Introduction To Ultrasound Physics PDF

Introduction to Ultrasound Physics Ultrasound  Describes sound waves with a frequencies that are higher than the range of human hearing which beyond 20,000 Hertz (Hz).  Sound is emitted from a transducer into the body at one or multiple frequencies.  Sound encounters various organs/tissues within...

Introduction to Ultrasound Physics Ultrasound  Describes sound waves with a frequencies that are higher than the range of human hearing which beyond 20,000 Hertz (Hz).  Sound is emitted from a transducer into the body at one or multiple frequencies.  Sound encounters various organs/tissues within the body.  Images are created when the echoes are reflected back to the transducer. Echolocation Similar phenomenon that occurs within our patients’ bodies when an ultrasound examination is performed. Ultrasound  Frequency –the number of times a cycle or wave is repeated per second.  Expressed as Hertz.  Wavelength - distance traveled by the sound waves and is expressed in millimeters (mm).  Velocity – speed at which sound travels through a medium.  Sound travels fastest in solid objects (closeness of the molecules) and slowest in gases (molecules further apart). Ultrasound Ultrasound Ultrasound  Resolution – being able to detect 2 structures separately located in a parallel beam.  Penetration – how far the sound waves are allowed to travel.  Increased frequency and short wavelength, resolution is increased.  Decreased frequency and longer wavelength, penetration is increased. Ultrasound interaction with tissue Attenuation – sound waves lose strength as they travel through a medium. Absorption – conversion of sound energy to heat. Very low in ultrasound machines. Reflection – sound waves encountering tissues of different acoustic impedance. Only sound waves that get back to the transducer contribute to the image. Scattering – sound waves encounter small and uneven surfaces and would regenerate weak echoes. Parenchymal appearance of organs. Ultrasound interaction with tissue Refraction – bending of the beam when encountering a change in medium when the beam strikes the interface at an oblique angle. As a result the bending of the beam leaves a shadow at the edges of curved structures such as the gall bladder and cysts. Attenuation  Increased with  increased distance from the transducer  More heterogenous medium with increased acoustic impedance mismatch.  Higher frequency transducers. Acoustic Impedance  Product of the tissue density and the velocity of the sound within that tissue.  Changes in impedance from one tissue to the next determines how much sound is reflected back to the transducer and also how much is transmitted to the next tissue.  If large difference, much sound is reflected.  If small difference, little sound is reflected.  If no difference, no sound is reflected. Acoustic Impedance Air and bone have the strongest interface Image display  Based on the pulse echo principle:  Sound is emitted 1% of the time and transducer listens 99% of the time.  Electric signals from returning echoes enhanced to display image.  Transit time directly related to depth.  Amount of reflected sound depends on the tissue impedance.  Ultrasound machine assumes a constant speed in tissue at 1540 m/s. Artifacts  Certain assumptions are made by the ultrasound machine when generating an image:  Sound waves travel in a straight line  All echoes originate from objects in the beam axis.  Echoes return to transducer after single reflection.  Speed of sound in the tissues is constant.  The strength of the echoes is directly related to the reflecting/scattering properties of the objects.  The depth to the reflecting or scattering object is proportional to the round trip time of the sound wave.  The strength of the sound wave is attenuated evenly. Artifacts  In ultrasound ,they can be helpful or confusing.  May be present in ultrasound study.  Acoustic shadowing  Acoustic enhancement  Edge shadowing  Slice thickness artifact  Mirror image artifact Acoustic shadowing Structures of high reflectivity appear white and distal to them a shadow is created. Interface absorbs or reflect all the sound. Acoustic shadow Clean acoustic shadowing Clean shadow is the result of all the sound being absorbed or reflected. No reverberation artifact Anechoic or black Dirty acoustic shadow Tends to happen at the tissue-gas interface eg. Bowel Most of the sound is reflected Shadow is more gray as a result of inhomogeneous/reverberation artifact Dirty acoustic shadow, air in bowel Acoustic enhancement Sound waves are less attenuated when transmitting through the fluid. Machine processing compensates and overcompensate resulting in a hyperchoic area distal to the fluid filled structure. Acoustic enhancement seen distal to urinary bladder Edge Shadowing Small shadows on the edge of rounded structures. Slice thickness artefact The sound beam hits the gall bladder wall and also the bile within. The computer combines the two and makes a false sludge image which is curved. Real sludge usually creates a straight edge. Slice thickness artefact. Mirror Image artefact Mirror image of liver and gall bladder Some of the sound is reflected from the liver back to the diaphragm-lung interface before going to the transducer. Increased time travel, computer places the artificial image distal to original. Transducers  Converts electrical current into sound waves and vice versa.  Piezo electrical crystals.  Emits sound waves less than 1% of the time and receives sound waves about 99% of the time.  Don’t drop the transducer!  Different shapes and sizes – selection depends on properties of the transducer and anatomical region being imaged.  Multifrequency. Transducers A – Linear transducer produces a rectangular image. Emits highest frequency and used for small parts. B – Convex transducer produces fanned out image with a “piece taken out”. General purpose. C – Sector transducer produces a pie shaped image. Echocardiology. A B C Transducers  General – choose the highest frequency that will penetrate the area of the patient during your exam.  Small dogs and cats: 7.5-10 MHz  Medium-sized dogs: 5-7.5 MHz  Large breed dogs: 5 MHz  Large animals: 2-5 MHz  Tendons and small parts, eg. eye: >10 MHz Ultrasound machine Ultrasound machine controls  Power – intensity of the sound output.  Absolute gain – amplification of the returning echoes.  Time gain/depth compensation  Focus  Mode  Measurements  Freeze Power/gain controls Increased gain/power causes increased brightness of the image. Time-Gain compensation (TGC) Can selectively amplify weakened echoes from deeper structures and from the different image fields. Increased far field gain. Focus Sound waves can be focused. Focus can be adjusted on the image manually. This is the area of sharpest sound, therefore one wants to place the focus at the level of the organ that you are scanning. Modes of echo display  B-mode – brightness mode. Echoes are displayed as dots in proportion to the amplitude of the returning echo.  M-mode – motion mode. Used in echocardiography. Records images in respect to time. Modes of echo display B-mode M-mode Doppler mode Measures blood flow velocity within a blood vessel. Colour flow Doppler also measures the direction of the blood flow. Computer assigns colour for direction. Blue – flow away from the transducer Red – flow towards the transducer (BART). Scanning Patient should be fasted. Stress should be avoided. Fur should be shaved Dorsal recumbency Ultrasound machine and examiner on right side of patient. Patient’s head in the direction of the machine. Acoustic gel. Acoustic gel Sound does not travel through air well. Gel provides a medium for the sound waves to travel. Scan planes  Sagittal or dorsal plane  Transverse plane  Can be referred to the organ or patient.  Each organ should be scanned in two planes. Sagittal plane Ventral caudal Cranial Transducer is pointed cranially. Dorsal Transverse plane Ventral Transducer is turned towards the examiner or the patient’s right. Cross-section. Left Right Dorsal Evaluation of structures  Size  Shape  Number  Location  Margination  Echogenicity=Opacity (on a radiograph). Evaluation of structures  In addition:  Homogeneity  Texture  Compressibility  Surrounding tissue  Vascularity  Through-transmission Echogenicity  Anechoic  Hypoechoic  Isoechoic  Hyperechoic  Normoechoic Anechoic Homogeneously black Pure fluids without cellular content. Very low intensity of echoes returning to the transducer. Hypoechoic Relative to other tissues Dark gray tones Low intensity of returning signals. Isoechoic Relative Same echogenicity as another structure. Same echogenicity as another structure Hyperechoic Relative White structures High intensity of signals going back to the transducer Diaphragm is hyperechoic compared to the liver Normoechoic The expected echogenicity for a certain structure. The expected returning signal. Scanning  Pick a starting point and go in a clockwise fashion.  Have to be consistent  Liver  Spleen  Left kidney  Left adrenal  Left ovary in intact females  Urinary bladder  Prostate Scanning  Uterus in intact females  Area of the internal/external iliacs  Right kidney  Right adrenal  Pancreas  Stomach and the GI tract Scanning  My Cat Loves Sunny Places Scanning Hepatic portal vein Scanning Hepatic vein Scanning - spleen Spleen with splenic vein seen at the hilus Scanning – left kidney Sagittal view Transverse view of left kidney Scanning – Urinary bladder Scanning – Intact male prostate Prostate Scanning – Right kidney Scanning - stomach Scanning – small intestines Lumen mucosa submucosa muscularis serosa