CT head‐scan dosimetry in an anthropomorphic phantom and associated measurement of ACR accreditation‐phantom imaging metrics under clinically representative scan conditions. Issue 8 (24th July 2013)

• Record Type: Journal Article
• Authors: Brunner, Claudia C.; Stern, Stanley H.; Minniti, Ronaldo; Parry, Marie I.; Skopec, Marlene; Chakrabarti, Kish
• Is Part Of: Medical physics. Volume 40:Issue 8(2013)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2013-07-24
• Subjects: Dosimetry/exposure assessment -- Dosimetry/exposure assessment -- Noise -- Reconstruction -- Edge enhancement -- Computed tomography; accreditation -- biological tissues -- computerised tomography -- data acquisition -- dosimeters -- dosimetry -- edge detection -- filtering theory -- image denoising -- image enhancement -- image reconstruction -- image representation -- image resolution -- iterative methods -- medical image processing -- optical transfer function -- phantoms -- statistical analysis; CT -- dose -- image quality -- OSLDs; Computerised tomographs -- Dosimeters -- Digital computing or data processing equipment or methods, specially adapted for specific applications -- Image data processing or generation, in general -- Image enhancement or restoration, e.g. from bit‐mapped to bit‐mapped creating a similar image; Medical imaging -- Image scanners -- Medical image noise -- Dosimetry -- Computed tomography -- Medical image reconstruction -- Germanium -- Modulation transfer functions -- Image reconstruction -- Image analysis; Medical physics -- Periodicals; Medical physics; Geneeskunde; Natuurkunde; Toepassingen; Biophysics; Periodicals; Periodicals; Electronic journals; 610.153
• DOI: 10.1118/1.4815964
• Languages: English
• ISSNs: 0094-2405

Abstract

Abstract : Purpose: : To measure radiation absorbed dose and its distribution in an anthropomorphic head phantom under clinically representative scan conditions in three widely used computed tomography (CT) scanners, and to relate those dose values to metrics such as high‐contrast resolution, noise, and contrast‐to‐noise ratio (CNR) in the American College of Radiology CT accreditation phantom. Methods: : By inserting optically stimulated luminescence dosimeters (OSLDs) in the head of an anthropomorphic phantom specially developed for CT dosimetry (University of Florida, Gainesville), we measured dose with three commonly used scanners (GE Discovery CT750 HD, Siemens Definition, Philips Brilliance 64) at two different clinical sites (Walter Reed National Military Medical Center, National Institutes of Health). The scanners were set to operate with the same data‐acquisition and image‐reconstruction protocols as used clinically for typical head scans, respective of the practices of each facility for each scanner. We also analyzed images of the ACR CT accreditation phantom with the corresponding protocols. While the Siemens Definition and the Philips Brilliance protocols utilized only conventional, filtered back‐projection (FBP) image‐reconstruction methods, the GE Discovery also employed its particular version of an adaptive statistical iterative reconstruction (ASIR) algorithm that can be blended in desired proportions with the FBP algorithm. We did an objective image‐metrics analysis evaluating the modulation transfer function (MTF), noise power spectrum (NPS), and CNR for images reconstructed with FBP. For images reconstructed with ASIR, we only analyzed the CNR, since MTF and NPS results are expected to depend on the object for iterative reconstruction algorithms. Results: : The OSLD measurements showed that the Siemens Definition and the Philips Brilliance scanners (located at two different clinical facilities) yield average absorbed doses in tissue of 42.6 and 43.1 mGy, respectively. The GE Discovery delivers about the same amount of dose (43.7 mGy) when run under similar operating and image‐reconstruction conditions, i.e., without tube current modulation and ASIR. The image‐metrics analysis likewise showed that the MTF, NPS, and CNR associated with the reconstructed images are mutually comparable when the three scanners are run with similar settings, and differences can be attributed to different edge‐enhancement properties of the applied reconstruction filters. Moreover, when the GE scanner was operated with the facility's scanner settings for routine head exams, which apply 50% ASIR and use only approximately half of the 100%‐FBP dose, the CNR of the images showed no significant change. Even though the CNR alone is not sufficient to characterize the image quality and justify any dose reduction claims, it can be useful as a constancy test metric. Conclusions: : This work presents a straightforward method to connect direct measurements of CT dose with objective image metrics such as high‐contrast resolution, noise, and CNR. It demonstrates that OSLD measurements in an anthropomorphic head phantom allow a realistic and locally precise estimation of magnitude and spatial distribution of dose in tissue delivered during a typical CT head scan. Additional objective analysis of the images of the ACR accreditation phantom can be used to relate the measured doses to high contrast resolution, noise, and CNR.