Hjärnavbildning Flashcards
Vilka två typer kan man dela in hjärnavbildningsmetoder i, baserat på vad det är de mäter?
Structural brain imaging - measures anatomy(CT, MRI)
Functional brain imaging - measures activity(EEG, fMRI, fNIRS, PET)
Vad menas med direkt och indirekt mätning av hjärnaktivitet?
Direkta mätningar mäter den faktiska hjärnaktiviteten, indirekta mäter andra saker som är korrelerade med hjärnaktivitet. Enda metoderna för direkt mätning är EEG och single cell recording(i föreläsningen sa hon att EEG var den enda men hon tog inte upp single cell jag antar att det är det också?)
Förklara vad CT scan är och vad det används till
Computerized/computered tomography: En hjärnavbildningsmetod som skapar en 3-dimensionell av hjärnan med hjälp av elektromagnetisk strålning(röntgenstrålning). Tätare vävnad(t.ex ben) absorberar mer av strålningen, neural vävnad mindre och vätskor såsom blod och CSF ännu mindre. Därför är ben vitt på bilderna, hjärnvävnad grå och ventriklarna svarta. I en CT-scan kör man röntgenstrålning från många olika vinklar så att det blir många olika avbildningar och man manipulerar sedan dessa genom computing och matte för att få en 3-dimensionell bild.
CT-scans är tillräckliga för att lokalisera hjärntumörer och lesioner, och är första valet vid akuta grejer pga snabbt(strokes och skador)
Vad är MRI och vad är de två sätten man kan använda för att få fram MRI-bilder?
Magnetic resonance imaging: brain imaging method that uses a large magnet and a specific radiofrequency pulse to generate a brain signal that produces an image. Used to study both brain anatomy and neural function noninvasively.
- Based on the principle that a hydrogen atom’s nucleus, which consists of a single proton, behaves like a spinning bar magnet. Each proton has a dipole: one end is a north pole and the other end a south pole. Each spinning proton produces an electrical current. Ordinarily, protons are oriented at random, so a given piece of tissue(all of which contain hydrogen in the form of water) has no net dipole and consequently generates no net electrical current. When placed in a magnetic field, the protons align in parallel(som en kompass som aligns med jordens magnetfält). This generates a summed electrical current that is strong enough to be measured.
- Proton density varies in different brain tissue(CSF, myelin, neurons), largely in proportion to its water content, the electrical currents produced by the protons are different: higher for some tissue and lower for others
- Measures of the electrical current are used to create the MRI image.
Another way to make an MRI image is to perturb the protons when they are aligned and record the changes occurring in the electrical field as they realign after the perturbation:
- A brief radiofrequency pulse is applied to a participant’s brain, horizontal to the magnetic field.
- The pulse forms a second magnetic field that pushes the aligned protons over onto their sides.
- The tipped protons now have two motions: they spin about their own axes, and they spin about their longitudinal(north-south) orientation. The protons wobble like a slowly spinning top, a motion called precession.
- When the horizontal magnetic field is turned off, the synchronously spinning protons begin to relax: they begin to “stand up” again and to fall out of synchrony with one another.
- Both relaxation processes are measured, using a current detector, by two time constants, T1 and T2:
- T1 measures how long it takes after the magnetic pulse is turned off for the protons to “right” themselves from their tipped positions and realign with the original magnetic field.
*T2 measures the rate at which protons lose synchrony about the horizontal axis after the horizontal pulse is turned off. - Protons in different tissue types have different relaxation rates and corresponding T1 and T2 time constants(ex. Relaxation rates for CSF are slower than for brain tissue). Therefore, at a set time(ex: at the midpoint of relaxation), differences in electrical current related to and indicating the composition of tissue can be measured.
- T1 and T2 can be translated into brain-image gradients that correspond to its different tissues, with darker gradients indicating low-density tissue and lighter gradients indicating high-density tissue. Either can be used, but one may be more suitable in a given situation(ex: T2 is more sensitive to differences between damaged and intact tissue and is therefore useful for detecting lesions)
Vad används MRI till?
MRI is best when the images need to be very detailed.
Används ofta för diagnostisering av:
* tumörer
*stroke
* hjärnskador
* MS
* Spinal cord conditions
* Conditions of the eye and inner ear.
