MRI pelvis: what the scan shows, how it works and what to expect

Magnetic resonance imaging, or MRI, creates pictures of the inside of the body without using X-rays. NHS inform explains that most of the body is made of water, and that the scanner uses powerful magnets and short bursts of radio waves to read signals from hydrogen atoms in that water. Different tissues give off slightly different signals, which is why MRI is good at telling one soft tissue from another.

A pelvic MRI focuses on the lower part of the abdomen, the region between the hip bones. Depending on the question being asked, the images can include the bladder, the rectum and the lower bowel, the prostate and seminal vesicles in men, the womb, cervix, vagina and ovaries in women, nearby lymph nodes and the muscles and bones of the pelvis. The scan is tailored to the clinical question, so the way it is performed for one person can differ from the way it is performed for another.

Cancer Research UK says MRI scans produce pictures from angles all around the body and show up soft tissues very clearly. That is the main reason an MRI is chosen for the pelvis, where many of the structures of interest are soft tissue that is hard to separate on an ordinary X-ray.

The NHS lists three broad reasons for an MRI scan: to find out what is causing symptoms, to check bones and organs, and to see how well a treatment is working, for example by checking the size of a tumour during and after cancer treatment. In the pelvis the question usually falls into one of the groups below.

• Cancer staging and planning. Cancer Research UK lists tumours in the pelvic organs, including prostate, bladder, womb and ovarian tumours, among the cancers that MRI is particularly good for, and says MRI can show how deeply a tumour has grown into body tissues.

• Unexplained pelvic pain or other symptoms where an ultrasound scan has not given the full answer. For example, the NHS says that for suspected fibroids a GP will usually refer you to a specialist for an ultrasound scan, which is a different test from MRI.

• Problems with the pelvic floor, bowel or the area around the anus, where the detailed soft tissue pictures help specialists plan treatment.

• Assessing joints and bones of the pelvis, such as the hips and the sacroiliac joints, when symptoms in the lower back or buttock need a closer look.

• Checking whether tissue left behind after treatment is scar or cancer. Cancer Research UK says an MRI can be particularly useful for this.

The hospital will tell you if there is anything you need to do before the scan. The NHS says you may be asked not to wear anything containing metal, such as jewellery, a bra, hair clips or hearing aids, to remove coins or keys from your pockets and to change into a hospital gown if your clothes have metal parts such as zips. You may also be asked not to wear make up.

NHS inform adds that for some scans you may be asked not to eat or drink for up to 4 hours beforehand, and that other patients are asked to drink quite a large amount of water. It depends on the area being scanned, which is why the instructions on your appointment letter matter. Take regular medicines as normal unless told otherwise.

On arrival you will fill in a safety questionnaire. The NHS says it asks whether you are pregnant or may be pregnant, are breastfeeding, have had surgery, have kidney problems, have metal parts in your body such as surgical clips, a pacemaker or dental work, and have any allergies including to contrast medium. Cancer Research UK notes that MRI is generally safe in pregnancy, but you should still tell staff beforehand so risks and benefits can be discussed. NHS inform lists items such as medicine patches, glucose monitors and insulin pumps among the things that may need to be removed.

An MRI scan is carried out by a radiographer. You lie on a flat bed that slides into a short tunnel that is open at both ends. NHS inform says the radiographer controls the scanner from another room, can see you through a window and monitor, and can talk to you through an intercom. You are given a squeeze alarm so you can attract attention if you have concerns.

The scanner makes loud tapping and banging noises, which NHS inform compares to standing next to roadworks. Ear plugs or headphones are provided, and you may be able to listen to music. For a pelvic scan you may be asked to lie on your back, and sometimes a flat receiver coil is placed over the area. Because movement blurs the pictures, you need to stay as still as you can, and you may be asked to hold your breath during short sequences.

You may be given an injection of contrast medium (dye) to show more detail. The NHS lists possible mild side effects such as feeling or being sick, a rash, headache, a metallic taste and dizziness, and says allergic reactions are rare. You may be asked to wait at the hospital for up to 30 minutes afterwards. NHS inform adds that contrast agents can cause tissue and organ damage in people with severe kidney disease, which is why a blood test of kidney function may be arranged first.

Preparation and technique vary between departments and between scan types, so follow the specific instructions you are given rather than general advice.

Many people feel uneasy about lying in a tunnel. The NHS advises talking to a healthcare professional at the hospital before your appointment if you are anxious or have a fear of small spaces, and says you may be able to have a medicine to help you relax and bring someone to support you. NHS inform notes that modern scanners have a wider tunnel, which can reduce claustrophobia, and that going in feet first can be easier although it is not always possible.

If you have a sedative you will need someone to take you home, and the NHS says you should not drive for 24 hours. If you have no sedative you can usually resume normal activities and eat and drink as normal immediately.

The NHS says it usually takes between 1 and 2 weeks to get MRI scan results because a specialist doctor called a radiologist has to look at the images and then write to the doctor who referred you. NHS inform adds that waiting times depend on the hospital and that the scan may need to be discussed with other specialists. If you have heard nothing after a few weeks, the NHS advises contacting the doctor who referred you.

The report describes what the radiologist sees, and it is the referring doctor who puts it in context with your symptoms, examination and other tests. Scans sometimes show things that look unusual but are harmless, and sometimes a further test such as a biopsy or a different type of scan is needed to be sure. Ask your doctor to explain anything in the report you do not understand.

NHS inform describes MRI as painless and says extensive research has found no evidence that the magnetic fields and radio waves pose a risk. Some people notice a tingling sensation or feel hot in the scanner, and this passes when the scan ends. The NHS says serious complications are rare. Because MRI does not use ionising radiation it is often chosen when repeated imaging is expected, though the decision always rests on the clinical question.

MRI is not suitable for everyone. NHS inform says some implants or foreign bodies mean the scan may not be recommended, so tell the person referring you about any metal in your body, and Cancer Research UK says you may still be scanned with some metal present if the doctor and radiographer decide it is safe. MRI also cannot answer every question. Bowel movement, a full bladder and breathing can all affect the pictures, and a CT scan or ultrasound may be better for some problems.

On the NHS, a pelvic MRI is requested by a GP or hospital specialist who has a clinical reason for it, and the images are reported by a radiologist who writes to the person who referred you. People who pay for a scan privately usually still need a referral or an agreed clinical indication, and the report goes to the doctor named on the request. The main practical differences are in appointment timing and in which location is used, not in how the scan works. If you have a scan outside the NHS, ask who will receive the report and how it will be shared with your GP so that the result reaches the person who can act on it.