Accurate Activity Quantification Research Articles

Effect of kilovoltage and quality reference mAs on CT-based attenuation correction in 177Lu SPECT/CT imaging: a phantom study

IntroductionCT-based attenuation correction (CT-AC) plays a major role in accurate activity quantification by SPECT/CT imaging. However, the effect of kilovoltage peak (kVp) and quality-reference mAs (QRM) on the attenuation coefficient image (μ-map) and volume CT dose index (CTDIvol) have not yet been systematically evaluated. Therefore, the aim of this study was to fill this gap and investigate the influence of kVp and QRM on CT-AC in 177Lu SPECT/CT imaging.MethodsSeventy low-dose CT acquisitions of an Electron Density Phantom (seventeen inserts of nine tissue-equivalent materials) were acquired using various kVp and QRM combinations on a Siemens Symbia Intevo Bold SPECT/CT system. Using manufacturer reconstruction software, 177Lu μ-maps were generated for each CT image, and three low-dose CT related aspects were examined. First, the μ-map-based attenuation values (μmeasured) were compared with theoretical values (μtheoretical). Second, changes in 177Lu activity expected due to changes in the μ-map were calculated using a modified Chang method. Third, the noise in the μ-map was assessed by measuring the coefficient of variation in a volume of interest in the homogeneous section of the Electron Density Phantom. Lastly, two phantoms were designed to simulate attenuation in four tissue-equivalent materials for two different source geometries (1-mL and 10-mL syringes). 177Lu SPECT/CT imaging was performed using three different reconstruction algorithms (xSPECT Quant, Flash3D, STIR), and the SPECT-based activities were compared against the nominal activities in the sources.ResultsThe largest relative errors between μmeasured and μtheoretical were observed in the lung inhale insert (range: 18%-36%), while it remained below 6% for all other inserts. The resulting changes in 177Lu activity quantification were -3.5% in the lung inhale insert and less than -2.3% in all other inserts. Coefficient of variation and CTDIvol ranged from 0.3% and 3.6 mGy (130 kVp, 35 mAs) to 0.4% and 0.9 mGy (80 kVp, 20 mAs), respectively. The SPECT-based activity quantification using xSPECT Quant reconstructions outperformed all other reconstruction algorithms.ConclusionThis study shows that kVp and QRM values in low-dose CT imaging have a minimum effect on quantitative 177Lu SPECT/CT imaging, while the selection of low values of kVp and QRM reduce the CTDIvol.

Impact of image reconstruction method on dose distributions derived from 90Y PET images: phantom and liver radioembolization patient studies

PET images acquired after liver 90Y radioembolization therapies are typically very noisy, which significantly challenges both visualization and quantification of activity distributions. To improve their noise characteristics, regularized iterative reconstruction algorithms such as block sequential regularized expectation maximization (Q.Clear for GE Healthcare, USA) have been proposed. In this study, we aimed to investigate the effects which different reconstruction algorithms may have on patient images, with reconstruction parameters initially narrowed down using phantom studies. Moreover, we evaluated the impact of these reconstruction methods on voxel-based dose distribution in phantom and patient studies (lesions and healthy livers). The International Electrotechnical Commission (IEC)/NEMA phantom, containing six spheres, was filled with 90Y and imaged using a GE Discovery 690 PET/CT scanner with time-of-flight enabled. The images were reconstructed using Q.Clear (with β parameter ranging from 0 to 8000) and ordered subsets expectation maximization. The image quality and quantification accuracy were evaluated by computing the hot () and cold () contrast recovery coefficients, background variability (BV) and activity bias. Next, dose distributions and dose volume histograms were generated using MIM® software’s SurePlan LiverY90 toolbox. Subsequently, parameters optimized in these phantom studies were applied to five patient datasets. Dose parameters, such as D max, D mean, D 70, and V 100Gy , were estimated, and their variability for different reconstruction methods was investigated. Based on phantom studies, the β parameter values optimized for image quality and quantification accuracy were 2500 and 300, respectively. When all investigated reconstructions were applied to patient studies, D mean, D50, D 70, and V 100Gy showed coefficients of variation below 8%; whereas the variability of D max was up to 30% for both phantom and patient images. Although β = 300−1000 would provide accurate activity quantification for a region of interest, when considering activity/dose voxelized distribution, higher β value (e.g. 4000–5000) would provide the greatest accuracy for dose distributions. In this 90Y radioembolization PET/CT study, the β parameter in regularized iterative (Q.Clear) reconstruction was investigated for image quality, accurate quantification and dose distributions based on phantom experiments and then applied to patient studies. Our results indicate that more accurate dose distribution can be achieved from smoother PET images, reconstructed with larger β values than those yielding the best activity quantifications but noisy images. Most importantly, these results suggest that quantitative measures, which are commonly used in clinics, such as SUVmax or SUVpeak( equivalent of D max), should not be employed for 90Y PET images, since their values would highly depend on the image reconstruction.

