Physics of Ultrasound Ultrasound Imaging and Artifacts รศ.นพ.เดโช จ กราพาน ชก ล สาขาหท ยว ทยา, ภาคว ชาอาย รศาสตร คณะแพทยศาสตร ศ ร ราชพยาบาล

Diagnosis TTE TEE ICE 3D 4D Evaluation of Cardiac Anatomy Hemodynamic Assessment 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 One hertz (Hz) = One cycle per second Frequency (f) time = number of cycles Frequency (f) 1 period = 1 cycle 1 cycle Period ( ) Time Frequency (f) = 1 period

Wavelength Wavelength (λ) times frequency (f) equals the propagation velocity (c): c f Propagation velocity in the heart is 1540 m/s, the wavelength for any transducer frequency can be calculated as 1.54 (mm) f (MHz)

Interaction with Tissue: Speed of Sound Compressed (high pressure) Expanded Compressed (high pressure) Expanded Distance As sound travels through tissue it compresses and expands the tissue. Where the tissue is compressed, the speed of sound is higher.

Nonlinear Propagation Compressed (high pressure) Expanded Compressed (hight pressure) Expanded Higher pressure portions of the wave travel faster Distance 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.

Harmonic Imaging Increasing Depth Near Field No harmonics being generated Signal has not traveled enough to be distorted Near Mid Field Harmonics Increasing Harmonics beginning to be produced as signal travels through tissue Mid Field Harmonics Unchanging Additional harmonics generated and attenuated in equal proportion Far Mid Field Harmonics Decreasing Harmonics being attenuated faster than being produced Far Field Fundamental Frequency Only Maximum Harmonic Effect

Constructive Interference Compressed (high pressure) Expanded Compressed (high pressure) Expanded Distance The waves are in phase with each other. The waves reinforce each other, resulting in an intensified sound.

Destructive Interference Distance The waves are out of phase with each other. The waves cancel each other, resulting in a diminished sound.

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) Fundamental signal Harmonic signal

Harmonic Imaging Amplitude Fundamental frequency bandwidth 2 nd Harmonic frequency bandwidth f 0 2f 0 (Velocity)

Ultrasound-Tissue Interaction Scatter Reflection Moving RBC Refraction Attenuation Specular reflector 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 1 C TIB: Bone thermal index TIC: Cranial-bone thermal index TIS: Soft tissue thermal index

Acoustic Output Limits Non-ophthalmic applications I spta.3 720 mw/cm 2 MI 1.9 TI 6.0 Ophthalmic applications I spta.3 50 mw/cm 2 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 pressure Frequency

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 Piezoelectric element Piezoelectric element 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. 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.

Transducer With 20 PZT Elements Piezoelectric elements

Transducer Mechanical annular array Electronic phased array

Ultrasound Beam Side lobe Side lobes Main lobe Main lobe

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

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 2 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)

Lateral Resolution Lateral resolution = F D

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 Amplitude High Pass Filter Tissue Blood Frequency (Velocity)

Doppler of Tissue Amplitude Low Pass Filter Tissue Blood Frequency (Velocity)

Doppler Filter Amplitude Transitional zone Pass Pass Boost Pass band Pass band Boost Cut Cut Low Pass Filter High Pass Filter (Wall Filter) Frequency

High Pass Filter (Wall Filter) Amplitude Transitional zone Pass Boost Cut Pass band Boost Cut Low Pass Filter High Pass Filter (Wall Filter) Frequency

Low Pass Filter Amplitude Transitional zone Pass Boost Pass band Cut Low Pass Filter Frequency

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 Frequency Low amplitude Time

Spectral Display Frequency Mid amplitude Time

Spectral Display Frequency High amplitude Time

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 line 2. Sound travels directly to a reflector and back 3. Sound travels in soft tissue at exactly 1540 m/s 4. Reflections arise only from structure positioned in the beam s main axis 5. The imaging plane is very thin 6. The strength of a reflection is related to the characteristics of the tissue creating the reflection (Understanding Ultrasound Physics. Sidney K Edelman, 4 th 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 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)

Ultrasound Beam Side lobes Main lobe (RadioGraphics 2009; 29:1179 1189) 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 lobe Main Side lobe

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 image Caused by a near shadow in the middle of the probe footprint

Enhancement