MR-Physics and Safety Flashcards
Whats the point?
- The point is that human body has H atoms and the MRI manipulates these atoms for ‘reading’ us.
- H atoms are magnetic, protons like to spin randomly,
- When you apply external magnetic field the protons align in the same directions (some face up some down but always more up than down), they are also spinning don’t forget
- When you apply a radio frequency they get energised and start spinning with matching frequencies
- You apply a gradient and that shows in the frequency reading, you reduce the number of protons, you see it in the strength of the frequency signal
What is Lamour telling us
- Lamour frequency equation: w= gB0
- It is the frequency at which nucleus will absorb energy
- Stronger the magnetic field (Bo) faster will be the rotation
- The constant g (gyromagnetic ratio) tells us how fast the rotation will be
Whats up with Magnetisation (Mo)
- Equation is too long, don’t need to know
- More the magnetisation better signal we have. Depends on three main factors:
- Stronger the magnetic field Bo better Mo
- Decreasing temp (T) is one way to increase magnetisation, but can only be done limitedly in human readings.
- Higher the Number of protons (N) stronger the magnetisation i.e. signal
What is the basic principle of MRI
We are trying to make images of the proton distribution in our body so with MRI we try to match the frequency of their spin to take images
What does Radio Frequency do?
- In order to read the signal from a proton, it needs to be excited
- For exciting the spin system, we send a radio signal that has the same frequency as the precesion frequency of protons
- Protons interact with this radio signal, they start spinning in the direction of the Magnetic field (Bo) in the transversal plane. Can only detect signals in this plane.
What is ‘phase’
When all protons are pointing in same direction it is called rotating in ‘phase’
What is important to know about the Frequency pulse strength
Longer the pulse, longer the rotation and more they rotate.
We need a 90deg rotation to be able to detect signal (because signals only in transversal plane are detected),
So, if we pulse for too long rotation will be 180 deg (longitudinal plane) and we wont be able to read the signal, so calculating the duration of pulse is super crucial for receiving readable signals
What happens after excitation?
Relaxation
(of NMR Signal)
Immediately after excitation the signal starts decaying to come back to ground state, aligning back to Bo (External magnetic field)
What is T1 and T1 weighted MRI
- As soon as signal stops, signal starts decaying
- The longitudinal relaxation also knows as T1 or spin-latice relaxation
- Restores from perpendicular alignment to parallel alignment (to the magnetic field)
- In T1 weighted MRI CSF appears dark & blood and fat are bright. This distinction is due to the different speed at which signals from different types of cells decay
Of course now we will talk about What is T2 or T2 weighted MRI
- Transversal relaxation or T2 or spin-spin relaxation
- After RF is applied, all spinning protons are in phase, when RF stops they start dephasing
- XY component decreases exponentially. This is the perpendicular plane
- In T2 weighted MRI, Fats appear intermediate bright & fluids & CSF appear bright
Reception of NMR signal
- the signals received are very small, the MRI machine amplifies it to be able to measure it
- the observable signal received is ‘Free Induction Decay’ (FID)
- voltages are 10,000 times smaller than needed for excitation
Slice Excitation
- MRI reading is done in slices
- Protons of the area of the body that needs to be measured are excited. Gradient coil creates a magnetics field that changes from left to right side
- Depending on RF pulse, can excite different slices of the brain , this helps create images of different slices of the brain at a time
Now we move to actual imaging.
What is Frequency encoding
To explain frequency encoding, imagine there are two scenarios, one where a constant magnetic field is applied and other where a gradient is applied
- Constant magnetic field will show a single frequency on frequency domain
- Varying magnetic field on its low magnetic strength side will show lower frequency on time, temporal & frequency domain
- And on its higher magnetic strength side will show higher frequency on time, temporal & frequency domain
- Number of protons affects the strength of the received frequency, so if you were to reduce the number of protons, the signal is weaker but at the same frequency
- Now that we know how a gradient behaves and how the frequency arising from it at different domains looks like, we know that this is how signals are localised and slices of brain are imaged
Whats the deal with Fourier and why does he have a whole flash card to himself?
(mainly because the prof said this is the single most important thing we must remember)
Fourier Transform
- Used to transform frequency from temporal domain to frequency domain. The math for it was found by Jean Baptiste Fourier (1768-1830)
- Temporal pattern can be explained by how sound waves are wavy but when we hear it, it is a continuous pitch, the math behind it is used to transform the wavy signals to a flat(ish) line
What is Slice excitation gradient?
This is step 1 of the entire imaging process.
A gradient used to excite a slice is a slice excitation gradient (duh!)
In this step, there is excitation of one slice (slice excitation) in the transversal plane
This is done by applying a gradient (slice excitation gradient, Gs) at a specific Radio Frequency (RF).
(When a gradient is applied a slice gets excited at different frequencies but we focus on one part of the slice for all future discussion, check image on slide 16 to understand)
Thus, all spins of this slice are now in the same phase and rotating with same frequency
