This is a basic article for medical students and other non-radiologists
Our medical student radiology curriculum provides links to investigations and core pathology that medical students will encounter in their training as well as pathology that they will be expected to diagnose on initial imaging come graduation.
Chest and abdomen
At graduation (and for final exams) you should be able to look at a chest x-ray and abdominal x-ray and be able to recognize important pathology. Having a standard approach to this is important and we have put together some resources to help with this:
It is important to understand when these tests should be requested and what information you should record on request forms.
You should also have an appreciation that these tests are often only the starting point of radiological investigation. You should have seen examples in clinical practice of ultrasound, CT and MRI in action and have an understanding of when they get used.
Neuroimaging is commonplace and a shift in A&E or the acute receiving unit will not be complete without a CT head request. You should understand the indications for requesting neuroimaging and understand that the information you include in your request will determine the type of test that is performed, e.g. whether contrast is given.
You should be able to look at a CT head and recognize common pathology.
You should be able to look at common appendicular plain films and recognize common fractures and dislocations.
An appreciation of the different imaging modalities, their uses and advantages and disadvantages is essential for medical students.
Plain radiographs, also known as x-rays or plain films, produce two-dimensional images. X-rays are generated by the machine and directed towards the subject (e.g. a wrist or chest). The detector on the other side of the subject is a piece of film or (more commonly) a digital plate. This records the magnitude of x-rays that have managed to make it to the detector and we can thereby infer where x-rays have been attenuated.
X-rays can be used in a wide variety of situations, such as investigating fractures, pneumonia or confirming nasogastric tube position.
They are quick and relatively simple to perform and compared to other imaging modalities, relatively inexpensive. The image is available almost immediately. However, they do make use of ionising radiation and their use is limited to the situations where there is a clinical need because of the risk of cancer induction.
The five basic densities
When x-rays meet the detector and create an image, there are five main densities that can be visualized. They are a direct result of how many x-rays have passed through the subject and arrived at the detector.
If all of the x-rays continue through (e.g. air), that area of the image has little density and is black. If the x-rays are blocked (e.g. bones), that area of the image is very dense and is therefore white. There are five basic densities you should be able to recognize - the differences between them can be subtle and require experience! Try to identify each of the five densities on the attached chest x-ray:
- air: the blackest part of the radiograph. May include areas outside the patient or air within the body (e.g. lungs).
- fat: lighter grey shade compared to air
- soft tissue or fluid: consists of denser organs and fluid within the body. More white than fatty tissue
- bones or calcium: bones are very dense and allow little x-rays to get through them. Calcifications elsewhere (e.g. in arteries) will also appear white.
- metal: extremely dense and white that will not allow any x-rays to pass. Not normally present in the body and may be placed on purpose (e.g. prosthetics, contrast media) or accidentally (e.g. ingested foreign object).
Computed tomography (CT)
CT scans also use x-rays to create a picture. The patient lies down on a table that passes into the CT scanner. A rotating x-ray source and detector spin around the outside of the patient gathering data similar to the plain radiograph above.
Once all the data has been gathered, a computer can build up the data and present it as a series of images. These two-dimensional cross-sectional images can be scrolled and viewed in the axial, sagittal or coronal planes. This also means that overlapping structures are not an issue, as they are in x-rays. Depending on the type of imaging, 3D reconstructions can be created.
As technology and techniques improve, the dose required to perform the scans is decreasing and the speed to acquire the images has decreased also.
The benefits of CT scans are their speed, accuracy and quantity of information. A CT can be taken within minutes of entering an emergency department and can help direct future management of the patient. Some disadvantages of CTs include their cost to purchase and maintain, and the high dose of radiation. Due to the potential effects on a fetus, a CT scan of the body is not usually permitted on pregnant women.
CT images are comprised of pixels or varying density. In the same manner as conventional radiographs, the density of each pixel corresponds to the type of tissue imaged. High density substances absorb more x-rays and appear whiter. Low density substances absorb few x-rays and appear darker.
The density of each pixel is measured in Hounsfield units (HU), where air is assigned -1000 HU, water is 0 HU and bone is around 500 HU. The range of Hounsfield units included in a study is called the window. Windowing is very important in diagnostic images at it allows optimization of the CT to identify different types of pathology - all without having to rescan the patient. A widely used example of this is in chest CTs - where different windows can show the bones, lung fields and mediastinum in detail. This may reveal fractures, emphysema or heart disease respectively. By adjusting the window you can highlight certain fields to maximize the diagnostic power of the CT.
Ultrasound probes produce high-frequency sound waves instead of x-rays to create images. Sound waves travel inside the patient and 'bounce back' off of internal structures such as bone or organs. The relative density of each substance varies and so does how much of the sound is reflected. These reflected waves are read by the same probe and are converted to produce a real-time image on the machine. Tissues are described by their echogenicity, with bone being hyperechoic and white, while fluid is hypoechoic and dark.
A Doppler ultrasound can interpret if an object is moving towards or away from the probe. This is especially useful for imaging blood flow and can determine the velocity and direction of blood in the heart or blood vessels. US can also be applied to increase the accuracy of biopsies (e.g. breast or thyroid mass) or can be used internally in transvaginal or transesophageal studies.
US is widely available and has advantages of being safe, inexpensive and portable. Since ionising radiation is not used, they are harmless in children and during pregnancy. They are especially good at differentiating between types of soft tissue, such as cystic (fluid-filled) or solid lesions. The main disadvantages are related to operator error and its inability to see past air and bone, as the sound waves are all reflected back and deeper structures cannot be visualized.
Magnetic resonance imaging (MRI)
MRI machines look similar to a CT scanner but utilize strong magnetic fields instead of a rotating x-ray. The physics involved are complicated but in a simplified manner, the magnetic fields cause certain atoms to release radio waves which can be picked up by the scanner. Hydrogen nuclei (comprised of one proton) have a positive electrical charge which makes a very tiny magnetic field. The MRI manipulates these protons to align with its own magnetic field and release energy that can be collected and turned into an image.
These machines are especially useful for visualizing soft tissue in detail. MRI is applied to view diseases in muscles, ligaments, brains, livers, masses and more. Another advantage is their absence of ionising radiation. Disadvantages of MRI include their cost and safety issues. Magnetic fields can manipulate ferromagnetic objects within the patient (e.g. shrapnel) or turn objects outside the patient in the room (e.g. scalpels) into high-velocity projectiles. Many prosthetic devices such as surgical staples or pacemakers are now made MRI compatible.
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