Sensitivity study of x-ray luminescence computed tomography

Superfast Scan of Focused X-Ray Luminescence Computed Tomography Imaging

X-ray luminescence computed tomography (XLCT) is a hybrid molecular imaging modality having the high spatial resolution of x-ray imaging and high measurement sensitivity of optical imaging. Narrow x-ray beam based XLCT imaging has shown promise for high spatial resolution imaging of luminescent targets in deep tissues, but the slow acquisition speed limits its applications. In this work, we have introduced a superfast XLCT scan scheme based on the photon counter detector and a fly-scanning method. The new scan scheme is compared with three other scan methods. We have also designed and built a single-pixel x-ray detector to detect object boundaries automatically. With the detector, we can perform the parallel beam CT imaging with the XLCT imaging simultaneously. We have built the prototype XLCT imaging system to verify the proposed scan scheme. A phantom embedded with a set of four side-by-side cylindrical targets was scanned. With the proposed superfast scan scheme, we have achieved 43 seconds per transverse scan, which is 28.6 times faster than before with slightly better XLCT image quality. The superfast scan allows us to perform 3D pencil beam XLCT imaging in the future.

Super-Fast Three-Dimensional Focused X-ray Luminescence Computed Tomography with a Gated Photon Counter

X-ray luminescence computed tomography (XLCT) is a hybrid molecular imaging modality combining the merits of both x-ray imaging (high spatial resolution) and optical imaging (high sensitivity to tracer nanophosphors). Narrow x-ray beam based XLCT imaging has shown promise for high spatial resolution imaging, but the slow acquisition speed limits its applications for in vivo imaging. We introduced a continuous scanning scheme to replace the selective excitation scheme to improve imaging speed in a previous study. Under the continuous scanning scheme, the main factor that limits the scanning speed is the data acquisition time at each interval position. In this work, we have used a gated photon counter (SR400, Stanford Research Systems) to replace the high-speed oscilloscope (MDO3104, Tektronix) to acquire measurement data. The gated photon counter only counts the photon peaks in each measurement interval, while the oscilloscope records the entire waveform including both background noise data and photon peak data. The photon counter records much less data without losing any relevant information, which makes it ideal for super-fast three-dimensional (3D) imaging. We have built prototype XLCT imaging systems of both types and performed both single target and multiple target phantom experiments in 3D. The results have verified the feasibility of our proposed photon counter based system and good 3D imaging capabilities of XLCT within a reasonable time, yielding a 14 times faster scanning time compared with the oscilloscope based XLCT system. Now, the total scan time is reduced to 27 seconds per transverse section.

Contrast agents for x-ray luminescence computed tomography

Imaging probes are an important consideration for any type of contrast agent-based imaging method. X-ray luminescence imaging (XLI) and x-ray luminescence computed tomography (XLCT) are both contrast agent-based imaging methods that employ x-ray excitable scintillating imaging probes that emit light to be measured for optical imaging. In this work, we compared the performance of several select imaging probes, both commercial and self-synthesized, for application in XLI/XLCT imaging. Commercially available cadmium telluride quantum dots (CdTe QDs) and europium-doped gadolinium oxysulfide (GOS:Eu) microphosphor as well as synthesized NaGdF4 nanophosphors doped with either europium or terbium were compared through their x-ray luminescence emission spectra, luminescence intensity, and also by performing XLCT scans using phantoms embedded with each of the imaging probes. Each imaging probe displayed a unique emission spectrum that was ideal for deep-tissue optical imaging. In terms of luminescence intensity, due to the large particle size, GOS:Eu had the brightest emission, followed by NaGdF4:Tb, NaGdF4:Eu, and finally the CdTe QDs. Lastly, XLCT scans showed that each imaging probe could be reconstructed with good shape and location accuracy.

Focused x-ray luminescence imaging system for small animals based on a rotary gantry

SIGNIFICANCE

The ability to detect and localize specific molecules through tissue is important for elucidating the molecular basis of disease and treatment. Unfortunately, most current molecular imaging tools in tissue either lack high spatial resolution (e.g., diffuse optical fluorescence tomography or positron emission tomography) or lack molecular sensitivity (e.g., micro-computed tomography, μCT). X-ray luminescence imaging emerged about 10 years ago to address this issue by combining the molecular sensitivity of optical probes with the high spatial resolution of x-ray imaging through tissue. In particular, x-ray luminescence computed tomography (XLCT) has been demonstrated as a powerful technique for the high-resolution imaging of deeply embedded contrast agents in three dimensions (3D) for small-animal imaging.

