ELEC5622 Chap.3 MRI Pulse Sequences, Spatial Encoding and k-Space
MRI Pulse Sequences, Spatial Encoding and k-Space
This chapter turns the MRI signal into an image. Spin echo timing, set by TR and TE, decides whether contrast follows T1 or T2. Three gradient coils then make frequency and phase depend on position, slice by slice, and the samples fill k-space, a spatial-frequency matrix that a 2D Fourier transform converts into the picture. The chapter ends with scan-time arithmetic and the strengths and limits of MRI.
The aim is to read any protocol, predict its contrast and estimate how long it will take, and to explain each gradient's job in a sentence. Every rule is tied back to the relaxation clocks of Chapter 2, so the weighting rules can be reasoned out rather than memorised blindly.
What this chapter covers
- 01
Free induction decay and the spin echo
- 02
Repetition time and echo time
- 03
Rules for T1-weighted and T2-weighted sequences
- 04
Gradient coils and the four scanner components
- 05
Slice selection, frequency encoding and phase encoding
- 06
The k-space matrix, the Fourier transform and scan time
- 07
Advantages and disadvantages of MRI
Worked example · free
Scan time and weighting from a protocol
- 1Time = 256 × 2.5 s = 640 s.
- 1640 s ÷ 60 = 10.7 minutes.
- 1TR above 2,000 ms and TE above 80 ms make the image T2-weighted.
Key terms
- Echo time
- Echo time, TE, is the interval between the 90° excitation pulse and the centre of the echo.
- Phase encoding
- Phase encoding is a brief gradient that gives each row a different phase so rows can be separated.
- K-space
- K-space is the matrix of raw MRI samples arranged by spatial frequency before Fourier transformation.
- Gradient coil
- A gradient coil adds a small, position-dependent field to B0 so that resonance frequency varies across space.
- Free induction decay
- A free induction decay is the fading signal that follows a single RF excitation pulse as spins dephase.
MRI Pulse Sequences, Spatial Encoding and k-Space FAQ
Why does a spin echo need a 180° pulse?
After the 90° pulse, spins drift out of phase and the signal fades. The 180° pulse flips their phase order so the faster spins catch the slower ones, and they come back into phase at the echo, recovering signal that seemed lost.
Why is phase encoding the slow part of an MRI scan?
Each phase-encoding step fills one line of k-space and needs its own excitation, separated by TR. A 256-row image needs 256 repetitions, so the number of rows times TR largely sets how long the patient stays in the scanner.
Which patients may not be suitable for MRI?
The lecture lists people with internal metal such as surgical clips, plates or screws, because of the strong magnet, and notes that the enclosed, noisy bore is difficult for claustrophobic patients.
What does frequency encoding do?
During readout a gradient along x makes the field, and therefore the Larmor frequency, vary with position. Each column of the slice then emits at its own frequency, so a Fourier transform of the received signal separates the columns.
What do the centre and edges of k-space control?
Samples near the centre hold low spatial frequencies, which set overall contrast and brightness. Samples near the edges hold high spatial frequencies, which carry edges and fine detail, so dropping outer lines blurs an image while keeping its contrast.
What are the main parts of an MRI scanner?
The lecture lists four: the magnet, usually superconducting, which produces B0; gradient coils for spatial encoding; RF coils that transmit to and receive from the tissue, in body, head and surface forms; and the computer, which includes the Fourier transformer that builds the image.
Exam move
Memorise the four TR and TE thresholds as two pairs, then test yourself with mixed protocols. Draw the encoding sequence as a flow from z to x to y to k-space to image and say what each gradient does in one sentence. Practise a scan-time calculation with different row counts and repetition times.
Add one line on each scanner component, magnet, gradient coils, RF coils and computer, and practise reconstructing two voxel values from a sum and a difference, the trick behind phase encoding. Practise explaining, without notes, why a long TR removes T1 contrast and why a long TE lets T2 differences appear.
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