MRI formulas

Question 1

Scan time: SE

Answer 1

TR x Phase Matrix x NEX

divide by 1000ms to convert to seconds

Question 2

Scan time: FSE

Answer 2

TR x Phase Matrix x NEX/ ETL

(divide by 1000 to convert to seconds

Question 3

Scan time: 3D Gradient echo

Answer 3

3D sequence scan time= TR x Phase Matrix x NEX x #of slices

divide by 1000 to convert to seconds

Question 4

Pixel size formula

Answer 4

FOV/Matrix=Pixel size

Question 5

Pixel size dimensions (phase)

Answer 5

FOV/phase matrix =phase dimension pixel

Question 6

Pixel size dimensions (frequency)

Answer 6

FOV/frequency matrix = frequency demension pixel

Question 7

Pixel area formula

Answer 7

multiply phase pixel size x frequency pixel size= pixel area (answer is squared^2)

Question 8

voxel volume

Answer 8

pixel area x slice thickness=voxel volume

answer is cubic/cubed^3

Question 9

NEX and SNR

Answer 9

SNR increase calculated by the square root of the increase

Question 10

bandwidth and SNR

Answer 10

decreasing BW by a factor of 2 increases SNR by a square root of 2

Question 11

FOV and SNR

Answer 11

reducing the FOV decreases SNR by a factor of the FOV^2

Question 12

FOV and SNR example: Decreasing FOV from 40cm to 20cm results in what?

Answer 12

SNR reduced to 1/4 the original SNR

is due to the voxel volume being reduced by a factor of 4

Question 13

Cardiac formula: Calculate the effective TR

Answer 13

60,000/heart rate

60 seconds per minute, 1000ms per second

Question 14

Calculate available imaging time in a cardiac MR

Answer 14

Effective TR -(trigger delay and trigger window)

Question 15

Calculate available imaging time in a cardiac MR example: 
Heart rate: 80bpm
Trigger delay: 50ms 
Trigger window: 10%
What is available imaging time?

Answer 15

60,000/80=750ms

10% of 750 = 75
so 75ms + 50ms(delay) = 125ms

So… 750ms - 125ms = 625ms of total imaging time