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British Journal of Radiology 74 (2001),836-840 © 2001 The British Institute of Radiology

Full paper

Application of European Commission reference dose levels in CT examinations in Crete, Greece

V Tsapaki, PhD1, S Kottou, PhD2 and D Papadimitriou, PhD2

1Euromedica Medical Center, 44 Demokratius Str., 176174 Iraklion, Crete and 2Athens University, Medical Physics Department, Medical School, 75M Asias Str., 11527 Athens, Greece


   Abstract
Top
 Abstract
Introduction
 Materials and methods
 Results
 Discussion
 References
 
The purpose of this study was to apply European Commission reference dose levels (EC RDLs) to routine CT examinations. The dosimetric quantities proposed in the European Guidelines (EG) for CT are weighted computed tomography dose index (CTDIw) for a single slice and dose–length product (DLP) for a complete examination. Patient-related data as well as technical parameters for brain, chest, abdomen and pelvis examinations were collected for four CT scanners in the Euromedica Medical Center. Computed tomography dose index (CTDI) measurements were performed on each scanner and CTDIw, DLP and effective dose E were estimated for each type of examination for a random sample of 10 typical patients. Mean values of CTDIw had a range of 27.0–52.0 mGy for brain and 13.9–26.9 mGy for chest, abdomen and pelvis examinations. Mean values of DLP had a range of 430–758 mGy cm for brain, 348–807 mGy cm for chest, 278–582 mGy cm for abdomen and 306–592 mGy cm for pelvis examinations. Mean values of E were 1.4 mSv for brain, 10.9 mSv for chest, 7.1 mSv for abdomen and 9.3 mSv for pelvis examinations. Results confirm that the Euromedica Medical Center meets EC RDLs for brain, abdomen and pelvis examinations, in terms of radiation dose and examination technique. As far as chest examination is concerned, although CTDIw of each scanner is within proposed values, the DLP is consistently exceeded, probably because of the large irradiation volume length L. It is anticipated that a reduction of L, or product mAs, or their combination, will reduce DLP without affecting image quality.


   Introduction
Top
 Abstract
 Introduction
Materials and methods
 Results
 Discussion
 References
 
It is well known that CT is related to high radiation dose to the patient. Many ways are found in the literature to describe [1, 2] and measure [3–6] radiation dose in CT. Recently, European Guidelines (EG) on quality criteria for CT [7] were published by the European Commission, in which two dose descriptors, weighted computed tomography dose index (CTDIw) and dose–length product (DLP), were proposed as reference dose levels (RDLs). CTDIw is derived from the principal dosimetric quantity computed tomography dose index (CTDI) [1], which is the integral along a line parallel to the axis of rotation z of the dose profile D(z) of a single slice, divided by the nominal slice thickness T. CTDI can be measured free-in-air (CTDIair) on or parallel with the axis of rotation of the scanner, at the centre of a head or body phantom (CTDIc), or 10 mm below the surface of the phantom (CTDIp). Measurements are carried out using a pencil shaped ionization chamber of active length 100 mm, orusing thermoluminescent dosemeters (TLDs). CTDIw is estimated by the following formula: Go


and provides the radiation dose from one slice at particular exposure settings. DLP characterizes the exposure from a complete examination and is estimated by the following formula: Go


where Ti is each different slice thickness used in the examination protocol, Ni is the number of Ti slices and CTDIwi is the value of CTDIw of each particular slice thickness Ti. Comparison of both CTDIw and DLP values for a specific examination using different protocols or scanners will provide information on relative performance.

To compare radiological examinations in terms of radiation risk, taking into account the relative radiosensitivities of body regions involved, it is necessary to estimate effective dose E, which is thesum of the products of organ doses and corresponding weighting factors [8]. Shrimpton et al [9, 10] calculated E from CTDI measurements using Monte Carlo conversion coefficients. Organ doses can also be measured using TLDs inside and at the surface of phantoms [4, 11]. As a practical alternative, EG [7] give region-specific normalized coefficients to calculate the risk of a particular examination protocol and to compare it with other CT protocols or different radiological examinations.

The purpose of this study was to investigate routine examination protocols utilized in the Euromedica Medical Center in terms of imaging technique and radiation dose and to compare results with European Commission reference dose levels (EC RDLs).


