In this chapter the physics of medical ultrasound will be discussed at an introductory level for users of the technology. Basic physics is also of central importance in considerations of safety.
We will see in the discussion of imaging techniques that this is very important since it means that pulses can be Feb 9, What is Ultrasound? Ultrasonic Probe. Identify the appropriate ultrasound probes for various bedside applications: cardiac imaging, pleural imaging, soft tissue imaging, central line placement, and paracentesis. Understand how to take and annotate ultrasound images. Understand basic ultrasound physics. Understand basic ultrasound language ie Nicholas J. Hangiandreou, PhD.
Ultrasonography US has been used in medical imaging for over half a century. Current US scanners are based largely on the Basic Physics and Imaging Characteristics of Ultrasound.
George Kossoff, D. The imaging characteristics of diagnostic ultrasound US are determined by the ultrasonic properties of tissue. The velocity of propa- gation of US and the attenuation Jun 20, Ultrasound imaging. View the article online for updates and Basics of ultrasonographic examination in A review of the state of the art of ultrasonics in medicine and biology was published in Physics. This review is provided to aid the physician in the understanding of the basic physics of ultrasound.
New ultrasound instrumentation and tech- niques are capable of producing sophisticated information. The dilemma is that the recent proliferation of ultrasound imaging instru-. Physics of Ultrasound and. Propagation m Tissue. MD, MS. Ultrasound is sound possessing a frequency Acoustic impedance is a property of the tissue, and is defined as the product of its tissue density and the propagation velocity of sound waves through that tissue.
If two tissue types have identical acoustic impedance, then no echo will be produced, as no sound waves will be reflected back. The brightness of the signal is directly related to the amount of reflection, and that the amount of reflection is proportional to the absolute difference in acoustic impedance between the two media.
It therefore follows that a large acoustic impedance mismatch between two tissue types will result in a bright echogenic signal, while a small acoustic impedance mismatch between another two tissue types will result in an echo-poor signal. Thus the interface between the kidney and the liver is somewhat harder to distinguish from one another Fig.
This results in minimal further propagation of sound waves. Therefore, beyond that interface, there is limited to no ability to further directly image structures [ 24 ]. This large acoustic impedance difference between air and skin is also the reason why coupling gel must be used for imaging purposes.
Application of gel eliminates any air present between the transducer and the skin, assisting in the transmission of sound waves, rather than having most of them reflected back.
Small acoustic impedance difference between liver and kidney results in a minimally echogenic interface between the two organs. Large acoustic impedance difference muscle and femur results in a bright echogenic interface between the two structures A second factor that determines the amount of reflection is the smoothness of the surface.
This results in diffuse reflection Fig. Because the returning echoes are in multiple directions, only a few of them are received back on the transducer. As a result, diffuse reflection results in a less echogenic signal. These differing velocities result in refraction, or change in the direction of the original or incident sound wave [ 25 ]. As can be seen from the equation, the higher the difference between the propagation velocities in the two media, the larger the magnitude of angle change of the refracted beam.
Without the need to modify any controls, the image resolution of the vascular structures is improved Absorption and Attenuation As sound waves propagate through tissue, part of the acoustic energy is absorbed and converted into heat. The amount of absorption that occurs is a function of the 1 sound wave frequency, 2 scanning depth, and 3 the nature of the tissue itself.
Higher frequency sound waves are absorbed more than lower frequency sound waves. As stated earlier in this chapter, although higher frequency sound waves yield better resolution than lower frequency sound waves, this improved resolution is gained at the expense of lower penetration [ 15 ]. The inability of high frequency sound waves to penetrate deeply into tissue is a direct result of high absorption and conversion of acoustic energy into heat. Thus, a shallower depth, provided it captures sufficiently the structure of interest in the field of view, will result in a better image than one at a deeper depth, as it results in less absorption.
The amount of absorption that occurs is also a function of the medium itself, with certain media resulting in higher attenuation than others. Overall attenuation through a particular medium is described by the attenuation coefficient, which is measured in decibel per cm per MHz see Table 1. As can be seen in Table 1.
All these described processes, such as diffuse reflection, scattering, refraction, and absorption, all serve to attenuate the strength of the returned echo signal, because they all ultimately in one way or another divert energy away from the main ultrasound beam [ 24 ]. Summary Increasing frequency results in less penetration and more detail: Use high-frequency probe for vascular access, soft tissue, and pleura. Use low-frequency probes for the chest and abdomen.
