








































Anesthesia eJournal
Volume 3 Issue 2 2015

AEJ

Educated Hand Publishing LLC 
“The Science Behind the Art”  

 Anesthesia eJournal - Online
ISSN 2333-2611

Ultrasound-guided regional anesthesia is now recognized as the evolving alternative to standard landmark-based 
techniques for nerve blockade and vascular access. The limitations of this technique begin to be recognized, however, 
as needle angles increase beyond 30 degrees, as commonly used for deep injections. This limitation remains difficult to 
overcome, especially for novice users.  Loss of needle visibility on the monitor screen remains a source of frustration for 
many providers, leaving them confused as to ultrasound’s true utility. This article explores a simple, reliable technique for 
improved needle visibility on any ultrasound machine through triangulation of 3 points using the law of sines. The three 
points of ultrasound triangulation are (1) the location of the ultrasound probe, (2) the nerve target, and (3) the site of 
needle entry. The location of the ultrasound probe is chosen by the user and the depth to the nerve is displayed by the 
ultrasound machine. The third point, the needle entry site, is based on the law of sines to ensure a needle entry angle of 
less than 30 degrees. This approach can simplify a challenging aspect of ultrasound-guided regional techniques.

KEYWORDS: anesthesiology, increased needle visualization under ultrasound, law of sines, needle visualization, 
needle enhancement, ultrasonography

Abstract

Reliable Needle Visualization During Ultrasound-Guided Regional Procedures: A Simple 
Solution to Steep-Angle Echogenicity Loss Based on Target Depth
Jonathan Kline, CRNA, MSNA
Affiliation:
Director of Education at Twin Oaks Anesthesia Services. contact - TwinOaksAnesthesia@gmail.com

Funding/Conflict of Interest Disclosure: 
None

INTRODUCTION
Ultrasound-guided regional anesthesia is now recognized as the evolving alternative to standard landmark-based techniques for 

nerve blockade and vascular access. Since the first documented anesthesia application of ultrasound in 1978 by La Grange,1 techniques 
have improved and clinical use has increased. Ultrasound use has shown increased success, and to some extent increased safety, versus 
blind techniques.2 It seems logical that the ability to actually visualize the needle during regional anesthetic placement allows for 
greater safety and success, although current literature is slow to support the former. The limitations of ultrasound-guided regional 
anesthesia began to be recognized, however, as needle angles increase, as is common for deep injections. This limitation remains 
difficult to overcome, especially for novice users.  Loss of needle visibility on the monitor screen remains a source of irritation for many 
providers, leaving them confused as to ultrasound’s true utility. If the angle at which the ultrasound waves hit the needle shaft is from 
a relatively parallel origin, such as in shallow injection techniques, the needle can be displayed as a hyperechoic (bright white) line that 
is easy to visualize. As the angle departs from parallel and exceeds greater than about 30 degrees, the ultrasound wave fails to return 
to the probe. This reliably happens in all ultrasound systems regardless of make or manufacturer. This in essence describes the physics 
problem associated with visualization during steep needle angle paths. 

We sought a simple, easily incorporated and cost-effective solution to this problem. One method would be to add software to the 
system allowing the probe to adjust the beam angle to compensate for needle approaches greater than 30 degrees. Another cheaper, 
more reliable, and less complex method would be to simply adjust the needle angle. Because depth readings are a fairly standard display 
in the ultrasound industry, we decided to use this single variable to provide a map for allowing increased needle visualization based on 
this piece of readable information. 



REVIEW OF THE LITERATURE
A current review of the literature was conducted by using the 

terms “increased needle visualization under ultrasound,” “law of 
sines,” “needle visualization,” and “needle enhancement.” 

