Magnetic Resonance Imaging (MRI)
Magnetic resonance imaging (MRI) is a tomographic imaging technique that exploits the interaction of powerful magnetic fields with biological tissue to generate detailed images of internal body structures. MRI images typically have high spatial resolution and good contrast between tissue types when compared to other common internal imaging techniques such as X-ray and positron emission tomography (PET). There is also very little energy transfer to the tissues during imaging, meaning that it does not transmit ionizing radiation as do X-ray and PET. Thus virtually no harmful effects have been observed using MRI. Because of the flexibility and acuity of the technique, MRI is exceptional at aiding physicians in identifying and diagnosing cancers and other diseases that involve gross morphological changes in internal tissues. Further, functional MRI (fMRI) allows researchers to uncover spatial dynamics of neurological function, which has significantly contributed to the understanding of brain function.
The history of MRI is dotted with Nobel laureates. The current imaging technology is based on the physics of nuclear magnetic resonance (NMR), which can be used to probe the magnetic properties of atomic nuclei. The NMR was discovered by Isidor Isaac Rabi (1898–1988) in 1937, for which he was awarded the Nobel Prize in Physics in 1944. In 1945, Felix Bloch (1905–1983) and Edward Mills Purcell (1912–1997) simultaneously and independently developed the technique of NMR for identifying molecular structures. By using specific frequencies of radio waves, Bloch and Purcell were able to provoke the molecules to release a small amount of energy based on their structural and chemical configuration. Bloch and Purcell were awarded the 1952 Nobel Prize in Physics for the development of modern NMR methods. NMR continued to be used to study intrinsic properties of chemicals until the 1970s, when imaging using NMR began to emerge. Significant contributions to imaging were made by chemist Paul Lauterbur (1929–2007) and physicist Sir Peter Mansfield (1933–), who developed the method of using magnetic field gradients to rapidly select and induce a signal from specific planes of the body and to resolve spatial localization, both of which are critical to modern MRI. Both Lauterbur and Mansfield were awarded the 2003 Nobel Prize in Physics. The first MRI scanner capable of scanning a human body was developed and used by Dr. Raymond Damadian (1936–) to perform the first human MRI scan on July 3, 1977.
Over the past four decades, MRI has become a staple in diagnostic imaging and research by employing a plethora of constantly evolving techniques. These techniques have been used for a variety of applications, from cancer diagnosis and monitoring to functional brain and cardiac imaging. In the United States alone, there are nearly 100,000 clinical MRI scans performed per year. This number does not include MRI used for research purposes. Although MRI is considered safer than X-ray techniques, its hardware and maintenance are more expensive and usage still lags behind that of computed tomography (CT) scans.
A tomographic imaging technique for clinical use needs a method of developing contrast between tissues of interest, a method of selecting a slice or region of interest to examine, and a method of capturing and encoding the image.
The basic principles of how MRI generates contrast are rooted in the physics of NMR. Permanent magnets, such as those that adorn refrigerators, are strongly magnetic in that they contain ferromagnetic materials, such as iron and nickel. However, there are also weakly magnetic paramagnetic and diamagnetic materials. Examples of these are water, deoxygenated blood, and biological tissue. The weakly magnetic properties of these materials are due to an uneven number of protons and neutrons, which results in a net magnetic spin. While not physically equivalent, one can picture the nucleus of an atom as a spinning top, which spins at a high speed but also has some precession around the axis of rotation at a much lower frequency. When a person is placed in the strong magnetic field of an MRI scanner, a number of the atomic nuclei align the axis of their spin to the direction of the magnetic field. When a pulse of electromagnetic energy at a specific frequency is briefly turned on, the nuclei that resonate precisely with that frequency are flipped onto their sides. This puts the nuclei into a very unfavorable state since they prefer to be aligned with the large magnetic field of the MRI scanner. This is similar to a spinning top or bicycle wheel that has been pushed over trying to realign itself with gravity. Thus, the nuclei attempt to realign themselves with the main magnetic field. At the same time, the previously aligned nuclei that have been flipped onto their sides begin to rapidly lose alignment with each other. A second electromagnetic pulse arrives just in time to reverse this process and kicks them back into alignment briefly. These two processes, the recovery of alignment of the fields to the original magnetic field (T1 or spin-lattice relaxation) and the kickback of the individual nuclei to be briefly aligned with each other (T2 or spin-spin relaxation), result in a signal emitted from the nuclei to be detected by receiver coils in the scanner.
Contrast is a function of how quickly these relaxation processes occur in different tissues. The length of T1 relaxation is based on the environment of the nuclei since the energy of the first electromagnetic pulse needs to be dissipated into the surroundings. Tissues such as fat or gray matter have short T1 relaxation times (100–150 milliseconds) and appear brighter in MRI and liquids such as the cerebrospinal fluid have long T1 relaxation times (1–1.5 seconds) and appear darker. T2 relaxation times are only dependent on intrinsic interactions between protons and so are affected by diffusion and molecular size. In T2 relaxation, gray matter appears darker than cerebrospinal fluid. The respective weighting of the each of these techniques of generating contrast (and others that have not been mentioned), is dependent on how the radiologist designs the pulse sequence, or the timing of when the previously described pulses arrive during a scan. The pulse sequence is critical to developing the correct contrast to diagnose or examine the tissues of interest.
