Abstract
PET (positron emission tomography) with its high sensitivity in combination with MRI (magnetic resonance imaging) providing anatomic information with good soft-tissue contrast is considered to be a promising hybrid imaging modality. However, the integration of a PET detector into an MRI system is a challenging task since the MRI system is a sensitive device for external disturbances and provides a harsh environment for electronic devices. Consequently, the PET detector has to be transparent for the MRI system and insensitive to electromagnetic disturbances. Due to the variety of MRI protocols imposing a wide range of requirements regarding the MR-compatibility, an extensive study is mandatory to reliably assess worst-case interference phenomena between the PET detector and the MRI scanner. We have built the first preclinical PET insert, designed for a clinical 3 T MRI, using digital silicon photomultipliers (digital SiPM, type DPC 3200-22, Philips Digital Photon Counting). Since no thorough interference investigation with this new digital sensor has been reported so far, we present in this work such a comprehensive MR-compatibility study.Acceptable distortion of the B0 field homogeneity (volume RMS = 0.08 ppm, peak-to-peak value = 0.71 ppm) has been found for the PET detector installed. The signal-to-noise ratio degradation stays between 2–15% for activities up to 21 MBq. Ghosting artifacts were only found for demanding EPI (echo planar imaging) sequences with read-out gradients in Z direction caused by additional eddy currents originated from the PET detector. On the PET side, interference mainly between the gradient system and the PET detector occurred: extreme gradient tests were executed using synthetic sequences with triangular pulse shape and maximum slew rate. Under this condition, a relative degradation of the energy (⩽10%) and timing (⩽15%) resolution was noticed. However, barely measurable performance deterioration occurred when morphological MRI protocols are conducted certifying that the overall PET performance parameters remain unharmed.
Highlights
Positron emission tomography (PET) distinguishes itself from other imaging technologies with its high sensitivity enabling the visualization of metabolic processes
The results of the B0 field characterization are shown in figure 4: representative slices in sagittal slice orientation from the center of the field of view (FOV) are shown for all four scenarios
We have presented such an MR-compatibility study of a preclinical, digital PET/MRI insert which is designed to be operated inside a 3 T clinical MRI system
Summary
Positron emission tomography (PET) distinguishes itself from other imaging technologies with its high sensitivity enabling the visualization of metabolic processes. The combination of PET with magnetic resonance imaging (MRI) gains interest since MRI has several advantages over CT such as a better soft tissue contrast, the lack of ionizing radiation as well as the possibility to apply the entire range of all different contrast mechanism developed in the field of MRI (e.g. dynamic contrast enhancement (DCE) MRI (Jackson et al 2005), functional MRI (fMRI), BOLD (Ogawa et al 1990), CEST (Walker-Samuel et al 2013), IRON (Stuber et al 2007), chemical shift imaging (Brateman 1986)) To benefit from this combination to the highest extent, both imaging devices have to work simultaneously to facilitate e.g. the best possible registration quality (in a spatial and temporal sense) (Torigian et al 2013) or to enable other features such as motion compensation (Soultanidis et al 2013) or economy of time. Several combined PET/MRI devices were developed and presented in recent years
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