Research On Myocardial Elastography Based On Ultrasound RF-Data

With the improvement of people’s eating habits and living standard, myocardial infarction has become a serious harm to human health. And myocardial infarction is coronary atherosclerosis, which can lead to a severe stenosis of the vascular lumen, will result in Myocardial ischemia. In this situation, if the blood supply to the blood vessel is further reduced or interrupted directly, then the acute myocardial ischemia time will be 1 hours or more, and the myocardial infarction will occur. As we know, the incidence of myocardial infarction increases with age, the people in 65-75 ages has the highest proportion, but now the proportion in young people is getting higher. By the way, the myocardial infarction seriously affects people’s normal life, limit people’s activities, reduce the quality of life, and even cause sudden death.The pathological change of human biological tissue has a great relationship with the change of tissue hardness. So, tissue elasticity has important reference value in the diagnosis and treatment of diseases. Usually, the soft tissue elasticity can be measured by palpation, a very important diagnosis technology. The palpation employs the hardness of diseased tissue as a judgement of benign and malignant. However, it mainly relies on the subjective judgment of the physician, and only can be used on the relatively larger lesions and surface tissue, which makes it works not so effective. With the development of the technology of ultrasonic elastography, the ultrasonic elastography gradually replaces the palpation. The ultrasound elastography was proposed by J. Ophir et al. in 1991, from that moment, it always be paid many attention in the field of medical imaging. It changes the soft tissue lesion detection from qualitative to quantitative judgment, and has a great performance in the diagnosis of the lesions of soft tissue, also there are some clinical experiments to verify its accuracy The basic principle of elasticity imaging is:using the ultrasonic probe for vertical pressing surface, collecting the organization of RF signal before and after compression, analyzing the signal, calculating the displacement field image in the organization to get the strain distribution in the organization. Different from the traditional medical imaging technologies, such as CT, MRI, which can’t directly provide information of tissue elasticity, the elastography utilizes the information of tissue elasticity. Due to the advantages of Noninvasive, simple, easy operation and low cost, the elastography now widely be used in the clinical. As one of the most important topic in the field of medical imaging, the elastography now can be used to detect the lesion in thyroid, breast, prostate, liver and blood vessel.With the development of hardware and software, the ultrasonic myocardial elastography imaging technology is gradually growing up. Many researchers try to calculate myocardial elastography by using radio frequency signal. And the experiments show that compared to the gray map, the RF signal has a great advantage in the myocardial elastography. The image information contained in the RF is more original, and can save more information of the organization. In order to verify the authenticity of the RF signal, it usually needs to reconstruct the RF signal. The direct method that translating the RF signal to gray images is a very populate.The concept of dynamic programming method is to solve the subproblem of unresolved issues. Dynamic programming method can save counting time, so the total experiment has a short operating time. Dynamic programming method is limited to the organization’s integer displacement calculation, which cannot satisfy the small deformation of the organization. Rivaz proposed the analytic minimization method based on the dynamic programming method to improve the precision of displacement calculation.Color strain image can display the biomechanics of information of organization, but the strain diagram shows the results of the whole image. In the strain map, myocardial tissue and other tissues cannot be distinguished, which cannot display the elasticity of myocardial area. The overall mapping method was proposed based on the above background to achieve the goal of observing the myocardium strain in real-time.Because of the disease model accuracy is related to the accuracy of the experimental study, the animal disease modeling process needs to verify the credibility of the result. By the way, many clinical studies scholars ligate the left anterior descending of rats to induce myocardial infarction. And experimental results show that this method can be applied to clinical practice. Using the dynamic programming and analytical method to calculation the displacement of normal rat and myocardial infarction based on ultrasonic RF signal to track the elasticity of myocardial infarction area dynamically.This paper makes experiments about the ultrasonic myocardial elasticity imaging in the following three aspects.①The study of the algorithm in tracking the displacement and strain of myocardial tissue.Ultrasonix Sonix RP system is an open integrated ultrasound system, which employs the Microsoft(?) Visual Studio(?) in Personal Computer (PC), and can save the B type ultrasound gray-scale image, ultrasound video signal and ultrasonic signal (RF signal). The ultrasonic probe model is C5-2/60, its center frequency is 5 MHz, and the detection depth is 15 cm. In this experience, the experimental object is a normal adult male,24 years old, with supine position. To get the left ventricular long axis image of heart, the ultrasonic probe is placed on the left side of the sternum after the experimental object has a stable heartbeat.After using software to import ultrasound RF signal, first I re-construct the B type ultrasound image, then down sample the RF signal on the longitudinal direction and up sample on the transverse direction by interpolation, to make sure that the re-constructed image is same with the B type ultrasound image shown on the Ultrasonic equipment.To ensure the accuracy of the data, the data was detecting by a clinical ultrasound doctor. Then I adopt the dynamic programming method to calculate the difference of myocardial positions before and during the myocardial contraction. After that, to calculate the displacement, I choose the analytic minimization method, which can accurate to the decimal level. At last, the displacement image can be got by the polynomial fitting method. By using the proposed method, the displacement value of the region of myocardial can be calculated in real-time, and the method can mark the region of myocardial in the gray-color image with the value which can give provide a clear and real-time image to observe the myocardial elasticity.