Principles of magnetisation and relaxation in mri

T1-weighted MRI images present a good contrast between fat, which appears dark, and water, which appears brighter. This type of contrast is used, for instance, in brain imaging to distinguish gray matter from white matter. Pathologies are often revealed by T2-weighted MRI. Edemas abnormal accumulation of fluids appear bright, while tumors often appear darker than normal tissues.

Principles of magnetisation and relaxation in mri

Because nuclei are charged particles, this precession produces a small magnetic moment. When a human body is placed in a large magnetic field, many of the free hydrogen nuclei align themselves with the direction of the magnetic field.

The nuclei precess about the magnetic field direction like gyroscopes. This behavior is termed Larmor precession. The frequency of Larmor precession is proportional to the applied magnetic field strength as defined by the Larmor frequency,: The gyromagnetic ratio is a nuclei specific constant.

To obtain an MR image of an object, the object is placed in a uniform magnetic field,of between 0. This behavior is illustrated in Figure 2.

In the absence of a strong magnetic field, hydrogen nuclei are randomly aligned as in a. When the strong magnetic field,is applied, the hydrogen nuclei precess about the direction of the field as in b.

Principles of magnetisation and relaxation in mri

Next, a radio-frequency RF pulse,is applied perpendicular to. This pulse, with a frequency equal to the Larmor frequency, causes to tilt away from as in Figure 2. During realignment, the nuclei lose energy and a measurable RF signal Once the RF signal is removed, the nuclei realign themselves such that their net magnetic moment,is again parallel with.

This return to equilibrium is referred to as relaxation. During relaxation, the nuclei lose energy by emitting their own RF signal see Figure 2. This signal is referred to as the free-induction decay FID response signal.

The FID response signal is measured by a conductive field coil placed around the object being imaged. This measurement is processed or reconstructed to obtain 3D grey-scale MR images. To produce a 3D image, the FID resonance signal must be encoded for each dimension.

The encoding in the axial direction, the direction ofis accomplished by adding a gradient magnetic field to.

Net magnetization - Questions and Answers ​in MRI

This gradient causes the Larmor frequency to change linearly in the axial direction. Thus, an axial slice can be selected by choosing the frequency of to correspond to the Larmor frequency of that slice. The 2D spatial reconstruction in each axial slice is accomplished using frequency and phase encoding.

As a result, the resonant frequencies of the nuclei vary in the -direction due to and have a phase variation in the -direction due to the previously applied. Thus, -direction samples are encoded by frequency and -direction samples are encoded by phase.The rate of relaxation is a characteristic of each specific tissue and is expressed by the T2 values, the transverse relaxation time.

A tissue with a short T2 will lose its transverse magnetization rapidly and will appear relatively dark in T2-weighted images. The physics of magnetic resonance imaging (MRI) involves the interaction of biological tissue with electromagnetic is a medical imaging technique used in radiology to investigate the anatomy and physiology of the body.

The human body is largely composed of water molecules, each containing two hydrogen nuclei, or inside the magnetic field (B 0) of the scanner, the.

Mina Kim, Mara Cercignani, in Quantitative MRI of the Spinal Cord, Abstract.

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Magnetization transfer (MT) imaging is a technique that indirectly assesses the status of hydrogen protons bounded to macromolecules such as lipid constituted of axons myelin sheet in tissues.

It is especially useful for studying the integrity of white matter, which contains large amounts of myelin. Transverse magnetisation begins to disappear, a process called transverse (or T2 (‘Time’ 2)) relaxation and the longitudinal magnetisation starts to return to its original value, a process termed longitudinal (or T1 (‘Time’ 1)) relaxation.

Development of net magnetization (M) when a sample is first placed in a magnetic field (Bo). M grows as a simple exponential with time constant T1. T1 is also called the spin-lattice, thermal or longitudinal relaxation time.

Net magnetization (M) behaves as a "regular" vector using the principles of classical physics. From this point forth we will almost exclusively use the vector M rather than individual nuclear spins when explaining aspects of NMR and MRI.

Basic Principles of MRI