Effect Of Coagulative Necrosis In Tissue On Temperature Measurement Of Longitudinal Relaxation Time T1

BackgroundHigh Intensity Focused Ultrasound(HIFU)is a non-invasive treatment for tumors.Because it is a non-invasive treatment from in vitro to in vivo,HIFU requires imaging technology to determine the focal position,determine the degree of coagulation necrosis at the focal point,and whether the boundary is damaged or not,in order to ensure the safety and effectiveness.Magnetic Resonance Imaging(MRI)has high resolution and is rich in temperature-sensitive imaging parameters.Therefore,MRI can non-invasive monitor the temperature distribution during HIFU treatment to determine the focus position,determine the degree of coagulation necrosis at the focus,and whether the boundary is damaged.Currently,existing research assumes that the temperature sensitive parameter has a monotonous and reversible linear relationship with the temperature throughout the HIFU treatment temperature range,and the unique temperature value can be determined based on the temperature sensitive parameter,thereby detecting the tissue temperature distribution.However,studies have found that MRI temperature measurement does not follow a monotonous and reversible linear relationship before and after certain temperature points,and there is an irreversible phenomenon.Under HIFU irradiation conditions,the properties of the tissue will change from one phase to another.And although the deposited energy in tissue continues to increase during the phase transition,the temperature does not increase and the temperature is unable to measure.In the course of HIFU treatment,the tissue undergoes phase transitions around 43°C and60°C,and the temperature measurement near 43°C and 60°C is inaccurate,and the linear relationship of temperature measurement in the tissues before and after phase change will change.Obviously,the existing MRI measurement temperature study did not consider the effect of coagulative necrosis phase change.In order to guide HIFU treatment more effectively,this study will use the longitudinal relaxation time T1 as an example to study the effect of tissue coagulation necrosis phase transition on MRI temperature measurement.ObjectiveThis article will combine the phase transition of the tissue,set up the T1 temperature measurement theory for the phase transition of the tissue,and discuss the effect of the phase transition of the HIFU therapy on the MRI temperature measurement.Methods1.Theoretical analysisThe traditional theory of temperature measurement in the tissue believes that the ratio of free water and bound water does not change in the two-state rapid exchange model.Therefore,the temperature measurement parameter bound water ratio b is regarded as a constant,and activation energy is seen as a constant.Therefore,the linear relationship between temperature-sensitive parameter T1 and temperature is consistent throughout the entire temperature range of HIFU therapy.Considering the tissue phase transition during HIFU treatment,the ratio of free water and bound water in the two-state rapid exchange model will change,so this study combines the parameters in the two-state rapid exchange model based on traditional temperature measurement theory in tissue.The bound water ratio b is regarded as the amount of change,and the activation energy E_a in the theoretical relationship between the T1 of free water and temperature is regarded as the amount of change.A temperature measurement theoretical model for the phase transition of the tissue is established to obtain the tissue temperature.The theoretical relationship is established about the proportion b of the combined water?activation E_a and T1.According to the phase transition of tissue coagulation necrosis,the effects of changes in the ratio of bound water b and activation energy E_a on the linear temperature relationship of T1 parameters were analyzed.It was pointed out that before and after the phase change of the tissue,the T1parameters have different relations with the temperature.And far away from the time of phase change in tissue,Temperature measurement is feasible.The change of tissue-associated bound water ratio b and activation energy E_a during the phase transition has an effect on temperature measurement.2.Experimental verificationBased on the principle of T1 temperature measurement,this study verified the feasibility of T1 weighted image signal temperature measurement of the experimental platform using water mode,and selected suitable magnetic resonance imaging parameters.In vitro tissue experiments were performed to verify theoretical analysis.ResultsThis article shows that during the phase transition of HIFU treatment tissue,the parameters of temperature measurement combined with the proportion b of bound water and the activation energy E_a are in change,and the relationship of temperature measurement is indeterminate.Therefore,there is difficulty in MRI temperature measurement.However,far from the phase change range,the temperature measurement parameters combined with the bound water ratio b and the activation energy E_a can be regarded as constants.The linear correlation between the T1 parameter and the temperature is more than 94%.MRI temperature measurement is feasible.The theoretical analysis is consistent with the experimental results.The tissue heat-denaturation process during HIFU treatment involves two phase transitions around 43°C and 60°C.In order to achieve the goal of guiding HIFU therapy in real-time,it is only possible to rely on measured temperature on the phase transition temperature range to predict.ConclusionThe results of the study showed that the phase transition of tissue coagulation necrosis affects the temperature measurement.In the phase transition process,the temperature measurement parameters the bound water ratio b and the activation energy E_a are in change,the temperature measurement relationship is uncertain,and the MRI temperature measurement is difficult.Before and after coagulation necrosis,b and E_a change from one constant to another constant,and the linear relationship between temperature-sensitive parameters and temperature in tissue has changed.There is a different relationship between temperature-sensitive parameters and temperature before and after phase transition in tissue.