RESUMEN
The Fletcher Suit Delclos (FSD) ovoids employed in intracavitary brachytherapy (ICB) for cervical cancer contain shields to reduce dose to the bladder and rectum. Many treatment planning systems (TPS) do not include the shields and other ovoid structures in the dose calculation. Instead, TPSs calculate dose by summing the dose contributions from the individual sources and ignoring ovoid structures such as the shields. The goal of this work was to calculate the dose distribution with Monte Carlo around a Selectron FSD ovoid and compare these calculations with radiochromic film (RCF) and normoxic polymer gel dosimetry. Monte Carlo calculations were performed with MCNPX 2.5.c for a single Selectron FSD ovoid with and without shields. RCF measurements were performed in a plane parallel to and displaced laterally 1.25 cm from the long axis of the ovoid. MAGIC gel measurements were performed in a polymethylmethacrylate phantom. RCF and MAGIC gel were irradiated with four 33 microGy m2 h(-1) Cs-137 pellets for a period of 24 h. Results indicated that MCNPX calculated dose to within +/- 2% or 2 mm for 98% of points compared with RCF measurements and to within +/- 3% or 3 mm for 98% of points compared with MAGIC gel measurements. It is concluded that MCNPX 2.5.c can calculate dose accurately in the presence of the ovoid shields, that RCF and MAGIC gel can demonstrate the effect of ovoid shields on the dose distribution and the ovoid shields reduce the dose by as much as 50%.
Asunto(s)
Braquiterapia/instrumentación , Protección Radiológica/instrumentación , Radiometría/métodos , Planificación de la Radioterapia Asistida por Computador/métodos , Neoplasias del Cuello Uterino/radioterapia , Braquiterapia/métodos , Diseño de Equipo , Análisis de Falla de Equipo , Femenino , Dosimetría por Película , Geles/efectos de la radiación , Humanos , Método de Montecarlo , Polímeros/efectos de la radiación , Traumatismos por Radiación/prevención & control , Protección Radiológica/métodos , Dosificación Radioterapéutica , Reproducibilidad de los Resultados , Sensibilidad y EspecificidadRESUMEN
Continuous arterial spin labeling (CASL) is a technique for performing quantitative perfusion measurements without the need for exogenous contrast agent administration. This technique has seen limited use in the clinic due to problems of poor sensitivity and the potential for artifacts. In addition, CASL requires the application of long-duration radiofrequency pulses and the acquisition of a large number of images, which can cause difficulties when implemented on commercial MR scanners. This work details our experience in implementing CASL on a commercial MR scanner for the measurement of cerebral blood flow, including pitfalls regarding hardware, radiofrequency energy deposition, and practical application in human subjects. Results of studies to determine the optimal acquisition procedures are also presented.
Asunto(s)
Encéfalo/anatomía & histología , Encéfalo/irrigación sanguínea , Circulación Cerebrovascular/fisiología , Aumento de la Imagen/métodos , Interpretación de Imagen Asistida por Computador/métodos , Imagen por Resonancia Magnética/métodos , Adulto , Arterias/anatomía & histología , Arterias/fisiología , Humanos , Imagen por Resonancia Magnética/instrumentación , Reproducibilidad de los Resultados , Sensibilidad y Especificidad , Marcadores de SpinRESUMEN
The objective of this work was to implement a motion-detection algorithm on a commercial real-time functional magnetic resonance imaging (fMRI) processing package for neurosurgical planning applications. A real-time motion detection module was implemented on a commercial real-time processing package. Simulated functional data sets with introduced translational, in-plane rotational, and through-plane rotational motion were created. The coefficient of variation (COV) of the center of intensity was used as a motion quantification metric. Coefficients of variation were calculated before and after image registration to determine the effectiveness of the motion correction; the limits of correctability were also determined. The motion-detection module allowed for real-time quantification of the motion in an fMRI experiment. Along with knowledge of the limits of correctability, this enables determination of whether an experiment needs to be reacquired while the patient is in the scanner. This study establishes the feasibility of using real-time motion detection for presurgical planning fMRI and establishes the limits of correctable motion.
Asunto(s)
Encéfalo/anatomía & histología , Encéfalo/fisiología , Aumento de la Imagen/métodos , Interpretación de Imagen Asistida por Computador/métodos , Imagen por Resonancia Magnética/métodos , Movimiento/fisiología , Técnica de Sustracción , Algoritmos , Artefactos , Encéfalo/cirugía , Sistemas de Computación , Humanos , Imagenología Tridimensional/métodos , Imagen por Resonancia Magnética/instrumentación , Modelos Biológicos , Procedimientos Neuroquirúrgicos/métodos , Reconocimiento de Normas Patrones Automatizadas/métodos , Fantasmas de Imagen , Reproducibilidad de los Resultados , Sensibilidad y Especificidad , Cirugía Asistida por Computador/métodosRESUMEN
The regional cerebral blood flow (rCBF) values determined using continuous arterial spin labeling (CASL) are subject to several sources of variability, including natural physiologic variations, sensitivity to the input parameters, and the use of different quantification models. To date, a thorough analysis of the impact of input parameters and the choice of quantification model has not been performed. These sources of variability were investigated through computer simulations using bootstrap techniques on actual CASL data. Coefficients of variation for representative single voxels were 6.7% for gray matter and 29% for white matter, and for eight-voxel regions of interest they were 4.5% for gray matter and 23% for white matter. Comparison of nine CASL quantification models showed differences in gray matter rCBF values of up to 42%. An analysis of the sensitivity of the rCBF to input parameters for each of the nine quantification models demonstrated that accurate quantification of the inversion efficiency, tissue and arterial blood longitudinal relaxation times, and transit times were critical in calculating precise rCBF values. The large potential variations in rCBF and the effect of the choice of quantification model suggest that interpreting absolute rCBF values in CASL studies can be challenging and requires great care.
Asunto(s)
Circulación Cerebrovascular , Imagen por Resonancia Magnética/métodos , Marcadores de Spin , Adulto , Simulación por Computador , Femenino , Humanos , Procesamiento de Imagen Asistido por Computador , Sensibilidad y Especificidad , Procesamiento de Señales Asistido por ComputadorRESUMEN
The Fletcher Suit Delclos (FSD) ovoids employed in intracavitary brachytherapy (ICB) for cervical cancer contain shields to reduce dose to the bladder and rectum. Many treatment planning systems (TPS) do not include the shields and other ovoid structures in the dose calculation. Instead, TPSs calculate dose by summing the dose contributions from the individual sources and ignoring ovoid structures such as the shields. The goal of this work was to calculate the dose distribution with Monte Carlo around a Selectron FSD ovoid and compare these calculations with radiochromic film (RCF) and normoxic polymer gel dosimetry. Monte Carlo calculations were performed with MCNPX 2.5.c for a single Selectron FSD ovoid with and without shields. RCF measurements were performed in a plane parallel to and displaced laterally 1.25 cm from the long axis of the ovoid. MAGIC gel measurements were performed in a polymethylmethacrylate phantom. RCF and MAGIC gel were irradiated with four 33µGym2h-1 Cs-137 pellets for a period of 24 h. Results indicated that MCNPX calculated dose to within ±2% or 2 mm for 98% of points compared with RCF measurements and to within ±3% or 3 mm for 98% of points compared with MAGIC gel measurements. It is concluded that MCNPX 2.5.c can calculate dose accurately in the presence of the ovoid shields, that RCF and MAGIC gel can demonstrate the effect of ovoid shields on the dose distribution and the ovoid shields reduce the dose by as much as 50%.