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Magnetic Resonance (MR) Defecography

Magnetic resonance (MR) defecography is a noninvasive test that uses magnetic resonance imaging to obtain images at various stages of defecation to evaluate how well the pelvic muscles are working and provide insight into rectal function. It is used to help determine the cause of fecal incontinence, constipation, and other conditions such as pelvic organ prolapse that may interfere with a person's ability to pass stool.

Tell your doctor about any health problems, recent surgeries or allergies and whether there's a possibility you are pregnant. The magnetic field is not harmful, but it may cause some medical devices to malfunction. Most orthopedic implants pose no risk, but you should always tell the technologist if you have any devices or metal in your body. Guidelines about eating and drinking before your exam vary between facilities. Unless you are told otherwise, take your regular medications as usual. Leave jewelry at home and wear comfortable clothing. You may be asked to wear a gown. If you have claustrophobia or anxiety, you may want to ask your doctor for a mild sedative prior to the exam.

What is magnetic resonance (MR) defecography?

Magnetic resonance imaging (MRI) is a noninvasive test used to diagnose medical conditions.

MRI uses a powerful magnetic field, radio waves and a computer to produce detailed pictures of internal body structures. MRI does not use radiation (x-rays).

Detailed MR images allow doctors to examine the body and detect disease. The images can be reviewed on a computer monitor. They may also be sent electronically, printed or copied to a CD, or uploaded to a digital cloud server.

Magnetic resonance (MR) defecography is a special type of MR imaging, where images are obtained at various stages of defecation. It produces detailed images during a bowel movement and provides information about the structure and function of the rectum and the pelvic floor, a network of muscles that stretches between the pubic bone and spine and the abdominal organs it supports.

What are some common uses of the procedure?

Physicians use MR defecography to:

  • obtain information about how well the pelvic muscles are working during a bowel movement.
  • provide insight into rectal function.
  • determine the cause of accidental or involuntary bowel motion/passage of feces or the inability to control the passage of waste material from the body.
  • determine the cause of constipation, or difficulty passing waste material from the body.
  • diagnose and evaluate diseases affecting rectal function and pelvic floor disorders (also called pelvic floor dysfunction), such as hernia, pelvic organ prolapse or rectal prolapse, a condition where part or all of the rectum wall slides out of place.
  • provide information for surgical and treatment planning.

How should I prepare?

You may need to wear a hospital gown. Or, you may be allowed to wear your own clothing if it is loose-fitting and has no metal fasteners.

Guidelines about eating and drinking before an MRI vary between specific exams and facilities. Unless you are told otherwise, take food and medications as usual.

Some MRI exams use an injection of contrast material. You may be asked if you have asthma or allergies to iodine contrast material, drugs, food, or the environment. MRI exams commonly use a contrast material called gadolinium. Gadolinium can be used in patients with iodine contrast allergy. A patient is much less likely to be allergic to gadolinium contrast than to iodine contrast. However, even if the patient has a known allergy to gadolinium, it may be possible to use it after appropriate pre-medication. For more information on allergic reactions to gadolinium contrast, please consult the ACR Manual on Contrast Media.

Tell the technologist or radiologist if you have any serious health problems or recently had surgery. Some conditions, such as severe kidney disease, may require the use of specific types of gadolinium contrast that are considered safe for patients with kidney disease. You may need a blood test to determine whether your kidneys are functioning normally.

Women should always tell their doctor and technologist if there is a chance they are pregnant. MRI has been used since the 1980s with no reports of any ill effects on pregnant women or their unborn babies. However, the baby will be in a strong magnetic field. Therefore, pregnant women should not have an MRI in the first trimester unless the benefit of the exam clearly outweighs any potential risks. Pregnant women should not receive gadolinium contrast unless absolutely necessary. See the MRI Safety During Pregnancy page for more information about pregnancy and MRI.

If you have claustrophobia (fear of enclosed spaces) or anxiety, you may want to ask your doctor to prescribe a mild sedative prior to your exam.

Leave all jewelry and other accessories at home or remove them prior to the MRI scan. Metal and electronic items can interfere with the magnetic field of the MRI unit, and they are not allowed in the exam room. They may cause burns or become harmful projectiles within the MRI scanner room. These items include:

  • jewelry, watches, credit cards and hearing aids, all of which can be damaged
  • pins, hairpins, metal zippers and similar metallic items, which can distort MRI images
  • removable dental work
  • pens, pocket knives and eyeglasses
  • body piercings
  • mobile phones, electronic watches and tracking devices.

In most cases, an MRI exam is safe for patients with metal implants, except for a few types. People with the following implants may not be scanned and should not enter the MRI scanning area without first being evaluated for safety:

Tell the technologist if you have medical or electronic devices in your body. These devices may interfere with the exam or pose a risk. Many implanted devices will have a pamphlet explaining the MRI risks for that particular device. If you have the pamphlet, bring it to the attention of the scheduler before the exam. MRI cannot be performed without confirmation and documentation of the type of implant and MRI compatibility. You should also bring any pamphlet to your exam in case the radiologist or technologist has any questions.

If there is any question, an x-ray can detect and identify any metal objects. Metal objects used in orthopedic surgery generally pose no risk during MRI. However, a recently placed artificial joint may require the use of a different imaging exam.

Tell the technologist or radiologist about any shrapnel, bullets, or other metal that may be in your body. Foreign bodies near and especially lodged in the eyes are very important because they may move or heat up during the scan and cause blindness. Dyes used in tattoos may contain iron and could heat up during an MRI scan. This is rare. Tooth fillings, braces, eyeshadows and other cosmetics usually are not affected by the magnetic field. However, they may distort images of the facial area or brain. Tell the radiologist about them.

What does the equipment look like?

The traditional MRI unit is a large cylinder-shaped tube surrounded by a circular magnet. You will lie on a table that slides into the center of the magnet.

Some MRI units, called short-bore systems, are designed so that the magnet does not completely surround you. Some newer MRI machines have a larger diameter bore, which can be more comfortable for larger patients or those with claustrophobia. "Open" MRI units are open on the sides. They are especially helpful for examining larger patients or those with claustrophobia. Open MRI units can provide high quality images for many types of exams. Certain exams cannot be performed using open MRI. For more information, consult your radiologist.

MR defecography may be performed in either the traditional MRI unit (a large cylinder-shaped tube surrounded by a circular magnet) or in an open unit. In an open MRI unit, two large magnets surround the patient and a removable chair that simulates a toilet is located in the space between the large vertical magnets.

How does the procedure work?

Unlike x-ray and computed tomography (CT) exams, MRI does not use radiation. Instead, radio waves re-align hydrogen atoms that naturally exist within the body. This does not cause any chemical changes in the tissues. As the hydrogen atoms return to their usual alignment, they emit different amounts of energy depending on the type of body tissue they are in. The scanner captures this energy and creates a picture using this information.

In most MRI units, the magnetic field is produced by passing an electric current through wire coils. Other coils are located in the machine and, in some cases, are placed around the part of the body being imaged. These coils send and receive radio waves, producing signals that are detected by the machine. The electric current does not come in contact with the patient.

A computer processes the signals and creates a series of images, each of which shows a thin slice of the body. These images can be studied from different angles by the radiologist.

MRI is able to tell the difference between diseased tissue and normal tissue better than x-ray, CT and ultrasound.

How is the procedure performed?

MRI exams may be done on an outpatient basis.

You will be asked to drink water during a period of 30 minutes prior to the exam. Your rectum will be filled with a soft substance that is similar to the consistency of feces and contains a contrast material. A towel will be placed underneath you to absorb any urine or feces that may leak out during the exam.

