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A Homegrown View of Ultrasound

Ultrasound, well it's a large part of this web-site so this page should deal mostly with general topics related to it.  I received an e-mail through this site from someone asking why ultrasound shows in blacks and whites and shades of gray and whether there was a "contrast" equivalent usable with ultrasound like the injectable types of contrast used in x-ray.  I was forced to organize my thoughts for the reply (which I'm not sure got through to the guy who wrote me the e-mail).

 Anyway, I thought I'd post it here in order to; let newcomers have a simplified view of the principles of ultrasound, and to let those more knowledgeable in the ways of ultrasound see the extent of my ignorance.

Firstly, I'm not an expert on ultrasound principle or theory, there are plenty of good explanations of ultrasound to be found on the  internet or in the library. 

Having said this, I will try to tell you what I know.  Ultrasound is high frequency sound generated in specific frequency ranges and sent through tissues.  Penetration into tissue is based in large part on the range of the frequency produced.  Lower frequencies (e.g- 2Mhz) penetrate deeper than high freq. (e.g.- 10mHz).  As the sound passes through tissues it is either absorbed, reflected or allowed to pass through, depending on the density ("echo"-density) of the tissue.  All ultrasound dissipates in tissue producing heat.  The "listening" part of the probe (a piezo-electric crystal just like the generating part of the probe) "listens" for reflections (echos) of the sound waves sent out and passes the information to the processing unit.  Time between sending and receiving equals distance.

The amount of energy reflected (not absorbed or propagated) equals density.  Substances containing a lot of water (cerebro-spinal fluid, blood) are very good conductors of sound and reflect very little, they are called echo-lucent.  Since they reflect very little of the sound they appear as dark areas.  Substances which contain little water (or made up of material that otherwise is a poor sound conductor) such as bone or a worse conductor, (e.g.- air),  reflect almost all the energy and appear very bright.  Substances which conduct sound to a degree in between these 2 extremes, appear darker to lighter depending on the amount of energy they reflect.  Most useful reflections (echos) occur at boundaries between tissues of differing densities. Mostly we are seeing outlines of things which have different reflecting properties in ultrasound.

Remember also that just like regular sound and light can reflect off of objects in their path, they don't necessarily reflect at an angle which returns to their source.  (this is the principle behind much of armed forces  "stealth" designs which reflect very little of the incident radar (light) energy back of the source).  So tissue surfaces which may be echo-reflective but lie at in a non-perpendicular plane to the "listening" probe will have little if any energy returning from them and therefore won't show up on ultrasound.

Obviously, there are many other issues involved in producing an image but I can sum it up in this way,  the implementation of the theory and physics of ultrasound to produce a meaningful, near-real-time, image of the inside of a living object is my idea of a miracle of human engineering and the neatest thing since sliced bread.  This is mostly due to a part I glossed neatly over with the words "processing unit" in the second paragraph.  The information contained in the "return signal", the echo, is heavily (and cleverly) processed to reduce the amount of artifact and produce a picture which corresponds to what we expect to see.

 

Contrast in Ultrasound

Contrast in ultrasound terms can be kind of a problem in some applications.  The greatest "contrast" will be introducing air into the scene.  However, while  injecting air into tissue (muscle, connective tissue, subcutaneous tissue, etc.) will definitely show you where you are, air will continue to reflect energy until it is absorbed which can take some time.  During that time you won't be able to see past the "contrast". 

If you are doing vascular ultrasound, specifically cardiac preferably on the right side of the heart, cold IV solution is briskly agitated with a small amount of air and injected in the venous side of the heart.  The small air bubbles come out solution in the warmer blood and appear as bright specks in the blood on ultrasound.  These bubbles are carried to the vascular system of the lungs and are harmlessly diffused in to the air passages and blown off  through respiration.  Aerated solution could possibly be used in tissues I suppose, might have to try that.  Mostly for the effect of contrast, to find your needle position, you are left mostly with moving the needle slightly and watching on ultrasound for the tissue to move, or to inject a small amount of saline solution to see the change in tissue density resulting from the introduction of an echo-lucent substance to the field of vision (so you would see tissue deformation from the solution bubble moving things and the bubble of solution itself should appear a bit darker).

 

Recognizing Nerves on Ultrasound

Tips for Successful Ultrasound Surveys

  • Use lots of ultrasound gel.
  • Adjust the gain, frequency, and focus on you ultrasound machine so that muscle tissue in the target area is fairly dim, the nerve tissue will be denser than the muscle and should appear brighter as long as the return from the muscle doesn't obscure the reflection of the nerve. Don't let a bright background obscure discrete structures in the scene.
     
