MRI technical information

Magnetic Resonance Imaging is a technique for imaging soft tissues (not solid bone) within the human body. It is unrivalled for its ability to show neurological tissues (brains and spinal cords) as they are surrounded by bone, making them difficult to see with X-rays. It is also unique in that image appearances can be changed by using different pulse sequences. This allows radiologists to look for different things with different sequences, eg one might show anatomy best and another one might show fluids better. From a patient point of view, the sequences sound very different.

MRI can acquire images from any plane in the body without image processing (CT and PET acquire their data in one plane and then reformat it to be 3D). Here are some typical MR images of a sagittal view of a normal brain, showing how the image contrast can be changed.

And here is a sagittal view of another volunteer whole spine and brain, which we can image in one session on our state-of-the-art scanners: this is called a T2-weighted image.

There is a complementary technique called Magnetic Resonance Spectroscopy, or MRS. This allows us to look at molecules within the body, and see if they are functioning normally, or if there are some unexpected molecules present or absent, as there often is in the case of cancer. This technique is not as commonly used as MRI, and needs a good, very high field magnet like our 3T Trio.

MR physics is complex, and a fun treatment with animations of the basic physics can be found here www.simplyphysics.com

A more comprehensive theory-based treatment is given here www.fmrib.ox.ac.uk/~stuart/lectures/index.html
Some lectures made for Radiographers on basic aspects of MR physics are here

MR Safety

Obviously using a hugely powerful magnet can cause some safety concerns: we have a questionnaire (link) which we require all patients to fill out, as well as visitors who may be coming into the controlled area around the scanners. We have to make sure that there is no metal within someone's body which may be pulled out or moved by the magnet. For example, these include shrapnel for service personnel, tiny fragments of metal in the eyes, or certain medical devices such as pacemakers. Even if the metal lodged in someone decades previously, if it is still there, it is a risk and we need to make sure a patient is safe.

Large metal objects can also go flying into the magnet if they are within a certain distance: for this reason we have to ensure that patients and visitors to the magnet room remove all metal from their person, including scissors, keys, coins and mobile phones. The magnetic field will destroy mobiles' circuits in any case, and also wipe credit cards. We have safe areas where patients may leave their valuables.

Anyone concerned about any aspect of MR safety, and scheduled for a scan, should telephone the radiographers on the number given on their appointment letter or questionnaire.