Atlas of Pain Medicine Procedures 1st Edition

SECTION I

BASIC APPLICATIONS

CHAPTER 3

Ultrasound Guidance for Interventional Pain Management

Hariharan Shankar and Kanishka Rajput

INTRODUCTION

Lazzaro Spallanzani is credited with the initial discovery of ultrasound (US) navigation by bats in 1790. But it was Pierre Curie’s invention of the US-generating piezoelectric crystal that heralded further development of US technology. Following its utilization in medical imaging and guidance, US imaging has seen tremendous progress in its applications and technology in the last 5 decades. Recent years have seen a surge in the use of US for diagnosis and therapeutic interventions in regional anesthesia. US is now making progress in the field of pain medicine because of its utility both in the diagnosis of several nerve, muscle, and joint pathologies and for the ability to see the target and the needle for injection of therapeutic substances (Table 3-1).

TABLE 3-1. Advantages of Ultrasound Guidance

• Watch real time injectate spread
• Avoid structures, eg, pleura
• Decrease injectate volume
• Lack of radiation
• May avoid multiple passes
• Detect pathology in the target area
• Avoid painful muscle contractions 2° to stimulation
• Good educational tool on clinical anatomy

Following its initial use in regional anesthesia, US was quickly adopted for a variety of pain medicine interventions (Table 3-2). Many feasibility studies have been published that attest to its safety and convenience. Studies have also documented elimination of radiation exposure secondary to the use of US imaging for pain medicine interventions.

TABLE 3-2. List of Common Procedures Performed in Pain Medicine Where US Guidance is Utilized

• Musculoskeletal

Joint injections

Bursa injections

Trigger point injections

Piriformis injections

Tendon injections

Plantar fasciitis injections

• Neuraxial

Preprocedural scanning

Caudal epidural

Facet and medial branch injections

Spinal root injections

• Sympathetic blocks

Stellate ganglion block

Celiac plexus block

Hypogastric plexus block

Ganglion impar

• Peripheral nerve injections

BASIC PHYSICS OF ULTRASOUND

• Sound waves at frequencies greater than 20 kHz are called US.
• Medical US uses frequencies in the range of 2 to 20 MHz.
• When an electrical current is passed through piezoelectric crystals, they vibrate and produce ultrasound waves.
• The US waves travel through tissues at a velocity that depends on the tissue with an average velocity of 1540 m/s assumed for all biologic tissues.
• This velocity is utilized to calculate the depth (the time from pulse generation to detection times half the velocity).
• The distance from one crest of the waveform to the next is the wavelength.

Attenuation

• As the waves travel through tissues, there is a loss in intensity or attenuation. This is due to the wave-induced motion of the tissues, absorption, reflection, and scattering. Attenuation is directly proportional to the frequency and the length of the path. This is described in decibels per centimeter of tissue traversed per megahertz, and they range from 0.3 to 0.8 dB/cm/MHz for most tissues. Attenuation results in conversion of the mechanical energy of the waveform into thermal and nonthermal energy.

Acoustic Impedance

• Acoustic impedance between tissues, which is density times the average velocity, determines the amount of reflection. Increasing impedance difference increases the intensity of reflection with no echoes occurring with identical impedances. Impedance matching between the transducers and skin is improved by applying a liberal amount of a water-soluble gel. In addition, the face of the transducer is coated with a quarter-wave matching layer to decrease the impedance difference.

PRINCIPLES OF ULTRASOUND

• Modern US transducers have arrays of piezoelectric crystals, which are electrically excited in small successive groups to create a sweeping effect of the beam.
• The transducers serve the dual function of generating and receiving signals reflected back from the tissues.
• US used in medical imaging is delivered in pulses and uses a brightness mode for display.
• The image is then displayed on the screen in shades from black to white.
• When the waves are completely reflected by a tissue structure they appear white or hyperechoic, and when none is reflected, they appear black or anechoic on the display. Bone and fascia are hyperechoic, while blood vessels are anechoic.
• Nerves and muscles have hyperechoic structures in a bed of hypoechogenicity to anechogenicity creating a stippled or starry sky appearance.
• Only a small percentage of the waves are returned back to the transducer, with the majority either travelling further into the tissues or scattered because of refraction.
• The US waves move through tissues and create rarefaction and compression.

Two distinct patterns of reflection give rise to the echoes that make up an US image—specular reflection and scattering.

1. Specular reflection. It is responsible for the bright appearance of fibrous structures such as tendons.
2. Scattering. It gives rise to the characteristic texture (echo texture) of the image seen within soft tissue in a manner that loosely resembles the waves created by a pebble dropped into a pond.

The ultrasonic beam in modern machines scans the tissues by electronic control such that each element is excited with a time delay, creating a sweeping motion for the image. Beam focusing is achieved by the design of the transducer: lenses placed in the front of the transducer or with the use of phased array.

