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kV Scaling (Leeds)

David Kirkaldie
  • Updated

Topics covered in this article include:

Leeds_Edge_Contouring.png

Introduction

This test is used to determine scaling discrepancies on the kV imager by comparing a known diameter of the Leeds phantom against the measured distance in a DICOM image. The outer Leeds casing is contoured in red to act as a visual confirmation on the measurements.

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

Measurement Summary

The analysis provides the following results:

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

kV 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 leeds anywhere in the filename or selecting the "Planar kV Imaging (Leeds)" in the Type dropdown 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 leeds (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

600px-Upload_auto.png

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.

 600px-Upload_auto_off.png

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. 

 600px-File_series_being_uploaded.png

While files are being processed users may perform other tasks such as data entry.

Available Tests

Recommended Tests
Template Section Subsection Tests

Monthly Imaging QA [Tg142 Table VI]

Planar kV Imaging (Leeds)

kV Image Scaling

Monthly Imaging QA [Tg142 Table VI]

Planar kV Scaling (FC-2 Phantom)

 

Image Acquisition Suggestions

Below are the recommended steps to acquire the kV scaling images:

  1. Place the phantom on the couch and center the mid-plane of phantom at isocenter using the lasers or ODI. image001.png
  2. Ensure that the phantom edge is unobstructed.
  3. Adjust the collimation so that the full phantom is exposed and so that there is a gap between the field edge and the phantom edge. 
  4. Acquire the image.

Detailed Discussion of Measurements

The scaling is reported in mm 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 SID by SID gives the pixel size projected to the isocenter. pxISO = pixel size (SID / SAD). SAD is assumed to be 1000. 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 if present, if not 1000 is used
      • SID is pulled from the 3002,0026 RT Image SID tag
  2. The algorithm finds the edge of the phantom and calculates the width of the phantom in pixels in the x and y direction and multiplies by pxISO to produce the widths in millimeters.
  3. Users can compare the measured X and Y phantom widths in the image against the known size of the phantom. Note that the most reliable results are achieved when the phantom is at isocenter. We recommend setting a baseline based on historical results.

The outer Leeds casing is contoured so that the edge of the phantom can be confirmed visually.

Below is an example of the report generated: 

kV_Scaling_Leeds.png

Example Files

Below is an example file for use in testing:

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