Advanced Laboratory Course on Positron Emission Tomography

Abstract

Positron-Emission-Tomography or short PET is the state-of-the-art technique for imaging physiological processes inside humans or animals. This happens in a noninvasive fashion as the distribution of a radioactive, positron emitting radiopharmaceutical inside the body is monitored by surrounding detectors. With the help of mathematical algorithms, the tracer distribution is reconstructed to an image. PET has become an indispensable tool for ensuring the correct treatment of patients and assured diagnostics by the attending doctor, e.g. for cancer treatment.
PET is heavily used in medicine, biology, neurology and pharmaceutical research as e.g. brain activity, blood flow or glucose flow can be monitored. Several interdisciplinary fields are merged within a running PET system, ranging from physics, communication technology, electrical engineering and image reconstruction up to the application in medicine. Thus there is a need for modern physicists to understand not only the underlying physics but also how the system works and is operated.

Motivation

The one day laboratory course gives an introduction to PET, starting from the physical background up to the image reconstruction. An insight is given to detector techniques, modern readout electronics, data aquisition and analysis. Furthermore a short introduction to some standard tools in particle physics, e.g. Linux or the data analysis framework ROOT are part of this course. The PET scanner is a refurbished former small animal prototype, featuring 96 readout channels and set up with an up to date readout system. Data analysis and image reconstruction is done using a standard PC. The radioactive distributions which have to be reconstructed are different symbols milled in plexiglass, which are filled with a β+ decaying radionuclide. The available radioactive tracers are either 18F or 22Na.

Physical Background

Annihilation

The β+ decaying radionuclide emits a positron, which immediately annihilates with an electron from the surrounding material by creating two annihilation photons with 511 keV each. Due to momentum and energy conservation, these two photons are emitted 180° apart. This geometric precise release ist the fundament for the image reconstruction. If the two photons are detected in coincidence in two opposing detectors, this results in a line between the detectors. Somewhere on this so called line of response LOR, the annihilation must have taken place. By detecting a large number of these lines with detectors surrounding the object, this gives the spatial information of the annihilation namely the decaying radionuclide.

Detection schematics for the annihilation photons.

Photon Detection and Signal Processing

The scanner is equipped with 96 single Avalanche Photodiodes APDs coupled to a scintillating LSO (Lutetiumoxyorthosilicate) crystals, which convert the high energetic 511 keV annihilation photons to visible light, now easily detectable by the APD. The analog signal pulses from the APDs are digitized with three Sampling Analog to Digital Converters SADCs. Each SADC handles 32 channels and is read out via USB.

2x8 APDs per detector module, each with an active area of 4 mm x 4 mm.
LSO crystals assembled in the white PTFE matrix to prevent the incoming photons from inter crystal scattering.
Sampling ADC for the digitization of the analogous signals from the APDs.
Readout scheme of the detector system.

Setup

Scanner ring.
Complete PET system.

The scanner ring with 6 detector modules, each with two layers of 8 readout channels. In the field of view, different radioactive sources with varying tracer distributions can be placed. To cover all projection angles for the reconstruction, the sources are rotated by a stepping motor for a full revolution. The step sizes can be specified. The complete system is very compact with three main components: the detector ring, the power crate which houses all electronics and the Linux PC used for readout, data analysis and the image reconstruction.

Learning Targets and Goals

  • Understand the physics behind PET
  • Get to know the principles of a PET scanner from startup up to the reconstructed image
  • Get in touch with detector techniques, electronics and signal processing
  • Set up and calibration of the PET system
  • Taking first spectra, data processing with ROOT on a Linux machine
  • Determining energy and time resolution of the scanner
  • Learn the basic algorithms for the image reconstruction
  • Reconstruct different tracer distributions

Location

The laboratory course takes place at the

Nuklearmedizinische Klinik und Poliklinik der TU München
Klinikum Rechts der Isar München
Ismaninger Str. 22
81675 München
U4/U5, Max-Weber-Platz
http://www.nuk.med.tu-muenchen.de/

The userguide and contact details can be found here.

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Summer Term 2012
Date Group Number Names
27/06/2012 406 Huber, Rips, Kienzle
28/06/2012 306 Cervantes, Sato,Stasinopoulos
29/06/2012 313 Kunze, Schwarz-Schilling
02/07/2012 403 Fürsich, Gradl, Schnell
06/07/2012 208 Shopov, Zhang, Machacek
17/10/2012 64 Berner, Mittag, Wegscheider