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Physica Medica (2008) 24, 149e158 available at www.sciencedirect.com journal homepage: http://intl.elsevierhealth.com/journals/ejmp ORIGINAL PAPER Polymer gel dosimetry on a multislice computed tomography scanner: Effect of changing parameters on CTDI B. Hill a,b,*, A.J. Venning c, C. Baldock b a Medical Physics, The Canberra Hospital, ACT Health, P.O. Box 11, Woden, Canberra, ACT 2606, Australia Institute of Medical Physics, School of Physics, University of Sydney, NSW 2006, Australia c Wellington Blood and Cancer Centre, Wellington Hospital, Private Bag 7902, Wellington South, New Zealand b Received 23 July 2007; received in revised form 20 November 2007; accepted 22 November 2007 Available online 4 March 2008 KEYWORDS Computed tomography; Normoxic polymer gel dosimetry; Radiation; Dosimeter; CT; MRI; Dose profile Abstract Polymer gel dosimetry undertaken on a multislice CT scanner provides an alternative method to conventional dosimetry measurements. Polymer gel dosimeters were used to measure CT radiation doses and compared to TLD and ionization chamber measurements in different diameter phantoms. CTDI was investigated for each of these phantoms for a range of mAs (100e400 mAs), tube voltage (100e135 kV) and nominal slice width (2e32 mm). Linear fits of the CTDI values for mAs show for the smallest phantom diameter an increase in CTDI of 60% for both TLD and polymer gel dosimeters. A similar increase in CTDI of 50% at 100 kVp and 100% for 135 kVp was also noted. It was also shown that slice width variation measured with either polymer gel or TLD was greatest with the smallest slice widths. In summary, it was found that polymer gels can be used as an alternative dosimeter to TLD for the determination of SWDP and subsequent CTDI calculations. ª 2007 Published by Elsevier Ltd on behalf of Associazione Italiana di Fisica Medica. Introduction Polymer systems for the use of radiation dosimetry were first proposed as early as 1954, where Alexander et al. discussed the effects of ionising radiation on polymethylmethacrylate * Corresponding author. Medical Physics, The Canberra Hospital, ACT Health, P.O. Box 11, Woden, Canberra, ACT 2606, Australia. Tel.: þ61 2 62442256. E-mail address: [email protected] (B. Hill). [1]. Following this, Hoecker and Watkins [2] in 1958 investigated the dosimetry of radiation-induced polymerization in liquids, and in 1961, Boni [3] used polyacrylamide as a gamma dosimeter. In 1992, a polymer formulation was suggested that consisted of acrylamide (AA) and N,N0 -methylene-bis-acrylamide (bis) monomers infused in an aqueous agarose matrix. This polymer gel formulation did not have the diffusion limitations of Fricke gels [4]. In 1994, Maryanski et al. modified the formulation by replacing agarose with gelatine and named the, now commercial, product BANG 1120-1797/$ - see front matter ª 2007 Published by Elsevier Ltd on behalf of Associazione Italiana di Fisica Medica. doi:10.1016/j.ejmp.2007.11.005 150 gel [5]. A procedure for manufacturing what is now commonly referred to in the literature as PAG, an acronym for polyacrylamide gel, was later described [6]. Upon exposure of the radiological tissue equivalent [7] PAG dosimeter to radiation, polymerization of the co-monomers is induced by the free radical products of water radiolysis resulting in a 3D insoluble polymer network infused within the gel matrix. Since the extent of the resulting polymer structure is a function of dose, MRI can be used to evaluate the relaxation rates which can be related to dose by means of an R2 (1/T2) versus dose calibration curve. The polymerization in the irradiated PAG is known to be inhibited by oxygen and therefore, requires that the polymer gel be produced in an oxygen free or hypoxic environment [6]. Over the years many formulations of polymer gels have been published [8e10] demonstrating its use mainly in radiotherapy dosimetry [11e14]. In 2001, a method was described for developing polymer gels in an oxygen or normoxic environment. The formulation became known as MAGIC polymer gel which is an acronym for Methacrylic acid (MAA), Ascorbic acid (AscA), Gelatine, Initiated by Copper [15]. The addition of the AscA oxygen scavenger into the formulation resulted in one of the major limitations of polymer gel