314 Q1
ULTRASOUND
1. Ultrasound
- Coined term:
- Greek word Graphene — to write and record
- Latin word Sonus — sound
- A.K.A. sonography
- Uses non-ionizing radiation, exposing POI with very high frequency sound waves.
- Real-time evaluation of soft tissues to rule out something in the POI.
Advantages
- Availability
- Most common modality to be used
- Low cost
- Cheaper from other modalities
- Patient friendliness
Some Applications
- Obstetrics and gynecology
- Urology
- Cardiology
- Internal medicine
- MSK system
ULTRASOUND BASICS
2. Waves
- Oscillation accompanied by a transfer of energy that travels through medium or vacuum.
- Has cyclical variations.
Cycle
- Any sequence of changes in molecular motion.
Propagation
- Transmittal to distant regions remote from the sound source.
Two Kinds of Propagation
- Rarefaction
- Compression
- These form sound waves.
Two Types of Waves
Mechanical Wave
- Needs a medium in order to be transmitted.
- Requires a medium to travel.
- Cannot travel in a vacuum.
- Moves through matter by causing molecules to vibrate successively along their path.
- Carries energy, not matter.
- Travels in straight lines.
Electromagnetic Wave
- Works in both a medium or vacuum.
SOUNDWAVES
3. Sound
- Sound is a mechanical energy that is transmitted by pressure waves through a medium.
- Sound is a mechanical energy that propagates through a medium through compression and rarefaction.
Propagation
- Transmittal to distant regions remote from the sound source.
Types of Sound Waves
Longitudinal or Compression Waves
- Motion of the particles in a medium is parallel to the direction of wave propagation.
- Particles in a medium oscillate in the same direction as the wave.
Transverse or Shear Waves
- Motion of particle in a medium is perpendicular to the direction of the wave propagation.
- Particles oscillate perpendicular to the direction of the wave.
Types of Mechanical Waves
- Longitudinal wave
- Particles in a medium oscillate in the same direction as the wave.
- Transverse wave
- Oscillates perpendicular to the direction of the wave.
Compression and Rarefaction
Compression
- Mechanical deformation
- Increase in pressure
- Positive amplitude of wave
- Compressed particles transfer energy.
Rarefaction
- Occurs after compression.
- Reduction in pressure.
- Negative amplitude of wave.
Series of Compressions and Rarefactions
- Wave front
- Longitudinal wave
Echoes
- Reflections of the incident energy pulse.
Medium of Sound
- Solid
- Liquid
- Gas
Speed of Sound
- Solid > Liquid > Gas
- Sound is transmitted faster in solid than in gas.
ULTRASOUND VS. EMR
- Ultrasound
- Non-ionizing
- Mechanical energy
- Longitudinal waves
- Needs matter in order to be transported
- Cannot travel in a vacuum
- X-ray / EMR
- X-ray is ionizing radiation.
- Electromagnetic wave
- Can travel through a vacuum.
- Can travel through solid, liquid, and gas.
HUYGENS' PRINCIPLE
- All transmitted waves emit smaller wavelengths that also emit sound waves.
- Huygens' principle states that every point in the surface can be modeled as a source emitting ultrasonic waves.
ACOUSTIC VARIABLES
Pressure
- Units of pressure:
- Pascal (Pa)
- N/m²
- atm or atmosphere
- 760 mmHg
- mmHg or millimeter mercury
- kg/m²
- pounds/inches squared
- 14.7 psi or atmospheric pressure at sea level is equal to 100,000 pressure amplitude.
- Formula:
- PA = 1 N/m²
- Diagnostic ultrasound beam:
- 1 MPa
Density
- Mass of the medium per unit volume.
- Concentration of medium particles.
Units
- kg/m³
- g/cm³
Temperature
Units
- Celsius
- Fahrenheit
- Kelvin
Frequency
- Normal audible sound:
- 20 to 20,000 Hz
- Cycles per second
- Infrasound:
- Low-frequency sound below 20 Hz.
- Ultrasound:
- Above 20,000 Hz or 20 kHz.
- Frequency greater than the upper limit of the human hearing range.
- Diagnostic ultrasound:
- 3 to 10 MHz
Particle Motion
- Distance
- Particle motion
- Particle displacement within a wave propagation
PROPERTIES OF SOUND WAVES
Wavelength
- Distance traveled by one cycle.
- Distance between rarefaction and compression.
- Extent of one complete wave cycle.
- Distance between two successive equivalent density zones:
- Two compression zones, or
- Two rarefaction zones.
Units
- mm
- micrometer
Ultrasound Wavelength
- Determined by frequency and speed.
- Determines spatial resolution along the direction of the beam.
Formula
- Wavelength = propagation velocity / frequency
Frequency
- Frequency, or pressure oscillations:
- Number of complete cycles per second.
- Number of cycles or pressure oscillations occurring at a given point in one unit of time, usually one second.
Unit
- Hertz (Hz)
Formula
- f = 1/T
Relationship
- As time increases, frequency decreases.
Period
- Time duration of one cycle.
- Inversely proportional to frequency.
