physics of ultrasound ultrasoundimaging and artifacts · ultrasound physics. wave motion vs...
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Physics of UltrasoundUltrasound Imaging and Artifacts
รศ.นพ.เดโช จกัราพานิชกุลสาขาหทยัวทิยา, ภาควชิาอายรุศาสตร์ คณะแพทยศาสตรศ์ริริาชพยาบาล
Diagnosis‐TTE‐ TEE‐ ICE‐3D‐ 4D
Evaluation ofCardiac Anatomy
HemodynamicAssessment
Multidimensional Echocardiography
Ultrasound Physics
Wave Motion VS Circular Motion
Ultrasound Waves
•Diagnostic medical ultrasound typically uses transducers with a frequency between 1-20 MHz
• Humans can hear sound waves with frequencies between 20 Hz and 20 KHz• Adult echocardiogram: 2-4 MHz• Pediatric echocardiogram: 4-8 MHz
Frequency
Frequency (f) time = number of cyclesFrequency (f) 1 period = 1 cycle
One hertz (Hz) = One cycle per second
Frequency (f) = 1period
Time
Period ()1 cycle
Wavelength
Wavelength (λ) times frequency (f) equals the propagation velocity (c):
c f
1.54 (mm) f (MHz)
Propagation velocity in the heart is 1540 m/s, the wavelength for any transducer frequency can be calculated as
Interaction with Tissue:Speed of Sound
• As sound travels through tissue it compresses and expands the tissue. • Where the tissue is compressed, the speed of sound is higher.
Compressed(high pressure)
ExpandedCompressed(high pressure)
Expanded
Distance
Nonlinear Propagation
• As the wave passes through tissue, the top of the waveform gets pulled forward to be non‐sinusoidal shape.• Propagation in which speed depends on pressure and the wave shape changes is called “nonlinear
propagation”. • Harmonic frequency is generated from this nonlinear propagation.
Compressed(high pressure) ExpandedCompressed
(hight pressure)Expanded
Distance
Higher‐pressure portionsof the wave travel faster
Harmonic Imaging
Near FieldNo harmonics being generated
Signal has not traveled enough to be distorted
Near Mid FieldHarmonics Increasing
Harmonics beginning to be produced as signal travels through tissue
Mid FieldHarmonics Unchanging
Additional harmonics generated and attenuated in equal proportion
Far Mid FieldHarmonics Decreasing
Harmonics being attenuated faster than being produced
Far FieldFundamental Frequency Only
Increa
sing De
pthMaxim
um
Harmonic Effect
Constructive Interference
• The waves are “in phase” with each other.• The waves reinforce each other, resulting in an “intensified” sound.
Distance
Compressed(high pressure)
ExpandedCompressed(high pressure)
Expanded
Destructive Interference
• The waves are “out of phase” with each other.• The waves cancel each other, resulting in a “diminished” sound.
Distance
Harmonic Signal• Fundamental ultrasound signals are generated from the transducer passing the fat
layer twice (transmit and receive)• Harmonic signals are generated from the tissue and transmitted to transducer
passing through the fat layer once (on receive)
Harmonic signal
Fundamental signal
Harmonic Imaging
Amplitude
(Velocity)f0 2f0
Fundamental frequency bandwidth
2nd Harmonic frequency bandwidth
Ultrasound-Tissue Interaction
Specular reflector
Moving RBC
Reflection
AttenuationRefraction
Scatter
Small structures (< 1 wavelength in lateral dimension) result in scattering
Biologic Effect of Ultrasound
Medical Ultrasound Safety:Adverse Biological Effects
•Thermal bioeffects•Heating of soft tissue and bone
•Nonthermal bioeffects•Cavitation
Thermal Index (TI)
•The ratio of attenuated acoustic power at a specified point to the attenuated acoustic power required to raise the temperature at that point in a specific tissue model by 1C
• TIB: Bone thermal index• TIC: Cranial-bone thermal index• TIS: Soft tissue thermal index
Acoustic Output Limits
•Non-ophthalmic applications• Ispta.3 720 mW/cm2
•MI 1.9•TI 6.0
•Ophthalmic applications• Ispta.3 50 mW/cm2
•MI 0.23•TI 1.0
Mechanical Index (MI)
•Mechanical bioeffects that occur when a certain level of output is exceed
Mechanical index (MI) = Peak negative pressureFrequency√
Output Display•Thermal index (TI)
•Soft tissue (TIS)•Bone (TIB)•Cranial bone (TIC)
•Mechanical index (MI)•Range 0.0 to 1.9
Acoustic Impedance (Z)
•Measure of how ultrasound traverses the medium and depends on• Density of the medium (p)• Propagation velocity of ultrasound through the medium (v)
Z = pv
Acoustic Impedance
Fluid Soft tissue
Fibrous tissue Solid (calcium)
Applied Ultrasound
Ultrasound Transducer
•Device that converts a signal in one form of energy to another form of energy•Mechanical rotating transducer (single element)•Electronic phased array transducer (multiple elements)
• Linear array• Annular array• Matrix array• Circular array• Curved array
Piezoelectric Effect
Piezoelectricelement
Piezoelectricelement
Inverse piezoelectric effect converts electrical energy into mechanical energy.
