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QUESTIONS
» Image Creation

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

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Scan Time Calculation

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For a tissue which has a T1 relaxation time of 900 ms, how long will a 256×256 pixel image take to acquire if 90° RF excitation pulses are used?

4.5 seconds.

230.4 seconds.

19.2 minutes.

INCORRECT. Advanced pulse sequences used in MRI can achieve even subsecond image scan-times. However, we are considering only basic pulse sequences.

Think about how many signal measurements are needed for a single MRI image, as well as how long you must wait after one signal measurement before the next signal measurement is possible.

Try again.

INCORRECT.

Think about how long you must wait after one signal measurement before the next signal measurement is possible, as well as how many signal measurements are needed for a single MRI image.

Try again.

CORRECT. The repetition time (TR) must be equal to 5·T1, which in this case is 4.5 seconds. This must be multiplied by the number of signal measurements required before the image can be calculated using the fast Fourier transform (256 measurements in this case). This equals 1152 seconds (19.2 minutes).

If the TR is less than 5·T1, there will be significantly less magnetisation available in the z-axis than we require for a "full" signal in the next MRI signal measurement.

Advanced pulse sequences used in MRI can achieve even subsecond image scan-times. However, we are considering only basic pulse sequences.

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