What is MRI and what is it used for?
Patients and their family members who have either been scheduled for or have undergone an MRI are likely to have a simple, yet understandable question. What is MRI and what is it used for? This article will overview the fundamentals of what an MRI is in easily understandable language and explain the basics of what MRI is used for and it’s numerous diagnostic purposes.
What is MRI?
MRI or Magnetic Resonance Imaging is a diagnostic exam in which a machine uses non-ionizing radiation to produce high-quality images of a specific region of a patient’s body. This radiation is not the harmful radiation used in CT or X-Ray, and while there is ongoing research on the safety and long-term effects of all imaging modalities, to this point there have been no proven long-term effects from Non-Contrast MRI.
With that being said there are numerous safety concerns that both the technologist and patient need to address as a team prior to beginning the exam or entering the exam room. There are a variety of reasons that a patient may not be a viable candidate for MRI, and while the screening may seem excessive prior to passing through the doors, this is due to the magnet within the room being constantly on, and its hazardous effects on numerous implanted devices and personal belongings.
Prior to entering the room, the patient will likely fill out a screening form which will ask about numerous implanted devices or the presence of metallic fragments. Marking yes to these preliminary questions may not necessarily exclude you from undergoing an MRI, but will likely warrant further questioning prior to your entrance into the exam room. If you or anyone planning on entering the exam room has an implanted device of some kind it is always the best bet to bring your implant card for the technologist and physicians on staff to approve your device prior to entering. Patient safety is everyone’s number one concern, particularly with unknown implanted devices, and while it can be mildly irritating to undergo such scrupulous processes prior to entering the exam room, the well-being of the patient is the heart of everyone’s concern.
The patient will likely have to dress down into a gown, having removed wired undergarments, and clothing. This is once again for patient safety, as any metal object, once it enters the ever-present magnetic environment can become a projectile. This includes hairpins, keys, cell phones, and will not only throw devices but can damage or remove information from credit and debit cards as well as other electronic devices containing information.
Once introduced to the external magnetic field within the exam room the protons in the nucleus of the hydrogen atoms inside of your body will align themselves either with or against the magnetic field and begin to spin or precess at a particular frequency depending on the strength of the magnet.
The patient will be positioned on the exam table either head or feet first and depending on the exam and anatomy of the patient, and the patient may be positioned on either their back or stomach. It is the technologist’s responsibility to make the patient as comfortable as possible, but as the patient, it is important to remember that the technologist cannot fix chronic pains, and has limits on how comfortable they are able to make an uncomfortable table.
The patient will then have an object placed either on or around the anatomical region of interest. This can be thought of as an antenna, which can receive and/or transmit and receive information from the body. The area is then secured to ensure that the region is as motionless as possible for the duration of the exam, which depending on the region and type of exam ordered can be anywhere from 10 minutes to multiple hours if numerous orders have been placed by your ordering physician.
The patient will then be given an emergency squeeze ball, which can be used throughout the exam to reach the technologist should an emergency or urgent matter arise. The technologist will be able to have two-way communication with the patient inside of the machine, but during the exam, there will be several minute episodes in which the noise within the room will be too loud for communication to occur. This is the purpose of the squeeze ball. The patient and anyone intending on remaining within the room during the duration of the exam will also receive hearing protection at this time, as the machine will get tremendously loud. (Typically construction zone, or rock concert level volumes.)
For some exam types, you will be asked to be an active participant and may have to hold your breath for 15-20 seconds at a time. The more uniform your breath holds are, and the more still you remain during those breath holds will have a great impact on image quality. Jerking motions or huge fluctuations in breathing patterns should be avoided as they will degrade image quality, and may result in repeat sequences (pictures) further increasing exam time.
The patient is introduced into the bore of the magnet until the region of interest is at the very center of the bore of the magnet (called isocenter). This will typically mean that if you are getting an MRI of your Brain, there will be an equal distance above your head to the end of the bore, or tunnel and below your chin to the end of the bore. While it may feel like the magnetic bore surrounds your entire body, depending on your height for most exam types some amount of your body will be sticking out of the end of the bore. This can allow for a reassuring touch from a family member should anxieties arise, but rules on family members within the MRI area vary from center to center.
The machine may be quiet for some time as the technologist gets your information into the computer. The technologist will likely confirm two way communication is working, further reassuring you that you are not alone, which will be followed by the loud noises generated by the machine, marking the beginning of the exam.
