Evaluation of six scatter correction methods based on spectral analysis in (99m)Tc SPECT imaging using SIMIND Monte Carlo simulation

Exploration of Coincidence Detection of Cascade Photons to Enhance Preclinical Multi-Radionuclide SPECT Imaging

We proposed a technique of coincidence detection of cascade photons (CDCP) to enhance preclinical SPECT imaging of therapeutic radionuclides emitting cascade photons, such as Lu-177, Ac-225, Ra-223, and In-111. We have carried out experimental studies to evaluate the proposed CDCP-SPECT imaging of low-activity radionuclides using a prototype coincidence detection system constructed with large-volume cadmium zinc telluride (CZT) imaging spectrometers and a pinhole collimator. With In-111 in experimental studies, the CDCP technique allows us to improve the signal-to-contamination in the projection (Projection-SCR) by ~53 times and reduce ~98% of the normalized contamination. Compared to traditional scatter correction, which achieves a Projection-SCR of 1.00, our CDCP method boosts it to 15.91, showing enhanced efficacy in reducing down-scattered contamination, especially at lower activities. The reconstructed images of a line source demonstrated the dramatic enhancement of the image quality with CDCP-SPECT compared to conventional and triple-energy-window-corrected SPECT data acquisition. We also introduced artificial energy blurring and Monte Carlo simulation to quantify the impact of detector performance, especially its energy resolution and timing resolution, on the enhancement through the CDCP technique. We have further demonstrated the benefits of the CDCP technique with simulation studies, which shows the potential of improving the signal-to-contamination ratio by 300 times with Ac-225, which emits cascade photons with a decay constant of ~0.1 ns. These results have demonstrated the potential of CDCP-enhanced SPECT for imaging a super-low level of therapeutic radionuclides in small animals.

An anthropomorphic body phantom for the determination of calibration factor in radionuclide treatment dosimetry

The aim of this study is to create an inhomogeneous human-like phantom, whose attenuation and scattering effects are similar to the human body, as an alternative to the homogeneous phantoms traditionally used in calibration factor (CF) determination. The phantom was designed to include the thorax, abdomen and upper pelvis regions sized to represent a 75-kg male with a body mass index of 25. Measurements using Lu-177 with 50- and 100-mL lesion volumes were performed using inhomogeneous anthropomorphic body phantom (ABP) and homogeneous NEMA PET body phantom. There was a difference of 5.7% of Calibration Factor including attenuation and scatter effect between ABP and NEMA PET body phantom. Because it better reflects the attenuation and scatter effect, it is recommended to use a human-like inhomogeneous phantom for determination of CF instead of a homogeneous phantom.

Investigation of Response of the Pixelated CZT SPECT Imaging System and Comparison with the Conventional SPECT System

Purpose

The aim of this study is to investigate the factors affecting the response of the pixelated CZT SPECT imaging systems and to compare the performance of these systems with the conventional SPECT imaging systems.

Materials and Methods

By using the simulation technique, the effect of applied voltage, gap size between the anode pixels, and electron cloud mobility on the response of a pixelated CZT SPECT system are investigated. Then, the response of this system is compared with the conventional SPECT system in both single- and dual-radioisotope imaging.

Results

The results of this study show that increasing the applied voltage, electron cloud mobility or decreasing the gap size, in the optimal range of these parameters obtained in this study, leads to reducing the lateral charge diffusion and consequently improving the probability of the complete charge collection by the target anode pixel. In dual-radioisotope imaging by the pixelated CZT SPECT system, although higher energy resolution results in better separation of photopeaks, the presence of a low-energy tail leads to an overestimation of counts in the low-energy photopeak.

Conclusion

The use of the optimal values for the applied voltage, gap size, and electron cloud mobility strongly affect the performance of the pixelated CZT SPECT systems. In addition, the presence of a tail restricts the use of these systems for dual-radioisotope imaging and also, the use of the conventional methods for scatter correction.

