Abstract
To assess the performance of two approaches to the system response matrix (SRM) calculation in pinhole single photon emission computed tomography (SPECT) reconstruction. Evaluation was performed using experimental data from a low magnification pinhole SPECT system that consisted of a rotating flat detector with a monolithic scintillator crystal. The SRM was computed following two approaches, which were based on Monte Carlo simulations (MC-SRM) and analytical techniques in combination with an experimental characterization (AE-SRM). The spatial response of the system, obtained by using the two approaches, was compared with experimental data. The effect of the MC-SRM and AE-SRM approaches on the reconstructed image was assessed in terms of image contrast, signal-to-noise ratio, image quality, and spatial resolution. To this end, acquisitions were carried out using a hot cylinder phantom (consisting of five fillable rods with diameters of 5, 4, 3, 2, and 1 mm and a uniform cylindrical chamber) and a custom-made Derenzo phantom, with center-to-center distances between adjacent rods of 1.5, 2.0, and 3.0 mm. Good agreement was found for the spatial response of the system between measured data and results derived from MC-SRM and AE-SRM. Only minor differences for point sources at distances smaller than the radius of rotation and large incidence angles were found. Assessment of the effect on the reconstructed image showed a similar contrast for both approaches, with values higher than 0.9 for rod diameters greater than 1 mm and higher than 0.8 for rod diameter of 1 mm. The comparison in terms of image quality showed that all rods in the different sections of a custom-made Derenzo phantom could be distinguished. The spatial resolution (FWHM) was 0.7 mm at iteration 100 using both approaches. The SNR was lower for reconstructed images using MC-SRM than for those reconstructed using AE-SRM, indicating that AE-SRM deals better with the projection noise than MC-SRM. The authors' findings show that both approaches provide good solutions to the problem of calculating the SRM in pinhole SPECT reconstruction. The AE-SRM was faster to create and handle the projection noise better than MC-SRM. Nevertheless, the AE-SRM required a tedious experimental characterization of the intrinsic detector response. Creation of the MC-SRM required longer computation time and handled the projection noise worse than the AE-SRM.Nevertheless, the MC-SRM inherently incorporates extensive modeling of the system and therefore experimental characterization was not required.
Highlights
Molecular imaging techniques play a valuable role in preclinical areas such as drug development, gene expression, and nanoparticle-based cell therapy monitoring.1–6 In particular, single photon emission computed tomography (SPECT) has become an essential tool in this field7 thanks to its ability to provide images of peptides, antibodies, and hormones labeled with Technetium (Tc-99m) and other radio-isotopes
point spread function (PSF) obtained from the AE-system response matrix (SRM) model were slightly narrower than those corresponding to experimental measurements, whereas PSFs obtained from the Monte Carlo (MC)-SRM were slightly wider than those obtained experimentally
The mean bias between AE-SRM and experimental PSFs was −0.13 mm in FWHM and −0.60 mm in FWTM. These results demonstrate that PSF is slightly overestimated in terms of FWHM and FWTM when using MC-SRM and that AE-SRM causes FWTM to be significantly lower than the experimental measurements
Summary
Molecular imaging techniques play a valuable role in preclinical areas such as drug development, gene expression, and nanoparticle-based cell therapy monitoring. In particular, single photon emission computed tomography (SPECT) has become an essential tool in this field thanks to its ability to provide images of peptides, antibodies, and hormones labeled with Technetium (Tc-99m) and other radio-isotopes. Single photon emission computed tomography (SPECT) has become an essential tool in this field thanks to its ability to provide images of peptides, antibodies, and hormones labeled with Technetium (Tc-99m) and other radio-isotopes. The relatively slow diffusion of these molecules allows the imaging of processes such as cell division, infection, and inflammation. Pinhole SPECT provides high resolution images thanks to the use of pinhole instead of parallelhole collimators. This, enables us to achieve submillimeter spatial resolution when the object is positioned close to the pinhole.. This, enables us to achieve submillimeter spatial resolution when the object is positioned close to the pinhole.8–11 This advantage comes at the expense of a reduced field of view which restricts the clinical use of these imaging systems This, enables us to achieve submillimeter spatial resolution when the object is positioned close to the pinhole. This advantage comes at the expense of a reduced field of view which restricts the clinical use of these imaging systems
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