The effect of tumour geometry on the quantification accuracy of planar 123I phantom images

IntroductionAccurate activity quantification is applied in radiation dosimetry. Planar images are important for quantification of whole-body images, enabling assessment of biodistribution from radionuclide administrations. We evaluated the effect of tumour geometry on quantification accuracy of ¹²³I planar phantom studies, including various tumour sizes, tumour-liver distances and two tumour-background ratios.Methods and materialsAn in-house manufactured abdominal phantom was equipped with a liver, different size cylindrical tumours, and a rod for tumour-liver distance variation. The geometric mean method with scatter and attenuation corrections was used for image processing. Scatter and attenuation corrections were made using the triple energy window scatter correction technique and a printed transmission sheet source, respectively. Region definitions for tumour activity distribution compensated for the partial volume effect (PVE). Activity measured in the dose calibrator served as reference for determining quantification accuracy.ResultsThe smallest tumour had the largest percentage deviation with an average activity underestimation of 34.6 ± 1.2%. Activity values for the largest tumour were overestimated by 3.1 ± 3.0%. PVE compensation improved quantification accuracy for all tumour sizes yielding accuracies of <12.4%. Scatter contribution to the tumours from the liver had minimal effect on quantification accuracy at tumour-liver distances >3 cm. With PVE compensation, increased tumour-background ratio resulted in a percentage increase of up to 26.3%.ConclusionWhen applying relevant corrections for scatter, attenuation and PVE without background activity, quantification accuracy of <13% was obtained. We demonstrated the successful implementation of a practical technique to obtain quantitative information from ¹²³I planar images.     

Characteristics of Bremsstrahlung emissions of 177Lu, 188Re,and 90Y for SPECT/CT quantification in radionuclide therapy

PurposeBeta particles emitted by radioisotopes used in targeted radionuclide therapies (TRT) create Bremsstrahlung (BRS) which may affect SPECT quantification when imaging these isotopes. The purpose of the current study was to investigate the characteristics of Bremsstrahlung produced in tissue by three β-emitting radioisotopes used in TRT.MethodsMonte Carlo simulations of ¹⁷⁷Lu, ¹⁸⁸Re, and ⁹⁰Y sources placed in water filled cylinders were performed. BRS yields, mean energies and energy spectra for (a) all photons generated in the decays, (b) photons that were not absorbed and leave the cylinder, and (c) photons detected by the camera were analyzed. Next, the results of simulations were compared with those from experiments performed on a clinical SPECT camera using same acquisition conditions and phantom configurations as in simulations.ResultsSimulations reproduced relatively well the shapes of the measured spectra, except for ⁹⁰Y which showed an overestimation in the low energy range. Detailed analysis of the results allowed us to suggest best collimators and imaging conditions for each of the investigated isotopes. Finally, our simulations confirmed that the BRS contribution to the energy spectra in quantitative imaging of ¹⁷⁷Lu and ¹⁸⁸Re could be ignored.ConclusionsFor ¹⁷⁷Lu and ¹⁸⁸Re, BRS contributes only marginally to the total spectra recorded by the camera. Our analysis shows that MELP and HE collimators are the best for imaging these two isotopes. For ⁹⁰Y, HE collimator should be used.     

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 (208 keV) using a Philips IRIX gamma camera provided with medium-energy collimators. System calibration was performed using a 16 mL 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 208 keV photopeak using medium-energy collimators.     

The effect of scatter correction and radius of rotation on semiquantitative measurements in SPECT 123I-FP-CIT imaging. A phantom study

