Biomedical subjects

Michael Ljungberg

Publications and source records attributed to Michael Ljungberg.

6 recordsLinked to original sources

PubMed2003

3D absorbed dose calculations based on SPECT: evaluation for 111-In/90-Y therapy using Monte Carlo simulations.

A general method is presented for patient-specific three-dimensional (3D) absorbed dose calculations based on quantitative SPECT activity measurements. The computational scheme includes a method for registration of the CT study to the SPECT image, and compensation for attenuation, scatter, and collimator-detector response including septal penetration, performed as part of an iterative reconstruction method. From SPECT images, the absorbed dose rate is calculated using an EGS4 Monte Carlo code, which converts the activity distribution to an absorbed dose rate distribution. Evaluation of the accuracy in the activity quantification and the absorbed dose calculation is based on realistic Monte Carlo simulated SPECT data of a voxel-computer phantom and (111)In and (90)Y. Septal penetration was not included in this study. The SPECT-based activity concentrations and absorbed dose distributions are compared to the actual values; the results imply that the corrections for attenuation and scatter yield results of high accuracy. The presented method includes compensation for most parameters deteriorating the quantitative image information. Inaccuracies are, however, introduced by the limited spatial resolution of the SPECT system, which are not fully compensated by the collimator-response correction. The proposed evaluation methodology may be used as a basis for future inter-comparison of different dosimetry calculation schemes.

Humans↗

PubMed2003

Crypt cell dosimetry for 99Tcm-sestamibi in a new small intestinal dosimetry model.

The aim of the study was to calculate the absorbed dose to the crypt cells in the small intestine from (99)Tc(m)-sestamibi excreted through the intestinal tract. The absorbed dose was calculated taking into consideration the biodistribution of the radiopharmaceutical in the small intestinal wall and its contents, based on data gathered in rats. Absorbed dose calculations were performed using a new intestinal model in which S values for crypt cells are given both for the intestinal wall and for the intestinal contents as source organs. A maximum of 6% of the injected activity was found to be located in the intestinal wall at 30 minutes after injection and 13% in the intestinal contents at 2 h, resulting in an absorbed dose of 8.9 microGy/MBq to the crypt cells. Assuming the activity to be located only in the wall, we calculate an absorbed dose to the crypt cells 2.5 times higher than if all the activity is assumed to be present in the intestinal contents. Using the new intestinal dosimetry model, together with detailed biokinetic data for the radiopharmaceutical from animal studies, it is possible to calculate the absorbed dose to the crypt cells, which is not possible when using external imaging.

Animals↗

PubMed2002

A parallel Monte Carlo code for planar and SPECT imaging: implementation, verification and applications in (131)I SPECT.

This paper reports the implementation of the SIMIND Monte Carlo code on an IBM SP2 distributed memory parallel computer. Basic aspects of running Monte Carlo particle transport calculations on parallel architectures are described. Our parallelization is based on equally partitioning photons among the processors and uses the Message Passing Interface (MPI) library for interprocessor communication and the Scalable Parallel Random Number Generator (SPRNG) to generate uncorrelated random number streams. These parallelization techniques are also applicable to other distributed memory architectures. A linear increase in computing speed with the number of processors is demonstrated for up to 32 processors. This speed-up is especially significant in Single Photon Emission Computed Tomography (SPECT) simulations involving higher energy photon emitters, where explicit modeling of the phantom and collimator is required. For (131)I, the accuracy of the parallel code is demonstrated by comparing simulated and experimental SPECT images from a heart/thorax phantom. Clinically realistic SPECT simulations using the voxel-man phantom are carried out to assess scatter and attenuation correction.

Computer Simulation↗

PubMed2002

An activity quantification method based on registration of CT and whole-body scintillation camera images, with application to 131I.

