So, I?d like to welcome you to our radiology course.
We?re going to begin this course by discussing some background of radiology
and how it all began and then I?d like to give you
some basics of an introduction to imaging.
We?ll talk about some basic things that you?ll see throughout the course
including the densities that are seen radiographically.
And we'll give you an introduction about radiographs and about CTs as well.
So radiology was discovered by Wilhelm Roentgen in 1895 in Germany.
He was actually studying the effects of a vacuum tube equipment
when an electrical charge is passed through
and he noticed that a cardboard box painted with a fluorescent material
glowed when it was placed adjacent to a cathode ray tube.
He realized that this was due to a type of ray which he called an x-ray.
So there are four basic radiological densities.
They are air, fat, fluid, and calcium.
These are the four that you?ll see over and over again
and all of the different types of imaging modalities that you?ll encounter.
There?s also a 5th type of density which is metallic
and we see this more and more frequently
as patients have multiple metallic objects placed within them
such as prosthesis and valves.
So conventional radiography is also called an x-ray
and it's pretty used by ionizing radiation.
You see two examples here of a normal chest x-ray
and a normal abdominal radiograph or an abdominal x-ray.
These are also called plain films.
So there are multiple terms that really signify the same thing.
A radiograph or a plain film or an x-ray are really all the same thing.
There are two basic principles of conventional radiography.
There?s what?s called summation of shadows
so that means that images are created by multiple overlapping tissue densities
and the density of the object increases as you have more overlap.
There?s also the silhouette sign which will come up over and over again
and that just means that the edges of an object are seen,
it only if it interfaces with an object of a different density.
So as you can see here, this area appears very dense.
And that?s because you have overlapping of multiple objects
or a summation of shadows.
We have the thoracic spine, we have the heart
and we have the mediastinal structures all overlapping with each other here.
You can also see there's actually a little bit difficult to differentiate here
between something that?s going on in the lung
and the right heart border and this is an example of the silhouette sign.
So whatever is going on here which is actually an area of pneumonia
is silhouetting the right heart border so we don?t see it any longer.
If you look at the left heart border that actually interfaces with the normal lung
and so we do see this well because it?s two objects of different densities.
Orthogonal imaging is actually very important when it comes to radiographs.
Radiographs really should be obtained in at least two different projections
whenever possible because what you?re doing is,
you?re looking at a three-dimensional structure
and you?re using a two-dimensional image to take a look at it.
So if you don?t perform it in two different views,
it?s hard for you to identify the location of an abnormality.
So let?s take a look at an example.
Here we see what appears to be a metallic column
within the midline of the chest on the single 2D frontal view.
So where in the body is this located?
Is this anterior to the heart?
Or is this posterior to the heart?
It?s actually very difficult or impossible to say just based on the single 2D image.
So what we need is a lateral image to show us the exact position.
These two images together will tell us exactly where this object is located.
So it helps us identify that this object is located posteriorly
and this is actually a spinal stimulator.
So it?s very important whenever you do any kind of plain film imaging
to do at least two different views whenever possible.
So how about Computer Tomography or CT?
This is again known as a CT scan or a CAT scan.
The official term is Computed Tomography.
This actually uses higher levels of ionizing radiation than a radiograph does
and a CT image is created and is actually a matrix
made up of thousands of different pixels.
Pixels are measured in Hounsfeld units.
So this is again an important concept that may come up over and over again
with CT scans. Hounsfeld units are based on how much x-ray beam
is actually absorbed by the object
and it reflects the density of that object
so it ranges from a scale of -1000 to +1000.
Air has the least Hounsfeld units.
It measures about -1000 Hounsfeld units.
Bone is close to one of the highest Hounsfeld units
so it measures anywhere between 4 and 600 Hounsfeld units
and again, this is somewhat of a range.
These aren?t exact numbers and they can vary a little bit by individual.
This is an example of what a CT scanner looks like.
So you can see that the patient will lie right here
and this table actually goes inside the bore.
This rotates and creates an image.
CT scans can be evaluated using multiple different window levels.
So CT scans are obtained in just one projection
so we have an axial image right here which is a single slice
cut through the body and it?s performed only in one set of windows.
However, post processing allows you to evaluate this
in multiple other window levels.
So we can see here, this is called a lung window
because you could see the lungs very well.
This one here is called a bone window because you can see the bones very well
and this is all done actually by the radiologist at their own work station.
So it involves no additional radiation to the patient.
CTs can also be evaluated in multiple planes.
So CT images are post processed again into different planes
which doesn?t require re-acquisition of different images.
So the axial is the plane that is most commonly done
and that is looking at the patient from their feet up to their head.
So the images that we were just looking at are axial CT images.
Sagittal images actually look at the patient from one side to the other
and I?ll show you an example.
Coronal images or the third different plane is looking at the patient
from the front to the back.
So let?s take a look at an example of sagittal and coronal images.
So this here is an example of a sagittal image.
You can see that it?s looking at the patient from one side to the other.
This is anterior and this is actually the sternum.
This here is posterior so you can see the thoracic vertebrae
and you can see the spinous processes.
On this side, we have a coronal image,
so looking at the patient from front to back.
You can see here both lungs.
You can see the heart right here and you can see a portion of the liver.
By convention, this side of the CT scan is always the right side of the patient
and this side is always the left side of the patient.
So again, it?s as if you are looking at the patient and the patient is facing you.
CTs can be performed with or without intravenous contrast.
Contrast is an iodinated solution and it?s administered really for vascular evaluation
or any structures that contain vascularity.
It?s helpful in determining enhancement and differentiating
between different adjacent structures.
So it?s almost like the silhouette sign seen in radiography.
If you take a look at this non-contrast image of the chest,
you can see that the heart is pretty much of equal density all throughout.
It?s very difficult to identify the different chambers of the heart.
However, if you look at this contrast-enhanced examination right adjacent to it
at the same level, you can see that now we can identify the different chambers
of the heart very easily because in between the chambers of the heart,
you have the walls of each chamber which help you differentiate
between a contrast-filled structure and a structure that doesn?t filled with contrast.
So intravenous contrast is a low ionic, low osmolar solution.
It contains high levels of iodine which is what?s absorbed by the x-rays
and that?s what makes the image appear white.
It?s excreted by the kidneys and because of that, there?s a contraindication in patients
who have renal failure because it can cause acute tubular necrosis
which may or may not be reversible.
Patients may have an allergy to CT contrast
and the symptoms can range quite significantly from hives to anaphylaxis.
So if a patient reports an allergy to CT contrast,
then CT contrast cannot be administered in that patient
usually even with medications on board, depending on the level of allergy
that they?ve had in the past.
Oral contrast is another way of performing a CT
and this actually helps distend and define the bowel.
What?s most commonly used is dilute barium sulfate
and the patient drinks the solution approximately 1000-1500 mL of it orally
and it?s usually performed about, it?s usually given about 60-120 minutes
prior to the examination. So the patient drinks the solution
and then they wait for approximately 2 hours or so
and then the exam is performed.
And this allows the oral solution to get down into the small and large bowel.