Ultrasound Physics Quiz Cards

Name four additional terms that are synonymous with axial resolution.

Longitudinal, axial, range, radial and depth (LARRD)

What is the Curie temperature or Curie point?

Materials that possess piezoelectric properties are sensitive to high temperatures. If piezoelectric material is heated to temperatures in excess of the Curie temperature or Curie point, it will permanently lose its piezoelectric properties. The Curie temperature is in the range of 300 to 400 degrees celsius.

Name three methods that may be used to focus an ultrasound beam.

Methods of focusing include the following: 1. Use of an acoustic lens (external focusing) 2. Use of a curved active element (internal focusing) 3. Electronic focusing (phased array). Note: Phased array means adjustable focus or multi-focus.

What material is most commonly used to construct the active element?

Most active elements are fabricated from a material known as lead zirconate titanate or PZT.

What us the overall shape of a linear phased array transducer?

Most linear phased array transducers are small and have a rectangular or square acoustic footprint. The area of transducer-skin contact is often no larger than 1 cm X 1 cm.

What is focusing?

Focusing is any process that narrows the diameter of an ultrasound beam, thereby improving lateral resolution.

Why is focusing important in diagnostic imaging?

Focusing is important in diagnostic imaging because it can improve image quality. Remember that lateral resolution is determined by the diameter of a sound beam. As a result of focusing, the diameter of the beam is reduced, the lateral resolution is improved, and the ability of the ultrasound system to identify side-by-side structures in close proximity to one another is enhanced.

How is focusing achieved when using an annular phased array transducer?

Focusing with an annular phased array transducer is accomplished electronically. We previously learned that the focal depth of an ultrasound beam is partially determined by the diameter of the ultrasound crystal. Small diameter crystals has a shallow focus, whereas large-diameter crystals have deeper foci. Annular phased arrays use this physical principle. First, the inner ring of the array is fired, and information from the shallow depths is obtained. Then the next outer ring is fired, and data is obtained from an intermediate depth. This process continues, with larger, outer rings acquiring data from ever-increasing depths.

The part of the beam that continues beyond the focus is called the

Fraunhofer or far zone.

What units are used to report frequency?

Frequency is reported in the following units: per second, or hertz (Hz).

What is the frequency of a sound wave?

Frequency refers to the number of times per second that the particles in a medium oscillate back and forth as a sound wave propagates through the medium.

The part of the ultrasound bea,m between the transducer face and the point where the beam reaches its smallest diameter is called the

Fresnel zone is also known as the near zone.

How is the focal zone affected by focusing?

Generally, the greater the degree of focusing, the greater the divergence of the sound beam beyond the focal zone. Thus, although strongly focused transducers will provide superb lateral resolution at the focal depth, image quality will degrade beyond this point. The greater the degree of focusing, the smaller the region of the focal zone.

How is attenuation related to the frequency of a sound wave?

Higher frequency sound is attenuated to a greater degree than low frequency sound. This is why lower frequency transducers are more successful at imaging structures that lie deep within the body than higher frequency transducers.

Describe Huygen's principle.

Huygen's principle explains why a beam produced by an ultrasound transducer has the shape of an hourglass rather that that of a wedge. Huygen predicted that a sound beam created by a transducer is actually made up of thousands of tiny "wavelets" of sound, each of which is wedge-shaped. As these wavelets travel, they interfere with each other in a manner that produces an hourglass-shaped sound beam.

What are the units of the Q-factor?

It is calculated by dividing the primary operating frequency of a transducer by its bandwidth. Therefore, the Q-factor is unitless.

While traveling through the body, ultrasound waves encounter many different media, including bone, fat, lung and soft tissue. Rank these four media according to their propagation speed (i.e., how fast sound travels through them), in increasing order.

1. Lung 2. Fat 3. Soft tissue 4. Bone The propagation speed of sound is slowest in the air-filled lungs, then fat, soft tissue and fastest in bone. Remember, all sound travels through soft tissue at exactly 1,540 meters per second.

What are the three most important facts about the quality factor?

1. The Q-factor is a unitless number 2. The Q-factor for diagnostic imaging transducers has a value in the range of 2 to 4. 3. The Q-factor from imaging transducers has a lower value than that for transducers used in therapeutic ultrasound.

What is the meaning of 10 dB? What is the meaning of -10 dB?

10 dB means that the beam's intensity is 10 times greater. -10 dB means that the beam's intensity is only 1/10th of the original intensity.

What is the meaning of 3 dB? What is the meaning of -3 dB?

3 dB means that the beam's intensity has doubled. -3 dB means that the beam's intensity is only half of the original intensity.

How is focusing and steering of a sound beam accomplished when using a convex phased array system?

