title: Importance of Attenuation Correction (AC) for Small Animal PET Imaging.
abstract: The purpose of this study was to investigate whether a correction for annihilation photon attenuation in small objects such as mice is necessary. The attenuation recovery for specific organs and subcutaneous tumors was investigated. A comparison between different attenuation correction methods was performed.
  
  
  METHODS
  Ten NMRI nude mice with subcutaneous implantation of human breast cancer cells (MCF-7) were scanned consecutively in small animal PET and CT scanners (MicroPET(TM) Focus 120 and ImTek's MicroCAT(TM) II). CT-based AC, PET-based AC and uniform AC methods were compared.
  
  
  RESULTS
  The activity concentration in the same organ with and without AC revealed an overall attenuation recovery of 9-21% for MAP reconstructed images, i.e., SUV without AC could underestimate the true activity at this level. For subcutaneous tumors, the attenuation was 13 ± 4% (9-17%), for kidneys 20 ± 1% (19-21%), and for bladder 18 ± 3% (15-21%). The FBP reconstructed images showed almost the same attenuation levels as the MAP reconstructed images for all organs.
  
  
  CONCLUSIONS
  The annihilation photons are suffering attenuation even in small subjects. Both PET-based and CT-based are adequate as AC methods. The amplitude of the AC recovery could be overestimated using the uniform map. Therefore, application of a global attenuation factor on PET data might not be accurate for attenuation correction.
authors: Cluster for Molecular Imaging, Faculty of Health Sciences & Department of Clinical Physiology, Nuclear Medicine & PET, Rigshospitalet, University of Copenhagen, Blegdamsvej 3, Copenhagen DK-2200, Denmark. helali@sund.ku.dk.
authors: Henrik H
authors: HH
authors: El Ali
authors: Cluster for Molecular Imaging, Faculty of Health Sciences & Department of Clinical Physiology, Nuclear Medicine & PET, Rigshospitalet, University of Copenhagen, Blegdamsvej 3, Copenhagen DK-2200, Denmark. rasmuspoul@sund.ku.dk.
authors: Rasmus Poul
authors: RP
authors: Bodholdt
authors: Cluster for Molecular Imaging, Faculty of Health Sciences & Department of Clinical Physiology, Nuclear Medicine & PET, Rigshospitalet, University of Copenhagen, Blegdamsvej 3, Copenhagen DK-2200, Denmark. jtran@sund.ku.dk.
authors: Jesper Tranekjær
authors: JT
authors: Jørgensen
authors: Cluster for Molecular Imaging, Faculty of Health Sciences & Department of Clinical Physiology, Nuclear Medicine & PET, Rigshospitalet, University of Copenhagen, Blegdamsvej 3, Copenhagen DK-2200, Denmark. rebeccam@sund.ku.dk.
authors: Rebecca
authors: R
authors: Myschetzky
authors: Cluster for Molecular Imaging, Faculty of Health Sciences & Department of Clinical Physiology, Nuclear Medicine & PET, Rigshospitalet, University of Copenhagen, Blegdamsvej 3, Copenhagen DK-2200, Denmark. akjaer@sund.ku.dk.
authors: Andreas
authors: A
authors: Kjaer
keywords: MicroPET; PET imaging; attenuation correction; molecular imaging; small animal