Probe selection

The first step in acquiring an ultrasound image is selecting the appropriate probe. As previously mentioned, there are different ultrasound probes that vary both in shape and in the frequency of waves transmitted to tissues. When selecting a transducer keep the size of the patient and location of the area of interest in mind (figure 9).

Figure 9: Probe selection

Probe orientation:

After selecting which probe to use the next step is to place the probe correctly on the patient to acquire and interpret images. Every ultrasound probe has an indicator that helps relate the position of the probe on the patient with the image on the screen (Figure 10). By convention the indicator is oriented to either the patient’s right or towards the patient’s head. There is a dot on the ultrasound screen which corresponds to the indicator on the probe. In PoCUS we generally leave this in the top left of the screen.

Figure 10: Probe marker and screen indicator

The exception to this convention is procedural ultrasound or scans that require approaching from the patient’s posterior. When performing these scans, keep the probe marker to your left. This will make both image generation and procedural movements most intuitive.

Hand Position and Movements

When performing an ultrasound scan hold the distal end of the probe and ensure you anchor your hand on the patient as you scan. To do this use the base of your hand, either the hypothenar eminence or the fourth or fifth digits to steady your hand as well as the probe on the patient (Figure 11).

Figure 11: Hand position

Bracing the probe like this will keep your probe from slipping on slick gel and will keep the image steady on a patient who is moving. Certain scans will require specific hand positioning that may make it impossible to hold the probe in this manner. We will discuss these specifically in the relevant modules. Proper hand positioning will allow you to make the fine motor movements required to get the best view of the area you are trying to visualize.

Probe movement requires subtle maneuvers to allow practitioners to make slight adjustments to their image. Large or rapid hand movements will result in scanning through multiple planes quickly, making it difficult to maintain orientation and lead to missing more subtle findings. While moving the probe to adjust and optimize the area of interest we speak of certain cardinal probe movements (Figure 12).

Figure 12: Probe movements

Slide Motion in the long axis of the probe across the body while maintaining contact of the probe to the patient at 90°

Sweep Motion in the short axis of the probe across the body while maintaining contact of the probe to the patient at 90°

Heel Motion in the long axis of the probe along a fixed point on the body while changing the angle of the probe to the patient from 90° to 180/0° in both directions. The body of the probe may make contact with the patient and remains there to generate the image

Fan Motion in the short axis of the probe along a fixed point on the body while changing the angle of the probe to the patient from 90° to 180/ 0° in both directions

Rotate Movement around the compression axis in a clockwise or counterclockwise direction. In other words, changing the probe orientation from transverse to longitudinal and vice-versa.

Translating sound into light

Ultrasound images are formed by detecting the amplitude of waves that are reflected back to the transducer as the transmitted waves travel through different tissues. Distances are judged by the time it takes for the waves to return to the probe.

Sound waves pass freely through fluid and very few are reflected back to the transducer—thus fluid appears black on the screen.  As waves travel through tissues that offer some resistance such as solid organs, the energy of these waves is reduced or attenuated. A small amount of this energy is lost to heat, while the rest is reflected back to the transducer and displayed on the image as brightness. The denser a tissue the more waves are reflected back to the probe and the brighter the image appears on the screen.

Overall, the brightness of images on the screen corresponds to the intensity of the waves that are reflected back to the probe as they encounter different tissues. When describing ultrasound images and the varying shades of black, grey and white we use the terms anechoic, hypoechoic, isoechoic and hyperechoic (figure 2)

Figure 2: Describing ultrasound images

Anechoic: when no or minimal waves are reflected back to the probe, the area appears as black

Hypoechoic: when there is less intense return of waves compared to other structures, thus appearing darker than surrounding tissue

Isoechoic: when there is a similar intensity of returning waves compared to surrounding structures, thus appearing a similar shade to surrounding tissue

Hyperechoic: when there is a more intense reflection of ultrasound waves compared to surrounding structures, appearing as brighter than surrounding tissue

Generally speaking fluid transmits US waves freely and therefore appears as black or anechoic areas. Tissues with a high-water content or low density such as muscle or fat appear as darker in comparison to solid organs such as the liver. Finally, dense structures such as bone completely reflect ultrasound waves and appear as very bright, hyperechoic structures compared to surrounding tissues.

