No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Comparison of TurboFLASH- and Spinecho-R1 measurements of FeMRI-Gel-Phantoms for Verification of 3D-dose Distributions
Abstract
Recently, MR-imaging techniques have been developed to quantify radiation doses with use of Fricke-gels. Fricke-gel-dosimetry requires a precise measurement of longitudinal relaxation rates R1. In this study two MR-methods for R1-determination of irradiated Fricke-gels were compared using a spinecho- and a TurboFLASH-technique. R1 measurements were repeatedly performed in calibration vials and a Fricke-gel glassphantom which was irradiated by a bremsstrahl-spectrum of 15 MeV electrons of a linear accelerator. Measured dose distributions in the phantom were compared with pre-calculated treatment planning dose distributions. The gel was irradiated with a maximumdose of 30 Gy. An average uncertainty of less than 2.5 Gy (TurboFLASH) and 10.4 Gy (spinecho) was measured at a spatial resolution of 2×2×6mm3 (TurboFLASH) and 1×1×6mm3 (spinecho). The reproducibility and the precision of these techniques were determined.
ResearchGate has not been able to resolve any citations for this publication.
Two gels have been found to be suitable to load with ferrous sulphate solution. In these soft tissue equivalent phantoms, the absorbed dose distribution can be measured after irradiation in clinically used MR imaging equipment. The present studies were carried out using a 0.25 T NMR analyser without imaging properties. A ferrous sulphate solution, 0.05 M with respect to sulphuric acid, can be gelled with 4% gelatin to give a dosemeter which has a response which is linearly correlated (r = 0.998) with the absorbed dose in the interval 0-40 Gy. Ferrous sulphate solution can also be gelled with 1% agarose, but this gel has to be purged with oxygen to obtain a linear relationship (r = 0.997) in the same absorbed dose interval. The ferrous sulphate loaded gels have a sensitivity which is a factor of 2.2 or 4.0 times higher for gelatin and agarose, respectively, than the ordinary dosemeter solution. Because the standard deviation of background measurements is higher for the gels than for the dosemeter solution, the minimum detectable absorbed dose is about the same, or 1.0 Gy, for the two gels and the dosemeter solution. The sensitivity of the ferrous sulphate loaded gels shows no dependence on dose rate if the mean dose rate and the absorbed dose per pulse are within the limits normally used by accelerators for radiotherapy.
Rapid measurements of in vivo proton spin-lattice relaxation times (T1) in human tissues were performed by magnetic resonance imaging in a 1.5 T whole-body super-conducting MR scanner. The measurements employ serial TurboFLASH imaging (Snapshot-FLASH) with scan times for a single experiment below 4 s. Using centric phase encoding order, an appropriate fitling of the T1-parameter from images with minimum motion artifacts is possible. Comparative T1-determination with a multipoint inversion recovery and spin-echo technique was performed on phantoms containing Gd-DTPA solutions with different T1-values. We found a maximum deviation of 3.3% for T1 < 1100 ms and of 12.1% for 1100 ms < T1 < 1700 ms from the results obtained by the IR technique. In vivo measurements of T1-relaxation times were performed in white and grey matter, cerebrospinal fluid, kidney, liver, spleen, muscle, and bone marrow, and yielded values that are in good agreement with reported data.
Most algorithms for the least-squares estimation of non-linear parameters have centered about either of two approaches. On the one hand, the model may be expanded as a Taylor series and corrections to the several parameters calculated at each iteration on the assumption of local linearity. On the other hand, various modifications of the method of steepest-descent have been used. Both methods not infrequently run aground, the Taylor series method because of divergence of the successive iterates, the steepest-descent (or gradient) methods because of agonizingly slow convergence after the first few iterations. In this paper a maximum neighborhood method is developed which, in effect, performs an optimum interpolation between the Taylor series method and the gradient method, the interpolation being based upon the maximum neighborhood in which the truncated Taylor series gives an adequate representation of the nonlinear model. The results are extended to the problem of solving a set of nonlinear algebraic e
The spin-lattice relaxation rate R1(= T1(-1) of irradiated Fricke solution was studied as a function of the absorbed dose D. The R1 increases linearly with dose up to D approximately 400 Gy after which the response saturates. A model describing the R1 of a solution of either ferrous (Fe2+) or ferric (Fe3+) ions is presented; it is based on the spin relaxation of protons on water molecules in the bulk and protons on water molecules in the coordination shells of the ions with fast exchange occurring between the two water environments. All inherent relaxation parameters of the different proton groups are determined empirically at NMR frequencies of 9 and 25 MHz. An extension of the model is made to describe the spin-lattice relaxation behavior of irradiated Fricke solution. Good agreement between model predictions and experimental results is observed. The model relates the spin-lattice relaxation rate of a Fricke dosimeter to the chemical yield of ferric ion, thus potentially creating an absolute NMR dosimetry technique. Various practical aspects of the NMR-Fricke system, such as the optimal initial ferrous concentration and the NMR frequency dependence of the sensitivity, are described.
