Imaging performance of the hybrid pixel detectors XPAD3-S

Tracking Dynamics of Spontaneous Tumors in Mice Using Photon-Counting Computed Tomography

Computed tomography is a powerful medical imaging modality for longitudinal studies in cancer to follow neoplasia progression and evaluate anticancer therapies. Here, we report the generation of a photon-counting micro-computed tomography (PC-CT) method based on hybrid pixel detectors with enhanced sensitivity and precision of tumor imaging. We then applied PC-CT for longitudinal imaging in a clinically relevant liver cancer model, the Alb-R26^Met mice, and found a remarkable heterogeneity in the dynamics for tumors at the initiation phases. Instead, the growth curve of evolving tumors exhibited a comparable exponential growth, with a constant doubling time. Furthermore, longitudinal PC-CT imaging in mice treated with a combination of MEK and BCL-XL inhibitors revealed a drastic tumor regression accompanied by a striking remodeling of macrophages in the tumor microenvironment. Thus, PC-CT is a powerful system to detect cancer initiation and progression, and to monitor its evolution during treatment.

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Development of photon-counting micro-computed tomography (PC-CT) with hybrid pixels

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PC-CT allows longitudinal imaging of tumor dynamics in mouse cancer models

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RTK-driven tumors are heterogeneous at onset, but grow steadily during progression

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MEK + BCL-XL targeting leads to tumor regression and microenvironment remodeling

Photon-counting hexagonal pixel array CdTe detector: Spatial resolution characteristics for image-guided interventional applications

PURPOSE

High-resolution, photon-counting, energy-resolved detector with fast-framing capability can facilitate simultaneous acquisition of precontrast and postcontrast images for subtraction angiography without pixel registration artifacts and can facilitate high-resolution real-time imaging during image-guided interventions. Hence, this study was conducted to determine the spatial resolution characteristics of a hexagonal pixel array photon-counting cadmium telluride (CdTe) detector.

METHODS

A 650 μm thick CdTe Schottky photon-counting detector capable of concurrently acquiring up to two energy-windowed images was operated in a single energy-window mode to include photons of 10 keV or higher. The detector had hexagonal pixels with apothem of 30 μm resulting in pixel pitch of 60 and 51.96 μm along the two orthogonal directions. The detector was characterized at IEC-RQA5 spectral conditions. Linear response of the detector was determined over the air kerma rate relevant to image-guided interventional procedures ranging from 1.3 nGy/frame to 91.4 μGy/frame. Presampled modulation transfer was determined using a tungsten edge test device. The edge-spread function and the finely sampled line spread function accounted for hexagonal sampling, from which the presampled modulation transfer function (MTF) was determined. Since detectors with hexagonal pixels require resampling to square pixels for distortion-free display, the optimal square pixel size was determined by minimizing the root-mean-squared-error of the aperture functions for the square and hexagonal pixels up to the Nyquist limit.

RESULTS

At Nyquist frequencies of 8.33 and 9.62 cycles/mm along the apothem and orthogonal to the apothem directions, the modulation factors were 0.397 and 0.228, respectively. For the corresponding axis, the limiting resolution defined as 10% MTF occurred at 13.3 and 12 cycles/mm, respectively. Evaluation of the aperture functions yielded an optimal square pixel size of 54 μm. After resampling to 54 μm square pixels using trilinear interpolation, the presampled MTF at Nyquist frequency of 9.26 cycles/mm was 0.29 and 0.24 along the orthogonal directions and the limiting resolution (10% MTF) occurred at approximately 12 cycles/mm. Visual analysis of a bar pattern image showed the ability to resolve close to 12 line-pairs/mm and qualitative evaluation of a neurovascular nitinol-stent showed the ability to visualize its struts at clinically relevant conditions.

CONCLUSIONS

Hexagonal pixel array photon-counting CdTe detector provides high spatial resolution in single-photon counting mode. After resampling to optimal square pixel size for distortion-free display, the spatial resolution is preserved. The dual-energy capabilities of the detector could allow for artifact-free subtraction angiography and basis material decomposition. The proposed high-resolution photon-counting detector with energy-resolving capability can be of importance for several image-guided interventional procedures as well as for pediatric applications.

K-edge imaging with the XPAD3 hybrid pixel detector, direct comparison of CdTe and Si sensors

We investigate the improvement from the use of high-Z CdTe sensors for pre-clinical K-edge imaging with the hybrid pixel detectors XPAD3. We compare XPAD3 chips bump bonded to Si or CdTe sensors in identical experimental conditions. Image performance for narrow energy bin acquisitions and contrast-to-noise ratios of K-edge images are presented and compared. CdTe sensors achieve signal-to-noise ratios at least three times higher than Si sensors within narrow energy bins, thanks to their much higher detection efficiency. Nevertheless Si sensors provide better contrast-to-noise ratios in K-edge imaging when working at equivalent counting statistics, due to their better estimation of the attenuation coefficient of the contrast agent. Results are compared to simulated data in the case of the XPAD3/Si detector. Good agreement is observed when including charge sharing between pixels, which have a strong impact on contrast-to-noise ratios in K-edge images.

Spectroscopic (multi-energy) CT distinguishes iodine and barium contrast material in MICE

OBJECTIVE

Spectral CT differs from dual-energy CT by using a conventional X-ray tube and a photon-counting detector. We wished to produce 3D spectroscopic images of mice that distinguished calcium, iodine and barium.

METHODS

We developed a desktop spectral CT, dubbed MARS, based around the Medipix2 photon-counting energy-discriminating detector. The single conventional X-ray tube operated at constant voltage (75 kVp) and constant current (150 microA). We anaesthetised with ketamine six black mice (C57BL/6). We introduced iodinated contrast material and barium sulphate into the vascular system, alimentary tract and respiratory tract as we euthanised them. The mice were preserved in resin and imaged at four detector energy levels from 12 keV to 42 keV to include the K-edges of iodine (33.0 keV) and barium (37.4 keV). Principal component analysis was applied to reconstructed images to identify components with independent energy response, then displayed in 2D and 3D.

RESULTS

Iodinated and barium contrast material was spectrally distinct from soft tissue and bone in all six mice. Calcium, iodine and barium were displayed as separate channels on 3D colour images at <55 microm isotropic voxels.

CONCLUSION

Spectral CT distinguishes contrast agents with K-edges only 4 keV apart. Multi-contrast imaging and molecular CT are potential future applications.