INTRODUCTION
Ultrasound (US) is the most essential modality for a gynecologist. A well done US may give maximum information about the anatomy and pathology of the female pelvic organs as well as the growing fetus. It is, therefore, essential that the operator is well-acquainted with the machine and can use its different functions for the best advantage of the patient. It is true that majority of us are reluctant to operate the different knobs available for obtaining the most expected image quality probably because we fear ruining the already made settings on the machine. This is well understood by the manufacturers and therefore several presets are available on the machines, with the names of the presets suggesting, what are these best for. These presets are made keeping in mind majority of the patients, but naturally would not suit to all and several adjustments are required to produce good quality images. Good quality images are essential for correct diagnosis.
This chapter will be divided under following heads:
- Optimizing B-mode image
- Optimizing Doppler image
- Artifacts
- Safety of US
- Tips to make a good setup
OPTIMIZING B-MODE IMAGE
The image can be optimized by manipulating and adjusting the knobs on the control panel of the scanner (Fig. 1). It is essential to know about the knobs, what they do, and when to use them.
The settings that need to be often done during each scan to get an optimum information out of the scans are scanning angle, scanning depth, probe frequency, focal zone, zoom, gains, contrast, and probe power.
- Scanning angle: Each probe has a maximum scanning angle. This is the maximum angle up to which the US beam can fan out. It indicates as to how far sideward can an US beam see. Maximum scanning angle for transvaginal probes usually vary from 80° to 180°. Large scanning angle is very convenient for obtaining the bird's eye view of the pelvis to see the entire uterus or first trimester fetus on a single image (Fig. 2), but it decreases the speed of scanning which is indicated by frame rate.What is frame rate?The real time B-mode images that we are seeing on the scanner screen are several static images seen quickly one after the other. Faster the change in frames, more real time it looks. Frame rate is the number of static images displayed in a unit time by the scanner. This means that if the frame rate is low, the image less real time.2
Fig. 3: Diagrammatic explanation of the sound beam direction in the central zone and in the side lobes.
Therefore, once the area of interest has been located, the scanning angle should be narrowed down to just a little larger than the area of interest. Narrowing the scanning angle increases frame rate. Moreover, when one decreases the scanning angle, area of interest is brought in the center. The sound beam from the transducer head then hits it at closer to right angle and gives sharper margin definition. When sound beam is emitted from the convex probe surface, the sound beam is vertical in the central part but is oblique on the sides (Fig. 3). This means that the structures that are not in the center of the image field are hit by the sound waves tangentially and lead to unsharp margins due to refraction.
- Scanning depth: Each probe has a limit of maximum depth up to which the US beam can penetrate. This, of course, depends on frequency of the probe. Maximum achievable depth by any probe may be used for the initial survey, but then depending on the depth at which the area of interest, the scanning depth, is to be decreased. This also increases the frame rate, and improves resolution (Figs. 4A and B). The correct depth setting is when the organ/lesion of interest fills up two-thirds of the image.
- Setting the focal zone: The US beam behaves like a light wave passing through the convex lens and therefore converges at a point called focal point, but since the probe (transducer) produces a series of similar sound beams the converging points of all make a plane called focal zone. Focal zone is the level at which the image is the sharpest because of the narrowest beam width. Therefore, focal zone is always set at the level of area of interest. The arrow head on right side of the image indicates the focal zone (Fig. 5). There is also an option of having more than one focal zone, when there are multiple levels at which the image needs to be sharp. But increasing the number of focal zones decreases the frame rate, and therefore usually single focal zone is selected.3
- Zoom: After the overview, decreasing the scanning angle and depth and setting the focal zone, further improvement in the image quality can be achieved by zooming the image. One may zoom the whole image (panzoom) (Figs. 6A to C) or may use a zoom box to decide and define which part of the image needs to be zoomed [high-definition (HD) zoom] (Figs. 6A to C). Panzoom only enlarges the image and therefore increases the distance between the image pixels and deteriorates the image quality unless it is used after freezing the image, whereas HD zoom concentrates pixels into a smaller box and thus improves image resolution. Larger image can depict more anatomical details. Image should be zoomed large enough to fill up at least two-thirds of the screen.