In addition to varying degrees of reflection and brightness there are several visual artifacts that result from the interaction between waves and various tissues.  These are important to understand as they can help identify key structures as well as avoid mistaking them for pathological areas. Common artefacts include:

Shadowing occurs deep to hyperechoic structures that strongly reflect ultrasound waves such as bones. The image that results is a brightly hyperechoic structure with an anechoic/hypoechoic tail in the far-field as ultrasound waves are blocked from the deep structures (Figure 3). Acoustic shadowing can be used to help identify the ribs and pleural line in lung imaging or looking for calcified structures such as stones.

Figure 3: Shadowing artifact

Reverberation occurs when ultrasound waves reflect between two surfaces in parallel resulting in recurrent, regularly spaced bright arcs on the screen (Figure 4). This is commonly seen in normal lung as waves bounce back and forth between the reflective pleural line and the ultrasound probe, these regularly spaced repeated reflections are known as A-lines.

Figure 4: Reverberation artifact

Refraction occurs when the US waves hit the interface of two tissues that transmit the waves at different speeds causing the waves to change direction at an oblique angle. The ultrasound waves are deflected at this interface and, therefore, do not return to the probe causing a dark tail beyond the interface. This is commonly seen at the edge of the bladder (Figure 5).

Figure 5: Refraction artifact

Mirroring is an artifact caused by a curved reflector. Outgoing ultrasound waves bounce off the reflector and encounter nearby tissue which in turn reflect these waves back to the curved reflector and ultimately back to the probe. As incoming waves reach the ultrasound probe, the probe cannot discern the path each wave took to reach it, only the time. Waves that take longer to reach the probe are thought to have come from a farther distance. This results in a mirror image of the structure superficial to the structure appearing deep to the reflector. Mirroring artifact is commonly seen at the level of the diaphragm when the liver or spleen’s mirror image is projected in the far-screen above the diagram (Figure 6).

Figure 6: Mirror image artifact

Enhancement is essentially the opposite of shadowing. When ultrasound waves travel through a fluid medium the tissue encountered in the far-field appears brighter than the surrounding tissues. This is because fluids reflect back very little energy. As such the far field tissues are being hit with higher energy waves than the neighboring tissues that are encountering waves which have already lost some degree of energy moving through non-fluid tissues (Figure 7). This enhancement can easily be appreciated when imaging the bladder.

Figure 7: Posterior enhancement artifact

Scatter is encountered when ultrasound waves encounter an air-filled structure. The deflection of the waves in various directions results in a diffuse grey “hazy” appearance (figure 8). This is seen in normal aerated lung, in gas filled bowel, and when air pathologically enters tissues.

Figure 8: Scatter artifact

Now that we have covered a basic understanding of how ultrasound images are formed we will focus on the basics how they are acquired and interpreted.

Introduction

Point-of-care ultrasound (PoCUS) is increasingly being used in the care of patients as it can lead to narrowed diagnostic possibilities in acutely unwell patients, increase safety of invasive procedures, improve efficiency of care and limit exposure to ionizing radiation. Improved access to bedside ultrasound has led to a demand for training as doctors are now reaching for an ultrasound probe to enhance their bedside assessments of patients.

The first step in learning to use PoCUS safely and effectively at the bedside involves a basic understanding of (1) how an ultrasound machine works, (2) how to operate it and (3) how to understand and interpret the images on the screen.

Ultrasound waves

In order to successfully operate the machine and interpret images one must have a basic understanding of the physics of sound. This is because an ultrasound machine uses sound waves to produce images. In essence, an ultrasound probe transmits sound waves into tissues. These waves are then reflected back to the ultrasound probe in varying amounts and intensity depending on tissue characteristics. In turn, the machine translates this signal into an image on the screen which can then be interpreted at bedside.

Understanding the properties of ultrasound waves and how they interact with tissues helps us acquire the best images as well as understand the images on the screen. The basic properties of sound waves are frequency, wavelength, and amplitude (Figure 1).

Figure 1: Anatomy of ultrasound waves

 

Wavelength is the distance between successive peaks or valleys of a waveform.

Frequency is the number of times a wave repeats or cycles in a one second period.

Amplitude refers to the height or intensity of a waveform.

Frequency and wavelength are important for image quality. High frequency ultrasound waves oscillate more quickly in tissues, resulting in a shorter wavelength and better resolution or clearer images. But as these short waves enter tissue their energy is quickly dissipated. As such, they do not penetrate well into deeper structures. On the contrary a low frequency ultrasound wave oscillates less vigorously with longer waves. This results in poorer resolution or clarity of structures, but the long waves have a better ability to penetrate into tissues and visualize deeper structures.

