Physiological Monitoring and Recording, part II

In this second physio-centric post I'll be discussing the sensors and equipment used to record pulse and respiration data from our subjects, as well as offer advice regarding their placement. Setting up the pulse oximeter and respirator belt takes only a couple minutes, meaning very little extra effort is needed in order to capture this valuable data. There's literally no excuse not to do it!

Pulse oximeter

Although the Biopac system we use does have a pulse oximeter unit available for purchase, we use an MRI-compatible Nonin 8600FO (see Note 1) and feed the analog signal to a UIM100C add-on module. The UIM100C has multiple analog input channels, so we use a separate one for the TTL signal input from the MRI equipment as well.

Figure 1 - Nonin 8600 FO pulse ox unit

Placement of the fiber optic sensor can be tricky, especially if your subject has a weak pulse, but generally speaking the sensor should be placed on the subject's index finger and secured firmly with medical tape. If you wrap it too tight then the sensor can have trouble getting consistent readings.

Figure 2 - Pulse oximeter sensor on finger

If using a button box or one of our other response devices then you'll want the subject to use the hand that doesn't have the pulse ox attached to it. The more they move the more likely the sensor could slip and give faulty readings. The image below shows a sample of the data we've recorded using this setup.

Figure 3 - Pulse data and TTL pulses recorded in Acknowledge 

Respirator belt

For acquiring respiration data we use the DA100C module along with a transducer that then attaches to a pneumogram sensor. The module attaches directly to our MP150 and UIM100C units thanks to the piecewise nature of the Biopac system.

Figure 4 - Biopac pneumogram sensor

Figure 5 - Respiratory sensor and elastic belt

The sensor is a self-inflating pressure sensing pad that connects to the transducer on the DA100C unit via plastic tubing and measures changes in the abdominal circumference of the subject during respiration. The unit should be placed on the subject's body at the location of maximum respiratory expansion. Speaking from experience, I typically find that to be around the subject's lower ribs and slightly off to the side (see Figure 5). The elastic belt keeps the sensor in place but care must be taken so as not to make the subject too uncomfortable (See Note 2). Below are two examples of data collected with this setup. The first image is normal respiration while the second shows a subject holding their breath and then inhaling. The pressure sensor is sensitive enough to pick up even the slightest fluctuations in respiration, which is why its placement and the tightness of the elastic strap are critical for successfully acquiring data.

Figure 6 - Respiratory data and TTL pulses in Acqknowledge

Figure 7 - Breath hold and recovery data with TTL pulses in Acqknowledge

Hopefully these last two posts have convinced you to incorporate acquiring physio data into your scanning protocol. It's easy to set up and well worth the effort. Even if you're not sure you'll end up using the data, it's better to have it stored away in a hard drive somewhere than not have it at all.

Note 1: Our Nonin unit has been discontinued for several years. It is a legacy device that's at least a decade old. We've purchased several newer pulse oximeters, even another one from Nonin, and none of them worked as effectively as the 8600FO. We currently have another pulse oximeter on order and I plan on updating this post once we fully test it.

Note 2:The adjustable elastic band did not come with the Biopac system but with the Siemens 3T Trio instead.

Physiological Monitoring and Recording, part I

Do you collect physiology data from subjects during an MRI scan? If so -- great job! If not -- you totally should! I strongly recommend collecting physiological data during your MRI study and these next two blog posts should shed some light not only on how important it is, but how easily it can be done with the proper setup and equipment.

The importance of collecting physio data

The main reason to collect physiological data while scanning is noise. Or rather to be able to identify potential noise and remove it in order to boost the signal-to-noise (SNR) ratio of your scan. Noise can be generated by both the subject's cardiac and respiratory cycles. (See Note 1) 

Blood vessels expand and contract as blood is pumped through them and into the brain. The resulting artifact, referred to as the pulsatility artifact, can be unpredictable and the effect varies depending on the region of the brain you are looking at. 

Respiration is in some ways trickier since a subject's breathing pattern isn't usually as predictable as their heart rate. As they breath in, their head (and by extension their brain) will move to varying degrees. If you're doing breath hold studies then it's even more likely to result in movement artifacts and noise. While we highly recommend the use of Caseforge head restraints to lessen the effect of subject head motion, collecting and accounting for respiratory data is still crucial. This is because movement of the chest modulates the magnetic field across the head even if it is perfectly restrained, and this B0 modulation appears like motion in the EPI data. Check out this PractiCAL post for more information. 

Physiological recording using Biopac and Acqknowledge

There are several different options for collecting physic data in an MRI environment. I'll be discussing the use of the Biopac MP150 system and Acknowledge 4.1, the software that came bundled with it, because it's what we have in our facility and we're quite happy with the performance.

Figure 1 - Biopac hardware with various modules added on

The Biopac unit consist of the central MP150 unit and, as you can see, we've purchase several add-ons over the years in order to record more than just respiration and pulse. We're currently able to record oxygen and carbon dioxide levels, galvanic skin response, respiration and pulse data. We've also set it up so the Biopac receives TTL pulses from the scanner equipment. This allows researchers to sync up the recorded physio data with the MRI data; making isolating and removing noise and artifacts easier.

Figure 2 - Example physio data collected using Biopac and Acqknowledge

Above you'll see an example of what data collection looks like with Biopac and the Acqknowledge software. The signal traces (from top to bottom) are: pulse, respiration (chest motion) and TTL (see Note 2). We use a sampling rate of 125 Hz for all recording channels, though the software does let you set the rates for channels individually. The fastest physiological signal of interest is the heart rate (pulse oximetry) which occurs at around 1 Hz for most people. 125 Hz digital sampling may be overkill, but it’s nice to have some fine detail in case someone needs the resolution for an advanced processing method at some point in the future. We try to “specify for tomorrow" and future-proof our facility whenever possible. In my next blog post I'll be going into the equipment used to acquire this data in more detail. 


Note 1: Respiration and cardiac data are not the only sources of noise and movement that you can encounter when doing an MRI, but I chose to focus on those two specifically because they are the bare minimum of what our facility recommends researchers acquire and account for.

Note 2: Our TTL pulses coming from the scanner are originally 10 μs long but we lengthened them to 60 ms via a custom box, so a sampling rate of 125 Hz easily captures them. Folks using shorter TTL pulses might need faster sampling. I plan on doing a post detailing our custom box in the near future so stay tuned for that.