Bremsstrahlung SPECT Imaging Research Articles

Outcomes following transarterial radioembolization with 90Y and nanoparticles loaded resin microspheres

90Y bremsstrahlung Single-Photon Emission Tomography (SPECT) imaging is employed to check the possibility of extrahepatic uptake and the quantification of delivered dose in Transarterial Radioembolization (TARE). 90Y bremsstrahlung SPECT imaging is challenging due to the nature of bremsstrahlung photons. We reported a Monte Carlo study using the resin microspheres loaded with 90Y and Nanoparticles (NPs) in the TARE. By injection of Bismuth (Bi) and Europium (Eu) NPs into the resin microspheres, the sensitivity and the contrast to noise ratio increased for the bremsstrahlung planar images. The highest signal to background ratio was observed in the characteristic X-ray planar images taken with the energy window at the Kα1 ± 10 keV when Eu NPs were incorporated into the microsphere. The dose enhancement ratio decreased dramatically at NP concentrations >2.4 M. Incorporating NPs into the resin microspheres improves the quality of post-treatment images and establishes a standardized imaging protocol for post-treatment imaging by characteristic X-rays.

Comparison of 90Y SIRT predicted and delivered absorbed doses using a PSF conversion method.

The aims of this study were to develop and apply a method to correct for the differences in partial volume effects of pre-therapy Technetium-99m (99mTc)-MAA SPECT and post-therapy Yttrium-90 (90Y) bremsstrahlung SPECT imaging in selective internal radiation therapy, and to use this method to improve quantitative comparison of predicted and delivered 90Y absorbed doses. The spatial resolution of 99mTc SPECT data was converted to that of 90Y SPECT data using a function calculated from 99mTc and 90Y point spread functions. This resolution conversion method (RCM) was first applied to 99mTc and 90Y SPECT phantom data to validate the method, and then to clinical data to assess the power of 99mTc SPECT imaging to predict the therapeutic absorbed dose. The maximum difference between absorbed doses to phantom spheres was 178%. This was reduced to 27% after the RCM was applied. The clinical data demonstrated differences within 38% for mean absorbed doses delivered to the normal liver, which were reduced to 20% after application of the RCM. Analysis of clinical data showed that therapeutic absorbed doses delivered to tumours greater than 100cm3 were predicted to within 52%, although there were differences of up to 210% for smaller tumours, even after the RCM was applied. The RCM was successfully verified using phantom data. Analysis of the clinical data established that the 99mTc pre-therapy imaging was predictive of the 90Y absorbed dose to the normal liver to within 20%, but had poor predictability for tumours smaller than 100cm3.

Post-administration dosimetry in yttrium-90 radioembolization through micro-CT imaging of radiopaque microspheres in a porcine renal model

The purpose of this study is to perform post-administration dosimetry in yttrium-90 radioembolization through micro-CT imaging of radiopaque microsphere distributions in a porcine renal model and explore the impact of spatial resolution of an imaging system on the extraction of specific dose metrics. Following the administration of radiopaque microspheres to the kidney of a hybrid farm pig, the kidney was explanted and imaged with micro-CT. To produce an activity distribution, 400 MBq of yttrium-90 activity was distributed throughout segmented voxels of the embolized vasculature based on an established linear relationship between microsphere concentration and CT voxel value. This distribution was down-sampled to coarser isotropic grids ranging in voxel size from 2.5 to 15 mm to emulate nominal resolutions comparable to those found in yttrium-90 PET and Bremsstrahlung SPECT imaging. Dose distributions were calculated through the convolution of activity distributions with dose-voxel kernels generated using the GATE Monte Carlo toolkit. Contours were computed to represent normal tissue and target volumes. Dose-volume histograms, dose metrics, and dose profiles were compared to a ground truth dose distribution computed with GATE. The mean dose to the target for all studied voxel sizes was found to be within 5.7% of the ground truth mean dose. was shown to be strongly correlated with image voxel size of the dose distribution (r 2 = 0.90). is cited in the literature as an important dose metric and its dependence on voxel size suggests higher resolution dose distributions may provide new perspectives on dose-response relationships in yttrium-90 radioembolization. This study demonstrates that dose distributions with large voxels incorrectly homogenize the dose by attributing escalated doses to normal tissues and reduced doses in high-dose target regions. High-resolution micro-CT imaging of radiopaque microsphere distributions can provide increased confidence in characterizing the absorbed dose heterogeneity in yttrium-90 radioembolization.

