Chapter 14 Bushberg

Front 1

what is NMR?

Back 1

spectroscopic study of the magnetic properties of the nucleus of an atom; protons and neutrons have a magnetic field associated with them

Front 2

What is magnetic suceptability?

Back 2

Extent to which a material becomes magnetized when placed in a magnetic field

Front 3

What are the 3 categories of susceptability?

Back 3

Diamagnetic
Paramagnetic
Ferromagnetic

Front 4

What do you positive and negative susceptability mean?

Back 4

Positive-enhance local magnetic field (paramagnetic-have to measurable self-magnetism); Negative (diamagtetic)-oppose local magnetic field

Front 5

Why are most organic materials diamagnetic?

Back 5

Diagmetic properties of C and H

Front 6

what kind of substance is gadolinium contrast?

Back 6

Paramagnetic

Front 7

Explain dipoles?

Back 7

North pole is the origin of the magnetic field lines and the south pole is the return

Front 8

What is the magnetic field strength?

Back 8

Number of magnetic lines of foce per unit area; B=Telsa and IT=10,000 G (gauss)

Front 9

How can you induce magnetic fields?

Back 9

Moving charge in a wire

Front 10

How do you determine direction of magnetic field?

Back 10

Fingers point in the direction of the magnetic field and thumb points in the direction of the moving positive charge (ie opp. the movement of the e-)

Front 11

How do you augment the magnetic field?

Back 11

Wrap the current carrying wire around many times in a coil

Front 12

What is the magnetic moment?

Back 12

Describes the magnetic field strength of a nucleus

Front 13

What are magnetic properties of portons and neutrons in a nucleus by?

Back 13

spin and charge distribution intrinsic to the proton and neutron

Front 14

What is pairing?

Back 14

onstituent protons and neutrons determine the nuclear magnetic moment

Front 15

When is magnetic moment zero?When do you generate a magnetic moment?

Back 15

1. if the total number of protons and neutrons in the nucelus is even MM=0
2. If N is even and P odd or visa versa=magnetic moment

Front 16

What is the signal generated by an MRI?

Back 16

conglomerate signal of billions of atoms magnetic moments

Front 17

Compare the magnetic moments of neutrons and protons:

Back 17

1. Neutron=-9.66x10-27
2. Proton=1.41x10-26

Front 18

What are the key features of elements capable of producing an MR image?

Back 18

1. Magnetic moment strength
2. Physiologic concentration
3. Isotropic abudundance

Front 19

What is the principle element used for MR imaging?

Back 19

proton

Front 20

How do you get net tissue magnetization?

Back 20

Thermal energy agitates and randomizes the direction of the proton spins in the tissue samples-->net magnetization

Front 21

What happens under the influence of a strong magnetic field?

Back 21

Spins distributed in two energy states: alignment with (parallel to) the applied field at a low energy state and alignment against the applied field at a higher energy state; a slightly greater # of protons exist in the parallel direction-->yields a measurable magnetic moment in direction Bo

Front 22

What is the energy state majority of spins?

Back 22

LSight majority of spins exist in the low-energy state-->determined by the thermal energy of the sample

Front 23

What happens to the distribution of energy states at higher magnetic field strengths?

Back 23

Energy seperation of low and high energy levels and the # of protons in the low energy state are >

Front 24

Give the number of excess low energy protons in a 1 T:

Back 24

1T is 3 spins per million 3x10^-6= 3x10^15 more spins in the low energy state in a typical voxel volume

Front 25

What is precession?

Back 25

The torque from the applied magnetic field that a proton experiences; direction of spin axis is perpendicular to the torques tiwisitng; occurs at an angular frequency (rotations/sec) proprtional to magnetic field strength

Front 26

What is the larmor equation?

Back 26

Describes the dependence b/w
magnetic field Bo and the precessional angular frequency wo
where wo=gamma * Bo

w=angular frequency

Front 27

What do larger magnetic fields do to precessional frequency?

Back 27

Produce larger precessional frequencies

Front 28

How is an MR signal produced?

Back 28

Energy (in form of pulse of radiogreqency EM radiation) at precessional frequency-->is absorbed-->converts spins from the low-energy prallel drection ot higher-energy antiparalle direction-->perterbed system goes back to eq. state-->produces the MR signal

Front 29

What are the typical magnetic field strenghts for MR imaging? what is the precessional frequency for portons in a 1T magnetic field?

Back 29

1. 0.1-4.0 T
2. 42.58 mHz in a 1T

Front 30

What allows ofr the selective excitation of one elemental species over another for a given magnetic field strength?

Back 30

differences in precessional frequency

Front 31

What is the larmor frequency wrt to linear frequency equation?

Back 31

fo=(gamma/2pi) * Bo

Front 32

What direction is the applied magnetic field directed?

Back 32

Parallel to the z axis

Front 33

What are the 3 components of net magnetization vector?

Back 33

Mz=component of the magnetic moment that is parallel to the applied magnetic field (longitudinal magnetization)
Mxy=transverse magnitization perpendicular to the applied magnetic field

Front 34

What is the value of Mz at equilibrium?

