Did you know?

Summer 2014

From CT scans to X-rays and MRIs, you may be familiar with the terms, but do you know how the technology differs? The array of diagnostic medical imaging tests - and what each is used for - can sometimes be confusing.

Dr. Andrea Lum, Chair/Chief of Diagnostic Radiology at London Health Sciences Centre, describes medical imaging technology and illuminates when each test may or may not be useful.

The Department of Diagnostic Radiology at LHSC provides medical diagnostic imaging services and serves both adult and paediatric patients at University and Victoria hopsitals. Through the department's digital technology, imaging can be accessed by all members of LHSC's radiology and clincal care teams.


How it works: An ultrasound machine’s probesends a sound wave into the body that bounces off internal structures and is recaptured by the probe.  The sound (or echo) is then processed through computer technology into a real time-type video during the examination with the ability to freeze frame still images that show different organs. 

Benefits: Ultrasound is very safe, relatively inexpensive, and the equipment is mobile, meaning it can be available in a number of settings, including the patient’s bedside.  

While ultrasounds are known for being used during pregnancy, they are also a useful tool for assessment of organs because they provide a real time look inside the body. Ultrasounds are excellent for distinguishing a fluid (e.g. a cyst) from a solid (e.g. a mass) and to assess movement (e.g. blood flow, bowel contraction). They can also be used to help insert catheters and IV’s, or guide needles when conducting a biopsy (i.e. retrieval of cells).

Considerations: Ultrasound may not be able to “see” through everything, including bone and gas.


How it works: X-rays are a form of electromagnetic radiation. They are projected through the body where tissue will either transmit or absorb them. Dense tissues like bone absorb the rays, whereas the rays pass through less-dense tissues like lungs or bowel. The image is formed in the same way a shadow is created when a hand is held in front of a light. Dense tissues cast a white “shadow” on the negative black image.

X-rays are widely available and usually provide a first look at a range of issues, including broken bones, infections in lungs such as pneumonia, and bowel gas or obstruction. Some soft tissue changes such as swelling can be seen, but subtle differences in soft tissue changes do not show clearly.

Considerations: X-rays carry the risks associated with radiation (see our sidebar on the left to learn more about radiation).  Risk is always weighed against the benefit when X-rays are considered and patients are asked to declare any underlying conditions before any exam.

Computerized Tomography (CT)

How it works: CT scans (or CAT scans) use X-rays. The patient is scanned while lying on a CT scan table, typically on their back.  By using powerful computer programs, images taken during a single scan can be stacked to create additional images. This allows the medical team to see internal structures from many different viewpoints. For exams involving the gut, patients may drink water or a solution containing a contrast agent, which helps show the difference between tissues. Sometimes, a special dye is injected into a vein, so that different organs are better viewed. 

Benefits: CT scans provide high-quality images of bone and lungs, and are able to provide a detailed view of internal organs and abnormalities (e.g. tumours, blood clots, etc.). The injectable dye can also help detect abnormalities or dysfunction, for instance, when the dye doesn’t pass through organs it is a signal that there is an obstruction.

CT scans can target specific areas of the body and process in only about 10-15 minutes, making them very effective in trauma-patient imaging. 

The different points of view provided by a single CT scan give radiologists high-resolution and 3-D images to help with making diagnoses. 

When CT images are superimposed (overlaid) onto other images taken using different technologies, such as MRI images, they help guide therapies (e.g. provide targeted radiation treatments of cancer). 


CT scans deliver a radiation dose that is higher than an X-ray scan, therefore, the risk-versus-benefit tradeoff of each individual patient is carefully considered. 

When dye has been used, there are potential effects to a patient’s kidneys, particularly if kidneys are not functioning well. Patients with kidney issues must undergo a blood test to determine kidney function before the scan.

Magnetic Resonance Imaging (MRI)

How it works: The MRI machine’s superconducting magnet creates a magnetic field that allows the machine to see interactions between molecules of the body.  The interactions are processed, revealing a very detailed image.  

Typically, the exam takes half an hour and the patient must stay very still inside the MRI machine. To help patients, particularly children, relax, they are given video goggles so they can watch a DVD. Similar to a CT scan, a dye can be injected to provide additional detail in the images. 

Benefits: An MRI does not use radiation and its main advantage is that it shows subtle changes in different organs and soft tissues (e.g. brain, breast, prostate, etc.). Radiologists can see fine detail, such as space between discs in the spine and view through bone to the marrow inside.

Considerations: Exposure to the magnetic field can be inappropriate and even dangerous for those patients with implantable metal medical devices such as artificial joints, pins and pacemakers.

Because MRIs may take a full hour to complete, it is a poor choice for trauma patients.

MRIs do not capture the fine detail of bone that a CT scan does.

Also, when a dye is used, patients with reduced kidney function face a risk of damage to soft tissues and skin. Any patient with known kidney disease must complete a blood test to check if the dye can safely be used.

Previous Article
Next Article
LHSC's Dr. Andrea Lum, Chair/Chief of Diagnostic Radiology, reviews a CT scan
An ultrasound works in a similar way as a submarine's sonar, sending sound waves out and 'listening' for the echo
An image taken with a Magnetic Image Resonance (MRI) machine.
X-rays are excellent for viewing bones, infections and bowel gas or obstruction.
CT scans take cross section images and can show internal structures from many different angles.