Technical note
Validation of CT doses of SPECT/CT and PET/CT hybrid devices: lessons learned Terez Seraa, Tamas Porubszkyc, Miklos Paposa, Richard Elekc, Zsuzsanna Besenyia, Katalin Gionb, Andras Barthac, Sandor Pelletc and Laszlo Pavicsa The aim of the study was to check the validity of computed tomographic (CT) doses exhibited by SPECT/CT and PET/ CT hybrid devices. Dose measurements were taken from four SPECT/CT and four PET/CT cameras commercially available from different manufacturers. A calibrated ionization chamber was placed in whole-body or head phantoms for the acquisition of CT images with clinically used parameters. Computed tomography dose index (CTDIvol) values were calculated according to the IEC 60601-2-44:1999 formula. The measured CTDIvol doses were compared with those preprogrammed by the manufacturer. In the case of the whole-body phantom, the differences between the measured and displayed values varied between – 31 and + 24% [European document RP162 (2012) sets up the limit for acceptance criterion as ±20%]. The head phantom data showed either an agreement between – 10 and + 24%, or an underestimation by two-fold. The latter seemed to be because, while preprogramming the doses, the manufacturer had used the whole-body phantom instead
of a proper head phantom. The results of the work demonstrate the need for individual dosimetric calibration of every single X-ray tube. Dosimetric checks should be included in the regular quality control programmes of the SPECT/CT and PET/CT devices. Special attention should be paid to head-and-neck and paediatric protocols, in which the use of a head phantom is recommended for c 2014 dose calibration. Nucl Med Commun 35:534–538 Wolters Kluwer Health | Lippincott Williams & Wilkins.
Introduction
Protection [1] (NCRP Report No. 160) revealed that the mean annual radiation exposure originating from medical applications per head of the public in the USA increased by a factor of about 6 between 1980 and 2006 – that is, from 0.53 to 3.0 mSv. As the number of examinations involving radiation exposure is continuously increasing, there is a real need to decrease the dose per examination. One possible way to achieve this is by manufacturing devices that subject patients to lower radiation burden by using detectors with higher efficiency and advanced software; another way is through optimum adjustment of devices.
Hybrid devices have become the state-of-the-art of nuclear medicine equipment. The developments were largely determined by the application of very specific radiopharmaceuticals and the need for better images. The new radiopharmaceuticals are used more for the characterization of biochemical processes than for the visualization of organs. In this process, the value of the anatomical information deteriorates and additional imaging is required. For the improvement of image quality in nuclear medicine, efforts are being made to develop more sensitive detectors and highperformance software and there is also a possibility of using computed tomographic (CT) imaging data for the attenuation correction of g-rays. The performance characteristics of SPECT and PET equipment supplemented with a CT modality, in the SPECT/CTand PET/CT hybrid devices, are undoubtedly better than those of SPECT or PET alone. The CT component can be low-dose CT, which is used only for attenuation and scatter correction, or contrast-enhanced CT with the possibility of X-ray diagnostic examinations as well. The radiation dose originating from medical applications to which the population is exposed, mainly from the widespread use of CTas well as SPECT/CTand PET/CT, is continuously increasing. The report of the National Council on Radiation c 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins 0143-3636
Nuclear Medicine Communications 2014, 35:534–538 Keywords: CT doses, CTDI, SPECT/CT and PET/CT hybrid devices, whole-body and head phantoms a Department of Nuclear Medicine, University of Szeged, bEuromedic Diagnostics, Szeged and c‘Fre´de´ric Joliot-Curie’ National Research Institute for Radiobiology and Radiohygiene, Budapest, Hungary
Correspondence to Terez Sera, PhD, Department of Nuclear Medicine, University of Szeged, H-6720 Szeged, Koranyi fasor 8, Hungary Tel: + 36 30 4750289; fax: + 36 62 544564; e-mail: [email protected] Received 24 July 2013 Revised 11 November 2013 Accepted 5 January 2014
Both international reports and the literature data indicate high deviations in patient doses among countries and institutions. Patient doses vary between different types of equipment and between departments [2]. In view of this, our team performed phantom measurements on the SPECT/CT and PET/CT equipment operating in Hungary in 2011 to check the doses resulting from the CT component and compare them with the values provided by the vendors of the CT equipment.