A convex phased array system is identical to a linear phased array system with one exception. In convex array, the elements are arranged in a curved line rather than a straight line. In spite of this difference, both linear phased arrays and convex phased arrays steer and focus the ultrasound beam by using electronic time delays that appear as a slope and curvature in the pattern of the electrical signals. All of the crystals of the array are excited by electrical voltages almost simultaneously, and each crystal produces a tiny sound wavelet. The wavelets interact with each other and produce a single acoustic pulse that is used to create an image.

What is the shape of the face of a convex sequential transducer array?

A convex sequential (also called convex switch) transducer is a large device, with long curved or arched dace that makes contact with the skin. The face of the transducer can be up to 10 cm in length.

What kind of image shape does a convex sequential transducer produce?

A convex sequential array transducer generates a two-dimensional image that is sector-shaped. Unlike images produced by use of mechanical and linear phased array systems, which starts at a narrow point, the shallow region of a convex sequential array image is blunted in the zone adjacent to the transducer face. At deeper imaging depths, the image appears identical to any other sector-shaped scan.

What is a decibel?

A decibel (dB) is a unit used to measure the relative change in the intensity or power of a sound beam. A relative change compares the current intensity of the sound beam with its original intensity. For example. if a sound beam's power has increased by 50%, compared with its original power, this is a relative change.

How is beam steering and focusing achieved with a linear phased array transducer?

A linear phased array system steers and focuses its ultrasound beams in the following manner. Basically, the ultrasound system transmits electrical pulses to all of the PZT crystals at nearly the same time. As a result, all of the crystals within the array are fired at almost the same time to produce each single acoustic pulse. Minuscule time delays exist between the electronic signals that strike the PZT crystals, however, and the pattern of these tiny time delays bring about steering and focusing.

What kind of image shape does a linear phased array transducer produce?

A linear phased array transducer produces images hat are sector shaped. In contrast, linear sequential arrays produce rectangular-shaped images.

What kind of image shape does linear sequential transducer produce?

A linear sequential array transducer generates a two-dimensional image that has a rectangular shape. The operator may select the maximum depth of the image. The length of the transducer's face limits the maximum width of the image. If the transducer face is 10 cm long, then the maximum width of the two-dimensional image will also be 10 cm.

What is the overall shape of a linear sequential trasnducer?

A linear sequential probe, also called a linear switched probe of, more commonly, a linear transducer, has a long flat face that makes contact with the patient's skin. The contact area, or "footprint," of the probe can be up to 4 inches in length. This ample contact area between transducer and skin is called a large acoustic footprint.

What is transducer array?

A transducer array is an ultrasound probe containing multiple active elements or piezoelectric crystals in a single housing. The combination of many PZT crystals within a single transducer gives the ultrasound systems the capability to produce a two-dimensional image. Depending on the transducer array, the ultrasound beam can be steered in various directions and focused at a variety of depths.

What is a transducer?

A transducer is any device that converts one form of energy into another. Examples of transducers include a light bulb (converts electrical energy into heat and light), and a stereo speaker (converts electrical energy into sound).

What is diffraction?

Diffraction causes an ultrasound beam to diverge or spread out as the wave travels away from the transducer. A diffraction pattern is a wedge shaped, similar to that of a wake produced by a motorboat when it moves across the surface of a calm lake. The divergence of a sound bean due to diffraction is more pronounced with smaller sound sources. The wedge-shaped wave is also called a Huygen's wavelet.

Sound beams with narrow diameters have the best lateral resolution. Therefore, imaging systems are designed to create the narrowest beams possible. How then does an ultrasound system create a two-dimensional image when the sound beam emitted by the transducer is extremely narrow?

A two-dimensional image, or frame, is created from a large number of individual image scan lines. An ultrasound pulse is transmitted, and a thin scan line of image data is acquired and stored. Then another ultrasound pulse, steered in a slightly different direction is emitted, and the reflected information is stored. When this process is repeated many times, a two-dimensional image is created. Therefore, a two-dimensional image is constructed from numerous, narrow pulses of ultrasound.

Does each piezoelectric element in the transducer require its own wire?

An annular phased array transducer has multiple active elements configured as a series of rings with a common center. Each piezoelectric ring within the probe must be acoustically and electrically isolated from all other rings. Therefore, each ring will have its own wire connecting it to the ultrasound system.

What is the overall shape of an annular phased array transducer?

An annular phased array transducer is quite similar in appearance to a mechanical transducer. The transducer face that makes contact with the skin is slightly curved and is generally quite small. Only a few square centimeters of the transducer face make contact with the skin. This is called the acoustic footprint.

What kind of image shape does an annular phased array transducer produce?

An annular phased array transducer is relatively small. The ultrasound beam must be mechanically steered or directed in a variety of directions in order to create a two-dimensional anatomic image. Such steering creates a sector-shaped image.

How does the damping material or backing material of a transducer affect an ultrasound system's longitudinal resolution?

