STRUCTURAL IMAGING
Structural imaging measures of the spatial configuration of different types of tissues in the brain (CT/MRI)
Functional imaging measures temporary changes in brain physiology associated with cognitive processing; the most common method is fMRI and is based on a hemodynamic measure
COMPUTERIZED TOMOGRAPHY
Computerized tomography (CT) scans are constructed according to the amount of X-ray absorption is related to tissue density: bone absorbs the most (so the skull appears white), cerebrospinal fluid absorbs the least (so the ventricles appear black) and the brain matter is intermediate (appears gray)
MAGNETIC RESONANCE IMAGING
Magnetic resonance imaging (MRI) has number of advantages over CT scanning:
- It does not use ionizing radiation and so is completely safe
- It provides a much better spatial resolution, which allows the folds of individual gyri to discerned
- It provides better discrimination between white and gray matter
- It can be adapted for use in detecting the changes in blood 02 associated with neural activity, called fMRI.
MRI physics for non-physicists
Most human tissue is water-based and the amount of water in each type of tissue varies. Different types of tissue will thus behave in slightly different ways when stimulated, and this can be used to construct a three-dimensional image of the layout of these tissue.
- A strong magnetic field is applied across the part of the body being scanned
- The single protons that are found in water molecules in the body have weak magnetic fields. Initially, these fields will be oriented randomly, but when the strong external field is applied a small fraction of them will align themselves with this.
- When the protons are in the aligned state a brief radio frequency pulse is applied that knocks the orientations of the aligned protons by 90 degrees to their original orientation.
- As the protons spins (or precess) in this new state, they produce a detectable charge in the magnetic field and this is what forms the basis of the MR signal.
- The protons will eventually be pulled back into their original alignment with the magnetic field (they "relax")
- The scanner repeats this process serially by sending the radio wave to excite different slices of the brain in turn.
The strength of the magnetic field is measured in units called tesla (T)
FUNCTIONAL IMAGING
Functional imaging is designed to measure the moment-to-moment variable characteristics of the brain that may be associated with changes in cognitive processing.
Functional imaging is designed to measure the moment-to-moment variable characteristics of the brain that may be associated with changes in cognitive processing.
Basic physiology underpinning functional imaging
The brain consumes 20% of the body's O2 uptake; it does not store O2 and it stores little glucose.Most of the brain's oxygen and energy needs are supplied from the local blood supply. When the metabolic activity of neurons increases, the blood supply to that region increases to meet the demand. When the metabolic activity of neurons increases, the bloody supply to that region increases to meet the demand. Techniques such as PET (positron emission tomography) measure the change in blood flow to a region directly, whereas fMRI and the emerging method of fNIRS (functional near-infrared spectroscopy) are sensitive to the concentration of O2 in the blood. All these methods are referred to as hemodynamic methods. PET requires administration of a radioactive tracer whereas fMRI uses a naturally occurring signal in the bloodstream.
Voxel-based morphometry (VBM) capitalizes on the ability of structural MRI to detect differences between gray matter and white matter. VBM divides the brain into tens of thousands of small regions, several cubic milimeters in size (called voxels)
Diffusion tensor imaging (DTI) is different from VBM in that it measures the white matter connectivity. It is able to do this because water molecules trapped in axons tend to diffuse in some directions but not others. Specifically a water molecule is free to travel down the length of the axon but is prevented from traveling out of the axon by the fetty membrane. When many such axons are arranged together it is possible to quantify this effect with MRI (using a measure called fractional anisotropy)
Fractional anisotropy (FA) A measure of the extent to which diffusion takes place in some direction more than others.
The basic requirement in all functional imaging studies involving cognitive tasks is that the physiological response must be compared with one or more baselines responses. Good experimental practice is needed to ensure that the baseline task is appropriately matched to the experimental task, otherwise the results will be very hard to interact.
Functional magnetic resonance imaging
The component of the MR signal that is used in fMRI is sensitive to the amount of deoxyhemoglobin in the blood.When neurons consume oxygen they convert oxyhemoglobin to deoxyhemoglobin. Deoxyhemoglobin has strong paramagnetic properties and this introduces distortions in the local magnetic field.
BOLD: Blood O2-level-dependent contrast; the signal measured in fMRI that relates to the concentration of deoxyhemoglobin in the blood
Hemodynamic response function (HRF): Changes in the BOLD signal over time.
The hemodynamic response function has three phases:
- Initial dip. As neurons consume oxygen there is small rise in the amount of deoxyhemoglobin, which results in a reduction of the BOLD signal.
- Overcompensation. In response to the increased consumption of oxygen, the blood flow to the region increases, The increase in blood flow is greater than the increased consumption, which means that the BOLD signal increases significantly.
- Undershoot. Finally the blood flow and oxygen consumptions dip before returning to their original level. This may reflect a relaxation of the venous system, causing a temporary increase in deoxyhemoglobin again.
Functional near infrared spectroscopy
The newer method of fNIRS measures the same BOLD signal as fMRI although it does it so in a completely different way. It dos not require the use of magnetic fields but, instead, it sends "light" of a particular wavelength to the brain; specifically, in the near infrared range, about 800 nanometers. This signal passes relatively freely through bone and skin but is more strongly scattered by oxy- and deoxyhemoglobin, each of which is sensitive to slightly different wavelengths in the near infrared range. The extent to which the signal is scattered by these different wavelengths is then used to compute the BOLD response.