Chapter 1: Physics Principles Examination Review as long as Ultrasound: Sonographic Pri

Chapter 1: Physics Principles Examination Review as long as Ultrasound: Sonographic Pri

Chapter 1: Physics Principles Examination Review as long as Ultrasound: Sonographic Pri

Buck, Martin, Formula One Editor has reference to this Academic Journal, PHwiki organized this Journal Chapter 1: Physics Principles Examination Review as long as Ultrasound: Sonographic Principles in addition to Instrumentation Steven M. Penny, B.S., RT(R), RDMS Traci B. Fox, MS, RT(R), RDMS, RVT Cathy Godwin, M.Ed, RT(R), RDMS, RDCS, RVT Basics of Sound Sound is a as long as m of energy. It is a pressure wave, created by a mechanical action, in addition to is there as long as e called a mechanical wave. Sound is produced when a vibrating source causes the molecules of a medium to move back in addition to as long as th. This backward in addition to as long as ward movement of the molecules creates waves of sound energy that travel, or propagate, through the medium. A medium is any as long as m of matter: solid, liquid, or gas. Sound requires a medium in which to propagate; there as long as e, it cannot travel in a vacuum. Basics of Sound When sound energy propagates through a medium, it does so in longitudinal waves, meaning that the molecules of the medium vibrate back in addition to as long as th in the same direction that the wave is traveling. In summary, sound is a mechanical, longitudinal wave. Longitudinal waves should not be confused with transverse waves where molecules in a medium vibrate at 90° to the direction of the traveling wave.

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Basics of Sound Acoustic variables are changes that occur within a medium as a result of sound traveling through that medium. The three primary acoustic variables are pressure, density, in addition to distance. When sound energy propagates through a medium, it causes the molecules to move back in addition to as long as th. Each back in addition to as long as th movement completes one wave or one cycle of movement. Each cycle consists of two parts: a compression, where the molecules are pushed closer together, in addition to a rarefaction, where they are spread wider apart. Basics of Sound The molecules, as they are squeezed together in addition to separated, cause changes in the pressure within the medium. Similarly, molecules undergoing compression in addition to rarefaction show variations in density. Density is defined as mass per unit volume. The Basics of Sound This movement of molecules, or particle motion, is due to propagating sound energy. Distance is defined as how far apart objects are, in addition to it is the measurement of particle motion. Distance may also be referred to as vibration or displacement.

Parameters of Sound A parameter is a measurable quantity. As this chapter progresses, the relationship that parameters of sound have with each other is discussed. Parameters may be described as directly related (directly proportional) or inversely related (inversely proportional) to each other. They are directly related when, if one parameter decreases, the other also decreases. Parameters are inversely related when one variable decreases, the other increases. (continued) Parameters of Sound Sound waves have several parameters that may be utilized to describe them. Parameters of sound waves include the period, frequency, amplitude, power, intensity, propagation speed, in addition to wavelength. Parameters of Sound Period (T) is defined as the time it takes as long as one cycle to occur. Since period is measured in time units, it is most often described in microseconds (s), or one millionth of a second. Frequency (f) is defined as the number of cycles per second. Frequency is measured in hertz (Hz), kilohertz (kHz), or megahertz (MHz).

Parameters of Sound Frequency in addition to period are inversely related. There as long as e, as frequency increases, the period decreases, in addition to as frequency decreases, the period increases. Their relationship is also said to be reciprocal. When two reciprocals are multiplied together, the product is 1. Consequently, period multiplied by frequency equals 1. Parameters of Sound Propagation speed (c) is defined as the speed at which a sound wave travels through a medium. All sound, regardless of its frequency, travels at the same speed through any particular medium. Parameters of Sound Propagation speeds tend to be the fastest in solids, such as bone, in addition to slowest in gases or gas-containing structures, such as the lungs. In the body, sound travels at slightly different speeds through the various organs in addition to tissues.

Parameters of Sound The units as long as propagation speed are meters per second (m/s) or millimeters per microsecond (mm/s). The average speed of sound in all soft tissue is considered to be 1540 m/s or 1.54 mm/s. This number was derived by averaging all of the actual propagation speeds of the tissues in the body. Parameters of Sound The propagation speed of sound in a medium is influenced by two properties: the stiffness (elasticity) in addition to the density (inertia) of the medium. Stiffness is defined as the ability of an object to resist compression in addition to relates to the hardness of a medium. Stiffness in addition to propagation speed are directly related: the stiffer the medium, the faster the propagation speed. Parameters of Sound Conversely, density, which can be defined as the amount of mass in an object, is inversely related to propagation speed. As the density of a medium increases, the propagation speed decreases.

Parameters of Sound The length of a single cycle of sound is called the wavelength (). It is the distance from the beginning of a cycle to the end of that cycle. Waves can be of any length, from several miles in some ocean waves to a few millimeters, as found in diagnostic ultrasound waves. In clinical imaging, the wavelengths measure between 0.1 in addition to 0.8 mm. Like period, wavelength in addition to frequency are inversely related. Parameters of Sound If frequency increases, wavelength decreases in addition to vice versa. However, the wavelength of a sound wave is also influenced by the propagation speed of the medium in which it is traveling. The faster the propagation speed, the longer the wavelength. In diagnostic imaging, because the average propagation speed of sound in soft tissue is treated as a constant of 1540 m/s, any change in the wavelength would be related only to changes in the frequency. Parameters of Sound Wavelength is in essence equal to the propagation speed divided by the frequency. It is important to note that the wavelength of a 1- in addition to 2-MHz transducer is 1.54 in addition to 0.77 mm, respectively.