Body habitus matters: Sound waves get absorbed and attenuated. With increasing soft tissue from skin to target organ, the quality of the image obtained decreases. Watch out for air and bone: Bone will result in almost complete reflecton, making it impossible to image structures under it. Air is a poor conductor of sound, and it will result in artifacts and failure to obtain a quality image. The Machine An ever increasing number and variety of commercially available ultrasound machines are available from multiple manufacturers, [ 27 ] and which unit to purchase depends on a variety of factors such as price, durability, ease of use, image quality, ergonomic design, boot-up time, lifespan of the battery, and portability [ 27 , 28 ].
The size of point-of-care devices is becoming smaller and with this trend, portability has correspondingly becoming better, with some of these point-of-care devices being no bigger or even smaller than the size of a laptop machine Fig. While each machine has its unique instrumentation , some of the basic components are universal, and many devices offer similar functionalities.
Image courtesy of Mobisante, with permission. Courtesy of GE Healthcare The critical components of all ultrasound machines include a transducer, a pulser, a beam former, a processor, a display, and a user interface [ 26 , 28 ]. The piezoelectric elements which generate the ultrasound waves are typically arranged within the transducer either sequentially in a linear fashion offering a rectangular field of view linear array , in an arch which offers a wider trapezoid field of view convex or curved array , or steered electronically from a transducer with a small footprint phased array Fig.
A curved array transducer middle where transducer elements are arranged in an arch, resulting in a trapezoid field of view.
A phased array transducer right where transducer elements are electronically steered, resulting in a sector or pie-shaped field of view. The pulser has two functions. Applying higher voltages will increase the overall brightness of the image. Practically however, the maximum resultant brightness is limited because the maximum voltage that can be applied and maximum acoustic output of ultrasound devices are restricted based on regulations by The FDA [ 29 ].
Second, the pulser controls the frequency of pulses emitted number of pulses per second , known as the pulse repetition frequency PRF. It is necessary that pulses of sound waves are delivered, instead of continuous emission of sound waves, so that in between the pulses, there is time for the reflected sound waves to travel back to the transducer [ 30 , 31 ]. The operator controls various parameters of color Doppler such as the velocity scale or pulse repetition frequency PRF , wall filter, size of the area within the field of B-mode and the angle of incidence that the ultrasound beam makes with the direction of blood flow.
Low velocity scales and filters are reserved for low impedance vascular beds such as ovarian flow in gynecology Figure 1. In order to optimize the display of color Doppler, the angle of insonation should be as parallel to the direction of blood flow as possible. Typically ovarian flow is low impedance and detected on low velocity scale with low filter setting. Blood flow in the fetal heart has high velocity and thus is detected on high velocity scale.
White arrows show the direction of blood flow. Note the absence of blood flow on color Doppler asterisk where the ultrasound beam grey arrow images the cord with an angle of insonation equal to 90 degrees. The black arrows represent blood flow with an angle of insonation almost parallel to the ultrasound beam and thus display the brightest color corresponding to the highest velocities.
In the spectral Doppler mode, or pulsed Doppler mode, quantitative assessment of vascular flow can be obtained at any point within a blood vessel by placing a sample volume or the gate within the vessel Figure 1. Similar to color Doppler, the operator controls the velocity scale, wall filter and the angle of incidence.
Flow towards the transducer is displayed above the baseline and flow away from the transducer is displayed below the baseline. In spectral Doppler mode, only one crystal is typically necessary and it alternates between sending and receiving ultrasound pulses. S corresponds to the frequency shift during peak systole and D corresponds to the frequency shift at end diastole. Doppler mode, or Energy mode, or High Definition Doppler mode is a sensitive mode of Doppler that is available on some high-end ultrasound equipment and is helpful in the detection of low velocity flow Figure 1.
The strength amplitude of the reflected signal is primarily processed. Power Doppler mode is less affected by the angle of insonation than the traditional color or spectral Doppler. Power Doppler mode is helpful in the detection of low velocity flow. Ultrasound is a form of mechanical energy and its output varies based upon the mode applied. In general B-mode has the lowest energy and pulsed Doppler has the highest energy. Given the presence of a theoretical and potential harm of ultrasound, the benefit to the patient must always outweigh the risk.
