title: A method to assess image quality for Low-dose PET: analysis of SNR, CNR, bias and image noise.
abstract: BACKGROUND
  Lowering injected dose will have an effect on PET image quality. In this article, we aim to investigate this effect in terms of signal-to-noise ratio (SNR) in the liver, contrast-to-noise ratio (CNR) in the lesion, bias and ensemble image noise.
  
  
  METHODS
  We present here our method and preliminary results using tuberculosis (TB) cases. Sixteen patients who underwent (18)F-FDG PET/MR scans covering the whole lung and portion of the liver were selected for the study. Reduced doses were simulated by randomly discarding events in the PET list mode data stream, and ten realizations at each simulated dose were generated and reconstructed. The volumes of interest (VOI) were delineated on the image reconstructed from the original full statistics data for each patient. Four thresholds (20, 40, 60 and 80 % of SUVmax) were used to quantify the effect of the threshold on CNR at the different count level. Image metrics were calculated for each VOI. This experiment allowed us to quantify the loss of SNR and CNR as a function of the counts in the scan, in turn related to dose injected. Reproducibility of mean and maximum standardized uptake value (SUVmean and SUVmax) measurement in the lesions was studied as standard deviation across 10 realizations.
  
  
  RESULTS
  At 5 × 10(6) counts in the scan, the average SNR in the liver in the observed samples is about 3, and the CNR is reduced to 60 % of the full statistics value. The CNR in the lesion and SNR in the liver decreased with reducing count data. The variation of CNR across the four thresholds does not significantly change until the count level of 5 × 10(6). After correcting the factor related to subject's weight, the square of the SNR in the liver was found to have a very good linear relationship with detected counts. Some quantitative bias appears with count reduction. At the count level of 5 × 10(6), bias and noise in terms of SUVmean and SUVmax are up to 10 and 20 %, respectively. To keep both bias and noise less than 10 %, 5 × 10(6) counts and 20 × 10(6) counts were required for SUVmean and SUVmax, respectively.
  
  
  CONCLUSIONS
  Initial results with the given data of 16 patients diagnosed as TB demonstrated that 5 × 10(6) counts in the scan could be sufficient to yield good images in terms of SNR, CNR, bias and noise. In the future, more work needs to be done to validate the proposed method with a larger population and lung cancer patient data.
authors: Department of Nuclear Medicine, First Hospital of Shanxi Medical University, 85 Jiefang S Rd, Yingze, Taiyuan, Shanxi, 030001, China. jianhua.yan@gmail.com.|Molecular Imaging Precision Medicine Collaborative Innovation Center, Shanxi Medical University, 85 Jiefang S Rd, Yingze, Taiyuan, Shanxi, 030001, China. jianhua.yan@gmail.com.|A*STAR-NUS, Clinical Imaging Research Center, Center for translational medicine, 14 medical drive, #B1-01, 17599, Singapore, Singapore. jianhua.yan@gmail.com.
authors: Jianhua
authors: J
authors: Yan
authors: A*STAR-NUS, Clinical Imaging Research Center, Center for translational medicine, 14 medical drive, #B1-01, 17599, Singapore, Singapore.|Department of Diagnostic Radiology, National University Hospital, Main Building, 5 Lower Kent Ridge Road, Level 3, 119074, Singapore, Singapore.
authors: Josh
authors: J
authors: Schaefferkoette
authors: Siemens Healthcare Molecular Imaging, 810 Innovation Drive, Knoxville, TN37932, USA.
authors: Maurizio
authors: M
authors: Conti
authors: A*STAR-NUS, Clinical Imaging Research Center, Center for translational medicine, 14 medical drive, #B1-01, 17599, Singapore, Singapore.|Department of Diagnostic Radiology, National University Hospital, Main Building, 5 Lower Kent Ridge Road, Level 3, 119074, Singapore, Singapore.
authors: David
authors: D
authors: Townsend
mesh_terms: D004307:Dose-Response Relationship, Radiation; D019788:Fluorodeoxyglucose F18 / Q000494:pharmacology*; D006801:Humans; D049268:Positron-Emission Tomography / Q000379:methods*; D019275:Radiopharmaceuticals / Q000494:pharmacology; D015203:Reproducibility of Results; D059629:Signal-To-Noise Ratio
chemical_list: D019275:Radiopharmaceuticals; D019788:Fluorodeoxyglucose F18
keywords: Image quality; Low dose; Lung; PET/CT; PET/MR