Altera Devices on the Cutting Edge of Medical Technology
Cutting-edge applications are using more and more programmable logic devices because of their flexibility and efficiency. Altera is proud to be at the forefront of medical research, as FLEX® 10K devices form an integral part of the latest positron emission tomography (PET) scanner from CTI PET Systems, Inc. CTI PET Systems is a joint venture by Siemens Medical Systems and CTI, Inc., the company that produced the first commercial PET scanner.
The ECAT High Resolution Research Tomograph (HRRT) offers very high resolution PET scanning for use in clinical diagnosis of brain disorders. With PET, doctors can effectively pinpoint the location of many common cancers, heart diseases, and neurological diseases, without resorting to exploratory surgery or ineffective medical treatments. PET reduces medical costs and makes treatment less traumatic and less costly for the patient.
What is PET?
In PET scanning, compounds tagged with radioactive isotopes are injected into patients to obtain images of metabolic or physiologic processes. The isotopes undergo radioactive decay, resulting in the emission of positively charged electrons or positrons. The positrons travel only a few millimeters in the body tissue before they collide with negatively charged electrons, and the total mass of the two is converted into two photons of pure energy. The two photons are emitted simultaneously at 180 degrees from each other toward the opposite sides of the body. Detectors in the PET scanner record the relative position of the pairs of photons, identifying and locating millions of positron-electron collisions per second. The scanner's computer then reconstructs this data as a visual image showing the distribution of the isotope inside the tissues being examined. The image shows both normal organ function and failure of organ systems due to disease.
Depending on the type of isotope used, different metabolic functions can be observed. PET scanning is particularly effective, for example, in diagnosing cancers, as it can follow the course of the cancer through the body and accurately show the extent of the disease. Another area of PET application that continues to develop is in the diagnosis of common neurological disorders, such as Alzheimer's disease, Hodgkins disease, and stroke.
The ECAT HRRT PET Scanner
The ECAT HRRT is composed of the main assembly, patient bed, computer, and operator's workstation. The main assembly, shown in Figure 1, has a gantry with a ring-shaped opening where the detectors and their electronics are located. The patient bed transports the patient through the detector ring, and the computer is used for data acquisition and the control system.
Figure 1. The ECAT HRRT
The ECAT HRRT is by far the most high-resolution PET scanner produced by CTI PET Systems. It contains 936 detectors, nearly four times the number of detectors found in the ECAT EXACT HR+, which was till now the highest performance PET scanner in the world. The HRRT detectors are arranged in a ring of eight panels formed of 9 × 13 detector blocks. Each detector block is composed of an 8 × 8 block of lutetium oxyorthosilicate (LSO) scintillating crystals, as shown in Figure 2. The crystals are in pairs, the one having a fast decay time and the other a slow decay time. Therefore, the HRRT has just under 120,000 crystals to detect the photon's gamma rays (compared to just 18,432 crystals in the ECAT EXACT HR+).
Figure 2. Detector Configuration
As shown in Figure 2, each of the eight detector panels has its own set of electronics. The detector head interface board distributes the system clocks, loads the configuration information, and performs event rate reporting and system diagnostics. It holds an embedded PC/104 computer to perform administrative and low-speed communications tasks, a bleeder board with four photomultiplier tubes (PMTs) to capture and convert the light signals detected by the crystals, and 39 analog subsection boards, each of which processes the events for three crystal blocks.
EPF10K10 Devices Provide a Flexible Solution
Because of the large number of detectors and the advanced features found in the HRRT, CTI PET Systems needed a fast, powerful, flexible, low-cost solution for the front-end processing. Rather than using the ASIC implementation found in the company's other products, the company chose to use 1,016 Altera® EPF10K10QC208-3 devices and 20 EPF10K30RC208-4 devices for the HRRT. "The Altera PLD architecture reduced our development time and risk compared to a mask-based ASIC architecture, while maintaining costs and density," said John Young, Senior Development Engineer.
The advantages of using PLDs in the HRRT rather than an application-specific integrated circuit (ASIC) were obvious to the CTI PET Systems engineers. The in-circuit reconfigurability (ICR) of FLEX 10K devices means that they can be configured for set-up, reconfigured for the actual detection process, and reconfigured as necessary for self-diagnostics. For example, during setup, the PC/104 computer programs the Altera devices with calibration algorithms that allow all of the block processing channels to perform calibration at once, reducing the setup time for the detectors.
"The EPF10K10 device's reconfigurability means it can easily handle all these functions. We just reconfigure it in-system at each step of the process. Also, signal processing time is kept to a minimum because the Altera device can simultaneously access all the other devices in the sub-system," said John Young.
