Using the oversampled frequency range for recording of non-MR signals is a new idea, but there are several established technologies of high relevance in that context. The EEG-fMRI literature is, for example, important when Magstripe MRI is used to record signals distorted by RF and gradient activity. Rather than trying to provide an exhaustive list, we will here simply reference a few of the classic papers that are of special relevance to Magstripe MRI. These papers of Allen, Cohen, Anami and their coworkers are described briefly below. Don't miss the the Anami paper that was our "stepping stone" for developing the method (see below).

"The first Allen Trick". Pulse artifact subtraction:

In combination with high magnetic field, the pulsation of the blood gives rise to strong artifacts in EEG recordings. As these are approximately repeated, they can be subtracted to make the EEGs appear more normal and suited for further analysis. This phenomenon is independent of the EEG recording method. Freely available software for pulse artifact template subtraction has proven to work well for Magstripe MRI.

Allen PJ, Polizzi G, Krakow K, Fish DR, Lemieux L. Identification of EEG events in the MR scanner: the problem of pulse artifact and a method for its subtraction. Neuroimage. 1998 Oct;8(3):229-39.

"The second Allen Trick". Gradient artifact template subtraction: 

Gradient artifacts that are magnitudes stronger than the weak electrical signals from neural activity are of major concern when doing EEG-fMRI. The gradient noise completely dominates the signals of interest but since the same gradient patterns are repeated over and over, the gradient noise can be measured and subtracted as shown by Allen and coworkers. Though this is conceptually nice, the method has significant problems. Since large signals are subtracted to isolate much smaller EEGs, the method is highly sensitive to any variation. If for example an electrode moves slightly, the gradient artifacts change significantly. Another problem is that the estimated gradient noise template must be free of EEG contributions. Otherwise artifactual EEG-like signals will appear after template subtraction. Magstripe MRI can record signals almost free of gradient artifacts and the residuals can be subtracted very easily and robustly using template artifact subtraction.

Allen PJ, Josephs O, Turner R. A method for removing imaging artifact from continuous EEG recorded during functional MRI.Neuroimage. 2000 Aug;12(2):230-9. 

"The Cohen trick". Syncronizing the scanner and EEG equipment to facilitate filtering.

Gradient artifact template subtraction is greatly facilitated if the scanner (being the source of the artifacts) and the EEG system are syncronized with microsecond precision. In that case the gradient artifacts appear similar between repetitions, even if the EEG sampling is performed with a relatively low bandwidth. This idea was introduced by Cohen and co-workers. The gradient artifacts are still dominant before filtering, however, and the robustness is consequently limited. Magstripe MRI almost avoids gradient artifacts in the first place (the Anami trick) and residuals can easily be filtered since perfect synchronization between the scanner and the EEG recording equipment is guaranteed when the scanner records the EEG.

Cohen MS, Goldman R, Stern J and Engel J. Simultaneous EEG and fMRI Made Easy. In Proceedings of the Human Brain Mapping Conference, 6, 2001.

"The Anami trick". Stepping stone sampling for EEG recording during fMRI:

Anami and co-workers showed that EEG with very limited gradient artifacts can be recorded even during echo planar imaging (EPI). The trick is to stop sampling whenever there is gradient noise, i.e. typically in sub-millisecond periods between readout-gradient switching. This was implemented by modifying a sequence so that a sample-hold circuit was triggered on gradient plateaus during readout. The method is demanding in terms of EEG hardware, interfacing to scanner and sequence programming, but the advantages are significant: As gradient artifacts are largely avoided, the recording becomes robust, even when patient motion is present. Filtering, range and linearity needs are limited and so is the danger of distorting the signals.

Anami K, Mori T, Tanaka F, Kawagoe Y, Okamoto J, Yarita M, Ohnishi T, Yumoto M, Matsuda H, Saitoh O. Stepping stone sampling for retrieving artifact-free electroencephalogram during functional magnetic resonance imaging. Neuroimage. 2003 Jun;19(2 Pt 1):281-95.

The Anami paper was the starting point for developing Magstripe MRI: If EEG can advantageously be measured only when the scanner records the MR-signal (normally done on gradient plateaus in EPI), why not use surplus capacity of scanners to record the EEGs simultaneously? Besides providing  a simple, inexpensive  implementation of the best EEG-fMRI schemes available, this would be advantageous in terms of data handling, real-time analysis, feedback and wirelessness. In order to facilitate the use of ramp sampling sequences and eliminate the need for sequence modification, the gradient activity sensor was introduced to trigger the highly beneficial plateau sampling.

Magstripe MRI can therefore be seen as an improvement of especially the techniques proposed by Anami, Cohen and their  coworkers. Similarly there is a vast literature with enhancements of the other mentioned techniques, many of which are applicable to Magstripe MRI with few or no modifications.

Alternative systems: The classical techniques mentioned above are implemented in commercially available systems, e.g. MagLink from NeuroScan and BrainAmp from BrainProducts. These systems do not offer all the advantages of Magstripe MRI, e.g. with respect to cost, size, real-time capabilities, scanner integration, artifact avoidance or synchronization. Significant advantages of these systems are their commercial availability.