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Competent performance of an ultrasound examination depends on an understanding of the basic physical principles that manage the production of ultrasound waves and their interaction with various biological tissues.
Ultrasound is defined as a sound with frequency above the range of human hearing (20Hz - 20000Hz). Quantities that are significant and described in ultrasound imaging contain: frequency, propagation speed, pulsed ultrasound, angle of incidence, interaction of ultrasound with tissue and attenuation. Images are generated by propagation of ultrasound throughout the tissues. The speed that sound travels through a medium is called "propagation speed" e.g. 1540m/sec for a soft tissue (Aldrich, 2007).
The sound beams are attenuated as they interact with tissues. The main interactions which contribute to the attenuation of the sound beam, are reflection, refraction, scattering and absorption (Aldrich 2007).When sound waves are transmitted into the body, various internal structures reflect and scatter it differently. Returning echoes can be collected and used to form an image of a structure.
During any ultrasound examination the operator should be continuously observing the image quality and trying to improve it. Often better scanning techniques are required and sometimes operator can optimise the image by changing system controls. Optimising the image is also an essential factor to eliminate artefacts.
The aim of this report is to describe the effect of main ultrasound controls on image optimisation and demonstrate this knowledge by using clinical ultrasound images.
Significant controls used for image optimisation:
The most important controls used for image optimisation consist of: frequency, depth, focus, time gain compensation (TGC), overall gain, zoom, dynamic range and frame averaging which will be discussed further in this section. The following method was used to demonstrate the effect of each control on the image.
Thick chicken breast was used as a phantom, cut a small slit and stuffed a large black olive in the centre. Then it was placed in a container filled with water. The container was left for an hour to settle then after the settlement period, the chicken breast was scanned. In this experiment the linear array transducer (12MHz) was used.
1. The impact of Frequency control on image optimisation:
The number of cycles per second performed by the particles of the medium in response to a wave passing through is called frequency. Ultrasound used for imaging usually has frequencies from 2-10 MHz (Aldrich. 2007).
The frequency and wave length of the ultrasound in a certain examination is determined by the fixed propagation speed of sound in a particular tissue. For instance, low frequency ultrasound which subsequently generates a larger wave length in soft tissues enables a deeper structure imaging. In contrast, high frequency ultrasound increases the resolution of image. The process decreases the sound wave so the examined area will be limited. It is desirable to choose the higher possible frequency for the depth of scan area.
Shows, higher frequency offers (Preset with frequency18MHz) better details resolution but poorer penetration and lower frequency provides less resolution with greater penetration. (A) 8MHz & (B) 13MHz
2. The impact of Depth setting on image optimisation:
Depth control adjusts the scanning range and can maximize the range of field of view. It also affects the frame rate and line density (Hedrick et al. 2005, P.132). The image area has a fixed number of pixels .Number of pixels in each square centimetre of tissue can be set by adjusting the field of view. This process consequently reduces the size of the relevant structures displayed on the monitor. Some parts of the scan area will be deleted from the image if the selected depth is too small. The lower image frame rate and smaller PRF are caused by the greater depth. The other factor to consider is, the arrival time of the echo from a structure being directly proportional to the depth of that structure.
The above images illustrate (A) how the image gets bigger and bigger as the depth decreases and (B) it gets smaller and smaller as the depth is increased.
3. The impact of Focus setting on image optimisation:
The ultrasound beam at first converges to a focal zone because the margins are unparallel. Then the beam diverges, distal to the focal zone (Taylor, 2003).The operator can change the depth and size of the focal zone viewed on the image. Some systems allow operator using more than one focal zone to observe the effects on frame rate.
The above images demonstrate that the focal point is positioned at the area of interest (2cm-Preset).
(A) & (B) indicate the single focal zone which does not correspond with the depth of interest.
4. The impact of Gain setting on image optimisation:
Overall gain generates amplification of the returning signal to the transducer. Therefore by increasing the overall gain, the amplification of signals will also increase. To avoid weak echoes or saturation, the right amount of amplification has to be selected (Frederick & Kremkau.2006, P.99).
