PET-MRI
Positron emission tomography–magnetic resonance imaging | |
---|---|
Purpose | used in clinical field of oncology |
Positron emission tomography–magnetic resonance imaging (PET–MRI) is a hybrid
The combination of PET and MRI was mentioned in a 1991 Phd thesis by R. Raylman.[2] Simultaneous PET/MR detection was first demonstrated in 1997, however it took another 13 years, and new detector technologies, for clinical systems to become commercially available.[3]
Applications
Presently, the main clinical fields of PET-MRI are oncology,[4][5][6] cardiology,[7] neurology,[8][9][10] and neuroscience.[11] Research studies are actively conducted at the moment to understand benefits of the new PET-MRI diagnostic method. The technology combines the exquisite structural and functional characterization of tissue provided by MRI with the extreme sensitivity of PET imaging of metabolism and tracking of uniquely labeled cell types or cell receptors.
Manufacturers
Several companies offer clinical and pre-clinical combined PET-MR system; clinical systems are available from United Imaging,
Clinical systems
The first two clinical
Siemens was the first company to offer simultaneous PET/MR acquisitions, with the first systems installed in 2010 based on avalanche photodiode detectors.[18][3]
Currently Siemens and GE are the only companies to offer a fully integrated whole body and simultaneous acquisition PET-MRI system. The Siemens system (Biograph mMR) received a
The GE system (SIGNA PET/MR) received its 510K & CE mark in 2014.[citation needed]
Preclinical systems
Currently, the combination of positron emission tomography (PET) and magnetic resonance imaging (MRI) as a hybrid imaging modality is receiving great attention not only in its emerging clinical applications but also in the preclinical field. Several designs based on several different types of PET detector technology have been developed in recent years, some of which have been used for first preclinical studies.[21][22][23]
Several companies offer MR-compatible preclinical PET scanner inserts for use in the bore of an existing MRI, enabling simultaneous PET/MR image acquisition.[24][25][26][27]
Comparison with PET-CT
The combination of PET with X-ray
The same clinical decisions that would influence the choice between stand-alone CT or MR imaging would also determine areas where PET-CT or PET-MR would be preferred.[14] For example, one advantage of MRI compared to CT is its superior soft tissue contrast, while CT has the advantage of being much faster than MRI.
One clear advantage of PET-MR compared to PET-CT is the lower total ionising radiation dose obtained. For body PET-CT applications, the CT part of the examination constitutes approximately 60-80% of the radiation dose, with the remaining radiation dose originating from the PET radiopharmaceutical.[30] In contrast, no ionising radiation dose is obtained from MRI. PET-MR is therefore appealing in children, in particularly for serial follow-up examinations as used in oncology or chronic inflammatory conditions.[31]
Attenuation correction
PET-MRI systems don't offer a direct way to obtain attenuation maps, unlike stand-alone PET or PET-CT systems.[32][33]
Stand alone PET systems' attenuation correction (AC) is based on a transmission scan (mu - map) acquired using a 68Ge (
There is no correlation between MR image intensity and electron intensity, therefore conversion of MR images into an attenuation map is difficult.
In areas of the body with predictable structures (e.g. the head), segmentation (where tissue is categorised using the MRI image data), or "atlas" methods can be used. In atlas methods a standard MR image, with associated CT attenuation data, can be warped to fit the actual patient anatomy. Disadvantages of this method include difficulty with unusual anatomy, a need for a suitable library of images, and the need to account for MR coil attenuation.[34] Synthetic, or Substitute CT (sCT) methods to generate CT like data from MRI are also of interest for radiotherapy planning, and have been primarily investigated for sites in the head. While some of these use an atlas technique, many take a voxel approach where actual voxel intensities (contrast data) are used in combination with machine learning (trained on MR/CT data) to assign electron density values.[34][38][39]
In many of the above methods, MRI artifacts (e.g. from physiological motion) can affect attenuation correction accuracy.[34][40]
See also
References
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