Sub-millimeter Spatial Resolution Research Articles

Development of an APD-Based PET Module and Preliminary Resolution Performance of an Experimental Prototype Gantry

The development of a high-resolution Positron Emission Tomography (PET) technique with sub-millimeter spatial resolution, which utilizes newly designed reverse-type APD-arrays, is uderway. All the detector blocks are modularized with the overall dimension of each module, including the APD array, LYSO scintillator matrix and Front-End Circuits (FECs), which are only 30 × 30 × 80 mm3. Each APD device also has a monolithic 16 × 16 pixel structure with an active area of 1.0 mm2 per pixel. The FEC includes two identical analog ASICs specifically designed for APDs with a noise characteristic of 560 + 30 e-/pF and a timing resolution of 460 ps (rms), respectively. An energy resolution of 13.7 ± 1.1% (FWHM) with 662 keV gamma-rays was measured using the 16 × 16 arrays. At this stage a pair of module and coincidence circuits has been assembled into an experimental prototype gantry. Spatial resolutions of 0.9, 1.4, and 1.3 mm (FWHM) were obtained from FBP reconstructed images in preliminary experiments with a point source positioned centrally, and 1 and 5 mm off-center, respectively. Comparison with a Monte-Carlo simulation of a fully-designed gantry over a wider range of field-of-view showed good correlation with the experimental data. A simple but conceptual design of a DOI configuration is also proposed as a test example of a future APD-PET scanner.

Phantom Experiments on a PSAPD-Based Compact Gamma Camera With Submillimeter Spatial Resolution for Small Animal SPECT

We demonstrate a position sensitive avalanche photodiode (PSAPD) based compact gamma camera for the application of small animal single photon emission computed tomography (SPECT). The silicon PSAPD with a two-dimensional resistive layer and four readout channels is implemented as a gamma ray detector to record the energy and position of radiation events from a radionuclide source. A 2 mm thick monolithic CsI:Tl scintillator is optically coupled to a PSAPD with a 8mm×8mm active area, providing submillimeter intrinsic spatial resolution, high energy resolution (16% full-width half maximum at 140 keV) and high gain. A mouse heart phantom filled with an aqueous solution of 370 MBq (99m)Tc-pertechnetate (140 keV) was imaged using the PSAPD detector module and a tungsten knife-edge pinhole collimator with a 0.5 mm diameter aperture. The PSAPD detector module was cooled with cold nitrogen gas to suppress dark current shot noise. For each projection image of the mouse heart phantom, a rotated diagonal readout algorithm was used to calculate the position of radiation events and correct for pincushion distortion. The reconstructed image of the mouse heart phantom demonstrated reproducible image quality with submillimeter spatial resolution (0.7 mm), showing the feasibility of using the compact PSAPD-based gamma camera for a small animal SPECT system.

Robust Capon beamforming for passive cavitation mapping during high-intensity focused ultrasound therapy.

A passive method that uses time exposure acoustics (TEA) to map inertial cavitation activity in real time during therapeutic ultrasound exposure was recently presented [Gyongy and Coussios, IEEE Trans. Biomed. Eng. 57, 48–56 (2010)]. While this approach provides sub-millimetric spatial resolution transversely to the high-intensity focused ultrasound (HIFU) beam, its axial resolution requires improvement. Furthermore, the TEA method is expected to have a lower overall resolution and diminished ability to reject interference and noise compared to an adaptive beamforming approach. To address these limitations, we propose the use of the adaptive, robust Capon beamformer (RCB), which has been previously shown in the context of active, 2-D ultrasound imaging to provide high resolution, good interference suppression, and robustness against steering vector errors. Using a multi-element tomographic cavitation sensor being developed for quality assessment of clinical HIFU transducers, the effectiveness of RCB is demonstrated for improved 3-D passive localization of inertial cavitation compared to TEA using simulated and experimental data.

Mapping the myoglobin concentration, oxygenation, and optical pathlength in heart ex vivo using near-infrared imaging

A method that provides maps of absolute concentrations of oxygenated and deoxygenated myoglobin (Mb), its oxygenation, and its near-infrared (NIR) optical pathlength in cardiac tissue was developed. These parameters are available simultaneously. The method is based on NIR diffuse reflectance spectroscopic imaging and specific processing of the NIR images, which included a first derivative of the diffuse reflectance spectrum. Mb oxygenation, total Mb concentration, and NIR light pathlength were found to be in the range of 92%, 0.3 mM, and 12.5 mm, respectively, in beating isolated buffer-perfused and arrested pig hearts. The charge-coupled device camera enables sub-millimeter spatial resolution and spectroscopic imaging in 1.5 to 2.0 min. The technique is noninvasive and nondestructive. The equipment has no mechanical contact with the tissue of interest, leaving it undisturbed.

