A method for the systematic selection of technique factors in paediatric ct c j kotre, ph d and s p willis, dcr_british journal of radiology (2003) 76, 51-56
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A method for the systematic selection of technique factors in paediatric ct c j kotre, ph d and s p willis, dcr_british journal of radiology (2003) 76, 51-56

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A method for the systematic selection of technique factors in paediatric ct c j kotre, ph d and s p willis, dcr_british journal of radiology (2003) 76, 51-56 A method for the systematic selection of technique factors in paediatric ct c j kotre, ph d and s p willis, dcr_british journal of radiology (2003) 76, 51-56 Document Transcript

  • The British Journal of Radiology, 76 (2003), 51–56 E 2003 The British Institute of RadiologyDOI: 10.1259/bjr/53215511A method for the systematic selection of technique factorsin paediatric CTC J KOTRE, PhD and S P WILLIS, DCRRegional Medical Physics Department, Newcastle General Hospital, Newcastle-upon-Tyne NE4 6BE, UK Abstract. A method for the systematic selection of paediatric CT technique factors is described. The approach is based on the assumption that the level of image noise acceptable for a given adult CT image is also acceptable for the equivalent paediatric examination. A simple exponential attenuation model is proposed. Effective linear attenuation coefficients were initially established from a series of phantom measurements simulating head, chest and abdomen examinations at 120 kVp, then extended for a range of tube potentials and beam qualities using a beam spectral model. Application of the method is demonstrated using phantoms representing head, chest and abdomen sections for neonate and ages 1 year, 5 years, 10 years, 15 years and adult. Paediatric CT examinations are performed relatively measure to determine the perimeter of the section required,rarely in most centres, and the range of patient sizes then dividing by p to give the diameter of the equivalentencountered is large. It is therefore difficult to make a circular section. All equivalent diameters used in this paperselection of technique factors that produces a consistently are measured in this way and it should be noted that theseoptimized balance between radiation dose and image are not directly interchangeable with those derived byquality. A number of recent publications have suggested different means, e.g. from patient height and weight, inthat paediatric CT doses may be unnecessarily high in other work [5–7]. By use of the equivalent diameter, thesome cases [1, 2], information that has been reported in the mAs per slice required to give a similar signal-to-noiseUK national news [3]. Recent UK legislation requires that ratio relative to that for the adult case in CT can bethe dose of ionizing radiation is kept as low as reasonably approximated using a simple monochromatic attenuationpracticable consistent with the diagnostic purpose, and model. The relationship isspecifically requires that special attention is paid tomedical exposures of children [4]. It is also required that mAsP SP ~mAsA SA e{keff ðdA {dP Þ ð1Þdetailed examination protocols are put in place for all where dA is the standard adult equivalent diameter, e.g.diagnostic X-ray work. In this paper a method for that of ‘‘Reference Man’’ [8], dP is the paediatricselecting technique factors is suggested such that the equivalent diameter, mAsA and SA are the tube current–images produced for paediatric patients will be similar in time product per slice and slice width, respectively, for theterms of signal-to-noise ratio to the images produced in adult examination, mAsP and SP are the same quantitiesthe equivalent adult examination, the technique factors for for the paediatric examination and meff is the effectivewhich will be much better established in most centres. linear attenuation coefficient for the body segment being The approach is based on the following assumptions: scanned at the tube potential being used. Equation 1(i) The technique factors for the equivalent adult includes the slice widths SA and SP specifically, as these may change between adult and paediatric examinations examination are known. given the smaller size of paediatric patients. In addition,(ii) The image signal-to-noise ratio in the equivalent manipulation of slice width is used to modify the total