Answer to Question #13577 Submitted to "Ask the Experts"
Category: Medical and Dental Patient Issues — MRI
The following question was answered by an expert in the appropriate field:
I understand MRI (magnetic resonance imaging) does not expose a person to ionizing radiation, but I also have concerns about the growing evidence about issues with even nonionizing radiation. I recently had an MRI scan of the brain and cervical neck and have a question regarding how this compares to the nonionizing radiation found via a cellphone. I came across this web page from a medical imaging facility. The page notes: "MRI will not expose you to any ionizing radiation. On the electromagnetic spectrum, AIM Medical Imaging's MRI is on the lowest end at 1–100 MHz, which is the wavelength equivalent of AM Radio. At this very low frequency, AIM's 90-minute Whole Body Diffusion MRI scan with a 1.5 T magnet is the radiation equivalent of four hours on a cell phone." I wonder if this comparison is valid and at all accurate. Can nonionizing radiation exposure from an MRI be compared to a cellphone call? If so, would a 30-minute MRI be similar to, say, being on a 1.3 hour-long cellphone call?
This is a tricky question to answer without getting too technical, partly because of the comparison to cell phones, which muddies the waters.
First, it is true that there is no ionizing radiation used in MRI. Ionizing radiation is the stuff people think about with regards to causing cancer or mutations. The electromagnetic radiation used in both MRI and cellphones is in the radio frequency (RF) range. The concern about using RF in both cell phones and MRI has to do with the tissue’s absorption of the RF can cause tissue to heat up. The metric most often used for this is called the specific absorption rate (SAR). It measures the amount of radio-frequency power absorbed per amount of tissue. Thus, it depends on the amount of tissue exposed. Food and Drug Administration (FDA) standards sets standards for SAR based on how much tissue heating is acceptable, which is usually to limit the amount of tissue heating in the head to less than 1°C, which corresponds to 3.2 watts per kilogram (W kg-1) SAR. The calculation of SAR in an individual patient during an MRI exam is somewhat complicated, as a patient does not only just heat up, but also cools down though the body's natural processes for eliminating heat, and what parts of the body are exposed to the RF. Other factors such as patient height, weight, age, etc., have to be considered. The exact same MRI exam on the same scanner can have different SAR values for two different patients. For this reason, MRI scanners have built in methods to calculate and measure the SAR dose for individual patients and exams and built-in safeguards to ensure that safe limits, defined by regulatory bodies such as the FDA, are not exceeded for any patient. If they do exceed the limits, the scanner stops and will not scan for some period of time to allow the patient to cool down.
The comparison between cell phones and MRI is somewhat misleading, as tissue heating from a cell phone is confined to a relatively limited volume in the head near the phone, while an MRI deposits energy over a larger region of tissue. For example, the 90-minute whole-body diffusion exam in the AIM example you give distributes energy all over the body (not all at the same time), whereas four hours on a cellphone confines the heating to a very small area of the brain. So, it is not really valid to say that a 90-minute MRI exam is the same as a four-hour phone call, and that a 30-minute MRI would be the same as a 1.3-hour phone call. These comparisons are really just to give people a (very!) rough idea of the energies involved.
An MRI of the brain and cervical spine is a very widely performed routine exam. As long as the exam has been performed on a commercial medical MRI scanner, the tissue heating (which occurs only during the exam), should not be a concern for a patient.
Mark Brown, PhD