DMSA201: Ultrasound Physics & Instrumentation 1 (Chapters 3 & 4)

Comprehensive practice flashcards covering wave parameters, sound source vs. medium, continuous sound waves, and pulsed sound parameters from Chapters 3 & 4 of Ultrasound Physics.

What are the 7 parameters required to describe a continuous sound wave?

The 7 parameters are Period, Frequency, Amplitude, Power, Intensity, Wavelength, and Propagation Speed.

In ultrasound physics, what is the difference between the sound source and the medium?

The sound source is the ultrasound system and transducer, whereas the medium (media) is the biologic tissue through which the sound waves travel.

How is period defined, what are its units, and can it be adjusted by the sonographer?

Period is the time it takes a wave to vibrate a single cycle (or the time from the start of one cycle to the start of the next). Its units are time ($\text{seconds}$, $\mu\text{s}$, $\text{minutes}$), and it is not adjustable by the sonographer because it is determined solely by the sound source.

What are the typical values of period in diagnostic ultrasound?

Typical values are $0.06\,\mu\text{s}$ to $0.5\,\mu\text{s}$ ($6 \times 10^{-8}\text{ s}$ to $5 \times 10^{-7}\text{ s}$ or $0.00000006\text{ s}$ to $0.0000005\text{ s}$).

What is frequency, what are its typical clinical values, and is it adjustable?

Frequency is the number of events (cycles) that occur in a specific duration of time. Its units are per second ($\text{Hz}$, $\text{kHz}$, $\text{MHz}$), with typical clinical values of $2\,\text{MHz}$ to $15\,\text{MHz}$. It is determined by the sound source and is not adjustable by the sonographer.

How are ultrasound, audible sound, and infrasound categorized by frequency?

• Ultrasound: Sound with a frequency greater than $20\,\text{kHz}$ ($20{,}000\,\text{Hz}$
• Audible Sound: Sound with a frequency between $20\,\text{Hz}$ and $20\,\text{kHz}$
• Infrasound: Sound with a frequency less than $20\,\text{Hz}$

What mathematical relationship exists between period and frequency?

Period and frequency are inversely related and reciprocals. As one increases, the other decreases: $\text{Period} \times \text{Frequency} = 1$. For example, if frequency is $2\,\text{cycles/second}$, the period is $\frac{1}{2}\,\text{second}$.

Which 3 parameters are known as the "bigness" parameters?

Amplitude, Power, and Intensity describe the magnitude or strength of a sound wave.

Based on the provided wave diagram, what are the crest, trough, and amplitude of a wave?

The crest is the maximum positive displacement, the trough is the maximum negative displacement, and the amplitude is the difference between the maximum (or minimum) value and the average or undisturbed value of an acoustic variable.

What is peak-to-peak amplitude?

Peak-to-peak amplitude is the difference between the maximum and minimum values of an acoustic variable, which equals double the amplitude.

What are the units, typical values, determination, and adjustability of amplitude?

• Units: Units of any acoustic variable (Pascals, $\text{g/cm}^3$, $\text{cm}$, $\text{dB}$)
• Typical Values: $1\,\text{MPa}$ to $3\,\text{MPa}$
• Determined By: Sound source initially (changes as it travels through media)
• Adjustable: Yes, the sonographer can adjust it (by altering power)

How is power defined, what are its typical values, and can it be adjusted?

Power is the rate of energy transfer or the rate at which work is performed. Its units are Watts ($\text{W}$), typical values are $0.004\,\text{W}$ to $0.090\,\text{W}$ ($4\,\text{mW}$ to $90\,\text{mW}$), and it is adjustable by the sonographer.

How is power mathematically related to amplitude?

Power is proportional to amplitude squared: $\text{Power} \propto \text{Amplitude}^2$. For example, if a sonographer increases amplitude by a factor of $3$, power increases by a factor of $3^2 = 9$.

