Signal acquisition and processing for magnetic resonance imaging

An understanding of data acquisition and processing in magnetic resonance imaging (MRI) facilitates subsequent manipulation and analysis of the resultant images. Unlike other medical imaging modalities, MRI is not well described in terms of the transmission and reception of a propagating wave since the wavelength of the radiofrequency signals involved is on the order of meters. Rather, MRI exploits the fact that the magnetization vectors associated with certain atoms (notably protons in water) will, when placed in a strong magnetic field, oscillate at a frequency proportional to that field. By spatially varying the magnetic field, position is mapped by the frequency and phase of the field effects caused by the oscillating magnetization vectors at each location. The data acquisition associated with this spatial mapping can be described as temporally scanning the Fourier or spatial frequency domain of the volume of interest. Image reconstruction is achieved by an inverse Fourier transform of the received signal. The Fourier representation of the imaging process is developed from an overview of the physics underlying MRI signal behaviour. Under this formalism, trade-offs in acquisition time, resolution, and field-of-view as well as image degradation associated with noise are addressed. Finally, the ability in MRI to manipulate soft tissue contrast over a wide range of independent parameters is outlined