Understanding of the Physical Principles of Diagnostic Ultrasound - Essay Example

?Literature Review To demonstrate a number of artefacts encountered in the practice in ultrasound, which should enhance understanding of the physicalprinciples of diagnostic ultrasound. Introduction Artefacts are considered to be anomalies or errors in images and are usually caused by physical processes which impact on the ultrasound beam and often affect the assumptions which the operator would make on the beam. Artefact refers to something artificial or something which is produced beyond the forces outside one’s control. In ultrasound imaging, it is considered as artificial data on the image produced through physical imaging of which the operator also has little control. Artefacts are important because this data can be linked to variations in the “propagation speed of sound in different tissues; it may be related to bending and vibrations encountered by ultrasound energy as it traverses complex anatomical structures” (Prosono, 2009, p. 1). This study shall discuss the number of artefacts encountered in the practice in ultrasound, which should enhance understanding of the physical principles of diagnostic ultrasound. This literature review will focus on three artefacts which are chosen because of relevance; moreover, the limited word limit would curtail further discussion of other artefacts. Literature Review Ultrasound artefacts are therefore sonographic display errors which are caused by complicated physical interactions between the ultrasound and the human tissue; issues in the limits of the ultrasound process also impact on the manifestation of the artefact (Prosono, 2009). Failing to detect these artefacts can cause confusion and mistakes in diagnosis. Ensuring effective detection and interpretation of artefacts as well as the physical elements of their origin will allow identification, as well as promote correct diagnosis. In other words, the accurate interpretation of artefacts would likely help establish correct diagnosis, and provide effective tools in the identification of images within the imaging field (Hoff, et.al., 1989). The ultrasound machine has different assumptions in its image generation. These assumptions include the fact that ultrasound beams only traverse a straight line via a fixed rate of attenuation; the speed of the sound in all the body tissues measures at 1540 m/s; the beam is very thin, with most of its echoes coming from its central axis; and the impact of the reflector is based on the time which the sound would travel from the transducer to the reflector and back again (Pichamuthu, 2009). There are different types of artefacts. First, the reverberation artefact is seen when the ultrasound is constantly reflected within two different reflective surfaced (George, 2012). Second, is the mirror image artefact. This is a kind of reverberation artefact seen at highly reflective systems like the lungs and descending aorta. The false image is at the opposite side of the reflector due to the mirror-like effect (George, 2012). Third, is the side-lobe artefact where side lobe beams are seen at the edges of the transducer element and reflects in a direction different from the main beam (George, 2012). Fourth, the multiple artefact is seen when the direction of the ultrasound beam is different. Fifth, beamwidth artefacts are seen when echoes of the beam would manifest as though they come from the centre (George, 2012). This is seen when the beam goes through fluids. Propagation speed errors are also considered artefacts. These are seen when the media where the beam would penetrate would not propagate at 1540 m/s; as a result it causes echoes at wrong depths on the displayed image. Acoustic shadowing may be seen when the beam cannot pass through an area “deep to a strongly reflecting or attenuating structure” (George, 2012). Finally, near field clutter can also cause artefacts. This is caused by acoustic noise in the areas of the transducer which then leads to high amplitude oscillations of related elements (George, 2012). These artefacts usually constantly change in appearance and sometimes drift in and out depending on the view. The real structures would often stay constant and can be viewed in different ways. With the application of the correct settings, along with the understanding of the imaging concepts, and being knowledgeable in echocardiographic anatomy, it may be possible to distinguish artefacts from the actual images being evaluated. Acoustic Shadowing Shadowing is seen where there is reduced amplitude in the echoes which are lying beyond a highly reflective structure (Baun, 2009). Such reduction is caused by the increased attenuation of ultrasound energy as it goes through an interface of significantly differentiated acoustic interference. As a significant part of the beam is bounced or absorbed at such an interface, not much is left to go through into deeper tissue to create echo feedback (Einstein, et.al., 1984). As a result, the reduction in beam impact within the vicinity of a high level attenuator manifests as a shadow on the reflected image. Acoustic shadows are seen when there is no “sound distal to a reflector” (Baun, 2009, p. 