Echo Basics (M mode, Chamber Assessment, Systolic Function...) Flashcards
Afterload
The force that opposes LV ejection
Preload
The force that acts to stretch the myocardial fibers at end diastole and is related to end diastolic volume
What conditions cause paradoxical (rightward) motion of the septum in early systole?
-RV volume overload
-LBBB
What is the benefit of M mode?
Higher temporal resolution (1000-200 frames per second vs 30-100 frames)
What is the average velocity of sound in soft tissue?
1540 m/s
Frequency
Number of waves/ sec (Hz)
Wavelength
Distance between wave peaks or wave nadir
Velocity (formula)
Velocity = frequency x wavelength
What is the wavelength of 3 MHz frequency?
0.5mm in soft tissue
Amplitude
Difference between maximum and minimum value of a wave
Power
Total energy produced each second (W, watts)
Intensity
Power / cross-sectional area (W/cm2)
Decibel (dB)
10 log (I/Io)
I = measured intensity
Io = is defined as the reference intensity (may be maximum for that machine/probe)
Mechanical index
-Measure of potential to produce cavitation (the formation of bubbles within a liquid)
Acoustic impedance
-Product of density and velocity of sound (Z = pv)
-Poor transmission of sound from one medium to another if impedances differ significantly
-Interface between materials with dissimilar acoustic impedances reflect sound
Specular reflection
-Structures > 1 wavelength
-Angle of reflection = angle of incidence
-Independent of frequency
Diffuse reflection
-Specular reflection from an irregular surface
-Sound reflected in may directions
Scattering
-Structures < 1 wavelength in diameter
-Omnidirectional
-Frequency dependent (up to fourth power of the frequency)
Interference
Algebraic sum of the wave amplitude
Speckle
Noise in ultrasound images produced by interference pattern from multiple small reflectors (scatters)
Refraction
-Bending of wavefront as sound passes between media with differing propagation velocities
-Snell’s law
Diffraction
-Spreading or divergence of a sound beam
-Greater with a smaller source
Absorption
-Sound energy is converted to other forms of energy (mainly heat)
-Related to sound frequency, viscosity of medium, and relaxation time of medium
Attenuation
-Loss of intensity as the sound wave passes through medium
-Summation of losses due to reflection, scattering, and absorption
What is the expected attenuation in soft tissue?
0.5-1.0 dB/ cm/ MHz
Ex: For 3MHz signal, assuming 1dB/cm/MHz attenuation -> Half (3dB) of the signal strength is lost for each cm of travel
Resonance
Persistent oscillation produces continued ultrasound signal
Piezoelectric crystal
-Vibrates with applied alternating current (most efficient when stimulated at its resonant frequency (determined by the crystal thickness)
-Also generates current when vibrated
-Functions as both transmitter and receiver
Backing block
-Limits “ringing” of the crystal to create more discrete ultrasound pulse
-Absorbs sound waves directed into transducer
Quarter-wavelength Matching Layer
-Provides better acoustic impedance matching between crystal and skin
-Significantly improves transmission of ultrasound into tissues and reduces heat at skin surface
Narrow band
width transducer (high Q factor)
-Crystal allowed to ring freely
-More efficient (sensitive)
Ex: Choir bell
Wide bandwidth transducer (low Q factor)
-Wide range of frequencies sent and received
-Short ultrasound pulses
-Facilitates harmonic imaging
Ex: Cowbell
Continuous
-Separate send and receive transducers
-CW Doppler
Pulsed
-Single transducer alternately sends and receives
-PW Doppler, Grayscale imaging, Colorflow imaging
Pulse repetition period (PRP)
-Time between pulses
-Directly related to the imaging depth
Pulse repetition frequency (PRF)
-Inversely related to PRP
-Inversely related to imaging depth
Duty Factor
-Fraction of PRP that transducer is emitting sound
Ex: CW Doppler =1, all pulsed techniques < 1 (usually 0.001 to 0.01)
Temporal resolution
-Determined by the frame rate: number of frames/ second
-Dependent on the imaging depth (13 used/cm, roundtrip)
-Dependent on line density (typically 50-200 lines/frame)
Frame rate
-Number of frames per second
-Use of multiple beam formers increases frame rate
Contrast Resolution
-Ability to discern ‘target’ from background
-Dependent on intrinsic physical properties of target and surrounding tissue
-Dependent on acquisition and display parameters: frequency, gain, compression, color, etc…
Axial resolution
-Determined by spatial pulse length
-Resolution = (cycles x wavelength) /2
-Better with higher frequencies, fewer cycles
-Pulse length set by manufacturer for probe/frequency
-Longer pulse used in harmonic imaging (better separation of fundamental and harmonic signal, slight degradation in axial resolution)
Lateral resolution
-Better with narrower beam
-Dependent on aperture, frequency (improved at higher frequencies) and focusing
-Beam is narrowest at the focal point and diverges rapidly after focal point (poor lateral resolution in far field, especially with small aperture, low frequency, focusing)
