What is Magnetic Resonance Imaging and How Does it Work ?

By Aneta Chmielewski

What is MRI ?

 Magnetic resonance imaging (MRI) is an imaging technique used primarily in medical settings to produce high quality images of the inside of the human body.

 More specifically the images of protons in water. Hydrogen is used because it is plentiful in the human body and has the highest NMR signal of any atomic species.

 Based on the principles of Nuclear Magnetic Resonance



A Brief History of MRI

Felix Bloch and Edward Purcell were awarded the Nobel Prize in 1952, for discovering the magnetic resonance phenomenon independently in 1946.  1950-1970: NMR was developed and used for chemical and physical molecular analysis (uniform magnetic field).  In 1973 the x-ray-based computerized tomography (CT) was introduced by Hounsfield  Early 1970s Paul Lauterbur and Peter Mansfield utilization of gradients in magnetic field.  1975 Richard Ernst proposed magnetic resonance imaging using phase and frequency encoding, and the Fourier Transform  In 1993 functional MRI (fMRI) was developed. This technique allows the mapping of the function of the various regions of the human brain.

Nuclear Magnetic Resonance

   All atoms consist of outer shells of negatively charged particles called electrons buzzing around in diffuse clouds, and a dense central portion called the nucleus. Some of these nuclei behave like small bar magnets and when placed in a powerful magnetic field about half line up in the direction of the magnetic field and about half line up in the opposite direction. The nuclei in opposing directions will cancel each other out but a few out of a million will not.

By providing energy in the form of radio waves these tiny magnets can be caused to change orientation, to resonate absorbing energy at a resonance frequency that depends directly on the strength of the magnetic field. The frequency of this precession is described by the Larmor frequency. ωo = -γHo

Resonance Frequency

The frequency of the radiation necessary for absorption of energy depends on three things:  First, it is characteristic of the type of nucleus (e.g., 13 C). 1 H or  Second, the frequency depends on chemical environment of the nucleus. For example, the methyl and hydroxyl protons of methanol absorb at different frequencies, and amide protons of two different tryptophan residues in a native protein absorb at different frequencies since they are in different chemical environments.  The NMR frequency also depends on spatial location in the magnetic field if that field is not uniform everywhere.

NMR to MRI

     At first the magnetic resonance technique was used mainly as a research tool for determining the structures of molecules. It relied on using very uniform magnetic fields, so that every part of the sample was exposed to the same field. It was more than twenty-five years after the original discovery of the magnetic resonance phenomenon that the possibility of using non-uniform magnetic fields which varied in a know way through out the sample to produce images was first realised. This results in each distinct part of the sample experiencing its own unique magnetic field which is characteristic of its position. Nuclei each at a different position will have different characteristic resonance frequencies.

So, detecting the resonance frequencies of the nuclei becomes equivalent to detecting where they are in the sample, and detecting the size of the signal tells you how many nuclei there are at that position. With information relating number of atomic nuclei with position in the sample it is possible using computer programs to reconstruct a detailed three-dimensional image of the whole sample which can then be examined on a monitor screen as cross-sectional slices in any direction.

The implementation of these ideas independently by Lauterbur in New York State University at Stonybrook and Mansfield and Grannell at Nottingham University signalled the beginning of a new imaging method.

So how would this work in the patient..

    When a patient is subject to a magnetic field (located straight down the center of the tube the patient is placed into) the H atoms in his/her body will line up in the direction of either his/her head or feet.

The vast majority of the H each other out, but a couple out of a million will not.

+ will cancel When an RF pulse specific to only H is applied to a specific part of the body being examined, the protons not cancelled out will absorb the energy required to make them spin or precess in a different direction a specific frequency called the Larmour frequency.

The RF pulses are applied through a coil, designed for different parts of the body and conform to the contour of the body.

In the patient..

      By switching three small gradient magnets (18-27mT) on and off a variable magnetic field is formed.

The large magnet immerses the patient in a stable and very intense magnetic field.

By altering the gradient magnets, we can choose exactly which specific area of the body we want to analyze in slices.

When the RF pulse is turned off, the H protons begin to slowly return to their natural alignment within the magnetic field and release their excess stored energy.

The released energy, gives off a signal that the coil now picks up and sends to the computer system.

The mathematical data is converted through the use of a Fourier transform, into a picture that we can put on film. Electronics and Computer Processing

Stable Magnets For MRI

 Commonly in the range of 0.5 to 2.0 T Three types of magnets used: a)

Resistive

operate.

– many windings or coils of wire wrapped around a cylinder through which an electric current is passed, generating a magnetic field. Good for field of less than 0.3T, require large amount of money to b)

Permanent

- Very heavy, weight is proportional to magnetic field strength.

c)

Superconducting

– most commonly used. Similar to resistive magnet but continually bathed in He at -452.4 °C, insulated by a vacuum. The cold temperature decreases the resistance in the wires, allowing for easy generation of 0.5-2.0 tesla fields. Very expensive.

Visualization

 Most imaging modalities such as CT and X-ray scan use injectable contrasts or dyes for certain procedures. These agents work by blocking the X-ray photons from passing through the area where they are located and reaching the X-ray film. This results in differing levels of density on the X-ray/CT film.  MRI contrast works by altering the local magnetic field in the tissue being examined.  Normal and abnormal tissue will respond differently to this slight alteration, giving different signals.

 These varied signals are transferred to the images, allowing us to visualize many different types of tissue abnormalities and disease processes better than we could without the contrast.

Advantages and Disadvantages of MRI

 Advantages: - do not use ionizing radiation - very low incidence of side effects - Ability to image any plane: axial, sagitall, coronally - Ideal for orthopaedic and neurological applications.

 Disadvantages: - Lower sensitivity then CT and X-Ray scans.

Many people who can not be scanned by MRI because they have metal in their body or are too big to be scanned or are claustrophobic.

Make a tremendous amount of noise. The stronger the main field, the louder the gradient noise.

MRI scans require patients to hold still from 20 to 90 minutes or more. Very slight movement can cause very distorted images that will have to be repeated.

Orphopedic hardware (screws, plates, artificial joints) in the area of a scan can cause severe distortions on the images. The hardware causes a significant alteration in the main magnetic field.

MRI systems are very expensive to purchase.

Current and Future developments of MRI

      Functional MRI (FMRI) is a technique that has recently been introduced to obtain functional information from the central nervous system. FMRI detects subtle increases in blood flow associated with activation of parts of the brain. FMRI may be useful for preoperative neurosurgical planning, epilepsy evaluation, and "mapping" of the brain. Looking at Hydrogen atoms in fat.

Detection of other atoms: recently, excellent MRI images of the airways in human lungs have been obtained by detecting inert gases such as helium or xenon inhaled by the patient. Improvements in strength of superconducting magnets at lower costs. A less claustrophobic design, such that the patient does not have to lie on the magnet bore. MRI for pregnant patients.

References

Gould, T.A. (2004) How MRI works.

January 24, 2004. www.howstuffworks.com/mri.htm

Hoole, P.R.P. Electromagnetic Imaging in Science and Medicine with wavelet applications. Wit Press: Boston, 2000.

Hornak, J.P. (2004) The Basics of MRI.

January, 24, 2004. www.cis.rit.edu/htbooks/mri/