| 1. |
- Baldoc, C, et al.
(författare)
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Dose resolution in radiotherapy polymer gel dosimetry: effect of echo spacing in MRI pulse sequence
- 2001
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Ingår i: Physics in Medicine and Biology. - : IOP Publishing. - 1361-6560 .- 0031-9155. ; 46:2, s. 449-460
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Tidskriftsartikel (refereegranskat)abstract
- In polymer gel dosimetry using magnetic resonance imaging, the uncertainty in absorbed dose is dependent on the experimental determination of T2. The concept of dose resolution (Dpdelta) of polymer gel dosimeters is developed and applied to the uncertainty in dose related to the uncertainty in T2 from a range of T4 encountered in polymer gel dosimetry. Dpdelta is defined as the minimal separation between two absorbed doses such that they may be distinguished with a given level of confidence, p. The minimum detectable dose (MDD) is Dpdelta as the dose approaches zero. Dpdelta and the minimum detectable dose both give a quantifiable indication of the likely practical limitations and usefulness of the dosimeter. Dpdelta of a polyacrylamide polymer gel dosimeter is presented for customized 32-echo and standard multiple-spin-echo sequences on a clinical MRI scanner. In evaluating uncertainties in T2, a parameter of particular significance in the pulse sequence is the echo spacing (ES). For optimal results, ES should be selected to minimize Dpdelta over a range of doses of interest in polymer gel dosimetry.
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| 2. |
- Bäck, Sven, et al.
(författare)
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Investigation of the NMR relaxation rate dose-response of a ceric sulphate dosimeter
- 2002
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Ingår i: Applied Radiation and Isotopes. - 0969-8043. ; 56:6, s. 895-899
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Tidskriftsartikel (refereegranskat)abstract
- The relationship between the radiation absorbed dose and the NMR longitudinal and transversal relaxation rates, R-1 and R-2, respectively, of a ceric sulphate dosimeter was examined. By adding copper sulphate, the R-1 and R-2 dose-responses were found to be linear up to 60 kGy with dose sensitivities of 13 x 10(-6) and 15 x 10(-6) s(-1) Gy-1, respectively. There is thus the potential for a three-dimensional ceric dosimeter for high dose applications. provided a suitable gelling substance is used.
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| 3. |
- Engert, JC, et al.
(författare)
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5 ' flanking variants of resistin are associated with obesity
- 2002
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Ingår i: Diabetes. - : American Diabetes Association. - 1939-327X .- 0012-1797. ; 51:5, s. 1629-1634
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Tidskriftsartikel (refereegranskat)abstract
- Diabetes and obesity have long been known to be related. The recently characterized adipocyte hormone resistin (also called FIZZ3/ADSF) has been implicated as a molecular link between impaired glucose tolerance (IGT) and obesity in mice. A search for sequence variants at the human resistin locus identified nine single-nucleotide polymorphisms (SNPs) but no coding variants. An investigation into the association of these SNPs with diabetes and obesity revealed two 5' flanking variants (g.-537 and g.-420), in strong linkage disequilibrium, that are associated with BMI. In nondiabetic individuals from the Quebec City area and the Saguenay-Lac-St-Jean region of Quebec, the g.-537 mutation (allelic frequency = 0.04) was significantly associated with an increase in BMI (P = 0.03 and P = 0.01, respectively). When the data from these two populations were combined and adjusted for age and sex, both the g.-537 (odds ratio [OR] 2.72, 95% Cl 1.28-5.81) and the g.-420 variants (1.58, 1.06-2.35) were associated with an increased risk for a BMI greater than or equal to30 kg/m(2). In contrast, in case/control and family-based study populations from Scandinavia, we saw no effect on BMI with either of these promoter variants. No association was seen with diabetes in any of the population samples.
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| 4. |
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| 5. |
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| 6. |
- Lepage, M, et al.
(författare)
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Magnetization transfer imaging for polymer gel dosimetry
- 2002
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Ingår i: Physics in Medicine and Biology. - 1361-6560. ; 47:11, s. 1881-1890
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Tidskriftsartikel (refereegranskat)abstract
- Off-resonance RF pre-saturation was used to obtain contrast in MRI images of polymer gel dosimeters irradiated to doses up to 50 Gy. Two different polymer gel dosimeters composed of 2-hydroxyethyl-acryl ate or methacrylic acid monomers mixed with N, N'-methylene-bisacrylamide (BIS), dispersed in an aqueous gelatin matrix were evaluated. Radiation-induced polymerization of the co-monomers generates a fast-relaxing insoluble polymer. Saturation of the polymer using off-resonance Gaussian RF pulses prior to a spin-echo read-out with a short echo time leads to contrast that is dependent on the absorbed dose. This contrast is attributed to magnetization transfer (MT) between free water and the polymer, and direct saturation of water was found to be negligible under the prevailing experimental conditions. The usefulness of MT imaging was assessed by computing the dose resolution obtained with this technique. We found a low value of dose resolution over a wide range of doses could be obtained with a single experiment. This is an advantage over multiple spin echo (MSE) experiments using a single echo spacing where an optimal dose resolution is achieved over only very limited ranges of doses. The results suggest MT imaging protocols may be developed into a useful tool for polymer gel dosimetry.
