Positron Emission Tomography/Magnetic Resonance Imaging: The Next Generation of Multimodality Imaging?
Section snippets
The Limitations of PET/CT and the Potential of PET/MRI
Although PET/CT is rapidly becoming a critical component of clinical diagnoses, it has certain limitations, largely related to the fact that the CT and PET scans are acquired sequentially rather than simultaneously. Especially problematic are artifacts caused by intra- and interscan patient and organ motion in addition to the differences between the breathing protocols used in PET and CT. These artifacts affect the accuracy of the registration and attenuation correction, seriously compromising
Technical Challenges of Combined PET and MRI
Combining 2 advanced imaging technologies without degrading the original optimum performance of either is challenging. PET and MRI are 2 such modalities that route and otherwise process electronic signal pulses that are prone to distortion. Besides avoiding any such signal distortion and associated degradation of performance when combining PET and MRI, the main challenges of merging the hardware into a single device are space constraints and the fact that conventional PET detectors21 are based
Different Approaches for Combined PET and MRI
There are different conceivable options for combining PET and MR. The easiest and most straightforward approach would be the placement of the 2 scanners in series in a manner analogous to current PET/CT scanners (Fig. 2A). However, this approach would require significant modifications, especially of the PET detectors to make them insensitive to the magnetic fields and to construct them in such a way as to negligibly affect the performance of the MRI scanner. As with the approach to PET/CT,
Applications of Combined PET/MRI
Considering the fact that the full integration of PET/MRI is technically feasible without compromising the performance of either of the individual modalities, PET/MRI offers considerable potential for novel imaging applications far beyond simply correlation of functional and anatomic images. First, performance evaluation tests have shown that the combination of PET/MRI allows the simultaneous acquisition of multifunctional data such as PET tracer uptake, MR spectroscopy, or fMRI along with
PET/MRI in Preclinical Research
High-resolution small-animal CT exposes the rodents to a significantly high radiation doses than those associated with clinical CT and, for vascular contrast-based imaging, requires relatively large amounts of dedicated iodine-based media to improve visualization and discrimination of soft tissues. Thus, other than for bone and lung, CT is generally not the modality of choice for anatomic imaging of small laboratory animals. Combining PET/MRI, thereby providing marked soft-tissue contrast
Clinical PET/MRI
Although all initial PET/MRI work was based on small-animal imagers, many clinicians see a promising future for PET/MRI. At least one manufacturer of medical imaging equipment is focusing on combining PET and MRI in a single device capable of simultaneous data acquisition. As a first step, a PET scanner based on LSO scintillation crystals and APDs was developed to be inserted into the imaging tunnel of a 3-T clinical MRI system (Fig. 6). Initial performance testing on phantoms as well as the
PET Attenuation Correction Based on MR Images
To achieve an accurate activity quantitation in PET, attenuation correction must be applied to the emission data to compensate for the absorption of γ-rays.44 In PET/CT, this is usually corrected by using a CT-derived attenuation-factor map.45 This is feasible because CT is based on a measure of x-ray attenuation, which is related to electron density of the stopping medium. MR, however, provides semiquantitative information on proton density. Thus, PET attenuation correction based only on MRI
Conclusion
The initial results achieved with various prototype animal PET/MRI systems as well as the first clinical PET/MRI brain-imaging devices demonstrate the feasibility of simultaneous data acquisition with negligible mutual interference between the 2 subsystems. The potential of fully integrated PET/MRI goes far beyond simple registration of morphological and functional images and holds the promise of temporal as well as spatial correlation of multiparameter functional data derived by MR
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