Lec2 Methods for assessing psychological dysfunction B Flashcards
Diffusion-weighted images (DWI) is the most sensitive to _______
stroke
what is diffusion-weighted images measuring
the intensity of each voxel reglects the best estimate of the rate of water molecules.
because the mobility of water is driven by the thermal agitation and highly dependent on cellular environment, DWI hypothesis that findings may indicate early pathologic changes
what is diffusion-tensor imaging (DTI)
diffusion-tensor imaging is a non invasive way of undestanding brain structural connectivity
contrast based on the directional rate of diffusion of water molecule
statistical methods for DTI still in infancy
because it can reveal abnormalities in white matter fiber structure and provide models of brain connectivity, it is rapidly becoming a standard for white matter disorders
what is isotropic diffusion and what method is most applicable when tissue of interest is dominated by isotrpic water diffusion
DWI is most applicable when the tissue of interst is dominated by isotopic water movement
e.g., grey matter in cerebral cortex and major brain nuclei, or in the body where diffusion rate appears to be the smae when measured along any axis
what is anistropic diffusion and what method is most applicable when tissue of interest is dominated by anisotrpic diffusion
DTI is important when a tissue (e.g., neural axons of white matter in the brain or muscle fibres in the heart) has an internal fibrous structure analogous to the anisotropicy of some crystals
water will the ndiffuse more rapidly in the direction aligned with the internal structure and more slowly as it moves perpendicular to the preferred direction
this means measurement rate of diffusion will differ deoending on the direction from which an observer is looked
Voxel based morphometry
take a group of brain MR-images and their asssociated variable,
normalize each brain to a template,
smooth to deal with individual differences in brain anatomy
compare groups or single subject to the group using gaussian statistics in cluding a correction for multiple comparisons
output the result as a statistical map
advantages of Voxel-ased morphometry(VBM)
objective criterion for a lesion
the model can account for variables of no interest (age, gender, etc)
the model can investigate neural correlates of continuous behavioural variables
what is Voxel-based morphometry assuming
can multiple brains be averaged into a standard brain to allow voxel by voxel comparisons?
is there a linear relationship between the neural loss and the intensity of the MR-image
describe voxel-wise behaviour mapping (VLBM)
approaches the same question as VBM using a binary classification -lesion vs. non-lesions
manually delineated lesions (does not assume linearity between neural loss and MR signal intensity)
non-parametric statistics (does not assume normal distribution in the variance in neuronal density from the mean)
e.g., spatial neglect, measure CoC score, delineate the lesion on MR scans
for each voxel, compare behaviour in patients with a lesion with behaviour in patients without a lesion, output a map showing voxels whose lesion is associated with a higher (or lower) behavioural scores
Pros and cons of VLBM
advantages: fewer assumptions
disadvantages: more laborious, subjective, assumption that brain normalisation works
what is the mechanism of fMRI
increasing neuronal activity requires more oxygen;
the amount of oxygen delivered by blood flow actually tends to exceed demand, creasting a ratio of oxygenated to deoxygenated blood that is known as the BOLD signal
BOLD signal can be precisely and accurately measured and quantifies, it is highly localizable (normally by mapping BOLD respose onto a structural MRI) at an individual subject level, giving fMRI a remarkbly high deree of spatial resolution which permits visualization of brain areas that are ‘activated’ during various cognitive tasks
describe BOLD signal used in fMRI
oxygenated haemoglobin (HbO2) has same magnetic propoerties as surrounding tissue, no signal disruption, normal flow, basal level Hbr, basal CBV, normal MRI signal)
de-oxygenated haemoglobin(Hbr) has different ‘paramagnetic’ properties and disrupts signal from surrounding tissue, increased flow, decreased Hbr (lower field gradients around vessels), increased CBV, increased MRI signal (from lower field gradients)
why is fMRI analysis of stroke is difficult
Hemodynamic response odften dirupted
Misery perfusion: system always compensating for reduced bloodflow, so no dynamic ability to increase
luxury perfusion: destroyed tissue no longer requires blook, so regulation not required for surviving tissue
evaluation of fMRI
advantages: non-invasive, spatial resolution temporal resolution, the typical interval between events ~2s, who brain can be imaged t the same time
Disadvantages: expensive, noisy, strictly no metal in the scanner room, correlation not causation, requires careful statistical analysis
describe positron emission tmography (PET)
intravenous injection of a radioactive tracer (specifically position-emitting substances) and subsequent scanning of the brain for radioactivity
investigating the dynamic properties of the brain
radioactive tracer decays in the emission of two photons travelling in oppositite direction, generating energy that detectors around the scalp can measure
PET functions by calculating millions of counts from detectors and estimating their origin