  • Nerves generally run along borders of other structures especially between different muscle groups.  Watch for muscle boundaries and junctions.
     
  • When scanning transversely for nerves, slightly changing the angle of the ultrasound probe along any axis may result in a much better quality nerve image.  What seem like minor changes in angle may cause the ultrasound beam to encounter the nerve at a much more advantageous angle for reflection.
     
  • The trunks of the brachial plexus appear hollow on ultrasound (like vessels without flow) TrunkNerveLook when viewed in the interscalene space.  I suppose they contain few if any dividing layers of perineurium and mostly axonic tissue. These nerves take on a characteristic mottled look PeripheralNerveLook as they continue peripherally.
  • The characteristic appearance of a peripheral nerve, in ideal ultrasound cross section, looks like a bundle of drinking straws viewed end-on.PeripheralNerveLook
  • You should have a practiced survey pattern for each nerve using landmarks and borders that you can follow every time.  Survey patterns are suggested in this site for each type of regional block on the corresponding site pages.  Many times more than one survey pattern is suggested for a single block.  This is valuable because there are times when a single survey technique will not work on a given patient’s body habitus especially for a deeper nerve.  We routinely use 2 or 3 different survey techniques for finding the sciatic nerve with a 5-10 Mhz probe, depending on the patient size and muscularity and we almost always are able to visualize and mark the nerves position. (more about this on the Sciatic Page).
  • It's often very easy to lose your orientation when looking for nerves on an ultrasound screen.  Be sure to find your landmarks and keep track of them.  If you get lost, start your survey from the beginning again.  Arrange your supplies and assistants in a way that allows you keep your eyes on the screen continuously while you position the needle.
     
  • If you're seeing too many potential targets on the screen and you can't tell which are nerves and which are other structures, it's helpful sometimes to move the ultrasound probe more quickly back and forth along the suspected track of the nerve.  Observing the moving image can give an almost 3 dimensional sense of the structures involved.  You may notice which of the image targets disappear after a short distance (a fairly sure sign it's not a nerve), or bifurcate (favors the target being a nerve), or just continues on seemingly unchanged (a good sign).
     
  • Tendons and ligaments can resemble nerves on ultrasound.  Both generally appearance much more echodense from more angles than nerves, but that’s hardly a help if there’s only 1 target in your view.  Both tendons and ligaments tend to be located below the level of nerves, closer to the bone.   Moving the limb a little may produce noticeable movement in a tendon on ultrasound (while a nerve would not move much, if at all).  Establishing what plane your looking at by identifying muscles should allow you to rule out ligaments.
     
  • Vascular structures are usually easily distinguished from other targets,  if your ultrasound unit has color doppler it can used to detect flow.  If you have no color flow doppler, hold the probe still and you will see pulsation in arteries, pressing with the probe will cause patent veins to collapse.  
     
  • In the neck and groin you will come across lymph nodes, they are clear (echolucent) in the center.  Lymph nodes usually cause confusion when they are mistaken for a vascular landmark (like the femoral artery or carotid).  Since they are spherical and not tubular, the space they contain will disappear when the probe is moved.  Obviously color flow doppler will show no flow.
     
  • Repetition is the key to comfort with the technique.  Practice by looking at a lot of nerves.  Look in on other people's scans.  Practice on volunteers.

Ultrasound Guidance versus Nerve Stimulator Guidance for Needle Placement for Regional Anesthesia

Nerve Stimulator Settings and Proximity to the Target Nerve(s)

Conventional wisdom goes that if you get a twitch at 0.5mA, you're close enough and you should inject there to avoid an intraneuronal injection with the attendant risk of nerve injury. The obvious truth however is that nerve stimulator settings have no consistent relationship to proximity to the nerve and can be misleading.

Conditions such as the neuropathy associated with diabetes can render the nerve stimulator almost worthless. Other peripheral and more central neuropathic conditions such as toxic neuropathies from chemotherapy or radiation, demyelinating conditions, multiple sclerosis, even peripheral vascular and advanced age can mute the response to the nerve stimulator.  Poor return electrode placement and inconsistent contact in the active electrode can further muddy the issues.

For the last 6 months or so we have been injecting at 0.3mA with no problems. More recently, after reviewing other published techniques 0.2mA has been used as well. Indeed recently injection was performed at 0.02mA in an awake patient for sciatic block  with no problem other than a persistent (more than 24 hours) motor and sensory block with a fairly low volume of local anesthetic.

