|
|
|||||||||
Brief Communication |
1 Division of Medical Radiation Hygiene and Dosimetry, Federal Office for Radiation Protection, Neuherberg, Germany
2 Department of Nuclear Medicine, University of Ulm, Ulm, Germany
3 Department of Nuclear Medicine, Technical University Munich, Munich, Germany
4 Department of Nuclear Medicine, University of Munich, Munich, Germany
5 Department of Nuclear Medicine, University Hospital Essen, Essen, Germany
| ABSTRACT |
|---|
|
|
|---|
Key Words: PET/CT patient exposure dosimetry dose reduction
| INTRODUCTION |
|---|
|
|
|---|
The development of dual-modality PET/CT systems has addressed these problems (2). These systems allow the quasisimultaneous acquisition of anatomic (CT) and functional (PET) information of a patient within a single examination and, thus, provide intrinsically coregistered images of the 2 modalities (35). In addition, the "hardware" fusion concept offers the possibility of CT-based attenuation correction of the emission scans instead of using noisy transmission data measured separately by means of an external positron-emitting source (6). The use of CT-based attenuation correction results not only in a marked reduction of the total examination time but also in an improved quality of the corrected PET scans (5,7,8).
On the other hand, whole-body PET/CT examinations incur an increased patient exposure compared with an individual CT or PET examination (8). Thus, patient referral for PET/CT studies must be justified in each case to avoid repeated exposure or overexposure of patients (9). Besides justification, optimization is the second general principle in radiologic protection (10). It was, therefore, the aim of the present study (a) to evaluate radiation exposure of patients undergoing whole-body PET/CT examinations after administration of 18F-FDG, (b) to derive a practical dosimetric concept for dose estimation in whole-body CT, and (c) to discuss strategies for dose reduction to decrease radiation risks to patients.
| MATERIALS AND METHODS |
|---|
|
|
|---|
|
TFDG provided by the International Commission on Radiological Protection (ICRP) in its Publication 80 (11) for a variety of organs and tissues of the adult hermaphrodite MIRD phantomthat is, DT = A·
TFDG. Effective doses were estimated by:
![]() | (Eq. 1) |
EFDG = 19 µSv/MBq is the dose coefficient for the effective dose and wT are the tissue weighting factors (
T wT = 1) given in ICRP Publication 60 (12).
External Exposure
To estimate radiation exposure of patients resulting from the acquisition of topograms and scans in CT, dose measurements were performed on an anthropomorphic whole-body Alderson RANDO phantom (Alderson Research Laboratories Inc.) using thermoluminescent dosimeters (TLDs). The method has been described in detail in a previous article (13). In brief, at least 180 dosimeters (TLD-100; Bicron-Harshaw) were suitably distributed inside and at the surface of the phantom. For smaller organs, absorbed doses were obtained by averaging the TLD values measured within the specified organs, whereas, for extended organs (e.g., skin and bone), they were estimated using specific weighting factors for the various cross sections of the Alderson phantom. The effective dose E was calculated from the absorbed doses DT according to Equation 1.
In analogy to the formalism presented for the case of internal dosimetry, organ doses were described by:
![]() | (Eq. 2) |
TCT is an organ-specific dose coefficient that relates the volume CT dose index CTDIvolthat is, the average dose for a standardized CT dosimetry phantomwith the organ dose DT. Variations in the organ doses with tube potential are considered by using the CTDIvol value indicated for the specific CT scan on the operatorss console of the scanner. Organ-specific dose coefficients were estimated according to Equation 2, using the organ doses derived from the TLD measurements on the Alderson phantom and the corresponding CTDIvol values. | RESULTS |
|---|
|
|
|---|
|
|
|
|
ECT = 1.47 ± 0.02 mSv/mGy given in Table 4. The resulting dose values are plotted versus the corresponding values determined from the TLD measurements on the Alderson phantom in Figure 2. Linear regression analysis (SigmaPlot, version 7.101; SPSS Inc.) yielded a slope of 1.03.
|
| DISCUSSION |
|---|
|
|
|---|
At 2 hospitals (H2 and H4), separate low-dose CT scans were acquired for attenuation correction of emission data in addition to a contrast-enhanced CT scan. At the other 2 sites (H1 and H3), a single, contrast-enhanced CT scan was used both for a fully diagnostic evaluation and for CT-based attenuation correction. This may imply the question of whether the administration of an intravenous CT contrast agent leads to serious artifacts in the attenuation-corrected PET images, since structures with a strong enhancement in the CT scans may be considered as bone by the attenuation correction algorithm, thus resulting in an overestimation of regional attenuation coefficients (14). However, recent evidence indicates that these artifacts rarely cause a diagnostic challenge in the clinical setting (15) and that these artifacts can be avoided prospectively when using adapted contrast administration protocols (16).
Nevertheless, if a contrast-enhanced diagnostic CT scan has already been performed on a conventional CT system as part of the regular clinical work-up, it is in general acceptable to acquire only a low-dose CT scan as part of the combined PET/CT study (17). The image quality of this scan is certainly adequate for anatomic correlation and attenuation correction (18). In the present study, the effective dose determined for 3 low-dose scans was <5 mSv (Table 2).
