Biomedical subjects

Anne Larsson

Publications and source records attributed to Anne Larsson.

11 recordsLinked to original sources

PubMed2006

Correction for scatter and septal penetration using convolution subtraction methods and model-based compensation in 123I brain SPECT imaging-a Monte Carlo study.

Scatter and septal penetration deteriorate contrast and quantitative accuracy in single photon emission computed tomography (SPECT). In this study four different correction techniques for scatter and septal penetration are evaluated for 123I brain SPECT. One of the methods is a form of model-based compensation which uses the effective source scatter estimation (ESSE) for modelling scatter, and collimator-detector response (CDR) including both geometric and penetration components. The other methods, which operate on the 2D projection images, are convolution scatter subtraction (CSS) and two versions of transmission dependent convolution subtraction (TDCS), one of them proposed by us. This method uses CSS for correction for septal penetration, with a separate kernel, and TDCS for scatter correction. The corrections are evaluated for a dopamine transporter (DAT) study and a study of the regional cerebral blood flow (rCBF), performed with 123I. The images are produced using a recently developed Monte Carlo collimator routine added to the program SIMIND which can include interactions in the collimator. The results show that the method included in the iterative reconstruction is preferable to the other methods and that the new TDCS version gives better results compared with the other 2D methods.

Algorithms↗

PubMed2006

Reduced functional brain activity response in cognitively intact apolipoprotein E epsilon4 carriers.

The apolipoprotein E epsilon4 (APOE epsilon4) is the main known genetic risk factor for Alzheimer's disease. Genetic assessments in combination with other diagnostic tools, such as neuroimaging, have the potential to facilitate early diagnosis. In this large-scale functional MRI (fMRI) study, we have contrasted 30 APOE epsilon4 carriers (age range: 49-74 years; 19 females), of which 10 were homozygous for the epsilon4 allele, and 30 non-carriers with regard to brain activity during a semantic categorization task. Test groups were closely matched for sex, age and education. Critically, both groups were cognitively intact and thus symptom-free of Alzheimer's disease. APOE epsilon4 carriers showed reduced task-related responses in the left inferior parietal cortex, and bilaterally in the anterior cingulate region. A dose-related response was observed in the parietal area such that diminution was most pronounced in homozygous compared with heterozygous carriers. In addition, contrasts of processing novel versus familiar items revealed an abnormal response in the right hippocampus in the APOE epsilon4 group, mainly expressed as diminished sensitivity to the relative novelty of stimuli. Collectively, these findings indicate that genetic risk translates into reduced functional brain activity, in regions pertinent to Alzheimer's disease, well before alterations can be detected at the behavioural level.

Aged↗

PubMed2006

Reduced hippocampal volume in non-demented carriers of the apolipoprotein E epsilon4: relation to chronological age and recognition memory.

Apolipoprotein E epsilon4 (APOE epsilon4) is the main known genetic risk factor for Alzheimer's disease (AD). Some previous studies have reported structural brain changes as well as cognitive deficits in non-demented APOE epsilon4 carriers, but the pattern of results is inconsistent and studies with larger sample sizes have been called for. Here we compared hippocampal volume and recognition-memory performance between AD-symptom-free carriers (N=30) and non-carriers (N=30) of the APOE epsilon4 (age range: 49-79 years). We observed reduced right hippocampal volume in APOE epsilon4 carriers, and found that the difference was most pronounced before the age of 65. Further, the APOE epsilon4 carriers made significantly more false alarms in the recognition-memory test, and the number of false alarms correlated significantly with right hippocampus volume. These results indicate that relatively young individuals at genetic risk for AD have smaller hippocampal volume and lower performance on hippocampal-dependent cognitive tasks. A question for the future is whether smaller hippocampal volume represents early-onset hippocampal volume reduction or an inherent trait.

Age Factors↗

PubMed2005

Learning by doing versus learning by thinking: An fMRI study of motor and mental training.

