Performance assessment of a slat gamma camera collimator for 511 keV imaging.

The physical performance of a prototype slat collimator is described for gamma camera planar imaging at 511 keV. Measurements were made of sensitivity, spatial resolution and a septal penetration index at 511 keV. These measurements were repeated with a commercial parallel hole collimator designed for 511 keV imaging. The slat collimator sensitivity was 22.9 times that of the parallel hole collimator with 10 cm tissue equivalent scatter material, and 16.8 times the parallel hole collimator sensitivity in air. Spatial resolution was also better for the slat collimator than the parallel hole collimator (FWHM at 10 cm in air 17.9 mm and 21.2 mm respectively). Septal penetration was compared by a single value for the counts at 120 mm from the point source profile peak, expressed as a percentage of the peak counts, showing less penetration for the slat collimator than the parallel hole collimator (1.9% versus 3.6% respectively). In conclusion, these results show that the slat collimator may have advantages over the parallel hole collimator for 511 keV imaging, though the greater complexity of operation of the slat collimator and potential sources of artefact in slat collimator imaging are recognized.     

Image reconstruction algorithm for a spinning strip CZT SPECT camera with a parallel slat collimator and small pixels

This paper discusses the use of small pixels in a spinning CdZnTe single photon emission computed tomography (SPECT) camera that is mounted with a parallel slat collimator. In a conventional slat collimation configuration, there is a detector pixel between two adjacent collimator slats. In our design, the pixel size is halved. That is, there are two smaller pixels to replace a regular pixel between two adjacent slats while the collimator remains unchanged. It has an advantage over our older design that uses tilted slats. In order to acquire a complete data set the tilted-slat collimator must spin 360 degrees at each SPECT view while the proposed design requires only 180 degrees at each SPECT view. Computer simulations and phantom experiments have been carried out to investigate the performance of the small-pixel configuration. It is observed that this design has the potential to increase the spatial resolution of the detector while keeping photon counts the same.     

Image reconstruction algorithm for a rotating slat collimator

A slat collimator in single photon emission computed tomography consists of a set of parallel slats. As the collimator spins, the detector measures a one-dimensional projection data set. A complete data set can be obtained by rotating the detector/collimator assembly around the object (patient) while the collimator spins continuously. The measured projection data are assumed to be weighted planar integrals of the object. This paper describes the development of an approximate three-dimensional image reconstruction algorithm for a rotating/spinning slat collimator. This algorithm is in filtered backprojection form. Computer simulations were performed to verify the effectiveness of the algorithm.     

Characterization of a rotating slat collimator system dedicated to small animal imaging

Some current investigations based on small animal models are dedicated to functional cerebral imaging. They represent a fundamental tool to understand the mechanisms involved in neurodegenerative diseases. In the radiopharmaceutical development approach, the main challenge is to measure the radioactivity distribution in the brain of a subject with good temporal and spatial resolutions. Classical SPECT systems mainly use parallel hole or pinhole collimators. In this paper we investigate the use of a rotating slat collimator system for small animal brain imaging. The proposed prototype consists of a 64-channel multi-anode photomultiplier tube (H8804, Hamamatsu Corp.) coupled to a YAP:Ce crystal highly segmented into 32 strips of 0.575 × 18.4 × 10 mm(3). The parameters of the rotating slat collimator are optimized using GATE Monte Carlo simulations. The performance of the proposed prototype in terms of spatial resolution, detection efficiency and signal-to-noise ratio is compared to that obtained with a gamma camera equipped with a parallel hole collimator. Preliminary experimental results demonstrate that a spatial resolution of 1.54 mm can be achieved with a detection efficiency of 0.012% for a source located at 20 mm, corresponding to the position of the brain in the prototype field of view.     

