Further tests

If the hospital tests show cancer cells in your colon or rectum, you will need more tests to stage the cancer. This means finding out the size of the cancer, where it is and whether it has spread to anywhere else in the body.

You may have one or more of the following tests.

• CT scan
• MRI scan
• Ultrasound scan
• PET-CT scan

Your doctor will give you the test results at your follow-up appointment. Your healthcare team will describe the stage of your cancer using the TNM, number or Dukes’ staging systems. 

CT scan

A CT (computerised tomography) scan shows the size of the cancer and whether it has spread to other parts of the body. It uses x-rays to produce 3D pictures of the inside of the body.

The scanning department will give you instructions on how to prepare for your scan. These are normally explained in your appointment letter.

You won’t be able to eat or drink for a few hours before the scan. A nurse will give you a drink or injection of a dye that makes the scan pictures clearer. If you are having a rectal scan, you may have a similar liquid passed into your rectum through a tube. Once you are in position you will need to keep as still as possible.

The scan takes between ten and 30 minutes and is painless. Most people are able to go home soon after the scan and you can eat and drink normally.

MRI scan

An MRI (magnetic resonance imaging) scan uses strong magnetic and radio waves to build up a picture of the inside of your body. It can give more detailed pictures than a CT scan.

You will have an MRI scan if you have rectal cancer. The results will show whether all of the cancer can be removed with surgery or whether you need to have radiotherapy first.

You may also have an MRI scan if you have colon or rectal cancer and your CT scan shows the cancer has spread.

You won’t be able to have an MRI scan if you have a heart monitor, pacemaker or some types of surgical clips. This is because they contain metal and will be affected by the magnets in the scanner.

The scanning department will tell you if you need to do anything to prepare for the scan. They may ask you not to eat or drink before the scan.

You may have an injection of dye into your arm to improve the image during the test. You will lie on the scanning table, which will pass into the scanner. The machine is very noisy and close to your body. Tell your radiographer if you don’t like closed-in spaces. They can give you ear plugs or headphones and an eye mask to wear. You will be able to speak to the radiographer through an intercom during the scan, which can take up to 45 minutes.

You should be able to go home soon after the scan.

Ultrasound scan

If you have rectal cancer, you may have an ultrasound scan of the back passage (rectum). This scan uses sound waves to build up a picture of the body. It can show the size of the cancer and where it is.

You will need to lie on your side while the ultrasound probe is passed into your back passage. The scan will take about ten minutes and you should be able to go home straight afterwards.

You might also have an ultrasound scan of your tummy, to see if the cancer has spread. There isn’t anything you need to do to prepare for this. You will have to lie on your back and the person doing the ultrasound (a sonographer) will move a probe over your skin.

PET-CT scan

If the cancer has spread to your liver or lungs, you may have a PET-CT scan. This uses a PET (positron emission tomography) scan and a CT scan together. It gives more detailed pictures of the body than a CT scan alone. A PET scan uses a low dose of radiation to take pictures of the whole body.

You may have a PET-CT scan if the results of other scans aren’t clear or if the cancer has spread to your liver or lungs. Doctors sometimes use PET-CT scans if there are signs that bowel cancer has come back after treatment.

Usually, you will not be able to eat for six hours before the scan but you will be able to drink water. Your appointment letter will tell you how to prepare for your scan.

At the hospital, you will have an injection of a mildly radioactive drug. You will need to wait for about an hour while the drug spreads throughout your body. You will then have your scan, which will take between 30 minutes and an hour. You will be able to go home straight after the scan.

More information

Cancer Research UK has detailed information on scans and tests for cancer.

Reviewed December 2019