This is important because certain ultrasound (US) transducers emit high-frequency ultrasound waves and therefore are best at looking at superficial structures in fine detail whereas other low-frequency transducers can penetrate more deeply into tissue. This is at the cost of a coarser image.

Simply put frequency and penetration are inversely related while frequency and resolution are directly related:

Frequency α 1/Penetration

Frequency α Resolution

Summary

• PoCUS is both sensitive and specific for the diagnosis of pneumothorax.
• Use the linear probe to scan the anterior chest in a supine patient.
• Absence of shimmering, comet tails and lung pulse indicate pneumothorax.
• Don’t forget to document your findings in the chart and save images fro QA.

References

1. Alrajhi K, Woo MY, Vaillancourt C. Test characteristics of ultrasonography for the detection of pneumothorax: a systematic review and meta-analysis. Chest 2012;141:703-8. doi:10.1378/chest.11-0131.

2. Seow et al. Comparison of Upright Inspiratory and Expieratory Chest Radiographs for Detecting Pneumothoraces. AJR 1996; 166:313-316. doi: 10.2214/ajr.166.2.8553937.

3. Murphy et al. CT and Chest Radiography are Equally Sensitive in the Detection of Pneumothorax After CTGuided Pulmonary Interventional Procedures. AJR 1990;154:45-46. doi: 10.2214/ajr.154.1.2104723.

4. Raimondi et al. Lung Ultrasound for Diagnosing Pneumothorax in the Critically Ill Neonate. J Pediatr 2016;175:74-8. doi:10.1016/j.jpeds.201.04.018

5. Cattarossi et al. Lung Ultrasound Diagnostic Accuracy in Neonatal Pneumothorax. Canad Resp J. 2016. doi 10.1155/2016/6515069

6. Liu et al. Lung ultrasonography to diagnose pneumothorax of the newborn. AJEM 2017;35:1298-1302. doi: 10.1016/j.ajem.2017.04.001.

7. Volpicelli et al. Semi-quantification of pneumothorax volume by lung ultrasound. Int Care Med 2016;40:14607. doi:10.1007/s00134-014-3402-9.

Quantifying Pneumothorax

In the unstable trauma patient, the absence of lung sliding is indication enough for chest tube placement due to pneumothorax. In the stable trauma or medical patient with absent lung sliding identification of the lung point confirms the presence of the pneumothorax and can help estimate its size. The lung point is the abrupt change from normal lung sliding to absence of lung sliding. It represents the edge of the pneumothorax (figure 6, video 6).

The lung point can be identified by rotating the probe to be in line with a rib space and following the pleural line laterally until the junction is identified.

A more posterolateral lung point corresponds to a larger pneumothorax. A lung point posterior to the mid-axillary line has an 82% sensitivity and 83% specificity for greater than 15% lung collapse on CT which is generally considered the recommended size for chest tube placement or drainage in a stable patient but again clinical judgement is warranted [7].

Note: The lung point may move subtly with respiration.

Figure 6: Lung point – CT illustration

Video 6: Lung point

Pitfalls – Cardiac Lung Point and Diaphragmatic Lung Point

There are two important mimics of the lung point, which should be recognized. The visceral and parietal pleura separate around the heart and at the edge of the diaphragm. These create a normal lung point at the left chest and inferiorly at the diaphragm (video 7&8). These are normal findings.

Care must be taken to not confuse the normal pleura at the heart or diaphragms as a lung point. In both cases, be sure to recognize the anatomy that the normal lung interfaces with. If normal lung touches pulsatile tissue in the left chest, it is likely the heart. If solid organ is visible consider that the diaphragmatic lung point and not a pathologic lung point is being identified.

Video 7: Cardiac lung point

Video 8: Diaphragmatic lung point

 

Pitfalls:

In the absence of lung sliding one must still consider the clinical context as other conditions can cause decreased or absent sliding, including:

• Large blebs
• Pleural adhesions
• Apnea
• Right main stem intubation (decreased movement on left chest)
• Dense consolidation: atelectasis, pneumonia, contusion
• Pleural effusion

In the case of apnea, main stem intubation and atelectasis often cause an artefact called the lung pulse that can be seen, differentiating these conditions from true pneumothoraces. A lung pulse is the slight motion of the pleura caused by the movement of the heart transmitted to the pleura and can only appear when the parietal and visceral pleura are opposed (video 4).