Investigation of Different Components of Parallel-Hole Collimator Response to Different Radioisotope Energies Used in Nuclear Medicine Imaging

Introduction

The quality of images obtained from the nuclear medicine imaging systems depends on different factors. One of the most important of these factors is the geometrical and physical characteristics of collimator used for imaging with a given radioisotope.

Aims and Objectives

The aim of this study is to investigate the contribution of different components of collimator response for determining the most suitable parallel-hole collimator for the different radioisotope energies used in nuclear medicine imaging.

Materials and Methods

In this study, the SIMIND Monte Carlo simulation program is used to determine the contribution of geometrical, penetrating and scattered response components of four hexagonal parallel-hole collimators including low-energy high-resolution (LEHR), low-energy general-purpose (LEGP), medium-energy general-purpose (MEGP), and high-energy general-purpose (HEGP) collimators, for 12 different energies used in nuclear medicine imaging.

Results

According to the simulation results, the use of both the LEHR and LEGP collimators leads to a geometrical component above 60% for energies between 69 and 171 keV. On the other hand, for energies between 185 and 245 keV, the MEGP collimator and for energy of 364 keV, the HEGP collimator gives the geometrical components above 70% and 60%, respectively, while for energy of 511 keV, the geometrical response of all four collimators is below 20%.

Conclusion

The results of this study show that for two low-energy single-photopeak radioisotopes, Tc-99m and I-123, the LEHR and LEGP collimators, and for high-energy single-photopeak radioisotope, I-131, the HEGP collimator are most suitable collimators. For dual-photopeak In-111 radioisotope and triple-photopeak Ga-67 radioisotope, the MEGP and HEGP collimators and for triple-photopeak Tl-201 radioisotopes, the LEHR and LEGP collimators are proposed as most suitable collimators.

Evaluation of quantitative accuracy among different scatter corrections for quantitative bone SPECT/CT imaging

Although scatter correction improves SPECT image contrast and thus image quality, the effects of quantitation accuracy under various conditions remain unclear. The present study aimed to empirically define the conditions for the optimal scatter correction of quantitative bone SPECT/CT images. Scatter correction was performed by applying dual and triple energy windows (DEW and TEW) with different sub-energy window widths, and effective scatter source estimation (ESSE) to CT-based scatter correction. Scattered radiation was corrected on images acquired using a triple line source (TLSP) phantom and an uniform cylinder phantom. The TLSP consisted of a line source containing 74.0 MBq of 99mTc in the middle, and a background component containing air, water or a K2HPO4 solution with a density equivalent to that of bone. The sum of all pixels in air, water and the K2HPO4 solution was measured on SPECT images. Scatter fraction (SF) and normalized mean square error (NMSE) based on counts from the air background as a reference were then calculated to assess quantitative errors due to scatter correction. The uniform cylinder phantom contained the same K2HPO4 solution and 222.0 MBq of 99mTc. The coefficient of variation (CV) was calculated from the count profile of this phantom to assess the uniformity of SPECT images across scatter correction under various conditions. Both SF and NMSE in SPECT images of phantoms containing water in the background were lower at a TEW sub-window of 3% (TEW3%), than in other scatter corrections, whereas those in K2HPO4 were lower at a DEW sub-window of 20% (DEW20%). Larger DEW and smaller TEW sub-energy windows allowed more effective correction. The CV of the uniform cylinder phantom, DEW20%, was inferior to all other tested scatter corrections. The quantitative accuracy of bone SPECT images substantially differed according to the method of scatter correction. The optimal scatter correction for quantitative bone SPECT was DEW20% (k = 1), but at the cost of slightly decreased image uniformity.

