ULTRASONOGRAPHY IN ANAESTHESIOLOGY

SPEAKER: DR. RAJESH CHOUDHURI,PGT

MODERATOR: DR. B. SUTRADHAR, ASST. PROF

DEPT. OF ANAESTHESIOLOGY, AGMC& GBP HOSPITAL

USG image generation•  Ultrasound waves are generated by piezoelectric crystals.• The piezoelectric crystals convert the mechanical energy of the returning echoes into an electrical current, which is processed by the machine to produce a two-dimensional grayscale image that is seen on the screen.• Audible sounds spread out in all directions, whereas ultrasound beams are well collimated• The machine spend 100x time to listen to returning echos compared to sent ones.

Ultrasound Image Modes• A-mode : the oldest mode(1930), a simple one- dimensional ultrasound image is generated as a series of vertical peaks corresponding to the depth of the structures at which the ultrasound beam encounters different tissues.

•  B Mode : a two dimensional mode and provide a cross sectional image through the area of interest and is the primary mode currently used in regional anesthesia.

• M mode(motion) : one dimensional mode against time mostly used for cardiac imaging.

• B mode image M mode image

• Doppler mode: measures the shift in the frequency between the incident and the reflected wave after hitting a moving object.

• Colour -flow: the probe should be slightly tilted to be near the long axis of the vessel. Blood coming toward the probe appears red and appear blue if it is moving away from the probe , increasing gain improves view for smaller vessels.

• Pulsed doppler : Continuous doppler.

DOPPLER COLOUR FLOW MAPPING

ECHOGENICITY

•Hyperechoic areas have a great amount of energy from returning echoes and are seen as bright &white.

• Hypoechoic areas have less energy from returning echoes and are seen as dark gray and black.

• Anechoic areas without returning echoes are seen as black.

Ultrasound image of various tissues for regional anesthesia• Veins : Anechoic/hypoechoic, non- pulsatile ,compressible,

vary in size with respiration.• Arteries: Round in cross section, pulsatile not easily

compressible.• Fat: Hypoechoic, compressible.• Muscles: Heterogeneous (mixture of hyperechoic lines within a

hypoechoic tissue background)• Tendons & Fascia: Hyperechoic .• Bone : Very hyperechoic with acoustic shadowing behind.• Nerves: Honeycomb structures with hypoechoic fascicles

surrounded by hyperechoic tissues.• Air bubbles: Hyperechoic .• Pleura : Hyperechoic line.• Local anesthetic : Hypoechoic, expanding hypoechoic region.

A sonogram showing a round anechoic artery (A) and an oval shaped anechoic vein (V); a vein is collapsible while an artery is not.

Fat has a hypoechoic background containing streaks of hyperechoic lines that are often irregular in texture and length; the fat layer is most superficial; note the difference in appearance between fat and muscle.

Muscle also has a hypoechoic background containing short streaks of hyperechoic lines; the outline of a muscle layer (the fascial sheath) is highly hyperechoic. Bone often shows a hyperechoic outline (arrows) and a hypoechoic bony shadow underneath due to a lack of beam penetration.

A nerve (arrowhead, N) and a tendon (arrow, T) of the forearm in cross section; the nerve is oval and the tendon has an irregular shape; note that the tendon will merge into a muscle proximally while the nerve does not.

A nerve in longitudinal section (arrowheads) showing internal echotexture consisting of continuous hypoechoic longitudinal elements (fascicle groups) interspersed with hyperechoic perineural connective tissues.

A tendon in longitudinal section (arrows); a tendon appears very much like a nerve in this view although the tendon has a fibrillar internal echotexture and discontinuous hyperechoic speckles.

A hypoechoic nerve root (arrowhead) in the low interscalene region; nerves are generally hypoechoic in the interscalene and supraclavicular regions; the hypoechoic component represents the neural tissue.

Nerves below the clavicle and in the lower limbs are predominantly hyperechoic and have a honey comb appearance. For example, the median nerve in the elbow region (arrowhead) is predominantly hyperechoic. The degree of hyperechogenicity likely reflects the amount of connective tissue within the nerve.

 REFLECTION •  Reflection occurs at the boundary or interface of different types of tissue.

The reflection is inversely related to water content of tissue .

• Substances low in water content or high in materials that are poor sound conductors (e.g., air, bone) reflect almost all the sound and appear very bright.

•  SCATTERED VERSUS SPECULAR REFLECTION

• • In specular reflection incident angle equals the reflected angle & usually the reflection is from smooth surface(e.g. needle ,diaphragm or blood vessels , pleura and peritoneum).

• . Scattering is the redirection of ultrasound in any direction caused by rough surfaces or by heterogeneous media

ULTRASOUND WAVE INTERACTION WITH TISSUES

•Reflection• SPECULAR (large

smooth objects like a needle) (d)• SCATTERING (most

neural images) (a)

•Refraction (c)•Transmission (b)

ATTENUATION

• Attenuation is the loss of mechanical energy of ultrasound.

