Showing papers on "Imaging phantom published in 2001"

A novel approach to overcome hypoxic tumor resistance: Cu-ATSM-guided intensity-modulated radiation therapy.

Abstract: Purpose: Locoregional tumor control for locally advanced cancers with radiation therapy has been unsatisfactory. This is in part associated with the phenomenon of tumor hypoxia. Assessing hypoxia in human tumors has been difficult due to the lack of clinically noninvasive and reproducible methods. A recently developed positron emission tomography (PET) imaging-based hypoxia measurement technique which employs a Cu(II)-diacetyl-bis( N 4 -methylthiosemicarbazone) (Cu-ATSM) tracer is of great interest. Oxygen electrode measurements in animal experiments have demonstrated a strong correlation between low tumor pO 2 and excess 60 Cu-ATSM accumulation. Intensity-modulated radiation therapy (IMRT) allows selective targeting of tumor and sparing of normal tissues. In this study, we examined the feasibility of combining these novel technologies to develop hypoxia imaging (Cu-ATSM)-guided IMRT, which may potentially deliver higher dose of radiation to the hypoxic tumor subvolume to overcome inherent hypoxia-induced radioresistance without compromising normal tissue sparing. Methods and Materials: A custom-designed anthropomorphic head phantom containing computed tomography (CT) and positron emitting tomography (PET) visible targets consisting of plastic balls and rods distributed throughout the "cranium" was fabricated to assess the spatial accuracy of target volume mapping after multimodality image coregistration. For head-and-neck cancer patients, a CT and PET imaging fiducial marker coregistration system was integrated into the thermoplastic immobilization head mask with four CT and PET compatible markers to assist image fusion on a Voxel-Q treatment-planning computer. This system was implemented on head-and-neck cancer patients, and the gross tumor volume (GTV) was delineated based on physical and radiologic findings. Within GTV, regions with a 60 Cu-ATSM uptake twice that of contralateral normal neck muscle were operationally designated as ATSM-avid or hypoxic tumor volume ( h GTV) for this feasibility study. These target volumes along with other normal organs contours were defined and transferred to an inverse planning computer (Corvus, NOMOS) to create a hypoxia imaging-guided IMRT treatment plan. Results: A study of the accuracy of target volume mapping showed that the spatial fidelity and imaging distortion after CT and PET image coregistration and fusion were within 2 mm in phantom study. Using fiducial markers to assist CT/PET imaging fusion in patients with carcinoma of the head-and-neck area, a heterogeneous distribution of 60 Cu-ATSM within the GTV illustrated the success of 60 Cu-ATSM PET to select an ATSM-avid or hypoxic tumor subvolume ( hGTV ). We further demonstrated the feasibility of Cu-ATSM-guided IMRT by showing an example in which radiation dose to the hGTV could be escalated without compromising normal tissue (parotid glands and spinal cord) sparing. The plan delivers 80 Gy in 35 fractions to the ATSM-avid tumor subvolume and the GTV simultaneously receives 70 Gy in 35 fractions while more than one-half of the parotid glands are spared to less than 30 Gy. Conclusion: We demonstrated the feasibility of a novel Cu-ATSM-guided IMRT approach through coregistering hypoxia 60 Cu-ATSM PET to the corresponding CT images for IMRT planning. Future investigation is needed to establish a clinical-pathologic correlation between 60 Cu-ATSM retention and radiation curability, to understand tumor re-oxygenation kinetics, and tumor target uncertainty during a course of radiation therapy before implementing this therapeutic approach to patients with locally advanced tumor.

Rigid registration of 3-D ultrasound with MR images: a new approach combining intensity and gradient information

Abstract: Presents a new image-based technique to rigidly register intraoperative three-dimensional ultrasound (US) with preoperative magnetic resonance (MR) images. Automatic registration is achieved by maximization of a similarity measure which generalizes the correlation ratio, and whose novelty is to incorporate multivariate information from the MR data (intensity and gradient). In addition, the similarity measure is built upon a robust intensity-based distance measure, which makes it possible to handle a variety of US artifacts. A cross-validation study has been carried out using a number of phantom and clinical data. This indicates that the method is quite robust and that the worst registration errors are of the order of the MR image resolution.

