title: How reliable are Hounsfield-unit measurements in forensic radiology?
abstract: OBJECTIVE
  To assess the reliability of computed tomography (CT) numbers, also known as Hounsfield-units (HU) in the differentiation and identification of forensically relevant materials and to provide instructions to improve the reproducibility of HU measurements in daily forensic practice.
  
  
  METHODS
  We scanned a phantom containing non-organic materials (glass, rocks and metals) on three different CT scanners with standardized parameters. The t-test was used to assess the influence of the scanner, the size and shape of different types of regions-of-interest (ROI), the composition and shape of the object, and the reader performance on HU measurements. Intra-class correlation coefficient was used to assess intra- and inter-reader reliability.
  
  
  RESULTS
  HU values did not change significantly as a function of ROI-shape or -size (p>0.05). Intra-reader reliability reached ICC values >0.929 (p<0.001). Inter-reader reliability was also excellent with an ICC of 0.994 (p<0.001). Four of seven objects yielded significantly different CT numbers at different levels within the object (p<0.05). In 6/7 objects the HU changed significantly from CT scanner to CT scanner (p<0.05).
  
  
  CONCLUSIONS
  Reproducible CT number measurements can be achieved through correct ROI-placement and repeat measurements within the object of interest. However, HU may differ from CT-scanner to CT-scanner. In order to obtain comparable CT numbers we suggest that a dedicated Forensic Reference Phantom be developed.
authors.affiliation: Center of Forensic Imaging and Virtopsy, Institute of Forensic Medicine, University of Bern, Buehlstrasse 20, CH-3012 Bern, Switzerland. thomas.ruder@irm.uzh.ch
authors.forename: Thomas D
authors.initials: TD
authors.lastname: Ruder
authors.forename: Yannick
authors.initials: Y
authors.lastname: Thali
authors.forename: Sebastian T
authors.initials: ST
authors.lastname: Schindera
authors.forename: Simon A
authors.initials: SA
authors.lastname: Dalla Torre
authors.forename: Wolf-Dieter
authors.initials: WD
authors.lastname: Zech
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Thali
authors.forename: Steffen
authors.initials: S
authors.lastname: Ross
authors.forename: Gary M
authors.initials: GM
authors.lastname: Hatch