A Rectal Magnetic Resonance Imaging (MRI) is a specialized medical imaging procedure that uses a powerful magnetic field and radio waves to create detailed, cross-sectional pictures of the rectum and the surrounding tissues in the pelvis. This non-invasive scan is primarily used by physicians to accurately stage rectal cancer, assess the extent of the tumor, and evaluate the response to treatments like chemotherapy or radiation. The high-resolution images allow for clear differentiation between healthy soft tissue, muscle, and abnormal growths. It provides a precise map of the disease, which is important in planning the most effective course of treatment.
Preparing for the Rectal MRI
Preparation for a rectal MRI begins before you arrive at the imaging center, focusing on ensuring the rectum is empty and the body is safe for the magnetic environment. You will typically be asked to fast, avoiding solid foods for about four hours before the scheduled exam time. Clear liquids are often permitted to maintain hydration, which can help with the contrast injection later.
A critical step is bowel preparation, which involves using a laxative or enema on the morning of the exam to clear the rectum of stool. This cleaning process is necessary because residual material can create artifacts, or shadows, on the images that interfere with the radiologist’s ability to see the rectal wall clearly. The medical team will also review your medical history, focusing on any implanted metallic devices or electronics. Since the MRI machine generates a strong magnetic field, objects like pacemakers, certain aneurysm clips, or metal fragments in the eye can pose a serious risk or distort the images, making them a contraindication for the scan.
The Examination Process
Once you arrive for the exam, you will be asked to change into a hospital gown and remove all metallic objects, including jewelry, hairpins, and clothing with metal zippers or snaps. The technologist will place a small intravenous (IV) line, typically in your arm or hand, for the injection of a contrast agent later in the procedure. You will then be positioned on the narrow, movable table that slides into the MRI scanner, usually lying on your back or stomach.
Endorectal Coil Insertion
For a rectal MRI, a specialized component called an endorectal coil may be used to improve image resolution, particularly for structures close to the rectal wall. This coil, which acts as a small receiver antenna, is encased in a lubricated balloon and is gently inserted a short distance into the rectum by a doctor or trained technologist. The balloon is then inflated with a small amount of air, often around 50 cubic centimeters, to stabilize the coil against the rectal wall. This inflation can cause a sensation of rectal fullness or pressure. This proximity allows the coil to capture highly detailed signals, which is beneficial for accurately staging tumors.
After the coil is positioned, the table will slide into the main body of the MRI machine, which is a large, tube-shaped magnet open at both ends. During the scan, the machine produces loud, repetitive knocking and buzzing sounds as the magnetic field is rapidly adjusted, so you will be provided with earplugs or headphones. You must remain completely still throughout the imaging process, as any movement can blur the images, and you may be asked to hold your breath for short intervals.
The intravenous contrast agent, most commonly a gadolinium-based compound, is administered through the IV line at a specific point during the scan. This contrast material circulates through the bloodstream and enhances the visibility of certain tissues, such as blood vessels and areas of inflammation or tumor. The technologist will monitor the entire process from a separate control room and can communicate with you through an intercom system. The total duration of the scan typically ranges from 30 to 60 minutes, depending on the specific imaging sequences required.
Post-Procedure and Results
Immediately following the completion of the imaging sequences, the technologist or doctor will gently deflate the balloon and remove the endorectal coil. The IV line used for the contrast agent will also be taken out, and a small dressing will be placed on the injection site. You are generally able to resume your normal daily activities, including eating and driving, unless you were given a sedative before the procedure.
It is important to drink extra fluids, such as water, after the exam to help your body flush the gadolinium contrast material out of your system through urination. While side effects are uncommon, minor discomfort at the injection site or a brief feeling of coolness during the contrast injection may occur. The images captured during the MRI are then interpreted by a board-certified radiologist, a physician who specializes in medical imaging. The radiologist prepares a detailed written report, which is sent to the physician who originally ordered the rectal MRI. Your ordering physician will discuss the results with you and determine the next steps for your care, which usually occurs within a few days of the scan.