Strain rate imaging using two-dimensional speckle tracking

Abstract: Strain rate images (SRI) of the beating heart have been proposed to identify non-contracting regions of myocardium. Initial attempts used spatial derivatives of tissue velocity (Doppler) signals. Here, an alternate method is proposed based on two-dimensional phase-sensitive speckle tracking applied to very high frame rate, real-time images. This processing can produce high resolution maps of the time derivative of the strain magnitude (i.e., square root of the strain intensity). Such images complement traditional tissue velocity images (TVI), providing a more complete description of cardiac mechanics. To test the proposed approach, SRI were both simulated and measured on a thick-walled, cylindrical, tissue-equivalent phantom modeling cardiac deformations. Real-time ultrasound images were captured during periodic phantom deformation, where the period was matched to the data capture rate of a commercial scanner mimicking high frame rate imaging of the heart. Simulation results show that SRI with spatial resolution between 1 and 2 mm are possible with an array system operating at 5 MHz. Moreover, these images are virtually free of angle-dependent artifacts present in TVI and simple strain rate maps derived from these images. Measured results clearly show that phantom regions of low deformation, which are difficult to identify on tissue velocity-derived SRI, are readily apparent with SRI generated from two-dimensional phase-sensitive speckle tracking.

Real-time spatial compound imaging : Application to breast, vascular, and musculoskeletal ultrasound

Abstract: Real-time spatial compound imaging (SonoCT) is an ultrasound technique that uses electronic beam steering of a transducer array to rapidly acquire several (three to nine) overlapping scans of an object from different view angles These single-angle scans are averaged to form a multiangle compound image that is updated in real time with each subsequent scan Compound imaging shows improved image quality compared with conventional ultrasound, primarily because of reduction of speckle, clutter, and other acoustic artifacts Early clinical experience suggests that real-time spatial compound imaging can provide improved contrast resolution and tissue differentiation that is beneficial for imaging the breast, peripheral blood vessels, and musculoskeletal injuries Future development of real-time spatial compound imaging will help address the bulk of general imaging applications by extending this technology to curved array transducers, tissue harmonics, panoramic imaging, and three-dimensional sonography

Specific coil design for SENSE: a six-element cardiac array.

Abstract: In sensitivity encoding (SENSE), the effects of inhomogeneous spatial sensitivity of surface coils are utilized for signal localization in addition to common Fourier encoding using magnetic field gradients. Unlike standard Fourier MRI, SENSE images exhibit an inhomogeneous noise distribution, which crucially depends on the geometrical sensitivity relations of the coils used. Thus, for optimum signal-to-noise-ratio (SNR) and noise homogeneity, specialized coil configurations are called for. In this article we study the implications of SENSE imaging for coil layout by means of simulations and imaging experiments in a phantom and in vivo. New, specific design principles are identified. For SENSE imaging, the elements of a coil array should be smaller than for common phased-array imaging. Furthermore, adjacent coil elements should not overlap. Based on the findings of initial investigations, a configuration of six coils was designed and built specifically for cardiac applications. The in vivo evaluation of this array showed a considerable SNR increase in SENSE images, as compared with a conventional array. Magn Reson Med 45:495–504, 2001. © 2001 Wiley-Liss, Inc.

Computer controlled guidance of a biopsy needle

Abstract: A computer controlled system for guiding the needle device, such as a biopsy needle, by reference to a single mode medical imaging system employing any one of computed tomography imaging (CTI) equipment, magnetic resonance imaging equipment (MRI), fluoroscopic imaging equipment, or 3D ultrasound system, or alternatively, by reference to a multi-modal imaging system, which includes any combination of the aforementioned systems. The 3D ultrasound system includes a combination of an ultrasound probe and both passive and active infrared tracking systems so that the combined system enables a real time image display of the entire region of interest without probe movement.

Dosimetric investigation and portal dose image prediction using an amorphous silicon electronic portal imaging device.

Abstract: A two step algorithm to predict portal dose images in arbitrary detector systems has been developed recently. The current work provides a validation of this algorithm on a clinically available, amorphous silicon flat panel imager. The high-atomic number, indirect amorphous silicon detector incorporates a gadolinium oxysulfide phosphor scintillating screen to convert deposited radiation energy to optical photons which form the portal image. A water equivalent solid slab phantom and an anthropomorphic phantom were examined at beam energies of 6 and 18 MV and over a range of air gaps (approximately 20-50 cm). In the many examples presented here, portal dose images in the phosphor were predicted to within 5% in low-dose gradient regions, and to within 5 mm (isodose line shift) in high-dose gradient regions. Other basic dosimetric characteristics of the amorphous silicon detector were investigated, such as linearity with dose rate (+/- 0.5%), repeatability (+/- 2%), and response with variations in gantry rotation and source to detector distance. The latter investigation revealed a significant contribution to the image from optical photon spread in the phosphor layer of the detector. This phenomenon is generally known as "glare," and has been characterized and modeled here as a radially symmetric blurring kernel. This kernel is applied to the calculated dose images as a convolution, and is successfully demonstrated to account for the optical photon spread. This work demonstrates the flexibility and accuracy of the two step algorithm for a high-atomic number detector. The algorithm may be applied to improve performance of dosimetric treatment verification applications, such as direct image comparison, backprojected patient dose calculation, and scatter correction in megavoltage computed tomography. The algorithm allows for dosimetric applications of the new, flat panel portal imager technology in the indirect configuration, taking advantage of a greater than tenfold increase in detector sensitivity over a direct configuration.

