Imaging Modality: Ultrasound - Article Example

Imaging Modality – Ultrasound Accurately describe and explain how the technology in question produces images. Ultrasound is one of the commonly employed Image-Guided Radiation Therapy (IGRT) technologies that use high-frequency sound waves to produce images of the internal structures in the human body. In ultrasound a transducer is used to produce high-frequency sound waves, which are encased in a probe and applied to the surface of the skin. Depending on the impendence of the internal structures based on the density of the tissues these waves are partially reflected as an echo. The time taken by the echo to be reflected back to the source is used to measure the depth of the tissue interface. Images are generated along the beam line of the probe. 2D or 3D images of the area of interest can be generated by sweeping the probe over the area of interest (Mell et al, 2008). The probe in the ultra sound scanner has within it several high-frequency sound transmitters that are arranged in a line along the length of the probe. Usually five of these transmitters are fired at the same time resulting in a short pulse of ultrasound in a narrow beam moving away from the probe. These transmitters then become receivers for recording the intensity of the reflected sound wave. This action is repeated in a sequence along the length of the probe. The time taken for receiving the echo helps to determine the depth from the probe, as sound is taken to travel at a constant speed (1540m/s). The intensity of the echoes received from any point is reflected through the brightness of that point on the screen of the scanner (Dynamic Ultrasound Group, 2009). A single pulse travels along a path that is called the beam. Lateral resolution of the image received is dependent on the width of the beam. The axial resolution is determined by the length of the pulse. The use of higher frequencies enables the generation of shorter pulses and so normally the highest frequency that is practical is used (Dynamic Ultrasound Group, 2009). The echoes from within the body are generated by two very distinct patterns of reflection of the sound waves, namely specular reflection and scattering, which are used to create the ultrasound image. It is specular reflection that causes the bright appearance of fibrous structures like tendons and the boundaries amidst various other tissues. This happens as a result of the sound wave coming in contact with a distinct surface that is larger than the wave length of the ultrasound. The intensity of the sound that is reflected between two different tissues like fat and muscle is dependent on their acoustic impedance, which varies depending on the density and compressibility characteristics of the tissue. It is scattering that is responsible for the characteristic texture of ultrasound images seen within the soft tissue. This is the result of the small and subtle boundaries that are present in tissues, which are responsible for the absorption and retransmission in all directions of the sound wave, mimicking a point source of sound. Thus it is physics and the technology involved in ultrasound imaging that has a strong influence on the appearance of structures in an ultrasound image (Dynamic Ultrasound Group, 2009). 2) Explain the key components of image quality for the system in question, and how the operator’s actions / selections / settings impact upon image quality and diagnostic efficacy. The key components of image quality in ultrasound are depth, enhancement, attenuation, anisotropy and frame rate. The normal depth that can be seen through the ultrasound scanner is usually indicated on the side of the image, along a scale that is present. The depth can be adjusted by the operator using the depth adjustor, which controls the frequency of the sound waves used. Lower frequencies enable seeing deeper, but come at the cost of resolution of the image (Dynamic Ultrasound Group, 2009). Ultrasound imaging has an inherent disadvantage in that as the sound wave pass through each layer of tissue a portion of the sound wave is absorbed or reflecting, resulting in the signal getting weaker, as it passes deeper into the tissue. This calls for the operator to make the necessary compensation through the application of a standard correction in proportion to the weaker sound signals received back from the deeper tissues, when the image is processed. In addition some structures in the body such as watery fluids, cysts and effusions, being translucent, allow sound waves to pass through with greater ease than in the case of other and more opaque structures. Consequently regions that lie behind such structures receive more sound than is expected at that depth, which leads to these regions appearing uniformly brighter in the image. This feature is called enhancement. Just the opposite of this can happen in the case of some tissues absorbing relatively more sound waves and causing the regions lying behind such