Question

Why do some gradients change frequencu while others change phase in MRI?

Answer 1

The MRI image that you see has some bright pixels and some gray pixels and some black pixels. The brightness of the pixels is determined by the strength of the MRI signal that’s picked up from those specific locations. But how does the MRI scanner ‘know’ what parts of the signal to assign to the strong signals and the weak signals? The frequency and phase encoding.

The frequency of the MRI signal received from the patient is directly proportional to the strength of the underlying magnetic field, so a 1.000 Tesla magnet has a signal frequency of 42 MHz. A 1.5 Tesla magnet has a signal frequency of 63 MHz.

The MRI machine superimposes additional (small) magnetic fields on top of the underlying main field during the image acquisition process, and those supplemental fields, called gradient fields, vary from left-right, up-down, and front-back, creating a unique magnetic field strength in each element of the patient being scanned. Therefore, the mishmash of signals that come out of the patient can be decoded by the computer to assign a signal strength (pixel brightness) to each part of the 3-D anatomy. This is frequency encoding. It results the computer ‘knowing’ the relative brightness of the signal in various parts of the body.

In practical use, frequency encoding is used by the MRI to determine slice thickness and location as well as the signal brightness in either top/bottom of the display screen, or else side/side on the screen.

Phase encoding is similar to the frequency encoding process. The MRI computer can tell not only the frequencies of the various signals, but also their relative positions to one another, similar to a wall of clocks on display at a clock shop. In the clock display, even though all the clocks might be running at virtually the same speed, the seconds hands will all be at a different position at any given time. So it is with the MRI signal phase encoding process.

Phase encoding is used in the MRI images to determine the relative pixel brightness in the opposite direction used by frequency encoding; if the frequency encoding direction is L/R, the the phase encoding direction is up/dn on the screen. This is selectable by the MRI technologist.

The application of frequency and phase encoding was first done by Mansfield and Lauterbur, who received the Nobel Prize for their work.

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