Frame Rate, Line Density, Sector Angle, and Image Depth, and their Interrelationship
According to Gibbs, Cole, & Sassano (2009), Frame rate refers to the number of frames or images that are displayed or projected per second by an imaging device. A sector angle is a central angle that a circle center forms. An imaging depth is a distance between an images’ corresponding scene point and the pinhole of a camera. The camera is often not in harmony with the perception of the depth of a human vision. Line density is the number of acoustic scan lines for every sector in a two-dimensional sector image.
These terms are applied in the working and selection of a diagnostic ultrasound machine and equipment. The images produced by the machine are optimized by the use of focal zones, line density, frame rate, frequency manipulation, and other image processing options. Thus, the terms refer to image processing options applied by a diagnostic ultrasound machine. A diagnostic ultrasound machine is made up of several components, each having a separate function to perform.
Tissue Harmonic Imaging and its advantages and limitations for B-mode imaging
Dogra & Rubens (2003) think that tissue harmonic imaging in the context of ultrasound technology is a signal processing technique whereby an ultrasonic beam is used to insonate body tissues. This insonation helps in the generation of harmonic waves as a result of nonlinear distortion in the pulse-echo cycle’s transmission stage. As time elapses, the ultrasonic sound moves through the tissues of the body nonlinearly.
The peaks present in the pulse waveform move at a faster speed than the troughs. Similar to the compressed sections of the tissues, it is transmitted faster than in areas that have undergone expansion courtesy of the passing pressure wave. The level of such an acoustic signal distortion within a tissue is dependent on the emitted pulse’s amplitude and the distance traveled by the pulse in the tissue (Dogra & Rubens 2003, p. 132).
Tissue harmonic imaging has numerous advantages for B-mode imaging. These benefits include improved cystic resolution and clearing, an aberration of the beam, and a decrease in reverberation. Others are an improved range penetration and axial resolution as a result of high frequencies. Side lobes experience a noise reduction, thereby reducing artifacts and the signal to noise ratio. One limitation is that it leads to substantial degradation of the axial resolution, making it unable to distinguish between tissue planes. Tissue harmonic imaging is most useful clinically for endocardiac visualization, mitral valve visualization, and EF reproducibility (Edelman 2003, p.173).
Assumptions Made by Ultrasound Equipment that give rise to artifacts
The basic assumptions made by ultrasound equipment that gives rise to artifacts are, according to Kohzaki (2003), as follows:
- The acoustic waves travel in straight lines and are unable to bend under any circumstances (Kohzaki 2003, p. 438).
- The waves in their lateral extent are infinitely thin since thick or medium waves will not have the same attributes as thin ones (Kohzaki 2003, p. 438).
- Each interface generates one reflection or echoes only (Kohzaki 2003, p. 438).
- The returning echo’s intensity is directly proportional to the imaged objects’ scattering strength (Kohzaki 2003, p. 438). This means that as intensity increases, the image’s scattering strength also increases.
- Sound attenuation and speed are homogenous and are known a priori (Kohzaki 2003, p. 438).
- Any echo that is detected is a result of the acoustic pulse that was transmitted most recently (Kohzaki 2003, p. 438).
How the Reverberation Artifact is Formed and Areas of the Body where it is Most Prominent
Reverberation artifacts are formed from an object’s multiple reflections if tissue layers’ acoustical impedance is overly different. They are also formed when the detected echo does not run the shortest path of sound as it bounces back and forth between the transducer and the object. Middleton, Kurtz & Hertzberg (2004) think that the body receives sound waves in reverberation artifacts from the transducer skin’s interface.
This artifact is prominent in the liver, skin, and kidney. When the sound waves strike a strong interface, a lot of sounds is reflected in the transducer, leading to a secondary echo being sent into the tissue, forming a series of duplicate, parallel, and ever-deepening interfaces at a depth that is directly proportional to the period that has passed since the signal emission. These artifacts are most common between the skin and the transducer, leading to the formation of the main bang artifact.