Retrospective SPECT/CT dosimetry following transarterial radioembolization

Transarterial radioembolization (TARE) effectively treats unresectable primary and metastatic liver tumors through intra‐arterial injection of Yttrium‐90 (90Y) beta particle emitting microspheres which implant around the tumor. Current dosimetry models are highly simplistic and there is a large need for an image‐based dosimetry post‐TARE, which would improve treatment safety and efficacy. Current post‐TARE imaging is 90Y bremsstrahlung SPECT/CT and we study the use of these images for dosimetry. Retrospective image review of ten patients having a Philips HealthcareTM SPECT/CT following TARE SIR‐Spheres® implantation. Emission series with attenuation correction were resampled to 3 mm resolution and used to create image‐based dose distributions. Dose distributions and analysis were performed in MIM Software SurePlanTM utilizing SurePlanTM Local Deposition Method (LDM) and a dose convolution method (WFBH). We sought to implement a patient‐specific background subtraction prior to dose calculation to make these noisy bremsstrahlung SPECT images suitable for post‐TARE dosimetry. On average the percentage of mean background counts to maximum count in the image across all patients was 9.4 ± 4.9% (maximum = 7.6%, minimum = 2.3%). Absolute dose increased and profile line width decreased as background subtraction value increased. The average value of the LDM and WFBH dose methods was statistically the same. As background subtraction value increased, the DVH curves become unrealistic and distorted. Background subtraction on bremsstrahlung SPECT image has a large effect on post‐TARE dosimetry. The background contour defined provides a systematic estimate to the activity background that accounts for the scanner and patient conditions at the time of the image study and is easily implemented using commercially available software. Using the mean count in the background contour as a subtraction across the entire image gave the most realistic dose distributions. This methodology is independent of microsphere and software manufacturer allowing for use with any available products or tools.

Collimator and energy window optimization for YTTRIUM-90 bremsstrahlung SPECT imaging

The optimal collimator and energy window for Yttrium-90 bremsstrahlung SPECT imaging was investigated in the study. Yttrium-90 images were acquired with a dual-head gamma camera, equipped with parallel hole collimators and 90Y vial for different energy windows ranging from 56 to 232 keV. Image quality parameters (sensitivity, %FOV, and S/B) were examined for the energy window and collimator combinations. It is concluded that the optimal SPECT imaging was achieved using FBP Method with a HEGP collimator and the energy window of 90–110 keV.

The Impact of Radiobiologically Informed Dose Prescription on the Clinical Benefit of 90Y SIRT in Colorectal Cancer Patients.