3D RBI-EM reconstruction with spherically-symmetric basis function for SPECT rotating slat collimator

A single photon emission computed tomography (SPECT) rotating slat collimator with strip detector acquires distance-weighted plane integral data, along with the attenuation factor and distance-dependent detector response. In order to image a 3D object, the slat collimator device has first to spin around its axis and then rotate around the object to produce 3D projection measurements. Compared to the slice-by-slice 2D reconstruction for the parallel-hole collimator and line integral data, a more complex 3D reconstruction is needed for the slat collimator and plane integral data. In this paper, we propose a 3D RBI-EM reconstruction algorithm with spherically-symmetric basis function, also called 'blobs', for the slat collimator. It has a closed and spherically symmetric analytical expression for the 3D Radon transform, which makes it easier to compute the plane integral than the voxel. It is completely localized in the spatial domain and nearly band-limited in the frequency domain. Its size and shape can be controlled by several parameters to have desired reconstructed image quality. A mathematical lesion phantom study has demonstrated that the blob reconstruction can achieve better contrast-noise trade-offs than the voxel reconstruction without greatly degrading the image resolution. A real lesion phantom study further confirmed this and showed that a slat collimator with CZT detector has better image quality than the conventional parallel-hole collimator with NaI detector. The improvement might be due to both the slat collimation and the better energy resolution of the CZT detector.     

Tomographic image quality of rotating slat versus parallel hole-collimated SPECT

Parallel and converging hole collimators are most frequently used in nuclear medicine. Less common is the use of rotating slat collimators for single photon emission computed tomography (SPECT). The higher photon collection efficiency, inherent to the geometry of rotating slat collimators, results in much lower noise in the data. However, plane integrals contain spatial information in only one direction, whereas line integrals provide two-dimensional information. It is not a trivial question whether the initial gain in efficiency will compensate for the lower information content in the plane integrals. Therefore, a comparison of the performance of parallel hole and rotating slat collimation is needed. This study compares SPECT with rotating slat and parallel hole collimation in combination with MLEM reconstruction with accurate system modeling and correction for scatter and attenuation. A contrast-to-noise study revealed an improvement of a factor 2-3 for hot lesions and more than a factor of 4 for cold lesion. Furthermore, a clinically relevant case of heart lesion detection is simulated for rotating slat and parallel hole collimators. In this case, rotating slat collimators outperform the traditional parallel hole collimators. We conclude that rotating slat collimators are a valuable alternative for parallel hole collimators.     

Comparing planar image quality of rotating slat and parallel hole collimation: influence of system modeling

The main remaining challenge for a gamma camera is to overcome the existing trade-off between collimator spatial resolution and system sensitivity. This problem, strongly limiting the performance of parallel hole collimated gamma cameras, can be overcome by applying new collimator designs such as rotating slat (RS) collimators which have a much higher photon collection efficiency. The drawback of a RS collimated gamma camera is that, even for obtaining planar images, image reconstruction is needed, resulting in noise accumulation. However, nowadays iterative reconstruction techniques with accurate system modeling can provide better image quality. Because the impact of this modeling on image quality differs from one system to another, an objective assessment of the image quality obtained with a RS collimator is needed in comparison to classical projection images obtained using a parallel hole (PH) collimator. In this paper, a comparative study of image quality, achieved with system modeling, is presented. RS data are reconstructed to planar images using maximum likelihood expectation maximization (MLEM) with an accurate Monte Carlo derived system matrix while PH projections are deconvolved using a Monte Carlo derived point-spread function. Contrast-to-noise characteristics are used to show image quality for cold and hot spots of varying size. Influence of the object size and contrast is investigated using the optimal contrast-to-noise ratio (CNR(o)). For a typical phantom setup, results show that cold spot imaging is slightly better for a PH collimator. For hot spot imaging, the CNR(o) of the RS images is found to increase with increasing lesion diameter and lesion contrast while it decreases when background dimensions become larger. Only for very large background dimensions in combination with low contrast lesions, the use of a PH collimator could be beneficial for hot spot imaging. In all other cases, the RS collimator scores better. Finally, the simulation of a planar bone scan on a RS collimator revealed a hot spot contrast improvement up to 54% compared to a classical PH bone scan.     

