Basics of Radiology and Computed Tomography (CT)

by Hetal Verma, MD

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    00:01 So, I'd like to welcome you to our radiology course.

    00:03 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.

    00:13 We'll talk about some basic things that you'll see throughout the course including the densities that are seen radiographically.

    00:19 And we'll give you an introduction about radiographs and about CTs as well.

    00:23 So radiology was discovered by Wilhelm Roentgen in 1895 in Germany.

    00:28 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.

    00:41 He realized that this was due to a type of ray which he called an x-ray.

    00:46 So there are four basic radiological densities.

    00:50 They are air, fat, fluid, and calcium.

    00:56 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.

    01:01 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.

    01:12 So conventional radiography is also called an x-ray and it's pretty used by ionizing radiation.

    01:19 You see two examples here of a normal chest x-ray and a normal abdominal radiograph or an abdominal x-ray.

    01:26 These are also called plain films.

    01:29 So there are multiple terms that really signify the same thing.

    01:32 A radiograph or a plain film or an x-ray are really all the same thing.

    01:36 There are two basic principles of conventional radiography.

    01:41 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.

    01:52 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.

    02:03 So as you can see here, this area appears very dense.

    02:07 And that's because you have overlapping of multiple objects or a summation of shadows.

    02:12 We have the thoracic spine, we have the heart and we have the mediastinal structures all overlapping with each other here.

    02:19 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.

    02:30 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.

    02:38 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.

    02:48 Orthogonal imaging is actually very important when it comes to radiographs.

    02:54 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.

    03:05 So if you don't perform it in two different views, it's hard for you to identify the location of an abnormality.

    03:11 So let's take a look at an example.

    03:13 Here we see what appears to be a metallic column within the midline of the chest on the single 2D frontal view.

    03:24 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.

    03:34 So what we need is a lateral image to show us the exact position.

    03:39 These two images together will tell us exactly where this object is located.

    03:43 So it helps us identify that this object is located posteriorly and this is actually a spinal stimulator.

    03:49 So it's very important whenever you do any kind of plain film imaging to do at least two different views whenever possible.

    03:56 So how about Computer Tomography or CT? This is again known as a CT scan or a CAT scan.

    04:05 The official term is Computed Tomography.

    04:07 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.

    04:18 Pixels are measured in Hounsfeld units.

    04:22 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.

    04:38 Air has the least Hounsfeld units.

    04:41 It measures about -1000 Hounsfeld units.

    04:44 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.

    04:52 These aren't exact numbers and they can vary a little bit by individual.

    04:55 This is an example of what a CT scanner looks like.

    05:00 So you can see that the patient will lie right here and this table actually goes inside the bore.

    05:06 This rotates and creates an image.

    05:08 CT scans can be evaluated using multiple different window levels.

    05:16 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.

    05:28 However, post processing allows you to evaluate this in multiple other window levels.

    05:33 So we can see here, this is called a lung window because you could see the lungs very well.

    05:37 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.

    05:46 So it involves no additional radiation to the patient.

    05:48 CTs can also be evaluated in multiple planes.

    05:54 So CT images are post processed again into different planes which doesn't require re-acquisition of different images.

    06:01 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.

    06:08 So the images that we were just looking at are axial CT images.

    06:11 Sagittal images actually look at the patient from one side to the other and I'll show you an example.

    06:17 Coronal images or the third different plane is looking at the patient from the front to the back.

    06:23 So let's take a look at an example of sagittal and coronal images.

    06:27 So this here is an example of a sagittal image.

    06:30 You can see that it's looking at the patient from one side to the other.

    06:34 This is anterior and this is actually the sternum.

    06:38 This here is posterior so you can see the thoracic vertebrae and you can see the spinous processes.

    06:46 On this side, we have a coronal image, so looking at the patient from front to back.

    06:51 You can see here both lungs.

    06:54 You can see the heart right here and you can see a portion of the liver.

    07:00 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.

