How to create the best optode template for your fNIRS setup

You have probably seen that there are many different templates (or as others also call it, layouts or montages) possible for the OxyMon. How do I choose the one best for me?

How many channels can I create?

First you need to know how many receivers and transmitters your system has. The easiest way is to simply count it in the device itself. The receivers (blue sticker) are on the left of the OxyMon and the transmitters (yellow sticker) on the right. Be aware that two lasers form one transmitter. So, if you count the number of connectors here, divide this by two. This can be seen in Figure 1.

Figure 1: OxyMon cabinet, left: two receivers ("Rx") labelled with a blue sticker and right four transmitters ("Tx") labeled with a yellow sticker

The maximum number of channels that could theoretically be formed with your device is the number of receivers * number of transmitters.

However, this assumes that every receiver is near every transmitter. Especially with systems with a large amount of optodes this is not feasible as some transmitters and receivers will have a distance that is not sufficient to get a signal. For example, a receiver on one side of the head and a transmitter on the other side will not be able to form a channel as the distance is too large to get sufficient light.

Split versus unsplit optical fibers

To make optimal use of your number of receivers and transmitters we offer the possibility to split your optical fiber.

What does this mean?

A simple example using a 2 channel system is shown in the figure below. A 2 channel system has 1 receiver and 2 transmitters. In case you use only unsplit fibers you will get the template as shown at the top. The receiver forms a channel with each transmitter. If you would replace the unsplit fiber for the receiver by a split fiber (see Figure 2), which has 2 ends at the subject end, you will be able to use the template shown on the bottom.

 

Figure 2:  Two channels using only unsplit fibers (top) versus two channels using an unsplit receiver fiber (bottom)

The advantage of the template on the top is that you now have two channels which can be placed independently on the body. This enables you to have a large amount of channels without the need to buy many receivers. You are required to not place two receiver ends closer than 50mm from each other as this causes crosstalk. The standard multichannel templates available in our Oxysoft software all fulfil this requirement.

The advantage of using an unsplit fiber is that you will have more light. General rule of thumb is that with a full fiber you can increase the interoptode distance with about 5mm in comparison with a split fiber, enabling you to measure deeper in the brain.

By using split fibers it is easier to have a large number of channels of your system. In general, you can never form more channels than ‘receivers’ x ‘transmitters’. If you developed your own template, and you do have more channels, than you have used a (number of) channel(s) multiple times.

Special intermezzo: We also offer a special receiver fiber which is split 8 times! This this fiber can be used to form shallow (or reference or superficial) channels with a short interoptode distance (usually between 5-15mm). Using this fiber, you only need 1 receiver to form up to 8 reference channels!

Choose your template

In the dropdown menu of the optode-template selection screen you will find many different templates you could use. To select the right one for you, you need to know the number of receivers and transmitters you have and the type of fibers (split vs unsplit) you have to your availability. On the bottom right (Figure 3) of the optode-template selection screen you see how many receivers and transmitters are required and which fibers (either 1-end (=unsplit fiber) or 2-end (=split fiber)).

Depending on the template you chose, the software will show you the corresponding NIRS signals for each channel. Do not worry if you chose a template in the software of which afterwards you found that it did not correspond with the actual template that you used on your subject. Oxysoft stores all data from every possible channel. So, it is possible to change the template in the software after finishing a measurement as well.

Tip: If you find a template which uses 1 split fiber, while you only have unsplit fibers, this 1 receiver with 1 split fiber can be replaced by 2 receivers with 2 unsplit fibers.

Tip: If you only have split fibers and the template of your choice has unsplit fibers, it is possible to use just one end of a split fiber by capping off the unused end.

Knowing all this, you can now select the template for your system. If you wish to use another template that can be made with your system or fibers, you can contact us and we can explain what you need to do to use that template.

Figure 3: Template screen from Oxysoft software, an 8 channel split template is chosen, with 2 unsplit receivers (Rx1 & Rx2), 2 unsplit transmitters (Tx1 & Tx2) and 2 split transmitters (Tx3a, Tx3b, Tx4a, Tx4b)

What if the template I want is not in the list?

You can create one yourself! If you go to the folder where Oxysoft is installed (usually C:\Program Files (x86)\Artinis Medical Systems BV\Oxysoft XXX). Here you will find a file named optodetemplate.xml. This file can be edited with any text editor. It includes templates with descriptions which should help you to get on your way. Before you start editing it, do not forget to make a backup. If you need any help, please contact us and we will gladly help you.

How do I choose the best holder for my optodes?

The OxyMon, the fibers and the holders are all separate items therefore a  system can be configured for any application.

As discussed before, the OxyMon can come in many combinations of number of receivers and transmitters, as well as the optical fibers can come split or unsplit. We also offer many options for the holder.For smaller number of channels we usually recommend to use our hard plastic holders. These do not cover large areas of the subjects’ skin and are therefore comfortable and easily accessible.

For larger number of channels (commonly with brain measurements) we advise to use a headcap. We developed these headcaps for optimal use with NIRS. These caps differ in a number of points compared to standard caps. For example they cover a larger area of the prefrontal cortex, have a better optode to skin contact and more coverage of the brain. These caps come with premade holes or without in case you want to determine the exact locations yourself. These caps can be used to in combination with any of the templates predesigned in the software but are also perfect to use when you want to create your own template.

If you need any assistance in determining the right selection for your specific research, please contact Artinis and we are always happy to help you!