Energy Window and Collimator Optimization in Lutetium-177 Single-photon Emission Computed Tomography Imaging using Monte Carlo Simulation.

Introduction:In lutetium-177 (Lu-177) single-photon emission computed tomography (SPECT) imaging, the accuracy of activity quantification is degraded by penetrated and scattered photons. We assessed the scattered photon fractions in order to determine the optimal situation and development of correction method. This study proposes to compare the image quality that can be achieved by three collimators.Materials and Methods:Siemens Medical System Symbia fitted with high-energy (HE), medium-energy (ME), and low-energy high-resolution collimators was simulated using the SIMIND Monte Carlo code simulation code. Counts were collected in three different main-energy window widths (20%, 15%, and 10%) for Lu-177 point source. Primary and scattered point spread functions and also geometric, penetration, scattering were drawn and analyzed.Results:In Lu-177 imaging, a 20% of main-energy window and ME collimator were found to be optimal. HE collimator can be used when the resolution is not required.Conclusion:These results provide the optimal energy window and collimator in Lu-177 SPECT imaging and will help the quantification of Lu-177.

Dual SPECT imaging of 111In and 67Ga to simultaneously determine in vivo the pharmacokinetics of different radiopharmaceuticals: a quantitative tool in pre-clinical research

Dual-isotope (DI) studies offer a number of advantages in pre-clinical imaging. These include: reducing study times when compared with sequential scans, reducing the number of animals required for any given study, and most importantly, producing images perfectly registered in space and time that provide simultaneous information about two distinct body functions. The ability of single photon emission computed tomography (SPECT) to measure and differentiate energies of the emitted photons makes it well suited for DI imaging. However, since scattered photons originating from one radioisotope may be detected in the energy window of the other and thus degrade image quality and quantitative accuracy, scatter and crosstalk corrections must be applied.The decay characteristics of 111In and 67Ga, which are suitable for quantitative DI imaging for up to 2 weeks post-injection, led us to investigate the performance of simultaneous 111In/67Ga SPECT imaging using a small-animal pre-clinical scanner. A series of phantom experiments were performed to investigate image quality and accuracy of activity quantification in 111In/67Ga images acquired with three different collimators and reconstructed from different photopeak combinations. The triple energy window (TEW) method was used to correct for scatter and crosstalk. Based on these phantom studies, the optimal selection of collimator and energy window settings was determined. When using these optimal settings, submillimeter-size structures were distinguishable in the reconstructed images and quantification errors below 20% were achieved for both isotopes. The optimal parameters were subsequently applied to an in vivo animal study. The determination of the distinct pharmacokinetic profiles of two polymer radiopharmaceuticals injected simultaneously, but by different administration routes (intravenously and intraperitoneally) into a single animal demonstrated the feasibility of simultaneous 111In/67Ga SPECT.

