ultrasound4 prelim

ultrasound4 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 3 Lecture •Transducer materials: Quartz, Barium Titanate, PZT •The basic concept of the piezoelectric effect •Transmit and Receive constants—d (m/V), g (V-m/N) •Resonant frequency f T =c T /2d T •Q of a transducer (losses, f/Δf), need for damping •Transmit/Receive switching, diode switches •Single transducer, sector scanners, linear arrays •Used knowledge of narrowness of pulse to simplify r(t) (received signal) to identify compensatable signal loss vs “z” (depth), using range equation: z=ct/2 •TGC, Time Gain Compensation •Key reflectivity equation: • Resolution cell, speckle •A-mode, M-mode, B-mode
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Bushberg, Essential Physics of Medical Imaging, 2 nd Ed,2002.
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Bushberg, Essential Physics of Medical Imaging, 2 nd Ed,2002.
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B-Mode Scan Makes an image by displaying R(x,y,z) using knowledge of the position and angle of the transducer at each pulse-echo. -manual modes >linear scanning moves the transducer in one direction >compound scanning allows angling the transducer while moving to see behind specular reflectors, building up an image from several directions -mechanical modes >linear array >mechanical sector scanner >phased array -sector scanners and compound scanning require conversion from polar to rectangular coordinates scan conversion
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Bushberg, Essential Physics of Medical Imaging, 2 nd Ed,2002.
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(80dB, typical) Frequency (MHz) Depth of Penetration (cm) 1 40 2 20 3 13 5 8 10 4 20 2 (based on a~1dB/cm/MHz)
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f R =afc/L=1dB/cm/MHz*2MHz*150000cm/s/80=3750/sec=3750Hz
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for f R =3750 pulses/sec and N=256, f F =14.6 frames/sec restricts depth of penetration……. .
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example: In a B-mode system, with N=256, 80dB sensitivity, c=1540m/s, a=1dB/cm/MHz, what choice of working frequency will allow a frame rate of 15/sec. ans: The required pulse repetition rate is T
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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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ultrasound4 prelim - 11.0 Ultrasound Imaging Systems After...

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