Showing papers on "Imaging phantom published in 1994"

The PHANToM Haptic Interface: A Device for Probing Virtual Objects

Abstract: 1. Abstract This paper describes the PHANToM haptic interface - a device which measures a user’s finger tip position and exerts a precisely controlled force vector on the finger tip. The device has enabled users to interact with and feel a wide variety of virtual objects and will be used for control of remote manipulators. This paper discusses the design rationale, novel kinematics and mechanics of the PHANToM. A brief description of the programming of basic shape elements and contact interactions is also given.

Power Doppler US : a potentially useful alternative to mean frequency-based color Doppler US

Abstract: PURPOSE: The authors present a preliminary report to demonstrate a new color Doppler (CD) ultrasonography (US) technique called power Doppler (PD), which displays the total integrated Doppler power in color, and to compare PD with CD imaging, which generally displays an estimate of the mean Doppler frequency shift. MATERIALS AND METHODS: Two standard commercial US scanners that encode the integrated power in the Doppler signal in color were used to demonstrate PD. A standard nonflow-containing US phantom, a normal right kidney, and a torsive and normal contralateral testis were scanned in the power mode. In the phantom and kidney, results with CD and PD were directly compared. RESULTS: PD does not alias, is relatively angle independent, and displays background noise in a way that increases the usable dynamic range of a US scanner. This extended dynamic range should increase machine sensitivity and may demonstrate increased flow in certain circumstances. CONCLUSION: PD is a new CD imaging imaging mode that...

The Ecat Exact Hr - Performance of a New High-resolution Positron Scanner

Abstract: Objective: The ECAT EXACT HR is a newly designed CTI-Siemens PET scanner with high spatial resolution. Its physical performance with respect to resolution, count rate efficiency, and scatter was investigated and evaluated with phantom studies. Materials and Methods: The new tomograph consists of three rings of 112 BGO block detectors (50 mm x 23 mm x 30 mm deep) each, covering an axial field of view of 15 cm with a patient port of 56 cm diameter. Each block is sawed into an 8 x 7 matrix giving 24 detector rings with 784 crystals each. Results: Total sensitivity for a 20 cm cylinder phantom is 177 kcps/mu Ci/ml in two-dimensional (2D) mode and increases to 1.46 Mcps/mu Ci/ml in 3D mode. Count rate performance was investigated for different low energy discriminator thresholds. Smaller detector blocks improve noise equivalent counts by similar to 50% compared with the EXACT system both in 2D and in 3D mode. Scatter fractions vary in 2D from 0.09 to 0.13 for energy thresholds from 450 to 250 keV for line sources in a 20 cm diameter phantom. In 3D mode an increase of scatter by a factor of 3 is observed. Transaxial spatial resolution varies from 3.6 mm full width at half-maximum (FWHM) at the center to 4.5 mm FWHM tangentially and 7.4 mm FWHM radially at R = 20 cm. Average axial resolution changes from 4.0 mm FWHM at center to 6.7 mm FWHM at R = 20 cm. Conclusion: Due to its special properties, the EXACT HR can be equally applied to routine clinical brain and whole-body imaging and to noninvasive experimental studies of regional tracer concentrations in medium-sized animals

Pinhole collimation for ultra-high-resolution, small-field-of-view SPECT

Abstract: The objective of this investigation was to evaluate small-field-of-view, ultra-high-resolution pinhole collimation for a rotating-camera SPECT system that could be used to image small laboratory animals. Pinhole collimation offers distinct advantages over conventional parallel-hole collimation when used to image small objects. Since geometric sensitivity increases markedly for points close to the pinhole, small-diameter and high-magnification pinhole geometries may be useful for selected imaging tasks when used with large-field-of-view scintillation cameras. The use of large magnifications can minimize the loss of system resolution caused by the intrinsic resolution of the scintillation camera. A pinhole collimator has been designed and built that can be mounted on one of the scintillation cameras of a triple-head SPECT system. Three pinhole inserts with approximate aperture diameters of 0.6, 1.2 and 2.0 mm have been built and can be mounted individually on the collimator housing. When a ramp filter is used with a three-dimensional (3D) filtered backprojection (FBP) algorithm, the three apertures have in-plane SPECT spatial resolutions (FWHM) at 4 cm of 1.5, 1.9 and 2.8 mm, respectively. In-air point source sensitivities at 4 cm from the apertures are 0.9, 2.6 and 5.7 counts s(-1) microCi(-1) (24, 70 and 154 counts s(-1) MBq(-1)) for the 0.6, 1.2 and 2.0 mm apertures, respectively. In vitro image quality was evaluated with a micro-cold-rod phantom and a micro-Defrise phantom using both the 3D FBP algorithm and a 3D maximum likelihood-expectation maximization (ML-EM) algorithm. In vivo image quality was evaluated using two (315 and 325 g) rats. Ultra-high-resolution pinhole SPECT is an inexpensive and simple approach for imaging small animals that can be used with existing rotating-camera SPECT system.

Realization and verification of three-dimensional conformal radiotherapy with modulated fields

Abstract: Purpose : We describe the experimental demonstration of the delivery of a three-dimensional conformal radiotherapy dose distribution using in-field modulation of nine fixed-gantry fields. Methods and Materials : Two-dimensional in-field modulation profiles, varying from field to field, were realized by quasi-dynamic multilaf collimation using the prototype of a commercially available multileaf collimator installed on a medical linear accelerator. The profiles were calculated to deliver an optimal dose distribution for a patient with a prostate carcinoma. The target volume surface was invaginated and bifurcated. The calculated dose distribution was delivered to a homogeneous polystyrene phantom consisting of 1 cm thick slices that were cut to match the patient's outer contour. Seven therapy verification films were placed between the phantom slices. Results : Analysis of the films revealed a degree of conformation of the high-dose region to the target shape that would not be possible with unmodulated conformal therapy. However, small observed spatial displacements of the dose distribution confirm the need for very accurate positioning. Conclusions : It is feasible to deliver clinically relevant, three-dimensional dose distributions that conform to in-vaginated and bifurcated target volumes using fields modulated by multileaf collimators.

A comparison of conventional and spiral CT : an experimental study on the detection of spherical lesions

Abstract: Objective It is accepted that spiral CT scanning may offer significant advantages in a number of clinical applications. There is still some concern with respect to image quality, however, since slice sensitivity profiles are slightly broadened due to the table motion. We carried out theoretical analysis, phantom measurements, and computer simulations to evaluate and to compare contrast and spatial resolution for conventional and for spiral scanning. Special emphasis was put on the task of detecting spherical lesions. Materials and methods For standard test objects that measure only resolution in the scan plane, no significant difference between conventional and spiral scanning was observed. We therefore designed a phantom setup that allowed us to place spheres of arbitrary diameter and contrast in arbitrary positions to test three-dimensional (3D) resolution. Results For conventional CT, both lesion contrast and the degree of spatial separation of lesions observed depend on the relation of the start position of the scan series to the random location of a sphere or lesion. Spiral CT offers space-invariant resolution due to its continuous scanning. Small lesion contrast may be improved by up to a factor of 1.8 when compared with conventional CT since slices can be centered retrospectively. Measurements and simulations were in excellent agreement. Conclusion We conclude that spiral CT can offer improved 3D contrast and spatial resolution. To exploit these advantages, images should be reconstructed in spiral CT at increments of less than half the distance traveled during one 360 degrees tube rotation. With four to five images per such interval, usually equal to the slice width, results very close to the theoretical optimum are achieved. Many of the presented considerations and results apply to other slice imaging modalities like MRI in analogous fashion.

