A rectal magnetic resonance imaging (MRI) uses a strong magnetic field and radio waves to create detailed pictures of the rectum and surrounding soft tissues. This imaging technique is frequently used to assess disease, especially for staging rectal cancer, because it provides exceptional clarity of tissue layers. The actual time spent inside the MRI machine typically ranges from 30 to 45 minutes, though the total appointment time is significantly longer.
Preparing for the Rectal MRI
Preparation requires careful steps to ensure high image quality. Patients are instructed to follow specific dietary restrictions, such as fasting from solid foods for four to six hours before the examination. This fasting period helps reduce motion artifacts that can blur the images.
Hydration is important, as is taking routine medications with a small sip of water, unless otherwise directed. Bowel preparation is often required to empty the rectum, which may involve using a micro-enema about three hours before the appointment. A clean rectum allows the scanner to clearly visualize the rectal wall and surrounding structures without interference.
Contrast agents are used to enhance the visibility of certain tissues and blood vessels. This may include an intravenous (IV) injection of a Gadolinium-based contrast dye, usually administered halfway through the scan. Some protocols also involve inserting a small amount of rectal gel or fluid to slightly distend the rectum, improving the distinction between the rectal wall and the tumor. The preparation process also involves checking in, completing paperwork, and changing into a hospital gown.
The Time Spent Inside the Scanner
The 30 to 45 minutes inside the scanner is dedicated to acquiring multiple imaging sequences while the patient is positioned on the table. This time is divided into several sequences, each lasting from a few seconds to several minutes, which together build a complete picture of the pelvic area. The most important sequence is the high-resolution T2-weighted imaging, which is the standard for evaluating the local extent of rectal disease.
Other sequences, such as diffusion-weighted imaging (DWI), provide information about the cellular density of tissues. The technologist uses these sequences to create images in different planes, including sagittal and oblique axial views, to accurately measure the distance of any abnormality from the surgical margins.
Remaining perfectly still is essential throughout these sequences. Even minor movement can distort the images, requiring a sequence to be repeated and lengthening the total scan time. Patients will be asked to follow specific breath-holding instructions, typically lasting 15 to 20 seconds, to minimize involuntary motion. A medication like glucagon may be administered during the procedure to suppress natural bowel movement.
Total Appointment Time vs. Scan Time
The 30 to 45 minutes spent scanning is only a fraction of the total appointment time. Patients should plan for a total commitment ranging from 90 to 120 minutes, accounting for necessary procedures before and after the scan.
Upon arrival, several steps must be completed before the scan begins:
- Completing the check-in process and safety screening for metal implants.
- Changing into a gown.
- Placing an IV line for contrast administration.
- Positioning the patient correctly on the table and placing the pelvic surface coil, which enhances image quality.
After the scanning is complete, the patient is asked to wait briefly while the technologist checks the initial images for quality and completeness. This ensures all required diagnostic information has been captured before the patient is released. This logistical process results in the two-hour window for the overall appointment.
Receiving the Results
Once the rectal MRI scan is finished, the images are sent to a specialized radiologist for interpretation. The MRI staff cannot provide immediate results or a diagnosis at the time of the scan. The radiologist reviews the complex images, often comparing them to previous scans, to create a formal written report.
The time it takes to receive the results varies depending on the facility’s workload and the complexity of the case. Generally, the referring physician receives the comprehensive report within 24 hours to several business days. In non-urgent cases, the patient should plan to discuss the findings with their ordering physician at a follow-up appointment, typically scheduled one to two weeks after the scan.