Activity quantitation in SPECT: a study of prereconstruction Metz filtering and use of the scatter degradation factor

Development of a scatter correction technique for planar 99mTc-MAA imaging to improve accuracy in lung shunt fraction estimation

PURPOSE

Prior to ⁹⁰Y selective internal radiation therapy (SIRT) treatment, ^99mTc-MAA scintigraphy imaging is used in the estimation of the lung shunt fraction (LSF). Planar imaging is recommended for determining a LSF ratio. However, the estimate may be affected by scatter contributions, attenuation and respiratory motion. The objective of this study was to correct for the effects of scatter in the LSF, towards the determination of a more accurate estimation method of LSF derived from planar scintigraphy imaging, which is recommended by international guidelines.

METHODS

The open access SIMIND Monte Carlo modelling software was used to estimate an optimum scatter window (SW) for scatter correction. The uncertainties associated with scatter and scatter contributions from the liver on the LSF were evaluated using an anthropomorphic thorax phantom and a virtual Vox-Man phantom. A brief retrospective examination of patient scans and tumour location investigated the impact that the inclusion of the simulated scatter corrections had on the LSF estimation.

RESULTS

The percentage overestimation of the manufacturer recommended method of LSF estimation was 192%. SW corrections improved the uncertainty to within 19% for the range of known LSFs. Similar findings were observed for our patient and tumour location studies.

CONCLUSION

The incorporated scatter corrections can significantly improve the accuracy of the LSF estimation, thereby providing a robust gamma camera, patient and tumour depth specific correction which is easily implementable. This is supported by Monte Carlo, phantom and preliminary patient studies.

Application of Monte Carlo Algorithms to Cardiac Imaging Reconstruction

Monte Carlo algorithms have a growing impact on nuclear medicine reconstruction processes. One of the main limitations of myocardial perfusion imaging (MPI) is the effective mitigation of the scattering component, which is particularly challenging in Single Photon Emission Computed Tomography (SPECT). In SPECT, no timing information can be retrieved to locate the primary source photons. Monte Carlo methods allow an event-by-event simulation of the scattering kinematics, which can be incorporated into a model of the imaging system response. This approach was adopted in the late Nineties by several authors, and recently took advantage of the increased computational power made available by high-performance CPUs and GPUs. These recent developments enable a fast image reconstruction with improved image quality, compared to deterministic approaches. Deterministic approaches are based on energy-windowing of the detector response, and on the cumulative estimate and subtraction of the scattering component. In this paper, we review the main strategies and algorithms to correct the scattering effect in SPECT and focus on Monte Carlo developments, which nowadays allow the threedimensional reconstruction of SPECT cardiac images in a few seconds.

Digital Solid-State SPECT/CT Quantitation of Absolute 177Lu Radiotracer Concentration: In Vivo and In Vitro Validation

The accuracy of ¹⁷⁷Lu radiotracer concentration measurements using quantitative clinical software was determined by comparing in vivo results for a digital solid-state cadmium-zinc-telluride SPECT/CT system with in vitro sampling. Methods: First, image acquisition parameters were assessed for an International Electrotechnical Commission body phantom emulating clinical count rates loaded with a lung insert and 6 hot spheres with a 12:1 target-to-background ratio of ¹⁷⁷Lu solution. Then, the data of 28 whole-body SPECT/CT scans of 7 patients who underwent ¹⁷⁷Lu prostate-specific membrane antigen radioligand therapy were retrospectively analyzed. Three users analyzed SPECT/CT images for in vivo urinary bladder radiotracer uptake using quantitative software. In vitro radiopharmaceutical concentrations were calculated using urine sampling obtained immediately after each scan, scaled to SUVs. Any in vivo or in vitro identity relations were determined by linear regression (ideally, slope = 1 and intercept = 0), within a 95% confidence interval. Results: Phantom results demonstrated lower quantitative error for acquisitions using the 113-keV ¹⁷⁷Lu energy peak rather than including the 208-keV peak, given that only low-energy collimation was available in this camera configuration. In the clinical study, 24 in vivo-in vitro pairs were eligible for further analysis, with 4 having been rejected as outliers (via Cook distance calculations). All linear regressions (R ² ≥ 0.82, P < 0.0001) provided identity in vivo-in vitro relations (95% confidence interval), with SUV averages from all users giving a slope of 0.96 ± 0.13, an intercept of -0.07 ± 0.46 g/mL, and an average residual difference of 19.5%. In acquisitions with the lower-energy ¹⁷⁷Lu energy peak, solid-state SPECT/CT imaging provided an accuracy to within approximately 20% of in vivo urinary bladder radiotracer concentrations. Conclusion: This noninvasive in vivo quantitation method can potentially improve diagnosis, patient management, and treatment response assessment and provide data essential to ¹⁷⁷Lu dosimetry.

