Srinivas Kappadath

Practical reconstruction protocol for quantitative 90Y bremsstrahlung SPECT/CT

PURPOSE To develop a practical background compensation (BC) technique to improve quantitative (90)Y-bremsstrahlung single-photon emission computed tomography (SPECT)/computed tomography (CT) using a commercially available imaging system. METHODS All images were acquired using medium-energy collimation in six energy windows (EWs), ranging from 70 to 410 keV. The EWs were determined based on the signal-to-background ratio in planar images of an acrylic phantom of different thicknesses (2-16 cm) positioned below a (90)Y source and set at different distances (15-35 cm) from a gamma camera. The authors adapted the widely used EW-based scatter-correction technique by modeling the BC as scaled images. The BC EW was determined empirically in SPECT/CT studies using an IEC phantom based on the sphere activity recovery and residual activity in the cold lung insert. The scaling factor was calculated from 20 clinical planar (90)Y images. Reconstruction parameters were optimized in the same SPECT images for improved image quantification and contrast. A count-to-activity calibration factor was calculated from 30 clinical (90)Y images. RESULTS The authors found that the most appropriate imaging EW range was 90-125 keV. BC was modeled as 0.53× images in the EW of 310-410 keV. The background-compensated clinical images had higher image contrast than uncompensated images. The maximum deviation of their SPECT calibration in clinical studies was lowest (<10%) for SPECT with attenuation correction (AC) and SPECT with AC + BC. Using the proposed SPECT-with-AC + BC reconstruction protocol, the authors found that the recovery coefficient of a 37-mm sphere (in a 10-mm volume of interest) increased from 39% to 90% and that the residual activity in the lung insert decreased from 44% to 14% over that of SPECT images with AC alone. CONCLUSIONS The proposed EW-based BC model was developed for (90)Y bremsstrahlung imaging. SPECT with AC + BC gave improved lesion detectability and activity quantification compared to SPECT with AC only. The proposed methodology can readily be used to tailor (90)Y SPECT/CT acquisition and reconstruction protocols with different SPECT/CT systems for quantification and improved image quality in clinical settings.

SU‐GG‐I‐04: Effect of CT Scan Parameters on Adult and Pediatric CT Dose When Using Automatic Tube Current Modulation

Purpose: To develop pediatric CT protocols by characterizing effects of CT parameters on adult and pediatric CT dose using automatic tube‐current modulation (ATCM). Method and Materials: Adult and 10‐year‐old (10yo) ATOM (CIRS) phantoms were scanned on a 16‐channel CT system (Emotion 16, Siemens) using ATCM (CareDose4D) to characterize the relationships between CT scan parameters and patient dose. The scan parameters investigated include scan‐kVp, reference‐mAs, scout‐kVp, patient‐size selection (adult/pediatric), and rotation‐time. Effective‐mAs delivered by ATCM was extracted from the DICOM header of each CTimage.Results: Effective‐mAs delivered by ATCM varied with patient‐size selection, relative to reference‐mAs, it decreased for adult but increased for pediatric. Using ATCM the average effective‐mAs for 80, 100, 130 scan‐kVp changed by 81%&124%, 61%&110%, 55%&103% relative to reference‐mAs for the adult&10yo phantoms. However, when the 10yo was selected as adult, the average effective‐mAs decreased to 34%, 29%, and 26% of reference‐mAs. When selected as adult, dose decreased for all anatomical regions but when selected as pediatric, dose increase was observed for abdomen and pelvis (120–155%). ATCM effective‐mAs per slice scaled linearly (1±0.06) with input reference‐mAs. However, the minimum mA limited the modulation of effective‐mAs at low reference‐mAs. For both phantoms, 80 kVp scouts reduced the effective‐mAs by 5–7% on average (up to 20% for thorax) compared to 130 kVp scouts. For identical reference‐mAs, ATCM increased effective‐mAs for lower scan‐kVp; however, patient dose also decreased for lower scan‐kVp. For the adult&10yo phantoms, average dose at 80 and 110 scan‐kVp was 42%&39% and 72%&72% of that at 130 scan‐kVp, respectively. Conclusion: Patient dose with ATCM decreased for adult and increased for pediatric patients. The scout‐kVp affects ATCM schema with lower effective‐mAs for lower scout‐kVp. ATCM effective‐mAs is linear with input reference‐mAs. Net patient dose is reduced despite increased effective‐mAs at lower scan‐kVp using ATCM.