PurposeThe high energy emissions of ¹²³I and the suboptimal radius of rotation affect the semiquantitative measurements performed during ¹²³I-FP-CIT tomographic imaging. An in-house extra low cost striatum phantom with brain and striatum compartments was constructed and was used to study the effects of Triple Energy Window scatter correction (TEW-SC) and radius of rotation on the Specific Binding Ratio (SBR) measurements.Materials and methodsThe phantom compartments were filled with radioactive ¹²³I solutions with varying concentrations, in a series of experiments. Tomographic images were acquired at six different radii of rotation, with and without TEW-SC and the SBRs were calculated using appropriate regions of interest, as in clinical imaging.ResultsSBRs decreased with increasing radius of rotation in both non-SC and TEW-SC images, the decrease being more pronounced in the latter. The application of TEW-SC increases SBR values by 40% on average. A maximum %Recovery of 42.7% of the true SBR value was achieved in the non-SC images, which increased to 64.6% after TEW-SC. Appropriate correction factors (CF) were calculated in order to make the SBR values independent on the radius of rotation, which could be used to correct SBR values obtained from tomographic acquisitions with suboptimal radius of rotation.ConclusionThe use of appropriate CF can provide more consistent SBR values and a more meaningful comparison between SBRs calculated from images acquired at different radii of rotation.     

Gamma densitometry for the measurement of skeletal density

A method is described for the measurement of the density of calcium carbonate materials from the attenuation of a narrow, collimated beam of gamma photons. For the measurement of density for slices, approximately 0.5 to 1.0 cm thick, from the skeletons of reef building corals, the optimum beam energy is 30–34 keV; and measurement is practical from approximately 22 to 100 keV. The potential utilities of five commercially available isotopic sources (¹⁰⁹Cd,¹²⁵I,²⁵³Gd,²¹⁰Pb and²⁴¹Am) are evaluated. Methods and results are presented for gamma densitometry using²¹⁰Pb and²⁴¹Am. The²¹⁰Pb point source had its principal gamma emission at 46.5 keV. Bremsstrahlung and high energy (800 keV) gamma emissions associated with the²¹⁰Pb decay grand-daughter were detected, and procedures were developed to accommodate the contribution of these emissions to the overall count rate. The attenuation of count rate by aluminium and aragonite absorbers closely followed simple theoretical considerations provided that narrow energy window settings were used at the radiation monitor. These theoretical considerations take account of the density of the material absorbing the radiation, and hence the density could be determined from the attenuation of the gamma beam. Increased accuracy was achieved by the use of²⁴¹Am and high speed counting equipment.²⁴¹Am has its principal gamma emission at 59.6 keV. The attenuation of this gamma beam follows simple theoretical considerations for targets with mass thicknesses from 0 to 6 g cm^-2. Aragonite from the shell of a giant clam was found to have slightly different properties in the absorption of gamma photons to aragonite from a coral skeleton. The differences were small but statistically significant.     

Evaluation of three scatter correction methods based on estimation of photopeak scatter spectrum in SPECT imaging: A simulation study

Three practical methods for scatter correction of Tc-99m SPECT images are evaluated. Among these, two methods, three-energy window (TEW) methods using the trapezoidal and triangular approximations, have been described previously by investigators, and a new approximation is offered in this work. The SIMIND (SIMulation of Imaging Nuclear Detectors) Monte Carlo program is used to simulate a line source placed at on-axis and 5 cm off-axis locations, a cold-sphere/hot-background phantom, a hot-sphere/cold-background phantom, and a more clinically realistic NCAT (Nonuniform Rational B-spline-based CArdiac-Torso) phantom. For evaluation of these methods, the scatter line-spread functions and scatter fractions for the on- and off-axis line source, image contrast, signal-to-noise ratio and relative noise for the cold spheres, and recovery coefficient for the hot spheres of different diameters are compared. For the NCAT phantom, a line profile through a slice of the reconstructed image is considered before and after scatter correction, and also image contrast defined by this profile is used to compare the correction methods. The results of this study indicate that for the line source simulation the scatter fractions obtained from the proposed method are a better estimation of true scatter fractions. Also, for both the sphere simulation and NCAT simulation, the proposed method improves the image contrast as compared to the two other methods.     