Absolute radiotracer concentration measurement using whole-body solid-state SPECT/CT technology: in vivo/in vitro validation

The accuracy of recently approved quantitative clinical software was determined by comparing in vivo/in vitro measurements for a solid-state cadmium-zinc-telluride SPECT/CT (single photon emission computed tomography/x-ray computed tomography) camera. Bone SPECT/CT, including the pelvic region in the field of view, was performed on 16 patients using technetium-99m methylene diphosphonic acid as a radiotracer. After imaging, urine samples from each patient provided for the measurement of in vitro radiopharmaceutical concentrations. From the SPECT/CT images, three users measured in vivo radiotracer concentration and standardized uptake value (SUV) for the bladder using quantitative software (Q.Metrix, GE Healthcare). Linear regression was used to validate any in vivo/in vitro identity relations (ideally slope = 1, intercept = 0), within a 95% confidence interval (CI). Thirteen in vivo/in vitro pairs were available for further analysis, after rejecting two as clinically irrelevant (SUVs > 100 g/mL) and one as an outlier (via Cook's distance calculations). All linear regressions (R² ≥ 0.85, P < 0.0001) provided identity in vivo/in vitro relations (95% CI), with SUV averages from all users giving a slope of 0.99 ± 0.25 and intercept of 0.14 ± 5.15 g/mL. The average in vivo/in vitro residual difference was < 20%. Solid-state SPECT/CT imaging can reliably provide in vivo urinary bladder radiotracer concentrations within approximately 20% accuracy. This practical, non-invasive, in vivo quantitation method can potentially improve diagnosis and assessment of response to treatment. Graphical abstract.

Quantification and reduction of the collimator-detector response effect in SPECT by applying a system model during iterative image reconstruction: a simulation study

BACKGROUND

Detector blurring and non-ideal collimation decrease the spatial resolution of the single-photon emission computed tomography (SPECT) images. Iterative reconstruction algorithms such as ordered subsets expectation maximization (OSEM) can incorporate degrading factors during reconstruction. We investigated the quantitative errors associated with poor SPECT resolution and evaluated the importance of two-dimensional (2D) and three-dimensional (3D) resolution recovery by modelling system response during iterative image reconstruction.

METHODS

Different phantoms consisted of the NURBS-based cardiac-torso (NCAT) liver phantom with small tumors, the Zubal brain phantom and the NCAT heart phantom were used in this study. Monte Carlo simulation was used to create SPECT projections. Gaussian functions were used to model collimator detector response (CDR). Modeled CDRs were applied during OSEM. Both noise-free and noisy projections were created.

RESULTS

Even with noise-free projections, conventional OSEM algorithm provided limited quantitative accuracy compared to both 2D and 3D resolution recovery. The 3D implementation of resolution recovery, however, yielded superior results compared to its 2D implementation. For the liver phantom, the ability to distinguish small tumors in both transverse and axial planes was improved. For the brain phantom, gray to white matter activity ratio was increased from 3.14 ± 0.04 in simple OSEM to 3.84 ± 0.06 in 3D OSEM. For the NCAT heart phantom, 3D resolution recovery, results in images with thinner wall and higher contrast for different noise levels.

CONCLUSION

There are considerable quantitative errors associated with CDR, especially when the size of the target is comparable with the spatial resolution of the system. Between different reconstruction algorithms, 3D OSEM that consider the 3D nature of CDR, improve both the visual quality and the quantitative accuracy of any SPECT studies.

Space-domain non-iterative approach for SPECT/CT systems considering attenuation and space-variant detector response

A quantitative analysis of emission planar image reconstruction from projections by an object dependent, exact, direct approach in the space-domain considering both object attenuation and space-variant impulse response of SPECT/CT systems is proposed. That approach is compared with iterative methods and non-object-dependent exact methods in both the space domain and the frequency one. Since the mean-projection precorrection method is the concern of some actual 3D methods of compensation for distance-dependent spatial resolution and is thought right for competing with different methods able to quantify the tracer density in the object of interest, it is also examined in the course of the analysis. The direct approach may also augment the simulation power of the Matlab Image Processing Toolbox concerning the direct and inverse Radon transform from parallel projection data, the Toolbox being actually restricted to the ideal transform in the frequency domain.