adult examination is suitable for diagnosis in the number of incident X-ray photons in the experiments paediatric case. reported below. It should be noted, however, that reducing(iii) The attenuation and relative proportions of body slice width without reducing mAs per slice does not result tissues are similar in adults and paediatrics. in a dose saving where contiguous scanning is employed,(iv) The tube voltage is not altered between the adult indeed the patient dose may well increase on single-slice and paediatric versions of the examination. scanners owing to the increased proportion of penumbra(v) The reconstruction filter used in the paediatric present at small slice widths. In all cases the slice widths examination is not radically different from that refer to the reconstructed image slice thickness rather than used in the adult examination (this point is the collimated thickness of the fan beam. Equation 1 is based upon the assumption that the discussed further below). problem may be adequately modelled in terms of simple A useful approximation to patient cross-sectional size is exponential attenuation. Clearly the patient exit spectrumthe equivalent diameter, d. For the purposes of this paper, is not monochromatic, but the heavy beam filtrationthis is measured directly on the subject using a tape normally used in CT plus the filtering effect of the patient makes the monochromatic assumption more appro-Received 4 December 2000 and in final form 6 August 2002, accepted priate for CT than for some other radiological imaging12 August 2002. techniques.The British Journal of Radiology, January 2003 51
  • C J Kotre and S P WillisInitial check of model validity technique factors are not commonly altered for gender. Axial scans of the phantoms were carried out on the A simple initial experiment was carried out to confirm Siemens Somatom Plus 4 scanner at 120 kVp, 12 mm Althe validity of the attenuation model proposed above. total filtration, using nominal 10 mm slice widths. Effec-Uniform Perspex CT dosimetry phantoms of 32 cm and tive attenuation coefficients for other tube potentials and16 cm were axially scanned on a Siemens Somatom Plus 4 scanner filtrations are derived below.scanner (Siemens AG, Munich, Germany) at 120 kVp. The To obtain values of meff for each body segment, the levelbeam filtration of this scanner was estimated from measure- of quantum noise within a region of uniform tissue-ments of half value layer (HVL) using data generated with equivalent material was measured for each of the sevena beam spectral simulation programme [9]. The filtration phantoms using fixed technique factors. For the chest,was estimated to be 12 mm Aluminium (AL), comparable noise measurements were confined to the near water-with the 10 mm Al minimum quoted by the manufacturer.Substituting the phantom diameters into Equation 1 and equivalent material of the phantom heart region. Fiveusing a value of 0.22 cm21 obtained from the spectral circular areas of interest were analysed on each image andsimulation programme for the linear attenuation coeffi- the geometric mean value of the pixel SD calculated. Grosscient of Perspex at a depth of 16 cm, gave a ratio between non-uniformities in the phantoms were avoided and effortsthe paediatric (16 cm) mAs–slice width product and the were made to use the same location for areas of interestadult (32 cm) mAs–slice width product of 0.03. This large when comparing noise measurements from the sameratio was approximated in practice using 300 mAs, 10 mm section.slice width for the 32 cm phantom and 50 mAs, 2 mm slice To measure the level of quantum noise within thewidth for the 16 cm phantom. Multiple noise samples were phantoms accurately, any additional noise componentstaken from each image to obtain a mean noise level in due to inhomogeneity of the phantom material at smallterms of pixel standard deviation (SD) and an estimate of scales and system noise had to be eliminated. Initialthe standard error on the mean (sem). For the 32 cm measurements showed that the material of the commercialdiameter phantom, the SD was 13.1 (sem50.25) and for paediatric phantoms was indeed inhomogeneous atthe 16 cm diameter phantom scanned at 0.03 of the adult small scales, giving rise to an additional noise component.mAs–slice width product, the SD was 13.6 (sem50.16). Although the adult phantoms were more uniform, theyThis level of agreement was