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Ultrasound: Fundamental Physics & Knobology Ioannis Gerogiannis MD, PhD, PGDipMedEd, FFMLM, FACS, FICS, FRCS, MFSTEd Consultant General & Emergency Surgeon Specialist Interest in Upper Gastrointestinal, Bariatric & Abdominal Wall Surgery Department of Surgery, Kingston Hospital London (KHFT) Honorary Senior Lecturer - St George's University London (SGUL) College Tutor - Royal College of Surgeons of England (RCSEng) Educational Committee Member - European Society for Trauma & Emergency Surgery (ESTES) Educational Committee Member - European Hernia Society (EHS) ATLS, PHTLS, DSTC, CCrISP, BSS, MUSEC InstructorNo conflict of interest Images, illustrations and short videos are taken from the website POCUS 101 and Glasgow Caledonian UniversityContents • Terms • Physics • Knobology • MCQs • Take home messages • Second presentations with clinical scenariosUltrasound • Waves of sound are sent into the body and ‘bounce’ off structures • The returning echoes are then collected & used to produce an imageSound waves are produced within the transducer Returning sound is captured by (Piezoelectric effect) the transducer, changed to electrical signal and passed to the processing unit for display on the monitor. Sound propagates into tissue when the transducer is applied to the patient. Coupling gel is required to combat air between interfaces.Piezoelectric Effect • for a short period of time.ed through the layer of crystals in the transducer • The shape of the crystal is changed by the current. • As soon as the current is switched off, the crystal snaps back into it's normal shape and produces a sound wave. • To produce continuous scanning, this cycle is repeated.Piezoelectric Effect Electrical charge applied – compression of crystals Electrical charge off – rarefaction of crystals Normal crystal Sound wave emitted Acoustic impedance (z) • Refers to the resistance offered by the tissue to Z= pc the sound wave as it travels through it • Controls how much sound is transmitted forward Original (reflected sound) and how much is reflected back sound wave Interface • Determined by tissue density (p) and sound between tissue velocity (c) within that tissue type (transmitted sound) The amount of sound allowed to pass through a border of two differing tissue types will depend on how closely matched the impedance values are – i.e. the greater the mismatch the less sound throughput Sound propagation • The more similar the neighbouring tissues are, the more sound is transmitted. • This is known as the intensity reflection coefficient (R). • Determines the ratio of energy reflected - where 0 = full transmission and 1 = full reflection. Tissue borders Intensity reflection coefficient Sound reflected - percentage (R) Soft tissue/ water 0.002 0.2 Fat/muscle 0.0108 1.08 Bone/muscle 0.412 41.2 Bone/fat 0.49 49.0 Air/soft tissue 0.999 99.9 Reflection • When a large amount of sound is reflected, the echo will be brighter. It will appear hyper echoic on screen. • Trade-off - there is very little in the way of imaging capability beyond this powerful reflector. • e.g. very bright appearance of gallstones, bones and an acoustic shadow posterior to them Reflection • The strength of the sound reflected back to the transducer will also depend on the type of interface it meets • A smooth surface will return a stronger echo (specular reflection) • An irregular surface will cause the beam to scatter into more, smaller echoes -therefore reducing the overall strength (diffuse reflection) Attenuation • The process by which a sound beam will lose • Reflection it’s intensity • Forming of echo § Absorption • Reflection is one of the main methods of • Transformation of sound energy into heat attenuation energy – stored within tissue • A beam is attenuated on it’s way in to the • Scattering • Form of reflection where the majority of body but also on it’s return path back to the the echoes are not returned to the transducer transducer –spread throughout the organ • Directly proportional to frequency – i.e. higher frequencies attenuate more therefore cannot travel as far. Not generally a problem for superficial MSK scanning.Image generation • Returning echoes are received by the piezoelectric crystal within the transducer • Crystal again is vibrated– this time converts the vibration to an electrical signal • The processing unit within the machine recognises the position of each crystal – i.e. this will allow the computer to plot the echo on the screen depending on where on the transducer it was received and the strength of returning signalImage generation The area of interest lies within the left field. As this is detected by crystals towards the left of the transducer this has been correctly plotted on the screenImage generation • The system must additionally plot the correct depth an echo has returned from • The time taken for a returning echo to be detected will determine how far it has travelled • E.g. a longer time will translate to a greater depth displayed on screenImage generation Time for pulse Time for pulse and receive 2X and receive X seconds secondsImage generation - artefacts • Acoustic shadow • Acoustic enhancement • Reverberation • Edge shadow • Mirror image Image display • Each echo is displayed as a grey scale • Level of grey determined by the amplitude of the echo received • Strong echo = white • No echo = black • Approximately 64 grey scales available on most scanners – can alter manuallyDoppler imaging • Frequency of a sound wave is changed when it encounters a moving object • Increases when object moves closer to the wave • Decreases when object moves further from the wave • Proportional to the velocity of the moving target – Doppler shift Choosing the Right Ultrasound Probe • Choosing based on Application • What application am I using the ultrasound machine for? • How deep are the structures I’m trying to visualize? • How big or small of a footprint do I need? • Does it involve a procedure? • Does it involve a cavity or not? (collection)Ultrasound Probe Frequency Applications Soft tissue, Musculoskeletal, Pediatric, Ocular, Linear 5-15 MHz Trachea, Thyroid, Thoracic, Most Procedures, DVT, Appendicitis, Testicular General Abdominal (Gallbladder, Liver, Curvilinear 2-5 MHz etc), eFAST, renal, aorta, IVC, Bladder, Bowel, OB/Gyn Cardiac, Abdominal, Phased Array/Sector 1-5 MHz eFAST, Renal, Bladder, Bowel, IVC OB/Gyn, Peritonsillar Endocavitary 8-13 MHz AbscessLinear Ultrasound Probe • High-frequency transducer (5-15 MHz) • High resolution • < 8cm Good view • > 8cm won’t be able to see much • Rectangular field of viewCurvilinear Ultrasound Probe • Frequency range of 2-5MHz • Low-frequency probe • Large/wide footprint • Better lateral resolution (compared to the phased array probe) • For abdominal and pelvic ultrasound exams • Can also be used for cardiac and thoracic ultrasound examse.g. Trying to avoid the ribs for the view of the liver and gallbladdere.g. Going from Short to Long Axis of Artery• DVT • Artery vs Vein • Appendicitis (non-compressible)Questions?Let’s listen to the sound… with MCQs An obese patient presents with erythema and pain around his umbilicus – you suspect a collection or a small incancerated umbilical hernia. After clinical examination which was inconclusive you decide to perform an USS. Which probe will you use? a. Linear b. Curvilinear c. Both d. None of them, I will not perform an USS!Let’s listen to the sound… with MCQs anastomosis) complains of abdominal distension and pain. After clinical examination (abdomen soft, distended, tympanic to percussion, no obvious bowel sounds) you decide to perform a USS to check for SB peristalsis or intra-abdominal fluid. Which probe will you use? a. Linear b. Curvilinear c. Both d. None of them, I will not perform an USS!Let’s listen to the sound… with MCQs An 80 yo patient post fall is brought by ambulance, and you decide during the primary survey after ”B” to perform a FAST scan. Which probe will you use? a. Linear b. Curvilinear c. Both d. None of them, I will not perform an USS! I will send for CT asap!Let’s listen to the sound… with MCQs An patient of a BMI of 50 is difficult to cannulate and after several failed attempts you decide to perform an USS guided cannulation. Which probe will you use? a. Linear b. Curvilinear c. Both d. None of them, I will not perform an USS! I will call an anaesthetist!T ake home messages… • USS is sound! If you realise that then the related physics are logical • Understand the USS and its principles before starting performing POCUS • Choose the right probe! Ask the questions before starting • Techniques of manipulation of the prove are crucial • Spend time and perform as many as you can! • To use it, you have to believe it's of value!