A parallel-detection frequency-domain near-infrared tomography system for hemoglobin imaging of the breast in vivo

Abstract: A novel near-infrared frequency-domain system designed for tomographic breast imaging is described. The setup utilizes five optical wavelengths, from 660 to 826 nm, and parallel detection with 16 photomultiplier tubes. Direct fiberoptic coupling with the tissue is achieved with a high precision positioning device using 16 motorized actuators (0.5 μm precision) arranged radially in a circular geometry. Images of breast tissue optical absorption and reduced scattering coefficients are obtained using a Newton-type reconstruction algorithm to solve for the optimal fit between the measurement data and predicted data from a finite element solution to the frequency-domain diffusion equation. The design, calibration, and performance of the tomographic imaging system are detailed. Data acquisition from the system requires under 30 s for a single tomographic slice at one optical wavelength with a measurement repeatability for a single phantom on average of 0.5% in ac intensity and 0.4° in phase. Absorbing and scatt...

Determination of the presampled MTF in computed tomography.

Abstract: A technique for measuring the presampled MTF in CT scanners is described. The technique uses a simple phantom consisting of approximately 0.050 mm aluminum foil sandwiched by flat plastic or tissue-equivalent slabs. The aluminum foil is slightly angled with respect to the reconstruction matrix, and CT images are acquired. The acquired CT image yields an angled slit image that can be used to synthesize the presampled line spread function (LSF). The presampled MTF is calculated from the presampled LSF. The technique is a direct extension of that proposed by Fujita et al. [IEEE Trans. Med. Imaging 11, 34-39 (1992)] for MTF calculation on digital radiography images. While the MTF in clinical CT scanners often reaches negligible amplitude below the Nyquist frequency, the technique is easy to implement, requires inexpensive materials, is robust to aliasing, and is more resilient to noise due to greater data averaging than conventional PSF-integration techniques. Use of the proposed technique is illustrated on a clinical multiple detector array scanner, and MTFs are shown for several common reconstruction kernels. It is likely that the proposed technique would be useful for all tomographic imaging systems, including single photon emission computed tomography, positron emission tomography, magnetic resonance imaging and ultrasound scanners.

Vessel surface reconstruction with a tubular deformable model

Abstract: Three-dimensional (3-D) angiographic methods are gaining acceptance for evaluation of atherosclerotic disease. However, measurement of vessel stenosis from 3-D angiographic methods can be problematic due to limited image resolution and contrast. We present a method for reconstructing vessel surfaces from 3-D angiographic methods that allows for objective measurement of vessel stenosis. The method is a deformable model that employs a tubular coordinate system. Vertex merging is incorporated into the coordinate system to maintain even vertex spacing and to avoid problems of self-intersection of the surface. The deformable model was evaluated on clinical magnetic resonance (MR) images of the carotid (n=6) and renal (n=2) arteries, on an MR image of a physical vascular phantom and on a digital vascular phantom. Only one gross error occurred for all clinical images. All reconstructed surfaces had a realistic, smooth appearance. For all segments of the physical vascular phantom, vessel radii from the surface reconstruction had an error of less than 0.2 of the average voxel dimension. Variability of manual initialization of the deformable model had negligible effect on the measurement of the degree of stenosis of the digital vascular phantom.

Development and characteristics of a biological tissue-equivalent phantom for microwaves

Abstract: Various phantoms (simulated biological bodies) have been proposed as a biological model for studies of electromagnetic effects on the human body. This paper reports the characteristics of the phantom developed by the authors that realized electrical characteristics equivalent to the biological body. Examples of its communication and clinical applications are presented. The present phantom is made of agar, polyethylene powder, sodium chloride, TX-151, preservative, and deionized water and simulates the relative permittivity and conductivity of a high-water-content tissue. In the present phantom, electrical characteristics almost equal to those in the biological tissue are realized with a single composition ratio over the frequency range of 300 MHz to 2.5 GHz. It is also possible to simulate the electrical characteristics of an arbitrary high-water-content tissue by adjustment of the composition. No special equipment is needed for fabrication and the preservation is easy. Further, as examples of applications of this phantom to the human body, the SAR measurement examples are presented in the COST 244 human head model and coaxial-slot antenna for hyperthermia. The present phantom is useful as a human model to study the mutual effects of the human body and electromagnetic waves. © 2000 Scripta Technica, Electron Comm Jpn Pt 1, 84(4): 67–77, 2001

Sensitivity-encoded spectroscopic imaging.