dosimeters being overcome, allowing the polymer gel to be manufactured under normal atmospheric conditions upon the bench top. De Deene et al. investigated the various components of the original MAGIC polymer gel formulation and proposed some alternative oxygen scavengers, in particular tetrakis (hydroxymethyl) phosphonium chloride (THPC), which was originally proposed in 1996 by Billingham [16] and from which MAGAT polymer gel was developed [17,18]. Further formulations were also proposed such as MAGAS and PAGAS [17,18] for which radiological tissue equivalence was investigated [19]. In 2005, Venning et al. investigated a normoxic formulation based on the original hypoxic PAG polymer gel formulation of AA and bis dissolved in a gel matrix with the novel addition of THPC and hydroquinone (HQ). This formulation was given the acronym PAGAT polymer gel [7]. PAGAT polymer gel was subsequently shown to be useful for both radiotherapy and diagnostic dosimetry with high spatial resolution being achieved when imaged with MRI in the relevant clinical dose ranges [14]. In addition to the MRI evaluation of polymer gels other evaluation modalities such as optical tomography [20e23], vibrational spectroscopy [24,25], ultrasound [26e30] and Xray CT [31e33] have been studied. Further investigations of normoxic polymer gel dosimeters using X-ray CT were subsequently pursued due to the potential advantage, due to the availability and ease of use of X-ray CT compared to MRI [34,35]. It was whilst undertaking these studies that the discovery was made that normoxic polymer gels polymerize due to diagnostic X-ray CT doses. A further investigation demonstrated that polymer gel dosimeters could be used for the measurement of the diagnostic X-ray slice width dose profile (SWDP) on helical CT scanners [36]. It was shown that normoxic polymer gel dosimeters potentially provide an alternative to conventional dosimeters for diagnostic CT scanner SWDP measurements and subsequent CT dose index (CTDI) calculation. Furthermore, polymer gel dosimeters also provide an advantage over conventional dosimeters since the dose profile can potentially be acquired in any direction allowing evaluation of B. Hill et al. the complete volume when the delivered CT scanner dose profile is recorded in 3D in the irradiated phantom. Given the potential application of this dosimeter in acceptance testing and for research purposes a review of various CT dosimetry studies was assessed in order to further compare polymer gels to more conventional methods of dosimetry on a multislice CT scanner. It is an accepted requirement to measure CTDI from CT scanners as a part of acceptance and quality assurance procedures [37e39]. Current protocols recommend the use of simulated adult head and body phantoms (of perspex diameters 16 and 32 cm, respectively). This may be a limitation in the current protocols as it does not consider the relationship between diameter and dose (or CTDI) and does not take into account dose determination at alternative diameters. Several authors have identified the importance of the CT scanning parameters, such as tube current time (mAs), tube voltage (kV), nominal slice width, pitch and length of scan on radiation dose when measured with a 100 mm ionization chamber as a function of phantom diameter. Siegel et al. evaluated the effects of varying the CT parameters, mAs and kV on radiation dose and image quality on a multislice CT scanner with a 100 mm ionization chamber placed in phantoms of diameters 8, 16, 24, 32 cm and a 28 18 cm oval phantom [40]. Boone et al. characterised CT contrast-to-noise (CNR) factors and thus image quality for similar CT parameters using an ionization chamber for various containers of diameters 10, 13, 16, 20, 25, 28 and 32 cm also on a multislice CT scanner [41]. Nickoloff et al. identified the exponential relationship of CTDI with an ionization chamber placed in phantoms of diameters 6, 10, 16, 24 and 32 cm for consecutive axial scans and helical scans of variable pitch [42]. Huda et al. identified the relationships of CT CNR ratios for CT parameters, mAs and kV in phantoms of varying weight and diameters [43]. Generally these studies found that through the optimisation of CT protocols for varying phantom diameters significant dose reductions could be achieved. Traditionally