- Gains: The US wave when travels into the body hit several tissue planes on its onward as well as return path. With each plane that it hits, the sound beam attenuates due to absorption of the energy and refraction of the sound wave. This leads to darker (hypoechoic) image of the structures that are far from the probe. Image can be made brighter by increasing the gains (Figs. 7A and B). This is potentiating of the returning beam to make the image brighter, but changing gains inadvertently may lead to erroneous diagnosis. Therefore, in all doubtful, difficult situations, the gains once set on the preset should then not be changed. If done, one must revert back to preset as soon as the scanning field is changed. Gains can be adjusted in two ways. Total gains can be increased/decreased, or gains can be adjusted layerwise, depending on the distance of the tissue plane from the probe. The latter one is known as time gain compensation (TGC) (Fig. 8). US wave returning from deeper structures takes longer time to return and is attenuated. This control compensates for the gains lost by time.
- Probe power: Brightness of the image can also be adjusted by increasing the power of the incident beam. Gain adjustment potentiates the returning beam, and power affects the incident beam. Probe power can be increased maximum to 100%, but usually set at between 80% and 90%. This is to control total mechanical and thermal energy transmitted to the tissues. Power may be increased only when, in spite of all adjustments, an optimum image brightness is not achieved.
- Contrast: Increased contrast means more black and white image with less shades of gray, and less contrast means more shades of gray in an image (Figs. 9A and B). Unoptimized contrast settings may, therefore, mask the details of soft tissues. Contrast setting may also be presented as dynamic contrast on the US machine.
OPTIMIZING DOPPLER IMAGE
What is Doppler?
Doppler effect is the change frequency of a sound wave when it hits a moving object. The difference in the emitted and the received frequency is known as Doppler shift. The shift depends on the angle at which the sound beam hits the moving object, velocity of the moving object, and the frequency of the incident beam. Looking into the equation used for calculation of the velocity from the frequency change on Doppler:
where,
fd = Doppler shift
ft = Transmitted beam
c = Speed of sound in tissue
V = Velocity of blood flow
θ = Angle of incidence between the ultrasound beam and the direction of flow
The Doppler effect can be displayed as color Doppler, power Doppler, and spectral Doppler.
Color Doppler
It displays the blood flow in two colors, and these are conventionally red and blue. The color indicates the direction of the flow. Flow toward the probe is red and away from the probe is blue (Fig. 10), but these can be interchanged by using the invert switch. When the flow is perpendicular to the sound beam, no color will be displayed in spite of presence of the flow. The arterial flow is pulsatile and the venous flow is nonpulsatile. The higher flow velocities display bright colors and the lower flow velocities display dull colors (Fig. 11).4
Figs. 6A to C: (A) B-mode image of uterus; (B) B-mode image of uterus with panzoom; and (C) B-mode image of uterus with high-definition (HD) zoom.
Since color Doppler does not give exact velocity values, so it is a directional semiquantitative Doppler.
Power Doppler
Power Doppler is nonangle-dependent technology. It is known that movement of any object produces energy and this is used to depict the blood flow signals in power Doppler. This means that wherever there is a movement of blood or of body tissues, color signals will be generated. It displays color signals even in vessels that are perpendicular to the sound beam, but the disadvantage is that it is a single color display and does not show the flow direction (Fig. 12). It indigenously potentiates the signals and therefore is a useful technology for documentation of low velocity blood flows. The main application of the power Doppler, therefore, is to pick up flow in low velocity blood vessels and the blood flows in the vessels are perpendicular to the sound beam. High velocity movements show a bright color and the low velocity movements display dull color (Fig. 12).
High-definition flow is a new addition to the basic power Doppler technology. It is a directional power Doppler. Apart from high flow sensitivity, HD flow also has a color coding for the flow toward or away from the probe as in color Doppler (Fig. 13).
Spectral Doppler
Spectral Doppler is the spectral display of the blood flow/movement of a moving object. Trace above the baseline in the spectrum is the flow toward the probe and the trace below the baseline is the flow away from the probe (Fig. 14) on the spectrum. On the spectral Doppler, the arterial flow appears spiky and the venous flow appears flat. There is a scale on the side of the spectrum that calculates exact velocities of the flows can be calculated (Fig. 14).
