Your CT scan, and the one that started all this

The fast scan hospitals reach for when they need an answer now. What it is, and why you had one instead of an MRI. What the dye and the x-rays actually mean. And why being sent for an MRI afterward is not somebody correcting a mistake.

The short version

A CT scan uses x-rays to build a detailed picture of the inside of your head. It is quick, it does not hurt, and it is the scan a hospital reaches for when it needs an answer now. That is usually why you had one. A CT can show that something is there. It cannot say for certain what it is. An MRI shows the soft tissue of the brain in far more detail, and naming a tumor takes tissue rather than pictures. So being sent for an MRI after a CT is not somebody correcting a mistake. It is the next question being asked.

What is a CT scan?

It is a scan that puts a lot of x-ray pictures together into one detailed view of a part of the body. You may hear it called a CAT scan. Both words mean the same machine.

The machine is a ring, not a tunnel. You lie on a table, and the table moves you through the ring. The ring is short, so most of you stays out in the open room the whole time.

It is painless.

Why a CT and not an MRI?

Because it is fast, and because it works on anybody.

For looking at a brain tumor, a CT is not as good as an MRI. Fine detail in the brain shows up less clearly on it, including the detail of disease, and that is exactly the detail your team needs next. None of that is a secret, and it is why the MRI gets booked. A CT gets used anyway when an MRI is not possible, or when a scan is needed right away.

Right away is the part that matters here. A CT is fast and simple, and in an emergency it can find bleeding quickly enough to save somebody's life. It is the scan that answers the frightening questions first. Is there bleeding? Is there a clot? Are the fluid spaces inside the brain bigger than they should be, the way they are with hydrocephalushydrocephalus: Fluid building up inside the brain because it cannot drain the usual way. It raises the pressure in the skull. There are treatments to let it drain again. See the glossary? Hospitals use a head CT to look for all of those, and for a tumor.

So if yours was done in an emergency room after a seizure or a bad headache, the fast scan was the right scan. Nobody chose the weaker test. They chose the one that could rule out the things that needed ruling out that night.

There are other things a CT is better at than an MRI:

• Bone. A CT shows the bone near a tumor in more detail than an MRI does. That matters when a tumor sits close to the skull.
• Blood vessels, sometimes. A CT can be used to make a picture of the vessels in and around a tumor, and in some cases it shows them better than the MRI version does.
• Devices. Having a device fitted does not stop you having a CT. It can stop you having an MRI, or at least slow it down while the staff check.

What no scan can do is name it. Pictures narrow the list, and sometimes they narrow it a long way. Only a piece of the tumor itself, looked at in a lab, can say for certain. How tissue is taken is the step after the pictures, when they have gone as far as pictures go.

What happens, step by step

1. Getting changed. You may be asked to put on a gown and take off anything metal. What to tell them before the day is the short list of things worth saying at the desk.
2. A thin tube into a vein, but only if dye is being used. Not everybody has it. It goes in with a small needle, the same as having blood taken.
3. Lying down. You lie on your back on a table. For a head scan, your head may be held still in a padded holder. It is there to stop small movements spoiling the pictures.
4. Through the ring. The table moves you through. You are asked to hold still while it runs.
5. The dye. Where one is used, it goes in through that tube.
6. Afterward. There is nothing to recover from, and no radiation stays in your body after a CT.

How long does it take?

A CT scan usually takes less than 30 minutes, and the scan of your head is the short part of that. Most of the time is getting you ready and getting you into position.

The appointment is longer than the scan. That is normal, and it is not a sign that anything difficult is happening.

What does it feel like?

Mostly it feels like lying still on a hard table for a few minutes.

It does not hurt. It is much quieter than an MRI, and you are not shut inside anything. If the MRI was the part you dreaded, that is worth knowing before you go in.

Where dye is used, the needle that puts the tube in feels like a blood test.

The hard part is usually not the scan

If you are reading this after the scan rather than before it, the scan itself is probably not the part that is bothering you. Somebody said there was something there, and then everything slowed down.

Those days have a page of their own. Something showed up on your scan was written for them: what you have really been told, the seven steps and which one you are on, and what is worth asking for this week.

And if the waiting is bigger than that, it is not something to carry quietly until the next appointment. Get help now is people, today.

The dye they may put in your arm

It is not the same dye as the one used for an MRI. The CT one contains iodine. The MRI one does not. If somebody has told you that you reacted to one of them, it is worth knowing which, because they are different liquids with different rules.

Not every CT uses dye. When one does, it is there to make certain things stand out that would otherwise blend in.

What it feels like. As it goes in you may feel warmth spreading through your body. Some people say it feels like they have wet themselves. It is only a feeling and it passes quickly. You may also get a bitter or metallic taste in your mouth, and that one can last a few hours.