• Electrical voltage causes a change in length or vibration of piezoelectric material to generate a sound wave.
Direct piezoelectric effect, also called generator or sensor effect, converts mechanical energy into electrical energy.
• Mechanical stress generates an electric charge proportional to that stress.
Transducer With 20 PZT Elements
Piezoelectricelements
Transducer
Electronic phased array
Mechanical annular array
Ultrasound Beam
Main lobe
Side lobe
Main lobeSide lobes
Ultrasound Beam
(RadioGraphics 2009; 29:1179–1189)
Side Lobes
•Secondary and smaller acoustic beams falling outside at predictable angle located around the main lobe
•Created by a single crystal transducer
Ultrasound Images
Motion mode Brightness mode
Amplitude mode
Ultrasound Images
Motion mode Brightness mode
Amplitude mode
Creating a Two-dimensional Image
• Ultrasound beam is electronically steered through a sector arc of 80°
• Speed of imaging rate of 25 frames/s.
Creating a Two-dimensional Image
The time needed to acquire all the data for one image frame is directly related to the number of scan lines
There is tradeoff between scan line density and image frame rate
Depth VS Time elapsed
Timing is proportional to distance from the transducer
Resolution
•The ability to distinguish two objects that are close together• Temporal resolution
• The ability to accurately locate structures or events at a particular instant in time• Spatial resolution
• Axial resolution: the ability to distinguish two objects that are close together along the axis of the ultrasound beam
• Lateral resolution: in the direction perpendicular to the beam’s axis
Temporal Resolution
•Dependent on frame rate •Can be improved by
• Minimizing depth - the maximum distance from the transducer as this affects the PRF
• Narrowing the sector to the area of interest - narrowing the sector angle• Minimize line density (but at the expense of lateral resolution)
Axial Resolution
• The ability to recognize two different objects at slightly different depths from the transducer along the axis of the ultrasound beam
where SPL = λ × no. of cycles
• It is improved by• Higher frequency (shorter wavelength) transducers but at the expense of penetration• Maximize line density (at the expense of frame rate, i.e. temporal resolution)
2
Lateral Resolution
Lateral resolution = FD
Ultrasound Imaging
Medical Image Orientation
Gain Control
•The degree of amplification of the returning ultrasound signal•Affects all parts of the image equally•Seen as a change in “brightness” of the images on the entire screen
Time Gain Compensation (TGC)
Depth
Signal Amplitude
Reject•Filters out low signal decrease noise in the image
No reject High reject
Dynamic Range (Compression)
•Determines the number of gray shades used to map the gray scale image on the display
• Higher more shades of gray (softer looking image)
• Lower fewer shades of gray (sharp looking image)
Wide Dynamic Range
Narrow Dynamic Range
High Compression
Low Compression
Harmonic Imaging
Type of Doppler Study
•Pulse wave Doppler•Doppler information is generated from a small “gate” that is interrogated
•Continuous wave Doppler•One crystal constantly sends, another constantly receives•Allows evaluation of very high velocity
Color flow Doppler
• This is a variety of pulse wave Doppler. Multiple points in the ‘region of interest’ are analysed and colour coded rapidly.
