Riccardo Marano Institute of Radiology, Dept. Agostino Meduri Institute of Radiology, Dept.
Giancarlo Savino Institute of Radiology, Dept. Luigi Natale Institute of Radiology, Dept. Thursday June 13th 8. Manfredi, Rome, I 9. Basics 9. Lamb Leiden, NL Savino Rome, I Natale Rome, I Meduri Rome, I Galea Rome, I It has strong underpinnings in physics which must be understood before any real sense of 'how it works' is gained.
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What follows is a very abbreviated, 'broad strokes' description of the process. Essentially, the process can be broken down into four parts:.
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For a more detailed description of each part of the process, please refer to the links scattered throughout this introduction and at the bottom of the page. The patient is placed in a static magnetic field produced by the magnet of the MR scanner. In living tissues there are a lot of hydrogen atoms included in water molecules or in many different other molecules.
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The spin magnetization vector precesses rotates around the magnetic field at a frequency called the Larmor frequency , which is proportional to the magnetic field intensity. The resulting magnetization of all protons inside the tissues aligns parallel to the magnetic field. The parallel magnetization scales with the magnetic field intensity, basically at 3T it will be twice the value obtained at 1. Additional preparation sequences can also be performed to manipulate the magnetization and so the image contrast, e.
When tuned to the Larmor frequency, the RF pulse is at resonance: it creates a phase coherence in the precession of all the proton spins.
MRI: A Guided Tour
The duration of the RF pulse is chosen such that it tilts the spin magnetization perpendicularly to the magnetic field. When a receiving coil an electrical conductor is put in the vicinity of the tissue, the transverse magnetization, that still rotates as the Larmor precession, will generate an electric current in the coil by Faraday induction: this is the nuclear magnetic resonance NMR signal.
The NMR signal is attenuated due to two simultaneous relaxation processes.
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The loss of coherence of the spin system attenuates the NMR signal with a time constant called the transverse relaxation time T2. The contrast in MR images originates from the fact that different tissues have, in general, different T1 and T2 relaxation times; as this is especially true for soft tissues, it explains the excellent soft tissue contrast of MR images. The portion of the gradient coils and the associated current that is perpendicular to the main magnetic field cause a force Lorentz force on the coils.
The gradients are turned on and off very quickly in this process causing them to vibrate and producing the majority of the acoustic noise during an MR image acquisition.
When using magnetic field gradients, the obtained NMR signal contains different frequencies corresponding to the different tissue spin positions and is called the MRI signal. The protons absorb the energy from the magnetic field and flip their spins.
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When the field is turned off, the protons gradually return to their normal spin, a process called precession. The return process produces a radio signal that can be measured by receivers in the scanner and made into an image, Filippi explained. Protons in different body tissues return to their normal spins at different rates, so the scanner can distinguish among various types of tissue.
The scanner settings can be adjusted to produce contrasts between different body tissues. Additional magnetic fields are used to produce 3-dimensional images that may be viewed from different angles. This form of MRI measures how water molecules diffuse through body tissues. Certain disease processes — such as a stroke or tumor — can restrict this diffusion, so this method is often used to diagnose them, Filippi said. Diffusion MRI has only been around for about 15 to 20 years, he added. In addition to structural imaging, MRI can also be used to visualize functional activity in the brain.
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It is used to observe brain structures and to determine which parts of the brain are handling critical functions. Functional MRI may also be used to evaluate damage from a head injury or Alzheimer's disease. MRI is increasingly being used to image fetuses during pregnancy, and no adverse effects on the fetus have been demonstrated, Filippi said. Still, the procedure can have risks, and medical societies don't recommend using MRI as the first stage of diagnosis.