- Elapsed time between compression zones as the wave passes through one point in the medium.
Relationship
- As f increases, T decreases.
Units
- Second
- Microsecond
- Millisecond
Formula
- T = 1/f
Amplitude
- Change in magnitude of physical entity.
Pressure Amplitude
- Caused by particle displacements and pressure variations in the propagation medium.
- Peak, maximum or peak minimum value from the average pressure on the medium.
Unit
- Pa
Formula
- PA = 1 N/m²
PROPAGATION VELOCITY
- Speed at which the pressure wave moves through the medium.
- Also called acoustic velocity.
Formula
- c = wavelength × frequency
- c = bulk modulus / density
Bulk Modulus
- Stiffness of a medium and resistance to compression.
- Build modulus.
- Negative ratio of stress and strain.
Compressibility and Propagation Velocity
- High compressibility → low propagation velocity.
- Gas has the highest compressibility.
- The atoms are widely spaced.
- Stiffness:
- Low stiffness
- High compressibility
- Low stiffness, high propagation velocity.
- This relationship is true if the stiffness is held constant.
Density and Propagation Velocity
- High density → low propagation velocity.
- Because of the law of inertia.
Materials and Their Propagation Velocity
Material| Propagation Velocity
Air| 330 m/s
Lung| 600 m/s
Fat| 1450 m/s
Water| 1480 m/s
Soft tissues| 1540 m/s
Kidney| 1565 m/s
Liver| 1555 m/s
Muscle| 600 m/s
Bone| 4080 m/s
- The speed at which a wave propagates through a medium depends on the compressibility and density of a medium.
OTHER RELEVANT TERMINOLOGIES
Power
- Rate of energy production, absorption, or flow.
- Unit is watt (W).
- 1 watt equals 1 joule of energy per second.
- Energy over time.
- Rate at which ultrasound energy is transmitted or transferred.
Intensity
- Amount of power per unit area.
- The larger the area evaluated, the lower the intensity of the beam.
- Amount of ultrasound power transmitted to an area.
- Proportional to the square of pressure amplitude.
Formula
- I = Power / Area
PHYSICAL PRINCIPLES OF ULTRASOUND
Wave Interference Patterns
Constructive
- Two waves with the same frequency and phase result in a higher amplitude output wave.
- The two interfering waves have a displacement in the same direction.
- Compression lines up with compression.
- Rarefaction lines up with another rarefaction.
Destructive
- Waves out of phase result in a lower amplitude output wave.
- The two interfering waves have a displacement in opposite direction.
- A compression from one wave may coincide with a rarefaction from another.
Complex
- Wave of different frequencies interact, resulting in areas of higher and lower amplitude.
- Waves with different frequencies continuously change their relationship.
- Combination of constructive and destructive.
Summary
- Constructive — combined
- Destructive — diminished
- Complex — changing
Importance
- Destructive, constructive, and complex are the factors why there is a sound wave.
- This forms a non-uniform ultrasound beam.
PIEZOELECTRIC EFFECT
Piezoelectric Effect
- Conversion of electrical energy to mechanical energy.
- It is the transmission of the sound beam.
- Conversion of mechanical energy:
- Receiving the reflected beam information.
- Piezoelectric crystal emits sound waves.
- Ultrasound waves return from the organ.
- Creation of sonogram on screen.
Piezoelectric Materials
- Crystalline materials composed of dipolar molecules.
Natural Materials
- Quartz
- Naturally occurring crystals
- Commonly used
- Tourmaline
- Rochelle salt
Synthetic Materials
- Lead zirconate titanate (PZT)
- Most commonly used synthetic
- Barium titanate
- Lead metaniobate
- Ammonium dihydrogen phosphate
- Lithium sulfate
CONTINUOUS WAVE
- Requires a minimum of two transducer elements.
- One element is always transmitting while the second is always receiving.
- Ultrasound is always on.
- In ultrasound, we use pulse waves because continuous wave sometimes cancel out the wave, but we use its concept.
PULSE WAVE
- Pulse waves require only one transducer element.
- Acts both as receiver and transmitter.
- Ultrasound beam is intermittently transmitted.
- Majority of its time is listening time.
- Creates anatomic images.
- Uses short bursts of ultrasound.
- Collection of cycles that travel together:
- On time or transmit time
- Off time or receive time
Characteristics
Characteristics described by all of the continuous wave description plus:
- Pulse duration
- Spatial pulse length
- Pulse repetition period
- Pulse repetition frequency
- Duty factor
CONTINUOUS WAVE CHARACTERISTICS
Characteristics described by:
- Frequency
- Period
- Propagation speed or acoustic velocity
- Wavelength
- Amplitude
- Power
- Intensity
PULSE DURATION
- Pulse duration is equal to period × number of cycles in the pulse.
- Time from the beginning to the end of the pulse.
Formula
- PD = Period × Number of cycles in the pulse
SPATIAL PULSE LENGTH
- SPL is equal to wavelength × number of cycles in the pulse.
- Physical length of the pulse.
Formula
- SPL = Wavelength × Number of cycles in the pulse
PULSE REPETITION PERIOD
- Time from the beginning to the end plus listening time.
