chapter  13
Cardiovascular Computed Tomography and Magnetic Resonance Imaging
Pages 20

Image formation in clinical computed tomography (CT) relies on the mathematical conversion, by filtered back-projection, of projection data that have been obtained by measuring with detectors the attenuation of a fan-shaped x-ray beam from many angles around the patient. Image formation in clinical magnetic resonance imaging (MRI) relies on the alignment of hydrogen nuclei or protons along an external magnetic field.The alignment and angular momentum (spin) of these particles can be excited if radiofrequency pulses are applied at the so-called Lamor frequency. The relaxation of the particles toward their original alignment in the magnetic field produces an MRI signal that can measured by external receiver coils. Each excitation-relaxation sequence results in one line of data to be used for image reconstruction, typically by Fourier transformation.