Magnetic resonance Imaging (MRI)

Introduction MRI is a way of looking inside the body and is especially good at producing images of soft tissues such as muscle, fat, cartilage and the brain. It does this by producing a map which depend on the density of hydrogen in the body.


Basic Principles of MRI:

MRI is based on the principles of nuclear magnetic resonance (NMR), The name Nuclear Magnetic Resonance (NMR) was changed into Magnetic Resonance Imaging (MRI) because it was believed that the word nuclear would not find wide acceptance amongst the public.
Nuclei with an odd number of protons and neutrons possess a property called spin. In quantum mechanics spin is represented by a magnetic spin quantum number. Spin can be visualised as a rotating motion of the nucleus about its own axis. As atomic nuclei are charged, the spinning motion causes a magnetic moment in the direction of the spin axis. This phenomenon is shown in Figure 1. The strength of the magnetic moment is a property of the type of nucleus. Hydrogen nuclei (1H), as well as possessing the strongest magnetic moment, are in high abundance in biological material. Consequently hydrogen imaging is the most widely used MRI procedure.




Figure 1: A charged, spinning nucleus creates a magnetic moment which acts like a bar magnet (dipole).
Consider a collection of 1H nuclei (spinning protons) as in Figure 2(a). In the absence of an externally applied magnetic field, the magnetic moments have random orientations. However, if an externally supplied magnetic field B0 is imposed, the magnetic moments have a tendency to align with the external field (see Figure 2(b)).




Figure 2: (a) A collection of 1H nuclei (spinning protons) in the absence of an externally applied magnetic field. The magnetic moments have random orientations. (b) An external magnetic field B0 is applied which causes the nuclei to align themselves in one of two orientations with respect to B0 (denoted parallel and anti-parallel).
The magnetic moments or spins are constrained to adopt one of two orientations with respect to B0, denoted parallel and anti-parallel. The angles subtended by these orientations and the direction of B0 are labelled theta in Figure 3(a). The spin axes are not exactly aligned with B0, they precess around B0 with a characteristic frequency as shown in Figure 3(b). This is analogous to the motion of a spinning top precessing in the earth's gravitational field. Atomic nuclei with the same magnetic spin quantum number as 1H will exhibit the same effects - spins adopt one of two orientations in an externally applied magnetic field. Elements whose nuclei have the same magnetic spin quantum number include 13C, 19E and 31P. Nuclei with higher magnetic spin quantum number will adopt more than two orientations.




(a) (b)



Figure 3: (a) In the presence of an externally applied magnetic field, B0, nuclei are constrained to adopt one of two orientations with respect to B0. As the nuclei possess spin, these orientations are not exactly at 0 and 180 degrees to B0. (b) A magnetic moment precessing around B0. Its path describes the surface of a cone.
The Larmor equation expresses the relationship between the strength of a magnetic field, B0, and the precessional frequency, F, of an individual spin.

The proportionality constant to the left of B0 is known as the gyromagnetic ratio of the nucleus. The precessional frequency, F, is also known as the Larmor frequency. For a hydrogen nucleus, the gyromagnetic ratio is 4257 Hz/Gauss. Thus at 1.5 Tesla (15,000 Gauss), F = 63.855 Megahertz

Correcting Artifacts

Artifacts are signal intensities that have no relation to the spatial distribution of the tissues being imaged. There are four types of artifacts (based on appearance): (a) edge artifacts (ghosting, chemical shifts, and ringing), (b) distortions, (c) aliasing (wraparound) artifacts, and (d) flow artifacts.
Motion Artifacts (ghosting and smearing): Artifacts often result from involuntary movements (eg, respiration, cardiac motion and blood flow, eye movements and swallowing) and minor subject movements. Motion artifacts appear only in the phase encoding direction and appear as ghosts or smears. Motion artifacts can be flipped 90 degrees by swapping the phase/frequency encoding directions. Flow effects can be reduced by using Gradient Motion Rephasing (GMR) or synchronisation of acquisition with motion rhythms. Increasing the number of acquisitions (NEX).
Chemical Shift Artifacts: ...
Summary: Optimising MRI Quality

SNR – voxel size, acquisitions, field strength, coil, bandwidth
Resolution – slice thickness, FOV, matrix
Contrast – TR, TE, flip angle, contrast media, sequence
Measurement Time – acquisitions (NEX)

Conclusion:
Magnetic Resonance Imaging (MRI), or nuclear magnetic resonance imaging (NMRI), is primarily a medical imaging technique most commonly used in radiology to visualize detailed internal structure and limited function of the body. MRI provides much greater contrast between the different soft tissues of the body than computed tomography (CT) does, making it especially useful in neurological (brain), musculoskeletal, cardiovascular, and oncological (cancer) imaging. Unlike CT, it uses no ionizing radiation, but uses a powerful magnetic field to align the nuclear magnetization of (usually) hydrogenatoms in water in the body. Radio frequency (RF) fields are used to systematically alter the alignment of this magnetization, causing the hydrogen nuclei to produce a rotating magnetic field detectable by the scanner. This signal can be manipulated by additional magnetic fields to build up enough information to construct an image of the body.




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