Harmonics, Contrast Agents, Hemodynamics Flashcards
What is fundamental frequency?
Frequency created by the transducer and transmitted into the body
What is harmonic frequency?
Twice the fundamental frequency
What is harmonic imaging?
Creation of an image using sound reflections at twice the frequency of the transmitted sound
What kind of behavior do harmonic waves display?
Nonlinear behavior
What is a fundamental image?
Imaged created by processing reflections that have the same frequency as the transmitted wave
What are the benefits of harmonic imaging?
- Improves poor image quality
- Waves undergo less distortion
What are the two forms of harmonics?
Tissue and contrast
What is linear?
- Proportional or symmetrical
- Behaves in an even manner
What is nonlinear?
- Irregular or disproportionate
- Asymmetric
- Behaves unevenly
- Harmonics utilize nonlinear behavior
What is tissue harmonics?
- Sound waves travel into the body
- Small amount of energy converted from fundamental freq to harmonic freq
How is the shape of the sound beam altered?
By variations in the speed of sound traveling through tissue
What happens to the harmonic wave strength as sound travels through tissue?
Strength grows
Where are the best harmonics produced?
Mid field
Where are little harmonic components generated?
Near field
Where are harmonics attenuated faster than produced?
Far field
Why is harmonic imaging most useful in improving poor image quality?
Harmonic frequency waves undergo less distortion than fundamental sound waves
What nonlinear behavior creates tissue harmonics?
Sound travels faster through compressions, slower through rarefactions in soft tissue
What occurs in the first few centimeters of tissue during fundamental imaging?
Significant amounts of artifact
Why does significant artifact occur in the first few cm of tissue during fundamental imaging?
- Beam is very strong
- Many different superficial anatomic layers distort the sound beam
Where do tissue harmonics develop? What is the benefit of this?
- Develop deeper in tissues
- Do not distort
- “free ride” through superficial tissues & don’t contain noise
How do harmonics affect signal-to-noise ratio?
Increase it
Is there another form of nonlinear behavior that further minimizes distortion with tissue harmonics?
- Relationship b/w beam strength & harmonic creation also nonlinear
- Weak beams do not create harmonics
- Intermediate strength beams create small amount of harmonics
- strong beams create significant harmonics
What are contrast agents?
Microbubbles, gas bubbles in a shell
IV or swallowed
How do the acoustic properties of contrast agents compare to bio tissues?
Contrast agents create strong reflections that light up blood chambers, vessels, etc
What five requirements do contrast agents need to meet?
- safe
- able to be metabolized
- long lasting
- strong reflector
- small enough to pass through capillaries
How are contrast harmonics created?
- Microbubbles act in a nonlinear manner when struck by sound waves
- Created during reflection as energy is converted from fundamental to harmonic freq
What nonlinear behavior of a microbubble creates contrast harmonics?
Resonance: when a microbubble is within a sound beam, it grows & shrinks in relation to pressure variations
How does the size of a microbubble change based on pressure?
Compression: bubble shrinks & pressure inside increases, stabilizes and resists further compression
Rarefaction: bubble expands to a greater extent than it shrinks
What is most important in creating contrast harmonics?
Peak rarefaction pressure
What is the mechanical index?
Estimates amount of contrast harmonics produced, depends on freq of transmitted sound & rarefaction pressure of sound wave
The mechanical index increases with what?
Lower freq sound and stronger sound waves (large pressure variation)
What is the relationship between low MI (<0.1) and harmonics?
- No harmonics
- Backscatter
- Linear behavior
- Higher frequency sound
- Low beam strength
- Bubble expands very little
What is the relationship between MI between 0.1-1.0 and harmonics?
- Some harmonics
- Resonance
- Nonlinear behavior
- Lower frequency sound
- Higher beam strength
- Bubble expands moderately
What is the relationship between MI > 1 and harmonics?
- Strongest harmonics
- Bubble disruption
- Extreme nonlinear behavior
- Lowest frequency sound
- Highest beam strength
- Bubble expands greatly
What two characteristics of contrasts agents are important when used with harmonic imaging?
- Nature of outer shell
- Gas that fills microbubble
How do the outer shell and the type of gas in the microbubble determine the contrast agents stability?
- Shells trap gas
- Shells are flexible, rigid shells fracture
- Larger gas molecules remain trapped, cannot permeate shell as easily
How do the strengths of tissue and contrast harmonics compare?
Contrast harmonics are much stronger than tissue harmonics
Summarize tissue harmonics (4)
- Created during transmission in tissue
- Occurs as sound propagates in tissue
- Results from nonlinear behavior of transmitted sound beam
- Weaker harmonic signal
Summarize contrast harmonics (4)
- Created during reflection off of microbubble
- Occurs only when contrast agents are present & with MIs > 0.1
- Results from nonlinear behavior of microbubble
- Stronger harmonic signal
What is hemodynamics?