Performance and limitations of positron emission tomography (PET) scanners for imaging very low activity sources

Emerging applications for positron emission tomography (PET) may require the ability to image very low activity source distributions in the body. The performance of clinical PET scanners in the regime where activity in the field of view is <1 MBq has not previously been explored. In this study, we compared the counting rate performance of two clinical PET/CT scanners, the Siemens Biograph Reveal 16 scanner which is based on lutetium oxyorthosilicate (LSO) detectors and the GE Discovery-ST scanner which is based on bismuth germanate (BGO) detectors using a modified National Electrical Manufacturers Association (NEMA) NU 2-2007 protocol. Across the activity range studied (2–100 kBq/mL in a 5.5 mL line source in the NEMA scatter phantom), the BGO-based scanner significantly outperformed the LSO-based scanner. This was largely due to the effect of background counts emanating from naturally occurring but radioactive ¹⁷⁶Lu within the LSO detector material, which dominates the observed counting rate at the lowest activities. Increasing the lower energy threshold from 350 keV to 425 keV in an attempt to reduce this background did not significantly improve the measured NECR performance. The measured singles rate due to ¹⁷⁶Lu emissions within the scanner energy window was also found to be dependent on temperature, and to be affected by the operation of the CT component, making approaches to correct or compensate for the background more challenging. We conclude that for PET studies in a very low activity range, BGO-based scanners are likely to have better performance because of the lack of significant background.     

Validation of irtGPUMCD,a GPU-based Monte Carlo internal dosimetry framework for radionuclide therapy

PurposeMonte Carlo (MC) simulations are highly desirable for dose treatment planning and evaluation in radiation oncology. This is true also in emerging nuclear medicine applications such as internal radiotherapy with radionuclides. The purpose of this study is the validation of irtGPUMCD, a GPU-based MC code for dose calculations in internal radiotherapy.MethodsThe female and male phantoms of the International Commission on Radiological Protection (ICRP 110) were used as benchmarking geometries for this study focused on ¹⁷⁷Lu and including ^99mTc and ¹³¹I. Dose calculations were also conducted for a real patient. For phantoms, twelve anatomical structures were considered as target/source organs. The S-values were evaluated with irtGPUMCD simulations (10⁸ photons), with gamma branching ratios of ICRP 107 publication. The ¹⁷⁷Lu electrons S-values were calculated for source organs only, based on local deposition of dose in irtGPUMCD. The S-value relative difference between irtGPUMCD and IDAC-DOSE were evaluated for all targets/sources considered. A DVHs comparison with GATE was conducted. An exponential track length estimator was introduced in irtGPUMCD to increase computational efficiency.ResultsThe relative S-value differences between irtGPUMCD and IDAC-DOSE were <5% while this comparison with GATE was <1%. The DVHs dosimetric indices comparison between GATE and irtGPUMCD for the patient led to an excellent agreement (<2%). The time required for the simulation of 10⁸ photons was 1.5 min for the female phantom, and one minute for the real patient (<1% uncertainty). These results are promising and let envision the use of irtGPUMCD for internal dosimetry in clinical applications.     

Enhancement of soft-tissue contrast in cone-beam CT using an anti-scatter grid with a sparse sampling approach

PurposeAnti-scatter grids suppress the scatter substantially thus improving image contrast in radiography. However, its active use in cone-beam CT for the purpose of improving contrast-to-noise ratio (CNR) has not been successful mainly due to the increased noise related to Poisson statistics of photons. This paper proposes a sparse-view scanning approach to address the above issue.MethodCompared to the conventional cone-beam CT imaging framework, the proposed method reduces the number of projections and increases exposure in each projection to enhance image quality without an additional cost of radiation dose to patients. For image reconstruction from sparse-view data, an adaptive-steepest-descent projection-onto-convex-sets (ASD POCS) algorithm regularized by total-variation (TV) minimization was adopted. Contrast and CNR with various scattering conditions were evaluated in projection domain by a simulation study using GATE. Then we evaluated contrast, resolution, and image uniformity in CT image domain with Catphan phantom. A head phantom with soft-tissue structures was also employed for demonstrating a realistic application. A virtual grid-based estimation and reduction of scatter has also been implemented for comparison with the real anti-scatter grid.ResultsIn the projection domain evaluation, contrast and CNR enhancement was observed when using an anti-scatter grid compared to the virtual grid. In the CT image domain, the proposed method produced substantially higher contrast and CNR of the low-contrast structures with much improved image uniformity.ConclusionWe have shown that the proposed method can provide high-quality CBCT images particularly with an increased contrast of soft-tissue at a neutral dose for image-guidance.     

Minimum acceptable sensitivity of intraoperative gamma probes used for sentinel lymph node detection in melanoma patients

The aim of this study was to determine the suspension level for the sensitivity of an intraoperative scintillation gamma probe in the detection of the sentinel lymph node (SLN) in melanoma patients.Thirty-eight consecutive patients with melanoma were enrolled in the study during a 12-month period and underwent lymphatic scintigraphy after the peritumoral intradermal administration of about 14 MBq of ^99mTc-nanocolloids. The SLNs were successfully removed during the surgical intervention about 4 h later.To identify and localize the SLN, a scintillation NaI(Tl) collimated probe was used. Predictably, the probe sensitivity decreased as the photopeak energy window was progressively narrowed, from 6.9 ± 0.7 counts per second (cps)/kBq (designated as the ‘optimum,’ or ‘OPT,’ sensitivity) to 2.5 ± 0.3 cps/kBq (LOW sensitivity) and to 1.4 ± 0.2 cps/kBq (VLOW sensitivity).Maximum lymph node count rates (cps) were determined for the foregoing energy windows prior to skin incision (PRE_OPT, PRE_LOW, PRE_VLOW, respectively) and in vivo after incision (INV_OPT, INV_LOW, INV_VLOW).Forty-three SLNs were removed with a mean source-to-detector distance of 46 ± 24 mm (min 12 mm, max 92 mm). Four SLNs could not have been detected using PRE_LOW. This figure would have decreased to 34, with nine undetectable lymph nodes, with PRE_VLOW. One SLN could not have been identified using INV_LOW and four could not have be identified using INV_VLOW.In the clinical scenario of SLN detection in melanoma patients, a system sensitivity of 2.5 cps/kBq represents a suspension level, that is, a level under which the equipment must be suspended from clinical use and the poor performance must be investigated.     

GATE Validation of Standard Dual Energy Corrections in Small Animal SPECT-CT

This paper addresses ¹²³I and ¹²⁵I dual isotope SPECT imaging, which can be challenging because of spectrum overlap in the low energy spectrums of these isotopes. We first quantify the contribution of low-energy photons from each isotope using GATE-based Monte Carlo simulations for the MOBY mouse phantom. We then describe and analyze a simple, but effective method that uses the ratio of detected low and high energy ¹²³I activity to separate the mixed low energy ¹²³I and ¹²⁵I activities. Performance is compared with correction methods used in conventional tissue biodistribution techniques. The results indicate that the spectrum overlap effects can be significantly reduced, if not entirely eliminated, when attenuation and scatter is either absent or corrected for using standard methods. In particular, we show that relative activity levels of the two isotopes can be accurately estimated for a wide range of organs and provide quantitative validation that standard methods for spectrum overlap correction provide reasonable estimates for reasonable corrections in small-animal SPECT/CT imaging.     

Simplified patient-specific renal dosimetry in 177Lu therapy: a proof of concept

PurposeThe aim of this proof-of-concept study is to propose a simplified personalized kidney dosimetry procedure in ¹⁷⁷Lu peptide receptor radionuclide therapy (PRRT) for neuroendocrine tumors and metastatic prostate cancer. It relies on a single quantitative SPECT/CT acquisition and multiple radiometric measurements executed with a collimated external probe, properly directed on kidneys.MethodsWe conducted a phantom study involving external count-rate measurements in an abdominal phantom setup filled with activity concentrations of ^99mTc, reproducing patient-relevant organ effective half-lives occurring in ¹⁷⁷Lu PRRT. GATE Monte Carlo (MC) simulations of the experiment, using ^99mTc and ¹⁷⁷Lu as sources, were performed. Furthermore, we tested this method via MC on a clinical case of ¹⁷⁷Lu-DOTATATE PRRT with SPECT/CT images at three time points (2, 20 and 70 hrs), comparing a simplified kidney dosimetry, employing a single SPECT/CT and probe measurements at three time points, with the complete MC dosimetry.ResultsThe experimentally estimated kidney half-life with background subtraction applied was compatible within 3% with the expected value. The MC simulations of the phantom study, both with ^99mTc and ¹⁷⁷Lu, confirmed a similar level of accuracy. Concerning the clinical case, the simplified dosimetric method led to a kidney dose estimation compatible with the complete MC dosimetry within 6%, 12% and 2%, using respectively the SPECT/CT at 2, 20 and 70 hrs.ConclusionsThe proposed simplified procedure provided a satisfactory accuracy and would reduce the imaging required to derive the kidney absorbed dose to a unique quantitative SPECT/CT, with consequent benefits in terms of clinic workflows and patient comfort.     

