Jörg Peter

A 3D gantry single photon emission tomograph with hemispherical coverage for dedicated breast imaging

Abstract A novel tomographic gantry was designed, built and initially evaluated for single photon emission imaging of metabolically active lesions in the pendant breast and near chest wall. Initial emission imaging measurements with breast lesions of various uptake ratios are presented. Methods: A prototype tomograph was constructed utilizing a compact gamma camera having a field-of-view of Results: As iteration number increased for the tomographically measured data at all polar angles, contrasts increased while signal-to-noise ratios (SNRs) decreased in the expected way with OSEM reconstruction. The rollover between contrast improvement and SNR degradation of the lesion occurred at two to three iterations. The reconstructed tomographic data yielded SNRs with or without scatter correction that were >9 times better than the planar scans. There was up to a factor of ∼2.5 increase in total primary and scatter contamination in the photopeak window with increasing tilt angle from 15° to 45°, consistent with more direct line-of-sight of myocardial and liver activity with increased camera polar angle. Conclusion: This new, ultra-compact, dedicated tomographic imaging system has the potential of providing valuable, fully 3D functional information about small, otherwise indeterminate breast lesions as an adjunct to diagnostic mammography.

Analytical versus voxelized phantom representation for Monte Carlo simulation in radiological imaging

Monte Carlo simulations in nuclear medicine, with accurately modeled photon transport and high-quality random number generators, require precisely defined and often detailed phantoms as an important component in the simulation process. Contemporary simulation models predominantly employ voxel-driven algorithms, but analytical models offer important advantages. The authors discuss the implementation of ray-solid intersection algorithms in analytical superquadric-based complex phantoms with additional speed-up rejection testing for use in nuclear medicine imaging simulations, and we make comparisons with voxelized counterparts. Comparisons are made with well-known cold rod:sphere and anthropomorphic phantoms. For these complex phantoms, the analytical phantom representations are nominally several orders of magnitude smaller in memory requirements than are voxelized versions. Analytical phantoms facilitate constant distribution parameters. As a consequence of discretizing a continuous surface into finite bins, for example, time-dependent voxelized phantoms can have difficulties preserving accurate volumes of a beating heart. Although virtually no inaccuracy is associated with path calculations in analytical phantoms, the discretization can negatively impact the simulation process and results. Discretization errors are apparent in reconstructed images of cold rod:sphere voxel-based phantoms because of a redistribution of the count densities in the simulated objects. These problems are entirely avoided in analytical phantoms. Voxelized phantoms can accurately model detailed human shapes based on segmented computed tomography (CT) or magnetic resonance imaging (MRI) images, but analytical phantoms offer advantages in time and accuracy for evaluation and investigation of imaging physics and reconstruction algorithms in a straightforward and efficient manner.

Comparison of noncontact and fiber-based fluorescence-mediated tomography

We present a comparative experimental phantom study of fiber-based and noncontact fluorescence tomography with respect to quantitation and localization of reconstructed fluorescent inclusions in turbid media such as tissue. Noncontact acquisition is usually considered potentially superior to fiber-based techniques because of the availability of a large number of detector readouts through a CCD. Our results indicate, however, that noncontact acquisition itself might improve the quality of reconstructions significantly, even without increasing the number of detectors and thus keeping the inverse problem moderately complex.

Four-dimensional superquadric-based cardiac phantom for Monte Carlo simulation of radiological imaging systems

A four-dimensional (x,y,z,t) composite superquadric-based object model of the human heart for Monte Carlo simulation of radiological imaging systems has been developed. The phantom models the real temporal geometric conditions of a beating heart for frame rates up to 32 per cardiac cycle. Phantom objects are described by boolean combinations of superquadric ellipsoid sections. Moving spherical coordinate systems are chosen to model wall movement whereby points of the ventricle and atria walls are assumed to move towards a moving center-of-gravity point. Due to the non-static coordinate systems, the atrial/ventricular valve plane of the mathematical heart phantom moves up and down along the left ventricular long axis resulting in reciprocal emptying and filling of atria and ventricles. Compared to the base movement, the epicardial apex as well as the superior atria area are almost fixed in space. Since geometric parameters of the objects are directly applied on intersection calculations of the photon ray with object boundaries during Monte Carlo simulation, no phantom discretization artifacts are involved.

