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16-detector multislice CT: dosimetry estimation by TLD measurement compared with Monte Carlo simulation

Departments of ¹ Radiology and ² Medical Physics, Addenbrooke's Hospital NHS Trust and the University of Cambridge, Hills Road, Cambridge CB2 2QQ and ³ Department of Nuclear Medicine, Guys Hospital, St Thomas Street, London SE1 9RT, UK

	Abstract

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Computer simulations are widely used to estimate effective doses from CT examinations. The raw data often used in their estimations were obtained some years ago and made certain assumptions regarding CT unit design. At that time multidetector CT units were unavailable. Changes in design will limit the accuracy of computer simulated dosimetry on these machines. We therefore estimated CT dose on a 16-detector unit directly using thermoluminescent dosemeters (TLDs) and an anthropomorphic phantom. We found that the dose measured directly was 18% higher than the computer simulated dosimetry, in keeping with the previously recognised underestimation by computer simulation techniques compared with TLD measurements.

	Introduction

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16-detector multislice CT has been a recent introduction in the UK. Its advent has made it possible to perform 0.75 mm thin sections from vertex to knee within a minute [1]. Such thin slice widths with isometric voxels allow high quality 3D reconstruction with improved resolution in the coronal and sagittal planes. The reconstructed images can then be displayed in different formats such as maximal intensity projections, surface shaded rendering and volume rendering. These attributes have considerably increased the versatility and repertoire of CT giving rise to multiple applications for the diagnosis and treatment of disease.

At present there are few dosimetry data from 16-detector multislice CT. Dose estimation is most often made indirectly by combining CT dose index (CTDI) measurements and normalized doses obtained from data tables produced by Monte Carlo simulations. These tables are published by the National Radiation Protection Board (NRPB) and were employed in the National Survey of CT Practice in the UK [2]. The tables were derived from simulations using a derivative of the Medical Internal Radiation Dosimetry (MIRD) mathematical phantom [3], for a variety of CT machines. Although the Monte Carlo method is convenient, it is dependent on the CT unit design. 16-detector multislice CT units were not available when the Monte Carlo tables were generated and consequently dose estimation has to rely on "best fitting" the attributes of the new machine to those of older designs. For these reasons we decided to measure the effective dose from a 16-detector multislice whole body CT study, directly using thermoluminescent dosemeters (TLDs) and an anthropomorphic (Rando Alderson) humanoid phantom.

	Methods and materials

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Measurement of the organ doses with TLDs and the anthropomorphic phantom
Doses were measured during a whole body examination, from vertex to mid thigh with a 1.5 mm detector width helical acquisition on a Siemens Somatom Sensation 16 CT machine (Siemens, Forchheim, Germany). The exposure parameters used are shown in Table 1. The exposure was repeated 4 times to improve the counting statistics. The CT protocol performed was based on a skeletal CT study being undertaken in the department.

The Rando Alderson phantom consists of 35 axial segments containing human skeleton and material with similar properties to soft tissues including lung. It has been validated as suitable for CT dose measurements [6]. After the TLDs had been exposed, they were removed from the phantom and were read 24 h later. The effective dose was then calculated from the organ and tissue doses using the weighting factors published in ICRP 60 [4].

The TLD batch was calibrated by irradiating three groups of five TLDs to known doses. These TLDs were irradiated free-in-air at the centre of rotation of the CT scanner. In the axial direction, the TLDs were positioned in the centre of a 24 mm wide radiation beam. The dose delivered to these TLDs was derived in two stages. First, the shape of the radiation beam profile was measured using 54 TLDs stacked in a 4 cm row and orientated in the axial direction. Second, absolute values of dose were assigned to the radiation beam profile by measuring the integral of the dose distribution in the axial direction using a 100 mm long pencil ionization chamber. This chamber had an air-kerma calibration traceable to national standards via a therapy-level secondary standard. To account for differences in the mass energy absorption coefficients of air and tissue, a factor of 1.07 was applied to doses measured using the ionization chamber. This factor has been shown to be appropriate for typical CT X-ray spectra, and results in doses expressed in terms of absorbed dose to ICRU muscle [7].

Monte Carlo estimation
Dose estimation methodology was based on the NRPB Monte Carlo simulations as mentioned above [2]. The exposure factors used in Table 1 were entered into the "ImPACT" spreadsheet [8], along with CTDI measurements made with an ionization chamber. The spreadsheet used this data and also determined which Monte Carlo table of normalized organ doses to employ when using the Siemens Sensation 16 CT machine, based on CT system matching performed by ImPACT.

	Results

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A summary of the results and errors are shown in Tables 2 and 3.

For most organs listed in Table 2 three individual TLD measurements were available. These measurements were used to calculate the coefficient of variation (CV) for each organ, and from these data the root mean square CV was calculated after weighting for the number of degrees of freedom for each organ. With a total of 39 degrees of freedom the error in each individual TLD measurement was estimated at 12.3%. This figure was used to calculate the standard error of the mean (SEM) for the weighted dose for each organ allowing for the number of individual TLD measurements contributing to each organ dose. After adding the errors for the individual organs in quadrature, the overall error in calculating the effective dose was 2.7% (the mean of the errors between the male and female results, see Table 2).

