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Magnetic Resonance Imaging

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Magnetic resonance image (MRI) is a cross-sectional imaging technology that uses a magnetic field and radiofrequency signals to cause hydrogen nuclei to emit their own signals, which then are converted to images by a computer.

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History

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MRI presents a radical departure from imaging methods based on the absorption of x-rays (conventional radiography and computed tomography [CT]) or reflection from tissues (ultrasound). The magnetic resonance phenomenon has been known since the 1940s. Initially employed to determine the structures of molecules, it was discovered by Felix Bloch and Edward Purcell, two American scientists, who were awarded the Nobel Prize in physics in 1952 for their discovery. In 2003, Paul C. Lauterbur and Sir Peter Mansfield were awarded the Nobel Prize in medicine for their contribution to the development of MRI for medical purposes.

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Principles of MRI

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Simply stated, MRI is based on measurements of energy emitted from hydrogen nuclei following their stimulation by radiofrequency signals. The energy emitted varies according to the tissues from which the signals emanate. This allows MRI to distinguish between different tissues.

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The Magnetic Resonance Phenomenon
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Magnetic resonance is the process by which nuclei, aligned with an external magnetic field, absorb and release energy. Many molecules display magnetic resonance, but for all practical purposes MRI is based on signals from hydrogen nuclei in water molecules. Because these nuclei consist of only a proton, the hydrogen nucleus is referred to simply as the proton in the context of MRI.

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Alignment of Protons in an External Magnetic Field
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The process of image acquisition begins by placing the patient in the scanner, which contains coils that produce an extremely strong magnetic field. In the magnetic field, protons line up like tiny bar magnets, either in the direction of the magnetic field or in the opposite direction.1 There is a slight difference between the number of protons lining up parallel with the magnetic field and the number lining up in the opposite direction. This difference gives rise to net magnetization, parallel with the external magnetic field, referred to as longitudinal magnetization.

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Altering the Alignment of Protons
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A pulse of radiofrequency (RF) waves is applied at right angles to the longitudinal magnetization. This pulse alters the alignment of the protons to a transverse plane, and the energy absorbed in the process brings them to a higher energy state: transverse magnetization (Fig. 5-1).

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Figure 5-1

Displacement and gradual realignment of protons. In this image, the protons are aligned with the main magnetic field (1) when a RF pulse is applied at right angles (2), resulting in an altered alignment of the protons. After the cessation of the RF pulse, the protons gradually return to alignment with the main magnetic field (3).

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