The effective doses determined for the 4 high-quality CT scans listed in Table 3 varied between 14.1 and 18.6 mSv. These values are somewhat higher than the dose estimates (mean ± SD) of 14.5 ± 4.9 mSv from a recent survey on whole-body, multislice CT examinations (19), which is mainly due to the inclusion of the thyroid in the whole-body scan range covered in this study.
The dose coefficients listed in Table 4 make it possible to estimate organ doses andusing the corresponding tissue weighting factorseffective doses related to whole-body 18F-FDG PET and CT scans. All data presented are for a standard person with a body weight of about 70 kg and are generic rather than patient specific since the age, sex, and constitution of individual patients are not considered. Nevertheless, they provide a reasonably good indicator of the relative radiation risks to patients (12) resulting from nonuniform exposures related to whole-body PET and CT procedures and, thus, for protocol optimization.
PET/CT users should note that the CTDIvol value displayed on the operators console is the principal descriptor to characterize patient exposure in CT on a local dose level. It represents an estimate of the average dose within an irradiated slice of a standardized CT dosimetry phantom and, thus, reflects not only the combined effect of the selected scan parameters but also of scanner-specific factors such as beam filtration, beam-shaping filter, geometry, and overbeaming. A detailed discussion of the various scan parameters and system features determining patient exposure in CT as well as strategies for dose reduction can be found elsewhere (19,20). Besides the CTDIvol, the length of the scan region is the second parameter that determines the effective dose and, thus, the integrated detriment to patients related to a CT examination. Whenever clinically justifiable, the range of whole-body scans should be limited by the symphysis at the lower limit and should exclude the eye lenses from the cranial imaging range.
However, adaptation of the scan length to the individual body size may not be possible at current PET/CT systems because the axial CT range can be set up only in integer multiples of the fixed axial field of view of the PET system. This technical limitation can be overcome in the future, for example, by the implementation of continuous bed motion for PET measurements. In general, noncongruent imaging ranges of PET and CT scans, as well as multiple contiguous spirals with different CT scan parameters, should become available with the clinical PET/CT acquisition software. This flexibility would open the possibility of acquiring a high-quality CT scan for only part of the body and imaging the remaining axial ranges with a low-dose CT, or even without attenuation correction. Moreover, prospective measures that offer the potential for dose reduction in CT without a considerable loss in image qualitysuch as automatic tube current modulation or adaptive filteringshould be adopted for routine PET/CT.
| CONCLUSION |
|---|
|
|
|---|
| FOOTNOTES |
|---|
For correspondence or reprints contact: Gunnar Brix, PhD, Bundesamt für Strahlenschutz, Abteilung für Medizinische Strahlenhygiene und Dosimetrie, Ingolstädter Landstraße 1, D-85764 Neuherberg, Germany.
E-mail: gbrix{at}bfs.de
| REFERENCES |
|---|
|
|
|---|
Related articles in JNM:
This article has been cited by other articles:
![]() |
M. Klein, M. Cohen-Cymberknoh, S. Armoni, D. Shoseyov, R. Chisin, M. Orevi, N. Freedman, and E. Kerem 18F-Fluorodeoxyglucose-PET/CT Imaging of Lungs in Patients With Cystic Fibrosis Chest, November 1, 2009; 136(5): 1220 - 1228. [Abstract] [Full Text] [PDF] |
||||
![]() |
A. Rominger, T. Saam, S. Wolpers, C. C. Cyran, M. Schmidt, S. Foerster, K. Nikolaou, M. F. Reiser, P. Bartenstein, and M. Hacker 18F-FDG PET/CT Identifies Patients at Risk for Future Vascular Events in an Otherwise Asymptomatic Cohort with Neoplastic Disease J. Nucl. Med., October 1, 2009; 50(10): 1611 - 1620. [Abstract] [Full Text] [PDF] |
||||
![]() |
A. Shammas, R. Lim, and M. Charron Pediatric FDG PET/CT: Physiologic Uptake, Normal Variants, and Benign Conditions RadioGraphics, September 1, 2009; 29(5): 1467 - 1486. [Abstract] [Full Text] [PDF] |
||||
![]() |