Energy response of radiophotoluminescent glass dosimeter for diagnostic kilovoltage x-ray beams

PurposeThis study aimed to investigate the energy response of a radiophotoluminescent glass dosimeter (RGD) for diagnostic kilovoltage x-ray beams by Monte Carlo (MC) calculations and measurements.MethodsThe uniformity and reproducibility of GD-352M (with Sn filter) and GD-302M (no filter) were tested with 45 RGDs in free air. Subsequently, the RGD response was obtained as a function of an Al-HVL using the parameter, quality index (QI), which is defined as the ratio of the effective energy (keV) to the maximum energy (keV) of the photons. The x-ray fluence spectra with QI of 0.4, 0.5, and 0.6 were set for tube voltages of 50 ~ 137.6 kVp. The RGD response was calculated in free air using the MC method and verified by the air kerma, K_air, measured using an ionization chamber.ResultsThe uniformity and reproducibility of the 45 RGDs were ± 2.3% and ± 2.7% for GD-352M and ± 0.7% and ± 1.6% for GD-302M at the one standard deviation level, respectively. The calculated RGD response was 0.965 to 1.062 at Al-HVL 2.73 mm or more for GD-352M and varied from 3.9 to 2.8 for GD-302M. Both RGD responses exhibited a good correlation with the Al-HVL for the given QI. K_air measured by RGDs for each beam quality with a QI of 0.5 was in the range of −5%~0.8% for GD-352M and −1.8%~3% for GD-302M, relative to the chamber measurements.ConclusionsThe RGD response was indicated as a function of the Al-HVL for the given QI, and it presented a good correlation with the Al-HVL.     

Fluorescence detection of carbofuran in aqueous extracts based on dual-emission SiO2@Y2O3:(Eu3+,Tb3+)@MIP core-shell structural nanoparticles

A novel double-windows fluorescence sensor for carbofuran (CF) detection was successfully developed based on rare-earth Eu,Tb-doped Y₂O₃@SiO₂-based molecularly imprinted nanoparticles (MINs) with a multilayer core-shell structure. The recognition process of the MINs for CF was fairly fast and needed only ~8 min to reach a dynamic equilibrium. Interestingly, one fluorescence attenuation window was found with an increase in CF concentration (Q) from 0.1 to 10 μg ml⁻¹ and with a limit of detection (LOD) of 0.04 μg ml⁻¹ at 544 nm belonging to the Tb³⁺ emission, as well as another fluorescence enhanced window within the CF concentration range 10–100 μg ml⁻¹ (LOD = 4 μg ml⁻¹) at 617 nm of Eu³⁺ emission in the dispersed rare-earth-doped MIN colloidal aqueous solution. Luminescence resonance energy transfer from CF to Eu³⁺ and an inner filter effect of CF towards Tb³⁺, as well from the two independent detection windows were clearly observed simultaneously. The competition experiment displayed hardly any marked interference during detection of CF following addition of its analogues (carbaryl, isoprocarb, aldicarb, methomyl, and etofenprox). Moreover, the MINs could also be applied to accurately detect CF in rhubarb and wolfberry samples with recoveries of 85.7–92.2%. This sensing system has high specific recognition and a wide detection range for CF and provides new opportunities for pesticide detection.     