Materials and methods Radiation dose measurements were taken from four SPECT/ CT and four PET/CT cameras. Among the manufacturers DOI: 10.1097/MNM.0000000000000087
Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
Accuracy of CTDI on SPECT/CT and PET/CT devices Sera et al. 535
Table 1
List of tested SPECT/CT and PET/CT equipment
Equipment
Manufacturer
Year of installation
Type (number of slices)
PET/CT PET/CT PET/CT PET/CT SPECT/CT SPECT/CT SPECT/CT SPECT/CT
Philips, Amsterdam, the Netherlands GE, Milwaukee, Wisconsin, USA Siemens, Erlangen, Germany Siemens Mediso, Budapest, Hungary Mediso Mediso Siemens
2007 2005 2008 2006 2007 2009 2011 2010
Gemini TF-64 PET/CT (64) Discovery PET/CT (8) Biograph Truepoint PET/CT (6) Biograph Truepoint PET/CT (6) Anyscan Nucline DHV/CT (16) Anyscan SC DHV/CT (16) Anyscan SC DHV/CT (16) Symbia TruePoint SPECT CT (2)
CT, computed tomography.
of the investigated cameras were numerous companies who had a presence on the international market (Table 1).
Fig. 1
In our investigations, quantities such as the computed tomography dose index (CTDIw and CTDIvol) and the derived dose-length product (DLP) were used. CTDIw represents the CTDI average over the cross-section of the standardized PMMA CTDI phantom and CTDIvol is the pitch-corrected CTDIw; their unit is mGy. The DLP is the product of CTDIvol and scan lengths (cm); its unit is mGy cm. For the estimation of patient-effective doses the CTDIvol and the DLP have to be used with the software provided by the manufacturer, containing the body region-specific parameters for the given scanner model. For the CTDI measurements, standardized PMMA CTDI whole-body (for modelling the whole-body patient investigations) and head phantoms (for modelling the head-and-neck and paediatric examinations) [3] developed by Wellho¨fer (Schwarzenbruck, Germany) were used. The head phantom is standardized as a 16-cm-diameter cylinder and the wholebody phantom as a 32-cm-diameter cylinder, both 15 cm in length. The cylinders have five 12.4-mm-diameter holes, one of them at the centre and the other four, 1 cm deep, positioned symmetrically at 901. CTDI measurements were taken by consecutive insertion of an ionization chamber with an active length of 10 cm into each hole (Fig. 1). While taking the measurements the remaining holes were filled with homogeneous PMMA inserts. The DCT-10 ionization chamber was produced by Wellho¨fer, calibrated by an accredited laboratory and had an accuracy of ±2.4%. The reproducibility of the ionization chamber was checked by taking repeated measurements with the whole-body phantom and detector placed in the middle of it; 10 dose measurements were recorded with the same acquisition parameters. The weighted CTDI values (CTDIweighted) were calculated from the measured data, according to the specified (IEC 60601-2-44:1999) formula: 1 2 CTDIw ¼ CTDI100;c þ CTDI100;p ; 3 3 where CTDI100,c is the dose measured at the central hole of the phantom and CTDI100;p is the mean of the doses measured at the peripheral holes.
Measuring arrangement of the whole-body computed tomographic dosimetry phantom with the pencil ionization chamber positioned in its central hole.
The acquisition parameters selected for the CTDI phantom measurements were mostly the same as applied in clinical practice, but in some cases additional settings were selected too. The differences in clinical parameters between the different sets of equipment are partly due to the preferences of the sites and partly due to the differences in the ranges of the selectable parameters. The measured dose values were compared with the displayed values of the equipment provided by the vendors (Tables 3 and 4). Special attention was paid to paediatric examinations, which are performed following either paediatric or headand-neck protocols. In these cases measurements were recorded using both phantoms with identical parameter selection. For details of the CT dosimetry, we refer to the basic review publications [4–9].
Results and discussion The reproducibility of the ionization chamber measurement was excellent (Table 2). The mean value and the SD were 8.9034 and 0.0142, respectively, which gives a
Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
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Nuclear Medicine Communications 2014, Vol 35 No 5
relative SD of less than 0.15%. These results led us to conclude that our device is suitable for the accurate measurement of CTDI doses. In the following part of our work, the CTDI doses were determined by using the ionization chamber placed into the whole-body phantom. To avoid the possibility of identifying the devices and sites, the sequencing of the equipment in the various tables differs. The CTDIvol values were measured by using different settings of the acquisition parameters. Every SPECT/CT and PET/CT measurement was checked with the wholebody phantom and the displayed values were recorded (Table 3). Two sets of equipment, SPECT/CT 1 and SPECT/CT 2, which were of the same type and with identical Table 2 Reproducibility of measurements with the ionization chamber Measured CTDI (mGy)
Measured irradiation time (ms)
8.901 8.888 8.881 8.909 8.885 8.921 8.912 8.911 8.907 8.919
1040 1032 1032 1032 1032 1032 1032 1032 1032 1032
CTDI, computed tomography dose index.