What is intensity, what is its mathematical formula, and what are its units?

Intensity is the concentration of energy in a sound beam, calculated as: $\text{Intensity} = \frac{\text{Power}}{\text{Area}}$. Its units are watts per square centimeter ($\text{W/cm}^2$), and typical values range from $0.01\,\text{W/cm}^2$ to $300\,\text{W/cm}^2$.

How does intensity change when power or amplitude is modified?

• Intensity is directly proportional to power: doubling power doubles intensity; quartering power quarters intensity.
• Intensity is proportional to amplitude squared: doubling amplitude quadruples intensity ($2^2 = 4$); halving amplitude reduces intensity to one-fourth ($\left(\frac{1}{2}\right)^2 = \frac{1}{4}$).

What is wavelength, and what makes its determination unique among the 7 wave parameters?

Wavelength is the distance or length of one complete cycle in space. Its units are distance ($\text{mm}$, $\text{m}$, $\text{cm}$), with typical values of $0.1\,\text{mm}$ to $0.8\,\text{mm}$. It is unique because it is the only parameter determined by BOTH the sound source and the medium. It is not adjustable.

According to the wave graphs shown, what is the fundamental difference between wavelength and period?

Wavelength is measured along the distance axis (representing physical length of a cycle), whereas period is measured along the time axis (representing time taken to complete a cycle).

What is the formula for calculating wavelength in soft tissue, and what is the wavelength of a $2\,\text{MHz}$ sound wave?

In soft tissue: \text{Wavelength (mm)} = \frac{1.54\,\text{mm/\mu s}}{\text{Frequency (MHz)}}. For a $2\,\text{MHz}$ sound wave, the wavelength is $\frac{1.54}{2} = 0.77\,\text{mm}$.

What is propagation speed, what is it determined by, and is it adjustable?

Propagation speed is the rate at which a sound wave travels through a medium, measured in distance divided by time ($\text{m/s}$). Typical values are $500\,\text{m/s}$ to $4{,}000\,\text{m/s}$. It is determined ONLY by the medium and is not adjustable by the sonographer.

What is the average speed of sound in soft tissue expressed in four different metric units?

• $1{,}540\,\text{m/s}$
• 1.54\,\text{mm/\mu s}
• $1.54\,\text{km/s}$
• $154{,}000\,\text{cm/s}$

What are the propagation speeds of sound in lung, fat, liver, muscle, tendon, and bone?

• Lung: $500\,\text{m/s}$
• Fat: $1{,}450\,\text{m/s}$
• Liver: $1{,}560\,\text{m/s}$
• Muscle: $1{,}600\,\text{m/s}$
• Tendon: $1{,}700\,\text{m/s}$
• Bone: $3{,}500\,\text{m/s}$

What are the propagation speeds of sound in air, water, and metals, and in which state of matter does sound travel fastest?

• Air: $330\,\text{m/s}$
• Water: $1{,}480\,\text{m/s}$
• Metals: $2{,}000\,\text{m/s}$ to $7{,}000\,\text{m/s}$ Sound travels fastest in solids, slower in liquids, and slowest in gases.

Which two characteristics of a medium determine its propagation speed, and how do they affect it?

Stiffness (Bulk Modulus) and Density. Stiffness is directly related to speed ($\uparrow\text{stiffness} = \uparrow\text{speed}$) and has more influence. Density is inversely related to speed ($\uparrow\text{density} = \downarrow\text{speed}$). Rule: "Stiff = Speed, Heavy = Slow".

What are the 5 additional parameters used to describe pulsed sound?

Pulse Duration, Pulse Repetition Period (PRP), Pulse Repetition Frequency (PRF), Duty Factor, and Spatial Pulse Length (SPL).

What two components make up pulsed ultrasound as illustrated in this diagram?

Transmit time (talking or "on" time / pulse duration) and Receive time (listening or "off" time).

What is pulse duration, its units, typical values, determination, and adjustability?

