1394b Delivers the Performance and Features Required for Advanced PET Imaging System

Data Acquisition System Designed by University of Washington Scientists Minimizes Development Time, Support Requirements

When a team of scientists at the University of Washington decided to develop a data acquisition system for the micro crystal elements system (MiCES) small animal positron emissions tomography (PET) scanner, one of their top priorities was a fast, high bandwidth, reliable interface technology. According to Dr. Tom K. Lewellen, University of Washington Professor of Radiology and one of the project directors, the choice was simple: 1394b (FireWire 800) running on an Apple Mac, with OS X.

The project began as the university group looked for ways to design a series of imaging systems for biomedical research, ‘one-off’ systems to push the limits of technology and provide unique tools for investigators at the university.  The basic challenge: achieve these goals within the funding limits common to university research.

The team wanted to devise a data acquisition system that reduced development effort and subsequent support requirements. FireWire’s support within the Macintosh OS X system proved a vital benefit, allowing users to reach FireWire devices under OS X without having to write and maintain specific kernel drivers. It also achieves the required performance with no concern about changes in the underlying operating system, such as upgrades. 

Once they decided on the MacPro platform running OS X, Tom Lewellen and the team developed software that acquires data from PET scans using a MacPro, then sends it via FireWire to a Mac xserve cluster for advanced image reconstruction and data storage within an image database server. For a typical scan of a mouse, this requires between 2GB to 4GB.

By setting up configuration ROMs to state that the in-house built nodes were SBP-2 devices with non-standard use of the SCSI instruction set, the OS X system registers them as FireWire devices. But it does not assign any higher-level drivers to them, such as trying to make them into a disk device. That enables direct SBP-2 level access to them from the user code space with no need to do anything in the kernel space. The result: acquisition software has remained fully operational through four major upgrades of the OS X system (10.1 to 10.5).

There are restrictions for any design using a serial bus for sending data to the host including data bandwidth and the number of nodes that can be supported. 1394b reduces those challenges, with its support of a bus bandwidth of 800 megabits/second, which is best utilized in data packets of 4096 bytes. The group is implementing an updated version of the system using a FPGA with large buffers to allow collection of data into 4096 byte packets for transmission – and it continues collecting data while a data packet is being transmitted. Each event is timed-stamped so the host computer can rebin the data into coincidence pairs. Normally, all data is acquired in list mode and rebinned into coincidence pairs with final timing and energy windows after acquisition. Depending on the detector system being supported, event positioning is done either in the FPGA (cMiCES) [12] or during the image reconstruction process (dMiCES). One of the FPGAs (with embedded processor) acts as a master controller for the scanner. Commands from the host are sent via the FireWire bus (eliminating the serial link between the host and the master controller in the original MiCES scanner system). As in the original MiCES system, each FireWire node is configured with two logical units - one for control and one for data. In this way, asynchronous command/response exchanges between the nodes and the host can be performed while large data blocks are being transferred.

Development work has continued on new imaging scanners that require more data channels and need to be able to operate within a MRI imaging system. To support these scanners, the team designed a new version of the data acquisition system that leverages the capabilities of FPGAs. The new design preserves the basic approach of the original system, but puts almost all functions into the FPGA, including the FireWire elements, the embedded processor, and pulse timing and pulse integration.

The design has been extended to support implementation of the position estimation and DOI algorithms developed for the cMiCE detector module. The design includes an acquisition node board (ANB) with 65 ADC channels, FireWire support, the FPGA, a serial command bus (for low speed messaging between FPGAs) and signal lines to support a rough coincidence window implementation to reject single events from being sent on the FireWire bus. Adapter boards convert detector signals into differential paired signals to connect to the acquisition node board.

When the team began, according to Dr. Lewellen, “We had hoped to adapt a commercial data acquisition system, but there were no such systems that offered the flexibility we needed.” 

Now there is one. 

For a complete technical paper on the PET scanner project, visit www.ee.washington.edu/people/faculty/hauck/publications/MICES_Phase2.pdf