An APD-based iterative reconstruction method for simultaneous technetium-99m/iodine-123 SPECT imaging

Dual-isotope SPECT (DI-SPECT) studies offer significant advantages over sequential scans, foremost among them faster acquisition and perfect image registration. However, reconstructed images may be affected by substantial cross-talk contamination rendering them inadequate for diagnosis. This effect is especially strong for isotopes with close photopeak energies, such as ^99mTc (140 keV) and ¹²³I (159 keV). In this paper we present an iterative DI-SPECT reconstruction method which includes accurate, analytically computed scatter corrections provided by the APD (analytical photon distribution) algorithm. This algorithm calculates first and second order Compton scatter (based on the Klein–Nishina formula) and first order Rayleigh scatter. Both self-scatter and cross-talk between the two isotopes are evaluated using patient specific attenuation maps and an initial activity distribution estimate. To validate our method we performed experiments using the Data Spectrum, Inc. thorax phantom and a SPECT/CT camera system. Reconstructed images demonstrate significant improvement in data quantitation. Their quantitative accuracy increases up to a factor of two, even for activity ratios which strongly enhance cross-talk effects and seriously degrade projections.     

An X-ray spectrum estimation method from transmission measurement combined with scatter correction

PurposeConventional x-ray spectrum estimation methods from transmission measurement often lead to inaccurate results when extensive x-ray scatter is present in the measured projection. This study aims to apply the weighted L1-norm scatter correction algorithm in spectrum estimation for reducing residual differences between the estimated and true spectrum.MethodThe scatter correction algorithm is based on a simple radiographic scattering model where the intensity of scattered x-ray is directly estimated from a transmission measurement. Then, the scatter-corrected measurement is used for the spectrum estimation method that consists of deciding the weights of predefined spectra and representing the spectrum as a linear combination of the predefined spectra with the weights. The performances of the estimation method combined with scatter correction are evaluated on both simulated and experimental data.ResultsThe results show that the estimated spectra using the scatter-corrected projection nearly match the true spectra. The normalized-root-mean-square-error and the mean energy difference between the estimated spectra and corresponding true spectra are reduced from 5.8% and 1.33 keV without the scatter correction to 3.2% and 0.73 keV with the scatter correction for both simulation and experimental data, respectively.ConclusionsThe proposed method is more accurate for the acquisition of x-ray spectrum than the estimation method without scatter correction and the spectrum can be successfully estimated even the materials of the filters and their thicknesses are unknown. The proposed method has the potential to be used in several diagnostic x-ray imaging applications.     

Energy spectra of secondary electrons in water vapour

The purpose of this work is to present a method for the calculation of secondary electron spectra generated by photons in water vapour in the energy region from 10 eV to 10 MeV. The cross sections below and above 1 keV have been treated separately. Examples are given for secondary electron spectra for low-energy photons, <100 eV, in which all electrons are photoelectrons, and at higher energy regions, such as for ⁶⁰Co photons. The spectrum of the first generation of secondary electrons, produced by ⁶⁰Co photons, which are mainly due to incoherent scattering, was fitted with a set of polynomial functions which can be used as input for electron radiation action calculations.     

Impact of iterative reconstructions on image quality and detectability of focal liver lesions in low-energy monochromatic images

PurposeTo assess the impact of iterative reconstructions on image quality and detectability of focal liver lesions in low-energy monochromatic images from a Fast kV-Switching Dual Energy CT (KVSCT) platform.MethodsAcquisitions on an image-quality phantom were performed using a KVSCT for three dose levels (CTDI_vol:12.72/10.76/8.79 mGy). Raw data were reconstructed for five energy levels (40/50/60/70/80 keV) using Filtered Back Projection (FBP) and four levels of ASIR (ASIR30/ASIR50/ASIR70/ASIR100). Noise power spectrum (NPS) and task-based transfer function (TTF) were measured before computing a Detectability index (d′) to model the detection task of liver metastasis (LM) and hepatocellular carcinoma (HCC) as function of keV.ResultsFrom 40 to 70 keV, noise-magnitude was reduced on average by −68% ± 1% with FBP; −61% ± 3% with ASIR50 and −52% ± 6% with ASIR100. The mean spatial frequency of the NPS decreased when the energy level decreased and the iterative level increased. TTF values at 50% decreased as the energy level increased and as the percentage of ASIR increased. The detectability of both lesions increased with increasing dose level and percentage of ASIR. For the LM, d′ peaked at 70 keV for all reconstruction types, except for ASIR70 at 12.72 mGy and ASIR100, where d' peaked at 50 keV. For HCC, d’ peaked at 60 keV for FBP and ASIR30 but peaked at 50 keV for ASIR50, ASIR70 and ASIR100.ConclusionsUsing percentage of ASIR above 50% at low-energy monochromatic images could limit the increase of noise-magnitude, benefit from spatial resolution improvement and hence enhance detectability of subtle low contrast focal liver lesions such as HCC.     