The ultrasound system processor displays a needle by way of 
specular reflection. Chan and Perlas3 describe specular reflection 
as the processing of ultrasound wave return from an object that 
is long and smooth. The wave return to the probe for processing 
is in a single direction. The object must be in an orientation to 
the ultrasound beam that is relatively similar to the angle of 
origin. “There is increased signal wave return as the needle is 
more parallel to the ultrasound’s probe face”. This orientation 
ensures that the majority of the emitted waves are received and 
processed.4 Specular reflection differs greatly from what is known 
as diffuse reflection. Chan and Perlas3 describe diffuse reflection 
as poor return of ultrasound waves from an object that is irregular 
in nature or has steep angles. Steep needle angles encourage wave 
refraction, or diffusion of ultrasound waves away from the probe. 
Because these waves fail to return to the probe, less information is 
available for processing, thus resulting in poor image resolution.4 

There is a unique element in the processing of hollow needle 
ultrasound return waves. The beam strength exceeds the strength 
of the needle shaft on the away path but not on the return path 
to the probe. Ultrasound waves become trapped within the 
lumen of the needle and return to the probe in a delayed fashion. 
The delay occurs because the waves easily penetrate the outer 
lumen of the needle on the away path but are reflected back to 
the lumen multiple times on the return path. This happens in a 
repeated fashion until the waves manage to penetrate the needle’s 
superficial wall and return to the probe. The bouncing around 
of these waves produces a delay in the return time. These wave 
bursts return in a staggered fashion, producing an image of a 
repeating needle on the screen. This was described by Reusz et 
al4 as an artifact occurring at increasing depths until the needle 
angle changes or the wave strength is decreased. This is known as 
reverberation artifact.5 

As early as 2004, needle visualization loss was reported.  
Schafhalter-Zoppoth and colleagues5 noted that, as the needle 
angle increased (or became steeper), needle visibility decreased. 
This was a reproducible finding in human tissue, but those 
authors did not use a variety of ultrasound equipment. Miura et 
al6 also described needle visualization loss at particular angles. 
An important element from this article was the clear loss of 
visualization with a linear probe at between 30 and 45 degrees.6 
This article was probably the first report making the link between 
linear probe use and steep needle angle visualization loss. This 
is important to clinical practice because most scans and needle 
techniques commonly used in anesthesia care are performed 
with a linear probe. The characteristic of increased insertion and 
visualization loss is not unique to needle insertion. Junji et al7 
observed that peri-neural catheters also exhibit similar visual loss 
during ultrasound techniques when placed at steeper angles. 

Interestingly, the visual loss is less pronounced with a 
curvilinear probe at steeper angles. Similar results were reported 
by Abbal and colleagues.8 Hocking et al9 observed that the use 
of cadaveric tissue versus commercially available alternatives to 

assess needle visibility differed in echogenicity. He noted that 
phantoms and gel mediums had a low background echogenicity 
and would therefore exaggerate needle visibility during skills 
acquisition. This differed from fresh-frozen cadavers in that they 
retained greater lifelike clinical echogenicity. This is important 
when evaluating needle angle visual loss, because the medium 
in which the evaluation occurs influences needle visibility.9 In 
essence, this means that techniques are easier to perform on 
simulators and gel mediums than on actual human tissue. It has 
yet to be evaluated whether ultrasound-guided skills acquisition 
is made more efficient by the addition of formal training on 
simulators prior to actual patient care, although this seems to 
make sense. 

Hebard and colleagues10 quantifiably linked steep needle angles 
to visual loss. In this important article, they also concluded that 
every 10-degree increase in needle angle steepness resulted in a 
12% visual loss. Stated another way, if the needle angle exceeds 
about 40 degrees, visualization of that needle decreases by about 
half. Weismann et al11 confirmed earlier findings that correlated 
visual loss with steep angle insertion. Those authors also observed 
that this visual loss was less severe if the needle was visualized by 
use of ultrasound beam compound imaging.11 

This literature review points to a few interesting correlations. 
If needle angles exceed about 30 to 40 degrees, visualization 
ultimately decreases. The only way described in the current 
literature to alleviate this problem is to use some sort of 
ultrasound beam alteration or enhancement, or to change to a 
curved probe (which is usually unsuitable for most anesthesia-
related procedures).  

