Magnetic resonance imaging

Magnetic Resonance Imaging, or MRI, is a method of imaging the interior of structures noninvasively. An MRI device consists of a magnet, magnetic gradient coils, an RF (radio frequency) transmitter and receiver, and a computer that controls the acquisition of signals and computes the MR images. The full name, Nuclear Magnetic Resonance Imaging, usually shortened to MRI, describes the technique. If an atomic Nucleus is exposed to a static Magnetic field, it Resonates when a varying electromagnetic field is applied at the proper frequency. An Image is computed from the resonance signals of which the frequency and phase (timing) contain space information. MRI is important because it is noninvasive, safe, and yields information that cannot be obtained with any other techniques. Its most common use by far is in diagnostic medicine but MRI has other applications, particularly in the oil and food industries.

 Basic principles

The nuclei of many kinds of atoms, commonly hydrogen, are tiny magnets. In the earth’s magnetic field they line up to some extent just as you walk around. When you walk past a piece of iron they’ll flop around in different directions. Think of us as having microscopic compass needles precessing (spinning on their axes like gyroscopes) in an orderly direction. To make an MR image, this tendency of the nuclei to line up in the direction of a magnetic field can be manipulated and measured. Since the nuclei from different regions of the body can be made to precess at different frequencies (their magneto-resonance frequencies), the electromagnetic energy at these frequencies yields signals that are location dependent. Computer images can be calculated, enhanced, and displayed. MRI is safe because only a very tiny amount of energy is absorbed or emitted, corresponding to the amount of energy in radio waves, to which we are constantly exposed. MRI does not affect any chemical processes. It doesn’t change molecules at all. The atomic nuclei within the molecules just report what is happening.
In the presence of a static magnetic field (B0), the atomic nuclei possess energy which differs depending on their orientation (ΔE). ΔE determines the strength of the signal and is related to the resonance frequencies (ν), by Planck’s constant (h).

ΔE=hν(1)

The size of ΔE and ν depend on the size of the static magnetic field, (B0), and the magnetogyric ratio, (γ), a characteristic of each kind of atomic nucleus, as shown in equation 2. This is the Larmor (Joseph Larmor, 1857 - 1942) relationship. The Larmor equation (2) is fundamental to all of nuclear magnetic resonance (NMR) and its subfield, MRI.

ν=(γ2π)B0(2)

Together these two equations determine that

ΔE=hν=hγ2πB0(3)

I.I. Rabi (Nobel Prize in Physics, 1944) demonstrated the phenomenon of nuclear magnetic resonance in 1937, and Felix Bloch and Edward Mills Purcell, working independently and within one month of each other (December 1945 and January 1946), demonstrated the use of radio waves to detect nuclear magnetic resonance signals, for which they jointly received the Nobel Prize for Physics in 1952. With their discovery, nuclear magnetic spectroscopy was born, without which chemistry would not be modern chemistry and without which MRI could never have been invented.
In the static magnetic field, B0 , the nuclei are primed for a response to a transient magnetic field B1 at their resonance frequency, ν , which changes their energy status. When the transient field is switched off, the nuclei emit radio waves as they return to their steady state condition. These reradiated signals yield NMR spectra or MR images. Many different atomic nuclei (those that possess a net nuclear spin) are susceptible to NMR, and a few produce signals that are strong enough for diagnostic MRI. The nucleus of the hydrogen atom (a single proton) has the largest magnetogyric ratio and, according to equation 3, has the highest energy and therefore the largest signal at any given field strength. Hydrogen is abundant as an element of water. The human body is about two thirds water, so the combination of natural abundance and signal strength determines that imaging with the hydrogen nucleus gives the highest possible resolution. Nearly all clinical MRI is proton or hydrogen MRI. Other nuclei such as ²³Na and ³¹P are also used, although until now primarily in research.
For MRI, as opposed to NMR, spatial gradients of magnetic field (G) are also needed. G makes the field experienced by each nucleus dependent upon its location within an object. For example Gx is a linear gradient along the x-axis and produces an extra field, GxX at the point of X. Together Gx, Gy, and Gz determine a unique point (X,Y,Z) in three-dimensional space. Equations 2 and 3 are modified to:

ν=γ2π(B0+G)(4)

E=h2πγ(B0+G)(5)

where for two-dimensional imaging G is usually Gx and Gy, with Gz added for three-dimensional imaging.
Since the atomic nuclei in different physical positions experience different values of (B0+G) the problem of making an image is now transformed into a problem of interpreting the spectral frequencies.

