34. Radio spectroscopies Flashcards
What did the stern gerlach experiment prove?
quantization of e- spin in 2 orientations
What happens to the dipole in a homogenous magnetic field?
opposite sides of a dipole cancel each other out
Why were silver atoms used in the S-G experiment?
because AG atoms have one single outer shell electron, so we can study the properties of 1 electron
When does the magnetic moment emerge?
when there is a charge and a net spin
What is magnetic resonance?
Resonance-absorbtion of EM energy by a material placed in a magnetic field
Magnetic moment formula
M = gamma * L
gamma : gyromagnetic ratio
L : nuclear spin (quantum number)
What is the gyromagnetic ratio?
ratio of magnetic moment and angular momentum
What is the Zeeman effect?
splitting of a spectrum line into several components by the application of a magnetic field. Distortion of e- orbitals by the magnetic field
What is angular momentum?
the quantity of rotation of a body. Product of its moment of inertia (a tendency to remain unchanged) and its angular velocity
In how many levels does each E level split into?
2l+1 levels (E of atom changes when put in a magnetic field)
What is a precession?
in a magnetic field, protons spin around the long axis of the primary magnetic field (change in the orientation of the rotational axis)
What happens to the protons when there is no external magnetic field applied?
They are oriented randomly (still have a spin bc of their charge)
What is the precession rate?
The rate of precession of the magntic moment of the proton around the external magnetic field
Which relationship does the Larmor equation describe?
Relationship between rate of precession (larmor frequency) and strength of magnetic field
What is paramagnetism?
magnetism emerging in external magnetic field (caused by dipole orientation)
What is net magnetization (also called macroscopic magnetization)?
Vector sum of parallel and antiparallel orientation i
What determines the ratio of low and high energy spin populations?
Boltzman distribution : Nantipar/Npar = e^(-dE/kT)
Which electromagnetic radiations are employed for resonance?
radiofrequencies (electromagnetic)
What is resonance?
- A perturbation of the nuclear spin orientation will occur when an oscillating magnetic field is applied whose frequency closely matches the larmor precession frequency
- applying an EM radiation of a certain freq, we can induce transitions between the E levels of the proton
What is the basic process of NMR?
Magnetic pulses are applied to excite transitions between Zeeman energy levels, and then they relax
What are T1 and T2 relaxation?
T1 : relaxation in long axis (spin-lattice)
T2 : relaxation in transverse axis (spin-spin)
What is relaxation?
an excited magnetic moment back to equilibrium on the Z axis
What happens during T2 relaxation?
After the 90° excitation pulse all spins will be pointing in the Y axis direction, and rotating in synchronized phase.
Spin vectors will disperse and distribute themselves uniformly in the the plane and lose their phase.
What does T2 relaxation time depend on?
on interaction between elementary magnets (spins, protons)
What is the net magnetization at T2?
0
What happens during T1 relaxation?
relaxation of the Z-axis vectorial component towards the direction of the external magnetic field. Give up the extra energy to their surroundings : the lattice.
What does T1 time depend on?
on the interaction between elementary magnet and its environment
What is FID?
Free induction decay. Magnetic moments dephase due to magnetic field inhomogeneity, creating a damped sine wave (Mr signal does not continue forever)
What is NMR spectroscopy used for?
determining the content and purity of a sample, as well as its molecular structure
What does NMR measure?
- measures local magnetic field around a nuclei by measuring resonance frequency
- measures energy emitted when the spin returns to its base level
What is the NMR spectrum?
intensity of absorbed radiation as a function of frequency
What does ESR stand for?
electron spin resonance
What is the area under the NMR line proportional to?
proportional to the number of absorbing nuclei
What does the energy gap bw parallel and antiparallel correspond to?
corresponds to radiofrequency
What is “chemical shift”?
- electron cloud surrounding nuclei influences the external magnetic field, so the nuclei will experience a slightly different local magnetic field than the external.
- changes the resonance frequency of given nucleus : NMR lines shift slightly
What does ESR measure?
the absorbtion of microwave radiation corresponding to the energy splitting of an unpaired electron when it is placed in a strong magnetic field
What is the ESR spectrum?
intensity of EM radiation as a function of magnetic field
What are the differences in magnetic field and frequencies bw ESR and MSR?
ESR : lower magnetic field, higher frequency radiation
How does space encoding work?
using phase shift to determine the contribution of different voxels
How does proton density contrast work?
signal is weighted to reflect the actual density of protons, since nuclear magnetic resonance is the basis of MRI
What is proton density contrast ideal for?
for distinction between fluid, cartilages… so for joints
What does T1 reflect? How does this affect the brightness/darkness?
T1 reflects time it takes for regrowth of Mz signal. Tissues with short T1 recover more quickly. Their Mz value is larger so it is a stronger signal (brighter)
What does T2 reflect? How does this affect the brightness/darkness?
T2 reflects time it takes for MR signal to decay in the transverse plane. Short T2 means signal decays rapidly. Shorter T2s have smaller signals and appear darker.
What is pixel brightness or darkness related to?
To magnitude of MR signal detected
In T1 weighted images, which will be darker, and which will be brighter?
long T1 materials will be darker
In t2 weighted images which will b darker and which will be brighter?
Long T2 materials will be brighter
How do we get T1 weighted image?
short TR/ short TE
How do we get T2 weighted image?
long TR/ long TE
How do we get PD weighted image?
long TR / short TE
