MRI magnet strength is measured in a scientific unit called a Tesla. For short-hand we use the abbreviation 'T' for this. So a 1.5T magnet is one-and-a-half-times stronger than a 1.0T magnet. There are MRI's used in experimental research that are 17.0T as of 2014 and there are even stronger magnetic fields that have been created! The earth's magnetic field at the equator is only about 3 micro-Tesla by comparison.
So what does this little crash-course in electromagnetism mean for my pet?
Well, the strength of the magnet is one of the more important variables in MR imaging. It has a major impact on the resolution of the image and the speed with which we're able to acquire that image. For comparison sake: with our 1.5T magnet we are able to scan the brain in a domestic feline in less than 20 minutes with excellent resolution and detail. Previously, I've worked with a 1.0T magnet that would take closer to an hour and the resolution wouldn't be nearly as good.
Of course, there are other variables that can be adjusted to gain better resolution in a weaker magnet, but it takes a longer time. And, when anesthesia is involved, less time is always safer! Also, one has to be careful to consider the computer post-processing that goes into making these weaker magnet's images look good. Just like Photo-shop can make anyone look like a super-slim super-model; images from these weaker magnets can be sharpened and manipulated to make them look as good as those from the stronger magnets. But this can lead to false results when we try to interpret them. It's a little like trying to gauge what's real when looking at the cover of a magazine!