Showing papers on "Imaging phantom published in 2002"

Flat-panel cone-beam computed tomography for image-guided radiation therapy

Abstract: Purpose: Geometric uncertainties in the process of radiation planning and delivery constrain dose escalation and induce normal tissue complications. An imaging system has been developed to generate high-resolution, soft-tissue images of the patient at the time of treatment for the purpose of guiding therapy and reducing such uncertainties. The performance of the imaging system is evaluated and the application to image-guided radiation therapy is discussed. Methods and Materials: A kilovoltage imaging system capable of radiography, fluoroscopy, and cone-beam computed tomography (CT) has been integrated with a medical linear accelerator. Kilovoltage X-rays are generated by a conventional X-ray tube mounted on a retractable arm at 90° to the treatment source. A 41 × 41 cm 2 flat-panel X-ray detector is mounted opposite the kV tube. The entire imaging system operates under computer control, with a single application providing calibration, image acquisition, processing, and cone-beam CT reconstruction. Cone-beam CT imaging involves acquiring multiple kV radiographs as the gantry rotates through 360° of rotation. A filtered back-projection algorithm is employed to reconstruct the volumetric images. Geometric nonidealities in the rotation of the gantry system are measured and corrected during reconstruction. Qualitative evaluation of imaging performance is performed using an anthropomorphic head phantom and a coronal contrast phantom. The influence of geometric nonidealities is examined. Results: Images of the head phantom were acquired and illustrate the submillimeter spatial resolution that is achieved with the cone-beam approach. High-resolution sagittal and coronal views demonstrate nearly isotropic spatial resolution. Flex corrections on the order of 0.2 cm were required to compensate gravity-induced flex in the support arms of the source and detector, as well as slight axial movements of the entire gantry structure. Images reconstructed without flex correction suffered from loss of detail, misregistration, and streak artifacts. Reconstructions of the contrast phantom demonstrate the soft-tissue imaging capability of the system. A contrast of 47 Hounsfield units was easily detected in a 0.1-cm-thick reconstruction for an imaging exposure of 1.2 R (in-air, in absence of phantom). The comparison with a conventional CT scan of the phantom further demonstrates the spatial resolution advantages of the cone-beam CT approach. Conclusions: A kV cone-beam CT imaging system based on a large-area, flat-panel detector has been successfully adapted to a medical linear accelerator. The system is capable of producing images of soft tissue with excellent spatial resolution at acceptable imaging doses. Integration of this technology with the medical accelerator will result in an ideal platform for high-precision, image-guided radiation therapy.

Capacitive micromachined ultrasonic transducers: next-generation arrays for acoustic imaging?

Abstract: Piezoelectric materials have dominated the ultrasonic transducer technology. Recently, capacitive micromachined ultrasonic transducers (CMUTs) have emerged as an alternative technology offering advantages such as wide bandwidth, ease of fabricating large arrays, and potential for integration with electronics. The aim of this paper is to demonstrate the viability of CMUTs for ultrasound imaging. We present the first pulse-echo phased array B-scan sector images using a 128-element, one-dimensional (1-D) linear CMUT array. We fabricated 64- and 128-element 1-D CMUT arrays with 100% yield and uniform element response across the arrays. These arrays have been operated in immersion with no failure or degradation in performance over the time. For imaging experiments, we built a resolution test phantom roughly mimicking the attenuation properties of soft tissue. We used a PC-based experimental system, including custom-designed electronic circuits to acquire the complete set of 128/spl times/128 RF A-scans from all transmit-receive element combinations. We obtained the pulse-echo frequency response by analyzing the echo signals from wire targets. These echo signals presented an 80% fractional bandwidth around 3 MHz, including the effect of attenuation in the propagating medium. We reconstructed the B-scan images with a sector angle of 90 degrees and an image depth of 210 mm through offline processing by using RF beamforming and synthetic phased array approaches. The measured 6-dB lateral and axial resolutions at 135 mm depth were 0.0144 radians and 0.3 mm, respectively. The electronic noise floor of the image was more than 50 dB below the maximum mainlobe magnitude. We also performed preliminary investigations on the effects of crosstalk among array elements on the image quality. In the near field, some artifacts were observable extending out from the array to a depth of 2 cm. A tail also was observed in the point spread function (PSF) in the axial direction, indicating the existence of crosstalk. The relative amplitude of this tail with respect to the mainlobe was less than -20 dB.

Respiration-correlated spiral CT: a method of measuring respiratory-induced anatomic motion for radiation treatment planning.