Monte Carlo modelling of the performance of a rotating slit-collimator for improved planar gamma-camera imaging.

Planar imaging with a gamma camera is currently limited by the performance of the collimator. Spatial resolution and sensitivity trade off against each other; it is not possible with conventional parallel-hole collimation to have high geometric sensitivity and at the same time excellent spatial resolution unless field-of-view is sacrificed by using fan- or cone-beam collimators. We propose a rotating slit-collimator which collects one-dimensional projections from which the planar image may be reconstructed by the theory of computed tomography. The performance of such a collimator is modelled by Monte Carlo methods and images are reconstructed by a convolution and backprojection technique. The performance is compared with that of a conventional parallel-hole collimator and it is shown that higher spatial resolution with increased sensitivity is possible with the slit-collimator. For a point source a spatial resolution of some 6 mm at a distance of 100 mm from the collimator with a x7 sensitivity compared with a parallel-hole collimator was achieved. Applications to bone scintigraphy are modelled and an improved performance in hot-spot imaging is demonstrated. The expected performance in cold-spot imaging is analytically investigated. The slit-collimator is not expected to improve cold-spot imaging. Practical design considerations are discussed.     

Study of noise propagation and the effects of insufficient numbers of projection angles and detector samplings for iterative reconstruction using planar-integral data

A rotating slat collimator can be used to acquire planar-integral data. It achieves higher geometric efficiency than a parallel-hole collimator by accepting more photons, but the planar-integral data contain less tomographic information that may result in larger noise amplification in the reconstruction. Lodge evaluated the rotating slat system and the parallel-hole system based on noise behavior for an FBP reconstruction. Here, we evaluate the noise propagation properties of the two collimation systems for iterative reconstruction. We extend Huesman's noise propagation analysis of the line-integral system to the planar-integral case, and show that approximately 2.0(D/dp) SPECT angles, 2.5(D/dp) self-spinning angles at each detector position, and a 0.5dp detector sampling interval are required in order for the planar-integral data to be efficiently utilized. Here, D is the diameter of the object and dp is the linear dimension of the voxels that subdivide the object. The noise propagation behaviors of the two systems are then compared based on a least-square reconstruction using the ratio of the SNR in the image reconstructed using a planar-integral system to that reconstructed using a line-integral system. The ratio is found to be proportional to the square root of F/D, where F is a geometric efficiency factor. This result has been verified by computer simulations. It confirms that for an iterative reconstruction, the noise tradeoff of the two systems is not only dependent on the increase of the geometric efficiency afforded by the planar projection method, but also dependent on the size of the object. The planar-integral system works better for small objects, while the line-integral system performs better for large ones. This result is consistent with Lodge's results based on the FBP method.     

Experimental results from a prototype slit-slat collimator with mixed multiplexed and non-multiplexed data

We have previously shown with simulations that a gain in signal-to-noise ratio (SNR) can be obtained by using mixed multiplexed (MX) and non-MX data in a slit-slat SPECT system as compared to using non-MX data only. We have now developed a prototype slit-slat collimator for a conventional gamma camera in order to validate these simulation results. The prototype collimator consists of seven slits and multiple parallel slats. Image reconstruction is performed using a modified OSEM algorithm, which takes into account geometric sensitivity variations and attenuation, but not scatter or resolution effects. Here, we first describe the calibration of the system and then we present the experimental validation with phantom experiments. SPECT acquisitions using different geometric and anthropomorphic phantoms were performed with and without multiplexing. The results show that reconstruction of the MX projections with the non-MX-projections eliminates artefacts caused by multiplexing. SNR gains obtained using the mixed MX and non-MX configurations were in the range of 26% to 51% for different phantoms. The results were in agreement with our previously published simulation work, proving that combining MX and non-MX data can result in artefact-free reconstructed images with improved SNR.     