The purpose of this study was to establish the dose-response relationship of selective internal radiation therapy (SIRT) in patients with metastatic colorectal cancer (mCRC), when informed by radiobiological sensitivity parameters derived from mCRC cell lines exposed to 90Y. Methods: Twenty-three mCRC patients with liver metastases refractory to chemotherapy were included. 90Y bremsstrahlung SPECT images were transformed into dose maps assuming the local dose deposition method. Baseline and follow-up CT scans were segmented to derive liver and tumor volumes. Mean, median, and D70 (minimum dose to 70% of tumor volume) values determined from dose maps were correlated with change in tumor volume and volumetric RECIST response using linear and logistic regression, respectively. Radiosensitivity parameters determined by clonogenic assays of mCRC cell lines HT-29 and DLD-1 after exposure to 90Y or external beam radiotherapy (EBRT; 6 MV photons) were used in biologically effective dose (BED) calculations. Results: Mean administered radioactivity was 1,469 ± 428 MBq (range, 847-2,185 MBq), achieving a mean absorbed radiation dose to tumor of 35.5 ± 9.4 Gy and mean normal liver dose of 26.4 ± 6.8 Gy. A 1.0 Gy increase in mean, median, and D70 absorbed dose was associated with a reduction in tumor volume of 1.8%, 1.8%, and 1.5%, respectively, and an increased probability of a volumetric RECIST response (odds ratio, 1.09, 1.09, and 1.10, respectively). Threshold mean, median and D70 doses for response were 48.3, 48.8, and 41.8 Gy, respectively. EBRT-equivalent BEDs for 90Y are up to 50% smaller than those calculated by applying protraction-corrected radiobiological parameters derived from EBRT alone. Conclusion: Dosimetric studies have assumed equivalence between 90Y SIRT and EBRT, leading to inflation of BED for SIRT and possible undertreatment. Radiobiological parameters for 90Y were applied to a BED model, providing a calculation method that has the potential to improve assessment of tumor control.

Effect of ME Collimator Characteristic, Energy Window Width, and Reconstruction Algorithm Selection on Imaging Performance of Yttrium-90: Simulation Study.

The treatment efficiency of 90Y and providing reliable estimates of activity are evaluated by SPECT imaging of bremsstrahlung radiation released during beta therapy. In this technique, the resulting spectrum from 90Y is very complex and continuous, which creates difficulties on the imaging protocol. Moreover, collimator geometry has an impressive effect on the spatial resolution, system sensitivity, image contrast, and the signal-to-noise ratio (SNR), which should be optimized. We evaluated the effect of energy window width, reconstruction algorithms, and different geometries of a medium-energy (ME) parallel-hole collimator on the image contrast and SNR of 90Y SPECT images. The Siemens E.Cam gamma camera equipped with a ME collimator and a digital Jaszczak phantom were simulated by SIMIND Monte Carlo program to generate the 90Y bremsstrahlung SPECT images. Our results showed that optimal image quality can be acquired by the reconstruction algorithm of OS-EM in the energy window width of 60 to 400 keV for 90Y bremsstrahlung SPECT imaging. Furthermore, the optimal values of the hole diameter and hole length of a ME collimator were obtained 0.235 and 4.4 cm, respectively. The acquired optimal ME collimator and energy window along with using a suitable reconstruction algorithm lead to improved contrast and SNR of 90Y bremsstrahlung images of hot spheres of the digital Jaszczak phantom. This can improve the accuracy and precision of the 90Y activity distribution estimation after radioembolization in targeted radionuclide therapy.

04:21 PM Abstract No. 121 Quantitative and qualitative 99mTc MAA SPECT/CT imaging predicts response to therapy for patients treated with Yttrium-90 (90Y) radioembolization

Selective 90Y radioembolization therapy for hepatic malignancies involves planning angiography and 99mTc macroaggregated albumin scintigraphy (99mTc -MAA) to assess lung shunt fraction and extrahepatic perfusion. The purpose of this study is to assess the relationship between tumor MAA uptake by SPECT/CT and response following radioembolization. 22 patients with primary hepatic malignancies (non-colorectal) or hepatic colorectal metastases underwent 99mTc -MAA hepatic artery perfusion with SPECT/CT. Qualitative evaluation and quantitative evaluation with tumor to disease free liver parenchyma background ratios (TBR) of SPECT were correlated with CT response. Post-radioembolization Bremsstrahlung SPECT imaging was also performed to assess for similar correlations. The mean MAA tumor TBR in the patients that did not progress was 11.0 ±13.8 (n=14), significantly higher than the TBR of patients that progressed (2.7±1.3 (n=8) (P<0.05). The pattern of 99mTc -MAA distribution was predictive of therapy response. In all cases, intense homogeneous uptake was seen in patients who responded; absent uptake was seen in those who progressed. Uptake on post-radioembolization Bremsstrahlung SPECT did not correlate with response. Our findings suggest that the pattern of uptake on planning 99mTc -MAA imaging can predict response to subsequent radioembolization. Bremsstrahlung SPECT imaging, which is not typically performed, did not correlate with response. Evaluation of 99mTc -MAA uptake on routine pre-therapy studies could potentially serve as a critical decision node in the pre-treatment assessment of patients for radioembolization. Such analysis would extend the utility of this routine study beyond a simple assessment for extrahepatic perfusion, thereby leveraging information inherent to the scan.

