MRI Data Acquisition, k-Space and Image Processing/Storage

Question 1

The central lines of k-space mainly determine which property of the final MR image?

Accepted Answer

Image signal and contrast

Answer

The low spatial-frequency data near the center of k-space is acquired with the weakest phase-encoding gradients, so it carries the most signal. It sets the overall signal-to-noise and contrast of the image. The outer (high spatial-frequency) lines carry fine detail and spatial resolution.

Question 2

A 2D spin-echo scan uses TR = 500 ms, a 256 (phase) x 256 (frequency) matrix, and 2 NEX. What is the approximate scan time?

Accepted Answer

4 minutes 16 seconds

Answer

2D scan time = TR x number of phase-encoding steps x NEX. Here that is 0.5 s x 256 x 2 = 256 s, or about 4 minutes 16 seconds. The frequency matrix does not affect scan time.

Question 3

Which mathematical process converts the raw data stored in k-space into the MR image?

Accepted Answer

Fourier transform

Answer

The MR signal is recorded as frequency and phase information in k-space. A 2D or 3D Fourier transform breaks that data into its component frequencies and maps them to pixel locations and intensities. Filtered back projection is mainly a CT method, and MIP is a post-processing display technique.

Question 4

Parallel imaging techniques such as SENSE or GRAPPA shorten scan time mainly by:

Accepted Answer

Skipping some phase-encoding lines and using multi-element coil sensitivity data to fill them in

Answer

Parallel imaging undersamples k-space in the phase-encoding direction by an acceleration factor (R). It then uses the spatial sensitivity profiles of a multi-channel phased-array coil to unfold the aliased image (SENSE) or rebuild the missing lines (GRAPPA). The trade-off is lower SNR, roughly by the square root of R multiplied by the coil geometry (g) factor.

Question 5

Partial Fourier (half-NEX) imaging cuts scan time by acquiring a little more than half of k-space. What property of k-space makes this possible?

Accepted Answer

Conjugate (Hermitian) symmetry

Answer

In theory k-space is conjugate-symmetric: data in one half mirrors the data in the opposite half. So a little more than half can be acquired and the rest calculated. This saves time but lowers SNR, because less data is actually sampled.

MRI - Magnetic Resonance Imaging Practice Test

MRI - Magnetic Resonance Imaging Practice Test

MRI Data Acquisition, k-Space and Image Processing/Storage