Abstract: Sensitivity encoding (SENSE) offers a new, highly effective approach to reducing the acquisition time in spectroscopic imaging (SI). In contrast to conventional fast SI techniques, which accelerate k-space sampling, this method permits reducing the number of phase encoding steps in each phase encoding dimension of conventional SI. Using a coil array for data acquisition, the missing encoding information is recovered exploiting knowledge of the distinct spatial sensitivities of the individual coil elements. In this work, SENSE is applied to 2D spectroscopic imaging. Fourfold reduction of scan time is achieved at preserved spectral and spatial resolution, maintaining a reasonable SNR. The basic properties of the proposed method are demonstrated by phantom experiments. The in vivo feasibility of SENSE-SI is verified by metabolic imaging of N-acetylaspartate, creatine, and choline in the human brain. These results are compared to conventional SI, with special attention to the spatial response and the SNR.

Evaluation of inverse methods and head models for EEG source localization using a human skull phantom.

Abstract: We used a real-skull phantom head to investigate the performances of representative methods for EEG source localization when considering various head models. We describe several experiments using a montage with current sources located at multiple positions and orientations inside a human skull filled with a conductive medium. The robustness of selected methods based on distributed source models is evaluated as various solutions to the forward problem (from the sphere to the finite element method) are considered. Experimental results indicate that inverse methods using appropriate cortex-based source models are almost always able to locate the active source with excellent precision, with little or no spurious activity in close or distant regions, even when two sources are simultaneously active. Superior regularization schemes for solving the inverse problem can dramatically help the estimation of sparse and focal active zones, despite significant approximation of the head geometry and the conductivity properties of the head tissues. Realistic head models are necessary, though, to fit the data with a reasonable level of residual variance.

Study of the efficacy of respiratory gating in myocardial SPECT using the new 4-D NCAT phantom

Abstract: Respiratory motion can cause artifacts in myocardial single photon emission computed tomography (SPECT) images, which can lead to the misdiagnosis of cardiac diseases. One method to correct for respiratory artifacts is through respiratory gating. We study the effectiveness of respiratory gating through a simulation study using the newly developed four-dimensional (4-D) NURBS-based cardiac-torso (NCAT) phantom. The organ shapes in the 4-D NCAT phantom are formed using nonuniform rational b-splines (NURBS) and are based on detailed human image data. With its basis on actual human data, the 4-D NCAT phantom realistically simulates human anatomy and motions such as the cardiac and respiratory motions. With the 4-D NCAT phantom, we generated 128 phantoms over one respiratory cycle (5 s per cycle) with the diaphragm and heart set to move a total of 4 cm from end-inspiration to end-expiration. The heart was set to beat with a normal contractile motion at a rate of I beat per second resulting in a total of five heart cycles. We divide the respiratory cycle into different numbers of respiratory gates (16, 8, and 4) by summing the phantoms. For each gate, we generate its projection data using an analytical projection algorithm simulating the effects of attenuation, scatter, and detector response. We then reconstruct the projections using an iterative OS-EM algorithm compensating for the three effects. The reconstructed images for each gating method were examined for artifacts due to the respiratory motion during that gate. We found that respiratory artifacts are significantly reduced if the respiratory motion of the heart that occurs during a gating time period is 1 cm or less. We conclude that respiratory gating is an effective method for reducing effects due to respiration. The timing of the respiratory gates for reduced image artifacts is dependent on the extent of the heart's motion during respiration. Index Terms-Biomedical image processing, biomedical nuclear imaging, image analysis, motion compensation, respiratory system, simulation software, single photon emission computed tomography (SPECT).

Three-dimensional time-resolved optical tomography of a conical breast phantom

Abstract: A 32-channel time-resolved imaging device for medical optical tomography has been employed to evaluate a scheme for imaging the human female breast. The fully automated instrument and the reconstruction procedure have been tested on a conical phantom with tissue-equivalent optical properties. The imaging protocol has been designed to obviate compression of the breast and the need for coupling fluids. Images are generated from experimental data with an iterative reconstruction algorithm that employs a three-dimensional (3D) finite-element diffusion-based forward model. Embedded regions with twice the background optical properties are revealed in separate 3D absorption and scattering images of the phantom. The implications for 3D time-resolved optical tomography of the breast are discussed.

Construction of a computed tomographic phantom for a Japanese male adult and dose calculation system.

