MRI Flashcards

1
Q

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

A

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

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2
Q

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

A

19 cm, +-2mm

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3
Q

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

A

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

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4
Q

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

A

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

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5
Q

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

A

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)

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6
Q

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

A

= (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

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7
Q

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

A

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

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8
Q

How do you calculate percent signal ghosting (PSG)?

A

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

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9
Q

How does the MRI receive coil bandwidth affect SNR?

A

SNR proportional to 1/sqrt(BW)

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10
Q

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

A

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.

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