MR physics

Question 1

What in wheel physics terms is precession?

Answer 1

When you have a wheel with handles through the axle. If you spin the wheel so that the axis is perpendicular to the force of gravity, & then let oneside go, the wheel sort of spins around the axis of the gravitational force. It takes the same amount of time for the wheel to rotate once around the gravity axis. You cant see precession if the wheel axis is parallel to gravity

Question 2

What in wheel terms is resonance?

Answer 2

the movement of torque (finger) follows the precession of the wheel exactly

Question 3

How is torque applied to atomic nuclei?

Answer 3

You have to apply a magnetic field which is orthogonal to the primary field & oscillating at the exact same frequency of the nuclei

Question 4

What is the frequency of precession of 1H?

Answer 4

63.8 MHz at 1T,
127.6 MHz at 3 T
300 MHz at 7T

Question 5

What are the two main parts of relaxation of a proton back to its resting, magnetized state?

Answer 5

Decay of traverse(observable) component T2 and recovery of parallel component T1

Question 6

How is contrast achieved in MRI images?

Answer 6

Different tissues have different relaxation rates.

Question 7

what two processes occur during relaxation of a proton back to it’s equilibrium state?

Answer 7

Decay of traverse(observable)component T2 and the recovery of the parallel component (T1)

Question 8

How do you got contrast in an MR image?

Answer 8

Rates of relaxation very for different types of tissue.

Question 9

What is echo time?(TE)

Answer 9

The interval between an RF pulse and data acquisition.

Question 10

In a pulsed MR experiment, what is the repetition time?(TR)

Answer 10

The time between R F pulses (IPI)

Question 11

what three parameters control T1 contrast? what is the main contributor?

Answer 11

TR, Excitation or flip angle

Question 12

What kind of contrast does TE control?

Answer 12

T2contrast

Question 13

which contrast type takes advantage of the decaying traverse component?

Answer 13

T2

Question 14

Which contrast type takes advantage of the recovery o the parallel cmponent?

Answer 14

T1

Question 15

put the following tissues in order from brightest to darkest for short TR imaging:grey matter, fat, CSF, white matter (which recover most rapidly and have the shortest recovery times)?

Answer 15

Fat>white matter>grey matter>CSF

Question 16

Rank tissues from brightest to darkest forT2 imaging (long TE)

Answer 16

CSF>Grey matter>white matter

Question 17

what relative lengths(long or short) of TR and TE give you a proton density image?

Answer 17

Long TR, short TE

Question 18

What lengths of TR and TE give you a T1weighted image?

Answer 18

Short TR and short TE

Question 19

What lengths of TR and TE give you a T2weighted image?

Answer 19

Long TR and long TE

Question 20

What is T1 in math/physics terms

Answer 20

T1 is the time constant which describes the relaxation of the z component (direction of static magnetic field) back to equilibrium(Mo). It is represented mathematically by Mz = Mo(1-e^-t/T1) and is the amount of time it takes to reduce the difference between Mz and Mo by e.

Question 21

What is precession in physics terms?

Answer 21

If the net magnetization is placed in the xy plane (around the z of the static magnetic field), The proton will rotate around the z axis at the frequency of the photon which would cause a transition between the two energy levels of the spin. T

Question 22

What is the larmour frequency?

Answer 22

The rate at which a nucleus/proton precesses after excitation.

Question 23

What is shimming

Answer 23

You can pass current through a coil which can produce gradients in three axes to compensate for inhomogeneities in the static magnetic field.

Question 24

For free induction decay, what change in frequency spectrum is associated with a faster rate of decay?

Answer 24

A faster rate of decay is associated with a broader frequency spectrum and smaller peak

Question 25

What is the area under the fourier transform graph associated with

Answer 25

The area under the fourier transform is associated with the amount of magnetization remaining after an echo

Question 26

What is T2 in physics terms?

Answer 26

Because of field inhomogeneity, precessing molecules tend to get out of phase with each other (not precessing at the same rates). The longer the time from the pre-polarizing pulse, the greater the phase difference. T2 relaxes back to zero and the magnetization vector grows along z until the z vector equals Mo

Question 27

The magnitude of T2 is always ______than T1

Answer 27

Less than

Question 28

The x and y axes of a rotating frame of reference are denoted by_____

Answer 28

x’ and y’

Question 29

Why is applying an alternating magnetization factor in the x’ y’ plane the same as rotating the net magnetization vector around z at the larmour frequency?

Answer 29

When a magnetic field is applied by, for example, placing a loop of wire around the x axis, the alternating current will produce a magnetic field that alternates in direction from B1 to -B1. In a rotating frame of reference, rotating at the alternation frequency, the magnetization vector B1 appears stationary (the B1 vector moving at v0 appears stationary, the B1 vector moving at -v0 is moving at 2v0 and is so far removed from the spin frequency that it doesn’t affect them…..)

Question 30

What is the angle a spin will rotate around a pulsed magnetic field equal to?

Answer 30

2(pi)(gamma)(tau)(B1), with tau being the amount of time the field is on, which is much smaller than either T1 or T2

Question 31

Which pulse rotates the equilibrium magnetization vector down to the y’ axis?

Answer 31

90 degree pulse

Question 32

Which pulse rotates the equilibrium magnetization vector down to the -z axis?

Answer 32

180 degree pulse

Question 33

What is the energy of the photon that will be absorbed by a 1H nucleus in a 1.5 Tesla magnetic field? How does this compare in energy to a 2x1019 Hz x-ray photon? What is the ionization potential for a typical organic molecule? Which of the two photons will ionize the molecule

Answer 33

Bo=1.5T
g = 42.58 MHz/T

The energy absorbed by a 1H nucleus is:
EH = h ν
EH = h γ Bo
EH = 6.626x10-34 Js * 42.58x106 Hz/T * 1.5 T
EH = 4.23x10-26 J

The energy of a ν=2x1019 Hz X-ray photon is:
EX = hν EX = 6.626 x 10-34 Js * 2x1019 Hz = 1.33x10-14 J

How the two energies

Question 34

A sample has a T1 of 1.0 seconds. If the net magnetization is set equal to zero, how long will it take for the net magnetization to recover to 98% of its equilibrium value?

Answer 34

Given: T1 = 1.0s

The relationship between the equilibrium net magnetization, Mo, and the net magnetization, Mz(t), at time t is:
Mz(t) = Mo(1 - e-t/T1).

When Mz(t) / M0 = 98%:

0.98 = Mz(t) / M0 = (1 - e-t/T1)
0.98 = 1 - e-t/T1
1 - 0.98 = e-t/T1
ln( 0.02 ) = -t/T1
-T1 * ln( 0,02 ) = t
-1.0s * ln( 0,02 ) = 3.9s = t

Question 35

A sample has a T2 of 100 ms. How long will it take for any transverse magnetization to decay to 37% of its starting value?

Answer 35

Given: T2 = 100ms.

The relationship between the initial transverse magnetization (Mxyo) and the transverse magnetization at any time t, Mxy(t), is:

Mxy(t) = Mxyoe-t/T2

When the transverse magnetization (Mxy) has decayed to 37% of its starting value:
0.37 = Mxy(t) / Mxy0 = e-t/T2

0.37 = e-t/T2

ln( 0.37 ) = -t/T2

• T2 * ln( 0.37 ) = t
• 100ms * ln( 0.37 ) = 99.4ms = t

Question 36

A hydrogen sample is at equilibrium in a 1.5 Tesla magnetic field. A constant B1 field of 1.17x10-4 Tesla is applied along the +x’-axis for 50 microseconds. What is the direction of the net magnetization vector after the B1 field is turned off?

Answer 36

Given:
B1 = 1.17x10-4 T,
τ = 50x10-6 s, and

B1 is applied along the +x’ axis.

The relationship between the rotation angle in radians (θ) and the length, in seconds, that the B1 field is applied (τ) is:
θ = 2π τ B1 γ

where: γ = gymagnetic ratio for the nuclei in question.

θ = 2π τ B1 γ

θ = 2π * 50x10-6s * 1.17x10-4T * 42.58 MHz/T

θ = 1.565 rad.

θ = 1.565 rad. * 180o/π rad = 89.673o

The net magnetization vector will be almost along the +y’ axis: 89.673o from the +z axis after the clockwise about the +x’ axis.