Abstract: Computational human phantoms have been widely used to estimate organ doses and other dosimetric quantities related to the human body where direct measurements are difficult to perform. In recent years, voxel phantoms (voxel = volume element) based on computed tomographic (CT) data of real persons have been constructed which provide a realistic description of the human anatomy. A CT phantom of a Japanese male adult with an average body size was developed as the first Asian voxel phantom. The segmented phantom consists of more than 100 regions enabling the calculation of doses for various parts of the body. The bone marrow distribution was precisely modelled according to the CT values. The EGS4 Monte Carlo transport code was combined with the phantom to calculate organ doses for external exposure due to photons and electrons up to 1 TeV. The calculated organ doses were compared with respective data using MIRD-type mathematical phantoms. In some cases, significant discrepancies in doses were observed, demonstrating the necessity of sophisticated models for accurate dose calculations.

Evaluation of high-resolution pinhole SPECT using a small rotating animal.

Abstract: Ex vivo measurements in animals are used frequently in the field of nuclear medicine for the characterization of newly developed radioligands and for drug development. In vivo SPECT would replace these ex vivo measurements in a relatively large number of cases if one were able to adequately image small organs. The pinhole collimator has been used extensively to obtain greater detail in planar imaging. However, using a pinhole collimator for SPECT is difficult because it requires a heavy collimated detector to rotate around a small object with a constant radius of rotation. Methods: We have developed a mechanism in which the gantry and collimator are fixed and the animal rotates. Hollow cylinders of different sizes were made to enable imaging of small animals of different sizes: mice, hamsters, and rats. The cylinder is mounted on a stepping motor-driven system and positioned exactly above the pinhole collimator of an ARC3000 camera with a 1-mm pinhole insert. The stepping motor is controlled by the Hermes acquisition/ processing system. After imaging each projection, a signal is given to rotate the stepping motor with the desired number of angular degrees. Filtered backprojection, adapted to pinhole SPECT, was used for reconstruction. The system allows adjustments of the radius of rotation and along the axis of the cylinder to select the field of view. Calibration experiments were performed to ensure that the axis of rotation was exactly in the middle of the cylinder. Phantom experiments were performed to assess sensitivity, spatial resolution, and uniformity of the system and to test the system for distortion artifacts. In addition, a brain dopamine transporter rat study and a hamster myocardial study were performed to test the clinical feasibility of the entire system. Results: In the line source experiment, the spatial resolution obtained in air was 1.3 mm full width at half maximum, with a radius of rotation of 33 mm. Furthermore, the system has good uniformity and is capable of detecting cold spots of 2-mm diameter. The animal studies showed that it was feasible to image receptors or transporters and organs with sufficient detail in a practical setup. Conclusion: A rotating cylinder mechanism for pinhole SPECT is feasible and shows the same characteristics as conventional pinhole SPECT with a rotating camera head, without distortion artifacts. This mechanism permits pinhole SPECT to replace many ex vivo animal experiments. During the development of new radiopharmaceuticals, the biologic behavior and dosimetry are studied frequently by ex vivo measurements in animals (1,2). However, the benefits of an in vivo technique, such as SPECT, are numerous. SPECT imaging offers the opportunity to obtain measurements at more than 1 point in time after administration of a radiopharmaceutical, which is a significant benefit for the determination of temporal behavior. Likewise, SPECT imaging can be repeated over a long time period on the same animal using serial injections of the radiopharmaceutical, which simplifies the assessment of drug therapy effects. The number of animals needed with an in vivo approach is far less than the number required for ex vivo measurements, in which the animal has to be killed for a single measurement. The smaller number of animals required with an in vivo approach is beneficial from an ethical point of view and may also save research time and, therefore, may be more cost-effective. The cost-effectiveness is an important aspect, especially when genetically manipulated animals are used.

LV volume quantification via spatiotemporal analysis of real-time 3-D echocardiography

Abstract: This paper presents a method of four-dimensional (4-D) (3-D+Time) space-frequency analysis for directional denoising and enhancement of real-time three-dimensional (RT3D) ultrasound and quantitative measures in diagnostic cardiac ultrasound. Expansion of echocardiographic volumes is performed with complex exponential wavelet-like basis functions called brushlets. These functions offer good localization in time and frequency and decompose a signal into distinct patterns of oriented harmonics, which are invariant to intensity and contrast range. Deformable-model segmentation is carried out on denoised data after thresholding of transform coefficients. This process attenuates speckle noise while preserving cardiac structure location. The superiority of 4-D over 3-D analysis for decorrelating additive white noise and multiplicative speckle noise on a 4-D phantom volume expanding in time is demonstrated. Quantitative validation, computed for contours and volumes, is performed on in vitro balloon phantoms. Clinical applications of this spatiotemporal analysis tool are reported for six patient cases providing measures of left ventricular volumes and ejection fraction.

Beam calibration without a phantom for creating a 3-D freehand ultrasound system.

