For research purposes, postmortem MRI of the human brain offers an advantage over in vivo imaging in that a postmortem sample can be sliced for histological examination immediately after the MRI scan (1-13), or otherwise tested in ways that are not appropriate for living subjects (14). However, postmortem imaging of the brain also presents new challenges that have not been dealt with for in vivo imaging. In particular, the MRI properties of postmortem tissue can change rapidly as a result of decomposition and chemical fixation. These largely uncharacterized changes in the tissue properties of the postmortem brain can lead to errors in the interpretation of MR findings, and can pose complications in the selection of appropriate data acquisition parameters. The purpose of this work was to investigate the MR-related changes that occur in a human brain hemisphere during formaldehyde fixation.The goal of chemical fixation is to preserve postmortem tissue in a state similar to that found in vivo (15). This can be accomplished by immersing the brain in a solution that contains a fixative agent such as formaldehyde. Over time, the fixative agent diffuses inward from the surface of the brain, slowing or stopping tissue decomposition. However, until complete fixation occurs, postmortem tissue remains vulnerable to bacterial degradation and autolysis (15). Formaldehyde fixation of the tissue promotes protein cross-linking (16 -18) and immobilization of water molecules and may lead to a reduction of the T 2 relaxation time (19). Conversely, decomposition of the tissue may lead to increased water content or increased water mobility which may increase T 2 values. Thus, T 2 relaxation can potentially provide important information about the changes that occur in brain tissue during fixation.Postmortem brain MRI studies have demonstrated that T 2 values are lower in a fixed postmortem brain than in vivo, for both white and gray matter (20 -23). One investigation indicated that T 2 values at locations near the surface of the brain decreased sharply within the first 7 days of formaldehyde fixation, reaching a plateau after that (up to 5 weeks postmortem) (20). In another study, three fixed brains were each scanned up to 21 times over the course of 3 weeks, and it was shown that T 2 values of tissue located within 1.4 cm of the brain surface decreased sharply and reached a plateau within approximately 5 days postmortem (21). In an 11-week-long study, the T 2 values of frontal gray matter in formaldehyde-fixed brains were observed to decrease sharply in the first 10 h postmortem (22). None of the aforementioned studies explicitly reported on T 2 changes in deep brain tissue, nor have prior studies examined fixed tissue over a period of more than 3 months.Sustained T 2 increases in any kind of fixed postmortem tissue have seldom been reported (14,16). In the most recent study to report such a T 2 increase, a small sample of bovine nasal cartilage was immersed for 9 weeks in 0.1% formalin (ϳ0.04% formaldehyde) solution (16). During th...

Background Phase analysis of cardiac arrhythmias, particularly atrial fibrillation (AF), has gained interest due to the ability to detect organized stable drivers (rotors) and target them for therapy. However, the lack of methodology details in publications on the topic has resulted in ongoing debate over the phase mapping technique. By comparing phase maps and activation maps we examined advantages and limitations of phase mapping. Methods and Results 7 subjects were enrolled. We generated phase maps and activation maps from ECGI-reconstructed epicardial unipolar electrograms (EGMs). For ventricular signals, phase was computed with: i) pseudo-empirical mode decomposition (pEMD) detrending, and ii) a novel Moving Average (MVG) detrending approach. For AF signals, MVG was modified to incorporate cycle length (CL) changes (MVG-DCL). Phase maps were visually analyzed to study phase singularity points and rotors. Results show that phase is sensitive to CL choice, a limitation that was addressed by the MVG-DCL algorithm. MVG-DCL was optimal for AF analysis. Phase maps helped to highlight high-curvature wavefronts and rotors. However, for some activation patterns phase generated non-rotational singularity points and false rotors. Conclusions Phase mapping computes singularity points and visually highlights rotors. As such, it can help to provide a clearer picture of the spatiotemporal activation characteristics during AF. However, it is advisable to incorporate EGM characteristics and the time-domain AT sequence in the analysis, to prevent misinterpretation and false rotor detection. Therefore, for mapping complex arrhythmias, a combined time-domain activation and phase mapping with variable CL appears to be the most reliable method.