Table 3
acquisition parameters, displayed exactly the same doses, whereas the measured values were different (first six rows of Table 3). However, the ratio of the doses measured on the two cameras (using the same sets of acquisition parameters) was constant at 1.34±0.02. The radiation output of the various X-ray tubes varies because of factors such as the type and usage level, the radiation filtration applied, the features of the X-ray generator, the geometry of irradiation, and selected loading factors, and therefore the exactly identical values displayed are difficult to believe. We presume that the doses of the various equipment sets were set up by using factorymeasured data on only one of the tubes, and not individually for each tube. In the case of the other cameras, the ratio of the displayed and measured CTDIvol doses varied between 0.69 and 1.24 (two of 19 values were 1.2). The requirements concerning the accuracy of doses displayed in CT are formulated by the IAEA in a document dealing with the quality assurance programme for CT equipment [10]. According to this, the acceptance criterion is ±20%. This requirement is also adopted by European Commission document RP162 [11]. Furthermore, the IAEA considers even ±10% to be an achievable level. Dose measurements were taken by members of our working team on several sets of contrast-enhanced CT equipment as well, and in some cases ±5% accuracy was indeed achieved. These results, however, are relatively rare; more frequently, differences greater than 20% were found. A factor possibly influencing the differences may be the accuracy (calibrated state) of the instrumentation used by the vendors for dose measurements.
CTDI measurements with the whole-body phantom
Equipment code SPECT/CT SPECT/CT SPECT/CT SPECT/CT SPECT/CT SPECT/CT PET/CT 1 PET/CT 4 PET/CT 2 SPECT/CT SPECT/CT PET/CT 1 PET/CT 2 SPECT/CT PET/CT 1 PET/CT 1 PET/CT 1 PET/CT 4 PET/CT 4 PET/CT 4 PET/CT 3 SPECT/CT
1 2 1 2 1 2
3 3
4
4
X-ray tube voltage (kV)
Tube current time product (mAs)
120 120 120 120 120 120 120 130 120 120 120 110 120 130 130 130 130 130 130 130 120 130
80 80 50 50 40 40 71 70 100 50 100 50 80 17 70 50 80 91 86 50 80 19
Pitch
Beam collimation (mm)
Axial Axial 1.5 1.5 Axial Axial 0.8 1.5 0.82 1.5 1.5 1.5 Axial 1.5 1.5 1.5 1.5 1.5 1.5 0.8 Axial 1.5
20 20 10 10 10 10 3 5 40 2.5 2.5 5 25 5 5 1.25 3 5 5 3 5 3
Rotation time (s)
CTDIvol displayed (mGy)
CTDIvol measured (mGy)
CTDIvol displayed/ CTDIvol measured
1 1 1 1 1 1 0.8 0.6 0.5 1 1 0.6 0.8 – 0.6 0.8 0.6 0.6 0.6 0.8 0.5 –
9.59 9.59 7.20 7.20 5.76 5.76 8.09 7.89 5.90 3.50 7.00 3.50 10.90 1.84 7.42 6.62 9.12 9.67 9.80 5.70 9.65 2.07
9.38 12.71 5.14 6.81 5.88 7.98 11.70 10.41 6.62 3.90 7.61 3.67 10.86 1.83 7.37 6.24 8.52 8.88 8.61 4.93 8.09 1.67
1.02 0.75 1.40 1.06 0.98 0.72 0.69 0.76 0.89 0.90 0.92 0.95 1.00 1.01 1.01 1.06 1.07 1.09 1.14 1.16 1.19 1.24
CT, computed tomography; CTDI, computed tomography dose index.
Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
Accuracy of CTDI on SPECT/CT and PET/CT devices Sera et al. 537
We subsequently extended the dose measurements with particular attention to paediatric examinations, for which the use of a head phantom is recommended (Table 4).
It seems that the manufacturers, and especially their maintenance services, do not pay sufficient attention to dosimetric calibration and check only whether it is enforced by users or by professional organizations. As a consequence of the latter, dose display is nowadays strictly demanded at both international [3,12] and European [13] levels. After each X-ray tube replacement in SPECT/CT and PET/CT equipment, a dosimetric calibration is required. Moreover, we consider it justified that dosimetric checks should be performed by users when conditions are given. Otherwise, the dose measurements should be ordered from independent organizations.
The clinical acquisition parameters were those set up for head-and-neck and paediatric examinations. The SPECT/CT 1 and SPECT/CT 2 cameras exhibited the same pattern as in the case of the body phantom (first six rows of Table 4). The other cameras either underestimated the doses by a factor of B2 or the exhibited values were very close to those measured. The two-fold underestimation gave rise to the suspicion that, instead of the head phantom (which was required by the clinical protocol), the whole-body phantom was used by the vendor when the doses were preprogrammed.
The results of the work clearly demonstrate the need for the individual dosimetric calibration of every single X-ray tube. Dosimetric checks should be included in regular quality control programmes.
To check on this supposition, measurements were repeated by using the body phantom on selected cameras (Table 5). The results demonstrated that the ratios between the displayed and measured CTDIvol values improved from 0.45 to 0.95, from 0.49 to 1.15, from 0.61 to 1.24, and from 0.47 to 0.90, respectively.
Table 4
The CT component of hybrid SPECT/CT and PET/CT equipment makes a significant contribution to the total doses of examinations and should therefore be taken into consideration [14].
CTDI measurements with the head phantom
Equipment code SPECT/CT SPECT/CT SPECT/CT SPECT/CT SPECT/CT SPECT/CT PET/CT 1 SPECT/CT PET/CT 4 PET/CT 1 PET/CT 2 PET/CT 3 PET/CT 4
1 2 1 2 1 2 3
X-ray tube voltage (kV)
Tube current time product (mAs)
120 120 120 120 120 120 110 120 130 130 120 120 130
100 100 80 80 20 20 50 50 50 47 80 80 50
Pitch
Beam collimation (mm)
Axial Axial Axial Axial 1.5 1.5 1.5 1.5 0.8 0.8 Axial Axial 0.55
20 20 20 20 5 5 5 2.5 3 3 25 10 3
Rotation time (s)
CTDIvol displayed (mGy)
CTDIvol measured (mGy)
CTDIvol displayed/ CTDIvol measured
1 1 1 1 0.8 1 0.6 0.8 0.8 0.8 1 1 1
11.99 11.99 9.59 9.59 3.73 3.73 3.50 3.50 5.70 3.08 5.60 7.50 11.33
23.25 31.33 18.36 24.90 5.27 7.21 7.80 7.46 11.49 5.54 5.49 7.34 10.92
0.52 0.38 0.52 0.39 0.71 0.52 0.45 0.47 0.49 0.56 1.02 1.02 1.04
CT, computed tomography; CTDI, computed tomography dose index.
Table 5
Comparison of measurements made with the whole-body and head phantoms
Equipment code PET/CT 1 head phantom PET/CT 1 whole-body phantom PET/CT 4 head phantom PET/CT 4 whole-body phantom SPECT/CT 4 head phantom SPECT/CT 4 whole-body phantom SPECT/CT 3 head phantom SPECT/CT 3 whole-body phantom
Rotation time (s)
CTDIvol displayed (mGy)
CTDIvol measured (mGy)
CTDIvol displayed/ CTDIvol measured
5 5
0.6 0.6
3.50 3.50
7.80 3.68
0.45 0.95
0.8 0.8
3 3
0.8 0.8
5.70 5.70
11.49 4.93
0.49 1.15
19 19
1.5 1.5
3 3
– –
2.07 2.07
3.41 1.67
0.61 1.24
50 50
1.5 1.5
2.5 2.5
0.8 1
3.50 3.50
7.46 3.90
0.47 0.90
X-ray tube voltage (kV)
Tube current time product (mAs)
Pitch
Beam collimation (mm)
110 110
50 50
1.5 1.5
130 130
50 50
130 130 120 120
CT, computed tomography; CTDI, computed tomography dose index.
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538
Nuclear Medicine Communications 2014, Vol 35 No 5
Special attention should be paid to head-and-neck and paediatric protocols, in which it is essential that dose calibration be performed with a head phantom; otherwise, patient doses could be underestimated significantly, altering the decision-making process in children’s examinations.
4 5 6 7 8
Acknowledgements The authors express their thanks to all departments that participated in the study.
9
10
Conflicts of interest
There are no conflicts of interest.
11
References
12
1
2
3
National Council on Radiation Protection & Measurements. Ionizing radiation exposure of the population of the United States. NCRP Report No. 160. Bethesda: NCRP; 2009. Verdun FR, Gutierrez D, Vader JP, Aroua A, Alamo-Maestre LT, Bochud F. CT radiation dose in children: a survey to establish age-based diagnostic reference levels in Switzerland. Eur Radiol 2008; 18:1980–1986. International Electrotechnical Commission. Medical electrical equipment – part 2-44 Ed 2: particular requirements for basic safety of X-ray equipment for computed tomography. IEC 60601-2-44. Geneva: IEC; 1999.
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Kalender WA. Computed tomography. 3rd ed. Munich: Publicis MCD VerlagNew York, Wiley, 2011; 2000. pp. 119–149. Nagel HD. Radiation exposure in computed tomography. 4th ed. Hamburg: CTB Publications; 2002. Goldman LW. Principles of CT: radiation dose and image quality. J Nucl Med Technol 2007; 35:213–225. Boone JM. The trouble with CTDI100. Med Phys 2007; 34:1364–1371. American Association of Physicists in Medicine. Size-specific dose estimates (SSDE) in pediatric and adult body CT examinations. AAPM Rep 204. College Park: AAPM; 2011. McCollough CH, Leng S, Yu L, Boone JM, McNitt-Gray MF. CT dose index and patient dose: they are not the same thing. Radiology 2011; 259:311–316. International Atomic Energy Agency. Quality assurance program for computed tomography: diagnostic and therapy applications. 73, HHS 19. Vienna: IAEA; 2012. pp. 19–26. European Commission. Criteria for acceptability of medical radiological equipment used in diagnostic radiology, nuclear medicine and radiotherapy. Radiation Protection 162. Luxembourg: EC; 2012. pp. 42–44. International Electrotechnical Commission. Medical electrical equipment – part 2-44 Ed 3: particular requirements for basic safety and essential performance of X-ray equipment for computed tomography. IEC 60601-244. Geneva: IEC; 2009. European Commission. Council Directive 97/43/Euratom of 30 June 1997 on health protection of individuals against the dangers of ionizing radiation in relation to medical exposure, and repealing Directive 84/466/Euratom. Off J Eur Comm 1997; L 180:22. Mattsson S, So¨derberg M. Radiation dose management in CT, SPECT/CT and PET/CT techniques. Radiat Prot Dosimetry 2011; 147:13–21.
Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
Validation of CT doses of SPECT/CT and PET/CT hybrid devices: lessons learned Terez Seraa, Tamas Porubszkyc, Miklos Paposa, Richard Elekc, Zsuzsanna Besenyia, Katalin Gionb, Andras Barthac, Sandor Pelletc and Laszlo Pavicsa The aim of the study was to check the validity of computed tomographic (CT) doses exhibited by SPECT/CT and PET/ CT hybrid devices. Dose measurements were taken from four SPECT/CT and four PET/CT cameras commercially available from different manufacturers. A calibrated ionization chamber was placed in whole-body or head phantoms for the acquisition of CT images with clinically used parameters. Computed tomography dose index (CTDIvol) values were calculated according to the IEC 60601-2-44:1999 formula. The measured CTDIvol doses were compared with those preprogrammed by the manufacturer. In the case of the whole-body phantom, the differences between the measured and displayed values varied between – 31 and + 24% [European document RP162 (2012) sets up the limit for acceptance criterion as ±20%]. The head phantom data showed either an agreement between – 10 and + 24%, or an underestimation by two-fold. The latter seemed to be because, while preprogramming the doses, the manufacturer had used the whole-body phantom instead
of a proper head phantom. The results of the work demonstrate the need for individual dosimetric calibration of every single X-ray tube. Dosimetric checks should be included in the regular quality control programmes of the SPECT/CT and PET/CT devices. Special attention should be paid to head-and-neck and paediatric protocols, in which the use of a head phantom is recommended for c 2014 dose calibration. Nucl Med Commun 35:534–538 Wolters Kluwer Health | Lippincott Williams & Wilkins.
Introduction
Protection [1] (NCRP Report No. 160) revealed that the mean annual radiation exposure originating from medical applications per head of the public in the USA increased by a factor of about 6 between 1980 and 2006 – that is, from 0.53 to 3.0 mSv. As the number of examinations involving radiation exposure is continuously increasing, there is a real need to decrease the dose per examination. One possible way to achieve this is by manufacturing devices that subject patients to lower radiation burden by using detectors with higher efficiency and advanced software; another way is through optimum adjustment of devices.
Hybrid devices have become the state-of-the-art of nuclear medicine equipment. The developments were largely determined by the application of very specific radiopharmaceuticals and the need for better images. The new radiopharmaceuticals are used more for the characterization of biochemical processes than for the visualization of organs. In this process, the value of the anatomical information deteriorates and additional imaging is required. For the improvement of image quality in nuclear medicine, efforts are being made to develop more sensitive detectors and highperformance software and there is also a possibility of using computed tomographic (CT) imaging data for the attenuation correction of g-rays. The performance characteristics of SPECT and PET equipment supplemented with a CT modality, in the SPECT/CTand PET/CT hybrid devices, are undoubtedly better than those of SPECT or PET alone. The CT component can be low-dose CT, which is used only for attenuation and scatter correction, or contrast-enhanced CT with the possibility of X-ray diagnostic examinations as well. The radiation dose originating from medical applications to which the population is exposed, mainly from the widespread use of CTas well as SPECT/CTand PET/CT, is continuously increasing. The report of the National Council on Radiation c 2014 Wolters Kluwer Health | Lippincott Williams & Wilkins 0143-3636
Nuclear Medicine Communications 2014, 35:534–538 Keywords: CT doses, CTDI, SPECT/CT and PET/CT hybrid devices, whole-body and head phantoms a Department of Nuclear Medicine, University of Szeged, bEuromedic Diagnostics, Szeged and c‘Fre´de´ric Joliot-Curie’ National Research Institute for Radiobiology and Radiohygiene, Budapest, Hungary
Correspondence to Terez Sera, PhD, Department of Nuclear Medicine, University of Szeged, H-6720 Szeged, Koranyi fasor 8, Hungary Tel: + 36 30 4750289; fax: + 36 62 544564; e-mail: [email protected] Received 24 July 2013 Revised 11 November 2013 Accepted 5 January 2014
Both international reports and the literature data indicate high deviations in patient doses among countries and institutions. Patient doses vary between different types of equipment and between departments [2]. In view of this, our team performed phantom measurements on the SPECT/CT and PET/CT equipment operating in Hungary in 2011 to check the doses resulting from the CT component and compare them with the values provided by the vendors of the CT equipment.
