Frame Rate & Acoustic Impedance

Frame Rate

• Frame rate indicates the system's capacity to generate multiple frames per second.
• Frame rate is directly proportional to temporal resolution; increasing frame rate improves temporal resolution, while decreasing it worsens it.
• A minimum of 15 frames per second are needed for a real-time, flicker-free image.
• Actions improving image quality can decrease frame rate and worsen temporal resolution.

Acoustic Impedance

• Acoustic impedance is the resistance sound encounters while traveling through a medium.
• Density and stiffness of the medium affect impedance.
• Impedance is calculated as density multiplied by propagation speed.
• PZT has higher impedance than the matching layer, gel, and skin.
• Sound waves transmit without reflection through mediums with the same impedance.
• Impedance mismatch is necessary for reflection.
• A greater impedance mismatch between mediums results in more reflection, while less mismatch results in more transmission.

Pulse Duration, Pulse Repetition Period, and Pulse Repetition Frequency

• Pulse duration (PD) equals the period multiplied by the number of cycles in a pulse.
• Pulse Repetition Period (PRP) is the time from the start of one pulse to the next (PRP = 1/PRF).
• Pulse Repetition Frequency (PRF) is the number of pulses an ultrasound system transmits per second (PRF = 1/PRP).
• PRP and PRF are inversely related.
• Depth is directly proportional to PRP/PRF.
• Increasing depth increases PRP and decreases PRF.

Spatial Pulse Length

• Spatial pulse length (SPL) is the length of space over which a pulse occurs.
• SPL is important for axial resolution and distinguishing separate structures front to back.
• SPL is calculated as wavelength multiplied by the number of cycles.

Axial and Lateral Resolution

• Axial resolution distinguishes two structures parallel to the sound beam (front to back).
• Smaller axial resolution is desired
• Axial Resolution Formula: AR = SPL/2
• Synonyms for axial resolution: LARRD (Longitudinal, Axial, Range, Radial, Depth).
• Increased damping improves axial resolution by reducing SPL.
• Axial resolution is improved with increased damping
• Axial resolution is improved by frequency
• Lateral resolution distinguishes structures perpendicular to the sound beam (side by side).
• Smaller lateral resolution is desired
• Synonyms for lateral resolution: LATA (Lateral, Azimuthal, Transverse, Angular).
• Lateral resolution varies with distance from the transducer.
• Transducer diameter affects lateral resolution.
• Focusing improves lateral resolution.
• Lateral resolution is optimal at the focus.
• LR = Beam Diameter

Velocity

• Velocity is the speed and direction of fluid movement.
• Stiffness (bulk modulus), density, and elasticity determine velocity.

Bandwidth

• Bandwidth is the range of frequencies within an ultrasonic pulse.
• Continuous wave transducers have a narrow bandwidth.
• Imaging transducers have a broad bandwidth, allowing for harmonics.

Doppler Shift

• Doppler shift is the changes in frequency due to motion.
• Minimal Doppler shift occurs at a 90-degree angle between motion and the sound beam, which is to be avoided.
• FFT (Fast Fourier Transform) derives Doppler shift information from returning echoes, plotting velocity and amplitude over time.
• Doppler shift provides information about velocity.
• A range of Doppler shift frequencies creates a frequency spectrum.
  • Y-axis represents frequency or velocity.
  • X-axis represents time.
  • Z-axis represents amplitude.
• Waveforms above the baseline indicate a positive Doppler shift; waveforms below indicate a negative shift.
• Positive shift indicates blood traveling toward the transducer; negative, away.

Wavelength

• Wavelength is the distance between two consecutive points in the same phase on a wave.
• Wavelength is calculated as propagation speed divided by frequency.

Frequency and Period

• Frequency is the number of events occurring in a specific time.
• Period is the time from the start of one cycle to the start of the next.
• Frequency and period are inversely related.
• Higher frequencies are used for superficial structures, lower frequencies for deep structures.
• Increasing frequency decreases penetration depth, and decreasing frequency increases penetration depth.
• Period is the time to complete a single cycle.

Intensity

• Intensity is the concentration of power in an area.
• Intensity is directly related to power and area.
• Intensity Formula: Power / Area
• Spatial Average, Temporal Average (SATA) has the lowest intensity.
• Intensity is proportional to amplitude squared.
• Power is proportional to amplitude squared.

Cavitation

• Cavitation involves the production and behavior of gas bubbles.
• Microbubbles form in tissues, undergoing rarefaction and compression.
• Bubbles are more likely to rupture during rarefaction.
• The Mechanical Index (MI) indicates the likelihood of cavitation.
• Very little likelihood of cavitation exists if MI is less than 1.0.

Beam Steering

• Beam steering can be electronic or mechanical.
• Beam steering involves angling the beam by sequentially firing the elements.
• Current transducers are phased array and steered electronically.

Threshold

• Threshold or rejection level is the level below which signals are not transmitted through an ultrasound receiver.
• Reject, threshold, and suppression eliminate "junk" or low-level echoes on the display.