An ultrasound system's ability to produce high-quality images depends, in part, upon its longitudinal resolution. Excellent longitudinal resolution is obtained when the transducer creates very short pulses. The damping material or backing material of a transducer has the important function of inhibiting the ringing of the active element. Therefore, damping shortens the spatial pulse length and the pulse duration. Damping also decreases the numerical value of the longitudinal resolution, resulting in better quality images. Damping is analogous to a person grasping a cymbal, after it has been struck, to stop its ringing. Without damping material, a system would not produce high-quality images. The active element would generate a sustained vibration, creating a long acoustic pulse. This would severely degrade the system's longitudinal resolution. Note: longitudinal resolution is also called axial, range, radial and depth resolution.

What happens to the image when one or more of the crystals in an annular phased array transducer malfunction?

Annular phased array systems use each ring-shaped element of the transducer to acquire anatomic information from a particular depth. Small inner rings obtain data from shallow depths, while the large outer rings acquire information from deeper regions. If a single active element is inoperative, the system will be unable to acquire information from that depth, and a region of signal dropout extending horizontally across the imager will appear. As an example, there may be signal dropout extending side-to-side in the image within a region from 5 to 7 cm in depth, while the image is intact in both shallower and deeper regions.

A single sound pulse from mechanical, linear sequential, and linear phased array transducers normally creates a single scan line on the ultrasound image. Annular phased array systems are different. How do annular array systems create a single scan line on the image?

Annular phased array systems use multiple acoustic pulses to create a single scan line on a two-dimensional image. The first sound pulse originates from the innermost ring and provides information from superficial depths. The next outermost ring of the array produces the second sound pulse, and data is acquired from an even deeper region. Therefore, a single scan line on the image is actually constructed from a number of acoustic pulses.

What is the meaning of the term pressure? Which units are used to measure pressure?

As a sound wave travels through a medium, the molecules of the medium are rhythmically compressed and expanded. This compression and rarefaction creates a pressure wave. Pressure is defined as concentration of force within a particular area, and is reported in units called Pascals (Pa).

How does focusing affect the depth of the focus (also known as the near zone length)?

As the degree of focusing increases, the depth at which the focus is located becomes shallower. Thus, a strongly focused transducer will have a shallower focal depth than an otherwise identical transducer that is weakly focused.

What is attenuation?

Attenuation refers to the decrease in strength of an ultrasound wave as it travels through tissue.

What is audible sound?

Audible sound is defined as a sound wave that can be heard by man. The frequency range of audible sound is from 20 Hz to 20,000 Hz.

What is axial resolution?

Axial resolution describes the ability of an ultrasound system to identify two adjacent structures or reflectors that lie front-to-back or parallel to the main axis of the sound beam.

What units are used to express axial resolution?

Axial resolution is expressed in units of distance, such as millimeters. The axial resolution is reported as the minimum distance by which two structures located front-to-back along the axis of the beam must be separated in order for the system to display them as two distinct reflections. Thus, the smaller the numerical value for the axial resolution, the better the image quality.

Example: the impedance of PZT is approximately 30,000,000 rayls, and the impedance of skin is about 1,600,000 rayls. (A rayl is a unit used to measure impedance.) For the purpose of ultrasonic imaging, which of the following represents the optimal choice for the impedances of the matching layer and the ultrasound gel? A. Matching layer 14,000,000 rayls. Gel 25,000,000 rayls. B. Matching layer 20,000,000 rayls. Gel 15,000,000 rayls. C. Matching layer 14,000,000 rayls. Gel 45,000,000 rayls. D. Matching layer 14,000,000 rayls. Gel 1,000,000 rayls.

B. Matching layer 20,000,000 rayls. Gel 15,000,000 rayls. In order for the maximum amount of sound energy to be transmitted from one medium to another, the acoustic impedances of adjacent media should be as similar as possible. In this example, maximum sound transmission will occur when the impedances of all the media, from the active element to the skin, are in decreasing order. B is appropriate because the impedances from the active element to the matching layer to the gel and, finally, to the skin decrease sequentially.

What effects does the backing material have on the bandwidth?

Because acoustic pulses from imaging systems are very short (in order to improve axial resolution), they have a wide bandwidth. It is the backing, or damping, material that is responsible for keeping the pulse short. The bandwidth produced by ultrasound systems from continuous wave Doppler evaluation of blood flow or for therapeutic applications are narrower than those produced by diagnostic imaging systems.

Does each piezoelectric element in a convex sequential transducer array require its own wire?

Behind the matching layer of the transducer are multiple active elements arranged in a curved arc. There may be hundreds of individual PZT crystals in a convex sequential transducer. In order to operate effectively, each crystal within the transduce must be electrically and acoustically isolated from its neighboring crystals. Therefore, each crystal is attached to its own wire and each crystal is backed with damping material.

