Unit 2 Physical Principles Flashcards

Comprehensive vocabulary flashcards covering the physical principles of ultrasound, including wave properties, pulsed parameters, attenuation, resolution, and real-time imaging.

Reciprocal relationship

A relationship where two values are inversely related and, when multiplied together, they equal 1.

Base Units

Meters, Seconds, Hertz, Liters, Pascals, Rayls, and Watts.

Sound Waves

Traveling variations of energy that carry it from one place to another; they require a medium and cannot travel in a vacuum.

Longitudinal Waves

Mechanical pressure waves where particle motion is parallel to the direction of wave travel.

Transverse Waves

Waves where particle motion occurs perpendicular to the direction of wave propagation.

Compressions

Areas of high pressure and density where medium molecules are squeezed together; associated with positive amplitude and media stiffness.

Rarefactions

Areas of low pressure and density where medium molecules are stretched apart; associated with negative amplitude and media elasticity.

Pressure

An acoustic variable defined as the concentration of force in an area, measured in Pascals ($Pa$).

Density

An acoustic variable defined as the concentration of mass or matter in a volume ($g/cm^3$ or $kg/cm^3$).

Bioeffects

The effects and potential damage to the medium or biologic tissue caused by high-energy sound waves.

In-Phase Waves

Waves where the peaks and troughs occur at the same time and same location.

Constructive Interference

Created by in-phase waves resulting in a single wave with a larger amplitude than either of the individual waves.

Destructive Interference

Created by out-of-phase waves resulting in a single wave with a smaller amplitude than the individual waves.

Period (T)

The time it takes for one cycle to occur; determined by the sound source and calculated as T (\text{s}) = \frac{1}{\text{frequency (MHz)}}.

Frequency

The number of cycles per second ($Hz$); in diagnostic ultrasound, it typically ranges from $2$ to $15$ MHz.

Amplitude

The maximum variation that occurs in an acoustic variable, indicating the relative strength of the wave.

Power

The rate of energy transfer or work performed, measured in Watts ($W$); it is proportional to amplitude squared.

Intensity

The concentration of energy in a sound beam, calculated as $\text{intensity (W/cm}^2\text{)} = \frac{\text{power (W)}}{\text{area (cm}^2\text{)}}$.

Wavelength ($λ$)

The length of one cycle, determined by both the source and the medium; calculated as λ\text{ (mm)} = \frac{\text{propagation speed (mm/s)}}{\text{frequency (MHz)}}.

Propagation Speed (C)

The speed at which a wave moves through a medium; it is determined by the medium's stiffness (Bulk Modulus) and density.

Stiffness

The ability of a medium to resist compression; it is directly related to propagation speed.

Average Propagation Speed in Soft Tissue

$1540\text{ m/sec}$ or 1.54\text{ mm/s}.

Spatial Pulse Length (SPL)

The length of space occupied by one pulse; calculated as $\text{SPL (mm)} = \text{Number of cycles} \times \text{wavelength (mm)}$.

Pulse Duration (PD)

The transmit or "talking" time for one pulse to occur; calculated as \text{PD (s)} = \text{Number of cycles} \times \text{Period (s)}.

Pulse Repetition Period (PRP)

The time from the start of one pulse to the start of the next, including both transmit and receive time; directly proportional to imaging depth.

Pulse Repetition Frequency (PRF)

The number of pulses occurring in one second; it is inversely related to imaging depth.

Duty Factor (DF)

The percentage or fraction of time that the ultrasound system is transmitting a pulse; calculated as $\text{DF} = \frac{\text{Pulse Duration}}{\text{Pulse Repetition Period}} \times 100$.

SPTA (Spatial Peak, Temporal Average)

The intensity measurement method most relevant to thermal bioeffects.

Logarithm

The exponent to which a base (usually 10) must be raised to produce a specific number.

Decibel (dB)

A unit used to measure attenuation or amplification based on a logarithmic ratio; $+3\text{ dB}$ means intensity has doubled.

Attenuation

The weakening of sound intensity, power, and amplitude as it propagates; determined by path length and frequency.

Specular Reflection

Reflection that occurs at a large, smooth boundary where the sound is reflected in only one direction.

Diffuse Reflection (Backscatter)

Reflection that occurs at a large, rough boundary, resulting in disorganized reflected sound.

Scattering

The redirection of sound in many directions when the tissue interface is small or the reflector size is smaller than the beam's wavelength.

Rayleigh Scattering

A special form of scattering where sound is distributed equally in all directions, occurring when reflectors (like RBCs) are much smaller than the wavelength; it is proportional to $\text{frequency}^4$.

Absorption

The conversion of sound energy into heat; it is the primary component of attenuation in soft tissue.

Attenuation Coefficient

The amount of attenuation per centimeter of travel; in soft tissue, it is estimated as $\frac{\text{frequency (MHz)}}{2}$.

Half Value Layer Thickness

The distance sound travels that reduces the intensity to half its original value ($-3\text{ dB}$).

Impedance (z)

The acoustic resistance to sound traveling in a medium, calculated as $\text{Impedance (rayls)} = \text{Density} \times \text{Propagation Speed}$.

Normal Incidence

Occurs when the sound beam strikes a boundary at exactly $90$ degrees; also known as perpendicular, orthogonal, or right angle incidence.

Oblique Incidence

Occurs when the sound beam strikes a boundary at any angle other than $90$ degrees.

Refraction

A change in direction of sound travel when passing from one medium to another; requires oblique incidence and different propagation speeds.

Snell's Law

The formula used to predict the amount of refraction: $\frac{\text{sin (transmission angle)}}{\text{sin (incident angle)}} = \frac{\text{propagation speed 2}}{\text{propagation speed 1}}$.

13-Microsecond Rule

In soft tissue, for every 13 \text{ s} of go-return time, the reflector is $1\text{ cm}$ deeper in the body.

Axial Resolution

The ability to distinguish reflectors parallel to the beam's axis (front-to-back); also known by the mnemonic LARRD (Longitudinal, Axial, Range, Radial, Depth).

Lateral Resolution

The ability to distinguish reflectors perpendicular to the beam's axis (side-to-side); also known by the mnemonic LATA (Lateral, Angular, Transverse, Azimuthal).

Frame Rate

The number of image frames stored or displayed per second; determined by imaging depth and pulses per image.

Temporal Resolution

The ability to accurately locate moving structures at any particular instance in time; improved by a higher frame rate.

Line Density

The number of scan lines per degree of sector or per centimeter; high line density improves spatial resolution but decreases temporal resolution.

Elevation Resolution

Resolution measured perpendicular to the imaging plane; also known as slice thickness resolution.