Intracranial electrical impedance tomography: a method of continuous monitoring in an animal model of head trauma

Abstract

Background: Electrical impedance tomography (EIT) is a method that can render continuous graphical cross-sectional images of the brain's electrical properties. Because these properties can be altered by variations in water content, shifts in sodium concentration, bleeding, and mass deformation, EIT has promise as a sensitive instrument for head injury monitoring to improve early recognition of deterioration and to observe the benefits of therapeutic intervention. This study presents a swine model of head injury used to determine the detection capabilities of an inexpensive bedside EIT monitoring system with a novel intracranial pressure (ICP)/EIT electrode combination sensor on induced intraparenchymal mass effect, intraparenchymal hemorrhage, and cessation of brain blood flow. Conductivity difference images are shown in conjunction with ICP data, confirming the effects.

Methods: Eight domestic piglets (3-4 weeks of age, mean 10 kg), under general anesthesia, were subjected to 4 injuries: induced intraparenchymal mass effect using an inflated, and later, deflated 0.15-mL Fogarty catheter; hemorrhage by intraparenchymal injection of 1-mL arterial blood; and ischemia/infarction by euthanasia. EIT and ICP data were recorded 10 minutes before inducing the injury until 10 minutes after injury. Continuous EIT and ICP monitoring were facilitated by a ring of circumferentially disposed cranial Ag/AgCl electrodes and 1 intraparenchymal ICP/EIT sensor electrode combination. Data were recorded at 100 Hz. Two-dimensional tomographic conductivity difference (Δσ) images, rendered using data before and after an injury, were displayed in real time on an axial circular mesh. Regions of interest (ROI) within the images were automatically selected as the upper or lower 5% of conductivity data depending on the nature of the injury. Mean Δσ within the ROIs and background were statistically analyzed. ROI Δσ was compared with the background Δσ after an injury event using an unpaired, unequal variance t test. Conductivity change within an ROI after injury was likewise compared with the same ROI before the injury making use of unpaired t tests with unequal variance.

Results: Eight animal subjects were studied, each undergoing 4 injury events including euthanasia. Changes in conductivity due to injury showed expected pathophysiologic effects in an ROI identified within the middle of the left hemisphere; this localization is reasonable given the actual site of injury (left hemisphere) and spatial warping associated with estimating a 3-dimensional conductivity distribution in 2-dimensional space. Results are shown as mean ± 1 SD. When averaged across all 8 animals, balloon inflation caused the mean Δσ within the ROI to shift by -11.4 ± 10.9 mS/m; balloon deflation by +9.4 ± 8.8 mS/m; blood injection by +19.5 ± 11.5 mS/m; death by -12.6 ± 13.2 mS/m. All induced injuries were detectable to statistical significance (P < 0.0001).

Conclusion: This study confirms that the bedside EIT system with ICP/EIT combination sensor can detect induced trauma. Such a technique may hold promise for further research in the monitoring and management of traumatically brain-injured individuals.

Figure 1

A modified cranial bolt for simultaneous deep parenchymal ICP monitoring and EIT sensing. The PTFE insulated tube (except at the tip) forms a central electrode for impedance measurement at a depth of 3 cm. The conductor emerging at the upper right is an electrical contact point for interfacing with the EIT hardware.

Figure 2

The instrumented cranium of the piglet with an array of 8 circumferential electrodes and one central electrode with integrated ICP sensor. The contralateral left cranial catheter for intraventricular blood instillation is also seen. For clarity, all EIT reconstructions and radiographs reported here share this same caudal-rostral orientation. The same orientation is employed for difference imaging tomograms.

Figure 3

Timeline of experiment progression (a) and an example data stream recorded intraoperatively along with a reconstructed Δσ image (b). The green and red vertical lines in the ICP plot represent the time at which the baseline and post-injury data are taken, respectively, for the post-injury image reconstruction. Each of these data vectors is computed by averaging 100 consecutive EIT frame acquisitions to improve signal to noise ratio. In this example, the green and red lines straddle an increase in ICP caused by the inflation of the Fogarty balloon catheter. The large amplitude oscillations in the ICP waveforms are caused by positive-pressure ventilation. The smaller perturbations are ICP variations due to blood pumping. The figure in the right column shows the EIT reconstruction. The slightly off-centered large insulative (blue) region in the Δσ image represents the effect of the balloon inflation. The snout is oriented toward the 3 o’clock position. A pre-injury difference image would similarly be created by utilizing an average of data at that location (i.e. from time = ~1290 s to ~1300 s) minus the baseline.

Figure 4

Axial electrical impedance tomogram after balloon deflation. The baseline image was taken just prior to deflation. Once the insulative inclusion is removed, the resulting void fills with the more conductive parenchyma or fluid such as CSF thereby presenting a more conductive (red) region in the reconstructed image. The X’s represent electrode locations within the mesh. The snout direction is at the 3 o’clock position as in figure A.

Figure 5

The reconstruction of the injection of 1 ml of blood (arrow) into the left hemisphere of the brain. Also shown is the simultaneously recorded increase in ICP and ROI Δσ vs. time. Similar graphs are readily created for all manipulations, confirming that the effects are genuine. While physically co-located with the Fogarty inflations, blood injections do not always appear co-located in the reconstructions. This is probably due to blood dissecting out of the region into which the catheter has been placed.

Figure 6

A lateral cross table radiograph of the instrumented cranium. The left arrow points to the inflated Fogarty catheter with 0.15mL contrast dye. The right arrow points to the position of the central electrode. The tip of the electrode is lower than the balloon. The dark objects above are the cranial bolts.

Figure 7

Simultaneous ICP, ECG, and ABP synchronous to EIT during euthanasia. Note that as the pentobarbital sodium cocktail takes effect (arrow), ECG stops and ICP and ABP immediately begin to drop. The small excursions seen after drop in ICP are due to positive pressure ventilation, which ceases at time 8835s. During this time there is a near symmetric loss of perfusion (seen as a decrease in conductivity in both hemispheres) as the brain becomes more ischemic post heart stoppage.

Figure 8

Difference conductivity image with the baseline and later frame data created by averaging EIT acquisitions from diastolic and systolic phases of the heart cycle while excluding ventilator induced breathing artifact. The arrows point to effects caused by the presumed and “unmasked” anterior (2) and middle cerebral (1) artery blood distributions. The scale has been adjusted to enhance the conductive regions.