Does each piezoelectric element in a linear sequential transducer requires its own wire?

Behind the matching layer of the transducer is an array of active elements arranged in a line. A linear sequential array may have hundreds of distinct PZT crystals. In order to operate effectively, each crystal within the transducer must be electrically and acoustically isolated from its neighboring crystals. Therefore, each crystal is attached to the ultrasound system by its own wire. Each wire is connected to an electronic circuit within the system called a channel. Backing material is located behind each PZT crystal and a matching layer is located in front.

Does each piezoelectric element in a linear sequential transducer require its own wire?

Behind the matching layer, located on the face of the transducer, is an array of active elements arranged in a line. Like the linear sequential array, there may be hundreds of distinct PZT crystals on a linear phased array. In order to operate effectively, each crystal must be electrically and acoustically isolated from its neighboring crystals. Therefore, each crystal is attached to its own wire and is backed with damping material. Architecturally, one may think of linear phased array transducers as miniaturized linear sequential array transducers.

Two sound waves are traveling through an opera house. One wave was produced by a soprano, while a baritone produced the other. Which sound wave travels faster?

Both waves travel at exactly the same speed. Only the characteristics of the medium through which sound travels determines the speed of sound.l The speed of sound is not affected by the characteristics of the wave. Since both waves are traveling through the same medium (i.e., the air), they must have identical speeds.

A single sound pulse from mechanical, linear sequential, and linear phased array transducers normally creates a single scan line on the ultrasound image. Annular phased array systems are different. What are the disadvantages of using annular phased array systems?

Compared with other types of transducers, the primary disadvantage of an annular phased array system is that it requires longer time to generate each two-dimensional image. In other systems, a single sound pulse creates each scan line. In annular phased arrays, numerous sound pulses are required to construct each scan line. This process is time consuming. Therefore, when using an annular array system, the sonographer may trade reduced temporal resolution for improved lateral resolution.

Compared with soft tissue, is ultrasound attenuated more or less when it travels through air?

Compared with soft tissue, ultrasound experiences enormously more attenuation when it travels through air.

Compared with soft tissue, is ultrasound attenuated more or less when it travels through bone?

Compared with soft tissue, ultrasound experiences much more attenuation when it travels through bone because bone absorbs ultrasonic energy. However, attenuation in bone is less than in air.

Compared with soft tissue, is ultrasound attenuated more or less with it travels through lung?

Compared with soft tissue, ultrasound experiences much more attenuation when it travels through lung because lung scatters ultrasonic energy. However, attenuation in lung is less than air.

Compared with soft tissue, is ultrasound attenuated more or less with it travels through water?

Compared with soft tissue, ultrasound experiences substantially less attenuation when it travels through water.

Which of the following ultrasound transducers will have the shortest near zone (or the shallowest focus)? A. 4-mm active elements diameter, 5-mm active element thickness, and 5-MHz frequency B. 6-mm active element diameter, 8-mm active element thickness, and 4-MHz frequency C. 10-mm active element diameter, 2-mm active element thickness, and 7-MHz frequency D. 3-mm active element, 7-mm active element thickness, and 2-MHz frequency

D. 3-mm active element, 7-mm active element thickness, and 2-MHz frequency. The transducer with the smallest diameter and the lowest frequency will have the shortest near zone length (the deepest focus). Note that in this example, the thickness of the crystal does not affect the near zone length in any way.

What material is used to fabricate the damping material?

Damping material is often made of tungsten fibers embedded into an epoxy mixture. The epoxy mixture adheres to the backside of the active element. The active element is sandwiched between the damping material and the matching layer.

What is the importance of duty factor?

Duty factor is very important in the study of biological effects of US.

What is the function of the matching layer of an ultrasound transducer?

If a sound pulse was allowed to travel from the active element of a transducer directly to the skin, a giant reflection called the "main bang" would occur at the boundary between the active element and the skin. Such large reflection would occur because of the dramatic difference in the acoustic impedances of these two structures. The function of the matching layer, which is located in front of the active element, is to minimize the main bang. This is accomplished by constructing the matching layer out of material that has an acoustic impedance in between the active element and skin. Thus, the matching layer permits more sound energy to travel into the body during transmission and reflected out of the body during reception.

What characteristics of the electrical pattern within a phased array transducer result in steering of the ultrasound beam?

If the line that connects the electrical signals has a slope, then the resulting sound beam will be steered.

What characteristics of the electrical pattern within a phased array transducer result in focusing of the ultrasound beam?

If the line that connects the electrical signals is curved or cupped, then the resulting sound beam will be focused.

What is the shape of the active elements of convex array transducer? How are they arranged?

In a convex array transducer, the active elements are rectangular-shaped and arranged in an arc, and thus, the face of the transducer has a dome-shaped appearance.

What is the shape of the active element of a linear array transducer? How are they arranged?

