Validation of an Adaptive Signal Enhancer in Intraoperative Somatosensory Evoked Potentials Monitoring

Signal-to-noise ratio of intraoperative tibial nerve somatosensory-evoked potentials

To reveal the intrinsic signal-to-noise ratio (SNR) of single-trial somatosensory-evoked potentials (SEP). SEP was recorded from 13 scoliosis patients during surgery. The power of SEP was estimated with least-square fitting to obtain the most accurate value and then to estimate the SNR of every trial of SEP. The SNR of cortical SEP from 13 cases presented individual difference among each other. According to the mean and standard deviation, the coefficients of variation of cortical and subcortical SEP were 4.2% and 23%, respectively. The SNR of SEP was estimated to be -24 +/- 1 dB in cortical SEP and -22 +/- 5 dB in subcortical SEP. The lowest SNR of individual case was found to be -30 dB in cortical SEP and -53 dB in subcortical SEP. The results showed that SNR of intraoperative SEP recordings varies from person to person and presents a higher variability in subcortical than that in cortical, with a broad range from -53 to -5 dB. The results from this study can be used to understand the nature of SEP signals, which could guide researchers and designers on SEP denoising method selection, extraction, and measurement, as well as equipment development.

Real-time somatosensory evoked potential monitoring using FPGA-based adaptive filter

This study was to analyze the performance of fixed-point adaptive signal enhancer (ASE) for somsatosensory evoked potential (SEP) measurement. An animal study was conducted to evaluate the usefulness of this technique in detection of spinal cord injury. The ASE technique has been reported powerful to improve signal to noise ratio of SEP. Therefore, we proposed an integrated circuit. module to perform real time ASE for fast SEP measurement, and a fast fixed-point algorithm was employed in the Field Programmable Gate Array design. We compared the SEP results from FPGA-beased ASE methods and the conventional ensemble averaging (EA) method during an animal experiment. Experimental results on a spinal cord compressive injury model showed that the FPGA-based ASE methods have superior performance over the EA method. It would be helpful to design fixed-point ASE based on FPGA for real-time monitoring of intraoperative SEP.

Tibial somatosensory evoked potential intraoperative monitoring: recommendations based on signal to noise ratio analysis of popliteal fossa, optimized P37, standard P37, and P31 potentials

OBJECTIVE

To compare the intraoperative signal-to-noise ratio (SNR), reproducibility and rapidity of popliteal fossa (PF), optimized P37, standard P37 and P31 potentials.

METHODS

Raw sweeps and 11 averages doubling sweep number from 2 to 2048 were compared in 37 patients undergoing scoliosis surgery. Optimized (highest amplitude or SNR) P37 derivations were Cz-CPc (22), CPz-CPc (27), Pz-CPc (7), iCPi-CPc (8), CPi-CPc (1), Cz-Pz (2) or Pz-FPz (3), and in two patients with non-decussation, Cz-CPi (1) or CPz-CPi (3). Standard P37 and P31 derivations were CPz-FPz and FPz-C5S. Signal amplitude was measured in 2048-sweep averages; peak noise was measured in raw sweeps and +/- averages; SNR was amplitude/noise. Visual superimposability and < 20-30% amplitude variation determined reproducibility. Sweeps to reproducibility determined rapidity.

RESULTS

The SNR order was PF >> optimized P37 > standard P37 > P31. Mean optimized P37 SNR advantages over the standard P37 and P31 were 2.1:1 and 4.9:1. SNR had powerful non-linear correlations to reproducibility and rapidity. Median sweeps to reproducibility were PF: 2, optimized P37: 128, standard P37: 512 and P31: 1024. EEG noise was greatest in FPz derivations. Burst-suppression increased scalp potential SNR and rapidity.

CONCLUSIONS

Optimized P37 and PF recordings are most rapidly reproducible due to superior SNRs and are recommended. FPz should be avoided. Burst-suppression may be desirable.

SIGNIFICANCE

CPz-FPz and FPz-C5S should no longer be standard.