Point-of-care ultrasonography

Sound wave physics

Sound is vibration that travels as a wave of alternating compression and rarefaction through a medium ( ) ( Fig. 20.e1 ). Sound waves are defined by their frequency, or the number of oscillations of the wave that pass a point per second. One hertz is defined as one oscillation per second. The frequencies of audible sound are 20 hertz to 20 kilohertz. Ultrasound is any sound frequency above the audible range. Diagnostic ultrasound, as discussed in this chapter, is 1 to 20 megahertz (MHz) ( ).

Diagnostic ultrasound waves are created through the piezoelectric effect. Ultrasound transducers, also called probes, are composed of piezoelectric crystals that convert electrical energy into mechanical energy and vice versa. When electricity is applied to the transducer, the piezoelectric crystals vibrate, which passes the vibration into the adjacent medium, creating the ultrasound wave. The ultrasound waves reflect, or bounce back, after coming into contact with a medium. The ultrasound transducer turns the reflected waves, or echoes, back into electrical energy. The ultrasound machine uses the electrical impulses from the echoes and their time delay to create ultrasound images.

Ultrasound transducers

The ultrasound transducers that will be discussed in this chapter include both high-frequency linear transducers and low-frequency phased array transducers. The words linear and phased refer to the distribution of the crystals within the transducer footprint and, therefore, the direction in which they emit ultrasound waves ( Fig. 20.e2 ). The precise frequency emitted by a transducer can be controlled within a set range specific to that particular transducer. Frequency and wavelength are inversely related ( Fig. 20.e3 ).

High-frequency ultrasound waves have short wavelengths. They are quickly reflected in the body and therefore produce high-resolution, low-depth images. These are optimal for superficial structure visualization and for procedural guidance. Their images are rectangular in shape because of the linear array of crystals ( Fig. 20.e4 ). Low-frequency ultrasound waves can reach deeper into the body and have more depth potential but at a cost of decreased resolution. Low-frequency phased array transducers create a sector or pie-shaped image that allows deeper and wider visualization from a small footprint ( Fig. 20.e5 ).

Ultrasound modes

The primary mode of ultrasound used in this chapter is brightness mode (B-mode), also known as two-dimensional (2D) ultrasound. We will also touch on motion-mode (M-mode) ultrasound for a few applications.

In 2D ultrasound, the tissues and objects can be distinguished based on their echogenicity or the degree to which the ultrasound waves are reflected back. Tissues and objects that are hyperechoic appear bright white, because they reflect most of the ultrasound waves—examples include bone, gas, and metal (needles). Liquids are anechoic and appear black, because the ultrasound waves travel through them without being reflected back to the transducer—examples include blood, effusions, and injected local anesthetic. Isoechoic means the same echogenicity, and such objects appear as a medium gray on the screen. Liver and normal soft tissue are typically considered isoechoic. Hypoechoic, or less echoic, refers to tissues that are darker gray than those considered isoechoic, or reflect less ultrasound waves than isoechoic tissues ( Fig. 20.1 ). Motion-mode ultrasound takes the gray scale in one vertical slice of the ultrasound screen and plots it over time.

Ultrasound controls

When beginning any ultrasound examination, the depth and gain are optimized. The gain controls the overall amplitude/brightness of the ultrasound screen. The gain can be adjusted to optimally visualize the structure of interest ( Fig. 20.2 ). Gain should be decreased so that things that are known to be anechoic appear black, and it should increase until there is enough contrast to allow differentiation and visualization. Depth should be adjusted to maximize visualization of the structure of interest. The ultrasound controls also allow for measurements to be taken and for images and video clips to be stored.

Ultrasound orientation

One of the most challenging aspects of ultrasound for the novice sonologist is developing an understanding of ultrasound orientation. The orientation of the ultrasound image on the screen can be understood by noting the position of both the location of the transducer on the patient and the direction of the transducer’s indicator. The top of the ultrasound screen always corresponds to where the ultrasound transducer is placed on the patient’s body. The indicator, or notch, on the ultrasound transducer corresponds to the side of the screen that the indicator marker is on. By convention in point-of-care ultrasound, the screen marker is on the left side of the ultrasound screen ( Fig. 20.3 ). In general, structures can be imaged in transverse (axial), longitudinal (sagittal), or coronal planes.

Scanning technique

The ultrasound transducer, for most applications, is held with the sonologist’s hand low on the transducer with the first and second digit, similar to a pen, with the other three fingers on the patient to stabilize. The right hand, regardless of hand dominance, is used to scan. For ultrasound-guided procedures, however, the transducer is held with the nondominant hand so that the dominant hand can be used to perform the procedure.

There are several methods of transducer manipulation that are important when performing an ultrasound examination, including sliding ( Video 20.1 ), fanning ( Video 20.2 ), rotating ( Video 20.3 ), rocking ( Video 20.4 ), and compression.

Ultrasound artifacts

Ultrasound artifacts are sonographic findings that do not correspond to true anatomic structure. Common artifacts are summarized in Table 20.1 .

