Sepideh Shokouhi

Multi-pinhole collimator design for small-object imaging with SiliSPECT: a high-resolution SPECT

We have designed a multi-pinhole collimator for a dual-headed, stationary SPECT system that incorporates high-resolution silicon double-sided strip detectors. The compact camera design of our system enables imaging at source-collimator distances between 20 and 30 mm. Our analytical calculations show that using knife-edge pinholes with small-opening angles or cylindrically shaped pinholes in a focused, multi-pinhole configuration in combination with this camera geometry can generate narrow sensitivity profiles across the field of view that can be useful for imaging small objects at high sensitivity and resolution. The current prototype system uses two collimators each containing 127 cylindrically shaped pinholes that are focused toward a target volume. Our goal is imaging objects such as a mouse brain, which could find potential applications in molecular imaging.

Longitudinal Progression of Cognitive Decline Correlates with Changes in the Spatial Pattern of Brain 18F-FDG PET

Evaluating the symptomatic progression of mild cognitive impairment (MCI) caused by Alzheimer disease (AD) is practically accomplished by tracking performance on cognitive tasks, such as the Alzheimer Disease Assessment Scale’s cognitive subscale (ADAS_cog), the Mini-Mental Status Examination (MMSE), and the Functional Activities Questionnaire (FAQ). The longitudinal relationships between cognitive decline and metabolic function as assessed using 18F-FDG PET are needed to address both the cognitive and the biologic progression of disease state in individual subjects. We conducted an exploratory investigation to evaluate longitudinal changes in brain glucose metabolism of individual subjects and their relationship to the subject’s changes of cognitive status. Methods: We describe a method to determine correlations in 18F-FDG spatial distribution over time. This parameter is termed the regional 18F-FDG time correlation coefficient (rFTC). By using linear mixed-effects models, we determined the difference in the rFTC decline rate between controls and subjects at high risk of developing AD, such as individuals with MCI or the presence of apolipoprotein E (APOE)–ε4 allele. The association between each subject’s rFTC and performance on cognitive tests (ADAS_cog, MMSE, and FAQ) was determined with 2 different correlation methods. All subject data were downloaded from the Alzheimer Disease Neuroimaging Initiative. Results: The rFTC values of controls remained fairly constant over time (−0.003 annual change; 95% confidence interval, −0.010–0.004). In MCI patients, the rFTC declined faster than in controls by an additional annual change of −0.02 (95% confidence interval, −0.030 to −0.010). In MCI patients, the decline in rFTC was associated with cognitive decline (ADAS_cog, P = 0.011; FAQ, P = 0.0016; MMSE, P = 0.004). After a linear effect of time was accounted for, visit-to-visit changes in rFTC correlated with visit-to-visit changes in all 3 cognitive tests. Conclusion: Longitudinal changes in rFTC detect subtle metabolic changes in individuals associated with variations in their cognition. This analytic tool may be useful for a patient-based monitoring of cognitive decline.

Multi-pinhole SPECT imaging with silicon strip detectors

Silicon double-sided strip detectors offer outstanding instrinsic spatial resolution with reasonable detection efficiency for iodine-125 emissions. This spatial resolution allows for multiple-pinhole imaging at low magnification, minimizing the problem of multiplexing. We have conducted imaging studies using a prototype system that utilizes a detector of 300-micrometer thickness and 50-micrometer strip pitch together with a 23-pinhole collimator. These studies include an investigation of the synthetic-collimator imaging approach, which combines multiple-pinhole projections acquired at multiple magnifications to obtain tomographic reconstructions from limited-angle data using the ML-EM algorithm. Sub-millimeter spatial resolution was obtained, demonstrating the basic validity of this approach.

Advances in Preclinical SPECT Instrumentation

Preclinical SPECT of rodents is both in demand and very demanding. The need for high spatial resolution in combination with good sensitivity has given rise to considerable innovation in the areas of detectors, collimation, acquisition geometry, and image reconstruction. Some of the developments described herein are beginning to carry over into clinical imaging as well.

