| 1. |
- Dewaraja, Y K, et al.
(författare)
-
Accuracy of 131I tumor quantification in radioimmunotherapy using SPECT imaging with an ultra-high-energy collimator: Monte Carlo study
- 2000
-
Ingår i: Journal of Nuclear Medicine. - 0161-5505. ; 41:10, s. 1760-1760
-
Tidskriftsartikel (refereegranskat)abstract
- Accuracy of 131I tumor quantification after radioimmunotherapy (RIT) was investigated for SPECT imaging with an ultra-high-energy (UHE) collimator designed for imaging 511-keV photons. METHODS: First, measurements and Monte Carlo simulations were carried out to compare the UHE collimator with a conventionally used, high-energy collimator. On the basis of this comparison, the UHE collimator was selected for this investigation, which was carried out by simulation of spherical tumors in a phantom. Reconstruction was by an expectation-maximization algorithm that included scatter and attenuation correction. Keeping the tumor activity constant, simulations were carried out to assess how volume-of-interest (VOI) counts vary with background activity, radius of rotation (ROR), tumor location, and size. The constant calibration factor for quantification was determined from VOI counts corresponding to a 3.63-cm-radius sphere of known activity. Tight VOIs corresponding to the physical size of the spheres or tumors were used. RESULTS: Use of the UHE collimator resulted in a large reduction in 131I penetration, which is especially significant in RIT where background uptake is high. With the UHE collimator, typical patient images showed an improvement in contrast. Considering the desired geometric events, sensitivity was reduced, but only by a factor of 1.6. Simulation results for a 3.63-cm-radius tumor showed that VOI counts vary with background, location, and ROR by less than 3.2%, 3%, and 5.3%, respectively. The variation with tumor size was more significant and was a function of the background. Good quantification accuracy (<6.5% error) was achieved when tumor size was the same as the sphere size used in the calibration, irrespective of the other parameters. For smaller tumors, activities were underestimated by up to -15% for the 2.88-cm-radius sphere, -23% for the 2.29-cm-radius sphere, and -47% for the 1.68-cm-radius sphere. CONCLUSION: Reasonable accuracy can be achieved for VOI quantification of 131I using SPECT with an UHE collimator and a constant calibration factor. Difference in tumor size relative to the size of the calibration sphere had the biggest effect on accuracy, and recovery coefficients are needed to improve quantification of small tumors.
|
|
| 2. |
- Dewaraja, Y K, et al.
(författare)
-
Accurate dosimetry in I-131 radionuclide therapy using patient-specific, 3-dimensional methods for SPECT reconstruction and absorbed dose calculation
- 2005
-
Ingår i: Journal of Nuclear Medicine. - 0161-5505. ; 46:5, s. 840-849
-
Tidskriftsartikel (refereegranskat)abstract
- I-131 radionuclide therapy studies have not shown a strong relationship between tumor absorbed dose and response, possibly due to inaccuracies in activity quantification and dose estimation. The goal of this work was to establish the accuracy of I-131 activity quantification and absorbed dose estimation when patient-specific, 3-dimensional (3D) methods are used for SPECT reconstruction and for absorbed dose calculation. Methods: Clinically realistic voxel-phantom simulations were used in the evaluation of activity quantification and dosimetry. SPECT reconstruction was performed using an ordered-subsets expectation maximization (OSEM) algorithm with compensation for scatter, attenuation, and 3D detector response. Based on the SPECT image and a patient-specific density map derived from CT, 3D dosimetry was performed using a newly implemented Monte Carlo code. Dosimetry was evaluated by comparing mean absorbed dose estimates calculated directly from the defined phantom activity map with those calculated from the SPECT image of the phantom. Finally, the 3D methods were applied to a radioimmunotherapy patient, and the mean tumor absorbed dose from the new calculation was compared with that from conventional dosimetry obtained from conjugate-view imaging. Results: Overall, the accuracy of the SPECT-based absorbed dose estimates in the phantom was > 12% for targets down to 16 mL and up to 35% for the smallest 7-mL tumor. To improve accuracy in the smallest tumor, more OSEM iterations may be needed. The relative SD from multiple realizations was < 3% for all targets except for the smallest tumor. For the patient, the mean tumor absorbed dose estimate from the new Monte Carlo calculation was 7% higher than that from conventional dosimetry. Conclusion: For target sizes down to 16 mL, highly accurate and precise dosimetry can be obtained with 3D methods for SPECT reconstruction and absorbed dose estimation. In the future, these methods can be applied to patients to potentially establish correlations between tumor regression and the absorbed dose statistics from 3D dosimetry.
|
|
| 3. |
- Dewaraja, Y K, et al.