TABLE 20.1

Ultrasound Artifacts

Artifact	Artifact Description	Ultrasound Image	Example
Contact artifact	Without gel or liquid, the ultrasound waves reflect back from air immediately next to the transducer, creating repeated hyperechoic lines		When trying to place a large transducer on an extremity, the part not touching gel or the patient will show up as contact artifact
Reverberation	Movement of the ultrasound waves between two highly reflective surfaces or within a highly reflective medium before reflecting back to the US transducer creates repeated hyperechoic lines at equal distance due to time delay		A-lines (repeated horizontal lines deep to the pleural line on lung ultrasound)
Posterior acoustic enhancement	Ultrasound waves lose energy as they travel through any medium but the least through liquids, which makes the area behind fluid collections appear relatively hyperechoic compared with the same surrounding impedance tissue that is not posterior to fluid		Bright white artifact noted behind the bladder or any fluid-filled structure
Acoustic shadowing	Anechoic area posterior to a strongly hyperechoic structure because all of the ultrasound waves were reflected back to the transducer and none are left to reflect from the deep space		The shadowing can be seen deep to bone, notable as rib shadows in lung ultrasound, or posterior to a foreign body as seen here
Mirror artifact	Ultrasound waves are first redirected by a highly reflective surface (the diaphragm) before encountering other tissue (here, liver) where they are reflected back toward the diaphragm before returning to the ultrasound transducer. The returning echoes are interpreted and displayed on the screen as a mirror of the liver tissue beyond the diaphragm (in the pleural space) because of the time delay.		Seen in normal right and left upper quadrant abdominal ultrasound images, where the liver/spleen density appears to be present superior to the diaphragm because of waves redirected off of the highly reflective diaphragm
		1. Ultrasound wave from transducer 2. Wave redirected by the diaphragm 3. Wave reflected back toward diaphragm 4. Wave redirected toward the transducer

Image acquisition

Image acquisition is an important part of diagnostic ultrasound and procedural guidance. It ensures proper ultrasound documentation and storage and it allows for quality assurance review. Depending on the application, still images and/or video clips may be indicated.

Introduction

Ultrasound guidance is standard of care for central venous access and is an asset for peripheral venous and arterial access ( ; ; ; ). This chapter will discuss specifics of ultrasound-guided vascular access. A critical part of performing any ultrasound-guided procedure is practice, preferably through simulation first and then with direct supervision prior to performing the procedure independently.

Transducer selection

High-frequency linear transducers are used for ultrasound-guided vascular access. These transducers allow for high-resolution imaging of the target vessel and needle.

Equipment

Supplies necessary for ultrasound-guided vascular access include a transducer cover, skin cleansing scrub, ultrasound gel, access kits or intravenous catheter, and gauze. There are commercially available sterile transducer covers. A skin cleansing scrub should be used, as appropriate for the procedure. Sterile ultrasound gel is needed. For central and arterial access, the same access kits are used whether or not ultrasound is employed for guidance. For peripheral access, the size and depth of the vessel chosen for access dictate the selection of the catheter. Standard length peripheral intravenous (PIV) catheters are appropriate for superficial access; longer PIV catheters are available for deeper vessels. Extra gauze and alcohol wipes should be immediately available so that the ultrasound gel can be wiped away prior to securing the vascular access device and dressing.

Transducer orientation

The two methods of ultrasound guidance for vascular access are the out-of-plane and in-plane techniques. The out-of-plane approach uses transverse transducer placement with the indicator pointing toward the patient’s right ( Fig. 20.4 ). For the in-plane approach, the vessel is imaged in its sagittal/longitudinal orientation, with the indicator pointing toward the patient’s head ( Fig. 20.5 ).

Sonographic anatomy

This section reviews the sonographic appearance of skin and common structures visualized during ultrasound-guided vascular access ( Fig. 20.6 ).

Veins can be visualized in the transverse orientation as anechoic circles or ovals. They can be approximately located by knowing their anatomy. Veins, compared with arteries, are more superficial, are more collapsible with compression, have thinner walls, and are less pulsatile ( Fig. 20.7 ; Video 20.5 ). Arteries have thicker walls, are less compressible (they will compress with enough pressure), and are more pulsatile ( Fig. 20.8 ; Video 20.6 ). Arteries and veins both appear as long anechoic tubes in the longitudinal view ( Fig. 20.5 ). Nerves appear on ultrasound as a cluster of hyperechoic dots in the transverse view, sometimes described as a honeycomb appearance ( Fig. 20.9 ). In the longitudinal view, nerves appear as a long hyperechoic fiber bundle.

Needle appearance and image optimization

Needles are hyperechoic and appear on the ultrasound screen as bright white objects with posterior shadowing. Additionally, as metal structures, reverberation artifacts may be seen posteriorly. In the transverse view, needles appear as a hyperechoic dot on the screen ( Fig. 20.10 ). Every part of the needle course (tip, middle, proximal) appears identical on the transverse ultrasound view ( Fig. 20.e6 ). Care must be taken to always know the location of the needle tip. Failure to track the needle tip may result in penetration through the posterior wall of the vessel. If the needle is not visualized as an echogenic dot because it is not seen or it is weakly hyperechoic, the transducer angle is adjusted. The optimal angle of insonation, or angle between the ultrasound beam and the object of interest (needle), is 90 degrees. Assuming the needle enters the skin at 45 degrees, the best image of the needle will occur when the transducer is held tilted away from the operator at 45 degrees ( Fig. 20.e7 ; Video 20.7 ). In the longitudinal view, the needle appears as a long hyperechoic linear structure with posterior shadowing and can have reverberation artifact ( Fig. 20.11 ).