Development of a miniature scintillation camera using an NaI(Tl) scintillator and PSPMT for scintimammography

We have developed a small scintillation camera dedicated to breast imaging and have evaluated the performance of the system. In order to increase the limited field of view (FOV) determined by the size of a position-sensitive photomultiplier tube (PSPMT), the imaging characteristics of a diverging hole collimator (DHC) were also investigated. The small scintillation camera system consists of an NaI(Tl) crystal (60 mm x 60 mm x 6 mm) coupled to a Hamamatsu R3941 PSPMT, a resistor chain circuit, preamplifiers, nuclear instrument modules, an analogue to digital converter and a PC for control and display. The intrinsic energy resolution of the system was 12.9% FWHM at 140 keV. The spatial resolution was measured using a line-slit mask and 99mTc point sources and was 3.1 mm FWHM. The intrinsic sensitivity of the system was approximately 162 counts/s kBq(-1). The DHC made it possible to image a larger FOV (75 x 75 mm2 at the surface of collimator) than a parallel-hole collimator (60 x 60 mm2). The system sensitivity obtained using the DHC gradually decreased with distance (3% at 1 cm, 6% at 2 cm and 9% at 3 cm). The results demonstrate that the system developed in this study could be utilized clinically to image malignant breast tumours. A DHC can be employed to expand the FOV of the system confined by the size of PSPMT with a modest compromise in the performance of the system.     

CdZnTe strip detector SPECT imaging with a slit collimator

In this paper, we propose a CdZnTe rotating and spinning gamma camera attached with a slit collimator. This imaging system acquires convergent planar integrals of a radioactive distribution. Two analytical image reconstruction algorithms are proposed. Preliminary phantom studies show that our small CdZnTe camera with a slit collimator outperforms a larger NaI(Tl) camera with a pinhole collimator in terms of spatial resolution in the reconstructed images. The main application of this system is small animal SPECT imaging.     

Survey of parallel slat collimator designs for hybrid PET imaging

Hybrid PET gamma cameras with coincidence detection electronics are commonly equipped with parallel slat collimators in order to reduce detection of singles and scattered photons, and create a pseudo-2D imaging geometry. The objective of this work was to survey a broad range of parallel slat collimator designs using a series of Monte Carlo simulated PET acquisitions. Collimator properties including septal height, septal thickness and pitch were independently examined over a wide range of values. Simulations were performed for hybrid PET imaging of a long cylindrical phantom uniformly filled with water and radioactivity. The performance for each collimator design was evaluated in terms of the trues-to-singles ratio, scatter fraction, and noise equivalent count rate for a wide range of camera trigger rates. Results indicate that increasing septal height offers the biggest performance gain. Septal thickness should be at least 0.5 mm, and should be optimized in conjunction with pitch to obtain the best performance. This survey provides the groundwork necessary for optimizing slat collimators, and provides a starting point for investigating new slat collimator designs.     

Inverse-geometry volumetric CT system with multiple detector arrays for wide field-of-view imaging

Current volumetric computed tomography (CT) methods require seconds to acquire a thick volume (>8 cm) with high resolution. Inverse-geometry CT (IGCT) is a new system geometry under investigation that is anticipated to be able to image a thick volume in a single gantry rotation with isotropic resolution and no cone-beam artifacts. IGCT employs a large array of source spots opposite a smaller detector array. The in-plane field of view (FOV) is primarily determined by the size of the source array, in much the same way that the FOV is determined by the size of the detector array in a conventional CT system. Thus, the size of the source array can be a limitation on the achievable FOV. We propose adding additional detector arrays, spaced apart laterally, to increase the in-plane FOV while still using a modestly sized source array. We determine optimal detector placement to maximize the FOV while obtaining relatively uniform sampling. We also demonstrate low wasted radiation of the proposed system through design and simulation of a pre-patient collimator. Reconstructions from simulated projection data show no artifacts when combining the data from the detector arrays. Finally, to demonstrate feasibility of the concept, an anthropomorphic thorax phantom containing a porcine heart was scanned on a prototype table-top system. The reconstructed axial images demonstrate a 45 cm in-plane FOV using a 23 cm source array.     