   Materials and methods
Top
 Abstract
 Introduction
 Materials and methods
Results
 Discussion
 References
 
The CT scanners investigated in this study were located in four subcentres of the Euromedica Medical Center in different cities on the island of Crete. Two subcentres had a Siemens AR scanner (Siemens, Erlargen, Germany) (Sara, Sarb), the third had a Philips LX scanner (Philips Medical Systems, The Netherlands) (PLX) and the fourth had a Toshiba TCT-804 scanner (Toshiba, Tokyo, Japan) (TTCT). Examinations were categorized as follows: (1) brain; (2) chest; (3) abdomen; and (4) pelvis. Technique parameters such as kilovoltage (kV), tube current–exposure time product (mAs), slice thickness T, slice increment I, window width and window level were fixed for standard sized patients. None of the above examinations included spiral scanning. Patient data were collected for 10 standard sized patients for each type of examination and each scanner. 160 patients were included in total. All available technique and equipment parameters were recorded. CTDIair measurements were made free-in-air using a pencil shaped ionization chamber (Model 20x5-10.3CT; Radcal Corporation, Monrovia, CA) connected to a radiation measuring device (Model 2025AC; Radcal Corporation, Monrovia, CA) on the axis of rotation of each scanner. The system was calibrated according to International Electrical Commission standards [12]. Overall accuracy of ionization chamber measurements was estimated to be ±5%. For comparison purposes, CTDIair measurements were also undertaken using an array of 30 TLDs (TLD-100, LiF:Mg, Ti, 3 mm x 3 mm x 0.9 mm; Harshaw, Bicron-NE Solon, OH) stacked side by side with the major planar surface oriented perpendicular to the axis of rotation, in a hollow cylindrical Perspex capsule. TLD annealing treatment was performed in a PTW TLD oven (PTW-Freiberg, Freiburg, Germany). TLDs were read using a Victoreen Model 2800M manual reader (Victoreen Inc., Cleveland, OH). Four groups of TLDs of different sensitivities were used. The group standard deviation was below 5% when irradiated by 60Co-gamma radiation [13]. Correction factors were calculated and applied toeach group of TLDs according to suitable calibration performed in terms of kerma-in-air, using 60Co-gamma radiation free-in-air for doses of 0.5–100 mGy. Energy response calculations were made with an X-ray Philips-Optimus 65 unit (Philips Medical Systems, The Netherlands) with a three-phase generator at 60 kV, 80 kV, 90 kV, 100 kV, 120 kV and 130 kV. The TLD response to X-rays, divided by their response to 60Co irradiation, was estimated in order to correct for differences in radiation quality between the CT filtered X-ray beam used in our study and the calibration radiation.

CTDIc and CTDIp were measured with the ionization chamber in a head phantom for brain examination and in a body phantom for chest, abdomen and pelvis examinations. The head phantom was a cylindrical (16 cm diameter, 14 cm length) solid Perspex phantom with five 13 mm diameter holes drilled parallel to its long axis, one at the axial centre and four around the perimeter, 90° apart and 1 cm from the edge. Each of the holes could be plugged with a cylindrical solid Perspex rod. The body phantom was identical to the head phantom, with a 32 cm diameter. CTDIw, DLP and E were then calculated according to EG to check compliance with dose criteria.


   Results
Top
 Abstract
 Introduction
 Materials and methods
 Results
Discussion
 References
 
Examination protocol details are shown in Table 1Go. Examinations were performed with fixed kV, mAs, T and I at each scanner, for standard sized patients. The variable parameter between scanners was mAs, with the TTCT using the lowest value (160 mAs) and the PLX using the highest value (332 mAs).


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  		Table 1. Details of examination protocols, including kilovoltage (kVp), tube current–exposure time product (mAs), slice thickness T and slice increment I

 
CTDIair measurements made using the ionization chamber and using TLDs are shown in Table 2Go. There was good correlation between the two measurements methods (r=0.994) and, since the chamber method was direct and less time consuming, this was chosen to continue the study. Table 3Go presents values of total number of slices N and irradiation length L for each examination. It appears that the four subcentres tended to have comparable L for brain, abdomen and pelvis examinations. In the chest protocol, L had a broader range of values (25–32 cm), probably depending on the size of the thorax.