51). Acoustic shadowing manifests as a portion of low amplitude echoes lying behind an area with strongly attenuated tissue. It is the result of the strong attenuation of the beam at an interface, causing little sound transmitted behind such interference (Baun, 2009). Such attenuation can be the result of the absorption or the reflection of the sound waves or both. This shadowing will also manifest at interface where there is a significant acoustic mismatch as seen in soft tissue and gas, soft tissue and bone or calculus. Sometimes it is not possible to distinguish between structures where shadowing manifests, as in gas present in the duodenum or calculi with such acoustic shadowing (Baun, 2009). As acoustic shadowing may block the sonographic field of the structures, it is also an important diagnostic tool (Baun, 2009). There are various physical elements which may lead to shadowing, and this may include reflection, attenuation, and refraction. The acoustic shadows can be seen where the beam passes through ribs, calcified masses or barium in the GI tract (Baum, 2009). This phenomenon is caused by the reflection of a significant percentage of the sound to the transducer as well as the absorption of the sound by the structure which is hit by the beam. These two actions prevent the passage of sound through the structure, thereby causing the shadow (Sommer and Taylor, 1980). Such shadows may also be seen at the edges of cysts and other curved objects. The process of creating such shadowing is mostly based on the combined elements of refraction and reflection (Sommer and Taylor, 1980). As the sound goes through the rounded structure near its edge, it is refracted and its path is changed, mainly because of the difference in the impact of the sound on soft tissue and fluids. The beam is then redirected from its usual course through the reflection at the edges of the cyst. The overall impact of the refraction and reflection is the “acoustic shadow at the margin of the cyst” (Baun, 2009, p. 53). Such shadow is seen at the gallbladder as well as cysts within solid organs. This type of shadowing can manifest at the margins of round and non-cystic structures and is mostly seen in obstetric scanning where acoustic shadows are seen at the margins of certain angles of the foetus’s head (Baun, 2009). This would be most likely caused by the reflected sound on the curved edge away from the transducer. Acoustic shadows forming from gallstones are often seen where the gallstone lies within the primary zone of the transducer, meaning, the width of the beam is limited in relation to the size of the gallstone. Where the gallstone is within proximal or distal distance to the primary zone of the transducer, the wider beam within these locations would mask the shadow (Baun, 2009). As a result, in order to diagnose gallstones correctly, it is important to utilise a transducer having a sufficient focal length or for some operators, to position the patient in such a way as to ensure that the gallbladder is found within the transducer’s focal zone. Gallbladder with gallstones causing acoustic shadowing Acoustic Enhancement This artefact usually manifests as a specific area which has an increased echo impact against an area of low attenuation (Prosono, 2009). This artefact would manifest as a space with increased brightness which can be seen distal to structures which are filled with fluids. These artefacts come about due to the impact of the TGC to spaces with low attenuating structures, like in fluids. This is basically the opposite of shadowing. When the sound beam would be made to pass through a structure which is less attenuating than the nearest soft tissue, it would then continue to pass through the structure without any interference (Baun, 2009). As the beam would then exit the structure, the impact of the beam stays strong enough to continue, thereby causing amplitude echoes as it goes through interfaces distal to the object. In effect, the echogenicity of the space behind the structure would be more enhanced as compared to the adjacent tissue structures (Baun, 2009). The space of higher echogenicity is considered an area of acoustic enhancement. For these artefacts, various studies indicate that fluid filled structures, like cysts or a full urinary bladder which would manifest as acoustic enhancement (Mendelson, et.al., 2001). Structures which cause the sound beam to go through without interference would manifest as enhanced transmission or acoustic enhancement. As was mentioned, this quality is seen where the beam goes through fluid-filled spaces or cystic areas (Mendelson, et.al., 2001). As the beam enters the fluid-filled structure, it speeds up and goes through the space with practically no interference. As a result, the speed and the intensity of the beam passing through the structure are almost as strong as when the beam enters the structure (Dayton and Ferrara, 2002). Normally, the ultrasound beam going through tissues adjacent to the fluid-filled area passes through at a normal or standard rate; in effect, under the same depth, it does not appear as echogenic as the area outside the fluid (Dayton and Ferrara, 2002). The manifestation of the bright region past the