MRI Flashcards
The scan time of a 2D MRI sequence is calculated by ______.
Number of phase encoding steps times Number of acquisitions times TR divided by Echo train length (ETL)
What is the diameter of the ACR large MRI phantom, and to what tolerance does it have to be measured to from the image?
19 cm, +-2mm
What are the technique specifications for the ACR T1-weighted protocol?
TE = 20ms, TR = 500 ms, Slice width = 5 mm, slice gap = 5 mm, 11 slices, 1 NEX,
What are the technique specifications for the ACR T2-weighted protocol?
TE = 80ms, TR = 2000 ms, Slice width = 5 mm, slice gap = 5 mm, 11 slices, 1 NEX,
How do you calculate the slice width from the ACR phantom?
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)
How do you calculate the SNR in the ACR MRI phantom?
= (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
How do you calculate percent image uniformity (PIU) of the MRI ACR phantom?
PIU = 100 * (1 - (Max - Min)/(Max + Min))
How do you calculate percent signal ghosting (PSG)?
= 100 * |(Left+Right)-(Top+Bottom) | / (2 * Mean phantom signal)
How does the MRI receive coil bandwidth affect SNR?
SNR proportional to 1/sqrt(BW)
What is a Gibbs (truncation) artifact, and how can it be resolved?
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.