A Bayesian spatiotemporal model for very large data sets.

Functional MRI provides a unique perspective of neuronal organization; however, these data include many complex sources of spatiotemporal variability, which require spatial preprocessing and statistical analysis. For the latter, Bayesian models provide a promising alternative to classical inference, which uses results from Gaussian random field theory to assess the significance of spatially correlated statistic images. A Bayesian approach generalizes the application of these ideas in that (1) random fields are used to model all spatial parameters, not solely observation error, (2) their smoothness is optimized, and (3) a broader class of models can be compared. The main problem, however, is computational, due to the large number of voxels in a brain volume. Sampling methods are time-consuming; however, approximate inference using variational Bayes (VB) offers a principled and transparent way to specify assumptions necessary for computational tractability. Penny et al. (2005b) described such a scheme using a joint spatial prior and approximated the joint posterior density with one that factorized over voxels. However, a further computational bottleneck is encountered when evaluating the log model evidence used to compare models. This has lead to dividing a brain volume into slices and treating each independently. This amounts to approximating the spatial prior over a full volume with stacked 2D priors. That is, smoothness along the z-axis is not included in the model. Here we describe a VB scheme that approximates the zero mean joint spatial prior with a non-zero mean empirical prior that factors over voxels, thereby overcoming this problem. We do this by modifying the original VB algorithm of Penny et al. using the conditional form of a so-called conditional autoregressive (CAR) prior to update a marginal prior over voxels. We refer to this as a spatially-informed voxel-wise prior (SVP) and use them to spatially regularise general linear model (GLM) and autoregressive (AR) coefficients (over time to model serial correlations). This algorithm scales more favourably with the number of voxels providing a truly 3D spatiotemporal model over volumes containing tens of thousands of voxels. We compare the scaling of compute times with the number of voxels and performance with a joint prior applied to synthetic and single-subject data.

SABRE hyperpolarisation of vitamin B3 as a function of pH.

In this work we describe how the signal enhancements obtained through the SABRE process in methanol-d4 solution are significantly affected by pH. Nicotinic acid (vitamin B3, NA) is used as the agent, and changing pH is shown to modify the level of polarisation transfer by over an order of magnitude, with significant improvements being seen in terms of the signal amplitude and relaxation rate at high pH values. These observations reveal that manipulating pH to improve SABRE enhancements levels may improve the potential of this method to quantify low concentrations of analytes in mixtures. 1H NMR spectroscopy results link this change to the form of the SABRE catalyst, which changes with pH, resulting in dramatic changes in the magnitude of the ligand exchange rates. The presented data also uses the fact that the chemical shifts of the nicotinic acids NMR resonances are affected by pH to establish that hyperpolarised 1H-based pH mapping with SABRE is possible. Moreover, the strong polarisation transfer field dependence shown in the amplitudes of the associated higher order longitudinal terms offers significant opportunities for the rapid detection of hyperpolarised NA in H2O itself without solvent suppression. 1H and 13C MRI images of hyperpolarised vitamin B3 in a series of test phantoms are presented that show pH dependent intensity and contrast. This study therefore establishes that when the pH sensitivity of NA is combined with the increase in signal gain provided for by SABRE hyperpolarisation, a versatile pH probe results.

Utilisation of water soluble iridium catalysts for signal amplification by reversible exchange.

The catalytic hyperpolarisation of pyridine, 3-hydroxypyridine and oxazole by the Signal Amplification By Reversible Exchange (SABRE) process is achieved by a series of water soluble iridium phosphine and N-heterocyclic carbene dihydride complexes. While the efficiency of the SABRE process in methanol-d4 solution or ethanol-d6 solution is high, with over 400-fold (1)H polarisation of pyridine being produced by [Ir(H)2(NCMe)(py)(IMes)(monosulfonated-triphenylphosphine)]BF4, changing to a D2O or a D2O-ethanol solvent mixture leads to dramatically reduced activity which is rationalised in terms of low H2 solubility.

Detection and direction-discrimination of diotic and dichotic ramp modulations in amplitude and phase.

When the source of a tone moves with respect to a listener's ears, dichotic (or interaural) phase and amplitude modulations (PM and AM) are produced. Two experiments investigated the psychophysical characteristics of dichotic linear ramp modulations in phase and amplitude, and compared them with the psychophysics of diotic PM and AM. In experiment 1, subjects were substantially more sensitive to dichotic PM than diotic PM, but AM sensitivity was equivalent in the dichotic and diotic conditions. Thresholds for discriminating modulation direction were smaller than detection thresholds for dichotic AM, and both diotic AM and PM. Dichotic PM discrimination thresholds were similar to detection thresholds. In experiment 2, the effects of ramp duration were examined. Sensitivity to dichotic AM and PM, and diotic AM increased as duration was increased from 20 ms to 200 ms. The functions relating sensitivity to ramp duration differed across the stimuli; sensitivity to dichotic PM increased more rapidly than sensitivity to dichotic or diotic AM. This was also reflected in shorter time-constants and minimum integration times for dichotic PM detection. These findings support the hypothesis that the analysis of dichotic PM and AM rely on separate mechanisms.