Bayesian reconstruction strategy of fluorescence-mediated tomography using an integrated SPECT-CT-OT system

Following the assembly of a triple-modality SPECT-CT-OT small animal imaging system providing intrinsically co-registered projection data of all three submodalities and under the assumption and investigation of dual-labeled probes consisting of both fluorophores and radionuclides, a novel multi-modal reconstruction strategy is presented in this paper aimed at improving fluorescence-mediated tomography (FMT). The following reconstruction procedure is proposed: firstly, standard x-ray CT image reconstruction is performed employing the FDK algorithm. Secondly, standard SPECT image reconstruction is performed using OSEM. Thirdly, from the reconstructed CT volume data the surface boundary of the imaged object is extracted for finite element definition. Finally, the reconstructed SPECT data are used as a priori information within a Bayesian reconstruction framework for optical (FMT) reconstruction. We provide results of this multi-modal approach using phantom experimental data and illustrate that this strategy does suppress artifacts and facilitates quantitative analysis for optical imaging studies.

Geometrical co-calibration of a tomographic optical system with CT for intrinsically co-registered imaging

A mathematical approach for geometric co-calibration of a dual-modal small-animal imaging system is presented. The system comprises an optical imaging setup for in vivo bioluminescence and fluorescence detection, as well as an x-ray CT, both mounted on a common rotatable gantry enabling fully simultaneous imaging at axially overlapping fields-of-view. Geometric co-calibration is performed once by imaging a single cylindrical light-emitting source with both modalities over 360 degrees at two axial positions, respectively. Given the three-dimensional coordinates of the source positions in the reconstructed CT volume data along with their two-dimensional locations projected at the optical detector plane, the following intrinsic system parameters are calculated: (i) the intrinsic geometric parameters of the optical detection system-five parameters for each view and (ii) the relative positional relationship between the optical and CT systems-two parameters for each view. After co-calibration is performed, experimental studies using phantoms demonstrate the high degree of intrinsic positional accuracy between the optical and CT measurements. The most important advantage of this approach is that dual-modal data fusion is accomplished without any post-registration strategies.

Development and Initial Results of a Tomographic Dual-Modality Positron/Optical Small Animal Imager

Herein, we present a novel concept for fully integrated dual-modality in vivo tomographic imaging yielding simultaneous detection of positron and optically labeled probes in small animals. The imager consists of an allocation of optical detector modules and, in radial extension, the allocation of positron emission detector modules. Laser scanning and large-field light sources are integrated to facilitate fluorescence imaging in addition to bioluminescence imaging. Each optical detector unit consists of a large-area photon sensor for light detection, a microlens array for field-of-view definition, a septum mask for cross-talk suppression, and a transferable filter for wavelength selection. To prove the working principle of the dual-modality detector system a pair of optical detectors along with a large-field excitation source was placed inside the bore of a Siemens EXACT HR+ scanner, performing simultaneous imaging. The imaging characteristics of the optical detector were evaluated experimentally using a prototypical setup with geometrical phantoms. The sensitivity of the optical detector prototype was found less than that of a reference CCD camera. We propose several ways of increasing optical detector sensitivity.

