US Probe Learning Notes

*** US Probe Manipulation Notes - 2013dec16 ***

Ultrasound: Basic understanding and learning the language - Int J Shoulder Surg. 2010 Jul-Sep

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3063344/

Abstract

... We stress that for the best use of US, one should venture beyond the “pattern recognition” mode to the more advanced systematic approach and use US as a tool to visualize structures beyond the skin.

... cover basic machine “knobology” and fundamentals of US probe selection and manipulation.

INTRODUCTION

... It should be clearly understood from the outset that the ability to “see” the targeted structure with US does not preclude a thorough knowledge of gross or micro-anatomy. Some experts agree that proper utilization of US requires an even better knowledge of applied anatomy ...

... For the best use of US ... , one should venture beyond the “pattern recognition” mode to the more advanced systematic approach and use US as a tool to visualize structures beyond the skin.

‘Pattern recognition’ refers to memorization of an US image of a targeted structure (textbook picture) and learning the maneuvers and techniques necessary to acquire the image. This is, however, not sufficient in the presence of anatomical variations or if US is used for diagnostic purposes.

To advance beyond the pattern recognition mode, a thorough knowledge of applied anatomy combined with a basic understanding of how a 2-dimensional (2-D) US image represents a 3-dimensional (3-D) anatomical structure is needed;

This article ... cover basic machine “knobology” and fundamentals of manipulation of the US probe ...


IDENTIFYING DIFFERENT TISSUE TYPES AND UNDERSTANDING ULTRASOUND TERMINOLOGY

Echogenicity

Echogenicity of the tissue refers to the ability to reflect or transmit US waves in the context of surrounding tissues.[7–9] Whenever there is an interface of structures with different echogenicities, a visible difference in contrast will be apparent on the screen. Based on echogenicity, a structure can be characterized as hyperechoic (white on the screen), hypoechoic (gray on the screen) and anechoic (black on the screen) ...

... Bone appears black or anechoic on US, with a bright hyperechoic rim. Because the US beam cannot penetrate bone, it casts an acoustic shadow beyond it. Cartilage appears hypoechoic, and is more penetrable by US than bone. Blood vessels also appear black or anechoic . Veins are usually easily collapsible upon external pressure by the transducer, while arteries are pulsatile and do not collapse with moderate pressure.

Blood vessels have a distinct appearance on color Doppler mode: flow toward the probe appears red, while flow away from the probe appears blue. A useful mnemonic used by radiologists is BART, i.e., Blue Away, Red Toward. Muscles are hypoechoic with striate structure; fat is almost anechoic, while fascia and other connective tissue strands and fascicles appear as hyperechoic lines ...

... Ligaments and tendons have a similar appearance to distal nerves (hyperechoic, but not “honeycomb”). If in doubt, one can trace the “target structure” proximally or distally in order to distinguish the nerve from a tendon based on anatomy ...

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... Scanning planes are similar to the well-known anatomical planes: axial (transverse), sagittal, parasagittal, and coronal. ...

Ultrasound views

All subjects, except cubes and spheres (that are absolutely symmetrical in all directions), have a long axis and a short axis when viewing them from a 2-D approach. Viewing a structure in the long axis will provide a long-axis view, and vice versa; an oblique view is also possible.

Anatomical structures, such as vessels or nerves, are more commonly viewed in the short axis (round shape on the screen) than long axis when the operator loses the lateral–medial perspective. Rotating an US probe to 90° will change a short-axis view into a long-axis view, and vice versa. An oblique view can be appreciated during rotation of the probe between the true short axis view and the long-axis view.

Angle of incidence

The angle at which the US waves encounter the surface of the structure, termed, the angle of incidence, affects the way it is presented on the screen. If the angle is perpendicular, or close to perpendicular, more US waves will be reflected back to the transducer and fewer will be “scattered” away, resulting in a better image. If the US waves are more parallel to the surface of the object (more than a 45° angle of incidence), the image will have less definition. The operator can improve the image of the target by tilting or rotating the probe, thus adjusting the angle of incidence ...

Schematic illustration of improving the angle of incidence by tilting the probe. By tilting the probe from position 1 to position 2, we obtained the true axial short-axis view of the artery and the nerve. The shape of the image of the artery and the nerve ...

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Anisotropy

Anisotropy in ultrasonography could be defined as a tissue property that is responsible for changes in the US reflection dramatically, even with mild changes in the angle of incidence. It creates the phenomenon known as “now-you-see-me-now-you-don’t”. Different tissues have varying degrees of anisotropy. ...


ULTRASOUND WAVE FREQUENCY, IMAGE RESOLUTION, AND PENETRATION

High frequency probes (10–15 MHz) and midrange frequency probes (5–10 MHz) provide better resolution but have less penetration. High frequency probes are, therefore, preferred for US imaging of superficial structures (2–4 cm), while midrange frequency probes are preferred for slightly deeper structures (5–6 cm). However, when US imaging of deep structures (for example, a proximal sciatic nerve that can be as much as 10 cm deep) is required, a low frequency probe (2–5 MHz) is preferred, although the quality of the image will be substantially poorer. When determining the correct choice between probes with different US frequencies, choose the one that will provide the best resolution for the required depth. Most practitioners have several different probes for more flexibility.