tissues to appear uniformly darker or when all the sound is absorbed causing a very dark shadow behind the structure. This feature is called attenuation. These are useful features that can be used to structures like calculi that cause strong acoustic shadows (Dynamic Ultrasound Group, 2009). At particularly smooth boundaries the angle of reflection and incidence of the sound waves are the same and hence the probe receives reflected sound waves only when the beam strikes the surface of a tissue at a right angle. The consequence of this is that a tendon appears bright when it runs at an angle of ninety degrees to the incident ultrasound beam and darker, when the angle is altered. This affect is known as anisotropy. The frame rate describes the quickness with which the image is updated and is determined by the length of time taken to acquire the image. When the time taken is too slow the image becomes jerky, making it difficult to view. The key factors involved in slowing down the frame rate are the number of focus levels and the depth of the image (Dynamic Ultrasound Group, 2009). There are several operator actions and controls that assist in improving the quality of the image in ultrasound scanning and through that the diagnostic assistance provided by the ultrasound image. Operating the probe and positioning the patient are the first two of such actions. Coordination of hands and eyes are skills that need to be acquired by the operator for properly operating the probe. Keeping the probe and screen synchronised reduces the stress on hand eye coordination. Synchronisation of the probe and screen involves ensuring that when the probe is moved to the left by the operator, the anatomy as seen on the screen appears to pass from left to right and vice versa when the probe is moved to the right by the operator. There is no suggested most comfortable positioning of the patient that is recommended. However, the thumb rule for the operator is to ensure that the patient is comfortable, so that discomfort does not cause the patient to struggle in maintaining the positioning and distorting the quality of the image (Dynamic Ultrasound Group, 2009). Several settings or controls are present for the operator to make adjustments for improving the quality of the image acquired. Increasing the depth enhances the quality of images of the deeper structures, but results in reducing the scale and slowing the frame rate. When the images appear too bright or too dark making it difficult to discern subtle difference in the structure, this aspect can be addressed by adjusting the overall brightness of the image through the adjusting the gain. It is also possible to adjust gain selectively at different depths through depth adjustments in what is known as time gain compensation and is a setting used to address the artefact effects of strong attenuation or enhancement that lead to poor image quality and employed when evaluating a larger area than a specific structure (Dynamic Ultrasound Group, 2009). Another setting that an operator can manipulate is the pulse of the ultrasound. By manipulating the pulse so that it is at its narrowest for a particular depth, the quality of the image including lateral resolution can be maximised at that level. Such manipulation is done to allow more detailed examination of a particular area. It is also possible to select more than one focus level, but doing this causes significant slowing of the frame rate. Zooming into an image is another manipulation, which can be done while scanning or with an image that has been frozen, which involves taking a portion of the screen and magnifying it. Orientation becomes an issue if zooming is done while scanning. This assists in examination of deep structures (Dynamic Ultrasound Group, 2009). 3) Discuss the risk benefit trade-offs for the particular technology you have chosen. Ultrasound is generally considered safe within the prescribed regulations of use of the frequency employed for ultrasound scanning. Nevertheless from the physics involved in ultrasound technology becomes clear that the frequency has to be adjusted to enhance the quality of images of structures deeper in the human body. Thus there is trade off involved in quality of the image of deep lying structures or structures blocked by other structures and the intensity and the frequency of the sound waves employed in the scanning. Higher ultrasound frequencies can cause heating of the tissues and hence possibly damage the tissues scanned. Thus the use of higher frequencies for better image quality is likely to increase the risk of tissue damage and this is the risk benefit trade off with ultrasound (Duck & Shaw, 2003). Ultrasound is most frequently used in the study of the foetus in the womb of a woman in the early stages of pregnancy. This also happens to be the period of time, when there is rapid development in the growing foetus. This rapid development of the embryo also involves organ creation and cell migration. There