Recent investigations have shown that nuclear magnetic resonance (NMR) can be used in conjunction with a suitable chemical dosimeter to estimate the dose from ionizing radiation (Gore et al., Phys Med. Biol. 29, 1189-1197, 1984). Based on this fact it was proposed that spatial dose distributions can be measured in gels infused with the chemical dosimeter using NMR imaging. There have been few such attempts and they provided only qualitative results. In this paper, we report results demonstrating the feasibility of obtaining quantitative dose distribution measurements by this technique. It is shown that quantitative dose distribution measurements necessitate the calculation of relaxation rate maps. We have determined that the spin-spin relaxation rate is a more sensitive parameter than the spin-lattice relaxation rate. It is also demonstrated that the addition of chemical sensitizers could improve the dose sensitivity of the measured NMR parameters. The two features characterizing a photon beam, depth-dose relationship, and beam profile as measured by this technique are in good agreement with the measurements using conventional methods, ionization chambers, and film dosimetry.
The nuclear magnetic resonance (NMR) longitudinal relaxation rate R1 dose-response characteristics of a ferrous-sulphate-doped chemical dosimeter system (Fe MRI) immobilized in a gelatin matrix were explored. Samples containing various concentrations of the FeSO4 dosimeter were irradiated to absorbed doses of 0-150 Gy. R1 relaxation rates were determined by imaging the samples at a field strength of 1.5T(1H Lamor frequency of 63.8 MHz). The response of the system was found to be approximately linear up to doses of 50 Gy for all FeSO4 concentrations studied (0.1-2.0 mM). Changing concentrations in the range of 0.1-0.5 mM affected both the slope and intercept of the dose-response curve. For concentrations of 0.5-2.0 mM, the slope of the dose-response curves remained constant at approximately 0.0423 s-1 Gy-1 in the dose range of 0-50 Gy. However, the intercept of the curve continued to increase in that region, as expected, because of the additional paramagnetic ions. The reproducibility of the absorbed dose estimates for measurements made over a 22 cm field of view was found to be 5% in the range of 20-50 Gy (an uncertainty of 0.81 Gy on average), decreasing to approximately 10% in the dose range of 5-10 Gy.
In vivo measurements of proton relaxation processes in human brain tumors have been performed by magnetic resonance (MR) imaging using a whole-body superconductive MR scanner, operating at 1.5 T. The T1 and T2 relaxation time measurements were based on a combined Carr-Purcell/Carr-Purcell-Meiboom-Gill sequence with two interleaved repetition times and 32 echoes. First, comparative measurements in the imager and with the spec-trometer of relaxation times were performed on phantoms containing fluids of different T1 and T2 to evaluate accuracy. A maximum deviation of ~10% was found. Multislicing with a gap width of one slice thickness influenced the accuracy of T1 relaxation measurement. A gap width of at least two times the slice thickness was necessary for reliable determination of Tl. No influence on T2 values was observed by multislicing. Second, in human head imaging the multiexponential behavior of the T2 decay curves has been analyzed in each pixel, where the mean square deviation has been used as a criterion to discriminate between mono- and biexponential behavior. Mean values of monoexponential Tl and multiexponential T2 relaxation data for white matter, gray matter, CSF, edema, and tumor were sampled in 12 patients with brain tumors. T2 showed monoexponential behavior in white and gray matter, whereas CSF, edema, and tumor showed distinct biexponentiality. The biexponential analysis generally yields “fast” and “slow” components with T2f= 80 ±17 ms and T2S = 2,030 ± 210 ms for CSF (partial volume effect), T2f= 104 ± 25 ms and T2S= 677 ± 152 ms for edematous tissues, T2f= 97 ± 19 ms and T2S = 756 ± 99 ms for tumor tissues, respectively. Using a stepwise discriminant analysis by forward selection, the two best discriminating parameters of the multiexponential relaxation analysis for each pair of classification groups have been selected. For the discrimination of edematous and tumor tissues a retrospective overall accuracy of 94% has been found.
Snapshot fast low angle shot (FLASH) magnetic resonance (MR) imaging techniques have been developed to enable real time imaging of MR parameters. The method is based on a 64 x 128 FLASH tomogram acquired within less than 200 ms. This work describes snapshot FLASH MR using a single 180 degrees pulse prior to the acquisition of a series of FLASH images. The experiment creates continuous dynamic inversion recovery (IR) T1 contrast in successive images. The total acquisition time of 16 images displaying the IR behavior is less than 4 s. Representative snapshot FLASH IR MR images of the abdomen of healthy rats and of an implanted hepatic tumor are illustrated.