felt to be encouraging, given also produced a small additional noise component. Allthe very large reduction in patient dose per slice implied by noise measurements were therefore made at two widelythis choice of technique factors. separated values of mAs so that the quantum noise and phantom inhomogeneity components could be separated using the relationshipExperimental derivation of effective linearattenuation coefficients  1 Ap2 {p2 1 2 2 ps ~ ð2Þ Measurements of perimeter were made directly on a set A{1of male and female adult anthropomorphic phantoms(Alderson Research Labs, Stamford, CT) based on the where sS is the pixel SD representing the non-quantumdimensions of ‘‘Reference Man’’ [8] and paediatricphantoms (ATOM Ltd, Riga, Latvia) [10] constructed to noise sources (including both phantom and system noise),the same dimensions as the software phantoms of Christy s1 is the pixel SD measured at the high value of mAs, s2 is[11]. These were then converted to the diameter of a circle the pixel SD measured at the low value of mAs and A iswith the same circumference. These dimensions are taken the ratio between the high and low mAs values used. Theas typical for neonate and ages 1 year, 5 years, 10 years, 15 value of this ratio in the experiments was 10.4. The pureyears and adult in the experimental work that follows quantum noise contribution for each measurement was(Table 1). then calculated by quadrature subtraction of the phantom Head, chest and abdomen examinations were performed and system noise, ss established for that phantom fromon the set of phantoms in order to obtain meff values for Equation 2, from the total noise SD. As the meanhead, chest and abdomen sections. Adult male and female Hounsfield unit (HU) value for the paediatric phantomsresults were combined to produce a result equivalent to was found to be significantly above the expected value foran adult hermaphrodite on the assumption that adult CT water equivalent tissue (in the range 70–90 HU), an additional correction for this anomalously high attenua- tion was necessary. The relative numbers of photonsTable 1. Typical values of equivalent diameter for use in contributing to the image were corrected to what theyEquation 1 would have been for zero HU and the quantum noise estimations were adjusted accordingly.Age (years) Equivalent diameter (cm) The reciprocal of the fractional change in quantum Head Abdomen Chest noise with equivalent diameter for each body segment was then squared to obtain the fractional change in number ofAdult 17.6 23.4 27.6 X-ray photons forming the image. Finally, the values of15 17.2 20.5 23.2 meff were obtained by plotting the natural logarithm of the10 15.9 17.5 19.45 15.3 16.9 16.9 relative number of photons forming the image against the1 13.7 13.4 13.2 equivalent diameter of the phantom section. The slopes ofNeonate 11.6 9.9 9.9 the fits to these experimental points gave the required effective attenuation coefficients.52 The British Journal of Radiology, January 2003
  • Selection of technique factors in paediatric CTExtension of meff values using an X-ray spectral scanners were used, a Siemens Somatom Plus 4 (estimatedsimulation programme total filtration 12 mm Al) and a Toshiba Asteion (Toshiba Corporation, Tochigi, Japan) (estimated total filtration A beam spectrum simulation programme [9] was 4 mm Al). Head, chest and abdomen sections for each ofemployed to extend the experimentally derived meff the available ages were scanned at 80 kVp, 120 kVp andvalues over a more useful range of tube potentials and 140 kVp (Somatom Plus 4) and 120 kVp (Asteion), usingbeam filtrations. The programme was used to model the a 10 mm nominal slice width. Adult mA-slice widthincident X-ray beam and the attenuation of the various product values were based on those found in a regionalbody sections in terms of thicknesses of water and bone CT dosimetry survey, and the values for the paediatricfor the head, water for the abdomen and water and air in scans were obtained from Equation 1 using the equivalentthe case of the chest. The parameters of the model were diameters in Table 1 and the effective linear attenuationadjusted to match the measured beam HVL and coefficients from Tables 2–4. A realistic field of view toexperimentally derived effective attenuation coefficient suit the size of section being scanned was chosen in eachfor the head, chest and abdomen sections at 120 kVp case. As