Positron Emission Tomography/Magnetic Resonance Imaging: The Next Generation of Multimodality Imaging?

doi:10.1053/j.semnuclmed.2008.02.001

Seminars in Nuclear Medicine

Volume 38, Issue 3, May 2008, Pages 199-208

https://doi.org/10.1053/j.semnuclmed.2008.02.001 Get rights and content

Multimodal imaging is now well-established in routine clinical practice. Especially in the field of nuclear medicine, new positron emission tomography (PET) installations comprise almost exclusively combined PET/computed tomography (CT) scanners rather than PET-only systems. However, PET/CT has certain notable shortcomings, including the inability to perform simultaneous data acquisition and the significant radiation dose to the patient contributed by CT. Magnetic resonance imaging (MRI) offers, compared with CT, better contrast among soft tissues as well as functional-imaging capabilities. Therefore, the combination of PET with MRI provides many advantages that go far beyond simply combining functional PET information with structural MRI information. Many technical challenges, including possible interference between these modalities, have to be solved when combining PET and MRI, and various approaches have been adapted to resolving these issues. Here, we present an overview of current working prototypes of combined PET/MRI scanners from different groups. In addition, besides PET/MRI images of mice, the first such images of a rat acquired with the first commercial clinical PET/MRI scanner, are presented. The combination of PET and MRI is a promising tool in preclinical research and will certainly progress to clinical application.

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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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

Technical Challenges of Combined PET and MRI

Different Approaches for Combined PET and MRI

Applications of Combined PET/MRI

PET/MRI in Preclinical Research

Clinical PET/MRI

PET Attenuation Correction Based on MR Images

Conclusion

Magn Reson Imaging

Eur J Cancer

A combined PET/CT scanner for clinical oncology

J Nucl Med

A combined PET/CT scanner: The choices

J Nucl Med

Accuracy of whole-body dual-modality fluorine-18-2-fluoro-2-deoxy-D-glucose positron emission tomography and computed tomography (FDG-PET/CT) for tumor staging in solid tumors: Comparison with CT and PET

J Clin Oncol

Clinical performance of PET/CT in evaluation of cancer: Additional value for diagnostic imaging and patient management

J Nucl Med

Prospective feasibility trial of radiotherapy target definition for head and neck cancer using 3-dimensional PET and CT imaging

J Nucl Med

Radiation oncology and functional imaging

Nuklearmedizin

Value of contrast-enhanced multi-phase CT in combined PET/CT protocols for oncological imaging

Br J Radiol

Dual-modality PET/CT imaging: the effect of respiratory motion on combined image quality in clinical oncology

Eur J Nucl Med Mol Imaging

Optimized contrast-enhanced CT protocols for diagnostic whole-body 18F-FDG PET/CT: Technical aspects of single-phase versus multiphase CT imaging

J Nucl Med

PET/CT of the abdomen: Optimizing the patient breathing pattern

Eur Radiol

PET-CT image co-registration in the thorax: Influence of respiration

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Primary staging of urinary bladder carcinoma: The role of MRI and a comparison with CT

Eur Radiol

The clinical efficacy of magnetic resonance imaging in neuroimaging

Ann Intern Med

Comparison of computed tomography and magnetic resonance based target volume in brain tumors

J Cancer Res Ther

Radiation exposure of patients undergoing whole-body dual-modality 18F-FDG PET/CT examinations

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Optical fiber readout of scintillator arrays using a multi-channel PMT: A high resolution PET detector for animal imaging

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Performance test of a LSO-APD PET module in a 9.4 Tesla magnet

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The road to the common PET/CT detector

IEEE Trans Nucl Sci

Highly resolved in vivo 1H NMR spectroscopy of the mouse brain at 9.4 T

Magn Reson Med

A multicrystal two dimensional BGO detector system for positron emission tomography

IEEE Trans Nucl Sci

Measured attenuation correction methods

Eur J Nucl Med