Design and performance tests of the calorimetric tract of a Compton Camera for small-animals imaging

The bio-distribution and targeting capability of pharmaceuticals may be assessed in small animals by imaging gamma-rays emitted from radio-isotope markers. Detectors that exploit the Compton concept allow higher gamma-ray efficiency compared to conventional Anger cameras employing collimators, and feature sub-millimeter spatial resolution and compact geometry. We are developing a Compton Camera that has to address several requirements: the high rates typical of the Compton concept; detection of gamma-rays of different energies that may range from 140 keV ( 99 m Tc) to 511 keV ( β + emitters); presence of gamma and beta radiation with energies up to 2 MeV in case of 188Re. The camera consists of a thin position-sensitive Tracker that scatters the gamma ray, and a second position-sensitive detection system to totally absorb the energy of the scattered photons (Calorimeter). In this paper we present the design and discuss the realization of the calorimetric tract, including the choice of scintillator crystal, pixel size, and detector geometry. Simulations of the gamma-ray trajectories from source to detectors have helped to assess the accuracy of the system and decide on camera design. Crystals of different materials, such as LaBr 3 GSO and YAP, and of different size, in continuous or segmented geometry, have been optically coupled to a multi-anode Hamamatsu H8500 detector, allowing measurements of spatial resolution and efficiency.

Deadtime calibration in high-rate microstrip gas counters

The advent of the microstrip gas chamber (MSGC) has permitted the development of position-sensitive gas avalanche detectors, which combine sub-millimeter spatial resolution with multi-MHz global rate capabilities not attainable with traditional types of gas counters. Based on massively parallel electronic readout, the need to keep down electronic channel costs imposes a simple front-end design without in-built deadtime compensation. In this report we show that insitu calibration of the deadtime of the detector system is straight-forward and reliable. Methods are described for applications to X-ray and neutron detection.

A simple indentation device for measuring micrometer-scale tissue stiffness

Mechanical properties of cells and extracellular matrices are critical determinants offunction in contexts including oncogenic transformation, neuronal synapse formation,hepatic fibrosis and stem cell differentiation. The size and heterogeneity of biologicalspecimens and the importance of measuring their mechanical properties under conditionsthat resemble their environments in vivo present a challenge for quantitative measurement.Centimeter-scale tissue samples can be measured by commercial instruments, whereasproperties at the subcellular (nm) scale are accessible by atomic force microscopy, opticaltrapping, or magnetic bead microrheometry; however many tissues are heterogeneous on alength scale between micrometers and millimeters which is not accessible to most currentinstrumentation. The device described here combines two commercially availabletechnologies, a micronewton resolution force probe and a micromanipulator for probing softbiological samples at sub-millimeter spatial resolution. Several applications of thedevice are described. These include the first measurement of the stiffness of anintact, isolated mouse glomerulus, quantification of the inner wall stiffness ofhealthy and diseased mouse aortas, and evaluation of the lateral heterogeneity inthe stiffness of mouse mammary glands and rat livers with correlation of thisheterogeneity with malignant or fibrotic pathology as evaluated by histology.

Silicon drift photodetector arrays for the HICAM gamma camera

Silicon drift detectors (SDDs) have shown to be a competitive device for the readout of scintillators with respect to conventional photodetectors, thanks to their high quantum efficiency and low electronics noise. Recently, they have been successfully employed in first small prototypes of Anger cameras to achieve sub-millimeter spatial resolution in gamma-ray imaging. To cover larger formats of Anger cameras, in particular in the framework of the HICAM project, specially focused on human imaging, we have developed new SDD arrays of larger active areas. To assemble photodetector planes of several cm 2, we have designed a basic unit composed by a linear array of 5 SDDs of 1 cm 2 active area each. In this work, we present the results of the experimental characterization of these photodetector arrays in direct X-ray detection to evaluate the electronics noise, as well as gamma-ray detection with a scintillator.

Fast Whole-Brain 4D Contrast-Enhanced MR Angiography with Velocity Encoding Using Undersampled Radial Acquisition and Highly Constrained Projection Reconstruction: Image-Quality Assessment in Volunteer Subjects: Fig 1.

