Understanding NIRS Optodes

 

"A principal challenge encountered with fNIRS studies is the difficulty to create an efficient, stable and comfortable optical interface with the scalp, especially in the presence of hair. Hair and hair follicles can act as strong attenuators leading to poor signal quality. Often, significant time and effort can be invested to ensure that a minimum amount of hair remains under each optode especially for subjects with thick, darkly colored hair." Gemignani J., Schmitz C. H., 2016.

 

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Dual tip Vs Single Tip

 

When choosing a fNIRS equipment for your experiment, the user can have some trouble to decide whether to use with single or dual-tip LED. In this post we will briefly describe their differences to help you choose better.

 

The dual-tip optode has an increase detection area of 2x when compared to the single one. This means that there is an increase around 2x on sensitivity and a reduction in the gain average which improves signal quality.

 

 

Each LED emits two wavelengths (760 nm and 850 nm) with 25 mW power and has 3 mm tip. The increase on sensitivity increases slightly the dark noise level. Taking this into consideration, for NIRSport the acquisition software (NIRStar) requires that the user chooses when using the dual tip which adjusts the thresholds for dark noise (see image bellow).

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APD vs. SiPD Detectors

NIRx offers both ‘standard’ silicon photodiode (SiPD) and ‘high-sensitivity’ avalanche photodiode (APD) fNIRS detectors. The APD detectors are up to 8x as sensitive as our SiPD detectors on average (sensitivity differences vary with gain settings). This does give a much better signal overall, though detector sensitivity is not the only factor in determining signal level and quality.

 

The NIRSport ‘Portable’ fNIRS System may only use the APD detectors with an additional coupling unit- the APDs in this case would not be for portable applications, and would furthermore require a different probe set from the standard probe sets generally. By contrast, the NIRScout ‘Lab-Based’ fNIRS System may use either the SiPD or APD detectors with identical probe sets.

 

Dynamic fNIRS Gain Settings

While considering detector sensitivity is critical, the system’s dynamic gain is also important when evaluating overall system performance. NIRx offers an industry-leading 8 levels of automated/programmable detector gain for both our SiPD and APD detectors (using NIRScout). So, even though the SiPDs have less sensitivity than the APDs, good signal quality can be achieved with both types of detectors. Bellow we present the threhold for dark-noise evaluation for eah type of detector, in each gain. This gives an ideia of the increased sensitivity in comparison of dual-tip against single-tip; and also when comparing SiPD and APD. 

 

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Which Detector to Choose: APD or SiPD?

 

It may come as no surprise, but the APD detectors are quite a bit more expensive than the SiPDs. That said, their value is unquestionable: a NIRx APD detector system, along with the innovative probe systems, will work on the vast majority of subjects, measuring from any part of the head, with an incredibly easy/fast setup.

 

The APDs also are best for end-users that wish to do fMRI/fNIRS or fMRI/MEG studies.

The SiPD detectors will work excellent on child and geriatric subjects on any part of the head, and will work with ~60-70% of college-aged subjects with thick  black hair (thicker, darker hair is more difficult than thinner, lighter hair) during measurements on top of their head (e.g., the motor, somatosensory cortex, etc.). 

 

All that said, this still could be a major factor for you. It is important to remember: with NIRx systems you can start with SiPD detectors and switch over to APD detectors. We offer one of the most versatile fNIRS platforms out there in terms of upgrade options and flexibility. Please do let us know what your decision context is and we will be happy to work with you to provide you the best-possible solution.

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Ideal NIRScap Size for your subject

NIRx offers comfortable and properly-fitting caps for all subject ages. You can use the following table to know the number of slits for cap size / age.

References: This post is composed of informations from NIRx newsletters or postages.

 

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