Describing Sound Waves. Period. Frequency. Parameters used to completely characterize a sound wave. Chapter 3. Period Frequency Amplitude Power

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1 Parameters used to completely characterize a sound wave Describing Sound Waves Chapter 3 Period Frequency Amplitude Power Intensity Speed Wave Length Period Defined as the time it take one wave vibrate a single cycle. The start of one cycle to the start of the next cycle Measured in units of time. ex. Milliseconds, seconds. Determined by the source only and not the medium It is not adjustable by the sonographer Frequency As pertains to the ultrasound wave Defined as the number of complete cycles in one second. The unit of measure is the hertz. ex 1 cycle in 1 second = 1 hertz Typical clinical values range from 2 to 15MHz Determined by the source and not the medium not adjustable with some ultrasound systems and transducer combinations 1

2 Source and Medium The source of a sound wave is the ultrasound system and transducer combination. The sonographer can adjust some of the system parameters while others are fixed. The material or tissue through which the sound travels is refereed to as the medium. Where did the term ultrasound come from The human ear can only hear sound when it is within an audible range. 20 Hz to 20 khz sound that has a frequency below 20 Hz is referred to as infrasound Sound that is greater than 20 khz is referred to as ultrasound Frequency is important to the very essence of ultrasound imaging. It has pronounced effects on both image quality and depth of penetration Period and frequency are inversely related to each other. As frequency increases the period will decrease As frequency decreases the period will increase. Period and frequency also have a reciprocal relationship When the two parameters are multiplied together the result is 1 If a wave takes 1 / 5 of a second to complete one cycle the period is 1 / 5 second and the frequency is 5 per second. 1 / 5 x 5 = 1 The frequency of this wave is 8Hz and the period is 1/8 of a second 2

3 Parameters that describe the magnitude of a sound wave are: amplitude power intensity Amplitude Defined as the of a given wave either above the baseline or below the baseline The unit of measure is variable and includes pressure - pascals density -g/cm 3 particle motion - cm, inches - any distance Amplitude can also be expressed in decibels (db) Peak to peak amplitude Amplitude continued Typically in clinical imaging pressure amplitude ranges from 1 million pascals (1MPa) to 3 million pascals (MPa). Determined only by the source the amplitude however does decrease as it propagates through the body and is also affected by the density of the medium as well Adjustable with a control on ultrasound systems allowing the sonographer to alter the initial amplitude Power Is the rate of energy transfer, or the rate at which work is done The unit of measure is watts Typical clinical imaging values range from to watts also expressed as 4 to 90 milliwatts Determined by the source and not the medium not adjustable with some ultrasound systems and transducer combinations 3

4 Power continued Determined by the source and not the medium However power does decrease as it propagates through the body and is also affected by the density of the medium as well It is adjustable with a control on the ultrasound systems allowing the sonographer to alter the initial power of the wave Power continued When power increase amplitude increases and when power decreases amplitude decreases. Power is proportional to amplitude 2 as you increase the amplitude of a wave by a factor of 4 you have increased the power by 16. 4x4=16 as you decrease the amplitude of a wave to ½ of the original value you have changed to power to ¼ its original value, ½ x ½ = ¼ Intensity Defined as the concentration of energy in a sound beam. Calculated by dividing the beams power by the beams cross-sectional area The units of intensity are watts/square centimeter (W/cm 2 ) watts from the power and cm 2 from the beam area intensity (W/cm2) = power (w) area (cm 2 ) It depends on the power of the beam and over how big an areas it is spread out on Intensity continued Typical clinical intensity values range from 0.01 to 300 W/cm 2 Determined by the source initially but its characteristics changes as it propagates through the medium and if one adjusts the beam shape. It is adjustable with ultrasound systems allowing the sonographer to either increase or decrease the initial intensity wave. Intensity continued Relationship between intensity and power Intensity is proportional to power if a the power of a wave doubles the intensity also doubles if the power of a wave is cut in half so too the intensity is cut in half Intensity continued Relationship between intensity and amplitude Intensity is proportional to the amplitude of the wave squared. if the wave s intensity is doubled the amplitude is increased four times 4

5 Wavelength Wavelength The distance or length of one complete cycle Units of measurements include but not limited to mm, meters In diagnostic imaging the wavelength in soft tissue range form 0.15 to 0.8 mm Is determined by both the source and medium The sonographer cannot adjust the wavelength of a particular transducer. (even a multihertz transducer has fixed wavelengths) Wavelength continued Wavelength is inversely related to frequency In soft tissue sound with a frequency of 1 MHz has a wavelength of 1.54mm This can be converted into a formula to calculate the wavelength of may MHz transducer wavelength (mm) = 1.54 mm/us frequency (MHz) Wavelength continued Wavelength plays an intricate role in role in ultrasound imaging The higher the frequency the shorter the wavelength and the higher the image quality The lower the frequency the longer the wavelength and the lower the image quality Wave length of 3 different frequencies Propagation speed Defined as the distance a sound wave travels in 1 second through a medium Speed is measured in meters per second, mm/μs, or distance divided by time Depending on the type of tissue the sound is traveling through the typical speed ranges from 500 m/s to 4000m/s 5

6 Propagation speed continued Speed is affected only by the medium through which it travels Irrespective of the frequency all sound travels at the same speed through the same medium Speed changes only when it travels form one medium to another medium of different density Propagation speed continued The average speed of sound in soft tissue is 1,540 m/s or 1.54 mm/μs or 1/54 km/s Sound travels fastest in solids slower in liquids and the slowest is gasses Speed(m/s) = frequency (Hz) x wavelength (m) Propagation speed continued Propagation speed continued Tissue Type Lung Fat Speed (m/s) 500 1,450 Material Air Speed (m/s 330 Soft Tissue (average) Liver 1,540 1,560 Water 1,480 Blood Muscle 1,560 1,600 Metals 2,000 to 7,000 Tendon 1,700 Bone 3,500 Characteristics of a medium Stiffness is the ability of an object to resist compression ex. a golf ball is stiff and Jell-O is not Stiffness and speed are directly related If you stiffness you speed Elasticity and compressibility are terms used to describe a type of stiffness The skin is elastic The appendix is compressible Characteristics of a medium continued Density is the weight of a material When equal volumes of material are compared the dense material will weight the most Ex bone is more dense than muscle Density and speed are inversely related As density speed will 6