MRI vs. CT Scan (CAT Scan), what's the difference?

By Dr. Alexander Flint , Chief Medical Officer at image32

An MRI and a CT Scan (also referred to as a CAT Scan) are two different ways to create pictures of the inside of the body using medical imaging technology.

MRI is an abbreviation for “Magnetic Resonance Imaging.” CT stands for “Computerized Tomography”, and CAT Scan stands for “Computerized Axial Tomography.” We’ll unpack these terms in just a bit.

When compared to traditional X-Ray images, both MRIs and CT Scans yield a much better degree of contrast between the various tissues of the body.

The main limitation of traditional X-Rays is that when X-Rays are sent through the body, they scatter significantly and they can only distinguish between dense structures like bones and less dense structures like the lungs, as is this Chest X-Ray image:

MRI VS CAT Scan SimilaritiesBoth MRIs and CT Scans produce cross-sectional imaging—in both cases, the scanner saves various two-dimensional (2D) ‘slices’ of the three dimensional (3D) body part. Unlike an X-Ray, in which a single ‘snapshot’ is taken of a body part, with MRIs and CTs, as many as hundreds, even thousands, of 2D slices can be created. For an example, have a look at the 27 slices of a person’s brain shown across 3 rows in this image:

In many other ways, MRIs and CT Scans are completely different.

MRI VS CAT Scan DifferencesCT Scans are basically an improvement upon the traditional X-ray technology. Instead of shining X-Rays in a diffuse way through a body part and detecting where the X-Rays end up on the other side, CT Scanners spin the X-Ray’s source and the X-Ray detector around the patient—this is why CT Scanners look like a donut:

All of the data obtained from spinning the detector and source around the patient is stored in memory, and then a computer is used to reconstruct what the tissues looked like to create the pattern of data obtained. This is where the name Computerized Tomography comes from: a computer is used to create a map (a tomograph) of the tissues through which the X-Rays passed. The older term Computerized Axial Tomography comes from the fact that the original CT Scanners would only create slices or sections in the Axial plane (like slicing bread). Modern CT Scanners can use even more complex reconstruction software to produce slices in other planes, like the Sagittal and Coronal planes:

Modern CT Scanners also typically have many detectors and X-Ray sources that work in parallel, improving scan performance and reducing scan times. To improve tissue contrast with CT Scanning, a special type of X-Ray dense contrast dye can be administered, either intravenously or by mouth, which makes any tissues or blood vessels it is in appear brighter on the resulting CT Scan.

Although CT Scans represent a major technological and diagnostic advance compared with simple X-Rays, the advent of CT Scanning has also dramatically increased the amount of radiation that patients are exposed to. (See “What are the Radiation Risks of a CT (CAT) Scan?”)

An MRI is made in a completely different way, without X-Rays or any other type of ionizing radiation. Magnetic Resonance Imaging involves some truly amazing (and pretty hard to follow) physical principles. Protons inside your body are constantly spinning around, and the axis of this spin (like the axis of a planet’s spin) is reasonably random. However, if you place a body part (and it’s protons) inside a really strong magnetic field, the protons start spinning with their axes aligned relative to the magnetic field. As if this isn’t complex enough, the next step is to send in a pulse of radio frequency (RF) energy, that causes some of the protons to shift the angle of their spin axis. When this “RF pulse” is turned off, the shifted protons “relax” their spin axes back to the original state, and when they do this, RF energy is emitted that can be measured by the MRI machine. The ‘coils’ that pull off this trick in the MRI machine often have to be arranged in a fashion that closely surrounds the body part of interest, as shown in the picture:

So how does all of this actually make a picture of the body? Well, it turns out that the rate at which the protons relax back to the “home state” after the RF pulse is turned off is dependent on the local environment—protons in water or fat or bone are relaxing at different rates, and this difference is used to create a picture that distinguishes between the various types of tissues in the body.

Because the technique is so complex, there are a lot of factors that can be varied. Stronger magnetic fields generally yield better pictures, and the magnetic strength is often referred to with labels like 1.5T or 3T, where the T stands for Tesla, after the inventor Nikola Tesla. The type of RF pulses used can be varied, and the weighting of various properties of the RF energy returned during relaxation can also be varied.

All of these variations mean that a large number of stacks of imaging slices are typically made in a single session in a MRI machine. An MRI may have many sequences stored as different series in the overall study (see How are Medical Imaging Studies Organized?) These sequences tend to have interesting abbreviations like T1, T2, FLAIR, GRE, and our personal favorite, FIESTA. The bottom line is that the same body part is imaged again and again with different sequences in order to ask different questions about what going on inside the body. This image shows four different sequences at the same level of a brain MRI:

As with CT Scans, MRIs can also use contrast when needed. In the case of MRIs, iodine-based contrast is not used. Instead, a special MRI contrast agent containing the element gadolinium is used.

Because MRI doesn’t use ionizing radiation, it is generally felt to be safer than CT Scanning. However, the very strong magnetic fields mean that MRI can’t be safely done for patients with certain types of metal in their bodies, like all but the most recent generations of pacemakers and some other metal implants.

By Dr. Alexander Flint, Chief Medical Officer at image32.

Alex Flint is a Neurointensivist, stroke specialist, medical device inventor and Chief Medical Officer of image32. He's the kind of doctor who isn't satisfied with improving care for his own patients-he wants to make things better for everyone. Find him on Doximity.

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