both thermoluminescent detectors (TLD) and radiographic film have been used for determination of SWDP [37,44]. In the case of TLD, SWDP can be determined by stacking the 1D dosimeters in the z-direction of the CT gantry to enable 2D measurement of the X-ray beam profile and therefore SWDP. SWDP measurements at depth in a phantom provide a means of determining the dose profile for a given CT X-ray collimation setting. The SWDP can be directly compared to the X-ray detection systems effective acquisition width in order to determine co-incidence. This is undertaken to determine z-efficiency of the CT scanner and is useful in ensuring overshoot or misalignment of the X-ray beam to X-ray detector system is minimized [44]. TLDs can also be used to measure the relative or absolute distribution of dose as a result of phantom diameter and other CT parameters. Cheung et al. assessed the effects of integrating lengths on the calculation of CTDI in standard head and body phantoms of 16 and 32 cm diameters [45]. McNitt-Gray et al. investigated the relative effects of collimation and pitch in a cylindrical body phantom of 32 cm diameter and found that the use of smaller collimations increases dose and that higher pitch lowers dose [46]. Tsai et al. investigated the relationship of scatter-to-primary dose from TLD SWDP measurements in standard head and Polymer gel dosimetry on a multislice CT scanner body phantoms from which a method of modeling organ dose using Monte Carlo was reported [47]. TLDs have also been used to determine relative dose in anthropomorphic phantoms of different diameters identifying the relationship between phantom diameter or patient size and dose [48,49]. Dixon used TLDs to measure SWDP for multislice CT scanners in standard head and body phantoms providing experimental evidence to support observed limitations in the use of 100 mm length ionization chambers for making measurements and subsequent calculation of CTDI [50]. In the case of radiographic film, a 2D dosimeter, SWDP is acceptable for use either in air or sandwiched in plastic phantom material [37,44]. More recently computer radiography equipment has been used in measuring SWDP on a CT scanner [51]. Other types of detectors have also been used experimentally for CT SWDP measurements. Nakonechny et al. demonstrated SWDP measurement using a diamond detector [52]. The purpose of the current research was to compare the use of polymer gel dosimeters to measure SWDP and subsequently calculate CTDI and compare with other conventional dosimeters. Changes in CTDI were investigated as a function of mAs, tube voltage and nominal slice width for different diameter phantoms. Materials and methods Each of the polymer gel dosimeters were prepared, poured into cylindrical phantom containers and allowed to set. Each polymer gel dosimeter was then irradiated using the CT scanner in axial mode, and left to polymerize before evaluation using MRI. The polymerized area of the polymer gels were visualized and evaluated using a single MRI slice and from that slice an associated R2 map calculated. Relative dose profiles were determined in any direction within the MRI R2 map from which the integrated dose under the profile was used to calculate CTDI. In order to calibrate the R2 map as a function of dose in the SWDP plane of the CT scanner an ionization chamber was used. By placing the ionization chamber at the centre of a ‘non-polymer’ gelatine gel phantom identical in geometry to the irradiated polymer gel phantom and exposing it to the same CT protocol the dose at the centre of polymer gel could be determined. At this point the method of calibration of the polymer gel varies from that previously described [36]. This method does not require the use of a water filled phantom, instead has the ionization chamber at the centre of the gelatine phantom for which dose measured with the ionization chamber is determined and directly related to the number of single axial slices required to deliver that particular dose to the phantom (Table 1). For all subsequent polymer gels the calibration result obtained using the ionization chamber for cumulative single axial slices was used to relate the R2 map of the exposed polymer gel to dose and therefore enable the calculation of CTDI. Polymer gel dosimeter preparation Polymer gel dosimeters