Parameters of Sound Amplitude, power, in addition to intensity all relate to the size or strength of the sound wave. All three of these decrease as sound travels through a medium. Parameters of Sound Amplitude is defined as the maximum or minimum deviation of an acoustic variable from the average value of that variable. As sound propagates through a medium, the acoustic variables (distance, density, in addition to pressure) will vary, in addition to there as long as e, they may increase or decrease. The amplitude of these changes can be measured. When amplitude is discussed in ultrasound physics, it is commonly the pressure amplitude that is being referenced. The units of amplitude are Pascals (Pa). Parameters of Sound Power (P) is defined as the rate at which work is per as long as med or energy is transmitted. As a sound wave travels through the body, it loses some of its energy. There as long as e, power decreases as the sound wave moves through the body. The power of a sound wave is typically described in units of watts (W) or milliwatts (mW). Power is proportional to the amplitude squared. There as long as e, if the amplitude doubles, the power quadruples.

Parameters of Sound The intensity of a sound wave is defined as the power of the wave divided by the area (a) over which it is spread, or the energy per unit area. Intensity is proportional both to power in addition to to amplitude squared. Intensity is measured in units of watts per centimeter squared (W/cm2) or milliwatts per centimeter squared (mW/cm2). Intensities typically range from 0.01 to 100 mW/cm2 as long as diagnostic ultrasound. Parameters of Sound Any medium through which sound is traveling will offer some amount of resistance to the sound. The resistance to the propagation of sound through a medium is called impedance (z). The amount of impedance depends on the density () in addition to the propagation speed (c) of the medium. Keep in mind that the density in addition to stiffness are the controlling factors of propagation speed. Impedance is measured in units called Rayls. Rayls are the product of the density of the medium in addition to the propagation speed of sound in the medium. Parameters of Sound There are slight variations in the density of the various tissues in the body just as there are slight variations in the propagation speed. Recall that 1540 m/s is used as the average speed of sound in all soft tissue. As a result, many of the tissues will have different impedance values. It is these variations in impedance that help create reflections at the interface between adjacent tissues.

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Parameters of Sound Assuming the beam strikes the interface at a 90° angle in addition to there exists a large impedance difference between two tissues, there will be a strong reflection in addition to a well-defined boundary displayed on the imaging screen. If the impedance difference between two media is more subtle, there will be a weaker reflection. If the impedances are the same, no reflection occurs. Continuous Wave Ultrasound Thus far in this chapter, we have been describing properties of all sound waves, which certainly apply to ultrasound waves as well. Sound that is continuously transmitted is termed continuous wave (CW) sound. We cannot image using CW ultrasound, though it is often employed as long as Doppler studies. Pulse–Echo Technique In order as long as an image to be created using sound, the sound waves must not only be sent into the body, but the sound returning from the body must be timed to determine the reflector’s distance from the transducer; this describes the pulse–echo technique. After a pulse is sent out, the machine listens as long as the sound to come back in addition to calculates how long it takes as long as the pulse to come back to the transducer. As a result of waiting as long as the pulse of sound to come back in addition to timing its travel, the machine is able to plot the location of the reflectors on the display.

Pulse–Echo Technique Transducers have material within them that, when electronically stimulated, produces ultrasound waves. These are referred to as piezoelectric materials (PZT) in addition to most often consist of some as long as m of lead zirconate titanate. PZT materials operate according to the principle of piezoelectricity, which states that pressure is created when voltage is applied to the material in addition to electricity is created when a pressure is applied to the material. Pulse–Echo Technique Piezo literally means to squeeze or press. Within the transducer, the element is electronically stimulated or stressed, which results in a pressure wave (sound) as a result of the vibration of the material. Diagnostic ultrasound uses high-frequency sound waves that are sent into the body by the transducer (transmission), in addition to then the transducer momentarily listens as long as returning echoes (reflection). The characteristics of the returning echoes are utilized by the ultrasound machine to create an image. Parameters of Pulsed Sound Remember that frequency is defined as the number of cycles of sound produced in 1 second. The number of pulses of sound produced in 1 second is called the pulse repetition frequency (PRF). Frequency in addition to PRF are not the same.

More about Intensity When grouped together, the spatial in addition to temporal intensities provide a specific explanation as long as the measurement of the intensity of the sound beam in both space in addition to time. It is most important to note that SATA is the lowest of the intensities, SPTP is the highest, in addition to the SPTA intensity is used when describing thermal bioeffects. More about Intensity The hydrophone, or microprobe, is a device used to measure output intensity of the transducer. It can be a needle-type device or a broad, disk-shaped device. Both types of hydrophones consist of a transducer that is placed into the path of the beam to measure PRP, PD, in addition to period. From these measurements, other parameters, such as frequency, wavelength, SPL, PRF, in addition to DF, can be derived. The hydrophone is also used to determine pressure amplitude in addition to intensities, which are important as long as patient safety.

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