Monte Carlo simulations of a scintillation camera using GATE: validation and application modelling

Geant4 application for tomographic emission (GATE) is a recently developed simulation platform based on Geant4, specifically designed for PET and SPECT studies. In this paper we present validation results of GATE based on the comparison of simulations against experimental data, acquired with a standard SPECT camera. The most important components of the scintillation camera were modelled. The photoelectric effect. Compton and Rayleigh scatter are included in the gamma transport process. Special attention was paid to the processes involved in the collimator: scatter, penetration and lead fluorescence. A LEHR and a MEGP collimator were modelled as closely as possible to their shape and dimensions. In the validation study, we compared the simulated and measured energy spectra of different isotopes: 99mTc, 22Na, 57Co and 67Ga. The sensitivity was evaluated by using sources at varying distances from the detector surface. Scatter component analysis was performed in different energy windows at different distances from the detector and for different attenuation geometries. Spatial resolution was evaluated using a 99mTc source at various distances. Overall results showed very good agreement between the acquisitions and the simulations. The clinical usefulness of GATE depends on its ability to use voxelized datasets. Therefore, a clinical extension was written so that digital patient data can be read in by the simulator as a source distribution or as an attenuating geometry. Following this validation we modelled two additional camera designs: the Beacon transmission device for attenuation correction and the Solstice scanner prototype with a rotating collimator. For the first setup a scatter analysis was performed and for the latter design. the simulated sensitivity results were compared against theoretical predictions. Both case studies demonstrated the flexibility and accuracy of GATE and exemplified its potential benefits in protocol optimization and in system design.     

A comparison of rotating slant hole collimator and rotating camera for single photon emission tomography of the heart

Measurements were made on the tomographic imaging abilities of a rotating slant hole collimator and a rotating scintillation camera system using a standard heart phantom and thallium-201. This allowed a comparison to be made between these and published results for conventional imaging and the seven pinhole technique under standardised conditions. Although better intraplanar resolutions were obtained with the rotating slant hole, the rotating camera was found to give superior definition of a defect and less propagation between reconstructed sections of the phantom.     

Performance of a PSPMT based detector for scintimammography

In breast scintigraphy, compact detectors with high intrinsic spatial resolution and small inactive peripheries can provide improvements in extrinsic spatial resolution, efficiency and contrast for small lesions relative to larger conventional cameras. We are developing a pixelated small field-of-view gamma camera for scintimammography. Extensive measurements of the imaging properties of a prototype system have been made, including spatial resolution, sensitivity, uniformity of response, geometric linearity and energy resolution. An anthropomorphic torso phantom providing a realistic breast exit gamma spectrum has been used in a qualitative study of lesion detectability. A new type of breast imaging system that combines scintimammography and digital mammography in a single upright unit has also been developed. The system provides automatic co-registration between the scintigram and the digital mammogram, obtained with the breast in a single configuration. Intrinsic spatial resolution was evaluated via calculation of the phase-dependent modulation transfer function (MTF). Measurements of extrinsic spatial resolution, sensitivity and uniformity of response were made for two types of parallel hole collimator using NEMA (National Electrical Manufacturers Association) protocols. Geometric linearity was quantified using a line input and least squares analysis of the measured line shape. Energy resolution was measured for seven different crystal types, and the effectiveness of optical grease coupling was assessed. Exit gamma spectra were obtained using a cadmium zinc telluride based spectrometer. These were used to identify appropriate radioisotope concentrations for the various regions of an anthropomorphic torso phantom, such that realistic scatter conditions could be obtained during phantom measurements. For prone scintimammography, a special imaging table was constructed that permits simultaneous imaging of both breasts, as well as craniocaudal views. A dedicated breast imaging system was also developed that permits simultaneous acquisition and superposition of planar gamma images and digital x-ray images. The intrinsic MTF is nonstationary, and is dependent on the phase relationship between the signal and the crystal array matrix. Averaged over all phases, the MTF is approximately 0.75, 0.57 and 0.40 at spatial frequencies of 1.0, 1.5 and 2.0 cycles per cm, respectively. The phase averaged line spread function (LSF) has a FWHM value of 2.6 mm. Following uniformity corrections, the RMS deviations in flood images are only slightly greater than is predicted from counting statistics. Across an 80 mm section of the active area, the differential linearity is 0.83 mm and the absolute linearity 2.0 mm. Using an anthropomorphic torso phantom with detachable breasts, scatter radiation similar to that observed exiting the breast of scintimammography patients was observed. It was observed that scattered gamma rays can constitute the majority of the radiation incident on the detector, but that the scatter-to-primary ratio varies significantly across the field of view, being greatest in the caudal portion of the breast, where scatter from the liver is high. Using a lesion-to-breast concentration ratio of 6:1, a 1.0 cm3 simulated breast lesion was detectable in lateral images obtained with both the developmental camera and with a clinical camera, while a 0.35 cm3 lesion was detectable in neither. Utilization of the dual x-ray transmission, gamma emission breast imaging system greatly increases the conspicuity of scintimammographic lesions relative to prone imaging, as well as greatly facilitating the localization and identification of structures in the gamma image. The prototype imaging gamma detector exhibits spatial resolution superior to that of conventional cameras, and comparable uniformity of response and geometric linearity. Because of light losses in the crystals, the energy resolution is inferior to that of single crystal NaI(Tl) came     

