NYQUIST FREQUENCY

Physics

NYQUIST FREQUENCY

1. Fourier analysis is a mathematical method of deconstructing any wave into a series of sine waves that vary in frequency and wavelength.

2. If our sampling frequency is too low then the high frequency components of the wave (which carry information about small structures with sharp edges) will be lost resulting in a low resolution image.

3. For an accurate representation, we must adhere to the Nyquist criterion - the signal must be sampled at at least twice the highest frequency present in the signal (i.e. sampled 2x to detect 2 points along its sine wave).

4. If we do not adhere to this criterion, high frequency signals will be erroneously recorded as low frequency (a phenomenon known as aliasing).
5. The maximum signal frequency that can be accurately sampled is called the Nyquist frequency: $F_{N}=\frac{1}{2}\Delta$ (where Δ is the size of a detector element).

REFERENCES

1. Bushberg, J. T. (2012). The essential physics of medical imaging. Philadelphia: Wolters Kluwer Health/Lippincott Williams & Wilkins. Find it at Amazon
2. Heggie, J. C., Liddell, N. A., & Maher, K. P. (1997). Applied imaging technology. Melbourne: St. Vincents Hospital.

Ⓒ A. Manickam 2018