Point-of-Care Ultrasound: An Introduction

Transcription

1 Point-of-Care Ultrasound: An Introduction Delegation Teaching Package for Registered Respiratory Therapists and Anesthesia Assistants Developed by: Rob Bryan RRT, AA Edited by: Kelly Hassall RRT, FCSRT, MEd This document has been developed by the RTSO with assistance from Mackenzie Health and is intended to serve as a sample of a medical delegation learning package. The RTSO and Mackenzie Health are not responsible for any of its content.

2 Objectives: To discuss the theory behind the use of Point of Care (POC) ultrasound To review the different types of probes and their clinical applications To familiarize the bedside clinician with the POC ultrasound unit Functionality of the controls Optimization of views

3 What are Sound Waves? Sound waves are mechanical vibrations and are measured in Hertz (Hz) Period is the time for one cycle to occur (µs) Hertz represents the number of cycles in a second Size of the amplitude is measured in decibels (bd) Wavelength is the distance between two excitations

4 What is Ultrasound? Ultrasound is a frequency above that which humans can hear, or more than 20,000Hz (20kHz) Therapeutic ultrasound is designed to create heat using mechanical sound waves Diagnostic ultrasound is in the millions of hertz and used to visualize structures

5 Point-Of-Care Applications for Ultrasound Point-of-care (POC) ultrasonography is defined as: ultrasonography brought to the patient and performed by the provider in real time Procedural guidance: the use of ultrasound to visualize anatomical structures to improve success and decrease complications of procedures (i.e. arterial line insertions) Diagnostic assessment: the use of ultrasound to diagnose or rule out certain conditions by examining a particular organ, disease or procedure directly relevant to their area of expertise (i.e. pneumothorax) Screening: the use of ultrasound as a means to identify patients at risk for certain diseases (i.e. abdominal aortic aneurysms)

6 Piezoelectric Effect and Ultrasound First described in 1880 by French physicists Pierre and Jacques Currie who observed the generation of an electrical charge as a result of a mechanical force being applied to certain crystals and materials and described it as the Piezoelectric Effect Paul Langevin later developed piezoelectric materials that can generate and receive mechanical vibration at high frequency known as ultrasound

7 How do we provide POC Ultrasound? Ultrasound acoustic waves are created by deformation of piezoelectric crystals or ceramic materials when stimulated by electrical energy producing high frequency mechanical pulses Ultrasound transducers produce and transmit sound waves and receive reflected sound waves or echoes converting them back into electrical energy that can be interpreted by a computer processor to produce an image

8 Principles of Ultrasound Imaging The density (concentration of medium) and stiffness (resistance of a medium to compress) directly influence the speed acoustic waves move through it Media with higher density will transmit mechanical waves with greater speed than lesser dense media Acoustic Impedance is the difficulty of sound moving through a medium as a result of its density, in practical terms penetration decreases as frequency increases Velocity of ultrasound beams are calculated by: Velocity=Wavelength (mm) x Frequency (Hz)

9 Principles of Ultrasound Imaging Acoustic speed of mechanical waves through different media: MEDIA VELOCITY AIR WATER SOFT TISSUE BONE 331 meters/second 1495 meters/second 1540 meters/second 4080 meters/second

10 Interaction of Ultrasound Waves with Tissue Ultrasound will travel a straight path through homogeneous medium The path ultrasound that travels through in a heterogeneous medium will be altered The relationship between ultrasound and tissue can best be described in terms of reflection, scattering, refraction, and attenuation The interaction of ultrasound waves through the media and the direction in which they travel is complex Note: Even the slightest amount of air between the probe and the scan site will severely degrade the image quality always use a coupling medium (acoustic gel) to maintain image quality.

11 Reflection Occurs when the ultrasound beam rebounds off a tissue interface and a certain amount returns back to the transducer The magnitude of the reflected wave is dependent on the acoustic impedance of the tissue The magnitude of the reflection is also dependent on the angle between the ultrasound beam and the tissue with the optimal reflection occurring at 90º perpendicular orientation The greater the acoustic impedance the greater the reflection, if the impedance is equal there will virtually be no echo or no reflection of the different boundaries.

12 Scattering Is the redirection of sound in any direction by a rough surfaces or heterogeneous mediums particularly on surfaces and boundaries that are smaller than the wavelength

13 Refraction Is a change in sound direction (bending) when crossing the boundary between two media

14 Attenuation During transmission ultrasound strength is progressively reduced as it converts from ultrasound energy to heat and absorbed by surrounding tissue Dependent on frequency and wavelength (higher frequencies are absorbed at a greater rate than lower frequencies) Acoustic impedance is also an influencing factor with higher impedance increasing attenuation of the ultrasound beam

15 Ultrasound Image and Resolution Resolution is the ability to distinguish between two closely related structures and varies directly with the frequency and inversely with the wavelength If two structures are closer then one wavelength apart then they will not seen as separate Higher frequency and shorter wavelengths improve resolution but decrease the penetration of the ultrasound beam

16 Aspects of Spatial Resolution Axial resolution is the ability to distinguish between two structures in the same direction of the acoustic wave Lateral resolution is the ability to distinguish between two adjacent structures that are perpendicular to the acoustic wave

17 Ultrasound frequency affecting resolution Resolution can be improved by increasing frequency and reducing beam width

18 Types of transducers Type Picture Image Clinical use Linear Array Phased Array Curved Array Rectangular shade display Covers the wide of the probe Idea for near field scans, nerves, vessels and soft tissue Frequency : 6-13 MHz Scan depth 6 cm Pie shaped tip towards the transducer Better for visualizing deeper structures and Organs Frequency : 1-5 MHz Scan depth 35 cm Combination of linear and phased array Frequency : 2-5 MHz Scan depth 30 cm Superficial tissue and anatomical landmarks, Musculoskeletal, Nerve, Small Parts, Vascular, Venous Abdominal, Cardiology, Obstetrics, Orbital, TCD Deep nerves, neuraxial imaging, Abdominal, Gynecology, Musculoskeletal, Obstetrics

19 Ultrasound Image Modes Electric signals of echoes are amplified and displayed on a monitor Can be displayed as a conventional imaging, compound image, tissue harmonic imaging (THI), and combined compound-thi imaging Images can be displayed in static form (allowing for measuring distance between two interfaces, tissue or anatomic landmarks) or in real time providing temporal relations of these interfaces Ultrasound Modes include: A-Mode, B-mode, Doppler- Mode, and M-Mode

20 Compound Imaging vs Conventional

21 Ultrasound Image Modes A-Mode is the oldest modality and sends a single pulse with vertical deflection into a medium and waits for a return of signal It is a one dimensional image and produces a series of peaks and valleys in relation to when the ultrasound beam reaches different tissues.

22 Ultrasound Image Modes B-Mode or brightness modulation and is the primary mode used in regional anaesthesia It creates a two dimensional image by simultaneously scanning piezoelectric elements compared to one in A-Mode. Converts amplitudes of echoes into various brightness of gray with horizontal and vertical direction representing real distances in tissue

23 Ultrasound Image Modes Doppler-Mode detects changes in frequency of a sound wave resulting from relative motion between a sound source and the receiver. Colour Doppler colour-codes Doppler movement onto the B-Mode and shows the direction of blood flow with red noting blood flowing towards the probe and blue noting blood flowing away from the probe Blue Away Red Towards

24 Ultrasound Image Modes M-Mode or Motion modulation is B-Mode with a continuous update of the returning echoes forming a sequence of B- Mode images that show change over time Useful in cardiology and obstetrics

25 Successful Imaging Optimizing an image is an essential skill in performing ultrasound guided procedures Involves complex understanding of the ultrasound system/equipment and the use of various transducers Requires in depth knowledge of anatomy, anatomical landmarks and location in the body Appreciation for the characteristics and appearance of tissues and structures viewed under ultrasound (specifically hyperechoic, hypoechoic, and honeycomb patterns and properties)

26 Setting Up for Ultrasound Procedures Steps Considerations S Supplies Gather supplies: ultrasound machine, transducer covers, nerve stimulator and needle, tray, LA drawn and labelled, plug in unit to avoid powering down during procedure C Comfortable positioning A Ambiance N Name and procedure N Nominate transducer Patient, ACT, equipment, ultrasound unit should be strategically position to optimize ergonomics and efficiency and infection control Ultrasound should be on the opposite side of the patient with the screen aimed at the operators eye Dim the lights to view the screen better time-out to ensure correct block, correct site and correct patient including any pt considerations and ensure all the equipment is present and functioning before starting Select a transducer that best suits the block or procedure being done (linear transducer for superficial scans and curved array for deeper scans I Infection control Disinfect the skin to reduce the risk of infection and contamination N Note lateral/medial side on screen G Gain depth Touch the side of the transducer to orientate the medial lateral side of the patient corresponds with the image on the screen Use a coupling gel to reduce the potential for poor image quality related to decreased reflection-absorption rates

27 knobology There are many knobs and functions on the modern POC ultrasound unit and can become overwhelming for the end user to navigate Most companies have programmed settings (or presets) and proprietary software to help optimize the view depending on the type of scanning required Each manufacturer has their own twist on standard features and image enhancement tools that can add to the confusion Some features require the end user to scroll to another page to access part of the menu they need to complete a task

28 Five Essential Function Keys in Ultrasound Key Function TIP Depth Frequency Focusing Gain Doppler First consideration in imaging Dependant on the location of the structure and body habitus Optimal depth is important for focusing Higher frequency ultrasound have a higher rate of absorption and attenuate at shallower depths Lower frequency ultrasound travel deeper but sacrifice resolution Lateral resolution can be adjusted by increasing the frequency or focusing the beam width by narrowing at the level of the nerve Is the ration of output to input electrical power Adjustments amplify small voltages received in the transducer into larger allowing for more information processing and storage Overall gain will affect the whole image Time-Gain compensation will affect the near field or far field specifically Used to detect vascular structures or location and spread of local anaesthetics The target structure should be at the centre of the image Choose the transducer with the best frequency range and depth to visualize the target structure Adjust the focus/focal zone at the same level or 0.5 cm below the target structure Too much gain will blur structures into one another, Useful in far field imaging to better identify structures that are damped by attenuation or acoustic impedance Can be decreased if scattering is a concern Power Doppler is more sensitive in identifying vessels

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30 artifact- UUUGGHHH!!

31 Optimal Imaging Depth Field Depth Transducer (frequency) Target Site < 2 cm Linear array (6-13 MHz) Wrist, ankle block cm Linear array (6-13 MHz) Interscalene, axillary brachial plexus Linear array (6-13 MHz) Femoral, superclavicular, abdominus plane block (TAP) cm Linear array (6-13 MHz) or Curved array (2-5 MHz) Infraclavicular, popliteal, subgluteal, sciatic nerve Curved array (2-5 MHz) Pundendal, gluteal sciatic nerve, lumbar plexus >10.0 cm Curved array (2-5 MHz) Anterior approach to sciatic nerve

32 References Moore CL, Copel JA. Point-of-Care Ultrasonography. NEJM [internet] [cited 2018 Oct 20];364(8) Available from: URL NEJM.org The New York School of Regional Anesthesia. Ultrasound Guided Techniques [Internet]. New York: The New York School of Regional Anesthesia;2018 [cited 2018 Oct 21]. Available from: Vincent JL, Abraham E, Moore F, Kochanek P, Fink M. Textbook of Critical Care. 7 th Edition. Canada:Elsevier;