Methods for studying CNS disorders Flashcards
whats the difference between fMRI and DTI?
The fMRI BOLD signal is based on the increased blood flow to a brain region that correlates with increased activation of that region (Logothetis, 2002), whereas DTI measures rely on the anisotropy (directionally dependent diffusion) of water molecules within myelinated axons (Emsell, Van Hecke, & Tournier, 2015)
what categories of methods are there for studying CNS disorders?
- Imaging techniques
- Indirect ‘markers’ for changes in neurotransmitter function
- Post-mortem studies on human brain
- Genetic-linkage analysis
- Animal models
What imaging techniqes can be used to study CNS disorders?
- Computerised tomography (CT)
- Magnetic resonance imaging (MRI)
- Functional Magnetic resonance imaging (fMRI)
- Diffuse tensor imaging (DTI)
- Positron emission tomography (PET)
- Single photon positron emission tomography (SPECT)
What is Computerised tomography?
CT scan
Computed tomography (CT) provides rapid, noninvasive imaging of the brain and skull
an imaging tool that combines X-rays with computer technology to produce a more detailed, cross-sectional image of your body.
What is Magnetic resonance imaging (MRI)?
imaging technique that produces cross-sectional images of your body.
Unlike X-rays and CT scans, MRI works without radiation. The MRI tool uses magnetic fields and a sophisticated computer to take high-resolution pictures of your bones and soft tissues
breifly compare CT and MRI
- MRI provides better resolution of neural structures than CT . This difference is most significant clinically for visualizing the following: Cranial nerves, Brain stem lesions, Abnormalities of the posterior fossa, Spinal cord - CT images of these regions are often marred by bony streak artifacts.
- CT is superior to magnetic resonance imaging (MRI) in visualizing fine bone detail in (but not the contents of) the posterior fossa, base of the skull, and spinal canal.
- CT may require a radioactive tracer which can have adverse effects, includign allergic reactions and contrast nephropathy.
- MRI is better for detecting demyelinating plaques, early infarction, subclinical brain edema, cerebral contusions, incipient transtentorial herniation, abnormalities of the craniocervical junction, and syringomyelia.
- MRI is contraindicated if patients
Have had a pacemaker or cardiac or carotid stents for < 6 weeks
Have ferromagnetic aneurysm clips or other metallic objects that may overheat or be displaced within the body by the intense magnetic field
what is functional MRI?
fMRI
a specialized form of MRI that is used for measuring and mapping the brain’s functional activity. evaluates blood flow in the brain called the blood oxygenation level dependent (BOLD) contrast technique.
based on the fact that oxyhaemoglobin has different magnetic resonance to deoxyhaemoglobin.
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.
The microvasculature responds by increasing blood flow – this increase the oxyHb to deoxyHb ratio
This reveals patterns of activity in the intact brain
What is Diffuse tensor imaging (DTI)?
While DWI refers to the contrast of the acquired images, DTI is a specific type of modeling of the DWI datasets
the basic concept behind DTI is that water molecules diffuse differently along the tissues depending on its type, integrity, architecture, and presence of barriers, giving information about its orientation and quantitative anisotropy
Mapping pathways and investigating aberrant connectivity
Uses water to get a more detailed Image than traditional MRI
What is Diffusion weighted imaging?
Diffusion weighted imaging (DWI) is a variant of conventional Magnetic Resonance Imaging based on the tissue water diffusion rate. It is a non-invasive method, with unparalleled sensitivity to water movements within the architecture of the tissues that uses existing MRI technology and requires no new equipment, contrast agents, or chemical tracers
What is Positron emission tomography?
An imaging test that can help reveal the metabolic or biochemical function of your tissues and organs. The PET scan uses a radioactive drug called a tracer to show both typical and atypical metabolic activity.
When the radiopharmaceutical undergoes beta plus decay, a positron is emitted, and when the positron interacts with an ordinary electron, the two particles annihilate and gamma rays are emitted. These gamma rays are detected by gamma cameras to form a three-dimensional image, in a similar way that an X-ray image is captured.
first unstable positron emission isotopes are created in an cyclotron e.g. O15 (half-life 2 min)
when these isotopes are injected they distribute according to the relative activity of the brain regions
how does SPECT differ from PET?
SPECT is very similar to PET in its use of radioactive tracer and detection of γ-rays. However, unlike PET the radioisotopes used for SPECT emit only a single γ-ray during decay that is measured directly. Moreover, SPECT scans are significantly less expensive than PET scans partly due to that the nuclides used in SPECT have a longer half-life and are relatively easily obtained than PET
what is the main advantage of SPECT compared to PET?
Main advantage does not require on site cyclotron (A cyclotron is a type of particle accelerator which repeatedly propels a beam of charged particles (protons) in a circular path)
what is Electroencephalography?
EEG
Twenty electrodes are distributed symmetrically over the scalp to detect electrical changes in the brain that are associated with Seizure disorders, Sleep disorders, Metabolic or structural encephalopathies.
what is Megnetoencephalography?
based on detection of the magnetic fields that arise because of the electrical activity
what are the differences between EEG and MEG?
- In EEG the electrodes are placed on the scalp. MEG is performed using a dewar that contains multiple sensor coils, which do not touch the patient’s head
- MEG primarily detects the magnetic fields induced by intracellular currents, whereas scalp EEG is sensitive to electrical fields generated by extracellular currents.
- Although MEG is more sensitive in detecting currents that are tangential to the surface of the scalp, EEG is sensitive to tangential and radial neuronal activities
- Magnetic fields are not distorted by the tissue conductivity of the scalp, skull, cerebrospinal fluid (CSF) and brain; in contrast, electrical fields may be distorted by the skull and CSF.
- MEG provides better spatial resolution of source localization (2-3 mm) than EEG (7-10 mm)