* Aneurysms of cerebral vessels
* causes of dementia or neurological diseases
If you are looking at places where bone might interfere, CT scan doesn’t work so you have to use MRI.
Används också ibland på patienter med komplexa psykologiska symtom
Hur funkar fMRI, vilka aspekter av hjärnfunktion mäter det och vad används det till?
- When neurons are active they use more oxygen, resulting in a temporary dip in oxygen amount in the blood. At the same time, active neurons signal the blood vessels to dilate to increase blood flow and bring more oxygen to the area.
-When brain activity increases, the increase in oxygen produced by increased blood flow actually exceeds the tissues need for oxygen. As a result, the amount of oxygen in an activated brain area increases.Increase in blood oxygen content alters the magnetic properties of the water in the blood. fMRI can accurately match these changes in magnetic properties to specific locations in the brain. A measure called the BOLD contrast(blood oxygen-level dependent contrast) provides an index of the brain’s relative activity.
Before neural activation: amounts of deoxyhemoglobin och oxyhemoglobin is about equal.
After: the amount of oxyhemoglobin increases.
- Oxyhemoglobin is less magnetic, and the T2 signal changes more slowly
- When superimposed on MRI-produced brain images, fMRI changes in activity can be attributed to particular structures.
Which aspects of neural functioning does fMRI measure? The most metabolically active part of a neuron is presumed to require the most oxygen. EPSPs, which ultimately results in cell firing, make the greatest energy demands. ISPS and the action potential itself have lower demands. Thus, fMRI is measuring EPSP and the probability that a neuron will fire - very much the same activity that ERP measures.
Används till: Pre-surgical assessment of function(epilepsy, tumors), surgical planning, follow-up, research
Vad är EEG, hur funkar det och vad används det till?
In a traditional EEG recording arrangement, one electrode (a small metal disc called the active electrode) is attached or pasted to the scalp to detect the electrical activity in the underlying brain area. A second electrode (the indifferent electrode) is attached elsewhere, on the earlobe perhaps, where no changing electrical activity exists. The two electrodes detect the difference in electrical potentials that might occur near the scalp electrode, thus revealing the underlying brain activity. Electrical fluctuations in the brain are small, usually much less than a millivolt, but when amplified, they can be displayed on a polygraph (vilket tidigare va en penna osv men nu är det i datorn).
Hur? If many neurons undergo graded potential changes at the same time(depolarisation and hyperpolarisation), the signal is large enough to be recorded from as far away as the skull surface. The signal recorded by the EEG is the sum of rhythmical graded potentials on the many thousands of cortical neurons that are closest to the recording electrode. Waves recorded from the skull are volume conducted through the brain and through the skull in the manner that waves travel through water. As the electrodes are moved farther from the source, the wave amplitude from a given generator grows smaller. Thus, if a number of electrodes are placed on the skull, amplitude differences can be used to estimate the approximate location of the generator that is producing a given set of waves. This triangulation method is used to locate the generators that produce brain waves associated with epilepsy and so locate the brain region responsible for the electrical abnormality.
Används i forskningssyfte t.ex för att studera medvetandetillstånd och kognitiva funktioner. Används kliniskt för att diagnostisera epilepsi, hjärnskador(man kan se hur hjärnaktiviteten/firing patterns har förändrats pga skadan), encefalit(hjärninflammation), encephalitis(disease), sömnstörningar, stroke och koma. Kan också användas för att se om nån är hjärndöd.
Vad är ERP och hur detekterar man en sån?
Event-related potentials: brief changes in an EEG signal in response to a discrete sensory stimulus. ERPs are largely the excitatory and inhibitory graded potentials, the EPSPs and IPSPs that a sensory stimulus triggers on dendrites. Not easy to detect because the signal is mixed in with many other EEG signals: one way to detect an ERP is by repeatedly producing the stimulus and then averaging the recorded responses.. This cancels out irregular and unrelated electrical activity, leaving only the graded potentials generated by the stimulus event. ERP recordings can also map the progress of the (stimulus) response as it makes its way through the nervous system. ERPS can reveal when and where in the brain actions are planned and executed.
Vad är Readiness potentials?
ERPS produced in the motor cortex later than 300 ms after stimulus presentation. Signal that the motor cortex is preparing a movement, as well as the part of the motor area involved in executing the impending movement.