335. Uncertainty in quantification of cumulative activity in bone metastasis dosimetry with 223Ra

Purpose Ra-223 therapy of prostate cancer bone metastases is being used to treat patients routinely. Accurate activity quantification through imaging is essential to calculate the absorbed dose in organs and sub-units in radiopharmaceutical therapy, enabling personalized absorbed dose-based treatment planning methodologies and more effective and optimal treatments. This work was aimed to estimate the error in quantifying cumulated activity in simulated case applying specific SPECT calibration procedure [1] , [2] . Methods Calibrations with Ra-223 were performed with the SPECT/CT Infinia Hawkeye 4 GE (3/8-inch crystal) acquiring planar static images with double-peak (82 and 154 keV, 20% wide) and MEGP collimator. The sensitivity was measured in air by varying activity, source–detector distance, and source diameter. Acquisition times were 10 min. Transmission curves were measured for attenuation and scatter correction with the pseudo-extrapolation number method. A real case has been simulated placing on the source a bone equivalent material and PMMA thickness simulating human condition. Results The calibration factors is 63.4 cps/MBq. The table attenuation affects less than 11.5%. Sensitivity values for each head varies by 0.4% respect to the mean value with the source-detector distance and the activity. The effect of the partial volume decreases the calibration factor of the smaller source by 10%. The quantified activity in the real case differs by less than 15% respect to the measured activity. Conclusions The calibration factor found is higher than the literature value (45.7 cps/MBq [2] ). This result and the variation of the sensibility factor with source and acquisition setup stress the needed to characterize the system to perform a proper dosimetry estimation for any SPECT scanner. The error in determining the cumulated activity is less than 15%. Further studies including variation in source depth and acquisition time are needed to confirm these results including analysis in anthropomorphic phantom or human.

Extension of the International Atomic Energy Agency phantom study in image quantification: results of multicentre evaluation in Croatia.

To make quantitative methods of nuclear medicine more available, four centres in Croatia participated in the national intercomparison study, following the materials and methods used in the previous international study organized by the International Atomic Energy Agency (IAEA). The study task was to calculate the activities of four Ba sources (T 1/2=10.54 years; E γ=356 keV) using planar and single-photon emission computed tomography (SPECT) or SPECT/CT acquisitions of the sources inside a water-filled cylindrical phantom. The sources were previously calibrated by the US National Institute of Standards and Technology. Triple-energy window was utilized for scatter correction. Planar studies were corrected for attenuation correction (AC) using the conjugate-view method. For SPECT/CT studies, data from X-ray computed tomography were used for attenuation correction (CT-AC), whereas for SPECT-only acquisition, the Chang-AC method was applied. Using the lessons learned from the IAEA study, data were acquired according to the harmonized data acquisition protocol, and the acquired images were then processed using centralized data analysis. The accuracy of the activity quantification was evaluated as the ratio R between the calculated activity and the value obtained from National Institute of Standards and Technology. For planar studies, R=1.06±0.08; for SPECT/CT study using CT-AC, R=1.00±0.08; and for Chang-AC, R=0.89±0.12. The results are in accordance with those obtained within the larger IAEA study and confirm that SPECT/CT method is the most appropriate for accurate activity quantification.