MO-E-I-609-02: Imaging Properties of Cone Beam Breast CT- Effects of Detector Properties and Imaging Conditions

Purpose: To investigates the effects of detector properties and imaging conditions on the imaging properties of cone‐beam breast CT with both computer simulations and imaging experiments. Method and Materials: Cone beam breast CT was simulated with the breast analytically modeled as cylinder embedded spherical shape soft tissue masses and calcifications. X‐ray spectrum, breast attenuation, geometric magnification, focal spot blurring, x‐ray detection,detector blurring, image pixelization and digitization were all incorporated in computing the projection images.Quantum noise, system noise,detector blurring were also simulated and incorporated in the model. Image filtering and reconstruction were then performed using the Feldkamp algorithm. Simulation was performed for two flat‐panel detectors, one CsI based and the other a‐Se based. Images of phantoms and breast specimens were also obtained to demonstrate the ability of our experimental cone beam breast CT system to image the 3‐D structures of the breast with embedded cancers and calcifications. Results: Our simulation results shows that the a‐Se detector performs slightly better at 30 and 40 kVps while the CsI detector performs better at 50 or higher kVps. ImageSNRs are optimized at 50 and 60 kVp for the s‐Se and CsI detector, respectively. Phantom images obtained with our experimental system show that with higher dose and smaller pixel size, calcifications as small as could be resolved. Images of breast specimens show excellent separation between glandular and adipose tissues. The speculated nature of the tumor masses can be clearly seen in selected projection while ambiguous in other projections or in regular mammograms. It was also found that inclusion of surgical clips (used to indicate tumor location) had caused detrimental reconstruction artifacts. Acknowledgment: This work was supported in part by a research grant EB000117 from the NIBIB and a research grant CA104759 from the NCI.

TH-AB-BRA-10: Clinical Implementation of a Grid-Based Boltzmann Solver with Adaptive Meshing for Nuclear Medicine Dosimetry

Purpose: We previously compared the Grid-Based Boltzmann Solver (GBBS) with DOSXYZnrc Monte Carlo (MC) for beta and gamma sources in uniform materials and along material interfaces. The current standard in nuclear medicine is MIRD, which only calculates mean organ doses assuming uniform uptake in anthropomorphic phantoms. This work extends GBBS to clinical nuclear medicine for fast and accurate patient-specific voxel-level dosimetry. Methods: We compared the GBBS with MC using a patient’s post-therapy quantitative 90Y microsphere bremsstrahlung SPECT/CT.The GBBS (Attila™ v8.0.0) requires a tetrahedral mesh as input instead of voxels. Adaptive tetrahedral meshes were generated with TetGen v1.5.0 by folding CT and SPECT images into a mesh sizing function; the function was defined at each node on a background mesh with nodes defined at each SPECT/CT voxel. Target tetrahedral edge lengths varied from 0.5 to 8 cm and were adapted on SPECT activity level, CT material, and material gradient.GBBS used 30 electron energy groups, 32 angles, and 67,000 tetrahedrons. GBBS reported point data aligned with the center of SPECT/CT voxels which match MC. We report: 1) run time of the GBBS, 2) average difference in non-zero absorbed dose voxels, and 3) percent difference in mean absorbed dose to tumor and normal liver. DVHs and isodose curves were visually compared, and the tetrahedral mesh was compared to SPECT/CT. Results: GBBS calculation required 2 minutes. The average difference over all non-zero absorbed dose voxels (N=318,000) was −0.2 Gy. GBBS mean doses agreed within 1% of MC for both normal liver (39.1 Gy vs 39.3 Gy) and tumor (317 Gy vs 320 Gy). Adaptive meshing allowed sufficient material assignment and yielded smaller tetrahedrons near high activity regions and bone. DVHs had excellent visual agreement. Conclusions: GBBS with adaptive meshing is practical for fast and accurate clinical nuclear medicine dosimetry. Research reported in this publication was supported by the National Cancer Institute of the National Institutes of Health under Award Number R01CA138986. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.