An alternative method for radioactivity measurement in quantitative bone SPECT/CT imaging

Bone scintigraphy with combined single-photon emission computed tomography (SPECT) and computed tomography (CT) has become widely used for the detection of bone metastases. However, calculation of the semi-quantitative standardized uptake value (SUV) requires measurement of the pre- and post-injection radioactivity of the radiopharmaceutical. This study aimed to compare measured and fixed input radioactivity values for quantitative SPECT/CT bone imaging to examine whether the fixed measurement method of radiopharmaceutical radioactivity could be used as an alternative method. Four different methods were used to quantify the Tc-99m hydroxymethylene diphosphonate input radioactivity: (A) measured pre- and post-injection radioactivity values; (B) measured pre-injection and fixed post-injection radioactivity values; (C) fixed pre-injection and measured post-injection radioactivity values; (D) fixed pre- and post-injection radioactivity values. All SPECT/CT acquisitions were analyzed using bone SPECT analysis software, and the semi-quantitative parameters (SUVpeak and SUVmean) were recorded and compared for each analytical method. Two semi-quantitative parameters showed significant differences between analytical methods A and B, A and D, and C and D. However, an additional subgroup analysis performed on patients whose median post-injection measured radioactivity value was <1.5 MBq showed no significant differences in parameters between all analytical methods. Measurement of the radiopharmaceutical radioactivity can be an alternative method because it reduces the volume of radioactivity post-injection. The simplified fixed measurement method of radiopharmaceutical radioactivity can be used as an alternative method in cases when measuring the radioactivity in quantitative bone SPECT/CT imaging is missed.

Variation of Contrast Values for Myocardial Perfusion Imaging in Single-photon Emission Computed Tomography/Computed Tomography Hybrid Systems with Different Correction Methods

Objectives:

Single-photon emission computed tomography/computed tomography (SPECT/CT) hybrid systems have the advantage of performing various scans using the same imaging setting. Absorption and scattering of the gamma rays by the patient’s body significantly affect images obtained from scintigraphy, especially in myocardial perfusion imaging. An important parameter for image quality in SPECT is image contrast which is defined as the difference in density between regions of the image corresponding to different levels of radioactive uptake in the patient. The objective of the study was to evaluate the influence of applying different correction methods on image contrast of myocardial SPECT/CT images.

Material and Methods:

A total of 114 patients, 43 females and 71 males, patient’s raw data were processed and analyzed using attenuation correction (AC), scatter correction (SC), both attenuation and scatter correction together (ACSC), and no correction (NC). The short axis (coronal) slices resulted from the raw data reconstruction were chosen to perform the processing for hot and cold spheres for contrast values measurement. Statistical analysis was made for the measured contrast values for AC, SC, ACSC, and NC to determine the best image contrast.

Results:

When applying SC alone, it yields better contrast value (0.834), compared to AC (0.677) and ACSC (0.739). Both ACSC and AC had better image contrast compared to NC (0.592).

Conclusion:

The intercomparison study between the correction conditions indicates that the counts in SPECT/ CT are highly affected by all correction methods. The image contrast has been significantly improved by using SC, AC, and ACSC when compared with the NC image. Furthermore, SC is superior in the image contrast than the other correction conditions in the reconstruction of SPECT/CT MPI.

Assessment of Four Scatter Correction Methods in In-111 SPECT Imaging: A Simulation Study

Introduction:

Detection of compton scattered photons is one of the most important factors affecting the quality of single-photon emission computed tomography (SPECT) images. In most cases, the multiple-energy window acquisition methods are used for estimation of the scatter contribution into the main energy window(s) used in imaging.

Aims and Objectives:

The purpose of this study is to evaluate and compare the performance of four different scatter correction methods in In-111 SPECT imaging. Due to the lack of sufficient studies in this field, it can be useful to perform a more detailed and comparative study.

Materials and Methods:

Four approximations for scatter correction of In-111 SPECT images are evaluated by using the Monte Carlo simulation. These methods are firstly applied on each of photopeak windows, separately. Then, the effect of the correction methods is investigated by considering both the photopeak windows. The images obtained from a simulated multiple-spheres phantom are used for the evaluation of the correction methods by using three assessment criteria, including the image contrast, relative noise, and the recovery coefficient.