Reactions. A serious allergic reaction to the iodine dye is extremely rare, and radiology departments are well set up to deal with one. If you have ever reacted to scan dye, say so before the day, because there is medicine that can be given first to lower the risk.

What about the radiation?

This is the question people ask last, quietly, and it deserves a straight answer in both directions.

A CT does use x-rays, and the amount of radiation from one is more than from a plain x-ray. There is always a slight chance of cancer from too much radiation. That is true, and it is why these are ordered for a reason rather than as a precaution.

The other half is just as true. The benefit of an accurate diagnosis far outweighs the risk of the scan. The dose from a head CT varies, which is exactly why this page will not put a number on it: a figure from one machine tells you nothing reliable about yours. And no radiation stays in your body afterward, so there is nothing left over from the scan you already had.

If you are going to have a lot of scans, the useful thing is not to worry about the one you already had. It is to ask two questions going forward: how many of these am I likely to need, and would an MRI answer the same question. Both are ordinary questions and neither of them is rude.

What to tell them before the day

Most of this is a short conversation at the desk, and all of it is easier before the scan than after it.

• Any kidney trouble. The dye can cause kidney problems, so they need to know.
• Any reaction to scan dye in the past, and which scan it was for, because the CT dye and the MRI dye are different liquids.
• Any chance you are pregnant. Say it before the scan rather than after.
• Whether you are breastfeeding, and here the advice genuinely splits. The companies that make the dye say not to feed for a day or two afterward. The American College of Radiology says studies show the amount a baby takes in through breastmilk is extremely low. Those are two real answers from two real sources, and you are allowed to know that before you decide. Ask which one the department follows. This is about the CT dye. Your MRI scan answers the same question about the MRI dye, and the answer there is not the same one.
• Anything you take, and any recent illness or allergy.
• Eating and drinking. If you are having dye, you may be told not to eat or drink for a few hours beforehand. Rules differ between hospitals, so go by what yours tells you.
• Leave jewelry at home and wear loose, comfortable clothes. You may be asked to change into a gown.

Why am I having another scan?

Because each scan is asked a different question. Being sent for more of them is ordinary, and each of the reasons below is a different question rather than a doubt about the last answer.

• An MRI, after the CT. The usual next step, and the reason is at the top of this page: the MRI shows the brain in far more detail. Your MRI scan is that appointment, start to finish.
• A scan just before an operation, and one just after it. Both trip people up and neither means something has changed. Your MRI scan explains the two together.
• A CT for radiation planning. If radiation is part of your treatment, one of these is done as part of a planning visit, with a mask made for you at the same appointment. Radiation therapy walks through that whole visit.
• A CT to guide a needle. A CT can be used to steer the needle when a piece of tissue is being taken. How tissue is taken covers that.
• Extra scans to plan an operation. Sometimes your team adds scans that answer one specific question about where things sit. The extra scans before treatment goes through them.
• Scans once treatment is over. A different job again: comparing, rather than finding. Follow-up scans, and what the results mean is that whole subject, vocabulary included.

Who reads it, and how do I get the result?

How long it takes depends on where you are, so ask before you leave. What follows is who does what, so you know what the wait is made of.

The pictures go to a radiologistradiologist: A doctor trained to read scans and write down what they show. The report goes to the doctor who asked for the scan, who then tells you. One who specializes in the brain and spine is called a neuroradiologist. See the glossary, who reads scans of every kind for a living. Their official report goes to whoever ordered the scan, and that person is the one who talks it through with you.

If your CT was done in an emergency room, the report may come back quickly and somebody may speak to you the same night. What they can tell you then is what the pictures show. It is usually not the whole answer, because the whole answer needs the MRI and often needs tissue.

Three things are worth asking before you go home: who reads this, who tells me, and what happens after that.

What to ask your team

• Why a CT today, and will I still need an MRI?
• Is this scan with dye or without, and why?
• I have reacted to scan dye before. Which one was it, and what do you do about that?
• I have kidney trouble. Does that change anything?
• There is a chance I am pregnant. What do you want to do?
• I am breastfeeding. What does this department advise, and why?
• How many scans like this am I likely to have?
• Could an MRI answer the same question instead?
• What should I do if I feel unwell after the dye, and who do I call?
• Who reads this scan, and who tells me what it says?
• Can I take away a disc or a link to the images, in case somebody else needs to look at them?

Where to go next

• Something showed up on your scan if you have been told there is something there and it does not have a name yet.
• Your MRI scan for the scan that usually comes next, and for the machine, the noise and the dye.
• The extra scans before treatment if your team has mentioned adding more scans to plan an operation or radiation.
• How tissue is taken for the step after the pictures.
• Radiation therapy for the planning CT in the mask, and for the treatment itself.
• Follow-up scans, and what the results mean for the scans that carry on afterward.
• Just diagnosed? Start here if this is all new.
• The glossary for any word on your report you do not recognize.
• Get help now if you need to talk to a person today.