the paediatric phantoms are hermaphrodite,(constant potential, 10 ˚ target angle) as measured above. averaged male and female adult measurements were usedThe kVp and total tube filtration were then varied to to produce an equivalent hermaphrodite adult.produce the required range of meff values, which are given Where the mAs value required for the smaller phantomin Tables 2–4 for the head, abdomen and chest examina- sections fell below the minimum mAs available for a fulltions, respectively. In each table the experimentally derivedvalue is shown in bold type and the values extrapolated rotation of the scanner (37.5 mAs for the Somatom Plus 4,from it in normal type. 30 mAs for the Asteion), the number of photons per slice was further decreased by reducing the slice width. The relationship between slice width and relative number ofExperimental validation photons detected was previously established for each scanner in a separate series of water calibration phantom A further series of scans of the anthropomorphic scans. Where slice width reduction was employed, aphantoms was made to verify that application of the further correction was required to account for the increasemethod of paediatric factor selection proposed above in measured phantom noise component as the effect ofresults in images with a similar signal-to-noise ratio to noise averaging across the slice width is reduced. Noisethose produced by the equivalent adult examination over a measurements were again corrected to remove the effectsrealistic range of tube potentials and beam filtrations. Two of phantom local inhomogeneity and the elevated attenuation of the paediatric phantom material.Table 2. Values of effective attenuation coefficient for use inEquation 1 (head examination)Tube potential Total filtration (mm Al)(kVp) Results 3 6 9 12 Figure 1 shows an example of the trend in mean noise 80 0.222 0.219 0.216 0.214 levels measured in the experimental validation for the100 0.208 0.205 0.203 0.201 abdominal examination. This example is for 120 kVp at120 0.198 0.196 0.194 0.192 12 mm Al total tube filtration. Error bars of ¡2 standard140 0.190 0.188 0.186 0.185 errors on the mean, calculated for each point, are shown together with the best fit line of zero gradient. The noise results are plotted against nominal phantom age, with theTable 3. Values of effective attenuation coefficient for use inEquation 1 (abdomen examination) hermaphrodite adult point being placed at age 30 years. This is an arbitrary choice but is consistent with the age atTube potential Total filtration (mm Al) which the relationship between body weight and age(kVp) reaches a plateau [8]. Complete results for the abdomen, 3 6 9 12 head and chest validations are given in Table 5. 80 0.210 0.208 0.207 0.206 Despite a careful analysis of propagation of errors100 0.197 0.195 0.194 0.194 through the experimental process, including the applica-120 0.187 0.186 0.184 0.184 tion of Equation 2 and the subtraction of the structure140 0.178 0.178 0.177 0.176 noise from the total measured noise, it was found that the zero gradient line did not pass through the error bars of ¡2 standard errors on the mean for a number of theTable 4. Values of effective attenuation coefficient for use in experimental points. This would seem to indicate thatEquation 1 (chest examination) some sources of error have not been adequately accounted for. Possible candidates for the ‘‘missing’’ error includeTube potential Total filtration (mm Al)(kVp) variations in the choice of noise sampling region between 3 6 9 12 the scans taken to establish the phantom structure noise 80 0.168 0.165 0.163 0.162 and the subsequent validation scans and variations in100 0.157 0.154 0.153 0.152 noise between individual scans. Despite this, no systematic120 0.149 0.147 0.146 0.145 trend in noise level with phantom age is seen across the140 0.143 0.141 0.140 0.138 results taken as a whole, indicating broad agreement with the expected flat noise levels.The British Journal of Radiology, January 2003 53