Modes: M-Mode, B-Mode, Color Doppler, Pulse Wave

• B-Mode (Brightness Mode) creates grayscale images.
• M-Mode measures time and motion, displaying B-mode in motion, often used for echocardiograms and fetal heart imaging.
• Color Doppler qualitatively shows blood flow direction and approximate speed, indicating positive and negative shifts, and utilizes autocorrelation.
• Pulse Wave Doppler provides quantitative velocity information displayed as a waveform in the spectral analysis window, and utilizes FFT for spectral analysis.

Ultrasound Bioeffects

• Ultrasound bioeffects are not confirmed below 100 mW/cm² with an unfocused beam and below 1 W/cm² with a focused beam.
• No human injuries have been reported from diagnostic medical ultrasound.

Artifacts

• Reverberation: Parallel, equally spaced lines from two strong reflectors close together.
• Comet Tail: Series of closely spaced echoes along the beam's axis and is a form of reverberation.
• Ring Down: Related to reverberation and associated with color Doppler.
• Shadow: Results from high reflection or absorption of US energy.
• Posterior Enhancement: Echoes distal to a weakly attenuating structure appear brighter.
• Speckle: Grainy appearance from constructive and destructive interference.
• Aliasing: Occurs when the Doppler shift exceeds ½ PRF.
• Mirror Image: A false image displayed distal to the real one.
• Electronic Noise: Specific to pulsed wave Doppler, appearing as a snowy appearance or vertical squiggly lines, and happens when a patient is connected to machines.

Duty Factor

• Duty factor is the fraction of time the transducer emits pulses.
• The duty factor of a pulsed echo system is normally less than 1%.
• Duty factor can be manipulated by adjusting the depth.
• Continuous wave has a duty factor of 100%.

Fresnel and Fraunhofer Zones

• Fraunhofer Zone: Also known as the far zone, the beam diverges.
• Fresnel Zone: The near zone, the beam narrows until it reaches the focus.

Transducer Steering and Focusing

• Linear Array: Electronic focus, fixed steer, rectangular image.
• Annular Array: Electronic focus, mechanical steer, sector image.
• Phased Array: Electronic focus, electronic steer, sector image.
• Mechanical Transducer: Fixed focus, mechanical steer, sector/fan shaped image.
• Beam steering angles the beam by sequentially firing the elements.
• Linear array transducers contain rectangular elements in a line.

Transducer Parts and Duties

• Case: Protects the transducer.
• Electric Shield: Lines the inside of the case.
• Damping Material: Reduces ringing & improves SPL.
• Acoustic Insulator: Prevents vibrations.
• PZT: Active element, ½ wavelength thick.
• Wire: Electrical connection between PZT and US system.
• Matching Layer: Material in front of the transducer face, ¼ wavelength thick.
• Backing Material: Bonded to the back of the element, reduces ringing and SPL.

Q Factor

• Quality factor is a unitless number inversely related to bandwidth.
• Wide bandwidth = low Q-factor, narrow bandwidth = high Q-factor.
• Imaging probes use backing material and have a wide bandwidth, so they are low-Q.
• Q Factor = Operating Frequency / Bandwidth.
• Continuous wave transducers have a high Q-factor.
• Q-factor is the "ringing time" or system sensitivity.
• Low range is an imaging transducer.
• High range is continuous wave.

Bits and Pixels

• The more bits per pixel, the better the contrast resolution.
• Pixel is the picture element.

Refraction and Snell's Law

• Snell's Law relates to refraction.
• Refraction occurs if there are different propagation speeds between media 1 and 2, and oblique incidence.
• If propagation speed 2 is faster than propagation speed 1, the refraction is away from normal incidence.
• If propagation speed 1 is faster than propagation speed 2, the refraction is toward normal incidence.

Mechanical Index (MI) and Thermal Index (TI)

• The MI is an indicator of possible acoustic effects in relation to cavitation effects.
• TI determines potential tissue heating based on the intensity of the sound waves, which tells us if we are going to cause harm to tissues.
• Reduce exposure time, use acoustic energy only when indicated, and increase gain instead of power to minimize potential risks.

Phantoms

• When using the AIUM phantom, the transducer, depth gain compensation, and output power should be kept constant for comparison.
• Propagation speed in an ultrasound phantom is 1.54 mm/microsecond.
• The AIUM test object measures the dead zone, lateral resolution, and depth calibration accuracy.
• A Doppler phantom determines if a machine can accurately measure blood velocities within a vessel and detect stenosis.
• The hydrophone measures the output power.

Transient & Stable Cavitation

• Transient cavitation is more violent than stable cavitation.
• Mechanical Index (MI) tells the likelihood of cavitation.
• Cavitation involves the production and behavior of gas bubbles, which are more likely to rupture during rarefaction.
• Very little likelihood of cavitation exists if MI is less than 1.0.

In Vitro, In Vivo, and Epidemiology

• In vivo research is performed within a living body of an animal or a plant.
• In vitro studies are not performed within the body but in an artificial environment.
• Epidemiology is a branch of medicine and exposure response method that utilizes clinical surveys, and it's studies performed retrospectively.