Abstract: We describe a method for generating CT images at multiple respiratory phases with a single spiral CT scan, referred to as respiratory-correlated spiral CT (RCCT). RCCT relies on a respiration wave form supplied by an external patient monitor. During acquisition this wave form is recorded along with the initiation time of the CT scan, so as to "time stamp" each reconstructed slice with the phase of the respiratory cycle. By selecting the appropriate slices, a full CT image set is generated at several phases, typically 7-11 per cycle. The CT parameters are chosen to optimize the temporal resolution while minimizing the spatial gap between slices at successive respiratory cycles. Using a pitch of 0.5, a gantry rotation period of 1.5 s, and a 180 degrees reconstruction algorithm results in approximately 5 mm slice spacing at a given phase for typical respiration periods, and a respiratory motion within each slice that is acceptably small, particularly near end expiration or end inspiration where gated radiotherapy is to occur. We have performed validation measurements on a phantom with a moving sphere designed to simulate respiration-induced tumor motion. RCCT scans of the phantom at respiratory periods of 4, 5, and 6 s show good agreement of the sphere's motion with that observed under fluoroscopic imaging. The positional deviations in the sphere's centroid between RCCT and fluoroscopy are 1.1+/-0.9 mm in the transaxial direction (average over all scans at all phases +/-1 s.d.) and 1.2+/-1.0 mm in the longitudinal direction. Reconstructed volumes match those expected on the basis of stationary-phantom scans to within 5% in all cases. The surface distortions of the reconstructed sphere, as quantified by deviations from a mathematical reference sphere, are similar to those from a stationary phantom scan and are correlated with the speed of the phantom. A RCCT scan of the phantom undergoing irregular motion, demonstrates that successful reconstruction can be achieved even with irregular respiration. Limitations from x-ray tube heating in our current CT unit restrict the length of the scan region to 9 cm for the RCCT settings used, though this will not be a limitation for a multislice scanner. RCCT offers an alternative to the current method of respiration-triggered axial scans. Multiple phases of respiration are imaged with RCCT in approximately the same scanning time required to image a single phase with a triggered axial scan. RCCT scans can be used in connection with respiratory-gated treatment to identify the patient-specific phase of minimum tumor motion, determine residual tumor motion within the gate interval, and compare treatment plans at different phases.

Real-time speckle reduction and coherence enhancement in ultrasound imaging via nonlinear anisotropic diffusion

Abstract: This paper presents a novel approach for speckle reduction and coherence enhancement of ultrasound images based on nonlinear coherent diffusion (NCD) model. The proposed NCD model combines three different models. According to speckle extent and image anisotropy, the NCD model changes progressively from isotropic diffusion through anisotropic coherent diffusion to, finally, mean curvature motion. This structure maximally low-pass filters those parts of the image that correspond to fully developed speckle, while substantially preserving information associated with resolved-object structures. The proposed implementation algorithm utilizes an efficient discretization scheme that allows for real-time implementation on commercial systems. The theory and implementation of the new technique are presented and verified using phantom and clinical ultrasound images. In addition, the results from previous techniques are compared with the new method to demonstrate its performance.

MR Elastography of Breast Cancer: Preliminary Results

Abstract: OBJECTIVE. Motivated by the long-recognized value of palpation in detecting breast cancer, we tested the feasibility of a technique for quantitatively evaluating the mechanical properties of breast tissues on the basis of direct MR imaging visualization of acoustic waves.SUBJECTS AND METHODS. The prototypic elasticity imaging technique consists of a device for generating acoustic shear waves in tissue, an MR imaging—based method for imaging the propagation of these waves, and an algorithm for processing the wave images to generate quantitative images depicting tissue stiffness. After tests with tissue-simulating phantom materials and breast cancer specimens, we used the prototypic breast MR elastography technique to image six healthy women and six patients with known breast cancer.RESULTS. Acoustic shear waves were clearly visualized in phantoms, breast cancer specimens, healthy volunteers, and patients with breast cancer. The elastograms of the tumor specimens showed focal areas of high shear stiffness. ...

Three-dimensional optical tomography of the premature infant brain

Abstract: For the first time, three-dimensional images of the newborn infant brain have been generated using measurements of transmitted light A 32-channel time-resolved imaging system was employed, and data were acquired using custom-made helmets which couple source fibres and detector bundles to the infant head Images have been reconstructed using measurements of mean flight time relative to those acquired on a homogeneous reference phantom, and using a head-shaped 3D finite-element-based forward model with an external boundary constrained to match the measured positions of the sources and detectors Results are presented for a premature infant with a cerebral haemorrhage predominantly located within the left ventricle Images representing the distribution of absorption at 780 nm and 815 nm reveal an asymmetry consistent with the haemorrhage, and corresponding maps of blood volume and fractional oxygen saturation are generally within expected physiological values

Time-resolved contrast-enhanced imaging with isotropic resolution and broad coverage using an undersampled 3D projection trajectory

Abstract: Time-resolved contrast-enhanced 3D MR angiography (MRA) methods have gained in popularity but are still limited by the tradeoff between spatial and temporal resolution. A method is presented that greatly reduces this tradeoff by employing undersampled 3D projection reconstruction trajectories. The variable density k-space sampling intrinsic to this sequence is combined with temporal k-space interpolation to provide time frames as short as 4 s. This time resolution reduces the need for exact contrast timing while also providing dynamic information. Spatial resolution is determined primarily by the projection readout resolution and is thus isotropic across the FOV, which is also isotropic. Although undersampling the outer regions of k-space introduces aliased energy into the image, which may compromise resolution, this is not a limiting factor in high-contrast applications such as MRA. Results from phantom and volunteer studies are presented demonstrating isotropic resolution, broad coverage with an isotropic field of view (FOV), minimal projection reconstruction artifacts, and temporal information. In one application, a single breath-hold exam covering the entire pulmonary vasculature generates high-resolution, isotropic imaging volumes depicting the bolus passage.

