Topic 25: T1 recovery + T2 decay

Question 1

What causes T1 recovery?

Answer 1

• Exchange energy from nuclei to surrounding environment/lattice = spin lattice energy transfer
• Rate of occurance is exponential = at different rates depending on tissue

Question 2

Describe T1 time

Answer 2

• Contrast parameter
• Inherent to tissue being imaged
• The time it takes for 63% of longitudinal magnetization to recover
• Period = 1 excitation pulse to next

Question 3

Define TR

Answer 3

• How much T1 recovery occurs in a cell

Question 4

Describe T1 relaxation

Answer 4

• Result of nuclei exchanging energy given by RF pulse → surrounding
• Efficiency of this process = T1 time of tissue situated
• E.g. fat absorbs energy quick = T1 time short
• E.g. water inefficient at receiving energy = T1 time long

Question 5

Describe TR

Answer 5

• Repetition time = controls how much of NMV recovered before next RF pulse
• Short = not full longitudinal recovery in tissue = causes contrast between fat/water due to different T1 recovery times
• Long = full recovery of longitudinal in tissue = no difference in longitudinal component magnitude = no contrast difference

Question 6

What causes T2 decay?

Answer 6

• Exchange of energy from 1 nucleus to another = spin-spin energy transfer
• Result of = MF of nuclei interacting with eachother
• Loss of phase coherence/dephasing
• Results in decay of NMV in transverse plane
• Exponential process = different rates in different tissue

Question 7

Describe T2 decay time

Answer 7

• Contrast parameter for tissue
  -Time it takes for 63% reduction of transverse magnetization of original value
• Period = time between excitation pulse + MR signal/echo time

Question 8

Define TE

Answer 8

• How much T2 decay in tissue

Question 9

Describe T2 relaxation

Answer 9

• Result of spins adjacent nuclei interacting + exchanging energy
• Efficiency = how closely packed molecules are to eachother
• E.g. fat molecules close = spin-spin efficient = T2 short
• E.g. water molecules spaced = spin-spin not efficient = T2 long

Question 10

Describe TE

Answer 10

• Echo time = controls how much transverse magnetization allowed to decay when signal read
• Short TE = not full dephasing = little contrast differnece between fat/water
• Long TE = dephasing transverse components = contrast difference

Question 11

Which tissues are dark on imaging?

Answer 11

1) Low proton density = always
2) Nuclei move = depends on velocity + pulse sequence

Question 12

Describe T1 weighted imaging

Answer 12

• Differences between T1 relaxation times in tissues
• TR selected = short
• Image contrasts differences in T1 recovery times
• Remove T2 effects = TE short
• If short T1 = bright
• If long = T1 = dark
• Best demonstrate anatomy + show pathology is used after contrast enhancement

Question 13

Describe T2 weighted imaging

Answer 13

• Differences between T2 relaxation times of tissues
• TE selected = long
• Image contrast due to differences in T2 decay time
• T1 effect removed = select long TR
• Short T2 = dark = air/fat
• Long T2 = bright
• Best demonstrate pathology

Question 14

Describe PD weighted imaging

Answer 14

• Differences in proton densities
• Both T1/T2 removed
• T1 effects reduced = long TR
• T2 effects reduced = short TE
• Image contrast due to differences in proton density of tissues
• Low = dark = cortical bone + air
• High = bright
• Best demonstrate = anatomy + some pathology