- Email: [email protected]
CT: An estimation of SSDE and effective dose
Accepted Manuscript Title: Radiation Dose in Cardiac SPECT/CT: An Estimation of SSDE and Effective Dose Author: Hamid Abdollahi Isaac Shiri Yazdan Salimi Maghsoud Sarebani Reza Mehdinia Mohammad Reza Deevband Seied Rabi Mahdavi Ahmad Sohrabi Ahmad Bitarafan-Rajabi PII: DOI: Reference:
S0720-048X(16)30326-6 http://dx.doi.org/doi:10.1016/j.ejrad.2016.10.021 EURR 7605
To appear in:
European Journal of Radiology
Received date: Revised date: Accepted date:
23-9-2016 11-10-2016 19-10-2016
Please cite this article as: Abdollahi Hamid, Shiri Isaac, Salimi Yazdan, Sarebani Maghsoud, Mehdinia Reza, Deevband Mohammad Reza, Mahdavi Seied Rabi, Sohrabi Ahmad, Bitarafan-Rajabi Ahmad.Radiation Dose in Cardiac SPECT/CT: An Estimation of SSDE and Effective Dose.European Journal of Radiology http://dx.doi.org/10.1016/j.ejrad.2016.10.021 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Radiation Dose in Cardiac SPECT/CT: An Estimation of SSDE and Effective Dose Proposed running title: Radiation Dose in Cardiac SPECT/CT: Hamid Abdollahi*1, Isaac Shiri1, Yazdan Salimi2, Maghsoud Sarebani1, Reza Mehdinia1, Mohammad Reza Deevband2, Seied Rabi Mahdavi1, 3, Ahmad Sohrabi4, Ahmad Bitarafan-Rajabi#1, 5
1. Department of Medical Physics, School of Medicine, Iran University of Medical Sciences, Tehran, Iran 2. Biomedical Engineering and Medical Physics Department, Faculty of Medicine, Shahid Beheshti University of Medical Sciences, Tehran, Iran 3. Radiation Biology Research Center, Iran University of Medical Sciences, Tehran, Iran 4. Department of Biostatistics, School of Public Health, Iran University of Medical Sciences, Tehran, Iran 5. Department of Nuclear Medicine, Rajaei Cardiovascular, Medical and Research Center, Iran University of Medical Sciences, Tehran, Iran *Corresponding information: Hamid Abdollahi Department of Medical Physics, School of Medicine, Iran University of Medical Sciences, Tehran, Iran, Address: Junction of Shahid Hemmat and Shahid Chamran Expressways, Tehran, Iran, Email: [email protected], Mobile Phone: +989014870748, Department Tell: +98218862247, Postal Code: 1449614525 Fax: +982188622647 #Co-corresponding information: Ahmad Bitarafan-Rajabi Department of Medical Physics, School of Medicine, Iran University of Medical Sciences, Tehran, Iran Address: Vali-Asr Avenue. Niyayesh Blvd. Rajaei Cardiovascular, Medical and Research Center, Iran University of Medical Sciences, Tehran, Iran, Email: [email protected], [email protected], Mobile Phone: +989121973266, Department Tell: +98218862247, Postal Code: 1449614525 Fax: +982188622647
Abstract
Aims: The dose levels for Computed Tomography (CT) localization and attenuation correction of Single Photon Emission Computed Tomography (SPECT) are limited and reported as Volume Computed Tomography Dose Index (CTDIvol) and Dose-Length Product (DLP). This work presents CT dose estimation from Cardiac SPECT/CT based on new American Association of Physicists in Medicine (AAPM) Size Specific Dose Estimation (SSDE) parameter, effective dose, organ doses and also emission dose from nuclear issue.
Material and methods: Myocardial perfusion SPECT/CT for 509 patients was included in the study. SSDE, effective dose and organ dose were calculated using AAPM guideline and ImpactDose software. Data were analyzed using R and SPSS statistical software. Spearman-Pearson correlation test and linear regression models were used for finding correlations and relationships among parameters.
Results: The mean CTDIvol was 1.34 mGy ± 0.19 and the mean SSDE was 1.7 mGy ± 0.16. The mean±SD of effective dose from emission, CT and total dose were 11.5±1.4, 0.49±0.11 and 12.67± 1.73 (mSv) respectively. The mean±SD of effective dose from emission, CT and total dose were 11.5±1.4, 0.49±0.11 and 12.67± 1.73 (mSv) respectively. The spearman test showed that correlation between body size and organ doses is significant except thyroid and red bone
marrow. CTDIvol was strongly dependent on patient size, but SSDE was not. Emission dose was strongly dependent on patient weight, but its dependency was lower to effective diameter.
Conclusion: The dose parameters including CTDIvol, DLP, SSDE, effective dose values reported here are very low and below the reference level. This data suggest that appropriate CT acquisition parameters in SPECT/CT localization and attenuation correction are very beneficial for patients and lowering cancer risks.
