The Research Of Myocardial Metabolism/Perfusion Imaging And Ultrastructural Damage In Experimental Dogs With Radiation Induced Heart Disease

Objective:RIHD（Radiation induced heart disease）is one of the delayed adverse reactions caused by thoracic radiotherapy.RIHD increased the rate of cardiac mortality,and had partly offset the survival benefit provided by adjuvant radiotherapy.In recent years,RIHD has attracted more and more attention,but there is no any specific diagnostic methods and monitoring indicators for RIHD.This study intends to explore the method for early diagnosis and monitoring of RIHD.1.To build the model of radiation induced heart disease（RIHD）in Beagle dogs and noninvasive method to evaluate it.2.To analyze the effects of different pre-scan preparation protocols on the physiologic myocardial uptake of ¹⁸F-FDG in Beagle dogs,and to determine the appropriate experimental conditions for this study.3.The changes of the myocardial metabolism after local heart irradiation were monitored by ¹⁸F-FDG PET/CT imaging under 12 h fasting followed by high fat diet preparation protocol,and the changes of myocardial ultrastructure were observed at the same time.Then the relationship between the changes of myocardial metabolism and pathological damage and the mechanism of myocardial metabolic changes were analyzed and to determine the potential value of ¹⁸F-FDG PET/CT in the monitoring of RIHD.4.Using the ¹³N-ammonia PET/CT MPI to monitor the changes of myocardial perfusion and left ventricular function.The corresponding myocardial pathological damage was observed.And to analyze the relationship between the changes of myocardial perfusion and pathological damage and the mechanism of RIHD,and to aessess the potential value of ¹³N-ammonia PET/CT MPI for detection ofmyocardial perfusion changes at early stage induced by radiation damage.Methods:1.Eight Beagle dogs were selected to perform ¹⁸F-FDG PET/CT myocardial metabolism imaging before irradiation.Then all animals had a control-enhanced CT simulation scan,and the anterior wall of the left ventricle was outlined as the radiation target,which received a single dose of 20 Gy irradiation with 6 MV-X-ray.Three months after irradiation,¹⁸F-FDG PET/CT myocardial metabolism imaging were performed again under the same conditions,and the changes of myocardial metabolism in the irradiation field were observed before and after irradiation.Then the ratio of the irradiated area to the non-irradiated area SUV（SUVig/SUVnig）was then computed.It is considered that the model is successful if the myocardial metabolism in the irradiation area is increased.The animals were sacrificed,and the gross pathological changes of the irradiated area and the non-irradiated area were observed by naked eyes.For each heart,myocardial tissue was taken from irradiated area and non-irradiated area respectively at the middle level of the left ventricle,myocardial and vascular injury was observed under light microscope.2.Twenty-four Beagle dogs were randomly divided into four group: short fasting group（SF,12 h）,short fasting followed by intravenous injection of glucose supplement group（SF-GS）,prolonged fasting group（PF,18 h）,or short fasting followed by high fat diet group（SF-HF）,and each group included six Beagle dogs.Then all dogs performed ¹⁸F-FDG PET/CT scan,the image qualityclassified into four grades: Grade 0,no FDG uptake;Grade 1,faint FDG uptake;Grade 2,definite FDG uptake;Grade 3,significant FDG uptake.The ROI was drawn on the myocardium of left ventricle and SUVmax was measured.The blood glucose 1evel was measured before ¹⁸F-FDG injection.The degree of myocardial ¹⁸F-FDG uptake was evaluated,and the differences of myocardial ¹⁸F-FDG PET/CT image quality,SUVmax and blood glucose level were compared between four groups.3.Thirty-six Beagle dogs were randomly divided into the control（n = 18）or the irradiation groups（n = 18）.The irradiation group underwent local irradiation to the left ventricular anterior cardiac wall with a single dose of 20 Gy,whereas the control group received sham irradiation.All dogs underwent ¹⁸F-FDG PET/CT myocardial metabolism imaging under 12 h fasting followed by high fatdiet preparation protocol at one week before irradiation and at three,six,and twelve months after sham or local irradiation.Then the ratio of the irradiated area to the non-irradiated area SUV（SUVig/SUVnig）was then computed.After completing ¹⁸F-FDG PET/CT examination,six randomly selected Beagle dogs from each group were sacrificed and used to detect myocardial ultrastructural injury at three,six,and twelve months after irradiation.4.Thirty-six Beagle dogs were randomly divided into the control（n = 18）or the irradiation groups（n = 18）.The irradiation group underwent local irradiation to the left ventricular anterior cardiac wall with a single dose of 20 Gy,whereas the control group received sham irradiation.All dogs underwent ¹³N-ammonia PET/CT MPI one week before irradiation and at three,six,and twelve months after sham or local irradiation.One week after completing ¹³N-ammonia PET/CT MPI examination,the irradiation group underwent coronary angiography examination（CAG）.Six randomly selected