EXAM #12 — PHYSICS UNIT 05

Question 1

RF pulse.

Answer 1

a burst of radio frequency energy

Question 2

net magnetization relaxation.

Answer 2

the return of net magnetization to equilibrium as it releases absorbed RF (Tl) and loses energy (T2)

Question 3

free induction decay.

Answer 3

the simple signal given off by the relaxation of net magnetization to equilibrium

Question 4

spin density or proton density.

Answer 4

the relative number of protons (spins) in a sample

Question 5

spin echo.

Answer 5

the signal given off by the relaxation of net magnetization to equilibrium after a refocusing pulse- is a mirror image of the FID

Question 6

Tl relaxation.

Answer 6

regrowth of net magnetization along the Z axis

Question 7

longitudinal recovery.

Answer 7

regrowth of net magnetization along the Z axis

Question 8

spin-lattice relaxation?

longitudinal recovery and spin-lattice relaxation are terms for ___ _____

Answer 8

spin-lattice relaxation. the process of RF being dissipated into the lattice surrounding the proton

longitudinal recovery and spin-lattice relaxation are terms for Tl relaxation

Question 9

T2 relaxation.

Answer 9

the decay of transverse magnetization

Question 10

transverse decay.

Answer 10

the decay of transverse magnetization

Question 11

spin-spin relaxation?

transverse decay and spin-spin relaxation are terms for ___ _____

Answer 11

spin-spin relaxation. the process of spins interacting with each other, which causes phase changes, and loss of transverse magnetization

transverse decay and spin-spin relaxation are terms for T2 relaxation

Question 12

T2* relaxation.

Answer 12

transverse decay in the presence of magnetic field inhomogeneities- spin-spin interaction plus magnetic field inhomogeneity

Question 13

phase.

Answer 13

the degree to which spins are precessing at the same position at a specific point in time- T2 relaxation is also known as phase coherence

Question 14

explain why T2* relaxation is shorter than T2 relaxation.

Answer 14

magnetic field inhomogeneities increase de-phasing and increase decay of transverse magnetization

Question 15

identify the type of relaxation most affected by magnetic field inhomogeneity. T2*

Answer 15

identify the type of relaxation most affected by magnetic field inhomogeneity.

T2*

Question 16

identify the amount of longitudinal regrowth that defines true Tl relaxation time.

Answer 16

Tl relaxation is defined as the time it takes for 63% of longitudinal magnetization to regrow

Question 17

identify the amount of transverse decay that defines true T2 relaxation time.

Answer 17

T2 relaxation is defined as the time it takes for 63% of transverse magnetization to decay

Question 18

explain the state of phase of all spins immediately after RF excitation.

Answer 18

all spins are in phase immediately after RF excitation

Question 19

recognize a diagram of longitudinal regrowth and the relative relaxation times of tissues on the diagram.

Answer 19

O.K.

Question 20

recognize a diagram of transverse decay and the relative relaxation times of tissues on the diagram.

Answer 20

O.K.

Question 21

describe the relative Tl and T2 relaxation times of fat and water.

Answer 21

fat has short Tl and short T2 relaxation times (relative to water) water has long Tl and T2 relaxation times (relative to fat)

Question 22

describe the relative brightness of fat and water on Tl and T2 weighted MR images.

Answer 22

Tl weighted images- fat is bright and water is dark T2 weighted images- fat is dark and water is bright

Question 23

describe the relative local magnet fields experienced by fat and water nuclei, and their resulting relative resonant frequencies.

Answer 23

nuclei of fat molecules experience a lower local magnetic field and therefore have a lower resonant frequency

Question 24

describe the effect of long and short Tl times on MR signal intensity and appearance on Tl weighted images.

Answer 24

tissues with long Tl times have low signal intensity, and appear dark; tissues with short Tl times have high signal intensity, and appear bright on Tl weighted images

Question 25

describe the effect of long and short T2 times on MR signal intensity and appearance on T2 weighted images.

Answer 25

tissues with long T2 times have high signal intensity and appear bright; tissues with short T2 times have low signal intensity and appear dark on T2 weighted images

Question 26

describe the relationship between M0 to M2 at equilibrium.

Answer 26

at equilibrium, M0 = Mz

Question 27

identify what flip angle RF pulse creates an FID.

Answer 27

90° or less

Question 28

identify what flip angle RF pulse creates a spin echo.

Answer 28

180°

Question 29

identify which type of relaxation is caused by spin-spin interaction and magnetic field inhomogeneity.

Answer 29

T2*

Question 30

identify which type of relaxation is caused by spin-spin interaction only.

Answer 30

T2