Hello, this is Mahan Mathur,
and this is the third video in our lecture series on introduction to radiology.
In this video, we'll cover ultrasound imaging,
and the objective for this video is to be able to
distinguish solid versus cystic masses on ultrasound.
Like the prior videos,
we'll start off with a brief foray into the historical context of ultrasound imaging,
go over some basic principles.
We'll then move on to basic orientation on ultrasound imaging,
followed by what cystic masses look like and what solid masses look like.
So, ultrasound imaging, as a concept,
has been around for a very long time.
In fact, if you look at bats,
this idea of seeing through sound waves has been found in nature for a long time.
And there were some Italian scientists who discovered that this actually occurs in bats,
which was back in 1794.
They actually blindfolded the bats,
and asked them to go from point A, to point B,
which was a food source, and they found that
the bats could get there with a blindfold on.
It had been used throughout the 20th century for submarine navigation,
and only came to the medical use around the 1950s,
particularly the mid 1950s,
where it was used for obstetrics.
And the reason that it was popular for use in obstetrics is because
unlike CT imaging and conventional radiography,
which uses x-rays, ultrasound uses sound waves.
So, there's no ionizing radiation,
so there's no risk of damage through the sound waves to the patient's DNA.
So how does ultrasound work?
Well, there's some equipment that's needed.
Firstly, you have a transducer,
and there are several different types of transducers,
and they're specific to the type of exam.
Some transducers will emit sound waves that allow you to penetrate deeper tissue,
while some will allow you to penetrate only more superficial tissue.
And depending on what you want to see,
whether it's a thyroid gland, which is superficial,
or some of the abdominal organs,
which are a little bit deeper, you can use that appropriate transducer.
Now, inside this transducer,
there's a special structure made up of piezoelectric crystals.
And what these crystals do,
is that they change shape with electrical impulse.
So, if we give an electrical impulse,
it'll start to vibrate,
get bigger, smaller, bigger,
smaller, and this generates sound waves.
Now, these sound waves go from 2-20 megahertz in frequency.
For a comparison, audible sound is 20-20,000 hertz,
so we don't hear these sound waves.
And as those sound waves go through our body,
they will travel differently in different structures.
At any interface that these sound waves interact with,
the sound waves may be transmitted,
they may be completely reflected or often,
something in between, where something gets reflected, something gets transmitted.
Whatever signal goes back to the transducer,
this deforms the piezoelectric crystal again,
and that results in electrical impulse,
that then gets recorded by a computer.
So, if we were to look at an ultrasound image, basic orientation,
you can either have the probe so that you slice through
the patient in an axial plane or through a sagittal plane,
in which case, this side of the patient will always be either the right side,
or cephalad side, depending on whether it's an axial or a sagittal cut.
While this side of the patient will be the left side,
or the caudal aspect of the patient.
The side over here is always the most superficial aspect,
so this is going to be the skin,
this is where the transducer will be placed.
And as you go deeper, these will be the deeper tissues.
On every ultrasound image,
there is a little marker,
called a probe marker,
that's marked by this small letter P over here.
And this can also be found on the transducer itself,
where there's a little bump that corresponds to where that probe marker should be.
You can see it over here in this linear transducer that's
useful to look at more superficial structures, such as the thyroid gland.
And we can see it here on this curve transducer,
that penetrates into deeper tissues,
and allows us to look at structures like the liver,
the pancreas, the spleen etc.
And so, if we go back to the concept
of the sound waves interacting with different tissues,
as we mentioned, they can be completely transmitted through that tissue,
they could be completely reflected or often, something in between.
Now, some things made up of pure fluid,
it turns out that sound waves will go directly through that fluid filled structure.
As a result, there will be a positive signal that goes back to the transducer,
and that'll appear black,
anechoic, dark on imaging,
as we'll see in the next few slides.
If something is completely reflected back,
as can be seen with the interaction of sound with bones and air interfaces,
well, there's lots of signal,
everything is going to go back to the transducer,
so it'll appear very bright or hyperechoic.
And everything behind that area
will not be seen because there's nothing that's going through,
so everything behind that area that's reflected will be very dark.
And if it goes in between,
most tissues like soft tissues, muscles, and fat.
Well, it can be either different shades of gray,
and it can be very bright or hyperechoic,
or a dark or hypoechoic,
or can look like the surrounding soft tissue,
in which case we call it isoechoic.
So, if you use those two concepts to evaluate this case,
look at these two renal masses.
Which one is cystic and which one is solid? And why do you think so?
Well, as you may have guessed, this structure here is cystic, while this one is solid.
As the sound waves goes through this structure,
nothing gets reflected back from the structure itself,
only from its interfaces,
or its peripheral borders.
Therefore, there is no signal inside this structure,
and therefore, we know that it's something that is cystic.
If we look at this case over here,
as the sound waves go through,
some of them may pass through,
but a lot of them will get also reflected backwards,
resulting in a very complex appearing solid mass in this kidney.
So, here are some ultrasound characteristics of a simple cyst,
and this applies to a cyst in any organ within the body.
It will be anechoic or black.
The walls that we'll see will be extremely smooth,
well circumscribed, you can trace them with a pencil.
The soft tissues immediately behind the cyst
will actually be brighter than the adjacent soft tissues.
And that's this idea of posterior acoustic enhancement.
Because if you have sound waves going through this simple cystic structure,
with nothing going by,
they'll go from the backside of it relatively unperturbed,
as opposed to the adjacent soft tissues where
some of the sound waves will go through, and some of them will go back.
So, the signal coming from there will be less than the signal
coming from immediately behind this simple cyst.
The wall posterior will be extremely sharp as well.
So again, that's a cystic mass,
and that's a solid mass.
How about this case over here.
This is a image taken from the patient's mid-abdomen.
This structure here is actually the pancreas,
that's the gallbladder, there's a portion of the liver we can see over here.
What do you think this structure represents?
If you look at it, as the sound waves go through,
there's a very sharp interface over here.
Behind it, it looks rather dark,
it doesn't look completely black like the simple cyst,
but it looks rather dark,
almost as if no sound waves are getting through the structure over here.
And so, if we look at this, it's certainly not fluid,
because it's not anechoic or completely dark.
If we look at sound waves that come reflected,
there's lots of bright signal,
and behind it there's a positive signal.
And so, it's either bones or air and inside somebody's abdomen,
it's probably going to be the bones,
and this turns out to be the vertebral body,
and that's what bones would look like on ultrasound.
So in summary, unlike conventional radiography and
CT imaging which uses x-rays to generate images,
ultrasound uses sound waves.
There's no ionizing radiation,
so it's a terrific modality to use for young patients,
and pregnant patients to look at the fetuses.
Now, the difference in the speed of sound within different tissues
results in roughly one of three actions.
The sound waves can go through the tissue completely,
in which case that tissue appears anechoic or black.
It can be completely reflected at that tissue interface,
in which that tissue looks hyperechoic or white.
And often, you'll see something in between,
so you see different shades of gray.
We've also covered what a cystic mass and a solid mass looks like on ultrasound.
Cystic masses will have sound waves that go right through them,
so they'll appear anechoic,
their borders will be very sharp,
and behind that cystic mass,
the soft tissues will appear relatively brighter.