In a linear array transducer, the active elements are rectangular-shaped and arranged in a straight line.

What is the shape of the active elements of an annular array transducer? How are they arranged?

In an annular array transducer, the active elements are ring-shaped and are arranged in a concentric pattern. Thus, an annular array appears as a collection of rings of increasing size.

Ultrasound beams must be steered in different directions to create a scan plane. How is the sound beam steered when using an annular phased array transducer?

In order to create a two-dimensional image, the sound beams produced by an annular phased array transducer must be steered in a variety of directions. This steering is accomplished by two techniques: 1) attaching the PZT crystals to a motor and moving them directly, or 2) firing stationary crystals toward a mirror that swivels, thereby steering the sound beam.

With respect to steering and focusing of the ultrasound beam, is a linear phased array transducer similar to a linear sequential array transducer?

In terms of steering and focusing of the ultrasound beam, the two types of transducers are not similar. If a linear phased array system functioned like a sequential array, the resulting images would be only 1 cm in width, which would of course be of little clinical value. Therefore, linear phased array systems operate in a fundamentally different way from sequential array systems.

What is infrasound?

Infrasound is defined as a sound wave with a frequency less than 20 hertz. The frequency of infrasound is so low that it is inaudible to the human ear.

What is the meaning of the term intensity? Which units are used to measure intensity?

Intensity is the concentration of power within a particular area of a sound beam. Intensity is measured in watts per square centimeter (watts/cm^2).

At what speed does sound travel through the body?

It is important to know that the speed of sound in soft tissue is 1,540 meters per second. That is, all sound waves travel nearly one mile per second through the body.

Since high frequency sound beams provide enhanced longitudinal and lateral resolution, why isn't ultrasound with a frequency of 100 MHz used in diagnostic imaging?

It is important to remember that the higher the frequency, the greater the degree of attenuation. (Attenuation is the decline of intensity in a sound beam as it travels through a medium.) Therefore, with use of a 100-MHz transducer, the energy of the sound beam would rapidly dissipate as it traveled through the body. Hence, the ability of the sound beam to produce imagers from any significant depth would be lost. The images would be of superb quality, but could only be obtained to a depth of approximately 0.2 cm!

Although low-frequency transducers are useful for obtaining images at substantial depths, what is their main disadvantage? What is the role of the sonographer in relation to this tradeoff?

It is indeed true that low-frequency transducers are more successful at imaging structures at greater depths, compared with high-frequency transducers. However, higher frequency transducers generally provide images of superior quality. This is due to higher frequency waves having shorter wavelengths. The role of the sonographer in relation to this tradeoff is to strike a balance between the depth that the ultrasound beam needs to travel to image the target area and the quality of the resultant image. As a rule, this is accomplished by using the highest frequency transducer that can successfully image the structure of clinical interest. However, when a higher frequency transducer is unsuccessful in imaging a structure located deep within the body, the only alternative may be to use a low-frequency transducer at the expense of image quality.

Why is the diameter of the sound beam at each particular depth important with respect to lateral resolution?

Lateral resolution is approximately equal to the width or diameter of the ultrasound beam. A beam created by a large diameter crystal will have poor lateral resolution, and thus will produce poorer quality images. In contrast, a beam from a small diameter crystal will have good lateral resolution, resulting in higher quality images. The diameter of the sound beam at each particular depth is important with regard to lateral resolution, because as the beam width changes with depth, so does the lateral resolution. Lateral resolution is best at the focus.

What types of units are used to express lateral resolution?

Lateral resolution is expressed in units of distance, such as millimeters. If we know that a particular system has a lateral resolution of 4mm, we can conclude that when the sound beam traverses two side-by-side structures separated by 4mm or less, two distinct reflectors will not be detected by the system. Rather this will result in only a single reflector being displayed. However, when two side-by-side structures are more than 4mm apart, two distinct echoes will be displayed. Thus, systems with smaller numerical values of lateral resolution are more capable of resolving structures in close proximity to one another than systems with larger numerical values.

What is lateral resolution?

Lateral resolution is the minimum distance perpendicular to the sound beam by which two side-by-side reflectors must be separated in order for them to be displayed as two separate echoes on the image.

In a linear phased array ultrasound system, are time delays used exclusively during transmission to steer and focus an ultrasound pulse?

No, although small time delays are critically important during transmission, the transducer also uses time delays during the reception phase. When reflected sound beams return to the transducer, similar time delays may be used to focus the beam further. The time delays are thus used both to steer and focus the sound beam during transmission and to focus the beam during reception. Changing the delay times during reception changes the depth of the focal zone. This is called dynamic focusing. Dynamic focusing is an important characteristic ph phased array technology ( as compared with more conventional focusing methods such as use of an acoustic lens and curvature of the active element). A single phased array transducer has the ability to focus at a variety of depths, whereas a conventionally focused transducer can only focus at a single depth.

