Lindsay C. Johnson

Longitudinal live animal micro-CT allows for quantitative analysis of tumor-induced bone destruction

The majority of breast cancer and prostate cancer patients with metastatic disease will go on to develop bone metastases, which contribute largely to the patients morbidity and mortality. Numerous small animal models of cancer metastasis to bone have been developed to study tumor-induced bone destruction, but the advancement of imaging modalities utilized for these models has lagged significantly behind clinical imaging. Therefore, there is a significant need for improvements to live small animal imaging, particularly when obtaining high-resolution images for longitudinal quantitative analyses. Recently, live animal micro-computed tomography (μCT) has gained popularity due to its ability to obtain high-resolution 3-dimensional images. However, the utility of μCT in bone metastasis models has been limited to end-point analyses due to off-target radiation effects on tumor cells. We hypothesized that live animal in vivo μCT can be utilized to perform reproducible and quantitative longitudinal analyses of bone volume in tumor-bearing mice, particularly in a drug treatment model of breast cancer metastasis to bone. To test this hypothesis, we utilized the MDA-MB-231 osteolytic breast cancer model in which the tumor cells are inoculated directly into the tibia of athymic nude mice and imaged mice weekly by Faxitron (radiography), Imtek μCT (in vivo), and Maestro (GFP-imaging). Exvivo μCT and histology were performed at end point for validation. After establishing a high-resolution scanning protocol for the Imtek CT, we determined whether clear, measurable differences in bone volume were detectable in mice undergoing bisphosphonate drug treatments. We found that in vivo μCT could be used to obtain quantifiable and longitudinal images of the progression of bone destruction over time without altering tumor cell growth. In addition, we found that we could detect lesions as early as week 1 and that this approach could be used to monitor the effect of drug treatment on bone. Taken together, these data indicate that in vivo μCT is an effective and reproducible method for longitudinal monitoring of tumor-associated bone destruction in mouse models of tumor-induced bone disease.

Osteoclast-derived matrix metalloproteinase-9 directly affects angiogenesis in the prostate tumor-bone microenvironment.

In human prostate to bone metastases and in a novel rodent model that recapitulates prostate tumor–induced osteolytic and osteogenic responses, we found that osteoclasts are a major source of the proteinase, matrix metalloproteinase (MMP)-9. Because MMPs are important mediators of tumor-host communication, we tested the effect of host-derived MMP-9 on prostate tumor progression in the bone. To this end, immunocompromised mice that were wild-type or null for MMP-9 received transplants of osteolytic/osteogenic-inducing prostate adenocarcinoma tumor tissue to the calvaria. Surprisingly, we found that that host MMP-9 significantly contributed to prostate tumor growth without affecting prostate tumor–induced osteolytic or osteogenic change as determined by microcomputed tomography, microsingle-photon emission computed tomography, and histomorphometry. Subsequent studies aimed at delineating the mechanism of MMP-9 action on tumor growth focused on angiogenesis because MMP-9 and osteoclasts have been implicated in this process. We observed (a) significantly fewer and smaller blood vessels in the MMP-9 null group by CD-31 immunohistochemistry; (b) MMP-9 null osteoclasts had significantly lower levels of bioavailable vascular endothelial growth factor-A164; and (c) using an aorta sprouting assay, conditioned media derived from wild-type osteoclasts was significantly more angiogenic than conditioned media derived from MMP-9 null osteoclasts. In conclusion, these studies show that osteoclast-derived MMP-9 affects prostate tumor growth in the bone microenvironment by contributing to angiogenesis without altering prostate tumor–induced osteolytic or osteogenic changes. Mol Cancer Res; 8(4); 459–70. ©2010 AACR.

Characterization of a high-purity germanium detector for small-animal SPECT

We present an initial evaluation of a mechanically-cooled, high-purity germanium double-sided strip detector as a potential gamma camera for small-animal SPECT. It is 90 mm in diameter and 10 mm thick with two sets of 16 orthogonal strips that have a 4.75 mm width with a 5 mm pitch. A sub-strip interpolation method is used to bin the data at a pixel size of 0.53mm × 0.53mm, while it is also possible to estimate the depth of interaction based on CFD time differences between the anode and cathode. The system has an energy resolution of 0.92% at 140 keV and an intrinsic efficiency of 55.40% at 122 keV. Simulations suggest that increases in the efficiency should be possible by altering signal processing to include Compton events in which charge is collected on more than one strip. Integral uniformity in the central field of view for strips was found to be less than 1% in a flood-corrected flood while pixel-level uniformity was 2.98%. Due to the excellent energy resolution, the presence of a scattering medium did not greatly alter the FWHM or FWTM of a pinhole projection when compared to an acquisition without a scattering medium. This high-purity germanium system offers many desirable properties for small-animal SPECT.