Slice selection is the process of choosing which part of the body is imaged. MRI, in reality, can be used to generate three-dimensional (3D) or two-dimensional (2D) images and theoretically image slices can be taken at any angle, unlike CT in which only coronal images can be taken (a direction from the top of the head to the base of the skull or feet). Slice selection in MRI involves having the radio frequency electromagnetic pulse affect only those molecules in the region of the tissue that the radiologist wants to image. The pulse only affects the nuclei that have a precession frequency (the wobble of a spinning body, such as a top, that creates a cone-shaped rotation) equal to the frequency of the pulse applied. The precession frequency of the nuclei is dependent on the strength of the main magnetic field, which can be made to vary across the body of the patient. For example, if the magnetic field is strongest near the top of the head and is weaker toward the jaw, the nuclei near the top of the head will have a higher resonant frequency and will be excited only by those high-frequency pulses. Therefore a combination of controlled pulse frequency and magnetic field variations is used to choose the region to be excited.
The signal recorded by the receiver coils is encoded in the frequency domain. In addition to the primary magnetic field and the receiver and transmitter coils there are three gradient coils in an MRI scanner, corresponding to the three spatial dimensions. These generate small variations in magnetic field across a slice that translate to differences in precession frequency based on position. The signal picked up by the receiver coils is therefore rich in different frequencies. Using mathematical tools, the frequency-encoded signal can be transformed into a familiar 2D image.
Applications and Techniques
MRI is used widely for clinical applications, especially for the diagnosis and monitoring of soft tissue diseases such as cancers and neurological conditions. This is because MRI has a unique ability to generate significant contrast between different types of soft tissue without the need for contrast agents, such as injected dyes. Many diseases of the brain that cause major changes in the soft tissue, such as multiple sclerosis, Alzheimer’s disease, and strokes, can be detected if the correct pulse sequence is used. For example, by using a pulse sequence known as Fluid Attenuation Inversion Recovery (FLAIR) the radiologist can effectively null the signal from fluids, such as the cerebrospinal fluid, while edematous tissue (swelling) signal remains strong to investigate pathologies such as multiple sclerosis plaques. Careful gating of the MRI image acquisition can allow for the imaging of rhythmically moving objects, such as the heart. This allows health care providers to investigate the physical condition of cardiac valves and coronary arteries that may be associated with disease conditions.
Beyond investigating structural properties of tissue, fMRI can be used to indirectly probe neural activity. Most fMRI is performed using the blood-oxygen-level-dependent (BOLD) contrast pulse sequence. This technique takes advantage of the differences in magnetic properties between oxygenated blood, which is weakly magnetic, and deoxygenated blood, which is more magnetic. The carrier of oxygen in blood is hemoglobin, which has iron-binding sites that are shielded from the magnetic field around them when bound to oxygen but interact more with the magnetic environment when unbound. The technique of BOLD correlates an increase in oxygenated blood in certain areas of the brain with an increase in neural activity and thus may be used to measure locations of gross neural activity. The clinical uses of fMRI studies extend to planning surgeries to avoid areas of important neural activity, diagnosing certain kinds of seizures, and for various behavioral therapies. Because the pulse sequence of high-resolution MRI can take many seconds to complete, fMRI is typically done at lower resolution, which sacrifices image quality for the speed required to detect fast neural activity.
Instrumentation and Safety
The MRI system is particularly cumbersome compared to other tomographic techniques such as CT or PET. The magnet, which is the heart of the system, requires the largest amount of apparatus, maintenance, and general cost for the instrumentation. Most magnets take advantage of superconductive coils of wire, which have virtually no resistance to electrical current at extremely low temperatures and thus allow for the amount of current that is necessary for generating magnetic field strengths of 0.3 to 3.0 Tesla (about 300 times stronger than a typical fridge magnet). The main magnet is often cooled by liquid helium and cannot be turned off without rapidly removing or quenching the helium, which is typically only done in an emergency. MRI systems can cost between 1 and 2 million dollars in initial capital and high consistent cryogen costs. The main magnet poses several safety concerns, as ferromagnetic objects can be violently attracted to the core of the scanner, where there may be a patient. Furthermore, if quenching is not properly controlled, the cryogen may boil explosively. This is why there are many rigid safety protocols and proper emergency quenching equipment to protect the patient.
Aside from the main magnet, MRI systems include smaller magnetic coils that control the gradient fields and radio transmitters, which are responsible for encoding spatial information and disturbing the magnetic spins of the nuclei respectively. During imaging the gradients can switch at very high rates causing large forces between the coils, which are the cause of the characteristic noise of the MRI system. Noise levels can exceed 120 dB, which comes close to the noise of a jet engine and patients are required to wear hearing protection during the scan. These rapidly changing gradients have the potential to cause minor peripheral nerve stimulation, manifesting as mild twitching of muscles, and heating of tissue. In practice, these effects are negligible and are rarely a concern for patients.
As long as proper safety considerations are practiced, MRI systems are considered much safer than CT or PET scans because of the lack of any harmful ionizing radiation. The image quality of an MRI is related to the length of the scan, so a large number of high quality images may take upward of an hour, which can be difficult for some patients. Functional imaging scans may take long extents of time to perform the various tasks required for the experiment as well.
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