Scatter modelling and compensation in emission tomography

In nuclear medicine, clinical assessment and diagnosis are generally based on qualitative assessment of the distribution pattern of radiotracers used. In addition, emission tomography (SPECT and PET) imaging methods offer the possibility of quantitative assessment of tracer concentration in vivo to quantify relevant parameters in clinical and research settings, provided accurate correction for the physical degrading factors (e.g. attenuation, scatter, partial volume effects) hampering their quantitative accuracy are applied. This review addresses the problem of Compton scattering as the dominant photon interaction phenomenon in emission tomography and discusses its impact on both the quality of reconstructed clinical images and the accuracy of quantitative analysis. After a general introduction, there is a section in which scatter modelling in uniform and non-uniform media is described in detail. This is followed by an overview of scatter compensation techniques and evaluation strategies used for the assessment of these correction methods. In the process, emphasis is placed on the clinical impact of image degradation due to Compton scattering. This, in turn, stresses the need for implementation of more accurate algorithms in software supplied by scanner manufacturers, although the choice of a general-purpose algorithm or algorithms may be difficult.

LROC analysis of detector-response compensation in SPECT

Localization ROC (LROC) observer studies examined whether detector response compensation (DRC) in ordered-subset, expectation-maximization (OSEM) reconstructions helps in the detection and localization of hot tumors. Simulated gallium (Ga-67) images of the thoracic region were used in the study. The projection data modeled the acquisition of attenuated 93- and 185-keV photons with a medium-energy parallel-hole collimator, but scatter was not modeled. Images were reconstructed with five strategies: 1) OSEM with no DRC; 2) OSEM preceded by restoration filtering; 3) OSEM with iterative DRC; 4) OSEM with an ideal DRC; and 5) filtered backprojection (FBP) with no DRC. All strategies included attenuation correction. There were four LROC studies conducted. In a study using a single tumor activity, the ideal DRC offered the best performance, followed by iterative DRC, restoration filtering, OSEM with no DRC, and FBP. Statistical significance at the 5% level was found between all pairs of strategies except for restoration filtering and OSEM with no DRC. A similar ranking was found for a more realistic study using multiple tumor activities. Additional studies considered the effects of OSEM iteration number and tumor activity on the detection improvement that iterative DRC offered with respect to OSEM with no DRC.

Attenuation and scatter compensation in myocardial perfusion SPECT

Nonuniform attenuation, Compton scatter, and limited, spatially varying resolution degrade both the qualitative and quantitative nature of myocardial perfusion SPECT. Physicians must recognize and understand the effects of these factors on myocardial perfusion SPECT for optimal interpretation and use of this important imaging technique. Recent developments in the design and implementation of compensation algorithms and transmission-based imaging promise to provide clinically realistic solutions to these effects and provide the framework for truly quantitative imaging. This achievement should improve the diagnostic accuracy and cost-effectiveness of myocardial perfusion SPECT.

Position-dependent scatter response functions: will they make a difference in SPECT conducted with homogeneous cylindrical phantoms?

This paper explains why it is possible to perform accurate quantitative SPECT when scatter correction is based on stationary and non-stationary scatter functions. This is achieved by comparing the variations of scatter parameters as a function of phantom thickness. The results show that the decrease of scatter fraction with phantom thickness and the decrease of values of scatter kernel inside the field of view are about equal. The deviation of the position-dependent slope from the average value is small for central distributions. These observations explain why estimations of scatter projection by non-stationary convolution and by stationary convolution are comparable when SPECT measurements are conducted with uniform cylindrical phantoms. It is concluded that investigations on the perceived superiority of non-stationary over stationary scatter subtraction in SPECT should be conducted with elliptic phantoms that deviate appreciably from cylindrical shape.