• About 75% of the attenuation is caused by conversion to heat.

• The amount of attenuation directly related to: 1-distance travelled . 2-wavelength used. 3-attenuation coefficient of tissue.

• Anisotropy :Anisotropy means that the backscatter echoes from a specimen depend on the directional orientation within the sound field. Anisotropy can be quantified by specifying the transducer frequency and the decibel change in backscatter echoes with perpendicular and parallel orientation of the specimen. Tissues like nerves, tendons, and muscle exhibit anisotropy , but most evident with tendon.

Overcoming Loss of Signal from Attenuation

•Artificial Enhancement (Adjusting Gain)• Time Gain Compensation•Adjusts gain independently

at specified depth intervals•Most modern U/S machines

do this automatically (autogain)

• Choosing lower frequencies for deeper tissues (posterior sciatic)

RESOLUTION• Resolution, the ability to distinguish two close objects as separate.

• • Axial resolution : the minimum separation of above-below planes along the beam axis. It is determined by spatial pulse length: Axial resolution = wavelength (λ) × number of cycle per pulse (n) ÷ 2

• Lateral resolution: is a parameter of sharpness to describe the minimum side-by-side distance between two objects. It is determined by both ultrasound frequency(higher frequency decreases beam width so it improves lateral resolution) .Beam width can be determined with focusing the ultrasound beam to the target 

Types of Ultrasound transducer used in regional anesthesia

1-linear: high frequency ,higher resolution used for superficial tissues. 2-curved: low frequency ,lower resolution ,used for deeper tissue

FACTORS DETERMINING RESOLUTION

• 1-Machine Setting:

• Setting Depth •Setting Gain •Setting Focus •Compound Imaging(sonoCT)

• 2-transducer handling: Transducer movement Transducer orientation.

• 3- nerve and needle localization.

Ultrasound machine controls

• • Gain : the wave looses energy and become attenuated deeper in tissue so time gain compensation is required.• • Depth• • Frequency:8-13 Mhz

• For shallow depth<4 cm use frequency>10 MHz. • For deeper structures >4cm preferably use frequency <8 Mhz

All Ultrasound Guided Blocks Involve Three Steps

•Choosing One of Two Imaging Views• Short Axis View• Long Axis View

•Scanning the Nerve Track for Image Optimization•Choosing a Needle Approach Technique• In Plane• Out of Plan

Imaging Plane Options

• Long Axis View or Longitudinal View• Rarely Used• Short Axis View or Transverse View• Most Commonly Used• Relatively Easy• Better Resolution of Fascial Barriers that Surrond Nerves• Dynamic Assessment of Circumferential Local Anesthetic Spread• Workable Image Even with Slight Movement of Transducer Probe

The ART of Scanning• Alignment:

Sliding Movement of Trandsducer Along the Course of the Nerve Lengthwise (Used in Short Axis View).

• Rotation:Clockwise/Counterclockwise Transducer Movement (Particularly

important when attempting a long axis view).

• Tilting:Angling Movement of Transducer to Optimize the Angle of

Incidence (90º).

Where is The Needle Coming From?• Out of Plane Technique: Inserting the needle so that it crosses

the plane of imaging near the target.• In Plane Technique: Inserted within the plane of imaging to

visualize the entire shaft and tip.

• Disadvantages• Out of Plane Technique• Unable to accurately track needle

tip• Difficulty finding the echogenic

“dot” as the needle crosses the US beam.

• In Plane Technique• More Time Consuming• More difficult to perform• False security when partial needle

lineups give appearance of a needle tip

• Can be more painful secondary to longer insertion paths

•Advantages• Out of Plane Technique• Shorter Needle Insertion Paths• Less Patient Discomfort• Easier to Perform

• In Plane Technique• Ability to Track the Needle Tip• Theoretically Safer

American society of Regional Anesthesiologists recommendations for performing an ultrasonography-guided block

• 1. Visualize key landmark structures including muscles, fascia, blood vessels, and bone. • 2. identify the nerves or plexus on short-axis imaging, • with the depth set 1 cm deep to

the target structures. • 3. Confirm normal anatomy or recognize anatomic variation(s). • 4. Plan for the safest and most effective needle approach. • 5. Use the aseptic needle insertion technique. • 6. Follow the needle under real-time visualization as it is advanced toward the target. • 7. Consider a secondary confirmation technique, such as nerve stimulation. • 8. When the needle tip is presumed to be in the correct position, inject a small volume

of a test solution. • 9. Make necessary needle adjustments to obtain optimal perineural spread of local

anesthesia. • 10. Maintain traditional safety guidelines of frequent aspiration, monitoring, patient

response, and assessment of resistance to injection.