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### QUESTIONS» Image Creation

How is the MRI image created? This section explores slice selection, spatial encoding and the FFT.

# Slice thickness

## How is the slice thickness increased?

Increase the transmitted RF bandwidth or the slice selection gradient strength.

Increase the transmitted RF bandwidth, or decrease the slice selection gradient strength.

Decrease the transmitted RF bandwidth, or increase the slice selection gradient strength.

Slice selection method

INCORRECT. If you increased the transmitted RF bandwidth, a larger range of Larmor frequencies will match the RF pulse frequencies, and the slice thickness will increase.

But if you increase the slice selection gradient strength, the range of frequencies along the patient will increase, and so the frequencies within an RF pulse will correspond with a smaller section of the patient and reduce the slice thickness.

Try again.

Slice selection method

CORRECT. The slice thickness is governed by the following equation:

thk = BWtrans / (γ0·GS)

where thk is the slice thickness, BWtrans is the transmitted RF bandwidth (the range of frequencies it covers), γ0 is the gyromagnetic ratio and GS is the magnitude of the slice selection magnetic field gradient.

For example, for a 10 mm slice thickness using a gradient magnetic field strength of 10 mTm-1, the transmitted RF pulse bandwidth would be about 4.3 kHz (using γ0=42.58 MHz T-1).

Slice selection method

INCORRECT. If you decrease the slice selection gradient strength, the range of frequencies along the patient will decrease, and so the frequencies within an RF pulse will correspond with a larger section of the patient and increase the slice thickness.

But if you decreased the transmitted RF bandwidth, a smaller range of Larmor frequencies will match the RF pulse frequencies, and the slice thickness will decrease.

Try again.