Ultrasound How It Produces Images Biology Essay

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Ultrasound is currently one of the more commonly used imaging modalities for the internal structures of the human body. The basic technology involves the production of high frequency sound waves from a transducer in a (usually) hand-held probe which is applied directly to the skin. (Merton D A 2000).

One can consider that the normal range of "acoustic" frequencies that can be heard by the young adult is approximately 20 Hz to 20,000 Hz (20 kHz). Ultrasound has a frequency considerably above this range. Frequencies in the range of 2 MHz to 20 MHz are typically used in diagnostic ultrasound. Ultrasound has the ability to be focused into small, well-defined beams that can probe the human body and interact with the tissue structures to form images.

Differential impedance of the internal body structures causes differential reflection of these sound waves and the time lapse between pulse generation and the detection of the echo is converted into a 2D picture, if static, or 3D picture if it is scanned over a target area. (Rawool N M et al. 2003).

There is minimal correlation between the actual physical density of a tissue and its ability to reflect the acoustic waves. The grey tone variation seen in the image is a function of the degree of acoustic reflection of the particular structure and its internal interfaces. Most modern machines have the ability to display the image as either white on a black background or the converse. There is no firm evidence that images are more accurately interpreted in either format, although the white image is generally the most commonly employed.

The commoner varieties of ultrasound probe have multiple transducers along their length (frequently five). The same transducers are used as receivers directly after emission of the pulse. They detect both the intensity of the received sound as well as the time lapse. The displayed image uses both variables with the former determining the brightness of the signal on the display screen and the later determining its spatial position with the longer time lapses being displayed as being further away from the probe. As sound travels at a constant 1540 m/s there is a linear relationship between elapsed time and distance between probe and reflecting tissue interface. (Ishii K et al. 2009)

The degree of lateral resolution of the image is determined by the width of the ultrasound beam whereas the length of the pulse determines the axial resolution of the image with the highest frequencies being able to produce the greatest degrees of image resolution. (Ukimura O et al. 2009)

There are two prime elements of acoustic reflection, scattering and specular reflection. Specular reflection is primarily responsible for the inter-surface delineation between areas of different acoustic impedance and will show up areas of bone, muscle, fascia etc. This is caused by a direct reflection of a proportion of the acoustic beam at the tissue interface. The brightness of the interface is determined by the differential degree of reflection at each tissue layer.

Scattering is the cause of the "texture" of the image seen with any particular tissue. It derives from acoustic echoes that arise from minor variations in density within soft tissue structures. It is generally not a strong signal, such as those received at a boundary interface, and may occur as a result of multidirectional scattering of the acoustic signal from other areas of the tissue acting as "point sources" of sound which are then reflected back to the probe. (Bamber J C 2006)

The intensity of the signal received by the probe form any given structure is determined by the initial energy level of the emitted signal, the magnitude of the change of the acoustic impedance at the tissue interface, the acoustic characteristics of any intervening tissue (see on) as well as the orientation of the surface to the acoustic source with surfaces at right angles to the source generally producing the greatest echo. The actual image is produced my electronic manipulation of the received signal and can be artificially enhanced and modified in various ways and these will be discussed in a later segment.

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.

If one accepts that the key element of the ultrasound scanner is the generation and subsequent detection of the ultrasound pulse by the transducers in the probe, then the image is generated from this basic information. The scan converter changes the format of the piezoelectric current from the receiver into a digital image matrix format which is suitable for processing and display. The image processor modifies the digital format in a number of ways to make it suitable for display (see on). This includes giving it specific, and operator definable, contrast characteristics as well as reformatting the image if necessary. The display is usually produced on a monitor screen which also has the ability to either record and playback real time scans or print (or digitally store) 'snapshots' of the ultrasound image for future reference.

There are a number of key components to the quality of the image which are described by Mulvagh as depth, attenuation, anisotropy, enhancement and frame rate. (Mulvagh S L. 2000). The depth scale can be varied by the operator with lower frequencies being used for greater penetration, but at the expense of image resolution. The deeper the acoustic beam penetrates, the weaker the signal becomes as it is both absorbed and reflected by successive tissue layers. This can be compensated during signal interpretation by artificial enhancement of the deeper signals. This effect can be modulated by the presence of tissue fluids which transmit the signal with greater efficiency and less acoustic loss and the compensation can therefore be varied. (Seidel G et al. 2000). The phenomenon of enhancement occurs below fluid filled cysts, as the greater acoustic conduction leads to higher levels of tissue reflection from the deeper layers. Attenuation is the converse of this phenomenon where particularly acoustically dense structures absorb the signal and the structures that lie deep to them are less well defined.

