Performance evaluation of a micro-CT system for laboratory animal imaging with iterative reconstruction capabilities

Development of a respiratory-gated computed tomography system for in-vivo murine imaging

BACKGROUND

Respiratory motion poses a critical challenge in small animal lung imaging with micro-computed tomography (µCT). Contact sensors, when utilized as respiratory gating devices, can introduce beam-hardening artifacts and degrade image quality.

PURPOSE

This study is to develop a respiration-gated computed tomography (CT) system utilizing a non-contact laser displacement sensor for in vivo murine imaging.

METHODS

The gating system comprises an x-ray beam shutter and a non-contact laser displacement sensor. The shutter controls the beam on and off during image acquisition, while the laser sensor converts thoracic surface displacement into a respiratory signal. The system's switch latency and measurement accuracy were assessed. Then, the gating system was utilized to analyze the respiratory patterns of animals (four groups and nine mice per group) anesthetized with varying isoflurane concentrations (1.0% to 2.5%). The external respiratory signal from the laser was compared with the diaphragm motion extracted from x-ray projections to analyze the delay between the two signals. Finally, eight mice were selected for retrospective and prospective gating imaging, respectively, and a variable number of landmarks, including the diaphragm, blood vessels, and bronchioles, were used to evaluate the image blur.

RESULTS

The system's turn-on and turn-off latencies were 31.4 ± 4.9 ms and 32.6 ± 2.8 ms, respectively. The Pearson correlation test showed a strong correlation between the laser signal and the trajectory of the dynamic phantom (R = 0.99). In all four groups, a delay of approximately 200 ms was observed for the internal signal entering the end-expiration (EE) phase when compared with the external signal and was accounted for by a "delayed gating" strategy. Retrospective gating studies demonstrated that the slopes of the intensity across the diaphragm in images obtained without gating, with traditional gating, and with delayed gating were 21.5 ± 5.5, 41.5 ± 6.0, and 72.5 ± 9.5 Hounsfield units (HUs) per pixel, respectively, with significant differences among them (p < 0.001). Compared to traditional gating, delayed gating reduced motion artifacts and improved the clarity of lung structures. In prospective gating studies, the intensity slope across the diaphragm for delayed gating was 72.4 ± 12.4 HU/pixel, significantly higher than in the no-gating condition, which was 20.9 ± 4.1 HU/pixel (p < 0.001).

CONCLUSIONS

The analysis of mouse respiratory patterns revealed a time delay between the internal and external respiratory signals. The non-contact respiratory gating system combined with the delayed gating strategy can effectively reduce motion blur and enhance the visibility of fine structures and therefore can be applied to enhance the ability of µCT in quantitative lung imaging, such as in the early detection and precise differentiation of lung lesions.

Next-Gen Poly(ε-Caprolactone) Scaffolds: Non-Destructive In Vivo Monitoring and Accelerated Biodegradation

Poly(ɛ-caprolactone) (PCL) is a biocompatible, biodegradable, and highly mechanically resilient FDA-approved material (for specific biomedical applications, e.g. as drug delivery devices, in sutures, or as an adhesion barrier), rendering it a promising candidate to serve bone tissue engineering. However, in vivo monitoring of PCL-based implants, as well as biodegradable implants in general, and their degradation profiles pose a significant challenge, hindering further development in the tissue engineering field and subsequent clinical adoption. To address this, photo-cross-linkable mechanically resilient PCL networks are developed and functionalized with a radiopaque monomer, 5-acrylamido-2,4,6-triiodoisophthalic acid (AATIPA), to enable non-destructive in vivo monitoring of PCL-based implants. The covalent incorporation of AATIPA into the crosslinked PCL networks does not significantly affect their crosslinking kinetics, mechanical properties, or thermal properties, but it increases their hydrolysis rate and radiopacity. Complex and porous 3D designs of radiopaque PCL networks can be effectively monitored in vivo. This work paves the way toward non-invasive monitoring of in vivo degradation profiles and early detection of potential implant malfunctions.

Evaluation of lesion contrast in the walk-through long axial FOV PET scanner simulated with XCAT anthropomorphic phantoms

BACKGROUND

This study evaluates the lesion contrast in a cost-effective long axial field of view (FOV) PET scanner, called the walk-through PET (WT-PET). The scanner consists of two flat detector panels covering the entire torso and head, scanning patients in an upright position for increased throughput. High-resolution, depth-of-interaction capable, monolithic detector technology is used to provide good spatial resolution and enable detection of smaller lesions.

METHODS

Monte Carlo GATE simulations are used in conjunction with XCAT anthropomorphic phantoms to evaluate lesion contrast in lung, liver and breast for various lesion diameters (10, 7 and 5 mm), activity concentration ratios (8:1, 4:1 and 2:1) and patient BMIs (18-37). Images were reconstructed iteratively with listmode maximum likelihood expectation maximization, and contrast recovery coefficients (CRCs) were obtained for the reconstructed lesions.

