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1) Acoustic characteristics of microbubble contrast agent The behavior of microbubble contrast agent in ultrasonic sound field is related to the size of microbubbles, the mechanical properties of the outer shell and the sound pressure of incident sound waves: when the applied sound pressure is weak, it mainly presents a linear back direction. Scattering; with the increase of applied sound pressure, the microbubble produces a rich second harmonic, the amplitude of which is close to the fundamental wave, which is more than 1000 times stronger than the second harmonic of human tissue. This feature can be used for second harmonic imaging. ; further increase the sound pressure, microbubbles rupture, gas overflow, showing instantaneous high-intensity signal scattering, called "stimulated sound wave emission".
A new type of microcirculation acoustic contrast agent (microbubble diameter less than 8μm, which can enter the systemic circulation through the pulmonary capillaries) can be divided into two generations: the first generation includes Levovist), Albunex, Echovist; the second generation includes Optison, SonoVue , Definity, Sonazoid, Imagent, PESDA, Aerosomes, Quanfism, etc.
Unlike the first-generation contrast agent in which the gas contained in the microbubble is air, most of the gases contained in the second-generation contrast agent are high-molecular-weight, low-solubility, low-dispersion fluorocarbon gas, so the properties are more stable. The currently used microbubble contrast agents have linear scattering properties, while the intensity of nonlinear scattering depends on the nature of the contrast agent: some contrast agents do not produce measurable second harmonics; all microbubble contrast agents have Excited acoustic emission characteristics.
2. Microbubble contrast imaging imaging using the fundamental wave signal of microbubble contrast agent In the basic gray mode, the contrast agent can increase the linear backscattering of the microvascular pool. In the basic Doppler mode, the ultrasound contrast agent can be Increase the Doppler signal in large and small blood vessels. Studies at home and abroad have shown that ultrasound contrast agents can be used to enhance Doppler signals in liver, kidney, pancreas and other organ lesions, so that blood flow in some lesions that were originally considered to be less vascular is also better displayed.
However, in the basic grayscale mode, the contrast agent does not increase the lesion or substantial microvascular display because the echo from the tissue is too strong and the microbubbles in the microcirculation are too little. In the basic Doppler mode, the image will have obvious bloom2ing artifacts, as well as respiratory and motion artifacts.
Harmonic signal imaging using microbubble contrast agents: Harmonic imaging technology extends the application of ultrasound contrast agents. These technologies mainly include the following.
Reverse pulse harmonic imaging technology: the probe continuously emits two sets of opposite ultrasonic waves. When returning, the fundamental wave (that is, the reflection mainly from the tissue) is cancelled by the opposite phase, and the nonlinear part (that is, the harmonic generated by the microbubble vibration) Wave) is enhanced, which not only improves the sensitivity of the contrast signal, but also obtains better image resolution and is not affected by the heart beat; on the other hand, the reverse pulse harmonic imaging technique is low in mechanical index. The emission of sound waves can prolong the life of microbubbles and avoid intermittent imaging, thus achieving continuous dynamic observation of the perfusion process of the lesions. At the same time, the proximity and remote are enhanced at the same time, and are not affected by the focus setting, so the penetration is significantly improved.
Energy contrast harmonic imaging technology: also known as power Doppler harmonic imaging. Energy Doppler technology can image blood flow at low speed and low energy. The flow velocity of microbubbles in small blood vessels and capillary networks is very low. With energy Doppler plus second harmonic technology, small blood vessels and capillaries can be made. The echo of the contrast agent in the net is easy to display. In the simple energy diagram, the Doppler signal generated by the tissue motion is stronger than the contrast agent when the frequency shift is the same, so the chaotic image is generated. The energy harmonic imaging combined with the energy map and the harmonic can solve the artifact and facilitate the detection of the tissue. Slow blood flow to small blood vessels.
Intermittent harmonic imaging technology: Evoke the probe intermittently or use other methods to intermittently emit ultrasound. Porter et al. found that intermittent imaging can achieve better contrast. The principle is that reducing the number of ultrasound pulse beam emission can reduce the damage of microbubbles by ultrasound, prolong the perfusion time of contrast agent, and increase accumulation in tissues. The intermittent imaging pulse beam emission interval can be controlled by an electrocardiographic trigger or by a gate circuit. The loss-correlation imaging using microbubble contrast agents is also called transient scattering technique and stimulated acoustic emission. The principle is to destroy bubbles with high-energy sound waves, produce strong and short-lived scattered echoes, which can be recorded by Doppler signals to display different color pixels.
3) Contrast injection method The method of intravenous injection of contrast agent is not uniform, including bolus injection and intravenous drip. In the past, when the microbubble contrast agent was used for imaging the fundamental wave signal, the intravenous drip method was used, and in the case of color Doppler imaging, flowering artifacts were avoided, which was favorable for quantitative analysis. Recently, the imaging of harmonic signals using microbubble contrast agents has been used, and the bolus injection method has been frequently used. Shanghai Medical Device Industry Association