Vector Model

Question 1

what is bulk magnetisation

Answer 1

the net orientation of all of the magnetic moments in a sample

Question 2

what is the right handed set rotation rule

Answer 2

point thumb along axis

fingers wrap in direction of vector movement

Question 3

how does equilibrium of magnetisation occur

Answer 3

initially moments are random
field is applied
moments stay random for a short time and eventually equilibrate parallel to the aligned field

Question 4

what is larmor precession

Answer 4

the cone shape loop that the vector follows when it is at an angle to the z axis

Question 5

what does the sign of the larmor frequency mean

Answer 5

direction of precession

Question 6

how in what direction does the vector preceed when nu>0

Answer 6

from x to y

Question 7

how is the larmor precession detected by the NMR machine

Answer 7

precession is cut by a wire

• generates current
• detected and amplified

Question 8

how can the size of the x component of a vector be calculated

Answer 8

= M(0)sin(beta)
M(0) = size of vector
beta = angle to z axis

Question 9

how is the vector moved away from the z axis

Answer 9

magnetisation in the xy plane is applied

- radio frequency pulse

Question 10

what is the apparent larmor frequency in a rotating frame known as

Answer 10

offset

Question 11

what are the 2 formulae for offset

Answer 11

offset = w(o) - w(rot field)
offset = gamma ^B

Question 12

what is the formula for effective field

Answer 12

Beff = root(B1^2 +(^B)^2)

Question 13

what is offset equal to when on resonance

Answer 13

0

Question 14

how does the length of a pulse of B1 affect the amount of rotation of the vector

Answer 14

beta = w(1) x tp
beta = amount of rotation
tp = pulse time

Question 15

what happens to the magnitude of w1 in hard pulses

Answer 15

w1&raquo_space; offset

w1 can cover multiple frequencies

Question 16

describe a pulse acquire experiment

Answer 16

sample comes to equilibrium
RF power is switched on to rotate magnetisation to 90 degrees
RF is switched off - detect signal as magnetisation moves back to equilibrium

Question 17

how does a 180 degree pulse transfer a vector

Answer 17

from + to -

Question 18

how does a 90 degree pulse transfer a vector

Answer 18

according to right hand rule

Question 19

describe the spin echo experiment

Answer 19

pulse RF 90 degrees along x
delay for tau
pulse RF 180 degrees along x
delay for tau
acquire spectrum

Question 20

what is key when carrying out soft pulses

Answer 20

to achieve 90% of a signal the offset must be 1.6x the field strength

Question 21

when is a weak RF field desired

Answer 21

when selective excitation of a spectrum is desired