Beagle dogs from each group were sacrificed and used to detect pathological cardiac injury at three,six,and twelve months after irradiation.Results:1.According to the results of ¹⁸F-FDG PET/CT myocardial metabolism imaging and pathological examination,eight Beagle dogs suffered from RIHD.Compared with before irradiation,three months after irradiation,the metabolism of the left ventricle anterior wall of the Beagle dogs was significantly increased,and the area of increased metabolism was consistent with the radiation target.Three months after irradiation,the cardiac surface was pale in the irradiated area of all the eight Beagle dogs,and one dog had the pericardial effusion,one dog had local pericardial adhesion in the irradiated area.Myocardium cut in cross-section,we found that the texture of the myocardium in the irradiated field was hard,and showed with greyish white scar.Pathological examination showed that myocardial degeneration,blood vessels swelling and increased peripheral permeability in the irradiated area.2.In the SF-GS group,myocardial imaging was clear and homogeneous（Grade 2: two dogs,Grade 3: four dogs）.While in the SF group,myocardial imaging was uneven,and the differences of imaging quality between these dogs in this group were significant（Grade 2: four dogs,Grade 1: one dog,Grade 3: one dog）.In the PF group（Grade 0: three dogs,Grade 1: two dogs,Grade 2: one dog）and SF-HF group（Grade 0: four dogs,Grade 1: two dogs）,there was nearly no FDG uptake.There were significant differences in the quality of myocardial ¹⁸F-FDG imaging between these groups（H = 16.83,P< 0.01）.The SUVmax in PF group（3.01±0.97）and SF-HF group（2.84±1.15）were significantly lower than those in SF-GS group（14.76±4.72）and SF group（10.91±2.48）（F = 69.84,P< 0.01）.The average blood glucose levels in PF group（4.18±0.27 mmo L/L）and SF-HF group（4.25±0.58 mmoL/L）were significantly lower than those in SF-GS group（5.80±0.56 mmoL/L）and SF group（4.91±0.51 mmoL/L）（F = 13.58,P < 0.01）.3.There was nearly no FDG uptake in the myocardium of all dogs in the control and irradiation groups before irradiation and in the control group at three,six,twelve months after irradiation.Compared with the control group and before irradiation,in the irradiation group,the myocardial metabolism was increased in the irradiated area and the SUVig/SUVnig was increased at three,six,twelve months after irradiation.The area of increased myocardial metabolism was located within the irradiation field,and its scope gradually reduced with time.Electron microscopic examination showed that the myocardial ultrastructure in the irradiated area was damaged at three months after irradiation,and the degree of ultrastructural damage was further aggravated at six months after irradiation,and reparative changes of ultrastructure were observed at twelve months after irradiation.4.Compared with the control group and baseline,the irradiation group showed significantly increased perfusion in the irradiated area of the heart at three months after irradiation,perfusion reduction at six months after irradiation,and perfusion defect at twelve months after irradiation.There was no significant difference in the left ventricular ejection fraction（LVEF）values between the control and irradiation groups at baseline and at three months after irradiation.The irradiation group showed a reduction of LVEF compared with the control group at six（50.0 ± 8.1% vs.59.3 ± 4.1%,P= 0.016）and twelve months（47.2 ± 6.7% vs.57.4 ± 3.3%,P = 0.002）after irradiation.No coronary stenosis was observed in the irradiation group.No obvious abnormal ventricular wall motion was observed in two group animals at three months after irradiation.Regional ventricular wall-motion abnormalities were commonly observed in or adjacent irradiation area,and involved 5/20 segments at six months after irradiation and involved 11/20 segments at twelve months after irradiation.Pathological changes were observed,radiation-induced myocardial degeneration,microvascular damage and interstitial fibrosis was progressively increased with time prolonged in the irradiated area.Conclusions:1.A stable and reliable RIHD model can be constructed in the left ventricular anterior wall with a single dose of 20 Gy irradiation.¹⁸F-FDG PET/CT can be used to verify whether the RIHD model was successfully constructed.2.The results of ¹⁸F-FDG PET/CT myocardial metabolism imaging were different depending on the different pre-scan preparation protocols.According to the purpose of the ¹⁸F-FDG PET/CT examination,different pre-scan preparation methods can be used to enhance or suppress the myocardial physiological ¹⁸F-FDG uptake,so as to improve the imaging quality and avoid the misdiagnosis and missed diagnosis.3.¹⁸F-FDG PET/CT examination was helpful to the early detection and diagnosis of RIHD,and the abnormal myocardial FDG uptake in the irradiated area suggested that there may be RIHD,which should be closely followed up.4.¹³N-ammonia PET/CT MPI can detect myocardial perfusion abnormalities and left ventricular dysfunction induced by irradiation at very early stage of RIHD.¹³N-ammonia PET/CT MPI may be a valuable method for monitoring and evaluating RIHD.