ultrasound3 prelim

ultrasound3 prelim - 11.0 Ultrasound Imaging Systems After...

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11.0 Ultrasound Imaging Systems After plane film x-ray, ultrasound is one of the most widely used medical imaging system due to low risk, low cost and portability. Most systems use a single transducer in the so-called pulse-echo format, where the transducer is coupled to the body with an “acoustic gel”. •Block Diagram •Transducer variations and considerations •Single elements, mechanical scanners, electronic scanners, phased arrays •Pulse-echo imaging, envelope detection •Transducer motion, estimation of reflector distribution •Resolution cell, speckle •A-mode, M-mode, B-mode scanning •Beam steering and focusing •Dynamic beamforming and focusing
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Important points from Ultrasound 2 Lecture •Sound attenuation coefficients (dB and Nepers), example of -32.4dB intensity loss at 5MHz in fat. •Frequency dependence of attenuation—typically increases linearly •Doppler shift of sound reflecting off a moving object (blood) •Naming of the various regions of an ultrasound ―beam‖: Near field, far field, Fresnel region, Fraunhofer region. D 2 /λ, w= λz/D •Diffraction formulation of the ultrasound beam pattern—how the beam pattern depends on small dipole radiators on the transducer face and the diffraction pattern they produce •An expression for the received signal involving a superposition of R (reflectivity), n(t) (the complex excitation) and q(x,y) (beam pattern) •Geometric, Paraxial, plane wave, Fresnel and Fraunhofer approximations for q(x,y) •The existence of beam ―sidelobes‖ •Making a focal point by curving the transducer in an arc—leads to more rapid divergence O D f c v f cos 2
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Diffraction Formulation of Beam Pattern ) ) ( ~ Re( ) ( 0 2 t f j e t n t n We model the transmitted pressure pulse as: j e e t n t n ) ( ) ( ~ where the complex envelope is In an ultrasound system, the modulation at f 0 is removed and the detected signal is the envelope, n e (t) z y r x (x,y,z) (x 0 ,y 0 ,0) r 0 Since the transducer is an extended source, we need to model it based on each point being an independent dipole radiator, with the beam pattern being the diffraction pattern of all of the interfering waves. For an individual point: ) ( ) , , , ( 0 1 2 0 r c t n r z t z y x p the dipole differs from the monopole by an extra factor of z/r r r=ct 2μsec 3mm
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0 0.2 0.4 0.6 0.8 1 1.2 The total pressure wave at x, y, z is obtained by integrating across the transducer: 0 0 0 1 2 0 0 0 ) ( ) , ( ) , , , ( dy dx r c t n r z y x s t z y x p  where s(x,y) is the transducer face indicator, { ) , ( y x s 1 for (x,y) in the face 0 otherwise For a dipole radiator the z/r term is an angular efficiency that modifies a spherical wave. As the angle varies from 90 0 to 0 0 the value of z/r (for fixed r) goes from 0 to 1. This shows 1/2 the ―beam‖ directivity from a single dipole point. This pattern is integrated over the face of the transducer to get the full beam. 0
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This note was uploaded on 10/30/2010 for the course MP 230 taught by Professor Macfall during the Fall '10 term at Duke.

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ultrasound3 prelim - 11.0 Ultrasound Imaging Systems After...

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