The Physics of Echo. The Physics of Echo. The Physics of Echo Is there pericardial calcification? 9/30/13

Transcription

1 Basic Ultrasound Physics Kirk Spencer MD Speaker has no disclosures to make Sound Audible range 20Khz Medical ultrasound Megahertz range Advantages of imaging with ultrasound Directed as a beam Tomographic Reflected from small objects Non-ionizing Disadvantages Propagates poorly through air Penetration poor (attenuation) λ = wavelength = v/f v= velocity f = frequency cycle Velocity of sound α density and temperature 1,540 m/sec soft tissue Frequency 3.5 MHz (1-7 MHz) λ = (1,540 m/sec)/(3.5 MHz) = 0.44 mm 1

2 Scattering echoes originating from relatively small, weakly reflective, irregularly shaped objects object size > λ/4 not angle dependant inefficient Reflection echoes originating from relatively large, regularly shaped objects with smooth surfaces objects large α wavelength angle dependant valve, endocardium, pericardium Is there pericardial calcification? 2

3 Resolution: Lateral resolution - the ability to resolve objects side by side Structures must be separated by more than the width of the ultrasound beam to be distinguished as separate Transducer size (larger better) Frequency (higher better) Focusing Gain (lower better) Beam width Low gain Medium gain High gain Resolution: Axial Resolution - Axial resolution is the ability to resolve objects that lie along the path of the ultrasound beam Related to frequency of transducer and pulse duration In practical terms, axial resolution is roughly twice the wavelength Frequency Wavelength 2.2 MHz 0.68 mm 3.5 MHz 0.43 mm 5.0 MHz 0.3 mm 3

4 Axial resolution vs penetration frequency leads to better resolution Penetration wavelength (1/ frequency) More scattering (more reflection by smaller scatterers) More attenuation Penetration Resolution Attenuation Loss of ultrasound energy as it passes through tissue (scattering and absorption) half-power (cm) Water 380 Blood 15 Soft tissue 1-5 Muscle Bone Air 0.08 Resolution vs penetration frequency leads to better resolution Penetration wavelength (1/frequency) Use the highest frequency probe that gives you an adequate image 2.5 Mhz MHz 4

5 Permanently-polarized material such as quartz (SiO2) will produce an electric field when the material changes dimensions as a result of an imposed mechanical force. These materials are piezoelectric, and this phenomenon is known as the piezoelectric effect. Conversely, an applied electric field can cause a piezoelectric material to change dimensions. This phenomenon is known as electrostriction, or the reverse piezoelectric effect This shape deformation creates ultrasound waveforms Scanning Mechanical transducers Rotating multiple elements, or a single element and set of acoustic mirrors to generate the sweeping beam for 2D imaging Electronic / array transducers Have the ability to be steered by sequentially stimulating each element. This feature creates the sector scan by rapidly steering the beam from left to right to give the two dimensional cross sectional image. 5

6 Electronic / array transducers Linear array Sequential firing or array elements moves beam linearly Require large acoustic window Creates a linear or rectangular shaped scan plane Electronic / array transducers Phased array Phased control of array firing controls beam direction (and thus scan line) Creates a sector or pie shaped scan plane. Imaging Electrical stimulate piezoelectric crystal which sends ultrasound pulse 0.4 µsec Transducer then listens for returning ultrasound signals Transducer listens 99 percent of time, which increases sensitivity 1-2 µsec 6

7 Modes: A Mode - amplitude mode. Where the signals are displayed as spikes that are dependent on the amplitude of the returning sound energy. B Mode - brightness mode. Where the signals are displayed as various points whose brightness depends on the amplitude of the returning sound energy. Modes: M Mode - motion mode. The application of B-mode and a strip chart recorder allows visualization of the structures as a function of depth and time. Modes: 2D Mode - 2 dimensional mode. The spatially oriented B-mode where structures are seen as a function of depth and width. The beam is rapidly swept back and forth to create a cross section of the imaged structures. 7

8 Shadowing The loss of information behind an object because the sound energy was reflected back by the object such that no signal passes beyond it Bone, metal valve, air Artifacts - sidelobe Ultrasound reflections off real objects, but from the ultrasound beam sidelobes, not the central beam Occurs because ultrasound beam has width to it Worse when gain is high Is there a catheter in RA? 8

9 Artifacts - reverberations Multipath artifacts Sound bounces back and forth between two interfaces. This prolongs the time of flight, producing an artifact deep to the interface. Artifacts - reverberation Results from ultrasound strikes a target composed of several highly reflective interfaces Appear as relatively parallel irregular bright lines extending from the structure Artifacts - reverberation Appears as a linear brightness in the direction of the sound beam and deep to a strong reflector Results from multiple back and forth reflections Appear as relatively parallel irregular bright lines extending from the structure 9

10 Harmonics Depth Mechanical Index Triggering Compression Frame rate Focus PRF Transducer frequency Packet size Gain Post-processing Grayscale / power Doppler Overall gain Increases the intensity of received echoes Makes image brighter Depth Use the least depth that fits the structure of interest on the screen 10

11 Nonlinear distortion of ultrasound Amplitude Fundamental Amplitude Fundamental Harmonics Frequency Frequency Harmonic imaging Lateral resolution Smaller harmonic beam width Clutter reduction Sidelobe levels decrease with increasing harmonic number Near field artifact reduction Amplitude On-Axis Reflectors Off-Axis Reflectors Fund Harm Frequency MHz Doppler Effect Christian Johann Doppler 1842 If a source of sound is stationary, the wavelength and frequency of sound emanating from the source are constant If a source of sound is moving toward you, it s wavelength is decreasing (frequency increasing) If a source of sound is moving away from you, it s wavelength is increasing (frequency decreasing ) 11

12 Dependence on angle between scatter and incident ultrasound beam Doppler echocardiography Continuous wave Separate transmit and receive transducer Continuously receiving No maximal velocity limit Range is ambiguous 12

13 Doppler echocardiography Continuous wave Doppler echocardiography Pulsed wave Range gated Doppler Inability to detect high frequency Doppler shifts Inability to detect high velocities 13

14 Doppler echocardiography Color Doppler Multiple pulsed Doppler samples along each scan line Doppler echocardiography Color Doppler Velocities colored coded Blue- away, Red - toward 14