Magnetic Resonance Imaging

Abstract

Magnetic resonance imaging (MRI) is a volumetric imaging modality that parallels, to a certain extent, computed tomography. However, the underlying physical principles are fundamentally different from CT. Where CT uses high-energy photons and the interaction of photons with electrons of the atomic shell for contrast generation, MRI is based on the orientation of protons inside a strong magnetic field. This orientation can be manipulated with resonant radiofrequency waves, and the return of the protons to their equilibrium state can be measured. The relaxation time constants are highly tissue-dependent, and MRI features superior soft tissue contrast, by far exceeding that of CT. On the other hand, MRI requires dramatically more time for image acquisition than CT, unless special high-speed protocols are used (which often suffer from poor image quality). In addition, modern MRI scanners require a superconductive magnet with liquid helium cooling infrastructure, extremely sensitive radiofrequency amplifiers, and a complete room shielded against electromagnetic interference. For this reason, MRI equipment is extremely expensive with costs of several million dollars for the scanner hardware and with accordingly high recurring costs for maintenance. However, MRI scanners provide images with a very high diagnostic value, and MRI can be used to monitor some physiological processes (e.g., water diffusion, blood oxygenation) and therefore partly overlaps with nuclear imaging modalities. Since MRI is a radiation-free modality, it is often used in clinical studies with volunteers.