In general, ultrasound is considered to be a safe imaging modality as compared to other imaging modalities that have ionizing radiation like X-ray and Computed Tomography CT. The Mechanical index MI gives an estimation of the cavitation effect of ultrasound, which results from the interaction of sound waves with microscopic, stabilized gas bubbles in the tissues. Other effects included in this category are physical shock wave and chemical release of free radicals effects of ultrasound on tissue.
In this ODS, the manufacturers are required to display in real time, the TI and the MI on the ultrasound screen with the intent of making the user aware of bioeffects of the ultrasound examination Figure 1. The user has to be aware of the power output and make sure that reasonable levels are maintained.
Despite the lack of scientific reports of confirmed harmful bioeffect from exposure to diagnostic ultrasound, the potential benefit and risk of the ultrasound examination should be assessed and the principle of ALARA should be always followed. Note the display of MI and TIb in white rectangle. Several national and international societies have official statements that relates to the use of medical ultrasound in obstetrics and gynecology. We have assembled in this chapter some of the relevant official statements along with the Internet link to their source.
Informed consent should be obtained. ISUOG- Safety Statement, reconfirmed 2 : The thermal index TI and the mechanical index MI are not perfect indicators of the risks of thermal and nonthermal bioeffects, but currently they should be accepted as the most practical and understandable methods of estimating the potential for such risks. B-mode and M-mode Acoustic outputs are generally not high enough to produce deleterious effects. Their use therefore appears to be safe, for all stages of pregnancy.
Doppler Ultrasound Significant temperature increase may be generated by spectral Doppler mode, particularly in the vicinity of bone. Caution is recommended when using color Doppler mode with a very small region of interest, since this mode produces the highest potential for bioeffects. When ultrasound examination is clinically indicated, there is no reason to withhold the use of scanners that have received current Food and Drug Administration clearance in tissues, which have no identifiable gas bodies.
Since ultrasound contrast agents are mostly gas-carriers, the risk of induction and sustenance of inertial cavitation is higher in circumstances when these agents are employed. Pregnancy Based on evidence currently available, routine clinical scanning of every woman during pregnancy using realtime B-mode imaging is not contraindicated. The risk of damage to the fetus by teratogenic agents is particularly great in the first trimester.
One has to remember that heat is generated at the transducer surface when using the transvaginal approach. Spectral and color Doppler may produce high intensities and routine examination by this modality during the embryonic period is rarely indicated.
In addition, because of high acoustic absorption by bone, the potential for heating adjacent tissues must also be kept in mind. Exposure time and acoustic output should be kept to the lowest levels consistent with obtaining diagnostic information and limited to medically indicated procedures, rather than for purely entertainment purposes. There have been no reported incidents of human fetal harm in over 40 years of extensive use of medically indicated and supervised diagnostic ultrasound.
Nevertheless, ultrasound involves exposure to a form of energy, so there is the potential to initiate biological effects. Some of these effects might, under certain circumstances, be detrimental to the developing fetus. Therefore, the uncontrolled use of ultrasound without medical benefit should be avoided. Furthermore, ultrasound should be employed only by health professionals who are trained and updated in the clinical usage and bioeffects of ultrasound.
AIUM-Conclusions Regarding Epidemiology for Obstetric Ultrasound 5 : Based on the epidemiologic data available and on current knowledge of interactive mechanisms, there is insufficient justification to warrant conclusion of a causal relationship between diagnostic ultrasound and recognized adverse effects in humans. Some studies have reported effects of exposure to diagnostic ultrasound during pregnancy, such as low birth weight, delayed speech, dyslexia and non-right-handedness.
Other studies have not demonstrated such effects. Biological effects such as localized pulmonary bleeding have been reported in mammalian systems at diagnostically relevant exposures but the clinical significance of such effects is not yet known. Ultrasound should be used by qualified health professionals to provide medical benefit to the patient. The use of ultrasound without a medical indication to view the fetus, obtain images of the fetus, or determine the fetal gender is inappropriate and contrary to responsible medical practice.
AIUM-Statement on Measurement of Fetal Heart Rate 8 : When attempting to obtain fetal heart rate with a diagnostic ultrasound system, AIUM recommends using M-mode at first, because the time-averaged acoustic intensity delivered to the fetus is lower with M-mode than with spectral Doppler.
If this is unsuccessful, spectral Doppler ultrasound may be used with the following guidelines: use spectral Doppler only briefly e. Being able to optimize the ultrasound image is much dependent on the understanding of the basic functionality of the ultrasound equipment. This chapter will focus on the review of various components of the ultrasound equipment and the basic elements of image optimization.
