Reducing motion artifacts for long-term clinical NIRS monitoring using collodion-fixed prism-based optical fibers

Abstract

As the applications of near-infrared spectroscopy (NIRS) continue to broaden and long-term clinical monitoring becomes more common, minimizing signal artifacts due to patient movement becomes more pressing. This is particularly true in applications where clinically and physiologically interesting events are intrinsically linked to patient movement, as is the case in the study of epileptic seizures. In this study, we apply an approach common in the application of EEG electrodes to the application of specialized NIRS optical fibers. The method provides improved optode-scalp coupling through the use of miniaturized optical fiber tips fixed to the scalp using collodion, a clinical adhesive. We investigate and quantify the performance of this new method in minimizing motion artifacts in healthy subjects, and apply the technique to allow continuous NIRS monitoring throughout epileptic seizures in two epileptic in-patients. Using collodion-fixed fibers reduces the percent signal change of motion artifacts by 90% and increases the SNR by 6 and 3 fold at 690 and 830 nm wavelengths respectively when compared to a standard Velcro-based array of optical fibers. The SNR has also increased by 2 fold during rest conditions without motion with the new probe design because of better light coupling between the fiber and scalp. The change in both HbO and HbR during motion artifacts is found to be statistically lower for the collodion-fixed fiber probe. The collodion-fixed optical fiber approach has also allowed us to obtain good quality NIRS recording of three epileptic seizures in two patients despite excessive motion in each case.

Keywords: Epilepsy; Motion artifact; Near-infra red spectroscopy.

Fig. 1

Diagram of prism-based fiber (left), collodion-fixed prism-based fiber with impregnated gauze on top (right)

Fig. 2

Optical probe for the motion artifact study. Symmetrical source detector localization on both hemispheres. The red and blue numbers indicate the position of sources and detectors respectively. The yellow lines indicate a source-detector pair, or channel.

Fig. 3

Probe arrangement in epilepsy patients for subject 1 (left, one side), subject 2 (right, symmetrical on both sides). The red and blue numbers indicate the position of sources and detectors respectively. The yellow lines indicate a source-detector pair, or channel.

Fig. 4

Raw NIRS signal from a pair of NIRS channel during a 6 minute run (A), examples of individual motion artifacts (B, C, D and E) as recorded from collodion-fixed fiber probe and Velcro probe at 690 nm.

Fig. 5

The mean and standard deviation of SNR values at 690 and 830 nm wavelengths averaged over five subjects (left), the mean and standard deviation of standard deviation of the changes in HbO and HbR averaged over five subjects (right) for collodion-fixed fiber probe and Velcro probe.

Fig. 6

Percent signal change during motion artifact averaged over trials for collodion-fixed probes (red) and Velcro-probes (green) at 690 and 830 nm wavelengths. The central line shows the median, the edges of the box are the 25^th and 75^th percentiles and the blue plus signs are outliers. (RL: reading loudly, NUD: nodding up and down, NS: nodding sideways, TR: twisting right, TL: twisting left, SHR: shaking head rapidly, RE: raising eyebrows.)

Fig. 6

Percent signal change during motion artifact averaged over trials for collodion-fixed probes (red) and Velcro-probes (green) at 690 and 830 nm wavelengths. The central line shows the median, the edges of the box are the 25^th and 75^th percentiles and the blue plus signs are outliers. (RL: reading loudly, NUD: nodding up and down, NS: nodding sideways, TR: twisting right, TL: twisting left, SHR: shaking head rapidly, RE: raising eyebrows.)

Fig. 7

Changes in HbO and HbR (in molars) during motion artifact averaged over trials for collodion-fixed probes (red) and Velcro-probes (green). The central line shows the median, the edges of the box are the 25^th and 75^th percentiles and the blue plus signs are outliers. (RL: reading loudly, NUD: nodding up and down, NS: nodding sideways, TR: twisting right, TL: twisting left, SHR: shaking head rapidly, RE: raising eyebrows.)

Fig. 7

Changes in HbO and HbR (in molars) during motion artifact averaged over trials for collodion-fixed probes (red) and Velcro-probes (green). The central line shows the median, the edges of the box are the 25^th and 75^th percentiles and the blue plus signs are outliers. (RL: reading loudly, NUD: nodding up and down, NS: nodding sideways, TR: twisting right, TL: twisting left, SHR: shaking head rapidly, RE: raising eyebrows.)

Fig. 8

EEG trace and NIRS recordings during seizure 1 of Subject 1. The time zero is the start of the seizure as defined by clinical inspection of the EEG trace (performed by SC). rHbO: relative oxyhemoglobin, rHbR: relative deoxyhemoglobin, rtHb: relative total hemoglobin, rCMRO₂: relative cerebral metabolic rate of oxygen, rCBF: relative cerebral blood flow. (F: frontal, T: temporal, C: central, P: parietal, O: occipital, EKG: electrocardiogram). The blue box indicates the start and the end of the epileptic seizure determined from electrophysiology as well as patient video recordings by an epileptologist.

Fig. 9

EEG trace and NIRS recordings during a seizure from Subject 1(7002). The time zero is the start of the seizure. rHbO: relative oxyhemoglobin, rHbR: relative deoxyhemoglobin, rtHb: relative total hemoglobin, rCMRO₂: relative cerebral metabolic rate of oxygen, rCBF: relative cerebral blood flow. (F: frontal, T: temporal, C: central, P: parietal, O: occipital, EKG: electrocardiogram). The blue box indicates the start and the end of the epileptic seizure determined from electrophysiology as well as patient video recordings by an epileptologist.

Fig. 10

EEG trace and NIRS recordings during a seizure from Subject 2 (7004). The time zero is the start of the seizure. rHbO: relative oxyhemoglobin, rHbR: relative deoxyhemoglobin, rtHb: relative total hemoglobin, rCMRO₂: relative cerebral metabolic rate of oxygen, rCBF: relative cerebral blood flow. (F: frontal, T: temporal, C: central, P: parietal, O: occipital, EKG: electrocardiogram). The blue box indicates the start and the end of the epileptic seizure determined from electrophysiology as well as patient video recordings by an epileptologist.