U/S System Overview

Most ultrasound systems have the same basic features.  It is important to have a firm grasp of what each function does and where it is located on the system before working with patients. One of the keys to any successful procedure is eliminating the extraneous tasks so all the focus can be directed at the goal (a successful block or vascular cannualation).  Familiarize yourself with the ultrasound system by scanning on a colleague or yourself numerous times until you feel comfortable.  This will reduce added stress and frustration that comes from not knowing what control to use, or where to find it when it really matters.

Know and understand these functions:

  • B-Mode imaging
  • Depth
  • Contrast adjustment (gain)
  • Color-flow Doppler
  • Image recording capability

Prior to performing any study, the practitioner should enter patient-specific information so that images and videos can be evaluated and critiqued at a later time.

B Mode Imaging

B mode imaging is the default screen of all ultrasound systems.  As discussed in the Ultrasound Physics section, it is the grey-scale image that appears on the screen as the result of sound/tissue interaction.  There are two common views used in B mode; cross-sectional and longitudinal.  The cross-sectional image of the internal jugular vein (IJV) in the low neck, shown below left shows the anatomy in the cross-sectional or transverse plane.  The cross-sectional image is the most common view used in regional anesthesia.  The image below right shows the IJV in the longitudinal view, along the sagittal plane of the body.  Longitudinal views are often used during vascular cannulation (discussed in Vascular Access).


Depth

Depth determines how far into the tissues echoes are interpreted.  Increasing the depth will decrease image resolution because it takes longer for the ultrasound system to send and receive information, thus fewer images are created each second.  In general, it is desirable to keep the structure of interest in the center of the screen.  Below are two images of the median nerve at the wrist.  The image on the left shows the correct depth for the median nerve, approximately 3cm.  This provides the anesthetist with a high-resolution image, which allows for better needle visualization, and avoidance of potential hazards such at the radial artery (RA) and veins (RV) at the top left.   The image on the right is of the same median nerve, but with the depth increased to approximately 5cm.  Notice that the resolution has slightly decreased, and all the structures appear smaller.


Gain

Gain compensates for attenuation of the ultrasound wave due to reflection, refraction and scattering that occurs during tissue interaction.  Gain amplifies only RETURNING echoes.  It does not increase the strength of the incident beam.  Gain should be adjusted so that sound/tissue interaction is consistent and readily identifiable, as in the image of the median nerve, below center.

Using too little gain, as shown below left, results in a dark image where it is nearly impossible to identify any of the structures.  By contrast, too much gain, as seen in the image below right, appears as a “snowstorm”.

To increase the strength of returning echoes attenuated by sound-tissue interaction, advanced ultrasound systems allow for gain to be adjusted at specific levels in the ultrasound image, known as time gain compensation (TGC).  By adjusting the TGC, an ultrasound image can be made to to uniform, despite imaging at increased depths.

In the image below, the TGC cursors (yellow arrow) are all in the same position, meaning that there is no compensation of the returning echoes .  Attenuation of the sound waves at deeper depths because of sound-tissue interaction results in a reduced image at the bottom of the screen.

 

To increase the strength of the weaker returning echoes, the gain is increased on the lower part of the TGC (red arrows) creating a uniform image.  This is compensation.

An artifact known as dropout can occur when using an ultrasound system that has TGC.  If the gain is not properely adjusted, areas of the screen will appear black . In the image of a thigh below, a mid-level TGC (red arrow) has been reduced causing a loss of the on the corresponding image.

Color-Flow Doppler

Many blocks are complicated by small superficial arteries and veins that often course over or near the nerves to be blocked.  Color-flow Doppler can be an invaluable tool when performing UGRA.  It provides a quick and easy method to determine if an anechoic structure being visualized is vascular by confirming the presence of flow using color representation.  Color-flow Doppler is superimposed over the B mode image, restricted to the area contained within a box on the screen.   One of the most common inaccuracies often discussed about color-flow Doppler is the assignment of color.  Some authors have mistakenly stated that arterial flow is red and venous flow is blue.  Others have stated that blood flowing toward the transducer (positive shift) is red, and blood flowing away from the transducer (negative shift) is blue.  This is also not entirely correct.  The only accurate statement that can be made regarding color-flow Doppler is that various hues of red and blue are assigned to demonstrate flow, and the operator is responsible for assigning which colors indicate a positive and negative shift.  The two images below are the same carotid artery using color-flow imaging.  The image on the left appears as one would expect it, red because it is an artery and there is a positive shift.  However, the image below right puts into question these two commonly held beliefs.  There are numerous reasons this could occur.  First, the transducer could have moved and we are now imaging venous flow from the IJV.  Second, the patient could have a pathological condition in which arterial blood flow is reversed.  Another reason could be that the transducer is not oriented properly to the patient (discussed later), resulting in a reversed image.  The fourth possibility, and correct answer, is that the color-flow orientation button was inadvertently reversed, causing the positive-shift appear blue and the negative-shift appear red.  This can be confirmed by reading the color-flow scale in the upper left (yellow star) of the screen that shows the different shades of blue from 0 to 45 and the red from 0 to -45.  This is opposite compared to the image on the left.

Doppler flow is dependent on the angle of insonation.  Unlike B mode imaging where placing the transducer perpendicular to the skin is generally desirable, the Doppler equation is reliant on the Cosine of the angle of the incident beam in relation to blood flow.  Since the Cosine of 90 degrees is zero, flow will appear absent or reduced when the Doppler wave is perpendicular to the blood vessel.  To compensate for this, the ultrasound system can “steer” the Doppler beam to create an angle that will detect flow.  Below are two images of the radial artery in the wrist.  Because the aretery courses superficial and almost perpendicular to the skin, color-flow is reduced when the color-flow box is at 0 degrees (indicated by yellow star).  To increase the color-flow, the box is steered 20 degrees (indicated by yellow star), which increases the angle of the incident beam to the blood flow less than 90 degrees, improving the Doppler Effect as shown by more visible color-flow.

Image and Video Recording

Currently there are no specific standards on what should be recorded when performing UGRA.  However, images and videos are invaluable tools for evaluating block performance, and should be taken whenever possible.  Videos supply more information about the block performace, while images offer only a “snapshot” of the procedure.  Compare the video and image below.  While the image appears to show a successful supraclavicular block, we have no insight into how many passes it took to place the needle in its final position, whether the needle punctured the nerve, artery or pleura, or spread of local anesthetic.  Now view the video.  Which one better documents the procedure?

 


References

Pollard BA, Chan VW. An introductory curriculum for ultrasound-guided regional anesthesia: a learner’s guide. Toronto. University of Toronto Press Inc.; 2009:15-18.

Gray AT. Ultrasound-guided regional anesthesia; current state of the art.  Anesthesiology. 2006; 104:368-373.

Sites B D, Brull R, Chan V W, et al. Artifacts and pitfall errors associated with ultrasound-guided regional anesthesia. part I: understanding the basic principles of ultrasound physics and machine operations. Reg Anesth Pain Med. 2007;32(5):412-418.

Falyar CR. Ultrasound in anesthesia: applying scientific principles to clinical practice. AANA J. 2010 Aug; 78(4):332-40.

Chan VW, Abbas S, Brull R, et al. Ultrasound imaging for regional anesthesia; a practical guide. 3rd ed. Toronto. Toronto Printing Company; 2010:20-23.

 

 

 

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