(författare)
-
Characterization of scatter and penetration using Monte Carlo simulation in 131I imaging
- 2000
-
Ingår i: Journal of Nuclear Medicine. - 0161-5505. ; 41:1, s. 123-130
-
Tidskriftsartikel (refereegranskat)abstract
- In 131I SPECT, image quality and quantification accuracy are degraded by object scatter as well as scatter and penetration in the collimator. The characterization of energy and spatial distributions of scatter and penetration performed in this study by Monte Carlo simulation will be useful for the development and evaluation of techniques that compensate for such events in 131I imaging. METHODS: First, to test the accuracy of the Monte Carlo model, simulated and measured data were compared for both a point source and a phantom. Next, simulations to investigate scatter and penetration were performed for four geometries: point source in air, point source in a water-filled cylinder, hot sphere in a cylinder filled with nonradioactive water, and hot sphere in a cylinder filled with radioactive water. Energy spectra were separated according to order of scatter, type of interaction, and gamma-ray emission energy. A preliminary evaluation of the triple-energy window (TEW) scatter correction method was performed. RESULTS: The accuracy of the Monte Carlo model was verified by the good agreement between measured and simulated energy spectra and radial point spread functions. For a point source in air, simulations show that 73% of events in the photopeak window had either scattered in or penetrated the collimator, indicating the significance of collimator interactions. For a point source in a water-filled phantom, the separated energy spectra showed that a 20% photopeak window can be used to eliminate events that scatter more than two times in the phantom. For the hot sphere phantoms, it was shown that in the photopeak region the spectrum shape of penetration events is very similar to that of primary (no scatter and no penetration) events. For the hot sphere regions of interest, the percentage difference between true scatter counts and the TEW estimate of scatter counts was <12%. CONCLUSION: In 131I SPECT, object scatter as well as collimator scatter and penetration are significant. The TEW method provides a reasonable correction for scatter, but the similarity between the 364-keV primary and penetration energy spectra makes it difficult to compensate for these penetration events using techniques that are based on spectral analysis.
|
|
| 4. |
- Dewaraja, Y. K., et al.
(författare)
-
MIRD Pamphlet No. 23: Quantitative SPECT for Patient-Specific 3-Dimensional Dosimetry in Internal Radionuclide Therapy
- 2012
-
Ingår i: Journal of Nuclear Medicine. - : Society of Nuclear Medicine. - 0161-5505 .- 2159-662X. ; 53:8, s. 1310-1325
-
Tidskriftsartikel (refereegranskat)abstract
- In internal radionuclide therapy, a growing interest in voxel-level estimates of tissue-absorbed dose has been driven by the desire to report radiobiologic quantities that account for the biologic consequences of both spatial and temporal nonuniformities in these dose estimates. This report presents an overview of 3-dimensional SPECT methods and requirements for internal dosimetry at both regional and voxel levels. Combined SPECT/CT image-based methods are emphasized, because the CT-derived anatomic information allows one to address multiple technical factors that affect SPECT quantification while facilitating the patient-specific voxel-level dosimetry calculation itself. SPECT imaging and reconstruction techniques for quantification in radionuclide therapy are not necessarily the same as those designed to optimize diagnostic imaging quality. The current overview is intended as an introduction to an upcoming series of MIRD pamphlets with detailed radionuclide-specific recommendations intended to provide best-practice SPECT quantification-based guidance for radionuclide dosimetry.
|
|
| 5. |
- Dewaraja, Yuni K., et al.
(författare)
-
MIRD Pamphlet No. 24: Guidelines for Quantitative I-131 SPECT in Dosimetry Applications
- 2013
-
Ingår i: Journal of Nuclear Medicine. - : Society of Nuclear Medicine. - 0161-5505 .- 2159-662X. ; 54:12, s. 2182-2188
-
Tidskriftsartikel (refereegranskat)abstract
- The reliability of radiation dose estimates in internal radionuclide therapy is directly related to the accuracy of activity estimates obtained at each imaging time point. The recently published MIRD pamphlet no. 23 provided a general overview of quantitative SPECT imaging for dosimetry. The present document is the first in a series of isotope-specific guidelines that will follow MIRD 23 and focuses on one of the most commonly used therapeutic radionuclides, I-131. The purpose of this document is to provide guidance on the development of protocols for quantitative I-131 SPECT in radionuclide therapy applications that require regional (normal organs, lesions) and 3-dimensional dosimetry.
|
|
| 6. |
|
|
| 7. |
|
|
| 8. |
- Jönsson, Lena M, et al.