Ultrasound synthetic aperture imaging: monostatic approach

Abstract: An ultrasound synthetic aperture imaging method based on a monostatic approach was studied experimentally. The proposed synthetic aperture method offers good dynamical resolution along with fast numerical reconstruction. In this study complex object data were recorded coherently in a two-dimensional hologram using a 3.5 MHz single transducer with a fairly wide-angle beam. Image reconstruction which applies the wavefront backward propagation method and the near-field curvature compensation was performed numerically in a microcomputer using the spatial frequency domain. This approach allows an efficient use of the FFT-algorithms. Because of the simple and fast scanning scheme and the efficient reconstruction algorithms the method can be made real-time. The image quality of the proposed method was studied by evaluating the spatial and dynamical resolution in a waterbath and in a typical tissue-mimicking phantom. The lateral as well as the range resolution (-6 dB) were approximately 1 mm in the depth range of 30-100 mm. The dynamical resolution could be improved considerably when the beam width was made narrower. Although it resulted in a slightly reduced spatial resolution this compromise has to be done for better resolution of low-contrast targets such as cysts. The study showed that cysts as small as 2 mm by diameter could be resolved. >

Feasibility study for positron emission mammography

Abstract: A feasibility study is presented for a small, low‐cost, dedicated device for positron emission mammography. Two detector arrays above and below the breast would be placed in a conventional mammography unit. These detectors are sensitive to positron annihilation radiation, and are connected to a coincidence circuit and a multiplane image memory. Images of the distribution of positron‐emitting isotope are obtained in real time by incrementing the memory location at the intersection of each line of response. Monte Carlo simulations of a breast phantom are compared with actual scans of this phantom in a conventional PET scanner. The simulations and experimental data are used to predict the performance of the proposed system. Spatial resolution experiments using very narrow bismuth germanate BGO crystals suggest that spatial resolutions of about 2 mm should be possible. The efficiency of the proposed device is about ten times that of a conventional brain scanner. The scatter fraction is greater, but the scattered radiation has a very flat distribution. By designing the device to fit in an existing mammography unit, conventional mammograms can be taken after the injection of the radio‐pharmaceutical allowing exact registration of the emission and conventional mammographicimages.

Time-gated transillumination of biological tissues and tissuelike phantoms

Abstract: The applicability and limits of time-resolved transillumination to determine the internal details of biological tissues are investigated by phantom experiments. By means of line scans across a sharp edge, the spatial resolution (Δx) and its dependence on the time-gate width (Δt) can be determined. Additionally, measurements of completely absorbing bead pairs embedded in a turbid medium demonstrate the physical resolution in a more realistic case. The benefit of time resolution is especially high for a turbid medium with a comparatively small reduced scattering coefficient of approximately µ(s)' = 0.12 mm(-1). Investigations with partially absorbing beads and filled plastic tubes demonstrate the high sensitivity of time-resolving techniques with respect to spatial variations in scattering or absorption coefficients that are due to the embedded disturber. In particular, it is shown that time gating is sensitive to variations in scattering coefficients.

Image fusion for stereotactic radiotherapy and radiosurgery treatment planning

Abstract: Purpose : We describe an image fusion application that addresses two basic problems that previously limited the use of magnetic resonance imaging (MRI) for geometric localization in stereotactic radiosurgery (SRS) and stereotactic radiotherapy (SRT). The first limitation is imposed by the use of a relocatable, MRI-incompatible, stereotactic frame for stereotactic radiotherapy. The second limitation is an inherent lack of geometric fidelity in current MRI scanners that invalidates the use of MRI for stereotactic localization. Methods and Materials : We recently developed and implemented a novel automated method for fusing computerized tomography (CT) and MRI volumetric image studies. The method is based on a chamfer matching algorithm, and provides a quality assurance procedure to verify the accuracy of the fused image set. The image fusion protocol removes the need for stereotactic fixation of the patient for the MRI study. Results : The image fusion protocol significantly improves on the spatial accuracy of the MRI study. We demonstrate the effect of distortion and the effectiveness of the fusion with a phantom study. We present two case studies, an acoustic neurinoma treated with SRS. and a pilocytic astrocytoma treated with SRT. Conclusion : The image fusion protocol significantly improves our logistical management of treating patients with radiosurgery and makes conformal therapy practical for treating patients with SRT. The image fusion protocol demonstrates both the superior diagnostic quality and the poor geometric fidelity of MRI. MRI is a required imaging modality in stereotactic therapy. Image fusion combines the superior MRI diagnostic quality with the superior CT geometric definition, and makes the use of MRI in stereotactic therapy possible and practical.

Accuracy of stereolithographic models of human anatomy

Abstract: A study was undertaken to determine the dimensional accuracy of anatomical replicas derived from X-ray 3D computed tomography (CT) images and produced using the rapid prototyping technique of stereolithography (SLA). A dry bone skull and geometric phantom were scanned, and replicas were produced. Distance measurements were obtained to compare the original objects and the resulting replicas. Repeated measurements between anatomical landmarks were used for comparison of the original skull and replica. Results for the geometric phantom demonstrate a mean difference of +0.47 mm, representing an accuracy of 97.7-99.12%. Measurements of the skull produced a range of absolute differences (maximum +4.62 mm, minimum +0.1 mm, mean +0.85 mm). These results support the use of SLA models of human anatomical structures in such areas as pre-operative planning of complex surgical procedures. For applications where higher accuracy is required, improvements can be expected by utilizing smaller pixel resolution in the CT images. Stereolithographic models can now be confidently employed as accurate, three-dimensional replicas of complex, anatomical structures.

Investigation of the use of X-ray CT images for attenuation compensation in SPECT

Abstract: This study investigates the general use of single-beam X-ray computed tomography (CT) images for generating attenuation maps for compensation of photon attenuation in SPECT images. A 3D mathematical thorax phantom is used to simulate both emission and transmission projection data for monoenergetic (radionuclide) and polyenergetic (X-ray) sources. Polyenergetic transmission projection data are simulated for a standard X-ray spectrum and fan-beam geometry. The projection data are reconstructed using filtered backprojection to form an X-ray CT image which is then scaled to produce an estimate of the attenuation map at the energy of the emission radionuclide. Emission projection data are simulated for a fan-beam geometry at the energies of /sup 201/Tl and /sup 99m/Tc, two radionuclides commonly used in cardiac SPECT. Detector response and scatter are not included in the model. Noiseless, emission projection data are iteratively reconstructed using the ML-EM algorithm with nonuniform attenuation compensation and attenuation maps derived from both the simulated X-ray CT image and from a simulated monoenergetic transmission CT image. The attenuation maps generated from the X-ray CT images accurately estimate the attenuation coefficient for muscle and lung tissues, but not for bone tissues, which show error in the attenuation coefficient of 21-42% for spinal bone and 34-58% for rib bone. However, despite the inaccurate estimate of bone attenuation, the reconstructed SPECT images provide estimates of myocardial radioactivity concentration to within 9% and show few artifacts. >

Device and automatic method for the geometrical calibration of an X-ray imaging system

Abstract: A phantom constituted by a helix is used. This helix enables an automatic geometrical calibration of any X-ray imaging system that uses a plane detector.

Stair-step artifacts in three-dimensional helical CT: an experimental study.

Abstract: PURPOSE: Stair-step artifacts in helical computed tomography (CT) are associated with inclined surfaces in longitudinal sections. The authors investigated the origin and the characteristics of the artifacts. MATERIALS AND METHODS: A cone phantom and a skull were dry-scanned with a helical CT scanner, and images were reconstructed by using the half-scan interpolation algorithm with combinations of detector collimation (1 and 5 mm), table feed (1, 2, 5, and 10 mm), and reconstruction interval (1, 2, 5, and 10 mm). RESULTS: Stair-step artifacts were perceived in most instances. Stair-step artifacts arose from two sources: large reconstruction intervals and asymmetric helix interpolation, forming isoclosed curves and spirallike patterns in three-dimensional axial views, respectively. CONCLUSION: To eliminate the stair-step artifacts, both the collimation and the table feed should be less than the longitudinal dimension of the important feature on inclined surfaces, and the reconstruction interval should be le...