A special reverberation case occurs when small gas bubbles act as reflectors, as can be commonly seen in the gastrointestinal tract. As the ultrasound reverberates back and forth amongst the gas bubbles, multiple echoes return to the transducer. Should the gas bubbles be in liquid form, little attenuation would take place, so the echo sequence returning to the transducer over time remains strong (Middleton, Kurtz & Hertzberg 2004, p. 271).
Helpful Artifacts in Diagnosis and Examples
Two artifacts that may be helpful in diagnosis include the imaging and reverberation artifacts. Middleton, Kurtz & Hertzberg (2004) cites examples of imaging artifacts as hypoechoic and anechoic artifacts. Examples of reverberation artifacts are the ring down artifact and the comet-tail artifact. In ultrasound images, artifacts are common, and it is vital to have the ability to recognize them.
An artifact can be ignored once it has been recognized because t s unlikely to have any negative impact on the diagnosis process, or because it is unlikely to lead to a misdiagnosis. In some instances, the artifacts are useful diagnostic signs themselves, as they can provide more information on tissues as well as assisting in highlighting tissues that may be of interest. However, in some cases the artifacts can be misleading, thus making it vital for them to be recognized and minimized, or eliminated if possible (Rumwell & McPharlin 2004, p. 298).
Pre-Processing and Post-Processing
Pre-processing is the process of adjusting or making changes to an image before the scan data is stored in the computer memory. Examples include edge enhancement and log compression. On the other hand, post-processing refers to the process used to adjust an image after storing the scan data in the computer memory. Examples include grayscale assignment, thresholding, black and white inversion, and freeze frame. Controls associated with preprocessing adjust the acquisition and transmission of the ultrasound signals, while its settings control the ultrasound signal formatting to enable its conversion to an electric signal.
Changes made to the controls of preprocessing affect the information that will be accessed by the scanner to create an image, with this formatted information being the basis of image creation. Postprocessing settings affect how the monitor displays the formatted information. In other words, post-processing, defines the ultrasound data’s cosmetic appearance, as displayed on the monitor (Zwiebel & Pellerito 2005, p. 285).
Selecting a High-Frequency Transducer
According to Nyland & Mattoon (2002), the selection of a transducer has been made easy with the emergence of broadband transducers that make it possible for fine-tuning to take place during image optimization. This is unlike in the past were no such tuning was possible. However, proper selection is a critical factor to ensure optimum performance in any ultrasonic gauging application. When selecting a transducer, one also selects the ultrasonic frequency to be applied in examinations. The frequency emitted by a given transducer depends on the piezoelectrical crystals’ characteristics contained in a scan-head.
Frequency change usually requires the ultrasonographer to select a different transducer since either multiple or single crystals produce sound at a certain frequency, depending on the design. Some transducers can operate using multi frequencies. Transducer technology advances now allow for simultaneous imaging of both far-fields and near-fields with different frequencies of sound waves (Nyland & Mattoon 2002). This ensures a maximum possible resolution for a given depth without having to switch transducers. The main objective of a transducer selection is choosing the right resolution and frequency.
The frequency should be capable of penetrating the required depth for a particular examination. Other factors to consider in selecting a transducer to affect its ability to separate adjacent structures and image resolution. They include the ultrasonic pulse length, the resolution, and the beam diameter of the video monitor. The sonographer usually cannot alter these parameters at the time of examination with a certain transducer. However, the focal point can be altered with a dynamic or selective focusing on the latest ultrasound equipment (Galiuto, Basdano, Fox, Sicari, & Zamorano 2011). Ultrasound imaging technology is based on the pulse-echo principle.
This, in essence, point to the fact that transducers do not produce continuous sound and instead produce it in pulses. The image is formed after each pulse, from the echoes reflected by the transducer. Thus, an appropriate time must be allowed for the return of all the echoes before the transducer being pulsed again. About three echoes are emitted in each pulse when the crystal is pulsed before a backing block within the transducer dampens the vibration (Nyland & Mattoon 2002 p. 2).