The following chapter chapter 3 will introduce some helpful scanning techniques. It is important to note that before you acquire ultrasound equipment, you should have an understanding of who will be using the equipment, for which medical purpose it is intended to be used, in which environment it will be used and how will it be serviced.
The answer to these important questions will help in guiding you to the appropriate type of ultrasound equipment for the right setting. For instance, ultrasound equipment destined for low-resource outreach settings should have special characteristics such as portability, sturdiness and a back-up battery in order to adjust to fluctuation in electricity. Furthermore, ultrasound equipment designed for the low-resource outreach setting should be easily shipped for repairs and service.
Ultrasound Transducers Ultrasound transducers are made of a transducer head, a connecting wire or cable and a connector, or a device that connects the transducer to the ultrasound machine. The transducer head has a footprint region Figure 2.
It is this footprint region of the transducer that needs to remain in contact with the body in order to transmit and receive ultrasound waves.
Each transducer also has a transducer probe marker located next to the head of the transducer in order to help identify its orientation Figure 2. The use of this probe marker in handling the transducer and its orientation will be further discussed in the following chapter Chapter 3. Figure 2. The footprint region is where the abdominal transducer. The probe marker is sound waves leave and return to the transducer.
Transducers are produced in an array of shapes, sizes and frequencies and are adapted for specific clinical applications. In general, transducers for cardiac applications have small footprints. Vascular transducers have high frequencies and are linear in shape and obstetric and abdominal transducers are curvilinear in footprint shape in order to conform to the shape of the abdomen Figure 2. Note the curvilinear shape of the footprint, which helps to conform to the abdominal curvature.
The width of the image and number of scan lines is uniform throughout all tissue levels Figure 2. This has the advantage of good near field resolution. Linear transducers are not well suited for curved parts of the body as air gaps are created between the skin and transducer Figure 2. Note the gap produced between the transducer footprint and the abdominal wall white arrows.
This can be eliminated by simply applying gentle pressure on the abdomen. Note the rectangular screen image and a good near-field resolution. Sector transducers produce a fan like image that is narrow near the transducer and increase in width with deeper penetration.
Sector transducers are useful when scanning in small anatomic sites, such as between the ribs as it fits in the intercostal space, or in the fontanel of the newborn Figure 2. Disadvantages of the sector transducer include its poor near field resolution and somewhat difficult manipulation. Curvilinear transducers are perfectly adapted for the abdominal scanning due to the curvature of the abdominal wall Figure 2. The frequency of the curvilinear transducers ranges between 2 and 7 MHz.
The density of the scan lines decreases with increasing distance from the transducer and the image produced on the screen is a curvilinear image, which allows for a wide field of view Figure 2. Note that the image is curvilinear in shape arrows and has a wide field of view. They are designed to fit in small endocavitary spaces with the footprint at the top of the transducer transvaginal or at the dorsal aspect of the transducer rectal.
When performing a transvaginal ultrasound examination, a clean condom, or the digit of a surgical rubber glove, should cover the transvaginal transducer. Ultrasound gel should be placed inside and outside the protective cover in order to facilitate the transmission of sound. Protocols for ultrasound transducer cleaning should be adhered to in order to reduce the spread of infectious agents.
Both the transabdominal and the transvaginal transducers should be wiped between ultrasound examinations and disinfection of the transvaginal transducer should be performed according to national or manufacturer guidelines 1. Controls of the Ultrasound Equipment Ultrasound equipment has a wide array of options and features. These features are typically operated from either the console of the ultrasound equipment, a touch screen monitor or a combination of both Figure 2.
Most ultrasound equipment have a keyboard and a trackball on their consoles. Power or Output Control: This controls the strength of the electrical voltage applied to the transducer crystal at pulse emission. Increasing the power output increases the intensity of the ultrasound beam emitting and returning to the transducer, thus resulting in increase in signal to noise ratio.
Increasing the power results in an increase in ultrasound energy delivered to the patient. It is therefore best practice to operate on the minimum power possible for the type of study needed. Resorting to lower frequency transducers can help achieve more depth while minimizing power output. Depth: The depth knob allows you to increase or decrease the depth of the field of view on the monitor.