In this context, one can note that discrete fluid collections, such as cysts and the bladder, will transmit the acoustic signal most efficiently whereas the majority of the soft tissues in the body have attenuation coefficient values of approximately 1 dB per cm per MHz, with the notable exception of both fat and muscle. Muscle has the characteristic that its attenuation coefficient covers a range of values that depends on the direction of the acoustic beam with respect to the muscle fibres. Lung tissue, for example, has a much higher attenuation rate than either air or soft tissue. This is largely due to the fact that the small pockets of air in the alveoli are very effective in scattering ultrasound energy. This makes the lung unsuitable for most forms of ultrasound imaging. In comparison with the soft tissues of the body, bone has a relatively high attenuation rate. Bone shields the deeper structures of the body against easy access by the ultrasound beam.

Anisotropy is the feature that refers to the ability of certain flat tissue boundaries to reflect the acoustic beam most strongly, and therefore appear brighter, when they are at right angles to the beam. (Czerwinski, R N et al. 2009)

Frame rate reflects the periodicity the updating of the image. High levels of frame rate result in a smoother image. The human eye plays an integral part in interpreting the overall picture from the scan and is very sensitive to flicker rates. Although image persistence in the retina may go some way towards reducing this effect, too slow rates of frame renewal can lead to significant information loss and thereby a reduction in diagnostic accuracy.

The operator is able to influence the quality of the examination in several ways. Optimal positioning of the patient so that they are comfortable and do not move is important, but the coordination of probe movement and screen image is an acquired skill for the operator and skill in this area can greatly enhance the diagnostic value of the investigation. (Joyner C R et al. 2003)

The brightness and contrast (gain) of the image can be altered to optimise structure definition. Coordination of frame rate and tissue penetration (wave length and frequency) can optimise examination of deep structures but this may be at the expense of definition. Time gain compensation allows for differential gain settings at different tissue depths when obtaining an image of a large field. (Mintz G 2001)

The pulse width can also be adjusted to optimise the image formed with best results being obtained with the narrowest beam. Focus depth can be set with multiple levels causing a compensatory reduction in frame rates.

The operator can also zoom into a specific picture segment which is arguably best done on a frozen image although it is possible to do this while scanning.

On a macro-scale, there are a number of commonly employed techniques to produce a variety of images. The classic "B" scan, also referred to as the contact compound scan, is generated by a rocking, or "compound" motion of the probe which generates a compound 2D picture of a segment of the body. A further sophistication of the "B scan" is the real time image which uses multiple transducers to produce a 2D image which is continuously updated and therefore allows a determination of tissue dynamics, but the field of view is not as global as with the contact compound scanner

3) Discuss the risk benefit trade-offs for the particular technology you have chosen.

Ultrasound has been shown to be a relatively safe imaging modality, particularly when compared to some other varieties of anatomical imaging, but no imaging method which relies on depositing additional energy into the body should be considered entirely risk free. (Miller D L 2001). As with any imaging technique, the clinician should always make a clear and considered judgement about whether the potential benefits of the imaging procedure are greater than any potential risk posed to the patient.

The risks of an ultrasound examination are minimal if used within the generally accepted guidelines. As has already been set out, there is effectively a trade-off between frequency and penetration of the signal. The higher the frequency, the greater is the propensity for tissue heating and the possibility of tissue damage. It therefore follows that there is the technical possibility of causing tissue damage by raising the frequency too high in order to try to improve the quality of the image of deep structures. (Abramowicz J S et al. 2008)

One of the most common ultrasound investigations is on the pregnant woman. As with all physical interventions at a time when organogenesis and cell differentiation is occurring, there is a theoretical restriction on unnecessary investigations. There is little evidence of either an increased risk of developmental malformations or biochemical disturbances in ultrasound-examined babies, but the absence of proof of adverse effects is not the same as the proof of absence of adverse effects. (Sladkevicius P et al. 2000)

Because of its great value in providing diagnostic information at all stages of pregnancy, ultrasound is frequently used, but must be regarded as a trade off of information against potential risks.

