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A magnetic resonance image (MRI) is a type of diagnostic scan that can show highly detailed pictures of the interior of the body. With their high contrast, MRIs are the tool of choice for mapping complex organs such as the brain and heart, as well as joints and muscles. Rather than using bursts of radiation like an X-ray, an MRI image is produced by using strong magnetic and radio frequency (RF) fields.
A MRI image is most often performed to discern the presence of potentially damaged or pathologic tissue. The reasons could range from a traumatic injury such as a muscle strain, to a subtler problem such as possible cancer. In these cases, a traditional X-ray or even a computed tomography (CT) scan is not ideal. An MRI image, which is produced through the use of RF waves as opposed to ionizing radiation, is better suited for renderings of soft, non-bone tissue.
Unlike a simple X-ray, the way an MRI image is taken can be tweaked to produce a variety of different results, depending on what a technician wants to highlight. Collectively, a particular group of settings is known as a pulse sequence. Pulse sequences can be equated to the way different shutter speeds and aperture sizes on a camera can produce different images of the same subject. Modern MRI machines store catalogs of pulse sequence settings for use in different situations.
Echo time and repetition time are two constituent parts of a pulse sequence, and can be adjusted up or down. A basic MRI image shows fat cells brighter than water, and is good for rendering joints and muscles. With what is known as a T2-weighted scan, the contrast is reversed and is ideal for scans of the brain and its highly fatty white-matter. A variety of other specialized scans are used to highlight different combinations of tissue.
In addition to varying levels of contrast, advanced MRI images can show time lapse frames, three-dimensional renders, and even close-to-live scans of the brain, known as functional MRI. A functional MRI takes a scan of the brain every several seconds while the patient is exposed to varying levels of stimuli. It can show whether or not a brain is functioning normally when compared to known patterns, as blood flow shows up as flares on the images.
Through the early 21st century, progress has been made in real-time MRI. This technique produces live images, making it ideal for scans of the heart and can show where valves may be malfunctioning as blood is pumped through. Real-time MRIs output movies as opposed to single-frame images, though screen captures can isolate individual frames for added scrutiny.