measured distance in the image, referred to as percentScalingErr, percentScalingErr = | bbDistance - measuredSideMM | / bbDistance * 100%.

The maximum discrepancy at isocenter is displayed in mm and is calculated by applying the scaling discrepancy against the longest axis of the imager. The maximum pixel dimension of the imager in pixels is referred to as maxImagerPx. Here is the formula:

maximum discrepancy at isocenter = maxImagerPx * pxISO * (percentScalingErr / 100)

The BB Location Detail Plot is included so that the BB position can be confirmed visually.

Below is an example of the report generated: 

Example Files

Below is an example file for use in testing: