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Medical Physics |
1 From the Department of Radiology (Strahlenklinik und Poliklinik), Charité, Virchow-Clinic of the Humboldt-University, Augustenburger Platz 1, 13353 Berlin, Germany (N.H., M.W., P.L., B.G., U.T., R.J.S., R.F.); and the Technical Supervisory Authority, Berlin, Germany (A.N.). Received November 24, 1999; revision requested January 21, 2000; final revision received June 16; accepted June 28. Address correspondence to N.H. (e-mail: hidajat@telda.net).
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONAPPENDIXREFERENCES |
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MATERIALS AND METHODS: CT dose index with an active length of 15 cm was measured in 16 different types of CT scanners by using ionization chamber dosimetry. Twenty-six holders (one who has legal responsibility under national law for a radiologic installation) operating a total of seven conventional and 20 spiral CT scanners were asked for their standard parameters for various CT examinations. Weighted CT dose index and dose-length product were determined for each examination.
RESULTS: For most examinations, the tube current time product was significantly higher for conventional CT than for spiral CT (.002 
P .05). The ratio of section distance to section thickness for conventional CT was significantly lower than the pitch for spiral CT (.001 P .05). The weighted CT dose index and dose-length product for spiral CT were about half of those for conventional CT. The third quartiles for weighted CT dose index and dose-length product for spiral CT were much lower than those recommended as reference doses.
CONCLUSION: CT examinations with conventional CT scanners are often performed with unnecessarily high radiation dose. For the establishment of reference doses, the radiation dose with spiral CT scanners should be taken into account.
Index terms: Computed tomography (CT), radiation exposure, **.121112, **.12112, **.12115 • Dosimetry, **.12111, **.12112, **.12115 • Radiations, exposure to patients and personnel, **.12111, **.12112, **.12115 • Radiations, measurement, **.12111, **.12112, **.12115
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONAPPENDIXREFERENCES |
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The International Commission on Radiological Protection recommended the establishment of investigation levels for radiologic procedures; for higher levels, the cause or the implications of the result should be examined (3). The Council Directive of June 30, 1997, requires the member states of the European Community to promote the establishment and use of diagnostic reference levels that are expected not to be exceeded for standard procedures (4). The European Commission suggests reference doses in which the weighted CT dose index and dose-length product are used for various CT examinations (5). However, these dose values are based on the results of older survey data from the late 1980s (6).
The technical improvements in CT, in particular use of the spiral technique, have offered new possibilities in both diagnostics and dose reduction (7–10). The tube current time product for spiral CT usually cannot be set as high as for conventional CT owing to the limited tube heat capacity; therefore, the radiation dose should be effectively lower for spiral than for conventional CT (11). The results of older surveys that were based on investigations of dose for conventional CT may not be representative of the present situation.
In this article, we present the results of our survey of radiation exposure in 1999 for different conventional and spiral CT examinations and techniques at various hospitals and practices in Berlin, Germany.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONAPPENDIXREFERENCES |
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The head phantom was placed on the head shell and the body phantom on the patient table of the CT scanner. CT dose index was determined according to the original definition of CT dose index (ie, for a single scan) and within the phantom at the center and the periphery at 12, 3, 6, and 9 o’clock (13). We found that some scanners produce overscanning, which means that the tube emits radiation for more than a full rotation. When there is substantial overscanning, the four peripheral CT dose index values depend on where the tube rotation begins and ends or some peripheral CT dose index values differ significantly from one another. With such scanners, each single scan was obtained with four rotations at constant table position to minimize the influence of overscanning. The weighted CT dose index was calculated as the sum of two-thirds of the peripheral dose and one-third of the central dose within the CT dose index phantom (14).
We asked 26 CT scanner holders in Berlin for their standard examination parameters for CT of the head, neck, chest, liver, pancreas, pelvis, and lumbar spine in cases of brain metastasis, cervical abscess, pulmonary metastasis, hepatocellular carcinoma, pancreatitis, rectal carcinoma, prolapse of vertebral disks, and fracture of lumbar vertebrae. The term "holder" is used in the European Council Directive for one who has the legal responsibility under national law for a given radiologic installation (4). The parameters included the scanning technique (ie, conventional or spiral), tube voltage, tube current time product per section for conventional CT or per rotation for spiral CT, nominal section thickness, distance between sections for conventional CT or table feed per rotation for spiral CT, upper and lower boundaries of the body region, and number of scan series (eg, nonenhanced CT and enhanced CT of the liver in the arterial and portal venous phases). The CT operators entered the values in a table after being thoroughly instructed by one member (N.H., P.L., U.T.) of the research team on the meaning of the values to resolve any ambiguity.
