Backscattering and secondary-electron emission from metal targets of various thicknesses

Backscattering and secondary electron emission from metal targets of various thicknesse

Empirical equations for electron backscattering coefficients

Empirical equations for electron backscattering coefficient

Recovery of total I-131 activity within focal volumes using SPECT and 3D OSEM

We experimentally investigated the SPECT recovery of I-131 activity in multiple spheres located simultaneously at different locations within a cylindrical phantom that had an elliptical cross section. The sphere volumes ranged from 209 cc down to 4.2 cc. A Prism 3000 camera and two types of parallel-hexagonal-hole collimation were employed: high energy (HE) and ultra high energy (UHE). Using appropriately-different 3D models of the point source response function for the two types of collimation, approximately the same recovery of activity could be achieved with either collimation by 3D OSEM reconstruction. The recovery coefficient was greater with no background activity in the phantom by 0.10, on average, compared to that with background. In the HE collimation case, the activity recovery was considerably better for all volumes using 3D OSEM reconstruction than it had been in the past using 1D SAGE reconstruction. Recovery-coefficient-based correction in a simulated patient case involving spherical tumours moderately improved the activity estimates (average error reduced from 14% to 9% for UHE collimation, and from 15% to 11% for HE collimation). For a test case with HE collimation, increasing the projection-image sampling density while decreasing the image voxel size increased the recovery coefficient by 0.075 on average, and, if used in a full set of calibration measurements of recovery coefficient versus volume, might lead to further improvement in accuracy for the patient case.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/58098/2/pmb7_3_017.pd

Quantitative I-131 SPECT Reconstruction using CT Side Information from Hybrid Imaging

A penalized-likelihood (PL) SPECT reconstruction method using a modified regularizer that accounts for anatomical boundary side information was implemented to achieve accurate estimates of both the total target activity and the activity distribution within targets. In both simulations and experimental I-131 phantom studies, reconstructions from 1) penalized likelihood employing CT-side information based regularization (PL-CT); 2) penalized likelihood with edge preserving regularization (no CT); 3) penalized likelihood with conventional spatially invariant quadratic regularization (no CT) were compared with 4) Ordered Subset Expectation Maximization (OSEM), which is the iterative algorithm conventionally used in clinics for quantitative SPECT. Evaluations included phantom studies with perfect and imperfect (misregistered) side information and studies with uniform and non-uniform activity distributions in the target. For targets with uniform activity, the PL-CT images and profiles were closest to the `truth', avoided the edge offshoots evident with OSEM and minimized the blurring across boundaries evident with regularization without CT information. Apart from visual comparison, reconstruction accuracy was evaluated using the bias and standard deviation (STD) of the total target activity estimate and the root mean square error (RMSE) of the activity distribution within the target. PL-CT reconstruction reduced both bias and RMSE compared with regularization without side information. When compared with unregularized OSEM, PL-CT reduced RMSE and STD while bias was comparable. For targets with non-uniform activity, these improvements with PL-CT were observed only when the change in activity was matched by a change in the anatomical image and the corresponding inner boundary was also used to control the regularization. In summary, the present work demonstrates the potential of using CT side information to obtain improved estimates of the activity distribution in targets wi- - thout sacrificing the accuracy of total target activity estimation.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/85862/1/Fessler243.pd

Determining Total I-131 Activity Within a VoI Using SPECT, a UHE Collimator, OSEM, and a Constant Conversion Factor