Materials and methods Radiation dose measurements were taken from four SPECT/ CT and four PET/CT cameras. Among the manufacturers DOI: 10.1097/MNM.0000000000000087
Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
Accuracy of CTDI on SPECT/CT and PET/CT devices Sera et al. 535
Table 1
List of tested SPECT/CT and PET/CT equipment
Equipment
Manufacturer
Year of installation
Type (number of slices)
PET/CT PET/CT PET/CT PET/CT SPECT/CT SPECT/CT SPECT/CT SPECT/CT
Philips, Amsterdam, the Netherlands GE, Milwaukee, Wisconsin, USA Siemens, Erlangen, Germany Siemens Mediso, Budapest, Hungary Mediso Mediso Siemens
2007 2005 2008 2006 2007 2009 2011 2010
Gemini TF-64 PET/CT (64) Discovery PET/CT (8) Biograph Truepoint PET/CT (6) Biograph Truepoint PET/CT (6) Anyscan Nucline DHV/CT (16) Anyscan SC DHV/CT (16) Anyscan SC DHV/CT (16) Symbia TruePoint SPECT CT (2)
CT, computed tomography.
of the investigated cameras were numerous companies who had a presence on the international market (Table 1).
Fig. 1
In our investigations, quantities such as the computed tomography dose index (CTDIw and CTDIvol) and the derived dose-length product (DLP) were used. CTDIw represents the CTDI average over the cross-section of the standardized PMMA CTDI phantom and CTDIvol is the pitch-corrected CTDIw; their unit is mGy. The DLP is the product of CTDIvol and scan lengths (cm); its unit is mGy cm. For the estimation of patient-effective doses the CTDIvol and the DLP have to be used with the software provided by the manufacturer, containing the body region-specific parameters for the given scanner model. For the CTDI measurements, standardized PMMA CTDI whole-body (for modelling the whole-body patient investigations) and head phantoms (for modelling the head-and-neck and paediatric examinations) [3] developed by Wellho¨fer (Schwarzenbruck, Germany) were used. The head phantom is standardized as a 16-cm-diameter cylinder and the wholebody phantom as a 32-cm-diameter cylinder, both 15 cm in length. The cylinders have five 12.4-mm-diameter holes, one of them at the centre and the other four, 1 cm deep, positioned symmetrically at 901. CTDI measurements were taken by consecutive insertion of an ionization chamber with an active length of 10 cm into each hole (Fig. 1). While taking the measurements the remaining holes were filled with homogeneous PMMA inserts. The DCT-10 ionization chamber was produced by Wellho¨fer, calibrated by an accredited laboratory and had an accuracy of ±2.4%. The reproducibility of the ionization chamber was checked by taking repeated measurements with the whole-body phantom and detector placed in the middle of it; 10 dose measurements were recorded with the same acquisition parameters. The weighted CTDI values (CTDIweighted) were calculated from the measured data, according to the specified (IEC 60601-2-44:1999) formula: 1 2 CTDIw ¼ CTDI100;c þ CTDI100;p ; 3 3 where CTDI100,c is the dose measured at the central hole of the phantom and CTDI100;p is the mean of the doses measured at the peripheral holes.
Measuring arrangement of the whole-body computed tomographic dosimetry phantom with the pencil ionization chamber positioned in its central hole.
The acquisition parameters selected for the CTDI phantom measurements were mostly the same as applied in clinical practice, but in some cases additional settings were selected too. The differences in clinical parameters between the different sets of equipment are partly due to the preferences of the sites and partly due to the differences in the ranges of the selectable parameters. The measured dose values were compared with the displayed values of the equipment provided by the vendors (Tables 3 and 4). Special attention was paid to paediatric examinations, which are performed following either paediatric or headand-neck protocols. In these cases measurements were recorded using both phantoms with identical parameter selection. For details of the CT dosimetry, we refer to the basic review publications [4–9].