    07:10 So again, it's as if you are looking at the patient and the patient is facing you.

    07:14 CTs can be performed with or without intravenous contrast.

    07:21 Contrast is an iodinated solution and it's administered really for vascular evaluation or any structures that contain vascularity.

    07:30 It's helpful in determining enhancement and differentiating between different adjacent structures.

    07:37 So it's almost like the silhouette sign seen in radiography.

    07:39 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.

    07:47 It's very difficult to identify the different chambers of the heart.

    07:50 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.

    08:10 So intravenous contrast is a low ionic, low osmolar solution.

    08:16 It contains high levels of iodine which is what's absorbed by the x-rays and that's what makes the image appear white.

    08:23 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.

    08:35 Patients may have an allergy to CT contrast and the symptoms can range quite significantly from hives to anaphylaxis.

    08:43 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.

    08:54 Oral contrast is another way of performing a CT and this actually helps distend and define the bowel.

    09:00 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.

    09:21 And this allows the oral solution to get down into the small and large bowel.

    About the Lecture

    The lecture Basics of Radiology and Computed Tomography (CT) by Hetal Verma, MD is from the course Introduction to Imaging. It contains the following chapters:

    • History and Basics of Radiology
    • Computed Tomography

    Included Quiz Questions

    1. Muscle
    2. Fat
    3. Air
    4. Calcium
    1. …when images are created by multiple overlapping tissue densities.
    2. …when edges of an object are seen in the interface with a different density.
    3. …when there is a loss of normal borders between thoracic structures.
    4. …as a crescentic and radiolucent finding due to lung cavity that is filled with air and has a round radiopaque mass.
    5. …due to bronchial wall thickening.
    1. It evaluates a 2-dimensional structure with 3 projections.
    2. It evaluates a 3-dimensional structure with 2-dimensional images.
    3. It helps to identify the exact location of an object in the body.
    4. Common views include antero-posterior and lateral.
    5. Orthogonal imaging with 2 projections is most commonly done whenever possible.
    1. Renal function tests
    2. Liver function tests
    3. Baseline EKG
    4. Chest X-ray
    5. Spirometry
    1. It uses lesser levels of ionizing radiation than radiographs.
    2. A CT image is a matrix of thousands of different pixels.
    3. Hounsfeld units is based on how much x-ray beam is absorbed by the object.
    4. Hounsfled units is a measure of pixels and reflect the density of an object.
    5. Air has the least Hounsfield units of -1000 HU.
    1. Sagittal: looking at the patient from the side.
    2. Coronal: looking at the patient from the side.
    3. Axial: looking at the patient from the front.
    4. Sagittal: looking at the patient from the feet up to the head.
    5. Axial: looking at the patient from the side.
    1. The contrast must be given about 15-30 minutes prior to the exam.
    2. Oral CT contrast helps in the distending and defining the bowel.
    3. Dilute barium sulphate is the most commonly used agent.
    4. 1000-1500 ml of the contrast is given to the patient orally.
    5. The patient should be asked if she/he is allergic to contrast.

    Author of lecture Basics of Radiology and Computed Tomography (CT)

     Hetal Verma, MD

    Hetal Verma, MD

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    By Rafael C. on 14. September 2020 for Basics of Radiology and Computed Tomography (CT)

    Nice lecture very complete and in very litlle time. keep up with the good job

    By Neuer d. on 06. August 2018 for Basics of Radiology and Computed Tomography (CT)

    Good for beginners... Explanation from scratch.. Easy language... Explanation with imagery examples is a highlight

    Very good!
    By Rhayan R. on 12. February 2018 for Basics of Radiology and Computed Tomography (CT)

    Very clear and objective explanation. I wouldn't mind if there were a few more details. :)

    Overal good but a little bit of more detail would be more helpfull!!
    By bnorn l. on 14. September 2017 for Basics of Radiology and Computed Tomography (CT)

    Overal good resource. But i think it could be a little bit more detailed to be really usefull for medical school.