Lecture 1 Notes

Attenuation 1 db cm mhz 60mhz attenuation 60 dbcm

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Unformatted text preview: uation leads to a decrease in amplitude of the ultrasound signal: Ultrasound radiated away from transducer Attenuation ~ 1 dB / cm / MHz Attenuation: An Example What relative amplitude of a 60 MHz ultrasound signal do you expect to receive from a depth of 5 mm? Attenuation ~ 1 dB / cm / MHz @ 60MHz: Attenuation ~ 60 dB/cm Attenuation: Consequences   Consequences of frequency dependent attenuation for imaging: –  Penetration of ultrasound is limited by frequency –  Frequency of ultrasound decreases with increasing depth of imaging Depth = 5 mm: Ultrasound propagates through 1 cm Attenuation ~ 60 x 1 = 60 dB 1/1000 of the transmitted signal is received! Resolution in Ultrasound Imaging   Axial Axial Resolution: –  Resolution in propagation direction –  Determined by length of pulse propagating in tissue   Lateral Resolution: Lateral Axial Resolution   Axial Resolution: Axial Resolution = pulse width (s) x speed of sound (m/s) /2 = N λ/2 –  Resolution orthogonal to propagation direction –  Determined by focusing properties of transducer Nλ Resolution vs Penetration Lateral Resolution   Lateral 2a Resolution: f-number = focal length/aperture = f/2a Lateral Resolution = wavelength x f-number = λf/2a     Resolution (axial and lateral) with frequency Penetration with frequency f Compromise between resolution and penetration Doppler Ultrasound: Basic Concepts   Ultrasound wave reflected from moving targets (Blood cells)   Frequency shift in received ultrasound wave compared to transmitted wave: Doppler Shift Fre...
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