is generally accepted evidence that points to the embryo at this stage of development being extremely sensitive to any external agents. The impact of these external agents may be such that it could lead to fatal developmental malformation or minor biochemical disturbances. It is for this reason that caution in the use of ultrasound for evaluation the foetus in early stages of pregnancy is recommended, till such time definite evidence is available on the safety of ultrasound in evaluating the foetus at an early stage. However, on the other hand ultrasound is an effective means to evaluate the foetus for normal development and an assurance that all will be well with the baby. Should it not be so, the parents could decide whether to progress with the pregnancy or to abort the pregnancy. Thus there is a trade-off in the use of ultrasound scan in the evaluation of the foetus in the early stages of pregnancy (Duck & Shaw, 2003). Another area of risk-benefit is the use of ultrasound scanning of the brain structure within the enclosing bony skull. There is the risk of temperature increase by a few degrees of the bony skull, when it is exposed to ultrasound. This increase in temperature can through thermal conduction pose a risk for damaging brain tissues that lie in proximity to the skull that could lead to dire consequences. Thus the use of ultrasound scanning for evaluation of brain tissues is a risk-benefit trade off between the risk of brain tissue damage and the benefit of such an evaluation (Duck & Shaw, 2003).  4) Explain how the operator’s actions directly affect the trade-offs in 3 Sonographer actions deeply influence the quality of the image through their knowledge of human anatomy and the physics involved in the technology and the skills and technique employed in the ultrasound examination. Trade-offs are involved in several aspects the quality of the image (Dynamic Ultrasound Group, 2009). For the purpose of examining structures deep inside the human body the sonographer adjusts the depth setting in the scanner, which decreases the frequency of sound waves that are employed for scanning. This has an impact on the resolution of the image received. If the frequency employed is too low then the poor resolution reduces the quality of the image rendering evaluation difficult. Then the operator has to make the best trade off between the depth or frequency used and the resolution of the image to obtain the best possible image of the deep structure that will enable an evaluation of the structure (Dynamic Ultrasound Group, 2009). In a similar way viewing deep structures reduces the scale as well as the frame rate, which again has an impact on poor image quality, by making the images uncomfortable to view. Thus the sonographer has to make a trade off between the frequency of the sound wave and the scale and frame rate to obtain an optimal image of the structure that is suitable for viewing. In essence means that the operator action lies in the selection of the right probe and the right frequency for images of particular structures in the human body. For example for viewing structures up to a depth of around 4cms probes using a frequency of about 10 MHz needs to be selected, while for viewing structures deeper suitable probes with appropriate lower frequency have to be selected (Dynamic Ultrasound Group, 2009). The brightness of an image impacts on the viewing quality of an image. If the image is either too dark or too bright than it makes viewing difficult. Trade offs between brightness and darkness is used by the sonographer to produce images that are adequate for examination (Dynamic Ultrasound Group, 2009). Image resolution quality including lateral resolution has to be maximised for a particular depth. A sonographer can address this aspect by manipulation of the pulse of the ultrasound frequency used. Manipulation of the pulse of the ultrasound to be at its narrowest at that particular depth causes the resolution including the lateral resolution to be the maximum for that depth to improve the quality of the image for that depth. However, the image quality is affected by the frame rate slowing done and making the image jerky and difficult for viewing and freezing of a clear image. The sonographer has to make the appropriate trade off between the pulse of the ultrasound to create an image with proper resolution for a structure at a specific depth and the frame rate to ensure that the image is not jerky and unsuitable for proper viewing and examination (Dynamic Ultrasound Group, 2009). Magnification may become essential for making available the appropriate images for examination of some structures in the human body. For this purpose zooming into the structure is possible and this can be done either while the structure is being scanned or after the image of the structure has been frozen. In the case of deeper structures zooming is frequently used for providing the suitable images for viewing. However orientation of such zoomed images is difficult while the scanning is in process. When such zoomed images are required in the scanning process, the sonographer has to make a trade off between the quanta of zooming done to allow for proper orientation, so that the quality of image is adequate for the required examination (Dynamic Ultrasound Group, 2009). 