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Vascular Medicine and Endovascular Interventions phần 4

Lỗi thuộc về tại: Eur J VASC Endovasc Surg. 1998; 16:456. Fiessinger JN, Schafer M. dùng thử của iloprost so với aspirin trong điều trị thiếu máu cục bộ chi quan trọng của thromboangiitis tắc nghẽn: Nghiên cứu TẠO. | CHAPTER 7 Arterial Testing in the Vascular Laboratory The Doppler shift is defined by the equation Af 2f V cos0 C where Af is the difference between the frequency of the transmitted and reflected ultrasound waves Doppler-shifted frequency f is the frequency of the transmitted ultrasound waves V is the velocity of the blood cells 0 is the angle between the incident ultrasound beam and the direction of blood cell motion Doppler angle and C is the speed of sound in tissue 1 540 m s . The constant 2 in this equation accounts for the round trip path traveled by the sound waves from the source to the reflectors and back to the source. Continuous-Wave and Pulsed-Wave Doppler In CW Doppler separate transmitting and receiving transducers operate simultaneously. Because a single transducer cannot transmit and receive at the same time a CW Doppler instrument must have two transducers. The simplest Doppler instruments used in vascular diagnosis are CW devices that provide an audio output of the frequency shift through earphones or a loudspeaker. These are satisfactory for most arterial physiologic tests but a direction-sensing Doppler instrument is necessary for more sophisticated testing. Although some CW instruments have directional capabilities they cannot identify flow at a specific depth or site in tissue. A CW Doppler signal can actually represent a combination of signals obtained from flow at various points along the path of the ultrasound beam. Interpretation of these Doppler signals can be difficult if vessels are superimposed within the ultrasound beam or if complex flow disturbances are present within a single vessel. These disadvantages have been overcome by the development of PW Doppler systems and more sophisticated techniques for Doppler signal analysis. With PW ultrasonography flow can be detected at discrete points along the ultrasound beam eliminating the problem of superimposed signals. In a PW Doppler system a single transducer alternates between .