Magnetic Resonance Imaging: Principles and Techniques: Lessons for Clinicians

Abstract

The development of magnetic resonance imaging (MRI) for use in medical investigation has provided a huge forward leap in the field of diagnosis, particularly with avoidance of exposure to potentially dangerous ionizing radiation. With decreasing costs and better availability, the use of MRI is becoming ever more pervasive throughout clinical practice. Understanding the principles underlying this imaging modality and its multiple applications can be used to appreciate the benefits and limitations of its use, further informing clinical decision-making. In this article, the principles of MRI are reviewed, with further discussion of specific clinical applications such as parallel, diffusion-weighted, and magnetization transfer imaging. MR spectroscopy is also considered, with an overview of key metabolites and how they may be interpreted. Finally, a brief view on how the use of MRI will change over the coming years is presented.

Keywords: ADC, apparent diffusion coefficient; CSI, Chemical shift imaging; DTI, diffusion tensor imaging; DWI, Diffusion-weighted imaging; FA, Fractional anisotropy; FID, free induction decay; MRI, magnetic resonance imaging; MTR, MT ratios; NMR, nuclear magnetic resonance; PRESS, Point-resolved spectroscopy; RA, relative anisotropy; RF, radiofrequency; SNR, signal-to-noise ratio; STEAM, Stimulated echo acquisition mode; TR, repetition time; magnetic resonance imaging; magnetic resonance spectroscopy; medical physics; nuclear magnetic resonance; nuclear medicine.

Figure 1

Nuclear spin. The “spinning” nucleus (a) induces a magnetic field, behaving like a bar magnet (b). N and S represent north and south respectively. The directions of the arrows represent the direction of the magnetic field.

Figure 2

Nucleus precessing around an external magnetic field (B₀). M₀ = direction of net magnetization. x, y and z represent the orthogonal Cartesian axes. ω₀ = γB₀

Figure 3

Larmor equation. ω₀ = angular frequency of the protons, γ is the gyromagnetic ratio, a constant fixed for a specific nucleus and B₀ is the field strength. ΔE = γhB₀/2π

Figure 4

Difference in energies of the two spin orientations, where h = Planck's constant.

Figure 5

The free induction decay (FID) and Fourier transformation to generate MR images or MR spectra.

Figure 6

Effect of field gradient on nuclei. (a) B₀ only, all nuclei precess at the same frequency. (b) B₀ with gradient Gx. Further along the x direction the field increases and thus the protons resonate faster- precession frequency depends upon position. Adapted from McRobbie, D et al. MRI From Picture to Proton. 1st edn. Cambridge. Cambridge University Press, 2003. MTR + 100 × (SI off − SI on)/SI

Figure 7

MTR formula. (SI off = signal intensity in the baseline proton density image, SI on = signal intensity in the image with the MT pulse applied).

Figure 8

Model demonstrating the concepts underlying the phenomenon of magnetization transfer.

Figure 9

Principles of diffusion. Isotropic (A) diffusion and restricted diffusion (B and C). See text for further explanation.

Figure 10

Color fractional anisotropy (FA) image from a 44-year-old healthy male volunteer. Diffusion tensor imaging (DTI) imaging performed on a 3 T Philips Intera™ using 32 different directions of diffusion sensitization. The different colors represent the principle diffusion directions and hence the direction of the white matter tracts: green represents anterior-posterior, blue represents caudo-cranial, red represents transverse.