Thoracic examinations with 16, 64, 128 and 256 slices CT: comparison of exposure doses measured with an anthropomorphic phantom and TLD dosimeters

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1 Thoracic examinations with 16, 64, 128 and 256 slices CT: comparison of exposure doses measured with an anthropomorphic phantom and TLD dosimeters Poster No.: C-2584 Congress: ECR 2015 Type: Scientific Exhibit Authors: F. Zito 1, M. Balbini 1, M. Palumbo 1, E. G. Cossa 2, G. Re 1, G. Keywords: DOI: Galetta 1, C. Canzi 1, F. Voltini 1 ; 1 Milan/IT, 2 Legnano/IT Radioprotection / Radiation dose, CT, Dosimetry, Dosimetric comparison /ecr2015/C-2584 Any information contained in this pdf file is automatically generated from digital material submitted to EPOS by third parties in the form of scientific presentations. References to any names, marks, products, or services of third parties or hypertext links to thirdparty sites or information are provided solely as a convenience to you and do not in any way constitute or imply ECR's endorsement, sponsorship or recommendation of the third party, information, product or service. ECR is not responsible for the content of these pages and does not make any representations regarding the content or accuracy of material in this file. As per copyright regulations, any unauthorised use of the material or parts thereof as well as commercial reproduction or multiple distribution by any traditional or electronically based reproduction/publication method ist strictly prohibited. You agree to defend, indemnify, and hold ECR harmless from and against any and all claims, damages, costs, and expenses, including attorneys' fees, arising from or related to your use of these pages. Please note: Links to movies, ppt slideshows and any other multimedia files are not available in the pdf version of presentations. Page 1 of 14

2 Aims and objectives With the introduction of multi-detector CT (MDCT) scanners the number of clinical applications of CT procedures has continued to increase due to the greater volume coverage and faster rotation times that have provided significantly higher performance [1]. Nevertheless, these diagnostic procedures can cause relatively high patient exposure and particular care is required in assessing and optimizing CT radiation doses to patients [2]. Radiation dose in CT is typically expressed with the CTDIvol index (mgy) measured by a pencil camera inside a uniform PMMA cylindrical phantom. However CTDIvol is not a dose to a point, but rather an average dose over a volume, furthermore it can be affected by large errors because the PMMA phantom has shape and size different from those of a patient body. The product of CTDIvol with the scan length, the dose length product (DLP), is another standard parameter (mgy.cm) to measure patient exposure in each CT examination; DLP includes overranging dose contributes. On modern CT scanners CTDIvol and DLP are supplied on the CT console [2-5]. An alternative CT dosimetry approach, even if more complex to apply, is that based on direct dose measurement by exposing an anthropomorphic phantom filled with TLDs (thermo-luminescent-dosimeters) from which punctual dose values can be derived. Aim of the work is: -to evaluate and compare five different MDCT scanners in terms of organ dose and effective dose specifically for standard thoracic examination. A patient with standard body size and same exposing parameters were used. To simulate the patient body, the Alderson-Rando anthropomorphic-phantom was used and filled with TLDs, direct organ doses were carried out ; -to evaluate the accuracy of the average lung dose determined by CTDIvol index; -to analyze the contribute on patient dose of the overranging effect. Methods and materials Page 2 of 14

3 Fig. 1: The Alderson Rando ready to be exposed References: Nuclear Medicine, Fondazione IRCCS Cà Granda Ospedale Maggiore Policlinico - Milan/IT The Alderson Rando, a phantom simulating an average sized person, formed by 25 mm thick slices. In each slice, organs and tissues of interest were determined and marked, based on an atlas of human anatomy. Different points were identified for each organ (87 in total) and in each of them 3 TLDs were positioned (261 in total), a summary is reported in Table 1. Table 1. Numbers of points selected for each organ and numbers of TLD positioned inside the Alderson Rando. Organ Number of points Number of TLDs Lung Brain 3 9 Thyroid 4 12 Bone-marrow 4 12 Stomach 3 9 Liver 6 18 Kidney 6 18 Page 3 of 14

4 Small intestine 6 18 Spleen 3 9 Utherus 3 9 Breast 4 12 Skin 2 6 Oral Mucosa 2 6 Lens 2 6 Total CT Scanner and acquisition parameter settings GE LightSpeed 16 slice CT GE LightSpeed VCT 64 slice CT Siemens Biograph 64 slice PET/CT Siemens Somatom Definition 128 Slice CT Philips ICT 256 slice CT All scans were performed in helical mode covering the same thoracic length (38,5 cm) of the phantom (description on Fig 2), and setting the following acquisition parameters: - Tube current: 200 ma - Tube voltage: 120 kvp - Rotation time: 0,5 s - FOV: 50 cm - Slice thickenss: 5 mm - Pitch:1.4 but 0.9 for the Philips scanner - Collimation: 0.6 mm Page 4 of 14

5 Fig. 2: Topogram of the Alderson Rando, the red box indicates the examinated thoracic region References: Nuclear Medicine, Fondazione IRCCS Cà Granda Ospedale Maggiore Policlinico - Milan/IT To increase TLD signal and to reduce the associated statistical error, thoracic region examinations were repeated three times for each scan. To evaluate accuracy of three different methods on estimating exposure dose, the following indices were determined and compared: the standard CTDIvol as reported by CT scanners after each phantom exposure (CTDIvol_ report ), the SSDE (size specific dose estimate) that is the CTDIvol_ report corrected for the effective diameter of the phantom Page 5 of 14

6 according to the formula and factors of AAPM report N. 204 (CTDIvol_ AAPM ) and the lung average dose measured with TLDs. By knowing scan length, CTDIvol and DLP, evaluation of the potential dose increase due to overranging of helical scan for organs positioned at the edge or external to the thoracic region of interest was carried out. Effective dose "E" (msv) were determined by weighting organ dose with ICRP-103 factors. For each scanner the overranging lenght (Lo) was calculated as: Lo (cm) = DLP (mgy.cm)/ctdivol(mgy) -Ls(cm); Ls is the length of the selected thorax region. Images for this section: Page 6 of 14

7 Fig. 2: Topogram of the Alderson Rando, the red box indicates the examinated thoracic region Page 7 of 14

8 Results Organ doses measured with TLDs by exposing the humanoid phantom with the 5 scanners are reported on Fig. 3 and Fig. 4. Fig. 3: Organ doses of the Alderson-Rando phantom assessed with TLDs exposed with the 5 CT scanners. References: Nuclear Medicine, Fondazione IRCCS Cà Granda Ospedale Maggiore Policlinico - Milan/IT As shown on Fig 3 there is no evidence of organ dose increase by increasing number of raw of CT scanners. Page 8 of 14

9 Fig. 4: Average lung doses estimated with the three methods applied to the 5 scanner data. References: Nuclear Medicine, Fondazione IRCCS Cà Granda Ospedale Maggiore Policlinico - Milan/IT Table 2. Percent differences of lung dose estimated with TLD and CTID volree methods. MDCT scanner TLD vs CTDIvol_AAPM %Difference GE GE Siemens Siemens Philips TLD vs CTDIvol_report %Difference Data reported on Table 2 show large underestimations of organ dose assessed with CTDIvol index with respect to TLD direct dose measure. Page 9 of 14

10 By considering only organ dose measured with TLD, Fig. 5 shows that dose exposure is higher for Siemens 64 than for GE 64 in those organs inside the selected thoracic volume; when organs outside that region are considered, GE 64 dose exposure is higher than Siemens 64 due to an higher overranging effect. To limit overranging effect some manufacturers (Philips and Siemens) introduced the dynamic collimation of the beam longitudinally at the beginning and at the end of the scan, others (GE) implemented ad hoc algorithm to interpolate and reconstruct data. Table 3. TLD organ dose (mgy) assessed for the 5 scanners; effective dose was calculated by using the ICRP 103 weighting factors. TLD dose (mgy) ORGAN GE 16 GE 64 Siemens 64 Siemens 128 Philips 256 Brain 0.14 ± ± ± ± ± 0.06 Oral mucosa 0.78 ± ± ± ± ± 0.03 Lens 0.19 ± ± ± ± ± 0.02 Utherus 0.08 ± ± ± ± ± 0.07 Thyroid 8.07 ± ± ± ± ± 0.29 Kidney 8.99 ± ± ± ± ± 0.22 Small intestine 8.68 ± ± ± ± ± 0.51 Lung 9.80 ± ± ± ± 0.66 Breast 8.42 ± ± ± ± ± 0.70 Skin 9.56 ± ± ± ± ± 1.28 Red marrow ± ± ± ± ± 0.87 Stomach ± ± ± ± ± 0.69 Liver ± ± ± ± ± 0.51 Spleen ± ± ± ± ± 0.29 E (msv) Table 4. DLP, CTDvol used to calculate the overranging length for each scanner MDCT DLP (mgy.cm) CTDI vol (mgy) Overranging (cm) GE Page 10 of 14

11 GE Siemens Siemens Philips Overranging increases with the number of detectors; higher the overranging length higher the direct beam irradiation of organs external to the selected volume of examination. Images for this section: Fig. 3: Organ doses of the Alderson-Rando phantom assessed with TLDs exposed with the 5 CT scanners. Page 11 of 14

12 Fig. 4: Average lung doses estimated with the three methods applied to the 5 scanner data. Page 12 of 14

13 Fig. 5: Comparison of TLD organ doses measured with GE 64 and Siemens 64. The red circle shows organs outside the selected thoracic volume. Page 13 of 14

14 Conclusion The present study, based on direct organ dose measurement by means of an antropomorfic phantom filled with TLDs, has shown that moving number of detctor raws from 16 to 256, does not correspond to an increase of exposure dose levels. For standard thorax examination, the scanner GE 16 slices demonstrated exposure dose levels comperable with those of Siemens 64. TLD direct organ dose measurements allowed also to verify that CTDIvol largely underestimated (16%-27%) thoracic average organ dose for a patient with standard size of body. Personal information References 1. R A. Douma, HMA Hofstee, C Schaefer-Prokop, JHTM. van Waesberghe, RLPW Kamphuisen, VEA Gerdes, MHH Kramer, H R Büller.Comparison of 4- and 64-slice CT scanning in the diagnosis of pulmonary embolism. New Technologies, Diagnostic Tools and Drugs Schattauer 2010 pag JA Bauhs, TJ Vrieze, AN. Primak,MR Bruesewitz, C. McCollough. CT Dosimetry: Comparison of Measurement Techniques and Devices. RadioGraphics 2008; 28: KP Antonios,E Papadakis, and J Damilakis.The effect of x-ray beam quality and geometry on radiation utilization efficiency in multidetector CT imaging. Medical Physics 2009, 36, JA Bauhs, TJ Vrieze, AN. Primak,MR Bruesewitz, C. McCollough. CT Dosimetry: Comparison of Measurement Techniques and Devices. RadioGraphics 2008; 28: A Tzedakis, J Damilakis, K Perisinakis, A Karantanas, S Karabekios, and N Gourtsoyiannis. Influence of z overscanning on normalized effective doses calculated for pediatric patients undergoing multidetector CT examinations Med. Phys. 2007; 34, Willi A Kalender. Dose in x-ray computed tomography. Phys. Med. Biol. 59 (2014) R129-R150 Page 14 of 14