The crystal frequency is inherent, and scanner controls cannot change it. It is vital to put several factors into consideration. These factors include part temperature and geometry, and the material under measurement. Evolution in technology has led to a reduction in transducer size and aperture. Smaller transducers require fewer acoustic windows than large ones and are easier to use than the latter. High-frequency transducers are required for a fine resolution imaging of shallow structures, such as children’s hearts and cardiac apex for adults (Galiuto, Basdano, Fox, Sicari, & Zamorano 2011, p. 4).
Factors Affecting the Near Field and Far Field Length of an Ultrasound Beam
According to Middleton (2002), one of the factors affecting the near-field and far-field length of an ultrasound beam is the size and shape of the ultrasound source. The ultrasound source size affects the width of the beam, the Fresnel zone length, and the divergence angle beyond the near-surface. Fresnel zone length is given in the below equation.
D =
In the equation, D is the Fresnel zone length, r the transducer radius and ϒ is the ultrasound beam wavelength. The breadth of the beam is more or less equal to the transducer diameter.
Inferring from the above equation, the Fresnel zone length increases rapidly as the width of the beam or diameter of the transducer is increased. Moreover, a rapid reduction in the Fresnel zone length is seen to lead to a decrease in the transducer diameter. Beam frequency is a factor known to affect both the far-field and near-field length of any ultrasound beam (Middleton 2002).
Now ϒ= v/f where ultrasound velocity is represented by v, while beam frequency is represented by f.
From D = 
Substituting D = 
From this equation, it can be seen that the Fresnel zone length increases as the frequency of the beam is increased. Additionally, the angle of divergence that is beyond the near field diminishes with an increase in frequency. The improved image resolution is thus not the only effect of higher frequencies but also increases the useful near field length (Middleton 2002).
Another factor that could significantly affect an ultrasound beam’s far-field and near-field lengths is how the ultrasound beam is focused. The ultrasound beam shape can be changed to varying extents through the application of different focusing methods. One of them is the crystal element shape whose crystal element can be suitably shaped by a concave curvature to focus on the ultrasound beam. This method is an internal focusing one, as it is affected by the crystal itself.
The focusing degree is dependent on the crystal curvature extent. Another focusing method is the use of acoustic lenses made from materials that propagate ultrasound at different velocities from those found in soft tissues. It can be applied in focusing the beam through refraction. The lens’ curvature will be concave, with the focusing degree, to a great extent, depending on the curvature radius. The ultrasound beam can also be focused using a concave mirror, with the focusing degree once again dependent on the curvature radius (Middleton 2002, p. 382).
References
Dogra, V & Rubens, DJ 2003, Ultrasound Secrets. Elsevier Health Sciences. New York.
Edelman, SK 2003, Understanding Ultrasound Physics, Esp, New York.
Galiuto, L, Basdano, L, Fox, K, Sicari, R & Zamorano, JS 2011, The EAE Textbook of Echocardiography, Oxford University Press, London.
Gibbs, V, Cole, D & Sassano, A 2009, Ultrasound Physics and Technology: How, Why and When. Elsevier Health Sciences, New York.
Kohzaki, S, Tsurusaki, K, Uetani, M, Nakanishi, K, & Hayashi, K 2003, The aurora sign: an ultrasonographic sign suggesting parenchymal lung disease’, British Journal of Radiology, vol 76, no, 2, pp 437- 443.
Middleton, WD 2002, General and Vascular Ultrasound: Case Review. Elsevier Health Sciences, New York.
Middleton, WD, Kurtz, AB, Hertzberg, BS 2004, Ultrasound: the requisites. Mosby, New Jersey.
Nyland, TG & Mattoon, JS 2002, Small Animal Diagnostic Ultrasound, Elsevier Health Sciences, New York.
Rumwell, C & McPharlin, M 2000,Vascular technology: an illustrated review, Davies Pub, New York.
Zwiebel, WJ & Pellerito, JS 2005, Introduction to vascular ultrasonography, Saunders, New York.