㏑esearch of rendering methods to draw the strain image of myocardiumBy using the mathematic formulation to decide if the point is in the manual delineation of myocardium region on the cardiac ultrasound gray image, we can get a binary image in which the values of the points in the region is 1 and else is 0. Then, we can get the positions of all points whose value is 1. After that, we translate the gray image to RGB image, via repeat the gray value in the red, green and blue channel. Follow that, we translate the RGB values in the strain image to gray values which have the same position with the gray image. Lastly, we cover the values in the cardiac ultrasound gray image of the myocardium region with the corresponding values in the strain image. Here, we get a composite image of gray image and strain image, which both can show the anatomical structure of the heart and can display the myocardial elasticity information.③The preliminary study on the ultrasonic elastography based on animal models of myocardial infarction animal.Vevo (?) 2100,a small animal real-time ultrasonic molecular imaging system, is produced by the Visual Sonics company.The imaging system is a high frequency and high resolution digital imaging platform with a linear array technology and color doppler technology. Vevo (?) 2100 can store a variety of data, including IQ signal, radio frequency signal, images and video signal.The experimental object is six healthy rats (200g-250 g). Firstly, the mixed gas anesthesia machine was used to anesthetize the healthy rat. Pour isoflurane into evaporation tank after connecting oxygen bomb for the induction of anesthesia (using the concentration of 2-3%). Then fix limbs of rats on ECG heating plate in supine position and keep rats body temperature at about 37℃ (using the concentration of 1.5-2%). Removing hair around the heart using small animal razor and shaving the small hair using depilate cream. Finally, probe MS-250 of Vevo (?) 2100 (frequency for the 21 st MHz, depth of 22 cm) was placed near the sternum of rats to get the left ventricular long axis image.Myocardial infarction operations were conducted for six healthy rats. Intraperitoneal injection method (0.4 mL/100 g) was used to hocus rats with a concentration of 10% chloral hydrate. Removing hair around the heart using small animal razor and shaving the small hair using depilate cream. Insert the breathing machine into the trachea to keep breathing. Cut up the skin and muscle of heart and fasten the blood vessels and muscle near the left anterior descending branch with surgery thread. The raising in ST wave of ECG suggests that the operation is a success. Then layered suture the muscles and skin, and keep breathing machine in the trachea at the same time. There hours later, probe MS-250 of Vevo (?) 2100 (frequency for the 21 st MHz, depth of 22 cm) was placed near the sternum of rats to get the left ventricular long axis image.The normal and myocardial infarction RF signal were processed to achieve the strain map based on the study①and②. Three hours after the scan, the rat was killed by neck fracture (or less than six hours natural death).Open the chest and cut off the heart completely. Then the heart was steeped in physiological saline to Wash away the blood clot and freezed in the fridge for 30 minutes. After freezing, the heart was cut in short axis into at least five pieces evenly (about 2 mm thickness of each piece), Put the piece of heart into 1% TTC solution and cover with foil to avoid light. About two or three minutes after the biopsy, take out the heart and put them into 10% formaldehyde solution in the fuming cupboard. Ten hours later, take out the sliced and observe the results. The result of the reaction in normal muscle is red and the result of the reaction in infarction muscle is white, infarcted myocardium can be due to the loss of dehydrogenase with a white, which can validate the myocardial infarction model in rats was successful or not.The following conclusions were achieved through the above research.① Dynamic myocardial elastography based on dynamic programming and analytic minimization methodBy using the ultrasound RF signal to reconstruct the gray images, the dynamic programming and the analytic minimization method to calculate the displacement, the polynomial fitting method to get the strain images, and the overall mapping method to color the gray images with the strain images of myocardial region, the proposed method can get a colored ultrasound RF strain image to track the course of heart contraction.② Using the global mapping method to draw the strain image of myocardium fastThe proposed global mapping method can render the irregular region of myocardium very fast. If there are 106 pixels in the myocardial region, it only cost 0.24s to draw the colorful strain image of myocardium. By the way, the rendering time relates with the size of target region. Cause in the contraction process, the myocardial elasticity is different in each stage, our method can track the change of myocardial elasticity.③ Establishing myocardial infarction rat model to evaluate the normal and infarction myocardial elasticityIn this experience, we detected the heart of normal rats and the rats which has myocardial infarction using the high frequency probe in the Visual Sonics Vevo(?)2010 Imaging system. By observing the result, it can be concluded that the myocardial infarcted rats have a narrower myocardial than normal rats,which is the same with the ultrasonic images. However, it is not obvious of the myocardial infarction region in myocardial elastic graph.In short, Ultrasonic imaging technology plays an important role in the prevention and treatment of heart disease. In this paper, the dynamic programming and analytic minimization methods were used to track small deformation of myocardial contraction process based on ultrasonic RF signal. The overall mapping method was proposed to draw grayscale and color ultrasound strain and evaluate normal and infarction myocardial elasticity. The research results laid a foundation for the further research in the future.