Results:

The results of this study show that the correction methods, when using the single photopeak windows, result in increase in image contrast with a significant level of noise. In return, when both the photopeak energy windows are used for imaging, it is possible to achieve the better image characteristics.

Conclusion:

The use of the proposed correction methods, by considering both the photopeak windows, leads to improve the image contrast with a reasonable level of image noise.

Optimization of Scatter Correction Method in Samarium-153 Single-photon Emission Computed Tomography using Triple-Energy Window: A Monte Carlo Simulation Study

PURPOSE

In single-photon emission computed tomography imaging, the presence of scatter degrades image quality. The goal of this study is to optimize the main- and sub-energy windows for triple-energy window (TEW) method using Monte Carlo SImulating Medical Imaging Nuclear Detectors (SIMIND) code for samarium-153 (Sm-153) imaging.

MATERIALS AND METHODS

The comparison is based on the Monte Carlo simulation data with the results estimated using TEW method. Siemens Symbia gamma-camera equipped with low-energy high-resolution collimator was simulated for Sm-153 point source located in seven positions in water cylindrical phantom. Three different main-energy window widths (10%, 15%, and 20%) and three different sub-energy window widths (2, 4, and 6 keV) were evaluated. We compared the true scatter fraction determined by SIMIND and scatter fraction estimated using TEW scatter correction method at each position. In order to evaluate the image quality, we used the full width at half maximum (FWHM) computed on the PSF and image contrast using Jaszczak phantom.

RESULTS

The scatter fraction using TEW method is similar to the true scatter fraction for 20% of the main-energy window and 6 keV sub-energy windows. For these windows, the results show that the resolution and contrast were improved.

CONCLUSION

TEW method could be a useful scatter correction method to remove the scatter event in the image for Sm-153 imaging.

Investigation of Different Factors Affecting the Quality of SPECT Images: A Simulation Study

Background:

Monte Carlo (MC) simulation codes are used extensively for modeling the nuclear medicine imaging systems, such as single photon emission computed tomography (SPECT) and positron emission tomography (PET). By using these codes, it is possible to set different imaging parameters and do various studies in the field of nuclear medicine imaging.

Aims and Objectives:

The aim of this study is to investigate the effective factors in improvement of the SPECT image quality by using MC simulation.

Materials and Methods:

In this study, we used the SIMIND MC simulation code and Jaszczak phantom containing six spheres with different diameters placed into a water-filled cylindrical phantom for consideration of the effects of different factors on quality of the images obtained from Tc-99m SPECT imaging system. The assessment criteria used to investigate these factors included image contrast, signal-to-noise ratio (SNR) and relative noise of the background (RNB).

Results:

The results of this study show that the right choice of the arc of rotation, the image matrix size, the number of angular views, type of the collimators, and also filters used in the image reconstruction affect the quality of SPECT images. Also, we show that use of scatter correction methods can improve the image quality.

Conclusion:

The MC simulation is a suitable tool for investigation of different factors affecting the quality of SPECT images, essentially in the studies based on the energy spectrum, such as the evaluation of the scatter correction methods.

Image quality parameters in brain imaging with fan-beam collimator: a Monte Carlo study on radiation scattering effects