Previous studies have documented that motor training improves performance on motor skill tasks and related this to altered functional brain activity in cerebellum, striatum, and frontal motor cortical areas. Mental training can also improve the performance on motor tasks, but the neural basis of such facilitation is unclear. The purpose of the present study was to identify neural correlates of training-related changes on a finger-tapping task. Subjects were scanned twice, 1 week apart, with fMRI while they performed two finger-tapping sequences with the left hand. In-between scans, they practiced daily on one of the sequences. Half of the participants received motor training and the other half received mental training (motor imagery). Both training procedures led to significant increases in tapping performance. This was seen for both the trained and the untrained sequence (non-specific effect), although the gain was larger for the trained sequence (sequence-specific effect). The non-specific training effect corresponded to a reduction in the number of activated areas from an extensive set of brain regions prior to training to mainly motor cortex and cerebellum after training. The sequence-specific training effect involved the supplementary motor area and the cerebellum for motor training and visual association cortex for mental training. We conclude that gains following motor and mental training are based on distinct neuroplastic changes in the brain.

Adult↗

PubMed2005

Structure-function correlates of cognitive decline in aging.

To explore neural correlates of cognitive decline in aging, we used longitudinal behavioral data to identify two groups of older adults (n = 40) that differed with regard to whether their performance on tests of episodic memory remained stable or declined over a decade. Analysis of structural and diffusion tensor imaging (DTI) revealed a heterogeneous set of differences associated with cognitive decline. Manual tracing of hippocampal volume showed significant reduction in those older adults with a declining memory performance as did DTI-measured fractional anisotropy in the anterior corpus callosum. Functional magnetic resonance imaging during incidental episodic encoding revealed increased activation in left prefrontal cortex for both groups and additional right prefrontal activation for the elderly subjects with the greatest decline in memory performance. Moreover, mean DTI measures in the anterior corpus callosum correlated negatively with activation in right prefrontal cortex. These results demonstrate that cognitive decline is associated with differences in the structure as well as function of the aging brain, and suggest that increased activation is either caused by structural disruption or is a compensatory response to such disruption.

Adaptation, Physiological↗

PubMed2005

Memory-provoked rCBF-SPECT as a diagnostic tool in Alzheimer's disease?

PURPOSE: Alzheimer's disease (AD) is a primary degenerative disease that progressively affects all brain functions, with devastating consequences for the patient, the patient's family and society. Rest regional cerebral blood flow (rCBF) could have a strategic role in differentiating between AD patients and normal controls, but its use for this purpose has a low discriminatory capacity. The purpose of this study was to evaluate whether the diagnostic sensitivity of rCBF single-photon emission computed tomography (SPECT) could be increased by using an episodic memory task provocation, i.e. memory-provoked rCBF-SPECT (MP-SPECT). METHODS: Eighteen persons (73.2+/-4.8 years) with mild AD and 18 healthy elderly (69.4+/-3.9 years) were included in the study. The subjects were injected with (99m)Tc-hexamethylpropylene amine oxime (HMPAO) during memory provocation with faces and names, followed by an rCBF-SPECT study. The rCBF (99m)Tc-HMPAO SPECT images were analysed using statistical parametric mapping (SPM2). Peaks with a false discovery rate corrected value of 0.05 were considered significant. RESULTS: On MP-SPECT, the AD group showed a significant rCBF reduction in the left parietal cortex in comparison with healthy elderly. At rest, no significant group differences were seen. CONCLUSION: Memory provocation increased the sensitivity of rCBF-SPECT for the detection of AD-related blood flow changes in the brain at the group level. Further studies are needed to evaluate MP-SPECT as a diagnostic tool at the individual level. If a higher sensitivity for AD at the individual level is verified in future studies, a single MP-SPECT study might be sufficient in the clinical setting.

Adult↗

PubMed2005

Treatment of chronic stress in employees: subjective, cognitive and neural correlates.

