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Addressing clock distribution challenges in medical imaging

Posted: 01 Sep 2011     Print Version  Bookmark and Share

Keywords:medical imaging  clock distribution  Positron Emission Tomography 

With the never ending drive to improve healthcare there is an increasing demand for higher resolution medical imaging to see better into the human body. With higher resolutions come issues of signal acquisition and distribution. Along with these requirements stable low-jitter clocking is required to improve both the signal acquisition accuracy and the transport of data through the system. In this article we will examine clock distribution systems on large scale imaging devices, which is a real challenge for design engineers.

X-Ray Computerized Axial Tomography (CAT) scanning has been available to the medical community since the mid to late 1970's. Improvements in computer processing power and acquisition times have greatly improved the scanning speed as well as the information content and resolution of the images. Today we have scanners that combine Positron Emission Tomography (PET) with Magnetic Resonance Imaging (MRI) or X-Ray Computed Tomography (CT) to provide better registration of information and improved images. These are called Dual Modality scanners and are some of the latest designs.

 detector rings

Figure 1: PET detector rings..

Timing, noise and resolution
In particular, PET scanners rely on radionuclide tracers that produce positrons as they decay. As the positrons lose momentum they combine through various methods with electrons and annihilate each other resulting in two 511 keV gamma rays moving in almost opposite directions. To register the chord or Line of Response (LOR) of the two gamma ray photons as they exit the patient, a detector ring is used that must accommodate the body of the patient making its diameter roughly one metre and may include 500 to over 1000 channels. The detectors must be able to correlate two gamma ray events into an LOR from positron-electron annihilations rather than from random events. Additionally, the channels must measure the energy of the gamma rays with some accuracy to detect errors caused by Compton Scattering which will lead to errors in the position of the source event. This is done by several ways that require accurate clocks to coordinate the detection window.

Creating an accurate and stable high frequency clock is fairly straight forward, however distributing that clock signal around large detector rings (figure) can be challenging since fast clock edges suffer from loss effects of the transmission medium. Some detectors are using fibre optics to move the output of the scintillation crystals to channel boards that hold the photoelectric components (PMTs or APDs). This places the detector electronics closer together, but distribution can still suffer from channel impairment, skew, jitter and other degradation effects which ultimately will affect the image noise and usable resolution.

Proper clock distribution
In CT imaging and many other similar data acquisition systems, the clock can limit the performance of time based systems as well as data converter performance. To help keep the high speed clocks clean, signals are distributed differentially and use either LVPECL or LVDS levels. As they are routed on channel boards, clock distribution can be challenging in both driving the loads (e.g. large FPGAs) and adjusting for board layout skew that can affect when edges arrive. To solve this problem semiconductor vendors have created clock distribution devices designed to allow engineers to "dial out" the skew through programmable delays as well as re-drive the clock signal. It is important to have in-system programmability for managing clocks as well as calibration to minimise timing errors.

When transporting clocks across channel boards, twisted pair wire or twin-axial cable may be used, however new problems will appear. As the high speed signals travel any distance, they will suffer from high frequency attenuation, group delay and other distortion caused by cross-talk and system noise especially in the presence of high-voltage supplies used to power PMTs. In most all of these cases, clock recovery is accomplished through retiming via a phase locked loop (PLL) device.

Conclusion
State of the art large scale data acquisition systems such as PET, CT, MRI and Dual Modality scanners require tightly controlled clocks to minimise distortion and noise as well as improve overall system performance. New scintillation crystals with high light output and fast decay times have made possible Time of Flight (TOF) detection in PET scanners. This new technology requires even higher frequency clocks and produces much higher resolution images. The future of CT scanning is bright with new developments continuously on the horizon, however as detection resolutions continue to improve, timing rates will also increase and clock distribution will persist in challenging designers.

- Richard F. Zarr
  National Semiconductor

To download the PDF version of this article, click here.





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