Hunting Bats. Diagnostic Ultrasound. Ultrasound Real-time modality

Transcription

1 Diagnostik Ultrasound Basic physics, image reconstruction and signal processing Per Åke Olofsson Dpt of Biomedical Engineering, Malmö University Hospital, Sweden Ultrasound Real-time modality 17-WEEK FETAL PROFILE Ultralyd kursus Føtal Medicin 5-7 oktober 2005 Rigshospitalet KøbenhavnK Severe hypertrophy Diagnostic Ultrasound - Introduction Diagnostic Ultrasound Hunting Bats Basic Physics Pulse Echo Method 2-D Acquisition and Image Reconstruction Doppler Principle CW and PW Doppler Colour Doppler Diagnostic Ultrasound - Biosonar 1

2 Sound propagation in a media Classification of sound Infrasound < 20 Hz Audible sound Ultrasound Hz > Hz Bats khz Surgical ultrasound khz Diagnostic ultrasound 2 15 MHz Longitudinal sound wave propagation wavelength λ 2

3 Biological Non biological Acoustic properties Medium Speed of sound (m/s) Blood 1566 Brain Fat 1446 Kidney 1567 Liver 1566 Muscle Bone Distilled water 1480 Air 333 Perpex 2670 Aluminium 6260 Brass MHz λ = 0,52 mm 6 MHz λ = 0,26 mm 10 MHz λ = 0,15 mm Wave equation λ = c / f λ = wavelength f = frequency c = speed of sound Acoustic Impedance Characteristic Impedance Reflection Z = ρc ρ = density c = propagation speed of sound I reflected = I incident Z Z Z 1 + Z1 ρ2c2 ρ1c 1 = ρ2c2 + ρ1c1 2 3

4 Different types of reflections Scattering Speckle or texture Destructive interference Echo interference Constructive interference 4

5 Attenuation DGC Depth Gain Compensation TGC Time Gain Compensation Amplitude Transducer Pulse Echo Principle Skin Tissue 1 Tissue 2 Tissue 3 d 1 d 2 t 1 c = 1540 m/s t = d x 2/c or d = t x c/2 t 2 1 cm s time/depth Pulse Echo Method A-mode B-mode M-mode Slow sweep Distance Time Reflection at tissue boundaries Distance proportional to time In B-mode amplitude controls intensity 5

6 Piezo electric effect Longitudinal / axial resolution Effect of beamwidth on lateral resolution Increased frequency decreased lateral width but.. Increased frequency more attenuation 6

7 Linear Array Transducer Multiple identical crystal elements Image Acquisition Multi element transducer array Sequential excitation of crystal elements Curved linear array Transmit focusing 7

8 Receive focusing 2-D D sector Transmit one line Receive along the same line Steer to second line Sequentially scans the complete sector Computer for control of tranmit and recieve Phased Array Phased Array Transducer Acoustic signals Electric signals Analog Digital Pulse amplitude Object Digital Time delay Transmitter Excitation pulses Array Steering controlled by time difference between excitation signals for adjacent elements Steering both in transmit and receive mode Variable steering angle Small footprint 8

9 Summation of frequencies - c + c - c + c + = Frequencies could be added and separated according to Fourier theory Tissue Second Harmonics generation P B 2 A + 2 π f ( ) 2ρν 3 d pac2 Second harmonic intensity in tissue A B parameter Water 4,2-6,1 Water 30C 1 atm 5,5 Blood 6,3 Liver 7,6 Spleen 7,8 P2 = second harmonic intensity A B f ρ ν d = nonlinerar parameter for media = frequency of insonification = density = acoustic velocity = distance Fat 11,1 pac = applied acoustic pressure 9

10 Non-linear relation Reverberation Applied pressure Fundamental Harmonic Harmonics Grating lobes fundamental vs harmonic Fundamental Tissue Harmonic Grating lobes fundamental Main lobe Grating lobes harmonic Fundamental beamprofile Harmonic beamprofile after amplification Harmonic beamprofile 10

11 Fundamental Tissue Harmonic Fundamental Tissue Harmonic 11