Keywords: Radiation Dose, Cardiac imaging, SPECT/CT, SSDE, Effective Dose
Introduction Technical advancements in nuclear imaging have paved the road of anatomical and functional imaging with a personalized manner. Revolution in hybrid imaging systems including Single Photon Emission Computed Tomography/Computed Tomography (SPECT/CT), Positron Emission Tomography/CT (PET/CT) and PET/Magnetic Resonance Imaging (PET/MRI) and also achievements in software based image reconstruction have changed clinicians’ interests from conventional imaging to nuclear issues (1-3). In this era, SPECT/CT plays a vital role to detect different diseases particularly cardiac. By high availability and low cost, SPECT/CT is a feasible choice for functional and anatomical cardiac studies (4, 5). The role of diagnostic CT scanners in cardiac SPECT/CT is attenuation correction, coronary artery calcium scoring and coronary CT angiogram at a fast pace (6). Integration of the SPECT
and CT information and image fusion also, has many clinical applications. One of the most important application of SPECT/CT is coronary artery disease (CAD) evaluation by myocardial perfusion imaging (MPI) using
201
Tl and
99m
Tc labelled radiopharmaceuticals in both stress and
rest conditions (7). Due to expansions of cardiac SPEC/CT worldwide, patient exposure to ionizing radiation form both CT and SPECT should be assessed and minimized as based on ALARA (as low as reasonably achievable) principle. In this light, correct dose estimation is of importance. In hybrid SPECT/CT systems, there are two different dosimetry issues based on emission and transmission imaging. In nuclear medicine modalities, dose can be assessed by Medical Internal Radiation Dosimetry (MIRD), International Commission on Radiological Protection (ICRP) and Monte Carlo (MC) methods (8) and there are different tools to calculate dose. Dose in CT is based on metrics such as CT dose index volume [(CTDIvol), measured in mGy] and dose length product (DLP, measured in mGy.cm) which according to International Electrochemical Commission (IEC) should be displayed before and after examination in the form of dose page or image (9). But as a major limitation, CTDIvol cannot represent real patient dose, because it does not take into account the size of individual patients and also the real heterogeneous attenuation of patient. To solve this problem, American Association of Physicists in Medicine (AAPM) in collaboration with the International Commission on Radiation Units (ICRU) and Measurements and the Image Gently campaign of the Alliance for Radiation Safety in Pediatric Imaging addressed a new method so called “Size Specific Dose Estimate” (SSDE) to assess patient doses more accurately (10). SSDE takes into account patient size based on patient’s physical dimensions. In the AAPM report conversion factors based on four different measurements, anterior-posterior (AP), lateral (Lat), AP + Lat and effective diameter which can be measured from either localizer radiograph
or transverse CT images can be applied on CTDIvol to calculate SSDE for appropriate phantom sizes (16 and 32 cm). In another issue, effective dose is of importance quantity to compare different imaging modalities and also, optimize radiation dose and risk assessment. To the best of our knowledge, there is no study to assess patient dose in SPECT/CT which taken into account patient size, SSDE and effective dose. The main aim of this study was to estimate patient emission and transmission dose from cardiac SPECT/CT using MIRD protocol in terms of effective dose and AAPM report on SSDE. Methods and materials Patients and scan protocol: In this retrospective study, at first, institutional review committee approval and informed consent were obtained. Five hundred and nine (509) patients undergoing myocardial perfusion imaging (MPI) using 99mTc SPECT/CT from 2013 to 2016 were included in the study. All patients were scanned in spiral mode with a pitch factor 1.5; 5 mm slice thickness and 50 cm field of view and arm-up position. The scan coverage was from the superior of aortic art to diaphragm. The following CT imaging parameters were used: 130 KVp, 30 mA, a 0.85 second rotation time and 11 second scan time. Emission imaging data was tabulated in table 1, but in brief, imaging was performed by a two-day stress-rest protocol for all patients. To induce stress, 0.56 mg/kg dipyridamole was injected intravenously and after 4 minutes, 15 - 20 mCi 99mTchnetium (Tc) Sestamibi was administrated. The rest phase was performed on the subsequent day with the same dose. Only the stress phase was done by cardiac gating and CT attenuation correction. All emission imaging data were acquired by Gamma Camera dual-head (Symbia T2; Siemens Healthcare). Each study was performed using parallel-hole low energy-
high resolution (LEHR) collimators, 32 projections, an acquisition time of 30 seconds per projection, a 64 × 64 matrix size, a 1.33 zoom factor in an elliptical orbit of 180 degrees (45degrees, right anterior oblique to left posterior oblique-RAO-to-LPO) in the step and shoot mode. Filter back projection (FBP) algorithm was applied to reconstruct images. For each patient, CTDIvol and DLP were obtained from CT scanner system. The injected radiopharmaceutical activity was recorded for stress and rest phases. To measure patient dimensions, CT images were extracted from the nuclear medicine imaging systems in DICOM format
and
viewed
by
open
source
DICOM-viewing
MicroDicom
software
(www.microdicom.com). Then, an axial CT image which heart had the largest size was selected and for each patient, AP and LAT dimensions at the mid-heart level were measured using digital calipers on the software console (Figure 1). Emission and transmission dose assessment: Administrated activity of Tc-Sestamibi doses were recorded. To calculate effective dose from internal exposure, MIRD software was used. The CTDIvol and DLP based on a 32cm diameter calibration phantom reported by CT scanner console were recorded. To measure SSDE, patient’s AP & Lateral diameters obtained from axial CT images were recorded and effective diameter and fsize, calculated by the analytic expressions (equations 1 and 2). Finally SSDE was computed by equation 3. Conversion factors (a, b) are based on AAPM Report 204, which derived from Monte Carlo or experimental measurements and normalized to patient size in terms of water- or tissue-equivalent materials. Effective diameter =
(AP⋅LAT)
fsize = a.e-b . (Effective diameter).
(1) (2)
(a = 3.70 and b = 0.0367). SSDE = fsize · CTDIvol.
(3).
For each patient, CT effective dose was calculated using Impact-Dose software version 2.3 (GMBH Company) (http://www.ct-imaging.de) based on effective diameter, pitch, KV and reported CTDIvol. To calculate CT organ doses, ICRP 103 tissue weighting factors were implemented on Impact-Dose for organs including breast (for women), thyroid, lung, heart, kidney, adrenal glands, red bone marrow and liver. Total patient dose, also was assessed by summation of emission and transmission doses. Statistical analysis Data were analyzed using R (version 3.3.1, the R Foundation for Statistical Computing) and SPSS version 16.0 statistical software (SPSS Inc., Chicago, IL). To find correlation between effective diameter and organ doses, Spearman and Pearson correlation test was used. Linear regression models were used to estimate separately the relationship of CTDIvol with size (AP+LAT), SSDE with size (AP+LAT) and emission dose with weight and effective diameter. Standard residual diagnostics including leverage and Cook’s D were used for evaluation of regression model to find best fit. For all statistical analysis, P-values lower than 0.05 were considered statistically significant. Results Data pertaining to 509 cardiac disease patients were evaluated. The mean age was 55.7±11.8 years (range: 22–87). Of these patients, 294 (58%), were male and 215 (42%) were female. Figure 2A and 2B show the frequency distributions of SSDE and emission dose respectively.
AP+LAT was 59.7 cm ± 5.3 (range, 38–76 cm; AP, 24.17 cm ± 2.3; LAT, 35.54 cm ± 3.6) corresponded with a range of fsize of 0.94–1.87 (mean=1.26±0.12). The mean scanner output as CTDIvol was 1.34 mGy ± 0.19 and the mean SSDE was 1.7 mGy ± 0.16. The mean±SD of effective dose from emission, CT and total dose were 11.5±1.4, 0.49±0.11 and 12.67± 1.73 (mSv) respectively. The mean±SD of effective dose from emission, CT and total dose were 11.5±1.4, 0.49±0.11 and 12.67± 1.73 (mSv) respectively. Table 2 shows the mean and SD of effective doses for different organs assessed in this study. The spearman test showed that correlation between body size and organ doses is significant except thyroid and red bone marrow. The figure 3 shows the mean CT organ doses for men and women. Results show heart and lung have the highest organ doses. Also, mean organ doses were same for both genders. But, effective dose was significantly higher for women (p=0.001, t= -11.74) due to taking mean breast dose into account. Result depicted in figure 4A show CTDIvol was strongly dependent on patient size, but SSDE was not. Our regression model showed that patient size had a 0.025 mGy/cm slope and R2=0.505 (95% CI: P<0.0001). According to SSDE linear regression model fit parameters (95% CI, R2=0.001, P<0.0001 and slope 0.00007 mGy/cm) SSDE was independent of size (figure 4B). Also our results as was seen in figure 5A show emission dose was strongly dependent on patient weight, but this dependency to effective diameter was lower (figure 5B). The regression model with R2=0.905 (95% CI: P<0.0001) indicated that patient weight explained 90% of the observed variability in emission dose. In regard to emission dose and effective diameter, 44% variability was observed. Discussion
SPECT myocardial perfusion is a sensitive modality to assess many cardiac diseases such as ischemia and infarctions. The diagnostic value of such SPECT images can be affected by attenuation of special organs including diaphragm and breast. Attenuation correction using CT overcomes this problem to the large extent and offers high quality, noiseless and fast attenuation map. The primary goal of this study was to estimate the SSDE and as secondary goal, we aimed to calculate organ and effective dose from CT in cardiac SPEC/CT imaging. Also, relationship of CTDIvol and SSDE with size and emission dose with weight and effective diameter were obtained. As shown in this study, the dose parameters including CTDIvol, DLP, SSDE, effective dose values are very low and below the reference level. This data suggest that appropriate CT acquisition parameters in SPECT/CT localization and attenuation correction are very beneficial for patients and lowering cancer risks. It has been shown that CTDI and DLP underestimate or overestimate patient dose and thereby cancer risk (11). In the other hand, effective dose is adjustable quantity for estimation of stochastic risks and comparing different modalities and also as an indicative parameter for national or international diagnostic dose references. There is an accumulating body of research evidence which show CT dose parameters are directly correlated to DNA damages (12, 13). In a new interesting study, Fukumoto et al showed that there is a correlation between the physical exposure parameters (CTDI, DLP and SSDE) and γH2AX in cardiac CT. They concluded that SSDE induce more DNA damages and may be the best parameter for estimating the γ-H2AX level (14). In another work, Franck et al. showed that
SSDE can estimate patient-specific organ and blood doses and lifetime attributable risks (LAR) in pediatric torso CT examinations. Their results showed SSDE have a significant strong linear correlation with organ dose (r>0.8) and blood dose (r>0.9) and LAR (r>0.9) (15). To date, there are no reports regarding CT attenuation correction dose in SPECT/CT. In a study by Jallow et al, CT dose was surveyed among different PET/CT centers and their results showed a wide range of CTDIvol (from 0.5 mGy to 27.6 mGy) and effective dose (from 0.6 mSv to 43.0 mSv) (16). Our results showed that mean CTDIvol was 1.34, which is significantly lower than diagnostic reference levels reported by Tsapaaki et al. Our effective doses is comparable with Rainer et al (0.36-0.9) (17). In myocardial perfusion SPECT/CT imaging, there are no further needs to have high-quality anatomic information and thereby high tube currents. In our department, although the image quality is very lower than diagnostic levels but are acceptable for physicians. A considerable amount of literature has been published on the SSDE and effective dose in diagnostic CT and there is lack of study regarding them in nuclear medicine. These studies have shown SSDE is independent of size and is a useful indicator for estimating the exposure dose Organ dose measurement showed that heart and lung received the highest dose in both genders. In women also, breast had the highest does (mean=1.47 mSv). Although organ doses in the present study was low, but it should be reminded that in cardiac studies which uses high dose techniques, sensitive organs including breast should be preserved. There are commercially available SPECT/CT or PET/CT scanners having organ based CT options in some scan areas such as thorax. In comparison, our results showed that dose from CT attenuation correction is lower than emission dose.
As a debating issue, our results indicated that obtained SSDE had low values and this data suggest in SPECT/CT scanners which use low tube current CT, doses are too low that there is no considerable difference between CTDIvol and SSDE. It may be concluded that in such systems, CTDIvol may be reported as patient dose and there is no need for further time consuming SSDE calculations. In this light, one can suggest CTDIvol as a dose reporting parameter for such departments. In regard to regression models, and obtained R2=0.9, our results indicated that correlations between emission dose and patient weight has a great dependency. It means that, radioactivity prescription in this department was based on valid guidelines and each patient received activity dose based on his/her weight. Also, SSDE was independent of patient size and this verify previous studies. It is due to care dose system and automatic exposure control (AEC) which modulate radiation beam based on region of tissue scanned. Conclusion Our data presented here show new physical dose parameter regard to CT in cardiac SPECT/CT imaging, SSDE, maybe a useful parameter to determine real patient dose and also predict risks to exposure to ionization radiation. In another way, organ doses due to CT radiation were as low as SSDE. It may be due to low dose protocol CT imaging including low tube current. Although obtained doses were low, but there are needs to optimize CT radiation dose in nuclear medicine.
Conflict of interest:None
The Conflict of interest: The author declares that has no conflict of interest.
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Figure 1: AP and Lateral diameters measured from the transverse CT images
Figure 2A: Frequency distribution SSDE
Figure 2B: Frequency distribution Emission Dose
Figure 3: Mean CT organ dose in both genders
Figure 4A: Scatterplots of CTDIvol as function of patient size, indicated by sum of AP thickness and LAT width (AP+LAT)
Figure 4B: Scatterplots of SSDE as function of patient size, indicated by sum of AP thickness and LAT width (AP+LAT)
Figure 5A: Scatterplots of Emission dose as function of patient weight
Figure 5B: Scatterplots of Emission dose as function of patient effective diameter
Table 1: Nuclear scan parameters and their characteristics RAO: Right Anterior Oblique, LPO: Left Posterior Oblique, FBP: Filter Back Projection, LEHR: Low Energy High Resolution Imaging System
Radiopharmaceutical
Gamma Camera dual-head (Symbia T2; Siemens Healthcare) Chicago, IL, USA
15 - 20 mCi 99 mTcSestamibi
No. of Reconstruction projections algorithm 32 RAO to LPO
FBP
Filter
Butterworth filter (order 5, cutoff frequency 0.40)
Matrix Collimator size 64 × 64
LEHR
Scan time
Gated data acquisition
30 seconds per projection
16 frames per cardiac cycle and 30% acceptance window for the R-R interval
Table 2: Descriptive Statistics of effective and organ doses (mSv) N
Minimu m
Maximu m
Mean
SD
Effective Dose
509
0.33
0.70
0.49
0.10
Breast Dose
215
0.87
1.83
1.47
0.17
Thyroid Dose
509
0.03
0.52
0.06
0.05
Lung Dose
509
0.89
1.87
1.46
0.18
Heart Dose
509
1.00
2.27
1.71
0.25
Kidney Dose
509
0.12
0.28
0.20
0.02
Adrenal Dose
509
0.00
1.64
1.21
0.22
Red Bone Marrow Dose
509
0.06
0.99
0.16
0.21
Liver Dose
509
0.47
0.99
0.78
0.09
S0720-048X(16)30326-6 http://dx.doi.org/doi:10.1016/j.ejrad.2016.10.021 EURR 7605
To appear in:
European Journal of Radiology
Received date: Revised date: Accepted date:
23-9-2016 11-10-2016 19-10-2016
Please cite this article as: Abdollahi Hamid, Shiri Isaac, Salimi Yazdan, Sarebani Maghsoud, Mehdinia Reza, Deevband Mohammad Reza, Mahdavi Seied Rabi, Sohrabi Ahmad, Bitarafan-Rajabi Ahmad.Radiation Dose in Cardiac SPECT/CT: An Estimation of SSDE and Effective Dose.European Journal of Radiology http://dx.doi.org/10.1016/j.ejrad.2016.10.021 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Radiation Dose in Cardiac SPECT/CT: An Estimation of SSDE and Effective Dose Proposed running title: Radiation Dose in Cardiac SPECT/CT: Hamid Abdollahi*1, Isaac Shiri1, Yazdan Salimi2, Maghsoud Sarebani1, Reza Mehdinia1, Mohammad Reza Deevband2, Seied Rabi Mahdavi1, 3, Ahmad Sohrabi4, Ahmad Bitarafan-Rajabi#1, 5
1. Department of Medical Physics, School of Medicine, Iran University of Medical Sciences, Tehran, Iran 2. Biomedical Engineering and Medical Physics Department, Faculty of Medicine, Shahid Beheshti University of Medical Sciences, Tehran, Iran 3. Radiation Biology Research Center, Iran University of Medical Sciences, Tehran, Iran 4. Department of Biostatistics, School of Public Health, Iran University of Medical Sciences, Tehran, Iran 5. Department of Nuclear Medicine, Rajaei Cardiovascular, Medical and Research Center, Iran University of Medical Sciences, Tehran, Iran *Corresponding information: Hamid Abdollahi Department of Medical Physics, School of Medicine, Iran University of Medical Sciences, Tehran, Iran, Address: Junction of Shahid Hemmat and Shahid Chamran Expressways, Tehran, Iran, Email: [email protected], Mobile Phone: +989014870748, Department Tell: +98218862247, Postal Code: 1449614525 Fax: +982188622647 #Co-corresponding information: Ahmad Bitarafan-Rajabi Department of Medical Physics, School of Medicine, Iran University of Medical Sciences, Tehran, Iran Address: Vali-Asr Avenue. Niyayesh Blvd. Rajaei Cardiovascular, Medical and Research Center, Iran University of Medical Sciences, Tehran, Iran, Email: [email protected], [email protected], Mobile Phone: +989121973266, Department Tell: +98218862247, Postal Code: 1449614525 Fax: +982188622647
Abstract
Aims: The dose levels for Computed Tomography (CT) localization and attenuation correction of Single Photon Emission Computed Tomography (SPECT) are limited and reported as Volume Computed Tomography Dose Index (CTDIvol) and Dose-Length Product (DLP). This work presents CT dose estimation from Cardiac SPECT/CT based on new American Association of Physicists in Medicine (AAPM) Size Specific Dose Estimation (SSDE) parameter, effective dose, organ doses and also emission dose from nuclear issue.
Material and methods: Myocardial perfusion SPECT/CT for 509 patients was included in the study. SSDE, effective dose and organ dose were calculated using AAPM guideline and ImpactDose software. Data were analyzed using R and SPSS statistical software. Spearman-Pearson correlation test and linear regression models were used for finding correlations and relationships among parameters.
Results: The mean CTDIvol was 1.34 mGy ± 0.19 and the mean SSDE was 1.7 mGy ± 0.16. The mean±SD of effective dose from emission, CT and total dose were 11.5±1.4, 0.49±0.11 and 12.67± 1.73 (mSv) respectively. The mean±SD of effective dose from emission, CT and total dose were 11.5±1.4, 0.49±0.11 and 12.67± 1.73 (mSv) respectively. The spearman test showed that correlation between body size and organ doses is significant except thyroid and red bone
marrow. CTDIvol was strongly dependent on patient size, but SSDE was not. Emission dose was strongly dependent on patient weight, but its dependency was lower to effective diameter.
Conclusion: The dose parameters including CTDIvol, DLP, SSDE, effective dose values reported here are very low and below the reference level. This data suggest that appropriate CT acquisition parameters in SPECT/CT localization and attenuation correction are very beneficial for patients and lowering cancer risks.
Keywords: Radiation Dose, Cardiac imaging, SPECT/CT, SSDE, Effective Dose
Introduction Technical advancements in nuclear imaging have paved the road of anatomical and functional imaging with a personalized manner. Revolution in hybrid imaging systems including Single Photon Emission Computed Tomography/Computed Tomography (SPECT/CT), Positron Emission Tomography/CT (PET/CT) and PET/Magnetic Resonance Imaging (PET/MRI) and also achievements in software based image reconstruction have changed clinicians’ interests from conventional imaging to nuclear issues (1-3). In this era, SPECT/CT plays a vital role to detect different diseases particularly cardiac. By high availability and low cost, SPECT/CT is a feasible choice for functional and anatomical cardiac studies (4, 5). The role of diagnostic CT scanners in cardiac SPECT/CT is attenuation correction, coronary artery calcium scoring and coronary CT angiogram at a fast pace (6). Integration of the SPECT
and CT information and image fusion also, has many clinical applications. One of the most important application of SPECT/CT is coronary artery disease (CAD) evaluation by myocardial perfusion imaging (MPI) using
201
Tl and
99m
Tc labelled radiopharmaceuticals in both stress and
rest conditions (7). Due to expansions of cardiac SPEC/CT worldwide, patient exposure to ionizing radiation form both CT and SPECT should be assessed and minimized as based on ALARA (as low as reasonably achievable) principle. In this light, correct dose estimation is of importance. In hybrid SPECT/CT systems, there are two different dosimetry issues based on emission and transmission imaging. In nuclear medicine modalities, dose can be assessed by Medical Internal Radiation Dosimetry (MIRD), International Commission on Radiological Protection (ICRP) and Monte Carlo (MC) methods (8) and there are different tools to calculate dose. Dose in CT is based on metrics such as CT dose index volume [(CTDIvol), measured in mGy] and dose length product (DLP, measured in mGy.cm) which according to International Electrochemical Commission (IEC) should be displayed before and after examination in the form of dose page or image (9). But as a major limitation, CTDIvol cannot represent real patient dose, because it does not take into account the size of individual patients and also the real heterogeneous attenuation of patient. To solve this problem, American Association of Physicists in Medicine (AAPM) in collaboration with the International Commission on Radiation Units (ICRU) and Measurements and the Image Gently campaign of the Alliance for Radiation Safety in Pediatric Imaging addressed a new method so called “Size Specific Dose Estimate” (SSDE) to assess patient doses more accurately (10). SSDE takes into account patient size based on patient’s physical dimensions. In the AAPM report conversion factors based on four different measurements, anterior-posterior (AP), lateral (Lat), AP + Lat and effective diameter which can be measured from either localizer radiograph
or transverse CT images can be applied on CTDIvol to calculate SSDE for appropriate phantom sizes (16 and 32 cm). In another issue, effective dose is of importance quantity to compare different imaging modalities and also, optimize radiation dose and risk assessment. To the best of our knowledge, there is no study to assess patient dose in SPECT/CT which taken into account patient size, SSDE and effective dose. The main aim of this study was to estimate patient emission and transmission dose from cardiac SPECT/CT using MIRD protocol in terms of effective dose and AAPM report on SSDE. Methods and materials Patients and scan protocol: In this retrospective study, at first, institutional review committee approval and informed consent were obtained. Five hundred and nine (509) patients undergoing myocardial perfusion imaging (MPI) using 99mTc SPECT/CT from 2013 to 2016 were included in the study. All patients were scanned in spiral mode with a pitch factor 1.5; 5 mm slice thickness and 50 cm field of view and arm-up position. The scan coverage was from the superior of aortic art to diaphragm. The following CT imaging parameters were used: 130 KVp, 30 mA, a 0.85 second rotation time and 11 second scan time. Emission imaging data was tabulated in table 1, but in brief, imaging was performed by a two-day stress-rest protocol for all patients. To induce stress, 0.56 mg/kg dipyridamole was injected intravenously and after 4 minutes, 15 - 20 mCi 99mTchnetium (Tc) Sestamibi was administrated. The rest phase was performed on the subsequent day with the same dose. Only the stress phase was done by cardiac gating and CT attenuation correction. All emission imaging data were acquired by Gamma Camera dual-head (Symbia T2; Siemens Healthcare). Each study was performed using parallel-hole low energy-
high resolution (LEHR) collimators, 32 projections, an acquisition time of 30 seconds per projection, a 64 × 64 matrix size, a 1.33 zoom factor in an elliptical orbit of 180 degrees (45degrees, right anterior oblique to left posterior oblique-RAO-to-LPO) in the step and shoot mode. Filter back projection (FBP) algorithm was applied to reconstruct images. For each patient, CTDIvol and DLP were obtained from CT scanner system. The injected radiopharmaceutical activity was recorded for stress and rest phases. To measure patient dimensions, CT images were extracted from the nuclear medicine imaging systems in DICOM format
and
viewed
by
open
source
DICOM-viewing
MicroDicom
software
(www.microdicom.com). Then, an axial CT image which heart had the largest size was selected and for each patient, AP and LAT dimensions at the mid-heart level were measured using digital calipers on the software console (Figure 1). Emission and transmission dose assessment: Administrated activity of Tc-Sestamibi doses were recorded. To calculate effective dose from internal exposure, MIRD software was used. The CTDIvol and DLP based on a 32cm diameter calibration phantom reported by CT scanner console were recorded. To measure SSDE, patient’s AP & Lateral diameters obtained from axial CT images were recorded and effective diameter and fsize, calculated by the analytic expressions (equations 1 and 2). Finally SSDE was computed by equation 3. Conversion factors (a, b) are based on AAPM Report 204, which derived from Monte Carlo or experimental measurements and normalized to patient size in terms of water- or tissue-equivalent materials. Effective diameter =
(AP⋅LAT)
fsize = a.e-b . (Effective diameter).
(1) (2)
(a = 3.70 and b = 0.0367). SSDE = fsize · CTDIvol.
(3).
For each patient, CT effective dose was calculated using Impact-Dose software version 2.3 (GMBH Company) (http://www.ct-imaging.de) based on effective diameter, pitch, KV and reported CTDIvol. To calculate CT organ doses, ICRP 103 tissue weighting factors were implemented on Impact-Dose for organs including breast (for women), thyroid, lung, heart, kidney, adrenal glands, red bone marrow and liver. Total patient dose, also was assessed by summation of emission and transmission doses. Statistical analysis Data were analyzed using R (version 3.3.1, the R Foundation for Statistical Computing) and SPSS version 16.0 statistical software (SPSS Inc., Chicago, IL). To find correlation between effective diameter and organ doses, Spearman and Pearson correlation test was used. Linear regression models were used to estimate separately the relationship of CTDIvol with size (AP+LAT), SSDE with size (AP+LAT) and emission dose with weight and effective diameter. Standard residual diagnostics including leverage and Cook’s D were used for evaluation of regression model to find best fit. For all statistical analysis, P-values lower than 0.05 were considered statistically significant. Results Data pertaining to 509 cardiac disease patients were evaluated. The mean age was 55.7±11.8 years (range: 22–87). Of these patients, 294 (58%), were male and 215 (42%) were female. Figure 2A and 2B show the frequency distributions of SSDE and emission dose respectively.
AP+LAT was 59.7 cm ± 5.3 (range, 38–76 cm; AP, 24.17 cm ± 2.3; LAT, 35.54 cm ± 3.6) corresponded with a range of fsize of 0.94–1.87 (mean=1.26±0.12). The mean scanner output as CTDIvol was 1.34 mGy ± 0.19 and the mean SSDE was 1.7 mGy ± 0.16. The mean±SD of effective dose from emission, CT and total dose were 11.5±1.4, 0.49±0.11 and 12.67± 1.73 (mSv) respectively. The mean±SD of effective dose from emission, CT and total dose were 11.5±1.4, 0.49±0.11 and 12.67± 1.73 (mSv) respectively. Table 2 shows the mean and SD of effective doses for different organs assessed in this study. The spearman test showed that correlation between body size and organ doses is significant except thyroid and red bone marrow. The figure 3 shows the mean CT organ doses for men and women. Results show heart and lung have the highest organ doses. Also, mean organ doses were same for both genders. But, effective dose was significantly higher for women (p=0.001, t= -11.74) due to taking mean breast dose into account. Result depicted in figure 4A show CTDIvol was strongly dependent on patient size, but SSDE was not. Our regression model showed that patient size had a 0.025 mGy/cm slope and R2=0.505 (95% CI: P<0.0001). According to SSDE linear regression model fit parameters (95% CI, R2=0.001, P<0.0001 and slope 0.00007 mGy/cm) SSDE was independent of size (figure 4B). Also our results as was seen in figure 5A show emission dose was strongly dependent on patient weight, but this dependency to effective diameter was lower (figure 5B). The regression model with R2=0.905 (95% CI: P<0.0001) indicated that patient weight explained 90% of the observed variability in emission dose. In regard to emission dose and effective diameter, 44% variability was observed. Discussion
SPECT myocardial perfusion is a sensitive modality to assess many cardiac diseases such as ischemia and infarctions. The diagnostic value of such SPECT images can be affected by attenuation of special organs including diaphragm and breast. Attenuation correction using CT overcomes this problem to the large extent and offers high quality, noiseless and fast attenuation map. The primary goal of this study was to estimate the SSDE and as secondary goal, we aimed to calculate organ and effective dose from CT in cardiac SPEC/CT imaging. Also, relationship of CTDIvol and SSDE with size and emission dose with weight and effective diameter were obtained. As shown in this study, the dose parameters including CTDIvol, DLP, SSDE, effective dose values are very low and below the reference level. This data suggest that appropriate CT acquisition parameters in SPECT/CT localization and attenuation correction are very beneficial for patients and lowering cancer risks. It has been shown that CTDI and DLP underestimate or overestimate patient dose and thereby cancer risk (11). In the other hand, effective dose is adjustable quantity for estimation of stochastic risks and comparing different modalities and also as an indicative parameter for national or international diagnostic dose references. There is an accumulating body of research evidence which show CT dose parameters are directly correlated to DNA damages (12, 13). In a new interesting study, Fukumoto et al showed that there is a correlation between the physical exposure parameters (CTDI, DLP and SSDE) and γH2AX in cardiac CT. They concluded that SSDE induce more DNA damages and may be the best parameter for estimating the γ-H2AX level (14). In another work, Franck et al. showed that
SSDE can estimate patient-specific organ and blood doses and lifetime attributable risks (LAR) in pediatric torso CT examinations. Their results showed SSDE have a significant strong linear correlation with organ dose (r>0.8) and blood dose (r>0.9) and LAR (r>0.9) (15). To date, there are no reports regarding CT attenuation correction dose in SPECT/CT. In a study by Jallow et al, CT dose was surveyed among different PET/CT centers and their results showed a wide range of CTDIvol (from 0.5 mGy to 27.6 mGy) and effective dose (from 0.6 mSv to 43.0 mSv) (16). Our results showed that mean CTDIvol was 1.34, which is significantly lower than diagnostic reference levels reported by Tsapaaki et al. Our effective doses is comparable with Rainer et al (0.36-0.9) (17). In myocardial perfusion SPECT/CT imaging, there are no further needs to have high-quality anatomic information and thereby high tube currents. In our department, although the image quality is very lower than diagnostic levels but are acceptable for physicians. A considerable amount of literature has been published on the SSDE and effective dose in diagnostic CT and there is lack of study regarding them in nuclear medicine. These studies have shown SSDE is independent of size and is a useful indicator for estimating the exposure dose Organ dose measurement showed that heart and lung received the highest dose in both genders. In women also, breast had the highest does (mean=1.47 mSv). Although organ doses in the present study was low, but it should be reminded that in cardiac studies which uses high dose techniques, sensitive organs including breast should be preserved. There are commercially available SPECT/CT or PET/CT scanners having organ based CT options in some scan areas such as thorax. In comparison, our results showed that dose from CT attenuation correction is lower than emission dose.
As a debating issue, our results indicated that obtained SSDE had low values and this data suggest in SPECT/CT scanners which use low tube current CT, doses are too low that there is no considerable difference between CTDIvol and SSDE. It may be concluded that in such systems, CTDIvol may be reported as patient dose and there is no need for further time consuming SSDE calculations. In this light, one can suggest CTDIvol as a dose reporting parameter for such departments. In regard to regression models, and obtained R2=0.9, our results indicated that correlations between emission dose and patient weight has a great dependency. It means that, radioactivity prescription in this department was based on valid guidelines and each patient received activity dose based on his/her weight. Also, SSDE was independent of patient size and this verify previous studies. It is due to care dose system and automatic exposure control (AEC) which modulate radiation beam based on region of tissue scanned. Conclusion Our data presented here show new physical dose parameter regard to CT in cardiac SPECT/CT imaging, SSDE, maybe a useful parameter to determine real patient dose and also predict risks to exposure to ionization radiation. In another way, organ doses due to CT radiation were as low as SSDE. It may be due to low dose protocol CT imaging including low tube current. Although obtained doses were low, but there are needs to optimize CT radiation dose in nuclear medicine.
Conflict of interest:None
The Conflict of interest: The author declares that has no conflict of interest.
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Figure 1: AP and Lateral diameters measured from the transverse CT images
Figure 2A: Frequency distribution SSDE
Figure 2B: Frequency distribution Emission Dose
Figure 3: Mean CT organ dose in both genders
Figure 4A: Scatterplots of CTDIvol as function of patient size, indicated by sum of AP thickness and LAT width (AP+LAT)
Figure 4B: Scatterplots of SSDE as function of patient size, indicated by sum of AP thickness and LAT width (AP+LAT)
Figure 5A: Scatterplots of Emission dose as function of patient weight
Figure 5B: Scatterplots of Emission dose as function of patient effective diameter
Table 1: Nuclear scan parameters and their characteristics RAO: Right Anterior Oblique, LPO: Left Posterior Oblique, FBP: Filter Back Projection, LEHR: Low Energy High Resolution Imaging System
Radiopharmaceutical
Gamma Camera dual-head (Symbia T2; Siemens Healthcare) Chicago, IL, USA
15 - 20 mCi 99 mTcSestamibi
No. of Reconstruction projections algorithm 32 RAO to LPO
FBP
Filter
Butterworth filter (order 5, cutoff frequency 0.40)
Matrix Collimator size 64 × 64
LEHR
Scan time
Gated data acquisition
30 seconds per projection
16 frames per cardiac cycle and 30% acceptance window for the R-R interval
Table 2: Descriptive Statistics of effective and organ doses (mSv) N
Minimu m
Maximu m
Mean
SD
Effective Dose
509
0.33
0.70
0.49
0.10
Breast Dose
215
0.87
1.83
1.47
0.17
Thyroid Dose
509
0.03
0.52
0.06
0.05
Lung Dose
509
0.89
1.87
1.46
0.18
Heart Dose
509
1.00
2.27
1.71
0.25
Kidney Dose
509
0.12
0.28
0.20
0.02
Adrenal Dose
509
0.00
1.64
1.21
0.22
Red Bone Marrow Dose
509
0.06
0.99
0.16
0.21
Liver Dose
509
0.47
0.99
0.78
0.09