With the use of ultrasound, marking surface landmarks has become less important. However, understanding relevant anatomy remains key to interpreting the sonographic anatomy which is crucial to procedural success.

 

Surface Landmarks

First, palpate the pubic tubercle (PT) in the midline and the anterior superior iliac spine (ASIS) which is found laterally over the hip and anterior to the iliac crest. A straight line drawn between the PT and the ASIS represents the inguinal ligament (IL) which serves as the upper border to the area of interest: the femoral triangle. Immediately inferior to the inguinal ligament is the fold of the inguinal crease (IC) where the femoral artery (FA) can be palpated. Medial to the FA lies the femoral vein (FV), and immediately lateral to the FA lies the femoral nerve (FN) (figure 1).

 

Figure 1: Surface Landmarks

PT = Pubic tubercle, ASIS = anterior superior iliac spine., IC = Iliac crest, FA = Femoral artery, FV = Femoral vein, FN = Femoral nerve, IC = inguinal crease

 

The Femoral Triangle

The femoral triangle holds the relevant structures needed to perform a successful femoral nerve block (figure 2). The superior boundary of the femoral triangle is the inguinal ligament, the lateral border is the sartorius muscle; and the medial border is the adductor longus muscle. The floor of the femoral triangle is made up of following muscles: Iliacus, psoas major, pectineus and adductor longus (lateral to medial). The key structures of interest which will need to be identified sonographically (from lateral to medial) include the femoral nerve, femoral artery, and femoral vein.

 

Figure 2: The femoral triangle

 

The Lumbar Plexus & Femoral Nerve

Four major nerves supply the lower extremity; the sciatic, the femoral, the obturator, and the lateral femoral cutaneous nerves. From the regional anesthesia perspective, this is important, in that an isolated femoral nerve block will not provide complete anesthesia for the lower leg. The femoral nerve supplies sensory innervation to the anterior thigh, anteromedial knee, medial lower leg and the medial aspect of the foot and ankle. Importantly, it also innervates the periosteum of the femur. In addition, it supplies motor innervation to the knee extensor muscles, including the quadriceps and sartorius (table 2).

 

Table 2: Distribution of effect of femoral nerve block

After its origin at the lumbar plexus the femoral nerve descends through the posterior third of the psoas major and iliacus muscle. It enters the thigh within the femoral triangle below the inguinal ligament and superior to the iliopsoas muscle (figure 3). Within a few centimeters of the inguinal ligament, the femoral nerve divides into anterior and posterior branches. It is important to place your blockade around the nerve prior to when it diverges, otherwise your block may be incomplete. If you are too distal, you may miss the posterior branch of the femoral nerve which provides motor innervation to the quadriceps muscles–an important consideration in patients with acute femur fractures who are experiencing quadriceps spasm (6).

 

Figure 3: The lumbar plexus

Goal

To provide safe and effective relief of acute pain for the following conditions:

• Femur (neck & shaft) fractures
• Patella injuries/fracture
• Anterior thigh wound care

 

Contraindications

• Patient refusal
• Allergy to local anaesthetic
• Infection over site of injection
• Coagulopathy (relative contraindication, use clinical judgement)

 

Complications

• Intravascular injection causing local anesthetic systemic toxicity (LAST)
• Intra-neuronal injection (can cause temporary or permanent nerve damage)
• Block failure
• Infection

 

The use of landmark-guided femoral nerve blocks for acute pain relief in pediatric patients with femur fractures was first described in 1979 (1). Regional blocks are used widely by anesthetists to manage intra and post-operative pain. However there has been variable uptake into other pediatric specialties, including pediatric emergency medicine.

 

Why Ultrasound?

Traditionally, femoral nerve blocks were performed using anatomic landmarks. However, in the late 1980’s ultrasound began to be used for procedural guidance and by the 2000’s, it’s use was commonplace (2, 3). Today, ultrasound-guidance for femoral nerve blocks is routine and provides several benefits over the traditional approach (table 1). In fact, many now consider it to be standard of care.

 

Table 1: Advantages of ultrasound-guided nerve blocks (4)

 

A retrospective pre- vs. post-implementation cohort study evaluating emergency physician performed femoral nerve blocks for pediatric patients presenting with acute femur fractures found that patients who received ultrasound-guided femoral nerve blocks for femur fracture pain, had longer duration of analgesia, required fewer doses of analgesic medications, and needed fewer nursing interventions than those patients receiving enteral or parenteral analgesic alone (5). When done appropriately, ultrasound-guided femoral nerve blocks are safe, effective and provide optimal pain relief for acute injuries.

Conclusion

Congratulations on taking the first step towards adopting PoCUS as a part of your practice! The key concepts in this chapter can be revisited regularly to help you understand how to generate and interpret different scans. Orienting yourself to a 2D representation of a 3D object will take some time, so take any opportunity you have to reach for an US probe to hone your skills. Image generation is the most difficult skill to obtain with respect to PoCUS but with a systematic approach you will be able to reliably create high-quality scans that can enhance your clinical decision-making.

Documentation

Our documentation recommendations are consistent with those from the Canadian Point of Care Ultrasound Society (CPoCUS). We share the belief that it is important for all physicians completing our modules to use consistent, unambiguous, and easily interpreted language when describing point of care ultrasound studies. It is important to chart using binary wording, and to limit the possible interpretations to those within our scope of practice. With this in mind, we endorse the following documentation recommendations from the Canadian Point of Care Ultrasound Society:

PoCUS for (insert indications here): negative/positive/indeterminate study: No/+ (insert pathology name here)

As an example, in a case of PoCUS for pneumothorax, in which a pneumothorax was found on the patient’s left side one would document the following in the chart:

PoCUS for PTX: positive: + L PTX

In addition to documenting your findings in the chart we strongly encourage users to record their images for QA review and to get ongoing feedback on your scans.

Tips for scanning children

Children, particularly young children can often be fearful of strangers and any medical examination can result in stress, fear and oppositional behavior. The use of a large, unfamiliar machine can add a level of fear and intimidation. There are several things one can do to maximize patient comfort and success of the scan in this challenging population.

Gain the trust of the child by respecting their fear and approaching slowly by first explaining to them what you are going to do in the simplest terms possible. In addition, giving them limited choices about how the scan will proceed (i.e. “Do you want to sit with Mom or Dad when I check your tummy?”) can help them feel more in control.

Normalize the US equipment by having the child handle the probe or having a caregiver hold the probe. Alternatively start by placing the probe on a non-invasive area such as their knee or hand to reassure them that the probe is harmless and engage them with the scan by watching the images on the screen. US gel can feel quite unpleasant when cold so warming the gel prior to use can mitigate this sensation. Having the child play with the gel before administration can prevent them from squirming when you apply it to their body for the scan. The low-frequency probe emits sounds that most children can hear. You can also let them listen to the probe and briefly explain how the machine works.

Use the parents to model the activity so the child is less afraid. You can also use the parents to help hold, comfort or even distract the anxious child.

Anticipate movement of the child either withdrawing from the sensation of the gel or probe or turning to face the screen. To minimize the effects of the child’s movement on the image, one should be sure to anchor one’s hand to the patient, so the probe remains in contact with the patient in the area of interest.

Scanning Modes

B-mode or “brightness mode” is used for the majority of PoCUS scans and was described well throughout this text. It produces a two-dimensional cross-sectional slice of the area where the probe is directed. This modality will be used for the majority of your scans.

M-mode– looks at motion over time in a slice of the ultrasound image on the screen. This is plotted as a waveform with time on the x axis and depth on the y axis. M-mode can be helpful in certain instances when looking at moving structures and will be addressed in individual modules when relevant.

Doppler mode relies on the Doppler shift principle of physics:  waves emitted from a source moving towards the transducer will be higher in frequency compared those emitted from a source moving away from the transducer. This frequency shift can be represented by color (red towards, blue away), or by audible/graphical peaks with spectral Doppler. It is important to keep in mind that the color Doppler modes do not discriminate between arteries and veins but rather the direction of the sound waves source. Therefore, you can have situations where arteries are displayed as blue and veins as red. Doppler will be addressed further in individual modules when relevant.

Understanding images

One of the tougher concepts to grasp is the orientation of the image on the screen in relation to the patient. Since the US probe can be moved and rotated to image patients in varying planes it is important to understand the relation of the image on the screen to the anatomy of the patient. In all cases the image on the top of the screen, or near-field relates to the superficial part of the patient that is touching the probe and the bottom of the screen, or far-field is deepest into the patient. Screen left corresponds to the direction the probe indicator is pointed on the patient, with the right screen being the opposite (Figure 13&14).

Figure 13: Transverse view orientation

 Figure 14: Longitudinal view orientation

It is critical to have a proper reference to know how to interpret what we are seeing. One way to think of it is to imagine the probe is a flashlight looking into the body with the closest structures to the probe at the top of the screen and farthest at the bottom. You aim the flashlight towards the area of interest by manipulating your hand movements. Remind yourself where you are starting by mentally noting the corresponding structures to near-field, far-field, screen right and screen left. This cueing mechanism will become very useful once you start manipulating the probe to optimize images and scan through structures.

Optimizing images

The typical ultrasound machine found in North American hospitals will contain a bevy of buttons. These can be quite intimidating. Fortunately, for the majority of our indications we can focus on the few critical controls and ignore the rest. The most important controls to optimize the image are gain and depth.

Gain controls how bright the image appears on the screen. Adjusting the gain essentially increases or decreases the sensitivity of the machine to ALL ultrasound waves sensed by the transducer. It acts as a “volume control” for the image brightness, turning up the gain makes everything appear brighter on the screen while turning it down makes everything darker.

Figure 15: Gain

Depth controls how deep the ultrasound waves returning to the probe are detected. Increasing depth will allow you to see deeper into the patient, while decreasing help you focus more superficially (Figure 16). There is a numeric scale on the right side of the screen to help orient you to the depth of the image you are viewing. Often it is best to begin imaging an area of interest using the maximum depth and then gradually reduce the depth to optimize the visualization of the area of interest. This helps to landmark surrounding structures and avoid missing anything due to a superficial scan.

Figure 16: Depth

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.