A Cardiac Magnetic Resonance Imaging (CMR) is a non-invasive medical procedure that uses a powerful magnetic field and radio waves to create detailed images of the heart’s structure and function. It provides clear pictures of the heart muscle, chambers, and blood flow without using ionizing radiation, unlike a CT scan or X-ray. While the actual time spent inside the scanner for image acquisition is generally between 45 and 90 minutes, the total appointment time is significantly longer due to necessary preparation and post-procedure steps. Most patients should plan for an overall appointment duration that can span from 90 minutes up to three hours, especially for more complex studies.
Preparing for the Cardiac MRI
The process begins with a comprehensive intake and safety screening long before the patient is positioned in the scanner. Patients must first complete a detailed questionnaire to screen for any metallic implants or electronic devices, such as pacemakers, certain neurostimulators, or metal fragments. These items could be dangerous in the strong magnetic field and must be identified before the procedure can begin.
After the screening, the patient changes into a hospital gown, removing all metal objects like jewelry, watches, and clothing with zippers. If the study requires a contrast agent, a technician will place an intravenous (IV) line to allow for the injection of the contrast material called gadolinium later in the scan. Multiple electrocardiogram (EKG) leads are then affixed to the chest to monitor the heart’s electrical activity and synchronize image acquisition with the patient’s heartbeat.
This preparation phase, including registration, safety checks, changing, and placing the IV and EKG leads, often takes 30 to 60 minutes. Finally, the patient is carefully positioned on the movable table of the MRI machine. A specialized coil is often placed over the chest to help capture the clearest signal before the image acquisition sequences begin.
Time Inside the Scanner
Once the patient is inside the bore of the MRI machine, the actual image acquisition begins, typically lasting from 45 to 90 minutes for a standard study. This duration is necessary because the heart is a constantly moving organ, requiring multiple, distinct imaging sequences to capture its anatomy and function. The images are synchronized with the heart’s movement using the EKG leads, allowing the machine to capture sharp pictures at specific points in the cardiac cycle.
Each imaging sequence, such as cine imaging to assess heart wall motion, lasts only a few minutes. To prevent blurring from breathing motion, the technologist instructs the patient to hold their breath for short intervals, usually 10 to 15 seconds, during the acquisition of each image set. This repeated need for breath-holding, combined with the time required to acquire dozens of unique sequences, accounts for the majority of the time spent in the scanner.
The technologist operates the machine from a nearby control room, communicating with the patient through an intercom and visual window. Patients must remain as still as possible during the sequences to maintain image quality, as small movements can necessitate repeating an image acquisition. The machine produces loud, repetitive knocking or buzzing noises due to the rapid switching of magnetic gradients, so earplugs or headphones are always provided.
Variables That Increase Scan Time
Increased scan time is primarily due to specific complexities in the requested study protocol and individual patient factors. One common factor is the necessity of a contrast agent, which requires additional imaging sequences both before and after the gadolinium injection. Delayed-enhancement sequences, which provide information on scar tissue or fibrosis, are often acquired 10 to 20 minutes after the contrast is administered, lengthening the overall time inside the scanner.
Specialized studies, such as a pharmacological stress MRI, can extend the total scan time to two hours or more. This involves administering medication to temporarily mimic the effects of physical exercise on the heart, requiring continuous monitoring and additional imaging sequences during the peak stress period. Patient-specific issues, such as an inability to follow breath-hold commands or an irregular heart rhythm, also increase duration as the technologist must repeat sequences to obtain diagnostic-quality images.
The specificity of the study also plays a role, as complex protocols require more data acquisition time than a simple assessment of heart function. For example, a comprehensive evaluation for conditions like myocarditis requires multiple tissue characterization sequences and will take longer. Additionally, older or lower-field strength MRI scanners have slower data acquisition capabilities, which can contribute to longer scan times compared to modern machines.
What Happens After the Procedure
Once all the necessary images have been acquired, the technologist slides the table out of the MRI machine. Immediate post-procedure steps involve removing the EKG leads and taking out the IV line if one was placed for contrast administration. The patient is then free to change back into their clothes and collect any personal belongings that were stored outside the magnetic field.
If a contrast agent was used, the patient is advised to drink plenty of fluids over the next 24 hours. Patients who received a sedative must remain in a recovery area until the effects of the medication wear off and are required to have a friend or relative drive them home. If a pharmacological stress agent was administered, a short period of post-procedure monitoring is mandatory to ensure the patient’s heart rate and blood pressure return to their baseline levels.
The patient will not receive the results immediately, as the acquired images must first be processed and reviewed by a specialized radiologist or cardiologist. This physician interprets the findings, generates a detailed report, and sends it to the ordering doctor, which typically takes a few business days. The ordering physician then discusses the results and any next steps with the patient during a follow-up appointment.