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

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

Answer: Image signal and contrast

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.

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?

Answer: 4 minutes 16 seconds

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.

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

Answer: Fourier transform

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.

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

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

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.

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?

Answer: Conjugate (Hermitian) symmetry

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.

Which standard governs the format and network transfer of MR images between the scanner, workstations, and PACS?

Answer: DICOM

DICOM (Digital Imaging and Communications in Medicine) is the standard for storing, formatting, and transmitting medical images and their metadata between modalities, PACS, and viewing stations. HL7 handles text-based clinical and administrative messages. HIPAA is a privacy law, not an image format.