Neurostimulation systems for deep brain stimulation: in vitro evaluation of magnetic resonance imaging-related heating at 1.5 tesla.

Abstract: Purpose To assess magnetic resonance imaging (MRI)-related heating for a neurostimulation system (Activa® Tremor Control System, Medtronic, Minneapolis, MN) used for chronic deep brain stimulation (DBS). Materials and Methods Different configurations were evaluated for bilateral neurostimulators (Soletra® Model 7426), extensions, and leads to assess worst-case and clinically relevant positioning scenarios. In vitro testing was performed using a 1.5-T/64-MHz MR system and a gel-filled phantom designed to approximate the head and upper torso of a human subject. MRI was conducted using the transmit/receive body and transmit/receive head radio frequency (RF) coils. Various levels of RF energy were applied with the transmit/receive body (whole-body averaged specific absorption rate (SAR); range, 0.98–3.90 W/kg) and transmit/receive head (whole-body averaged SAR; range, 0.07–0.24 W/kg) coils. A fluoroptic thermometry system was used to record temperatures at multiple locations before (1 minute) and during (15 minutes) MRI. Results Using the body RF coil, the highest temperature changes ranged from 2.5°–25.3° C. Using the head RF coil, the highest temperature changes ranged from 2.3°–7.1° C.Thus, these findings indicated that substantial heating occurs under certain conditions, while others produce relatively minor, physiologically inconsequential temperature increases. Conclusion The temperature increases were dependent on the type of RF coil, level of SAR used, and how the lead wires were positioned. Notably, the use of clinically relevant positioning techniques for the neurostimulation system and low SARs commonly used for imaging the brain generated little heating. Based on this information, MR safety guidelines are provided. These observations are restricted to the tested neurostimulation system. J. Magn. Reson. Imaging 2002;15:241–250. © 2002 Wiley-Liss, Inc.

Introduction to biomedical imaging

Abstract: Preface. Acknowledgments. 1. X-Ray Imaging and Computed Tomography. 1.1 General Principles of Imaging with X-Rays. 1.2 X-Ray Production. 1.3 Interactions of X-Rays with Tissue. 1.4 Linear and Mass Attenuation Coefficients of X-Rays in Tissue. 1.5 Instrumentation for Planar X-Ray Imaging. 1.6 X-Ray Image Characteristics. 1.7 X-Ray Contrast Agents. 1.8 X-Ray Imaging Methods. 1.9 Clinical Applications of X-Ray Imaging. 1.10 Computed Tomography. 1.11 Image Processing for Computed Tomography. 1.12 Spiral/Helical Computed Tomography. 1.13 Multislice Spiral Computed Tomography. 1.14 Radiation Dose. 1.15 Clinical Applications of Computed Tomography. 2. Nuclear Medicine. 2.1 General Principles of Nuclear Medicine. 2.2 Radioactivity. 2.3 The Production of Radionuclides. 2.4 Types of Radioactive Decay. 2.5 The Technetium Generator. 2.6 The Biodistribution of Technetium-Based Agents within the Body. 2.7 Instrumentation: The Gamma Camera. 2.8 Image Characteristics. 2.9 Single Photon Emission Computed Tomography. 2.10 Clinical Applications of Nuclear Medicine. 2.11 Positron Emission Tomography. 3. Ultrasonic Imaging. 3.1 General Principles of Ultrasonic Imaging. 3.2 Wave Propagation and Characteristic Acoustic Impedance. 3.3 Wave Reflection and Refraction. 3.4 Energy Loss Mechanisms in Tissue. 3.5 Instrumentation. 3.6 Diagnostic Scanning Modes. 3.7 Artifacts in Ultrasonic Imaging. 3.8 Image Characteristics. 3.9 Compound Imaging. 3.10 Blood Velocity Measurements Using Ultrasound. 3.11 Ultrasound Contrast Agents, Harmonic Imaging, and Pulse Inversion Techniques. 3.12 Safety and Bioeffects in Ultrasonic Imaging. 3.13 Clinical Applications of Ultrasound. 4. Magnetic Resonance Imaging. 4.1 General Principles of Magnetic Resonance Imaging. 4.2 Nuclear Magnetism. 4.3 Magnetic Resonance Imaging. 4.4 Instrumentation. 4.5 Imaging Sequences. 4.6 Image Characteristics. 4.7 MRI Contrast Agents. 4.8 Magnetic Resonance Angiography. 4.9 Diffusion-Weighted Imaging. 4.10 In Vivo Localized Spectroscopy. 4.11 Functional MRI. 4.12 Clinical Applications of MRI. 5. General Image Characteristics. 5.1 Introduction. 5.2 Spatial Resolution. 5.3 Signal-to-Noise Ratio. 5.4 Contrast-to-Noise Ratio. 5.5 Image Filtering. 5.6 The Receiver Operating Curve. Appendix A: The Fourier Transform. Appendix B: Backprojection and Filtered Backprojection. Abbreviations. Index.