A Method for Point Spread Function Estimation for Accurate Quantitative Imaging

Aim: A method to determine the point spread function (PSF) of an imaging system based on a set of 3-D Gaussian functions is presented for a robust estimation of the recovery corrections for accurate activity quantification in positron emission tomography (PET) and single-photon emission computed tomography systems. Materials and Methods: The spatial resolution of the ALBIRA II PET subsystem was determined using a 370-kBq 22Na point source. The measured data were reconstructed with a maximum-likelihood expectation-maximization algorithm. The PSF was calculated based on the National Electrical Manufacturing Association (NEMA) NU-4 2008 protocol and on alternative methods based on three 3-D fitting functions for the xyz-directions: 1) a 3-D Gaussian function (3-D 1-Gauss) and convolutions of this function with a pixel size (3-D ${\text {Gauss}}_{p}$ ) or source dimension of O 0.25 mm (3-D ${\text {Gauss}}_{s}$ ); 2) the sum of two Gaussian functions (3-D 2-Gauss); and 3) three Gaussian functions (3-D 3-Gauss). Goodness of fit and the method based on an Akaike information criterion were used for choosing the best function. A MATLAB-based mathematical source simulation study was performed to quantify the relevance of PSFs calculated from the different methods. Results: Based on the PSFs calculated from the NEMA protocol, the full-width at half-maximum (FWHM) in xyz-directions were 1.68, 1.51, and 1.50 mm. The corresponding results using 3-D Gauss and 3-D ${\text {Gauss}}_{s}$ functions both were (1.87 ± 0.01), (1.70 ± 0.01), and (1.50 ± 0.01) mm and for 3-D ${\text {Gauss}}_{p}$ were (1.84 ± 0.01), (1.67 ± 0.01), and (1.47 ± 0.01) mm. The FWHMs calculated with 3-D 2-Gauss and 3-D 3-Gauss were (1.78 ± 0.01), (1.74 ± 0.01), and (1.83 ± 0.01) mm and (1.76 ± 0.03), (1.72 ± 0.03), and (1.78 ± 0.03) mm, respectively. All coefficients of variations of the fit parameters were ≤29% and the adjusted $R^{2}$ were ≥0.99. Based on Akaike weights $w_{i}$ , the 3-D 3-Gauss method was best supported by the data ( $w_{i} = 100$ %). The simulation study showed a relative error in quantification of spherical lesions in the range of 15%–45% for lesions of diameters 1–5 mm compared to the PSFs based on the NEMA method. Conclusion: An alternative method to calculate the PSFs of imaging systems to accurately correct for recovery effects is presented. The proposed method includes choosing and fitting of 3-D functions, validation of fitting quality, and choosing the function best supported by the data along with an estimation of the uncertainty.

Accuracy of 177Lu activity quantification in SPECT imaging: a phantom study

BackgroundThe aim of the study is to assess accuracy of activity quantification of 177Lu studies performed according to recommendations provided by the committee on Medical Internal Radiation Dose (MIRD) pamphlets 23 and 26. The performances of two scatter correction and three segmentation methods were compared. Additionally, the accuracy of tomographic and planar methods for determination of the camera normalization factor (CNF) was evaluated.Eight phantoms containing inserts of different sizes and shapes placed in air, water, and radioactive background were scanned using a Siemens SymbiaT SPECT/CT camera. Planar and tomographic scans with 177Lu sources were used to measure CNF. Images were reconstructed with our SPEQToR software using resolution recovery, attenuation, and two scatter correction methods (analytical photon distribution interpolated (APDI) and triple energy window (TEW)). Segmentation was performed using a fixed threshold method for both air and cold water scans. For hot water experiments three segmentation methods were compared as folows: a 40% fixed threshold, segmentation based on CT images, and our iterative adaptive dual thresholding (IADT). Quantification error, defined as the percent difference between experimental and true activities, was evaluated.ResultsQuantification error for scans in air was better for TEW scatter correction (<6%) than for APDI (<11%). This trend was reversed for scans in water (<10% for APDI and <14% for TEW). For hot water, the best results (<18% for small objects and <5% for objects >100 ml) were obtained when APDI and IADT were used for scatter correction and segmentation, respectively. Additionally, we showed that planar acquisitions with scatter correction and tomographic scans provide similar CNF values. This is an important finding because planar acquisitions are easier to perform than tomographic scans. TEW and APDI resulted in similar quantification errors with APDI showing a small advantage for objects placed in medium with non-uniform density.ConclusionsFollowing the MIRD recommendations for data acquisition and reconstruction resulted in accurate activity quantification (errors <5% for large objects). However, techniques for better organ/tumor segmentation must still be developed.

Quantitative 177Lu SPECT imaging using advanced correction algorithms in non-reference geometry