AIM

To facilitate the translation of XLCT for small-animal imaging, we have designed and built a small-animal dedicated focused x-ray luminescence tomography (FXLT) scanner with a μCT scanner, synthesized bright and biocompatible nanophosphors as contrast agents, and have developed a deep-learning-based reconstruction algorithm.

APPROACH

The proposed FXLT imaging system was designed using computer-aided design software and built according to specifications. NaGdF4 nanophosphors doped with europium or terbium were synthesized with a silica shell for increased biocompatibility and functionalized with biotin. A deep-learning-based XLCT image reconstruction was also developed based on the residual neural network as a data synthesis method of projection views from few-view data to enhance the reconstructed image quality.

RESULTS

We have built the FXLT scanner for small-animal imaging based on a rotational gantry. With all major imaging components mounted, the motor controlling the gantry can be used to rotate the system with a high accuracy. The synthesized nanophosphors displayed distinct x-ray luminescence emission, which enables multi-color imaging, and has successfully been bound to streptavidin-coated substrates. Lastly, numerical simulations using the proposed deep-learning-based reconstruction algorithm has demonstrated a clear enhancement in the reconstructed image quality.

CONCLUSIONS

The designed FXLT scanner, synthesized nanophosphors, and deep-learning-based reconstruction algorithm show great potential for the high-resolution molecular imaging of small animals.

Review of in vivo optical molecular imaging and sensing from x-ray excitation

SIGNIFICANCE

Deep-tissue penetration by x-rays to induce optical responses of specific molecular reporters is a new way to sense and image features of tissue function in vivo. Advances in this field are emerging, as biocompatible probes are invented along with innovations in how to optimally utilize x-ray sources.

AIM

A comprehensive review is provided of the many tools and techniques developed for x-ray-induced optical molecular sensing, covering topics ranging from foundations of x-ray fluorescence imaging and x-ray tomography to the adaptation of these methods for sensing and imaging in vivo.

APPROACH

The ways in which x-rays can interact with molecules and lead to their optical luminescence are reviewed, including temporal methods based on gated acquisition and multipoint scanning for improved lateral or axial resolution.

RESULTS

While some known probes can generate light upon x-ray scintillation, there has been an emergent recognition that excitation of molecular probes by x-ray-induced Cherenkov light is also possible. Emission of Cherenkov radiation requires a threshold energy of x-rays in the high kV or MV range, but has the advantage of being able to excite a broad range of optical molecular probes. In comparison, most scintillating agents are more readily activated by lower keV x-ray energies but are composed of crystalline inorganic constituents, although some organic biocompatible agents have been designed as well. Methods to create high-resolution structured x-ray-optical images are now available, based upon unique scanning approaches and/or a priori knowledge of the scanned x-ray beam geometry. Further improvements in spatial resolution can be achieved by careful system design and algorithm optimization. Current applications of these hybrid x-ray-optical approaches include imaging of tissue oxygenation and pH as well as of certain fluorescent proteins.

CONCLUSIONS

Discovery of x-ray-excited reporters combined with optimized x-ray scan sequences can improve imaging resolution and sensitivity.

Fast method for computing a system matrix using a polar-coordinate pixel model with concentric annuluses of different radial widths

Despite better reconstruction quality for incomplete or noisy projection data compared to analytic reconstruction, computed tomography iterative techniques are time-consuming, mainly due to high system matrix computation. A polar-coordinate pixel model with concentric annuluses of different radial widths was established and a fast method for computing the system matrix was presented based on characteristics of this model. Compared with the Siddon algorithm and an efficient Cartesian algorithm introduced by Zhang, the proposed algorithm based on the simultaneous algebraic reconstruction technique shows speed advantages for both numerical simulation and experiment, without noticeable loss of image quality.