RBE of 25 kV X-rays for the survival and induction of micronuclei in the human mammary epithelial cell line MCF-12A

The broad application of low energy X-rays below about 50 keV in radiation therapy and diagnostics and especially in mammography substantiates the precise determination of their relative biological effectiveness (RBE). A quality factor of 1 is stated for photons of all energies in the International Commission on Radiological Protection Recommendations. However, the RBE of low-energy X-rays compared to high-energy photons was found to be dependent on photon energy, cell line and endpoints studied, hence varying from less than one up to about four. In the present study, the human mammary epithelial cell line MCF-12A has been chosen due to the implementation of the results in the estimation of risk from mammography procedures. The RBE of 25 kV X-rays (W anode, 0.3 mm Al filter) relative to 200 kV X-rays (W anode, 0.5 mm Cu filter) was determined for clonogenic survival in the dose range 1–10 Gy and micronuclei (MN) induction in the range 0.5–3.5 Gy. The RBE for clonogenic survival was found to be significantly higher than 1 for surviving fractions in the range 0.005 < S < 0.2. The RBE decreased with increasing survival, with an RBE_0.1 at 10% survival of 1.13 ± 0.03. The effectiveness of soft X-rays for MN induction was found to be 1.40 ± 0.07 for the fraction of binuclear cells (BNC) with MN and 1.44 ± 0.17 for the number of MN per BNC. In contrast, the RBE determined from the number of MN per MN-bearing BNC was found to be 1.08 ± 0.32. This indicates that the effectiveness of 25 kV X-rays results from an increase in the number of damaged cells, which, however, do not have higher number of MN per cell.     

Assessment of MIRD data for internal dosimetry using the GATE Monte Carlo code

GATE/GEANT is a Monte Carlo code dedicated to nuclear medicine that allows calculation of the dose to organs of voxel phantoms. On the other hand, MIRD is a well-developed system for estimation of the dose to human organs. In this study, results obtained from GATE/GEANT using Snyder phantom are compared to published MIRD data. For this, the mathematical Snyder phantom was discretized and converted to a digital phantom of 100 × 200 × 360 voxels. The activity was considered uniformly distributed within kidneys, liver, lungs, pancreas, spleen, and adrenals. The GATE/GEANT Monte Carlo code was used to calculate the dose to the organs of the phantom from mono-energetic photons of 10, 15, 20, 30, 50, 100, 200, 500, and 1000 keV. The dose was converted into specific absorbed fraction (SAF) and the results were compared to the corresponding published MIRD data. On average, there was a good correlation (r ²>0.99) between the two series of data. However, the GATE/GEANT data were on average −0.16 ± 6.22% lower than the corresponding MIRD data for self-absorption. Self-absorption in the lungs was considerably higher in the MIRD compared to the GATE/GEANT data, for photon energies of 10–20 keV. As for cross-irradiation to other organs, the GATE/GEANT data were on average +1.5 ± 8.1% higher than the MIRD data, for photon energies of 50–1000 keV. For photon energies of 10–30 keV, the relative difference was +7.5 ± 67%. It turned out that the agreement between the GATE/GEANT and the MIRD data depended upon absolute SAF values and photon energy. For 10–30 keV photons, where the absolute SAF values were small, the uncertainty was high and the effect of cross-section prominent, and there was no agreement between the GATE/GEANT results and the MIRD data. However, for photons of 50–1,000 keV, the bias was negligible and the agreement was acceptable.