title: Quantification of arterial plaque and lumen density with MDCT.
abstract: OBJECTIVE
  This study aimed to derive a mathematical correction function in order to normalize the CT number measurements for small volume arterial plaque and small vessel mimicking objects, imaged with multidetector CT (MDCT).
  
  
  METHODS
  A commercially available calcium plaque phantom (QRM GmbH, Moehrendorf, Germany) and a custom built cardiovascular phantom were scanned with 320 and 64 MDCT scanners. The calcium hydroxyapatite plaque phantom contained objects 0.5-5.0 mm in diameter with known CT attenuation nominal values ranging 50-800 HU. The cardiovascular phantom contained vessel mimicking objects 1.0-5.0 mm in diameter with different contrast media. Both phantoms were scanned using clinical protocols for CT angiography and images were reconstructed with different filter kernels. The measured CT number (HU) and diameter of each object were analyzed on three clinical postprocessing workstations. From the resultant data, a mathematical formula was derived based on absorption function exp(--micro.-d) to demonstrate the relation between measured CT numbers and object diameters.
  
  
  RESULTS
  The percentage reduction in measured CT number (HU) for the group of selected filter kernels, apparent during CT angiography, is dependent only on the object size (plaque or vessel diameter). The derived formula of the form 1-c.-exp(-a.-d--b) showed reduction in CT number for objects between 0.5 and 5 mm in diameter, with asymptote reaching background noise for small objects with diameters nearing the CT in-plane resolution (0.35 mm). No reduction was observed for the objects with diameters equal or larger than 5 mm.
  
  
  CONCLUSIONS
  A clear mathematical relationship exists between object diameter and reduction in measured CT number in HU. This function is independent of exposure parameters and inherent attenuation properties of the objects studied. Future developments include the incorporation of this mathematical model function into quantification software in order to automatically generate a true assessment of measured CT number (HU) corresponding to plaque physical density rho (g/cm(3)). This is a significant development for the accurate, noninvasive classification of noncalcified arterial plaque.
authors.affiliation: Department of Medical Imaging, Toronto General Hospital, Ontario, Canada. narinder.paul@uhn.on.ca
authors.forename: Narinder S
authors.initials: NS
authors.lastname: Paul
authors.forename: Joerg
authors.initials: J
authors.lastname: Blobel
authors.forename: Hany
authors.initials: H
authors.lastname: Kashani
authors.forename: Murray
authors.initials: M
authors.lastname: Rice
authors.forename: Ali
authors.initials: A
authors.lastname: Ursani
mesh_terms: D015502:Absorptiometry, Photon / Q000295:instrumentation / Q000379:methods*; D001158:Arteries / Q000503:physiopathology*; D050197:Atherosclerosis / Q000000981:diagnostic imaging* / Q000503:physiopathology*; D003198:Computer Simulation; D006801:Humans; D008955:Models, Cardiovascular; D019047:Phantoms, Imaging; D011857:Radiographic Image Interpretation, Computer-Assisted / Q000379:methods*; D014057:Tomography, X-Ray Computed / Q000295:instrumentation / Q000379:methods*