File Name: ultrasound physics and instrumentation .zip
Ultrasound application allows for noninvasive visualization of tissue structures. Real-time ultrasound images are integrated images resulting from reflection of organ surfaces and scattering within heterogeneous tissues. Ultrasound scanning is an interactive procedure involving the operator, patient, and ultrasound instruments.
Ultrasound application allows for noninvasive visualization of tissue structures. Real-time ultrasound images are integrated images resulting from reflection of organ surfaces and scattering within heterogeneous tissues. Ultrasound scanning is an interactive procedure involving the operator, patient, and ultrasound instruments. Although the physics behind ultrasound generation, propagation, detection, and transformation into practical information is rather complex, its clinical application is much simpler.
Because ultrasound imaging has improved tremendously over the last decade, it can provide anesthesiologists opportunity to directly visualize target nerve and relevant anatomical structures. Understanding the basic ultrasound physics presented in this section will be helpful for anesthesiologists to appropriately select the transducer, set the ultrasound system, and then obtain satisfactory imaging.
In , French physicists Pierre Curie and his elder brother, Paul-Jacques Curie, discovered the piezoelectric effect in certain crystals. Paul Langevin, a student of Pierre Curie, developed piezoelectric materials, which can generate and receive mechanical vibrations with high frequency therefore ultra sound. During World War I, ultrasound was introduced in the navy as a means to detect enemy submarines. In the medical field, however, ultrasound was initially used for therapeutic rather than diagnostic purposes.
In the late s, Paul Langevin discovered that high-power ultrasound could generate heat in bone and disrupt animal tissues. Diagnostic applications of ultrasound began through the collaboration of physicians and sonar sound navigation ranging engineers.
In , Karl Dussik, a neuropsychiatrist, and his brother, Friederich Dussik, a physicist, described ultrasound as a medical diagnostic tool to visualize neoplastic tissues in the brain and the cerebral ventricles. However, limitations of ultrasound instrumentation at the time prevented further development of clinical applications until the mids.
The real-time B-scanner was developed in and was first introduced in obstetrics. In , the first ultrasound machines coupled with Doppler measurements were commercially available. With regard to regional anesthesia, as early as , La Grange and his colleagues were the first anesthesiologists to publish a case series report of ultrasound application for peripheral nerve blockade.
In , Ting and Sivagnanaratnam reported the use of B-mode ultrasonography to demonstrate the anatomy of the axilla and to observe the spread of local anesthetics during axillary brachial plexus block. In , Stephan Kapral and colleagues systematically explored brachial plexus with B-mode ultrasound. Since that time, multiple teams worldwide have worked tirelessly to define and improve the application of ultrasound imaging in regional anesthesia.
Ultrasound-guided nerve blockade is currently used routinely in the practice of regional anesthesia in many centers worldwide. Sound travels as a mechanical longitudinal wave in which back-and-forth particle motion is parallel to the direction of wave travel. Ultrasound is high-frequency sound and refers to mechanical vibrations above 20 kHz.
Human ears can hear sounds with frequencies between 20 Hz and 20 kHz. Ultrasound frequencies commonly used for medical diagnosis are between 2 and 15 MHz. However, sounds with frequencies above kHz do not occur naturally; only human-developed devices can both generate and detect these frequencies, or ultrasounds.
Ultrasound waves can be generated by material with a piezoelectric effect. The piezoelectric effect is a phenomenon exhibited by the generation of an electric charge in response to a mechanical force squeeze or stretch applied on certain materials. Conversely, mechanical deformation can be produced when an electric field is applied to such material, also known as the piezoelectric effect Figure 3.
Both natural and human-made materials, including quartz crystals and ceramic materials, can demonstrate piezoelectric properties. Lead-free piezoelectric materials are also under development. Individual piezoelectric materials produce a small amount of energy. However, by stacking piezoelectric elements into layers in a transducer, the transducer can convert electric energy into mechanical oscillations more efficiently.
These mechanical oscillations are then converted into electric energy. Wavelength is the length of space over which one cycle occurs; it is equal to the travel distance from the beginning to the end of one cycle.
Frequency is the number of cycles repeated per second and measured in hertz Hz. Acoustic velocity is the speed at which a sound wave travels through a medium. It is equal to the frequency times the wavelength. Density is the concentration of a medium. Stiffness is the resistance of a material to compression. Propagation speed increases if the stiffness is increased or the density is decreased.
However, ultrasound cannot penetrate lung or bone tissues. It increases if the propagation speed or the density of the medium is increased. Attenuation coefficient is the parameter used to estimate the decrement of ultrasound amplitude in certain media as a function of ultrasound frequency.
The attenuation coefficient increases with increasing frequency; therefore, a practical consequence of attenuation is that the penetration decreases as frequency increases Figure 4. Ultrasound waves have a self-focusing effect, which refers to the natural narrowing of the ultrasound beam at a certain travel distance in the ultrasonic field.
