Combination Device from MRI and PET Faster and More Precise
How is simultaneous imaging from two medical pieces of equipment possible that were considered incompatible just a few years earlier? How can we ensure that the patient profiles presented correspond to reality? The Teaching and Research Area Molecular Imaging Systems, led by Univ.-Prof. Dr.-Ing. Volkmar Schulz at the Institute of Experimental Molecular Imaging at RWTH Aachen University Hospital, is one step closer to answering this question. Researchers developed a method that makes magnetic resonance imaging, MRI, and positron emission tomography, PET, simultaneously possible and more precise.
Nuclear Medicine in High Magnetic Fields
Such a combined machine is very technically sophisticated, because a MRI scanner employs strong magnetic fields that are 50000 times as strong as the Earth's magnetic field. PET scanners until now have not worked in such an environment. On the other hand, MRI scanners are sensitive to the electromagnetic radiation of PET scanners until now. Both processes play an important role, for example in diagnosing cancer.
Current PET scanners are still primarily combined for unity with computer tomography, CT, in order to simultaneously get pictures of the anatomic structures of the organism. In contrast, a MRI-PET scanner lacks a CT unit – both because of the strong magnetic field, but also in order to spare patients from the radiation dose of a CT examination. The CT images are almost always replaced by MRI pictures during diagnosis. MRI pictures provide a much broader range of image contrasts, thus enabling more widely differing interpretations. CT images, however, are still essential in this application: in order to correct the decrease in radioactive radiation through the organism, CT images are necessary that directly illustrate the attenuation characteristics of the body. If attenutation is not corrected, the inside of the body appears "dark" on PET images, because the radiation is decreased there more strongly than at the surface of the body. This greatly affects diagnoses for internal organs.
The Aachen work group has developed a method with which images similar to CT images can be obtained from MR images with a level of detail that was not previously possible. "These can be used for PET attenuation correction instead of a CT image," says Schulz. The work was recognized in June with the "JNM Editors' Choice Award" by the Journal of Nuclear Medicine. The primary author of the paper, Dipl.-Ing. Yannick Berker from the Institute of Experimental Molecular Imaging at Aachen University Hospital, is also being honored with the "Alavi-Mandell Award.“ The "Alavi-Mandell Award“ is awarded to authors who are still in training at the time of completion of the paper.
Four Tissue Classes for Head and Neck Examinations
In order to imitate CT images as accurately as possible, image regions in the MRI images are recognized and categorized, and they are corrected using the known attenuation values for the tissue class in question. It is difficult, however, to differentiate between bones and enclosed air which in many MR images look the same. It is similarly problematic to distinguish between fat and other soft tissue. There are, however, solutions to this problem: with the help of ultrashort echo time sequences, bones can be made visible, and so-called MR Dixon sequences are used to distinguish fat and soft tissue.
The Aachen scientists have now combined these methods for the first time. According to Schulz, “it just takes too much time to apply these methods in succession; further, the movements of the patient result in image registration problems.” The new development enhances both image accuracy and patient comfort, and by keeping the medical examination as short as possible it is also economically efficient. Only by using the combined PET-MRI method, the tissue classes enclosed air, fat tissue, soft tissue, and bones can be distinguished in a quick and efficient manner.
The interdisciplinary project was conducted in collaboration between University Hospital Aachen; the Philips Research Division, Aachen; and research teams from London, Maastricht, and New York. The RWTH work group focused on the integration of the MRI and PET image information, which requires research on new MRI-compatible PET scanners, new image reconstruction methods, as well as the testing of the new high-resolution pre-clinical MRI-PET imaging devices. Furthermore, the group is investigating innovative imaging methods, such as Magnetic Particle Imaging, MPI for short.
Univ.-Prof. Dr.-Ing. Volkmar Schulz
Helmholtz Institute for Biomedical Engineering
Tel. +49 241 80 80116