A multiresolution restoration method for cardiac SPECT imaging

In this study we present a multiresolution based method for restoring cardiac SPECT projections. Original projections were decomposed into a set of sub-band frequency images by using analyzing functions localized in both the space and frequency domain. This representation allows a simple denoising and restoration procedure by discarding high-frequency channels and performing inversion only in low frequencies. The method was evaluated in bull's eye reconstructions of a realistic cardiac chest phantom with a custom-made liver insert and 99mTc liver-to-heart activity ratios (LHAR) of 0:1, 1.5:1, 2.5:1, and 3.5:1. The cardiac phantom in free air was used as the reference standard. Reconstructions were performed by filtered backprojection using (1) no correction; (2) restoration without attenuation correction; (3) attenuation correction without restoration; and (4) restoration and attenuation correction. The attenuation correction was carried out with the Chang's method for one iteration. Results were compared with those obtained using an optimized prereconstruction Metz filter. Quantitative analysis was performed by calculating the normalized chi-square measure and mean +/- s.d. of bull's eye counts. In reconstructions with high liver activity (LHAR > 2), attenuation correction without restoration severely distorted the polar maps due to the spill-over of liver activity into the inferior myocardial wall. Both restoration methods when combined with an attenuation correction compensated this artifact and yielded uniform polar maps similar to that of the standard reference. There was no visual or quantitative difference between the performance of Metz filtering and multiresolution restoration. However, the main advantage of the multiresolution method is that it states a more concise and straightforward approach to the restoration problem. Multiresolution based methods does not require information about the object image or optimization processes, such as in conventional nuclear medicine restoration filters. In addition, the method is easy to implement using DFT techniques and potentially can be extended to noniterative spatially shift-invariant restorations in SPECT.

Quantitative assessment of regional myocardial blood flow with thallium-201 and SPECT

Thallium-201 has been used extensively as a myocardial perfusion agent and to assess myocardial viability. Unlike other 99mTc-labeled agents such as 99mTc-sestamibi and 99mTc-tetrofosmine, the regional concentration of 201Tl varies with time, and its kinetics make it a potential candidate for estimating absolute physiologic parameters with kinetic model analysis. This article outlines a strategy for quantitative assessment of regional myocardial blood flow in man using 201Tl and dynamic single photon emission computed tomography (SPECT). Quantitatively accurate SPECT images that are proportional to the true radioactivity distribution are prerequisites for model-based kinetic analysis. Our technique for quantitative SPECT includes ordered-subset maximum likelihood-expectation maximization (ML-EM) reconstruction with transmission data-based attenuation correction and transmission-dependent convolution subtraction scatter correction. A three-compartment model was found to reproduce the observed regional time-activity curves well, and dog experiments demonstrated that influx rate constant (K1) values estimated from the dynamic SPECT data correlated well with absolute myocardial blood flow determined by in vitro microspheres for a physiologically wide range of flows. Several possible strategies for simplifying the study procedures, without compromising accuracy, are also presented, which should make absolute quantitation of regional myocardial blood flow feasible using 201Tl and a conventional SPECT camera in a clinical setting.

Quantitative bremsstrahlung single photon emission computed tomographic imaging: use for volume, activity, and absorbed dose calculations

PURPOSE

To perform bremsstrahlung single photon emission computed tomographic (SPECT) imaging using 32P chronic phosphate for volume and activity quantitation to calculate absorbed dose estimates.

METHODS AND MATERIALS

Seven cancer patients enrolled in clinical Phase I therapeutic protocols were injected with 2.5 million particles of macroaggregated albumin, followed by colloidal 32P chromic phosphate by direct interstitial injection into the tumor-bearing region under computed tomographic (CT) guidance. SPECT images were obtained in these patients. The patient body contour was defined through the use of two externally placed Compton backscatter 99mTc sources. A computer algorithm was written to facilitate region-of-interest volume and activity determination on the reconstructed SPECT slices based on a fixed threshold method. Three sequential SPECT studies were acquired in two of these patients, to determine the accuracy of activity quantitation for bremsstrahlung SPECT studies using Chang's postprocessing method of attenuation compensation with a computer-generated body contour based on the Compton backscatter sources, and an experimentally measured effective linear attenuation coefficient for 32P. The serial data in these two patients were used to calculate absorbed dose estimates.

RESULTS

The 99mTc backscatter sources enabled the patient body outline to be clearly visualized in all the transaxial reconstructed slices and did not contribute significant counts to the patient 32P counts. The calculated activities from the SPECT studies were within 7.8% of the administered 32P activity. The two calculated patient absorbed doses were 4.2 x 10(3) Gy and 5.9 x 10(3) Gy for injected activities of 736 MBq and 920 MBq, respectively.

CONCLUSION

We conclude that accurate quantitative bremsstrahlung SPECT imaging, for the case of high contrast well-localized activity distributions, with a commercially available postprocessing attenuation correction algorithm, can be performed in a clinical setting. Entirely SPECT-based measurements can be used to generate absorbed dose estimates.