Optimization of Bremsstrahlung imaging in Yttrium internal radiation therapy

Introduction: Treatment efficacy of radiation therapy using Yttrium-90 radionuclide is evaluated by the bremsstrahlung SPECT imaging following radiation therapy. The radioisotopic images have the ability to provide reliable activity map of 90Y distribution. But these images have a low quantitative accuracy in the 90Y bremsstrahlung SPECT imaging, therefore the optimization of some imaging important indicators i.e. the energy window width and collimator geometry, which heavily affect acquisition images quality, is necessary. The aim of this paper is to estimate the effects of the energy window width and the hole diameter of a ME collimator on the image contrast of the 90Y bremsstrahlung SPECT images obtained using SIMIND Monte Carlo simulation program and improve image quality which could be used to estimate activity distribution after radiation therapy. Materials and Methods: In this paper we simulated the SPECT system and a Jaszczak phantom consist of six hot spheres in different diameters by use of the SIMIND Monte Carlo simulation program to study 90Y SPECT images by assessment of the holes diameter of a ME collimator including 2.35, 2.59, 2.82, 2.94, 3.06 and 3.3 mm with fixed values of the collimator and septa thicknesses (3.2 cm and 1.14mm, respectively) in the two energy windows including 60- 160 keV and 60-400 keV. SPECT imaging protocol is selected according to the clinical study. An activity of 1.5GBq 90Y was considered for the hot spheres without background activity. We used OS-EM algorithm to reconstruct projections and evaluated the contrast of the reconstructed images of six hot spheres of Jaszczak phantom with selection of proper ROIs. Results: The comparison of the images contrast of Jaszczak phantom spheres indicated that generally with increasing the hole diameter of collimator, the images contrast of the spheres were decreased. In order to, in comparison to the 1st energy window (60–160 keV), the 2nd energy window (60-400 keV) was increased the image contrast. The results showed that for a fixed collimator and septa thickness values (3.2 cm and 1.14mm, respectively), the optimal value of the hole diameter was found 2.35 mm. Also from the stand point of the image contrast, the optimal energy window was found ranging from 60 to 400 keV. Conclusion: By use of the evaluation of the images contrast of Jaszczak phantom by SIMIND Monte Carlo simulation study, the best contrast is obtained when by use of the wide energy window ranging from 60 to 400 keV, which increased the imaging system sensitivity and decreased variance of the activity estimation. Moreover, a ME collimator with fixed values of 3.2 cm and 1.14 mm for the collimator and septa thicknesses, respectively and a hole diameter of 2.35 mm can improve the 90Y activity estimation and provide a suitable image quality for the 90Y bremsstrahlung SPECT imaging.