THE PORTABLE ULTRASOUND MACHINE

The modern portable US machine has made it easy to perform bedside evaluations and interventions. The transducers, the most critical components of the US machine, contain the piezoelectric crystals and are responsible for the transmission and receiving of the US waves. The electronic circuitry of the central processing unit and the image display screen form the other major components. The image system has user interfaces including a computer keyboard to enter information and buttons, knobs, and sliders to control the various operations (Figure 3-1). Some newer machines have touch screen capabilities for adjusting the various parameters.

Figure 3-1. (A) and (B) Key pads and other control knobs of two portable ultrasound machines.

Transducers

• Transducers are manufactured in many different shapes and sizes.
• The commonly used transducers in pain medicine are the linear array and curved array transducers (Figure 3-2).
• The frequencies used for medical imaging are generally in the range of 1 to 18 MHz.
• Transducers have different frequency ranges to provide versatility in imaging at different depths and are centered around their resonant frequency.
• Based on the location of the target, a suitable frequency range transducer is selected.
• The lower-frequency transducers are optimal for viewing deeper structures, and the higher frequencies are used for more superficial structures.
• The choice of frequency is a trade-off between spatial resolution of the image and imaging depth: the deeper the penetration, the less the resolution.
• The frequency of the selected transducer may be further adjusted using a dial.

Figure 3-2. Transducers used in pain medicine interventions. (A) Curved array transducer producing lower ultrasound frequencies for use in imaging deeper structures. (B) Linear array transducer for use with superficial structures. (C) Phased array transducer for use with deeper structures and abdomen. (D) “Hockey stick” transducer for use with superficial structures and the small foot print facilitates use in narrow areas.

IMAGE OPTIMIZATION FUNCTIONS

Some of the important adjustable functions on the key pad are depth, gain, focus, and zoom. Most portable machines also provide color flow Doppler and power Doppler features with the ability to steer the beam. More recently, touch screen capabilities have also been introduced. Newer machines have dynamic US image optimization technology (Native Tissue Equalization [NTEQ]) that adjusts the gain and other parameters while scanning, thus optimizing in real time. The computer is programmed to perform advanced real-time motion analysis, in addition to accurately detecting and differentiating noise and artifacts from soft tissue.

Depth

• Depth is adjusted to optimally visualize the structure of interest. It should be adjusted to approximately 1 cm greater than the target (Figure 3-3).
• Increasing the depth reduces the scale and also slows the frame rate, as each line of the image takes slightly longer to acquire.

Figure 3-3. Setting an optimal depth. (A) The lower portion of this 3-dimensional image does not contribute any useful information to the image and hence the depth setting should have been set as in 3B, and (B) correct depth setting.

Gain

• Gain compensates for attenuation as sound travels deep into the body.
• The intensity of the returning signals is amplified by the receiver upon arrival so that the displayed image is brighter.
• Gain can be adjusted for the entire field or specifically for the near or far fields.
• Excessive increase in gain adds “noise” to the image (Figure 3-4).
• Gain can also be adjusted selectively at different depths by Time Gain Compensation or TGC where up to 10 separate depth adjustments are provided to adjust the gain at a particular depth.

Figure 3-4. Ultrasound images showing different gain settings. (A) High gain showing structures as very bright. (B) Low gain preventing visualization of most of the tissue. (C) Optimal gain setting allowing proper visualization.

Focus

• The pulse of US can be manipulated to be at its narrowest at a particular depth.
• This means that image quality, including lateral resolution, is maximized at that level.
• This can be manually adjusted so that a particular area can be examined with clarity.
• Selecting more than one focus level can significantly decrease the frame rate.

Zoom

• This feature magnifies a portion of the screen. This can be performed while scanning or after freezing the image.
• For superficial structures, it is easier to obtain magnification just by reducing the depth of the image.
• During scanning, orientation becomes difficult if zoom is used.

Tissue Harmonic Imaging

• As US waves pass through tissues, they get distorted by nonlinear propagation. This creates harmonic waves of the initial frequency.
• Out of the many harmonic frequencies, the second harmonic frequency provides better resolution and suffers from less distortion.
• The fundamental frequency of the received beam is filtered out, and only the higher harmonics are retained.
• Tissue harmonic imaging utilizes the second harmonic frequency of the received signals, thus avoiding clutter and artifacts.

Doppler Functions

• US machines use the Doppler principle extensively to measure flow, range, and velocity.
• Color flow imaging provides real-time blood velocity and direction in color. It is superimposed on the gray scale image with red, denoting flow toward the transducer, and blue, away from the transducer (Figure 3-5).

Figure 3-5. Ultrasound image showing cross-sectional view at the level of carotid and internal jugular vein in the neck. The use of color flow Doppler showing flow toward the transducer as red and away from the transducer as blue.

