Contact

Dr. Jens Maus
Research Associate / Head IT
Positron Emission Tomography
j.mausAthzdr.de
Phone: +49 351 260 - 2757, +49 351 4585056
Fax: +49 351 260 - 12757

Quantification and correction for head motion in PET

Problem

  • 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.

Method

  • 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.
Tracking cameras tracking cameras, view through PET scanner
ARTtrack 1 cameras View through PET scanner
glasses with tracking targets patient with tracking targets
Head fixation of tracking-body Patient with tracking-body

Results

  • 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.
plane of uncorrected and corrected brain image
Uncorrected (left) and corrected image of a volunteer patient

Conclusion

  • 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.

Contact

Dr. Jens Maus
Research Associate / Head IT
Positron Emission Tomography
j.mausAthzdr.de
Phone: +49 351 260 - 2757, +49 351 4585056
Fax: +49 351 260 - 12757