It is widely accepted that the superimposition of anatomical information improves the predictive value of functional and molecular imaging. This is reflected in the success of hybrid systems using combined PET and CT scanner . But the combination of positron tomography and magnetic resonance imaging can offer great advantages , such as higher contrast with the soft tissue anatomical MRI images with simultaneous exposure to a much lower radiation dose to the patient. Correlative imaging will open up exciting new applications in oncology, neurology and cardiology . At present, magnetic fields compatibility with PET detectors remains a technical challenge and the restriction of space on the magnet has almost been resolved. There is no fully developed and mature clinical MR / PET system in this market, although this time the three largest manufacturers indicate system availability MR-PET.
Introduction
Combining functional components of nuclear medicine with anatomical information from CT or MRI, details of PET tracers uptake aeas are becoming more specific. Combination of pictures and fusion techniques have been developed and optimized for the interpretation of PET, CT and MRI data. While the combination of PET and CT in one device is already being used on a regular basis in oncology, the combination of PET and MR applies very limited from two manufacturers and always at the research stage.
In this article we explain the added value of a combined MR/PET compared with advanced combination methods for images acquired separately.
In principle, a combined MR/PET examination with simultaneous measurements that will provide functional and anatomical information simultaneously in the same space and time. Combined inaccuracies, which usually occur when acquired separately datasets repositioning the patient in a different situation and place, generally can not be obtained using hybrid imaging system. Apart from the specific anatomical information, MRI can provide information from which to extract the attenuation coefficients of PET thus not requiring the use of radioactive sources or x-ray tube.
Simultaneous imaging
A further advantage of the execution of PET in a high magnetic field is to increase the resolution due to the limited movement of positrons. Although this was the motivation for the initial investigation of the combined PET and MR, now the focus has shifted to the simultaneous MR / PET, since they require very high magnetic fields to improve the spatial resolution for PET imaging component. The measurement time for a complete picture of the MRI is usually greater in comparison with physiological functions such as breathing or heartbeat . This is similar to PET, which usually takes several minutes per bed position. In MRI certainly various techniques have been developed to overcome this problem. Some of these are based on the ability to measure MR signals with respiratory or cardiac trigger (respiratory, cardiac triggering) or with the aid of navigation pulses (navigator). For example, the position mediastinal can be detected in real time.
This information can be used directly in the magnetic resonance tomography for adjusting the selected section or Menis dimensional imaged region , to follow the movement of the anatomical structure , which can not be
in CT because of the radiation and the process downloads the image. For PET, this information may be used retroactively for the best post treatment data always combined with movement data for tissue reduce ambiguity motion . Similar techniques also apply to cardiac applications. Techniques are currently being developed for PET / CT, can be exploited the situation in MR-PET. Optimized reconstruction programs with technical limiting the ambiguity of breath developed PET / CT systems. By development of reconstruction algorithms in clinical systems PET / CT the spatial resolution capacity of the order of only 4 mm in brain studies and less than 10 mm oncological studies of the chest and abdomen . The ability to combine PET and MRI in a single device provides greatly improved resolution compared with available PET / CT systems.
The main physical problem in PET imaging limiting the spatial resolution (> 2mm) is due to the motion positrons.
Clinical Applications of MR / PET.
Various sectors of diagnosis are sure to benefit from a hybrid system MR / PET. Some of them are presented below. The FDG-PET in the diagnosis of Alzheimer 's disease is an established technique, but corrections for atrophy today rely on estimates . With MR / PET, accurate correction is possible using MRI data aligned with the data of PET. Fusion studies of displacement of receptors with PET and functional magnetic tomography (BOLD fMRI) may be beneficial in the study of the interaction between receptor activation and regional perfusion . Even change effects of perfusion in areas that are remote areas receptor changes can be assessed. Using the same test for patient taking the exact same time of PET data and MR can simultaneous study by fMRI and PET radiotracer with O-15. Another important application area of clinical MR-PET in the neurological field is Simultaneous identification of functionally important parts of the brain and delineation of tumor localization before surgery and radiotherapy . Also, MR perfusion data of stroke patients could improved synergy with the operational information of the PET. The exact anatomical overlay of intelligence PET with the anatomical information In an MRI scan will greatly improve the accuracy of focal lesions.
In oncology, individualized treatment planning , treatment monitoring is a matter of great clinical interest . Especially functional elements must be accompanied by precise anatomical reference information with high contrast and SNR. H PET / CT imaging is already bearing fruit . The high soft tissue contrast of MRI and the lowest radiation exposure enables continuous monitoring and repeated examinations. The cardiac applications will definitely improve the MR / PET approach. Best estimate of the area of viable myocardium , studies nekromenon areas with perfect anatomical and functional information that has been obtained in a test would give great confidence to clinicians in the effort to detect and interpret the lesions kardias.Paradeigma an obvious strategy should combine high resolution images
of MRI showing the wall thickness and motion , scar tissue , coronary anatomy, and the weight of the plate , and syndyastoum very specific , quantitative data on the viability of the tissues and blood flow.
