Exam 1 Flashcards
Spatial resolution
the smallest distinguishable distance separating 2 objects
> size of pixel
Resolution in contrast
the smallest density difference distinguishable
> gray levels
Digital image
representation by a finite number of
« image elements» : the pixel (picture elements)
> 2D image: pixels
> 3D image: voxels
Temporal resolution
time interval between two images
What is a CT scan?
Computed tomography
Uses X-Rays
Measures attenuations: Energy is attenuated (it has a certain energy when it is being sent, crosses through the body, and a different energy is received)
2D slices imaging of different tissues: You can reconstruct 3D but its not a real 3D as it is acquired in 2D slices that are put together to get a 3D impression VS MRI which is actually 3D
> thinner slices = more resolution
Sectional image of the volume whose
contrast depends on the attenuation of an Rx beam according to the density of the structures crossed.
Reconstruction by pixels and
representation of densities by gray levels.
> you cannot know how much attenuation is caused by EACH organ because the attenuation value is for all the organs the energy crosses through at that angle
CT scan: spiral acquisition
Continuous rotation combined with displacement
> sending beams of energy at different angles = produces a complete constructed image of the whole body
Characteristics of MRI
Very strong magnetic field
Uses radio waves
Has an antenna
Different MRI sequences
T1 weighted images
T2 weighted images
> Flair
> DIR
How does MRI work?
9.5% of the body is hydrogen
Hydrogen atoms have specific spins which can be represented as a vector with 2 values, size + direction
MRI starts to send a magnetic field that makes all the protons align with that magnetic field (before this the protons are aligned randomly)
Antenna sends radio waves to the region that wants to be measured.
The radio waves move the protons into a different position
When the radio waves are turned off the protons release an energy that is proportional to the time needed for the proton to go back to the initial aligned state.
> antenna is a sender + receiver: sends B1 small magnetic field, is a receiver when it captures the energy released by the protons
Advantages & Disadvantages of MRI
Advantages:
Non-invasive
Non-ionising radiation
High soft-tissue resolution and
discrimination between tissue
types
Morphological information, as
well as functional information
Disadvantages:
Time-consuming
Contraindications for MRI
Noise
The sequence needs to be adapted to the question
T1 weighted images
+ how can you do contrast enhancement
- contrasts fat/water
– anatomy /enhancement - ventricles are hypointense
- contrast between cortex gray matter and white matter is not very clear
- lesions appear hypointense so not very clear either (BUT Optimal for atrophy, changes in fat, and contrast)
T1 image contrast enhancement: done via injection of gadolinium
> gadolinium can only penetrate the BBB if it is disrupted = shows regions where the barrier is opened/ruptured e.g tumors, MS etc
T2 weighted images
- Contrasts water/tissue
- Good for pathology imaging
> lesions appear as slightly more hyperintense
Optimal for lesions: changes related to water (appear more hyperintense) - ventricles are hyperintense
T2-flair
Fluid Attenuated Inversion Recovery
T2 based
> cancels the signal that is coming from the liquid (CSF) = CSF becomes hypointense = only edema/lesions remain
Optimal for lesions: changes related to water (appear more hyperintense) e.g edema
- ventricles are hypointense
DIR
Double Inversion Recovery: Also suppresses WM
T2 > FLAIR > DIR
For when you are interested in the cortical ribbon
Flair or T2 has almost no contrast at the cortical ribbon between gray and white matter = difficult to see lesions in this area
Cancelling signals coming from white matter = leaves cortical ribbon / gray matter = easier to spot lesions in this area
Unit for magnetic field
3T, 7T:
7T is clearer
DWI
Diffusion Weighted Imaging
Looks at water molecules + the direction they are moving in + amount of diffusion (how much they can move)
Informs us on tensor = how much the water molecules can move
In WM this suggests placement of axons
> Indirect way of looking at axons, based on this tensor you can have different measurements that you can calculate the extent of damage
Nonconstrained environment: isotropic, water molecule can move in all directions
Constrained environment: ansiotropic, there are specific directions where the water molecule can move
Axial diffusivity: along the axon
Radial diffusivity: across the axon
Mean diffusivity: diffusion strength
Fractional anisotropy: directionality
Can visualize structural connections in vivo
* E.g., investigate the effects of lesions on connectivity
how does fMRI work
functional MRI
stimulus = increase in neuronal activity = Active neurons consume all the oxygen present in the region via blood = all O2 consumed, more O2 required = more blood flow req. = arteries expand
> Identifies which neurons are more active than others by capturing the ratio between oxyhemoglobin and deoxyhemoglobin
fMRI BOLD response
Blood Oxygenation Level Dependent (BOLD) imaging
increase in neuronal activity→ increase in metabolism (oxygen use)
* Hemodynamic Response Function (HRF)
→Ratio oxygen-rich / oxygen-poor hemoglobin
(iron disturbs magnetic field!)
Initial dip = consumption of oxygen in surrounding area
Peak = increase in blood flow bringing oxyhemoglobin
X axis: time, slow recruitment of oxyhemoglobin
Y axis: 2-3% signal, very low
Bad temporal resolution but very good spatial resolution
Task-based fMRI
+ block design
Brain activation: task vs control
Task: Thinking, Emotion, Movement
Example: reaction to facial expressions
Subject performs a task whilst in fMRI machine = Image consequence of this task
Stimulus is repeated many times so that you get the same brain activity every single time = you can get a much stronger signal that can be captured
Block design:
Alternating block design: ABABAB
Controlled block design: ACBCACBC
> powerful in detecting activated voxels
> Weak ability to determine the time course of the response (summation of hemodynamic responses in time)
Resting-state fMRI
Brain activation: do nothing! with the eyes open or closed
More slow, weak signal = you have to repeat fMRI scan even more times
Memory task: CP vs CI
Increased functional activation in CP (cognitively preserved) compared to CI (cognitively impaired)
Increased thalamic connectivity →Related to cognitive impairment!
Graph analysis
1) T1 to locate brain regions
2) fMRI capture activity for each region
3) Region has a pattern of activity: if different regions have similar patterns it can suggest which regions are functionally connected and which are not
> FC functional connectivity, SC structural connectivity
Each region can be interpreted as a node and then do a map of connectivity
Important regions that are highly connected can be referred to as hubs
MR spectroscopy
Image specific signals come from specific molecules
You can look at a graph and see the amount of molecules in certain parts of the brain
> Not good spatial resolution
Structural vs Functional measurements
Structural measurements: lesions, atrophy, diffusion, …
Functional measurements: activation and connectivity