- Time from the onset of the pulse until the start of the next pulse.
DUTY FACTOR
- Percentage of the time that transducer is emitting sound waves.
Formula
- DF = PD / PRP
Continuous Wave
- Duty factor value of continuous wave is 1.
PULSE WAVE INTENSITIES
Most common intensity units for pulse-wave intensities:
- ISPTP
- Spatial peak temporal peak
- ISPTA
- Spatial peak temporal average
- ISATP
- Spatial average temporal peak
- ISATA
- Spatial average temporal average
- ISPPA
- Spatial peak pulse average
- ISAPA
- Spatial average pulse average
SOUND PROPAGATION MEDIA PROPERTIES
Elasticity
- Ability of an object to return to its original shape and volume after a force is no longer acting on it.
- In ultrasound, when there is no pressure affecting the sound waves, it will return to its original value.
- Demonstrated by a return to its original value or shape when the pressure is lowered to its initial value.
Density
- Mass of the medium per unit volume.
- Concentration of medium particle.
Compressibility
- Fractional decrease in a volume when pressure is applied to a material.
Bulk Modulus
- Negative ratio of stress and strain.
Stress
- Force per unit area applied to an object.
Stiffness
- Resistance of the medium to compression.
Decibel
- Relative sound intensity.
INTERACTIONS OF ULTRASOUND WITH MATTER
Absorption
- Conversion of energy of sound waves to heat within the medium.
- Increases when frequency increases.
- Reduces the amount of energy there is to reflect.
- Reduces penetration and signal strength because it is already converted to sound energy.
- Can cause thermal biological effects.
- It is the only process whereby sound energy is dissipated in a medium.
Reflection
- Return of incident ultrasound energy as an echo directly back to the transducer when interacting at a boundary with normal incidence.
- The way the ultrasound is reflected is determined by the size of the surface features of the interface.
Reflection Coefficient
- A fraction of the incident energy reflected by the acoustic interface.
- Occurs at an interface between two dissimilar materials.
- Caused by atomic number.
- Happens when there is acoustic impedance.
Acoustic Impedance
- Explains how particles of a substance behave when subject to pressure waves.
- Measures the resistance to sound passing through a medium.
- Related to stiffness and flexibility of a compressible medium.
Formula
- Z = ρc
Unit
- kg/m²·s
Impedance Mismatch
- Differences between acoustic impedances at interfaces determine the amount of energy reflected at the interface.
SPECULAR REFLECTORS
- Large and relatively smooth interfaces reflect sound like a mirror.
Examples
- Diaphragm
- Urine-filled bladder
- Endometrial stripe
Important Points
- Echoes return to the transducer only if the sound beam is perpendicular to the interface.
- If the interface is not at a 90-degree angle to the sound beam:
- The echo will be reflected away.
- It will not be detected.
DIFFUSE REFLECTORS
- Smaller interfaces in the body where most echoes arise.
- Echoes are scattered in all directions.
- Non-specular reflection.
- Medium is bigger than the sound beam.
- Almost same with scattering but different in the medium.
Echoes
- Sound reflected back to the transducer are called echoes.
- Return of incident ultrasound energy directly back to the transducer when interacting at a boundary.
- Transducer must be 90 degrees to the POI to avoid scattering.
- The way the ultrasound is reflected is determined.
- This principle is what allows the image to be generated by the ultrasound machine.
- More reflection means more hyperechoic images.
- The sound beam is directed at right angles to a smooth interface, larger than the width of the sound beam.
SCATTERING
- This is specular reflection, responsible for creating the internal echoes of the organ.
- Ultrasound speckle is produced by the constructive and destructive interference of sound scattered by small tissue reflectors.
- A weak reflection mechanism.
Examples
- Kidneys
- Pancreas
- Spleen
- Liver
- Smaller media than sound waves.
REFRACTION
- Changing direction of the sound wave when passing through tissues with different propagation velocities.
- Important because it is one of the causes of misregistration.
- The echo detected and displayed in the image is coming from a different depth or location than is shown in the display.
ATTENUATION
- Loss and transfer of energy to tissue as sound passes through it.
- Determines the efficiency with which ultrasound penetrates specific tissues.
Attenuation Coefficient
- dB/cm
- Relative intensity loss per centimeter of travel for a given mediuvm.
Attenuation Tissue Coefficienty
- Property of the degree of sound wave attenuation in specific tissues.
Formula
- Attenuation = Absorption + Scattering + Reflection
Important Factor
- We consider frequency.
ULTRASOUND TERMINOLOGIES
Anechoic
- No echo
- Fluid
- Bile
- Blood
Hypoechoic
- Low echogenicity
- Fats
Isoechoic
- Similar echogenicity
Hyperechoic or Echogenic
- Moderate to high echogenicity
- Stone and fibrous plaques
- Calcified
- Strongly echogenic with acoustic shadow
- Stones and bone
Inhomogeneous or Heterogeneous
- Not uniform
- Diverse in echogenicity
Complex
- Mixed echogenicity
- Solid and cystic components