The 26 holders used a total of 27 scanners; seven were conventional and 20 were spiral CT scanners. Twenty-four holders used only one scanner, one holder used two scanners, and one holder used four scanners. Thus, there were 30 examination protocols.
Dose-length product was calculated as follows: DLP = CTDIw x st x n, where DLP is dose-length product, CTDIw is weighted CT dose index, st is the nominal section thickness, and n the number of sections or rotations necessary to cover the body region. The number n was determined from the length of the body region and the distance between sections for conventional CT or the table feed per rotation for spiral CT. For spiral CT, two rotations were added to n because they had to be performed to calculate the planar data by means of interpolation at the beginning and end of the examined region. We determined the mean length of each body region from 20 CT examinations of adult patients (>18 years) who were randomly chosen from the patients in our department.
The weighted CT dose index and dose-length product were determined for each examination and protocol. For statistical comparisons, we used the Wilcoxon rank sum test. A P value less than .05 was considered to indicate a statistically significant difference.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONAPPENDIXREFERENCES |
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Table 1 shows the parameters for examination of the different body regions. All of the 26 holders examined the head by means of the conventional technique. Only one holder used the spiral technique for the skull base and summation of two adjacent sections to reduce the volume artifact. For both skull base and brain, tube voltage and tube current time product did not differ significantly from each other, although the skull base was scanned with a significantly smaller section thickness than was the brain (P < .001). In one protocol, the skull base was examined with 140 mAs and 3 mm, whereas the neurocranium was examined with 170 mAs and 10 mm at the same tube voltage. Both the ratio of section distance to section thickness and the pitch were 1.
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For examination of the chest, the examination was conducted eight times as conventional CT and 22 times as spiral CT. Conventional CT was performed with significantly higher tube current time product and also significantly greater section thickness than was spiral CT (P < .05). For conventional CT, the ratio of section distance to section thickness was always 1; for spiral CT, the pitch was 1.4 (0.8–2). This difference between ratio of section thickness to section distance and pitch was significant (P = .002).
For eight of the 30 protocols, the examination of the liver was conducted as conventional CT and for 22 as spiral CT. Again, conventional CT was performed with significantly higher tube current time product than was spiral CT (P < .01 for nonenhanced CT; P = .002 for enhanced CT). For nonenhanced spiral CT, the section thickness was significantly greater than it was for the arterial or portal venous phases (P < .01 and P < .05, respectively), although the tube current time products were similar. The ratio of section distance to section thickness for enhanced conventional CT was significantly lower than was the pitch for spiral CT for the arterial or portal venous phase (P < .05).
For CT of the pancreas, the examination was performed seven times as conventional CT and 22 times as spiral CT. Conventional CT was performed with significantly higher tube current time product than was spiral CT (P < .05 for nonenhanced CT; P < .02 for enhanced CT). For nonenhanced spiral CT, the section thicknesses were significantly greater than for the arterial or portal venous phase (P < .002 and P < .05, respectively), although the tube current time products did not show significant differences. The ratios of section distance to section thickness for conventional CT were smaller than was the pitch for spiral CT, but the differences were not significant.
The examination of the pelvis was conducted 10 times as conventional CT and 19 times as spiral CT. The tube current time product used for conventional CT was significantly higher than that used for spiral CT (P < .02 for nonenhanced CT; P < .002 for enhanced CT). Conventional CT was performed with greater section thickness than was spiral CT. This difference was significant for enhanced spiral CT and enhanced conventional CT (P < .01). The ratio of section distance to section thickness for conventional CT was lower than the pitch for spiral CT. This was significant for enhanced CT (P < .01).
The examination of the lumbar spine was conducted 29 times as conventional CT and one time as spiral CT in cases of disk disease and 21 times as conventional CT and eight times as spiral CT in cases of fracture. In cases of fracture, conventional CT was performed with significantly higher tube current time product than was spiral CT (P < .01). For conventional CT of disk disease, a significantly greater section thickness was chosen than for conventional CT of fracture (P < .001). The ratio of section distance to section thickness for conventional CT was significantly lower than was the pitch for spiral CT in cases of fracture (P < .05).