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  		Table 2. Computed tomography dose index free-in-air (CTDIair) measurements carried out using a pencil ionization chamber (ch) and using thermoluminescent dosemeters. Results are normalized by tube current–exposure time product (mAs) in order to compare scanners

 

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  		Table 3. Total number of slices N and irradiation length L of examined body region

 
CTDIw and DLP were then calculated for each examination; the mean results are shown in Table 4Go. CTDIw was calculated for each scanner from an average of three measurements in the head phantom and another three measurements in the body phantom. EC RDLs for CTDIw and DLP are also found in Table 4Go. Brain examination was performed either with or without iv contrast medium. If the patient was scanned using both methods, the radiation dose was doubled. CTDIw of each examination protocol investigated was below the EC RDL. Performance of all scanners was satisfactory as far as CTDIw is concerned. DLP was found to be within proposed EG for brain, abdomen and pelvis examinations. The pelvis examination on the PLX was slightly above the EC RDL because of the long L and higher value of CTDIw in comparison with the other scanners. Chest DLP exceeded EG, except on the TTCT in which the lowest mAs and L were found. Since RDLs act as parameters to help identify relatively poor or inadequate use of technique, the exposure settings and the extent of the scan should be further investigated to lower the dose without affecting image quality. Clarke et al [14] also presented CTDIw and DLP results for the same examinations. Their results have a broad range of values, probably because of the larger number of scanners included in the study. Their values were well within proposed EG for both CTDIw and DLP, apart from brain CTDIw in which 50% of their results exceeded the EC RDL. For chest examination, Clarke et al's scanned volume length (range 13.4–28.7 cm) was generally lower than in the current study (range 2–32 cm), which seems to have great implication for DLP. Verdun et al [15] presented DLP results for standard abdominal examinations in the range 421–904 mGy cm. Our corresponding range in the abdominal protocol was lower (278–582 mGy cm), probably because our scanning length was 20–23 cm, which is shorter than the scanning length of 38 cm presented in Verdun's study.


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  		Table 4. Mean weighted computed tomography dose index (CTDIw) and dose–length product (DLP) results compared with European Guidelines (EG)

 
There are no RDLs for E. However, since E provides a direct estimation of radiation risk and is useful for comparison with other radiological examinations, it should always be calculated. For the purpose of this study, broad estimation of Ewas deemed to be satisfactory, since DLP is shown to have a strong correlation with E [16]. Table 5Go presents mean values of E for the examination protocols included in this study. Mean values of E are 1.4±0.3 mSv for brain, 10.9±3.4 mSv for chest, 7.1±2.0 mSv for abdomen and 9.3±2.4 mSv for pelvis examinations. This table also presents E results of Shrimpton et al [9], Poletti [17] and Clarke et al [14], which are comparable to E values found in our study. Van Unnik et al [18] presented a study of E in the Netherlands. Their results have a broad range of values (brain 0.8–5 mSv, chest 6–18 mSv, abdomen 6–24 mSv). Verdun et al [15] presented results on E delivered by four scanners for abdominal examination. Their range of E was 5.1–10.8 mSv.


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  		Table 5. Mean values of effective dose E compared with those of Shrimpton et al [9], Clarke et al [14] and Poletti [17]

 

   Discussion
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
References
 
Chest CT examinations appear to have the highest DLPs in the Euromedica Medical Center, and also exceed EG dose criteria. The large irradiation volume of investigations seems to be an important factor since CTDIw is within RDLs. Reducing the extent of the scan as much as possible, without missing any vital anatomical regions, could be a first step to lower DLP and E. Furthermore, reducing mAs of the examination protocol is also important, especially for patients who are thinner than the standard sized patient. Mayo et al [19] presented a study regarding the minimum tube current required for good image quality with the least radiation dose on CT chest examination. Results of the study indicate that the lowest mAs can be used without affecting diagnosis, despite the fact that images may be noisier. As far as the other examinations (brain, abdomen and pelvis) are concerned, the protocols utilized in our centre have CTDIw and DLP values that are well within EG dose criteria. This is encouraging, since the most important aspect of radiation protection is to have the amount of dose absorbed by the patient as low as reasonably achievable, provided that this does not affect image quality and accurate diagnosis. The CTDIw and DLP values found for each CT scanner will be used as a local reference level for each subcentre. It should be noted that CT RDLs should be monitored at certain time intervals to constantly assure optimization of the procedure. Furthermore, all examination protocols performed in each subcentre will be investigated in terms of technique and radiation dose and compared with EG RDLs, since they appear to be a very useful tool in assessing standard CT performance.

