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