5) Identify and explain the nature of the image data the system produces, and what this may be able to indicate clinically Studies on tissue characterization using ultrasound have been going on for nearly four decades since 1970. Yet, tissue characterization has not a strong clinical influence. This situation has arisen from several factors in the nature of data received for imaging from ultrasound. The interaction between ultrasound and the different tissues are quite complex arising from the tissue attenuation and scattering being nonlinear functions of ultrasound frequency in the case of many tissues. Furthermore, the polychromatic nature of the ultrasound beam and the effects of the transducer focusing and diffraction is the cause for severe difficulty in separating one effect from the other. In addition, some of the ultrasonic features of certain tissues like attenuation do mot give sufficient difference between normal and abnormal tissues, making it difficult for discrimination between the two. Another issue that impacts on tissue characterization using ultrasound is the complexity involved in several of the calculations that are involved in the characterization of tissues. This complexity is responsible for delay in making the images ready for display, hindering real time evaluation of the tissues being examined ( In spite of these limitations caused by the nature of the image data produced through ultrasound scanning, it still has the potential to be clinically useful in several tissue characterizations features calculated through several methods, consisting of methods which analyze data of the images, methods which use estimation of ultrasonic properties of tissue like speed of sound and acoustic attenuation and methods that employ ultrasound for the purpose of detecting physical properties of a tissue like hardness of the tissue (Garra, 1993). Image data analysis makes use of statistical calculations based on the pixel grey level values or the intensity values ascertained from the envelope detected radio frequency signal. These statistical calculations essentially provide the spatial variations in signal intensity or the average signal intensity from the region of interest, but are seldom employed for the purpose of clinical evaluation of the nature of the underlying tissue (Garra, 1993). Methods which make use of estimates of acoustic properties, however, are singly concentrated on estimation of a single acoustic property from the backscattering of the ultrasound tissue through the elimination of system effects and any other inter actions. The specific acoustic properties include frequency dependent backscatter coefficients, acoustic attenuation and sound speed. The integral over all received frequencies of the back scatter coefficients has been used frequently for the clinical evaluation of normal and abnormal ischemic myocardium (Garra, 1993). Employing ultrasound for the purpose of determining the non-acoustic properties of tissues is based on the estimation of the elasticity of the tissue being evaluated from the motion of the tissue as a reaction to the application of sound vibrations generated from an external source or in the case of cardiac muscles from the internal motion. Abnormal tissues tend to be stiff and non-elastic and as a result move in a different manner in response to the external pressure in comparison to the movement of normal tissues. Detecting this motion is possible through the use of Doppler analysis and by applying correlation techniques to M-mode data. Ultrasound through the use of such techniques is clinically useful in the evaluation of foetal lung, myocardium, skeletal muscle, liver and the prostrate gland (Garra, 1993). Literary References Duck, F. A. & Shaw, A. 2003, ‘Safety of Diagnostic Ultrasound’, in Diagnostic Ultrasound: Physics and Equipment, eds. P. R. Hoskins, A Thrush, K. Martin & T. A. Whittingham, Greenwich Medical Media Limited, London, 179-205 Dynamic Ultrasound Group. 2009, ‘Physics, instrumentation and basic technique’, [Online] Available at: http://dynamicultrasound.org/dugphysics.html (Accessed December 14, 2009). Garra, B. 1993, ‘In Vitro Experimental Results on Ultrasound Scattering in Biological Tissues’, in Ultrasound Scattering in Biological Tissues, eds. K. Kirk Shung & Gary A. Thieme, CRC Press, Boca Raton, Florida, 291-312. Mell, L. K., Pawlicki, T., Jiang, S. B. & Brady, L. W. 2008, ‘Image-Guided Radiation Therapy’, in Perez and Brady’s Principles and Practice of Radiation Oncology’, Fifth Edition, eds. Edward C. Halperin, Carlos A. Perez & Luther W. Brady, Lippincott Williams & Wilkins, Philadelphia, PA, 263-299. Read More