1. PET-MRI: challenges and solutions in the development of integrated multimodality imaging
MSc medical imaging, Division of Medical Physics, University of Leeds
Introduction
Attenuation correction (AC)
They are considered as an insensitive to large magnetic fields.
However, it limited space for PET detectors inside MRI bore adds another major challenge. {3}
In MRI - longer acquisition times compared to CT: a CT scan lasts for 15 s–1 min, whereas, total acquisition time for MRI sequences is about 20–40 min
Magnetic Resonance Imaging (MRI) uses radio waves to produce detailed images that enhances soft tissues contrast accurately.
Positron emission tomography (PET) - detects pairs of gamma rays - emitted by a tracer(FDG), placed in the system on a biologically active molecule. {1}
Although that combining both PET and MRI technologies could provide crucial information that give an accurate diagnoses. They remain as a challenge to achieve full performance.
PET imaging sessions acquire a single image - minimal scope for optimising acquisition parameters.
MRI session involves several examinations with contrast sequences performed consecutively to observe different disease aspects.
Quantitative PET imaging necessitates linear attenuation coefficients (at 511 keV) of an object in order to scatter correction and perform attenuation.
In a PET-MRI system, derivation of attenuation correction from either emission data, MRI images, or transmission imaging is complicated.
It is crucial to apply (AC) in order to avoid artefacts.
MRI’s effect on PET
Photomultiplier tubes used in standard PET block detector design may not be function in very weak static magnetic fields.
Gradient fields switched rapidly at frequencies of 1 kHz due to greater skin depth at lower frequencies - more difficult to protect than higher frequency RF 3 T).
Electronics situated in magnet bore may be vulnerable to RF interference, generated by MRI transmit coil.
PET’s effect on MRI
Minute differences in magnetic susceptibility due to PET scanner components in magnet bore may cause inhomogeneity in main magnetic B0 field. {4}
NMRI signals generated within the human system in accordance with MRI B1 field excitation are extremely weak - necessitating MRI receive coils to be of highly sensitive and complete MRI scanning room with Faraday shielding.
(Stoney Brook, 2013):
Technical challenges
Image source :
Presence of magnetic fields.
Compact MRI-compatible PET detector technology is a challenge.
PET systems based on configuration of inorganic scintillation crystal array optically coupled to four photomultiplier tubes. {2}
Therefore, using strong and reliable MRI compatible solid state photodetectors could be a solution for this issue, for example avalanche photodiodes and silicon photomultipliers. As in figure (1)
Conclusion
Development of MRI compatible detectors relied on compact solid state PMTs led to complete real-time integrated PET-MRI systems for human imaging
One of the major challenges yet exists - attenuation correction, does not have equal accuracy.
MRI imaging is guiding image reconstruction and perform motion compensation to enhance PET image quality.
PET-MRI require Long acquisition time.
Future research focussed on quantitative and operational aspects of PET-MRI systems.
Compatible MRI with PET
For MRI compatibility, scintillator material does not lead to susceptibility artefacts.
PET scintillators LSO, BGO, LYSO have magnetic susceptibility close to human tissue - have minor effects on MRI.
Long optical fibres to combine scintillators to photomultipliers outside magnet.
Ingenuity TF sequential PET-MRI from the Philips Healthcare was the first commercially available human PET-MRI system molecule. {1}
High resolution anatomical data and motion information from MRI could be used to improve PET image quality.
PET-MRI offers possibility of obtaining high quality anatomical images uninterruptedly throughout PET acquisition at high dynamic rate, temporally spatially and correlated to PET emission data.
References
Cho, Z., Son, Y., Kim, H., Kim, K., Oh, S., Han, J., Hong, I. and Kim, Y. (2007). A hybrid PET-MRI: An integrated molecular-genetic imaging system with HRRT-PET and 7.0-T MRI. Int. J. Imaging Syst. Technol., 17(4), pp
Gambhir, A., Jena, A., Renjen, P., Taneja, S. and Negi, P. (2015). Integrated 18 F-fluorodeoxyglucose positron emission tomography magnetic resonance imaging ( 18 F-FDG PET/MRI), a multimodality approach for comprehensive evaluation of dementia patients: A pictorial essay.Indian Journal of Radiology and Imaging, 25(4), p.342.
Lord, M., Ratib, O. and Vallée, J. (2011). 18F-Fluorocholine integrated PET/MRI for the initial staging of prostate cancer. European Journal of Nuclear Medicine and Molecular Imaging, 38(12), pp
Ratib, O. (2013). PET/MRI: a new era in multimodality molecular imaging. Clin Transl Imaging, 1(1), pp.5-10.
Vandenberghe, S. and Marsden, P. (2015). PET-MRI: a review of challenges and solutions in the development of integrated multimodality imaging. Physics in Medicine and Biology, 60(4), pp.R115-R154
avalanche photodiodes
silicon photomultipliers
figure (1)
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Image source: