Lens Dose in Routine Head CT: Comparison of Different Optimization Methods With Anthropomorphic Phantoms

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1 Medical Physics and Informatics Original Research Nikupaavo et al. Lens Dose in Head CT Medical Physics and Informatics Original Research Ulla Nikupaavo 1,2 Touko Kaasalainen 1,3 Vappu Reijonen 1 Sanna-Mari Ahonen 2 Mika Kortesniemi 1,3 Nikupaavo U, Kaasalainen T, Reijonen V, Ahonen SM, Kortesniemi M Keywords: anthropomorphic phantom, CT optimization, image quality, lens dose, MOSFET DOI: /AJR Received February 24, 2014; accepted after revision March 31, HUS Medical Imaging Center, Helsinki University Central Hospital, POB 340 (Haartmaninkatu 4), Helsinki, Finland. Address correspondence to U. Nikupaavo (ulla.nikupaavo@hus.fi). 2 University of Oulu, Institute of Health Sciences, Oulu, Finland. 3 Department of Physics, University of Helsinki, Helsinki, Finland. AJR 2015; 204: X/15/ American Roentgen Ray Society Lens Dose in Routine Head CT: Comparison of Different Optimization Methods With Anthropomorphic Phantoms OBJECTIVE. The purpose of this study was to study different optimization methods for reducing eye lens dose in head CT. MATERIALS AND METHODS. Two anthropomorphic phantoms were scanned with a routine head CT protocol for evaluation of the brain that included bismuth shielding, gantry tilting, organ-based tube current modulation, or combinations of these techniques. Highsensitivity metal oxide semiconductor field effect transistor dosimeters were used to measure local equivalent doses in the head region. The relative changes in image noise and contrast were determined by ROI analysis. RESULTS. The mean absorbed lens doses varied from 4.9 to 19.7 mgy and from 10.8 to 16.9 mgy in the two phantoms. The most efficient method for reducing lens dose was gantry tilting, which left the lenses outside the primary radiation beam, resulting in an approximately 75% decrease in lens dose. Image noise decreased, especially in the anterior part of the brain. The use of organ-based tube current modulation resulted in an approximately 30% decrease in lens dose. However, image noise increased as much as 30% in the posterior and central parts of the brain. With bismuth shields, it was possible to reduce lens dose as much as 25%. CONCLUSION. Our results indicate that gantry tilt, when possible, is an effective method for reducing exposure of the eye lenses in CT of the brain without compromising image quality. Measurements in two different phantoms showed how patient geometry affects the optimization. When lenses can only partially be cropped outside the primary beam, organ-based tube current modulation or bismuth shields can be useful in lens dose reduction. I n CT of the head, the eye lens is always exposed to scattered radiation and, depending on the scan geometry, often to the direct beam. Results of epidemiologic studies of populations with low-dose radiation exposure have suggested that the lens is more sensitive to ionizing radiation than has previously been assumed and that the process of radiation-induced cataract formation may even be stochastic without a threshold dose [1 3]. Therefore, the International Commission on Radiological Protection has reevaluated the equivalent dose limit of the eye lens to ionizing radiation by lowering the recommended acute dose threshold for lens effects (cataracts and opacities) from 2 8 Gy to 0.50 Gy [4, 5]. Measurements in phantoms and patients have varied considerably in absorbed dose of the lens. The variation, ranging from a few to approximately 100 mgy per scan, depends on the scanner, imaging technique, and optimization methods used [6 16]. Analysis of data on occupational exposure and on patients undergoing diagnostic scanning of the head has had mixed results. Klein et al. [17] reported a significant correlation between CT scanning and increased risk of nuclear sclerosis and posterior subcapsular opacity. Hourihan et al. [18], however, found no evidence of the prevalence of any type of cataract in people with a history of head CT. However, a 2013 study by Yuan et al. [19] corroborated the connection between increased risk of cataract formation and CT of the head and neck region. The risk seems to increase gradually with repeated CT studies. The radiation exposure of the lens and of other selective radiation-sensitive tissues can be reduced by use of technical solutions such as organ-based tube current modulation (OBTCM) [6 8], local exterior shielding (bismuth shields) [7, 9, 10, 15, 16], and optimization of the gantry tilt angle [10, 11, 15, 20]. Users can also reduce radiation exposure and improve image quality by taking AJR:204, January

2 Nikupaavo et al. steps such as centering patients properly in the scan isocenter [21, 22]. Iterative reconstruction techniques have enabled major reductions in total dose, 20 30% in the case of head CT, while diagnostic image quality is maintained [23 25]. Our aim was to compare a group of methods developed to reduce the dose to the eye lens in head CT. We quantitatively assessed their effect on dose distribution and image quality in different anatomic locations by performing measurements in two anthropomorphic phantoms. A Fig. 1 Photographs show anthropomorphic phantoms used in study. A, ATOM model 702-D (CIRS). B, RANDO (Phantom Laboratory). Materials and Methods Dose Measurements and Calculations We imaged two tissue-equivalent anthropomorphic phantoms ATOM model 702-D (CIRS) and RANDO (The Phantom Laboratory) in the head-first supine position (Fig. 1) with a 128- MDCT scanner (Somatom Definition AS+, Siemens Healthcare). We performed CT head scanning in helical mode with eight settings: reference scan without an optimization method; gantry tilted according to clinical practice (baseline from skull base to radix nasi); gantry tilted at an angle one half of that used in clinical practice; 0.06-mm lead equivalent bismuth shield (AttenuRad Radiation Protection, F&L Medical Products) on the eyes; both a bismuth shield and gantry tilted according to clinical practice; OBTCM (X-CARE, Siemens Healthcare); both OBTCM and gantry tilted according to the clinical practice; and bismuth shield already set on the eyes in scout imaging (Fig. 2). The following scan parameters were identical for all studied scan settings and defined as applied in clinical practice: 120 kv, tube current modulation with quality reference tube current time product of 410 mas, 1-second rotation time, pitch of 0.6, and detector configuration of mm (with double z-sampling corresponding to a total detector width of 38.4 mm). Scan range was set for a routine head CT examination (from skull base to vertex). Volume CT dose index varied from to mgy for the ATOM phantom and from to mgy for the RANDO phantom. We measured local organ doses with high-sensitivity TN-1002RD metal oxide semiconductor field effect transistor (MOSFET) dosimeters (TN-1002RD, Best Medical) with high bias settings. Before the measurements were made, the dosimeters were calibrated in the CT beam on an axial scan obtained with 120-kVp tube voltage. In calibration, the reference air kerma was measured with a CT pencil ionization chamber (Ray- Safe Xi, Unfors RaySafe), and the calibration factor was defined separately for each MOSFET dosimeter. One of the dosimeters was discarded because of observed high instability. Thus a total of nine MOSFET dosimeters were inserted in the phantom head in different locations: left mandible, both lenses, skull base, posterior right and left hemispheres, occipital bone, anterior right hemisphere, and anterior aspect of the central part of the brain. The active part and epoxy bulb of the MOSFET dosimeters were located slightly below the midpoint of each phantom layer, the tips pointing toward the anterior part of the phantom. We read the MOSFETs after each acquisition, and scanning was performed five times for each setting. Average absorbed doses (in milligrays) were calculated for each organ in each scanning setting. B Fig. 2 Scan settings used for head CT and geometric profiles of phantoms. Top row, gantry tilting angles and reference scan setting in two phantoms (left, ATOM, CIRS; right, RANDO, Phantom Laboratory). Bottom row, scan settings with bismuth shields. Image Analysis Because the MOSFET dosimeter lead wires produce metal artifacts and interfere with image 118 AJR:204, January 2015

3 Lens Dose in Head CT TABLE 1: Absorbed Organ Doses in Milligrays and Relative Change Compared With Reference Scanning of ATOM (CIRS) Phantom Lens Right Left Anterior Right Posterior Left Anterior Part of Central Brain Occipital Bone Posterior Right Scan Setting Left Mandible Skull Base Reference 3.1 ± 0.2 (0.0) 18.1 ± 0.3 (0.0) 20.1 ± 0.9 (0.0) 15.4 ± 0.3 (0.0) 16.9 ± 1.0 (0.0) 18.9 ± 0.4 (0.0) 17.5 ± 0.7 (0.0) 20.1 ± 1.3 (0.0) 19.2 ± 1.5 (0.0) Bismuth shield after scout imaging 3.1 ± 0.2 (0.0) 18.0 ± 0.5 ( 0.8) 20.3 ± 0.5 (1.1) 15.2 ± 0.6 ( 1.3) 16.7 ± 0.8 ( 0.8) 19.2 ± 1.3 (1.6) 17.0 ± 0.5 ( 3.0) 14.9 ± 0.1 ( 26.1) 15.2 ± 1.5 ( 21.2) Bismuth shield before scout imaging 3.5 ± 0.4 (13.8) 18.1 ± 0.5 ( 0.1) 19.7 ± 1.0 ( 2.2) 15.0 ± 0.5 ( 2.5) 15.8 ± 0.8 ( 6.2) 18.7 ± 0.7 ( 1.1) 17.0 ± 0.8 ( 3.0) 15.1 ± 1.2 ( 24.8) 14.5 ± 1.2 ( 24.4) Bismuth shield with gantry tilt 2.2 ± 0.2 ( 29.5) 18.3 ± 0.4 (1.2) 19.2 ± 0.5 ( 4.5) 15.8 ± 0.8 (3.0) 15.5 ± 0.9 ( 8.3) 18.9 ± 0.8 (0.4) 17.4 ± 0.8 ( 0.5) 5.0 ± 0.4 ( 74.9) 4.8 ± 0.6 ( 74.9) Gantry tilt 2.1 ± 0.2 ( 31.1) 17.6 ± 0.7 ( 2.7) 19.7 ± 1.1 ( 2.2) 15.8 ± 0.5 (2.7) 15.5 ± 1.0 ( 8.4) 19.2 ± 0.9 (1.6) 17.1 ± 0.5 ( 2.0) 5.1 ± 0.2 ( 74.5) 4.7 ± 0.2 ( 75.6) Gantry tilt with one-half angle 2.4 ± 0.2 ( 20.9) 17.8 ± 0.6 ( 1.6) 20.0 ± 0.8 ( 0.8) 15.5 ± 0.3 (1.0) 15.3 ± 0.7 ( 9.6) 18.3 ± 0.5 ( 2.9) 17.3 ± 0.5 ( 1.0) 16.0 ± 2.0 ( 20.6) 15.3 ± 2.3 ( 20.3) OBTCM 3.5 ± 0.3 (14.5) 21.2 ± 0.4 (16.9) 22.8 ± 0.6 (13.5) 14.7 ± 0.2 ( 4.2) 18.8 ± 1.2 (11.5) 20.2 ± 1.3 (7.2) 15.7 ± 0.5 ( 10.3) 13.8 ± 0.4 ( 31.3) 13.1 ± 1.1 ( 32.1) OBTCM with gantry tilt 2.4 ± 0.2 ( 23.8) 20.4 ± 0.8 (12.8) 21.6 ± 0.7 (7.4) 14.1 ± 0.6 ( 8.3) 17.7 ± 0.8 (5.1) 19.9 ± 0.6 (5.6) 14.9 ± 0.9 ( 14.9) 6.2 ± 0.6 ( 69.1) 5.4 ± 0.4 ( 71.9) Note Values are mean ± SD. Values in parentheses are relative change as percentage. Negative relative change value indicates dose reduction compared with reference scan setting. OBTCM = organ-based tube current modulation. TABLE 2: Absorbed Organ Doses in Milligrays and Relative Change Compared With Reference Scan in RANDO Phantom (Phantom Laboratory) Lens Right Left Anterior Right Posterior Left Anterior Part of Central Brain Occipital Bone Posterior Right Scan Setting Left Mandible Skull Base Reference 3.8 ± 0.2 (0.0) 16.4 ± 0.7 (0.0) 17.6 ± 0.6 (0.0) 18.1 ± 0.7 (0.0) 15.0 ± 1.4 (0.0) 17.1 ± 0.9 (0.0) 18.0 ± 0.6 (0.0) 17.7 ± 1.8 (0.0) 16.1 ± 1.2 (0.0) Bismuth shield after scout imaging 3.9 ± 0.5 (3.5) 16.9 ± 0.5 (2.9) 18.5 ± 0.4 (5.2) 17.9 ± 0.5 ( 1.2) 14.8 ± 1.6 ( 1.5) 17.1 ± 0.7 (0.0) 17.6 ± 0.6 ( 2.6) 16.3 ± 1.3 ( 7.6) 14.1 ± 1.1 ( 12.3) Bismuth shield before scout imaging 4.4 ± 0.4 (15.1) 16.8 ± 0.4 (2.8) 17.5 ± 1.0 ( 0.3) 17.6 ± 0.5 ( 2.8) 15.0 ± 1.0 ( 0.4) 17.2 ± 0.9 (0.4) 17.4 ± 0.4 ( 3.7) 14.5 ± 1.2 ( 17.9) 15.2 ± 1.2 ( 15.5) Bismuth shield with gantry tilt 3.1 ± 0.3 ( 18.9) 17.8 ± 0.4 (8.9) 18.3 ± 0.4 (4.2) 17.7 ± 0.5 ( 2.4) 14.4 ± 1.2 ( 4.5) 17.5 ± 0.6 (2.2) 18.3 ± 0.5 (1.4) 12.9 ± 0.7 ( 27.2) 10.4 ± 0.7 ( 35.3) Gantry tilt 2.9 ± 0.1 ( 22.0) 18.4 ± 0.3 (12.3) 18.0 ± 1.2 (2.5) 17.7 ± 0.7 ( 2.2) 13.7 ± 1.2 ( 8.9) 17.0 ± 1.0 ( 0.9) 18.3 ± 0.7 (1.7) 14.8 ± 1.7 ( 16.1) 12.9 ± 1.5 ( 19.7) Gantry tilt with one-half angle 3.1 ± 0.2 ( 18.7) 17.7 ± 1.1 (7.8) 17.7 ± 0.5 (1.0) 17.9 ± 0.4 ( 1.0) 13.9 ± 1.6 ( 7.7) 17.0 ± 0.7 ( 0.8) 17.9 ± 0.5 ( 0.9) 16.5 ± 1.8 ( 6.7) 14.9 ± 1.3 ( 7.6) OBTCM 4.6 ± 0.3 (21.4) 19.0 ± 0.7 (16.0) 19.3 ± 0.5 (9.9) 17.3 ± 0.3 ( 4.5) 18.1 ± 1.6 (20.1) 19.6 ± 0.5 (14.5) 16.5 ± 0.4 ( 8.5) 14.4 ± 0.4 ( 18.6) 11.3 ± 0.5 ( 30.0) OBTCM with gantry tilt 3.5 ± 0.3 ( 9.6) 20.9 ± 0.8 (27.6) 20.6 ± 0.6 (17.1) 17.7 ± 0.5 ( 2.3) 15.9 ± 1.8 (5.6) 18.9 ± 1.0 (10.4) 17.3 ± 0.6 ( 4.0) 11.3 ± 0.7 ( 36.3) 10.4 ± 1.2 ( 35.4) Note Values are mean ± SD. Values in parentheses are relative change as percentage. Negative relative change value indicates dose reduction compared with reference scan setting. OBTCM = organ-based tube current modulation. quality analysis (noise, contrast), we repeated all scans with different settings without the dosimeters and with the dosimeter holes filled with tissue-equivalent plugs. Each time, the scan parameters were identical to the scans obtained with the dosimeters. We estimated that the accuracy of placement of the phantom for the image quality as opposed to dosimetry scans was ± 2 mm in the x, y, and z directions. The image quality in the case of the RANDO phantom was poor because of air gaps between the phantom layers, and we decided to use only image data from the ATOM phantom scans for image quality measurements. We determined image contrast by measuring the mean CT number and noise by measuring 1 SD of the CT number in five different ROIs. We selected locations of particular clinical significance for ROIs: ROI 1 was located in the region of the right cerebellum, ROIs 2 and 3 in the expected positions of the anterior temporal lobes, and ROIs 4 and 5 in the expected positions of the basal ganglia nuclei (Fig. 3). The size of ROI 1 was approximately 450 mm 2, and the size of ROIs 2 5 was 300 mm 2. We compared image noise and contrast at each setting with the noise and contrast of the reference images and calculated the relative changes. We studied image quality using both filtered back projection and iterative reconstruction with Safire level 2 (Siemens Healthcare), which is used at our clinic. Axial slices were reconstructed with 4-mm thickness and a 4-mm interslice interval. We performed the image analysis with ImageJ software (version 1.48, U.S. National Institutes of Health). Results Dosimetric Results In Tables 1 and 2, we present the results of the dose measurements in each scanning setting and their relative changes compared with the reference scanning performed without any optimization methods. The mean absorbed organ doses varied from 2.2 to 22.8 mgy in the case of the ATOM phantom and from 3.1 to 20.9 mgy in the case of the RANDO phantom. Depending on the scanning setting used, the mean lens dose varied from 4.9 to 19.7 mgy for the ATOM phantom and from 10.8 to 16.9 mgy for the RANDO phantom. In the case AJR:204, January

4 Nikupaavo et al. TABLE 3: Mean Image Noise (1 SD HU) and Relative Change Compared With Reference Setting in Five ROIs in Clinically Significant Areas ROI 1 ROI 2 ROI 3 ROI 4 ROI 5 Scan Setting of the ATOM phantom, the most efficient way to reduce eye lens dose appeared to be gantry tilt according to our clinical practice (baseline set from skull base to radix nasi), with or without bismuth shields, with an approximately 75% decrease in the absorbed dose compared with the reference setting. In addition, using the combination of OBTCM and gantry tilt produced a dose savings of 70%, whereas OBTCM alone was associated with a dose reduction of 32%. With the 6-mm lead equivalent bismuth shield, it was possible to reduce lens dose as much as 25%, whereas gantry tilted at an angle one half of that used in clinical practice produced 20% dose savings for the lens compared with the reference setting. In the case of the RANDO phantom, the dose reduction was less and occurred in different order than in the ATOM phantom owing to different phantom geometry. The highest lens dose reduction FBP Safire 2 FBP Safire 2 FBP Safire 2 FBP Safire 2 FBP Safire 2 Reference 5.0 (0.0) 4.1 (0.0) 5.8 (0.0) 4.9 (0.0) 5.8 (0.0) 4.9 (0.0) 4.7 (0.0) 3.9 (0.0) 4.5 (0.0) 3.7 (0.0) Bismuth shield after scout imaging 5.0 (0.0) 4.1 (0.0) 5.4 ( 7.4) 4.5 ( 8.6) 6.0 (4.0) 5.1 (4.9) 4.9 (4.3) 4.0 (3.6) 5.1 (14.5) 4.3 (17.4) Bismuth shield before scout imaging 5.4 (8.5) 4.6 (8.3) 5.5 ( 5.8) 4.6 ( 5.1) 5.4 ( 7.4) 4.5 ( 8.4) 4.9 (4.7) 4.1 (6.2) 4.7 (4.0) 3.8 (4.6) Bismuth shield with gantry tilt 5.1 (3.2) 4.2 (2.9) 4.4 ( 24.5) 3.6 ( 26.0) 4.6 ( 20.5) 3.8 ( 22.2) 4.4 ( 5.8) 3.7 ( 6.2) 4.4 ( 2.5) 3.6 ( 2.7) Gantry tilt 5.1 (2.4) 4.0 ( 2.4) 4.5 ( 22.6) 3.8 ( 22.8) 4.7 ( 19.8) 3.7 ( 23.9) 4.8 (2.4) 3.9 (0.0) 4.7 (5.6) 3.9 (5.2) Gantry tilt with half an angle 4.8 ( 3.6) 3.9 ( 6.1) 4.9 ( 17.0) 4.0 ( 18.7) 5.4 ( 6.7) 4.5 ( 9.0) 4.8 (2.2) 4.1 (4.1) 4.7 (5.4) 3.9 (5.7) OBTCM 6.3 (27.9) 5.3 (29.7) 6.2 (5.8) 4.8 ( 0.8) 6.5 (12.4) 5.2 (5.3) 5.7 (22.8) 4.7 (20.3) 5.8 (29.2) 4.7 (28.1) OBTCM with gantry tilt 6.4 (30.1) 5.1 (24.8) 5.6 ( 4.1) 4.8 ( 0.8) 6.3 (7.9) 4.9 (0.0) 5.7 (22.8) 4.3 (9.7) 5.5 (22.0) 4.0 (7.1) Note Values in parentheses are relative change as percentage. Negative relative change value indicates noise reduction compared with reference scan setting. ROI 1 = region of the right cerebellum, ROIs 2 and 3 = expected positions of the anterior temporal lobes, ROIs 4 and 5 = expected positions of the basal ganglia nuclei, FBP = filtered back projection, Safire 2 = Safire level 2 iterative reconstruction (Siemens Healthcare). OBTCM = organ-based tube current modulation. TABLE 4: Mean CT Numbers (HU) and Relative Change Compared With Reference Setting in Five ROIs in Clinically Significant Areas Scan Setting ROI 1 ROI 2 ROI 3 ROI 4 ROI 5 FBP Safire 2 FBP Safire 2 FBP Safire 2 FBP Safire 2 FBP Safire 2 Reference 51.0 (0.0) 50.8 (0.0) 51.9 (0.0) 51.7 (0.0) 53.2 (0.0) 53.3 (0.0) 49.4 (0.0) 49.2 (0.0) 49.7 (0.0) 49.7 (0.0) Bismuth after scout imaging 51.6 (1.2) 51.3 (1.0) 51.8 ( 0.2) 51.6 ( 0.3) 53.9 (1.2) 53.8 (0.9) 49.6 (0.4) 49.5 (0.6) 50.1 (0.9) 50.2 (1.1) Bismuth shield before scout imaging 51.7 (1.5) 51.8 (2.0) 53.8 (3.5) 53.4 (3.2) 55.4 (3.9) 54.4 (2.1) 49.3 ( 0.2) 49.4 (0.4) 49.8 (0.3) 49.9 (0.6) Bismuth shield with gantry tilt 48.3 ( 5.3) 48.3 ( 4.9) 52.3 (0.8) 52.3 (1.2) 53.2 ( 0.2) 53.1 ( 0.5) 49.4 (0.1) 49.3 (0.1) 49.7 ( 0.1) 49.7 (0.1) Gantry tilt 47.9 ( 6.0) 48.0 ( 5.5) 51.9 ( 0.2) 52.1 (0.7) 53.0 ( 0.5) 52.9 ( 0.7) 49.3 ( 0.2) 49.4 (0.3) 49.8 (0.2) 50.0 (0.7) Gantry tilt with one-half angle 48.2 ( 5.4) 47.9 ( 5.7) 49.0 ( 5.8) 49.9 ( 3.4) 53.5 (0.5) 53.5 (0.4) 49.0 ( 0.8) 48.8 ( 0.9) 49.8 (0.2) 49.6 ( 0.1) OBTCM 50.5 ( 1.0) 50.2 ( 1.2) 53.4 (2.8) 53.7 (3.9) 51.5 ( 3.3) 51.7 ( 3.1) 48.7 ( 1.4) 49.1 ( 0.3) 50.2 (1.1) 50.2 (1.2) OBTCM with gantry tilt 52.3 (2.7) 50.4 ( 0.9) 51.4 ( 1.1) 50.5 ( 2.3) 55.2 (3.7) 55.9 (4.8) 48.2 ( 2.5) 48.8 ( 1.0) 49.9 (0.3) 49.0 ( 1.3) Note Values in parentheses are relative change as percentage. Negative relative change value indicates noise reduction compared with reference scan setting. ROI 1 = region of the right cerebellum, ROIs 2 and 3 = expected positions of the anterior temporal lobes, ROIs 4 and 5 = expected positions of the basal ganglia nuclei, FBP = filtered back projection, Safire 2 = Safire level 2 iterative reconstruction (Siemens Healthcare). OBTCM = organ-based tube current modulation. (36%) was achieved with the RANDO phantom with the combination of OBTCM and gantry tilt. The other methods produced dose reductions of 31% (bismuth shield together with gantry tilt), 24% (OBTCM alone), 18% (gantry tilt), 12% (bismuth shield already set on eyes in scout imaging), and 10% (bismuth shield alone). Using gantry tilt at an angle one half of that used in clinical practice produced only approximately 7% dose savings for the lens compared with the results with the reference scan setting. When OBTCM was used, the dose distribution differed substantially from the distributions observed with the other settings: The absorbed dose increased in the posterior parts of the phantoms (up to 17% in the case of the ATOM phantom and 27% in the case of the RANDO phantom) owing to boosting of radiation output in the posterior angles (covering approximately 240 ) and decreased in the anterior tissues owing to reduced radiation output in the anterior angles (covering approximately 120 ). By tilting the gantry, we were able to reduce scan length by a few slices. Thus the dose-length product and therefore the effective dose calculated with International Commission on Radiological Protection publication 103 conversion factors was reduced approximately 6% compared with the reference scan with no gantry tilt. Image Analysis Results Mean image noise and CT numbers are shown in Tables 3 and 4. Image noise varied in the range of HU in ROI 1, HU in ROIs 2 and 3, and HU in ROIs 4 and 5 when iterative reconstruction was not used. When OBTCM was used with or without gantry tilt, the image noise increased as much as 30% in the bottom and posterior part 120 AJR:204, January 2015

5 Lens Dose in Head CT A Fig. 3 CT images show selected ROIs for measurement of image noise and contrast. A, ROI 1 is in region of right cerebellum. B, ROIs 2 and 3 are in expected positions of anterior temporal lobes. C, ROIs 4 and 5 are in expected positions of basal ganglia nuclei. of the brain (ROI 1) of the ATOM phantom. Similarly, OBTCM increased image noise approximately 12% in the anterior part of the brain (ROIs 2 and 3) and 29% in the central part of the brain (ROIs 4 and 5). The use of gantry tilt decreased image noise approximately a quarter in the anterior part of the brain compared with the reference scan setting. In other ROIs, the change was more moderate. The use of a bismuth shield increased image noise approximately 17% in the central part of the brain when the shield was set on the eyes after scout imaging. Image noise increased less when the bismuth shield was set, incorrectly, before scout imaging, because the tube current modulation compensated the increased x-ray attenuation of the scan object by increasing the tube current time product. The use of iterative reconstruction (Safire level 2) decreased image noise approximately 20% in each ROI and for all scan settings. Image contrast was maintained for each scan setting when Safire level 2 was used for image reconstruction. The mean CT number, and thus the image contrast, in the brain tissue of the ATOM phantom was approximately 50 HU (minimum and maximum, 48 and 56 HU). The greatest variation (5 6%) in image contrast was observed in the bottom posterior part of the brain when gantry tilting was used. Discussion CT scanner manufacturers and users have made an effort to minimize the dose to B the eye lenses since results of several studies have indicated that the lens is more sensitive to ionizing radiation than previously believed. The aim of our study was to explore the lens dose reduction capability of different optimization methods in head CT of the brain by use of two tissue-equivalent anthropomorphic phantoms, MOSFET dosimeters, and quantitative image quality analysis. In all, we used eight different scanning settings. In this study, we followed the gantry tilt practice at our hospital, aligning the scan plane along the skull base to the radix nasi. The European guidelines call for a gantry tilt above the orbitomeatal line to reduce exposure of the eye lenses [26]. Because of different geometric profiles of the phantoms, tilting in the case of the ATOM phantom was mainly comparable to the supraorbital baseline, whereas in the case of the RANDO phantom, the gantry tilt followed closely the orbitomeatal baseline, although the tilting angles (13 and 11.5 ) were about the same. Thus the lens dosimeter points were left outside of the primary radiation beam in the ATOM phantom, whereas in the case of the RANDO phantom, the lenses were partly inside the primary beam. According to results of previous studies, the use of supraorbital gantry tilt reduces exposure of the eye lenses by 78 88%, depending on the CT scanner and technique used [10, 12, 13, 15]. Our results for the ATOM phantom were similar, the lens dose reduction being up to 75%. In the case of the RANDO phantom, however, the lens dose was reduced only up to 18%. We studied the effect of tilting the gantry at an angle one half of that used in clinical practice (6.5 and 7 ) for lens doses, because tilting angles also vary remarkably in routine work owing to patient-specific physiologic limitations [20]. Although the lens dose reduction was considerably smaller (20% for the ATOM phantom, 7% for the RANDO phantom) than in the case of the original gantry tilt, using even a small tilting angle appears useful because the scanning range may be shortened with a corresponding decrease in total radiation dose. Bismuth shields are used in CT examinations to protect radiosensitive superficial tissues and organs such as the lens, thyroid, and mammary tissue from primary radiation within the scanning range. In our study, the reduction of lens dose due to bismuth shielding was approximately 10% in the case of the RANDO phantom and approximately 25% in the case of the ATOM phantom, which is moderate compared with earlier published results [7, 9, 10, 16, 27]. OBTCM can also be used for reduction of dose to superficial radiosensitive tissues. In this study, we found that the lens doses were reduced as much as one third in the case of the ATOM phantom and one fourth for the RANDO phantom when we used the OBTCM method. The results were similar to those in other studies of OBTCM tech- C AJR:204, January

6 Nikupaavo et al. niques, which have indicated dose reductions of 26 59% to the lenses [6 8]. We also studied whether combining bismuth shields or OBTCM with simultaneous gantry tilt brings any benefit. When the lenses were located completely outside the primary beam owing to tilting, neither bismuth shielding nor OBTCM resulted in extra dose reduction. In fact, adding OBTCM resulted in a slight increase in the lens dose, most probably due to higher scatter intensity originating from the elevated primary beam dose distribution on the occipital side of the skull. On the other hand, with more gradual tilting, we observed that both the bismuth shield and OBTCM resulted in clear dose reduction to the lenses. The primary beam dose contribution and close range scatter would otherwise become more prominent for the lenses. Wang et al. [7] studied the simultaneous use of bismuth shielding and OBTCM and reported considerable dose reduction (almost 50%) for the eye lenses. However, this combination should be used carefully because OBTCM boosts radiation output in the posterior part of the brain, and the bismuth shielding scatters this back to the patient [28]. Gantry tilting is not possible with all available CT scanner models. In such cases, the patient s head can be set on a head support with the chin turned to the chest to mimic gantry tilt. In practice, however, tilting the patient s head is not always conceivable because of anatomic properties and the physiologic condition of the patient. In these cases, the use of bismuth shields or OBTCM together with ordinary tube current modulation can be considered the primary means of reducing the dose to the eye lenses. The benefit of different dose reduction methods must be evaluated against the requirements for clinically adequate image quality. Gantry tilting has been reported to cause beam-hardening and partial volume artifacts in the base of the skull and to cause problems in visualization of the temporal lobes [15, 29]. However, in other reports [20, 30] it has been claimed that no substantial differences in clinical image quality of head CT can be observed with different gantry tilt angles. The use of bismuth shielding has also been questioned, because it can degrade image quality and accuracy by causing streak and beam-hardening artifacts