We report on the image quality obtained by using fast contrast-enhanced whole-brain 4D radial MRA with 0.75-second temporal resolution, isotropic submillimeter spatial resolution, and velocity encoding (HYPRFlow). Images generated by HYPR-LR by using the velocity-encoded data as the constraining image were of diagnostic quality. In addition, we demonstrate that measurements of shear stress within the middle cerebral artery can be derived from the high-resolution 3D velocity data.

PC HYPR flow: A technique for rapid imaging of contrast dynamics

To improve spatial and temporal resolution and signal-to-noise ratio (SNR) in three-dimensional (3D) radial contrast-enhanced (CE) time-resolved MR angiography by means of a novel hybrid phase contrast (PC) and CE MRA acquisition and HYPR reconstruction (PC HYPR Flow). PC HYPR Flow consists of a CE exam immediately followed by a PC scan used to constrain the HYPR reconstruction of the time series. Temporal resolution of the new method was studied in computer simulations. The feasibility of the new technique was studied in healthy subjects and patients with brain arteriovenous malformations and in a canine model of aneurysms. Simulations demonstrated preservation of contrast agent dynamics in proximal vessels, showing better performance than peer methods for acceleration up to 20 in 2D. In vivo, PC HYPR Flow yielded 3D time series with frame rate of 0.5 s and significantly outperformed two peer methods by means of a major increase in spatial resolution (0.8 x 0.8 x 0.8 mm(3)) and arterial/venous ratio, while maintaining necessary temporal waveform fidelity and high SNR. This initial study indicates that PC HYPR Flow simultaneously provides 3D isotropic sub-millimeter spatial resolution, sub-second temporal reconstruction windows and high SNR level, which may benefit a wide range of CE MRA applications.

Enhancement of in-plane spatial resolution in volumetric computed tomography with focal spot wobbling – Overcoming the constraint on number of projection views per gantry rotation

The spatial resolution of diagnostic Computed Tomography (CT) has increased substantially, and 3D isotropic sub-millimeter spatial resolution in both axial and helical scan modes is routinely available in the clinic. However, driven by advanced clinical applications, the pursuit for higher spatial resolution and free of aliasing artifacts in diagnostic CT has never stopped. A method to accommodate focal spot wobbling at an arbitrary number of projection views per gantry rotation in CT is presented and evaluated here. The method employs a beta-correction scheme in the row-wise fan-to-parallel rebinning to transform the native cone beam geometry into the cone-parallel geometry under which existing 3D weighted cone beam filtered backprojection algorithms can be utilized for image reconstruction. The experimental evaluation shows that the row-wise fan-to-parallel rebinning with the beta-correction can increase the quantitative in-plane spatial resolution (Modulation Transfer Function) substantially, while the visual spatial resolution can be enhanced significantly. Consequently, the architectural designers of CT scanners are no longer constrained to choosing the number of projection views per rotation determined by gantry geometry. Instead, they can choose the number of projection views per rotation to optimize the trade-offs between in-plane spatial resolution and noise characteristics. Therefore, the presented method is of practical relevance in the architectural design of state-of-the-art diagnostic CT.

LaBr 3:Ce Scintillation Imagers Optimization by GEANT4

The increasing demand of sophisticated devices for Medical Imaging, with submillimeter spatial resolution and high detection efficiency, motivates an in-depth study of detectors able to fulfill these requirements. In particular for Single Photon Emission Tomography applications, the on-going development of large LaBr3:Ce crystals makes very attractive their use as a system of gamma imaging. Single crystals can overcome the limitation in term of spatial resolution related to pixel size in scintillation arrays, once the problems of limited position linearity and image size are solved. In this work we use GEANT4 simulations in order to model different configurations based on LaBr3:Ce crystals coupled to a Hamamatsu H8500 Multi Anode Photomultiplier. The position linearity, the spatial and energy resolutions obtainable from optimum photodetection conditions are investigated. The values obtained by the Monte Carlo simulations are compared to the experimental results and found to be in good agreement with them upon tuning of some parameters.