Performance characterization of a high-purity germanium detector for small-animal SPECT

We present an initial evaluation of a mechanically-cooled, high-purity germanium double-sided strip detector as a potential gamma camera for small-animal SPECT. It is 90 mm in diameter and 10 mm thick with two sets of 16 orthogonal strips that have a 4.75 mm width with a 5 mm pitch. A sub-strip interpolation method is used to bin the data at a pixel size of 0.53mm × 0.53mm, while it is also possible to estimate the depth of interaction based on CFD time differences between the anode and cathode. The system has an energy resolution of 0.92% at 140 keV and an intrinsic efficiency of 55.40% at 122 keV. Simulations suggest that increases in the efficiency should be possible by altering signal processing to include Compton events in which charge is collected on more than one strip. Integral uniformity in the central field of view for strips was found to be less than 1% in a flood-corrected flood while pixel-level uniformity was 2.98%. Due to the excellent energy resolution, the presence of a scattering medium did not greatly alter the FWHM or FWTM of a pinhole projection when compared to an acquisition without a scattering medium. This high-purity germanium system offers many desirable properties for small-animal SPECT.

Effect of pinhole shape on projection resolution.

We are designing a dual-resolution pre-clinical SPECT system based on square-pinhole apertures for use in applications with a small field-of-view (FOV), such as cardiac imaging of mice. Square pinholes allow for increased sensitivity due to more efficient projection tiling on the detector compared to circular pinholes. Aperture fabrication techniques cannot produce a perfect square, giving the square pinholes some amount of roundedness at the corners. This work investigates how this roundedness affects the physical properties of projection images in terms of spatial resolution. Different pinhole full-acceptance angles and roundedness values were simulated. To facilitate a fair comparison, properties of the non-square pinholes were manipulated to yield pinholes with approximately the same sensitivity (to within 0.1%) and FOV (to within 0.5%) as those of the square pinholes, subsequently referred to as matched apertures. The aperture size (flat-to-flat edge length) of each non-square aperture was increased until its sensitivity was approximately equal to that of the square pinhole. Next, the full acceptance angle was increased until the FOV of each non-square aperture was approximately equivalent to that of the square pinhole. Sensitivity was calculated to include both the geometric and penetrative sensitivity of a point source, as well as the packing faction of the multi-pinhole collimator. Using the sensitivity-matched and FOV-matched apertures, spatial resolution was estimated. For the 0.3 mm, 0.5 mm, and 1 mm edge-length square apertures studied, the full-width at half-maximum widened as pinhole shape changed from square to circle, while full-width tenth-maximum showed little change. These results indicate that a perfect square pinhole shape is more desirable than a rounded-square pinhole with regard to spatial resolution when sensitivity and FOV-matched pinholes are compared.

Development of a Germanium Small-Animal SPECT System

Advances in fabrication techniques, electronics, and mechanical cooling systems have given rise to germanium detectors suitable for biomedical imaging. We are developing a small-animal SPECT system that uses a double-sided Ge strip detector. The detectors excellent energy resolution may help to reduce scatter and simplify processing of multi-isotope imaging, while its ability to measure depth of interaction has the potential to mitigate parallax error in pinhole imaging. The detectors energy resolution is <; 1% FWHM at 140 keV and its spatial resolution is approximately 1.5 mm FWHM. The prototype system described has a single-pinhole collimator with a 1-mm diameter and a 70-degree opening angle with a focal length variable between 4.5 and 9 cm. Phantom images from the gantry-mounted system are presented, including the NEMA NU-2008 phantom and a hot-rod phantom. Additionally, the benefit of energy resolution is demonstrated by imaging a dual-isotope phantom with 99mTc and 123I without cross-talk correction.

Simulation of the effects of multiplexing in multi-pinhole SPECT using stacked Si-HPGe detectors

Small-animal pinhole single photon emission computed tomography (SPECT) image quality is limited by a number of factors, including the systems detection efficiency. Efficiency can be improved upon by using multi-pinhole apertures, but when detector space is limited pinhole projections may overlap, or multiplex, which can cause artifacts in reconstruction. We are currently developing a SPECT system utilizing a high-purity germanium (HPGe) detector, but due to its limited size, in order to achieve reasonable magnification with multiple pinholes, projections on the HPGe are likely to have some amount of multiplexing. In this work we explore whether a stacked-detector configuration with both HPGe and a silicon detector, used with 1231 (27-32, 159 keY), where little or no multiplexing occurs in the Si projections, can help to compensate for the image degradation caused by the multiplexed HPGe projections. Simulations with varying pinhole configurations in conjunction with three different object phantoms (a hot-rod, cold-rod and cool-spot) are used to examine whether the additional projections from the Si detector help to overcome the artifacts from the multiplexing on the HPGe detector. The number of pinholes and the distance between pinholes are varied such that different amounts of multiplexing are seen on the HPGe detector. Reconstructed images using both Si and HPGe data are compared to those using HPGe data alone. Normalized mean square error provides a quantitative comparison of reconstructed images to the original phantoms and a means to evaluate the impact of the additional non-multiplexed data on image quality. For a qualitative comparison, the differential point response function is used to examine artifacts for each configuration. Results show that in cases of highly-multiplexed HPGe projections, the addition o