Three-dimensional image reconstruction for PET by multi-slice rebinning and axial image filtering

Abstract: A fast method is described for reconstructing volume images from three-dimensional (3D) coincidence data in positron emission tomography (PET). The reconstruction method makes use of all coincidence data acquired by high-sensitivity PET systems that do not have inter-slice absorbers (septa) to restrict the axial acceptance angle. The reconstruction method requires only a small amount of storage and computation, making it well suited for dynamic and whole-body studies. The method consists of three steps: (i) rebinning of coincidence data into a stack of 2D sinograms; (ii) slice-by-slice reconstruction of the sinogram associated with each slice to produce a preliminary 3D image having strong blurring in the axial (z) direction, but with different blurring at different z positions; and (iii) spatially variant filtering of the 3D image in the axial direction (i.e. 1D filtering in z for each x-y column) to produce the final image. The first step involves a new form of the rebinning operation in which multiple sinograms are incremented for each oblique coincidence line (multi-slice rebinning). The axial filtering step is formulated and implemented using the singular value decomposition. The method has been applied successfully to simulated data and to measured data for different kinds of phantom (multiple point sources, multiple discs, a cylinder with cold spheres, and a 3D brain phantom).

Assessment of colour Doppler tissue imaging using test-phantoms.

Abstract: An investigation has been carried out on the velocity resolution, spatial resolution and accuracy of Doppler images as part of a study into the Doppler display of cardiac tissue motion. Test-phantoms were designed to perform this work and images were captured on a computer. The characteristics of the phantom images and of the image capture process were studied. The smallest spatial detail that was observed in the Doppler image was 3 mm by 3 mm. Doppler receive gain and Doppler ensemble size both affected velocity resolution. Different target materials gave different measures for velocity resolution. This could be related to the different back-scatter intensities of the materials.

Fast Nonsupervised 3D Registration of PET and MR Images of the Brain

Abstract: We propose a fully nonsupervised methodology dedicated to the fast registration of positron emission tomography (PET) and magnetic resonance images of the brain. First, discrete representations of the surfaces of interest (head or brain surface) are automatically extracted from both images. Then, a shape-independent surface-matching algorithm gives a rigid body transformation, which allows the transfer of information between both modalities. A three-dimensional (3D) extension of the chamfer-matching principle makes up the core of this surface-matching algorithm. The optimal transformation is inferred from the minimization of a quadratic generalized distance between discrete surfaces, taking into account between-modality differences in the localization of the segmented surfaces. The minimization process is efficiently performed via the precomputation of a 3D distance map. Validation studies using a dedicated brain-shaped phantom have shown that the maximum registration error was of the order of the PET pix...

Automated 3D nonlinear deformation procedure for determination of gross morphometric variability in human brain

