MRI Glossary

This is a glossary of terms for MRI - feel free to add any more. Acquisition - One data set of n Fourier lines where n = the number of phase encoded lines of the matrix. Two or more sets may be averaged to improve signal to noise ratio. Amplitude - Maximum value - eg the peak of an echo. Angular Magnetic Moment - Magnetic property which produces a magnetic field and causes alignment with external magnetic fields. Bipolar Flow-Encoding Gradients - Gradients whose polarity is inverted ti encode velocities as charges of phase - a technique used in phase-contrast angiography. (1) Bloch Equations - Equations which approximate the signal strength received as a result of various imaging sequences. Cross Excitation - Radio Frequency (RF) interference between adjacent slices due to non rectangular RF pulse profiles. This may result in loss of signal and Repetition Time (TR) shortening. Echo - Characteristic signal received during spin-echo sequence following refocusing with 180 degree pulse - shaped like two back to back Free Induction Decay (FIDs). First Echo Image - Also known as a 'Proton Density' image generated with a long Repetition time (TR) and a short Echo Time (TE). Flow Related Enhancement - Phenomenon of increased intensity in moving tissues relative to stationary tissue. Fourier Transformation - Mathematical technique used to resolve the complex wave-form into phase and frequency components to allow spatial localization of the signal. Free Induction Decay (FID) - Signal produced by rotating transverse magnetization following a 90 degree pulse. Decays at a rate governed by T2*. Frequency Encoding - Method of spatially localizing an MR signal using a magnetic gradient to vary the frequency dependent on position. Gauss - Unit of magnetic field strength. 1 Gauss = 1/10,000 Tesla. The earths magnetic field varies from about 0.3 to 0.7 Gauss. (Pacemaker line = 5 Gauss). Gradient Echo Imaging - Method of producing an echo by reversing the direction of a magnetic gradient Signal Averaging - An SNR-enhancing technique in which the same MRI signal is repeatedly acquired tow or more times and then combined and averaged. (1) References 1. Bontrager, K. L., & Lampignano, J. P. (2005). Textbook of Radiographic Positioning and Related Anatomy 6th edition. Elsevier Mosby