Results and discussion The reproducibility of the ionization chamber measurement was excellent (Table 2). The mean value and the SD were 8.9034 and 0.0142, respectively, which gives a
Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
536
Nuclear Medicine Communications 2014, Vol 35 No 5
relative SD of less than 0.15%. These results led us to conclude that our device is suitable for the accurate measurement of CTDI doses. In the following part of our work, the CTDI doses were determined by using the ionization chamber placed into the whole-body phantom. To avoid the possibility of identifying the devices and sites, the sequencing of the equipment in the various tables differs. The CTDIvol values were measured by using different settings of the acquisition parameters. Every SPECT/CT and PET/CT measurement was checked with the wholebody phantom and the displayed values were recorded (Table 3). Two sets of equipment, SPECT/CT 1 and SPECT/CT 2, which were of the same type and with identical Table 2 Reproducibility of measurements with the ionization chamber Measured CTDI (mGy)
Measured irradiation time (ms)
8.901 8.888 8.881 8.909 8.885 8.921 8.912 8.911 8.907 8.919
1040 1032 1032 1032 1032 1032 1032 1032 1032 1032
CTDI, computed tomography dose index.
Table 3
acquisition parameters, displayed exactly the same doses, whereas the measured values were different (first six rows of Table 3). However, the ratio of the doses measured on the two cameras (using the same sets of acquisition parameters) was constant at 1.34±0.02. The radiation output of the various X-ray tubes varies because of factors such as the type and usage level, the radiation filtration applied, the features of the X-ray generator, the geometry of irradiation, and selected loading factors, and therefore the exactly identical values displayed are difficult to believe. We presume that the doses of the various equipment sets were set up by using factorymeasured data on only one of the tubes, and not individually for each tube. In the case of the other cameras, the ratio of the displayed and measured CTDIvol doses varied between 0.69 and 1.24 (two of 19 values were 1.2). The requirements concerning the accuracy of doses displayed in CT are formulated by the IAEA in a document dealing with the quality assurance programme for CT equipment [10]. According to this, the acceptance criterion is ±20%. This requirement is also adopted by European Commission document RP162 [11]. Furthermore, the IAEA considers even ±10% to be an achievable level. Dose measurements were taken by members of our working team on several sets of contrast-enhanced CT equipment as well, and in some cases ±5% accuracy was indeed achieved. These results, however, are relatively rare; more frequently, differences greater than 20% were found. A factor possibly influencing the differences may be the accuracy (calibrated state) of the instrumentation used by the vendors for dose measurements.
CTDI measurements with the whole-body phantom
Equipment code SPECT/CT SPECT/CT SPECT/CT SPECT/CT SPECT/CT SPECT/CT PET/CT 1 PET/CT 4 PET/CT 2 SPECT/CT SPECT/CT PET/CT 1 PET/CT 2 SPECT/CT PET/CT 1 PET/CT 1 PET/CT 1 PET/CT 4 PET/CT 4 PET/CT 4 PET/CT 3 SPECT/CT
1 2 1 2 1 2
3 3
4
4
X-ray tube voltage (kV)
Tube current time product (mAs)
120 120 120 120 120 120 120 130 120 120 120 110 120 130 130 130 130 130 130 130 120 130
80 80 50 50 40 40 71 70 100 50 100 50 80 17 70 50 80 91 86 50 80 19
Pitch
Beam collimation (mm)
Axial Axial 1.5 1.5 Axial Axial 0.8 1.5 0.82 1.5 1.5 1.5 Axial 1.5 1.5 1.5 1.5 1.5 1.5 0.8 Axial 1.5
20 20 10 10 10 10 3 5 40 2.5 2.5 5 25 5 5 1.25 3 5 5 3 5 3
Rotation time (s)
CTDIvol displayed (mGy)
CTDIvol measured (mGy)
CTDIvol displayed/ CTDIvol measured
1 1 1 1 1 1 0.8 0.6 0.5 1 1 0.6 0.8 – 0.6 0.8 0.6 0.6 0.6 0.8 0.5 –
9.59 9.59 7.20 7.20 5.76 5.76 8.09 7.89 5.90 3.50 7.00 3.50 10.90 1.84 7.42 6.62 9.12 9.67 9.80 5.70 9.65 2.07
9.38 12.71 5.14 6.81 5.88 7.98 11.70 10.41 6.62 3.90 7.61 3.67 10.86 1.83 7.37 6.24 8.52 8.88 8.61 4.93 8.09 1.67
1.02 0.75 1.40 1.06 0.98 0.72 0.69 0.76 0.89 0.90 0.92 0.95 1.00 1.01 1.01 1.06 1.07 1.09 1.14 1.16 1.19 1.24
CT, computed tomography; CTDI, computed tomography dose index.