The spectrum can be displayed for pulsed wave Doppler and the continuous wave Doppler. In pulsed wave Doppler, the sound waves are emitted in pulses and the frequency of this is called pulse repetition frequency (PRF).
To obtain the correct information about flow velocities with Doppler, certain settings and adjustments on the scanner are required.
Fig. 10: Color Doppler image showing flow toward the probe in red and flow away from the probe in blue.
Fig. 11: Color Doppler showing bright color for high velocity flow and dull color for low velocity flow.
Fig. 12: Power Doppler image showing single color flow and bright color for high velocity flow and dull color for low velocity flow.
Fig. 14: Trace above the baseline in the spectrum is the flow toward the probe and the trace below the baseline is the flow away from the probe and scale on the side of the spectrum calculates the velocities.
Though most of these are set on the dedicated presets, it is important to understand how we can manipulate certain switches/knobs to achieve best flow information.
These are the Doppler box size, color gains, PRF, wall motion filter (WMF) and balance on color and power Doppler and sample volume, gains, PRF, WMF, and angle correction for the pulsed wave Doppler.
- Color/power Doppler settings:
- Box size: When one switches on the color Doppler, a box appears on the screen, on the B-mode image. This box defines in which area of the B-mode image, the blood flow information will be looked for. It is important to consider here that when the Doppler is switched on, the machine has to process the B-mode information as well as the flow information and therefore the frame rate significantly decreases.7The color box size is planned just large enough to cover the area of interest. The color box can be moved all across the B-mode image and the size can be altered based on the requirement (Fig. 15).
- Gains: When the Doppler is switched on, it should show the blood vessels filled up with color but no color spilling out of the vessels. This is done by gain adjustment. When the gains are too high, the color will be seen spilling out of the vessels (Fig. 16). Whereas when the gains are low, the color will not completely fill up the vessel. This is because when the gains are low, the low velocity signals will not be picked up by Doppler. The correct gain, therefore, is when the entire lumen of the vessel is filled with color and there is no spill outside. Color gains can be adjusted by rotating the color knob.
- Pulse repetition frequency: It is important to select an optimum PRF for the velocity of the blood vessels flow studied. High PRF if used for low velocity flow, it will not be possible to pick up the color where there are flows (Fig. 17A). Instead if low PRF is used for high velocity flows, there will be aliasing mixing of red and blue colors—appears like turbulence (Fig. 17B). The PRF setting would be optimum when the color homogeneously fills the entire vessel with single color—red or blue.
- Wall motion filter: It is known that Doppler produces color signals wherever there is a movement and the brightness of the color depends on the velocity of the moving object.8This means that the color signals are produced by the red blood corpuscles in the blood, but are also produced by the wall movement of the artery and also by the pulsations transmitted to the surrounding tissues. The color signals of the blood flow are the brightest, those of wall motion are dull, and those due to transmitted pulsations from the surrounding tissues are the dullest, but these dull color signals corrupt the flow information and can be eliminated only if a low velocity filter is used. This filter is named as WMF. For larger vessels with high velocity flows, the arterial flow movement is more and higher WMF is required, whereas for small vessels with low velocity signals, the arterial wall movements are less and so low wall filters are required. Using higher wall filter for a low velocity blood, flow vessel will eliminate the slow flow information.
- Balance: As the name suggests, this is a balancing tool between the two modalities—the B-mode and the color Doppler. When color/power Doppler is switched on, a gray bar and a color bar appear on the left side of the screen. On the gray bar, is a green line. This line indicates the balance adjustment. When the brightness of the grayscale on the image matches the brightness below the green line on the gray bar, the color predominates and the color filling is normal, but when the brightness on the grayscale image matches the brightness above the green line on the gray bar, the B-mode predominates and therefore in these areas if the color is present to show the flows, the color will be patched up with white (Figs. 18A and B). When this happens, the correct thing to do is to change the balance to higher, or decrease the B-mode gains (Figs. 18A and B).