FLEX 10K Devices in the Detection Process
The HRRT scanner is a highly complex system in which Altera FLEX 10K devices play a vital role. This section concentrates on how these devices operate in the detection process. Figure 3 shows the analog subsection block diagram. Three EPF10K10QC208 devices are located on each analog sub-section board, as shown in Figure 4, for a total of 936 devices per system, one for each crystal block on the HRRT. The EPF10K10 device interfaces with three 8-bit FLASH analog-to-digital-converters (ADCs), a time-to-digital converter (TDC) circuit, and two 128-Kbyte × 8 look-up RAM devices. It also performs energy qualification, shape discrimination, and time correction.
Figure 3. Analog Subsection Block Diagram
Figure 4. Analog Subsection Board
During scanning, a crystal absorbs incoming photon energy (an event) and generates a light signal. The detector block's four photomultiplier tubes (PMTs) then convert the light signal into four digital signals, which are passed to an analog ASIC with a built-in constant fraction discriminator (CFD). The ASIC generates a time-mark signal indicating when a gamma ray is detected. This time-mark signal is sent to the TDC where it is digitized with respect to the master system clock. Meanwhile, the energy signal, consisting of the sum of the four PMT signals, is sampled twice by an 8-bit FLASH ADC to determine the decay shape of the energy. Two other 8-bit FLASH ADCs digitize the position of the event in the x and y directions. The output from the FLASH ADCs then passes to the EPF10K10 device.
The PCI/104 computer programs the EPF10K10 device with a control sequencer, which controls the integration time for the energy and the x and y signals. The sequencer starts when it detects the time mark from the analog ASIC. It clocks the FLASH ADCs and sequences the energy and x and y signals through the lookup RAM devices.
For this event processing to take place, the EPF10K10 device must be configured during block setup to allow writing to the analog ASIC setup registers and loading the look-up RAM blocks. The PC/104 computer then loads the RAM contents serially. To reduce the time to load the RAMs, the data is compressed using run-length encoding, and the EPF10K10 device is configured to uncompress the RAM data. To load the analog ASIC registers, the I2C bus operation is loaded into the control register and the analog ASIC data is shifted into the EPF10K10 device, which then controls the I2C bus signals.
The look-up RAM blocks are used to decrease event processing time and to reduce the amount of logic needed in the Altera devices. The RAM blocks perform the shape discrimination, energy qualification, crystal position determination, and time correction look-up. Two passes are made through the two RAM blocks, which have sufficient memory to hold the look-up data.
On the first pass, the energy signal is used as input to the shape discriminator. The shape discriminator determines the decay constant of the crystal that detected the event. Because the scanner uses crystals with fast and slow decay times, the shape of the energy integration can be used to report the depth of interaction; that is, to determine the location of the event more precisely, an important new feature in the HRRT. The x and y signal values are used to determine which of the crystals in the block detected the event, using the crystal position look-up table stored in the RAM.
On the second pass, energy qualification determines whether the energy signal level falls within the upper and lower settings given in the energy look-up table for that particular crystal. At the same time, because the signal transmit time may vary based on its location on the PMT, the event time for each crystal is corrected using a time correction look-up table and an adder circuit.
Next, the sequencer sends the event data serially to the detector head interface for priority selection and transmission to the coincidence processor.
The detector head interface prioritizes the multiple events sent by the analog subsection and passes them to the coincidence processor, which has twenty EPF10K30RC208-4 devices. Figure 5 shows a coincidence processor board.
Figure 5. Coincidence Processor Board with Ten EPF10K30RC208-4 Devices
The coincidence processor takes the time difference from the time-to-digital converter values of all the events detected and determines the events that are actually in coincidence (simultaneous emissions of photons) and those that are random. Next, the real-time sorter converts the raw coincidence data into sinograms, specifying the line along which the event apparently occurred, and accounting for the image plane, time gating, and time sequence selections. The array processor adds the necessary corrections to the sinograms and reconstructs them to form the emission images. The final step in the whole scanning process takes place on the workstation, where the images are displayed for evaluation.
Conclusion
CTI PET Systems chose EPF10K10 devices for their most advanced PET scanner because Altera programmable logic devices (PLDs) can easily be reconfigured in-system. They are able to implement the setup, the detection process, and the self-diagnosis in the same FLEX 10K devices, reducing development time, saving board space, and lowering power consumption and costs. As an added benefit, they are also assured of support for any future changes that may occur in the system requirements or configuration. The ECAT HRRT scanner shows how Altera products are not only being used in manufacturing and communications, but are also creating rapid and accurate solutions in the medical field.
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