How to verify the Alignment of PET and MRI
For a precise overlay and image fusion of PET and MRI images, the physical coordinate systems of the PET and MRI imaging devices needs to be exactly calibrated and a suitable rigid-body transformation matrix between the coordinate system is calculated and stored in the configuration of ParaVision 360. Based on this transformation matrix, images from PET and MRI can automatically be overlaid and fused. The validly of that approach depends on reproducible relative positioning of sample inside the PET and the MRI FOV by the ATS.
During the initial installation of the PET and MRI modality, the origins of the coordinate systems of both modalities are measured and the co-registration transformation matrix is calculated and stored in the Paravision configuration. It is recommended to verify the precise FOV positioning and the co-registration in regular intervals using a suitable phantom.
The calibration test must be carried out with a special phantom that is visible both in the MR and PET modality, e. g., a cylinder with three perpendicular narrow cylinders, filled with FDG.
- 1.
- Select:
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- PET Inline: Install the phantom on the ATS, select the cross as central position and move it into the center FOV position of the PET.
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- PET Insert: Make sure the PET insert is correctly in place. Place the phantom into the center of the system as precisely as possible
- 2.
- Create a New Study with Specimen set to 'Material'.
- 3.
- In the Exam Card
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- create a default PET scan: duration depending on the radioactivity in the phantom.
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- create a default MRI Scan: FLASH (3D, Sagittal, FOV 50mm, Matrix 100, min TE/TR)
- 4.
- Acquire first the PET scan, followed by the MR scan (for PET Inline you have to move the table to the MR center).
- 5.
- Open the MR image in “Image Fusion”.
- 6.
- Drag & Drop the PET image and use as first overlay.
- 7.
- Set color map of the PET overlay to grey and opaque.
- 8.
- In Palette > Viewing > Analysis > Orthogonal Slice Planes (Tab 3), select “Physical [mm]”.
- 9.
- In each 2D viewport, click into the center of the PET image. Zoom as necessary to obtain a good estimate.
- 10.
- Copy values from the palette >analysis into the PET center column of table below.
- 11.
- In the Palette > colormap, set PET image to “transparent”.
- 12.
- In each of the 2D viewports, click into the center of the MRI image, see figure below.
- 13.
- Copy the values into the MR results column.
- 14.
- For all coordinates, calculate the differences and note results in the table.
If PET and MR data are not correctly overlaid: