24 - 25 MRI Flashcards
What is Magnetic field?
lines of a magnet
What is magnetic Flux () I?
the magnetic field added up over a given area.
What is Magnetic moment (µ)?
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
A change in the angular momentum will caused by the ___
force acting on the rotating object.
THE LAW OF CONSERVATION OF ANGULAR MOMENTUM STATES THAT
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
What is Gyroscope?
a device used for measuring or maintaining orientation and angular velocity
What is precession?
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.
What is Larmor frequency?
the angular frequency of precession.
What is Electromagnetism?
A wire with current flowing will create a magnetic field.
What is The Stern-Gerlach experiment?
In an inhomogeneous magnetic field an ion beam can split into two.
The intrinsic magnetic moments comes from the __ of the particle. (behaves like a spin)
SPIN
What is spin?
The intrinsic angular momentum of elementary particles (and their composites)
Does every nucleus of odd mass number has spin?
Yes
What is the value of spin for odd mass number?
An integer multiple of 1/2
Describe spin of electrons
Half integer spin or spin 1/2 particles
What is Zeeman-splitting?
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
Spin quantum number of Proton (and electron) spins
ms = 1⁄2
Describe number of protons in both states
There are almost the same number of protons in both states even under a very strong ~T magnetic field.
What can we measure in NMR?
In NMR we can measure the magnetic field of the protons.
The excess spins are oriented to the magnetic field. If there is an RF irradiation ( eg. 127 MHz) then the spins will be
parallel
excited to the antiparallel state.
One RF photon can excite __.
→ Depending on the___ (2 THINGS), some or all of the spins can get to the excited state.
- one spin
- intensity (photons/s) and duration of the irradiation
What does the precession have?
The precession has a frequency and a phase
→ Both will depend on the magnetic field strength
NMR spectrometer setup
The high filed magnet is superconductive coil, it needs to be cooled with liquid He.
The field inhomogenities are compensated with the shim-coils
What is The conventional method of NMR?
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.
After a ___ we can measure the relaxation signal
pulsed excitation
How to measure the Free Induction Decay (FID) signal.?
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)
How to produce NMR spectrum?
Fourier-transformation of the FID signal will produce the NMR spectrum.
The frequencies are very sensitive to the presence of other atoms/electrons. Therefore the spectrum is different for every molecule.
→ Why does this happen?
electrons shield the magnetic field, so H=H0(1-).
→ The local field H will depend on the electrons, which is molecule and chemistry specific.
What is 90 pulse?
the duration+intensity is “enough” to excite 1⁄2 of the excess spins
What is spin-lattice relaxation?
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”)
What is spin-spin relaxation: T2?
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.
Spin-echo methods and MRI (Magnetic Resonance Imaging)
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.
What are the 3 contrast factors?
- No of spins / voxel volume (1H conc., Proton Density)
- T1 (spin-lattice) differences
- T2 (or T2*) (spin-spin) differences
What is Slice selection.?
By applying a gradient in the magnetic field the Larmor frequency of protons change in every layer.
How to create 3D images from NMR?
1: slice select
2: phase select: this has to be repeated multiple times with different gradients!
3: frequency select
Describe Gd-contrast
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
Describe Electron spin resonance
Electrons have spins, and spin+orbital magnetic moments.
The magnetic moment is larger -> GHz frequency photons are required.
What is spin echo?
A single RF pulse generates a free induction decay (FID), but two successive RF pulses produce aspin echo (SE).
→ The SE represents regeneration of spin phase information apparently lost during the decay of the FID.
The role of spin echo?
A single RF pulse generates a free induction decay (FID), but two successive RF pulses produce aspin echo (SE).
→ The SE represents regeneration of spin phase information apparently lost during the decay of the FID.