Accurate determination of activity within a volume of interest is needed during radiopharmaceutical therapies. Single-photon emission computed tomography(SPECT) is employed but requires a method to convert counts to activity. We use a phantom-based conversion; that is, we image an elliptical cylinder containing a sphere that has a known amount of 131-I activity inside. The regularized space alternating generalized expectation (SAGE) algorithm employing a strip-integral detector-response model was employed for reconstruction in previous patient evaluations. With that algorithm and a high-energy collimator, the estimates for sphere activity varied with changes in: 1) the level of uniform background activity in the cylinder; 2) the image resolution due to different values of the radius of rotation R; and 3) the volume of the sphere. When one used those to convert reconstructed counts within a patient tumor into an activity estimate, the resultant value may have been in error because of patient-phantom mismatch. As a potential remedy, in this paper, we use an ordered subsets expectation maximization (OSEM) algorithm with a 3-D depth-dependent detector-response model and an ultra-high-energy collimator. Results after 100 OSEM iterations and using a maximum counts registration show the estimates for sphere activity: 1) have a dependence on the level of background activity with a slope whose absolute magnitude is typically only 0.37 times that with SAGE; 2) are independent of R; and 3) are independent of sphere volume down to and including a sphere volume of 20 cm3. We conclude that using a global-average conversion factor to relate counts to activity and no volume-based correction might be reasonable with OSEM. For a test of that conclusion, target activity is estimated for an anthropomorphic phantom containing a 100 cm3 spherical tumor centrally located inferior to the lungs. With OSEM-based quantification, using: 1) a global-average conversion factor and 2) no volume-based correction, mean bias in the simulated-tumor activity estimate over 20 realizations is -7.37% (relative standard deviation =5.93%). With SAGE-based quantification using: 1) the conversion factor corresponding to the experimental estimate of ba- ckground and 2) volume-based correction, the mean bias is -10.7% (relative standard deviation =2.37%). The mean bias is smaller in a statistically significant way and relative standard deviation is not more than a factor of 2.5 bigger with OSEM compared to SAGE. In addition, with OSEM, a patient image apparently shows more highly resolved features, and the activity estimates for two tumors are increased by an average of 10%, relative to results with SAGE.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/85985/1/Fessler57.pd

Update on HE vs UHE Collimation for Focal Total-activity Quantification in I-131 SPECT Using 3D OSEM

We calibrated a scintillation camera for the counts-to-activity conversion factor, CF, by measuring a phantom consisting of a sphere containing a known 131-I activity placed within an elliptical cylinder. Within a 3D OSEM reconstruction algorithm, we employed a depth-dependent detector-response model based on smooth fits to the point-source-response function. Using the ultra-high-energy (UHE) collimator and 100 iterations, the recovery coefficient, RC, appeared to be 1 for any sphere volume down to 20 cm3. The CF changed only a small amount as the background-over-target activity concentration ratio, b, increased for both UHE and high-energy (HE) collimation. Tests of activity quantification were carried out with an anthropomorphic phantom simulating a 100 cm3 spherical tumor centrally located inferior to the lungs. With 3D OSEM reconstruction, using the global-average CF and no RC-based correction, mean bias in the simulated-tumor activity estimate over 20 realizations was -7.4% with UHE collimation, and -9.4% with HE collimation. For comparison, with 1D SAGE reconstruction, using the CF corresponding to the experimental estimate of b and RC-based correction, the mean bias was worse, -10.7% for UHE collimation, but better, -4.3 %, for HE collimation.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/85907/1/Fessler190.pd

Regularized reconstruction in quantitative SPECT using CT side information from hybrid imaging

A penalized-likelihood (PL) SPECT reconstruction method using a modified regularizer that accounts for anatomical boundary side information was implemented to achieve accurate estimates of both the total target activity and the activity distribution within targets. In both simulations and experimental I-131 phantom studies, reconstructions from (1) penalized likelihood employing CT-side information-based regularization (PL-CT), (2) penalized likelihood with edge preserving regularization (no CT) and (3) penalized likelihood with conventional spatially invariant quadratic regularization (no CT) were compared with (4) ordered subset expectation maximization (OSEM), which is the iterative algorithm conventionally used in clinics for quantitative SPECT. Evaluations included phantom studies with perfect and imperfect side information and studies with uniform and non-uniform activity distributions in the target. For targets with uniform activity, the PL-CT images and profiles were closest to the 'truth', avoided the edge offshoots evident with OSEM and minimized the blurring across boundaries evident with regularization without CT information. Apart from visual comparison, reconstruction accuracy was evaluated using the bias and standard deviation (STD) of the total target activity estimate and the root mean square error (RMSE) of the activity distribution within the target. PL-CT reconstruction reduced both bias and RMSE compared with regularization without side information. When compared with unregularized OSEM, PL-CT reduced RMSE and STD while bias was comparable. For targets with non-uniform activity, these improvements with PL-CT were observed only when the change in activity was matched by a change in the anatomical image and the corresponding inner boundary was also used to control the regularization. In summary, the present work demonstrates the potential of using CT side information to obtain improved estimates of the activity distribution in targets without sacrificing the accuracy of total target activity estimation. The method is best suited for data acquired on hybrid systems where SPECT-CT misregistration is minimized. To demonstrate clinical application, the PL reconstruction with CT-based regularization was applied to data from a patient who underwent SPECT/CT imaging for tumor dosimetry following I-131 radioimmunotherapy.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/85409/1/pmb10_9_007.pd