Simple model of MR imaging

Models the first MRI study ever done², and serves to illustrate the basic principle. Two capillaries of water are within a cylindrical test tube in the sample holder of an NMR spectrometer. This is not one of the large imaging systems of today, but a machine that will fit nothing larger than 5 mm in diameter. Magnetic field gradients are applied at 45⁰ intervals in the XZ-plane. As shown by the arrows, which represent changing magnetic field or signal frequency along each projection, humps of NMR signal appear at spectral frequencies corresponding to the positions of the water protons in the tubes. The physical positions of the nuclei are now encoded as spectral frequency. These are one-dimensional projections, from which a two-dimensional image can be made. Paul Lauterbur and Peter Mansfield shared the Nobel Prize in Physiology or Medicine in 2003 for Lauterbur’s discovery of MRI in 1972, and its enhancements by Mansfield. Since that time many complex techniques of encoding position and computing images have been developed to improve quality, speed, resolution and contrast in MRI. Richard Ernst, Nobel Laureate for Chemistry in 1991, and his colleagues³ provided an early enhancement by demonstrating that Fourier Transform methods (mathematical conversion of the time decaying signals to their frequencies) would greatly improve MR images. Another important early step was the realization that if one physical dimension is encoded by frequency, the phase, i.e. the offset or timing of signals at the same frequency, could be used to encode additional directions. In 1977 Peter Mansfield and his colleagues⁴ introduced Echo-Planar Imaging (EPI), a method that uses both frequency and phase encoding, thus allowing all physical directions to be observed simultaneously⁴. The increased speed of EPI makes possible imaging of structures that change quickly, such as the beating heart. New MRI methods are invented regularly, each having different advantages and disadvantages, depending on the type of image desired. While recent and current practices of MRI in medicine have used techniques to acquire two-dimensional images, in principle and increasingly in practice, the methodology can be extended to three-dimensions or more, the only limitation being the amount of resonance signal that can be acquired and the capability of the hardware and software used for image formation. MRI techniques are still evolving rapidly, as they are optimized for specific applications.

 Physiological MR imaging

The magnetic resonance behavior of the atomic nucleus is determined by the surrounding magnetic field it experiences, and thus by a large number of different parameters, including blood flow, chemistry, chemical exchange, diffusion and other physiological phenomena. An image that contains information about these parameters provides information on how tissues and organs function, both normally and in disease. Specific MRI techniques have been developed and continue to be developed that highlight changes in these phenomena and emphasis different physiological states or differential diagnosis of disease.

 Functional image of a finger moving task

Magnetic resonance angiography

Magnetic resonance angiography is used to generate pictures of arteries in order to evaluate them for potential ruptures or for abnormal narrowing. MR angiographpy was first introduced in the late 1980s, and a number of different specific methods are now used.

Functional imaging

Functional imaging is based Seiji Ogawa’s discovery with his colleagues⁸ in 1988 that small veins in the brain give extra contrast to the image. The phenomenon was named the BOLD effect, for Blood Oxygenation Level Dependent signal changes. Deoxyhaemoglobin in blood is paramagnetic and therefore distorts the magnetic environment of the surrounding water molecules. In general, the brain uses more oxygen when it is active, and the local blood flow increases to supply even more oxygen than is required. This over-supply leaves its carrier, haemoglobin, more oxygenated and the magnetic distortion by deoxyhemoglobin decreases. This is the basis of the BOLD effect and of most Functional Magnetic Resonance Imaging or fMRI, a noninvasive way to assess brain function. It has been shown that the BOLD effect correlates directly with electrical communication among nerve cells, the synaptic activity.

Diffusion tensor image of fibre tracks in the normal brain

Diffusion imaging

Diffusion imaging records the rate and direction of water (or sometimes of metabolites) diffusion within body organs. The technique is useful in observation of strokes, in which the water of edema diffuses particularly freely. A variant, 'diffusion tensor imaging' or 'diffusion tractology' provides spectacular images of tracts of muscle or nerve fiber bundles, because water diffusion is much faster along the length of the fibers than across them. These images are clinically useful in showing interuption of normal fiber anatomy by tumors or trauma.

Spectroscopic imaging

Spectroscopic imaging was first described⁹ by Paul Lauterbur in 1975. The technique combines the effect of molecular structure on the magnetic field experienced by an atomic nucleus, the Chemical Shift, with the effects the magnetic field gradients used in MRI. Chemical shifts show different chemical entities in a spectrum and are thus the basis of NMR in chemistry. Chemical shifts are combined with MRI to make physical maps of molecules that are important to cellular function.
This Spectroscopic MRI or Chemical Shift MRI has enormous potential because it allows direct observation of the chemical basis of disease. Spectroscopic MRI is difficult because of formidable sensitivity problems, and has not yet lived up to its promise. Metabolically important chemicals are best observed using insensitive atomic nuclei that are present in concentrations only one thousandth or less that of body water. The sensitivity may be improved by the use of 'a priori' techniques (for example, use of a high resolution proton image to constrain the computation of the spectroscopic image); these techniques appears promising.