Ultrasound Physics Have no fear Presentation by Alexis Palley MD Department of Emergency Medicine Cooper University Hospital

“How does it do that?” Lecture Objectives: Review basic physics vocabulary Explain the principles of sound waves Use ultrasound physics to explain how images are produced Teach how to use these principles to help your diagnostic abilities

Basics Sound is energy traveling though matter as a wave The wave travels by compressing and rarefacting matter Depending on the matter- the wave will travel at different velocities or directions U/S probes emit and receive the energy as waves to form pictures

Physical Principles

Cycle 1 Cycle = 1 repetitive periodic oscillation Cycle VOCAB

Frequency VOCAB # of cycles per second Measured in Hertz (Hz) -Human Hearing 20 - 20,000 Hz -Ultrasound > 20,000 Hz -Diagnostic Ultrasound 2.5 to 10 MHz (this is what we use!) VOCAB

frequency 1 cycle in 1 second = 1Hz 1 second = 1 Hertz VOCAB

High Frequency High frequency (5-10 MHz) greater resolution less penetration Shallow structures vascular, abscess, t/v gyn, testicular

Low Frequency Low frequency (2-3.5 MHz) greater penetration less resolution Deep structures Aorta, t/a gyn, card, gb, renal

Wavelength VOCAB The length of one complete cycle A measurable distance VOCAB

Wavelength Wavelength VOCAB

Amplitude The degree of variance from the norm Amplitude VOCAB

Producing an image Probe emits a sound wave pulse-measures the time from emission to return of the echo Wave travels by displacing matter, expanding and compressing adjacent tissues It generates an ultrasonic wave that is propagated, impeded, reflected, refracted, or attenuated by the tissues it encounters

Producing an image Important concepts in production of an U/S image: Propagation velocity Acoustic impedance Reflection Refraction Attenuation

Propagation Velocity Sound is energy transmitted through a medium- Each medium has a constant velocity of sound (c) Tissue’s resistance to compression density or stiffness Product of frequency (f) and wavelength (λ) c=fλ Frequency and Wavelength therefore are directly proportional- if the frequency increases the wavelength must decrease.

Propagation Velocity Propagation velocity Increased by increasing stiffness Reduced by increasing density Bone: 4,080 m/sec Air: 330 m/sec Soft Tissue Average: 1,540 m/sec

Impedance Acoustic impedance (z) of a material is the product of its density and propagation velocity Z= pc Differences in acoustic impedance create reflective interfaces that echo the u/s waves back at the probe Impedance mismatch = ΔZ

Acoustic Impedance Homogeneous mediums reflect no sound acoustic interfaces create visual boundaries between different tissues. Bone/tissue or air/tissue interfaces with large Δz values reflect almost all the sound Muscle/fat interfaces with smaller Δz values reflect only part of the energy

Refraction A change in direction of the sound wave as it passes from one tissue to a tissue of higher or lower sound velocity U/S scanners assume that an echo returns along a straight path Distorts depth reading by the probe Minimize refraction by scanning perpendicular to the interface that is causing the refraction

Reflection The production of echoes at reflecting interfaces between tissues of differing physical properties. Specular - large smooth surfaces Diffuse – small interfaces or nooks and crannies

Specular Reflection Large smooth interfaces (e.g. diaphragm, bladder wall) reflect sound like a mirror Only the echoes returning to the machine are displayed Specular reflectors will return echoes to the machine only if the sound beam is perpendicular to the interface

Diffuse Reflector Most echoes that are imaged arise from small interfaces within solid organs These interfaces may be smaller than the wavelength of the sound The echoes produced scatter in all directions These echoes form the characteristic pattern of solid organs and other tissues

Reflectors Specular Diffuse

Attenuation The intensity of sound waves diminish as they travel through a medium In ideal systems sound pressure (amplitude) is only reduced by the spreading of waves In real systems some waves are scattered and others are absorbed, or reflected This decrease in intensity (loss of amplitude) is called attenuation.

The Machine

Ultrasound scanners Anatomy of a scanner: Transmitter Transducer Receiver Processor Display Storage

Transmitter a crystal makes energy into sound waves and then receives sound waves and converts to energy This is the Piezoelectric effect u/s machines use time elapsed with a presumed velocity (1,540 m/s) to calculate depth of tissue interface Image accuracy is therefore dependent on accuracy of the presumed velocity.