Phantom and clinical evaluation of the effect of full Monte Carlo collimator modelling in post-SIRT yttrium-90 Bremsstrahlung SPECT imaging

BackgroundPost-therapy SPECT/CT imaging of 90Y microspheres delivered to hepatic malignancies is difficult, owing to the continuous, high-energy Bremsstrahlung spectrum emitted by 90Y. This study aimed to evaluate the utility of a commercially available software package (HybridRecon, Hermes Medical Solutions AB) which incorporates full Monte Carlo collimator modelling. Analysis of image quality was performed on both phantom and clinical images in order to ultimately provide a recommendation of an optimum reconstruction for post-therapy 90Y microsphere SPECT/CT imaging.A 3D-printed anthropomorphic liver phantom was filled with 90Y with a sphere-to-background ratio of 4:1 and imaged on a GE Discovery 670 SPECT/CT camera. Datasets were reconstructed using ordered-subsets expectation maximization (OSEM) 1–7 iterations in order to identify the optimal OSEM reconstruction (5 iterations, 15 subsets). Quantitative analysis was subsequently carried out on phantom datasets obtained using four reconstruction algorithms: the default OSEM protocol (2 iterations, 10 subsets) and the optimised OSEM protocol, both with and without full Monte Carlo collimator modelling. The quantitative metrics contrast recovery (CR) and background variability (BV) were calculated.The four algorithms were then used to retrospectively reconstruct 10 selective internal radiation therapy (SIRT) patient datasets which were subsequently blind scored for image quality by a consultant radiologist.ResultsThe optimised OSEM reconstruction (5 iterations, 15 subsets with full MC collimator modelling) increased the CR by 42% (p < 0.001) compared to the default OSEM protocol (2 iterations, 10 subsets). The use of full Monte Carlo collimator modelling was shown to further improve CR by 14% (30 mm sphere, CR = 90%, p < 0.05).The consultant radiologist had a significant preference for the optimised OSEM over the default OSEM protocol (p < 0.001), with the optimised OSEM being the favoured reconstruction in every one of the 10 clinical cases presented.ConclusionsOSEM (5 iterations, 15 subsets) with full Monte Carlo collimator modelling is quantitatively the optimal image reconstruction for post-SIRT 90Y Bremsstrahlung SPECT/CT imaging. The use of full Monte Carlo collimator modelling for correction of image-degrading effects significantly increases contrast recovery without degrading clinical image quality.

Advances in SPECT for Optimizing the Liver Tumors Radioembolization Using Yttrium-90 Microspheres.

Radioembolization (RE) with Yttrium-90 (90Y) microspheres is an effective treatment for unresectable liver tumors. The activity of the microspheres to be administered should be calculated based on the type of microspheres. Technetium-99m macroaggregated albumin (99mTc-MAA) single photon emission computed tomography/computed tomography (SPECT/CT) is a reliable assessment before RE to ensure the safe delivery of microspheres into the target. 90Y bremsstrahlung SPECT imaging as a posttherapeutic assessment approach enables the reliable determination of absorbed dose, which is indispensable for the verification of treatment efficacy. This article intends to provide a review of the methods of optimizing 90Y bremsstrahlung SPECT imaging to improve the treatment efficacy of liver tumor RE using 90Y microspheres.

Technical Considerations of Phosphorous-32 Bremsstrahlung SPECT Imaging after Radioembolization of Hepatic Tumors: A Clinical Assessment with a Review of Imaging Parameters.

Background. Bremsstrahlung (BS) imaging during radioembolization (RE) confirms the deposition of radiotracer in hepatic/extrahepatic tumors. The aim of this study is to demonstrate 32P images and to optimize the imaging parameters. Materials and Methods. Thirty-nine patients with variable types of hepatic tumors, treated with the intra-arterial injection of 32P, were included. All patients underwent BS SPECT imaging 24–72 h after tracer administration, using low energy high resolution (LEHR) (18 patients) or medium energy general purpose (MEGP) (21 patients) collimators. A grading scale from 1 to 4 was used to express the compatibility of the 32P images with those obtained from CT/MRI. Results. Although the image quality obtained with the MEGP collimator was visually and quantitatively better than with the LEHR (76% concordance score versus 71%, resp.), there was no statistically significant difference between them. Conclusion. The MEGP collimator is the first choice for BS SPECT imaging. However, if the collimator change is time consuming (as in a busy center) or an MEGP collimator is not available, the LEHR collimator could be practical with acceptable images, especially in a SPECT study. In addition, BS imaging is a useful method to confirm the proper distribution of radiotherapeutic agents and has good correlation with anatomical findings.

