Comparison of 3D-OP-OSEM and 3D-FBP reconstruction algorithms for High-Resolution Research Tomograph studies: effects of randoms estimation methods

The High-Resolution Research Tomograph (HRRT) is a dedicated human brain positron emission tomography (PET) scanner. Recently, a 3D filtered backprojection (3D-FBP) reconstruction method has been implemented to reduce bias in short duration frames, currently observed in 3D ordinary Poisson OSEM (3D-OP-OSEM) reconstructions. Further improvements might be expected using a new method of variance reduction on randoms (VRR) based on coincidence histograms instead of using the delayed window technique (DW) to estimate randoms. The goal of this study was to evaluate VRR in combination with 3D-OP-OSEM and 3D-FBP reconstruction techniques. To this end, several phantom studies and a human brain study were performed. For most phantom studies, 3D-OP-OSEM showed higher accuracy of observed activity concentrations with VRR than with DW. However, both positive and negative deviations in reconstructed activity concentrations and large biases of grey to white matter contrast ratio (up to 88%) were still observed as a function of scan statistics. Moreover 3D-OP-OSEM+VRR also showed bias up to 64% in clinical data, i.e. in some pharmacokinetic parameters as compared with those obtained with 3D-FBP+VRR. In the case of 3D-FBP, VRR showed similar results as DW for both phantom and clinical data, except that VRR showed a better standard deviation of 6-10%. Therefore, VRR should be used to correct for randoms in HRRT PET studies.

The neural correlates of verbal encoding and retrieval in monozygotic twins at low or high risk for depression and anxiety

Emotional processing and brain activation were examined during an encoding and recognition paradigm using emotionally salient words in a sample of monozygotic twin pairs at low or high risk for anxiety and depression. Discordant twin pairs were used to chart the effects of environmental risk factors and concordant twin pairs were used to chart the effects of genetic risk factors on performance and brain activation. Performance data did not support the existence of a negative response bias in subjects at high risk. At the neural level, however, increased left inferior frontal gyrus (LIFG) activation by negative words was found in high-risk subjects, most prominently during recognition. Increased LIFG activity was found in subjects at high risk through either genetic or environmental risk factors. These results suggest that fMRI activation of the LIFG in a verbal emotional memory task may be a useful vulnerability marker for anxiety and depression.

Intrapair differences in hippocampal volume in monozygotic twins discordant for the risk for anxiety and depression

BACKGROUND

Current biological psychiatric models assume that genetic and environmental risk factors for anxiety and depression act on the same brain structures.

METHODS

To test this assumption, we assessed brain anatomy by using optimized voxel-based morphometry on magnetic resonance images obtained in monozygotic twin pairs who were discordant for the risk of anxiety and depression (n = 10 pairs) and in monozygotic twin pairs who were concordant for high (n = 7 pairs) or low (n = 15 pairs) risk for anxiety and depression.

RESULTS

We observed volume reductions in the temporal lobe, most notably in the left posterior hippocampal region in subjects at high risk for anxiety and depression, but exclusively in the intrapair comparison of discordant monozygotic twins. Because monozygotic twins are genetically identical, any discordance in their risk for anxiety and depression and hippocampal volume must arise from differential exposure to environmental influences. A group comparison between pairs concordant for low or high risk, which is more likely to reflect differences in genetic vulnerability, did not show reduced temporal-lobe and posterior hippocampal volumes in the pairs at high risk for anxiety and depression.

CONCLUSIONS

This pattern of results suggests that damage to temporal-lobe structures may be specific to an environmentally driven etiology of anxiety and depression.