X-ray luminescence imaging for small animals

X-ray luminescence imaging emerged for about a decade and combines both the high spatial resolution of x-ray imaging with the high measurement sensitivity of optical imaging, which could result in a great molecular imaging tool for small animals. So far, there are two types of x-ray luminescence computed tomography (XLCT) imaging. One uses a pencil beam x-ray for high spatial resolution at a cost of longer measurement time. The other uses cone beam x-ray to cover the whole mouse to obtain XLCT images at a very short time but with a compromised spatial resolution. Here we review these two methods in this paper and highlight the synthesized nanophosphors by different research groups. We are building a focused x-ray luminescence tomography (FXLT) imaging system, developing a machine-learning based FXLT reconstruction algorithm, and synthesizing nanophosphors with different emission wavelengths. In this paper, we will report our current progress from these three aspects. Briefly, we mount all main components, including the focused x-ray tube, the fiber detector, and the x-ray tube and x-ray detector for a microCT system, on a rotary which is a heavy-duty ring track. A microCT scan will be performed before FXLT scan. For a FXLT scan, we will have four PMTs to measure four fiber detectors at two different wavelengths simultaneously for each linear scan position. We expect the spatial resolution of the FXLT imaging will be around 100 micrometers and a limit of detection of approximately 2 μg/mL (for Gd2O2S:Eu).

High-resolution x-ray luminescence computed tomography

High-resolution imaging modalities play a critical role for advancing biomedical sciences. Recently, x-ray luminescence computed tomography (XLCT) imaging was introduced as a hybrid molecular imaging modality that combines the high-spatial resolution of x-ray imaging and molecular sensitivity of optical imaging. The narrow x-ray beam based XLCT imaging has been demonstrated to achieve high spatial resolution, even at depth, with great molecular sensitivity. Using a focused x-ray beam as the excitation source, orders of magnitude of increased sensitivity has been verified compared with previous methods with a collimated x-ray beam. In this work, we demonstrate the high-spatial resolution capabilities of our focused x-ray beam based XLCT imaging system by scanning two sets of targets, differing in the target size, embedded inside of two tissue-mimicking cylindrical phantoms. Gd2O2S:Eu3+ targets of 200 µm and 150 µm diameters were created and embedded with the same edge-to-edge distances as their diameters respectively. We scanned and reconstructed a single transverse section and successfully demonstrated that a focused x-ray beam with an average dual-cone size of 125 µm could separate the targets in both phantoms with good shape and location accuracy. We have also improved the current XLCT imaging system to make it feasible for fast three-dimensional XLCT scanning.

Limited view cone-beam x-ray luminescence tomography based on depth compensation and group sparsity prior

Significance: As a promising hybrid imaging technique with x-ray excitable nanophosphors, cone-beam x-ray luminescence computed tomography (CB-XLCT) has been proposed for in-depth biological imaging applications. In situations in which the full rotation of the imaging object (or x-ray source) is inapplicable, the x-ray excitation is limited by geometry, or a lower x-ray excitation dose is mandatory, limited view CB-XLCT reconstruction would be essential. However, this will result in severe ill-posedness and poor image quality. <p> Aim: The aim is to develop a limited view CB-XLCT imaging strategy to reduce the scanning span and a corresponding reconstruction method to achieve robust imaging performance.</p> <p> Approach: In this study, a group sparsity-based reconstruction method is proposed with the consideration that nanophosphors usually cluster in certain regions, such as tumors or major organs such as the liver. In addition, depth compensation (DC) is adopted to avoid the depth inconsistency caused by a limited view strategy. </p> <p> Results: Experiments using numerical simulations and physical phantoms with different edge-to-edge distances were carried out to illustrate the validity of the proposed method. The reconstruction results showed that the proposed method outperforms conventional methods in terms of localization accuracy, target shape, image contrast, and spatial resolution with two perpendicular projections. </p> <p> Conclusions: A limited view CB-XLCT imaging strategy with two perpendicular projections and a reconstruction method based on DC and group sparsity, which is essential for fast CB-XLCT imaging and for some practical imaging applications, such as imaging-guided surgery, is proposed. </p>

Method for improving the spatial resolution of narrow x-ray beam-based x-ray luminescence computed tomography imaging

X-ray luminescence computed tomography (XLCT) is an emerging hybrid imaging modality which has the potential for achieving both high sensitivity and spatial resolution simultaneously. For the narrow x-ray beam-based XLCT imaging, based on previous work, a spatial resolution of about double the x-ray beam size can be achieved using a translate/rotate scanning scheme, taking step sizes equal to the x-ray beam width. To break the current spatial resolution limit, we propose a scanning strategy achieved by reducing the scanning step size to be smaller than the x-ray beam size. We performed four sets of numerical simulations and a phantom experiment using cylindrical phantoms and have demonstrated that our proposed scanning method can greatly improve the XLCT-reconstructed image quality compared with the traditional scanning approach. In our simulations, by using a fixed x-ray beam size of 0.8 mm, we were able to successfully reconstruct six embedded targets as small as 0.5 mm in diameter and with the same edge-to-edge distances by using a scanning step as small as 0.2 mm which is a 1.6 times improvement in the spatial resolution compared with the traditional approach. Lastly, the phantom experiment further demonstrated the efficacy of our proposed method in improving the XLCT image quality, with all image quality metrics improving as the step size decreased.