A similar trade off arises in ultrasound examinations of the brain where the risk of thermal damage of brain tissue in close proximity to the skull is more substantial and clearly could have profound consequences for the patient. (Schneider F et al. 2006)

Trade offs occur in other contexts of ultrasound use. For example ultrasound has been used in conjunction with mammography for the screening of breast malignancies. This is done with the evidence base that using ultrasound in this way reveals more breast malignancies than using mammography alone. The trade off, in this context, is that for the increased detection rate, one has to accept a higher level of false positive results which, inevitably results in a greater number of unnecessary surgical biopsies. (Berg W A et al. 2008)

To put this observation into context, it has been shown that, for every 1,000 women screened, mammography can be expected to detect 8 malignancies and will produce 25 false positive results. Adding ultrasound to this procedure detects 12 cancers but produces 93 false positive results. The trade off is a four fold increase in false positive results for the extra four malignancies picked up. In order to give a balanced argument, one should also note that the combination of mammography and ultrasound, whilst picking up 12 malignancies per 1,000 women, still fails to detect 20% of malignancies which are present in the pre-clinical state. (Evans J T 2010)

4) Explain how the operator's actions directly affect the trade-offs in 3).

There is little doubt that the skill of the operator can hugely influence the quality of the images obtained which, in turn, can affect the diagnostic quality of the investigation. Several illustrative examples of trade offs have already been given and the operator's knowledge of the anatomy of the body can be a fundamental factor in producing a diagnostically useful image. An illustrative example of how the operator's actions can affect the trade offs can be a consideration of the selection of the acoustic frequency for the investigation. If a particularly deep structure is required to be examined, then a typically low frequency will be selected. If the frequency is too low, then the increased degree of tissue penetration will be traded off against poorer image resolution and reduced diagnostic value. There is therefore an operator defined titration between these two factors to achieve the best diagnostic result for the patient. Further variables need to be selected in terms of frame renewal rate with faster rates being associated with better image quality but inversely associated with lower sound frequency. (Metcalfe S C et al. 2002)

Choice of the narrowest beam width at a specific depth increases lateral resolution and thereby improves diagnostic use of the investigation.

The ability of modern machines to zoom in and magnify a specific area have hugely improved their diagnostic value. The operator may well choose to scan an area to locate a specific area for scrutiny and then choose to zoom in on that area when the image has been frozen.

5) Identify and explain the nature of the image data the system produces, and what this may be able to indicate clinically.

The ability of the ultrasound examination to diagnostically determine the nature of the tissue being examined is not great. It is certainly capable of detecting differences in acoustic translucency, anatomical positioning and orientation of tissue layers with great accuracy, but the acoustic signature of some, quite disparate tissue types, is very similar and simply reflects the fact that some tissues reflect and scatter the acoustic signal in similar ways. Similarly, the presence of a tumour in a tissue mass may be clearly identified or may be hard to find if the acoustic properties of the tumour tissue are not significantly different from the normal tissue. The natural spectrum of tissue types in the normal human body is sufficiently varied, at least in terms of acoustic properties, to allow for the ultrasound examination, particularly in skilled hands, to offer a valuable and sensitive tool, which is painless to use and is virtually side effect-free. This can offer a variety of diagnostic possibilities including detailed repeated examination of pregnancies, the presence of structural abnormalities in the superficial layers of the body and the determination of the anatomical relationship of various tissue planes within the body. As has already been highlighted, the ability of the ultrasound technique to investigate lung pathology is severely limited due to the technical difficulties in acoustic scatter. And structures that are deep inside an obese patient may not be amenable to investigation due to the fall off in discrimination at significant tissue depth.

The advent of real-time scanning and polychromatic techniques to demonstrate rates of blood flow have revolutionised many of the area of cardiology and vascular medicine. (Mor-Avi V et al. 2000). Image processing technology allows for more subtle manipulations of the image than was the case even a decade ago with signal averaging technology playing a role in the improvement of the diagnostic capabilities of ultrasound. (Ren M et al. 2009)

Statistical computational techniques can be employed to enhance a single acoustic characteristic or to reduce "background noise" or signal scatter and thereby enhance the diagnostic capability further. (Pislaru C et al. 2001). This is certainly highlighted in the case of vascular imaging where degrees of ischaemia can be detected over very small areas of myocardial tissue. Further information can be gleaned, in this context, by the observations of the dynamic movements of mobile structures such as the identification of immobile segments in post-infarct myocardium where tissue elasticity and motility is significantly reduced in the affected areas of tissue. (Kondo H et al. 2005). In non-motile structures, dynamic tissue reaction can also be determined by the application of acoustic or direct pressure stimuli so that degrees of stiffness or elasticity can be assessed. This is a particularly useful technique in examinations of the prostate, the liver and skeletal muscle. .