Study of blood moving through the circulatory system
What is flow?
AKA volume flow rate
Indicates volume of blood moving during a particular time
Units/time
What is velocity?
Speed of fluid moving from one location to another
What is pulsatile flow?
When blood moves with variable velocity
Blood accelerates & decelerates with cardiac contraction
Arterial
What is phasic flow?
Blood moves with variable velocity
Blood accelerates & decelerates due to respiration
Venous
What is steady flow?
Fluid moves as a constant velocity
Venous circulation when breathing is stopped
What is laminar flow?
- Normal
- Flow streamlines are aligned and parallel
- Layers of blood traveling at individual speeds
What is plug flow?
All of the layers & blood cells travel at the same velocity
What is parabolic flow?
Bullet shaped, highest velocity in center of lumen, gradually decreases to its minimum at vessel wall
What is the Reynolds number?
Predicts whether flow is laminar or turbulent, for laminar flow it is less than 1500
What is turbulent flow?
Chaotic flow patterns, many directions, many speeds
Small hurricane like patterns
Assoc with CV pathos
What sounds are associated with turbulent flow?
Murmur or bruit, thrill
What is the Reynolds number for turbulent flow?
> 2000
How does blood flow based on an energy gradient?
Moves from a region of high energy to region of low energy
When is energy given to blood?
During cardiac contraction cycle
What are 3 types of energy?
Kinetic, pressure, gravitational
What is kinetic energy?
Associated with moving object
Determined by object’s mass & speed
Is the kinetic energy of a ping pong ball or a golf ball greater?
Golf ball because of its mass
What is pressure energy?
- Stored or potential energy of an object
- Ability to perform work
What is gravitational energy?
- Stored or potential energy of an object
- Associated with any elevated object
- Identical objects at the same height have the same gravitational energy
What happens to the energy of blood as it travels through the circulatory system?
It loses energy
What is viscous loss?
Thickness of a fluid, the thicker the fluid the more energy is lost
What is frictional loss?
- Occurs when flow energy is converted to heat
- One object rubs against another (RBC-RBC or vessel wall)
What is inertial loss?
- Objects in motion stay in motion, objects at rest stay at rest
- Energy is lost when speed changes (acceleration or deceleration)
When does inertial loss occur?
During pulsatile flow, phasic flow & in a stenosis
What is the effect of stenosis on flow?
- Changes in direction as blood flows in & out of narrow portion of vessel
- Increased velocity within stenosis
- Post-stenotic turbulence
- Pressure gradient: pressure is lower after stenosis than before
- Conversion of pulsatile to steady flow
Where is velocity highest in a vessel with stenosis?
Where the vessel is narrowest
What is Bernoulli’s Principle?
- Relationship between velocity & pressure in a moving fluid
- “With steady flow, the sum of all forms of energy is the same everywhere”
- Sum of kinetic & pressure energy remains constant
What factors are important in pressure-flow relationships?
Elasticity of vessels, composition of blood & pulsatility of heart contractions
How do you calculate pressure gradient?
flow x resistance
Pressure gradient increases when what increases?
flow or resistance
Flow increases when what increases OR what decreases?
When pressure gradient increases OR resistance decreases
What are characteristics of veins?
- Thin walled, easily compressible, easily respond to pressure changes
- Low pressure, partially blood-filled, partially expanded
- Low resistance, passive vessels
What are the effects of exercise on venous flow?
- Increases venous flow
- Change from hourglass to oval to round to accomodate volume increase
- Resistance to flow decreases even more, allows more flow back to heart
- Once volume passes, vessel collapses
Why does breathing affect venous flow?
- Venous system is low pressure
- Muscles responsible for respiration alter pressures in chest & abdomen
Breathing affects which two venous flows?
- Venous flow in legs
- Venous return to heart which compromises venous flow from the head & arms and flow from IVC to heart
What structure has a large role in the effects of breathing on venous flow?
Diaphragm
How does inspiration impact venous flow?
- Diaphragm moves down & chest expands
- Decreases pressure in chest, creates suction that increases venous return to heart
- Venous flow from head, arms, SVC increases
- Abdomen compresses, increases pressure in abdomen, decreases venous flow from legs
How does expiration affect venous flow?
- Diaphragm moves up
- Chest compresses
- Increases pressure in chest, reduces venous return to heart from head, arms, SVC
- Abdomen expands, decreases pressure in abdomen, increases venous flow from legs
What is hydrostatic pressure?
Pressure related to the weight of blood pressure on a vessel measured at a height above or below the heart
Where is hydrostatic pressure optimal?
At the level of the heart
When supine, how is hydrostatic pressure affected?
- All body parts are at same level as heart
- Hydrostatic pressure at every level/location is 0
How does standing affect hydrostatic pressure?
- At heart level: accurately represents true circulatory pressure
- Below heart: hydrostatic pressure is positive
- Above heart: hydrostatic pressure is negative
