Digital Data Processing

A typical scan protocol calls for a sequence of tailored RF and gradient pulses with duration controlled in steps of 0.1 msec. To achieve sufficient dynamic range in control of pulse amplitudes, 12- to 16-bit digital-to-analog converters are used. The RF signal at the Larmor frequency (usually in the range from 1 to 200 MHz) is mixed with a local oscillator to produce a baseband signal which typically has a bandwidth of 16–32 kHz. The data-acquisition system must digitize the baseband signal at the Nyquist rate, which requires sampling the detected RF signal at a rate one digital data point every 5–20 msec. Again, it is necessary to provide sufficient dynamic range. Analog-to-digital converters with 16–18 bits are used to produce the desired digitized signal data. During the data acquisition, information is acquired at a rate on the order of 800 kilobytes per second, and each image can contain up to a megabyte of digital data. The array processor (AP) is a specialized computer that is designed for the rapid performance of specific algorithms, such as the fast Fourier transform (FFT), which are used to convert the digitized time-domain data to image data. Two-dimensional images are typically displayed as 256 × 128, 256 × 256, or 512 × 512 pixel arrays. The images can be made available for viewing within about 1 sec after data acquisition. Three-dimensional imagining data, however, require more computer processing, and this results in longer delays between acquisition and display.

A brightness number, typically containing 16 bits of gray-scale information, is calculated for each pixel element of the image, and this corresponds to the signal intensity originating in each voxel of the object. To make the most effective use of the imaging information, sophisticated display techniques, such as multi-image displays, rapid sequential displays (cine loop), and three-dimensional renderings of anatomic surfaces, are frequently used. These techniques are often computationally intensive and require the use of specialized computer hardware. Interfaces to microprocessor-based workstations are frequently used to provide such additional display and analysis capabilities. MRI images are available as digital data; therefore, there is considerable utilization of local area networks (LANs) to distribute information throughout the hospital, and long-distance digital transmission of the images can be used for purposes of teleradiology.