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| We feel that this is unlikely to be of much benefit. PACE attempts online correction of subject movement, by tracking the apparent movement of the EPI volumes, and adjusting the gradient angles to try and make the angles of acquisition follow the movement of the EPI. Our worry about this technique is that estimation of movement parameters can be biased by large activations - see CbuImaging:PrinciplesSpatialProcessing. If PACE is disabled, we can look for this problem in the spatial processing phase of the analysis, and try and address it by using different realignment methods, or not doing motion correction. If PACE is enabled, the acquisition itself has changed as a result of the movement parameter estimates, and it is no longer possible to do post-hoc correction. An argument put forward sometimes for online motion correction is that by moving the excitation with the head, spin history effects will be reduced. However, as the PACE method involves deriving the motion parameters from the previous EPI, which takes some time, the parameters are adjusted with lag of 1 TR. Any spin history effects will not therefore be corrected in time. PACE might however be useful where there is the danger that the brain region of interest will leave the imaging field of view, for example, where only a few slices are being collected, or with a patient that might move substantially. The problem with assessing PACE is that we would expect the problems to depend on the location and size of activation, so a single study will be unable to answer the question as to whether this is worthwhile or not. |
We feel that this is unlikely to be of much benefit - see CbuImaging:TipsForDataAcquisition for a discussion. The problem with assessing PACE is that we would expect the problems to depend on the location and size of activation, so a single study will be unable to answer the question as to whether this is worthwhile or not. |
Requests for routine methods studies
Sometimes there will be a genuine empirical question about differences in acquisition or analysis methods that can only be well answered by acquiring data across several subjects and comparing techniques.
As with all studies, this has to be of significant interest and importance to someone, in order justify scanning and analysis time. In practice this means that if we think the parameters under study are likely to have a significant effect on our analysis pipeline, then we will perform the study. Otherwise, we are very happy to support both analysis and acquisition, but will expect the interested researcher to take the lead in analysis.
We will use this page to put up ideas for such studies, and comment on their importance. Please send requests for studies to MatthewBrett, or another member of the AdminGroup for this wiki.
Suggested studies
Comparison of acquisition with Siemens PACE enabled and disabled
We feel that this is unlikely to be of much benefit - see TipsForDataAcquisition for a discussion. The problem with assessing PACE is that we would expect the problems to depend on the location and size of activation, so a single study will be unable to answer the question as to whether this is worthwhile or not.
Comparison of interleaved and sequential slice acquisition in FMRI
As a result of work by RhodriCusack, we have recently changed the default FMRI slice acquisition from interleaved to sequential. See TipsForDataAcquisition for discussion. There are theoretical and practical reasons to think that the sequential method is less prone to artefacts, and we think it most unlikely that there would be an easily measurable difference between the techniques in favour of interleaved.
Optimum jittering for event related design
There are well worked out methods of assessing a design for optimum jittering - see [http://www.mrc-cbu.cam.ac.uk/~matthew/abstracts/ER/er_analysis.html my event-related analysis poster and abstract] and DesignEfficiency and references therein. A rule of thumb is that the effect of jittering is predictable from the design until events come close to 1 second apart, at which point there are significant non-linearities, which will render the standard models of design efficiency invalid.