retrospective-cardiac-gating-with-prototype-small-animal-x-ray

Retrospective Cardiac Gating with A Prototype Small-Animal X-ray Computed Tomograph

The CrumpCAT is a prototype small-animal X-ray computed tomography (CT) scanner developed at our research institution. The CMOS detector with a maximum ...

University of California, Los Angeles

The purpose of this work was to calculate radiation dose and its organ distribution in a realistic mouse phantom from micro-computed tomography (microCT) imaging protocols. CT dose was calculated using GATE and a voxelized, realistic phantom. The x-ray photon energy spectra used in simulations were precalculated with GATE and validated against previously published data. The number of photons required per simulated experiments was determined by direct exposure measurements. Simulated experiments were performed for three types of beams and two types of mouse beds. Dose-volume histograms and dose percentiles were calculated for each organ. For a typical microCT screening examination with a reconstruction voxel size of 200 microm, the average whole body dose varied from 80 mGy (at 80 kVp) to 160 mGy (at 50 kVp), showing a strong dependence on beam hardness. The average dose to the bone marrow is close to the soft tissue average. However, due to dose nonuniformity and higher radiation sensitivity, 5% of the marrow would receive an effective dose about four times higher than the average. If CT is performed longitudinally, a significant radiation dose can be given. The total absorbed radiation dose is a function of milliamperes-second, beam hardness, and desired image quality (resolution, noise and contrast). To reduce dose, it would be advisable to use the hardest beam possible while maintaining an acceptable contrast in the image.

Monte carlo simulations of dose from microCT imaging procedures in a realistic mouse phantom.

We report here on a technique to implement high-resolution objects with voxels having variable dimensions (compressed) for the reduction of memory and central processing unit (CPU) requirements in Monte Carlo simulations. The technique, which was implemented in GATE, the GEANT4 application for positron emission tomography/single photon emission computed tomography (PET/SPECT) imaging simulations, was developed in response to our need for realistic high-resolution phantoms for dosimetry calculations.

Compressed voxels for high-resolution phantom simulations in GATE.

Dual-energy x-ray computed tomography (DECT) has the capability to decompose attenuation coefficients using two basis functions and has proved its potential in reducing beam-hardening artifacts from reconstructed images. The method typically involves two successive scans with different x-ray tube voltage settings. This work proposes an approach to dual-energy imaging through x-ray beam filtration that requires only one scan and a single tube voltage setting. It has been implemented in a preclinical microCT tomograph with minor modifications. Retrofitting of the microCT scanner involved the addition of an automated filter wheel and modifications to the acquisition and reconstruction software. Results show that beam-hardening artifacts are reduced to noise level. Acquisition of a mu-Compton image is well suited for attenuation-correction of PET images while dynamic energy selection (4D viewing) offers flexibility in image viewing by adjusting contrast and noise levels to suit the task at hand. All dual-energy and single energy reference scans were acquired at the same soft tissue dose level of 50 mGy.

Dual-energy attenuation coefficient decomposition with differential filtration and application to a microCT scanner.

Laboratory mice routinely are housed at 20 to 22 °C-well below the murine thermoneutral zone of 29 to 34 °C. Chronic cold stress requires greater energy expenditure to maintain core body temperature and can lead to the failure of mouse models to emulate human physiology. We hypothesized that mice housed at ambient temperatures of 20 to 22 °C are chronically cold-stressed, have greater energy expenditure, and have high glucose utilization in brown adipose tissue. To test our hypotheses, we used indirect calorimetry to measure energy expenditure and substrate utilization in C57BL/6J and Crl:NU-Foxn1(nu) nude mice at routine vivarium (21 °C), intermediate (26 °C), and heated (31 °C) housing temperatures. We also examined the activation of interscapular brown adipose tissue, the primary site of nonshivering thermogenesis, via thermography and glucose uptake in this region by using positron emission tomography. Energy expenditure of mice was significantly higher at routine vivarium temperatures compared with intermediate and heated temperatures and was associated with a shift in metabolism toward glucose utilization. Brown adipose tissue showed significant activation at routine vivarium and intermediate temperatures in both hirsuite and nude mice. Crl:NU-Foxn1(nu) mice experienced greater cold stress than did C57BL/6J mice. Our data indicate mice housed under routine vivarium conditions are chronically cold stress. This novel use of thermography can measure cold stress in laboratory mice housed in vivaria, a key advantage over classic metabolic measurement tools. Therefore, thermography is an ideal tool to evaluate novel husbandry practices designed to alleviate murine cold stress.

The hidden cost of housing practices: using noninvasive imaging to quantify the metabolic demands of chronic cold stress of laboratory mice.

We have used simulations and measurements to investigate the feasibility of using slanted scintillator crystal geometries as means to provide depth-of-interaction (DOI) information for a pixelated gamma ray imaging detector. The simulations were performed to estimate the fraction of scintillation light detected by the photodetector as a function of interaction location along the height of crystals with different geometries. In addition, physical measurements of the light output for these crystal geometries were obtained from individual crystals coupled to a solid state photodetector (Philips digital-SiPM DPC-3200). In agreement with previous work, we found a change in light output in the slanted region of the crystals compared to the rectangular region. The results from this study indicate the potential of using slanted crystals to gather DOI information based on light output changes as a function of the location of interaction. An examination of the measured energy spectra for the geometries evaluated here, suggests that for BGO crystals somewhere between 2 or 3 DOI bins could be implemented. Based on these results, we conceived a design for a DOI detector module that consists of two slanted crystals, each read-out by separate SiPM pixels.

Article	Year
Monte carlo simulations of dose from microCT imaging procedures in a realistic mouse phantom. Medical physics\| PubMed ID: 16485428	2006
Compressed voxels for high-resolution phantom simulations in GATE. Molecular imaging and biology\| PubMed ID: 18046609	2008
Dual-energy attenuation coefficient decomposition with differential filtration and application to a microCT scanner. Physics in medicine and biology\| PubMed ID: 20107245	2010
The hidden cost of housing practices: using noninvasive imaging to quantify the metabolic demands of chronic cold stress of laboratory mice. Comparative medicine\| PubMed ID: 24210014	2013
Feasibility of Using Crystal Geometry for a DOI Scintillation Detector. IEEE transactions on radiation and plasma medical sciences\| PubMed ID: 31098432	2018

Richard Taschereau

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