A critical study of the mechanical and electrical properties of the PHANToM haptic interface and improvements for high-performance control

Abstract: This paper presents a critical study of the mechanical and electrical properties of the PHANToM haptic interface and improvements to overcome its limitations for applications requiring high-performance control. Target applications share the common requirements of low-noise/granularity/latency measurements, an accurate system model, high bandwidth, the need for an open architecture, and the ability to operate for long periods without interruption while exerting significant forces. To satisfy these requirements, the kinematics, dynamics, high-frequency dynamic response, and velocity estimation of the PHANToM system are studied. Furthermore, this paper presents the details of how the unknown subsystems of the stock PHANToM can be replaced with known, high-performance systems and how additional measurement electronics can be interfaced to compensate for some of the PHANToM's shortcomings. With these modifications, it is possible to increase the maximum achievable virtual wall stiffness by 35%, active viscous damping by 120%, and teleoperation loop gain by 50% over the original system. With the modified system, it is also possible to maintain higher forces for longer periods without causing motor overheating.

A 30-MHz piezo-composite ultrasound array for medical imaging applications

Abstract: Ultrasound imaging at frequencies above 20 MHz is capable of achieving improved resolution in clinical applications requiring limited penetration depth. High frequency arrays that allow real-time imaging are desired for these applications but are not yet currently available. In this work, a method for fabricating fine-scale 2-2 composites suitable for 30-MHz linear array transducers was successfully demonstrated. High thickness coupling, low mechanical loss, and moderate electrical loss were achieved. This piezo-composite was incorporated into a 30-MHz array that included acoustic matching, an elevation focusing lens, electrical matching, and an air-filled kerf between elements. Bandwidths near 60%, 15-dB insertion loss, and crosstalk less than -30 dB were measured. Images of both a phantom and an ex vivo human eye were acquired using a synthetic aperture reconstruction method, resulting in measured lateral and axial resolutions of approximately 100 /spl mu/m.

PET Performance Measurements Using the NEMA NU 2-2001 Standard

Abstract: The NU 2-1994 standard document for PET performance measurements has recently been updated. The updated document, NU 2-2001, includes revised measurements for spatial resolution, intrinsic scatter fraction, sensitivity, counting rate performance, and accuracy of count loss and randoms corrections. The revised measurements are designed to allow testing of dedicated PET systems in both 2-dimensional and 3-dimensional modes as well as coincidence gamma cameras, conditions not considered in the original NU 2-1994 standard. In addition, the updated measurements strive toward being more representative of clinical studies, in particular, whole-body imaging. Methods: Performance measurements following the NU 2-1994 and NU 2-2001 standards were performed on several different PET scanners. Differences between the procedures and resulting performance characteristics, as well as the rationale for these changes, were noted. Results: Spatial resolution is measured with a point source in all 3 directions, rather than a line source, as specified previously. For the measurements of intrinsic scatter fraction, sensitivity, and counting rate performance, a 70-cm line source is now specified, instead of a 19-cm-long cylindric phantom. The longer configuration permits measurement of these performance characteristics over the entire axial field of view of all current PET scanners and incorporates the effects of activity outside the scanner. A measurement of image quality has been added in an effort to measure overall image quality under clinically realistic conditions. This measurement replaces the individual measurements of uniformity and of the accuracy of corrections for attenuation and scatter. Conclusion: The changes from the NU 2-1994 standard to the NU 2-2001 standard strive toward establishing relevance with clinical studies. The tests in the updated standard also are, in general, simpler and less time-consuming to perform than those in the NU 2-1994 standard.

Spherical navigator echoes for full 3D rigid body motion measurement in MRI.

Abstract: We developed a 3D spherical navigator (SNAV) echo technique that can measure rigid body motion in all six degrees of freedom simultaneously by sampling a spherical shell in k-space. 3D rotations of an imaged object simply rotate the data on this shell and can be detected by registration of k-space magnitude values. 3D translations add phase shifts to the data on the shell and can be detected with a weighted least-squares fit to the phase differences at corresponding points. MRI pulse sequences were developed to study k-space sampling strategies on such a shell. Data collected with a computer-controlled motion phantom with known rotational and translational motions were used to evaluate the technique. The accuracy and precision of the technique depend on the sampling density. Roughly 2000 sample points were necessary for accurate detection to within the error limits of the motion phantom when using a prototype time-intensive sampling method. This number of samples can be captured in an approximately 27-ms double excitation SNAV pulse sequence with a 3D helical spiral trajectory. Preliminary results with the helical SNAV are encouraging and indicate that accurate motion measurement suitable for retrospective or prospective correction should be feasible with SNAV echoes.