It is important to always maximize the area of interest on your monitor and decrease the depth of your field of view, which enlarges the target anatomic organs under view. Figures 2. Gain: The gain knob adjusts the overall brightness of the image by amplifying the strength of the returning ultrasound echo. The overall brightness of the image can be increased or decreased by turning the gain knob clockwise or counterclockwise respectively.
In A, the depth white double arrow is increased, resulting in a small head, whose anatomic details are consequently difficult to see. In B, the depth is reduced, which allows for a larger head thus improving visualization.
In A, the gain is too low and in B, the gain is adequate. Note better visualization of intracranial anatomy with a higher gain B. Adjusting the gain to the correct level comes with experience. The upper knobs increase or decrease brightness closer to the transducer footprint and the lower knobs increase or decrease brightness farthest from the transducer footprint.
As a general rule, in transabdominal ultrasound, the upper field gain knobs should be kept slightly to the left than lower field ones in this way the eye of the operator can focus on the deeper part of the screen where the fetus is.
The reverse is true with transvaginal ultrasound, where the region of interest is often in the near field. The upper and lower knobs adjust brightness in the upper and lower fields respectively labeled. The overall knob labeled adjusts brightness in the whole image. Focal Zones: The focal zones should always be placed at the depth of interest on the ultrasound image in order to ensure the best possible lateral resolution.
Multiple focal zones can be used to maximize lateral resolution over depth; however this will result in a slower frame rate and is thus less desirable when scanning moving structures such as in obstetrics or the fetal heart specifically. Freeze: The freeze knob allows the image to be held frozen on the screen. While the image is frozen measurements can then be taken and organ annotations can be applied to the image before saving it. This is a very important function in obstetric ultrasound imaging, as it assists in capturing frames during fetal movements, such as measurement of long bones.
Trackball: The Trackball or Mouse pad is used for moving objects on the monitor and for scrolling back in freeze mode. It has a multi-function and can be used in conjunction with caliper placement, screen annotation, or moving the zoom or Doppler boxes to the desired location. Res or Zoom: Some ultrasound equipment has this function, which allows magnification of areas of the ultrasound image displayed on the monitor in real time.
B stands for brightness mode. When the operator presses this knob, the traditional 2-D image is activated. Note the various gradation of grey with the ribs being the brightest echogenic followed by the lungs and heart labeled. The amniotic fluid AF is black in color anechoic reflecting a weak intensity of the returning echo. M- Mode stands for Motion mode and in this function an M-Mode cursor line appears on the upper section of the image with an M-Mode display on the lower part of the image Figure 2.
The M-Mode display corresponds to the anatomic components that the M-Mode cursor intersects. The M-Mode is used primarily to document motion, such as cardiac activity of the fetus in early gestation Figure 2.
Note the corresponding M-Mode display large bracket in the lower image showing cardiac motion. Reflections in the M-Mode tracing asterisks represent cardiac motion. Calipers are measuring fetal heart rate FHR at beats per minute bpm. Color Flow: The color flow knob activates color flow or color Doppler, which adds a box superimposed on the 2-D real-time image on the screen.
The operator can control the size and location of the color box on the 2-D image. Color flow or color Doppler detects blood flow in the insonated tissue and assigns color to the blood flow based upon the direction of blood flow.
By convention, red is assigned for blood flow moving in the direction of the transducer up and blue is assigned for blood moving in the direction away from the transducer down. The operator can also control the velocity scale of blood flow pulse repetition frequency and the filter or threshold of flow.
These parameters are important in assessing various vascular beds. Note that the display of color flow follows the physical principles of Doppler flow and thus if the ultrasound beam is perpendicular to the direction of flow, color Doppler information will not be displayed on the monitor see chapter 1 for details.
Newer ultrasound equipment tries to overcome this limitation by providing other means for display of blood flow such as Power Doppler which primarily relies on wave amplitude and B-flow not to be confused with B- Mode both of which are relatively angle independent.
In this display a cursor line with a gate appears in the upper half of the screen and a pulse or spectral Doppler display appears in the lower half of the screen Figure 2. The pulsed wave Doppler gate can be moved by the operator and placed within a vessel as imaged by color Doppler. Typically, this mode is activated when a vessel is first identified or suspected and after color flow Doppler is activated. The operator has the option to invert the display of the Doppler spectrum in order to display the waveforms above the line Figure 2.