Abstract: To create a freehand three-dimensional (3-D) ultrasound (US) system for image-guided surgical procedures, an US beam calibration process must be performed. The calibration method presented in this work does not use a phantom to define in 3-D space the pixel locations in the beam. Rather, the described method is based on the spatial relationship between an optically tracked pointer and a similarly tracked US transducer. The pointer tip was placed into the US beam, and US images, physical coordinates of the pointer and the transducer location were simultaneously recorded. US image coordinates of the pointer were mapped to the physical points using two different registration methods. Two sensitivity studies were performed to determine the location and number of points needed to calibrate the beam accurately. Results showed that the beam is most efficiently calibrated with approximately 20 points collected from throughout the beam. This method of beam calibration proved to be highly accurate, yielding registration errors of approximately 0.4 mm.

A fast calibration method for 3-D tracking of ultrasound images using a spatial localizer

Abstract: We have developed a fast calibration method for computing the position and orientation of 2-D ultrasound (US) images in 3-D space where a position sensor is mounted on the US probe. This calibration is required in the fields of 3-D ultrasound and registration of ultrasound with other imaging modalities. Most of the existing calibration methods require a complex and tedious experimental procedure. Our method is simple and it is based on a custom-built phantom. Thirty N-fiducials (markers in the shape of the letter “N”) embedded in the phantom provide the basis for our calibration procedure. We calibrated a 3.5-MHz sector phased-array probe with a magnetic position sensor, and we studied the accuracy and precision of our method. A typical calibration procedure requires approximately 2 min. We conclude that we can achieve accurate and precise calibration using a single US image, provided that a large number (approximately ten) of N-fiducials are captured within the US image, enabling a representative sampling of the imaging plane. (E-mail: ykim@u.washington.edu)

Frequency decomposition and compounding of ultrasound medical images with wavelet packets

Abstract: Ultrasound beams propagating in biological tissues undergo distortions due to local inhomogeneities of the acoustic parameters and the nonlinearity of the medium. The spectral analysis of the radio-frequency (RF) backscattered signals may yield important clinical information in the field of tissue characterization, as well as enhancing the detectability of tissue parenchymal diseases. Here, the authors propose a new tissue spectral imaging technique based on the wavelet packets (WP) decomposition. In a conventional ultrasound imaging system, the received echo-signals are generally decimated to generate a medical image, with a loss of information. With the proposed approach, all the RF data are processed to generate a set of frequency subband images. The ultrasound echo signals are simultaneously frequency decomposed and decimated, by using two quadrature mirror filters, followed by a dyadic subsampling. In addition, to enhance the lesion detectability and the image quality, the authors apply a nonlinear filter to reduce noise in each subband image. The proposed method requires simple additional signal processing and it can be implemented on any real-time imaging system. The frequency subband images, which are available simultaneously, can be either used in a multispectral display or summed up together to reduce speckle noise. To localize the different frequency response in the tissues, the authors propose a multifrequency display method where 3 different subband images, chosen among those available, are encoded as red, green, and blue intensities (RGB) to create a false-colored RGB image. According to the clinical application, different choices can evidence different spectral proprieties in the biological tissue under investigation. To enhance the lesion contrast in a grey-level image, one of the possible methods is the summation of the images obtained from narrow frequency subbands, according to the frequency compounding technique. The authors show that by adding the denoised subband images created with the WP decomposition, the contrast-to-noise ratio in 2 phantom images is largely increased.

Multimodality image registration quality assurance for conformal three-dimensional treatment planning

Abstract: Purpose: We present a quality assurance methodology to determine the accuracy of multimodality image registration and fusion for the purpose of conformal three-dimensional and intensity-modulated radiation therapy treatment planning. Registration and fusion accuracy between any combination of computed tomography (CT), magnetic resonance (MR), and positron emission computed tomography (PET) imaging studies can be evaluated. Methods and Materials: A commercial anthropomorphic head phantom filled with water and containing CT, MR, and PET visible targets was modified to evaluate the accuracy of multimodality image registration and fusion software. For MR and PET imaging, the water inside the phantom was doped with CuNO3 and 18F-fluorodeoxyglucose (18F-FDG), respectively. Targets consisting of plastic spheres and pins were distributed throughout the cranium section of the phantom. Each target sphere had a conical-shaped bore with its apex at the center of the sphere. The pins had a conical extension or indentation at the free end. The contours of the spheres, sphere centers, and pin tips were used as anatomic landmark models for image registration, which was performed using affine coordinate-transformation tools provided in a commercial multimodality image registration/fusion software package. Four sets of phantom image studies were obtained: primary CT, secondary CT with different phantom immobilization, MR, and PET study. A novel CT, MR, and PET external fiducial marking system was also tested. Results: The registration of CT/CT, CT/MR, and CT/PET images allowed correlation of anatomic landmarks to within 2 mm, verifying the accuracy of the registration software and spatial fidelity of the four multimodality image sets. Conclusions: This straightforward phantom-based quality assurance of the image registration and fusion process can be used in a routine clinical setting or for providing a working image set for development of the image registration and fusion process and new software.