• As the received signal is angle dependent, many machines offer steering capabilities.
• Power Doppler is more sensitive to flow, but does not provide velocity direction. Small vessels can be adequately imaged with power Doppler, displaying the signal power over the gray scale B mode image.
• It is prudent to use Doppler to identify vessels in the vicinity of the target during a preliminary scout scan prior to any intervention.

Compound Spatial Imaging

• Compound imaging is a broad bandwidth technology that combines multiple coplanar images captured from different beam angles and from multiple US frequency spectra to form a single image in real time.
• This eliminates edge-shadowing effect, reduces speckle artifacts, and improves contrast resolution.

NEEDLE APPROACHES

There are 2 methods of orienting the needle relative to the US beam: the in-plane and out-of-plane approaches (Figure 3-6).

Figure 3-6. Commonly used techniques for needle orientation with regard to the transducer. (A) In plane technique with the needle being advanced from one end of the transducer. (B) Corresponding ultrasound image of in-plane technique. (C) Out-of-plane technique with the needle being introduced from the side of the transducer. (D) The corresponding ultrasound image of the out-of-plane technique showing the needle in cross section as a bright hyperechoic dot.

In-Plane Needle Approach

• The needle is inserted in the same plane as the US beam.
• The image captured will be a long axis view.
• Needle-beam alignment is critical to visualize the shaft of the needle in the in-plane approach.

Out-of-Plane Needle Approach

• The longitudinal axis of the needle is inserted in a plane perpendicular to that of the US beam.
• Visualizing the needle tip in this approach can be challenging, as only a cross-sectional area of the needle is imaged; in addition, the tip and the shaft have similar appearances.

NEEDLE VISIBILITY

It is critical to the successful performance of an intervention to be able to continuously visualize the needle shaft and tip. This is facilitated by the use of various techniques and devices. Needles used in US guided procedures are available in different sizes and materials. Larger needles may be better visualized.

• The acoustic impedance difference between the tissues and the needle aid visualization: the greater the difference, the better the visibility.
• Visibility of the needle tip has been shown to be better at more shallow rather than deeper needle-beam angles.
• One of the most commonly performed techniques in difficult situations is the injection of a small volume of 5% dextrose or saline, a technique described ashydrolocalization.
• When the needle tip is not visualized, further advancement should not be performed.

Troubleshooting if the Needle Is Not Visualized

• Exclude gross misalignment by visually inspecting needle and transducer position.
• The transducer should then be adjusted, using three basic movements (sliding, tilting, and rotating) until the needle shaft and tip have been brought back into view (Table 3-3).
• The transducer and needle should not be moved together, as this makes the task more difficult.
• A needle-to-beam angle of about 90° offers the best needle visibility when approaching in plane.
• When the target is deeper, a “heel-in” maneuver may be helpful. This technique involves pressing one end (the “heel”) of the transducer more deeply into the patient than the other end (the “toe”), thus increasing the needle-to-beam angle.

TABLE 3-3. Technique for Ultrasound Image Optimization During Scanning

Scanning PART

• Pressure—applying pressure with the transducer to see compressibility
• Alignment—sliding to trace the course lengthwise
• Rotation—aligning target to beam by turning clockwise and counterclockwise
• Tilting—optimizing the angle of incidence and maximizing the beam signal

SONOPATHOLOGY

This topic is slowly gathering interest among pain practitioners. Ultrasound may also be used to diagnose abnormal anatomy and fluid collections without extensive training on the part of the sonographer (Figure 3-7). Its use in musculoskeletal injuries is more complex because of the possibility of artifacts based on the angle of the US beam. Nevertheless, the use of portable US diagnostic imaging affords the ability to narrow down the differential diagnosis and facilitates management.

Figure 3-7. Ultrasound image of an intrathecal pump seroma with the pump creating a mirror image of the seroma beneath the pump’s surface.

LIMITATIONS OF ULTRASOUND IMAGING

• The major limitation of US imaging is the inability of US waves to penetrate through bone (Figure 3-8). This creates an acoustic shadow preventing visualization of structures beneath it.
• Air attenuates the US wave significantly and prevents visualization, especially around the intestinal loops or lung.
• As most commonly used, needles are not adequately visualized by US imaging, with a potential for intravascular and intraneuronal injection.
• US contrast agents have not gained widespread acceptance for regional anesthesia and pain interventions. Hence the reliance on techniques like “hydrolocalization” to ensure delivery to the intended target.

Figure 3-8. Ultrasound image showing an acoustic shadow created by bone, eg, rib, and loss of visualization through air in lungs. Structures beneath bone cannot be visualized.

