M1U2: Amplitude, Power, Intensity

What is amplitude?

- Max variation of variable from its mean (change from rest to max/min density of particles)

- How much compression and rarefraction

- Strength of sound (how loud)

What are the units of amplitude?

- Volts (V)

- Pressure (mmHg, atm, Pascals)

- Density (kg/m^3)

- Temperature (celsius/farenheit)

- Particle motion (nm)

What is transmit voltage?

- How much power we put into transducer (most cases this setting is at the max)

- U/S machine applies current to induce transducer crystals to convert electrical energy to sound energy (U/S), then convert returning echoes from sound to electrical energy)

What does increasing transmit voltage do? (in terms of amplitude and sonographic appearance)

- Increases sound wave amplitude (pressure, density, particle motion, and temperature), resulting in greater degree of compression and rarefraction

- Makes sonogram more echogenic (brighter)

How can you calculate the amplitude?

Method 1: Amp = max-mean

Method 2: Amp = mean - min

Method 3: Amp = (max - min)/2

TRUE OR FALSE: Frequency correlates to how loud/quiet a sound wave is, while amplitude references the pitch of a sound.

FALSE:

- Frequency = pitch

- Amplitude = strength/volume

TRUE or FALSE: You can have 2 sound waves that are the same amplitude, but different frequencies.

TRUE: This means the sound waves are at the same volume, but different pitches

TRUE or FALSE: Scanning at a higher frequency results in more attenuation and a weaker signal (lower amplitude)

TRUE

What is power? What are its units?

- Measure of how much work/energy is expended per time

- Watts or Joules/sec

What is the relationship between power and amplitude?

Power is proportional to amplitude squared

What happens to power and intensity if transmit voltage is tripled?

- Power and intensity increases by factor of 9

- Remember: transmit voltage = amplitude

- Power is proportional to amplitude squared

- Intensity is directly proportional to power. Therefore, increasing amplitude by 3 increases intensity by factor of 9

Higher transmit voltage results in increased acoustic energy (power). What are the pros and cons of this?

Pros:

- Increased penetration

- Higher signal to noise ratio, resulting in better images

Con:

- Potential bioeffects (ex. heat)

What is a good signal to noise ratio?

- Want more signal and less noise, as signals give good representation of tissue and noise makes image less clear

What is intensity?

Distribution of power over an area (power/area)

How can intensity be adjusted in ultrasound?

Focus knob can focus the sound beams to area of interest, increasing the intensity to this area

TRUE or FALSE: Sound intensity is uniform, as sound beams assume a straight line shape.

FALSE: Sound beams are an HOURGLASS shape, meaning the intensity of the beam is not uniform throughout. The sound beam is the most intense at the focal point (place of sound beam w/ smallest area)

What part of the sound beam is the most intense?

Focal point (section of sound beam w/ smallest area)

TRUE or FALSE: Intensity is inversely proportional to area.

TRUE: As area increases, intensity decreases, as there is more space for the sound beam to hit, resulting in less focus.

What is attenuation? What are its units?

- Weakening of sound as it propagates through a medium

- Units: decibels (dB)

TRUE or FALSE: Attenuation is a linear relationship.

FALSE: Attenuation is EXPONENTIAL and NON-linear. Intensity of the sound beam decreases exponentially as it goes deeper through different mediums

How do we account for attenuation on an U/S machine?

- Overall gains: making whole image bright/dark

- TGC (time gain compensation): making image bright/dark at different depths

Why are decibels used as the unit for attenuation (and gain)?

- dB use small numbers to represent drastic changes (ex. 20dB = change that is 100 times more)

- Attenuation is non-linear