Experimental Study Of Detection Of Brain Tissue With Electrical Impedance Tomography After Brain Injury

Brain injury (BI) is characterized by dangerous condition and highmortality. It is the most severe form of acute encephalopathy that has poorprognosis. It is also one of the fatal diseases of the elderly as well as the mortaldisease caused by accidents. The main reason for the unsatisfied prognosis isthat early detection and real-time monitoring of BI cannot be carried out, whichresults in the lack of timely treatment and interve ntion. At present, clinicaldiagnostic techniques of BI include X-ray, CT, MRI, ultrasound examination,cerebral angiography, intracranial pressure monitoring and spinal puncture, etc.Although these techniques are able to obtain some valuable diagnosticinformation, they are incapable of conducting real-time monitoring of BI toinform early warning. Thus the best therapeutic time window for patients isoften missed so that death and disability due to BI occur from time to time.Therefore an effective method for real-time dynamic monitoring of BI isurgently needed.Electrical impedance tomography (EIT) is a medical imaging technology which aims to visualize the distribution of resistivity (conductivity) insidehuman body. The main principle is to estimate the distribution of resistivity(conductivity) inside the target by measuring the boundary voltage or currentafter injecting driving signals (voltage or current) into the target. It has theadvantages of being non-invasive, functional imaging, low cost and smallvolume. EIT has fine potential and value of investigation in the application ofbedside dynamic imaging monitoring of BI.After near20years of research, our group has made breakthrough progressin EIT data acquisition, imaging algorithms and clinical trials, which made ourgroup the international leader in the field of brain dynamic imaging monitoring.Based on the preliminary work, this thesis focuses on the practical problems indynamic EIT imaging monitoring of BI and conducts research in the fo llowingtwo aspects to further promote the clinical application of EIT.(1) Experimental study on dynamic EIT imaging monitoring of animalmodel of cerebral edema induced by radiation injury.In previous animal experiments electrodes were embedded into the skull toeffectively reduce the contact impedance of the electrode system. But it easilyled to bleeding and damage of intracranial pressure environment, which wouldinfluence the results. Therefore, improved electrode system was made based onthe previous electrodes to meet the experimental requirements. The newcharacteristics included insulation board, external traction system and electrodeprobes set on the insulation board. The probes were free to adjust their lengthand were able to make tight contact with the top of the skull through down-pullcomponents. Besides, the performance of the new electrodes and that of Ag/Clelectrodes were compared by the two-electrode method. Animal model of cerebral edema induced by radiation injury was made withsingle high dose of Dt30Gy and dose rate of300cGy/min. Radiotherapy planwas designed in strict accordance with experiment requirements employingCADPLAN/HELIOS3D treatment planning system. The model was verified byanatomical and radiographic examinations as well as optical and electronmicroscopy. The relationship between EIT images and cerebral edema inducedby radiation injury was analyzed.Animal model of cerebral edema induced by radiation injury with accurate3Dpositioning was made using high-energy X-ray, which had the advantages ofaccurate positioning, closed property, controllable range of edema and bettersimulation of clinical conditions. Thus we proposed to simulate cerebral edemausing animal model of radiation injury.(2) Experimental study on dynamic EIT imaging monitoring of animalmodel of endogenous cerebral hemorrhage.The animal model of cerebral hemorrhage was made by the collagenaseinduction method, in which collagenase was injected at striatum. Theexperimental process included anesthesia, depilation, drilling, collagenaseinjection and model verification. The advantage of the model was that thevolume and range of bleeding could be controlled by regulation of concentrationand amount of collagenase. The process of the delayed bleeding could bemonitored by EIT. In addition, the injection of tiny amount of collagenase didnot form intracranial mass effect, which was closer to the actual cerebralhemorrhage. More importantly, the use of this model could guarantee thepresence of intracranial pressure by sealing the injection site, which made bettersimulation of clinical cerebral hemorrhage. The relationship between EIT images and cerebral hemorrhage was analyzed.Results:(1) Early impedance change induced by radiation BI could be detected by EIT,in which one-dimension value (P<0.05) and two-dimension image changedsignificantly. The impedance increased noticeably and the location was basicallyconsistent with that of the hemorrhage. By radiographic, pathological andanatomical examinations we found that radiation injury could not be discoveredanatomically in tissue sections12h after exposure. It was difficult to tellradiation injury from CT of brain3d after exposure. Cell damage was found byoptical microscope24h after exposure and radiation injury could be detected byelectron microscope10h after exposure. The preliminary results of this studyindicated that acute cerebral radiation injury, i.e. cerebral edema induced byradiation might be detected by EIT. The results also proved the sensitivity andfeasibility of EIT to detect cerebral edema and radiation injury.(2) Early impedance change induced by cerebral hemorrhage of animal couldbe detected by EIT. Via changes of one-dimension information and two-dimension images, combined with anatomical, pathological and radiographicexaminations as well as impedance analyzer, it was found that the cerebralimpedance increased with time as the hematoma aggravated and the extentenlarged. Preliminary results showed that the impedance increase in target areawas caused by cerebral hemorrhage and EIT was able to detect the change.Combined with CT scan results, it was indicated that when cerebral hemorrhageoccurred impedance change might precede density change and EIT mightbecame a more sensitive approach than radiography.This study verified the feasibility and sensitivity of monitoring BI by EIT.Although much more specific research work remains to be done before clinical application, the prospect of early BI detection using EIT has been proved.