R. L. Bridges Software Fusion: An Option Never Fully Explored J. Nucl. Med., May 1, 2009; 50(5): 834 - 836. [Full Text] [PDF] |
||||
![]() |
B. Huang, M. W.-M. Law, and P.-L. Khong Whole-Body PET/CT Scanning: Estimation of Radiation Dose and Cancer Risk Radiology, April 1, 2009; 251(1): 166 - 174. [Abstract] [Full Text] [PDF] |
||||
![]() |
W. De Wever, S. Stroobants, J. Coolen, and J. A. Verschakelen Integrated PET/CT in the staging of nonsmall cell lung cancer: technical aspects and clinical integration Eur. Respir. J., January 1, 2009; 33(1): 201 - 212. [Abstract] [Full Text] [PDF] |
||||
![]() |
M. Imbriaco, M. G. Caprio, G. Limite, L. Pace, T. De Falco, E. Capuano, and M. Salvatore Dual-Time-Point 18F-FDG PET/CT Versus Dynamic Breast MRI of Suspicious Breast Lesions Am. J. Roentgenol., November 1, 2008; 191(5): 1323 - 1330. [Abstract] [Full Text] [PDF] |
||||
![]() |
A. Del Guerra, N. Belcari, G. L. Llacer, S. Marcatili, S. Moehrs, and D. Panetta ADVANCED RADIATION MEASUREMENT TECHNIQUES IN DIAGNOSTIC RADIOLOGY AND MOLECULAR IMAGING Radiat Prot Dosimetry, August 29, 2008; (2008) ncn236v1. [Abstract] [Full Text] [PDF] |
||||
![]() |
C. A Yi, K. M. Shin, K. S. Lee, B.-T. Kim, H. Kim, O J. Kwon, J. Y. Choi, and M. J. Chung Non-Small Cell Lung Cancer Staging: Efficacy Comparison of Integrated PET/CT versus 3.0-T Whole-Body MR Imaging Radiology, August 1, 2008; 248(2): 632 - 642. [Abstract] [Full Text] [PDF] |
||||
![]() |
D. W. Townsend Dual-Modality Imaging: Combining Anatomy and Function J. Nucl. Med., June 1, 2008; 49(6): 938 - 955. [Abstract] [Full Text] [PDF] |
||||
![]() |
P. Zanotti-Fregonara, C. Champion, R. Trebossen, R. Maroy, J.-Y. Devaux, and E. Hindie Estimation of the {beta}+ Dose to the Embryo Resulting from 18F-FDG Administration During Early Pregnancy J. Nucl. Med., April 1, 2008; 49(4): 679 - 682. [Abstract] [Full Text] [PDF] |
||||
![]() |
T. C. Kwee, R. M. Kwee, and R. A. J. Nievelstein Imaging in staging of malignant lymphoma: a systematic review Blood, January 15, 2008; 111(2): 504 - 516. [Abstract] [Full Text] [PDF] |
||||
![]() |
N. Federman and S. A. Feig PET/CT in Evaluating Pediatric Malignancies: A Clinician's Perspective J. Nucl. Med., December 1, 2007; 48(12): 1920 - 1922. [Full Text] [PDF] |
||||
![]() |
M. B. Dolovich and D. P. Schuster Positron Emission Tomography and Computed Tomography versus Positron Emission Tomography Computed Tomography: Tools for Imaging the Lung Proceedings of the ATS, August 1, 2007; 4(4): 328 - 333. [Abstract] [Full Text] [PDF] |
||||
![]() |
K Strobel, N. Schaefer, C Renner, P Veit-Haibach, D Husarik, A. Koma, and T. Hany Cost-effective therapy remission assessment in lymphoma patients using 2-[fluorine-18]fluoro-2-deoxy-D-glucose-positron emission tomography/computed tomography: is an end of treatment exam necessary in all patients? Ann. Onc., April 1, 2007; 18(4): 658 - 664. [Abstract] [Full Text] [PDF] |
||||
![]() |
N Aide, M Benayoun, K Kerrou, A Khalil, J Cadranel, and J N Talbot Impact of [18F]-fluorodeoxyglucose ([18F]-FDG) imaging in sarcoidosis: unsuspected neurosarcoidosis discovered by [18F]-FDG PET and early metabolic response to corticosteroid therapy Br. J. Radiol., March 1, 2007; 80(951): e67 - e71. [Abstract] [Full Text] [PDF] |
||||
![]() |
H. Schoder and M. Gonen Screening for Cancer with PET and PET/CT: Potential and Limitations J. Nucl. Med., January 1, 2007; 48(1_suppl): 4S - 18S. [Abstract] [Full Text] [PDF] |
||||
![]() |
H. Kuehl, P. Veit, S. J. Rosenbaum, A. Bockisch, and G. Antoch Can PET/CT Replace Separate Diagnostic CT for Cancer Imaging? Optimizing CT Protocols for Imaging Cancers of the Chest and Abdomen J. Nucl. Med., January 1, 2007; 48(1_suppl): 45S - 57S. [Abstract] [Full Text] [PDF] |
||||
![]() |
K. Brechtel, M. Klein, M. Vogel, M. Mueller, P. Aschoff, T. Beyer, S. M. Eschmann, R. Bares, C. D. Claussen, and A. C. Pfannenberg Optimized Contrast-Enhanced CT Protocols for Diagnostic Whole-Body 18F-FDG PET/CT: Technical Aspects of Single-Phase Versus Multiphase CT Imaging J. Nucl. Med., March 1, 2006; 47(3): 470 - 476. [Abstract] [Full Text] [PDF] |
||||
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | RSS | TABLE OF CONTENTS |
| JOURNAL OF NUCLEAR MEDICINE TECHNOLOGY | THE JOURNAL OF NUCLEAR MEDICINE |