title: Flexible mini gamma camera reconstructions of extended sources using step and shoot and list mode.
abstract: OBJECTIVE
  Hand- and robot-guided mini gamma cameras have been introduced for the acquisition of single-photon emission computed tomography (SPECT) images. Less cumbersome than whole-body scanners, they allow for a fast acquisition of the radioactivity distribution, for example, to differentiate cancerous from hormonally hyperactive lesions inside the thyroid. This work compares acquisition protocols and reconstruction algorithms in an attempt to identify the most suitable approach for fast acquisition and efficient image reconstruction, suitable for localization of extended sources, such as lesions inside the thyroid.
  
  
  METHODS
  Our setup consists of a mini gamma camera with precise tracking information provided by a robotic arm, which also provides reproducible positioning for our experiments. Based on a realistic phantom of the thyroid including hot and cold nodules as well as background radioactivity, the authors compare "step and shoot" (SAS) and continuous data (CD) acquisition protocols in combination with two different statistical reconstruction methods: maximum-likelihood expectation-maximization (ML-EM) for time-integrated count values and list-mode expectation-maximization (LM-EM) for individually detected gamma rays. In addition, the authors simulate lower uptake values by statistically subsampling the experimental data in order to study the behavior of their approach without changing other aspects of the acquired data.
  
  
  RESULTS
  All compared methods yield suitable results, resolving the hot nodules and the cold nodule from the background. However, the CD acquisition is twice as fast as the SAS acquisition, while yielding better coverage of the thyroid phantom, resulting in qualitatively more accurate reconstructions of the isthmus between the lobes. For CD acquisitions, the LM-EM reconstruction method is preferable, as it yields comparable image quality to ML-EM at significantly higher speeds, on average by an order of magnitude.
  
  
  CONCLUSIONS
  This work identifies CD acquisition protocols combined with LM-EM reconstruction as a prime candidate for the wider introduction of SPECT imaging with flexible mini gamma cameras in the clinical practice.
authors.affiliation: Department of Informatics, Technische Universität München, München 80333, Germany and Klinikum Rechts der Isar, Technische Universität München, München 80333, Germany.
authors.forename: José
authors.initials: J
authors.lastname: Gardiazabal
authors.affiliation: Department of Informatics, Technische Universität München, München 80333, Germany.
authors.forename: Philipp
authors.initials: P
authors.lastname: Matthies
authors.affiliation: Department of Informatics, Technische Universität München, München 80333, Germany.
authors.forename: Jakob
authors.initials: J
authors.lastname: Vogel
authors.affiliation: Department of Informatics, Technische Universität München, München 80333, Germany.
authors.forename: Benjamin
authors.initials: B
authors.lastname: Frisch
authors.affiliation: School of Engineering, Johns Hopkins University, Baltimore, Maryland 21218 and Department of Informatics, Technische Universität München, München 80333, Germany.
authors.forename: Nassir
authors.initials: N
authors.lastname: Navab
authors.affiliation: Klinikum Rechts der Isar, Technische Universität München, München, München 80333,Germany.
authors.forename: Sibylle
authors.initials: S
authors.lastname: Ziegler
authors.affiliation: Department of Informatics, Technische Universität München, München 80333, Germany.
authors.forename: Tobias
authors.initials: T
authors.lastname: Lasser
mesh_terms: D000465:Algorithms; D015902:Gamma Cameras; D006801:Humans; D007091:Image Processing, Computer-Assisted; D019047:Phantoms, Imaging; D012371:Robotics; D013961:Thyroid Gland / Q000000981:diagnostic imaging; D015899:Tomography, Emission-Computed, Single-Photon / Q000295:instrumentation*