Image Contrast CH. 2

What types of contrast will the following produce? (a) TR 250 ms, TE 5 ms, flip angle 120°. (b) TR 50 ms, TE 15 ms, flip angle 35°.

(a) T1 weighting. (b) T2* weighting.

Why does fat have a short T1 and T2 relaxation time?

Because fat has a low inherent energy, a slow molecular tumbling rate and its molecules are packed together. This means energy exchange and spin-spin interactions are efficient and therefore T1 and T2 relaxation respectively occurs quickly.

Using a long TE enhances T2 and T2* contrast. a)true b)false

true Long TEs always give the spins more time to dephase before the echo is read, therefore increasing T2 contrast. This is true in spin echo and gradient echo sequences.

Why does water have a long T1 and T2 decay time?

Because water has a high inherent energy, a fast molecular tumbling rate and its molecules are spaced far apart. This means energy exchange and spin-spin interactions are inefficient and therefore T1 and T2 relaxation respectively occurs slowly.

What term is used to indicate gradient rephasing?

Rewinding.

What term is used to indicate gradient dephasing?

Spoiling.

What parameter controls T2 decay and why?

The TE controls T2 decay as it determines how much dephasing is allowed to occur before the signal is read.

What values of TR and TE are needed for PD weighting in a spin echo sequence and why?

The TR must be long so that neither fat nor water has had time to fully recover their longitudinal magnetization. The TE must also be short to minimize the T2 differences between the tissues.

In general, the T1 relaxation time: a. is longer for diseased tissue b. remains unchanged for diseased tissue c. is unrelated to diseased states d. is shorter for diseased tissue

a. is longer for diseased tissue

List the main factors that make gradient echo sequences different from spin echo.

Variable flip angles, gradient rephasing, shorter TRs and scan times.

Define the term weighting.

Weighting means that parameters are selected to make one contrast mechanism dominate over the others.

T1 relaxation occurs: a. along the Z axis b. in the XY plane c. along the Y axis d. along the X axis

a. along the Z axis

A spin echo appears: a. sometime following a 180°RF pulse b. immediately following a 90°RF pulse c. sometime following a 90°RF pulse d. immediately following a 180°RF pulse

a. sometime following a 180°RF pulse

Relative to T1 relaxation times, T2 relaxation times are: a. very much shorter b. just a little shorter c. about the same d. a little longer

a. very much shorter

A spin echo pulse sequence consists of the following train of RF pulses: a. 90°...180°...90°...180°... b. 90°...90°...90°... c. 180°...180°...180°... d. 180°...90°...180°...90°

a. 90°...180°...90°...180°...

Which of the following statements are TRUE? a. T2 weighted images are obtained using a long TE and long TR. b. T1 weighted images are obtained using a short TE and long TR. c. T2 weighted images are obtained using a short TE and a long TR. d. PD weighted images are obtained using a short TE and short TR.

a. T2 weighted images are obtained using a long TE and long TR.

Spin density can best be described as the number of hydrogen nuclei: a. in a voxel b. at equilibrium in a voxel c. passing through a voxel d. excited in a voxel

a. in a voxel

Net magnetization vector a. is the sum of the magnetizations of the excess parallel nuclei. b. is the result of precession c. points in the direction opposite to the vector. d. is not related to MRI.

a. is the sum of the magnetizations of the excess parallel nuclei.

T1 relaxation time is defined as when: a. 76% of the longitudinal magnetization has recovered b. 63% of the longitudinal magnetization has recovered c. 63% of the transverse magnetization has recovered d. 76% of the tissue's magnetization has recovered

b. 63% of the longitudinal magnetization has recovered

T1 relaxation time is related to the time required for: a. longitudinal saturation b. Mz to return to equilibrium c. Mxy to return to equilibrium d. transverse saturation

b. Mz to return to equilibrium

When placed in a large static magnetic field, hydrogen nuclei: a. align with the magnetic field b. align in either parallel or anti-parallel position c. relax d. all of the above

b. align in either parallel or anti-parallel position

A magnetic moment: a. only applies to hydrogen protons. b. has strength and direction. c. only occurs when the magnet is turned on. d. only applies to diamagnetic elements.

b. has strength and direction.

