title: Upper limits of the photon fluence rate on CT detectors: Case study on a commercial scanner.
abstract: OBJECTIVE
  The highest photon fluence rate that a computed tomography (CT) detector must be able to measure is an important parameter. The authors calculate the maximum transmitted fluence rate in a commercial CT scanner as a function of patient size for standard head, chest, and abdomen protocols.
  
  
  METHODS
  The authors scanned an anthropomorphic phantom (Kyoto Kagaku PBU-60) with the reference CT protocols provided by AAPM on a GE LightSpeed VCT scanner and noted the tube current applied with the tube current modulation (TCM) system. By rescaling this tube current using published measurements on the tube current modulation of a GE scanner [N. Keat, "CT scanner automatic exposure control systems," MHRA Evaluation Report 05016, ImPACT, London, UK, 2005], the authors could estimate the tube current that these protocols would have resulted in for other patient sizes. An ECG gated chest protocol was also simulated. Using measured dose rate profiles along the bowtie filters, the authors simulated imaging of anonymized patient images with a range of sizes on a GE VCT scanner and calculated the maximum transmitted fluence rate. In addition, the 99th and the 95th percentiles of the transmitted fluence rate distribution behind the patient are calculated and the effect of omitting projection lines passing just below the skin line is investigated.
  
  
  RESULTS
  The highest transmitted fluence rates on the detector for the AAPM reference protocols with centered patients are found for head images and for intermediate-sized chest images, both with a maximum of 3.4 ⋅ 10(8) mm(-2) s(-1), at 949 mm distance from the source. Miscentering the head by 50 mm downward increases the maximum transmitted fluence rate to 5.7 ⋅ 10(8) mm(-2) s(-1). The ECG gated chest protocol gives fluence rates up to 2.3 ⋅ 10(8) - 3.6 ⋅ 10(8) mm(-2) s(-1) depending on miscentering.
  
  
  CONCLUSIONS
  The fluence rate on a CT detector reaches 3 ⋅ 10(8) - 6 ⋅ 10(8) mm(-2) s(-1) in standard imaging protocols, with the highest rates occurring for ECG gated chest and miscentered head scans. These results will be useful to developers of CT detectors, in particular photon counting detectors.
authors.affiliation: Department of Physics, Royal Institute of Technology, Stockholm SE-10691, Sweden.
authors.forename: Mats
authors.initials: M
authors.lastname: Persson
authors.affiliation: Unit of X-ray Physics, Section of Imaging Physics Solna, Department of Medical Physics, Karolinska University Hospital, Stockholm SE-17176, Sweden.
authors.forename: Robert
authors.initials: R
authors.lastname: Bujila
authors.affiliation: Unit of X-ray Physics, Section of Imaging Physics Solna, Department of Medical Physics, Karolinska University Hospital, Stockholm SE-17176, Sweden.
authors.forename: Patrik
authors.initials: P
authors.lastname: Nowik
authors.affiliation: Unit of X-ray Physics, Section of Imaging Physics Solna, Department of Medical Physics, Karolinska University Hospital, Stockholm SE-17176, Sweden.
authors.forename: Henrik
authors.initials: H
authors.lastname: Andersson
authors.affiliation: Medical Radiation Physics, Sunderby Hospital, Luleå SE-97180, Sweden.
authors.forename: Love
authors.initials: L
authors.lastname: Kull
authors.affiliation: Department of Radiation Sciences, Radiation Physics, Umeå University, Umeå SE-90185, Sweden.
authors.forename: Jonas
authors.initials: J
authors.lastname: Andersson
authors.affiliation: Department of Physics, Royal Institute of Technology, Stockholm SE-10691, Sweden.
authors.forename: Hans
authors.initials: H
authors.lastname: Bornefalk
authors.affiliation: Department of Physics, Royal Institute of Technology, Stockholm SE-10691, Sweden.
authors.forename: Mats
authors.initials: M
authors.lastname: Danielsson
mesh_terms: D000388:Air; D000465:Algorithms; D003198:Computer Simulation; D004562:Electrocardiography / Q000295:instrumentation / Q000379:methods; D005260:Female; D006257:Head / Q000000981:diagnostic imaging; D006321:Heart / Q000000981:diagnostic imaging; D006801:Humans; D008297:Male; D008953:Models, Anatomic; D019047:Phantoms, Imaging; D017785:Photons; D011829:Radiation Dosage; D011860:Radiography, Abdominal / Q000295:instrumentation / Q000379:methods; D013902:Radiography, Thoracic / Q000295:instrumentation / Q000379:methods; D015898:Tomography Scanners, X-Ray Computed; D014057:Tomography, X-Ray Computed / Q000295:instrumentation* / Q000379:methods