Hybrid PT/MR Scanner May Offer Viable Alternative to PET/CT

A new technique for correcting data distortions could help make combined positron emission tomography (PET)/magnetic resonance (MR) imaging technology possible.

A major obstacle to developing a hybrid PET/MR imaging system could be eliminated by using an innovative approach for reconstructing data, according to researchers at SNM's (formerly the Society of Nuclear Medicine) 56th annual meeting in Toronto, Canada, held in June 2009. Many researchers view fused PET/MR as a significant next step in improving imaging capabilities and believe that PET/MR could become a viable alternative to hybrid PET/computed tomography (CT) systems.

"Providing both PET and MRI capabilities in a single room could be a powerful tool for research, and eventually, for patient care,” said André Salomon, molecular imaging systems, Philips Research (Aachen, Germany). "Our successful approach to addressing a major shortcoming in data reconstruction could be an important breakthrough in the development of such a system.”

A combined PET/MR system could provide the specific molecular information related to cell surface reactors, enzymes, and gene expression that PET provides. At the same time, clinicians could use MRI to see important anatomic data, soft-tissue contrast, and information about perfusion and permeability. However, a combined PET/MRI cannot provide accurate, effective images unless it includes a method to account for PET attenuation. Attenuation refers to the scattering of photons that should be detected by PET scanners, but fall out of their range or are absorbed by the body instead.

In combined PET/CT systems, attenuation mapping is performed routinely based on available CT transmission data. Researchers are working on many alternatives to provide attenuation for PET/MR imaging. Most of these involve methods of segmentation, atlas-based registration, and computer-learning techniques using databases with MR and corresponding CT images.

In this study, an innovative new method for estimating attenuation uses a data reconstruction approach that simultaneously computes the activity and the attenuation distribution using the MR image as a geometric reference. In this way, the true physical attenuation of the photons provided by the PET data is measured. Early findings from simulated and measured clinical data that were compared to reference data from CT attenuation maps indicate excellent agreement between the two techniques. Moreover, performing time-of-flight reconstruction, which measures how long gamma rays produced by radionuclides reach PET detectors, also improves accuracy.

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