DESCRIPTION OF TECHNIQUE FOR IMPROVED 
NEEDLE VISIBILITY

We developed a method whereby a clinician can place an in-
plane needle under any ultrasound beam and be able to visualize 
it, regardless of  depth of the target, age, or complexity of the 
machine. This method can allow even the most inexperienced 
providers access to readily viewable needles on the monitor 
during regional and vascular procedures. The criteria for this new 
method are as follows: 
1) The needle angle cannot exceed 30 degrees. 
2) The needle guidance software and enhancement cannot 

be engaged, as there are too many variables to consider 
regarding current and older systems. 

3) Only standard block needles, which are easy to obtain and 
cost-effective, should be used. 

We first determined the pieces of information easily accessible 
by virtually all ultrasound machines. The commonality was depth. 
Reading the depth on the screen was nearly universal among 
the surveyed machines in current practice. The second piece of 
information known from the literature review was that 30 degrees 
seemed to be the maximum allowable angle to produce perfectly 
readable needle echogenicity. This cutoff applies to needles placed 
in-plane. Although a great many other pieces of information were 
available, they were not common to virtually all machines. Thus, 
on the basis of these pieces of information, we sought to complete 

Anesthesia eJournal                         www.anesthesiaejournal.com
Volume 3 Issue 2 2015



a simple method to allow anyone to produce readable 
needles at any depth. 

The solution was described as a simple inverted triangle 
(Figure 1). The base of the triangle became the probe 
interface with the skin. The height of the triangle became 
the depth to the target of interest. This left only the 
distance from the probe to begin needle entry, keeping 
in mind that the needle could not exceed 30 degrees of 
approach to the target. 

Figure 1. Triangulation of 3 points using the law of sines: 
(1) the location of the ultrasound probe, (2) the nerve 
target, and (3) the site of needle entry.

Figure 2 shows images revealing the difficulty of needle 
visualization during steeper needle angles. Note that in image 
A, the needle shows up displaying strong echogenicity. This is 
an excellent example of an entry that is less than 30 degrees. 
Image B shows the same needle on the same patient, but 
visualization of that needle is markedly decreased. Image B 
shows the visual loss as reduced echogenicity of the needle 
shaft at a larger insertion angle.

Figure 1 
Needle visualization applying the Law of Sines   

A
B

C

Probe center

1 cm

2 cm

3 cm

?
Point of needle entry

30 degrees 

A/a sin= B/b sin= C/c sin 
Target

Jonathan P. Kline, CRNA

In solving this triangle problem, we used the law of sines. Although there are many variations of this solution, we 
used a simple technique that allowed for a simple formula to be used based solely on the depth of the target structure. 
For example, if the target structure is 4 cm in depth and the approach cannot exceed 30 degrees without signal loss 
(leading to decreased needle visualization), the skin must be entered at exactly 6.92 cm from the probe. This solution 
is reliable and reproducible every time. It allows for a simple formula to be incorporated to ensure successful needle 
visualization, regardless of the age or complexity of the ultrasound machine. It is important to note that the center of 
the probe (not the edge) is the beginning point for the triangle’s base measurement. Table 1 presents common needle 
angle distances developed on the basis of the law of sines.  The new total distance to the target that the needle will 
have to travel is also included.

Anesthesia eJournal                         www.anesthesiaejournal.com
Volume 3 Issue 2 2015

Image B

Figure 2. Images A and B are actual patient (in vivo) pictures captured during an ultrasound-guided regional 
procedure. (Images from Jonathan Kline, CRNA.)

Image A



REFERENCES
1. La Grange PDP, Foster PA, Pretorius LK. Application of the Doppler ultrasound blood flow detector in supraclavicular brachial 

plexus block. Br J Anaesth. 1978;50(9):965-967. http://dx.doi.org/10.1093/bja/50.9.965.
2. Marhofer P, Greher M, Kapral S. Ultrasound guidance in regional anaesthesia. Br J Anaesth. 2005;94(1):7–17. doi: http://dx.doi.

org/10.1093/bja/aei002. Published online July 26, 2004.
3. Chan V, Perlas A. Atlas of ultrasound-guided procedures in interventional pain management. In: Chan V, Perlas A. Basics of 

Ultrasound Imaging. New York: Springer Science Business Media; 2011:13-19. doi: http://dx.doi.org/10.1007/978-1-4419-
1681-5_2. 

