Topics covered in this article include:

Introduction

The CT/SIM Check daily CT Sim QA Phantom measures the critical elements of the CT Simulation process (registration hardware, couch, external lasers, internal lasers, scanner, and their collective coincidence) so that they remain within established conformance parameters.

Measurement Summary

The analysis provides the following results:

Uniformity and Noise (Daily)
Uniformity and Noise (Monthly)
Maximum Deviation From Center
Sensitometry
Slice Spacing
Slice Spacing Detail
Slice Thickness
Orientation
Orientation Correct?
CT Table top alignment
CT tabeltop rotation around the z-axis
X-Scaling (Distance)
X-Scaling (Pixel Size)

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 full contiguous scan of the phantom and each slice must be a separate DICOM file. The full width and depth of the uniformity disc in the Z direction must be in the scan for the analysis to work. The files must contain "ct_sim" somewhere in the file name or that "CT/SIM Check" 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.

Image Acquisition Suggestions

Reference the latest manufacturer's guidance for phantom placement and scan acquisition parameters. The image set should include a full, contiguous scan of the phantom.

Detailed Discussion of Measurements

Belo are example results for each section of the analysis.

Uniformity

Mean pixel values and standard deviations are reported for the center, top, right, bottom, and left ROIs as shown in the figure below. The center of each ROI is marked with the plus sign.

The mean pixel value is report for the area inside of the large outer circle ROI along with the standard deviation (noise) within that area.

The means for the top, right, bottom, and left ROI is compared to the mean for the center ROI and the difference is reported along with the standard deviation (noise) for each ROI.

The the outer small ROI with the greatest deviation from the small center ROI is reported separately.

Sensitometry

The analysis will repot on the mean HU value for the area inside the marked region of interest (ROI). When setting tolerances, please note that the sensitometry plugs are removable and may change positions. Ideally they should be kept in the same position so that any trends may be tracked.

The mean pixel value for each plug ROI is reported along with the standard deviation within that area. The intercept and rescale slope are used to convert the pixel values to HU.

Slice Thickness and Spacing

A sagittal view fit of the ramp is done to determine the the distance between slices.

The detail of the sagittal view of the ramp is provided for a visual confirmation of the slice spacing.

The visible section of the ramp is used to determine the slice thickness.

Geometry

The scan must be in the correct orientation for proper geometry results; the R should be on the left and L on the right. If these are mirrored then the scan direction is backwards.

The table top alignment is reported. If the horizontal alignment is even then the Left Y Position (left red plus sign) and Right Y Position (right red plus sign) would be the same. -Y indicates the above center while + Y indicates below center. The center of the image (via DICOM tag information) is where two red dashed lines intersect.

Rotation along the z axis is reported below.

The next table reports on the scaling in the axial axis and assumes the pixel spacing in the DICOM tags is correct. For the table below the following is how each column heading should be interpreted:

Nominal X Distance - the distance reported by the manufacturer of the phantom.
Nom. X Distance projected on X-Axis - is the nominal X distance projected on the X axis. This accounts for any tilt in the phantom.
Measured Distance X - the measured distance assuming the pixel spacing reported in the DICOM tags are accurate.
X-Scale Deviation - the difference between nominal and measured.
X-Scale Deviation Percentage - the difference between nominal and measured as a percentage of the nominal.

The next table reports on the pixel spacing and assumes the distance matches the nominal X distance (the expected distance specified by the phantom manufacturer). For the table below the following is how each column heading should be interpreted:

Nominal X Distance - the distance reported by the manufacturer of the phantom.
Nom. X Distance projected on X-Axis - is the nominal X distance projected on the X axis. This accounts for any tilt in the phantom.
Measured Pixels X Axis - the measured number of pixels assuming the x distance is the same as the nominal X distance.
Nominal Pixel Size - the pixel size reported in the DICOM pixel spacing tag.
Measured Pixel Size - the measured pixels X Axis divided by the number of pixels.
X-Scale Deviation Percentage - the difference between nominal and measured pixel size as a percentage of the nominal pixel size.

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