Abstract: We describe an automated method to register MRI volumetric datasets to a digital human brain model. The technique employs3D non-linear warping based on the estimation of local deformation fields using cross-correlation of invariant intensity featuresderived from image data. Results of the non-linear registration on a simple phantom, a complex brain phantom and real MRIdata are presented. Anatomical variability is expressed with respect to the Talairach-like standardized brain-based coordinatesystem of the model. We show that the automated non-linear registration reduces the inter-subject variability of homologouspoints in standardized space by 15% over linear registration methods. A 3D variability map is shown. 1 INTRODUCTION New imaging modalities and techniques, e.g., PET, functional MRI (fMRI), SPECT, magnetoencephalography (MEG), andEEG have made it possible to map functional areas of the human brain with respect to anatomy. Two aspects of this workrequire integration of data from different individuals: 1) The low signal associated with cognitive activation (e.g., a subtlechange in cerebral blood flow (CBF) as measured by PET) requires averaging across subjects to improve statistical significanceofmeasured CBF changes'4"°. 2) Although high resolution imaging techniques such as fMRI now make it possible to measureactivation within a single subject, it will still be necessary to compare results across individuals in order to fully understandthe relationship between functional areas and the underlying gross morphology such as gyral anatomy. For both situations weideally wish to remove all morphological differences between individual brains before considering the distribution of functionalinformation superimposed on the anatomical substrate. This requires a method for deforming one brain to match another atall points, and has typically been accomplished by mapping the volumetric data into a standardized brain-based coordinatesystem24. Until recently, most centers have used linear transformations only13'15'19'25. However, previous work24'23 has shownthat even after linear mapping, there is variability of up to 1.5 cm in the position of cortical structures, which may representa significant source of error when mapping activation foci. We have shown9, that on average for points throughout the brain(cortical and sub-cortical), there is a 6-7mm anatomical variability in 3D position not accounted for by linear registration.The objective of this paper is to present an automated method of establishing the non-linear morphometric variability ina population of normal brains with 3D MRI. Non-linear warping based on homologous landmark matching2'9 has not beenpractical for routine use as a deformation/warping model because of the subjectivity involved in selecting the precise locationand number of points that define the non-linear deformation. This has lead our group and others (e.g.,1"6'20) to consider fullyautomated, objective non-linear mapping techniques. Our method uses non-linear 3D warping of one data set to register itwith another, based on the estimation of local deformations derived from local neighbourhood correlation of invariant featurescalculated from image data3'4.To properly assess non-linear variability it is first essential to have a well-defined 3D coordinate space where the linearcomponent of the anatomical variation is removed by application of an affine transformation. Without a priori knowledgeof anatomical variability, the best minimum variance frame cannot be defined since it is wholly dependent on the former.Therefore, we have selected a brain-based coordinate system very similar to that proposed by Talairach24. Our implementationuses a single global affine transformation whereas Talairach employs 12 piece-wise linear transformations (as implemented in

Accelerated EM reconstruction in total-body PET: potential for improving tumour detectability.

Abstract: Total-body positron emission tomography (PET) is a useful diagnostic tool for evaluating malignant disease. However, tumour detection is limited by image artefacts due to the lack of attenuation correction and noise. Attenuation correction may be possible using transmission data acquired after or simultaneously with emission data. Despite the elimination of attenuation artefacts, however, tumour detection is still hampered by noise, which is amplified during image reconstruction by filtered backprojection (FBP). The authors have investigated, as an alternative to FBP, an accelerated expectation maximization (EM) algorithm for its potential to improve tumour detectability in total-body PET. Signal to noise ratio (SNR), calculated for a tumour with respect to the surrounding background, is used as a figure of merit. A software tumour phantom, with conditions typical of those encountered in a total-body PET study using simultaneous acquisition, is used to optimize and compare various reconstruction approaches. Accelerated EM reconstruction followed by two-dimensional filtering is shown to yield significantly higher SNR than FBP for a range of tumour sizes, concentrations and counting statistics ( Delta SNR=6.3+or-3.9, p<0.001). The methods developed are illustrated by examples derived from physical phantom and patient data.

The application of breath hold phase velocity mapping techniques to the measurement of coronary artery blood flow velocity: phantom data and initial in vivo results.

Abstract: A segmented kappa-space breath hold phase velocity mapping technique has been developed for the study of coronary artery blood flow velocity. In vitro validation has been performed using a number of pulsatile flow phantoms and the accuracy of the technique for determining the velocity increase at the site of a stenosis demonstrated in several phantom models. Examples of both in-plane and through-plane velocity maps of the left anterior descending and right coronary arteries of normal subjects in early diastole are presented. In one subject, through-plane velocity maps were obtained in the right and left anterior descending arteries throughout the cardiac cycle in order to generate flow velocity time curves. The problems associated with coronary artery velocity mapping are discussed.

Fat-suppressed spoiled GRASS imaging of knee hyaline cartilage: technique optimization and comparison with conventional MR imaging.