Assume that a sound wave travels through the body and strikes a boundary between two media. What is normal incidence? What other terms have a meaning similar to normal incidence?

Normal incidence refers to a sound wave striking the boundary between two media at an angle of exactly 90 degrees. Similar terms used to describe normal incidence are perpendicular, at right angles, at 90 degrees or orthogonal.

What is oblique incidence?

Oblique incidence refers to a sound wave striking a boundary between two media at an angle other than 90 degrees. Any angle different from 90 degrees (even an angle of 89.99 degrees) is oblique.

What is the meaning of the term power? Which units are used to measure power?

Power is the amount of energy per second that is delivered by an ultrasound beam. Power is measured in watts.

Three specific terms are used to describe the size of magnitude of a sound beam:

Pressure, power, and intensity.

What is meant by reflection of an ultrasound wave?

Reflection is the redirection or turning back of an ultrasound wave toward he transducer.

What causes reflection of a sound wave in the body?

Reflection occurs when a sound wave traveling through the body reaches a boundary between two media that have dissimilar characteristics, referred to as acoustic impedances. The degree of differences in the acoustic impedance between the two media at the boundary determines how much ultrasonic energy is reflected back toward the transducer. As a rule, when sound strikes a boundary between two types of soft tissue in the body, only a very small percentage of energy is reflected.

What two factors affect the degree of scattering?

Scattering is more likely to occur when the boundary struck by the ultrasound wave is rough or irregular. The degree of scattering is likely to be especially significant when the size of the irregular structures is smaller than the wavelength of the ultrasound beam. This is referred to as Raleigh scattering. The second factor that determines the degree of scattering is the frequency of the sound wave. Scattering is more substantial when the frequency of the sound wave is high rather than low. Therefore, the degree of scattering would be greater for an 8-MHz ultrasound beam than for a 3-MHz beam.

What is scattering?

Scattering is the reflection of a wave in many different directions after striking a boundary between two different media. Scattering is chaotic, disorganized, and random.

What determines the pulse repetition frequency of a diagnostic imaging system?

The PRF is determined by the maximum imaging depth or "depth-of-view" of the image. By selecting the maximum depth from which the ultrasound system will provide images, the sonographer in effect determines the PRF of the ultrasound system. When an ultrasound system images only from shallow depth, the system sends out a pulse and then "listens" for reflected echoes. Since the imaging depth is shallow, the system has to listen only for a brief time before sending out another wave. Thus, the PRF is high. In contrast, when the sonographer chooses to obtain images from a greater depth, the system must listen for reflected echoes for a longer time. Thus, the PRF is lows. In other words, when images are being obtained from deep within the body, the ultrasound system sends out fewer pulses each second, because it must stay in the listening or receiving mode for a longer time before transmitting the next pulse.

Is the value of the Q-factor higher for ultrasound transducers used for imaging of for transducers used for therapy?

The Q-factor for imaging transducers has a lower value than that for transducers used in therapeutic ultrasound.

How is focusing achieved with a convex sequential transducer?

Sound beams from convex sequential arrays are usually focused conventionally, by use of an acoustic lens (external focusing) or by fabricating each crystal into a cupped shape (internal focusing).

How is focusing achieved with a linear sequential array transducer?

Sound beams from linear sequential arrays are usually focused by means of an acoustic lens or by internal focusing (that is, by fabricating each crystal in a cupped shape).

What is sound?

Sound is a wave that travels through a medium and carries energy from one place to another. A sound wave consists of a series of orderly oscillations of particles within the medium. Those oscillations produce regions of compression and rarefaction (high and low pressures) throughout the medium.

What is specular reflection?

Specular reflection refers to a reflection that occurs when a wave strikes a very smooth boundary. Specular reflections are organized, regular, and predictable. One example of a specular reflection is that of a light wave being reflected off of a mirror.

What is ultrasound?

Ultrasound is defined as a sound wave with a frequency above 20,000 cycles per second or hertz (Hz). One hertz means one event per second. The frequency of an ultrasonic wave is so high that it is inaudible to humans.

What are the functions of the active element of an ultrasound transducer?

The active element of an ultrasound transducer has two functions. During the transmission phase, when the transducer is producing an acoustic pulse, an electrical signal from the ultrasound system travels down the wire and strikes the active element. Because of its piezoelectric properties, the crystal vibrates and creates an acoustic wave. During the reception phase an ultrasound pulse that reflects off of a structure in the body returns to the transducer and strikes the active element. The active element is deformed, and an electrical signal is produced. The electrical signal travels down the wire and returns to the ultrasound system where the signal is processed, resulting in the display of an anatomic image.

A single sound pulse from mechanical, linear sequential, and linear phased array transducers normally creates a single scan line on the ultrasound image. Annular phased array systems are different. What are the advantages of using annular phased array systems?

