MR Physics and Safety

Question 1

How good is fMRI in terms of spatial and temporal resolution?

Answer 1

• Temporal resolution — from seconds to hours/days
• Spatial resolution — from the whole brain to the columns (attempt to extend to layers)

Question 2

Name some physicist who directly or indirectly contributed to the physics of MRI

Answer 2

Fourier, Purcell, Bloch, Ernst, Mansfield, Lauterbur

Question 3

How can one deduce the activity responsible for a certain performed task?

Answer 3

A usual principle — subtracting two conditions. One assumes that the difference between the two conditions will be only attributed to the task one performs. However, there is always an additional signal (“noise”, actually brain adaptation)

Question 4

What is the most common chemical element in the Universe?

Answer 4

Hydrogen (proton’s spin = 1/2)

Question 5

MRI is based on the features of molecule of … and its … [subatomic particle]

can only see this particle inside the molecule

Answer 5

MRI is based on the features of molecule of water and its proton

Question 6

What happens with the protons in the absence of external magnetic field?

Answer 6

In the absence of an external magnetic field, protons in free space will have their spin axes aligned randomly

Question 7

What happens with the protons in the presence of external magnetic field?

Answer 7

In the presence of an external magnetic field, some protons will align parallel to or antiparallel to the magnetic field -> some will cancel out, but there will be some magnetisation left

Question 8

How to calculate the magnetisation ratio?

Answer 8

(N of parallel aligned — N of antiparallel aligned) / N of antiparallel aligned

Question 9

Can MRI detect other chemical elements? What is the main problem?

Answer 9

Yes, it can, e.g., 7T MRI can detect sodium and phosphorus molecules.
But usually the amount of other elements is too low compared to the hydrogen

Question 10

What is precession of the spins?

Answer 10

The precession of spins (Larmor precession) refers to the spinning or rotation of the magnetic dipoles associated with atomic nuclei in a magnetic field. Its frequency is proportional to the magnetic field strength B0.

Question 11

What is Larmor frequency. What is it proportional to?

Answer 11

Larmor frequency is the frequency at which the spins of particles or nuclei precess in a magnetic field.
ω0 = γ * B0,
where
ω is the Larmor frequency,
γ is the gyromagnetic ratio (how fast the spins will rotate per second)
B0 is the magnitude of the applied main static magnetic field
=> Larmor frequency is proportional to the magnetic field strength B0

1H (hydrogen isotope) has the highest gyromagnetic ratio, γ = = 4,2 * 10^7 Hz/T

Question 12

What is the net magnetisation M? What does it depend on?

Answer 12

The net magnetisation M is the sum of all magnetic moments. It depends on the number of protons, external magnetic field (manipulated in the experiment) and temperature (not manipulated)

Question 13

How strength of the magnet is measured? How strong are magnets used in MRI?

Answer 13

The strength of magnets is measured in units of tesla (T).
In MRI, the most common magnets have the strength of 1.5Tto 3T. However, more strong, 7T, magnets are also available.
B0(3-T magnet) = 60000 * Earth’s magnetic field

Question 14

What are the exclusion criteria for MRI? What is the danger?

Answer 14

• Claustrophobia
• Pacemaker
• Implantable cardioverter defibrillator (ICD)
• Neurostimulator
• Aneurysm clip
• Metal implant
• Implanted drug infusion device
• Foreign metal objects, especially if in or near the eye
• Shrapnel or bullet wounds
• Permanent cosmetics or tattoos
• Dentures/teeth with magnetic keepers
• Other implants that involve magnets
• Medication patch (i.e., transdermal patch) that contains metal foil

There is danger of localized burns due to metallic implants

Question 15

What happens after the RF excitation?

Answer 15

Longitudinal Relaxation (T1 Relaxation):
all nuclear spins return to their equilibrium alignment along the static magnetic field (B0) after being perturbed.
Transverse Relaxation (T2 Relaxation, or Spin-Spin Relaxation):
decay or loss of phase coherence among the spins in the transverse plane (x-y plane) perpendicular to the static magnetic field (B0):

after the excitation all the spins have the same phase, then, they will try to point in different directions, when they all finally point in different directions (dephasing), the sum of magnetisation will be 0

Question 16

How to reduce the number of locations we potentially receive the signal from?

