Image Formation

Question 1

What is slice selection? Which gradient is it represented by?

Answer 1

rf pulse at larmor frequency (frequencies before precess slower, after precess faster)

bandwidth and slice thickness

Equation:
B_z = B_0 + G_z * z

Question 2

What is phase encoding? Which gradient is it represented by?

Answer 2

Gradient causes larmor frequency to change; spin faster at the top of the patient, slower at bottom

Chooses k_y

Equation:
B_y = B_0 + G_y * y

Question 3

What is frequency encoding? Which gradient is it represented by?

Answer 3

Finds all k_x for chosen k_y

Equation:
B_x = B_0 + G_x * x

Question 4

How does gradient field affect slice thickness?

Answer 4

Higher field = lower thickness

Question 5

How is a new slice chosen?

Answer 5

shift the gradient field in z

Question 6

Calculate acquisition time for a 256 x 128 voxel image for an Incoherent GRE

Answer 6

** always use smaller voxel number
TE = 5 msec
TR = 40 msec

Tacq = 128 * 1 * 40 msec = 5120 ms = 5.12 sec

Question 7

What is NEX?

Answer 7

Number of excitations / number of repeats / number of acquisitions

Question 8

What is N_phase?

Answer 8

Smaller voxel number

Question 9

What is ETL? What is a normal ETL for EPI and FSE?

Answer 9

Echo train length

Number of 180 degree pulses in pulse sequences like EPI and FSE

EPI: ETL = 32 to 128
FSE: ETL ~= 10

Question 10

Calculate acquisition time for a 256 x 128 voxel image for a T1w SE?

Answer 10

TE = 5 msec
TR = 800 msec

Tacq = 128 * 1 * 800 msec = 102400 ms = 102.4 sec = 1.7 min

Question 11

Calculate acquisition time for a 256 x 192 voxel image for a T1w FSE?

Answer 11

TE = 10 msec
TR = 400 msec
ETL = 10

Tacq = 192 * 1 * 400 msec = 76800 ms = 76.8 sec
T_FSE = Tacq / ETL = 7.68 sec

Question 12

Calculate acquisition time for a 256 x 192 voxel image for a T1w EPI?

Answer 12

TE = 10 msec
TR = 400 msec
ETL = 64

Tacq = 192 * 1 * 400 msec = 76800 ms = 76.8 sec
T_EPI = Tacq / ETL = 1.2 sec