Spatial resolution of a noninvasive measurement of the arterial and venous input function using a wrist monitor

The current method for measuring the input function of a PET tomograph is by withdrawing arterial blood from a patients wrist In this study, the possibility of making a noninvasive measurement of the arterial blood is explored to determine the feasibility of using a planar set of detectors situated around the wrist. The arterial measurement was simulated using a peristaltic pump and an anatomically correct wrist phantom with attenuation. A step function was used to measure the activity flowing through arterial and venous tubing in the phantom. The detector was tested for spatial resolution and counting efficiency. The results showed the detector was able to discriminate the arterial and venous flows from noise when using planar coincidence images.

Evaluation of image reconstruction for mouse brain imaging with synthetic collimation from highly multiplexed SiliSPECT projections

We have performed a theoretical study to explore the potential and limitations of synthetic collimation for SPECT imaging with stacked-detector acquisition (dual magnification). This study will be used to optimize SiliSPECT, a small-animal SPECT for imaging small volumes such as a mouse brain at high sensitivity and resolution. The synthetic collimation enables image reconstruction with a limited number of camera views and in the presence of significant multiplexing. We also developed a new formulation to quantify the multiplexed object sensitivity and investigated how this changes for different acquisition parameters such as number of pinholes and combinations of front and back detector distances for imaging objects as small as the mouse brain. In our theoretical studies, we were not only able to demonstrate better reconstruction results by incorporating two detector magnifications in comparison to either one alone, but also observed an improved image reconstruction by optimizing the detector-collimator distances to change the multiplexing ratio between the front and back detectors.

Thick Silicon Double-Sided Strip Detectors for Low-Energy Small-Animal SPECT

This work presents characterization studies of thick silicon double-sided strip detectors for a high-resolution small-animal SPECT. The dimension of these detectors is 60.4 mm times 60.4 mm times 1 mm. There are 1024 strips on each side that give the coordinates of the photon interaction, with each strip processed by a separate ASIC channel. Our measurement shows that intrinsic spatial resolution equivalent to the 59 mum strip pitch is attainable. Good trigger uniformity can be achieved by proper setting of a 4-bit DAC in each ASIC channel to remove trigger threshold variations. This is particularly important for triggering at low energies. The thick silicon DSSD (Double-sided strip detector) shows high potential for small-animal SPECT.

A silicon SPECT system for molecular imaging of the mouse brain

We previously demonstrated the feasibility of using silicon double-sided strip detectors (DSSDs) for SPECT imaging of the activity distribution of iodine-125 using a 300-micrometer thick detector. Based on this experience, we now have developed fully customized silicon DSSDs and associated readout electronics with the intent of developing a multi-pinhole SPECT system. Each DSSD has a 60.4 mm x 60.4 mm active area and is 1 mm thick. The strip pitch is 59 micrometers, and the readout of the 1024 strips on each side gives rise to a detector with over one million pixels. Combining four high-resolution DSSDs into a SPECT system offers an unprecedented space-bandwidth product for the imaging of single-photon emitters. The system consists of two camera heads with two silicon detectors stacked one behind the other in each head. The collimator has a focused pinhole system with cylindrical-shaped pinholes that are laser-drilled in a 250 mum tungsten plate. The unique ability to collect projection data at two magnifications simultaneously allows for multiplexed data at high resolution to be combined with lower magnification data with little or no multiplexing. With the current multi-pinhole collimator design, our SPECT system will be capable of offering high spatial resolution, sensitivity and angular sampling for small field-of-view applications, such as molecular imaging of the mouse brain.

PDAQ - a fast data acquisition system for the RatCAP tomograph

We describe a custom data acquisition system for the RatCAP tomograph. It is a small, head-mounted PET detector designed to image the brain of an awake rat. At its core, the tomograph consists of a number of LSO crystals read out with an array of APDs. The data will be collected through a custom-designed ASIC, along with a custom VME board. We describe the design, implementation, and performance of a versatile VME-based data acquisition system which will be used to read out the VME board, as well as other off-the-shelf data acquisition electronics.

Light output measurements of LSO single crystals and 4/spl times/8 arrays: comparison of experiment with Monte Carlo simulations

We have made a comparison between experimentally determined light output from various size LSO crystals and the Monte Carlo simulations using the program Opticad. The results show good agreement between the predicted results and the experimental measurements. These results show that it is possible to accurately simulate the light output using these calculations. This allows testing of many parameters quickly and the determination of the critical parameters in maximizing the light output and photoelectron yield from these detector modules.