Quantification and correction for head motion in PET
- Correction for patient motion during PET investigations is a persistent issue.
- Patient movement can severely reduce the validity of the resulting images and derived quantities.
- Head movements can be very significant in individual studies and be of comparable size with the instrumental spatial resolution of the PET camera.
- Development of methods to correct for patient motion in PET data is therefore important especially as spatial resolution of new PET cameras is improving and movement artifacts become a limiting factor in quantitative PET.
- Common methods to correct head motion in PET are based on reorientation of multiple frame images to a common reference position. These methods allow the correction of movements between frames but can not account for motion occurring within frame boundaries.
- For this purpose a list mode-based method is needed.
- The PET data is acquired in list mode and simultaneously the head motion is tracked with a suitable device.
- According to the motion data each detected event is then spatially corrected to a reference point in time.
- Potentially this results in a fully corrected list mode stream and after binning and reconstruction to motion free images.
- At the PET center Rossendorf the head motion is tracked with an infrared camera system from A.R.T advanced realtime tracking GmbH.
- The system consists of two cameras which are equipped with CCD imaging sensors.
- Infrared light flashes integrated in each camera illuminate the tracking volume periodically.
- Light reflected back to the cameras from retro-reflecting (passive), spherical markers is imaged.
- Each camera contains a small PC running under Linux which is used to analyze the images and deduce the 2D-centers of the markers in the camera specific image plane.
- This information is transmitted via Ethernet to a central computing unit, which computes the full 3D-positions of the markers and the six degrees of freedom (translation, rotation) of the tracked bodies in the camera coordinate system.
- For PET brain investigations a body, consisting of 5 markers is mounted to a glasses frame which can be fixed to the patients head.
- In the actual setup, where the two cameras are mounted behind the PET scanner pointing into the scanner tube a spatial resolution of below 1 mm is achieved.
|ARTtrack 1 cameras||View through PET scanner|
|Head fixation of tracking-body||Patient with tracking-body|
- The following picture shows an example of a movement corrected PET brain measurement.
- A female patient was administered 250 MBq of FDG. Following the transmission a list mode acquisition of 6 minutes was started.
- The first half of the emission scan the patient was asked to remain quiet. Upon a given sign after 3 minutes the patient intentionally moved the head and remained quiet in the new position for the rest of the scan time.
- Throughout the entire investigation the head movement was monitored with the tracking system.
- The improvement in spatial resolution and contrast due to the movement correction is evident.
|Uncorrected (left) and corrected image of a volunteer patient|
- At the PET center Rossendorf the method is successfully implemented for an ECAT EXACT HR+ PET scanner.
- With this method "motion free" PET images of the brain are produced.
- The glasses frame with the tracking-body is well accepted by the patients.
- The method can be applied in routine operation.