Quantitative Comparison of PET and Bremsstrahlung SPECT for Imaging the In Vivo Yttrium-90 Microsphere Distribution after Liver Radioembolization

BackgroundAfter yttrium-90 (90Y) microsphere radioembolization (RE), evaluation of extrahepatic activity and liver dosimetry is typically performed on 90Y Bremsstrahlung SPECT images. Since these images demonstrate a low quantitative accuracy, 90Y PET has been suggested as an alternative. The aim of this study is to quantitatively compare SPECT and state-of-the-art PET on the ability to detect small accumulations of 90Y and on the accuracy of liver dosimetry.Methodology/Principal FindingsSPECT/CT and PET/CT phantom data were acquired using several acquisition and reconstruction protocols, including resolution recovery and Time-Of-Flight (TOF) PET. Image contrast and noise were compared using a torso-shaped phantom containing six hot spheres of various sizes. The ability to detect extra- and intrahepatic accumulations of activity was tested by quantitative evaluation of the visibility and unique detectability of the phantom hot spheres. Image-based dose estimates of the phantom were compared to the true dose. For clinical illustration, the SPECT and PET-based estimated liver dose distributions of five RE patients were compared. At equal noise level, PET showed higher contrast recovery coefficients than SPECT. The highest contrast recovery coefficients were obtained with TOF PET reconstruction including resolution recovery. All six spheres were consistently visible on SPECT and PET images, but PET was able to uniquely detect smaller spheres than SPECT. TOF PET-based estimates of the dose in the phantom spheres were more accurate than SPECT-based dose estimates, with underestimations ranging from 45% (10-mm sphere) to 11% (37-mm sphere) for PET, and 75% to 58% for SPECT, respectively. The differences between TOF PET and SPECT dose-estimates were supported by the patient data.Conclusions/SignificanceIn this study we quantitatively demonstrated that the image quality of state-of-the-art PET is superior over Bremsstrahlung SPECT for the assessment of the 90Y microsphere distribution after radioembolization.

Bioevaluation study of 32P-CP-PLLA particle brachytherapy in a rabbit VX2 lung tumor model

ObjectiveThe purpose of this study was to investigate the therapy effects of intratumoral administration of 32P-CP-PLLA particles in a rabbit VX2 lung tumor model. Methods16 rabbits with tumors were randomly divided into 4 groups. 4 rabbits served as untreated controls, and others received intratumoral administration of 32P-CP-PLLA particles with CT guidance. The total radioactivities in treated groups were as follows: a low activity was 93MBq (n=4) (group 1), a medium activity was 185MBq (n=4) (group 2) and a high activity was 370MBq (n=4) (group 3). Brachytherapy treated VX2 tumors underwent 18F-FDG PET/CT at 0 day, 3 day, 7 day and 14 day postinjection. In control group, 18F-FDG PET/CT images were acquired at the same time points but without any treatment. Bremsstrahlung SPECT images were performed at 14 days after intratumoral brachytherapy in treated groups. After Bremsstrahlung SPECT and last 18F-FDG PET/CT imagings, the rabbits were euthanized and the tumors were removed for histological examination. ResultsBremsstrahlung SPECT images study indicated that there was no leakage of 32P out of the injection site at 14 days after treatment. Compared with the control, the tumor volumes in treated groups significantly decreased, and 32P-CP-PLLA particle produced a reduction in maximum or mean SUV of VX2 tumor (p<0.05). The percentage changes in maximum and mean SUV gradually decreased in group 1 and group 2 from day 3 to day 14 (p<0.05). A transient increase in 18F-FDG accumulation at group 3 occurred due to the inflammatory reaction elements. Activity dependence was seen in HE and PCNA staining after 14 days treatment among three treated groups (p<0.05). ConclusionsOur data suggested that 32P-CP-PLLA particle localized on the injecting sites. This novel brachytherapy device efficiently suppressed the growth of the VX2 tumors implanted in the rabbit.

