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.