MRI

Question 1

The scan time of a 2D MRI sequence is calculated by ______.

Answer 1

Number of phase encoding steps times Number of acquisitions times TR divided by Echo train length (ETL)

Question 2

What is the diameter of the ACR large MRI phantom, and to what tolerance does it have to be measured to from the image?

Answer 2

19 cm, +-2mm

Question 3

What are the technique specifications for the ACR T1-weighted protocol?

Answer 3

TE = 20ms, TR = 500 ms, Slice width = 5 mm, slice gap = 5 mm, 11 slices, 1 NEX,

Question 4

What are the technique specifications for the ACR T2-weighted protocol?

Answer 4

TE = 80ms, TR = 2000 ms, Slice width = 5 mm, slice gap = 5 mm, 11 slices, 1 NEX,

Question 5

How do you calculate the slice width from the ACR phantom?

Answer 5

Set the display level to (1/2) of the average ramp signal value, and the display width to 0. Measure the length of the top and bottom ramps.

= (0.2) * (top * bottom) / (top + bottom)

Question 6

How do you calculate the SNR in the ACR MRI phantom?

Answer 6

= (Mean signal value in phantom ) divided by (standard deviation of air)

** Note: if using the subtraction method from two acquisitions, add in a sqrt(2) factor in the numerator and use the standard deviation of the difference image

Question 7

How do you calculate percent image uniformity (PIU) of the MRI ACR phantom?

Answer 7

PIU = 100 * (1 - (Max - Min)/(Max + Min))

Question 8

How do you calculate percent signal ghosting (PSG)?

Answer 8

= 100 * |(Left+Right)-(Top+Bottom) | / (2 * Mean phantom signal)

Question 9

How does the MRI receive coil bandwidth affect SNR?

Answer 9

SNR proportional to 1/sqrt(BW)

Question 10

What is a Gibbs (truncation) artifact, and how can it be resolved?

Answer 10

Truncation artifacts look like ringing / reverberations near a boundary of high and low signal intensity. These can be alleviated by increasing the sampling rate in the direction of the boundary.