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