Scatter correction in scintigraphy: the state of the art

In scintigraphy, the detection of scattered photons degrades both visual image analysis and quantitative accuracy. Many methods have been proposed and are still under investigation to cope with scattered photons. The main features of the problem of scattering in radionuclide imaging are presented first, to provide a sound foundation for a critical review of the existing scatter correction techniques. These are described using a classification relating to their aims and principles. Their theoretical potentials are analysed, as well as the difficulties of their practical implementation. Finally, the problems of their evaluation and comparison are discussed.

Two-dimensional restoration of single photon emission computed tomography images using the Kalman filter

The discrete filtered backprojection (DFBP) algorithm used for the reconstruction of single photon emission computed tomography (SPECT) images affects image quality because of the operations of filtering and discretization. The discretization of the filtered backprojection process can cause the modulation transfer function (MTF) of the SPECT imaging system to be anisotropic and nonstationary, especially near the edges of the camera's field of view. The use of shift-invariant restoration techniques fails to restore large images because these techniques do not account for such variations in the MTF. This study presents the application of a two-dimensional (2D) shift-variant Kalman filter for post-reconstruction restoration of SPECT slices. This filter was applied to SPECT images of a hollow cylinder phantom; a resolution phantom; and a large, truncated cone phantom containing two types of cold spots, a sphere, and a triangular prism. The images were acquired on an ADAC GENESYS camera. A comparison was performed between results obtained by the Kalman filter and those obtained by shift-invariant filters. Quantitative analysis of the restored images performed through measurement of root mean squared errors shows a considerable reduction in error of Kalman-filtered images over images restored using shift-invariant methods.

Order statistic-neural network hybrid filters for gamma camera-bremsstrahlung image restoration

An order statistic and neural network hybrid filter (OSNNH) is proposed for the restoration of gamma camera images using the measured modulation transfer function. Planar images of beta-emitting radionuclides are used to evaluate the filter because they exhibit higher degradation than images of single photon emitters due to increased photon scattering and collimator septal penetration. The filter performance is quantitatively evaluated and compared to that of the Wiener filter by investigating the relationship between the externally measured counts from sources of phosphorous-32 ((32)P) at various depths in water. An effective linear attenuation coefficient for (32)P is determined to be equal to 0.13 cm(-1) and 0.14 cm(-1) for the OSNNH and the Wiener filters, respectively. Evaluation of phantom and patient filtered images demonstrates that the OSNNH filter avoids ring effects caused by the ill-conditioned blur matrix and noise overriding caused by matrix inversion, typical of other restoration filters such as the Wiener.

Quantitative single photon emission tomography: verification for sources in an elliptical water phantom

Accurate absorbed dose calculations are important for a proper dose planning in internal radionuclide therapy. The activity distribution must be measured and the target volume defined. This can be done with single photon emission tomography (SPET) if proper attenuation and scatter correction are employed. This study investigated the calculation of the activity and the volume of different spherical sources. These two parameters are essential for a proper dose calculation. The scatter and attenuation correction method is based on spatially variant scatter functions and density maps. The volume calculation method is based on obtaining a threshold from a grey-level histogram. Both point sources and spheres of different diameters containing technetium-99m were placed in different locations in an elliptical water phantom and imaged by SPET. The activity and the volume of the spheres were calculated from the SPET images and compared with known activities. Results show a quantification of activity within 10% for most of the sources. Important influences on the quantification are (a) the presence of artefacts due to improper reconstruction and (b) the finite spatial resolution which affects the total number of counts within the determined volume.

Three-dimensional photon detection kernels and their application to SPECT reconstruction

In single photon emission computed tomography (SPECT), three-dimensional photon detection kernels characterize the probabilities that photons emitted by radio-isotopes in different parts of the source region will be detected at particular projection pixels of the projection images. Monte Carlo modelling is used to study these kernels for the case of parallel hole collimators. The use of vectorized Monte Carlo computer code speeds the modelling computations. The contributions of direct and scattered photons to projection data in a transverse plane from neighbouring planes are significant for the case of uniform activity within a water-filled cylinder. A reconstruction method using the 3D kernels is proposed in which projection measurements in three adjacent planes are used simultaneously to estimate the source activity of the center plane. This multiple slice method accounts for the fact that photons detected in a given transverse plane may have originated in other transverse planes with different activity distributions. The matrix equations for image reconstruction are solved using generalized matrix inverses. The new method shows compensation for 3D photon detection effects when applied to projection data from a numerical simulation and a cardiac phantom experiment. Quantitation for the numerical study is improved compared with results from a single slice reconstruction method.