<p>Basically, you have two unknowns, so you need two equations.</p>
<p>From <a href="https://en.wikipedia.org/wiki/Functional_near-infrared_spectroscopy" rel="nofollow noreferrer">Wikipedia</a>:</p>
<blockquote>
<p>Using a dual wavelength system, measurements for HbO2 and Hb can be solved from the matrix equation</p>
</blockquote>
<p><a href="https://i.sstatic.net/8SGx6.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/8SGx6.png" alt="fnirs matrix equation" /></a></p>
<p>With just one measurement, you could still describe relative changes in the signal over time, but you wouldn't be able to measure the ratio of the two. That might be sufficient within a given experiment, but wouldn't result in numbers that you could compare to other recordings. Each recording is going to have a distinct signal-to-noise ratio based on things like the amount of tissue you're imaging through: most of the light is going to be lost no matter what the signal is.</p>

CC BY-SA 4.0
2023-12-07T14:30:16.233Z
2023-12-07T14:30:16.233Z
Bryan Krause
<p>One of the most common techniques used for functional neuroimaging nowadays is functional near infra-red spectroscopy (fun fact: IIRC Natalie Portman worked on a research paper involving fNIRS as the modality), which shines near infrared light into the brain from a source to a detector (both called optodes) in a &quot;banana&quot; shape.</p>
<p>It's not uncommon to read that most of these devices, be they continuous wave (CW) or one of the two kinds that involve fast modulation, frequency domain (FD) or time domain (TD), require two separate frequencies to be emitted. For instance, NIRx explains it as follows on their <a href="https://nirx.net/cwfnirs" rel="nofollow noreferrer">website</a>:</p>
<p>&quot;For neuro-imaging applications it is by far most common to illuminate with two discrete wavelength, which is the minimum requirement to assess relative variations of both oxygenation states of the hemoglobin molecule independently.&quot;</p>
<p>Why is that the case?</p>
<p>I haven't delved into the intricacies of it, but no reason immediately jumps out at me. For instance, in the case of CW, the relative difference in intensity is all that matters, so why do we need two frequencies?</p>

CC BY-SA 4.0
2023-12-06T13:15:38.200Z
By "frequency" I assume you refer to (inverse of) different wavelengths of light?
Bryan Krause
CC BY-SA 4.0
2023-12-07T09:38:42.673Z
@BryanKrause yes, that is correct
David Cian
CC BY-SA 4.0
2023-12-07T09:40:52.667Z
@BryanKrause I added a source from the NIRx website
David Cian
<neuroimaging>
Why do fNIRS devices commonly use two different frequencies?
David Cian
# Why do fNIRS devices commonly use two different frequencies?

(Asked by: username_0 on 2023-12-05)

One of the most common techniques used for functional neuroimaging nowadays is functional near infra-red spectroscopy (fun fact: IIRC Natalie Portman worked on a research paper involving fNIRS as the modality), which shines near infrared light into the brain from a source to a detector (both called optodes) in a "banana" shape.

It's not uncommon to read that most of these devices, be they continuous wave (CW) or one of the two kinds that involve fast modulation, frequency domain (FD) or time domain (TD), require two separate frequencies to be emitted. For instance, NIRx explains it as follows on their [website](https://nirx.net/cwfnirs):

"For neuro-imaging applications it is by far most common to illuminate with two discrete wavelength, which is the minimum requirement to assess relative variations of both oxygenation states of the hemoglobin molecule independently."

Why is that the case?

I haven't delved into the intricacies of it, but no reason immediately jumps out at me. For instance, in the case of CW, the relative difference in intensity is all that matters, so why do we need two frequencies?

**Tags:** neuroimaging

> **Comments:**
> **username_1**: By "frequency" I assume you refer to (inverse of) different wavelengths of light?  
> **username_0**: @BryanKrause yes, that is correct  
> **username_0**: @BryanKrause I added a source from the NIRx website  

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## 1 answers

### Answer by username_1 on 2023-12-07. Score: 1 | Accepted answer

Basically, you have two unknowns, so you need two equations.

From [Wikipedia](https://en.wikipedia.org/wiki/Functional_near-infrared_spectroscopy):

> Using a dual wavelength system, measurements for HbO2 and Hb can be solved from the matrix equation

[![fnirs matrix equation](https://i.sstatic.net/8SGx6.png)](https://i.sstatic.net/8SGx6.png)

With just one measurement, you could still describe relative changes in the signal over time, but you wouldn't be able to measure the ratio of the two. That might be sufficient within a given experiment, but wouldn't result in numbers that you could compare to other recordings. Each recording is going to have a distinct signal-to-noise ratio based on things like the amount of tissue you're imaging through: most of the light is going to be lost no matter what the signal is.


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