Neuroimaging Flashcards
What is the process of computer assisted tomography?
The head is placed between a source which emits a narrow beam of X-rays and an X-ray detector. A series of measurements is made of X-ray transmission. The source and detector are rotated as a pair through a small angle and a further series of measurements taken. This is repeated until the source and detector have rotated through 180∞. The radiodensity of each region of the head is computed from the transmission data for all of the beams that have traversed that region, and the results visually displayed.
CAT scans provide a view through a single slice of brain lying at a known orientation. How is the whole brain able to be scanned?
By moving the head at right angles to the orientation plane for a short distance another section can be
imaged. This is repeated until the whole brain has been scanned.
What is computerized tomography?
the algorithm—and the computer software to implement it— that calculates the radiodensity for each point in the brain slice.
Computerized tomography is an example of an inverse problem, what does this mean?
starts with a data set from
which initial parameters, in this case source location, must be calculated. It contrasts with forward problems in which the source location is known and it is the data set which is calculated.
What is the difficulty with inverse problems?
they do not have unique solutions. Hence they have to be constrained by assumptions and prior modeling based on earlier
results to find the most likely solution.
What are the abilities of CAT scans?
CAT can distinguish tissues which differ in X-ray opacity by 1% (the lower the density the darker the image) with a spatial resolution of about 0.5 mm. Blood vessels can be seen by injection of radio-opaque dyes.
What information does positron emission tomography provide?
insights into the function of the living brain as well as its anatomy.
PET It uses the principles of computerized tomography in which g-ray detectors are located around the head and the source is a positron-emitting compound, either injected or inhaled, which enters the brain. What are these compounds?
Compounds used include neurotransmitters, receptor ligands, and glucose analogs which are used for studying brain activity. Typically they are radiolabeled with ** These isotopes have short half-lives, decaying to the element with atomic number one less
In PET what happens to the positron produced when isotopes decay?
The positron (e+ , the antiparticle of the electron) travels a short distance before colliding with an electron (e-). The two particles annihilate with the production of two g-ray pho-
tons that shoot off in exactly opposite directions. These are detected simultaneously by a pair of detectors 180∞ apart. This coincidence detection permits localization of the site of the g-ray emission, which is between 2 and 8 mm from the positron source, depending on the isotope used.
What is the spatial resolution of PET?
about 4–8 mm, not as good as CAT, but it can be used to
follow brain events over time.
How the nonmetabolizable analog of glucose, 2-deoxyglucose (2-DG) used in PET functional studies?
This molecule crosses the blood–brain barrier, is transported into neurons and phosphorylated to 2-DG-6-phosphate, so it remains in the cell. However, it cannot be metabolized further. This means it acts as a marker for local glucose uptake and therefore of neuron activity. Imaging the distribution
of [18/9F]2-DG while subjects engage in sensory, motor, or cognitive tasks reveals how these functions are localized in the brain.
What do PET studies show that implies that brief periods of brain activity can be supported by glycolysis?
these studies show that during transient increases
in neuronal activity, the rise in local cerebral oxygen consumption (as measured by 15OPET) does not match the increase in glucose utilization (as estimated from 2-DG PET).
What gives rise to a net longitudinal magnetic field parallel to the scanner field in MRI?
Nuclei with odd mass number, for example, 1
1H, generate a magnetic field along their spin axis. In the powerful magnetic field of an MRI scanner, hydrogen nuclei can adopt one of two orientations; with their magnetic fields either parallel or antiparallel to the external field. The parallel state has a slightly lower energy and normally a small excess of nuclei will be in this state
A cylindrical coil placed around the head broadcasts a radio frequency (rf) pulse to a slice of head at right angles to the main scanner field. What does this rf do to the nuclei in MRI?