Back 34

Long. mag. is max (Mo)=Mz amplitude determined by excess number of protons in the low-energy state

Front 35

What is the value of Mxy at eq?

Back 35

=0 b/c vector components of the spins are radomly oreinted about 360 defress in the x-y plane and cancel each other out

Front 36

What happens when the system absorbs energy?

Back 36

Mz is tipped into the transverse plane

Front 37

Describe resonance and excitation:

Back 37

RF pulse (B1 field) matched to precesional frequency of protons-->displacment of eq. magnetization-->resonance

Front 38

Give the quantum mechanics model for resonance and excitation:

Back 38

RF energy equal to energy difference b/w parallel and antiparallel spis appled to sample (lamor frequency; in eq. parallel>antiparrallel)-->discrete quanta absorption hcanges proton energy from parallel to antiparallel (antiparallel=parallel)-->with contineud app of RF energy at Larmor frequency Mz displaced from Eq through zero to opp. direction (high-energy state)-->now antiparallel>parallel and Mz is negative

Front 39

Describe the classical physics model for magnetic field compoenent fo the radiofrequency pulse:

Back 39

clockwise and countercolcowise rotating magnetic vectors produce magnetic field variation by constructive and desctructive interaction-->at larmor frequency one magnetic field vector rotates synhornously in the ortating frame and is therfore stationary (the other vector rotates in the opp. direction)

Front 40

What does the 90 degree RF pulse produce?

Back 40

Phase coherence of individual protons and generates max. possible transverse magnetization for a given sample volume

Front 41

As Mxy rotates at the Larmor frequency, what happens to the receiver antenna coil?

Back 41

-receiver antenna coil (in the laboratory frame) id induced (by magnetic induction) to produce damped sinusoidal electronic signal known as the FID (free induction decay)

Front 42

What does conversion of longitudinal magnetization (Mz) into transverse magnetization (Mx,y) result in?

Back 42

Initial phase coherence of individual spins in the sample-->magnetic moment vector precesses at the Larmor freq.-->dephases with time

Front 43

What happens in the laboratory frame?

Back 43

Mxy precesses and induces a singl in an atenna receiver that is sens. to TRANSVERSE magnetization-->produces a FID signal that oscillates at the larmor frequency and decays with time (loss of phase coherence)

Front 44

What is the "decay" of the FID envelope?

Back 44

Loss of phase coherence of the individual spins caused by magnetic field variations

Front 45

How does the loss of the Mxy phase coherence occur?

Back 45

Exponentially-->caused by intrinsic spin-spin interactions in the tissues and extrinsic magnetic field inhomogenities

Front 46

What is the exponential decay constant?

Back 46

T2-time over which the signal decays to 37% of max transverse magnetization (decay time that results from INTRINSIC magnetic properties of the sample)

Front 47

What is T2*?

Back 47

Decay time resulting from both INTRINSIC and EXTRINSIC magnetic field variations

Front 48

What value is greater T2 or T2*?

Back 48

T2

Front 49

When and why is the decay constant shortened to T2*?

Back 49

-in the prescence of extrinsic magnetic inhomogenities like an imperfect main magnetic field (Bo)
-susceptibility agents present in tissues (MR contrast, paramagnetic or feromagnetic objects)

Front 50

What time frame does the loss of transverse magnetization (T2) occur?

Back 50

relatively fast compared to return to eq (max long. magnetization Mz)

Front 51

Describe the process of spin-lattice recovery in comparison to spin-spin decay:

Back 51

After 90 degree pulse-->long mag. Mz is converted from max. value at eq to 0-->return of Mz to eq. occurs exponetially characterizaed by the spin-lattice relaxation constant T1

Front 52

What does the spin-lattice relaxation constant mean?

Back 52

After elapsed time equal to T1 63% of the long. magnetization is recovered

Front 53

Compare the T1 relaxation times for the following:
Medium and viscous, small and aqueous, large and stationary

Back 53

Medium, viscous (more structured lattice w/vibrational freq. more condusive to spin-lattice relaxtion)<small, acqueous (exhibits a broad range of freqencies)<large,stationary (little motion, low frequencies)

Ex for biologic tissue T1 ranges from 0.1-1 sec for ST and 1-4 seconds for acqueous tissue

Front 54

Which each scenario give the effect on T1 relation time:
1. higher field strength
2. Increase in larmor precesional freq.
3. Contrast agents (gad)

Back 54

1. Increase T1 relaxation time
2. Increase T1 relaxation time
3. Decrease T1 relaxation time

Front 55

Why do contrast agents decrease T1 relaxation times?

Back 55

allows free protons to become boudn and create a hydration layer-->creates a spin lattice energy sink-->rapid return to eq.
-Decreases from a few sec. to tens of milliseconds

Front 56

What does a shorter T1 relaxation time mean?

Back 56

Brighter on T1

Front 57

What is larger (longer) T1 or T2 relaxation?