Abstract: We studied healthy volunteers with fat-suppressed three-dimensional (3D) spoiled gradient-recalled acquisition in the steady state (SPGR) to determine parameters that maximize positive contrast between knee articular cartilage and fluid, marrow fat, and muscle; and we compared the technique with conventional MR imaging sequences. The purpose was to determine if fat-suppressed 3D SPGR imaging is useful for detecting abnormalities of the articular cartilages.The knees of 10 healthy volunteers were imaged in the axial plane. Fat-suppressed 3D SPGR imaging was performed with a TR of 60 msec, a TE ranging from 5 to 15 msec, and a flip angle ranging from 20 degrees to 80 degrees. This was followed by a similar set of fat-suppressed two-dimensional (2D) SPGR images, and conventional T1- and T2-weighted spin-echo and multiplanar gradient studies. Contrast-to-noise (C/N) ratios were determined for cartilage versus a saline fluid phantom, marrow fat, and muscle. Optimal parameters were determined both quantitativel...

Computer‐assisted identification and quantification of multiple sclerosis lesions in MR imaging volumes in the brain

Abstract: Magnetic resonance (MR) imaging is the principal imaging technique for the diagnosis of multiple sclerosis (MS). However, quantifying the number and extent of lesions on MR images manually is arduous. The authors have developed a computerized three-dimensional (3D) quantitative system to assist in the identification and analysis of MS lesions in proton-density (PD)- and T2-weighted volumes of the head. The system provides intuitive, interactive operations that allow flexible extraction of information from the data. Use of the system to analyze MR examinations of a phantom containing regular "lesions" showed that accurate (average error, 1 cm3) measurements of objects less than 7 cm3 is possible, and that an estimate of the quantization error predicted the uncertainty in the volume. Analysis of four MR examinations of a chronic-progressive MS patient conducted over an 18-month period was performed. A two-dimensional histogram showing the frequency of voxels with particular PD- and T2-weighted intensities revealed a distinct cluster only in histograms of sections that contained lesions. Measurements and 3D volume rendering of lesions clearly showed changes in lesion shape, position, and size.

Triple energy window scatter correction technique in PET

Abstract: A practical triple energy window technique (TEW) is proposed, which is based on using the information in two lower energy windows and one single calibration, to estimate the scatter within the photopeak window. The technique is basically a conventional dual-window technique plus a modification factor, which can partially compensate object-distribution dependent scatters. The modification factor is a function of two lower scatter windows of both the calibration phantom and the actual object. In order to evaluate the technique, a Monte Carlo simulation program, which simulates the PENN-PET scanner geometry, was used. Different phantom activity distributions and phantom sizes were tested to simulate brain studies, including uniform and nonuniform distributions. The results indicate that the TEW technique works well for a wide range of activity distributions and object sizes. The comparisons between the TEW and dual window techniques show better quantitative accuracy for the TEW, especially for different phantom sizes. The technique is also applied to experimental data from a PENN-PET scanner to test its practicality. >

Planar Gamma Camera Imaging and Quantitation of Yttrium-90 Bremsstrahlung

Abstract: Yttrium-90 is a promising radionuclide for radioimmunotherapy of cancer because of its energetic beta emissions. Therapeutic management requires quantitative imaging to assess the pharmacokinetics and radiation dosimetry of the {sup 90}Y-labeled antibody. Conventional gamma photon imaging methods cannot be easily applied to imaging of {sup 90}Y-bremsstrahlung because of its continuous energy spectrum. The sensitivity, resolution and source-to-background signal ratio (S/B) of the detector system for {sup 90}Y-bremsstrahlung were investigated for various collimators and energy windows in order to determine optimum conditions for quantitative imaging. After these conditions were determined, the accuracy of quantitation of {sup 90}Y activity in an Alderson abdominal phantom was examined. When the energy-window width was increased, the benefit of increased sensitivity outweighed degradation in resolution and S/B ratio until the manufacturer`s energy specifications for the collimator were exceeded. Using the same energy window, the authors improved resolution and S/B for the medium-energy (ME) collimator when compared to the low-energy, all-purpose (LEAP) collimator, and there was little additional improvement using the high-energy (HE) collimator. Camera sensitivity under tissue equivalent conditions was 4.2 times greater for the LEAP and 1.7 times greater for the ME collimators when compared to the HE collimator. Thus, the best, most practical selections more » were found to be the ME collimator and an energy window of 55-285 keV. When they used these optimal conditions for image acquisition, the estimation of {sup 90}Y activity in organs and tumors was within 15% of the true activities. The results for this study suggest that reasonable accuracy can be achieved in clinical radioimmunotherapy using {sup 90}Y-bremsstrahlung quantitation. 28 refs., 5 figs., 7 tabs. « less