The loud noises that you hear are what are called gradient coils within the machine itself. An electrical current running through these coils allow the machine to spatially localize the region of interest, and are an unfortunate, but necessary part of the exam. This spatial encoding allows protons within your body to spin at different rates, known as precessional frequencies, and allow the machine to select protons in a specific area of your body.
These same protons are the same described above and are the protons within hydrogen in your body. While these protons are spinning randomly throughout your body without you knowing it all of the time, when introduced to an external magnetic field they orient themselves either with or against the external magnetic field and precess or spin around the magnetic field as described above.
At this point, an RF Pulse is sent in at the same frequency to disrupt select protons and knock them from a low-energy state into a high-energy state. As they relax they are sampled, and the data is collected and digitized. This relaxation is sampled in numerous different ways which allow the physician to see tissue with several different contrast weightings. That is to say that tissues appear differently in different pictures; water being bright in one picture but dark in another for instance.
These pictures come out individually and when combined create a three-dimensional image that the physician can scroll through to see what is happening in select regions of the anatomy. These images are taken in three separate directions, or planes, and can be taken and viewed front to back, side to side, and up and down.
What Is MRI Used For?
The next fundamental question that numerous patients who have been prescribed or instructed to get an MRI will have is “What is MRI For?” This answer is highly dependent on what your symptoms are that have led up to your physician ordering your exam, and there are several exam types within MRI.
The most common exam type is an MRI without contrast. These exams are typically looking at the soft tissues within a given area to look for any abnormality and pathologic processes or structures within the area. For a knee exam without contrast, for example, the physician may have a suspicion leading up to the exam that they are attempting to confirm. These images will show all of the major structures within the knee including, menisci, tendons, ligaments, muscles, body structures, and vessels within this area. One common diagnosis being investigated with a knee MRI, for instance, would be an ACL tear.
Your physician may have ordered your exam for numerous reasons, be it to investigate or get an updated study of a known disease or pathologic process, to rule out a potential diagnosis, or to further investigate symptoms you have been experiencing. That is to say that being prescribed an MRI does not necessarily mean that your physician is certain something is wrong. It is very possible that they are using MRI as an investigative tool to check and confirm that nothing abnormal is happening.
The next most common is an MRI with contrast. MRI contrast is not the same as the Iodine based contrast used in CT, and if you have been on the receiving end of CT contrast MRI contrast does not have the same “feeling of peeing your pants” effect that comes with CT. In MRI we used Gadolinium contrast dye. This dye is a chelated heavy metal that when introduced into a vein gives the doctor a highlighted image of the vascular (veins, arteries etc.) structures within your body and any possible abnormal structures that these vessels are feeding, for instance, a tumor. Contrast dye can also differentiate surgical scar tissue from pathologies as well as numerous other uses within MRI. These procedures require the patient to experience a small IV poke which allows the introduction of the IV dye. If you are pregnant, think you may be pregnant, or have allergies to Gadolinium based contrast dye be sure to tell your ordering physician as well as your technologist upon your arrival to the center.
MRA is the next most common exam type. These exams are intended to look at the arteries within a specific region of the body, hence the A in MRA (Angiography). These exams are frequently ordered without contrast for the head, but will likely require an IV if ordered from the neck down to the feet. These exams can be used to confirm that flow is adequate within all of the major arteries within a specific area of the body and can be ordered in conjunction with other exam types for further evaluation.
MRV is less frequent still, but like an MRA is dealing with the vasculature. These exams are to view your veins hence the V in MRV (Venography). Once again these exams are intended to view the patency of these vessels or any abnormal pathways or pathologies.
There are a number of other exam types that are far less common and include spectroscopy, fetal MRI, surgical MRI, biopsy, cardiac MRI and more. These exams are far less commonly ordered, and an in depth conversation should be had with your ordering physician, and all physicians involved in your care regarding the protocols, procedures and what to expect with these exam types.
This article was designed to help answer the question “what is MRI and what is it used for.” We sincerely hope it has done so and has proven to be an informative and educational means to answer all of the questions you had coming in. If you have any unanswered questions regarding what to expect or want us to delve further into what MRI is for or the basics of what an MRI is and how to prepare for an MRI please feel free to contact us using the tab at the top of the page.