The rf pulse makes the nuclei wobble around their magnetic axis—rather like a spinning top as it slows down—with the rate of wobbling in resonance with the pulse frequency. The wobble generates an electric field
that is received by the coil, producing a transverse magnetic field at right angles to the scanner field. When the rf pulse is turned off the nuclei return to their original state, and the longitudinal and transverse fields decay with relaxation times that are characteristic for the nucleus and its chemical environment (e.g., lipid or aqueous).
How many coils is required to produce a MRI image?
actually requires a further three coils that produce magnetic field gradients in the x, y, and z directions.
What is the resolution of an MRI image?
MRI has a resolution < 1 mm.
An MRI method that records changes related to brain function in successive images is termed functional MRI (fMRI). Which is the most important one?
blood oxygen level
detection (BOLD) which provides a very sensitive measure of cerebral cortical activity with a voxel (volumetric pixel, the 3-D analog of a pixel in a 2-D image) size of 2 mm
on each side, following changes in activity with a time resolution of a few seconds.
What does BOLD depend on?
on the ratio of oxygenated to deoxygenated hemoglobin and this varies with
blood flow and metabolism.
What have BOLD studies shown about energy expenditure in the brain?
most of the energy expenditure of the brain is related to synaptic events
rather than the generation and propagation of action potentials. Indeed it seems that action potentials are produced using only 30% more energy than the calculated theoretical minimum.
What is electroencephalography?
Recording the net electrical activity of the brain by means of surface electrodes attached to the scalp
What are local field potentials?
Large numbers of cerebral cortical cells fire in synchrony and consequently their summed activity produces local field potentials (LFPs) big enough that they can be recorded with scalp electrodes. By using an
array of electrodes, activity of different brain areas can be examined simultaneously. The recording may be monopolar—each scalp electrode measures the potential with respect
to a distant indifferent electrode—or bipolar, in which the potential is measured between a pair of scalp electrodes
What are the groupings of the frequencies of the LFPs?
alpha (8–13 Hz), beta (13–30 Hz), delta (1–4 Hz), theta (4–7 Hz). Activity in these frequency bands correlates with behavioral state, for example, sleep, arousal, or learning.
What are evoked potentials (EPs) or event-related potentials (ERPs)?
They are brief fluctuations in the EEG generated by sensory, perceptual or cognitive stimuli. These potentials are used to
investigate the context, timing, and brain regions implicated in the process of interest.
What is is measured in magnetoencephalography (MEG)?
the synchronized flow of currents along dendrites of about 50 000 cortical pyramidal cells all oriented in the same direction is sufficient to set up a measurable, if weak,
magnetic field
How are these magnetic fields measured in MEG?
using an array of magnetometers called SQUIDS (superconducting quantum interference devices) which surround the head. The MEG signal is generated mostly by the flow of intracellular currents in dendrites generated by synaptic activity.
Why must MEG be done in a magnetically shielded environment?
Because cortical activity generates a field of order 10-13 tesla (T) compared with the Earth’s magnetic field of 3.1 x 10-5T. SQUIDs are sensitive to magnetic
fields as small as 10-18 T.
What is the major advantage of MEG over EEG?
its temporal resolution of better than 1 ms, which
is comparable to intracranial electrodes. Also, magnetic fields are less distorted by skull anatomy than electric fields, which gives MEG a better spatial resolution.
A magnetic field changing with time (in strength or direction or both) induces an electrical field how is this exploited In transcranial magnetic stimulation (TMS)?
by using electromagnetic coils to induce currents in the brain which influence firing of neurons directly beneath the coil. The currents attenuate with distance from the coil.
To what depths is TMS effective?
to depths of 1.5–3 cm so the cerebral and cerebellar cortices are the usual targets, but more powerful coils can be used to affect subcortical structures.
Why is the shape of the induced electrical field hard to model?
because of the nonuniform electrical properties of neural tissue
What is the resolution of TMS?
it has a high spatial resolution (< 1 cm) if a figure-of-eight coil is used which concentrates magnetic flux at the node of the coil, and a temporal resolution of tens of ms.