Off-resonance spin locking for MR imaging

Abstract: Off-resonance spin locking is investigated as a low power method for achieving low field spin-lattice relaxation contrast using high field clinical MR imaging systems (e.g., 1.5 tesla). Spin-lattice relaxation times and equilibrium magnetizations in the off-resonance rotating frame (T1 rho(off), beta) were measured for tissue-mimicking phantom materials as a function of the ratio of the amplitude to the resonance offset of the spin-locking pulse (f1/delta). The phantom materials consisted of vegetable oil to simulate fat and two different gels containing 2% and 4% agar to simulate nonfatty tissues with different macromolecular compositions. These measurements were used to verify a signal strength equation for a multislice off-resonance spin-locking technique implemented on a clinical MR imaging system operating at 1.5 tesla. Although the oil showed little change in image contrast with increasing f1/delta, the two gels demonstrated a strong variation which provided improved discrimination compared to T1-weighted imaging. Off-resonance spin locking is suggested as a method for improving delineation of breast lesions and a preliminary clinical example from a patient volunteer is presented.

Artifacts in computational optical-sectioning microscopy

Abstract: We tested the most complete optical model available for computational optical-sectioning microscopy and obtained four main results. First, we observed good agreement between experimental and theoretical point-spread functions (PSF's) under a variety of imaging conditions. Second, using these PSF's, we found that a linear restoration method yielded reconstructed images of a well-defined phantom object (a 10-microns-diameter fluorescent bead) that closely resembled the theoretically determined, best-possible linear reconstruction of the object. Third, this best linear reconstruction suffered from a (to our knowledge) previously undescribed artifactual axial elongation whose principal cause was not increased axial blur but rather the conical shape of the null space intrinsic to nonconfocal three-dimensional (3D) microscopy. Fourth, when 10-microns phantom beads were embedded at different depths in a transparent medium, reconstructed bead images were progressively degraded with depth unless they were reconstructed with use of a PSF determined at the bead's depth. We conclude that (1) the optical model for optical sectioning is reasonably accurate; (2) if PSF shift variance cannot be avoided by adjustment of the optics, then reconstruction methods must be modified to account for this effect; and (3) alternative microscopical or nonlinear algorithmic approaches are required for overcoming artifacts imposed by the missing cone of frequencies that is intrinsic to nonconfocal 3D microscopy.

Speckle motion artifact under tissue rotation

Abstract: Speckle patterns in ultrasound images may move in a way which bears no simple relationship to the motion of the corresponding tissues. In some instances the speckle motion replicates the underlying tissue motion, in others it does not. The authors name "speckle motion artifact" the difference between the speckle and the underlying tissue motion. An echographic image formation model is used to study the motion artifact produced by a rotating phantom and observed by a linear scan imaging system with a Gaussian beam. The authors propose that when the tissue is modeled as a random array of small and numerous scatterers, such motion aberration be accounted for by the 2D phase characteristics of the imaging system. An analytic prediction of this motion artifact in relation to the imaging system characteristics (beam width, transducer frequency, pulse duration) is presented. It is shown that the artifact results from the curvature of the system point spread function, which in turn determines the curvature of the 2D phase characteristics. To the authors' knowledge, it is the first time a comprehensive model of ultrasonic speckle motion artifact is presented. The model has been developed to study rotation-induced artifact; the method is however quite general and can be extended to study the effects of other tissue motion, in particular deformation and shear. >