A method is described for determining the spatial distribution of radiation dose in a tissue-equivalent phantom using nuclear magnetic resonance imaging. The conversion of ferrous ions to ferric by ionising radiation alters the magnetic moment and electron spin relaxation times of the metal ion. The spin relaxation times (T1 and T2) of the hydrogen nuclei in an aqueous solution of a ferrous salt are consequently reduced substantially. These changes in T1 and T2 can be measured using standard NMR techniques. The same conversion is used in conventional Fricke dosimetry, which can be used to calibrate the technique.
A detailed methodology has been developed to verify the three-dimensional (3D) radiation dose mapping of conformal therapy radiation treatment planning by FeMRI dosimetry. A phantom that consisted of a human skull filled with a 1-mM solution of ferrous sulfate and 0.1-N sulfuric acid gelled with 7.5% (by weight) gelatin was employed. With a spherical target volume in the head phantom, five noncoplanar conformal beams were designed through the use of a 3D treatment planning system developed in-house. The phantom was irradiated with a 6-MV linear accelerator to a total dose of 25 Gy delivered to the periphery of the target volume. The phantom and a set of calibration vials were scanned simultaneously in a GE 1.5T MR imager with six different multiscan inversion-recovery pulse sequences. The values of T1 were evaluated on a pixel-by-pixel basis through the use of custom-built software on a UNIX workstation and were converted to dose using calibration data. Comparisons of dose distributions between those measured by FeMRI and those calculated by 3D treatment planning show good agreement.
Dosimetric characteristics of the FeMRI gels for various energy photon and electron beams were investigated using the Integrated Tiger Series (ITS) Monte Carlo codes. The FeMRI gels have been reported as being tissue-substitutes which can be shaped to any desired contour for three-dimensional (3D) mapping of dose distribution using magnetic resonance (MR) scanning. This study indicated that the central axis depth dose, off-axis depth dose, transverse profile at the depth of 50% of maximum dose, and energy deposition for photons and electrons in the gels are close to those in water. From these comparisons, it is concluded that FeMRI gels are suitable substitutes for water for the purposes of dose measurements of both photons and electrons used in radiotherapy.
The treatment plans for stereotactic radiosurgery employ small, circular, noncoplanar fields applied in a series of arcs, or with synchronous rotation of the accelerator gantry and patient support assembly. Primary or metastatic brain tumors and arterial-venous malformations are localized in relation to a stereotactic head frame using CT, MRI, and angiography. As x-ray doses in the range of 20-40 Gy are delivered in a single treatment, it is critical that the dose distribution produced by the accelerator accurately reflect the one developed by the treatment planning computer. Until the advent of Fricke-infused gels, whose NMR characteristics are changed by irradiation, there was no practical method for assessing the accuracy of x-ray beam positioning on a target that was localized by both CT and MRI. A stereotactic head frame was attached to a hollow glass head filled with a Fricke-infused gel. A 2-mm target point at approximately the center of this manikin was localized by CT and MRI. The head frame was then mounted to the patient support assembly of a linear accelerator, and given a dose of 40 Gy to the isocenter from 6-MV x rays using a modified version of the dynamic stereotactic radiosurgery plan developed in Montreal. Subsequent MRI showed the target point at the center of the dose distribution, thus confirming the accuracy of the stereotactic radiosurgery procedure. This demonstrated the unique characteristics of the Fricke-infused gel for the simultaneous localization of x-ray beams in three dimensions.
The chemical action of roentgen rays on dilute ferrous sulphate solutions as a measure of dose
- H Fricke
- S Morse
Fricke. H.: Morse, S.: The chemical action of roentgen rays on dilute
ferrous sulphate solutions as a measure of dose. Am. J. Roemgenol.
Radium Therapy uel. Med. 18 (1927) 430-432
Assessment of the accuracy of stereotactic rddiosurgery using Fricke-infu ed gel and MRI
- R J Schulz
- M J Maryan Ki
- G Lbbott
- J E Bond
Schulz, R.J.; Maryan ki, M.J.; lbbott, G..; Bond. J. E.: Assessment
of the accuracy of stereotactic rddiosurgery using Fricke-infu ed gel
and MRI. Med.Phys. 20 (6) (1993) 1731-1734
Spin-lattice relaxarion time measurement by means of a TurboFLASH technique
- S Biumi
- L R Schad
- B Stepanow
- W J Lorenz
BIUmI. S.; Schad. L. R.; Stepanow, B.; Lorenz, W.J.: Spin-lattice relaxarion time measurement by means of a TurboFLASH technique.
Magn.Reson.Med. 30 (1993) 289-295