# How does MRI distinguish information between in-plane localization and slice selection, despite both using frequency encoding?

A MRI machine can generate 3D image of a human brain. A 3D image have three axis x (horizontal), y (vertical) and z (head-foot). First, a slice along z were selected using frequency encoding. Then, the slice (x, y plane) were imaged using phase encoding and frequency encoding and can be converted to human readable image using Fourier transformation. Finally, every slice along z-axis will be imaged to get the final 3D image.

So, "Frequency-encoding may be used to define location either: 1) within a slice, or 2) between slices." How can frequency encoding be used in two dimensions? How can they be distinguished? Why not be used in all three dimensions?

Because of their coherency these can be added and their interference be detected by an amplitude sensor. To measure distance one measures phase difference between these two signals, reference and echo. This is a very accurate method over a radial distance of one wavelength, the problem is the inherent $$2\pi$$ ambiguity of the phase measurement because the result is essentially independent of a range variation that is an integer multiple of the wavelength.
• With a single frequency (wavelength) you can measure relative phase shift between the local reference and the received echo within a wavelength but this results in $n \lambda$ range ambiguity. Now repeat the phase measurement with several wavelengths and use 1 to resolve the ambiguity. Mar 30, 2022 at 14:33