	Discussion

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The results demonstrate that the direct TLD dosimetry measurements were higher than those using the Monte Carlo technique. The underestimation by computer simulated techniques has also been noted by other studies [9, 10]. However in these studies the size of the discrepancy between the two dosimetry techniques was larger than in our study. The suggested underestimation by the Monte Carlo technique has been attributed to design differences between the Rando Alderson and the MIRD mathematical phantoms and also the accurate employment of normalized organ doses [9, 10]. The computer simulated technique employs the mathematical MIRD phantom, which approximates the organs by simplified geometric shapes, contrary to the situation in practical circumstances. Consequently, irradiation of the chest for example, is assumed to result in no exposure to the upper abdominal viscera and hence underestimation of radiation doses. Moreover the computer simulated dose estimation relies on the normalized organ doses acquired from Monte Carlo data. These are specific to the model of CT unit and are dependent on radiation quality and field geometry. Since information used for the Monte Carlo tables was acquired long ago [2], there is little current relevant information available for modern 16-detector CT units. It is necessary therefore to choose the data from the available CT model with the closest attributes. In the present study the ImPACT calculator utilized the organ conversion factors for the Philips Tomoscan 310 machine (Philips, Reigate, UK) [8]. There is however evidence that both beam geometry and dosimetry is significantly different in multidetector machines compared with the single detector units [11].

The direct TLD dose estimations have limitations as well. There are statistical uncertainties, which can adversely affect the dose calculations. We used 65 TLDs in the phantom and the resulting statistical uncertainty may limit the accuracy of the dose measurement. We found less discrepancy between the TLD and Monte Carlo simulated dose measurements than that obtained by other investigators [9, 10]. It could be postulated that the slight differences in the results between investigators might be due in part to the statistical uncertainties when using limited TLD numbers. In addition the Rando Alderson phantom has only limited sites drilled for TLD placement. This is not anatomically ideal, especially since there is only one hole drilled for TLD placement in the bone marrow (in the lumbar spine). Moreover, there is scope for misplacement of TLDs if there is poor knowledge of anatomy. None the less, as stated above this phantom has been shown to be a valid tool in dose estimation [6].

It should also be appreciated that there is a difference between the male and female doses. This is due to the more superficial anatomical location of the male gonads in the scrotum, rather than the deeper placement of the ovaries in the pelvis. As a consequence the female gonadal exposure is less since the surrounding tissues attenuate a significant proportion of the beam. Since the gonads are subject to the highest tissue-weighting factor, the overall effective dose is that much lower in women.

In conclusion, direct TLD CT dose estimation has some advantages over computer simulated dosimetry techniques. Unfortunately, it is a time consuming technique and as such, not practical for routine use. However, since most CT dose estimations have been made by simulations, the reader should be aware that they are likely to be underestimated.

	Acknowledgments

 
The Authors would like to thank Stefan Schaller (Siemens, Forchheim, DE) for his advice.

Received for publication June 19, 2003. Revision received November 6, 2003. Accepted for publication March 18, 2004.

	References

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1. Eight and sixteen slice CT scanner comparison report. MDA evaluation report. MDA 02059. Norwich: HMSO, 2002.
2. Jones DG, Shrimpton PC. Survey of CT practice in the UK. Part 3: normalised organ doses calculated using Monte Carlo techniques (NRPB-R250). Chilton: National Radiological Protection Board, 1991.
3. Snyder WS, Ford MR, Warner GG, Fisher HL. Estimates of absorbed fractions for monoenergetic photon sources uniformly distributed in various organs of a heterogeneous phantom. J Nucl Med 10 (Suppl. 3), Pamphlet No. 5, 1969.
4. ICRP. 1990 Recommendations of the International Commission of Radiological Protection (publication 60). Oxford: Pergamon Press, 1991.
5. Ellis H, Logan B, Dixon A. Human cross-sectional anatomy atlas of body sections and CT images. Oxford: Butterworth-Heinmann, 1991.
6. Shrimpton PC, Wall BF, Fischer ES. The tissue equivalence of the Alderson Rando anthropomorphic phantom for X-rays of diagnostic qualities. Phys Med Biol 1984;47:463–7.
7. Shrimpton PC, Jones DG, Hillier MC, Wall BF, Le Heron JC, Faulkner K. Survey of CT Practice in the UK. Part 2: Dosimetric Aspects (NRPB-R249). Chilton: National Radiological Protection Board, 1991.
8. ImPACT CT Patient Dosimetry Calculator version 0.99q. ImPACT. Medical Devices Agency. London. 2003.
9. Geleijns J, Van Unnik JG, Zoetelief J, Zweers D, Broerse JJ. Comparison of two methods for assessing patient dose from computed tomography. Br J Radiol 1994;67:360–5.[Abstract/Free Full Text]
10. Hashemi-Malayeri B, Williams JR. A practical approach for the assessment of patient doses from CT Examinations. Western General Hospital Edinburgh, 2003. www.dundee.ac.uk/medphys/documents/hashemi.pdf
11. Thornton FJ, Paulson EK, Yoshizumi TT, Frush DP, Nelson RC. Single versus multi-detector row CT: comparison of radiation doses and dose profiles. Acad Radiol 2003;10:379–85.[CrossRef][Medline]

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