and waste of image data by absorbing photons transmitted through the patient before they would appear on the detector [28, 31]. In addition, if the shields are placed erroneously before scout imaging in the case of automatic exposure control, the patient dose will be increased. These disadvantages have been observed in the case of bismuth shielding of the breast [32] and thyroid [33]. Authors of several studies [34 37] have recommended the use of tube current modulation with lowered tube current time values instead of bismuth breast shields. This method will not cause image quality impairments due to streak artifacts while resulting in a similar dose reduction. Nevertheless, in the use of bismuth shields on the eyes, artifacts seem to fall outside of the diagnostic area of interest whenever visualization of the orbits and sinuses is not important [16, 38]. In earlier studies, use of OBTCM techniques did not appear to affect image noise or CT numbers, or their influence was scant [6 8]. In this study, we assessed image quality by using five ROIs located in the posterior, anterior, and central parts of the brain. The RANDO phantom was not suitable for image quality evaluation because the air between its slices caused strong artifacts. Gantry tilt and partial gantry tilt reduced the noise level in the anterior parts of the brain (the former by almost one fourth), whereas in the case of the posterior and central ROIs, the changes in noise level were minor. The use of iterative reconstruction (Safire level 2) resulted systematically in an approximately 20% decrease in noise level. However, in this study we did not consider other image quality aspects related to the use of iterative reconstruction. According to our results, using bismuth shielding of the eyes appears advantageous with only little effect on image quality in the case of brain scans. No artifacts were observed, and the differences in noise level in the case of the reference scan and the scan with a bismuth shield were measured to be minor in all brain regions. Placing the bismuth shield before scout imaging decreased the exposure in all measuring points except for the mandible, where the increase in dose was 14% (resulting in a total increase in dose of 2.5% for the whole scan), whereas the change in the image quality was small compared with the reference scan. However, this feature is due to the real-time angular tube current modulation technique and cannot be generalized to automatic exposure control techniques in which the angular modulation profile is based on the scout image. In this study, we found that OBTCM increased the noise level in the posterior and central (right and left) regions of the brain both without (28%, 23%, and 29%) and with combined gantry tilt (30%, 23%, and 22%). However, the absolute change in measured noise level compared with the reference level was less than 1.5 HU in all of our study conditions. All the different imaging methods studied had only little effect on the measured contrast values (mean attenuation in HU measured in the ROIs). The mean attenuation changed the most in the posterior fossa when gantry tilt was used. The decrease was 5 6% from the reference image, corresponding to an absolute difference of approximately 3 HU. However, even such small changes in attenuation levels can be crucial in CT of the brain, in which, for example, the contrast between white and gray matter is approximately only 4 5 HU. Our study had limitations. First, we used only one CT scanner from a single vendor. Because focus-detector and focus-isocenter distances, tube current modulation techniques, and bowtie filters vary between scanners, the effects of different dose reduction methods on radiation dose distribution and image noise may vary between scanners. In addition, the OBTCM technique is not available with most of the scanners used today. 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