Peptide receptor therapy with 177Lu-labelled somatostatin analogues is a promising tool in the management of patients with inoperable or metastasized neuroendocrine tumours. The aim of this work was to perform accurate activity quantification of 177Lu in complex anthropomorphic geometry using advanced correction algorithms.Acquisitions were performed on the higher 177Lu photopeak (208keV) using a Philips IRIX gamma camera provided with medium-energy collimators. System calibration was performed using a 16mL Jaszczak sphere surrounded by non-radioactive water. Attenuation correction was performed using μ-maps derived from CT data, while scatter and septal penetration corrections were performed using the transmission-dependent convolution-subtraction method. SPECT acquisitions were finally corrected for dead time and partial volume effects. Image analysis was performed using the commercial QSPECT software.The quantitative SPECT approach was validated on an anthropomorphic phantom provided with a home-made insert simulating a hepatic lesion. Quantitative accuracy was studied using three tumour-to-background activity concentration ratios (6:1, 9:1, 14:1).For all acquisitions, the recovered total activity was within 12% of the calibrated activity both in the background region and in the tumour. Using a 6:1 tumour-to-background ratio the recovered total activity was within 2% in the tumour and within 5% in the background. Partial volume effects, if not properly accounted for, can lead to significant activity underestimations in clinical conditions.In conclusion, accurate activity quantification of 177Lu can be obtained if activity measurements are performed with equipment traceable to primary standards, advanced correction algorithms are used and acquisitions are performed at the 208keV photopeak using medium-energy collimators.

O32. Technetium-99m and Gold-198 activity quantification using SPECT/CT Monte Carlo Simulations

Introduction Accurate activity quantification is important for radiation dose calculations in Nuclear Medicine. Quantification accuracy varies due to inherent limitations of imaging methods, object shape, size and background levels. The aim of this study was to evaluate the accuracy of activity quantification for Tc-99m and Au-198 with a SPECT/CT system using Monte Carlo (MC) simulations. Materials and methods Segmented CT images of each phantom setup were used as input for the MC simulations. Three phantom studies and a patient study were simulated with Tc-99m and Au-198 respectively. A radioactive concentration of 0.27 MBq/ml* was placed in all cylinders or spheres mentioned below. (i) Cylinder phantom study: Six different sized cylinders filled with radioactivity* were simulated in the water filled large cylinder phantom (LCP), with two levels of background activity. (ii) Sphere phantom study: Six different sized spheres filled with radioactivity* were positioned in the LCP and simulated as mentioned above. (iii) RSD thorax phantom study: Two spheres filled with radioactivity* were positioned in the RSD phantom with three different levels of organ and background activity. This was repeated for three different sphere sizes. (iv) Patient study: A patient study with two spherical lesions was simulated. Corrections for attenuation, scatter and collimator response were applied during image reconstruction. Using CT regions of interest, counts from reconstructed SPECT images were converted to activity using a partial volume correction (PVC) and a calibration factor. Results For cylindrical sources, good accuracies were observed ( ⩽ 10% for Tc-99m and ⩽ 40% for Au-198). Similar results were obtained for the spherical sources in the LCP, RSD phantom and patient study ( ⩽ 10% for Tc-99m and ⩽ 20% for Au-198). Conclusion The accuracy of activity quantification varies with object size and from the results it is evident that a PVC remains necessary for the smallest sphere sizes.

Organ-specific SPECT activity calibration using 3D printed phantoms for molecular radiotherapy dosimetry.

BackgroundPatient-specific absorbed dose calculations for molecular radiotherapy require accurate activity quantification. This is commonly derived from Single-Photon Emission Computed Tomography (SPECT) imaging using a calibration factor relating detected counts to known activity in a phantom insert.MethodsA series of phantom inserts, based on the mathematical models underlying many clinical dosimetry calculations, have been produced using 3D printing techniques. SPECT/CT data for the phantom inserts has been used to calculate new organ-specific calibration factors for 99mTc and 177Lu. The measured calibration factors are compared to predicted values from calculations using a Gaussian kernel.ResultsMeasured SPECT calibration factors for 3D printed organs display a clear dependence on organ shape for 99mTc and 177Lu. The observed variation in calibration factor is reproduced using Gaussian kernel-based calculation over two orders of magnitude change in insert volume for 99mTc and 177Lu. These new organ-specific calibration factors show a 24, 11 and 8 % reduction in absorbed dose for the liver, spleen and kidneys, respectively.ConclusionsNon-spherical calibration factors from 3D printed phantom inserts can significantly improve the accuracy of whole organ activity quantification for molecular radiotherapy, providing a crucial step towards individualised activity quantification and patient-specific dosimetry. 3D printed inserts are found to provide a cost effective and efficient way for clinical centres to access more realistic phantom data.

