Topics covered in this article include:

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

Introduction

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

The Las Vegas phantom is a common phantom used for planar MV imaging. Below is a sample of the analysis identifying the holes in the phantom:

Note that in the title the UR stands for upper right and is identifying where the undrilled area to the right of the offset rows is located.

Green asterisks indicate holes that were identified (contrast is greater than zero) and red asterisks indicate that an expected hole was not seen (contrast is at or below zero).

The gridlines are not part of the original image; they are added for reference.

The rows have constant depths but varying diameters. The columns have constant diameters but varying depths.

Measurement Summary

The analysis provides the following results:

• Plot: Constant Contrast Vs Diameter by Depth
• Plot: Constant Contrast to Noise Ratio Vs Diameter by Depth
• Plot: Constant Contrast Vs Depth by Diameter
• Plot: Constant Contrast to Noise Ratio Vs Depth by Diameter
• Spatial and Contrast Resolution
• Constant Contrast to Noise Ratio

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

The test requires a DICOM image. It is necessary to use the file naming convention or to select "Planar MV Imaging (Las Vegas) in the Type field in the upload widget to identify the phantom. If using the filename to identify the image, use "lasvegas" somewhere in the file name. Below is an example of selecting the image type 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 "lasvegas" (case insensitive) is found the image set will be processed as MV Planar Imaging test series. 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

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

Image Acquisition Suggestions

Below are the recommended steps to acquire the images:

1. Ensure that the phantom is oriented so that it is aligned along a cardinal angle.
2. Ensure that the edge of the image is at least 50 mm away from the contrast objects.
3. When collimating the open field, ensure that the field size is 30x30 cm or larger.
4. Although there is limited capability to analyze an inverted phantom, the open end of the holes should face the source.

Detailed Discussion of Measurements

The following definitions and calculations are used in the analysis:

background = mean pixel value of the area around each disk
meanDisk = mean pixel value in the detected disk
contrast = (meanDisk - background) / (meanDisk + background) * 100;

sdDisk:  standard deviation of the disk pixels
sdBg:  standard deviation of the background pixels

CNR = contrast/noise;

constantContrast = contrast * diskDiameter
constantCNR = CNR * diskDiameter

Below is an example of the report generated:

Example Files

Below are example files for use in testing: