Ultrasound Principles cycle Frequency Wavelength Period Velocity

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1 ! Teresa S. Wu, MD, FACEP Director, EM Ultrasound Program & Fellowship Co-Director, Simulation Based Training Program & Fellowship Associate Program Director, EM Residency Program Maricopa Medical Center Associate Professor, Emergency Medicine University of Arizona, College of Medicine-Phoenix!! Ultrasound Principles! Understanding the basic physics of ultrasound will help you make sense of the information provided during your scans! Sound and Wave Theory o Sound is energy that is transmitted through a medium: air, liquid, or solid. o Sound is actually mechanical energy causing molecules to vibrate. o Human hearing detects sound in the frequency range of 20Hz to 20,000Hz. o Ultrasound is defined as sound with a frequency >20,000Hz. o Diagnostic ultrasound uses a frequency range of 1.0MHz to 10MHz.! Definitions/Terminology o In diagnostic ultrasound, sound waves are used to generate an anatomic picture. o Sound waves travel through a medium and transfer energy to that medium o The propagation of sound waves causes periodic changes in the pressure of that medium thereby causing molecular oscillations. o This repetitive periodic oscillation is termed a cycle. o Frequency = cycles/second (Hz) o Wavelength = one complete wave cycle o Period = time between a successive wave peak or trough o Changes in particle density during a cycle leaves areas of high density (compression) and areas of low density (rarefication) o Velocity = frequency x wavelength and remains constant for a given medium o Since velocity is constant, as the frequency increases, the wavelength decreases.

2 Page 2 of 12 Ultrasound Production! The generation of an ultrasonic wave impulse is achieved through the piezoelectric effect.! Piezoelectric effect = phenomena where a crystalline material vibrates at a given frequency when an alternating current is applied.! The vibration causes an expansion and contraction of the material.! When an ultrasound wave hits the crystal, it will cause a vibration and generate an electrical current.! The piezoelectric crystal can act as both a microphone and a speaker (it can receive and generate sound).! Two main types of piezoelectric crystals are found in diagnostic ultrasound transducers: continuous and pulsed-echo.! Continuous mode is generated when a constant current is placed across the crystals. (e.g. Doppler sonography to detect and measure blood flow)! In continuous mode, one crystal generates the signal while another acts as the receiver simultaneously.! With pulsed-echo mode, the crystal generates an ultrasound signal for a given period of time, and then switches over and acts as a receiver for another specified amount of time.! In pulsed ultrasound, only 1% of the entire pulsed cycle is spent on signal generation. The rest of the time, the crystal is receiving signals.! Spatial Pulse Length (SPL) = wavelength x the number of cycles! After the crystal emits a pulsed sound wave, the must listen for the return signal.! The time it takes for the sound to return to the transducer is proportional to the distance the sound must travel.! Things that affect the transmitted sound will result in alteration of the signal received and the subsequent image that is produced. Ultrasound Transmission! Multiple components can affect the quality of the image produced: frequency of the sound, type of transducer, medium, artifacts, etc.! The velocity of ultrasound in most body tissues is approximately 1540 meters/second.

3 Page 3 of 12 ² The actual velocity is based upon the density of the substance and the resulting impedance (or resistance to propagation) of the sound waves. ² As a sound wave crosses from a medium of one impedance to a medium of another impedance, an interface is crossed (e.g. from diaphragm to liver) ² Reflection of the sound occurs at this interface. ² The amount of reflection is dependent upon the acoustic impedance of the object. ² Very dense objects (e.g. bone) have high acoustic impedance and will reflect all of the sound waves thereby leaving an anechoic or echo-free image on the monitor. ² An object of lower impedance (e.g. liver) will reflect a proportion of the sound waves and transmit the remainder. ² The ability to discern an image by ultrasound is dependent upon the difference in acoustic impedance of various body tissues and interfaces. ² Refraction = bending of sound as it crosses an interface of two mediums at an oblique angle. ² Attenuation = diminution of the signal energy as it passes through a medium. ² Scatter = reflection of the sound off objects that are irregular or smaller than the ultrasound beam. ² The transducer will receive the largest proportion of the reflected sound when it is perpendicular to the interface being examined. ² Ultrasound beams hitting an interface at an angle will result in some refraction and loss of returned signal. ² Remember that only the sound that returns back to the transducer produces an image on the screen. ² Try to minimize the amount of scatter, refraction, and attenuation to achieve the best possible image. ² Resolution = minimum reflector separation required to produce separate distinct reflections in a pulse-echo system. ² Axial Resolution = minimum reflector separation along or parallel the sound path, and is dependent upon the SPL of the pulse. ² In high frequency transmission, the SPL pulse is shorter, and consequently, you can resolve objects that are closer together. Therefore, high frequency transducers have better resolution but lower penetration. Conversely, low frequency transducers have less axial resolution, but better tissue penetration. ² Lateral Resolution = minimum reflector separation across, or perpendicular to the beam path, and is dependent upon the beam width.

4 Page 4 of 12! The beam width depends on the focal zone for focusing.! Different transducers allow for varying degrees of control over the focal zone. Ultrasound Transducers! Ultrasound transducers are probes that contain a collection of piezoelectric elements used for ultrasound transmission.! There are two basic ultrasound scanning formats: linear and sector.! Linear format produces a rectangular field of view.! Sector format produces a pie-shaped image.! Linear array transducers have the piezoelectric elements arranged in a linear fashion and therefore produce a rectangular/linear screen image.

5 Page 5 of 12 ² Curvilinear transducers use the same linear sequential activation of piezoelectric elements arranged in a curved scanning surface. ² Phased array transducers group the piezoelectric elements in a smaller scanning surface. The elements are activated with small timing differences, which allows the steering of the ultrasound signal. ² Linear array transducer: superficial structures such as muscles, tendons, nerves, vessels, soft tissue applications, ocular, pneumothorax, testicles, etc. ² Curvilinear transducer: deeper structures and organs such as the liver, gallbladder, kidneys, aorta, IVC, appendix, bladder, uterus, ovaries, etc. ² Phased array transducer: better for getting in between ribs to evaluate the heart, pediatric organs, etc. Ultrasound Image Production ² Each transducer has an indicator marker on one side of the probe. ² This indicator marker corresponds to the orientation marker on the screen. ² When the transducer elements are aligned in the sagittal or cephaladcaudal orientation, this is termed the long-axis or longitudinal view. ² The indicator marker should be pointed towards the patient s head in the longitudinal view. ² If the transducer placed in a transverse fashion, you are obtaining a cross-sectional or transverse view. ² By convention, the indicator marker should be pointed towards the patient s right side in the transverse view.

6 Page 6 of 12! Aligning the ultrasound beam to separate the body into anterior-posterior sections is termed the coronal view. Ultrasound Image Modification! Objects closer to the skin and transducer are termed near-field.! Objects deeper to the surface are termed far-field.! As signal travels through a medium, it can become attenuated, scattered, and reflected, and therefore objects in the far-field are more difficult to visualize.! Power: modifies the amount of energy leaving the transducer.! Gain: modulates the received echo signal. Boosting gain will allow more incoming echoes to be processed and will make the image brighter. The downside is more scatter is generated.! Time Gain Compensation (TGC): allows for the control of echo amplification at various depths. Gain at a particular depth can be altered without changing the overall gain.! B Mode Scanning: brightness mode that provides real-time crosssectional images. Provides 2D structural images using varying shades of gray.! M Mode Scanning: motion mode that takes a unidimensional signal from one line of B Mode scanning and displays it against time.! Reverberation Artifact: occurs when the sound waves are repeatedly reflected between two highly reflective surfaces. Most commonly occurs at interfaces where the acoustic impedance is high.