CURVILINEAR VERSUS STRAIGHT PROBE

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ULTRASOUND PROBE FOOTPRINT SIZE

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COLOR DOPPLER FUNCTION

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KNOBOLOGY : GAIN, FOCUS, AND MODE OF SCANNING

Changing the gain will change the amount of white, black, and gray on the monitor. Adjusting the gain of the image may improve the operator’s ability to distinguish structures on the screen; the amount of gain to use depends on personal preference. Most US machines have an auto-gain knob, which is commonly used.

Modern US machines have useful “nerve”, “angio” or “general” modes. ...


DEPTH SETTINGS

It is wise to begin with a somewhat higher depth setting in order to first get a “big picture”, and then gradually decrease the depth when the targeted structure is found. ...


PROBE ORIENTATION

Probe orientation is important because the US probe can be easily rotated around (180°) while the position of the monitor remains unchanged, which may create confusion in the direction of probe manipulation ... Therefore, it is always useful to confirm which side of the probe corresponds to a particular side of the screen in order to identify the correct orientation of the image. All transducers have an orientation marker that corresponds to the marker on the screen.

PROBE MANIPULATION

When dealing with US probe manipulation, the mnemonic PART (Pressure, Alignment, Rotation and Tilt) is useful ... . It is important to understand that by manipulating the US probe, we primarily manipulate the direction of the beam, and, by changing the direction of the beam, slightly different US images of the same structures can be obtained.

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Correct pressure application can considerably improve the image quality. It affects the echogenicity of the tissue and shortens the distance to the structure of interest. Ordinarily, pressure must be applied evenly to get the correct direction of the scan; however, occasionally, the operator may intentionally need to apply more pressure on one side of the probe in order to direct the US beam in the desired manner (angling the probe). Pressure to the probe is also applied to compress ...

Alignment (sliding)

The main goal of this maneuver is to find the structure of interest and position it optimally on the screen for needle advancement (usually in the middle of the screen for an out-of-plane approach and somewhat on the opposite side of the screen for an in-plane approach). Sliding is also very useful for tracing the potential structure proximally and distally for better verification of pertinent anatomy during a “scout scan”.

Rotation

With rotation, one can achieve several goals. First, one can attain a true axial view of the target with its long axis parallel to the surface but not perpendicular to the current US plane. For example, if you image a blood vessel (that is parallel to the surface) in the short-axis view and slide a US probe along the vessel’s long axis, you must slightly rotate the probe when the vessel makes a turn in order to maintain true short axis view. Second, one can align the target into a more favorable trajectory for a safe needle pass (away from vessels or pleura, for example).

Rotation will affect the image if it brings the object out of the true axial view. If the long axis of the object remains parallel to the surface and the US probe is gradually rotated relative to the long axis of the structure, the round cross section of the true axial cut (of the normally round vessel or nerve, for example) will be replaced by a more oval shape. By continuous rotation of the probe of 90° from the initial probe position, one can change the view of the structure from its short axis to its long axis, and vice versa.

Tilt

There is no particular recognized terminology to define the direction of the tilt, and confusion can arise from the fact that when the probe is tilted in one direction, the US plane, in fact, sweeps to the opposite direction.

Several goals can be achieved by tilting the probe. First, by sweeping the US beam in the particular direction desired by tilting the probe, one can “preview” the image by sliding the probe in the opposite direction of the tilt. Second, by tilting the probe, a true short-axis view of the object can be obtained, the long axis of which is not perpendicular to the initial US beam plane.

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The distance from the surface to the target will increase and The shape of the target on the screen will be untrue (oval instead of round, for example).

ULTRASOUND ARTIFACTS

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SONOPATHOLOGY

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ERGONOMICS OF ULTRASOUND-GUIDED PROCEDURES

An US machine should be positioned on the contralateral side of the patient, with the operator standing on the ipsilateral side that needs to be blocked or examined. The transducer is usually held in the operator’s nondominant hand ...

The transducer should gently be held quite low on the probe, close to the scanning surface, rather than harshly gripped on the top of the handle. When planning to use the in-plane approach, it is preferable to place the probe directly perpendicular to the skin. (If it is possible to do without sacrificing image quality, try not to tilt the probe, as this will add difficulty with in-plane needle advancement; personal observation).

PROBE MANIPULATION DURING NEEDLE ADVANCEMENT

It is important to stabilize the US probe position after obtaining the desired image. This can be facilitated by gently bracing the hand holding the probe on the patient’s body. ...

... Eye-hand coordination is required for this maneuver, and phantom exercises are very helpful to enhance this particular skill.

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