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A prototype small animal positron emission tomography (PET) scanner for mouse brain imaging has been developed at UC Davis. The new scanner uses tapered detector arrays with depth of interaction (DOI) measurement. In this paper, we present an efficient system model for the tapered PET scanner using matrix factorization and a virtual scanner geometry. The factored system matrix mainly consists of two components: a sinogram blurring matrix and a geometrical matrix. The geometric matrix is based on a virtual scanner geometry. The sinogram blurring matrix is estimated by matrix factorization. We investigate the performance of different virtual scanner geometries. Both simulation study and real data experiments are performed in the fully 3D mode to study the image quality under different system models. The results indicate that the proposed matrix factorization can maintain image quality while substantially reduce the image reconstruction time and system matrix storage cost. The proposed method can be also applied to other PET scanners with DOI measurement.

A procedure for the measurement of spatial dose rate distribution of beta particles emitted by 186Re-liposomes in tumoral tissue, using HS GafChromic films, is presented. HNSCC xenografts were intratumorally injected with 3.7 or 11.1 MBq of 186Re-liposomes, and planar gamma camera images were acquired to determine the liposome retention in the tumor. After imaging, rats were sacrificed and tumors were excised and processed in slices; HS film sections were placed between slices and the tumor lobe was reassembled. Tumors and films were kept in the dark at 4°C for 18 h. After irradiation, films were removed and response was read using a transmission scanner. Films were analyzed to determine two-dimensional spatial dose rate distributions and cumulative dose volume histograms. Dose rate distributions were quantified using a 60Co calibration curve, the 186Re physical half-life, and a perturbation factor that takes into account the effect of the film protective layer. Dose rate distributions are highly heterogeneous with maximal dose rates about 0.4 Gy h−1 in tumors injected with 3.7 MBq and 1.3 Gy h−1 in tumors injected with 11.1 MBq. Dose volume histograms showed dose distributed in more than 95% and 80% of the tumor when injected with the lower and the higher activity, respectively. The described procedures and techniques have shown the potential and utility of HS GafChromic film for determination of dose rate distributions in solid tumors injected intratumorally with 186Re-liposomes. The film’s structure and the liposomes’ biodistribution must be taken into account to obtain quantitative dose measurements. [ABSTRACT FROM AUTHOR]

The optically stimulated luminescence (OSL) response of Al2O3:C to high doses of gamma or beta irradiation can be used to predict the response of this material to charged particles as a function of particle fluence, particle energy and/or linear energy transfer (LET). In particular, it is predicted that track interaction effects at high particle fluences should result in linear-sublinear growth of the OSL signal. Similar considerations also predict a dependence of the fluence at which sublinearity starts upon the energy of the particles. In this work the OSL response of Al2O3:C to low-energy charged particles was investigated using protons (1, 2 and 4 MeV), carbon ions (13 MeV) and oxygen ions (10 MeV). The sublinear growth predicted above was qualitatively confirmed, but the energy dependence prediction was not. Furthermore, the efficiency of OSL production in the material after charged particle irradiation, compared to that obtained for gamma irradiation, is determined from the dose-response curves by fitting to a simple saturating exponential function. The efficiency values so obtained using this method are compared with those obtained from a conventional single-point measurement in the linear portion of the curve and found to be in good agreement. In general, the efficiency decreases as the LET of the particle increases. The present data are compared with published data obtained using high-energy charged particles and the results show that the efficiency is not a unique function of LET. [ABSTRACT FROM AUTHOR]

Los sistemas más modernos de planeación de tratamiento en radiocirugía y radioterapia utilizan imágenes de tomografía computarizada (CT) para corregir el cálculo de la dosis por inhomogeneidades en los tejidos. Para garantizar un tratamiento de alta precisión es importante calibrar el tomógrafo, es decir, obtener la relación entre unidades Hounsfield (HU) y densidades electrónicas (re para un amplio número de tejidos. Una calibración adecuada de un sistema de CT requiere del uso de materiales equivalentes a tejidos cuya composición se conozca con un alto grado de precisión. El sistema de planeación BrainScan del acelerador Novalis de1 INNN incluye una opción para corregir por heterogeneidades. La conversión de números Hounsfield a densidades electrónicas se realiza suponiendo una relación lineal entre -1000 HU y 0 HU a los que se les asigna una densidad electrónica de 0 y 1 para aire y agua, respectivamente. Para HU>O también se utiliza una relación lineal pero con diferente pendiente. Sin embargo, se ha reportado en la literatura la necesidad de utilizar más de dos relaciones lineales para ajustar las relaciones entre re y HU en todo el intervalo de HU relevantes para radioterapia. El tomógrafo del INNN (General Electric HiSpeedCT/i SYS=CD01) fue incluido recientemente en un estudio de pruebas de control de calidad de acuerdo a la norma oficial mexicana NOM-158-SSA1 - 1996. En ese trabajo se reportan tres pruebas importantes que se refieren a la calibración del número CT en aire, a la constancia del número CT en agua y a la dependencia del número CT con el espesor del corte, mostrando que dichas pruebas no cumplían con los criterios de aceptación de la NOM. Adicionalmente, un trabajo realizado en el acelerador Novalis del INNN mostró grandes diferencias en la comparación de distribuciones espaciales de dosis (experimentales y calculadas con el sistema de planeación) en lesiones cercanas a cavidades de aire, indicando una posible necesidad de verificar la calibración del tomógrafo. En este trabajo se propone la calibración del sistema de tomografía computarizada del INNN en términos de densidades electrónicas. El trabajo incluye el diseño y construcción de un maniquí de calibración (para aplicaciones para estudios de cabeza) utilizando materiales cuya composición se conozca con alta precisión. También se evaluará dicha calibración al ser utilizados en el sistema de planeación de tratamiento BrainScan. Se utilizarán materiales comerciales cuya composición sea equivalente a músculo, grasa, hueso cortical y hueso trabecular. Adicionalmente, se caracterizarán otros materiales tales como lucita, teflón, nylamid, etcétera para poder ser utilizados en el maniquí. [ABSTRACT FROM AUTHOR]

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