Characterization of a small-animal high-purity germanium SPECT system

Attempts have been made to use segmented planar germanium detectors for nuclear medicine imaging since the early 1970s, but have been hindered by difficulties with detector fabrication, cumbersome electronics, and limited computing power, in addition to the need for a bulky liquid nitrogen dewar to achieve required operating temperatures. Advances in electronics in addition to compact mechanical cooling systems have made high-purity germanium (HPGe) systems appealing for biomedical applications. We are developing a small-animal single-photon emission computed tomography (SPECT) system based on an HPGe double-sided strip detector that is 90 mm in diameter, 10 mm thick, and comprised of two sets of 16 orthogonal strips that are each 4.75 mm wide with a 5 mm strip pitch. The detectors energy resolution is <;1% FWHM at 140 keV, and the spatial resolution is approximately 1.5 mm FWHM. The system is equipped with a single-pinhole collimator with a 1mm diameter and a 70-degree opening angle with a focal length variable between 4.5 and 9 cm. Preliminary phantom images from a bench-top HPGe SPECT system and from an integrated HPGe-SPECT-CT system are shown.

Task‐based design of a synthetic‐collimator SPECT system used for small animal imaging

Purpose In traditional multipinhole SPECT systems, image multiplexing — the overlapping of pinhole projection images — may occur on the detector, which can inhibit quality image reconstructions due to photon‐origin uncertainty. One proposed system to mitigate the effects of multiplexing is the synthetic‐collimator SPECT system. In this system, two detectors, a silicon detector and a germanium detector, are placed at different distances behind the multipinhole aperture, allowing for image detection to occur at different magnifications and photon energies, resulting in higher overall sensitivity while maintaining high resolution. The unwanted effects of multiplexing are reduced by utilizing the additional data collected from the front silicon detector. However, determining optimal system configurations for a given imaging task requires efficient parsing of the complex parameter space, to understand how pinhole spacings and the two detector distances influence system performance. Methods In our simulation studies, we use the ensemble mean‐squared error of the Wiener estimator (EMSEW) as the figure of merit to determine optimum system parameters for the task of estimating the uptake of an 123I‐labeled radiotracer in three different regions of a computer‐generated mouse brain phantom. The segmented phantom map is constructed by using data from the MRM NeAt database and allows for the reduction in dimensionality of the system matrix which improves the computational efficiency of scanning the systems parameter space. To contextualize our results, the Wiener estimator is also compared against a region of interest estimator using maximum‐likelihood reconstructed data. Results Our results show that the synthetic‐collimator SPECT system outperforms traditional multipinhole SPECT systems in this estimation task. We also find that image multiplexing plays an important role in the system design of the synthetic‐collimator SPECT system, with optimal germanium detector distances occurring at maxima in the derivative of the percent multiplexing function. Furthermore, we report that improved task performance can be achieved by using an adaptive system design in which the germanium detector distance may vary with projection angle. Finally, in our comparative study, we find that the Wiener estimator outperforms the conventional region of interest estimator. Conclusions Our work demonstrates how this optimization method has the potential to quickly and efficiently explore vast parameter spaces, providing insight into the behavior of competing factors, which are otherwise very difficult to calculate and study using other existing means.

Using the Wiener estimator to determine optimal imaging parameters in a synthetic-collimator SPECT system used for small animal imaging

In synthetic-collimator SPECT imaging, two detectors are placed at different distances behind a multi-pinhole aperture. This configuration allows for image detection at different magnifications and photon energies, resulting in higher overall sensitivity while maintaining high resolution. Image multiplexing the undesired overlapping between images due to photon origin uncertainty may occur in both detector planes and is often present in the second detector plane due to greater magnification. However, artifact-free image reconstruction is possible by combining data from both the front detector (little to no multiplexing) and the back detector (noticeable multiplexing). When the two detectors are used in tandem, spatial resolution is increased, allowing for a higher sensitivity-to-detector-area ratio. Due to variability in detector distances and pinhole spacings found in synthetic-collimator SPECT systems, a large parameter space must be examined to determine optimal imaging configurations. We chose to assess image quality based on the task of estimating activity in various regions of a mouse brain. Phantom objects were simulated using mouse brain data from the Magnetic Resonance Microimaging Neurological Atlas (MRM NeAt) and projected at different angles through models of a synthetic-collimator SPECT system, which was developed by collaborators at Vanderbilt University. Uptake in the different brain regions was modeled as being normally distributed about predetermined means and variances. We computed the performance of the Wiener estimator for the task of estimating activity in different regions of the mouse brain. Our results demonstrate the utility of the method for optimizing synthetic-collimator system design.