were manufactured according to a formulation and method described previously [7]. The PAGAT polymer gel formulation by % mass consisted of 151 Table 1 CTDI(100C) obtained using an ionization chamber in a 6 cm diameter phantom for cumulative single axial slices with the following CT parameters: 135 kV, 100 mAs and 16 mm slice width No of slices Ionization chamber dose (mGy) Ionization chamber CTDI (mGy) 0 1 2 5 10 20 40 0.0 20.8 41.6 103.8 208.0 416.5 832.9 0.0 34.3 68.7 172.9 345.6 691.0 1381.2 4.5% bis (Sigma Aldrich, Sydney), 4.5% AA (Sigma Aldrich, Sydney), 5% gelatine (300 bloom) (Sigma Aldrich, Sydney) and 86% de-ionized water, 10 mM THPC (Sigma Aldrich, Sydney) and 5 mM HQ (Sigma Aldrich, Sydney). Costs of amounts to make several batches of polymer gels are in the hundreds of dollars. All components were mixed on the bench top under a fume hood. The gelatine was left to soak in de-ionized water for 10 min before heating commenced. The gel was heated to 48 C and then bis was added and stirred until dissolved. The AA was added followed by THPC and HQ. The length of time for preparation was 3 h. The polymer gel was left to cool to 30 C and then poured into the cylindrical Barex containers (CIRS, Virginia, USA) and sealed with an airtight cap and placed in a refrigerator for 12 h before irradiation. The cylindrical Barex containers varied in length with the largest 14 cm, medium 12 cm and smallest 10 cm. They also varied in diameter with largest 16 cm, medium 9 cm and smallest 6 cm. TLD calibration and measurement LiF:MgTi TLD-100 was used to measure SWDP for the subsequent calculation of CTDI. Each TLD had dimensions of 0.35 0.35 0.9 mm3 and energy dependence to within 10.0% for the energy range of 60e140 keV [53]. Calibration of the TLD was achieved by exposing each TLD using an Xray machine (BuckyDiagnost, Philips, Netherlands) with additional filtration to make the beam quality similar to that of the CT scanner (6.8 mmAl). Each TLD was exposed under the same conditions (100 cm sourceeaxisedistance, 10 10 cm2 field size, 135 kV, 50 mAs). The output of the X-ray machine was determined with a 0.3 cc model 20 3 RadCal ionization chamber used in conjunction with a RadCal 2025 radiation monitor (RadCal Corp., Monrovia, CA, USA). The resulting exposure was converted to dose within the dosimeter making it possible to apply a sensitivity correction factor of 0.97 5.0% mGy to each TLD. The annealing of TLDs was performed with a Fimel automatic TLD oven (Vélizy, France) and readout with a Bicron 5500 (Saint-Gobain Crystals and Detectors, France) TLD reader which had a cycle time of 4 h for annealing and 2 h to read 50 TLD. For SWDP measurements using TLDs, the TLDs were stacked into a holder (Fig. 1) and placed centrally into each of the gelatine filled phantoms. Up to 25 TLDs at 152 B. Hill et al. 200 Ion chamber in gel phantom 100 mAs PAGAT phantom 100 mAs TLD in gel phantom 100 mAs Ion chamber in gel phantom 200 mAs PAGAT phantom 200 mAS TLD in gel phantom 200 mAs Ion chamber in gel phantom 400 mAs PAGAT phantom 400 mAs TLD in gel phantom 400 mAs 180 CTDI (mGy) 160 140 120 100 80 60 40 20 6 7 8 9 10 11 12 13 14 15 16 Diameter (cm) Figure 1 chamber. Image of containers, TLD holder and ionization a time were stacked about the central part of the TLD holder and a maximum of 3 at a time were stacked at equal intervals throughout the rest of the holder. The length of time preparation of TLD in holder was approximately 1 h. Each phantom was subsequently placed centrally in the CT scanner with the TLDs stacked in the direction of the z-axis and exposed. Three single exposure rotations from the CT scanner were made each time and the TLD results averaged. The individually determined TLD doses were used to calculate the integrated area under the SWDP using Origin (Microcal) and subsequently CTDI was calculated. Ionization chamber calibration and measurement The RadCal 2025 radiation monitor and model 20 5100 mm CT ionization chamber was placed centrally into each of the gel filled phantoms and each phantom was centrally placed in the CT scanner and exposed. The ionization chamber was calibrated to within 5% of its expected reading for the energy range of 60e140 kV by RadCal Corp. (Monrovia, CA, USA). Three measurements were made with the ionization chamber for each of the CT scanner protocols used. An average dose value and standard deviation was calculated from these measurements. CT scanner All CT doses were measured on a Toshiba Aquilion Multislice CT scanner (Toshiba Corporation, Japan). The total slice widths that a user can define in a single rotation are 2.0, 4.0, 8.0, 12.0, 16.0, 24.0, or 32.0 mm. Total slice widths will be used to describe the nominal slice widths from this point on in this paper. Further information about this specific CT scanner acquisition details can be obtained from a previous paper on optimisation of protocols for this CT scanner [35]. The HVL was measured with the tube in a fixed position under the table using a 0.3 cc model 20 3 RadCal ionization chamber used in conjunction with a RadCal 2025 Figure 2 CTDI(100C) as a function of diameter at currents 100, 200 and 400 mAs for an ionization chamber, TLD and PAGAT. Fitted lines are for PAGAT only. radiation monitor (RadCal Corp., Monrovia, CA, USA) and found to be 6.8 mmAl at 135 kV. When tube voltages were compared 100, 120 and 135 kV were used while 100 mAs and 16 mm nominal slice width were kept constant. When mAs were compared 100, 200 and 400 mAs were used while tube voltage 135 kV and 16 mm nominal slice width were kept constant. When nominal slice widths were compared, tube voltage of 135 kV and mAs of 100 mAs were kept constant. For the calibration of the polymer gel using the ionization chamber at the centre of the polymer gel a 16 mm nominal slice width was used with 135 kV and 100 mAs. Table 2 Mean CTDI(100C) and standard deviation (in brackets) of ionization chamber, TLD and PAGAT for different CT parameters in 6, 9 and 16 cm diameter phantoms Set CT parameters Diameter (cm) Slice Tube Current 6 9 16 width voltage time Mean CTDI(100C) values (SD) (mm) (kVp) (mAs) 16 16 16 135 135 135 100 200 400 34.4 (1.9) 27.1 (0.3) 20.3 (0.8) 73.1 (2.6) 56.5 (3.2) 49.7 (0.5) 135.4 (4.6) 107.9 (1.5) 79.1 (4.6) 16 16 16 100 120 135 100 100 100 21.5 (0.5) 30.1 (0.8) 39.3 (1.3) 12.5 (1.3) 11.6 (0.5) 19.9 (0.9) 18.4 (0.9) 26.9 (0.4) 20.3 (0.8) 2 4 8 12 16 24 32 135 135 135 135 135 135 135 100 100 100 100 100 100 100 97.7 66.3 44.9 39.1 34.0 34.8 31.6 94.8 53.5 38.9 31.6 28.7 26.9 20.6 (2.0) (1.4) (0.9) (0.8) (0.7) (0.7) (0.7) (1.7) (0.9) (0.5) (0.4) (0.5) (0.1) (0.3) 45.6 35.9 25.2 23.0 20.4 21.1 19.7 (0.9) (0.8) (0.6) (0.5) (0.4) (0.5) (0.4) Polymer gel dosimetry on a multislice CT scanner 153 Table 3 CTDI(100C) linear fits for the PAGAT polymer gel across the range of phantoms as a function of tube current time and nominal slice widths Set CT parameters Linear regression Slice width (mm) Tube voltage (kVp) Current time (mAs) Gradient Intercept Correlation co-efficient 16 16 16 135 135 135 100 200 400 1.2 0.2 2.4 1.0 5.5 0.5 38.8 2.0 85.5 10.8 161.1 5.2 0.99 0.92 0.99 2 4 8 12 16 24 32 135 135 135 135 135 135 135 100 100 100 100 100 100 100 1.8 0.3 1.2 0.1 1.8 0.3 1.6 0.1 1.3 0.1 1.3 0.4 1.2 0.1 54.4 3.3 38.2 0.4 54.4 3.4 46.7 1.3 40.6 0.9 40.5 4.3 38.2 0.4 0.99 0.99 0.99 0.99 0.99 0.96 0.99 MRI scanner and image processing the gel filled phantom and used to convert the relative R2 map profile to a relative dose profile [36]. Polymer gel phantoms were scanned in Siemens Vision and Avanto 1.5 T MRI whole body scanners using circularly polarised head coils between 12 and 24 h following irradiation. T2 weighted base images were acquired using a 64 echoes multiple slice multiple spin-echo pulse sequence with the following parameters: TE 20 e 1280 ms, TR 4 s, FOV 256 256 mm2 in the Siemens Vision and a 32 echoes multiple slice multiple spin-echo pulse sequence with the following parameters: TE 40e1280 ms, TR 4 s, FOV 256 256 mm2 in the Siemens Avanto. Acquisition time was 25 min with 5 min set up time. The T2 relaxation signal intensity base images of the coronal slices were transferred to a PC and the images were processed with modified Matlab (Mathworks) software to calculate T2 image maps [54]. The R2 image map was calculated and a profile of the slice was calculated and exported to Excel (Microsoft) and Origin (Microcal). An R2 versus dose calibration curve was obtained from the ionization chamber measurements at the centre of CTDI calculations CTDI100C for the PAGAT polymer gel dosimeter dose profile was calculated by integrating under the dose profile, converting to