Coronary Artery Calcium: Absolute Quantification in Nonenhanced and Contrast-enhanced Multi–Detector Row CT Studies

Abstract: PURPOSE:(a) To determine the accuracy of multi–detector row computed tomography (CT) in the measurement of the calcium concentration in a cardiac CT calibration phantom and (b) to assess the correlation of a traditional 3-mm section width CT coronary screening protocol and a 1.25-mm section width CT angiography imaging protocol in the quantification of the absolute mass of coronary calcium in patients who underwent both coronary screening and CT angiography with a multi–detector row CT scanner. MATERIALS AND METHODS: A heart phantom containing calcified cylinders was scanned to determine calibration factors and absolute calcium mass. In 50 patients, the variability (value 1 − value 2/mean value 1 − value 2), limit of agreement (±2SD value 1 − value 2), and systematic error (mean value 1 − value 2) of the total amount of coronary calcium calculated at traditional 3-mm section width CT and at 1.25-mm section width CT angiography were determined. RESULTS: The correlation coefficient between the 3-mm section ...

Near-infrared diffuse optical tomography.

Abstract: Diffuse optical tomography (DOT) is emerging as a viable new biomedical imaging modality. Using near-infrared (NIR) light, this technique probes absorption as well as scattering properties of biological tissues. First commercial instruments are now available that allow users to obtain cross-sectional and volumetric views of various body parts. Currently, the main applications are brain, breast, limb, joint, and fluorescence/bioluminescence imaging. Although the spatial resolution is limited when compared with other imaging modalities, such as magnetic resonance imaging (MRI) or X-ray computerized tomography (CT), DOT provides access to a variety of physiological parameters that otherwise are not accessible, including sub-second imaging of hemodynamics and other fast-changing processes. Furthermore, DOT can be realized in compact, portable instrumentation that allows for bedside monitoring at relatively low cost. In this paper, we present an overview of current state-of-the -art technology, including hardware and image-reconstruction algorithms, and focus on applications in brain and joint imaging. In addition, we present recent results of work on optical tomographic imaging in small animals.

Real-time in vivo color Doppler optical coherence tomography

Abstract: Color Doppler optical coherence tomography (CDOCT) is a functional extension of optical coherence tomography (OCT) that can image flow in turbid media. We have developed a CDOCT system capable of imaging flow in real time. Doppler processing of the ana- log signal is accomplished in hardware in the time domain using a novel autocorrelation technique. This Doppler processing method is compatible with a high speed OCT system capable of imaging in real time. Using this system, we demonstrate cross-sectional imaging of bidirectional flow with CDOCT at four frames per second in a tissue- simulating phantom consisting of intralipid solution flowing in glass capillaries. As a demonstration of real-time imaging of blood flow in vivo we imaged pulsatible blood flow in a rat femoral artery at eight frames per second. Issues of velocity sensitivity, imaging speed, and range of velocity measurement are discussed, as well as potential ap- plications of real-time CDOCT. © 2002 Society of Photo-Optical Instrumentation

Correction of Partial-Volume Effect for PET Striatal Imaging: Fast Implementation and Study of Robustness

Abstract: PET imaging of D2 receptors or 18F-L-dopa metabolism are reference protocols to follow and study neurodegenerative diseases, but the accuracy of striatal PET imaging is limited by the partialvolume effect (PVE). For such studies, the geometric transfer matrix (GTM) method has been proposed to correct the regional mean values for PVE and is now widely used. Methods: The GTM method models the geometric interactions induced by the PET system between the anatomic regions in which PVE correction is performed. This implies estimation of the corresponding regional spread function (RSF). The literature describes 2 implementations for the RSF calculation; they differ in the way the point spread function (PSF) of the imaging system is modeled, but no comparison or discussion has been given. The first and reference implementation uses an accurate intrinsic detector PSF that is applied in the sinogram space. The second uses a global PSF that is applied in the image space. In this work, we compared the 2 GTM implementations for 3-dimensional (3D) PET striatal imaging using Monte Carlo simulations and a phantom study. We studied the robustness of the GTM correction with respect to residual registration errors between PET and anatomy and with respect to residual segmentation errors. Results: Despite the differences in RSF calculation and computation cost between the 2 implementations, similar recovery results were obtained (between 95% and 100%). The study of robustness of the GTM correction yielded 2 results. A realistic residual misregistration between the anatomic and PET images did not modify the algorithm accuracy but decreased its precision. Conversely, a realistic residual missegmentation of the anatomic regions submitted to GTM correction decreased the correction accuracy. Conclusion: A simple but efficient implementation in the image space of the GTM method yields accurate PVE correction in striatal regions in studies with 3D PET and enables clinical use. The method is less sensitive to residual misregistration errors between PET and anatomy than to residual missegmentation of the anatomy. Special care should be taken with segmentation of the regions to correct for PVE.

Reproducibility of total cerebral blood flow measurements using phase contrast magnetic resonance imaging.

Abstract: Purpose To evaluate reproducibility of total cerebral blood flow (CBF) measurements with phase contrast magnetic resonance imaging (pcMRI). Materials and Methods We repeated total CBF measurements in 15 healthy volunteers with and without cardiac triggering, and with and without repositioning. In eight volunteers measurements were performed at two different occasions. In addition, measurement of flow in a phantom was performed to validate MR measurements. Results A difference of 40.4 ml/minute was found between CBF measurements performed with and without triggering (P < 0.05). For repeated triggered measurements, the coefficient of variation (CV) was 7.1%, and for nontriggered measurements 10.3%. For repeated measurements with repositioning, the CV was 7.1% with and 11.2% without triggering. Repeated measurements at different occasions showed a CV of 8.8%. Comparing measured with real flow in the phantom, the triggered differed 4.9% and the nontriggered 8.3%. Conclusion The findings of this study demonstrate that pcMRI is a reliable method to measure total CBF in terms of both accuracy and reproducibility. J. Magn. Reson. Imaging 2002;16:1–5. © 2002 Wiley-Liss, Inc.

Performance evaluation of A-SPECT: a high resolution desktop pinhole SPECT system for imaging small animals

Abstract: Pinhole collimation of gamma rays to image distributions of radiolabeled tracers is considered promising for use in small animal imaging. The recent availability of transgenic mice, coupled with the development of /sup 125/I and /sup 99m/Tc labeled tracers, has allowed the study of a range of human disease models while creating demand for ultrahigh resolution imaging devices. We have developed a compact gamma camera that, in combination with pinhole collimation, allows for accessible, ultrahigh resolution in vivo single photon emission computed tomography (SPECT) imaging of small animals. The system is based on a pixilated array of NaI(Tl) crystals coupled to an array of position sensitive photomultiplier tubes. Interchangeable tungsten pinholes with diameters ranging from 0.5 to 3 mm are available, allowing the camera to be optimized for a variety of imaging situations. We use a three dimensional maximum likelihood expectation maximization algorithm to reconstruct the images. Our evaluation indicates that high quality, submillimeter spatial resolution images can be achieved in living mice. Reconstructed axial spatial resolution was measured to be 0.53, 0.74, and 0.96 mm full width at half maximum (FWHM) for rotation radii of 1, 2, and 3 cm, respectively, using the 0.5-mm pinhole. In this configuration, sensitivity is comparable to that of a high-resolution parallel hole collimator. SPECT images of hot- and cold-rod phantoms and a highly structured monkey brain phantom illustrate that high quality images can be obtained with the system. Images of living mice demonstrate the ability of the system to obtain high-resolution images in vivo. The effect of object size on the quantitative assessment of isotope distributions in an image was also studied.

MR imaging-related heating of deep brain stimulation electrodes: in vitro study.

Abstract: BACKGROUND AND PURPOSE: Recent work has shown a potential for excessive heating of deep brain stimulation electrodes during MR imaging. This in vitro study investigates the relationship between electrode heating and the specific absorption rate (SAR) of several MR images. METHODS: In vitro testing was performed by using a 1.5-T MR imaging system and a head transmit-receive coil, with bilateral deep brain stimulation systems positioned in a gel salinefilled phantom, and temperature monitoring with a fluoroptic thermometry system. Standardized fast spin-echo sequences were performed over a range of high, medium, and low SAR values. Several additional, clinically important MR imaging techniques, including 3D magnetization prepared rapid acquisition gradient-echo imaging, echo-planar imaging, quantitative magnetization transfer imaging, and magnetization transfer-suppressed MR angiography, were also tested by using typical parameters. RESULTS: A significant, highly linear relationship between SAR and electrode heating was found, with the temperature elevation being approximately 0.9 times the local SAR value. Minor temperature elevations, <1°C, were found with the fast spin-echo, magnetization prepared rapid acquisition gradient-echo, and echo-planar clinical imaging sequences. The high dB/dt echo-planar imaging sequence had no significant heating independent of SAR considerations. Sequences with magnetization transfer pulses produced temperature elevations in the 1.0 to 2.0°C range, which was less than theoretically predicted for the relatively high SAR values. CONCLUSION: A potential exists for excessive MR imaging-related heating in patients with deep brain stimulation electrodes; however, the temperature increases are linearly related to SAR values. Clinical imaging sequences that are associated with tolerable temperature elevations in the <2.0°C range at the electrode tips can be performed safely within an SAR range <2.4 W/kg local (0.9 W/kg whole body averaged). Deep brain stimulation (DBS) treatment of refractory movement disorders and other conditions is a rapidly evolving neurosurgical field, with an ever increasing number of patients undergoing DBS surgery for Parkinson disease, essential tremor, dystonias, ep

Accuracy of CT-based thickness measurement of thin structures: modeling of limited spatial resolution in all three dimensions.

Abstract: Measurement of the width of thin structures such as the cortical shell of the vertebral body or femoral neck with computed tomography (CT) is limited by the spatial resolution of the CT system. Limited spatial resolution exists both within the CT image plane and perpendicular to it and can be described by the in-plane point spread function (PSF) and the across-plane slice sensitivity profile (SSP), respectively. The goal of this study was to confirm that errors of thickness measurement of thin structures critically depend on the spatial positioning of the object and the spatial resolution limitations of CT in all three dimensions, and to assess the size of the errors themselves. We compared computer models that incorporated both effects to experimentally assessed cortical thicknesses of the European Spine Phantom. Analysis included varying CT slice width, the orientation of measurement and angle beta of misalignment of longitudinal scanner and phantom axes. Agreement of models with measurements was good in all configurations with an overall error of 0.17 mm. This showed that PSF and SSP are adequate system characteristics to predict deviation of measured values from true widths. Errors between measurements and true cortical thickness values delta(true) averaged to 1.5 mm were strongly positively correlated with slice width d and beta. When the across-plane partial volume effect was eliminated, limited in-plane resolution still accounted for overestimation of delta(true) by 0.68 (137%), 0.27 (27%), and 0.06 mm (4%) for delta(true)=0.5, 1.0, and 1.5 mm, respectively. For delta(true) of 1.0 mm and above, it was shown that although the absolute cortical thickness values might not be accurately measurable, relative differences between two values are reflected in measurement. Implications for cortical thickness measurement are that the spinal cortical shell is too thin, whereas accurate assessment at locations of the femoral neck exhibiting a thicker cortical shell of both difference and absolute values should be possible with CT even for larger misalignment angles, especially when a smaller CT slice width is chosen.

Method and apparatus for calibration of radiation therapy equipment and verification of radiation treatment

Abstract: A method of calibration and verification of radiotherapy systems deduced radiation beam fluence profiles from the radiation source from a complete model of an extended radiation phantom together with dose information from a portal imaging device. The improved beam fluence profile characterization made with an iterative modeling which includes scatter effects may be used to compute dose profiles in the extended phantom or a patient who has been previously characterized with a CT scan. Deviations from the expected beam fluence profile can be used to detect patient misregistration.

Reproducibility and accuracy of coronary calcium measurements with multi-detector row versus electron-beam CT

Abstract: PURPOSE: To methodically evaluate the reproducibility and accuracy of coronary arterial calcification measurements by using spiral multi–detector row and electron-beam computed tomography (CT) with a beating heart phantom. MATERIALS AND METHODS: A phantom was built to mimic a beating heart with coronary arteries and calcified plaques. The simulated vessels moved in a pattern similar to that of a beating heart. The phantom operated at a variety of pulse rates (0–140 beats per minute). The phantom was repeatedly scanned in various positions by using various protocols with electron-beam and multi–detector row CT scanners to assess interexamination variability. Statistical analysis was performed to determine significant differences in interexamination variability for various acquisition protocols. RESULTS: Electrocardiographically (EKG) gated volume coverage with spiral multi–detector row CT (2.5-mm collimation) and overlapping image reconstruction (1-mm increment) was found to significantly improve the relia...

J-substitution algorithm in magnetic resonance electrical impedance tomography (MREIT): phantom experiments for static resistivity images

Abstract: Recently, a new static resistivity image reconstruction algorithm is proposed utilizing internal current density data obtained by magnetic resonance current density imaging technique. This new imaging method is called magnetic resonance electrical impedance tomography (MREIT). The derivation and performance of J-substitution algorithm in MREIT have been reported as a new accurate and high-resolution static impedance imaging technique via computer simulation methods. In this paper, we present experimental procedures, denoising techniques and image reconstructions using a 0.3-tesla (T) experimental MREIT system and saline phantoms. MREIT using J-substitution algorithm effectively utilizes the internal current density information resolving the problem inherent in a conventional EIT, that is, the low sensitivity of boundary measurements to any changes of internal tissue resistivity values. Resistivity images of saline phantoms show an accuracy of 6.8%-47.2% and spatial resolution of 64 /spl times/ 64. Both of them can be significantly improved by using an MRI system with a better signal-to-noise ratio.

Quantification of wall shear stress in large blood vessels using Lagrangian interpolation functions with cine phase-contrast magnetic resonance imaging.

Abstract: Arterial wall shear stress is hypothesized to be an important factor in the localization of atherosclerosis. Current methods to compute wall shear stress from magnetic resonance imaging (MRI) data do not account for flow profiles characteristic of pulsatile flow in noncircular vessel lumens. We describe a method to quantify wall shear stress in large blood vessels by differentiating velocity interpolation functions defined using cine phase-contrast MRI data on a band of elements in the neighborhood of the vessel wall. Validation was performed with software phantoms and an in vitro flow phantom. At an image resolution corresponding to in vivo imaging data of the human abdominal aorta, time-averaged, spatially averaged wall shear stress for steady and pulsatile flow were determined to be within 16% and 23% of the analytic solution, respectively. These errors were reduced to 5% and 8% with doubling in image resolution. For the pulsatile software phantom, the oscillation in shear stress was predicted to within 5%. The mean absolute error of circumferentially resolved shear stress for the nonaxisymmetric phantom decreased from 28% to 15% with a doubling in image resolution. The irregularly shaped phantom and in vitro investigation demonstrated convergence of the calculated values with increased image resolution. We quantified the shear stress at the supraceliac and infrarenal regions of a human abdominal aorta to be 3.4 and 2.3 dyn/cm2, respectively. © 2002 Biomedical Engineering Society.

X-ray micro-CT with a displaced detector array.

Abstract: Because the sizes of samples differ in x-ray micro-CT applications, it is desirable to have a mechanism to change the field of view of a micro-CT scanner. A well-known way to double the diameter of the field of view is to displace a detector array by 50%. In this paper, we propose to displace a detector array by an amount of greater than 0% but less than 50% for a continuously adjustable field of view, and formulate a weighting scheme for artifact-free reconstruction. Then, we perform numerical simulation with the Shepp-Logan phantom to demonstrate the feasibility in fan-beam and cone-beam geometry.

IMRT verification by three-dimensional dose reconstruction from portal beam measurements

Abstract: A method of reconstructing three-dimensional, in vivo dose distributions delivered by intensity-modulated radiotherapy (IMRT) is presented. A proof-of-principle experiment is described where an inverse-planned IMRT treatment is delivered to an anthropomorphic phantom. The exact position of the phantom at the time of treatment is measured by acquiring megavoltage CT data with the treatment beam and a research prototype, flat-panel, electronic portal imaging device. Immediately following CT imaging, the planned IMRT beams are delivered using the multiple-static field technique. The delivered fluence is sampled using the same detector as for the CT data. The signal measured by the portal imaging device is converted to primary fluence using an iterative phantom-scatter estimation technique. This primary fluence is back-projected through the previously acquired megavoltage CT model of the phantom, with inverse attenuation correction, to yield an input fluence map. The input fluence maps are used to calculate a "reconstructed" dose distribution using the same convolution/superposition algorithm as for the original planning dose calculation. Both relative and absolute dose reconstructions are shown. For the relative measurements, individual beam weights are taken from measurements but the total dose is normalized at the reference point. The absolute dose reconstructions do not use any dosimetric information from the original plan. Planned and reconstructed dose distributions are compared, with the reconstructed relative dose distribution also being compared to film measurements.

Rapid elastic image registration for 3-D ultrasound

Abstract: A Subvolume-based algorithm for elastic Ultrasound REgistration (SURE) was developed and evaluated. Designed primarily to improve spatial resolution in three-dimensional compound imaging, the algorithm registers individual image volumes nonlinearly before combination into compound volumes. SURE works in one or two stages, optionally using MIAMI Fuse/spl copy/ software first to determine a global affine registration before iteratively dividing the volume into subvolumes and computing local rigid registrations in the second stage. Connectivity of the entire volume is ensured by global interpolation using thin-plate splines after each iteration. The performance of SURE was quantified in 20 synthetically deformed in vivo ultrasound volumes, and in two phantom scans, one of which was distorted at acquisition by placing an aberrating layer in the sound path. The aberrating layer was designed to induce beam aberrations reported for the female breast. Synthetic deformations of 1.5-2.5 mm were reduced by over 85% when SURE was applied to register the distorted image volumes with the original ones. Registration times were below 5 min on a 500-MHz CPU for an average data set size of 13MB. In the aberrated phantom scans, SURE reduced the average deformation between the two volumes from 1.01 to 0.30mm. This was a statistically significant (P=0.01) improvement over rigid and affine registration transformations, which produced reductions to 0.59 and 0.50 mm, respectively.

Pulsed-microwave-induced thermoacoustic tomography: filtered backprojection in a circular measurement configuration.

Abstract: Our study on pulsed-microwave-induced thermoacoustic tomography in biological tissues is presented. A filtered backprojection algorithm based on rigorous theory is used to reconstruct the cross-sectional image from a thermoacoustic measurement in a circular configuration that encloses the sample under study. Specific details describing the measurement of thermoacoustic waves and the implementation of the reconstruction algorithm are discussed. A two-dimensional (2D) phantom sample with 2 mm features can be imaged faithfully. Through numerical simulation, the full width half-maximum (FWHM) of the point-spread function (PSF) is calculated to estimate the spatial resolution. The results demonstrate that the circular measurement configuration combined with the filtered backprojection method is a promising technique for detecting small tumors buried in biological tissues by utilizing microwave absorption contrast and ultrasound spatial resolution (∼mm).