Förklara vad PET är och hur det funkar
Positron Emission Tomography: Studerar metabolisk aktivitet i hjärnceller.
Funkar såhär:
When a radionuclide is combined with some substance within the metabolic pathway, the resulting “labeled” compound can be used as a radiotracer to detect the activity of that pathway. One widely used radiotracer is fluorodeoxyglucose/FDG(an analogue of glucose that is labeled with the radionucleide fluorine-18. This form of glucose(deoxyglucose) does not break down readily and so accumulates in the cell. Active cells accumulate more deoxyglucose than less active cell..
Hur man kan mäta det:
- Man injicerar vatten med radiotracer in i blodomloppet.
- De radioaktiva molekylerna(som flourine-18) utsöndrar positroner. Positronernaattraheras till den negativa laddningen av elektroner i hjärnan.
- När positronerna och elektronerna kolliderar förstörs dem
- Detta producerar energi i form av två fotoner som lämnar kollisionsplatsen i exakt motsatta riktningar
- i PET-scannern finns det par av radioaktivitetsdetektorer placerade mittemot varandra i en ring som är runt huvudet. Dessa detekterar fotonerna
- Det finns många par av detektorer, vilket gör att PET-kameran kan avbilda flera parallella “skivor” av hjärnan för att producera en bild
Ge exempel på vad PET kan användas till.
- PET can detect the decay of hundreds of radiochemicals to allow for mapping a wide range of brain changes and conditions, including changes in pH, glucose, oxygen, amino acids, neurotransmitters, and proteins. PET can detect relative amounts of a given neurotransmitter, the density of neurotransmitter receptors, and metabolic activities associated with learning, brain poisoning, and degenerative processes possibly related to aging.
- Diagnosticering och övervakning av tumörer(scanner can locate more active cells, which might indicate the presence of a tumor), är väldigt känslig för att detektera tumörer
- Diagnosticering av (vissa typer av??) demens(less active cells, which might indicate brain injury or degeneration)
- Diagnosticering av epilepsi
- Diagnosticering av infektion/inflammation(ex. vasculitis)
- Det är gold standard för att diagnosticera parkinsons(pga dopaminproducerande neuroner dör)
- PET is also used to study the “where” and “when” of brain areas involved in cognitive function(but is only a relatively crude detector of cognitive processes?)
Berätta om ett specifikt sätt man kan detektera symtomatisk alzheimers.
PET scan with florbetapir. Florbetapir binds to amyloid plaques, which are characteristic of alzheimers disease.
Hur kan man studera sambandet mellan blodtillförsel och mental aktivitet?
PET can indirectly detect changes in the brain’s blood flow because it is the blood that is bringing the radiotracer to the cell, and more active brain areas have higher blood flow.PET researchers studying the link between blood flow and mental activity resort to a statistical trick. They subtract the blood-flow pattern when a participant is in one experimental condition(or in a neutral condition) from the pattern imaged when the participant is engaged in a different experimental condition. This process images the difference in blood flow between two states. The difference can be averaged across participants to yield a representative, average image difference that reveals which brain areas are selectively active during a given experimental condition.
Förklara vad MEG är och hur det funkar(förklara också SQUID och isocontour map)
By generating an electrical field, neural activity also produces a magnetic field. A single neuron produces a micromagnetic field, but the field produced by many neurons can be recorded on the skull surface. MEG is the magnetic counterpart of the EEG or ERP.
SQUID(superconducting quantum interference device): sensing device containing the superconducting coils needed to detect the brain’s very weak magnetic field.
One or more probes are moved across the surface of the skulls and send signals to the SQUID.
Isocontour map: a chart with concentric circles(gradients) representing different intensities in the neural magnetic field(each probe produces one isocontour map). Investigators can use these maps to calculate the locations of the neurons generating the field in three dimensions. They can also convert MEG maps into a graph of electrical events very similar to electrical potentials recorded by EEG instruments.
Vad är Diffusion tensor imaging (DTI)?
Is an MRI method that detects the directional movements of water molecules to create virtual images of the brain’s nerve fiber pathways. Water molecules in ventricles and even in cell bodies move relatively unimpeded in random directions. In nerve fibers, however, their movement is restricted by the tract’s orientation and its content and tends to follow the direction of its longitudinal axis. DTI tractography is thus used to map the brain’s pathways and connectivity.
