Chapter 4: brain imaging Flashcards
Brain imaging
MRI and CT are mainly structural techniques, they investigate the anatom of the brain.
PET, SPECT, EEG and EMG are mailnly functional techniques, they investigate the underlying processes (like movement of water or blood).
Contrast agent
Other substance that enhances the contrast between different areas in the brain.
Computerized Tomography (CT)
Using X-rays to visualize brain tissue in detail. Detectors measure the strength of the radiation after it passed through the patient’s head. Then the computer measures where the absorption is strong and weak.
What do you see on a CT scan?
- Atrophy (loss of brain tissue), swelling of the brain, a stroke or other abnormalities is mostly accompanied by. more water content in the brain tissue, which you can see as darker parts in the images.
- The blood and calcium are white.
- Sometimes a contrast agent (fluids) are inserted into the brain to diagnose trombosis or tumor. The fluid is injected into the blood and makes it even more white. Normally the blood-brain barriers prevents this agent to enter the brain, but in a tumor the BBB is impaired and thus shows more white on the CT scan.
(This type of CT scan is quick and available, but can be very harmful and is not very detailed.)
MRI
This type of scan is detailes end less harmful than CT, but it is very expensive.
The water in the brain there are protons. These protons revolve around their own axis, which creates a tiny magnetic field The MRI scanner sends out a horizontal radio frequency pulse. This pulse tries to get every proton to move in the same direction as the radio frequency (horizontal). When the pulse stops, the protons that have absorbed it’s energy let this energy go (relaxation) and go back to revolvolving around their own axis. This signal is picked up by the radio frequency coils.
2 forms of relaxation of protons in MRI
- T1: the return of the protons to moving towards the longitudinal plane (vertical).
- T2: the protons stop rotating in sync because the radio wave has stopped.
3 types of MRI scans
- T1: Water-rich structures are colored dark and fat is colored light-grey/white. This shows a good contrast between the cerebrospinal fluid (CSF), the grey matter, and the white matter. Mainly used to view the anatomy of the brain.
- T2: CSF is colored white, and since cerebral pathology causes more water in the brain, there will be a lot of white seen. Mainly used for brain abnormalities/pathological changes.
- Flair: a type of T2 scan but here the water/CSF is colored dark. This is mainly used for detecting subtle changes around the hemispheres and in the periventricular area (people with a stroke or MS show white spots if there is inflammation).
- Susceptibility weighted imaging: a type of T2 scan but more sensitive, which makes microbleeds more visible. It can also differentiate between calcium and blood.
(T1 and Flair imaged are used to diagnose dimentia).
(Flair images are used to assess global cortical atrophy.)
(T1 is used to assess medial temporal lobe atrophy MTA).
Fazeska scale
This scale looks at the presence of white matter lesions via Flair or T2 images:
0 = no or single punctuate (white matter) lesions
1 = multiple
punctuate lesions
2 = onset of confluent white matter abnormalites
3 = large confluent white matter abnormalities
Diffusion-weighted imaging (DWI)
A form of MRI that measures the mulecular diffusion in the brain. This means that it measures the microscopic movement of water molecules that collide with each other and their environment. This is measured via T2 and it is a functional MRI. Measures the contrast between different brain areas determined by their degree of diffusion. This technique is mainly used for diagnosis tumors or early diagnosis of cerebral infarctions. There are 2 types of diffusion that can be measured: isotropic and anisotropic.
Isotropic diffusion
The water molecules move completely random.
Diffusion tensor imaging (DTI)
Measures the direction of the diffusion of the water molecules, also called tractography. This technique is used to diagnose Alzheimer’s and Parkinson’s disease at an early stage since it looks an microstructural changes and reduced white matterintegrity.
Anisotropic diffusion
In certain pathological conditions (like tumors or swelling in the brain), the environement in which the water molecules move is restricted and the cells are very densely packed.
Perfusion MRI
Perfusion is the delivery of oxygen and nutrients to the brain through the bloodstream. This measure is a form of functional MRI. It can determine the place of a tumor and even a stroke. There are 3 different kinds of perfusion techniques:
1. Dynamic susceptiblity contrast (DSC): uses a contrast agent and results in T1 or T2 image.
2. Dynamic contrast-enhanced (DCE): uses a contrast agent at results in T1 image.
3. Arterial spin labeling (ASL): uses naturally occuring water in arterial blood as contrast agent (non-invasive).
Magnetic resonance spectroscopy (MRS)
Studies the molecular composition of the brain and measures the biochemical changes, mainly in tumors.
Functional MRI (fMRI)
Measures the acitivity of the brain by comparing blood flow to the active areas. Areas that are more active will have more blood flow and thus more oxygenated hemoglobin, which causes a high signal. fMRI used teh radio of oxygenated to deoxygenated hemoglobin in the blood: blood oxygenated level dependent (BOLD) signal.
fMRI is used to understand brain functions, but also to look at the important areas for a specific patient. When someone has to undergo brain surgery, the surgeons can use fMRI to determine where the vital functions in the brain are for this particular patient. It can also show where an epeleptic seizure starts.