See chapter 1 for more details. Note that the Doppler gate is placed within the umbilical artery as seen in the upper part of the image and the spectral Doppler waveform is displayed in the lower part of the image. The spectral Doppler is inverted to display the waveforms above the line. Doppler waveforms are shown in blue color. S stands for flow at peak systole and D stands for flow at end diastole. Note the Doppler indices in the right upper corner of the image yellow.
For more details, refer to Chapter 1. This function allows the operator to measure, in different formats, various objects on the screen. When the measure button is pressed, a caliper appears on the screen.
Use the trackball to move the caliper to the desired location and set it. Once set, a second caliper appears, which can be set in similar fashion. Stored normograms within the ultrasound equipment allow for determination of gestational age and estimation of fetal weight when various fetal biometric parameters are measured.
If you do not enter this information or any other patient identifier at the initiation of your examination patient name ; most ultrasound systems will not allow you to print or save an image from your examination. Chapter 15 details the parameters of an ultrasound report in obstetrics and gynecology.
It is important to know that image documentation is an essential component of the ultrasound examination and report. Images can be produced in paper format or stored digitally on the ultrasound equipment.
Several ultrasound systems have knobs for images, which can be formatted to allow for printing on a thermal printer and for saving a digital copy in a DICOM format on the equipment hard drive. The operator also has the option of downloading and saving a study on an external hard drive or a USB jump drive. This is an important function in the low-resource setting as it allows for exchange of cases for educational and consultative function.
Typically these knobs can be formatted for these functions, such as for thermal paper printer, for saving on the hard drive and for downloading to the USB outlet. A permanent copy of the ultrasound report, including ultrasound images, should be kept and stored in accordance with national regulations. The technical aspects of the ultrasound examination in obstetrics and gynecology are not standardized, and the operators develop their own ultrasound skills and approach to the ultrasound examination based upon their own experience and habits.
Understanding some basic principles and technical aspects of the ultrasound examination will undoubtedly improve the quality of the examination and reduce repetitive stress injuries. In this chapter, we present the technical aspects of the ultrasound examination with a focus on obstetrics.
The approach to the performance of the transvaginal pelvic ultrasound examination is discussed in details in Chapters 11 and 14 and a standardized approach to the performance of the basic obstetric ultrasound examination is presented in chapter Typically ultrasound tables have a retractable section beneath the legs with stirrups, which allow for the performance of a transvaginal ultrasound examination if needed Figure 3. Figure 3.
This provides support and minimizes repetitive stress injuries. Patients do not need to wear a special gown for the ultrasound examination but they should be provided with a towel paper or linen or a sheet to protect their clothes and for modesty Figure 3. In some low-resource settings, patients may bring their own towels to the ultrasound examination. The ultrasound gel is water-based and typically does not stain, but it does wet clothes, which is unpleasant. Asking the patient to have a full bladder is generally not required anymore with modern ultrasound equipment.
Gels are more convenient than oil, because the latter tends to stain and is more difficult to wipe off. But in low-resource countries where obtaining ultrasound gel is either expensive or impractical, regular cooking oil does an excellent job. When applying the gel, remember to use as little as possible, as scanning through a thick layer of gel tends to degrade the quality of the image by interposing numerous micro bubbles that are contained within the gel.
All brands of gel are equally suited for sound transmission, but if you make lengthy examinations, try to select one that does not dry too fast. Other products that can degrade the ultrasound image include creams that the patient may have applied on her abdomen before the ultrasound examination.
For instance, anti-stretchmark creams may contain chemicals that deteriorate sound transmission. Manufacturers market gel heaters for the purpose of easing patient discomfort, but an inexpensive baby bottle warmer will do as well. The standing position Figure 3. Although this position minimizes repetitive stress injuries, it is somewhat uncomfortable for long examinations.
The sitting position Figure 3. When performing an ultrasound examination, face the screen as perpendicularly as possible in order to avoid perception and distortion artifacts, especially with newer ultrasound monitors. For example, a biparietal diameter can be difficult to measure when you look at the screen obliquely. Work in dim light to help avoid reflections on the screen. To avoid repetitive stress injuries pay attention to the following factors: Posture Position your ultrasound equipment and the patient such that the posture you hold is comfortable.
Do not lean or bend over the patient and, avoid reaching over, especially in obese patients, during transabdominal Figure 3. Stand close to the patient and if not sitting, use the bed as a support to lean onto. If you are sitting, use a chair high enough, with a footrest. It is also important to place your non-scanning hand typically left hand on the freeze knob in order to freeze the image immediately when the desired target anatomy is viewed.