Prediction of Therapy Tumor-Absorbed Dose Estimates in I-131 Radioimmunotherapy Using Tracer Data Via a Mixed-Model Fit to Time Activity

Abstract Background: For individualized treatment planning in radioimmunotherapy (RIT), correlations must be established between tracer-predicted and therapy-delivered absorbed doses. The focus of this work was to investigate this correlation for tumors. Methods: The study analyzed 57 tumors in 19 follicular lymphoma patients treated with I-131 tositumomab and imaged with SPECT/CT multiple times after tracer and therapy administrations. Instead of the typical least-squares fit to a single tumor's measured time-activity data, estimation was accomplished via a biexponential mixed model in which the curves from multiple subjects were jointly estimated. The tumor-absorbed dose estimates were determined by patient-specific Monte Carlo calculation. Results: The mixed model gave realistic tumor time-activity fits that showed the expected uptake and clearance phases even with noisy data or missing time points. Correlation between tracer and therapy tumor-residence times (r=0.98; p<0.0001) and correlation between tracer-predicted and therapy-delivered mean tumor-absorbed doses (r=0.86; p<0.0001) were very high. The predicted and delivered absorbed doses were within±25% (or within±75 cGy) for 80% of tumors. Conclusions: The mixed-model approach is feasible for fitting tumor time-activity data in RIT treatment planning when individual least-squares fitting is not possible due to inadequate sampling points. The good correlation between predicted and delivered tumor doses demonstrates the potential of using a pretherapy tracer study for tumor dosimetry-based treatment planning in RIT.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/98438/1/cbr%2E2011%2E1053.pd

Effect of Including Detector Response in SPECT Quantification of Focal I-131 Therapy

With a regularized strip-integral (1D) SAGE reconstruction, circular-orbit SPECT estimates of phantom focal 131-I activity vary with changes in the level of uniform background. They also vary with changes in image resolution due to different settings of the radius of rotation. To solve these problems, we investigated the effect of employing two different depth-dependent detector-response models. A regularized plane-by-plane (2D) SAGE algorithm reduced dependence of the counts-to-activity conversion factor on relative background concentration by 37% compared to the 1D SAGE. With unregularized multi-plane (3D) OSEM reconstruction, initial results showed: 1) a conversion factor that was independent of relative background concentration, and 2) a recovery coefficient that was approximately 1 for any sphere volume down to 20cc. We conclude that using a 3D detector-response model has the potential to eliminate bias problems. For a patient, the preliminary activity-estimate changes using 3D OSEM compared to 1D SAGE were: 1) +16% for a large tumor, and 2) -35% for a small tumor for which recovery-coefficient-based-correction-factor errors can be large.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/85991/1/Fessler170.pd

Use of Integrated SPECT/CT Imaging for Tumor Dosimetry in I-131 Radioimmunotherapy: A Pilot Patient Study

Abstract Integrated systems combining functional (single-photon emission computed tomography; SPECT) imaging with anatomic (computed tomography; CT) imaging have the potential to greatly improve the accuracy of dose estimation in radionuclide therapy. In this article, we present the methodology for highly patient-specific tumor dosimetry by utilizing such a system and apply it to a pilot study of 4 follicular lymphoma patients treated with I-131 tositumomab. SPECT quantification included three-dimensional ordered-subset expectation-maximization reconstruction and CT-defined tumor outlines at each time point. SPECT/CT images from multiple time points were coupled to a Monte Carlo algorithm to calculate a mean tumor dose that incorporated measured changes in tumor volume. The tumor shrinkage, defined as the difference between volumes drawn on the first and last CT scan (a typical time period of 15 days) was in the range 5%-49%. The therapy-delivered mean tumor-absorbed dose was in the range 146-334cGy. For comparison, the therapy dose was also calculated by assuming a static volume from the initial CT and was found to underestimate this dose by up to 47%. The agreement between tracer-predicted and therapy-delivered tumor-absorbed dose was in the range 7%-21%. In summary, malignant lymphomas can have dramatic tumor regression within days of treatment, and advanced imaging methods allow for a highly patient-specific tumor-dosimetry calculation that accounts for this regression.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/78152/1/cbr.2008.0568.pd