A phantom study of the geometric accuracy of computed tomographic and magnetic resonance imaging stereotactic localization with the Leksell stereotactic system.

Abstract: OBJECTIVE: To assess the spatial accuracy of magnetic resonance imaging (MRI) and computed tomographic stereotactic localization with the Leksell stereotactic system. METHODS: The phantom was constructed in the shape of a box, 164 mm in each dimension, with three perpendicular arrays of solid acrylic rod, 5 mm in diameter and spaced 30 mm apart within the phantom. In this study, images from two different MRI scanners and a computed tomographic scanner were obtained using the same Leksell (Elekta Instruments, Stockholm, Sweden) head frame placement. The coordinates of the rod images in the three principal planes were measured by using a tool provided with Leksell GammaPlan software (Elekta Instruments, Norcross, GA) and were compared with the physical phantom measurements. RESULTS: The greatest distortion was found around the periphery, and the least distortion (<1.5 mm) was present in the middle and most other areas of the phantom. In the phantom study using computed tomography, the mean values of the maximum errors for the x, y, and z axes were 1.0 mm (range, 0.2-1.3 mm), 0.4 mm (range, 0.1-0.8 mm), and 3.8 mm (range, 1.9-5.1 mm), respectively. The mean values of the maximum errors when using the Philips MRI scanner (Philips Medical Systems, Shelton, CT) were 0.9 mm (range, 0.4-1.7 mm), 0.2 mm (range, 0.0-0.7 mm), and 1.9 mm (range, 1.3-2.3 mm), respectively. Using the Siemens MRI scanner (Siemens Medical Systems, New York, NY), these values were 0.4 mm (range, 0.0-0.7 mm), 0.6 mm (range, 0.0-1.0 mm), and 1.6 mm (range, 0.8-2.0 mm), respectively. The geometric accuracy of the MRI scans when using the Siemens scanner was greatly improved after the implementation of a new software patch provided by the manufacturer. The accuracy also varied with the direction of phase encoding. CONCLUSION: The accuracy of target localization for most intracranial lesions during stereotactic radiosurgery can be achieved within the size of a voxel, especially by using the Siemens MRI scanner at current specifications and with a new software patch. However, caution is warranted when imaging peripheral lesions, where the distortion is greatest.

Intraluminal fiber-optic Doppler imaging catheter for structural and functional optical coherence tomography

Abstract: We describe a miniature fiber-optic Doppler imaging catheter for integrated functional and structural optical coherence tomography (OCT) imaging. The Doppler catheter can map blood flow within a vessel as well as image vessel wall structures. A prototype Doppler catheter has been developed and demonstrated for measuring the intraluminal velocity profile in a vessel phantom (conduit). A simple mathematical model is demonstrated to estimate the total flow rate. This estimation technique also enables the spatial range of flow measurements to be extended by approximately two times the normal OCT image-penetration depth. The Doppler OCT catheter could be a powerful device for cardiovascular imaging.

Tissue mimicking materials for a multi-imaging modality prostate phantom.

Abstract: Materials that simultaneously mimic soft tissue in vivo for magnetic resonance imaging (MRI), ultrasound (US), and computed tomography (CT) for use in a prostate phantom have been developed. Prostate and muscle mimicking materials contain water, agarose, lipid particles, protein, Cu++, EDTA, glass beads, and thimerosal (preservative). Fat was mimicked with safflower oil suffusing a random mesh (network) of polyurethane. Phantom material properties were measured at 22 degrees C. (22 degrees C is a typical room temperature at which phantoms are used.) The values of material properties should match, as well as possible, the values for tissues at body temperature, 37 degrees C. For MRI, the primary properties of interest are T1 and T2 relaxations times, for US they are the attenuation coefficient, propagation speed, and backscatter, and for CT, the x-ray attenuation. Considering the large number of parameters to be mimicked, rather good agreement was found with actual tissue values obtained from the literature. Using published values for prostate parenchyma, T1 and T2 at 37 degrees C and 40 MHz are estimated to be about 1,100 and 98 ms, respectively. The CT number for in vivo prostate is estimated to be 45 HU (Hounsfield units). The prostate mimicking material has a T1 of 937 ms and a T2 of 88 ms at 22 degrees C and 40 MHz; the propagation speed and attenuation coefficient slope are 1,540 m/s and 0.36 dB/cm/MHz, respectively, and the CT number of tissue mimicking prostate is 43 HU. Tissue mimicking (TM) muscle differs from TM prostate in the amount of dry weight agarose, Cu++, EDTA, and the quality and quantity of glass beads. The 18 microm glass beads used in TM muscle increase US backscatter and US attenuation; the presence of the beads also has some effect on T1 but no effect on T2. The composition of tissue-mimicking materials developed is such that different versions can be placed in direct contact with one another in a phantom with no long term change in US, MRI, or CT properties. Thus, anthropomorphic phantoms can be constructed.