WE‐AB‐204‐01: Performance Characterization of Regularized‐Reconstruction Algorithm for 90Y PET/CT Images

Purpose: 9⁰Y PET/CT imaging and quantification have recently been suggested as an approach of treatment verification. However, due to low positron yield (32ppm), the 9⁰Y-PET/CT images are very noisy. Iterative reconstruction techniques that employ regularization, e.g. block sequential regularized expectation maximization (BSREM) algorithm (recently implemented on GE scanners – QClear™), has the potential to increase quantitative accuracy with lower noise penalty compared to OSEM. Our aim is to investigate the performance of RR algorithms in 9⁰Y PET/CT studies. Methods: A NEMA IEC phantom filled with 3GBq 9⁰YCl₂ (to simulate patient treatment) was imaged on GE-D690 for 1800s/bed. The sphere-to-background ratio of 7. The data were reconstructed using OSEM and BSREM with PSF modeling and TOF correction while varying the iterations (IT) from 1–6 with fixed 24subsets. For BSREM, the edge-preservation parameter (γ ) was 2 and the penalty-parameters (β) was varied 350–950. In all cases a post-reconstruction filter of 5.2mm (2pixel) transaxial and standard z-axis were used. Sphere average activity concentration (AC) and background standard deviation (SD) were then calculated from VOIs drawn in the spheres and background. Results: Increasing IT from 1to6, the %SD in OSEM increased from 30% to 80%, whereas %SD in BSREM images increased by <5% for all βs. BSREM with β=350 didn’t offer any improvement over OSEM (convergence of mean achieved at 2 IT, in this study). Increasing β from 350 to 950 reduced the AC accuracy of small spheres (<20mm) by 10% and noise from 40% to 20%, which resulted in CNR increase from 11 to 17. Conclusion: In count-limited studies such as 9⁰Y PET/CT, BSREM can be used to suppress image noise and increase CNR at the expense of a relatively small decrease of quantitative accuracy. The BSREM parameters need to be optimized for each study depending on the radionuclides and count densities. Research reported in this publication was supported by the National Cancer Institute of the National Institutes of Health under Award Number R01CA13898 and was also partly supported by General Electric.

TU-A-18A-01: Basic Principles of PET/CT, Calibration Methods and Contrast Recovery Across Multiple Cameras

This continuing education session will discuss the physical principles of PET/CT imaging and characterization of contrast recovery using accreditation phantoms. A detailed overview will be given on the physical principles of PET including positron decay physics, 2D and 3D data acquisition, time-of-flight, scatter correction, CT attenuation correction, and image reconstruction. Instrument quality control and calibration procedures will be discussed. Technical challenges, common image artifacts and strategies to mitigate these issues will also be discussed. Data will be presented on acquisition techniques and reconstruction parameters affecting contrast recovery. The discussion will emphasize the minimization of reconstruction differences in quantification metrics such as SUV and contrast recovery coefficients for the NEMA and ACR clinical trial phantoms. Data from new and older generation scanners will be shown including comparison of contrast recovery measurements to their analytical solutions. The goal of this session is to update attendees on the quality control and calibration of PET/CT scanners, on methods to establish a common calibration for PET/CT scanners to control for instrument variance across multiple sites. LEARNING OBJECTIVES 1. Review the physical principles of PET/CT, quality control and calibration 2. Gain further understanding on how to apply techniques for improving quantitative agreement across multiple cameras 3. Describe the differences between measured and expected contrast recovery for the NEMA and ACR PET phantoms.