This should be avoided in order to minimize repetitive stress injury. Diming the ambient light is also important to allow you to optimize the ultrasound gain. The Monitor Position the monitor of the ultrasound equipment such that its display is at eye-level and perpendicular to your line of sight.
Newer ultrasound equipment have flat panels for monitors, which are commonly on adjustable arms. It is usually simple to add a second monitor for the patient to view the examination.
This will also avoid having the patient twist on the ultrasound table to look at the monitor on the ultrasound machine, as this may tense abdominal musculature and impair the examination. The second monitor may be connected either from a video port or a digital port.
In general, the curvilinear transducers are best adapted to obstetric scanning as they conform to the abdominal curvature in pregnancy Figure 3. Larger transducers are harder to manipulate then smaller ones, but when they provide special functions such as 3D, our experience has been that users will tolerate the added bulk. It is important that the transducer rests fills the palm of the hand and the fingers hug the body of the transducer with minimal pressure Figure 3. In this position, the thumb and fingers allow for the greatest precision of movements, such as sliding, rotating or angling, with minimal tension on the wrist.
Note that the transducer is held very close to its footprint. Holding the transducer with the thumb and fingers at mid-body Figure 3. Finally, holding the transducer near its cable end Figure 3. The transducer is held in the palm of the hand with minimal pressure on the wrist and phalangeal joints Figure 3. Besides facilitating interpretation of your ultrasound images by others, there are advantages to sticking to these simple rules: The position of the fetus and placenta can be evaluated with a quick glance at the ultrasound images and spatial orientation is greatly facilitated.
The transducer cable should be supported in order to provide minimal pull eliminate drag during scanning. On many occasions, the cable can be supported in the transducer holder during scanning Figure 3. Check that the cable is not too rigid, which may interfere with the ease of transducer manipulation. Applying abdominal pressure with the transducer will not improve image quality and is uncomfortable for the patient and for the operator.
Furthermore, transducer pressure on the abdomen may result in fetal bradycardia in some instances. The only instance where "digging" the transducer is justified is in late pregnancy, when the fetal head is low in the pelvis and evaluation of head anatomy and biometry is difficult.
We have selected here some scanning techniques that the authors use on a daily basis in their busy practice. Transducers have various footprint sizes and Megahertz MHz ranges. Some are adapted for the first trimester and others for the third trimester when depth is critical. For more details on properties of transducers, see chapter 2.
Furthermore, ultrasound machines have manufacturer-established presets that optimize resolution and frame rate for various types of study. It is important that you familiarize yourself with the presets and choose the right preset for the right study.
Furthermore, applying minimal pressure will allow for a film of amniotic fluid between the anterior uterine wall and the target organ, which enhances visualization Figures 3. In A, increased pressure is applied on maternal abdomen resulting in compression of the fetal abdomen arrows.
Minimal optimal pressure is applied in B resulting in improved imaging with a film of amniotic fluid between the uterine wall and the fetal abdomen broken arrows. Furthermore, minimal pressure results in no deformity of the abdominal perimeter, which improves abdominal circumference measurement in B. This will enhance resolution and frame rate. An image with greater depth requires more processing from the ultrasound equipment, which results in slower frame rate and reduced resolution.
Note how small the fetal heart is in A, as the depth of the image is not adjusted arrow. The depth is minimized in B same fetus resulting in image magnification. Minimizing depth also improves frame rate not shown. Minimize the sector width Most ultrasound machines have the ability to adjust the sector width on the ultrasound screen.
It is important to start your examination with a wide sector width Figure 3. This shows a wide sector width arrow , which is the initial approach to image optimization. Once the target organ is under view, reduce the sector width See Figure 3. Adequate sector width is applied arrow. This maneuver optimizes imaging and increases frame rate. Using multiple focal zones tend to reduce frame rate and thus should be avoided in obstetrical scanning.
In A, the focal zone is erroneously applied below the target organ circle. Note the improved lateral resolution arrows — compare A to B of the target organ abdomen in B where the focal zone is accurately applied circle. This can be achieved by zooming the whole image, or selecting an area of interest from the image to magnify. It is important to learn to scan with this feature, which allows for the identification of details within target organs.