If your exam is being performed in a traditional MRI unit, you will be positioned on a moveable examination table lying on your back with your knees bent. Straps and bolsters may be used to help you remain still and maintain the correct position during imaging. If your exam is being performed in an open MRI unit, you will be seated on an adjustable chair within the unit between two large magnets.

A device that contains coils capable of sending and receiving radio waves will be strapped around your pelvis in a traditional MRI unit or placed on the seat beneath you in an open MRI unit.

The examination will be performed by a radiologist working at a computer outside of the room.

Images will be obtained as you contract your muscles as you would during a bowel movement including squeezing, straining and defecating. Images will also be taken while your muscles are relaxed. The technologist will give you instructions during the exam.

The MR defecography exam typically includes two or more sets of images and is usually completed in 30 minutes to an hour.

What will I experience during and after procedure?

Most MRI exams are painless. However, some patients find it uncomfortable to remain still. Others may feel closed-in (claustrophobic) while in the MRI scanner. The scanner can be noisy. Sedation may be arranged for anxious patients, but fewer than one in 20 require it.

It is normal for the area of your body being imaged to feel slightly warm, but if it bothers you, notify the radiologist or technologist. It is important that you remain perfectly still while the images are being obtained, which is typically only a few seconds to a few minutes at a time. You will know when images are being recorded because you will hear and feel loud tapping or thumping sounds when the coils that generate the radiofrequency pulses are activated. Some centers provide earplugs, while others use headphones to reduce the intensity of the sounds made by the MRI machine. You will be able to relax between imaging sequences, but will be asked to maintain your position without movement as much as possible.

You will usually be alone in the exam room during the MRI procedure. However, the technologist will be able to see, hear and speak with you at all times using a two-way intercom. Many MRI centers allow a friend or parent to stay in the room as long as they are also screened for safety in the magnetic environment.

Children will be given appropriately sized earplugs or headphones during the exam. MRI scanners are air-conditioned and well-lit. Music may be played through the headphones to help you pass the time.

Some patients feel mild bloating or cramping when the substance and contrast material is inserted into the rectum.

You may resume your usual activities and normal diet immediately after the exam.

Who interprets the results and how do I get them?

A radiologist, a doctor trained to supervise and interpret radiology exams, will analyze the images. The radiologist will send a signed report to your primary care or referring physician, who will share the results with you.

What are the benefits vs. risks?

Benefits

  • MR defecography helps assess pelvic floor abnormalities that can be difficult to diagnose through physical examination and other tests such as computed tomography, colonoscopy and sigmoidoscopy.
  • MRI is a noninvasive imaging technique that does not involve exposure to radiation.
  • MR images of the soft-tissue structures of the body—such as the heart, liver and many other organs—are clearer and more detailed than with other imaging methods. This detail makes MRI an invaluable tool in early diagnosis and evaluation of cancer.
  • MRI has proven valuable in diagnosing a broad range of conditions, including heart and vascular disease, stroke, and joint and musculoskeletal disorders.
  • MRI can help physicians evaluate both the structure of an organ and how it is working.
  • MRI can detect abnormalities that might be obscured by bone with other imaging methods.

Risks

  • The MRI examination poses almost no risk to the average patient when appropriate safety guidelines are followed.
  • Although the strong magnetic field is not harmful in itself, implanted medical devices that contain metal may malfunction or cause problems during an MRI exam.

What are the limitations of magnetic resonance imaging (MR) defecography?

High-quality images depend on your ability to remain perfectly still and follow breath-holding instructions while the images are being recorded. If you are anxious, confused or in severe pain, you may find it difficult to lie still during imaging.

A person who is very large may not fit into certain types of MRI machines. There are weight limits on the scanners.

Implants and other metallic objects can make it difficult to obtain clear images. Patient movement can have the same effect.

A very irregular heartbeat may affect the quality of images. This is because some techniques time the imaging based on the electrical activity of the heart.

Although there is no reason to believe that MRI harms the fetus, pregnant women should not have an MRI exam during their first trimester unless medically necessary.

This page was reviewed on March 05, 2019

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