Magnetic Resonance Imaging
What is it? What does it measure?
Magnetic Resonance Imaging (MRI) uses the complex but well understood phenomena of Nuclear Magnetic Resonance (NMR) to allow us to map the distribution of water within the body. The NMR phenomena in bulk materials dates back to the 1940's and uses radio waves and large magnets to initiate an intricate "dance" of the nuclei of the hydrogen atom within water. This is possible since the hydrogen nucleus is magnetic, resembling a tiny compass needle. The characteristic motion started by the burst of radio waves is then detected as a faint radio signal emanating from the wobbling nuclei themselves. This is detected by a sensitive radio wave detector system.
Because the NMR phenomena is subtly altered by the environment of water, MRI can do far more than just map the density of water in the body. Almost every physical and chemical property we associate with an object, such as temperature, chemical content, molecular structure, fluidity, flow, electrical conductivity and many others leaves a subtle mark on the NMR signal we detect. This has allowed MRI to be remarkably flexible for imaging the body, were water is used in many complex ways that differ across organs in healthy tissue and are altered in disease. For example, MRI can detect the location of neuronal processing in the brain associated with a mental or physical task by detecting the effect of increased blood flow and blood oxygenation to that region of the brain.
How does it work? What equipment is needed?
MR imaging is fundamentally different from other types of imaging. Conventionally, when we refer to “imaging” we mean a light scattering experiment. Since our eyes “see” a rock by forming a 2 dimensional array of the light intensity being scattered off the rock, the instrumental equivalent of this has come to be synonymous with imaging. In the canonical scattering experiment, “rays” are aimed at an object and then detected as they either scatter off of the object or penetrate through it. The “rays” in question are usually electromagnetic radiation (radio waves, microwaves infra red light, visible light, ultraviolet light, x-rays, or gamma rays) but can, of course, be almost anything including sound waves, water waves, or matter waves (particles).
MR imaging uses a completely different principle; one that uses the complexity of the NMR phenomena and the fact that the water nuclei "answer" our radio wave pulse, which sets up the NMR motion, with a faint response detected some milliseconds later. The radio waves themselves are far too coarse to tell us much about the body simply from their scattering. As an illustration of the difference between MRI and conventional imaging, consider how you would "image" the location of everyone in the United States. One could image the location of all the people in North America by setting up a large camera in space with a giant flash bulb and wait for a clear day to snap a picture. This is the traditional scattering experiment approach to imaging. Light is the scatterer, and the film detects the scattered intensity as a function of position. A much more sensible approach is taken by the US Census Bureau. They send out a questionnaire to everyone asking them to respond with their current address. After waiting to receive this information, all of the responses are dutifully recorded and a plot of the population density of the US is reconstructed.
In MRI the hydrogen nuclei in water respond with their location encoded in the frequency and phase of the radio waves they emit. We supervise and enforce this encoding by controlling the spatial distribution of the applied magnetic field, and thus the frequency and phase of the emitted radio signal from the water. (There is always an enforcement/supervision step needed in these non-passive types of imaging!) We apply magnetic field gradients in which the magnetic field varies linearly with position, and therefore the frequency and phase of the detected NMR signal varies linearly with phase and position. It is the job of the image reconstruction computer to then tabulate the responses and reconstruct a plot of water density as a function of position within the body.
Thus the MRI method uses several devices for generating and manipulating a large magnetic field around the body, as well as a radio wave generator and detector. The large magnetic field is generated by a large superconducting magnet; a spool of wire with hundreds of turns of wire with hundreds of amperes of current generating a large magnetic field. This is the massive (>10 tonnes) imposing device into which the patient is slid. It generates a magnetic field tens of thousands of times larger than the earth's magnetic field. The radio wave detector is similar to an antenna, and is usually placed directly on the body.
What do the data look like? How is information extracted?
For a single image, the data are reconstructed as a 2 or 3 dimentional image matrix of the water density modulated by the physical environment of the water. The researcher chooses the type of physical information s/he is interested in by altering how the NMR signal is excited or from further analysis of the signal itself. For time-resolved imaging, a series of slices or volume images are acquired and analyzed for changes in their temporal time course.
A single MR image can be encoded in about 30ms. Including the excitation phase, the whole process can be as short as 50ms, allowing 20 frames per second. High resolution detailed volumes (of a brain, for instance) can take as long as 10 minutes to acquire.
The spatial resolution of MRI acheived in the human brain depends on the desired imaging time. For fast imaging (more than 10 images per second), resolutions of 2 to 3mm are common. For slowly acquired images, resolutions as fine as 400um have been obtained, although 1mm is more typical.
What are some features and benefits of MRI?
The principle feature of MRI is its ability to take high spatial resolution 2D or 3D images of the body in many different ways; each method providing different insight due to the different physical mechanisms which modulate the image intensity (see Research and Clinical Applications, below). Additionally, MRI is a truly non-invasive tool that has proven safe over millions of examinations.
What are its limitations?
MRI scanners are costly instruments to purchase and maintain. Because the technique is so flexible and constantly improving, the personnel involved must receive regular training. MRI is also a relatively slow technique; high resolution images require the patient to hold still for up to 10 minutes. Even the dynamic imaging methods of MRI are not always fast enough to capture biological motion (such as the heart) without considerable effort. Finally, the information in the MR image is often only an indirect measure of the physiological parameter of interest.
Research and clinical applications
MRI has proven to be the method of choice for detecting disease in soft tissue. It has found a wide range of applications in most parts of the body and for many diseases. In a ranking of how medical interventions and technology affected their ability to treat patients, physicians ranked MRI and CT as the most important innovation (Viagra was number 28). (Fuchs and Sox, Health Affairs 2001 20(5) p30-43.) While the MRI examination is expensive, it has nearly eliminated costly and potentially dangerous "exploratory surgery" procedures.
In research, MRI has provided the ability to study disease processes and the response of disease to therapeutic treatment in the only truly relevant laboratory setting; the human body. It has therefore proved a valuable tool for honing disease treatments as well aiding diagnosis. The diagnostic power of MRI has also allowed the sub-classification of complex disorders, giving hope that more specific treatments can be targeted at individuals.
In the last 10 years MRI has proved to be a powerful research tool for studying human brain function in disease and health. Using the local changes in blood flow and oxygenation that occur where the brain is relatively active, fMRI can localize brain activity and allow the mapping of function to location. More importantly, the fMRI experiment allows the study of the underlying networks within the brain.
Teamed with other imaging techologies MRI can provide very detailed information about the location and mechanisms of a variety of physiological functions throughout the body.
Many MR imaging techniques are being developed and applied to a variety of biomedical research areas at the Martinos Center. Click below to learn more about them.
by L.Wald, updated 2/2005
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