The above images reveal (A) Overall gain is too high, therefore causing saturation (B) Overall gain is too low, generating unclear image.
5. The impact of Time Gain Compensation (TGC) setting on image optimisation:
Ultrasound systems have user operated TGC control which also known as time-dependent exponential amplification, depth gain compensation [DGC], swept gain control and signal attenuation (Hedrick et al. 2005, P.77). TGC control is also utilized to compensate the signal strength which is exponentially decreased. It amplifies the processed signal with time or depth and facilitates signal compression (Hedrick et at. 2005, P.78).
The above images display, when correct TGC is applied (preset), compensation for attenuation at depth over time occurs thus equal brightness of echoes throughout depth of field can be seen. (A) & (B) when TGC not applied evenly, attenuation occurs and brightness of echo decreases with depth.
6. The impact of Zoom on image optimisation:
This is a control for structures magnification. It is better to be used in real time or write-zoom mode. The mode affects the image quality by increasing line density and pixels and consequently decreases the field of view without enlarging pixel sizes. Read -zoom is used to enlarge pixels, so a smaller number of them are viewed on the monitor (Frederick & Kremkau.2006, P.122).
The area of interest has been magnified.
7. The impact of Dynamic range on image optimisation:
Dynamic range is the ratio of the largest to the smallest signal held by a system. "Compression" is achieved by decreasing the dynamic range. Amplification has no effect on intensity of the ultrasound beam. Extra sensitivity is only obtained by detecting weaker signals (Hedrick et al. 2005, P.77).
(A) Decreasing dynamic range gives fewer greys and increases contrast (B) Increasing dynamic range gives a wider range of greys and decreases contrast.
8. The impact of Harmonics on image optimisation:
Harmonic waves are generated from the nonlinear distortion of an audio signal. In this method transmitted frequency is eliminated and the secondary harmonic frequency echoes go through (Frederick & Kremkau.2006, P.105). The harmonic frequency is higher on its return which improves the axial and lateral resolution. It also reduces the effect of side lobe artefacts.
A comparison between two images clearly shows a higher quality image as harmonics. Harmonics applied.
9. The impact of Compounding on image optimisation:
This method is a real-time transmit spatial imaging. It utilizes electronic beam to generate several overlapping lines. These lines begin at various angles from the transducer crystal. Compound imaging also reduces the image noise and scatter which improves resolution and tissue differentiation (Whitsett, 2009).
(A)Compound imaging improves the visibility of tissue edges and decreases the noise in the image (B) No compound imaging applied.
10. The impact of Edge enhancement on image optimisation:
This is a filtering technique that can be used for the line- of -sight and matrix image data to underline an alteration in signal levels. This is a technique suitable for detecting of boundaries and edges between structures. Undersized high-contrast structures- which are not so visible- will be clarified by using this technique. This process in turn increases image noise (Hedrick et al. 2005, P.157&167).
(A)When the Edge enhancement increases, a better edge and boundaries differentiation is visible. The image noise is higher (B) Edge enhancement reduced.
Other factors affecting the image optimisation:
It adjusts the energy, stimulating the crystal and consequently the strength of the ultrasound beam into patient. To achieve necessary depth, penetration and minimum exposure, this control has to be sustained to the minimum. By increasing intensity, Signal-to-Noise ratio and sensitivity will be improved (Hedrick et al. 2005, P.182).
Also called persistence, builds up the echo information over several frames by holding 4 or more consecutive frames in a buffer and adding them together to increase signal -to-noise ratio. Slight textural variations result in image blurriness which can be improved by increasing the frame averaging (Hedrick et al. 2005, P.163).Scanning highly mobile tissues like cardiac structure requires higher effective frame rate which is achieved by decreasing or removing frame averaging , where as scanning immobile structures such as the thyroid requires higher frame averaging (Taylor, 2003).
