Ultrasound Flashcards
Fundamental Physics of Ultrasound
Compression waves in a compressible media (tissue). Bulk Compressibility, density leads to speed of sound in medium. Piezoelectric effect, resonance condition of transducer, relation of resonant frequency to speed of sound and thickness in transducer material. Reflection, refraction, scattering, attenuation of sound. Acoustic Impedance. Doppler Effect. Intensity (energy) vs pressure.
What is the input and output of Ultrasound. How do we make the image?
For “pulse-echo” imaging: input is the ultrasound pulse and the array of scatterers. Output is a convolution of the PSF and a phase factor with R PSF is the product of the z shape of the pulse and the beam profile at the depth z Envelope detection and time gain compensation is applied.
What is the basic cascade of subsystems for ultrasound?
It is all in the transducer: transmit frequency, beam profile, pulse shape due to damping and electronics, sensitivity
What determines the contrast between regions in the system for ultrasound
Differences in acoustic impedance, Z, for larger regions and reflectivity for smaller point scatterers. Speckle pattern differences also provide contrast.
What limits the resolution of a system in ultrasound
The PSF is modeled by the resolution cell, pulse length in z and then by the main lobe width in x.
What is the noise source for ultrasound
Speckle due to resolution cell averaging over phases. For a collection of random scatterers, SNR~1.92. Speckle is the same if scan is repeated-not stochastic! Scanning from different directions and adding reduces speckle because the pattern depends on the angle of the beam. “compounding can reduce speckle.
What are the sampling requirements for ultrasound
The maximum signal frequency in z is limited by the averaging over the resolution cell. The sampling must be twice the width in z. The sampling rate of different lines in the image is dictated by the depth of penetration needed and speed of sound. Sampling requirements for pulsed Doppler are that we need to sample at least twice the Doppler shift frequency.
What parameters can be manipulated to effect tradeoffs of resolution, noise, and contrast in ultrasound?
Resolution is improved at higher frequencies and smaller transducers but limited by desired depth of penetration. Contrast is basically determined by impedance differences.
What bioeffects limit the safe use of ultrasound?
The bioeffects are heating due to power deposition and disruption of tissue do to cavitation (bubbles). Systems typically display a thermal index (TI) and a mechanical index (MI) that have safe regions of use.
What are the main artifacts of ultrasound and how can they be avoided/manipulated?
Artifacts are shadowing, refraction, reverberation, mirroring, sideline “clutter” or “sludge”. One of the main methods to determine if something is an artifact is to try a different transducer angle or placement or to perform “compounding” to incorporate a variety of angles. Some artifacts are useful to ultrasonographers to indicate tissue properties. For Doppler imaging, velocities can be aliased if higher than the sampling limits.
Bulk compressibility, density and how it relates to speed of sound in a medium
Piezoelectric effect, resonance condition for transducer, relation of resonant frequency to speed of sound and thickness in transducer material
Reflection, refraction
Pressure Reflectivity
Pressure Transmittivity
Intensity Reflectivity
Intensity Transmittivity
Acoustic Power
Characteristic Impedance
Attenuation
Absorption Coefficient
Absorption coefficient relation to mu
Doppler Shift
The general expression for the sound frequency observed by a stationary observer from a moving source
Derivation of Doppler Shift
Frequency
Wavelength
Relation of frequency and wavelength
Width of beam
