L4 - Neuroimaging and Brain Mapping Flashcards
1
Q
What are Hounsfield Units?
A
- Quantifies how much radiation passes through different tissues
- 4000 step scale that is standardized to water
- Interval scale where water = 0
- Different tissues convert x rays into photons in a different manner and device measures amount of photons produced
- Denser tissues have higher values
2
Q
List 10 tissues and their corresponding HU.
A
- Air: -1000
- Fat: -120
- Water: 0
- CSF: 15
- White matter: 20-30
- Blood: 30-45
- Grey matter: 37-45
- Muscle: 40
- Contrast: 130
- Bone: 400 - 3000
3
Q
How does Computerized Tomography work?
A
- X-ray beam results in creation of a data set that allows the 3D construction of the imaged body part
- Greyscale for each voxel is expressed in HU normalized to water. HU value at a given voxel refers to the amount of x-radiation generated at the x-ray source that passed through the imaged tissue to the detector (radiodensity)
- Involves back projection: Superimposing x-ray beams at target from different angles to create 3D image of tissue
- Sufficient number of 2D images from different angles allows superimposition of image set and the creation of a 3D image of the target
4
Q
List 9 common uses of CT scans
A
- Damage from TBI
- Fractures
- Metallic foreign bodies
- Intracranial bleeds
- Intracranial electrodes
- Calcified lesions
- Bony disease
- Perfusion angiography
- Contraindication of MRI (metallic implants)
5
Q
How does Magnetic Resonance Imaging work?
A
- Uses information obtained from magnetic moment of proton and applied magnetic and radiofrequency forces of apparatus
- Tissue is placed in field of strong magnet while gradient coils alter the magnetic field and RF coils apply strong radio frequency power to perturb proton’s magnetic moment
- Record radiofrequency signal when moment returns to equilibrium
6
Q
Compare how CT scanners and MRIs obtain their greyscale contrast.
A
- CT scanners: depends solely on tissue density
- MRI: Derived from details of how magnetic moments relax to their equilibrium state following perturbation
7
Q
What is Diffusion Weighted Imaging?
A
- Like MRI, uses sensitivity to proton motion
- Water usually diffuses isotropically (equally in all directions)
- Water diffusion in structured tissues (neural tracts) is anisotropic (water all flows in one direction) and this physical property is used to estimate the underlying direction of groups of axons
8
Q
What are contrast agents? Provide an example.
A
- Agents that are introduced into the intravascular space by oral or intravenous injection in order to increase the imaging contrast of MRI
- Example: Gadolinium
- Has paramagnetic properties (attracted to external magnetic fields) and can be used to identify BBB breakdown
- At points of break in the vascular system, the Gadolinium seeps through the capillary bed and into the surrounding tissues where it makes the leaked area in the MR image appear brighter than normal
- Can be used to identify brain tumours, vascular lesions, infections, inflammation (myelin plaques)
9
Q
What is Magnetic Resonance Spectroscopy?
A
- Used to identify the chemical composition of lateral within a voxel or set of voxels
- Displayed in histogram
- Focuses on 4 molecules:
- Creatine (cellular metabolism)
- Choline (cellular membrane synthesis)
- N-acetyl aspartate (neural density and integrity)
- Lactate (ischemia)
10
Q
What is Single Photon Emission Computed Tomography?
A
- Measures regional CBF, which is an index of neuronal activity
- Two chemicals combined and injected intravenously and taken up by brain tissue proportional to CBF
- Can be used to radiojlabel specific chemicals in NT pathways
11
Q
What is Positron Emission Tomography?
A
- FDG is radioactive pharmaceutical that is inhaled as a gas and enters blood supply where it binds to glucose
- Measures metabolic activity by detecting the presence of two gamma photons produced by annihilation of two colliding positrons
- Lower spatial resolution than fMRI: Statistical uncertainty of where collision occurs. The photons that are created move away from each other at 180 degrees. If they are picked up at the same time, it indicates that there was activity but spatial resolution is poor.
12
Q
How does fMRI work?
A
- Local neuronal activity indirectly affects the fraction of deoxygenated hemoglobin in the blood
- Oxygenated and deoxygenated hemoglobin have different magnetic properties
- As neural tissue becomes electrically active, elevation of local field potentials causes local arterioles to dilate and supply neural tissues with oxygenated blood (neurogenic control)
- Latency of about 5 seconds from neuronal response
- Result of difference between baseline and treatment conditions
13
Q
What are the 2 advantages and 1 disadvantage of using MEG over EEG?
A
- Advantages: Skin attachment electrodes unnecessary, magnetic signal strength is only attenuated by distance rather than tissues
- Disadvantage: Costs 1 million dollars to buy one
14
Q
What is the difference between structural and functional neuroimaging?
A
- Structural: Permits visualization of neural structures
- Functional: Maps physiologic states onto NS structures and links them via behaviour
15
Q
What is the difference between invasive and non-invasive neuroimaging?
A
- Invasive: Techniques that change the characteristics of the nervous tissue under study (e.g. slicing and staining, radioisotopes)
- Non-invasive: Techniques that do not cause changes to the characteristics of the nervous tissue under study (e.g. MRI, fMRI, PET, SPECT, EEG, MEG, NIR)
