CLINCAL SUBJECTSREVIEW SERIESSPECIALTY-MS/MD/FRCS

Duke Review of MRI Physics CASE REVIEW SERIES 2019 eBook

Duke Review of MRI Physics CASE REVIEW SERIES 2019 eBook

Duke Review  of MRI Physics CASE REVIEW SERIES 2019 eBook

Duke Review of MRI Physics CASE REVIEW SERIES 2019 eBook Because of its abundance in the human body, hydrogen is the protons is 63.8 MHz and approximately 127 MHz at 3.0 Tmost frequently imaged nucleus in clinical MRI. Hydrogen has The vector sum of the magnetic moments of the precessing a considerable angular magnetic moment, with its single, posi- protons (MZ and MXY) results in a net equilibrium magnetizatively charged proton acting as a tiny spinning bar magnet. tion (M0).

This magnetization vector is primarily in the longiProtons normally spin in random directions in the absence of an external magnetic field; because of this random movement, the magnetic vector sum of these protons is typically zero. When placed in a strong external magnetic field (B0), these protons align parallel (low energy) or antiparallel (high energy) with respect to B0; more protons tend to align parallel to B0 because less energy is required to do so. Because they possess magnetic and angular momentum, the protons precess, or wobble, around the axis of B0 instead of spinning in a tight circle; this precession motion confers both longitudinal (μz) and transverse (μxy) components in the magnetic moments of the protons.

Protons tend to precess at a certain frequency while under the influence of B0, which is called the Larmor frequency. The Larmor frequency defines the frequency at which the radiofrequency pulse is broadcast to induce proton resonance, or excitation. The Larmor frequency is defined as W = γB, where W is the Larmor frequency, γ is the gyromagnetic ratio in MHz/ tesla (T), and B is the strength of the static magnetic field in T. Thus the Larmor frequency is proportional to the strength of the  tudinal direction (MZ) because moreprotons align in parallel  with B0.

The transverse component (MXY) does not contribute significantly to M0 because the protons do not spin in
phase with each other and effectively cancel each other out.  As the energy of B0 increases, so does the energy differential  between protons in the low (parallel) and high (antiparallel) states, with increasing numbers of protons aligning parallel to  B0. This results in a significant directional (vector) component  of the net magnetization.

However, the receiver coil, which is the component of the MRI machine that detects signals, is  sensitive only to variations of the magnetization vector; the  original main net magnetization along the z direction, even  though it is precessing, is viewed as a stationary vector from  the receiver coil perspective. Given this, something must be  done to perturb the system (i.e., tip the magnetization away from the z-axis so that the precession motion is visible) and  generate detectable signal changes that can be picked up by
the receiver coils. This comes in the form of a radiofrequency
(RF) excitation pulse.

Duke Review of MRI Physics CASE REVIEW SERIES 2019 eBook

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