Magnetic Resonance Imaging MRI

Unlike radiography, which is based on the differential absorption of the radiation transmitted through a patient, MRI or simply MR detects the quantity and distribution of mobile hydrogen protons within the patient. Although hydrogen is found in a number of compounds within the body, in many instances it is locked into a crystalline structure and is not mobile. However, the hydrogen within the water molecule is quite mobile and is the primary source of the MR image.

A more complete description of the physical basis for the modality can be found in Westbrook and Kaut (1999). For the purpose of this text, a more generalized, less technical overview will be presented.

Hydrogen is an unusual atom. In its most abundant form, it has a nucleus containing only a single positively charged proton. Only in its isotopic forms are one or two neutrally charged neutrons included in the nucleus. In a magnetic field, the proton will spin on its axis, or precess. The rate of precession is proportional to the intensity of the magnetic field.

Before we go further into the imaging process, a review of the basic laws of magnetism is necessary. All magnets have north and south poles. There are lines of force that run from north to south outside the magnet and south to north inside the magnet. Poles with like charges or lines of force repel, whereas unlike poles or lines of force attract. The attraction or repulsion between two magnets is directly proportional to the intensity of the magnetic field and inversely proportional to the square of the distance between them; this is also known as the inverse square law as it applies to magnetism.

Materials can be classified according to their reaction to magnets. Ferromagnetic material, such as iron, is strongly attracted to magnets. Diamagnetic materials, such as gold and aluminum, are weakly repelled. Paramagnetic material, such as gadolinium, is weakly attracted. Nonmagnetic materials, such as glass, ceramic, and wood, are not affected at all by magnets.

The magnetic field that we experience when walking about on the Earth is relatively low and equivalent to about 0.5 gauss (G). A "gauss" is an older and less frequently used unit of magnetic field measurement in medical imaging. It is strong enough that a ferromagnetic compass needle can indicate the direction of the magnetic north pole; however, it is not of sufficient strength for imaging purposes. The unit of magnetism used for imaging purposes is the tesla (T), named after Nicoli Tesla, who did extensive research with electricity and magnetic fields. One tesla equals 10,000 G. For high-field MRI, the magnetic field strength is 1.5 to 3 T, or roughly 30,000 to 60,000 times stronger than the Earth's geomagnetic field.

Now back to the H1 protons. Normally, these protons are randomly aligned throughout the body (Figure 3.49A). When a strong external magnetic field is applied (B0), the protons are forced into alignment with the overwhelmingly more powerful external field. They tend to align either parallel (low energy) or antiparallel (higher energy) to the field (Figure 3.49B). Parallel and antiparallel pairs cancel each other out. Since nature favors a lower-energy state, there will be a greater number of parallel protons. This imbalance leaves a remainder of protons available for imaging (Figure 3.49C), and aligned with B0, but out of phase (Figure 3.50). In a 1.5 T magnetic field, approximately 7 out of every 1 x 105 hydrogen protons will remain. That may not seem like many, but since humans are made up of 70%-80% water and there are 1 x 1023 hydrogen atoms per cubic centimeter, there are literally millions of individual hydrogen protons available for imaging, though the overall number is dependent on the tissue type and the strength of the magnetic field.

When radiofrequency (RF) energy is transmitted into the sample, several things will happen. First, all the remaining protons will be gathered together from their naturally out-of-phase precessions and be placed in step with each other. Once organized in this fashion,

Figure 3.49A Randomly aligned hydrogen protons within the body.

Anti-parallel

Parallel

Anti-parallel

Parallel

Figure 3.49B When a strong external magnetic field (B0) is applied, the protons tend to align antiparallel or parallel.

Figure 3.49B When a strong external magnetic field (B0) is applied, the protons tend to align antiparallel or parallel.

Figure 3.49C After the antiparallels cancel out the parallels, the remaining protons are available for imaging.

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