Evaluation of quantitative 90Y SPECT based on experimental phantom studies

In SPECT imaging of pure beta emitters, such as 90Y, the acquired spectrum is very complex, which increases the demands on the imaging protocol and the reconstruction. In this work, we have evaluated the quantitative accuracy of bremsstrahlung SPECT with focus on the reconstruction algorithm including model-based attenuation, scatter and collimator–detector response (CDR) compensations. The scatter and CDR compensation methods require pre-calculated point-spread functions, which were generated with the SIMIND MC program. The SIMIND program is dedicated for simulation of scintillation camera imaging and only handles photons. The aim of this work was therefore twofold. The first aim was to implement simulation of bremsstrahlung imaging into the SIMIND code and to validate simulations against experimental measurements. The second was to investigate the quality of bremsstrahlung SPECT imaging and to evaluate the possibility of quantifying the activity in differently shaped sources. In addition, a feasibility test was performed on a patient that underwent treatment with 90Y-Ibritumomab tiuxetan (Zevalin®). The MCNPX MC program was used to generate bremsstrahlung photon spectra which were used as source input in the SIMIND program. The obtained bremsstrahlung spectra were separately validated by experimental measurement using a HPGe detector. Validation of the SIMIND generated images was done by a comparison to gamma camera measurements of a syringe containing 90Y. Results showed a slight deviation between simulations and measurements in image regions outside the source, but the agreement was sufficient for the purpose of generating scatter and CDR kernels. For the bremsstrahlung SPECT experiment, the RSD torso phantom with 90Y in the liver insert was measured with and without background activities. Projection data were obtained using a GE VH/Hawkeye system. Image reconstruction was performed by using the OSEM algorithm with and without different combinations of model-based attenuation, scatter and CDR compensations. The reconstructed images were then evaluated in terms of the accuracy of the total activity estimate in the liver insert. It was found that the activity in a large source such as the liver was estimated with a bias of around −70%, when no compensations were included in the reconstruction, whereas when compensations were included the bias obtained was between −10 and 16%. It is concluded that although the 90Y bremsstrahlung spectrum is continuous with no pronounced peak and the count rate is low, it is possible to achieve reasonably accurate activity estimates from bremsstrahlung SPECT images if proper compensations are applied in the reconstruction. This conclusion was also confirmed by the patient study.

Three‐dimensional dosimetric analysis and quantitative bremsstrahlung SPECT imaging for treatment of nonresectable pancreatic cancer using colloidal phosphorus‐32

A clinically applicable methodology was developed to: (1) co‐register the reconstructed three‐dimensional images from CT and SPECT, and (2) determine the 3‐D absorbed dose distribution from bremsstrahlung SPECT images for patients undergoing infusional brachytherapy using colloidal P‐32. The image co‐registration helps verify that the region of P‐32 activity distribution corresponds to the anatomic site selected for injection. For outer surfaces, the body contours obtained from bremsstrahlung SPECT data were co‐registered with corresponding contours from CT. For the administered activity, a 3‐D surface was generated by mapping the iso‐counts in the SPECT image in real coordinates. The corresponding surface objects from the CT then were matched with the SPECT using an outer surface‐fitting algorithm. The accuracy of the fit measured as the mean distance between the SPECT and CT surfaces was then calculated. Calibration curves were generated from phantom studies and activity per voxel from the attenuation corrected measured counts per voxel were determined. The cumulative activity at each voxel position was then converted to dose (Gy) using a Monte Carlo based P‐32 point dose kernel in water. Two‐ and three‐dimensional quantitative dose distributions were then generated and projected on the reconstructed SPECT or CT slices and dose‐volume histograms were analyzed.