  • C J Kotre and S P Willis Figure 1. Experimentally measured mean noise level plotted against phantom age for abdomen examina- tions at 120 kVp, 12 mm Aluminium. Error bars of ¡2 standard errors on the mean calculated for each point are shown together with the best fit line of zero gradient.Table 5. Summary of image noise results, (mean pixel standard deviation) for the experimental validation on phantoms for head,abdomen and chest sections, (standard error on the mean in parentheses) Phantom age (years) 0 1 5 10 15 AdultHead a aS80 kV 5.0 (0.13) 5.8 (0.16) 6.2 (0.15) 6.6 (0.14) 6.5 (0.19) 6.0 (0.08) aS120 kV 1.6 (0.21) 2.0 (0.33) 2.9 (0.18) 2.4 (0.27) 2.7 (0.21) 2.9 (0.09) aS140 kV 1.4 (0.23) 0.7 (0.80) 1.6 (0.28) 1.7 (0.32) 2.1 (0.27) 2.4 (0.10) aT120 kV 1.7 (0.56) 1.7 (0.72) 1.3 (0.68) 1.6 (0.72) n/e 3.1 (0.20)Abdomen aS80 kV 8.7 (0.41) 9.4 (0.13) 9.1 (0.44) 10.0 (0.28) 9.1 (0.31) 10.5 (0.29)S120 kV 3.5 (0.50) 4.2 (0.23) 4.5 (0.45) 4.2 (0.37) 4.6 (0.32) 4.2 (0.28)S140 kV 3.3 (0.30) 3.1 (0.19) 3.4 (0.34) 3.3 (0.38) 3.2 (0.41) 3.1 (0.29)T120 kV 2.4 (0.70) 2.7 (0.33) 2.0 (0.67) 2.4 (0.72) 2.2 (0.72) 2.4 (0.41)Chest a a aS80 kV 13.0 (0.53) 10.2 (0.31) 9.7 (0.98) 10.6 (0.13) 11.5 (0.26) 9.0 (0.29) aS120 kV 4.5 (0.37) 3.5 (0.36) 4.9 (0.51) 4.6 (0.56) 4.8 (0.22) 4.8 (0.24)S140 kV 3.1 (0.63) 3.8 (0.25) 3.9 (0.60) 3.9 (0.60) 4.0 (0.24) 4.2 (0.25)T120 kV 2.5 (0.51) 2.0 (0.47) 2.3 (1.06) 2.3 (1.06) 2.9 (0.34) 3.5 (0.30)S, Siemens Somaton Plus 4 scanner; T, Toshiba Asteion scanner; n/e, quadrative sun could not be evaluated.a Result falling outside two standard errors on the mean of a best fit line of zero gradient.Application of the method in practice ratio of paediatric to adult mAs–slice width product for abdominal examinations as a function of phantom It is envisaged that the relationship of Equation 1 would age, based on the average equivalent diameters given inbe applied as follows. Measure the perimeter of a Table 1. Curves are shown for 80 kVp, 100 kVp, 120 kVprepresentative part of the section to be scanned using a and 140 kVp, and are based on the values of meff fromtape measure on each individual patient and divide this Table 2 corresponding to a total filtration of 9 mm Al.by p to obtain the equivalent diameter. Substitute the The curves shown are spline fits to calculated values at agepaediatric equivalent diameter into Equation 1 along with 0 years, 1 year, 5 years, 10 years and 15 years and 100%the appropriate value of meff from Tables 2–4 and a value for the nominal adult 30-year-old. At first sight the curvesfor dA, taken from Table 1 for the appropriate body seg- are surprising both for the very small proportions of adultment. The results of this calculation could be tabulated orshown graphically as mAs against perimeter for standard mAs–slice width product required for younger childrenexaminations. and for the very rapid increase from the teens to adult- hood. The curve shapes do, however, seem more reason- able when the rates of change in the equivalent diametersDiscussion from Table 1 are considered together with the exaggera- The magnitude of the dose saving expected from the use tion of those changes resulting from the application of theof Equation 1 is illustrated in Figure 2, which shows the exponential relationship given in Equation 1. The noise54 The British Journal of Radiology, January 2003
  • Selection of technique factors in paediatric CT Figure 2. Fraction of adult mAs–slice width product per slice required for a paediatric abdomen CT image with the same noise level plotted against phantom age, using the standard dimensions from Table 2. The curves are spline fits to points for 80 kV, 100 kV, 120 kV and 140 kV at ages neonate, 1 year, 5 years, 10 years and 15 years, and adult placed at age 30 years.results obtained from phantom body sections of realistic findings are consistent with those of Rogella et al [12]dimensions illustrate that the choice of mAs–slice width who have reported no loss of diagnostic information forproduct value based on these relationships does produce paediatric chest CT performed at 25 mAs per slice, andimages with similar noise content to the adult case. Huda et al [13] who found that the dose per slice for head Figure 2 serves to demonstrate the relationship between scans of newborns could be reduced to 40% of the adultmAs–slice width product and age resulting from the use value.of Equation 1 for the standard phantoms used. It is not Difficulty was experienced in obtaining the low mAsrecommended that this kind of relationship be used as the values required for the experimental validation on one ofbasis of a clinical scheme to select CT technique factors the CT scanners used, and changes in slice width had to bedependent on age alone, as the variation in patient size for employed to simulate