Doppler Filter
•Doppler signal from tissue VS blood•Blood flow has low amplitude but high frequency•Myocardial motion has high amplitude but low frequency
•High pass filter (Wall filter)• Remove low frequency signals from the display• Display Doppler signal from blood flow only
•Low pass filter• Remove high frequency signals from the display• Display Doppler signal from tissue only
Doppler of Blood
Frequency
Amplitude
Tissue
Blood
(Velocity)
High‐Pass Filter
Doppler of Tissue
Frequency
Amplitude
Tissue
Blood
(Velocity)
Low‐Pass Filter
Doppler Filter
Frequency
Amplitude
Boost
Cut
Pass band Pass band
Transitional zone
Boost
Cut
High‐Pass Filter(Wall Filter)
Low‐Pass Filter
PassPass
High Pass Filter (Wall Filter)
Frequency
Amplitude
Boost
Cut
Pass band
Transitional zone
Boost
Cut
High‐Pass Filter(Wall Filter)
Low‐Pass Filter
Pass
Low Pass Filter
Frequency
Amplitude
Boost
Cut
Pass band
Transitional zone
Low‐Pass Filter
Pass
Doppler Display
• The perceived returning frequency is lower than the transmitted frequency, it will be plotted below the zero baseline as a negative Doppler shift
• The spectrum displays echo amplitude by varying the brightness of the display
Spectral Display
Low amplitude
Frequency
Time
Spectral Display
Mid amplitude
Frequency
Time
Spectral Display
Frequency
Time
High amplitude
Sampling of Received Doppler Signal
Sampling Frequency
Aliasing
Aliasing of Doppler Signal
•Erroneous display of velocities that have exceeded the Nyquist limit• The velocity exceeds the rate at which the pulsed wave system can record it properly
•Spectral trace is cut-off at a given velocity with placement of the cut section in the opposite channel or reverse flow direction
Nyquist Frequency
Measurements of frequency shifts (and, thus, velocity) will be appropriately displayed only if the pulse repetition frequency (PRF) is at least twice the maximum velocity (or Doppler shift frequency)
encountered in the sample volume
The Nyquist Limit
• A sampled waveform thus needs at least two sample points per cycle. • Thus the wave's frequency must not be above half the sampling frequency. • This limit is called the Nyquist limit of a given sampling frequency
Basic Assumption Used in Imaging Systems
1. Sound travels in a straight line2. Sound travels directly to a reflector and back3. Sound travels in soft tissue at exactly 1540 m/s4. Reflections arise only from structure positioned in the beam’s main axis5. The imaging plane is very thin6. The strength of a reflection is related to the characteristics of the tissue
creating the reflection
(Understanding Ultrasound Physics. Sidney K Edelman, 4th ed 2012)
Artifacts
•Caused by violation of the assumptions•Direction
• Ultrasound path between transducer and reflector is not in straight line• Transmit path• Receive path
•Reflection•Reflection from side lobe ultrasound beam•Speed is not correct•Attenuation
Artifacts due to Direction of Ultrasound
Refraction
Mirror Image
Reverberation• A is the true anatomical structure strongly reflective of ultrasound• A1 is the artefact generated by the returning ultrasound beam being re-reflected from the
transducer and again reflected from the true anatomical structure at A.
As this doubly reflected beam has travelled twice as far it is seen at twice the distance from the transducer. It is weaker but otherwise identical to the real structure at A
A A1
Reverberation
Reverberation
Reverberation
Reverberation and Shadowing
Comet Tail (Ring Down Artifact)
•Reverberation with the spaces squeezed
•The reflecting surfaces are located in a medium with a very high propagation speed, such as mechanical heart valve
•Appearance of “solid hyperechoic line” directed downward
Comet Tail (Ring Down Artifact)
Comet Tail (Ring Down Artifact)
Ultrasound Beam
(RadioGraphics 2009; 29:1179–1189)
Main lobe
Side lobes
http://folk.ntnu.no/stoylen/strainrate/Ultrasound/#Depth_res
Side Lobe Artifacts
•Created by the emitted side lobes reflecting to a strong reflector, laterally positioned targets
•Erroneously displayed and interpreted by the machine as if they originated from the central ultrasound beam
•The depth of the artifact depends on time taken to and from transducer•The intensity of the artefact decreases with distance. •Common sources of the artifact are
• The atrioventricular groove• The fibrous skeleton of the heart
Side Lobe Artifact
Side lobeSide lobe Main
Side Lobe Artifact
Side Lobe Artifact
Side Lobe Artifact
Side Lobe Artifact
Artifacts due to Speed of Ultrasound
Range Ambiguity Artifact
•“All reflections are received by the transducer before the next pulse is transmitted.”
• If a reflection is created by a very deep structure and arrives at the transducer after the next pulse has been transmitted
•This very deep reflection will be interpreted as being created by the second pulse and is placed at a very shallow depth
Range Ambiguity Artifact
Artifacts due to Attenuation of Ultrasound
Acoustic Shadowing
Edge Shadow
Spit Image•If the near shadow is in the center of the probe, the ultrasound beam is splitted in two, resulting in two apparent apertures
Spilt imageCaused by a near shadow in the middle of the probe footprint
Enhancement
Enhancement