Received for publication July 4, 2000. Revision received February 26, 2001. Accepted for publication April 24, 2001.


   References
Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 

  1. Spokas JJ. Dose descriptors for computed tomography. Med Phys 1982;9:288–92.[Medline]
  2. Shope TB, Gagne RM, Johnson GC. A method of describing the doses delivered by transmission X-ray computed tomography. Med Phys 1981;8:488–95.[Medline]
  3. Shope TB, Morgan TJ, Showalter CK. Radiation dosimetry survey of computed tomography systems from ten manufacturers. Br J Radiol 1982;55:60–9.[Abstract]
  4. Calzado A, Ruiz Sanz S, Melchor M, Vano E. A comparison of measured and calculated organ doses from CT examinations. Radiat Prot Dosim 1995;57:381–5.[Abstract]
  5. Jones AP, Mott DJ, Parkinson L. Experience with a new simple method for the determination of doses in computed tomography. Radiat Prot Dosim 1992;43:139–42.[Abstract]
  6. Shrimpton PC, Wall BF. Assessment of patient dose from computed tomography. Radiat Prot Dosim 1992;43:205–8.[Abstract]
  7. European Commission. European Guidelines on Quality Criteria for Computed Tomography, Report EUR 16262. Brussels: EC, 1999.
  8. International Commission on Radiological Protection. 1990 Recommendations of the International Commission on Radiological Protection, ICRP Publication 60. Annals of the ICRP 1991; 21 (1–3). Oxford: Pergamon Press, 1991: 6–8.
  9. 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: NRPB, 1991.
  10. Jones DG, Shrimpton PC. Survey of CT practice in the UK. Part 3. Normalised organ doses calculated using Monte Carlo techniques, NRPB R250. Chilton: NRPB, 1991.
  11. Nishizawa K, Maruyama T, Takayama M, Okada M, Hachiya J, Furuya Y. Determinations of organ doses and effective dose equivalents from computed tomographic examination. Br J Radiol 1991;64:20–8.[Abstract]
  12. International Electrotechnical Commission. Medical diagnostic X-ray equipment—radiation conditions for use in the determination of characteristics, IEC 61267. Geneva: IEC, 1994.
  13. IPSM, NRPB, CoR. National protocol for patient dose measurements in diagnostic radiology. Chilton: NRPB, 1992.
  14. Clarke J, Cranley K, Robinson J, Smith S, Workman A. Application of draft European Commission reference levels to a regional CT dose survey. Br J Radiol 2000;73:43–50.[Abstract]
  15. Verdun FR, Meuli RA, Bucher G, Noel A, Stines J, Schnyder P, et al. Dose and image quality characterisation of CT units. Radiat Prot Dosim 2000;90:193–6.[Abstract]
  16. Hidajat N, Maurer J, Schroder RJ, Nunnemann A, Wolf M, Pauli K, et al. Relationships between physical dose quantities and patient dose in CT. Br J Radiol 1999;72:556–61.[Abstract]
  17. Poletti JL. Patient doses from CT in New Zealand and a simple method for estimating effective dose. Br J Radiol 1996;69:432–6.[Abstract]
  18. Van Unnik JG, Broerse JJ, Geleijns J, Jansen JThM, Zoetelief J, Zweers D. Survey of CT techniques and absorbed dose in various Dutch hospitals. Br J Radiol 1997;70:367–71.[Abstract]
  19. Mayo JR, Hartman TE, Lee KS, Primack SL, Vedal S, Müller NL. CT of the chest: minimal tube current required for good image quality with the least radiation dose. AJR 1995;164:603–7.[Abstract/Free Full Text]



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