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Nuclear & Plasma Sciences Society

Distinguished Lectures

Dr. David W. Townsend

Director, A*STAR-NUS Clinical Imaging Research Centre Professor, National University of Singapore


The Evolution of Hybrid Imaging

From autoradiography to planar X-rays, Computed Tomography (CT) and Magnetic Resonance (MR), morphology and structure has been the mainstay of biological and medical imaging for over a century. While structural changes may suggest the presence of disease, functional changes are more sensitive indicators of early-stage pathology, and with cancer, early detection is the key to a favorable prognosis. Since molecular imaging offers the potential to quantitatively image functional changes in vivo, it is assuming an increasingly important role in the identification, staging and re-staging of human disease. Specifically, Positron Emission Tomography (PET) and Single Photon Emission Computed Tomography (SPECT) are sensitive techniques to map human physiology non-invasively through the use of high-resolution imaging devices and appropriate radioactively-labeled biomarkers. However, such metabolic maps do not offer the structural detail associated with anatomical imaging techniques such as CT and MR and therefore dual modality devices such as PET/CT, SPECT/CT or PET/MR that combine both structural and functional information offer a more complete and accurate assessment of the status of disease. PET/CT instrumentation, for example, was first introduced into the clinic in 2001 and since then, progress has been rapid. Technological advances in each modality, CT and PET, have been consistently incorporated into the combined device ensuring state-of-the-art performance for PET/CT. Recent advances in CT include an increase in the number of detector rows or slices (from 1 to 64), a reduction in rotation times (to less than 0.5 s), and the emergence of the first CT scanner incorporating dual X-ray sources. Paralleling these advances, PET instrumentation has witnessed the introduction of new faster scintillators, higher resolution detectors, increased sensitivity through extended axial coverage, and the resurgence of time-of-flight information to improve image signal-to-noise. A major advance in image reconstruction techniques has been the introduction of statistically-based algorithms into clinical routine, with progressive refinement of the system model to more accurately represent the imaging process. Most of the independent advances in CT and PET instrumentation have been rapidly incorporated into state-of-the-art PET/CT designs and over the past six years, the development, introduction and rapid adoption of PET/CT technology has significantly impacted the medical imaging field. For oncology in particular, PET/CT has become the preferred imaging modality with over 1600 scanners now installed in clinical practice worldwide, progressively replacing PET-only tomographs.

The development of high performance CT scanners (64-slice and 0.3 s rotation times) has been driven primarily by cardiac applications. Lower performance CT (16-slice, 0.5 s rotation times) combined with state-of-the-art PET components will in general be adequate for oncology applications such as diagnosis and staging of malignant disease and monitoring therapeutic response. With the improvements in spatial resolution and sensitivity of PET instrumentation, the potential exists for earlier diagnosis and assessment of response to treatment when it can still make a difference for the patient. While FDG-PET/CT has provided incremental improvements compared to FDG-PET in both sensitivity and specificity for many clinical studies, some applications such as mediastinal and cervical lymph node detection still lack good specificity. There are, therefore, opportunities to further improve the sensitivity and specificity of PET, although such improvement is more likely to be achieved through the use of new, novel biomarkers than advances in PET/CT instrumentation. For cardiac imaging, the identification of plaque formation and other associated inflammatory processes is an important, although challenging, goal. The application of PET/CT to cardiology is still in its infancy as issues related to respiration and cardiac motion are addressed, and especially those arising from the use of CT-based attenuation correction in this setting. Mismatch between the CT and PET images can create artifacts that may have diagnostic consequences and therefore appropriate respiration and cardiac gating strategies are currently being explored. The goal remains a single exam that can provide cardiac anatomy, angiography, perfusion and functional status of the myocardium. Incremental improvements and refinements in CT and PET instrumentation are to be anticipated in the future, including further increase in axial coverage, whereas major breakthroughs and insights are more likely to come from the introduction of novel PET biomarkers into clinical practice. Such biomarkers map physiological processes such as inflammation, cell proliferation, hypoxia, apoptosis and gene expression. As the specificity of these biomarkers increases, the requirement for the anatomical framework provided by CT will be essential. Thus, although currently the primary role of PET/CT is imaging FDG for oncology studies, the availability of other biomarkers will likely expand the use of PET/CT despite challenges from other developing hybrid modalities such as PET/MR.

This lecture will describe the evolution of multimodality instrumentation for the imaging of human disease, with particular emphasis on cancer. Some recent developments and future directions of multi-modality imaging technology will be highlighted.

Lost in Translation - From Basic Science to Clinical Reality

The transfer of technology from a basic science field such as particle physics to more applied areas like medical imaging, although offering promise is not always as straightforward as it may appear. While accelerator and particle physicists are presented with problems of extreme technical complexity requiring ingenious solutions, their techniques and instrumentation may not easily translate to other fields. The particular constraints imposed by one field may complicate or even invalidate the translation of a solution that appears promising from the perspective of the other field. Medical imaging instrumentation must be cost effective, offering adequate clinical performance for reasonable levels of cost and reliability; particle physics instrumentation is designed for extremely high levels of performance and reliability, with cost concerns often being secondary. In some limited areas, such as in the development of scintillators and detector electronics, translation of the technology has achieved a measured level of success. However, in attempting to facilitate this translation it is essential that one field understands the limitations, constraints and objectives of the other field. Without this bilateral understanding, promising advances in particle physics will have little or no impact on medical imaging; the advances will literally be lost in translation. This talk will discuss examples of techniques that originated from accelerator and particle physics and that should, or could have had a more significant impact on medical imaging, and critically examine the procedures by which the transfer of such technology might be accomplished.


photo of Dr. David W. Townsend

David W. Townsend obtained his Ph.D. in Particle Physics from the University of London and was a staff member for eight years at the European Centre for Nuclear Research (CERN) in Geneva, Switzerland. In 1980, Dr Townsend joined the faculty of Geneva University Hospital as a physicist in the Department of Nuclear Medicine. Working with Dr Alan Jeavons from CERN, he explored the use of the High Density Avalanche Chamber (HIDAC) for clinical PET imaging. He has worked on PET instrumentation development since the early eighties, and has been a senior consultant for CTI PET Systems (now Siemens Molecular Imaging) in Knoxville, Tennessee since 1992. In collaboration with Dr Terry Jones at the Cyclotron Unit of Hammersmith Hospital, London he participated in the development of 3D reconstruction and methodology for PET, and later designed and built the first rotating partial ring PET scanner using BGO block detectors.

In 1993, Dr Townsend moved to the University of Pittsburgh as an Associate Professor of Radiology and Senior PET Physicist. He was Co-Director of the Pittsburgh PET Facility from 1996-2002, and became Professor of Radiology in 2000. In 1995, Dr Townsend was Principal Investigator on the first proposal to design and build a combined PET/CT scanner. The PET/CT scanner, attributed to Dr Townsend and Dr Nutt, then President of CPS Innovations, was named by TIME Magazine as the medical invention of the year 2000. In recognition of his work on PET/CT, Dr Townsend received the 2004 Distinguished Clinical Scientist Award from the Academy of Molecular Imaging, and the 2008 Nuclear Medicine Pioneer Award from the Austrian Society of Nuclear Medicine. In 2006, he was elected a Fellow of the IEEE. Since February 2003, Dr Townsend has been at the University of Tennessee in Knoxville as Professor of Medicine and Radiology, and Director of the Molecular Imaging and Translational Research Program.

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