The Principles of MRI Flashcards
- For MR imaging we use -
Protons
The main particles we use in magnetic resonance are protons. Protons are small particles that have a positive electrical charge. Think of these protons as a little planets. Like earth, they are constantly turning, or spinning around an axis; or you could say each proton possess a spin. The positive electrical charge, being attached to the proton, naturally spins around with it. And we all know that a moving electrical charge is called an electrical current. We should also know that where there is an electrical current, there is also a magnetic field.
- The protons carry out the following movements -
Spinning (around their axis) and Precessing.
- So, to conclude - A proton has a spin, and thus the electrical charge of the proton also moves. This moving electrical charge is accompanied by a magnetic field. Thus, the proton has its own magnetic field and it can be seen as a little bar magnet.
- The protons may align with their South or North poles in the direction of the external field which we call parallel. Or they may point in the opposite direction which we call anti-parallel.
- So, now we know, that we have protons, which act as little magnets and that they are oriented parallel or anti-parallel. Unfortunately, it is not that simple. We will see that the protons do not just lay there, aligned parallel or anti-parallel to the magnetic field lines. Instead, they move around in a certain way. The type of movement is called precession.
*During this precession, the axis of the spinning top circles forming a cone shape. It is hard to draw a proton in precession as this is a very fast movement as we will see below. For the sake of simplicity, we will just make “freeze frame” pictures.
For reasons we will learn later, it is important to know how fast the protons precess. This speed can be measured as the precession frequency, means how many times the protons precess per second. This precession frequency is not constant. It depends upon the strength of the magnetic field, in which the protons are placed. The stronger the magnetic field, the faster the precession rate and the higher the precession frequency.
- The rate of precession of element cores in a magnetic field depends on -
The force of the magnetic field
For reasons we will learn later, it is important to know how fast the protons precess. This speed can be measured as the precession frequency, means how many times the protons precess per second. This precession frequency is not constant. It depends upon the strength of the magnetic field, in which the protons are placed. The stronger the magnetic field, the faster the precession rate and the higher the precession frequency.
- The precession frequency on MRI is defined as -
Larmor Frequency
For reasons we will learn later, it is important to know how fast the protons precess. This speed can be measured as the precession frequency, means how many times the protons precess per second. This precession frequency is not constant. It depends upon the strength of the magnetic field, in which the protons are placed. The stronger the magnetic field, the faster the precession rate and the higher the precession frequency.
It is possible and necessary to precisely calculate this frequency. This is done by using an equation called the Larmor Equation where the frequency is calculated as a product of the strength of a magnetic field (B0) and the gyro-magnetic ratio. The equation states that the precession frequency becomes higher when the magnetic field strength increases. The exact relationship is determined by the gyro-magnetic ratio. This gyro-magnetic ratio is different for different materials.
- Longitudinal magnetization in MRI appears –
By changing the orientation of the magnetic vector of the nuclei
- Transverse magnetization in MRI apperas –
After synchronization of the precession phase
- Select the correct statement about magnetization on MRI -
We can not measure longitudinal magnetization directly
As this magnetization is in direction along/longitudinal to the external magnetic field, it is also called longitudinal magnetization. And it is actually this new magnetic vector that may be used to get a signal. It would be nice if we could measure the magnetization of the patient, but there is a problem: we cannot measure this magnetic force, which is in the same direction or parallel to the external magnetic field.
To illustrate this: imagine that you are standing on a boat, floating down a river. You have a water hose in your hand and squirt water into the river. For somebody who is watching you from the shore, it is impossible to tell how much water you pour out (this represents how much new magnetization is added in the old direction). However, when you point the water hose to the shore (change the direction of the new magnetic field) then the water may perhaps be directly picked up and measured by an impartial observer on the shore. What we should learn from this is: only magnetization which is transverse to the external magnetic field can be measured directly.
- The synchronization of the precessing protons in MRI is made by using –
A radiofrequency pulse
- What frequency is the radiofrequency pulse commonly used in MRI? -
The frequency that matches the Larmor frequency
- When the original vector of longitudinal magnetization is 6 (6 proton vectors pointing up), what would the resulting vector be when two protons are reversed by the radiofrequency pulse? -
2
- What is the effect of radiofrequency pulse on protons in MRI? -
RF pulse reduces longitudinal magnetization and creates transverse magnetization.
- How do we choose the proper plane of imaging in MRI? -
By a magnetic field gradient
- What happens to protons in MRI after switching off the radiofrequency pulse? -
Relaxation
- The longitudinal relaxation on MRI appears -
by returning the magnetic vectors of the protons to the original arrangement.
- Longitudinal relaxation is also called -
Spin-lattice relaxation
- T1 relaxation time depends on -
Longitudinal magnetization
- T2 relaxation time depends on -
Transverse magnetization
- By synchronizing the protons after an RF pulse in MRI -
Transversal magnetization occurs