Calibration of three-dimensional ultrasound images for image-guided radiation therapy

Abstract: A new technique of patient positioning for radiotherapy/radiosurgery of extracranial tumours using three-dimensional (3D) ultrasound images has been developed. The ultrasound probe position is tracked within the treatment room via infrared light emitting diodes (IRLEDs) attached to the probe. In order to retrieve the corresponding room position of the ultrasound image, we developed an initial ultrasound probe calibration technique for both 2D and 3D ultrasound systems. This technique is based on knowledge of points in both room and image coordinates. We first tested the performance of three algorithms in retrieving geometrical transformations using synthetic data with different noise levels. Closed form solution algorithms (singular value decomposition and Horn's quaternion algorithms) were shown to outperform the Hooke and Jeeves iterative algorithm in both speed and accuracy. Furthermore, these simulations show that for a random noise level of 2.5, 5, 7.5 and 10 mm, the number of points required for a transformation accuracy better than 1 mm is 25, 100, 200 and 500 points respectively. Finally, we verified the tracking accuracy of this system using a specially designed ultrasound phantom. Since ultrasound images have a high noise level, we designed an ultrasound phantom that provides a large number of points for the calibration. This tissue equivalent phantom is made of nylon wires, and its room position is optically tracked using IRLEDs. By obtaining multiple images through the nylon wires, the calibration technique uses an average of 300 points for 3D ultrasound volumes and 200 for 2D ultrasound images, and its stability is very good for both rotation (standard deviation: 0.4°) and translation (standard deviation: 0.3 mm) transformations. After this initial calibration procedure, the position of any voxel in the ultrasound image volume can be determined in world space, thereby allowing real-time image guidance of therapeutic procedures. Finally, the overall tracking accuracy of our 3D ultrasound image-guided positioning system was measured to be on average 0.2 mm, 0.9 mm and 0.6 mm for the AP, lateral and axial directions respectively.

A phantom with tissue-like optical properties in the visible and near infrared for use in photomedicine.

Abstract: Background and objective Modeling of light transport in tissue requires development of theoretical models and experimental procedures, as well as tissue-simulating phantoms. Our purpose was to develop a phantom that matches the optical characteristics of human skin in the visible and near infrared spectral range. Study design/materials and methods The phantom consists of a transparent silicone rubber in which Al(2)O(3) particles and a cosmetic powder are embedded. Layers with thickness as thin as 0.1 mm can be made. The optical properties of Al(2)O(3) particles and cosmetic powder, i.e., total attenuation, absorption and scattering coefficients, and phase function, have been determined in the visible and near infrared spectral range, by using direct and indirect techniques. Results By varying the concentration of scattering and absorbing particles, tissue-like layers can be produced with predictable optical properties. In particular, mixing at suitable concentration Al(2)O(3) particles and cosmetic powder with the silicone rubber, the optical properties of human skin have been simulated over a range of wavelengths from 400 to 1,000 nm. The comparison between the phantom diffuse reflectance spectrum and that of human skin, averaged over a sample of 260 patients, showed a good agreement. Conclusion The proposed technique allows to produce a stable and reproducible phantom, with accurately predictable optical properties, easy to make and to handle. This phantom is a useful tool for numerous applications involving light interaction with biologic tissue.

Planar strip array (PSA) for MRI.

Abstract: Parallel, spatial-encoded MRI requires a large number of independent detectors that simultaneously acquire signals. The loop structure and mutual coupling in conventional phased arrays limit the number of coils and therefore the potential reduction in minimum scan time achievable by parallel MRI tchniques. A new near-field MRI detector array, the planar strip array (PSA), is presented that eliminates the coupling problems and can be extended to a very large number of detectors and high MRI frequencies. Its basic structure is an array of parallel microstrips with a high permittivity substrate and overlay. The electromagnetic (EM) wavelength can be adjusted with the permittivity, and the strip lengths tuned to a preselected fraction of the wavelength of the MRI frequency. EM wave analysis and measurements on a prototype four-element PSA reveal that the coupling between the strips vanishes when the strip length is either an integer times a quarter wavelength for a standing-wave PSA, or a half wavelength for a travelling-wave PSA, independent of the spacing between the strips. The analysis, as well as phantom and human MRI experiments performed by conventional and parallel-encoded MRI with the PSA at 1.5 T, show that the decoupled strips produce a relatively high-quality factor and signal-to-noise ratio, provided that the strips are properly terminated, tuned, and matched or coupled to the preamplifiers.