Answer 16

There are two steps to excite a slice in MRI:

1. Slice excitation (exciting spins in a specific area using a gradient): we are changing the external magnetic field -> the spins rotate in different frequencies -> then we use the rotation pulse of the specific area we are interested in (we need to know the exact magnetic field, e.g. head — 2.9-T, stomach — 3.1-T, heart — exactly 3-T)
2. Frequency encoding: constant magnetic field -> varying magnetic field (also, a gradient principle) = one superimposed frequency -> decomposed frequencies

In other words,
(1) a slice-select gradient is imposed along an axis perpendicular to the plane of the desired slice, resulting in a linear variation of potential resonance frequencies in that direction, and (2) a specially tailored RF-pulse is simultaneously applied, whose frequency components match the narrow range of frequencies contained in the desired slice.

Question 17

What is the purpose of Fourier transformation in MRI?

Answer 17

The purpose of Fourier transformation in MRI is to convert the acquired raw data into a frequency-domain representation known as the Fourier domain or k-space.

The raw data is acquired through the reception of RF signals emitted by the excited spins in the sample. These signals are typically collected in the time domain as a series of data points over a specific duration.
The Fourier transform analyzes the complex waveform of the acquired data and decomposes it into its constituent frequency components.

Question 18

How are frequency and phase coding applied in MR image acquisition?

Answer 18

1. Slice Selection: choose a slice and excite it by applying a slice-selective gradient and an RF pulse
2. Orient all the phases in the same direction
3. Phase Encoding: change the phases to establish a unique phase for each spatial location by applying a phase encoding gradient
4. Equalise the frequencies by switching the gradient on
5. Frequency Encoding: slow down some of the layers and amplify the other layers by switching the gradient up => variations in the Larmor frequency across the selected slice
6. Data Acquisition: acquiring RF signals emitted by the spins within the selected slice as a series of data points in the time domain
7. Fourier Transformation

Question 19

What is a k-space?

Answer 19

k-space is an array of numbers representing spatial frequencies obtained directly from the MR signal.

In practice, k-space often refers to the temporary image space, usually a matrix, in which data from digitized MR signals are stored during data acquisition. When k-space is full (at the end of the scan) the data are mathematically processed to produce a final image. Thus k-space holds raw data before reconstruction.

Question 20

Is it true that increasing number of voxel we can get better SNR?

Answer 20

No, it is the other way around. MRI scanner is not a microscope, the bigger voxel is, the more protons there are, while zooming in adds noise.
Number of protons is proportional to voxel volume and to signal-to-noise ratio

Question 21

How is the spin system excited?

Answer 21

1. Slice Selection
2. RF Pulse Application: RF pulse has a frequency that matches the Larmor frequency of the spins. This resonant frequency causes the spins to absorb energy from the RF pulse (resonance principle: sending energy to the protons, they are sending the energy back)
3. Flip Angle: The flip angle of the RF pulse determines the extent to which the net magnetization of the spins is rotated away from its aligned position with the static magnetic field. The maximum flip angle that can be achieved during RF excitation is 90 degrees (=> transverse, x-y-axes plane, we can only detect signal in a transversal plane, we cannot detect the longitudinal magnetisation, including z-axis planes)
4. Tipping the Spins: spins go from their equilibrium state, aligned parallel or antiparallel to B0, to a transverse or off-equilibrium state
5. Precession: after excitation, the spins start precessing around the B0 field => a detectable signal is generated

Question 22

What is echo time? How does it influence the image contrast?

Answer 22

Echo time refers to the time interval between the excitation pulse and the collection of the echo signal. It determines the time delay before the signal is acquired after the excitation of the spins.

The choice of TE can significantly impact the image contrast in MRI:
1. T1-weighted imaging: shorter TE values, bright signals from tissues with short T1 relaxation times (WM).
2. T2-weighted imaging: longer TE values => more time for transverse magnetization to dephase and decay, brighter signals from tissues with longer T2 relaxation times (CSF).
3. Proton density-weighted imaging: moderate TE to balance the contributions from T1 and T2 relaxation => a relatively balanced representation of tissue proton density

Also,
10 ms > 50 ms > 200 ms =high signal amplitude, low contrast > suffient amplitude, good contrast > decayed signal, only CSF visible