Scott Stephen Zelakiewicz

Evaluation of a position sensitive avalanche photodiode for PET

A gamma ray detector for PET, consisting of an array of mixed lutetium oxyorthosilicate (MLS) scintillator crystals coupled to a position sensitive avalanche photodiode (PSAPD), was evaluated. The scintillator array was constructed from individual MLS crystals with dimensions of 1.5 mm /spl times/ 1.5 mm /spl times/ 15 mm. The assembled 7 /spl times/ 7 array, including intercrystal reflector material, had a pitch of 1.79 mm. The low noise, high gain PSAPD had dimensions of 14 mm /spl times/ 14 mm. Peaks associated with each of the 49 scintillator crystals were readily identifiable in flood histograms, and most of the crystals demonstrated energy resolution in the range of 15% to 20% at 511 keV. Measurements of the timing of the PSAPD in coincidence with a fast-scintillator/PMT detector indicated a timing resolution of approximately 4 ns. The operating characteristics and design attributes, such as compactness and reduced readout channel requirements, of the PSAPD make it attractive for high resolution PET applications.

Digital Breast Tomosynthesis: Observer Performance of Clustered Microcalcification Detection on Breast Phantom Images Acquired with an Experimental System Using Variable Scan Angles, Angular Increments, and Number of Projection Views

PURPOSE To investigate the dependence of microcalcification cluster detectability on tomographic scan angle, angular increment, and number of projection views acquired at digital breast tomosynthesis ( DBT digital breast tomosynthesis ). MATERIALS AND METHODS A prototype DBT digital breast tomosynthesis system operated in step-and-shoot mode was used to image breast phantoms. Four 5-cm-thick phantoms embedded with 81 simulated microcalcification clusters of three speck sizes (subtle, medium, and obvious) were imaged by using a rhodium target and rhodium filter with 29 kV, 50 mAs, and seven acquisition protocols. Fixed angular increments were used in four protocols (denoted as scan angle, angular increment, and number of projection views, respectively: 16°, 1°, and 17; 24°, 3°, and nine; 30°, 3°, and 11; and 60°, 3°, and 21), and variable increments were used in three (40°, variable, and 13; 40°, variable, and 15; and 60°, variable, and 21). The reconstructed DBT digital breast tomosynthesis images were interpreted by six radiologists who located the microcalcification clusters and rated their conspicuity. RESULTS The mean sensitivity for detection of subtle clusters ranged from 80% (22.5 of 28) to 96% (26.8 of 28) for the seven DBT digital breast tomosynthesis protocols; the highest sensitivity was achieved with the 16°, 1°, and 17 protocol (96%), but the difference was significant only for the 60°, 3°, and 21 protocol (80%, P < .002) and did not reach significance for the other five protocols (P = .01-.15). The mean sensitivity for detection of medium and obvious clusters ranged from 97% (28.2 of 29) to 100% (24 of 24), but the differences fell short of significance (P = .08 to >.99). The conspicuity of subtle and medium clusters with the 16°, 1°, and 17 protocol was rated higher than those with other protocols; the differences were significant for subtle clusters with the 24°, 3°, and nine protocol and for medium clusters with 24°, 3°, and nine; 30°, 3°, and 11; 60°, 3° and 21; and 60°, variable, and 21 protocols (P < .002). CONCLUSION With imaging that did not include x-ray source motion or patient motion during acquisition of the projection views, narrow-angle DBT digital breast tomosynthesis provided higher sensitivity and conspicuity than wide-angle DBT digital breast tomosynthesis for subtle microcalcification clusters.

Digital breast tomosynthesis: studies of the effects of acquisition geometry on contrast-to-noise ratio and observer preference of low-contrast objects in breast phantom images

The effect of acquisition geometry in digital breast tomosynthesis was evaluated with studies of contrast-to-noise ratios (CNRs) and observer preference. Contrast-detail (CD) test objects in 5 cm thick phantoms with breast-like backgrounds were imaged. Twelve different angular acquisitions (average glandular dose for each ~1.1 mGy) were performed ranging from narrow angle 16° with 17 projection views (16d17p) to wide angle 64d17p. Focal slices of SART-reconstructed images of the CD arrays were selected for CNR computations and the reader preference study. For the latter, pairs of images obtained with different acquisition geometries were randomized and scored by 7 trained readers. The total scores for all images and readings for each acquisition geometry were compared as were the CNRs. In general, readers preferred images acquired with wide angle as opposed to narrow angle geometries. The mean percent preferred was highly correlated with tomosynthesis angle (R = 0.91). The highest scoring geometries were 60d21p (95%), 64d17p (80%), and 48d17p (72%); the lowest scoring were 16d17p (4%), 24d9p (17%) and 24d13p (33%). The measured CNRs for the various acquisitions showed much overlap but were overall highest for wide-angle acquisitions. Finally, the mean reader scores were well correlated with the mean CNRs (R = 0.83).

Dependence of timing resolution on crystal size for TOF PET

Recently, with the prospect of great improvement in image quality, the development of time of flight technology has become an exciting topic for positron emission tomography. The excitement was further accelerated by the introduction of various fast and high light output scintillators as well as photosensors. However, the development of improved time of flight detectors is not only about the selection of crystals and photosensors, but also about how detectors are assembled to optimize their performance. For example, depending on crystal block structure, photo-sensor layout, and coupling methods, a detectors timing resolution can be drastically different. Since the effect of block structure for timing resolution is complex and less understood it is essential to first dissect the block structure and understand the impact of its basic components on timing resolution. In this paper, we will present the dependence of timing resolution on varying the dimensions of the scintillator crystals that are the main component of a block detector.

