DIAGNOSING EPILEPSY: MRI SCAN OR CT SCAN

Both MRI (Magnetic Resonance Imaging) and CT (Computerized Tomography) are ways of producing images of the structure of the brain. CT does this by measuring the way X-rays are absorbed by the brain. MRI does it in a more complex way, by surrounding the brain with a strong magnetic field and assessing the way that the water in the brain (which is composed of charged particles called protons) responds to high-frequency pulses of radiomagnetic energy. Sometimes the radiologist will ask for an injection of ‘contrast’ material to be injected intravenously. This substance will outline the major blood vessels in the brain and will show up any abnormalities in them.
Neither of these procedures is either harmful or uncomfortable, and both give a very good picture of the main structures of the brain. The radiologist who interprets the films will be able to tell whether the fluid-filled cavities in the brain (the ventricles), are normal in size and position, whether the convolutions of the brain are normal in shape and form, and whether or not there is an abnormality in the substance of the brain such as a tumour, a vascular anomaly or a developmental abnormality.
Although all major hospitals now have CT scanners, MRI scans are only available in a few units. However, it is always worth having an MRI scan if you can, because it can give information about the structure of the brain which the CT scan cannot give. Twenty per cent of people who have normal CTs are found to have abnormalities when they are given MRI scans.
In particular, the MRI scan can show the structure of the hippocampus and amygdala, which is an area in the temporal lobe of the brain from which seizures very commonly arise. MRI scanning can also show developmental abnormalities which are difficult to see on the CT. This is a technology which is developing very rapidly. A new technique called MRI spectroscopy shows chemical abnormalities in some cells, indicating the site of the seizure focus.
Having a CT scan
Having a CT scan is rather like having to lie with your head in a huge washing machine. You lie on a bed with your head inside a metal structure which rotates around it, sending out X-rays all the time. Even if you dislike being in confined spaces, you probably will not mind the CT scanner. The machine feels very open and there is plenty of space around you so that you should not feel at all constricted.
The whole scanning process is very quick and should be over within five minutes. However, it will take considerably longer for the computers to build up the brain pictures for the radiologist. While the scan is being taken it is essential that you lie absolutely still as any movement distorts the image.
Because X-rays are dangerous for an unborn child, CT scans are never given to anyone who is pregnant. Women are always asked to fill in a questionnaire before being scanned, letting the radiologist know the date of their last period.
Having an MRI scan
Having an MRI scan is a more complicated procedure. Because very strong magnetic fields, several million times stronger than the earth’s magnetic field, are used during MRI scanning, all watches, credit cards and metal materials such as belt buckles have to be left outside the scanning room.
You will have to fill in a questionnaire before having the scan. If you have a heart pacemaker you will not be able to have an MRI scan. You will be asked about the work that you do, too. If your job involves working with metal there is a risk that metal filings may have worked their way into your eyes. In this case, scanning may be dangerous as the magnetic field is strong enough to pull out the filings and damage the eye. Metal clips in the brain from previous intercranial surgery will also stop you having a scan. Metal bridges in the teeth and metal pins in the shoulders or hips will not usually prevent you having a scan, but you must note these on the form.
An MRI scanner looks quite different from a CT scanner. It consists of a long tube into which your whole body slides. It does seem constricting, and makes many people feel claustrophobic. If you tend to feel panicky in confined spaces, it is worth mentioning this to the doctor. You can then be given a sedative half an hour before the scan to make you feel more relaxed.
The MRI scanner also makes a terrible racket – a very loud drumming, thumping noise — while it is working. Most units offer you headphones so that you can play a tape of your choice, so make sure you take one with you. In fact, the noise is often so loud that you cannot hear the tape, but it is nice to know it is playing!
MRI scans take much longer than CT scans. The whole process of filling in the form, emptying your pockets, going into the scanner, being scanned, coming out again and being reunited with your possessions seldom takes less than an hour. As with the CT scan, it is essential that you lie absolutely still during the scan, as any movement degrades the MRI images. You will have to lie still for at least 20 minutes.
There is as yet no evidence that strong magnetic fields are in any way dangerous. However, because this is still a fairly new method, and it is always best to be on the safe side, MRI scans are not given to pregnant women.
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