dose in soft tissue (correction factor 0.94 (R/cGy)) and multiplying by 100 mm to take into account the length of the dose profile and dividing this value by the nominal slice width [36]. CTDI will henceforth be used to refer to CTDI100C. CTDI for the TLD dose profile was also calculated by integrating under the dose profile, converting to dose in soft tissue and multiplying by 100 mm to take into account the length of the dose profile and dividing this value by the nominal slice width. CTDI for the ionization chamber, which is CTDI for the 100 mm ionization chamber in the centre of each of the phantoms and corrected for dose in soft tissue was determined according to the literature [38,55]. 45 2 mm nominal slice width 4 mm nominal slice width 8 mm nominal slice width 12 mm nominal slice width 16 mm nominal slice width 24 mm nominal slice width 32 mm nominal slice width 90 CTDI (mGy) 80 70 60 35 30 25 50 20 40 15 30 Ion chamber in gel phantom 100 kVp PAGAT phantom 100 kVp TLD in gel phantom 100 kVp Ion chamber in gel phantom 120 kVp PAGAT phantom 120 kVp TLD in gel phantom 120 kVp Ion chamber in gel phantom 135 kVp PAGAT phantom 135 kVp TLD in gel phantom 135 kVp 40 CTDI (mGy) 100 10 20 6 6 8 10 12 14 16 Diameter (cm) Figure 3 CTDI(100C) as a function of diameter at nominal slice widths 2, 4, 8, 12, 16, 24 and 32 mm for PAGAT phantoms. 7 8 9 10 11 12 13 14 15 16 Diameter (cm) Figure 4 CTDI(100C) as a function of diameter at voltages 100, 120 and 135 kVp for an ionization chamber, TLD and PAGAT. Fitted lines are for PAGAT only. 154 Table 4 B. Hill et al. CTDI(100C) linear fits for the PAGAT polymer gel across the range of phantoms as a function of tube voltage Set CT parameters Single exponential decay Slice width (mm) Tube voltage (kVp) Current time (mAs) Decay constant Offset Amplitude Correlation co-efficient 16 16 16 100 120 135 100 100 100 1.0 1.7 2.7 11.0 17.2 20.7 3027.0 407.0 186.0 1.0 1.0 1.0 z-Efficiency For each of the nominal slice widths a Catphan 500 (The Phantom Laboratory, Cambridge NY, USA) was used to measure the imaged slice width of the X-ray detector system. The Catphan 500 has angled lengths of wire that appear on the CT image and through calculation the imaged slice width could be obtained. The percentage difference of the imaged slice width obtained from the Catphan 500 and measured SWDP obtained from the full-width half-maximum (FWHM) values of either the TLD or polymer gel were used for comparison to obtain z-efficiency for each of the nominal slice widths for the multislice CT scanner. produce type A uncertainty for the polymer gel. Similar type A uncertainty can be considered for TLD e.g. evaluation of the reader and temperature drift in the annealing cycle [58]. The ionization chamber has much greater reproducibility and therefore, the type A uncertainty is reduced significantly compared to both the polymer gel and TLD. Type B uncertainty for all of the dosimeters was considered to approximate a Gaussian distribution. The combined estimated uncertainty for each of the dosimeters was 0.5% for the ionization chamber, 3% for TLD and 3% for the polymer gel. The plotted uncertainty on each of the data points is the absolute value of uncertainty for that data point. Results and discussion Uncertainty Tube current time (mAs) Uncertainty associated with the determination of dose for each of the dosimeters was estimated following the general guidelines of the International Organisation of Standardisation [56]. Standard uncertainties are classified into types A or B. Type A uncertainties are associated with the distribution of measured values around the mean and type B uncertainties are such that they cannot be determined from repeated measurements. The polymer gel uncertainties were from several sources e.g. the method of calibration from the ionization chamber, production of the polymer gel, the output of the CT scanner varying at the time the polymer gel data is recorded, evaluation variation from the MRI such as temperature drift or non-uniformity of the radiofrequency [14,57]. The combined uncertainty of these sources of uncertainty and others were combined to TLD PAGAT 100 80 b Normalised Dose Normalised Dose a Fig. 2 shows CTDI as a function of diameter for each of the dosimeters when exposed to varying mAs. The mean CTDI values for the dosimeters