Revisited position arithmetics for LaBr 3:Ce continuous crystals

The development of the molecular imaging technique based on radiopharmaceuticals is requiring sophisticated devices with sub-millimeter spatial resolution and high detection efficiency. Recently, in the field of scintillation gamma camera, it has been demonstrated that continuous crystals may improve spatial resolution with respect to the use of scintillation arrays. In this work we propose a new algorithm that improves spatial resolution. It calculates the position from a single scintillation event by squaring the charge collected on a multi-anodes photomultiplier tube (MA-PMT). It is able to remove the position linearity distortion due to the light reflections and the truncations at the crystal edge. We present measurements from a compact gamma camera based on a 51 mm × 51 mm × 4.0 mm LaBr 3 :Ce continuous scintillation crystal, coupled to a MA-PMT Hamamatsu H8500. The results show a strong improvement in spatial resolution and confirm the high potential in using of LaBr continuous crystal for molecular imaging applications.

Radiochromic leuco dye micelle hydrogels: II. Low diffusion rate leuco crystal violet gel

Radiation-sensitive hydrogels offer the capability of verifying intricate dose distributions in three-dimensional (3D) space conveniently in a single measurement with sub-millimetre spatial resolution. In this study, a new radiochromic hydrogel called leuco crystal violet (LCV) micelle gel is introduced. Upon irradiation, LCV converts to crystal violet (CV+). Triton X-100 micelles are used to provide the required hybrid-interfacing environment to dissolve LCV. The diffusion coefficient of the LCV gel has been measured to be 0.036 ± 0.001 mm2 h−1, which is a factor of 25 times less than the standard radiochromic ferrous xylenol-orange (FX) gel; LCV gels without Triton X-100 micelles have a diffusion coefficient of 0.33 ± 0.02 mm2 h−1. The LCV gel formulation contains: 1 mM LCV, 25 mM trichloroacetic acid, 4 mM Triton X-100 and 4% w/w gelatin. The primary innovative feature of this 3D hydrogel is that the radiation-induced CV+ dye is more soluble in the Triton X-100 micelles than in the surrounding water which consequently leads to more stable post-irradiation dose distributions. A dosimetric characterization revealed that the dose response is reproducible to within 1% over three separate batches, independent of energy, dose rate and dose fractionation but is affected by the temperature (∼4% per °C) during irradiation. LCV micelle gels scanned optically with a yellow light source are a promising system for 3D dose verification. They may prove to be, especially, useful for scanning large volume dosimeters (i.e. 20 cm) since they are easily manufactured, transparent and near colourless prior to irradiation.

Probing for local activity‐related modulation of the infrared backscattering of the brain cortex

The possibility to measure the metabolic activity of the brain cortex, with submillimeter spatial and subsecond temporal resolution, would open up enticing scenarios in addressing basic issues on the relation between different structural components of brain signal processing, and in providing an operational pathway to interaction with (dis)functional signal patterns. In the present article, we report the description of a simple system that allows the detection of the minute changes that occur in the optical backscattering of the cortex as a metabolic response to external stimuli. The simplicity of the system is compatible with scalability to an implantable probe. We validate the system on an animal model, and we propose an algorithm to extract meaningful data from the measured signal. We thus show the detection of individual haemodynamic cortical responses to individual stimulation events, and we provide operational considerations on the signal structure.

Use of cone beam computed tomography in the management of glass injuries to the face

Cone beam computed tomography (CBCT) is an emerging method of imaging in maxillofacial surgery. Its advantages include short scanning time and a lower radiation dose than conventional CT, and it produces images with isotropic sub-millimetre spatial resolution and a high diagnostic quality.1 To date it has been used for dental implant planning, evaluation of temporomandibular joints, and trauma of the facial skeleton.2,3

Three-Dimensional T2-Weighted MRI of the Human Femoral Arterial Vessel Wall at 3.0 Tesla

To evaluate the potential use of a novel 3D turbo spin-echo (TSE) T2-weighted (T2w) technique for assessing the vessel wall in the superficial femoral artery at 3.0 T. Magnetic resonance imaging can be used for the noninvasive assessment of atherosclerotic plaque burden in the peripheral circulation. Although black-blood 2D TSE techniques have been used for femoral arterial wall imaging, these techniques require prolonged imaging time to cover a large field of view required to cover the leg. Recently, variable-flip-angle 3D TSE T2w (SPACE) has been introduced as a fast vessel wall imaging technique with submillimeter spatial resolution. A systematic investigation of the application of this technique to femoral arterial wall imaging has yet to be performed. Fifteen healthy volunteers and 3 patients with peripheral arterial disease (PAD) underwent 3D SPACE imaging of the superficial femoral artery at 3.0 T, with the conventional 2D TSE T2w imaging as a reference. Muscle-lumen contrast to noise ratio (CNR) and wall/lumen volumes (WV, LV) were measured at the matched locations on the 3D and 2D image sets. Statistical comparison on a per-subject basis was conducted to determine the difference and agreement between 3D SPACE and the 2D TSE techniques. The 3D SPACE data sets enabled vessel visualization from arbitrary orientation through multiplanar reformation technique. Muscle-lumen CNR was significantly higher with 3D SPACE than with the 2D TSE (3.12 +/- 0.84 vs. 2.17 +/- 0.34, P < 0.01). This trend was confirmed when CNR efficiency (CNR(eff)) values were further compared. A similar trend was observed in PAD patients (SPACE vs. 2D TSE T2w: CNR 2.35 +/- 0.13 vs. 1.77 +/- 0.25; CNR(eff) 15.35 +/- 0.61 vs. 3.59 +/- 2.62. all P < 0.05). Measurements of WV and LV from the 3D and 2D techniques were highly correlated in volunteers and PAD patients (volunteers, WV: linear regression r2 = 0.98, LV: r2 = 0.98, P < 0.001 for both; patients, WV: linear regression r2 = 0.96, LV: r2 = 0.94, P < 0.001 for both). We established the feasibility of using the 3D SPACE technique for vessel wall imaging in the superficial femoral artery at 3.0T. High, isotropic-resolution SPACE images, with the aid of multiplanar reformation, enable superior vessel wall visualization. Superior blood signal suppression comparable to vessel wall morphologic measurements, and superior time efficiency compared to conventional 2D TSE imaging indicate the great potential of the SPACE method as a noninvasive imaging technique for the assessment of atherosclerotic plaque burden in PAD patients.

Conebeam computed tomography (CBCT): The new vista in oral and maxillofacial imaging

Cone beam computed tomography (CBCT) has the potential to reduce the size and cost of CT scanners. This emerging technology produces images with isotropic sub-millimeter spatial resolution with high diagnostic quality, short scanning times of about 10-30 seconds, and radiation dosages of up to 15 times lower than those of conventional CT scans. It is ideally suited for dedicated dentomaxillofacial CT scanning. This technology provides dental practitioners with complete solution for performing specific diagnostic and clinical tasks, including implant planning, temporomandibular joint evaluation, facial fractures, dentoalveolar surgery, orthognathic surgery and periodontal surgery. CBCT is capable of providing a 3-dimensional representation of the maxillofacial hard tissues with minimal distortion. The objective of this article is to highlight the clinical applications of CBCT to oral and maxillofacial surgeons and other dental specialists in Nigeria.

Appearance of Normal Cranial Nerves on Steady-State Free Precession MR Images

As radiologic imaging technology improves and more intricate details of the anatomy can be evaluated, images provide more precise diagnostic information and allow better localization of abnormalities. For example, standard T2-weighted magnetic resonance (MR) imaging sequences adequately depicted only the larger cranial nerves, whereas current steady-state free precession (SSFP) sequences are capable of depicting the cisternal segments of all 12 cranial nerves. SSFP sequences provide submillimetric spatial resolution and high contrast resolution between cerebrospinal fluid and solid structures, allowing the reconstruction of elegant multiplanar images that highlight the course of each nerve. These sequences have become a mainstay in the evaluation of the cerebellopontine angles and inner ear. Usually referred to by their trade names or acronyms (eg, constructive interference steady state, or CISS, and fast imaging employing steady-state acquisition, or FIESTA), SSFP sequences allow precise differentiation between branches of the facial and vestibulocochlear nerves, accurate detection of small masses in the cerebellopontine angles and internal auditory canals, and detailed evaluation of endolymph and perilymph within the inner ear. To take full advantage of these imaging sequences, radiologists must be familiar with the appearances of similar anatomic details of all 12 cranial nerves on SSFP MR images.

Multi-Pinhole SPECT Imaging With Silicon Strip Detectors

Silicon double-sided strip detectors offer outstanding instrinsic spatial resolution with reasonable detection efficiency for iodine-125 emissions. This spatial resolution allows for multiple-pinhole imaging at low magnification, minimizing the problem of multiplexing. We have conducted imaging studies using a prototype system that utilizes a detector of 300-micrometer thickness and 50-micrometer strip pitch together with a 23-pinhole collimator. These studies include an investigation of the synthetic-collimator imaging approach, which combines multiple-pinhole projections acquired at multiple magnifications to obtain tomographic reconstructions from limited-angle data using the ML-EM algorithm. Sub-millimeter spatial resolution was obtained, demonstrating the basic validity of this approach.