There are two modes of TMS, single pulse and repetitive pulse. What is the advantage and disadvantage of single pulse?
it can be time-locked to the delivery of a stimulus so can allow precise timing of any effect. However, single pulses may not always be effective and hence repetitive TMS
(rTMS) can be used.
What is repetitive TMS?
This delivers exponentially rising and falling magnetic pulses with a frequency up to 50 Hz for several seconds.
What is repetitive TMS?
This delivers exponentially rising and falling magnetic pulses with a frequency up to 50 Hz for several seconds.
What does TMS have effects on?
on visual perception,
movement control, attention, memory, language, and decision making. TMS can excite or inhibit depending on the stimulus characteristics. For example, 10 Hz rTMS to the motor cortex stimulates muscle contraction and improves performance in a motor learning task whereas I Hz rTMS impairs motor learning. When used to study cognition it is usually inhibitory so that it interferes with performance of cognitive tasks, either increasing reaction time or increasing the number of errors.
The firing patterns of either single neurons or clusters of neurons in living animals in
response to physiological stimuli are obtained by extracellular recording. What does this techniques use?
two fine electrodes usually of tungsten or stainless steel. One, the exploring (focal)
electrode is placed as close as possible to a neuron. The second, indifferent electrode is placed at a convenient distance. Neuron activity will cause currents to flow between the two electrodes. These currents are amplified and fed to a computer. By convention, if the exploring electrode is positive with respect to the indifferent electrode an upward deflection is recorded. The polarity, shape, amplitude, and timing of the recorded waveform generated by neural activity will depend on the position of the electrodes. The closer the
exploring electrode is to the neuron, the larger the measured signal. Changing the distance between the two electrodes or altering their relative positions will modify all the above parameters.
The technique can be used in brain slices or other in vitro preparations or in vivo, for example, in anesthetized animals. What is a useful technique in animals?
inserts electrodes into
the brain that are attached to a connector cemented into the skull. This is done under
anesthetic. The animal is allowed to recover. As required, the recording circuitry (amplifier to computer) is plugged into the connector. This allows electrophysiology in conscious, behaving animals, using sophisticated experimental protocols over long periods.
How does intracellular recording measure membrane potentials directly?
it is necessary to have two electrodes, one inside the cell, the other outside, both connected to a voltmeter of some description.
Because neurons are small the tip of the intracellular electrode impaling the cell needs to be very fine when doing intracellular recording. How is this achieved?
glass micropipettes are manufactured to have a tip diameter of less than 1 μm. The micropipette is filled with an electrolyte (commonly KCl at a concentration of 0.15–3 M) to carry the current, so forming the microelectrode.
Typically transmembrane potentials are less than 0.1 V and so must be amplified with an operational amplifier in intracellular recording. How does this work?
This has inputs from both the intracellular microelectrode that impales the cell and the reference (bath or indifferent) electrode,
which is placed in the solution bathing the cell. If no potential difference exists between the microelectrode and the reference electrode the amplifier output will be zero. If a potential difference exists between the electrodes, however, the amplifier generates a signal, the magnitude of which is proportional to the potential. The output of the amplifier goes to the analog-to-digital port of a computer running software which allows display, storage, and analysis of the data.
How do neurophysiologists stimulate neurons directly?
by injecting an electrical current into it via a stimulating microelectrode. The stimulator normally delivers a square wave
current pulse.
Which three variables can e altered at will on most neuron stimulators?
the duration of the pulse, the amplitude of the injected current, and the frequency of the pulses.
What determines the response of a neuron to a stimulator?
The direction of the current (which is defined as the flow of positive charge). If a small inward current is injected into a cell it will become a
little more inside positive. This is a decrease in the membrane potential because Vm gets closer to zero and is called a depolarization. If, on the other hand, an outward current is injected (that is, if current is withdrawn from the cell) then the membrane potential increases; this is called hyperpolarization.
How are the sizes and time courses of depolarizing and
hyperpolarizing potentials seen in nerve cell injected with small currents determined?
by the passive electrical properties of the neuron.