The resultant doses are shown in Tables 2 and 3. Concerning weighted CT dose index, the examinations of the head at the skull base and the brain did not differ significantly from each other. The dose-length product was significantly lower at the skull base than at the brain (P < .001) owing to the different lengths of these body regions. For examination of the neck, weighted CT dose index and dose-length product were higher for conventional CT than for spiral CT. This difference was not significant for the weighted CT dose index (P > .1) but was significant for dose-length product (P < .002). For examination of the chest, liver, pancreas, pelvis, and lumbar spine in cases of fracture, conventional CT led to significantly higher doses than did spiral CT: weighted CT dose index (P < .002, P < .001, P < .001, P < .001, and P < .01, respectively) and dose-length product (P < .002, P < .002, P < .05, P < .001, and P < .001, respectively).
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The mean weighted CT dose indexes were usually less than the values recommended as reference doses by the European Commission. The third quartiles of weighted CT dose indexes for conventional CT were in the range of the recommended values, but the third quartile for spiral CT was about half of these values.
The mean dose-length products were also less than the recommended reference doses, except for conventional CT of the pelvis. The third quartiles of dose-length product were less than these values for conventional CT of the head, neck, liver, pancreas, and lumbar spine in cases of disk hernia; in the range of these values for conventional CT of the chest; and higher than them for conventional CT of the pelvis. For spiral CT, the third quartile was less than the recommended reference dose values for all examinations.
The total dose-length products for the examination of the liver and pancreas were significantly higher for conventional CT than for spiral CT (P < .05), although conventional CT consists of two series of scans and spiral CT consists of three series of scans (Table 4). The ratio of the doses for spiral CT to those for conventional CT was in the range of 0.50–0.72 for weighted CT dose index and 0.36–0.54 for dose-length product. For each examination region, this ratio was always lower for dose-length product than for weighted CT dose index (Table 5).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONAPPENDIXREFERENCES |
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Despite technical differences between the scanners, the lower tube current time product associated with spiral CT seems to be the main reason that weighted CT dose index for spiral CT is lower than it is for conventional CT. This indicates that the radiation dose delivered at conventional CT should be reducible at least to a level similar to that at spiral CT. Moreover, the wide range of weighted CT dose indexes within a factor of up to 5.34 and the wide range of dose-length products within a factor of up to 6.90 for spiral CT reveals that even at spiral CT a substantial dose reduction can be achieved by using appropriate examination parameters.
The spiral technique is optimal in enabling an increase in the table feed and a decrease in the integral dose to the patient without causing gaps between two adjacent sections. This advantage was used by most of the operators. Together with the lower tube current time product, the increase in pitch for spiral CT leads to a dose-length product that is about half of the dose-length product for conventional CT scanners. Many spiral CT examinations of the liver and pancreas are performed as nonenhanced CT and enhanced CT in the arterial and portal venous phases, whereas conventional CT is performed in only two series—nonenhanced and enhanced. Nevertheless, in such cases, the dose-length product for spiral CT can remain lower than for conventional CT.
Several authors (19,20) show good correlation between dose-length product and effective dose and suggest this dose quantity as a measure of the effective dose. This would indicate that the effective dose for a single spiral CT examination is significantly lower than for a single conventional CT examination. To our knowledge, there has not been any other survey in which the differences between conventional and spiral CT have been analyzed concerning the radiation dose to the patient. Therefore, it is surprising that the dose for spiral CT is so much lower than for conventional CT for the examinations considered in this study, even if one takes the threefold scanning of the liver or pancreas into account. The similarity of weighted CT dose index and dose-length product for conventional CT obtained in this survey to those obtained in older investigations leads to the assumption that the attitude toward radiation dose for CT has not changed much and that spiral CT causes a lower radiation dose because spiral scanners force the operator to use a lower dose.
The similarity between the third quartile of weighted CT dose index for conventional CT in this study and the recommended reference dose values is not surprising, since these values are based on the results of a survey about conventional CT scanners (2) and represented the third quartile of the doses (5). The significantly lower weighted CT dose index for spiral CT indicates that there will be room for reference doses lower than those currently recommended by the European commission.
When weighted CT dose index is to be used as the reference dose quantity and the values are to be determined from the results of a survey, the weighted CT dose index values for spiral CT scanners should be considered. Users of conventional CT scanners should change their examination parameters to get weighted CT dose indexes similar to those of spiral CT scanners.
When dose-length product is to be used as the reference dose quantity, the following problems will arise. Because of the possibility of increasing the pitch at spiral CT, a reference dose that takes the dose-length product of spiral CT examinations into account would handicap the users of conventional CT more than it would those of spiral CT. Furthermore, the total length of the examination region depends on the underlying disease, findings, opinion, and experience of the CT operator.
