Unlike in CT and plain films in which localisation of the signal is simple (an x-ray beam travels through the material and where it hits the receptor is the physical location of what it has passed through) MRI is much more complicated. With MRI the signal is localised in the 3D space by manipulating the magnetic properties of the nuclei in a predictable way. The signals are then returned with a particular frequency and phase and these are slotted into their respective locations. The brightness of the pixel is the amplitude of the signal returned.
The key concept of spatial encoding is the use of gradients.
There are three steps involved in identifying where in a 3D location a signal is arising from:
- Slice selected along z-axis
- Segment of slice along x-axis selected by frequency encoding
- Part of segment along y-axis selected by phase encoding