In addition to spin density, signal intensity is also affected by: a. the mass of the spin b. how the spin is chemically bound c. how the spin is charged d. the distribution of the spin

b. how the spin is chemically bound

Spin density is most closely related to: a. transient hydrogen b. mobile hydrogen c. bound hydrogen d. ionized hydrogen

b. mobile hydrogen

In relation to the static magnetic field, the B1 magnetic field is oriented: a. parallel b. perpendicular c. at 180 degrees d. all of the above

b. perpendicular

T2* is a result of dephasing due to T2 decay and a. T1 b. inhomogeneities c. molecular weight d. a and b

b. inhomogeneities

Following the return of net magnetization to equilibrium, the MR signal: a. is constant b. is zero c. is decreasing d. oscillates at the Larmor frequency

b. is zero

T2* is: a. varies with tissue type b. shorter than T2 c. longer than T2 d. equal to T2

b. shorter than T2

Control of TE is exercised by control of: a. the T1 relaxation time b. the 180°RF pulse c. the T2 relaxation time d. the 90°RF pulse

b. the 180°RF pulse

Repetition time (TR) is best described as a a. time interval between a 90 degree RF pulse and a 180 degree pulse b. time interval between a 90 degree RF pulse and a subsequent 90 degree RF pulse c. time constant that characterizes FID d. time interval between a 90 degree RF pulse and the first echo

b. time interval between a 90 degree RF pulse and a subsequent 90 degree RF pulse

T2 relaxation time is defined as when: a. 76% of the longitudinal magnetization has decayed b. 63% of the longitudinal magnetization has decayed c. 63% of the transverse magnetization has decayed d. 76% of the tissue's magnetization has decayed

c. 63% of the transverse magnetization has decayed

All of the following are intrinsic contrast parameters except: a. T2 decay b. T1 recovery c. TR d. Proton density

c. TR

Following a 90°RF pulse, the signal received from the patient is: a. a gradient-refocused echo b. a spin echo c. a free induction decay d. none

c. a free induction decay

Generally, the appearance of tissue having a short T1 relaxation time on a T1 weighted image will appear: a. dark b. gray c. bright d. varies with tissue type

c. bright

Spin density can best be defined as hydrogen: a. configuration b. relaxation c. concentration d. charge

c. concentration

T2*: a. characterizes the rephrasing of transverse relaxation b. describes the decay of the second echo of an excitation pulse c. describes the decay of the FID d. all of the above

c. describes the decay of the FID

A free induction decay will be produced by a ____ RF pulse followed by a ____ RF pulse. a. 180°; 90° b. 180°; 180° c. 90°; 90° d. 90°; 180°

c. 90°; 90°

A 4T magnet would have a Larmor frequency of approximately: a. 80 MHz b. 20 MHz c. 160 MHz d. 40 MHz

c. 160 MHz

On a T2 weighted image, tissues having: a. high Bo will appear bright b. short T2 will appear bright c. long T2 will appear bright d. high spin density will appear dark

c. long T2 will appear bright

The term longitudinal, in longitudinal relaxation time refers to events occuring along the axis of the: a. patient b. magnetic field gradient c. static magnetic field d. radio antennae

c. static magnetic field

Which of the following is not a principle MRI parameter? a. T1 relaxation time b. spind density c. T2 relaxation time d. Larmor frequency

d. Larmor frequency

In general, as one increases Bo the T1 relaxation time will: a. remain the same b. vary from tissue to tissue c. decrease d. increase

d. increase

T1 relaxation times are generally: a. about the same as T2 relaxation times b. vary from tissue to tissue in relation to T2 relaxation times c. shorter than T2 relaxation times d. longer that T2 relaxation times

d. longer that T2 relaxation times

At equilibrium, Mz is undetectable because: a. the spin density is too low b. it relaxes too fast c. Mo is too high d. Bo is too high

d. Bo is too high

Transverse relaxation occurs because: a. energy is released by RF b. energy is absorbed from RF c. spins exist separately d. spins interact with each other

d. spins interact with each other

A long TR, short TE, and large flip angle produce a Proton Density (PD) weighted image. True False

false

Combining a low flip and long TR results in saturation. a)true b)false

false This combination results in the vectors being flipped through a small angle while giving them a long time to recover using a long TR. Hence the vectors never exist beyond the transverse plane in saturation.

What factors determine the T1 and T2 relaxation time of a tissue?

• Inherent energy of the tissue. • How well the molecular tumbling rate matches the Larmor frequency. • How closely spaced the molecules are.