US Vocabulary 2
Terms
undefined, object
copy deck
- Intensity Transmission Coefficient
- The percentage of intensity that passes in the forward direction when the beam strikes a boundary
- Refraction
- A change in direction of wave propogation when traveling from one medium to another, transmission with a bend
- Snell's Law
- A prediction of the amount of refraction that will occur, relates angle of transmission to propogation speed
- Time of Flight
- The elapsed time from pulse creation to pulse reception
- 13-Microsecond Rule
- For every 13 microseconds of round trip time, the object creating the reflection is 1 cm deeper in soft tissue
- Axial Resolution
- The ability of a system to display two structures that are very close together when the structures are PARALLEL to the sound beam's main axis
- Resolution
- The ability to image accurately
- Mnemonic for Axial Resolution
-
LARRD
Longitudinal
Axial
Range
Radial
Depth - Transducer
- Any device that converts one form of energy into another
- Piezoelectric Effect
- The property of certain materials to create a voltage when they are mechanically deformed
- Piezoelectric Materials
- Materials which convert sound into electricity and vice versa
- Commonly Used PZT in US transducers
- Lead Zirconate Titanate
- Transducer Components (7)
- Case, electrical shield, acoustic insulator, PZT or active element, wire, matching layer, backing material
- Transducer Case
- Protects the internal components of the transducer from damage
- Electric Shield
- Thin metallic barrier that line case and prevents spurious electrical signals in the air from entering the transducer
- Acoustic Insulator
- A thin barrier of cork or rubber that prevents vibrations in the case from inducing an electrical voltage in the PZT of the transducer
- PZT or Active Element
- Piezoelectric crystal itself, shaped like a coin, PZT is one-half wavelength thick
- Wire
- An electrical connection between PZT and the US system
- Matching Layer
- In front of the PZT, increases the efficiency of sound energy transfer and protects the active element, ML is 1/4 wavelength thick
- Backing Layer (Damping Element)
- Bonded to the back of the active element, it reduces the "ringing" of the pulse, enhances axial resolution
- Bandwidth
- The range or difference between the highest and lowest frequencies in the pulse
- Resonant Frequency
- A long vibration at a single frequency
- Q Factor
- The main frequency divided by the bandwidth
- Curie Temperature
- The temperature at whick PZT is polarized
- Continuous Wave Transducers
- Frequency of sound emitted by probe is equal to the frequency of the electrical signal
- Pulsed Wave Transducers
- Frequency of sound created determined by (1) speed of sound in PZT and (2)thickness of PZT
- High Frequency Transducers
-
Thinner PZT crystal
PZT with higher speeds
Thin & fast - Low Frequency Transducers
-
Thicker PZT crystal
PZT with slower speeds
Thick and slow - Focus
- The location where the beam diameter is narrowest, width at focus is 1/2 the width of the beam as it leaves the transducer
- Near Zone (Fresnel Zone)
- Region from the transducer to the focus (beam narrows)
- Focal Length
- Distance from the transducer to the focus
- Far Zone (Fraunhofer Zone)
- Region starting at the focus and extending deeper (beam diverges)
- Focal Zone
- Region around the focus where the beam is relatively narrow, superior image detail
- Huygens' Principle
- A large active element may be thought of as millins of tiny, distinct sound sources
- Spherical Waves
- Sound waves produced by very small pieces of PZT diverge in the shape of a "V"
- Lateral Resolution
- The ability to distinctly identify two structures that are very close together when the structures are PERPENDICULAR or side by side
- Mnemonic for Lateral Resolution
-
LATA
Lateral
Angular
Transverse
Aximuthal - Three Types of Focusing
-
External - with a lens
Internal - with a curved active element
Phased Array - with the electronics of the US system - Hemodynamics
- Study of blood circulation (flow of blood)
- Pulsation
- Blood moves through the body due to the pumping action of the heart
- Heartbeat
- Rythmic contraction of the heart
- Heart Rate
- # of times heart contracts per minute
- Pressure Gradient
- Blood flows from high pressure to low pressure
- Resistance is affected by
-
Length of vessel
Viscosity (thickness) of fluid
Radius of vessel - Two Energy Concepts (capacity to do work)
-
Kinetic - the energy of something in motion
Potential - stored energy or energy at rest - Total Fluid Energy
- Kinetic + Potential
- Blood Flow Characteristics
- Laminar Flow - flow pattern arranged in layers
- Parabolic Flow Profile
-
Bullet shaped pattern in vessel
fastest at center
slowest at vessel wall - Flattened Plug Profile
- All layers travel at the same velocity
- Arterial Hemodynamics
-
Pulsation
Heartbeat
Heart rate - Venous Hemodynamics
-
Pressure Gradient
Calf Muscle Pump
Valves
Respiration - Hydrostatic Pressure
- The effect of gravity on veins while standing; measured above or below the heart
- Doppler
- Used to detect and evaluate blood flow by measuring (1) speed and (2) direction
- Types of Doppler (3)
-
Spectral - waveform
Audio - hear flow
Color - Flow imaging - Doppler Effect
- Change in the frequency of sound because of movement of RBCs
- Doppler Shift
- The difference between the frequencies that are transmitted and received; dependent upon cosine of the angle
- Positive Shift
- Received frequency is greater than transmitted frequency, RBCs moving toward transducer, seen as doppler shift above baseline
- Negative Shift
- Received frequency is less than transmitted frequency, RBCs moving away from transducer, seen as doppler shift below baseline
- Doppler Equation
- Doppler Shift = 2 X speed of blood X transducer frequency X Cosine / Propogation speed