Pulse duration is the actual time from the start of a pulse to the end of that pulse (transmit/"on" time). Units are time ($\mu\text{s}$), with typical values of $0.3\,\mu\text{s}$ to $2.0\,\mu\text{s}$. It is determined ONLY by the sound source and is not adjustable.

What are the two formulas used to calculate pulse duration?

• $\text{Pulse Duration (}\mu\text{s)} = \text{number of cycles} \times \text{period}$
• $\text{Pulse Duration (}\mu\text{s)} = \frac{\text{number of cycles}}{\text{frequency (MHz)}}$

How many cycles are typically in a clinical imaging pulse, and why are shorter pulses preferred?

In clinical imaging, a pulse typically contains $2$ to $4$ cycles. Shorter duration pulses are preferred because they create more accurate images.

What concept is illustrated in the diagram, and how is it defined and calculated?

Spatial Pulse Length (SPL) is the distance a pulse occupies in space from start to end. It is calculated as: $\text{SPL (mm)} = \text{number of cycles in each pulse} \times \text{wavelength (mm)}$. It is determined by both the sound source and the medium, and is not adjustable.

What is the primary difference between pulse duration and spatial pulse length (SPL)?

Pulse duration is a measurement of time ($\mu\text{s}$), whereas spatial pulse length is a measurement of distance ($\text{mm}$).

What is Pulse Repetition Period (PRP), what are its typical values, and is it adjustable?

PRP is the time from the start of one pulse to the start of the next pulse (one pulse duration plus one listening time). Typical values are $100\,\mu\text{s}$ to $1\,\text{ms}$. It is determined by the sound source only and is adjustable by changing the depth of view.

How does changing imaging depth alter the Pulse Repetition Period (PRP)?

PRP and depth of view are directly related. As depth of view increases, PRP increases; as depth decreases, PRP decreases. Only the listening (receive) time of the PRP changes when depth is adjusted.

What is Pulse Repetition Frequency (PRF), what are its typical values, and is it adjustable?

PRF is the number of pulses that an ultrasound system transmits into the body each second. Typical values are $1{,}000\,\text{Hz}$ to $10{,}000\,\text{Hz}$ ($1{,}000$ to $10{,}000\,\text{pulses/second}$). It is determined solely by the sound source and is adjustable by changing the depth of view.

How are PRF and depth of view related?

PRF and depth of view are inversely related. When imaging shallow, PRF is higher; when imaging deep, PRF is lower.

What mathematical relationship exists between PRP and PRF?

PRP and PRF are inversely related and are reciprocals: $\text{PRF} \times \text{PRP} = 1$, $\text{PRF} = \frac{1}{\text{PRP}}$, and $\text{PRP} = \frac{1}{\text{PRF}}$.

What is duty factor, what are its units, and what is its typical value in clinical imaging?

Duty factor is the percentage or fraction of time that the ultrasound system transmits a pulse. It has no units (percentages are unitless). In anatomic imaging, typical values range from $0.002$ to $0.005$ ($0.2\%$ to $0.5\%$), meaning the system spends about $99.8\%$ of its time listening.

What is the duty factor of continuous wave ultrasound, and what is the formula to calculate duty factor?

For continuous wave ultrasound, the duty factor is $1.0$ ($100\%$). The formula is: $\text{Duty Factor (\%)} = \frac{\text{Pulse Duration}}{\text{PRP}} \times 100$.

How do listening time, PRP, PRF, and Duty Factor differ between shallow and deep imaging?

• Shallow Imaging: Less listening time, shorter PRP, higher PRF, higher duty factor
• Deep Imaging: More listening time, longer PRP, lower PRF, lower duty factor

Which parameters of pulsed sound change when the sonographer alters the depth of view?

Pulse Repetition Period (PRP), Pulse Repetition Frequency (PRF), and Duty Factor change with imaging depth. (Pulse Duration does not change with depth).