This is especially critical when you are scanning the fetal heart given its complex anatomy and small size Figure 3. Note that for many machines there are 2 forms of zoom. Familiarize yourself with both options if available on your ultrasound equipment. The detailed anatomic features of the heart can be easily recognized in B when compared to A. Image magnification and zoom are important features in cardiac imaging. Maintain the target anatomic area in the center of the screen It is important to keep the area of interest in the center of your screen in order to minimize the effect of lateral resolution as the ultrasound resolution decreases significantly from the central area of the image towards each lateral side.
Furthermore, this technique allows for the ultrasound beam to insonate the target area in a perpendicular orientation, which enhances visualization Figure 3. The slide technique involves sliding the transducer along its long axis as shown in Clip 3.
This brings target anatomy from the lateral to the center of the screen while maintaining the same anatomic view and orientation of the target image. Clip 3. In A, the femur is in the center of the image allowing for optimal imaging of its borders and thus measurements. In B, the distal portion of the femur is in the lateral aspect of the image resulting in reduced resolution broken arrow.
The solid arrow in A and B shows the direction of the ultrasound beam. Obese women are at increased risk for complications during pregnancy, including gestational diabetes, hypertension, and cesarean delivery 3. In addition to maternal complications, obesity also poses risks to the fetus including an increased risk of prematurity, stillbirth, macrosomia, and a higher rate of congenital anomalies 4.
Although sonographic screening during pregnancy is recommended for all women, it is particularly relevant in the obese population due to higher rates of structural abnormalities, specifically neural tube defects, heart defects, and abdominal wall defects 5. Sonographic assessment of fetal anatomy in the obese population is challenging with multiple studies confirming that maternal obesity significantly reduces the likelihood of completion of the anatomic survey, and ultrasound screening is associated with lower detection rates of fetal anomalies A recent fetal imaging consensus meeting in the United States, sponsored by multiple societies including the Eunice Kennedy Shriver National Institute of Child Health and Development NICHD , made specific recommendations for the pregnant obese population, including a targeted ultrasound at weeks gestation approximately 2 weeks later than the usual time period for anatomy survey in the non-obese patients , and a follow up ultrasound exam in 2 to 4 weeks if fetal anatomy could not be completely assessed The following is a list of techniques commonly used in ultrasound examinations of obese pregnant women: Figure 3.
Note the sub-optimal resolution of the right lateral fetal abdomen small arrows. Further studies however are needed to confirm the feasibility of this approach in the obese population. Scanning underneath the panniculus The operator can lift the panniculus with the left hand and scan underneath it with the right hand. This maneuver is tiring however and should not be employed for long scanning time. In this figure, the patient is holding the panniculus up arrow during the examination. Scanning above the panniculus The ultrasound examination can be performed above the panniculus in the region of the mid- abdomen while pushing the panniculus down, which may shorten the distance between the skin surface and the fetus Figure 3.
This maneuver may be improved by filling the patient's bladder, which displaces the uterus cephalad. In this figure, an assistant is pushing the panniculus down arrow during the examination. Alternatively, the transvaginal probe can be used through the umbilicus given its small footprint Figure 3.
This may allow you to see fetal anatomy more clearly in some obese patients. This technique may improve imaging in some obese patients. Placing the patient in the Sims position The Sims position involves a position in which the patient lies on the left side with the knee and thighs drawn upward toward the chest.
The chest and abdomen are allowed to fall forward. This approach for scanning allows the panniculus to be displaced to the left side. Insonating the uterus from the right lateral quadrant may improve imaging given the presence of less adipose tissue. JAMA ; Maternal morbid obesity and the risk of adverse pregnancy outcome.
Obstet Gynecol ; Maternal overweight and obesity and the risk of congenital anomalies: a systematic review and meta-analysis. Maternal obesity and risk for birth defects. Pediatrics ; Effect of maternal obesity on the ultrasound detection of anomalous fetuses.
American College of Obstetricians and Gynecologists ; Maternal obesity limits the ultrasound evaluation of fetal anatomy ; Factors affecting feasibility and quality of second-trimester ultrasound scans in obese pregnant women. Ultrasound Obstetric Gynecology ; Effect of maternal obesity on the ultrasound detection of anomalous fetuses, Obstetric Gynecology ; J Ultrasound Med ; — Sonography in obese and overweight pregnant women: clinical, medicolegal and technical issues. Accuracy of ultrasonography at weeks of gestation for detection of fetal structural anomalies: a systematic review.
Obstet Gynecol. Assessment of fetal anatomy at the week ultrasound examination.
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