and standard deviation of results are in Table 2. The fitted lines in Fig. 2 are for the PAGAT polymer gel as a function of phantom diameter with the linear fits summarized in Table 3. The slope of the linear fits increases with decreasing mAs. The effects of mAs become more significant for the smallest phantom diameter with an increase in CTDI from 16 to 6 cm diameter phantom of 60%. Optimisation of mAs based on weight or patient diameter has been suggested by several authors in order to reduce dose and maintain acceptable image quality [40,43,59]. The data from both PAGAT polymer gel and TLDs indicates that optimisation of mAs is necessary and a function of 60 40 80 60 40 20 20 0 0 0 20 40 60 Distance (mm) 80 100 TLD PAGAT 100 0 20 40 60 80 100 Distance (mm) Figure 5 (a) A 16 mm Slice Width Dose Profile for TLD and PAGAT polymer gel in the centre of a 6 cm, and (b) 16 cm diameter phantoms. Polymer gel dosimetry on a multislice CT scanner 155 Table 5 For each of the nominal slices the z-efficiency was determined from TLD and PAGAT FWHM compared to the Catphan 500 Nominal slice width (mm) TLD FWHM (mm) PAGAT FWHM (mm) Catphan FWHM (mm) z-Efficiency (%) 2 4 8 12 16 24 32 4.0 7.8 12.0 14.5 18.8 27.5 35.4 4.2 8.0 11.6 15.0 18.8 28.0 36.1 4.7 5.4 7.4 11.0 13.5 18.1 23.0 111.9 67.5 63.8 73.3 71.8 64.6 63.7 associated with the gel itself but with the evaluation method used, in this case MRI. The image resolution used for determining the R2 map and subsequent dose map was from a 1 mm pixel resolution limiting the dose measurement to 1 mm, similar to that of the TLD. This can be overcome by using a finer pixel resolution in the MRI evaluation or an alternative evaluation technique. The fitted lines shown in Fig. 3 are for the PAGAT polymer gel as a function of phantom diameter with the linear fits summarized in Table 3. The correlation co-efficient remain above 0.99 indicating a strong correlation for all but the 24 mm slice thickness. The gradient increases for smaller nominal slice widths and the CTDI increases by 45% for the smaller nominal slice widths and smallest phantom. Tube voltage patient diameter in order to achieve lower CTDI. Polymer gel dosimeters irradiated on a multislice CT scanner supports the previously demonstrated relationship of increasing CTDI as a result of increasing mAs shown with TLD [40]. Generally for Fig. 2 it was noted that with fixed tube voltage and increasing mAs, CTDI increases with decreasing phantom diameter as a result of less attenuating material. Nominal slice width Fig. 3 shows CTDI as a function of diameter for only the PAGAT polymer gel dosimeter when exposed to varying slice widths. The mean CTDI values and standard deviation for ionization chamber, TLD and PAGAT polymer gels are in Table 2. The greatest standard deviation occurs for the 2 mm slice width and at the smallest diameter. Considerable difference was noted between the ionization chamber, PAGAT polymer gel and TLD results at the smallest slice width for which PAGAT and TLD are both consistently lower than the ionization chamber values. This occurs because both TLD and polymer gel are limited in measuring dose at the smaller slice widths. TLDs were stacked together to obtain the dose profile with each TLD having a width of 0.9 mm that limits the overall dose measurement level to 0.9 mm or two stacked TLD across the FWHM of the 2 mm slice width. Two points were not enough to achieve an adequate representation of the SWDP and thinner TLD would have been more appropriate for this slice width. The polymer gel limitation with the 2 mm slice width is not Fig. 4 shows CTDI as a function of diameter for each of the dosimeters when exposed to varying tube voltages. The mean CTDI values for the dosimeters and standard deviation of results are in Table 2. The logarithmic relationship of CTDI to phantom diameter for tube voltage should hold true for mono-energetic energies due to linear attenuation [60]. As the CT scanner produces a spectrum of energy with a mean energy of approximately 70 kV for example at 120 kVp the logarithmic relationship may result in a weaker correlation. The logarithmic relationship shown in Fig. 4 and summarized in Table 4 was a good representation as the correlation co-efficient was 1.0. The results are consistent with a previous study that found it was