Sheep can be used as animal model of regional myocardial remodeling and controllable work

Background: Pacing the right heart has been shown to induce reversible conduction delay and subse­quent asymmetric remodeling of the left ventricle (LV) in dogs and pigs. Both species have disadvantages in animal experiments. Therefore the aim of this study was to develop a more feasible and easy-to-use animal model in sheep. Methods: Dual-chamber (DDD) pacemakers with epicardial leads on the right atrium and right ven­tricular free wall were implanted in 13 sheep. All animals underwent 8 weeks of chronic rapid pacing at 180 bpm. Reported observations were made at 110 bpm. Results: DDD pacing acutely induced a left bundle branch block (LBBB) — like pattern with almost doubling in QRS width and the appearance of a septal flash, indicating mechanical dyssynchrony. Atrial pacing (AAI) resulted in normal ventricular conduction and function. During 8 weeks of rapid DDD pacing, animals developed LV remodeling (confirmed with histology) with septal wall thinning (–30%, p < 0.05), lateral wall thickening (+22%, p < 0.05), LV volume increase (+32%, p < 0.05), decrease of LV ejection fraction (–31%, p < 0.05), and functional mitral regurgitation. After 8 weeks, segmental pressure-strain-loops, representing regional myocardial work, were recorded. Switching from AAI to DDD pacing decreased immediately work in the septum and increased it in the lateral wall (–69 and +41%, respectively, p < 0.05). Global LV stroke work and dP/dtmax decreased (–27% and -25%, respectively, p < 0.05). Conclusions: This study presents the development a new sheep model with an asymmetrically remod­eled LV. Simple pacemaker programing allows direct modulation of regional myocardial function and work. This animal model provides a new and valuable alternative for canine or porcine models and has the potential to become instrumental for investigating regional function and loading conditions on regional LV remodeling

High Quality Image Formation of Small Animals Using Emission Tomography (Hoge kwaliteitsbeeldvorming van kleine dieren door middel van emissietomografie)

De voorbije tien jaar zijn zowel SPECT als PET uitgebreid bestudeerd ter verbetering van de kwaliteit van functionele beeldvorming in kleine proefdieren. In dit werk hebben we ons toegespitst op SPECT van kleine dieren, wat veel toepassingsmogelijkheden heeft en de ruimtelijke resolutie van de micro-PET kan overtroeven. Het reconstructieprogramma werd verbeterd door het nauwkeuriger modelleren van het beeldvormingsproces, wat superieure beelden oplevert. Bovendien werd een efficiënte methode ontwikkeld voor beeldkwaliteitsevaluatie, die gevalideerd werd voor enkel- en meervoudige pinhole SPECT. Hiermee werd de invloed van vele ontwerpparameters voor pinholecollimatoren op de reconstructiebeeldkwaliteit onderzocht, de hoeveelheid overlap in multipinhole SPECT projecties incluis. Op basis van die resultaten werd een multipinhole-ontwerp voor muisbeeldvorming geoptimaliseerd, vervaardigd, en getest op een klinische gammacamera uitgerust met twee pinholecollimatoren. Dezelfde methode werd ook toegepast om de eigenschappen van time-of-flight PET te bestuderen. Tot slot werd een overzicht van onze belangrijkste pinhole SPECT toepassingen gegeven ter illustratie van de beeldkwaliteitsevolutie.status: publishe

Noise Propagation in Multipinhole SPECT Calibration

It has been shown previously that multipinhole single photon emission computed tomography (SPECT) calibration only requires a two-point source measurement, without knowledge of the distance between them, to uniquely determine the camera geometry. In this work, we studied how this method propagates measurement noise into errors on the estimated calibration parameters, which in turn will degrade the reconstruction image quality. Furthermore, the effect on the calibration and reconstruction accuracy of fixing either the inter-point or the inter-pinhole distance during calibration is assessed. As expected, including information about the distance between the point sources improved the robustness of the calibration against measurement noise. In addition, it was found that similar robustness and even a superior spatial resolution can be obtained by fixing the distance(s) between the pinhole apertures instead of fixing those between the point sources. Finally, optimal two-point source configurations were determined for calibrating our newly-designed 7-pinhole SPECT system for limited field of view mouse imaging while fixing the inter-pinhole distances.status: publishe

Multi-Pinhole SPECT Calibration: Influence of Data Noise and Systematic Orbit Deviations

The geometry of a single pinhole SPECT system with circular orbit can be uniquely determined from a measurement of three point sources, provided that at least two inter-point distances are known. In contrast, it has been shown mathematically that, for a multi-pinhole SPECT system with circular orbit, only two point sources are needed, and the knowledge of the distance between them is not required. In this paper, we report that this conclusion only holds if the motion of the camera is perfectly circular. In reality, the detector heads systematically slightly deviate from the circular orbit, which may introduce non-negligible bias in the estimated parameters and degrade the reconstructed image. An analytical linear model was extended to estimate the influence of both data noise and systematic deviations on the accuracy of the calibration and on the image quality of the reconstruction. It turns out that applying the knowledge of the distances greatly reduces the reconstruction error, especially in the presence of systematic deviations. In addition, we propose that instead of using the information about the distances between the point sources, it is more straightforward to use the knowledge about the distances between the pinhole apertures during multi-pinhole calibration. The two distance-fixing approaches yield similar reconstruction accuracy. Our theoretical results are supported by reconstruction images of a Jaszczak-type phantom scan.status: publishe

