Period and Frequency

Ultrasound is defined as a sound with a frequency of: A. greater than 20,000 kHz B. less than 1 kHz C. greater than 10 MHz D. greater than 0.02 MHz

D. Ultrasound is defined as an acoustic wave with a frequency so high that it is not audible to humans. Ultrasound is an inaudible wave with a frequency of at least 20,000 Hz, 20 kHz, or 0.02 MHz.

Which of the following cannot be considered a unit of frequency? A. per day B. cycles/sec C. Hz D. herz E. cycles

E. The term cycles informs us of the number of events, but does not inform us of the duration of time required for those events to occur. Choice E is incomplete, and is not a unit of frequency. All the other choices reveal that a number of events took place in a specific time span.

T/F With standard ultrasound pulses, the frequency of the ultrasound changes significantly as the wave propagates through the body

FALSE. In diagnostic imaging, the frequency of the sound wave generally remains constant and does not routinely change as the sound propagates through the body. Slight changes in frequency occur when sound strikes moving structures. This forms the basis for Doppler ultrasonography.

Which of the following units are appropriate to describe the period of an acoustic wave? (More than one answer may be correct.) A. minutes B. microseconds C. meters D. mm/us E. cubic centimeters

A and B. The period of a wave is defined as the time that elapses as a wave oscillates through a single cycle. The units for period must be a measure of time, such as minutes or seconds. A and B are units of time. The incorrect selections C, D, and E are units of distance, speed, and volume, respectively.

What determines the period of an ultrasound wave? A. the transducer B. the medium through which the sound travels C. both A and B D. neither choice A nor B

A. The sound source (the transducer) that produces an acoustic signal determines the period of a wave. The wave's period is unrelated to the medium through which the sound travels, and will not change as the wave moves from one medium to another.

What establishes the frequency of an ultrasound wave? A. the transducer B. the medium through which the sound travels C. both A and B D. neither choice A nor B

A. When created by a transducer, an ultrasound wave has a specific frequency. The frequency is not determined by the medium through which the sound travels. Only the sound source (the transducer) establishes the wave's frequency.

Infrasound is defined as a sound with a frequency of: A. greater than 20,000 kHz B. less than 20 Hz C. greater than 10 MHz D. less than 0.02 MHz

B. Infrasound is defined as an acoustic wave with a frequency so low that it is not audible to humans. Infrasound is an inaudible wave with a frequency of less than 20 Hz.

What term describes the number of cycles that an acoustic variable completes in a second? A. period B. frequency C. PRP D. variable rate

B. This is the definition of the term frequency. Frequency can also be thought of as the number of regularly occurring events in a specific time.

What is the range of periods commonly found in waves produced by ultrasound systems? A. 0.001 to 1 s B. 0.06 to 0.5 us C. 0.2 to 1 ms D. 10 to 100 us

B. Ultrasonic imaging waves have a period in the range of 0.06 to 0.5 us. The period is the time of a single cycle. Period is the reciprocal of frequency. A wave with a frequency of 2 MHz has a period of 0.5 usec. A wave with a frequency of 15 MHz has a period of 0.06 us.

With standard ultrasonic imaging, what happens to the period of a wave as it propagates? A. increases B. decreases C. remains the same

C. Certain parameters of a wave change as the wave travels through the body. However, the period and frequency of a wave typically remain constant as a sound wave propagates.

Compare two sound waves, A and B. The frequency of wave A is 1/3 that of wave B. How does the period of wave A compare with the period of wave B? A. A is 1/3 as long as B B. A is the same as wave B C. A is 3 times as long as B D. cannot be determined.

C. Frequency and period are reciprocals. If the frequency of one wave is 1/3 as large as another, then the period of the wave will be three times longer than the other.

____ is the reciprocal of period. A. inverse period B. PRP C. frequency D. propagation speed

C. Frequency is the reciprocal of period. Reciprocals are related in two ways: -As one increases, the other decreases. -When they are multiplied together, the result is unity. For example, a wave with a period of 0.01 of a second has a frequency of 100 per second or 100 Hz.

The frequency of a continuous acoustic wave is 5 MHz. The wave is then pulsed with a duty factor of 0.1. What is the new frequency? A. 0.5 B. 0.5 MHz C. 5 MHz D. 10 MHz

C. Frequency is the reciprocal of the period. The fact that a wave is pulsed rather than continuous does not alter the frequency of the signal. The new and old frequencies are the same, 5 million/second.

When a sonographer increases the maximum imaging depth during an exam, what happens to the frequency? A. increases B. decreases C. remains unchanged

C. Imaging depth and frequency are unrelated. When the depth of view is increased, the frequency of sound remains the same.

What is the range of frequencies emitted by transducers used in ultrasonic imaging? A. 1 to 3 MHz B. 1 to 1,000 kHz C. -10,000 to +10,000 Hz D. 2,000,000 to 15,000,000 Hz

D. Frequencies commonly used in diagnostic imaging range from approximately 2 to 15 megahertz or 2 to 15 million cycles per second.

What is characteristic of acoustic waves with frequencies exceeding 20,000 Hz when compared with waves having frequencies of less than 20,000 Hz? A. they travel more effectively in soft tissue B. they travel more rapidly C. they attenuate less when traveling in soft tissue D. humans can't hear them.

D. Waves with frequencies exceeding 20 kHz are inaudible to humans and are called ultrasonic. They travel at the same speed as waves with lower frequencies and attenuate at a faster rate than waves with lower frequencies.

T/F: The sonographer has the ability to alter the period of an ultrasound wave that is produced by a basic transducer.

FALSE. The sonographer cannot adjust the period (or frequency) of a wave produced by a basic transducer. Think of striking a key on a piano. The frequency of sound created by striking a single key is constant. If the sound beam's frequency and period are not suited for the particular type of imaging, the sonographer must select a new transducer with a different frequency.

T/F waves in the ultrasound range behave in the same general manner as sound waves that are audible

TRUE. As stated above, the primary difference between audible and ultrasonic waves is that humans can hear audible waves. A wave's behavior or adherence to physical laws and principles is generally the same, regardless of whether it can be heard by humans.

T/F: If the periods of two waves are the same, then the frequencies of the waves must also be the same.

TRUE. Frequency and period are reciprocals. When the periods of two waves are identical, the frequencies of the waves must also be identical.

T/F The period of an ultrasound wave is related to the frequency and is the same regardless of whether the wave is pulsed or continuous

TRUE. The period of a wave is a characteristic of each individual cycle in the wave. The period of a wave (as well as frequency and wavelength) remains unchanged, whether the wave is continuous or pulsed.