Topics covered in this article include:

• Introduction
• Measurement Summary
• Instructions
• Available Tests for Leeds Image Analysis
• Image Acquisition Suggestions
• Detailed Discussion of Measurements
  • Critical Frequencies
  • 2D Spatial Resolution (MTF) and 2D Low Contrast Visibility
  • Uniformity, Contrast, and Noise
• Sample File

Introduction

The Leeds TOR18 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 II: Imaging.

Measurement Summary

• 2D Spatial Resolution
• 2D Low Contrast Visibility
• Modulation Transfer Function (MTF)
• Line Pairs per Millimeter

Instructions

kV Imaging uploads must be DICOM files. If you have added the appropriate tests to your template, you will be able to select the test from the Type drop-down, as shown below.  There are limited options for encoding the analysis into the filename, but this is not the the suggested method. 

In cases where it is necessary to use manual naming conventions, please try this naming convention add "leeds" anywhere in the file name.

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 will be processed as a Leeds TOR18 phantom. 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 for Leeds Image Analysis

The following tables show the tests to select in the template builder corresponding to the supported analyses.

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.

• 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 so that there is a gap between the field edge and the phantom edge. 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:
  • Anatomy
  • SID = 150 cm
  • 50 kVp
  • 10 mAs (200 mA, 50 ms)
• If using the scaling tests, ensure that the phantom edge is unobstructed.
• When using a plate (not recommended), ensure that it does not cross the outside edge of the phantom. The phantom image should be inside the plate or the plate should be fully inside the outer edge of the phantom.
• In the imaging software, capture the image and export for use.

Detailed Discussion of Measurements

The Leeds analysis provides the following measurements and plots:

• Critical Frequencies
• 2D Spatial Resolution and 2D Low Contrast Visibility
• Uniformity, Contrast, and Noise

Critical Frequencies

The MTF curve is plotted and MTF values are reported for each bar pattern. For each bar pattern, the Contrast Transfer Function is calculated as follows:

Where

• 𝐼max(𝑓) is the local contrast maxima for a given frequency 𝑓
• 𝐼min(𝑓) is the local contrast minima for a given frequency 𝑓.²

The CTF is converted to MTF using the Colman formula

Where

• MTF(𝑓) is the MTF for a given frequency
• C(𝑓) is the the bar pattern CTF(𝑓) for the same given frequency.³

The MTF values are normalized and plotted. Frequency values at MTF values of 50%, 10%, 5%, and 2% are calculated. For each critical frequency (50%, 10%, 5%, and 2%) the lp/cm intercept is reported.

2D Spatial Resolution (MTF) and 2D Low Contrast Visibility

The 2D spatial resolution is determined by the number of MTF points above the critical frequency modulation of 2%.

The 2D low contrast visibility is determined by the number of disks that have a greater than 1 contrast to noise ratio.

On the plot individual items may be hidden/shown by clicking the the legend items below the plot. You may also hover over the fit line or individual points to see the numerical values.

The 2D Low Contrast Visibility Data chart shows the data underlying the 2D Low Contrast Visibility plot, including the test disk number, nominal contrast, measured CNR, the CNR linear fit, and if the disk is visible (greater that a 1/1 contrast to noise ratio).

Uniformity, Contrast, and Noise

Contrast is determined by comparing the background ROI (bg) to the dark square (d).

The formula used for contrast is:  

The formula used for noise is:  

The formula for the Contrast to Noise Ratio (CNR) is:

Uniformity is determined by the pixel values at the 10th (p10) and 90th (p90) percentiles of pixel values within the dark square ROI. The uniformity is computed with the following equation:

Sample File

Below is an example file for use in testing:

leeds.dcm