When does a CT give better information than an MRI

[MisterX]

In both cases there are contrast agents available. MRI has vastly superior location error but that doesn't mean everything. It also doesn't illuminate the tissue in the same way as x-ray optics. With MRI you can find where molecules are but wouldn't necessarily illuminate an equivalent density structure which happened to have different opacity to x-rays. But I am just speculating about this. Why use CT scans when non-ionizing MRI equipment is available?

 

[Ygggdrasil]

In general, CT is better for imaging bone and other dense structures while MRI gives better images of soft tissue. So while MRI would generally be the method of choice for looking at tumors in a cancer patient, you'd probably go with CT if you were looking at a patient with a bone cancer. CT is much faster (~ 5 min scans) than MRI (~ 30 min scans), so it is much better in emergency situations (also if you don't know whether the patient has any metal in them). CT scans are also generally cheaper than MRI, but modern medicine doesn't really seem concerned with keeping costs down these days.

 

[Choppy]

Why use CT scans when non-ionizing MRI equipment is available?

They both have their advantages and disadvantages.

One of the major disadvantages of MRI is the time that it takes to acquire a scan. A lot can depend on the specific sequence you're using and the volume you're trying to reconstruct, but it's fairly typical for MRI times to run on the order of 10 minutes or more. In some cases patients can be inside the bore for half an hour. With a modern multislice CT you can take a large scan in a matter of seconds.

You also have "4D" capabilities with conventional CT scanners - meaning you can make a movie of someone breathing. This can be really important if you want to develop a radiation therapy treatment plan. I'm sure this can be done with an MRI as well, but I'm pretty sure it's not a conventional capability yet.

Another point is spatial distortions. It can be very difficult to set up an MRI to be free of spatial distortions because you need a very homogenous magnetic field and/or a reliable map of distortions in it. CT on the other hand tends to give very accurate spatial information.

So what all of this means is that there are some people who can't be candidates for an MRI study - they can't lie still for long enough, or they have material in them with a high magnetic susceptibility that will distort images that are taken. Or even worse, the high magnetic field can shift a piece of shrapnel around inside the body and cause serious damage.

Another factor is just the information that you get out. CT easily maps electron density information in a one to one manner, but MRI doesn't do this. So although you can see contrast in soft tissues more readily on MRI scans than on CT scans, you run into situations where the same grayscale value can come from different physical quantities.

Edit: Ygggdrasil beat me to this one.

 

[Vanadium 50]

CT tells you where the heavy atoms are. MRI tells you where the hydrogen is. PET tells you where the metabolism is going on.