This study reports the effect of an affect-focused intervention program, the Affect School, on stress, psychological symptoms, cognitive functioning and neural activity. Fifty employees in social service and education, with high levels of chronic stress, were randomly divided into a treatment (N=27) and control (N=23) group. Complete sets of data were available in 20 participants in the treatment group and 17 in the control group. The Perceived Stress Questionnaire assessed stress and the Symptom Check List-90 psychological symptoms before and after treatment. Episodic-memory functioning under focused and divided attention conditions was also assessed. Prior and after the Affect School, seven participants in the treatment group were studied with functional magnetic resonance imaging (fMRI) during episodic memory processing. After the Affect School there was a reduction in stress and psychological symptoms for the treatment group but not in the control group. The controls showed a reduction in episodic memory functioning whereas the performance of the treatment group remained intact. The fMRI scanning indicated a qualitative change in the neural network subserving episodic memory. These preliminary results suggest that the Affect School is effective on individuals with high stress.

Adaptation, Psychological↗

PubMed2004

Visual consciousness: dissociating the neural correlates of perceptual transitions from sustained perception with fMRI.

To investigate the possible dichotomy between the neurophysiological bases of perceptual transitions versus sustaining a particular percept over time, an fMRI study was conducted with subjects viewing fragmented pictures. Unlike most other perceptually unstable stimuli, fragmented pictures give rise to only one perceptual transition and a continuous period of sustained perception. Earlier research is inconclusive on the subject of which anatomical regions should be attributed to what temporal aspect of perception, and the aim of the present study was to shed more light on the subject. In this study occipitotemporal and fronto-parietal regions were found to be activated for both aspects. However, regions in the medial-temporal lobe were activated specifically for transitions, whereas medial and dorsolateral prefrontal regions were activated specifically for sustained perception. These results provide further support for the theory that the initial creation of perceptual awareness and upholding perceptual awareness over time are separate processes involving different brain regions.

Adult↗

PubMed2003

Scatter-to-primary based scatter fractions for transmission-dependent convolution subtraction of SPECT images.

In single photon emission computed tomography (SPECT), transmission-dependent convolution subtraction has been shown to be useful when correcting for scattered events. The method is based on convolution subtraction, but includes a matrix of scatter fractions instead of a global scatter fraction. The method can be extended to iteratively improve the scatter estimate, but in this note we show that this requires a modification of the theory to use scatter-to-total scatter fractions for the first iteration only and scatter-to-primary fractions thereafter. To demonstrate this, scatter correction is performed on a Monte Carlo simulated image of a point source of activity in water. The modification of the theory is compared to corrections where the scatter fractions are based on the scatter-to-total ratio, using one and ten iterations. The resulting ratios of subtracted to original counts are compared to the true scatter-to-total ratio of the simulation and the most accurate result is found for our modification of the theory.

Computer Simulation↗

PubMed2003

Altered prefrontal brain activity in persons at risk for Alzheimer's disease: an fMRI study.

BACKGROUND: Early diagnosis of Alzheimer's disease (AD) is critical for adequate treatment and care. Recently it has been shown that functional magnetic resonance imaging (fMRI) can be important in preclinical detection of AD. The purpose of this study was to examine possible differences in memory-related brain activation between persons with high versus low risk for AD. This was achieved by combining a validated neurocognitive screening battery (the 7-minutes test) with memory assessment and fMRI. METHODS: One hundred two healthy community-living persons with subjective memory complaints were recruited through advertisement and tested with the 7-minutes test. Based on their test performance they were classified as having either high (n = 8) or low risk (n = 94) for AD. Six high-risk individuals and six age-, sex-, and education-matched low-risk individuals were investigated with fMRI while engaged in episodic memory tasks. RESULTS: The high-risk individuals performed worse than low-risk individuals on tests of episodic memory. Patterns of brain activity during episodic encoding and retrieval showed significant group differences (p < .05 corrected). During both encoding and retrieval, the low-risk persons showed increased activity relative to a baseline condition in prefrontal brain regions that previously have been implicated in episodic memory. By contrast, the high-risk persons did not significantly activate any prefrontal regions, but instead showed increased activity in visual occipito-temporal regions. CONCLUSION: Patterns of prefrontal brain activity related to episodic memory differ between persons with high versus low risk for AD, and lowered prefrontal activity may predict subsequent disease.

Aged↗