24 - 25 MRI

Question 1

What is Magnetic field?

Answer 1

lines of a magnet

Question 2

What is magnetic Flux (􏰥) I?

Answer 2

the magnetic field added up over a given area.

Question 3

What is Magnetic moment (µ)?

Answer 3

It is related to the torque experienced in a magnetic field:

→ Here every quantity is a vector, 𝔁 is the vector product.
Unit: Nm/T = J/T

Question 4

A change in the angular momentum will caused by the ___

Answer 4

force acting on the rotating object.

Question 5

THE LAW OF CONSERVATION OF ANGULAR MOMENTUM STATES THAT

Answer 5

When the net external torque (􏰦) acting on a system about a given axis is zero , the total angular momentum (L) of the system about that axis remains constant

Question 6

What is Gyroscope?

Answer 6

a device used for measuring or maintaining orientation and angular velocity

Question 7

What is precession?

Answer 7

A torque (􏰦) is created on the rotating object by the gravity force and the holding force of the stand (w).

Since this torque is perpendicular to the angular momentum, therefore 􏰛L is perpendicular to L itself causing precession:

the magnitude of the angular momentum will not change, just its direction.

Question 8

What is Larmor frequency?

Answer 8

the angular frequency of precession.

Question 9

What is Electromagnetism?

Answer 9

A wire with current flowing will create a magnetic field.

Question 10

What is The Stern-Gerlach experiment?

Answer 10

In an inhomogeneous magnetic field an ion beam can split into two.

Question 11

The intrinsic magnetic moments comes from the __ of the particle. (behaves like a spin)

Answer 11

SPIN

Question 12

What is spin?

Answer 12

The intrinsic angular momentum of elementary particles (and their composites)

Question 13

Does every nucleus of odd mass number has spin?

Answer 13

Yes

Question 14

What is the value of spin for odd mass number?

Answer 14

An integer multiple of 1/2

Question 15

Describe spin of electrons

Answer 15

Half integer spin or spin 1/2 particles

Question 16

What is Zeeman-splitting?

Answer 16

energy depends on magnetic field strength.

In the presence of an external magnetic field, the interaction of the magnetic moments and the field cause the energy levels of proton to split into 2 levels
-> these 2 levels correspond to the ground state and excited state of the particle, respectively -> The splitting depends on the magnitude of the applied external field

Question 17

Spin quantum number of Proton (and electron) spins

Answer 17

m_s = 1⁄2

Question 18

Describe number of protons in both states

Answer 18

There are almost the same number of protons in both states even under a very strong ~T magnetic field.

Question 19

What can we measure in NMR?

Answer 19

In NMR we can measure the magnetic field of the protons.

Question 20

The excess spins are oriented to the magnetic field. If there is an RF irradiation ( eg. 127 MHz) then the spins will be

Answer 20

parallel

excited to the antiparallel state.

Question 21

One RF photon can excite __.

→ Depending on the___ (2 THINGS), some or all of the spins can get to the excited state.

Answer 21

1. one spin
2. intensity (photons/s) and duration of the irradiation

Question 22

What does the precession have?

Answer 22

The precession has a frequency and a phase

→ Both will depend on the magnetic field strength

Question 23

NMR spectrometer setup

Answer 23

The high filed magnet is superconductive coil, it needs to be cooled with liquid He.

The field inhomogenities are compensated with the shim-coils

Question 24

What is The conventional method of NMR?

Answer 24

We measure the RF absorption spectrum of the sample. (with cw RF irradiation, varying frequency)

Drawback: absorption methods have a low sensitivity.

Emission methods have a high sensitivity-> FT-NMR We excite with an RF pulse, and measure the resulting emission.

Question 25

**After a ___ we can measure the relaxation signal**

Answer 25

**pulsed excitation**

Question 26

How to measure the Free Induction Decay (FID) signal.?

Answer 26

**After a pulsed excitation we can measure the relaxation signal** **→ We can measure the electro-magnetic field produced by the precessing spins (with a simple wire coil. Similar coils can send the RF pulses)**

Question 27

How to produce NMR spectrum?

Answer 27

Fourier-transformation of the FID signal will produce the NMR spectrum.

Question 28

The frequencies are very sensitive to the presence of other atoms/electrons. Therefore the spectrum is different for every molecule. → Why does this happen?

Answer 28

electrons shield the magnetic field, so H=H0(1-􏰨). → The local field H will depend on the electrons, which is molecule and chemistry specific.

Question 29

What is **90**􏰞 **pulse?**

Answer 29

**the duration+intensity is “enough” to excite 1⁄2 of the excess spins**

Question 30

What is **spin-lattice relaxation?**

Answer 30

The excited spins return to the ground state following an exponential decay. The decay rate is sensitive to the local magnetic field, which is influenced by the surrounding material (the “lattice”)

Question 31

What is **spin-spin relaxation: T2?**

Answer 31

In the xy plane ALL spins can contribute. They are synchronized by the RF pulse → but then due to local inhomogeneities (T2-\>T2\*) and random fluctuations (caused by nearby spin’s fields) of local H → a dephasing will start.

Question 32

**Spin-echo methods and MRI (Magnetic Resonance Imaging)**

Answer 32

For 3D imaging one needs to select a **voxel** of the reconstruction volume This can be done in general by varying the larmor frequency of the spins.

Question 33

What are the 3 contrast factors?

Answer 33

- No of spins / voxel volume (1H conc., Proton Density) - T1 (spin-lattice) differences - T2 (or T2\*) (spin-spin) differences

Question 34

What is Slice selection.?

Answer 34

By applying a gradient in the magnetic field the Larmor frequency of protons change in every layer.

Question 35

How to create 3D images from NMR?

Answer 35

1: slice select 2: phase select: this has to be repeated multiple times with different gradients! 3: frequency select

Question 36

Describe Gd-contrast

Answer 36

Gd: Gadolinium, has a lot of unpaired electrons, these have a large net magnetic moment. This can induce relaxation of nearby nuclei. (a paramagnetic effect) This shortens T1 relaxation times of nearby protons

Question 37

Describe Electron spin resonance

Answer 37

Electrons have spins, and spin+orbital magnetic moments. The magnetic moment is larger -\> GHz frequency photons are required.

Question 38

What is spin echo?

Answer 38

A single RF pulse generates a ***free induction decay (FID)***, but two successive RF pulses produce a***spin echo (SE)***. → The SE represents regeneration of spin phase information apparently lost during the decay of the FID.

Question 39

The role of spin echo?

Answer 39

A single RF pulse generates a ***free induction decay (FID)***, but two successive RF pulses produce a***spin echo (SE)***. → The SE represents regeneration of spin phase information apparently lost during the decay of the FID.