(författare)
-
A dosimetry model for the small intestine incorporating intestinal wall activity and cross-doses.
- 2002
-
Ingår i: Journal of Nuclear Medicine. - 0161-5505. ; 43:12, s. 1657-1664
-
Tidskriftsartikel (refereegranskat)abstract
- 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 99mTc, 111In, 131I, 67Ga, 90Y, and 211At. 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.
|
|
| 9. |
- Jönsson, Lena M, et al.
(författare)
-
Evaluation of accuracy in activity calculations for the conjugate view method from monte carlo simulated scintillation camera images using experimental data in an anthropomorphic phantom.
- 2005
-
Ingår i: Journal of Nuclear Medicine. - 0161-5505. ; 46:10, s. 1679-1686
-
Tidskriftsartikel (refereegranskat)abstract
- Activity determination from scintillation camera images using the conjugate view method may be inaccurate because of variation in scattered radiation from adjacent organs and activity from overlapping tissues. The aim of this study was to simulate patient scintillation camera images and from these evaluate the accuracy of 2 correction methods. The contribution from overlapping tissue activity was also calculated for some organs. Methods: Biokinetic data for Tc-99m-sestamibi obtained in rats was used as input to simulate scintillation camera images with a voxel-based computer phantom using the Monte Carlo method. The organ activity was calculated using the conjugate view method with either the effective attenuation coefficient method or scatter correction using the triple-energy window (TEW) method combined with attenuation correction with a transmission factor image. Images were simulated with activity in organs one by one to evaluate the accuracy of the 2 correction methods and to evaluate the activity contribution from activity in adjacent or overlapping tissues. To allow comparison with the clinical situation, the total activity distribution from the animal study was used to simulate scintillation camera images at different points in time and the calculated activity was compared with both the input data and some patient data from the literature. Results: The combination of scatter and attenuation correction gave the most accurate calculated activity, +/- 10% of the true activity from the images with activity in one organ at a time. In the images similar to the clinical situation, the kidney activity was overestimated up to a factor of 34, mainly because of excretion of activity through the intestines. Conclusion: The scatter correction using the TEW method in combination with attenuation correction with the measured transmission factor resulted in the most accurate activity determination of the methods used. This study also shows that organ activity data calculated from scintillation camera images may be overestimated by >90% because of activity in overlapping tissues.
|
|
| 10. |
- Lee, Zhenghong, et al.
(författare)
-
Usefulness and pitfalls of planar gamma-scintigraphy for measuring aerosol deposition in the lungs: a Monte Carlo investigation
- 2001
-
Ingår i: Journal of Nuclear Medicine. - 0161-5505. ; 42:7, s. 1077-1083
-
Tidskriftsartikel (refereegranskat)abstract
- Planar gamma-scintigraphy is often used to quantify pulmonary deposition patterns from aerosol inhalers. The results are quite different from those obtained using 3-dimensional PET and SPECT. The purpose of this study was to characterize the effects of scatter and tissue attenuation on the distribution of radiolabeled aerosol as measured by planar scintigraphy using Monte Carlo simulations. This study also investigated the applicability of a few correction methods used in inhalation studies. METHODS: Body density maps were derived from CT scans. Regions of interest-lungs, major airways, and esophagus-were defined from the same CT volume. Two radioactivity source distribution patterns in the lung, uniform and nonuniform, were used. A Monte Carlo program, SIMIND, was used to generate anterior and posterior gamma-images of the composed inhalation distributions for 2 energy windows, photopeak (127-153 keV) and scatter (92-125 keV). The effects of scatter and attenuation were estimated on the basis of the imaging components separated from the simulation. A scatter correction method and 2 attenuation correction methods, all applied to inhalation scintigraphy, were evaluated using the simulated images. RESULTS: The amount of scatter ranges from 24% to approximately 29% in the lungs and from 29% to approximately 35% in the central (airway or esophagus) region on the planar images. Significant differences were found among regions and between source distributions (P < 0.05). The fraction k used for dual-energy-based scatter correction also varied and was found to be less than the commonly used k = 0.5. The simplified narrow-beam attenuation correction and the effective (broad-beam) correction methods were found to either under- or overcorrect the regional activities. CONCLUSION: The amount of scatter and tissue attenuation in the thorax region depends on source distribution and body attenuation. In applying planar scintigraphy for aerosol inhalation studies, it is difficult to obtain precise quantitative measurements because of the uncertainties associated with scatter and attenuation corrections. Accurate corrections require knowledge of both source and density distributions.
|
|
