Magnetic Resonance Imaging (MRI) Practice Test

Which parameter adjustments will lengthen scan time in an MR pulse sequence?

1. Decrease TR, Increase ETL, Increase NEX, Decrease parallel imaging factor

2. Increase TR, Decrease ETL, Increase NEX, Decrease parallel imaging factor

3. Increase TR, Increase ETL, Decrease NEX, Increase parallel imaging factor

4. Decrease TR, Increase ETL, Decrease NEX, Increase parallel imaging factor

The correct answer is: Increase TR, Decrease ETL, Increase NEX, Decrease parallel imaging factor

Lengthening the scan time in an MRI pulse sequence is related to how various parameters influence the overall timing and repetition of the images acquired during the scan. Increasing the repetition time (TR) will extend the time between each radio frequency pulse for each slice, thus increasing the total scan time. This is crucial as longer TR values allow for more complete relaxation of the spins before the next excitation, improving image quality but at the cost of extended duration. Decreasing the echo train length (ETL) will also lengthen scan time since a shorter ETL leads to acquiring fewer echoes per slice per repetition. Fewer echoes mean that the overall imaging session requires more repetitions to gather the same amount of data. Increasing the number of excitations (NEX) enhances the signal-to-noise ratio (SNR) but also significantly increases scan time as each slice or acquisition is repeated multiple times. Finally, decreasing the parallel imaging factor will result in longer scan times because higher parallel imaging factors allow for faster data acquisition by effectively reducing the required number of phase-encoding steps. Lowering this factor means that more phase-encoding steps—and consequently more time—are necessary to collect the same amount of imaging data. Combining these changes—specifically increasing TR,