The advantage of annular arrays is that the information used to construct each part of a scan line originates only from a pulse's focal zone. The scan line is constructed of data from multiple focal zones, thereby providing outstanding lateral resolution at all depths.

What is the bandwidth of an acoustic pulse?

The bandwidth represents the range of frequencies that are present within an acoustic pulse. Although each transducer has a primary frequency, such as 3.5 MHz or 5MHz, the pulse that is produced by the transducer does not consist of only a single frequency. In fact, the pulse consists of sound with a range of frequencies. Generally, longer pulses have a narrow bandwidth, while shorter pulses have a wide bandwidth.

What is the function of the damping material of an ultrasound transducer?

The damping material, or backing material, prevents the active element from ringing, and absorbs much of the acoustic energy produced by the active element. Thus, the duration of the acoustic pulse produced by the PZT is shortened. Shortening the pulse improves the quality of the images produced by an ultrasound system.

What are the units of duty factor?

The duty factor has no units and is expressed merely by the percentages ranging from 0% to 100%.

What is the duty factor?

The duty factor is the fraction of percentage of time during which an ultrasound machines is transmitting sound or "talking." A continuous-wave ultrasound system is always transmitting sound and its duty factor is 1 or 100%. When a system is not in operation, and thus not transmitting sound, its duty factor is zero or 0%.

What is the focal zone or focal area?

The focal zone (or focal area) is the general region of the sound beam surrounding the focus where the diameter is small. As a result, image quality is superior in this region.

What determines the axial resolution of an ultrasound system?

The length of the pulse used to form the beam. This is known as the spatial pulse length (SPL). The shorter the pulse length, the better the axial resolution. In fact the axial resolution limit is defined as being one half of the SPL.

What is the piezoelectric effect?

The piezoelectric effect is the process by which pressure energy is converted into electrical energy. Ultrasound transducers use piezoelectric crystals to produce images. When a sound wave reflects off a structure in the body, the wave returns to the transducer and strikes piezoelectric crystals. The crystal vibrates, resulting in an electrical signal. The ultrasound system then processes the electrical signal into an image. During transmission, electrical signals produced by the ultrasound system excite the piezoelectric material in the transducer and create a sound wave. This is sometimes called the reverse piezoelectric effect.

What is pulse repetition frequency?

The pulse repetition frequency (PRF) is defined as the number of sound pulses emitted by a transducer in one second.

What determines the pulse repetition period of a diagnostic imaging system?

The pulse repetition period is determined by the sum of the talking listening durations. The transmitting time is determined only by the ultrasound system, and cannot be adjusted by the sonographer. Conversely, the listening time is determined by the maximum depth-of-view chosen by the sonographer. If the depth-of-view is shallow, then the listening time is brief. If the depth-of-view is deep, then the listening time is long. Hence, the pulse repetition period varies with imaging depth, which is decided on by the sonographer. Typically, the listening time is 100 to 1000 times longer that the pulse duration (talking time).

What is the pulse repetition period?

The pulse repetition period is the time span extending from the start of one sound pulse to the start of the next pulse. The pulse repetition period is composed of two durations: the duration of the pulse (the "on," "talking," or transmitting time) and the duration of silence (the "off," "listening," or receiving time).

What is the quality factor or Q-factor?

The quality factor or Q-factor relates to the degree to which a transducer dampens or shorten an ultrasound pulse.

Does the sonographer have the ability to alter the duty factor of an ultrasound system?

The sonographer does have the ability to alter the duty factor by changing the depth-of-view or maximizing imaging depth. When the maximum imaging depth is shallow, the system is listening less, and therefore is transmitting a greater percentage of the time (high duty factor). When the maximum imaging depth is deep, the system is listening more, and therefore has a low duty factor. The greater the imaging depth, the lesser the duty factor. In diagnostic imaging, the duty factor ranges from 0.001 to 0.01. In other words, imaging systems transmit less than 1% of the time, and receive over 99% of the time.

What is the frequency range of the ultrasound waves used in diagnostic imaging?

The sound waves used in diagnostic imaging have a frequency range of 2 million to 10 million cycles per second (2 to 10 MHz).

The location of the focus of an ultrasound beam is important because the focus yields the best lateral resolution. What two factors have an effect on the length of the near zone (or the depth of focus)?

The two factors that affect the length of the near zone (or depth of the focus) are (1) the diameter of the transducer's active element and (2) the frequency of the ultrasound wave. The greater the diameter of the PZT crystal, the longer the near zone and the deeper the focus. Likewise, the higher the frequency of the sound wave, the longer the near zone and the deeper the focus.

What are the units of the pulse repetition period?

The units of the pulse repetition period are any measure of time, such as milliseconds.

What are the units used to measure the pulse repetition frequency?

The units used to measure PRF are per second or hertz.