UNLABELLED: This article presents a new method for conjugate view activity quantification for 131I-labeled monoclonal antibody distribution. METHODS: The method is based on the combined use of images from 3 modalities: whole-body (WB) scintillation camera scanning, WB transmission scanning using 57Co, and CT. All images are coaligned using a recently developed program for the registration of WB images. Corrections for attenuation, scatter, and septal penetration are performed in image space. Compensation for scatter and septal penetration is performed by deconvolution, using point-response functions determined from Monte Carlo simulations. Attenuation correction is performed by applying a patient-specific 364-keV narrow-beam attenuation map obtained by combining information from the CT and the transmission scan. A relationship is presented for the conversion of the CT numbers to mass density. The attenuation- and scatter-compensated image is converted from counts to activity using a sensitivity value that was determined for 364-keV photons in air. This activity projection image is then analyzed for the activity of volumes of interest (VOI) using 2-dimensional regions of interest (ROIs) that are determined from the CT study. The CT is first resliced into coronal slices, and a maximum-extension ROI is outlined that encloses the VOI. Compensation for background activity and overlapping organs is performed on the basis of total patient thickness in the projection line, and on precalculated organ- background thickness fractions. RESULTS: Method evaluation was performed using data from both experimental measurements and Monte Carlo simulations. The use of an attenuation map derived directly from the CT study was also evaluated. For organ activity quantification, an accuracy of > or =10% was obtained. For small-diameter tumors, deviations were larger because of lack of correction for the background-dependent partial-volume effect. CONCLUSION: Registration of CT and WB scintillation camera images was successfully applied to improve activity quantification by the conjugate view method.

Antibodies, Monoclonal↗

PubMed2002

A 3-dimensional absorbed dose calculation method based on quantitative SPECT for radionuclide therapy: evaluation for (131)I using monte carlo simulation.

UNLABELLED: A general method is presented for patient-specific 3-dimensional absorbed dose calculations based on quantitative SPECT activity measurements. METHODS: The computational scheme includes a method for registration of the CT image to the SPECT image and position-dependent compensation for attenuation, scatter, and collimator detector response performed as part of an iterative reconstruction method. A method for conversion of the measured activity distribution to a 3-dimensional absorbed dose distribution, based on the EGS4 (electron-gamma shower, version 4) Monte Carlo code, is also included. The accuracy of the activity quantification and the absorbed dose calculation is evaluated on the basis of realistic Monte Carlo-simulated SPECT data, using the SIMIND (simulation of imaging nuclear detectors) program and a voxel-based computer phantom. CT images are obtained from the computer phantom, and realistic patient movements are added relative to the SPECT image. The SPECT-based activity concentration and absorbed dose distributions are compared with the true ones. RESULTS: Correction could be made for object scatter, photon attenuation, and scatter penetration in the collimator. However, inaccuracies were imposed by the limited spatial resolution of the SPECT system, for which the collimator response correction did not fully compensate. CONCLUSION: The presented method includes compensation for most parameters degrading the quantitative image information. The compensation methods are based on physical models and therefore are generally applicable to other radionuclides. The proposed evaluation methodology may be used as a basis for future intercomparison of different methods.

Humans↗

PubMed2002

A dosimetry model for the small intestine incorporating intestinal wall activity and cross-doses.

UNLABELLED: Current internal radiation dosimetry models for the small intestine, and for most walled organs, lack the ability to account for the activity uptake in the intestinal wall. In existing models the cross-dose from nearby loops of the small intestine is not taken into consideration. The aim of this investigation was to develop a general model for calculating the absorbed dose to the radiation-sensitive cells in the small intestinal mucosa from radionuclides located in the small intestinal wall or contents. METHODS: A model was developed for calculation of the self-dose and cross-dose from activity in the intestinal wall or contents. The small intestine was modeled as a cylinder with 2 different wall thicknesses and with an infinite length. Calculations were performed for various mucus thicknesses. S values were calculated using the EGS4 Monte Carlo simulation package with the PRESTA algorithm and the simulation results were integrated over the depth of the radiosensitive cells. The cross-organ dose was calculated by summing the dose contributions from other intestinal segments. Calculations of S values for self-dose and cross-dose were made for monoenergetic electrons, 0.050-10 MeV, and for the radionuclides (99m)Tc, (111)In, (131)I, (67)Ga, (90)Y, and (211)At. RESULTS: The self-dose S value from activity located in the small intestinal wall is considerably greater than the S values for self-dose from the contents and the cross-dose from wall and contents except for high electron energies. For all radionuclides investigated and for electrons 0.10-0.20 MeV and 8-10 MeV in energy, the cross-dose from activity in the contents is higher than the self-dose from the contents. The mucus thickness affects the S value when the activity is located in the contents. CONCLUSION: A dosimetric model for the small intestine was developed that takes into consideration the localization of the radiopharmaceutical in the intestinal wall or in the contents. It also calculates the contribution from self-dose and cross-dose. With this model, more accurate calculations of absorbed dose to radiation-sensitive cells in the intestine are possible.

Electrons↗