Multiplexing requirements for solid state photomultipliers in time-of-flight PET

The solid state photomultiplier (SSPM), an array of Geiger-mode APDs, developed by Hamamatsu Corp. has been evaluated for time-of-flight PET detectors. As it was demonstrated in previous work the SSPM is a promising photo sensor to achieve very good timing resolution for PET applications. Due to relatively small size of individual sensor (3×3 mm2) a large number of readout channels will be required in applications such as a whole body PET scanner. The obvious solution for this problem is multiplexing several devices into a single readout channel. To better understand the limits and trade offs involved for timing resolution a detailed analysis of the effects of dark current, amplifier noise, bandwidth and amplifier input impedance was done.

A new surface parameterization for modeling thin layers of reflector material in the DETECT2000 optical modeling program

A new surface parameterization has been implemented in DETECT2000 that allows for the efficient representation of a thin film of reflector material with non-negligible transmission. Material is usually modeled in DETECT by specifying the optical attenuation and scatter lengths. For a material such as Teflon, commonly used as a reflector material for nuclear medicine scintillators, the scatter length can be very short. Modeling the propagation of photons in a material with a very short scattering length can be computationally intensive as there can be many scatter interactions before the photon either exits the material or is absorbed. In a detector design that makes generous use of Teflon, most of the computations can be spent simply transporting photons in the Teflon. To address this issue, a parameterized surface has been implemented which is specified not by the bulk optical scatter and attenuation lengths, but rather by a reflection and transmission coefficient. This allows computationally efficient modeling of thin film reflectors where surface properties and transmission are relevant.

Evaluation of position sensitive avalanche photodiodes for PET

A gamma ray detector for PET, consisting of an array of mixed lutetium oxyorthosilicate (MLS) scintillator crystals coupled to a position sensitive avalanche photodiode (PSAPD), was evaluated. The scintillator array was constructed from individual MLS crystals with dimensions of 1.5 mm/spl times/1.5 mm/spl times/15 mm. The assembled 7/spl times/7 array, including inter-crystal reflector material, had a pitch of 1.79 mm. The low noise, high gain PSAPD had dimensions of 14 mm/spl times/14 mm. Peaks associated with each of the 49 scintillator crystals were readily identifiable in flood histograms, and most of the crystals demonstrated energy resolution in the range of 15% to 20% at 511 keV. Preliminary measurements of the timing of the PSAPD in coincidence with a fast-scintillator/PMT detector indicated a timing resolution of approximately 4 ns. The operating characteristics and design attributes, such as compactness and reduced readout channel requirements, of the PSAPD make it attractive for high resolution PET applications.

Gated Diode Design to Mitigate Radiation Damage in X-Ray Imagers

Radiation damage of amorphous silicon X-ray imagers leads to degradation of the detectors performance due to increased diode perimeter leakage. To reduce the effect of this damage, a novel pixel device based on a gated diode was fabricated. The additional gate metalization placed on the perimeter of the diode modulates the surface side-wall leakage and has been tested up to a 64 kGy absorbed dose in the diode. This new pixel design significantly reduces the increase in diode leakage and noise due to radiation damage, providing a more uniform performance and extending the lifetime of the imager.

RIVL: a radiation imager virtual laboratory

A software suite for the modeling of medical imaging detectors has been constructed that uses GEANT4 for particle tracking and DETECT2000 for the optical modeling of scintillation photons. The Radiation Imager Virtual Laboratory (RIVL) is a collection of modular, stand-alone programs that are interfaced to each other via format conversion utilities and is intended to model the energy deposition, scintillation conversion, optical photon transport, signal sensing, electronics and pulse processing. RIVL makes use of a GEANT4 application developed at General Electric Global Research called the Virtual Radiation Imager (VRAI). VRAI is based on GEANT4, a sophisticated and mature collection of C++ libraries that are commonly used in nuclear and particle physics and is seeing an increased use in the medical imaging community. With GEANT4, the physical interactions of particles are well modeled in the energy regime relevant for medical imaging. Modeling of the optical transport of scintillation photons is performed with the program DETECT2000, which enables detailed control over optical properties and is better suited than GEANT4 for modeling the transport of optical scintillation photons. Interfacing GEANT with DETECT2000 harnesses the strengths of each of these programs to create a complete model for various medical imaging applications. The simulation of the response of the photosensor to scintillation photons, as well as the logic applied to the photosensor signals to reconstruct the hit position and energy are accomplished with custom software modules written at the GE Global Research Center.

Toward a dose reduction strategy using model-based reconstruction with limited-angle tomosynthesis

Model-based iterative reconstruction (MBIR) is an emerging technique for several imaging modalities and appli- cations including medical CT, security CT, PET, and microscopy. Its success derives from an ability to preserve image resolution and perceived diagnostic quality under impressively reduced signal level. MBIR typically uses a cost optimization framework that models system geometry, photon statistics, and prior knowledge of the recon- structed volume. The challenge of tomosynthetic geometries is that the inverse problem becomes more ill-posed due to the limited angles, meaning the volumetric image solution is not uniquely determined by the incom- pletely sampled projection data. Furthermore, low signal level conditions introduce additional challenges due to noise. A fundamental strength of MBIR for limited-views and limited-angle is that it provides a framework for constraining the solution consistent with prior knowledge of expected image characteristics. In this study, we analyze through simulation the capability of MBIR with respect to prior modeling components for limited-views, limited-angle digital breast tomosynthesis (DBT) under low dose conditions. A comparison to ground truth phantoms shows that MBIR with regularization achieves a higher level of fidelity and lower level of blurring and streaking artifacts compared to other state of the art iterative reconstructions, especially for high contrast objects. The benefit of contrast preservation along with less artifacts may lead to detectability improvement of microcalcification for more accurate cancer diagnosis.