ABSTRACT
MR images are acquired slice by slice or by encoding the entire volume. I will use the slice-by-slice approach common to the spin-echo-style imaging to simplify the description. A slice is selected using a slice-encoding gradient that slightly alters the magnetic eld and frequencies in the slice direction. e slice-encoding gradient takes a short time to stabilize when turned on. en, an RF signal is transmitted into the body to excite spins at the desired slice position (based on center frequency) and thickness (based on bandwidth). Aer desired excitation, the RF and slice-encoding gradient are turned o. e timing between RF transmissions and timing for reception of RF signals from the slice provide the basis for contrast between tissues (Section 4.7.2). During reception, RF signals from the selected slice are spatially encoded with frequencies that vary across the slice using a second gradient (position-encoding gradient). A mixer circuit in the RF receiver removes the high-frequency carrier signal (~43 mHz at 1 T), resulting in a position-encoded lowerfrequency signal (e.g., −16 to +16 kHz). An ADC samples this lower-frequency positionencoded signal, and the samples are stored as values in one row of a 2-D image called a k-space image (usually 256 × 256). is is repeated, applying phase encoding in the direction perpendicular to the frequency encoding direction, to change the row location to ll in the k-space array. e 2-D k-space images (actually real and imaginary parts) are the Fourier transform of the signals from the slice. An inverse Fourier transform is used to convert the 2-D k-space image to real and imaginary 2-D images of the object. e real and imaginary images are transformed to magnitude and phase images, and the
2-D magnitude image serves as the MR image of signals from the slice. is is repeated to acquire multiple slices forming a multislice tomographic image of the object (Figure 16.1). Note: Phase encoding is a much slower process than frequency encoding such that motion tends to produce artifacts in the phase-encoded direction in the nal MR image.