structure (normal or abnormal) indicates that the structure is filled with fluid. Full bladders are classic examples of acoustic enhancement. For which reason, full bladders are often needed during trans-abdominal gynaecologic ultrasounds (Dayton and Ferrara, 2002). As the beam passes through a full bladder, the beam continues to the posterior pelvic tissues with an intensity sufficient to ensure echo information to ensure quality images. Without this acoustic window, the sound beam would be attenuated by the abdominal cavity tissues and good images would be impossible to produce (Baun, 2009). Acoustic enhancement is also favourable for readings on ovaries. During the years where menstruation occurs, the ovaries may produce small cyst follicles at varying sizes with each menstrual cycle. As follicles are often fluid-filled, they would often also be considered cysts (Baun, 2009). Even as the follicles would be clearly distinguished on the ultrasound, acoustic enhancement outside the ovoid structure in the adnexa would provide evidence of the imagery of the ovary (Baun, 2009). Bowel loops on the other hand would not manifest posterior acoustic enhancement. Acoustic enhancement (cyst) Mirror image artefacts Sound can usually bounce off when beamed through a highly reflective area, as in the diaphragm. The surface would likely act as a mirror and then bounce off the pulse into another interface (Baun, 2009). Mirror-image artefacts have origins similar to multipath artefacts and the structure is usually wrongly indicated on the display because of redirection of the sound beam as it relates with strong reflectors (Kremkau and Taylor, 1980). The incident beam comes across strong reflectors which would then cause it to reflect to other structures. The reflection from the second structure would then manifest as if they were actually part of the true path of the original beam (Baun, 2009). Mirror image artefacts are created when an object is found in front of a significantly reflective surface where the total reflection is seen. The usual example for this artefact is when the beam goes through a highly reflective surface like the diaphragm and the beam is then bounced off toward another object (Baun, 2009). Some of the beam or energy is reflected back to the diaphragm and some is also bounced off to the transducer. This process would often be repeated and would cause multiple echoes to manifest, usually separated in time (Baun, 2009). Such separation is equal to the distance between the object and the diaphragm. Mirror-image artefact Ultrasound phantoms These are test objects which usually have tissue mimicking material (TMN) which imitates some acoustic and physical qualities of tissues. They may also have different kinds of embedded objects and may be used to evaluate diagnostic ultrasound system performance over time (Pierce, 2010). These are usually two types. One type would mimic the acoustic qualities of tissue, in relation to the speed of sound and its average attenuation. The primary goal of the other type is to make an approximation of the sonographic manifestation of tissue (Pierce, 2010). The first type is used as a tool in training for biopsies. Those imitating the acoustic qualities of tissue have been made of agar with suspended graphite, polyurethane foam, and magnesium silicate gels; these are often used as test phantoms for evaluating the interaction of sound with tissue (Pierce, 2010). References Baun, J., 2009. Sonographic image interpretation. Prosono [online]. Available at: http://prosono.ieasysite.com/03_sono_interp_mar_09.pdf [Accessed 25 October 2012]. Dayton, P. and Ferrara, K., 2002. Targeted imaging using ultrasound. Journal of Magnetic Resonance Imaging, 16(4), 362–377. Einstein, D., Lapin, S., Ralls, P., and Halls, J., 1983. The insensitivity of sonography in the detection of choledocholithiasis. AJR, 142, 725-728. George, L., 2012. Ultrasound physics: AACA Pre-conference Workshop Perioperative Ultrasound [online]. Available at: http://www.bats.ac.nz/resources/pdfs/physics.pdf [Accessed 27 October 2012]. Hoff, H., Korbijn, A., Smit, T., and Klinkhammer, J., 1989. Imaging artifacts in mechanically driven ultrasound catheters. The International Journal of Cardiac Imaging, 4(2-4), 195-199. Kremkau, F. and Taylor, K., 1986. Artifacts in ultrasound imaging. JUM, 5(4), 227-237. Prosono, 2009. Image artifacts [online]. Available at: http://prosono.ieasysite.com/14_image_artifacts_mar_2009.pdf [Accessed 25 October 2012]. Mendelson, E., Berg, W., and Merritt, C., 2001. Toward a standardized breast ultrasound lexicon, BI-RADS: Ultrasound. Seminars in Roentgenology, 36(3), 217-225 Pichamuthu, K., 2009. Tutorial 1 - Basic physics of ultrasound and the Doppler phenomenon. ICU Sonography [online]. Available at: http://www.criticalecho.com/content/tutorial-1-basic-physics-ultrasound-and-doppler-phenomenon [Accessed 26 October 2012]. Pierce, H., 2010. Ultrasound Phantoms: B-Mode, Doppler and Others [online]. Available at: http://www.aapm.org/meetings/amos2/pdf/49-14379-35435-483.pdf [Accessed 24 October 2012]. Sommer, F. and Taylor, K., 1980. Differentiation of acoustic shadowing due to calculi and gas collections. Radiology, 135, 399-403. Read More