18.2. Imaging Flashcards

Question

What is the normal presentation of a proton MR spectrum of the brain? What are the metabolites most commonly used as signals? [EXTRA]

Answer 1

* N-acetyl aspartate (NAA) * Chemically hidden * Marker of **viable** neurons, but is not present in glial cells * Levels decrease when there is neuronal damage * *myo*-inositol * Marker of glial cells * Choline * Marker of membrane changes/disturbances * Creatine * Often used as an internal reference for energy/metabolic measure

Answer 2

* MRI visible lesion load is a relatively poor predictor of clinical disability * This suggests that widespread axonal dysfunction or atrophy may not observed on MRI * If this “Axonal Hypothesis” is correct, measurement of NAA in normal appearing brain should correlate with function

Answer 3

* Large voxel studies indicate that metabolite concentrations are altered in normal appearing brain * NAA is decreased and Choline is increased * Reduced NAA correlates well with a disability score (EDSS) * De Stefano et al, Brain 121, 1469-1477, (1998) * Biochemical changes seen before those in imaging

Answer 4

* Can see dramatic reduction in NAA * As this marker is only found in neurons, these results suggest that there are now fewer neurons

Answer 5

* Phosphorous (31P) * Carbon (13C) * Isotope 31P is 100% naturally abundant, detect signals from various metabolites (see diagram) * ALSO use 31P MRS to measure internal pH of cells in the brain - frequency shift between PCr and inorganic phosphate (Pi) * Contrasting with 31P, the 13C isotope is not abundant naturally (1.1%) * But does have uses and can enrich individual carbons selectively with 13C and fate of the 13C followed as it passes from molecule to molecule * “Labelling” with 13C allows information about metabolic pathways * e.g. start with pyruvate labelled at carbon 2, if it is metabolised via pyruvate dehydrogenase it will label glutamate at carbon 5 - OR if it enters glia and is metabolised via pyruvate carboxylase it will be incorporated into glutamate at carbon 3 * Also use 13C to measure FLUX of carbons into metabolites * In addition to use as a chemical microscope, there is a functional variant - fMRS * fMRS used to look at chemical changes in the brain AS THEY OCCUR

Answer 6

* More GABA, less distraction: a neurochemical predictor of motor decision speed * Sumner et al., Nat Neurosci. 2010;13(7):825-7. * People vary markedly in efficiency they resolve competitive action decisions - even simple ones such as shifting gaze to one stimulus rather than another * Found that individual's ability to rapidly resolve such competition is predicted by the concentration of GABA – the main inhibitory neurotransmitter - in region of frontal cortex relevant for eye movements, but not in a control region (occipital cortex) * Healthy individuals showed variance, and this in turn indicated that subtle differences may alter behaviour

Answer 7

* Positron is an anti-electron; given off during decay of the nuclei of specific radioisotopes (e.g. radioactive fluorine 18F) * When positron meets electron, collision produces two gamma rays with same energy but going opposite directions * Gamma rays leave patient’s body - detected by PET scanner * Use positron emitting radioligands introduced into subject’ s blood and their resultant BINDING or METABOLISM/SEQUESTERING in the brain measured using computerised tomography to reconstruct spatial location of emission * e.g. Local cerebral glucose consumption measured this way using FLUORODEOXYGLUCOSE (18FDOG) * Glucose used as tumours need this and so will readily take up the radioligand * Can also make radioligands specific to certain receptors for more targeted imaging

Answer 8

* Cerebral blood flow * Cerebral blood volume * Oxygen Metabolism * Oxygen extraction * Glucose consumption

Answer 9

* These can measure more precise receptor distribution * Can then compare these between disease and health to further understand pathology

Answer 10

* PK11195 binds to inflammatory mediators, e.g. those involved in macrophage-microglia interactions * Can see huge inflammatory response in the AD patient * Radioligands allow us to visualise this *in vivo*

Answer 11

* There is no one perfect technique * For the highly invasive procedures, they frequently cannot be carried out on humans (or, if they are, can only be done once)

Answer 12

* We are taught to think of the brain in a 'bottom-up' manner, using inputs such as light, sound, taste, touch, smell, nociception to then create responses * However, this is not necessarily the case - the brain is NOT a simple ‘receipt’ organ producing perceptions and experiences by processing bottom up sensory inputs as sole contributor * This led to the conception of 'priors' and the Bayesian view of the brain * This is a probalistic explanation of how events are contingent/dependent on how things were before, and it is these factors that shape what happens next * Priors act almost like a probability and can bias how something might be perceived subsequently * This involves a reasonable degree of anticipation and memory * If the priors are wrong/biased, then they can have a huge impact on the perception of actual inputs - this is true for senses such as pain and vision * E.g. if you expect something to hurt, pain can be exacerbated * Videos were taken during inactivity, and it could be seen that there was still a lot of activity * See lecture for full video, but activity was colour coded and it was shown that spatial areas were activated together * These were the resting state/housekeeping networks * We are not yet sure of their exact function, but they do help to indicate where different networks lie * Experiment showed that even without inputs, functional networks prefer to fire together - indicates independance from sensory input

Answer 13

* EEG = electroencephalography * MEG = magnetoencephalography * NB PET has been largely superceded by fMRI in functional imaging

Answer 14

Roy and Sherrington, 1890

Answer 15

* Deoxyhaemoglobin becomes paramagnetic/similar to a contrast agent, therefore influencing the relaxation of protons in MRI

Answer 16

* Functional magnetic resonance imaging measures brain activity by detecting changes associated with blood flow. * This technique relies on the fact that cerebral blood flow and neuronal activation are coupled. When an area of the brain is in use, blood flow to that region also increases. * Electrical activity is stimulated in pulses * It is possible to detect a difference in the image intensity between 'on' and 'off' states * The contrast created by this can be used to create an image

Answer 17

* Air -\> Black * Fat -\> Black * Bone -\> White * Metal -\> White * Calcium -\> White * Organs, Muscles, Soft tissues -\> Shades of grey

Answer 18

A nuclear medicine imaging technique that uses both PET and CT scanning simultaneously. The images are superimposed.

Answer 19

* Barium sulphate -\> Gastrointestinal imaging * Iodinated contrasts -\> Vascular snd lymphatic imaging, Contrast CTs, Cavities * Air -\> Used alongside other contrast to provide double contrast, since the air is less opaque than the contrast tissue

Answer 20

Magnetically active agents, such as gadolinium.

Answer 21

* Air -\> Black * Fat -\> White (or grey in T2) * Bone -\> Dark * Bone Marrow -\> White (or grey in T2) * Organs, Muscles, Soft tissues -\> Shades of grey * Gadolinium -\> White * Water -\> Dark (but WHITE in T2)

Answer 22

Although fat will appear darker in T2, water will appear much brighter, so that, for example, fluids will be much lighter. Overall, T2 scans frequently seen lighter in general.

Answer 23

Ultrasound

Answer 24

Magnetoencephalography (MEG) is a functional neuroimaging technique for mapping brain activity by recording magnetic fields produced by electrical currents occurring naturally in the brain, using very sensitive magnetometers.

18.2. Imaging Flashcards

(53 cards)