Quantitative analysis of intraspinal cerebrospinal fluid flow in normal adults

Tissue Temperature Increases by a 10 kHz Spinal Cord Stimulation System: Phantom and Bioheat Model

OBJECTIVE

A recently introduced Spinal Cord Stimulation (SCS) system operates at 10 kHz, faster than conventional SCS systems, resulting in significantly more power delivered to tissues. Using a SCS heat phantom and bioheat multi-physics model, we characterized tissue temperature increases by this 10 kHz system. We also evaluated its Implanted Pulse Generator (IPG) output compliance and the role of impedance in temperature increases.

MATERIALS AND METHODS

The 10 kHz SCS system output was characterized under resistive loads (1-10 KΩ). Separately, fiber optic temperature probes quantified temperature increases (ΔTs) around the SCS lead in specially developed heat phantoms. The role of stimulation Level (1-7; ideal pulse peak-to-peak of 1-7mA) was considered, specifically in the context of stimulation current Root Mean Square (RMS). Data from the heat phantom were verified with the SCS heat-transfer models. A custom high-bandwidth stimulator provided 10 kHz pulses and sinusoidal stimulation for control experiments.

RESULTS

The 10 kHz SCS system delivers 10 kHz biphasic pulses (30-20-30 μs). Voltage compliance was 15.6V. Even below voltage compliance, IPG bandwidth attenuated pulse waveform, limiting applied RMS. Temperature increased supralinearly with stimulation Level in a manner predicted by applied RMS. ΔT increases with Level and impedance until stimulator compliance was reached. Therefore, IPG bandwidth and compliance dampen peak heating. Nonetheless, temperature increases predicted by bioheat multi-physic models (ΔT = 0.64°C and 1.42°C respectively at Level 4 and 7 at the cervical segment; ΔT = 0.68°C and 1.72°C respectively at Level 4 and 7 at the thoracic spinal cord)-within ranges previously reported to effect neurophysiology.

CONCLUSIONS

Heating of spinal tissues by this 10 kHz SCS system theoretically increases quickly with stimulation level and load impedance, while dampened by IPG pulse bandwidth and voltage compliance limitations. If validated in vivo as a mechanism of kHz SCS, bioheat models informed by IPG limitations allow prediction and optimization of temperature changes.

The relationships among spinal CSF flows, spinal cord geometry, and vascular correlations: evidence of intrathecal sources and sinks

We studied relationships of cerebral spinal fluid (CSF) pulsatile flow at cervical, thoracic, and lumbar levels using phase-contrast cine MRI (PCCMRI) to determine the following: 1) instantaneous and average net flows at cervical, thoracic, and lumbar levels, 2) stochastic correlations of CSF flow with major arterial supplies and major draining veins, and 3) whether adjustments of cord-flow curves-using cord cross-sectional areas, caudal lengths, and caudal volumes-would normalize flow curves from different levels. We scanned 15 healthy volunteers without anesthesia, ages 23-46 yr, using external, retrocardiac-gated, two-dimensional PCCMRI at 3T. Transverse scans of the subarachnoid space, arteries, and veins were acquired and analyzed at cervical, thoracic, and lumbar levels. Instantaneous CSF flow decreased craniocaudally along the full time course of a cardiac cycle. Downward net flow generally increased craniocaudally. During diastole, instantaneous CSF flow decreased proportionally to cross-sectional area, caudal residual length, and caudal residual volume of the cord. The proportionalities were less consistent during systole. CSF, internal carotid artery (ICA), vertebral artery, and lower aorta temporal correlations were highest in systole and decreased craniocaudally. CSF flow temporally correlated better with lower aorta flow than with the ICA at T7 and L2 during systole but not diastole. Inferior vena cava temporal correlation increased craniocaudally. We conclude that whereas instantaneous flow is attenuated cranial caudally, net downward flow, per cardiac cycle, increases caudally, becoming statistically significant at T7 and below the conus medullaris. We can explain the results with the assumption of cord CSF production and peripheral-dominated CSF absorption.