Modes of operation
The purpose of this apparatus is to characterize particles. An immediate difficulty arises when asked to define what "characterizing" truly means: Is it is the spectral density of the signal when excited into saturation? is it the m-H curve? Or is it the dynamics the particles undergo during imaging? The diversity of literature would suggest the true answer is all of the above, and luckily for the designer, the hardware required to answer these questions is for the most part quite similar. The difference arises both in the data analysis and the excitation waveform; to answer each of those three questions there is a different driving waveform, and these correspond to the three modes of operation-- spectroscopy, magnetometry, and relaxometry, respectively. We reviewed the existing literature, and assigned this nomenclature in an effort to categorize the numerous systems and schemes by their primary function and commonly used name. In no way are these the only modes of particle characterization, though, and especially for the "magnetometry mode" there is no singular agreed-upon name ("spectroscopy" and "relaxometry" appear to be consistent across the literature).
Figure 1 Left: An illustration of the Fourier transform of the excitation waveform showing in spectroscopy mode the system operates by employing a high amplitude, high-frequency wave to excite the particles. Right: The region of the magnetization curve that the particles are driven through.
Figure 2 Left: An illustration of the Fourier transform of the excitation waveform showing in magnetometry mode the system operates by employing a low amplitude, high-frequency wave to excite the particles with a second low frequency, high amplitude field superimposed. Right: A select set of regions of the magnetization which particles would be driven through in "magnetometry mode". Because the high-frequency component is very low amplitude you can make a linear approximation at any given section of the curve.
Figure 3 Left: An illustration of the Fourier transform of the excitation waveform showing in relaxometry mode the system operates by employing a high amplitude, high-frequency wave to excite the particles with a high amplitude near DC component superimposed. Right: An illustration of the particles being driven through large segments of the magnetization curve at any given time in this mode of operation which simulates the external fields the particles undergo during imaging.