case 1 - neuroimaging Flashcards
MRI precursor
- Origin of MRI - Nuclear Magnetic Resonance (NMR):
o MRI originally stood for “Nuclear Magnetic Resonance,” but the term “nuclear” was dropped due to concerns about public perception. The basic principles remain the same. - The term can be broken down as follows:
o Nuclear: Refers to the nuclei of atoms with a non-zero spin, such as hydrogen (H), phosphorus (P), carbon (C), and fluorine (F).
o Magnetic: MRI utilizes an external magnetic field to interact with the magnetic moments of these atomic nuclei.
o Resonance: It exploits the phenomenon of resonance to disturb and measure the magnetic properties of atomic nuclei.
o Imaging: In MRI, the external magnetic field is varied as a function of position to localize and capture signals from specific regions of the body.
steps of MRI
- net longitudinal magnetization by alignement of protons with external magnetic field.
- radiofrequentie pulse
- resonance of precession
- spin-spin relaxation( de-phasing) and spin-lattice relaxation (flipping back to z-axis) –> radio pulse emission
- detection by receiver coils
slice selection
using a - Gradient of magnetism to align only a number of atoms at a time (only align when their Larmor frequency matches to the tesla of the MRI – therefor you only get reaction of certain tissues) so you know which ones will react at what time –> spatial resolution
o The steeper the gradient of the slice selection the better the spatial resolution the MRI will have
magnetisation and resonance
- The process begins with the generation of a strong magnetic field. In advanced MRI machines, this magnetic field is produced by superconducting coils cooled to extremely low temperatures using liquid helium.
- This powerful magnetic field aligns the nuclei of hydrogen atoms (usually found in water molecules) within the body. It does so by matching the magnetic properties of these nuclei (spin), causing them to align like tiny magnets. This alignment is crucial for the subsequent steps.
- When the magnetic field matches the intrinsic precession frequency of these hydrogen nuclei (called the Larmor frequency), a phenomenon known as resonance occurs. This resonance essentially prepares the hydrogen nuclei to respond to the next step.
excitation
- To create an image, we need to introduce a bit of chaos into the orderly alignment of the hydrogen nuclei. This is done through an electromagnetic pulse, called a radiofrequency (RF) pulse.
- The RF pulse is applied perpendicular to the magnetic field, disrupting the alignment of the hydrogen nuclei. This temporarily flips some of them into higher-energy states.
signal encoding
- After the RF pulse is turned off, the hydrogen nuclei gradually return to their equilibrium states (spin-spin and spin lattice), releasing energy in the form of radio waves. This emitted signal is detected by specialized coils in the MRI machine.
- By varying the strength of magnetic field gradients, the MRI machine encodes spatial information into the returning signals. This encoding is essential for determining where in the body the signals originated.
fMRI
- We use MRI to measure an indirect affect of local neuroactivity on the magnetic properties of blood –> BOLD signal (blood oxygen level depended signal)
- We do not measure the neurons themselves but the capillary bed around them so there is a 3 to 4 mm error in the measurements –> spatial limitation
- Temporal limitation –> BOLD comes after the neural activity
- Resting state fMRI signals correlation between areas and for detection of deterioration of an area
hemodynamic response
When a specific area of the brain becomes active, it requires more oxygen and glucose to function. In response, blood flow to that area increases to deliver these resources. This is known as the hemodynamic response.
BOLD
BOLD (Blood Oxygenation Level Dependent) signal is a key principle of fMRI. It relies on the fact that oxygenated and deoxygenated hemoglobin have different magnetic properties. When neurons become active and blood flow increases, the ratio of oxygenated to deoxygenated hemoglobin changes, leading to changes in the MRI signal
1. Neural activity
2. Increased metabolic rate
3. Local vasodilation
4. Increased blood flow
* Fresh oxygenated blood arrives at places of activity
5. Oxygen-Hb overshoot
* CO2-Hb / O2-Hb ratio decreases
6. Less magnetic disturbances
* When there is more oxygen the iron in the haemoglobin is less disturbed and the MR signal increases in those areas
T1
- The desire of the signal to go back to the initial Z axis (the lowest energy state)
- After the RF pulse, the protons gradually return to their original alignment with the magnetic field. This process is described by the T1 relaxation time. Different tissues have different T1 values, which determine how quickly they recover
- longitudinal relaxation time
- fat is bright
T2
- Dephasing in x-y plane = horizontal/transverse relaxation = spin-spin relaxation
- The individual magnetisations will interact in the xy-plane (precession) and start to deviate from each other as time goes on (lowering the net magnetisation) = T2 relaxation
- Different tissues do this at different speeds
- water is bright
gradient coils
When current is passed through these coils a secondary magnetic field is created. This gradient field slightly distorts the main magnetic field in a predictable pattern, causing the resonance frequency of protons to vary in as a function of position. The primary function of gradients, therefore, is to allow spatial encoding of the MR signal.
