Ask Dr. M: What’s the difference between an X-ray, CT scan, ultrasound, and MRI?

Question: I’m confused — what’s the difference between an x-ray, MRI, CT scan, and ultrasound? Don’t they all do the same thing?

These days, there are many ways to medically image your body, inside and out. For centuries, doctors had to guess what was going on beneath the skin. Today, diagnostic imaging plays an important role in maintaining and improving your health by providing a view of the human body not seen by the naked eye, before your doctor ever needs to pick up a scalpel.

Let’s break down the types of imaging technology used today, and why they’re all equally important.

X-Ray

Diagnostic x-ray procedures use radiation in the form of x-rays (a form of electromagnetic radiation) to help diagnose disease and injury. They penetrate body tissues, displaying black and white images that can be seen on a TV monitor. A lot of the time, your x-ray technologist will give you a lead vest to wear, to protect you from over-exposure.

If you’ve ever broken a bone or gone to the dentist, you’ve probably had an x-ray done. Some common types of x-ray procedures include:

  • Bones
  • Teeth
  • Chest (lungs, heart, bones)
  • Abdomen (bowels)
  • Fluoroscopy (continuous x-rays)
  • Mammogram
  • Angiogram (blood flow using contrast dye)
  • Arthrogram (joint movement using contrast dye)
  • Urogram (kidneys, bladder etc.)

Chest x-ray

Ultrasound

Known in healthcare as medical ultrasound, diagnostic sonography or ultrasonography, ultrasound diagnostic imaging uses sound waves to create an image of internal body structures such as tendons, muscles, joints, blood vessels, and organs. These high-energy waves pass through the body and capture shadows and reflections that are seen on a computer screen in real time. Unlike x-rays, there is no radiation used.

While it’s widely known that ultrasound imaging is commonly used in obstetrics to examine pregnant women, the technology is used in many other ways, including:

  • Vascular (arteries or veins)
  • Interventional (biopsy of tissues)
  • General (abdomen, pelvic, breast, groin)
  • Small parts (thyroid, scrotum, lumps, etc.)
  • Musculoskeletal (joints, muscles)
  • Echocardiogram (heart)

Formal ultrasound scans are generally performed by a licensed technician, read by a radiologist, and then discussed with you at your doctor’s appointment. Some doctors also perform “point-of-care” ultrasound scans, which are informal (not always logged or read by a radiologist) but help doctors figure out what might be causing your symptoms and to guide further testing.

Ultrasound of an unborn infant

CT scan

A computed tomography (CT) scan combines many x-ray measurements, taken from different angles, to produce cross-sectional (tomographic) images of specific areas of a scanned object. CT scans allow your doctor to see inside parts of your body like your head or bladder, without having to cut.

Common types of CT scans:

  • Head (brain and its vessels, eyes, inner ears, sinuses)
  • Neck (soft tissues)
  • Chest (heart and lungs)
  • Abdomen/pelvis (gastrointestinal tract, bladder, reproductive systems)
  • Skeletal system (extremities)
  • Spine (cervical, thoracic, lumbar)

Abdominal CT Scan

MRI

Magnetic Resonance Imaging (MRI), like x-rays and CT scans, is a non-invasive imaging technology; unlike x-rays and CT scans, MRI produces three dimensional, detailed anatomical images without the use of radiation. It uses powerful magnetic fields and radio frequency pulses to produce detailed images of organs, soft tissues, bone and other internal body structures. To obtain an MRI image, a patient is placed inside a large magnet and must remain very still during the imaging process in order not to blur the image. Often MRIs image the same body parts as CT scans, but in finer detail to look more closely at certain structures.

Common Types of MRI tests:

  • Visualizing abnormalities in tissues, organs or bones
  • Atherosclerosis screening
  • Masses (e.g. tumours, fat deposits etc.) identification
  • Cancer screening
  • Results analysis

Cranial MRI

Electrocardiogram (ECG)

An Electrocardiogram or ECG is a non-invasive scan used to look at the heart. In this test, thin wires go from a computer or portable monitor to small sticky patches on your chest (called leads). The monitor then picks up your heartbeat and records it as electrical waves.

Applications of ECG tests include:

  • Bedside test (usually done as a screening tool, or when you’re having symptoms)
  • Holter Monitor (short term, portable monitor, usually used for 24–72 hours)
  • Telemetry (continuous monitoring in hospital)

Bone Mineral Density (BMD) scan

In a bone mineral density scan, special x-ray machines called bone densitometers measure the calcium content of your bones, telling you whether your bones are weak (low density) or strong (high density). The most common BMD scans are on your lower back and hips.

For all of the scans I mentioned, your technologist (the person administering the test) will guide you through the process. Sometimes multiple images might be required to get the exact image you doctor needs to make an informed decision about your health.

Dr. Shaun Mehta is a medical resident and Dot Health’s team physician
shaun@dothealth.ca

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