Synopsis

This talk is aimed at physicists, engineers, and others who would like a better appreciation of the challenges involved in integrating a PET detector inside a 3T MR scanner. The main challenges involved in integrating a PET detector inside an MR scanner will be explained, and various approaches will be discussed for solving them.

OUTLINE OF TOPICS

Challenges involved in integrating a PET detector inside an MR scanner include:

• Space: where do you put the detector? How do you make it small enough? A conventional (analog, PMT-based) PET detector is about 15 cm tall: a new approach is needed to fit the detector in the space between the gradient coil and the patient.
• Magnetic field: Conventional (vacuum tube) photomultipliers are very sensitive to magnetic fields, and do not produce signal in a 3T magnetic field. A completely new technology is needed that provides sufficient photoelectric gain without being sensitive to magnetic fields. This is provided by solid state devices: avalanche photodiodes and silicon photomultipliers.
• RF interference: The RF transmit field can induce signals in the PET detector that might saturate the electronics. We discuss methods for shielding the PET detector from the MR RF transmit power; and explore how to prevent emissions from the PET detector from interfering with the MR reception - Gradients: Strong, rapidly slewing gradient fields induce eddy currents in nearby conductors. This results in thermal loads and mechanical forces (vibration). We discuss techniques for protecting the PET detector from these effects
• Stability and cooling: PET detector gain stability is essential for quantitative imaging: if the peak shifts, so does the scatter fraction. Since model-based scatter correction (MBSC) attempts to model the physics that leads to scatter, feeding this algorithm the “wrong” value for the LLD (lower level discriminator) will lead to errors in scatter correction. Solid state detectors are sensitive to temperature. You will learn the source of this sensitivity, and techniques used to maintain temperature and control gain – with the goal to keep the PET detector stable in the MR environment
• Attenuation correction: Attenuation in the body can reduce the observed signal by 90% or more. To correct for this we need an accurate determination of the electron density – information that is not directly available from MR. Techniques will be discussed for indirectly measuring the attenuation from MR, or directly inferring it from the PET data itself.
• TOF PET: when the detector provides timing information about each line of response, the reconstructed image will converge more rapidly, and with better SNR. TOF information has additional benefits that are unique to the PET/MR application: a non-attenuation corrected (NAC) image has less blurring due to scatter, and can be used to estimate subject outline; also, TOF reconstruction is significantly more robust to errors in the attenuation map. These things combine to give greater quantitative accuracy.
• Challenges of registration: Since the PET and MR systems are independent, care must be taken to ensure that images are properly registered
• Challenges of simultaneity: Some MR sequences introduce particular challenges to the PET detector system. Making sure that there is no mutual interference so the systems can truly operate simultaneously requires particular precautions
• Challenges of time scales: depending on the PET tracer used, the processed that are being imaged with PET may evolve over a period of minutes or hours. Functional MRI tends to look at processes that are changing on the second scale. When and how is simultaneity useful?
• Challenges of workflow: operating a scanner with both PET and MR components presents unique challenges to operators who need training in both modalities. Some of the problems that need to be addressed in the user interface will be touched upon.

CONCLUSION

Acknowledgements

References