- Pulsed wave Doppler:
- Sample volume: Sample volume is the selected length of the vessel to assess the flow. When pulsed wave Doppler is switched on, a dotted line appears on the screen. This line is parallel to the sound beam that can be swapped across the entire image. Two parallel short horizontal lines (=sign) appear on this line and is called sample volume/gate size (Fig. 19), this “=sign” can be moved up and down on the dotted line anywhere. This sign is to be placed on the vessel in which the flow is to be measured. The distance between the two lines decide, what length of the vessel will be evaluated for the flow assessment. If the vessel is not absolutely parallel to the sound beam (overlapping on the dotted line), the distance between the two line (sample volume) should be equal to the diameter of the vessel.9A sample volume smaller than the diameter will lead to error in the velocity assessment.
- Gains: Gain settings on the pulsed wave Doppler should be such that it produces a clear well-defined bold spectrum of blood flows (Figs. 20A to C). If the gains are too high, the flow information will be corrupted by lot of noise (Figs. 20A to C). If the gains are too low, the entire spectrum will appear scarce and scattered (Figs. 20A to C).
- Pulse repetition frequency: PRF is adjusted according to flow velocity to be assessed, high PRF for high velocity flow and low PRF for low velocity flow. If high PRF if used for low velocity flow, it will not be possible to differentiate between the systolic and diastolic flows as the systolic flow recordings will be subdued (Figs. 21A and B). If low PRF is selected for high-velocity blood flows, there will be overlapping of systolic and diastolic signals and is known as aliasing (Figs. 21A and B).
- Wall motion filter: Like in color and power Doppler, the function of wall motion filter in pulsed Doppler also is to eliminate signals from low velocity movements, not to corrupt the image with wall motions. Again like color and power Doppler, the settings are low for low velocity vessels and high for high velocity vessels, but the wall filter setting in a pulsed Doppler spectrum is known to be correct only if the spectrum touches the baseline. When there is a black line or a gap between the baseline and the spectrum (Fig. 22), this trace is not to be accepted as this clearly indicates high wall filter for the case and may erroneously diagnose absence of diastolic flows. In that case if we say it eliminates low velocity information, means it interferes with the diastolic flow information and may lead to false diagnosis of absent end-diastolic flow and naturally then wrong interpretations.
- Doppler angle: As is discussed earlier considering the equation for calculation of the blood flow velocity from frequency of incident sound beam, frequency of received sound beam, and cos of the angle of incidence, if the angle of incidence is 90°, then the cos θ being 0, the velocity value will be 0 and also that with increasing angle from more than 60°, the percentage of error in calculation is highly significant and so the Doppler angle is always set between 0° and 60°, preferably <30°.10Figs. 21A and B: (A) Spectral Doppler image with high pulse repetition frequency (PRF) for low velocity flow, hardly any differentiation seen between systolic and diastolic velocities; and (B) Spectral Doppler image with low PRF for high velocity flow, showing overshooting of systolic flow on the other side of the spectrum (aliasing).The Doppler angle can be set at 0° when the vessel is parallel to the dotted line. This is often times possible because the dotted line can be swapped across the entire B-mode image and the probe manipulation may also help in the alignment of the two. But if it is still not possible, after achieving the smallest angle between the vessel and the dotted line, angle correction is used. This deviates out a short line from the dotted line, and is tried to align this short line to the vessel (Fig. 23). In trying to do this, the angle between the dotted line and the short line is the Doppler angle. It is displayed on the screen or the touch pad of the scanner.
- Setting the speed of the trace: An ideal spectral trace is when there are four to five cardiac cycles (Fig. 24) recorded on any one spectrum image. This can be done by setting the speed of the trace. For most scans, this is possible when the speed is set as 4 or 5. Higher speed gives trace of too few cardiac cycles and lesser speed gives too many cardiac cycles traced.
UNDERSTANDING THE ARTIFACTS
In spite of all these settings used to optimize the Doppler images, certain artifacts still cannot be completely eliminated. These are aliasing, mirror image artifact, and artifacts due to electrical interferences.11
Fig. 24: Spectral Doppler showing five complete cardiac cycles on the baseline, suggesting normal speed adjustment.
- Aliasing: This is overlapping effect of systolic and diastolic velocities, across the baseline on both the sides of the spectrum (see Figs. 21A and B). This effect is similar to what we have often observed especially in movies. The car wheels suddenly appear to start rotating in the opposite direction when the car speeds up. Adjusting the PRF can eliminate this artifact.