Transfer of the sFLT-1 Gene in Morris Hepatoma Results in Decreased Growth and Perfusion and Induction of Genes Associated with Stress Response

Purpose: Inhibition of tumor angiogenesis is emerging as a promising target in the treatment of malignancies. Therefore, monitoring of antiangiogenic approaches with functional imaging and histomorphometrical analyses are desirable to evaluate the biological effects caused by this treatment modality. Experimental Design: Using a bicistronic retroviral vector for transfer of the soluble receptor for the vascular endothelial growth factor (sFLT) hepatoma (MH3924A) cell lines with sFLT expression were generated. In human umbilical vein endothelial cells cultured with conditioned medium of sFLT-expressing hepatoma cells, the inhibitory action of secreted sFLT was determined using a Coulter counter and a thymidine incorporation assay. Furthermore, in vivo experiments were done to measure the effects on tumor growth and perfusion. Finally, the tumors were examined by immunohistochemistry (including computer-assisted morphometry) and DNA chip analysis. Results: Stable sFLT-expressing hepatoma cells inhibited endothelial cell proliferation in vitro. In vivo, growth and perfusion, as measured by H215O positron emission tomography, were reduced in genetically modified tumors. However, the immunohistochemically quantified microvascularization and macrovascularization, as indicated by CD31- and α-actin-positive area, revealed no significant changes, whereas the number of apoptotic cells was increased in sFLT-expressing tumors, although not significantly. DNA chip analysis of tumors with gene transfer showed an increase of genes related to apoptosis, signal transduction, and oxidative stress. Conclusion: Our results suggest that sFLT expression inhibits tumor growth and perfusion and enhances expression of apoptosis-related genes in this model. Enhanced expression of genes for signal transduction, stress, and metabolism indicates tumor defense reactions.

Fully adaptive temporal regression smoothing in gated cardiac SPECT image reconstruction

A data-driven weighted polynomial regression estimator with variable bandwidth and adaptation of the order of the polynomial can enhance the diagnosis of both ventricular function and myocardial perfusion in gated cardiac SPECT. The authors propose a temporal approach which estimates a mean regression function of either time frame projections or reconstructed images. They investigated several implementations of the regression estimation procedure: (a) prior to the reconstruction, applied on the projection data, (b) iteratively, as a semi-parametric reconstruction method, and (c) applied as a post-reconstruction regression smoother. Implementation of the regression method in step (a) and (c) is independent from the algorithm used to reconstruct the projection data; (b) requires an iterative image reconstruction algorithm. Monte Carlo simulated projection data as well as clinical data have been processed to evaluate the technique. Unique regression smoothing of projection data, (a), or reconstructed images, (c), improves markedly the image statistics while preserving spatial and temporal resolution. Iterative application, (b), without the use of a relaxation parameter may result in over-smoothed time activity curves.

List-mode maximum-likelihood reconstruction for the ClearPEM system

A dedicated implementation of list-mode maximum-likelihood expectation-maximization (MLEM) reconstruction for the ClearPEM system is presented. The system is composed of two face-to-face detectors, which can be rotated to acquire data from different angular positions. Due to the specific design with irregular sampling and depth of interaction capability, the possible number of lines of response (LOR) is significantly greater than the number of detected events in a standard clinical study. Because reconstruction methods based on data histogramming to sinogram lead to a high computational cost and/or a loss of the intrinsical system resolution, it is necessary to consider the processing of events in list-mode during the reconstruction. The presented method adopted EM algorithm to maximize the logarithmic likelihood function that is expressed in list-mode. The voxel efficiency is corrected by pre-calculated efficiency maps based on flood phantom acquisitions. The method is also implemented with parallelization by distributing the calculation of the acquired events into different threads for significantly increasing computational speed. The results of a Derenzo phantom study show that the presented algorithm can achieve a similar result as 3D-OSEM reconstruction based on data histogramming with significantly lower reconstruction time (6 times faster with one thread, 20 times faster with 8 threads distributed in 8 CPU cores). In clinical studies with lower acquired events, the acceleration ratio can be even higher. The result from a breast phantom study shows that lesions with 15 mm in diameter, each, as well as a small lesion with 5 mm in diameter are clearly visible and can be characterized. The mouse imaging studies show also great potential of the system in small animal applications.