The technical challenge
The main problem in MR-PET systems arises from the fact that the photomultipliers can not operate in a magnetic field , without degradation of performance. Moreover , there is very limited space for mounting magnet detector PET. Therefore , it is possible to use the standard detector PET, consisting of crystals spinthyrismou , arrays photomultiplier , a combined MR-PET scanner. At UCLA team of Cherry et al started developing compatible detectors PET. The operating principle of the detector based on the use of long fiber , which direct the light from the scintillation crystal of the magnetic field outside the photomultipliers , wherein the field has fallen below 10 mT. Based on this technology , the first simultaneous PET and MR imaging of the phantom at 1.5 T MRI scanner with a PET that was actually a ring of detectors LSO 54 mm. The PET had poor performance. The MR / PET which was also developed by the research team of Dr. Cherry comprised a PET system in a magnet to facilitate simultaneous imaging . although scintillator arrays continue to be used , the scintillation light transmitted through optical fibers to silicon detector that is not affected by magnetic fields . This new system delivers good results with little interference. In the first study in vivo mouse imaging held recently, the MR / PET yielded high quality images and demonstrated the feasibility of the combined MR / PET.
Left. The combined system RF coil and detectors. The mouse and guinea pig tube anesthesia chamber is also evident in the picture. Right: illustration of the apparatus of PET positioned within a MRI 3 T.
For greater spatial coverage of gadgets PET in experimental systems appeared another more useful form of detector, the avalanche photodiodes (Avalance photodiodes - APDs). Have been used in prototypes imaging small animal PET to detect light or BGO scintillator crystals LSO crystals. it main advantage is that they operate in high magnetic fields without degradation performance. Furthermore, the semiconductor detectors is very compact, thus offering the opportunity to build very compact modules with potentially minimal intervention design of the hybrid system. The figure below shows a pipe photomultiplier, such systems are usually in a PET matrix which APD used for reading of scintillation crystals.
Photomultiplier tube, such systems are usually in a PET matrix and APD detector. This image clearly depicts the difference in size of the two systems.
The Siemens initially in 2007 presented the first results of this approach using a coil brain with combined LSO crystals and APD detectors.
Brain PET system from Siemens which is placed in 3T MRI scanner. In the bottom row observe brain imaging with this combined method.
The combination of PET and MRI presents five technical challenges :
1. The need for sensors operating at high magnetic field
2. Field uniformity better than 1 ppm
3. mitigate interference RF.
4. design, material and size of the shield
5. and of course the cost.
The first two clinics installed MR-PET systems installed by U.S. Philips (Mount Sinai Medical http://www.mssm.edu/research/labs/imaging-sciencelaboratories/facilities) and in Europe (Uninversity General Hospital of Geneva http://www.hug-ge.ch/hug_cite/inauguration_PET_IRM.html). Major clinical applications MR-PET is oncology, cardiology and neurology . The research studies conducted at this time are primarily designed to understand the benefits of new diagnostic method.
At present , Siemens is the only company that offers a comprehensive set of body and simultaneous acquisition PET / MRI system . This system was approved by the FDA and released for customer purchase in 2011. More than a dozen facilities have done with this technology. In the U.S., three facilities already use this technology for the care of research and patient. In Europe , there are many most systems, with most of them in Germany.
The General Electric (GE Trimodality) essentially promotes a system with PET / CT and MRI in two scanners and a mobile examination table.
It is worth noting that the Siemens located near the completion of the development system MR / PET with section 7 and 9.4 Tesla.
Depiction of the structure of a single hybrid MR / PET scanner.
The single hybrid modality MR / PET of the Siemens Biograph mMR.
Open structure - serial hybrid scanner MR / PET.
The serial hybrid modality MR / PET of Philips Ingenuity TF.
Desktop control system Siemens Biograph mMR and superimpose PET images and MRI (right panel).
Imaging of the liver and breast from the MR/ PET system Philips Ingenuity TF.
Imaging of the chest and whole body dataset from the MR / PET system from Siemens Biograph mMR.
The role of MRI
Although the PET / CT has shown clinical value may not be the ultimate diagnostic tool relative to the MRI, which offers many advantages imaging and quantification level :
1. The MRI does not burden the patient with radiation exposure.
2. The administration of potentially nephrotoxic iodinated contrast media through or radioactive tracers
is not necessary.
3. MRI provides much greater contrast soft tissues. This has proved to be necessary in imaging applications in neurology , the musculoskeletal, in cardiology and oncology (eg detection and characterization of focal lesions the liver) .
4. MRI allows additional techniques such as angiography, functional MRI ( e.g. brain activation studies ), spectroscopic imaging, whole body imaging (DWIBS, T1 , T2 , STIR), diffusion (DWI, DTI) and technical
dynamic gadolinium infusion studies in a single examination at times scan ranging from 15 minutes to one hour.
5. Finally , MRI is emerging as a highly efficient method for molecular imaging . This technique , possibly in combination with the rational use targeted therapies , could profoundly affect the practice of clinical diagnosis and treatment course as these technologies continue to mature .
The combination of MRI and PET devices into a single hybrid system offers several advantages compared with the PET / CT and MRI alone.
Comparison of DWIBS and PET / CT in oncology. Whole-body imaging DWIBS show significantly higher sensitivity and specificity, spatial resolution, lower scanning time, no need for contrast agents and radiation dose (MRI 92% / 88% - PET / CT 85% / 78%, sensitivity / specificity).
Conclusions
The MR / PET is a valuable clinical tool with the advantage of a minimum radiation exposure compared to the PET / CT. Many innovative methods for the examination are now available in oncology, neurology, and cardiology. The construction of clinical systems MR / PET is now possible, offering the possibility for true simultaneous PET and MR imaging, opening new avenues of diagnosis.
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