- Mirror image artifact: Mirror image artifact is when a similar spectrum is seen on both the sides of the baseline. This is especially possible when the sample volume is large and is tracing the flow in two vessels or two loops of the same vessel positioned, side by side (Fig. 25), or a large sample volume is placed on the curve of the loop, when in the proximal half of the loop the blood flow is observed away from the probe by the transducer and in the distal half of the sample volume the flow is perceived toward the probe. Decreasing the sample volume and planning to place it on one vessel only sorts out this problem.
- Electrical interferences: These may appear as random signals on color, power, or spectral Doppler, especially when the scanner is sharing the same electrical line as some high voltage gadget and the only way to get rid of this is to plan the electrical supply to the scanner wisely.
SAFETY OF DOPPLER
The two major effects of sound wave when it passes through the human body are:
- Thermal effect: Production of heat that may damage the cells.
- Mechanical effect: Due to pressure changes on the molecule.
Thermal effect: As the sound waves pass through the body tissues, there is absorption of energy and transformation of US energy into heat. The energy absorption is minimal in fluid and maximum in bones. It is also dependent on the frequency of the US waves. The absorption is higher with high frequency waves and lower with low-frequency sound waves. A temperature rise of up to 1°C is considered absolutely safe, whereas if it is >2.5°C, it can lead to significant tissue damage. This thermal effect is measured and displayed as thermal index on the screen. It is generally found that the temperature rise of 2°C is thermal index 2. The thermal index should be limited at maximum 1.
Mechanical effect: When the sound wave passes through the body tissues, it leads to oscillations of the body molecules, resulting in cavitating (low pressure) phase and a compressing (high pressure) phase. The mechanical damage caused by the sound waves is quantified as mechanical index (MI). MI is defined as “maximum estimated in situ rarefaction pressure or maximum negative pressure (in MPa) divided by the square root of the frequency (in MHz)”. MI of up to 0.3 can be considered safe and more than 0.7 can lead to cavitation.1
APART FROM THIS A FEW PRACTICAL TIPS ON PLANNING YOUR SETUP
Ordering the Scanner
Apart from optimizing the image, it is essential to make the correct choice of the machine according to your requirements and also correctly ordering the probes and the softwares.
There is a huge choice available now in the market as there are several different brands available with a range of lower end only B-mode or portable machines to high-end 3D-4D machines loaded with lot of automations and softwares. Though the image quality, even on the B-mode, significantly improves as one moves on from lower end to the higher end scanners, 3D and 4D scanners are not essential for all. It depends on one's type of practice and the amount of work that the scanner should be selected. But as far as obstetrics and gynecology practice is con-cerned, in our opinion, a good quality scanner with a good quality Doppler is essential. Check for the service facilities of a particular brand around your place.12
Probes must also be selected according to one's practice. It should be kept in mind always that low frequency probes are better for penetration but have a poor resolution, whereas high frequency probes have poor penetration but high resolution. Always check the B-mode and the Doppler image quality with the same probes that is to be bought during presale demonstrations. Instead of depending on the demonstration images in presentations by the company, depend on the live scans that are being done, preferably by a colleague.
When ordering the machine, confirm which softwares are optional and decide which of those are required for one's practice and specifically order for those and confirm when the written order comes to you for signature.
It is also important that in your setup, the electrical power point supplying the scanner should not be in the same line as any other equipment that consumes high power voltage. Moreover, do not fit any light-emitting diode (LED) lights in the scanning room.
CONCLUSION
Doppler is a very useful modality for assessment of circulation in the human body. Correct settings on the scanner only can give optimum results and therefore it is very important to understand the basic principles and settings of the US scanner before starting to use Doppler for interpretation of vascular flows and information of oxygenation in human fetus. US and Doppler are generally safe modalities. Their safety can be related to frequency used and the length of exposure. Therefore, Doppler should not be used for long time on a single focus and therefore as low as reasonably achievable (ALARA)2 principle is now applied for all US scan.
REFERENCES
- The British Medical Ultrasound Society (BMUS) (2019). Safety of ultrasound. [online] Available from www.bmus.org/public-info/pi-safety01.asp. [Last accessed November, 2019].
- Auxier JA, Dickson HW. Guest editorial: concern over recent use of the ALARA philosophy. Health Phys. 1983;44(6):595–600.