Mechanical Characterization Of Brain Tissues Using Ultrasound Elastography

Determining the mechanical properties of brain tissues is essential in such cases as the trauma research of traumatic brain injury,surgical training using virtual reality based simulators,and the diagnosis of some diseases that can alter the elastic properties of brain tissues.Mechanical characterization of brain tissues should consider the effect of the post-mortem interval(PMI),preservation temperature,working frequency of the specified testing methods and complex microstructures of brain itself;and therefore remains a challenging issue.Given that shear wave elastography(SWE)can acquire the shear moduli of brain tissues by measuring the shear wave speed in brain,we suggested a protocol based on the SWE to measure both in-vivo and ex-vivo elastic properties of brain tissues and explore the effects of PMI and the temperature on the measurement results.Firstly,the brain tissue was assumed to be a poroelastic solid and the cerebral perfusion pressure caused by blood and the cerebrospinal fluid led to the prestress in brain.Shear waves propagating in a pre-stressed poroelastic solid based on the poroelasticity theory were disscussed;and analytical solution has been suggested to revel the quantitative relationship between the shear wave speed and the cerebral perfusion pressure.Secondly,we proposed a shear wave elastography method to measure both in vivo and ex vivo elastic properties of brain tissues.Based on the method and the protocol,we measured the shear moduli of rabbit brain tissues.Our results showed that the shear modulus from in-vivo measurement(4.756 ?0.394 k Pa)is about 47% higher than that given by the ex-vivo measurement(3.227 ?0.483 k Pa).Using the theoretical model above,we successfully explained the physical mechanism underlying the measurement results.The cerebral perfusion pressure was released in ex vivo measurements with the lose of blood and the cerebrospinal fluid,which made the shear wave speed obtained ex-vivo be lower than that obtained in-vivo.At the same time,with the analytical shear wave speed formula,we can identify the cerebral perfusion pressure based on the shear wave speeds measured in-vivo.Finally,using the shear wave elastography method and the protocol,experiments were conducted on rabbit and pig brains to investigate the effects of PMI and the temperature on brain tissues elasticity.The results showed that the viriation of the brian shear moduli within 60-minites post-mortem is negligible.We found that the shear moduli of brain tissues decreased approximately linearly when the temperature increased from the room temperature(about 23 ℃)to body temperature(about 38 ℃),with the slops of-0.192 ?0.014 k Pa/℃(for rabbit)and-0.041?0.006 k Pa/℃(for pig).When the temperature varied from 23 ℃ to 53 ℃,the brain tissues was the softest at the body temperature.