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| 7. |
- Lepage, M, et al.
(författare)
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Modelling of post-irradiation events in polymer gel dosimeters
- 2001
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Ingår i: Physics in Medicine and Biology. - : IOP Publishing. - 1361-6560 .- 0031-9155. ; 46:11, s. 2827-2839
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Tidskriftsartikel (refereegranskat)abstract
- The nuclear magnetic resonance (NMR) spin-spin relaxation time (T2) is related to the radiation-dependent concentration of polymer formed in polymer gel dosimeters manufactured from monomers in an aqueous gelatin matrix. Changes in T2 with time post-irradiation have been reported in the literature but their nature is not fully understood. We investigated those changes with time after irradiation using FT-Raman spectroscopy and the precise determination of T2 at high magnetic field in a polymer gel dosimeter. A model of fast exchange of magnetization taking into account ongoing gelation and strengthening of the gelatin matrix as well as the polymerization of the monomers with time is presented. Published data on the changes of T2 in gelatin gels as a function of post-manufacture time are used and fitted closely by the model presented. The same set of parameters characterizing the variations of T2 in gelatin gels and the increasing concentration of polymer determined from FT-Raman spectroscopy are used successfully in the modelling of irradiated polymer gel dosimeters. Minimal variations in T2 in an irradiated PAG dosimeter are observed after 13 h.
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| 8. |
- Lepage, M, et al.
(författare)
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The relationship between radiation-induced chemical processes and transverse relaxation times in polymer gel dosimeters
- 2001
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Ingår i: Physics in Medicine and Biology. - : IOP Publishing. - 1361-6560 .- 0031-9155. ; 46:4, s. 1061-1074
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Tidskriftsartikel (refereegranskat)abstract
- The effects of ionizing radiation in different compositions of polymer gel dosimeters are investigated using FT-Raman spectroscopy and NMR T2 relaxation times. The dosimeters are manufactured from different concentrations of comonomers (acrylamide and N,N'-methylene-bis-acrylamide) dispersed in different concentrations of an aqueous gelatin matrix. Results are analysed using a model of fast exchange of magnetization between three proton pools. The fraction of protons in each pool is determined using the known chemical composition of the dosimeter and FT-Raman spectroscopy. Based on these results, the physical and chemical processes in interplay in the dosimeters are examined in view of their effect on the changes in T2. The precipitation of growing macroradicals and the scavenging of free radicals by gelatin are used to explain the rate of polymerization. The model describes the changes in T2 as a function of the absorbed dose up to 50 Gy for the different compositions. This is expected to aid the theoretical design of new, more efficient dosimeters, since it was demonstrated that the optimum dosimeter (i.e, with the lowest dose resolution) must have a range of relaxation times which match the range of T2 values which can be determined with the lowest uncertainty using an MRI scanner.
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| 9. |
- Trapp, J V, et al.
(författare)
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An experimental study of the dose response of polymer gel dosimeters imaged with x-ray computed tomography
- 2001
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Ingår i: Physics in Medicine and Biology. - : IOP Publishing. - 1361-6560 .- 0031-9155. ; 46:11, s. 2939-2951
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Tidskriftsartikel (refereegranskat)abstract
- Changes in the linear attenuation coefficient of polymer gel dosimeters post-irradiation enable the imaging of dose distributions by x-ray computed tomography (CT). Various compositions of polymer gel dosimeters manufactured from acrylamide (AA), and N,N'-methylene-bis-acrylamide (BIS) comonomers and gelatin or agarose gelling agents were investigated. This work shows that increasing the comonomer concentration increases the CT-dose sensitivity of the polymer gel dosimeter. This can be further increased by replacing gelatin with agarose. Varying the gelatin concentration however does not significantly change the CT-dose sensitivity. Among the compositions studied, dose resolution (D(delta)95%) was found to be optimal for polymer gel dosimeters comprising 5% gelatin, 3% AA, 3% BIS and 89% water.
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