What is the wavelength of 1 MHz sound in soft tissue?

The wavelength of 1 MHz sound in soft tissue is 1.54 mm.

Beam width helps to determine an ultrasound system's lateral resolution, as well as overall image quality. Name three factors that can affect beam width.

Three factors that can affect beam width include: 1. Diameter of the active element. Active elements with small diameters produce more narrow sound beams in the focal zone. 2. Focusing. Within the focal area, focusing narrows the diameter of the ultrasound beam and improves image quality. 3. Frequency of the sound beam. High-frequency sound pulses are narrower than low-frequency pulses. Thus, high-frequency pulsed ultrasound systems have better lateral resolution and produce higher quality images.

Name three additional terms that are synonymous with lateral resolution.

Three other terms for lateral resolution are angular, transverse, and azimuthal resolution.

What three processes contribute to attenuation?

Three processes that contribute to attenuation are reflection, scattering, and absorption. Absorption is the conversion of sound energy into heat energy.

Ultrasound beams must be steered in different directions to create a scan plan. How is the sound beam steered with a convex sequential transducer?

To generate a two-dimensional image with a convex sequential array, the ultrasound system sends electrical spikes down a small number of wires that excites a small number of elements. An acoustic pulse is emitted outward, directly from that small group of crystals, and then the next, in a sweeping manner, which produces a two-dimensional image. The sound beam produced by a convex sequential array transducer are not steered. The beams are merely emitted from the crystals and directed straight ahead. The curved architecture of the probe sends the sound beams out in different directions, thereby creating what is called a blunted sector image.

How is the sound beam steering using a linear sequential array transducer?

To generate a two-dimensional image with a linear sequential array, the ultrasound system excites a small number of PZT crystals at the same time by sending electrical spikes down a number of wires. Each electrical spike excites a single crystal element in the array, and an acoustic pulse is emitted outward, directly from that crystal. Since a group of pulses are emitted simultaneously, they combine to form a single sound wave. The system then fires the next group of crystals, and then the next, resulting in a two-dimensional image. The sound beams produced by a linear sequential array transducer are not steered. The beams are merely emitted from the crystals and directed straight ahead.

How is the wavelength determined?

Wavelength is determined both by the frequency of sound and the medium through which the sound is traveling.

What is the importance of wavelength in diagnostic imaging?

Wavelength is important in diagnostic imaging because it helps to determine image quality. Shorter wavelengths produce higher quality images. The wavelengths of ultrasound traveling through soft tissue range from 0.2 to 0.8 mm. There is an inverse relationship between frequency and wavelength. Higher frequency sound waves have shorter wavelengths, while lower frequency waves have longer wavelengths.

What units are used to report wavelength?

Wavelength is measured in units of distance, most commonly in millimeters.

What is the definition of wavelength?

Wavelength is the distance occupied by a single cycle in a wave.

What happens to an image when one or more of the active elements in a linear sequential array transducer malfunction?

When a crystal in a linear sequential array transducer malfunctions, the result is a line of echo dropout extending from the top to the bottom of the image. The position of the dropout on the image corresponds to the location of the malfunctioning crystal within the linear array transducer.

What happens to the image when a single element in a convex phased array system malfunctions?

When a single crystal in a convex phased array system malfunctions, the steering and focusing of all acoustic pulses emitted by the transducer are affected, As a result, the entire image may be negatively impacted by the malfunction.

What happens to the image when a single element in a convex sequential array system malfunctions?

When a single crystal in a convex sequential array system malfunctions, a line of dropout, corresponding to the location of that particular crystal and directed downward from the top toward the bottom of the image, is created. Thus, a single scan line on the image is lost while the remainder of the image is unaffected.

Approximately how much of the energy contained in a sound wave is reflected at the boundary between muscle and blood?

When a sound wave strikes the boundary between muscle and blood, less than 1/10th of 1% of the energy is reflected. The remaining 99.9% continues to propagate in the forward direction. This is called transmission. This small reflection indicates that the impedances of blood and muscle are only slightly different.

Approximately how much of the energy contained in a sound wave is reflected at the boundary between muscle and bone?

When ha sound wave strikes the boundary between muscle and bone, approximately 50% of the energy is reflected. The remaining 50% continues to transmit in the forward direction. This large reflection indicated that the impedances of muscle and bone and significantly different.

What happens to the image when one or more of the active piezoelectric crystals in a linear sequential phased array transducer malfunctions?

With phased array technology, all of the PZT crystals in the transducer are fired nearly simultaneously to produce a single acoustic pulse. The acoustic pulse is a result of the interaction of many individual sound wavelets produced by the tiny PZT crystals. If one or more of the crystals malfunctions, then the steering and focusing of the resultant sound beam become erratic.

The point where the beam reaches its smallest diameter is called the

focus or focal zone.