Echo-planar imaging (EPI) allows rapid, near real time, acquisition and display of magnetic resonance (MR) images. While many of the commercial imagers are technically capable of producing EPI scquences. only few offer EPI Functionality in respect to data acquisition and processing. To circumvent this commercial limitation. we have developed an inexpensive instrument allowing sampling to be performed from a PC-class computer. Designed as a standard expansion board for personal computer, our device permits implementation of a non-uniform sampling pattern with minimum sample time resolution of 1OOns. and sustained data uansfer rate nearing 1.6 M samples/sec. Introduction Any MR system capable of producing a complete k-space trajectory hac passes through all points needed for image reconstruction, bcfore deterioration of RD signal, may be potentially used lor creating an image f" a single RF excitation. While there are many altemative implementations of such process (commonly known as echo-planar imaging, or EPI techniques), lhey all face rcchnological challenges related to bandwidth limitations of the gradient system. and problems with image sampling and image reconstruction based on inverse FFT pmcess. Since the latter operation expects equidistant discribution of data samples in kspace, gradient waveform generalion and data acquisition musk be tightly synchronized, placing frequently unrealistic demands on standard MR imagers. Common compromises include: For the systems with limited rise/fall times of gradient amplifiers. a constant phase encode gradient may be. used in combinauon with trapezoidal readout [l]. The resulting zigzag trajectory misses most of desired points in k-space, and therefore time-consuming 2-D interpolation of the acquired data set is needed, which for all practical reasons makes this technique obsolete. For the systems with high-performance gradients. pulsed (blipped) phase encode gradient [2] can be. used in combination with rectangular. triangular. or sinusoidal readout gradient which forces the systems trajectory to traverse k-space in a piecewise-linear fashion. passing all of the desired points in time intervals which in general fluctuate with nonlinearities of the readout gradient. To compensate for the inter-sample timing fluctuations observed in blipped EPI. and to assure that the acquired data falls Onto a rectangular grid in k-space, one of the two following strategies may be implemented: Sample piecewise-finear sections of k-space aajectory at a fixed frequency, and use I -D interpolation to obtain desired data points located in between actual samples. 0-78031377-1/93 $3.00 01993 IEEE Measure actual gradient waveforms, determine the exact timing of desired data. and obtain image samples at timevarying intervals without further need for post-processing. In both cases, data sampling rates and size of data sets which need to be buffered prior to image reconstruction hequently exceed capabilities of the commercial scanners. In this paper, the design principle of a system-independent, EPI-capable upgrade m d u l e is presented, along with some of the sample results obtained for Picker-HF'Q 1.5T scanner used in OUT EPI research. Methods Direct modifications of the imaging system required to provide EPI functionality are generally not possible either due to unknown workings of its proprietary hardware and software. or because of problems with manufacturer warranties. technical suppon contracts. etc. Therefore, an autonomous EPI-capable subsystem, which is designed for parallel operation with the host imager wihout affecting its normal operations becomes a desirable design option. To support such EPI subsystem, the host imager must offer rudimental hook-ups for data acquisition and inter-system synchronization. These include (1) start of sequence pulse required for synchnization. (2) RF. IF or Real/Imaginary signal, (3) gradient waveform monitor, and (4) connection to the image base maintained by the host imager (if re-insertion of image into the normal archival data flow is required). Fortunately, most commercial imagers satisfy these minimum requirements. In the Picker HPQ imaging system used for our research, the Front panel provides a 'Main Pulse' which identifies the start of sequence. In addition, the imager offers an intermediate frequency (IF) data signal connection centered at 125 KHz, a gradient current monitor port, and an erhemet connection for external access to systems data structures. The advantages of an autonomous WI subsystem are even more obvious. once it is realized that i t can be builr around practically any computing platform. thus allowing for a cost-sensilive ophiza t ion of the performance, technology, and function. In OUT case, this led to the selection of a standard personal computer (K). which "bines very low cost with well-documented interface definition, networking capabilities, and the abundance of readily available prototype boards and software developments mls. From a functional smdpoint. the FC platform offers 8MHz expansion bus, with a 16-bit per bus cycle data transfer capability. and a minimum of 1 wait state on all port accesses. Working within this constraints, and dealing with the processor's segmenmion of memory into 64 Kbyte blocks, we have devised a data uansfer scheme which uses REP INSW (repeat input s m n g word) insuuction for burst transfer of blocks of 32K samples of data (up to 16-bit per samples), and may be reexecuted at very shon