7 Page 7 of 12! Comet Tail Artifact: occurs when multiple internal reflections resonate in a small highly reflective surface. Usually occur in relatively echo-free areas.! Ring Down Artifact: produced when small structures resonate at the ultrasound frequency and emit sound. Because the sound is emitted after the transducer receives the initial reflection, the system thinks the emitted sound is coming from structures deeper in the body. Appear similar to comet tails.! Shadowing Artifact: caused by partial or total reflection or absorption of the sound energy. A much weaker signal returns from behind a strong reflector or sound-absorbing structure.

8 Page 8 of 12! Posterior Enhancement: the area behind an echo-free or echo-weak structure appears brighter or more echogenic than surrounding structures. This occurs because neighboring signals had to pass through more attenuating structures and will return with weaker echoes.! Edge Artifact: results from the bending of the signal beam as it makes contact with large curved surfaces.! Side Lobe Artifact: refraction artifact that occurs when an off-axis signal interrogates a reflective surface away from the primary beam. As multiple piezoelectric point surfaces generate a pulsed ultrasound signal, multiple secondary wavelets are created. These secondary waves are termed sidelobes and can generate a false image of an interface.

9 Page 9 of 12! Mirror Artifact: occurs when a strong reflector at an angle to the probe causes structures that lie around it to appear as if they lie behind it. The reflected sound waves have a longer travel time and are perceived as an additional anatomic structure duplicated on the other side of the strong reflector.!!!!

10 Page 10 of 12 Basic Knobology The most important buttons on the ultrasound machine: ² On/Off button: self-explanatory ² 2D: This button returns you to your initial screen. Think of this as the home key.

11 Page 11 of 12 ² Arrow button (shaped like an arrow): Pressing this button will bring up an arrow on your screen which you may move using the touchpad. Use this to highlight an area of interest on the screen. ² Calculator: This button enables you to calculate various measurements depending on which type of ultrasound you are doing and what presents you are under. Use this function when you want to do more than just a simple measurement (for example CRL, HC, FL, etc.) ² Caliper: This button brings up a set of calipers and enables you to measure the absolute distance between two objects. The select key is used to toggle between one end of the caliper and the other. The touchpad is used to move each end of the caliper. The green caliper is the active one. ² Color: Enables you to use color and power Doppler to evaluate flow ² Depth: These two buttons adjust how deep into the body you are able to see. Increasing your depth (the bottom button) will show you deeper into the tissue. Decreasing the depth will concentrate your scan on a more shallow area. ² Doppler: Enables you to apply Doppler flow within your image ² Farfield Gain: This knob adjusts the gain in the far (or deep) field. ² Freeze: This key will freeze the image on screen and allow you to take measurements, save an image, etc. The two buttons of either side of the freeze key enable you to scroll forward or backward through your recent images. This is helpful if you saw something but didn t hit freeze in time to catch it. Simply hit freeze and then repeatedly press the button to the left of freeze to scroll backwards until you see the image you want. ² Gain: Think of this as the volume button. If you have a clear image, turning this clockwise will make your image brighter. If you have an image with a lot of static, this will make your image AND the static brighter. ² M-Mode: Activates M-Mode scanning. ² Nearfield Gain: This knob adjusts the gain in the near (or superficial) field. ² Patient: This key brings up your home screen where you may enter patient information and select the type of exam being performed. ² Report: This key brings up your report screen where you may input your findings. ² Review: This key brings up the studies performed recently and enables you to review the saved images. ² Save: This button enables you to save a copy of a still image. ² Save Clip: This button enables you to save a video clip of images. When you press the button, there is a beep preceded by a slight delay. When the beep begins, that is your signal to begin recording. The machine can be adjusted to record any length of clips up to 60 seconds prospectively. ² Setup: This key brings up the master settings for the machine. ² Text: This button on the keyboard will bring up a cursor on your screen which enable you to type text onto your screen. You may use the touchpad to move the cursor over the area of interest.

12 Page 12 of 12 ² Zoom: This button enables you to magnify an area of interest. Press the button once to bring up a green box. Place the box over the area of interest and press zoom again. This will highlight the area you have selected. ² Auto Gain: This button takes you back to factory preset settings. Consider it a general reset button for gain settings.