The effects of scattered radiation on image quality in brain imaging with fan-beam collimator were quantitatively evaluated. A commercial gamma camera in conjunction with a fan-beam collimator was simulated using MCNPX code. The effects of radiation scattering on image quality were evaluated by employing the Snyder phantom and comparing the system response to an isotropic ^99mTc point source in both spatial and frequency domains. The trans-axial spatial resolution of the obtained point spread functions were studied in the spatial domain, at source-to-collimator distances of 2, 4, 6, 8, and 10 cm. At the same distances, the spatial frequencies at 90% (SF_0.9) and 10% (SF_0.1) of the maximum modulation transfer function were considered in the frequency domain. The maximum difference between the obtained full width at half-maximum in presence and absence of phantom was approximately 5%, while this difference was 14% for full width at tenth maximum. An analysis of system response in the frequency domain demonstrated a large difference of 43% between the obtained SF_0.9 values in presence and absence of phantom. In contrast, this difference was a mere ~ 2% between the obtained SF_0.1 values. Radiation scattering mainly degrades the image contrast resolution and has no considerable effect on the spatial resolution of the images acquired by the fan-beam collimator. Accordingly, the impact of radiation scattering on image quality was more obvious in frequency domain, and SF_0.9 can be considered as an operational parameter for the quantitative assessment of radiation scattering effects on image quality in the frequency domain.

Improved scatter correction with factor analysis for planar and SPECT imaging

Quantitative nuclear medicine imaging is an increasingly important frontier. In order to achieve quantitative imaging, various interactions of photons with matter have to be modeled and compensated. Although correction for photon attenuation has been addressed by including x-ray CT scans (accurate), correction for Compton scatter remains an open issue. The inclusion of scattered photons within the energy window used for planar or SPECT data acquisition decreases the contrast of the image. While a number of methods for scatter correction have been proposed in the past, in this work, we propose and assess a novel, user-independent framework applying factor analysis (FA). Extensive Monte Carlo simulations for planar and tomographic imaging were performed using the SIMIND software. Furthermore, planar acquisition of two Petri dishes filled with ^99mTc solutions and a Jaszczak phantom study (Data Spectrum Corporation, Durham, NC, USA) using a dual head gamma camera were performed. In order to use FA for scatter correction, we subdivided the applied energy window into a number of sub-windows, serving as input data. FA results in two factor images (photo-peak, scatter) and two corresponding factor curves (energy spectra). Planar and tomographic Jaszczak phantom gamma camera measurements were recorded. The tomographic data (simulations and measurements) were processed for each angular position resulting in a photo-peak and a scatter data set. The reconstructed transaxial slices of the Jaszczak phantom were quantified using an ImageJ plugin. The data obtained by FA showed good agreement with the energy spectra, photo-peak, and scatter images obtained in all Monte Carlo simulated data sets. For comparison, the standard dual-energy window (DEW) approach was additionally applied for scatter correction. FA in comparison with the DEW method results in significant improvements in image accuracy for both planar and tomographic data sets. FA can be used as a user-independent approach for scatter correction in nuclear medicine.

Evaluation of Compton attenuation and photoelectric absorption coefficients by convolution of scattering and primary functions and counts ratio on energy spectra

Purpose:

Estimation of Compton attenuation and the photoelectric absorption coefficients were explored at various depths.

Methods:

A new method was proposed for estimating the depth based on the convolution of two exponential functions, namely convolution of scattering and primary functions (CSPF), which the convolved result will conform to the photopeak region of energy spectrum with the variable energy-window widths (EWWs) and a theory on the scattering cross-section. The triple energy-windows (TEW) and extended triple energy-windows scatter correction (ETEW) methods were used to estimate the scattered and primary photons according to the energy spectra at various depths due to a better performance than the other methods in nuclear medicine. For this purpose, the energy spectra were employed, and a distinct phantom along with a technetium-99 m source was simulated by Monte Carlo method.

Results:

The simulated results indicate that the EWW, used to calculate the scattered and primary counts in terms of the integral operators on the functions, was proportional to the depth as an exponential function. The depth will be calculated by the combination of either TEW or ETEW and proposed method resulting in the distinct energy-window. The EWWs for primary photons were in good agreement with those of scattered photons at the same as depths. The average errors between these windows for both methods TEW, and ETEW were 7.25% and 6.03% at different depths, respectively. The EWW value for functions of scattered and primary photons was reduced by increasing the depth in the CSPF method.

Conclusions:

This coefficient may be an index for the scattering cross-section.