Video 4: Lung Pulse

Another cause of confusion is subcutaneous emphysema. Subcutaneous emphysema is generally associated with pneumothorax but the air in the subcutaneous tissues scatters the ultrasound beams creating a grey haze similar to lung. In this case it is often difficult to visualize the ribs or pleural line but pneumothorax can be assumed in the right clinical context (video 5).

Video 5: Subcutaneous emphysema

Rarely absent lung slide can be caused by pathology other than pneumothorax. Looking deep to the pleural line can also help distinguish absent lung sliding due to pneumothorax from absent slide due to other pathologies. On ultrasound, air appears as homogenous grey and regularly-spaced, echogenic, horizontal A-lines appear deep to the pleural line (figure 5). This pattern would be seen deep to a motionless pleural line in pneumothorax, large blebs and adhesions so the clinical context must be considered.

Figure 5: A-lines

Pleural effusions can be differentiated as a cause of absent lung sliding by anechoic fluid deep to the relatively hyperechoic pleural line. In dense consolidation caused by pneumonia, contusions and atelectasis not only is a lung pulse likely present but the lung deep to the pleural line will have absent a-lines and take on the appearance of a hypoechoic or organ-like structure.

What is NOT normal?

When air enters the pleural space separating the parietal and visceral pleura the air cannot be seen directly. The absence of shimmering and comet tails indicates the two pleural layers are not in contact as is the case with pneumothorax (video 3).

Video 3: Absent pleural motion: pneumothorax

If no shimmering or comet tails are visualized, pneumothorax is likely but the clinical context should be considered. Some conditions can mimic pneumothorax with absent or minimal pleural motion.

A pneumothorax at the level of the probe is indicated by the ABSENCE OF BOTH:

• Lung sliding
• Comet tail artifacts

Confirm with M-mode

When pleural motion is minimal it can be difficult to appreciate. If there is uncertainty M-mode can be used to help identify the presence or absence of a pneumothorax. M-mode, or motion mode, plots structures visualized on a thin line of the ultrasound image across time on the x-axis and depth on the y-axis. To use M-mode, press the “M” button on the machine and make sure the vertical marker is over a section where the pleural line is not shadowed by a rib. Identify the pleura in the M-mode image identify the brightest white line on the, M-mode view at the corresponding depth.

In normal lung, the motionless chest wall appears as horizontal lines and the and movement of pleural and lung beneath the pleura creates a grainy, coarse pattern (figure 3). When obtaining an M-mode image it is imperative to hold the probe very still on the chest wall. The movement of an agitated or very dyspneic patient can make the image difficult to interpret.

Figure 3: Seashore sign (normal)

In pneumothorax motion is absent in the chest wall, pleural line and below. This lack of motion creates smooth, horizontal lines throughout when viewed in M-mode (figure 4).

Figure 4: Barcode sign (pneumothorax)

What is normal?

Our thorax is lined by a continuous serous membrane called the pleura. The pleura is further divided into the visceral pleura which is attached to the lung and the parieta pleural which is attached to the chest wall. The pleural space is a potential space between the visceral and parietal pleura. It is normally filled with small amounts of physiologic fluid. On ultrasound, this interface is seen as a hyperechoic line running deep to the ribs. As the patient breathes the fluid moves between the pleura and is called lung sliding or “shimmering” (video 2).

Video 2: Normal Lung Sliding

In addition to shimmering, normal lung sliding leads to a reverberation artefact called a comet tail. Comet tails are formed by ultrasound pulses bouncing between the reflective surfaces of the visceral and parietal pleura and appear as vertical echogenic lines originating from the pleura which move with respiration (figure 2). Because the surfaces are not parallel many of the bouncing beams are scattered so the artefact fades as it gets further from the pleural line.

Figure 2: Comet tail artifact

A normal pleural interface at the level of the probe is indicated by the PRESENCE OF EITHER:

• Lung sliding
• Comet tail artifacts

What am I looking at?

In order to recognize a pneumothorax, it is first important to note the following normal anatomic structures and their sonographic appearance (video 1).

Chest wall:

• Most superficial structure
• Hypoechoic with irregular fascial lines

Ribs:

• Oval, hyperechoic periosteum
• Dark shadow behind

Pleural line:

• Hyperechoic horizontal line
• Runs between and deep to the ribs

Lung:

• Deep to the pleural line
• Uniform grey haze

Video 1: Normal anatomy