Parallel Hole and Rotating Slat Collimators: Comparative Study Using Digital Contrast Phantoms

The parallel hole (PH) collimator is the most commonly used collimator in single photon emission computed tomography (SPECT) imaging. A well-known limitation of the PH collimator is its low geometric efficiency. For better geometric efficiency without compromising the resolution, a rotating slat (RS) collimator is a potential alternative. In our previous work, a linear relation between the image resolution and the optimal collimator aperture was derived for both the PH and the RS collimator systems, and the two collimator systems were compared with the optimized geometries using different uniform phantoms. In this study, the PH and the RS collimator system were further compared using digital contrast phantoms. Three figures of merit (FOMs) were calculated for comparison, i.e., the contrast-to-noise ratio (CNR) in a pixel of interest, the CNR in a region of interest, and the signal-to-noise ratio (SNR) of a channelized Hotelling observer (CHO). All the FOMs were calculated analytically using an efficient approximation method. © 1963-2012 IEEE.status: publishe

Comparison Between Parallel Hole and Rotating Slat Collimation: Analytical Noise Propagation Models

We have previously proposed a method to compare tomographic systems. It is assumed that each system acquires a tomographic scan of a certain tracer distribution in the same acquisition time. From this scan, each system is forced to reconstruct an image with a predefined spatial resolution. The system that can perform this task with the "most favorable" noise propagation is considered as the best system. The variance on pixel values or region-of-interest (ROI) values is used to assess the noise in the reconstructed image. In this paper, we extend this idea to compare the performance of parallel hole (PH) and rotating slat (RS) collimations. Two different analytical approaches were used to analyze the variance of the reconstructed pixel/ROI values. The first method is based on the filtered-backprojection (FBP) theory, and was applied to the central point of a uniform symmetrical phantom. It yields analytical expressions for the optimal collimator aperture and the corresponding variance of the reconstructed pixel values, but it can only be applied to highly symmetrical configurations. The second method is based on approximations for the Fisher information matrix. It provides numerical results, and it is more general and can be applied to nonsymmetrical objects and shift-variant tomographic systems. The collimations were compared for both planar imaging and volume imaging. The main results are as follows. 1) For cases where both methods are valid, they are in excellent agreement. 2a) The optimal collimator aperture varies linearly with the target resolution. 2b) For a fixed target resolution, the optimal collimator aperture depends on the collimator type and the imaging mode (planar or volume). 2c) The optimal aperture of PH is a factor of root 2 larger than that of RS. 3a) The relative performance of the two collimators is determined by both the object size and the object-to-detector distance. 3b) Pixel variance and variances of ROIs with varying sizes yield very similar relative performance for RS versus PH.status: publishe

Fisher Information-Based Evaluation of Image Quality for Time-of-Flight PET

The use of time-of-flight (TOF) information during positron emission tomography (PET) reconstruction has been found to improve the image quality. In this work we quantified this improvement using two existing methods: 1) a very simple analytical expression only valid for a central point in a large uniform disk source and 2) efficient analytical approximations for postfiltered maximum likelihood expectation maximization (MLEM) reconstruction with a fixed target resolution, predicting the image quality in a pixel or in a small region of interest based on the Fisher information matrix. Using this latter method the weighting function for filtered backprojection reconstruction of TOF PET data proposed by C. Watson can be derived. The image quality was investigated at different locations in various software phantoms. Simplified as well as realistic phantoms, measured both with TOF PET systems and with a conventional PET system, were simulated. Since the time resolution of the system is not always accurately known, the effect on the image quality of using an inaccurate kernel during reconstruction was also examined with the Fisher information-based method. First, we confirmed with this method that the variance improvement in the center of a large uniform disk source is proportional to the disk diameter and inversely proportional to the time resolution. Next, image quality improvement was observed in all pixels, but in eccentric and high-count regions the contrast-to-noise ratio (CNR) increased less than in central and low- or medium-count regions. Finally, the CNR was seen to decrease when the time resolution was inaccurately modeled (too narrow or too wide) during reconstruction. Although the maximum CNR is not very sensitive to the time resolution error, using an inaccurate TOF kernel tends to introduce artifacts in the reconstructed image.status: publishe

Fisher information-based evaluation of image quality for time-of-flight PET

The use of time-of-flight (TOF) information during reconstruction is generally considered to improve the image quality. In this work we quantified this improvement using two existing methods: (1) a very simple analytical expression only valid for a central point in a large uniform disk source, and (2) efficient analytical approximations for post-filtered maximum likelihood expectation maximization (MLEM) reconstruction with a fixed target resolution, predicting the image quality in a pixel or in a small region based on the Fisher information matrix. The image quality was investigated at different locations in various software phantoms. Simplified as well as realistic phantoms, measured both with TOF positron emission tomography (PET) systems and with a conventional PET system, were simulated. Since the time resolution of the system is not always accurately known, the effect on the image quality of using an inaccurate kernel during reconstruction was also examined with the Fisher information-based method. First, we confirmed with this method that the variance improvement in the center of a large uniform disk source is proportional to the disk diameter and inversely proportional to the time resolution. Next, image quality improvement was observed in all pixels, but in eccentric and high-count regions the contrast-to-noise ratio (CNR) increased slower than in central and low- or medium-count regions. Finally, the CNR was seen to decrease when the time resolution was inaccurately modeled (too narrow or too wide) during reconstruction. Although the optimum is rather flat, using an inaccurate TOF kernel might introduce artifacts in the reconstructed image.status: publishe