Transducers Continuous mode Pulsed mode continuous alternating current doppler or theraputic u/s 2 crystals –1 talks, 1 listens Pulsed mode Diagnostic u/s Crystal talks and then listens

Receiver Sound waves hit and make voltage across the crystal- The receiver detects and amplifies these voltages Compensates for attenuation

Signal Amplification TGC (time gain compensation) Gain Manual control Selective enhancement or suppression of sectors of the image enhance deep and suppress superficial *blinders Gain Manual control Affects all parts of the image equally Seen as a change in “brightness” of the images on the entire screen *glasses

Changing the TGC

Changing the Gain

Displays B-mode M-mode Real time gray scale, 2D Flip book- 15-60 images per second M-mode Echo amplitude and position of moving targets Valves, vessels, chambers

“B” Mode

“M” Mode

Image properties Echogenicity- amount of energy reflected back from tissue interface Hyperechoic - greatest intensity - white Anechoic - no signal - black Hypoechoic – Intermediate - shades of gray

Anechoic Hyperechoic Hypoechoic

Image Resolution Image quality is dependent on Axial Resolution Lateral Resolution Focal Zone Probe Selection Frequency Selection Recognition of Artifacts

Axial Resolution Ability to differentiate two objects along the long axis of the ultrasound beam Determined by the pulse length Product of wavelength λ and # of cycles in pulse Decreases as frequency f increases Higher frequencies produce better resolution

Axial Resolution 5 MHz transducer 10 MHz transducer Wavelength 0.308mm Pulse of 3 cycles Pulse length approximately 1mm Maximum resolution distance of two objects = 1 mm 10 MHz transducer Wavelength 0.15mm Pulse of 3 cycles Pulse length approximately 0.5mm Maximum resolution distance of two objects = 0.5mm

Axial Resolution screen body

Lateral Resolution The ultrasound beam is made up of multiple individual beams The individual beams are fused to appear as one beam The distances between the single beams determines the lateral resolution

Lateral resolution Ability to differentiate objects along an axis perpendicular to the ultrasound beam Dependent on the width of the ultrasound beam, which can be controlled by focusing the beam Dependent on the distance between the objects

Lateral Resolution screen body

Focal Zone Objects within the focal zone Objects outside of focal zone

Probe options Linear Array Curved Array

Ultrasound Artifacts Can be falsely interpreted as real pathology May obscure pathology Important to understand and appreciate

Ultrasound Artifacts Acoustic enhancement Acoustic shadowing Lateral cystic shadowing (edge artifact) Wide beam artifact Side lobe artifact Reverberation artifact Gain artifact Contact artifact

Acoustic Enhancement Opposite of acoustic shadowing Better ultrasound transmission allows enhancement of the ultrasound signal distal to that region

Acoustic Enhancement

Acoustic Shadowing Occurs distal to any highly reflective or highly attenuating surface Important diagnostic clue seen in a large number of medical conditions Biliary stones Renal stones Tissue calcifications

Acoustic Shadowing Shadow may be more prominent than the object causing it Failure to visualize the source of a shadow is usually caused by the object being outside the plane of the ultrasound beam

Acoustic Shadowing

Acoustic Shadowing

Lateral Cystic Shadowing A type of refraction artifact Can be falsely interpreted as an acoustic shadow (similar to gallstone)

Lateral Cystic Shadowing X

Beam-Width Artifact Gas bubbles in the duodenum can simulate a gall stone Does not assume a dependent posture Do not conform precisely to the walls of the gallbladder

Beam-Width Artifact Gas in the duodenum simulating stones

Side Lobe Artifact More than one ultrasound beam is generated at the transducer head The beams other than the central axis beam are referred to as side lobes Side lobes are of low intensity

Side Lobe Artifact Occasionally cause artifacts The artifact by be obviated by alternating the angle of the transducer head

Side Lobe Artifact

Reverberation Artifacts Several types Caused by the echo bouncing back and forth between two or more highly reflective surfaces

Reverberation Artifacts On the monitor parallel bands of reverberation echoes are seen This causes a “comet-tail” pattern Common reflective layers Abdominal wall Foreign bodies Gas

Reverberation Artifacts

Reverberation Artifacts

Gain Artifact

Contact artifact Caused by poor probe-patient interface

Take homes!

review u/s uses waves echoed off reflective interfaces to display the structures of the body- works by “talking and listening” Must be careful when interpreting images We have more control than we thought!

Think before you ultrasound!

Think before you ultrasound! Choose the right frequency probe – frequency = shallow & detailed Hold the probe perpendicular to the organ wall being studied Adjust the depth, tgc and gain to help your image Artifact may be affecting your image Use knowledge of physical properties of tissues to help with positioning- ie. use bladder for acoustic window to ta u/s.

Thank you Any questions?