The influence of triple energy window scatter correction on activity quantification for 1 7 7Lu molecular radiotherapy

Accurate activity quantification is the foundation for all methods of radiation dosimetry for molecular radiotherapy (MRT). The requirements for patient-specific dosimetry using single photon emission computed tomography (SPECT) are challenging, particularly with respect to scatter correction. In this paper data from phantom studies, combined with results from a fully validated Monte Carlo (MC) SPECT camera simulation, are used to investigate the influence of the triple energy window (TEW) scatter correction on SPECT activity quantification for Lu MRT.Results from phantom data show that; (1) activity quantification for the total counts in the SPECT field-of-view demonstrates a significant overestimation in total activity recovery when TEW scatter correction is applied at low activities (200 MBq). (2) Applying the TEW scatter correction to activity quantification within a volume-of-interest with no background activity provides minimal benefit. (3) In the case of activity distributions with background activity, an overestimation of recovered activity of up to 30% is observed when using the TEW scatter correction.Data from MC simulation were used to perform a full analysis of the composition of events in a clinically reconstructed volume of interest. This allowed, for the first time, the separation of the relative contributions of partial volume effects (PVE) and inaccuracies in TEW scatter compensation to the observed overestimation of activity recovery. It is shown, that even with perfect partial volume compensation, TEW scatter correction can overestimate activity recovery by up to 11%. MC data is used to demonstrate that even a localized and optimized isotope-specific TEW correction cannot reflect a patient specific activity distribution without prior knowledge of the complete activity distribution. This highlights the important role of MC simulation in SPECT activity quantification.

SU‐F‐J‐08: Quantitative SPECT Imaging of Ra‐223 in a Phantom

Purpose:Ra‐223 therapy of prostate cancer bone metastases is being used to treat patients routinely. However, the absorbed dose distribution at the macroscopic and microscopic scales remains elusive, due to the inability to image the small activities injected. Accurate activity quantification through imaging is essential to calculate the absorbed dose in organs and sub‐units in radiopharmaceutical therapy, enabling personalized absorbed dose‐based treatment planning methodologies and more effective and optimal treatments.Methods:A 22 cm diameter by 20 cm long cylindrical phantom, containing a 3.52 cm diameter sphere, was used. A total of 2.01 MBq of Ra‐223 was placed in the phantom with 177.6 kBq in the sphere. Images were acquired on a dual‐head Siemens Symbia T16 gamma camera using three 20% full‐width energy windows and centered at 84, 154, and 269 keV (120 projections, 360° rotation, 45 s per view). We have implemented reconstruction of Ra‐223 SPECT projections using OS‐EM (up to 20 iterations of 10 subsets) with compensation for attenuation using CT‐based attenuation maps, collimator‐detector response (CDR) (including septal penetration, scatter and Pb x‐ray modeling), and scatter in the patient using the effective source scatter estimation (ESSE) method. The CDR functions and scatter kernels required for ESSE were computed using the SIMIND MC simulation code. All Ra‐223 photon emissions as well as gamma rays from the daughters Rn‐219 and Bi‐211 were modeled.Results:The sensitivity of the camera in the three combined windows was 107.3 cps/MBq. The visual quality of the SPECT images was reasonably good and the activity in the sphere was 27% smaller than the true activity. This underestimation is likely due to partial volume effect.Conclusion:Absolute quantitative Ra‐223 SPECT imaging is achievable with careful attention to compensate for image degrading factors and system calibration.

Quantitative imaging of 223Ra-chloride (Alpharadin) for targeted alpha-emitting radionuclide therapy of bone metastases