Background luminescence in x-ray luminescence computed tomography (XLCT) imaging

X-ray luminescence computed tomography (XLCT) is an emerging hybrid imaging modality. It has been recently reported that materials such as water, tissue, or even air can generate optical photons upon x-ray irradiation, which can increase the noises in measurements of XLCT. In this study, we have investigated the x-ray luminescence from water, air, as well as tissue mimicking phantoms, including one embedded with a 0.01 mg/mL GOS:Eu3+ microphosphor target. We have measured the optical emission spectrum from each sample, including samples of meat and fat, using a spectrograph. Our results indicate that there are plenty of optical photons emitted by x-ray irradiation, and a small nanophosphor concentration, as low as 5.28 μM in a deep background, can provide enough contrast for XLCT imaging.

Focused x-ray luminescence computed tomography: experimental studies

X-ray luminescence computed tomography (XLCT) is an emerging hybrid molecular imaging modality and has shown great promises in overcoming the strong optical scattering in deep tissues. Though the narrow x-ray beam based XLCT imaging has been demonstrated to obtain high spatial resolution at depth, it suffers from a relatively long measurement time, hindering its practical applications. Recently, we have designed a focused x-ray beam based XLCT imaging system and have successfully performed imaging in about 7.5 seconds per section for a mouse sized object. However, its high spatial resolution capacity has not been fully implemented yet. In this paper, with a superfine focused x-ray beam we design a focused-x-ray luminescence tomography (FXLT) system for spatial resolution up to 94 μm. First, we have described our design in details. Then, we estimate the performance of the designed FXLT imaging system. Lastly, we have found that the spatial resolution of FXLT can be further improved by reducing the scan step size, which has been demonstrated by numerical simulations.

Time domain X-ray luminescence computed tomography: numerical simulations

X-ray luminescence computed tomography (XLCT) has the potential to image the biodistribution of nanoparticles inside deep tissues. In XLCT, X-ray excitable nanophosphors emit optical photons for tomographic imaging. The lifetime of the nanophosphor signal, rather than its intensity, could be used to extract biological microenvironment information such as oxygenation in deep tumors. In this study, we propose the design, the forward model, and the reconstruction algorithm of a time domain XLCT for lifetime imaging with high spatial resolution. We have investigated the feasibility of the proposed design with numerical simulations. We found that the reconstructed lifetime images are robust to noise levels up to 5% and to unknown optical properties up to 4 times of absorption and scattering coefficients.

Sparse view cone beam X-ray luminescence tomography based on truncated singular value decomposition

Cone beam X-ray luminescence computed tomography (CB-XLCT) has been proposed as a promising hybrid imaging technique. Though it has the advantage of fast imaging, the inverse problem of CB-XLCT is seriously ill-conditioned, making the image quality quite poor, especially for imaging multi-targets. To achieve fast imaging of multi-targets, which is essential for in vivo applications, a truncated singular value decomposition (TSVD) based sparse view CB-XLCT reconstruction method is proposed in this study. With the weight matrix of the CB-XLCT system being converted to orthogonal by TSVD, the compressed sensing (CS) based L₁-norm method could be applied for fast reconstruction from fewer projection views. Numerical simulations and phantom experiments demonstrate that by using the proposed method, two targets with different edge-to-edge distances (EEDs) could be resolved effectively. It indicates that the proposed method could improve the imaging quality of multi-targets significantly in terms of localization accuracy, target shape, image contrast, and spatial resolution, when compared with conventional methods.

X-ray luminescence computed tomography using a focused x-ray beam

Due to the low x-ray photon utilization efficiency and low measurement sensitivity of the electron multiplying charge coupled device camera setup, the collimator-based narrow beam x-ray luminescence computed tomography (XLCT) usually requires a long measurement time. We, for the first time, report a focused x-ray beam-based XLCT imaging system with measurements by a single optical fiber bundle and a photomultiplier tube (PMT). An x-ray tube with a polycapillary lens was used to generate a focused x-ray beam whose x-ray photon density is 1200 times larger than a collimated x-ray beam. An optical fiber bundle was employed to collect and deliver the emitted photons on the phantom surface to the PMT. The total measurement time was reduced to 12.5 min. For numerical simulations of both single and six fiber bundle cases, we were able to reconstruct six targets successfully. For the phantom experiment, two targets with an edge-to-edge distance of 0.4 mm and a center-to-center distance of 0.8 mm were successfully reconstructed by the measurement setup with a single fiber bundle and a PMT.

Bright X-ray and up-conversion nanophosphors annealed using encapsulated sintering agents for bioimaging applications

Nanophosphors are promising contrast agents for deep tissue optical imaging applications because they can be excited by X-ray and near infrared light that penetrates deeply through tissue and generates almost no autofluorescence background in the tissue. For these bioimaging applications, the nanophosophors should ideally be small, monodispersed and brightly luminescent. However, most methods used to improve luminescence yield by annealing the particles to reduce crystal and surface defects (e.g. using flux or sintering agents) also cause particle fusion or require multiple component core-shell structures. Here, we report a novel method to prepare bright, uniformly sized X-ray nanophosphors (Gd2O2S:Eu or Tb) and upconversion nanophosphors (Y2O2S: Yb/Er, or Yb/Tm) with large crystal domain size without causing aggregation. A core-shell nanoparticle is formed, with NaF only in the core. We observe that increasing the NaF sintering agent concentration up to 7.6 mol% increases both crystal domain size and luminescence intensity (up to 40% of commercial microphosphors) without affecting the physical particticle diameter. Above 7.6 mol%, particle fusion is observed. The annealing is insensitive to the cation (Na+ or K+) but varies strongly with anion, with F->Cl->CO32->Br->I-. The luminescence depends strongly on crystal domain size. The data agree reasonably well with a simple domain surface quenching model, although the size-dependence suggests additional quenching mechanisms within small domains. The prepared bright nanophosphors were subsequently functionalized with PEG-folic acid to target MCF-7 breast cancer cells which overexpress folic acid receptors. Both X-ray and upconversion nanophosphors provided low background and bright luminescence which was imaged through 1 cm chicken breast tissue at a low dose of nanophosphors 200 µL (0.1 mg/mL). We anticipate these highly monodispersed and bright X-ray and upconversion nanophosphors will have significant potential for tumor targeted imaging.

Anatomical image-guided fluorescence molecular tomography reconstruction using kernel method

Fluorescence molecular tomography (FMT) is an important in vivo imaging modality to visualize physiological and pathological processes in small animals. However, FMT reconstruction is ill-posed and ill-conditioned due to strong optical scattering in deep tissues, which results in poor spatial resolution. It is well known that FMT image quality can be improved substantially by applying the structural guidance in the FMT reconstruction. An approach to introducing anatomical information into the FMT reconstruction is presented using the kernel method. In contrast to conventional methods that incorporate anatomical information with a Laplacian-type regularization matrix, the proposed method introduces the anatomical guidance into the projection model of FMT. The primary advantage of the proposed method is that it does not require segmentation of targets in the anatomical images. Numerical simulations and phantom experiments have been performed to demonstrate the proposed approach’s feasibility. Numerical simulation results indicate that the proposed kernel method can separate two FMT targets with an edge-to-edge distance of 1 mm and is robust to false-positive guidance and inhomogeneity in the anatomical image. For the phantom experiments with two FMT targets, the kernel method has reconstructed both targets successfully, which further validates the proposed kernel method.

Collimated superfine x-ray beam based x-ray luminescence computed tomography

X-ray luminescence computed tomography (XLCT) is a hybrid imaging modality with the potential to achieve a spatial resolution up to several hundred micrometers for targets embedded in turbid media with a depth larger than several millimeters. In this paper, we report a high spatial resolution XLCT imaging system with a collimated superfine x-ray beam in imaging the deeply embedded targets. A collimator with a 100 micrometer pinhole was mounted in the front of a powerful x-ray tube to generate a superfine x-ray pencil beam with a beam diameter of 0.175 mm. For the phantom experiment of four capillary targets with an edge-to-edge distance of 400 micrometers, we were able to reconstruct the targets in a depth of 5 mm successfully, which were validated with microCT images. We have further investigated the effect of different x-ray beam diameters on the reconstructed XLCT images with numerical simulations. Our results indicate that XLCT has the ability to image successfully multiple deeply embedded targets when the collimated x-ray beam diameter is less than or equal to the target edge-to-edge distance. Our numerical simulations also demonstrate that XLCT can achieve a spatial resolution of 200 micrometers for targets embedded at a depth of 5 mm if the scanning beam has a diameter of 100 micrometers.