25-48 Flashcards
What is diffraction?
Describe Huygen’s principle.
Diffraction causes an ultrasound beam to diverge or spread out as the wave travels away from the transducer. A diffraction pattern is 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 beam due to diffraction is more pronounced with smaller sound sources. This wedge-shaped wave is also called a Huygen’s wavelet.
Huygen’s principle explains why a beam produced by an ultrasound transducer has the shape of an hourglass rather than 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 is lateral resolution?
What types of units are used to express lateral resolution?
Name 3 additional terms that are synonymous with lateral resolution.
Why is the diameter of the sound beam at each particular important with respect to 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.
Lateral resolution is expressed in units of distance, such as millimeters. If we know that a particular system has a lateral resolution of 4 mm, we can conclude that when the sound beam traverses two side-by-side structures separated by 4 mm 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 4 mm 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.
Three other terms for lateral resolution are angular, transverse, and azimuthal 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.
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 the focus?
The two factors that affect the length of the near zone (or the 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.
Which of the following ultrasound transducers will have the shortest near zone (or the shallowest focus)?
A. 4-mm active element 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 diameter, 7-mm active element thickness, and 2-MHz frequency
The answer is “D”. The transducer with the smallest diameter and the lowest frequency will have the shortest near zone length (the shallowest focus. Note that in this example, the thickness of the crystal does not affect the near zone length in any way.
Note: Using the same logic, you should be able to explain why the transducer with the characteristics described in selection “C” has the longest near zone length and the deepest focus.
What is focusing?
Why is focusing important in diagnostic imaging?
Name 3 methods that may be used to focused an ultrasound beam.
Focusing is any process that narrows the diameter of an ultrasound beam, thereby improving the lateral resolution.
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.
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.
How does focusing affect the depth of the focus (also known as the near zone length)?
What is the focal zone or focal area?
How is the focal zone affected by focusing?
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.
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. 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.
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.
What is axial resolution?
What units are used to express axial resolution?
Name four additional terms that are synonymous with axial resolution.
What determines the axial resolution of an ultrasound system?
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.
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.
As an example, if a system has an axial resolution of 1.7 mm, then any two structures that lie closer together than this distance will be displayed as only a single reflection on the image. In contrast, if a distance of more than 1.7 mm separates two structures, each will produce their own distinct reflection on the image.
Assume that an ultrasound system with a lateral resolution of 4 mm and a longitudinal resolution of 2 mm is used to image the different reflectors embedded in the test object shown below. Based on their distance from one another (the numbers in the image), how will each pair of pins in the test object be displayed by the ultrasound system?
Since the system’s longitudinal resolution is 2 mm, pins B and C will be combined into a single reflection, because the two pins are only 1 mm apart. The same is true for pins F and G. Likewise, pins E and F will appear as a single horizontal reflection on the image, because the system’s lateral resolution is 4 mm and the pins are only 2 mm apart.
Every other pair of pins will be displayed as two distinct echoes on the image in the appropriate position.
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 sound 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.
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 indeed true that very high ultrasonic frequencies will result in images of exceptional quality. It is important to remember, however, 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 images 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!
What is the bandwidth of an acoustic pulse?
What effect does the backing material have on the bandwidth?
The bandwidth represents the range of frequencies that are present within an acoustic pulse. Although each transducer has a primary frequency, such as 3.25 MHz or 5 MHz, 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.
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 bandwidths produced by ultrasound systems used for continuous-wave Doppler evaluation of blood flow or for therapeutic applications are narrower than those produced by diagnostic imaging systems.
What is the quality factor or Q-factor?
What are the units of the Q-factor?
Is the value of the Q-factor higher for ultrasound transducers used for imaging or for transducers used for therapy?
The quality factor or Q-factor relates to the degree to which a transducer dampens or shortens an ultrasound pulse. It is calculated by dividing the primary operating frequency of a transducer by its bandwidth. Therefore, the Q-factor is unitless.
The Q-factor for imaging transducers has a lower value than that for transducers used in therapeutic ultrasound.
In summary, the three most important facts about the quality factor:
A. The Q-factor is a unitless number.
B. The Q-factor for diagnostic imaging transducers has a value in the range of 2 to 4.
C. The Q-factor for imaging transducers has a lower value than that for transducers used in therapeutic ultrasound.
Sound beams with narrow diameters have the best lateral resolution. Therefore, imaging systems are designed to create the narrowest beams possible. How theft 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.
What is a transducer array?
What is the shape of the active elements of a linear array transducer? How are they arranged?
What is the shape of the active elements of an annular array transducer? How are they arranged?
What is the shape of the active elements of a convex array transducer? How are they arranged?
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 system 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.
In a linear array transducer, the active elements are rectangular-shaped and arranged in a straight line.
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.
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.