Considering all the knowledge obtained by previous tests, a comparison is carried out between default settings and optimised image. To optimise the image (Figure 11), the depth was increased so the size of the relevant structure on the monitor was
maximised. Therefore, the focus needed to be reduced and re-positioned to the area of interest which increased intensity. These factors led to overall gain adjustment, to obtain a clearer image. When Harmonics utilized, it improved resolution and decreased penetration.
However, decreased penetration caused by harmonics is not suitable for patients with abdominal and pelvic imaging. Harmonic imaging is useful in 2-D imaging to detect cystic or fluid-filled structures or solid masses in a limited area. A typical application is breast examination and it is not recommended for obese patients due to lack of penetration (Whitsett,2009). In optimised image the compounding was elevated (Figure 12). This feature improved the visibility of tissue edges and decreased the noise in the image. Compound imaging is useful and effective in round surfaces and imaging of calcifications. This method is recommended for the cortex of the kidney, ovaries, fibroids, thyroid nodules and masses within some tissues. Using various scan line angles generates a clear image of plaque on vessels walls, so makes it useful in vascular imaging for plaque characterization. (Whitsett,2009).
Figure 11: The posterior tibial veins are more visible in optimised image than default setting.
The above images reveal the difference between default setting and optimised image.
Preset parameters: Dynamic range:65,Edge enhancement:1,Compound:4,Focal no:1,Frequency:14 MHz
Parameters applied for optimization: Depth decreased, Compound: 5, Edge enhancement: 2, Dynamic range: 65, Gain: increased, Frequency: 12 MHz, Harmonic on
Other important factor which needs to be considered for optimising the image is to prevent artefacts. Artifacts are defined as an image appearance that does not accurately correspond to anatomical features or characteristics in a patient (Blond&Buczinski,2009). Types of artifacts include:
Acoustic artifacts arise from beam interaction with tissues
Electronic artifacts relate to noise and electrical interference inherent in electronic systems
Equipment setting artifacts many of which are produced by operator.
For instance, inappropriate setting of overall gain, TGC and lack of contact can cause confusion. This problem can be solved by cautious manipulation of these controls. The operator tends to turn up the gain to make things more clear. However, excessive use of gain will purely add noise to the image. Equally, the slope will be uneven, when one of the TGC slider controls is misaligned, resulting in a band of bright or low level echoes within the field of view.
Transducer frequency is another crucial element in image quality. A better image resolution can be achieved by higher frequency. Multi frequency transducers enable the operator to image with increased axial and lateral resolution. This also eliminates the need for multiple transducers (Whitsett,2009).
On ultrasound system there are other controls depending on manufacturers. Pre and post processing are two of these controls discussed frequently in ultrasound image production.Pre-processing is used to form the images before they are digitally memorized in the scan converter. For instance, variation in strength of neighbouring echoes can be improved and create a type of edge effect. To make the final images more homogenous, connections between scan lines can be found. This is a modifiable function accomplished by interpolation between values of echoes and filling the gap between nearby lines of view. These echoes are situated in chronologically order on the same line.
Post-processing manipulates the data after storing in memory and optimizes the final image by affecting the intensity of echoes in transitional stage between the scan converter and monitor (Frederick & Kremkau.2006, P.122).
Some ultrasound machines have an automatic optimisation controls such as, Q-scan on Toshiba and I-scan on Philips. Quick scan, in 2D mode, automatically adjusts parameters like STC, 2D Gain and lateral gain (Toshiba operation manual,2008).
I-Scan was developed by some manufacturers to adjust some controls individually or in groups automatically such as setting gain, TGC, LGC, ABO. This is a time-saving function for a user who manually adjusts controls. These controls have two main disadvantages. First , a button has to be pressed by user to activate it and secondly, it has unreliable performance.
Figure 13: The improvement in image quality is clearly seen where Q-Scan is applied.
Hence,a good understanding of physics maximizes the benefit of diagnostic ultrasound and minimizes errors. Generating high quality ultrasound images depends on numerous factors such as equipment, technique used by operator and physical principles.