MRI Facility-Based Molecular Imaging

Abstract

Based on principle, magnetic resonance imaging (MRI) is a direct molecular imaging modality because all of the possible contrasts of MRI are formed by the decay process after excitation of the polarized spin nuclei in resonance in an imaged subject. The spin nuclei exist in a molecule, such as a proton in a water molecule. The polarized and excited spin nuclei have the same spin frequency fixed by the Larmor equation, which also determines the relation between the center frequency ω and B ₀:

$$ \omega = \gamma B_0 $$

((8.1))

where γ is a constant of the gyromagnetic ratio with a value of γ=2.68×10₈ rad/s/T or after normalization by 2π, then the value of γ=42.6 MHz/T for proton spin. The center frequency ω is usually in a radio frequency (RF) range and the polarized spin nucleus balanced in B ₀ can be excited by the RF wave, which is defined as B ₁. In order to position the interaction event of the excited spin nucleus, a gradient field of G is needed, which fixes the positions in a Cartesian coordinate system for the imaged subject. The details of the principle, scheme and key technologies for MRI can be found in many references, and some of these can be seen in [1].