It is a transition level between near field and far field. The beam width at the transition level is equal to half the diameter of the transducer. At the distance of two times the near-field length, the beam width reaches the transducer diameter. The self-focusing effect amplifies ultrasound signals by increasing acoustic pressure.
In ultrasound imaging, there are two aspects of spatial resolution: axial and lateral. Axial resolution is the minimum separation of above-below planes along the beam axis.
It is determined by spatial pulse length, which is equal to the product of wavelength and the number of cycles within a pulse.
It can be presented in the following formula:. The number of cycles within a pulse is determined by the damping characteristics of the transducer. The number of cycles within a pulse is usually set between 2 and 4 by the manufacturer of the ultrasound machines. As an example, if a 2-MHz ultrasound transducer is theoretically used to do the scanning, the axial resolution would be between 0. For constant acoustic velocity, higher-frequency ultrasound can detect smaller objects and provide an image with better resolution.
The axial resolution of current ultrasound systems is between 0. Lateral resolution is another parameter of sharpness to describe the minimum side-by-side distance between two objects. It is determined by both ultrasound frequency and beam width. The higher frequencies have a narrower focus and provide better axial and lateral resolution. Lateral resolution can also be improved by adjusting focus to reduce the beam width. Temporal resolution is also important for observing a moving object such as blood vessels and heart.
Like a movie or cartoon video, the human eye requires that the image is updated at a rate of approximately 25 times a second or higher for an ultrasound image to appear continuous. However, imaging resolution will be compromised by increasing the frame rate. Optimizing the ratio of resolution to the frame rate is essential for providing the best possible image. As the ultrasound wave travels through tissues, it is subject to a number of interactions.
The most important features are as follows:. When ultrasound encounters boundaries between different media, part of the ultrasound is reflected and the other part is transmitted. Refection of sound waves is similar to optical reflection. Some of its energy is sent back into the medium from which it came. The strength of the reflection from an interface is variable and depends on the difference of impedances between two affinitive media and the incident angle at the boundary.
If the media impedances are equal, there is no reflection no echo. If there is a significant difference between media impedances, there will be nearly complete reflection. For example, an interface between soft tissues and either lung or bone involves a considerable change in acoustic impedance and creates strong echoes. This reflection intensity is also highly angle dependent. In practical terms, it means that the ultrasound transducer must be placed perpendicular to the target nerve to visualize it clearly.
A change in sound direction when crossing the boundary between two media is called refraction. If the propagation speed through the second medium is slower than that through the first medium, the refraction angle is smaller than the incident angle. Refraction can cause the artifact that occurs beneath large vessels on the image. During ultrasound scanning, a coupling medium must be used between the transducer and the skin to displace air from the transducer-skin interface.
A variety of gels and oils are applied for this purpose. Moreover, they can act as lubricants, making a smooth scanning performance possible.
Most scanned interfaces are somewhat irregular and curved. If the boundary dimensions are significantly less than the wavelength or not smooth, the reflected waves will be diffused. Scattering is the redirection of sound in any directions by rough surfaces or by heterogeneous media Figure 7. Normally, scattering intensity is much less than mirror-like reflection intensities and is relatively independent of the direction of the incident sound wave; therefore, the visualization of the target nerve is not significantly influenced by another nearby scattering.
Absorption is defined as the direct conversion of the sound energy into heat. In other words, ultrasound scanning generates heat in the tissue. Higher frequencies are absorbed in a greater rate than lower frequencies. However, a higher scanning frequency gives a better axial resolution. If the ultrasound penetration is not sufficient to visualize the structures of interest, a lower frequency is selected to increase the penetration. The use of longer wavelengths lower frequency results in lower resolution because the resolution of ultrasound imaging is proportional to the wavelength of the imaging wave.
Frequencies between 6 and 12 MHz typically yield adequate resolution for imaging in peripheral nerve blockade, whereas frequencies between 2 and 5 MHz are usually needed for imaging of neuraxial structures. Frequencies of less than 2 MHz or higher than 15 MHz are rarely used because of insufficient resolution or the insufficient penetration depth in most clinical applications. The A-mode is the oldest ultrasound technique and was invented in The transducer sends a single pulse of ultrasound into the medium.
Consequently, a one-dimensional simplest ultrasound image is created on which a series of vertical peaks is generated after ultrasound beams encounter the boundary of the different tissue.
Illustration 2: Hearing us animals and humans. Illustration 3: Absorption, reflection, refraction and scatter. In this still image the M-mode captures the movement of a particular part of the heart. Figure 6: Attenuation shadowing artifact caused by gallstones. Figure 8: Edge artifact.
Full-Text · PDF · Evaluation and Management of Abnormal Uterine Bleeding. Mayo Clinic ProceedingsVol.
Bedside ultrasound has become an important modality for obtaining critical information in the acute care of patients. It is important to understand the physics of ultrasound in order to perform and interpret images at the bedside. The physics of both continuous wave and pulsed wave sound underlies diagnostic ultrasound. The instrumentation, including transducers and image processing, is important in the acquisition of appropriate sonographic images.
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