1. Principals and Technology of 2D Echo Flashcards
What is intensity?
The level of sound energy in an area of tissue
What is rarefaction?
Areas where molecules of a sound wave are dispersed (opposite of compression)
Maximum intensity of cardiac ultrasound systems?
720 W/cm^2
What are the determinants of the propogation velocity of sound?
Only the medium through which it passes
What is the velocity of sound through soft tissue?
1540 m/s
Velocity Equation
Velocity = Wavelength (mm) x Frequency (MHz)
In human tissue, velocity is constant = 1540 m/s
Acoustic Impedance Equation
Acoustic Impedance = Density x Velocity
What is acoustic impedance?
The capacity of a tissue to transmit sound
Higher acoustic impedance = greater ability to transmit sound
(Air = 0.00004; Water = 1.52)
What determines the percentage of ultrasound energy reflected from a tissue interface?
The absolute difference in the levels of acoustic impedance between two materials
(When difference is large, more energy is reflected back to the transducer)
Reflection Coefficient Equation
RC = (Z2-Z1)^2/(Z2+Z1)^2
What is Specular Reflection?
= normal reflection back to probe
Greatest when beam encounters large object with smooth surface
What angle of incidence maximizes specular reflection?
90 degrees (or beam perpendicular to surface)
What is Scattering Reflection?
Ultrasound energy is scattered in multiple directions when encountering small or irregular shaped surfaces
- Basis of Doppler
What is refraction
Ultrasound beam that is not reflected, but propagates through the medium with an altered direction
At what angle of incidence is refraction minimized?
90 degrees
What are the two types of attenuation?
1) Dispersion: ultrasound beam diverges over a greater area
2) Absorption: frictional forces convert ultrasound energy to heat
Components of Transducer
1) Ceramic piezoelectric crystal
2) Electrodes: conduct electric energy
3) Backing: dampens crystal vibrations
4) Insulation: prevent unwanted vibration
5) Faceplate: Optimizes contact between crystal and esophagus
What is the near field of ultrasound?
Area of highest beam manipulation and image resolution before area of focus
Characteristics of near field
1) Ultrasound beam is narrow
2) Proportional to diameter of transducer face
3) Inversely proportional to wavelength
Length = Diameter^2/4*Wavelength
What is the far field of ultrasound?
Distal to the near field zone the ultrasound beam diverges, forming the far field zone
Characteristics of far field
1) Angle of divergence inversely related to diameter of transducer face
What type of transducer produces the most desirable beam profile?
Large transducers with high-frequency (low wavelength) signals
= A long, narrow field with less divergent far field
Definition of Axial Resolution
Ability of the ultrasound to identify two separate objects that lie along the path of the ultrasound beam axis
Improved by high bandwidth pulses (ie high frequency of short duration)
Axial Resolution Equation
Axial resolution = 1.5 * Wavelength
Definition of Lateral Resolution
Ability of the ultrasound to distinguish between objects that are horizontally aligned perpendicular to the path of the ultrasound beam
Lateral Resolution Equation
Lateral Resolution = Depth/50
Definition of Elevational Resolution
Ability of the ultrasound to distinguish between objects that are vertically aligned and perpendicular to the ultrasound beam
Types of Resolution
1) Axial
2) Lateral (Azimuth)
3) Elevational
Gain
Allows amplification of the electrical signal
Time Gain Compensation
Selective amplification of structures of varying distance from the transducer
Distance Equation
Distance = Velocity * Time
Display Formats
1) Amplitude Mode
2) Brightness Mode
3) Motion Mode (M-Mode)
Amplitude Mode Display
Amplitudes of the returning signals are represented as a series of horizontal spikes along a vertical axis
Brightness Mode Display
Amplitudes of returning echo are represented as pixels of varying brightness. Brightness corresponds with signal strength.
Motion Mode (M-Mode)
1) Applies temporal information to B-Mode
2) Frame rate >/= 2,000 frames/sec
3) Most superior display of dynamic motion
* Best mode for examining timing of cardiac events when displayed with ECG
4) Displays only axial motion
Pulse Repetition Frequency
Rate at which sound pulses are transmitted per second
1) Greater pulse repetition frequency = greater number of scan lines emitted in a given period of time
2) PRF inversely related to sector depth
Frame Rate
Frequency at which a sector is rescanned
Dependent on:
1) Sector depth
2) Sector width
