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Applied Radiological Anatomy for Medical Students Applied - part 2

Những thuận lợi và hạn chế của siêu âm Siêu âm cung cấp hình ảnh trong thời gian thực như vậy có thể được sử dụng để di chuyển hình ảnh của cấu trúc như các van tim và các mô hình của lưu lượng máu trong mạch. | An introduction to the technology of imaging THOMAS h. BRYANT and ADAM d. waldman Fig. 1.12. A stone within the gall bladder shows as a bright echo with black acoustic shadow behind it the result of almost complete reflection of the ultrasound hitting it. The fluid in the gall bladder appears black as the contents of the gall bladder are homogeneous and there are no internal structures to cause echoes or changes in attenuation the adjacent liver is more complex in structure and causes more reflection of sound so appears gray. and velocity of flow. Spectral Doppler is a graphical display with time on the horizontal axis frequency on the vertical axis and brightness of the tracing indicating the number of echoes at each specific frequency and therefore blood cell velocity . A combined gray-scale and spectral Doppler display is known as a duplex scan. Power Doppler imaging discards the direction and velocity information but is about 10 X more sensitive to flow than normal color Doppler. Doppler ultrasound is used to image blood vessels and to examine tissues for vascularity fig. 1.13 - see color plate section . Ultrasound contrast agents Contrast agents have been developed for ultrasound consisting of tiny microbubbles of gas small enough to cross the capillary bed of the lungs. These are safe for injection into the bloodstream and are very highly reflective they can be used to improve the imaging of blood vessels and to examine the filling patterns of liver lesions. Ultrasound artifacts Acoustic shadowing Produced by near complete absorption or reflection of the ultrasound beam obscuring deeper structures. Acoustic shadows are produced by bone calcified structures such as gall bladder and kidney stones gas in bowel and metallic structures. Acoustic enhancement Structures that transmit sound well such as fluid-filled structures bladder cysts cause an increased intensity of echoes deep to the structure. Reverberation artifact Repeated bouncing echoes between strong .