Monte Carlo simulations of pinhole imaging accelerated by kernel-based forced detection

Pinhole collimation can provide both higher sensitivity and resolution than parallel hole collimation when used to image small objects. When objects are placed close to the pinhole, small pinhole diameters combined with high-magnification pinhole geometries yield ultra high resolution images. With Monte Carlo (MC) calculations it is possible to simulate accurately a wide range of features of pinhole imaging. The aim of the present work is to accelerate MC simulations of pinhole SPECT projections. To achieve speed-up, forced detection (FD), a commonly used acceleration technique, is replaced by a kernel-based forced detection (KFD) step. In KFD, instead of tracing individual photons from the source or last scatter position to the detector, a position dependent kernel (point spread function (PSF)) is projected on the detector. The PSFs for channel and knife edge pinhole apertures model the penetration effects through the aperture material. For simulations, the PSFs are pre-calculated and stored in tables. The speed-up and accuracy achieved by using KFD were validated by means of digital phantoms. MC simulations with FD and with KFD converge to almost identical images. However, KFD converges to an equal image noise level one to four orders of magnitude faster than FD, depending on the number of photons simulated. A simulator accelerated by KFD could serve as a practical tool to improve iterative image reconstruction.     

Development of a high-resolution Si-PM-based gamma camera system

A silicon photomultiplier (Si-PM) is a promising photodetector for PET, especially for PET/MRI combined systems, due to its high gain, small size, and lower sensitivity to static magnetic fields. However, these properties are also promising for gamma camera systems for single-photon imaging. We developed an ultra-high-resolution Si-PM-based compact gamma camera system for small animals. Y(2)SiO(5):Ce (YSO) was selected as scintillators because of its high light output and no natural radioactivity. The gamma camera consists of 0.6 mm × 0.6 mm × 6 mm YSO pixels combined with a 0.1 mm thick reflector to form a 17 × 17 matrix that was optically coupled to a Si-PM array (Hamamatsu multi-pixel photon counter S11064-050P) with a 2 mm thick light guide. The YSO block size was 12 mm × 12 mm. The YSO gamma camera was encased in a 5 mm thick gamma shield, and a parallel hole collimator was mounted in front of the camera (0.5 mm hole, 0.7 mm separation, 5 mm thick). The two-dimensional distribution for the Co-57 gamma photons (122 keV) was almost resolved. The energy resolution was 24.4% full-width at half-maximum (FWHM) for the Co-57 gamma photons. The spatial resolution at 1.5 mm from the collimator surface was 1.25 mm FWHM measured using a 1 mm diameter Co-57 point source. Phantom and small animal images were successfully obtained. We conclude that a Si-PM-based gamma camera is promising for molecular imaging research.