COMMON ARTIFACTS

• Enhancement. Fluid, as it is less attenuating, permits US waves to pass through with little distortion. The area beneath the fluid column will, therefore, appear brighter (Figure 3-9).
• Anisotropy. The transducer will only receive the reflected sound if the beam strikes the surface at a right angle. Any change in the angle of insonationalters the echogenicity. This effect is called anisotropy and is commonly used to differentiate between tendons and nerves as tendons are more anisotropic.
• Mirror images. A large difference in acoustic impedance can reflect an image to create a mirror image beneath the hyperechoic tissue (refer toFigure 3-7).
• Reverberation. A superficial strong reflector creates reverberation of the sound signal. This creates evenly spaced hyperechoic lines at increasing depths (Figure 3-10).
• Comet tail. Similar to reverberation, hyperechoic lines are seen deeper to the highly reflective structure, which gradually peters off (Figure 3-11).

Figure 3-9. Ultrasound image showing enhancement (increased brightness) beneath fluid filled structures, eg, carotid artery and internal jugular vein.

Figure 3-10. Reverberation artifact of the needle over a hyperechoic surface during aspiration.

Figure 3-11. Comet tail artifact created by the pleural lining with the hyperechoic lines gradually diminishing in size.

ERGONOMICS DURING ULTRASOUND IMAGING

Proper attention to body ergonomics of both the patient and the provider avoids injury. The patient should be resting comfortably in a bed. The operator should be seated comfortably on the side of the procedure, and the machine image display should be located on the contralateral side at an eye level to avoid neck strain. The transducer head should be held comfortably in the palm of the nondependent hand, and contact with the skin of the patient with the operator’s hand should be maintained at all times to avoid inadvertent movement of the transducer. The hand should be resting comfortably without extension at the wrist. The needle should be held in the dominant hand.

ADVANCES IN ULTRASONOGRAPHY

Capabilities for three-dimensional (3D) US images are available in most machines. Their utility in pain interventions is yet to be realized. One of the major difficulties is the size of the transducer. Until the recent development of the matrix array transducers, 3D technology rested on mechanically steered transducers. The target area is scanned for a predetermined duration, usually in seconds. The computer then collates all the images and renders the final 3D image (Figure 3-12). This can be further manipulated, trimmed, and edited using manufacturer’s software. Four-dimensional US transducers afford real-time 3D images by incorporating time as the fourth dimension.

Figure 3-12. Ultrasound image displayed during rendering of a 3D image by the computer program.

Mechanical and adjustable needle guides have been developed—with their own advantages and limitations. Despite facilitating needle and transducer alignment for better visualization, they hinder finer adjustments during target localization. Optical needle guides have a laser-sighting apparatus that facilitates in plane needle-beam alignment. However, a portion of the needle shaft has to protrude from the skin surface at all times to allow alignment with the laser. This may require the use of longer needles making manipulation technically challenging. By embedding a piezoelectric crystal at the needle tip, the needle tip may be visualized by stimulating it while using the Doppler function. In order to improve the visibility of needles, echogenic needles have been manufactured with indentations, and polymer encasement with a bubbling agent to facilitate needle visualization or nanoparticles (Figure 3-13). More recently, GPS technology and electromagnetic tracking have been introduced in some machines, by adding a sensor to the needle tip and the hub, to facilitate tracking and trajectory planning. Newer US machines have introduced automatic needle optimization technology to enhance needle visibility (Figure 3-14).

Figure 3-13. Some echogenic needles provide optimal visualization of needle by a coating of nanoparticles. (A) Long axis view of an echogenic needle. (B) Long axis view of an echogenic needle with an echogenic catheter.

Figure 3-14. Needle visualization technology. (A) Image of a phantom with needle in place but not visualized. (B) Same image with needle visualization technology showing the needle clearly.

RECOMMENDATIONS FOR ULTRASOUND IMAGING GUIDANCE

Recommendations proposed by the American Society of Regional Anesthesia on proper technique of US imaging may be utilized for ultrasound guidance in pain medicine interventions.

• Phantom practice
• Review relevant anatomy
• Start with simple superficial procedures
• Scan wide the area of interest
• Check with color Doppler
• Watch injectate spread

SUMMARY

US imaging, with its unique advantages of real-time imaging with clarity and facilitating needle guidance, is proving to be a useful tool in pain medicine without any radiation exposure. A basic understanding of the US technology improves its proper utilization. Adequate knowledge of anatomy, systematic learning of the use of US machine, and regular practice is likely to enhance the practitioner’ skills.

Suggested Reading

Gibbs V, Cole D, Sassano A, eds. Ultrasound Physics and Technology How, Why and When. Elsevier; 2009.

Kremkau FW, ed. Sonography Principles and Instruments. 8th ed. Saunders; 2011.

Narouze SN, ed. Atlas of Ultrasound Guided Procedures in Interventional Pain Management. Springer; 2010.

CHAPTER 4