Physics Chapters 1-12 Review

Physics Chapters 1-12 Review

• All numerical values require corresponding units
• Macro = Bigger, Micro = Smaller
• Common metric prefixes are 10⁹ (billion; giga), and 10⁻⁶ (millionth; micro)
• Sound waves carry energy, characterized by compressions (increased pressure and density) and rarefactions (decreased pressure and density)
• Sound cannot travel in a vacuum
• Sound is a mechanical, longitudinal wave (not transverse), traveling in a straight line
• Three acoustic variables for sound waves include pressure (force per area, measured in Pascals), density (mass per volume, measured in kg/cm³), and distance (particle motion, measured in mm or cm)
• Transverse waves: particle movement is perpendicular to the wave's direction (90°)
• Longitudinal waves: particle movement is parallel to the wave's direction
• Parameters for sound waves include period, frequency, amplitude, power, intensity, wavelength, and speed
• Period and frequency are inversely related to each other, determined by the sound source
• Amplitude, power, and intensity are directly related and adjustable by the sonographer
• Wavelength depends on both the sound source and the medium
• Speed depends only on the medium

Three Bigness Parameters

• Amplitude, power, and intensity describe the strength of a sound beam
• All three are adjustable by the sonographer
• Period: time for one complete cycle (measured in microseconds); unchangeable by the sonographer
• Frequency: number of cycles per unit time (measured in Hertz or Hz); determined by the sound source and unchangeable
• Audible frequencies range from 20 Hz to 20,000 Hz
• Ultrasound frequencies are greater than 20,000 Hz
• Infrasound frequencies are less than 20 Hz
• Period and frequency are inversely related; if period decreases, frequency increases, and vice-versa.

Propagation Speed

• Determined solely by the medium
• Speed and wavelength are directly related

Phase Relationships

• Constructive interference: combined wave amplitude is greater than the original waves
• Destructive interference: combined wave amplitude is less than one of the original waves

Pulse Duration & Spatial Pulse Length

• Pulse duration is the time from the start to the end of a pulse
• Spatial pulse length is the distance from start to end of the pulse
• Shorter pulses create higher image quality
• The relationship between pulse duration and spatial pulse length is that shorter pulses have a shorter spatial pulse length

Pulse Repetition Period and Pulse Repetition Frequency

• PRP is the time from the start of one pulse to the start of the next pulse
• PRF is the number of pulses per unit time
• Increasing PRP decreases imaging depth, and vice-versa

Intensity

• The concentration of power in a sound beam
• Two types: Spatial and Temporal
• Key words related to intensity: peak, average, spatial peak, temporal peak, spatial average, temporal average
• SPTA (spatial peak, temporal average) is the most relevant to thermal bioeffects.

Attenuation

• Attenuation is the decrease in intensity as sound travels through a medium
• Three components of attenuation: absorption, scattering, and reflection
• Attenuation is different in air, lung, bone, water, and soft tissue, with soft tissue typically having medium attenuation
• Lower frequency sound penetrates further in soft tissue than higher frequency sound, attenuating less
• Reflectors create specular or diffuse reflection depending on the shape.

Refraction

• Refraction is the bending of sound at a boundary between two different media
• Refraction occurs when sound travels from one medium to another at an oblique angle.
• Snell's law describes the physics of refraction

Time of Flight

• Time needed for a pulse to travel to and from a reflector
• Can be used to determine reflector depth

Basic Transducers

• Piezoelectric materials (e.g., PZT) convert energy into sound waves
• PZT crystal properties are destroyed if heated above Curie temperature
• The active element is a piezoelectric crystal; it is ½ wavelength thick
• Don't use a transducer with a cracked case to avoid patient electrical shock
• Matching layer is a quarter wavelength thick; between the skin and active element for increased transmission

Bandwidth and Quality Factor

• Bandwidth is the range of frequencies
• Imaging transducers use backing material, but therapeutic transducers do not
• Quality factor is a unitless number related to damping

Pulsed Transducers

• The main and central frequency depend on crystal thickness and propagation speed

Anatomy of a Sound Beam

• Sound beam width changes during travel, narrowing to a focus and then diverging
• Near zone (Fresnel zone) is where the beam is relatively narrow
• Far zone (Fraunhofer zone) is where the beam diverges
• Focal zone is where the beam is narrow
• Focal depth is determined by the transducer diameter and frequency
• Beam divergence describes the spread of sound in the deep far zone
• Diffraction is the wave-like behavior of sound

Axial and Lateral Resolution

• Axial resolution is the ability to distinguish two structures positioned along the beam axis
• Lateral resolution is the ability to distinguish two structures positioned side-by-side perpendicular to the beam axis
• Lateral resolution is equal to the beam diameter

Two-Dimensional Imaging

• Mechanical scanning moves the active element or a mirror, creating a scan plane
• Phased arrays achieve adjustable focus electronically.
• Malfunctional elements in phased arrays can distort the image.

Contrast, Spatial, and Temporal Resolution

• Contrast is visualized in gray shades
• Spatial resolution is image detail determined by axial, lateral resolution, and line density
• Temporal resolution is the time for creating an image frame; limited by imaging depth and speed of sound in medium; higher frame rates for shallow imaging
• Frame rate is determined by imaging depth and number of pulses per image

Frame Rate vs. Image Quality

• As temporal resolution improves, image quality may degrade, and vice versa
• Factors affecting frame rate and temporal resolution include line density, number of focal points, and sector angle