So is the lesson here, even poor needle placement (not particularly close to the nerve) can produce an adequate block if a large enough volume of local anesthetic is instilled? Probably. It follows then that if the needle placement is good (particularly close to the nerve) and the same volume of local is injected, you may expect a longer block.

Without a way to accurately and consistently place the needle close to the nerve, a given volume of a given concentration of a local anesthetic will always produce unpredictable results. Since the nerve stimulator cannot give consistent, predictable results across a patient population it is not the ideal instrument for placing needles in proximity to nerves and therefore nerve blocks performed using needle placement by this method are doomed to inconsistent results.

 

Modes of failure in ultrasound guided regional nerve block

Imagine this, you've located a nerve on ultrasound, you're staring right at it, you inject an adequate amount of local anesthetic which you see accumulate around the nerve on ultrasound.  And... the block fails.  How can this be?

The most logical answer is... voodoo, mojo, bad vibes, gypsy curse.  Anyway you want to say it.

Then again, it could be caused by injection of the local into an adjacent compartment which is unseen or at least not obvious on ultrasound.  Between the two compartments, the one which holds your target nerve and the one which acts as a drain to another dimension for your local, is a dividing membrane of some kind.  Peripheral nerves generally run along spaces between muscles in proximity to vessels.  Muscular compartments are divided from these potential spaces by fascial membranes.  Even within the potential spaces carrying neurovascular bundles there can be septa forming sub compartments. If the needle placement at injection is on the wrong side of one of these dividers the local anesthetic can be carried away or held separate from the space actually containing the target nerve.  Little if any of the medication actually comes in contact with the nerve and little if any block occurs.

A common mode of failure in interscalene blocks of the brachial plexus are under ultrasound guidance is injection into the body of the anterior or middle scalene muscle.  This can happen easily.  As the needle is positioned between the two scalene muscles the tip of the needle can catch the fascia covering the muscle and further advancement of the needle places the bevel into the muscle body.  Then as injection takes place the solution spreads apart the muscle fascicles mimicking the appearance of injection around the trunks of the brachial plexus.  Here again is seen the importance of not taking your eyes from the ultrasound screen as the injection takes place.  The injection into the interscalene space should clearly show the scalene muscle borders being pushed apart by the solution and the brachial plexus trunks being floated in the expanding space created by the local anesthetic.  The floating fascicular bundles caused by injection into the body of the scalene muscle can be recognized as multiple floating echo-densities arranged in a roughly circular pattern, as well as by the observation that the border of the involved scalene muscle was not pushed away from the interscalene space by the injection, but was instead pushed across the interscalene space by the injection.

In femoral nerve blocks there are many "adjacent compartments" that can funnel off local anesthetic solutions.  The mass of fibers making up the femoral nerve lie in a compartment lateral to that containing the femoral artery.  Placement of the needle too close to the artery may cause injection of the local anesthetic solution into the sheath containing the femoral artery keeping it out of contact with the femoral nerve.  Misplacement of the block needle in other directions around the femoral nerve are easily noted once injection has begun.

Most modes of failure of regional nerve blocks under ultrasound guidance can be avoided by following two simple rules.

 1 -- organize your procedure so that your attention can be focused on your target shown on the ultrasound screen during placement of the needle and the initial injection of the local solution.

 2 -- try not to inject all your local solution in a single needle placement, under ultrasound observation reposition your needle at least twice during intermittent injection.

The second point above brings out an interesting observation on essential differences between ultrasound guided nerve blocks and placement of the needle using either nerve stimulation or parasthesias.  Common wisdom holds that after correctly placing the needle using parasthesias or nerve stimulation, the needle should be held rigidly in place while the injection occurs.  This is a maneuver designed to increase safety by minimizing the chance of needle migration into a vascular space or other dangerous or counterproductive position.  Under ultrasound observation one often notes that rigidly holding a needle in place during injection of a significant volume of solution sometimes results in the displacement of the needle to into a disadvantageous position by deformation of the tissue space in which the needle resides.  In fact, this has been seen so many times in our series it seems clear that an adequate nerve block requires much less volume and was previously assumed. 

In conclusion most failed nerve blocks under ultrasound guidance can be avoided by focusing on your target, and repositioning the needle at least once during injection to create the most advantageous pattern of local anesthetic delivery possible.

 

 

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