RESULTS

Results shows notable variations in contrast recovery coefficients (CRC) across different lesion sizes and organ locations within the XCAT phantoms. Specifically, our findings reveal that 10 mm lesions consistently exhibit higher CRC compared to 7 mm and 5 mm lesions, with increases of approximately 54% and 330%, respectively, across all investigated organs. Moreover, high contrast recovery is observed in most liver lesions regardless of diameter or activity ratio (average CRC = 42%), as well as in the 10 mm lesions in the lung. Notably, for the 10 mm lesions, the liver demonstrates 42% and 62% higher CRC compared to the lung and breast, respectively. This trend remains consistent across lesion sizes, with the liver consistently exhibiting higher CRC values compared to the lung and breast: 7 mm lesions show an increase of 96% and 41%, while 5 mm lesions exhibit approximately 294% and 302% higher CRC compared to the lung and breast, respectively.

CONCLUSION

A comparison with a conventional pixelated LSO long axial FOV PET shows similar performance, achieved at a reduced cost for the WT-PET due to a reduction in required number of detectors.

Cone-beam x-ray luminescence computed tomography (CB-XLCT) prototype development and performance evaluation

This study developed a prototype for a rotational cone-beam x-ray luminescence computed tomography (CB-XLCT) system, considering its potential application in pre-clinical theranostic imaging. A geometric calibration method applicable to both imaging chains (XL and CT) was also developed to enhance image quality. The results of systematic performance evaluations were presented to assess the feasibility of commercializing XLCT technology. Monte Carlo GATE simulation was performed to determine the optimal imaging conditions for nanophosphor particles (NPs) irradiated by 70 kV x-rays. We acquired a low-dose transmission x-ray tube and designed a prone positioning platform and a rotating gantry, using mice as targets from commercial small animalμ-CT systems. We then employed the image cross-correlation (ICC) automatic geometric calibration method to calibrate XL and CT images. The performance of the system was evaluated through a series of phantom experiments with a linearity of 0.99, and the contrast-to-noise ratio (CNR) between hydroxyl-apatite (HA) and based epoxy resin is 19.5. The XL images of the CB-XLCT prototype achieved a Dice similarity coefficient (DICE) of 0.149 for a distance of 1 mm between the two light sources. Finally, the final XLCT imaging results were demonstrated using the Letter phantoms with NPs. In summary, the CB-XLCT prototype developed in this study showed the potential to achieve high-quality imaging with acceptable radiation doses for small animals. The performance of CT images was comparable to current commercial machines, while the XL images exhibited promising results in phantom imaging, but further efforts are needed for biomedical applications.

Dose reduction and image enhancement in micro-CT using deep learning

BACKGROUND

In preclinical settings, micro-computed tomography (CT) provides a powerful tool to acquire high resolution anatomical images of rodents and offers the advantage to in vivo non-invasively assess disease progression and therapy efficacy. Much higher resolutions are needed to achieve scale-equivalent discriminatory capabilities in rodents as those in humans. High resolution imaging however comes at the expense of increased scan times and higher doses. Specifically, with preclinical longitudinal imaging, there are concerns that dose accumulation may affect experimental outcomes of animal models.

PURPOSE

Dose reduction efforts under the ALARA (as low as reasonably achievable) principles are thus a key point of attention. However, low dose CT acquisitions inherently induce higher noise levels which deteriorate image quality and negatively impact diagnostic performance. Many denoising techniques already exist, and deep learning (DL) has become increasingly popular for image denoising, but research has mostly focused on clinical CT with limited studies conducted on preclinical CT imaging. We investigate the potential of convolutional neural networks (CNN) for restoring high quality micro-CT images from low dose (noisy) images. The novelty of the CNN denoising frameworks presented in this work consists of utilizing image pairs with realistic CT noise present in the input as well as the target image used for the model training; a noisier image acquired with a low dose protocol is matched to a less noisy image acquired with a higher dose scan of the same mouse.

METHODS

Low and high dose ex vivo micro-CT scans of 38 mice were acquired. Two CNN models, based on a 2D and 3D four-layer U-Net, were trained with mean absolute error (30 training, 4 validation and 4 test sets). To assess denoising performance, ex vivo mice and phantom data were used. Both CNN approaches were compared to existing methods, like spatial filtering (Gaussian, Median, Wiener) and iterative total variation image reconstruction algorithm. Image quality metrics were derived from the phantom images. A first observer study (n = 23) was set-up to rank overall quality of differently denoised images. A second observer study (n = 18) estimated the dose reduction factor of the investigated 2D CNN method.

RESULTS

Visual and quantitative results show that both CNN algorithms exhibit superior performance in terms of noise suppression, structural preservation and contrast enhancement over comparator methods. The quality scoring by 23 medical imaging experts also indicates that the investigated 2D CNN approach is consistently evaluated as the best performing denoising method. Results from the second observer study and quantitative measurements suggest that CNN-based denoising could offer a 2-4× dose reduction, with an estimated dose reduction factor of about 3.2 for the considered 2D network.

CONCLUSIONS

Our results demonstrate the potential of DL in micro-CT for higher quality imaging at low dose acquisition settings. In the context of preclinical research, this offers promising future prospects for managing the cumulative severity effects of radiation in longitudinal studies.