Topics covered in this article include:

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

Introduction

This phantom measures aspects of image quality:  contrast scale, high contrast spatial resolution, low contrast detectability, noise and uniformity.

The QA phantom contains three sections, each corresponding to a single scan plane:   Section 1: Resolution block S0mm scan location, Section 2: Contrast membrane S40mm scan location, and Section 3: Water bath S60mm scan location.

Measurement Summary

The analysis provides the following results:

• Contrast Scale
• Contrast
• Contrast Detail
• Uniformity
• Bar Pattern Standard Deviation
• Bar Pattern Standard Deviation
• Bar Pattern Standard Deviation 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

The test requires at least one image and it must be a DICOM file. The files must contain "ct_ge" somewhere in the file name or that "GE CT QA Phantom" is selected in the Type drop down as shown below:

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

Reference the GE Technical Reference Manual for phantom placement and scan acquisition parameters. The tolerance suggestions in the measurement discussion match the tolerances in the GE technical Reference Manual.

Detailed Discussion of Measurements

Contrast Scale

The Contrast Scale is determined by subtracting the mean HU value of a water region from the mean HU value of a plastic region. The expected value is 120 HU ± 12 HU. The regions selected are shown below:

Contrast

Contrast is determined by taking the difference between the plastic area and water area.

Contrast Top = Area A Mean - Area B Mean

Contrast Bottom = Area C Mean - Area D Mean

Contrast Detail

The following definitions and calculations are used in the contrast detail analysis:

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

sdHole standard deviation of the hole pixels
sdBg:  standard deviation of the background pixels

CNR = contrast/noise;

constantContrast = contrast * holeDiameter
constantCNR = CNR * holeDiameter

Uniformity and Noise

Uniformity is determined by the mean pixel value of the given 15 mm ROI. The uniformity difference between the center ROI and the average of the edge ROIs should be 0 ± 3.

Noise is the standard deviation of the mean pixel value of the central 25 mm ROI. Noise should equal 3.2 ± 0.3,

Resolution

An ROI is fitted to each bar pattern and the standard deviation across the region is reported for each bar pattern. Only the largest 5 bar patterns are used.

Sample Report

Below is an example of the report generated from the asymmetric field test:

Example File

Below is an example image set for use in testing: