mr physics

Question 1

Hardware components of an MRI system

Answer 1

B0 magnet: alignment of spins, longitudinal magnetization Mz

transmit RF coil: excitation (signal generation)

gradient coils: spatial encoding of MR signal

dedicated receive RF coils: signal detection, image quality (SNR)

Question 2

Nuclear spin

Answer 2

Individual unpaired protons
and unpaired neutrons each possesses a spin of 1/2

All nuclei with non-zero spin possess an angular momentum (J)

Question 3

spin and momentum relation
larmor freq

Answer 3

µ = γ J

ωL = γB0

Question 4

Bulk magnetization generation

Answer 4

relatively: very small net alignment of the protons 1T Bo creates 7 difference in 1.000.000

Question 5

Only ….? magnetization can be measured

Answer 5

Only transverse magnetization can be measured
we generate transverse magnetization by RF excitation

Question 6

MR signal intensity =?

Answer 6

MR signal intensity = amount of transverse magnetization Mxy precessing at the given location in space

Question 7

Basic sequence parameters:

Answer 7

– time between excitation pulses = repetition time TR

– time between excitation and measurement of Mxy = echo time TE

Question 8

measured signal formula

Answer 8

S = PD exp(−TE / T2)(1− exp(−TR / T1))

Question 9

Contrast in an MR image

Answer 9

Contrast = difference in signal intensity between different tissues

Question 10

T2*

Answer 10

Dephasing due to off-resonance effects: T2’

1/T2* =1/T2 + 1/T2’

Question 11

Spatial encoding is achieved by

Answer 11

Use of gradient coil magnetic fields

Question 12

SNR considerations

Answer 12

Requirements for ideal imaging:
– high spatial resolution (detect small structures)
– short acquisition times
– high SNR (good image quality)