Generation of ultrasound

Generation of ultrasound

 In this blog, we shall learn about the basic physic behind the generation of ultrasound. Ultrasound is basically used sound waves for diagnostic purposes.

• The physics behind ultrasound image generation
•  How an ultrasound image is created   
• The impact that different transducers have on image quality
• Use one transducer over another and describe some of the basic users controlled features that can impact image quality

 Ultrasound waves and how they produce an ultrasound image an ultrasound wave is a form of

mechanical energy as it passes through tissue causes compression of the tissue and what we call rarefaction of the tissue which is sort of like expansion of these ultrasound waves that are used in medical imaging.

Occur within a frequency range of about 1 to 20 megahertz and then your average soft tissue

travels at a velocity of 1540 meters per second however that velocity will change based

on the tissue types and that change does contribute to our image production as the waves pass

through the tissue, many different things can happen to those waves but the most important thing is the reflection of the wave off of a tissue interface back to the probe and those reflected waves are what produce your image waves that are not reflected but are scattered refracted or absorbed reduce the signal within your image and create noise and artifacts

The strength of a reflection off of a tissue interface is based on a few different basic factors

one the acoustic impedance or the difference in the way the wave travels through two

different adjacent tissue types the surface of that tissue interface and whether it's very smooth or irregular the size of that tissue interface is it a large uniform interface or a small

interface.

 The orientation of that interface to our probe or to our our insulation so is it oriented

perpendicular to the probe and thereby a strong reflector or is it oriented parallel to our ultrasound wave and therefore a poor reflector specular reflector is a term that is used to describe the optimal reflector it's going to be something very bright in our image, it tends to be smooth with a perpendicular orientation to the sound waves and a high acoustic impedance difference between the two adjacent tissues.

Acoustic impedance is a physical principle that is defined as

                                                     The product of the speed of sound or how quickly our ultrasound wave passes through the tissue and the density of the tissue and it's

interfaces between two tissues of different acoustic impedance that generate

the echoes bone allows sound to pass very rapidly through it soft tissue is a little bit slower fluid a little slower and gas is the slowest and these interfaces particularly the interfaces between different types of soft tissue are going to contribute greatly to our image

 Reflection here we have a smooth surface this smooth surface is at least right under our probe oriented relatively perpendicular to the orientation of our sound waves so when the sound wave comes down it hits that smooth surface and because it is perpendicular at that location, we get a nice strong reflection back to our probe to generate our image

Scattering is best demonstrated on an irregular surface and with an irregular surface

instead of reflecting back to the probe, your waves get redirected away from the probe and since those sound waves do not return to the probe they cannot contribute to our image generation and they result in loss of signal refraction is a slightly more advanced concept but essentially any form of wave whether it's light passing through a prism or sound passing through a tissue interface can undergo refraction if it passes through that interface at

an angle.

We have sound waves passing quickly through tissue and then they hit a tissue that the sound passes more slowly through the waves that hit earlier in the process will begin to slow down

more quickly than the waves that travel further through that fast tissue and that difference between this part of the wave will cause an angle or angulation of the wave and again refraction is going to often reduce the signal intensity of our image

and can result in artifacts absorption is the conversion of that wave which is mechanical energy that is compressing and rare affecting the tissue into thermal energy so that mechanical

energy, as it passes through the tissue, will convert to thermal energy and slightly raise the temperature of that tissue and bone absorbs.

Ultrasound waves are better than soft tissue absorbs ultrasound waves

better than fluid and that absorption of that mechanical energy again reduces the signal

within our image and can contribute to artifacts.

 The ultrasound pulse generated traditionally we use piezoelectric crystals so within each probe there are these specialized crystals when you apply a certain electrical current to the crystal the

crystal will resonate at a specific frequency that resonating crystal creates mechanical energy that is transmitted as an ultrasound wave into the tissue when that mechanical energy or ultrasound wave reflects off of the tissue and comes back to the probe the mechanical energy within the wave distorts the piezoelectric crystal and generates an electric impulse so these crystals are very unique in that they can both resonate when stimulated with electricity and can generate electricity when distorted by mechanical energy image resolution on an ultrasound image is a little bit harder to conceptualize than the resolution of ct images the characteristics of our pulse both.

How many sound waves are being generated and what their wavelength and frequency are

we're going to determine our resolution the axial and lateral resolution are what you typically think of as the resolution of your image so the up and down resolution and the side-to-side resolution with your elevation resolution referring to the thickness of your image

Axial resolution the shorter the wavelength and the higher the frequency the better your axial resolution is so how fine are your pixels right in your superficial to deep orientation

 An ultrasound wave with a large wavelength we are going to have difficulty discriminating between saying line one and line four because the wavelength is greater than the distance between each of these lines if we reduce the wavelength now our wavelength is actually less

than lines the distance between line one and line three so it will be able to discriminate

between line one and line three but it probably will not be able to discriminate between line one and line two as we continue to decrease our wavelength and increase our frequency

now our wavelength is less than the distance between these two interfaces so

A curved probe and instead of like in the diagram where we're focusing

more waves into a smaller space where we're actually directing the waves into a cone or a curved field of view and you can see that we lose resolution as the density of our ultrasound waves

decreases as we move deeper into the tissue so we have a high lateral resolution here

where the number of waves relative to the volume of tissue being imaged is higher

here we're imaging much more tissue with a similar number of waves so the density of the

waves are less and therefore our lateral resolution will be decreased

Elevation resolution is the third type of resolution in our image and you can think of it as

slice thickness or similar to volume averaging so the thicker the slice the more tissue is being averaged together to generate our 2d image the thinner the slice the less tissue is being averaged together to generate a single pixel in our two-dimensional image.