Back 57

T1

Front 58

Describe whether each of the following has long/short T1 and long/short T2:
1. Small molecules
2. Med. sized molecules
3. Large molecules

Back 58

1. Long T1, Long T2
2. Short T1, Short T2
3. Long T1, short T2

Front 59

What are the usual sizes of molecules in MRI imaging?

Back 59

Small or Med. sized

Front 60

What confers the high contrast in MRI?

Back 60

Difference in T1, T2 and T2*

Front 61

Does magnetic field strength affect T1 relaxation and T2 decay?

Back 61

Yes affects T1 relaxation (not T2 decay)

Front 62

Why does magnetic field strenght affect T1 relaxation but not T2 decay?

Back 62

Dependence of the Larmor frequency on magnetic field strength and the degree of overlap with molecular vibration schedule

Front 63

What does a higher magnetic field strenght do to the lamor frequency and spectral overlap?

Back 63

Increases Larmor frequency and decreases spectral overlap-->longer T1

Front 64

What are situations when T2* is decreased? T1 decreased?

Back 64

-T2* decreased agents that disrupt local magnetic fields like paramagnetic blood degredation products, elements with unpaired electrons (Gad), or ferrogmagnetic materials
-T1 decreased when macromolecule bind free water to hydration layer

Front 65

What are the specific relaxation characteristics of T1, T2, T2* dependent on?

Back 65

tissue charactersitics: T1>T2>T2*

Front 66

What is the key to acquiring great contrast sensitivity of MR images?

Back 66

Emphasizing differences in spin density, T1 and T2

Front 67

What makes the emitted frequencies dependent on T1, T2, or spin density relaxation characterisitcs?

Back 67

The timing, order, polarity, and repetition of RF pulses and applied magnetic field gradients

Front 68

What are the 3 major pulse sequences of MR?

Back 68

Spin echo, inversion recovery, gradient recalled echo

Front 69

What is spin echo?

Back 69

-Excitation of magnetized proton wth an RF pulse-->prodcution of a FID-->second RF pulse to produce an echo
-Timing b/w RF pulses allow sep. of initial FID and echo and ability to adjust tissue contrast

Front 70

Describe the spin echo pulse sequence:

Back 70

1. 90 degree RF pulse-->FID
2. FID decays according to T2* relaxation properties
3. After delay of TE/2 (echo time/2)-->180 degree RF pulse inverts spine-->reestabilishes phase coherence
4. Echo produced at time TE
5. Inhomogeneities of ext. magnetic field are canceled
6. Peak amplitude determined by T2 decay

Front 71

When does digital sampling and acquisition of the signl in spin echo pulse sequence occur?

Back 71

Just before and after peak amplitude (centered at time TE)

Front 72

How is contrast in a spin echo image produced?

Back 72

Different tissues relax differently (based on their T1 and T2 characteristics)

Front 73

How is the "true T2" decay determined?

Back 73

From multiple 180 degree refocusing pulses; signal amplitude measured at several points in time-->exponential curve is fit to this measured data

Front 74

In spin echo, how are the 90 degree RF pulses seperated?

Back 74

By TR (time of repeition) 300-3000 msec range; after the TR interval-->next 90 degree pulse is applied usually before complete longitiduinal (Mz) recovery so 2nd FID is <1st;

Front 75

Why after the second 90 degree pulse is FID amplitude from each additional 90 degree pulse stable?

Back 75

steday state longitudinal magnetization produces same FID amplitude from each susbsequent 90 degree pulse

Front 76

What does it mean to become partially saturated?

Back 76

full transverse magnetization is decreased from eq.-->amount of saturation depends on T1 relaxation time

Front 77

What does a short T1 tissue mean and long T1 tissue mean in relation saturation?

Back 77

Short T1 tissue <saturation than Long T1 tissue

Front 78

What does partial saturation impact?

Back 78

tissue contrast

Front 79

What variables are an MR signal dependent on?

Back 79

spine proton density, signal arising from fluid flow, T1 and T2 tissue properties, TR and TE pulse sequence controls

Front 80

If TR and TE are kept constant, what will change the MR signal?

Back 80

different T1 and T2; singal in adjacent voxels will be different when T1 or T2 changes b/w those two voxels-->describes how contrast is formed in MR

Front 81

what happens if you change the pulse sequence parameters TR and TE?

Back 81

Contrast dependence of the image can be weighted to T1 or T2

Front 82

What does T1 weighting mean?

Back 82

Designed to produce contrast based chiefly on the T1 characteristics of the tissue by de-emphasizing T2

Front 83

How do you T1 weight an image?

Back 83

Use a relatively short TR to maximize differences in longitudinal magnetization during the return to eq. and a short TE to minimize T2 dependency during signal acquistion
TR=500 msec and TE<15 msec;

Front 84

Describe the brightness and darkness of tissues in relation to T1 relaxation times:

Back 84

short T1 relaxation tissues (rapid recovery of the Mz vector) are bright and long T1 relaxation tissues are dark

Front 85

What does short T1 relaxation time mean?

Back 85

spins rapidly reasumme their equilibrium conditions