okay well we’re going to dive right into
ultrasound physics and instrumentation and while this may not be the most
exciting topic to start off with I assure you it is foundational to apply
ultrasound clinically and even though physics is in the title of the
presentation do not let that frighten you in classic med cram style we will
break down the concepts so that they are easy for you to understand in this
course we are going to define ultrasound we will discuss basic ultrasound physics
will touch on ultrasound biosafety and the lraa principle we will discover
different ultrasound transducers and their applications we will look at
ultrasound orientation and common ultrasound terminology will explore
ultrasound neurology or the different knobs and controls that you need to know
on the ultrasound machine we will look at the ultrasound modes will discuss
ultrasound artifacts and we’ll finish up with a step-by-step guide on how to
perform an exam on a patient well let’s start off by defining ultrasound and if
this line represents all of the possible audio frequencies that exist then a
portion of those frequencies are audible or those that can be heard by the human
ear and this is said to be 20 Hertz to 20,000 Hertz well then everything below
the hearing threshold is said to be infra sound or subsonic then everything
above the hearing threshold is said to be ultra sound so ultrasound is simply
sound waves that are above the hearing threshold in diagnostic medical
ultrasound the frequency ranges typically are one to twenty plus
megahertz that are being used before we dive into ultrasound physics I want to
briefly explain how an ultrasound machine works and while you may not be
familiar with an ultrasound machine you’re likely familiar with the speaker
and since they both generate sound wave it provides a good example the speaker
uses electrical energy in the electrical waves that travel through the speaker
cable cause the speaker to pump in and out this generates sound waves in the
air which are then heard by the human ear well similarly the ultrasound
machine uses electrical energy the electrical waves generated by the
ultrasound machine traveled down the transducer cable into the transducer and
the transducer produces sound waves but in this case some of these sound waves
reflect off of the patient’s tissues and are transmitted back to the transducer
the transducer then at this point converts the sound waves that it’s
receiving back into electrical energy which is then processed to form a visual
image on the ultrasound machine so let’s look at this another way
you have your ultrasound transducer in your patient the ultrasound transducer
generates a sound wave it’s transmitted through the patient’s tissues reflected
back to the transducer and then those returning sound waves or those returning
echoes are processed to form a single scan line now there’s a few components
of the scan line that I’d like to discuss in one is depth depth is a
function of distance to the ultrasound machine meaning the longer that it takes
to return to the ultrasound machine means it had to penetrate deeper into
the tissues and the ultrasound machine knows to place those returning echoes
deep in the scan line conversely the more shallow that the ultrasound wave
must penetrate and reflect back the less time it takes to return and therefore
the ultrasound machine knows to place that more shallow on the scan line the
second component that you’ll notice is the brightness and think of the
brightness as the volume of the returning echo if you will or the
intensity of the returning echo the louder the returning echo
the brighter it will appear the quieter the returnee echo the less bright it
will appear so a more intense returning echo will appear brighter in a less
intense returning echo will appear darker
well this same process happens again to form another scan line and this repeats
in a sequential fashion to form a 2d image now on this 2d image you can pick
out the individual scan lines and that’s because it’s an old ultrasound image of
a fetal skull but in modern ultrasound images you will no longer be able to see
the individual scan lines that will appear as a homogeneous 2d image but the
image is still generated in the same fashion so now that we understand how an
ultrasound machine works let’s return to our discussion of ultrasound physics as
we touched on before as the speaker pumps in and out it generates sound
waves well how does it really do this well as it pumps out it compresses air
molecules and these areas of highly compressed air molecules are called
compressions and as it goes back in creates this
vacuum effect and there is areas of low pressure called rare fractions and with
this alternating in-out movement it creates alternating compressions in
rare fractions and for this course we will illustrate compressions as a sound
wave like this in the space in between them as rare fractions so for us to
discuss the characteristics of a sound wave it’s much easier to depict the
compressions as Peaks on a sine wave in the rare fractions as troughs on a sine
wave although we understand that a sound wave is not a sine leaf but the first
characteristic of a sound wave we want to discuss is the wavelength in the
wavelength is the distance between a peak and a peak and a trough in a trough
and similarly the time between a peak and a peak in a trough in a trough is
called a period so wavelength is a function of distance and period as a
function of time the next characteristics will see is the
amplitude an amplitude is the height of the peak of a sound wave or how high
pressure the compression is this is the loudness or in the intensity of the
sound so the higher the amplitude the louder the sound or the more intense the
sound the lower the amplitude the less intense the sound is and then lastly
we’ll look at frequency and frequency is how many sound waves travel past a
certain point in a certain period of time and frequency is measured in Hertz
and Hertz are cycles per second so the more waves that pass a given point in a
second the higher the frequency the less sound waves that pass a given point in a
second the lower the frequency okay well to make sure we understand these
characteristics of sound waves I want to do a few examples so we’re going to
compare these various sound waves and if we compare the top wave to the middle
wave will notice the wavelength of the top wave is longer compared with the
wavelength of the middle wave therefore since the wavelength is longer the
frequency is going to be lower and therefore the pitch of this sound wave
on top will be lower than the middle wave if we look at the amplitude the
amplitude of the top wave compared to the middle wave however is lower
therefore the volume or the amplitude of the top wave will be quieter than the
volume or amplitude of the middle sound wave in the bottom wave has an even
longer wavelength therefore a lower frequency and it also has a lower
amplitude than both the above waves so therefore it will be lower in quieter
and if you look at this you’ll notice a key concept that frequency is inversely
proportional to wavelength meaning the bigger the wavelength the lower the
frequency and the smaller the wavelength the higher the frequency so what does
this have to do with diagnostic medical ultrasound well it primarily boils down
to these concepts of resolution versus penetration now resolution simply is how
pretty your picture is how defined in detail the ultrasound image is
penetration simply is how deep the ultrasound wave is able to penetrate
into the patient’s body and therefore generate an image and these things are
primarily a function of frequency see as you increase the frequency you improve
the resolution and as you decrease the frequency you decrease the resolution or
the resolution gets poorer now the converse is true of penetration as you
increase the frequency you decrease your ability to penetrate into the tissue and
the opposite is true where you decrease your frequency you actually increase
your ability to penetrate into the tissue now if this doesn’t make sense or
seem to abstract let me give you two illustrations what is in photography in
the age of smartphones all having cameras many people know that the
cameras resolution is rated in megapixels so the higher the megapixel
on your camera the better that resolution is or the prettier the
picture will be but as you know if the object you are trying to take a picture
of is a far distance away your resolution decreases you’ve all zoomed
in on your smartphone when the object is far away and the picture gets all grainy
and you can’t see it as well it’s because the better the resolution is
when the object is closer to you or another illustration
be in music you’ve all driven by a stadium concert and when you’re half a
mile a mile away from that concert you can actually hear the bass you don’t
hear any of the rest of the music you just hear those very low frequencies
it’s because those low frequencies those large wavelengths can travel a long
distance they can penetrate very well but they’re not well defined you can’t
hear the detail of the low frequency where if you were to go into that
stadium and put your ear right up against the speaker it would be the high
frequencies that pierce your ear because they’re very well defined but they
cannot travel a far distance all right well that wraps up how an ultrasound
machine works in the basics of ultrasound physics next we will look at
the interactions of ultrasound in tissue

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