4. Reusz G, Sarkany J, Gal J, Csomos A. Needle-related ultrasound artifacts and their importance in anaesthetic practice. Br J 
Anaesth. 2014;112(5):794-802. http://dx.doi.org/10.1093/bja/aet585.

5. Schafhalter-Zoppoth I, McCulloch CE, Gray AT. Ultrasound visibility of needles used for regional nerve block: An in vitro 
study. Reg Anesth Pain Med. 2004;29(5):480-488. http://dx.doi.org/10.1097/00115550-200409000-00014.

6. Miura M, Kazuhide T, Toshiyasu S. Visibility of ultrasound-guided echogenic needle and its potential in clinical delivery of 
regional anesthesia. Tokai J Exp Clin Med. 2014;39(2):80-86.

7. Takatani J, Takeshima N, Okuda K, Uchino T, Noguchi T. Ultrasound visibility of regional anesthesia catheters: An in vitro study. 
Korean J Anesthesiol. 2012;63(1):59-64. http://dx.doi.org/10.4097/kjae.2012.63.1.59.

Anesthesia eJournal                         www.anesthesiaejournal.com
Volume 3 Issue 2 2015

Table 1. Application Chart of the Law of Sines for Increased 
Needle Visibility During Ultrasound-Guided Regional 
Procedures

DISCUSSION
The benefits of incorporating this chart into clinical use are 

straightforward. Allowing users to see needles, even during 
deep structure targeting, may allow for increased safety and 
effectiveness for every technique. The formula is simple to 
incorporate and requires only one piece of information to use. 
This piece of information is the depth of the target structure. The 
depth is available on most clinical ultrasounds in use today. As 
an added, albeit unforeseen benefit of this discovery, the chart 
also reveals not only the point of entry that will enhance needle 
visibility but also the total distance the needle will be required to 
travel. This will simplify needle length selection before the start of 
a procedure.

 Some problems may be associated with this technique. This 
chart or technique is difficult to use if the probe is placed in a 
position that may be difficult to measure accurately. Also, the 
region must be relatively flat in order to ensure relative accuracy 
of the formula. The technique described to increase visualization 

dictates the distance that the needle must be placed from the probe. 
As the target depth increases (albeit beyond most common clinical 
depths), such as beyond 6.5 cm, needles greater than 10 cm must be 
used. This may represent a special order item and may therefore 
increase cost or exclude use of the technique by centers that lack 
the necessary needle length. Increased needle distances may 
become uncomfortable for the patient, because it will become 
difficult to localize an area at great depth from the projected 
needle insertion site. It is difficult to assess or predict patient 
comfort in this arena, but an adequate volume of a fast-acting, 
dilute local anesthetic to the projected needle path may prove 
more comfortable for patients having deep injections. Another 
obvious deficit in this chart is the lack of necessary information 
regarding targets greater than 10 cm. However, it would be rare 
for a patient to require an injection or vessel access at this depth 
without another more feasible alternative. We recommend a 
formulated study to formalize these theoretical suggestions. 

CONCLUSION
Ultrasound guidance has promoted increased effectiveness, and 

to some degree safety, during regional anesthesia techniques and 
vascular access.12 We sought to produce and describe a technique 
that would simplify anesthesia providers’ attempts to visualize 
needles during any regional or vascular procedure. We determined 
that by use of the law of sines, a simple distance could reliably be 
used to increase needle visualization during ultrasound-guided 
regional procedures. To use this chart, one only needs to know 
the target depth, and this seems to be available on most clinically 
used ultrasound systems. As shown in the table, many clinically 
relevant depths to target structures (ranging from 2 to 10 cm) 
can be used with reliable results. These results lead to increased 
needle visualization with the most primitive to the most complex 
ultrasound systems. 
Acknowledgments

Special thanks to Bill Gordan, CRNA, for his assistance with 
this article.