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Accuracy of CTDI on SPECT/CT and PET/CT devices Sera et al. 537
We subsequently extended the dose measurements with particular attention to paediatric examinations, for which the use of a head phantom is recommended (Table 4).
It seems that the manufacturers, and especially their maintenance services, do not pay sufficient attention to dosimetric calibration and check only whether it is enforced by users or by professional organizations. As a consequence of the latter, dose display is nowadays strictly demanded at both international [3,12] and European [13] levels. After each X-ray tube replacement in SPECT/CT and PET/CT equipment, a dosimetric calibration is required. Moreover, we consider it justified that dosimetric checks should be performed by users when conditions are given. Otherwise, the dose measurements should be ordered from independent organizations.
The clinical acquisition parameters were those set up for head-and-neck and paediatric examinations. The SPECT/CT 1 and SPECT/CT 2 cameras exhibited the same pattern as in the case of the body phantom (first six rows of Table 4). The other cameras either underestimated the doses by a factor of B2 or the exhibited values were very close to those measured. The two-fold underestimation gave rise to the suspicion that, instead of the head phantom (which was required by the clinical protocol), the whole-body phantom was used by the vendor when the doses were preprogrammed.
The results of the work clearly demonstrate the need for the individual dosimetric calibration of every single X-ray tube. Dosimetric checks should be included in regular quality control programmes.
To check on this supposition, measurements were repeated by using the body phantom on selected cameras (Table 5). The results demonstrated that the ratios between the displayed and measured CTDIvol values improved from 0.45 to 0.95, from 0.49 to 1.15, from 0.61 to 1.24, and from 0.47 to 0.90, respectively.
Table 4
The CT component of hybrid SPECT/CT and PET/CT equipment makes a significant contribution to the total doses of examinations and should therefore be taken into consideration [14].
CTDI measurements with the head phantom
Equipment code SPECT/CT SPECT/CT SPECT/CT SPECT/CT SPECT/CT SPECT/CT PET/CT 1 SPECT/CT PET/CT 4 PET/CT 1 PET/CT 2 PET/CT 3 PET/CT 4
1 2 1 2 1 2 3
X-ray tube voltage (kV)
Tube current time product (mAs)
120 120 120 120 120 120 110 120 130 130 120 120 130
100 100 80 80 20 20 50 50 50 47 80 80 50
Pitch
Beam collimation (mm)
Axial Axial Axial Axial 1.5 1.5 1.5 1.5 0.8 0.8 Axial Axial 0.55
20 20 20 20 5 5 5 2.5 3 3 25 10 3
Rotation time (s)
CTDIvol displayed (mGy)
CTDIvol measured (mGy)
CTDIvol displayed/ CTDIvol measured
1 1 1 1 0.8 1 0.6 0.8 0.8 0.8 1 1 1
11.99 11.99 9.59 9.59 3.73 3.73 3.50 3.50 5.70 3.08 5.60 7.50 11.33
23.25 31.33 18.36 24.90 5.27 7.21 7.80 7.46 11.49 5.54 5.49 7.34 10.92
0.52 0.38 0.52 0.39 0.71 0.52 0.45 0.47 0.49 0.56 1.02 1.02 1.04
CT, computed tomography; CTDI, computed tomography dose index.
Table 5
Comparison of measurements made with the whole-body and head phantoms
Equipment code PET/CT 1 head phantom PET/CT 1 whole-body phantom PET/CT 4 head phantom PET/CT 4 whole-body phantom SPECT/CT 4 head phantom SPECT/CT 4 whole-body phantom SPECT/CT 3 head phantom SPECT/CT 3 whole-body phantom
Rotation time (s)
CTDIvol displayed (mGy)
CTDIvol measured (mGy)
CTDIvol displayed/ CTDIvol measured
5 5
0.6 0.6
3.50 3.50
7.80 3.68
0.45 0.95
0.8 0.8
3 3
0.8 0.8
5.70 5.70
11.49 4.93
0.49 1.15
19 19
1.5 1.5
3 3
– –
2.07 2.07
3.41 1.67
0.61 1.24
50 50
1.5 1.5
2.5 2.5
0.8 1
3.50 3.50
7.46 3.90
0.47 0.90
X-ray tube voltage (kV)
Tube current time product (mAs)
Pitch
Beam collimation (mm)
110 110
50 50
1.5 1.5
130 130
50 50
130 130 120 120
CT, computed tomography; CTDI, computed tomography dose index.
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Special attention should be paid to head-and-neck and paediatric protocols, in which it is essential that dose calibration be performed with a head phantom; otherwise, patient doses could be underestimated significantly, altering the decision-making process in children’s examinations.
4 5 6 7 8
Acknowledgements The authors express their thanks to all departments that participated in the study.
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Conflicts of interest
There are no conflicts of interest.
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