For example, examination of the neck is performed by some operators from the skull base down to the tracheal bifurcation to depict enlarged mediastinal lymph nodes, whereas other operators choose the jugulum as the lower scan level. For diagnosing hepatic and pancreatic diseases, it can be sufficient to perform nonenhanced CT, but it can be necessary to scan the organ additionally with intravenous contrast media during the arterial and portal venous perfusion phases to detect and characterize lesions with vascularization different from that of the surrounding parenchyma (8–10). Some operators examine the upper abdomen only, whereas others generally examine the whole abdomen, which means the upper abdomen and the pelvis, in all cases of hepatic and pancreatic disease. Strictly speaking, clinical studies have to be performed to gain a consensus for the optimal examination technique before dose-length product is established definitively as the reference dose quantity.
Another dose quantity proposed by some authors (17) is the Dave*, which is equal to the weighted CT dose index divided by the pitch. However, this would handicap the conventional CT operator much more than it would the spiral CT operator because of the inability to increase the ratio of section distance to section thickness at conventional CT to greater than 1 without causing gaps between two adjacent sections.
Another quantity used by the German Federal Chamber of Physicians is the dose free in air on the axis of rotation (21). Because the patient dose depends strongly on the beam filtration, especially on the use of beam shaping filter, this dose quantity is less accurate than is the weighted CT dose index in characterizing the patient dose (5,20).
The lower radiation dose delivered at spiral CT for a single examination compared with that at conventional CT should not obscure the fact that the collective effective dose of CT may still increase because of the increasing number of CT scanners and CT examinations (22–24). These numbers and the wide range of doses delivered at CT for similar examinations support the need for reference doses. Our results may contribute to the subject of dose reduction and to the establishment of reference dose values. This survey includes only 30 examination protocols. Nevertheless, the similarity of the results to those of other authors (Tables 6, 7) show that they should be representative for the radiation dose at CT. At the time this article was written, a larger survey was being performed by the German Roentgen Society; the results will be helpful not only for European radiologists but for all radiologists.
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APPENDIX |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONAPPENDIXREFERENCES |
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Dose-length product is the product of the weighted CT dose index, the nominal section thickness, and the number of sections for conventional CT or rotations for spiral CT. Monitoring the dose-length product provides control of the volume of irradiation and overall exposure for an examination (5).
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Author contributions: Guarantor of integrity of entire study, N.H.; study concepts and design, N.H., M.W., A.N.; definition of intellectual content, N.H.; literature research, N.H.; clinical studies, N.H.; experimental studies, N.H., M.W., A.N., P.L., B.G.; data acquisition, N.H., M.W., A.N., P.L., B.G., R.J.S., U.T.; data analysis, N.H., M.W., A.N., P.L.; statistical analysis, N.H.; manuscript preparation, N.H.; manuscript editing, N.H.; manuscript review, N.H., M.W., A.N., R.J.S., R.F.; manuscript final version approval, N.H., M.W.
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REFERENCES |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONAPPENDIXREFERENCES |
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1.5 cm) malignant neoplasms. AJR Am J Roentgenol 1995; 164:879-884.This article has been cited by other articles:
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  V. Tsapaki, J. E. Aldrich, R. Sharma, M. A. Staniszewska, A. Krisanachinda, M. Rehani, A. Hufton, C. Triantopoulou, P. N. Maniatis, J. Papailiou, et al. Dose Reduction in CT while Maintaining Diagnostic Confidence: Diagnostic Reference Levels at Routine Head, Chest, and Abdominal CT--IAEA-coordinated Research Project Radiology, September 1, 2006; 240(3): 828 - 834. [Abstract] [Full Text] [PDF]
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 M. Das, A. H. Mahnken, G. Muhlenbruch, A. Stargardt, C. Weiss, D.-A. Sennst, T. G. Flohr, R. W. Gunther, and J. E. Wildberger Individually Adapted Examination Protocols for Reduction of Radiation Exposure for 16-MDCT Chest Examinations Am. J. Roentgenol., May 1, 2005; 184(5): 1437 - 1443. [Abstract] [Full Text] [PDF]
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M. Mahesh, J. C. Scatarige, J. Cooper, and E. K. Fishman Dose and Pitch Relationship for a Particular Multislice CT Scanner Am. J. Roentgenol., December 1, 2001; 177(6): 1273 - 1275. [Abstract] [Full Text] [PDF]
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