Ra is an alpha particle emitter that targets areas of increased bone turnover in bone metastases. Alpha particles account for 95% of the 27.8 MeV emitted per decay. Less than 2% of the emissions are from photons. This means that a high absorbed dose will be delivered locally, although the number of photons for imaging will be low. The purpose of this study was to investigate the possibility of quantitative imaging of Ra to enable biodistribution studies. A Philips Forte gamma camera, equipped with a medium-energy collimator, was used. Basic imaging parameters were determined from phantom studies, and the accuracy of activity quantification was tested in a phantom study and within a patient study. Imaging parameters were determined for the three most suitable photon peaks from the acquired energy spectrum (82, 154 and 270 keV). Camera sensitivity is constant for circular sources with areas greater than 10 cm. The spatial resolution (full-width at half-maximum) was 1.1 cm for each of the three energy windows. The possibility for quantitative imaging was further investigated for the 82 keV energy window, which showed the highest sensitivity and spatial resolution. A phantom study showed that activity could be quantified to within 10% for a 200 ml volume placed within water containing background activity and to within 50% for a 0.5 ml phantom. Quantification of activity in bone after administrations of 100 kBq/kg of Ra-chloride proved the feasibility of quantitative imaging of patients who have received radionuclide therapy. Because of the high-energy deposition of Ra, only a low injected activity is required for therapy, which results in a low count rate for the gamma camera. Nevertheless, this study has demonstrated that it is possible to quantify uptake with a sufficient degree of accuracy to obtain clinically relevant information.

DAcquisition setting optimization and quantitative imaging for 124I studies with the Inveon microPET-CT system

BackgroundNoninvasive multimodality imaging is essential for preclinical evaluation of the biodistribution and pharmacokinetics of radionuclide therapy and for monitoring tumor response. Imaging with nonstandard positron-emission tomography [PET] isotopes such as 124I is promising in that context but requires accurate activity quantification. The decay scheme of 124I implies an optimization of both acquisition settings and correction processing. The PET scanner investigated in this study was the Inveon PET/CT system dedicated to small animal imaging.MethodsThe noise equivalent count rate [NECR], the scatter fraction [SF], and the gamma-prompt fraction [GF] were used to determine the best acquisition parameters for mouse- and rat-sized phantoms filled with 124I. An image-quality phantom as specified by the National Electrical Manufacturers Association NU 4-2008 protocol was acquired and reconstructed with two-dimensional filtered back projection, 2D ordered-subset expectation maximization [2DOSEM], and 3DOSEM with maximum a posteriori [3DOSEM/MAP] algorithms, with and without attenuation correction, scatter correction, and gamma-prompt correction (weighted uniform distribution subtraction).ResultsOptimal energy windows were established for the rat phantom (390 to 550 keV) and the mouse phantom (400 to 590 keV) by combining the NECR, SF, and GF results. The coincidence time window had no significant impact regarding the NECR curve variation. Activity concentration of 124I measured in the uniform region of an image-quality phantom was underestimated by 9.9% for the 3DOSEM/MAP algorithm with attenuation and scatter corrections, and by 23% with the gamma-prompt correction. Attenuation, scatter, and gamma-prompt corrections decreased the residual signal in the cold insert.ConclusionsThe optimal energy windows were chosen with the NECR, SF, and GF evaluation. Nevertheless, an image quality and an activity quantification assessment were required to establish the most suitable reconstruction algorithm and corrections for 124I small animal imaging.

Planar imaging quantification using 3D attenuation correction data and Monte Carlo simulated buildup factors

A new method to correct for attenuation and the buildup of scatter in planar imaging quantification is presented. The method is based on the combined use of 3D density information provided by computed tomography to correct for attenuation and the application of Monte Carlo simulated buildup factors to correct for buildup in the projection pixels. CT and nuclear medicine images were obtained for a purpose-built nonhomogeneous phantom that models the human anatomy in the thoracic and abdominal regions. The CT transverse slices of the phantom were converted to a set of consecutive density maps. An algorithm was developed that projects the 3D information contained in the set of density maps to create opposing pairs of accurate 2D correction maps that were subsequently applied to planar images acquired from a dual-head gamma camera. A comparison of results obtained by the new method and the geometric mean approach based on published techniques is presented for some of the source arrangements used. Excellent results were obtained for various source - phantom configurations used to evaluate the method. Activity quantification of a line source at most locations in the nonhomogeneous phantom produced errors of less than 2%. Additionally, knowledge of the actual source depth is not required for accurate activity quantification. Quantification of volume sources placed in foam, Perspex and aluminium produced errors of less than 7% for the abdominal and thoracic configurations of the phantom.