the effect of applying lower doses toany given age could result in non-diagnostic images for the phantoms. For reduction in paediatric techniquelarger children. factors to the low levels proposed, an extended lower Additional factors that could affect the noise content in range of tube current settings would be of value.the paediatric image include choice of a different imagereconstruction filter, use of zoom reconstruction and useof different choices of beam hardening correction. The Conclusionvariations offered will change depending on the model ofCT scanner used, but experimental tests on choice of adult A method for the systematic selection of paediatricand paediatric filters (with the same ‘‘sharpness’’), zoom CT technique factors has been described using a simplereconstruction and a range of fields of view showed no attenuation model based on measured patient equivalentsignificant difference in image noise content. Some diameter. Images produced for paediatric patients scannedscanners change beam filtration for small fields of view. using technique factors selected in this manner will beSince the rates of change of meff with total filtration are similar, in terms of signal-to-noise ratio, to the imagessmall (Table 2), differences in filtration between the adult produced for the equivalent adult examination, theand paediatric cases within the range 3–12 mm Al will technique factors for which will be much better establishedproduce a maximum error on the estimated mAs–slice in most centres. Using this method, a significant reductionwidth product of 12%. from present patient dose levels for paediatric CT would All experimental work described employed single slice be expected.axial scanning. The method derived can, however, beapplied directly to spiral scanning protocols, as thequantum noise level in these images will still be governed Referencesby the mAs–slice width product. Changes to the pitch ofthe spiral run will alter the average patient dose per slice, 1. Donnelly LF, Emery KH, Brody AS, Laor T, Gylys-Morin VM,but not the number of photons collected per tube rotation. Anton CG, et al. Minimising radiation dose for paediatric body applications of helical CT. AJR 2001;176:303–6.For multislice spiral scanners the method is also applicable 2. Paterson A, Frush DP, Donnelly LF. Helical CT of theprovided the same combination of multiple slices and pitch body: are settings adjusted for paediatric patients? AJRis maintained between the adult and paediatric cases. 2001;176:297–301. The results obtained indicate that significant dose 3. BBC News Online. Worry over childrens CT scans. BBC,savings could be made in paediatric CT without loss of 2001. http://news.bbc.co.uk/hi/english/health/newsid_1126000/diagnostic quality by using the approach described. These 1126776.stmThe British Journal of Radiology, January 2003 55
  • C J Kotre and S P Willis4. The Ionising Radiations (Medical Exposure) Regulations 10. Varchenya V, Gubatova D, Sidorin V, Kalnitsky S. 2000. London: HMSO, 2000. Children’s heterogeneous phantoms and their application in5. Lindskoug BA. The Reference Man in diagnostic radiology roentgenology. Radiat Prot Dosim 1993;49:353–6. dosimetry. Br J Radiol 1992;65:431–7. 11. Christy M. Mathematical phantoms representing children of6. Hart D, Wall BF, Shrimpton PC, Dance DR. The establish- various ages for use in estimates of internal dose. NUREG/ ment of reference doses in paediatric radiology as a function CR-1159, ORNL/NUREG/TM-367. Oak Ridge, TN: Oak of patient size. Radiat Prot Dosim 2000;90:235–8. Ridge National Laboratory, 1980.7. Chapple C-L, Broadhead D, Faulkner K. A phantom based 12. Rogella P, Stover B, Scheer I, Juran R, Gaedicke G, Hamm B. method for deriving typical patient doses from measurements Low-dose spiral CT: applicability to paediatric chest imaging. of dose–area product on populations of patients. Br J Radiol Pediatr Radiol 1999;29:565–9. 1995;68:1083–6. 13. Huda W, Chamberlin CC, Rosenbaum AE, Garrisi W.8. ICRP. Report of the task group on Reference Man. Radiation doses to infants and adults undergoing head CT Publication 23. Oxford, UK: Pergamon Press, 1975. examinations. Med Phys 2001;28:393–9.9. Institute of Physics and Engineering in Medicine. Catalogue of diagnostic x-ray spectra and other data. Institute of Physics and Engineering in Medicine Report 78. York, UK: IPEM, 1997.56 The British Journal of Radiology, January 2003