About MRI and Related Technologies
What is magnetic resonance imaging (MRI)?
Magnetic resonance imaging (MRI) is a technology for visualizing body structures that depends on harmless magnetic fields and radio waves rather than X-rays and other forms of ionizing radiation. MRI's diagnostic and research value lies in its ability to provide high-quality images of soft tissue structures that X-rays cannot distinguish.
MRI depends on the fact that many nuclei, such as the hydrogen atoms in water, in the presence of a powerful magnetic field become aligned either with or against the field and rotate like spinning tops.
The AIRC has MRI scanners that operate at fields up to 9.4 Tesla for animal studies or 7 Tesla for human studies. One Tesla equals 10,000 gauss, and the earth's magnetic field is about 0.5 gauss. Thus, a 7 Tesla magnet generates a magnetic field that is about 140,000 times the earth's magnetic field. The higher the magnetic fields used in MRI, the better the sensitivity and resolution of the images produced.
Once the MRI machine aligns the spinning nuclei in its intense magnetic field, it probes those atoms with pulses of radio frequency (RF) waves that perturb this alignment, like flicking spinning tops off balance. When this RF field is turned off, the atoms realign their spins and in the process reemit a radio frequency signal that is measured by a detector.
MRI can distinguish biological structures from one another because atoms in different biological environments show a characteristic frequency and timing of this reemitted RF signal. To further enhance the image of specific structures, researchers can inject chemicals called contrast agents that react strongly to magnetic fields to highlight structures selectively, such as tumors or other tissue with specific characteristics.
To create MRI images of an individual tissue or the entire body, the MRI machine uses magnetic field gradients that spatially localize the atoms reemitting RF frequencies from the tissue or whole patient.
What is functional magnetic resonance imaging (fMRI)?
In functional MRI, the brain is scanned about every second or so as a human subject performs some mental task. The MRI machine’s parameters are adjusted so that it can distinguish oxygenated hemoglobin from deoxygenated hemoglobin, called the blood oxygen-level dependent (BOLD) signal. Since more active brain areas use more oxygen, researchers can thus distinguish brain regions that activate during the task by an increase in flow of oxygenated blood to that region. By detecting such activated brain regions, researchers gain insights into the neural mechanisms responsible for particular mental tasks.
What is magnetic resonance spectroscopy (MRS)?
While MRI scans enable clinicians and researchers to image body structures, magnetic resonance spectroscopy (MRS) gives them insights into the biochemical characteristics of a selected region or tissue—for example, information on unique molecules that are present in higher concentrations in a tumor that indicate its malignancy. MRS detects molecules that are normally present in brain tissue at much lower concentrations than water. An MRS spectrum consists of an array of peaks that show the identity and number of biomolecules present in the brain in any particular region.
What are MRI RF coils?
In MRI, RF coils transmit radio frequency energy into tissues and receive the resulting reemitted radio frequency signals that arise from realignment of the perturbed nuclei. The geometries of such coils are tailored to image particular regions, for example the brain or a whole body. Multiple coils may also be used to receive parallel channels of information, providing even more information about broader regions of tissues and other structures.
What is hyperpolarization?
Hyperpolarizing atoms within a molecule means "supercharging" their spin polarization. Hyperpolarizing a molecule involves first freezing a sample to the liquid helium temperature of -452 degrees Fahrenheit to eliminate thermal motion. The sample also contains a polarizing agent, a chemical that is naturally highly polarized at these temperatures. In the hyperpolarization instrument, the sample is subjected to a high magnetic field to align the spin of its atomic nuclei, and when it is bombarded with microwave radiation, the polarizing agent absorbs energy and transfers it to the molecule, hyperpolarizing it. This process is called dynamic nuclear polarization (DNP).