SU‐E‐I‐20: Dead Time Count Loss Compensation in SPECT/CT: Projection Versus Global Correction

PURPOSE To compare projection-based versus global correction that compensate for deadtime count loss in SPECT/CT images. METHODS SPECT/CT images of an IEC phantom (2.3GBq 99mTc) with ∼10% deadtime loss containing the 37mm (uptake 3), 28 and 22mm (uptake 6) spheres were acquired using a 2 detector SPECT/CT system with 64 projections/detector and 15 s/projection. The deadtime, Ti and the true count rate, Ni at each projection, i was calculated using the monitor-source method. Deadtime corrected SPECT were reconstructed twice: (1) with projections that were individually-corrected for deadtime-losses; and (2) with original projections with losses and then correcting the reconstructed SPECT images using a scaling factor equal to the inverse of the average fractional loss for 5 projections/detector. For both cases, the SPECT images were reconstructed using OSEM with attenuation and scatter corrections. The two SPECT datasets were assessed by comparing line profiles in xyplane and z-axis, evaluating the count recoveries, and comparing ROI statistics. Higher deadtime losses (up to 50%) were also simulated to the individually corrected projections by multiplying each projection i by exp(-a*Ni*Ti), where a is a scalar. Additionally, deadtime corrections in phantoms with different geometries and deadtime losses were also explored. The same two correction methods were carried for all these data sets. RESULTS Averaging the deadtime losses in 5 projections/detector suffices to recover >99% of the loss counts in most clinical cases. The line profiles (xyplane and z-axis) and the statistics in the ROIs drawn in the SPECT images corrected using both methods showed agreement within the statistical noise. The count-loss recoveries in the two methods also agree within >99%. CONCLUSION The projection-based and the global correction yield visually indistinguishable SPECT images. The global correction based on sparse sampling of projections losses allows for accurate SPECT deadtime loss correction while keeping the study duration reasonable.

WE-A-17A-07: Evaluation of a Grid-Based Boltzmann Solver for Nuclear Medicine Voxel-Based Dose Calculations

PURPOSE Grid-based Boltzmann solvers (GBBS) have been successfully implemented in radiation oncology clinics as dose calculations for e×ternal photon beams and 192Ir sealed-source brachytherapy. We report on the evaluation of a GBBS for nuclear medicine vo×el-based absorbed doses. METHODS Vo×el-S-values were calculated for monoenergetic betas and photons (1, 0.1, 0.01 MeV), 90Y, and 131I for 3 mm vo×el sizes using Monte Carlo (DOS×YZnrc) and GBBS (Attila 8.1-beta5, Transpire). The source distribution was uniform throughout a single vo×el. The material was an infinite 1.04 g/cc soft tissue slab. To e×plore convergence properties of the GBBS 3 tetrahedral meshes, 3 energy group structures, 3 different square Chebyschev-Legendre quadrature set orders (Sn), and 4×2013;7 spherical harmonic e×pansion terms (Pn) were investigated for a total of 168 discretizations per source. The mesh, energy group, and quadrature sets are 8×, 3×, and 16×, respectively, finer than the corresponding coarse discretization. GBBS cross sections were generated with full electronphoton-coupling using the vendors e×tended CEP×S code. For accuracy, percent differences (%Δ) in source vo×el absorbed doses between MC and GBBS are reported for the coarsest and finest discretization. For convergence, ratios of the two finest discretization solutions are reported along each variable. RESULTS For 1 MeV, 0.1 MeV, 0.01 MeV, Y90, and I-131 beta sources the %Δ in the source vo×el for (coarsest,finest) discretization were (+2.0,-6.4), (-8.0, -7.5), (-13.8, -13.4), (+0.9,-5.5), and (- 10.1,-9.0) respectively. The corresponding %Δ for photons were (+33.7,-7.1), (-9.4, -9.8), (-17.4, -15.2), and (-1.7,-7.7), respectively. For betas, the convergence ratio of mesh, energy, Sn, and Pn ranged from 0.991-1.000. For gammas, the convergence ratio of mesh, Sn, and Pn ranged from 0.998-1.003 while the ratio for energy ranged from 0.964-1.001. CONCLUSIONS GBBS is promising for nuclear medicine vo×el-based dose calculations. Ongoing work includes evaluating GBBS in bone, lung, and realistic clinical PET/SPECT-based activity distributions. Research reported in this publication was supported by the National Cancer Institute of the National Institutes of Health under Award Number R01CA138986. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.

