Why is MRI important?
MRI does not use x-rays.
MRI can lead to early detection and treatment of disease.
The MRI images are extremely precise.
Is sensitive for a variety of diseases, cancers tumours, injuries and other abnormalities.
MRI Uses?
A Large Magnet
Radio waves
Special Receiver Coils
A powerful Computer
The Receiver Coils…..? (explain)
Specific coil for each region
The coil measures the energy released by the protons as well as the time it takes to return to their aligned positions.
Different coils with different elements/receivers.
The fundamentals of MRI can be explained via:
Quantum physics as well as
Classical physics.
The most abundant atom in the human body is?
Hydrogen; most commonly found in molecules of water (H2O).
The atom consists of….
a central nucleus with orbiting electrons
The electrons spin around the nucleus in a…
cloud
The atom consists of the following particles: (3)
Protons
In the nucleus with a positive charge
Neutrons
In the nucleus with no net charge
Electrons
Orbiting the nucleus with a negative charge.
Three motions within the atom:
- Electrons spinning on their own axis
- Electrons orbiting around the nucleus
- Nucleus spins on its own axis
why is the Hydrogen atom Used in clinical MRI?
because it contains a single proton (atomic and mass number of 1).
It is used because of its abundance in the human body and its solitary proton gives it a relatively large magnetic moment.
The laws of electromagnetism… ( 4 )
- A magnetic field is created when a charged particle moves.
- The nucleus of Hydrogen has one positively charged proton that spines or moves! It has a North and South Pole of equal strength.
- This north/south axis of each nucleus is represented by a magnetic moment and is used in the principles of classic theory of MRI.
- The magnetic moment of each nucleus has size and direction (vector properties) and is denoted by an arrow.
describe Alignment…
- If there is no magnetic field the magnetic moments of the hydrogen nuclei are randomly orientated, but when placed in a strong externally applied magnetic field, the magnetic moments align with the magnetic field
- Quantum theory describes discrete quantities of energy of Hydrogen. Low-energy nuclei align their nuclei to the external magnetic field (spin-up) nuclei. High-energy nuclei go in the opposite direction and are called (spin-down) nuclei.
NMV stands for…
The Net Magnetic Moment of hydrogen (Net Magnetization Vector)
Bo stands for…
The Static External Magnetic Field
Basis of MRI
The interaction between the NMV with the Bo
Unit of Bo
Tesla or Gauss. 1 Tesla (T) =
10 000 gauss (G)
what is Precession
Each hydrogen nucleus is spinning on its own axis. Bo has got an influence on this spin in that it produces an addition spin or wobble of the magnetic moments of hydrogen around the external applied magnetic field. This secondary spin is called precession and it follows a circular path around Bo.
This path is called the precessional path and the speed at which they wobble around B0 is called the precessional frequency and is measured in megahertz (MHz).
what is The Larmor equation
The Lamor equation governs the precessional frequency, it states:
ῳo = Bo x у
ῳo = Precessional frequency
Bo = Magnetic field strength of the magnet
у = Gyro-magnetic ratio
what is the Gyro-magnetic ratio?
Gyro-magnetic ratio is constant and expresses the relationship between the angular momentum and the magnetic moment of each MR active nucleus. It is expressed as the precessional frequency of a specific MR active nucleus at 1T and therefore the unit of gyro-magnetic ration is MHz/T.
what is The (у) gyro-magnetic ration of hydrogen ?
42.57 MHz/T.
Precessional frequency is often called….?
the Larmor frequency.
Two important facts to remember about the Larmor equation.
- Hydrogen precesses at different frequency than other MR active nuclei, because they have their own gyro-magnetic constant even when they are exposed to the same field strength. Hydrogen can therefore specifically being imaged in MR by ignoring the other MR active nuclei
- “The gyro-magnetic ratio is a constant of proportionality. Bo is proportional to the Larmor frequency. Therefore if Bo increases the Larmor frequency increases and vice versa.”
what is Resonance?
“Resonance is a phenomenon that occurs when an object is exposed to an oscillating perturbation that has a frequency close to its own natural frequency of oscillation.”
When this process happens to a nucleus, it gains energy from the external force. The frequency delivered must be exactly the same as the as the precessional frequency of the nucleus otherwise resonance does not occur.
what is excitation?
Excitation is the term used when resonance occur if an RF pulse is applied that causes resonance.
There is an increase in spin-up nuclei because of the absorption of energy and if the field strength increases the energy difference between the two populations (spin-up and spin-down) also increases.