reasonable to model the relationship of CTDI to phantom diameter as a logarithmic function [40]. Both TLDs and PAGAT polymer gels demonstrate this relationship with an increase in CTDI with tube voltage. The increase in CTDI at 100 kVp was 50% from the 16 to 6 cm phantom diameter and 100% for 135 kVp. The effect of tube voltage becomes more significant with lower tube voltages due to increasing scatter and has the greatest impact on the smallest diameter phantom. SWDP and z-efficiency Fig. 5 shows SWDP measured centrally with TLD and PAGAT polymer gel in a 6 and 16 cm diameter phantom for the 16 mm nominal slice. The 16 cm diameter phantom shows a greater scatter contribution relative to the 6 cm Table 6 For each of the nominal slices the CTDI(100C) was determined from an ionization chamber and TLD in a 16 cm diameter gel phantom and from PAGAT Nominal slice width (mm) Uncertainty TLD CTDI(100C) Uncertainty PAGAT CTDI(100C) (mGy) Uncertainty Ion chamber (mGy) CTDI(100C) (mGy) 2 4 8 12 16 24 32 52.7 35.0 25.2 23.5 21.2 21.3 20.0 0.3 0.2 0.1 0.1 0.1 0.1 0.1 44.5 36.1 24.7 24.1 20.1 20.9 20.1 1.3 1.0 0.7 0.7 0.6 0.6 0.6 39.5 36.6 25.7 21.3 19.7 21.0 19.1 1.2 1.1 0.8 0.6 0.6 0.6 0.6 156 B. Hill et al. 120 b 120 100 100 Normalised Dose Normalised Dose a 80 60 40 80 60 40 20 20 0 0 0 10 20 30 40 50 60 0 20 Figure 6 40 60 80 100 120 140 160 180 Distance (mm) Distance (mm) (a) PAGAT polymer gel dose profile across the 16 mm nominal slice width in a 6 cm, and (b) 16 cm diameter phantoms. diameter phantom. Table 5 summarises the results for both TLD and PAGAT polymer gel SWDP for all nominal slice widths and shows the calculated z-efficiency of polymer gel to range between 63.7 and 73.3% for all but the nominal 2 mm slice width. For the 2 mm nominal slice width the z-efficiency was 111.9% with the polymer gel measured slice width less than the imaged. The maximum difference of the TLD and polymer gel FWHM was 5% for the 2 mm nominal slice width and less than that for all other nominal slice widths. Both TLD and polymer gel overestimate the slice width for the 2 mm nominal slice due to the size of the TLD used and the evaluation parameters of the polymer gel. The TLD and polymer gel overlie each other in most instances. The benefit of the polymer gel over the TLD is that the number of data points that can be obtained is dependent on the evaluation image resolution not the number of physically stacked TLD. Some of the nominal slice widths were identified as outside of the manufacturers specifications and subsequently corrected as a result of this work. Table 6 shows the results of the calculated CTDI data obtained with TLD and polymer gel for each of the nominal slice widths as determined from the SWDP and for comparison CTDI calculated for an ionization chamber in air is also shown. The uncertainty approximately doubles for decreasing nominal slice for all dosimeters. The maximum difference of 33.4% between CTDI occurs for the smallest nominal slice width. The CTDI values for the other nominal slice widths vary between 2 and 8%. Dose profile Fig. 6 shows a dose profile measured centrally with PAGAT polymer gel in a 6 and 16 cm diameter phantom for the 16 mm nominal slice across the axial slices in the xey plane. Variation in relative dose occurs across both of the phantoms. For the 16 cm diameter phantom the variation increases from 3.0 to 10.0%, respectively, from 32 to 2 mm nominal slice width. This effect is reduced for the smaller diameter phantom. For the 6 cm diameter phantom the difference increases from 1.0 to 5.0%, respectively, from the 32 to 2 mm nominal slice width. Conclusion The aim of this paper was to compare PAGAT polymer gel dosimeters to conventional TLD and ionization chamber measurements for the purpose of multislice CT scanner dosimetry and to investigate CTDI as a function of chosen CT parameters. Polymer gel dosimeters were shown to support the previously demonstrated relationship of increasing CTDI with increasing mAs measured with TLD [40]. It has also been shown to be suitable for measurement of dose and subsequently CTDI for changing mAs. The effect of tube voltage was shown to become more significant with lower tube voltages. This was also consistent with TLD measurements under the same conditions. 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