SU‐DD‐A4‐05: Effects of Pixel Size and OSEM Iteration Parameters On Tc‐99m SPECT Resolution

Purpose: To evaluate the effect of pixel size and OSEM iterative reconstruction parameters on radial (RR) and tangential (TR) Tc‐99m SPECT resolution versus distance from isocenter. Method and Materials: Ten high‐concentration Tc‐99m point sources of size <2mm3 were positioned coplanar 0–9 cm from isocenter in a cylindrical phantom with low‐concentration background. Emission scans were acquired on a SPECT/CT system (Symbia T6, Siemens Medical Solutions) with LEHR collimation in continuous (C) and step‐and‐shoot (SS) modes for 360 views over 360° at 0.9, 1.8 and 3.6 mm/pixel. Data were iteratively reconstructed with 3D‐OSEM incorporating resolution, CT‐based attenuation, and scatter modeling, for different combinations of iterations and subsets (IT_SUB): 1_18, 10_18, 20_18, 30_18, 30_36, 30_60, 30_90. SPECT resolution was estimated using a Gaussian fit of the radial and tangential profiles through each point source. Results: TR was consistently better than RR. Anisotropy was independent of pixel size and scan mode but decreased with IT times SUB (e.g., TR/RR=0.78 and 0.62 for 1_18 and 30_90 with 0.9 mm/pixel in SS). Both TR and RR improved linearly with distance away from isocenter. The center‐to‐periphery resolution differences decreased with IT times SUB (e.g., slopes of resolution versus radius were −0.74 and −0.45 for 20_18 and 30_36 with 0.9 mm/pixel in SS) and with smaller pixel sizes (e.g., slopes of resolution versus radius were −0.89, −0.82 and −0.74 for 3.6, 1.8 and 0.9 mm/pixel for 20_18 in SS). TR and RR improved as a power‐law with IT times SUB. The rate of improvement showed no obvious dependence on pixel size. TR and RR were similar between SS and C. Conclusion:Spatial resolution of SPECTimagesreconstructed iteratively exhibited power‐law dependence on IT times SUB, linear dependence on radial position, and exhibited TR/RR anisotropy — modeling of which are important for accurate SPECT quantification.

MO‐E‐I‐609‐07: Dual‐Energy Digital Mammography for Calcification Imaging: Improvement by Image Processing

Purpose: We have developed and implemented a full-field dual-energy digital mammography (DEDM) technique, with total mean-glandular dose similar to screening-examination levels, to detect and visualize calcifications over tissue structures on mammograms. The suppression of tissue structures by DEDM comes at the cost of increased DE image noise. We report on the effects of image processing techniques on the DE calcification images. Method and Materials: To evaluate different image processing techniques, we have designed a special phantom with calcium carbonate crystals simulating calcifications of various sizes superimposed with a 5 cm thick breast-tissue phantom with continuously varying glandular ratio from 0.0 to 1.0. We report on the effects of three noise reduction techniques: (1) a simple smoothing filter applied to the DE image, (2) a median filter applied to the HE image, and (3) a correlated-noise reduction technique (KNR), where the scaled correlated noise from the DE tissue image was added to the DE calcification image (originally developed for DE computed tomography by Kalender et al., IEEE Transactions on Medical Physics 7, 218, 1988). Results: Most calcifications larger than 355 microns were visible in the unprocessed DE calcification image. The simple smoothing filter, although effective in noise suppression, does not improve calcification visibility due to loss of spatial resolution. Calcifications larger than 300 microns were visible with the median filter for kernel sizes of 3 and 5 pixels; while calcifications larger than 250 microns were visible with the the KNR technique for a scale facor of 0.00145 and kernal sizes of 25 and 51 pixels. Conclusion: The median and KNR image processing techniques improved calcification visibility while reducing the image noise in the DE calcification images. (Supported in part by research grants DAMD 17-00-1-0316 from the US Army BCRP, CA 104759 from the NCI, and EB000117 from the NIBIB)