Towards cardiac C-arm computed tomography

Cardiac interventional procedures would benefit tremendously from sophisticated three-dimensional image guidance. Such procedures are typically performed with C-arm angiography systems, and tomographic imaging is currently available only by using preprocedural computed tomography (CT) or magnetic resonance imaging (MRI) scans. Recent developments in C-arm CT (Angiographic CT) allow three-dimensional (3-D) imaging of low contrast details with angiography imaging systems for noncardiac applications. We propose a new approach for cardiac imaging that takes advantage of this improved contrast resolution and is based on intravenous contrast injection. The method is an analogue to multisegment reconstruction in cardiac CT adapted to the much slower rotational speed of C-arm CT. Motion of the heart is considered in the reconstruction process by retrospective electrocardiogram (ECG)-gating, using only projections acquired at a similar heart phase. A series of N almost identical rotational acquisitions is performed at different heart phases to obtain a complete data set at a minimum temporal resolution of 1/N of the heart cycle time. First results in simulation, using an experimental phantom, and in preclinical in vivo studies showed that excellent image quality can be achieved.     

Adaptive segmentation of MRI data

Intensity-based classification of MR images has proven problematic, even when advanced techniques are used. Intrascan and interscan intensity inhomogeneities are a common source of difficulty. While reported methods have had some success in correcting intrascan inhomogeneities, such methods require supervision for the individual scan. This paper describes a new method called adaptive segmentation that uses knowledge of tissue intensity properties and intensity inhomogeneities to correct and segment MR images. Use of the expectation-maximization (EM) algorithm leads to a method that allows for more accurate segmentation of tissue types as well as better visualization of magnetic resonance imaging (MRI) data, that has proven to be effective in a study that includes more than 1000 brain scans. Implementation and results are described for segmenting the brain in the following types of images: axial (dual-echo spin-echo), coronal [three dimensional Fourier transform (3-DFT) gradient-echo T1-weighted] all using a conventional head coil, and a sagittal section acquired using a surface coil. The accuracy of adaptive segmentation was found to be comparable with manual segmentation, and closer to manual segmentation than supervised multivariant classification while segmenting gray and white matter.     

The signal-to-noise ratio of nuclear magnetic resonance surfacecoils and application to a lossy dielectric cylinder model. II. The caseof cylindrical window coils

For pt. I see ibid., vol. 42, no. 5, p. 497-506 (1995). In pt. I the authors developed complete expressions for the power dissipated by and for the signal-to-noise ratio (S/N) of a coil of arbitrary geometry facing an infinite lossy dielectric cylinder. They now consider an example coil geometry, a “cylindrical window”, and demonstrate the effects of coil size and position, tissue properties, and source location on the S/N. Frequencies ranging from 1 to 170 MHz are investigated for two coil sizes, the larger having four times the surface area of the smaller. For a dipole source of strength assumed independent of frequency, the S/N is constant for frequencies up to about 10 MHz. Both coil sizes yield similar optimal S/N values when imaging structures deep within the body, the larger coil showing less dependence on the source location. For more superficial structures, the smaller coil has a better performance at all frequencies investigated while still being more sensitive to source position. Hence, when imaging superficial structures the choice of coil size should be balanced between image uniformity and the need for a higher S/N. For each coil size, there is an optimal position away from the tissues which yields the highest S/N when imaging deep. By contrast, the coil should be placed as close as possible to the body when the source is near the surface. From an electromagnetic standpoint and aside from the increased equilibrium magnetization in the tissues, the S/N of both coils is actually improved by operating at a higher frequency when imaging superficially, whereas it is degraded when imaging deep. Experimental results gathered on a saline-filled cylinder correlate very well with these simulations and show the model will also predict with good accuracy the S/N for a finite length cylinder as long as that length is at least three or four times the coil longitudinal dimension     

T1 fast acquisition relaxation mapping(T1-FARM): an optimized reconstruction

Maps of spin lattice relaxation time (T₁) can be reconstructed directly from magnetic resonance imaging (MRI) k-space data measured with very short data acquisition times, e.g., a data set for a 128×128 T₁ map can be acquired in less than 3 s using gradients with 10-T/m/s slew rate. In principle, this approach could be extended to quantitate other MRI parameters but current use is limited by the lack of precise, accurate and fast reconstruction. Using theoretical calculations, computer simulations, and experiments the authors have optimized a parametric reconstruction method using a Leverberg-Marquardt (L-M) algorithm and compared it to the quasi-Newton method originally used. The authors have found significant improvement using the L-M method provided T₁ is solved for directly without linearization. Reconstruction time was reduced by a factor of 60. Computer simulations show that the method has acceptable accuracy even in signals with 5% noise. Optimization included the investigation of the signal-to-noise (S/N) of each k-space data point and its impact on relative error of the reconstruction. This result indicates that rectangular L-space data could be collected for further reduction of data acquisition times. Determination of T₁ maps by direct parametric reconstruction of k-space data appears feasible and may stimulate further application of quantitative MRI     

The signal-to-noise ratio of nuclear magnetic resonance surfacecoils and application to a lossy dielectric cylinder model. I. Theory

Surface coils are used in magnetic resonance imaging (MRI) for their high signal-to-noise ratio (S/N) when placed near the-region to be imaged. However, their optimization for high field MRI systems is hampered by the lack of understanding of the electromagnetic effects taking place at high frequencies when a coil is placed near the human body. The aim of this work was to calculate the S/N of surface coils using complete solutions to Maxwell's equations and also to study the high frequency effects and parameters determining the S/N. Here the authors present a general approach to the computation of the S/N of surface coils using the reciprocity principal and the complex Poynting vector for arbitrary coil and body geometries. This approach is then applied to the case of the human body modeled as an infinitely long homogeneous dielectric cylinder exhibiting both conductive and dielectric losses. The S/N of a coil of unspecified geometry facing the cylinder is derived using a dyadic Green's function. Complete solutions for the fields of a dipolar source arbitrarily located in the cylinder are first derived, and applying the reciprocity principle, the authors deduce the fields created at the dipole position by a coil excited with a unit radiofrequency current. These yield the expressions for the power dissipated in the cylinder, for its reciprocal the noise picked up by the coil, and also for the signal received. Any coil geometry and any coil or source position can be evaluated with this infinite cylinder model. It is valid at all frequencies and for any tissue parameter. The general approach to the computation of the S/N of MRI coils can be applied to other body geometries as well     

Simultaneous ultrasound and MRI system for breast biopsy: compatibility assessment and demonstration in a dual modality phantom

Simultaneous capturing of ultrasound (US) and magnetic resonance (MR) images allows fusion of information obtained from both modalities. We propose an MR-compatible US system where MR images are acquired in a known orientation with respect to the US imaging plane and concurrent real-time imaging can be achieved. Compatibility of the two imaging devices is a major issue in the physical setup. Tests were performed to quantify the radio frequency (RF) noise introduced in MR and US images, with the US system used in conjunction with MRI scanner of different field strengths (0.5 T and 3 T). Furthermore, simultaneous imaging was performed on a dual modality breast phantom in the 0.5 T open bore and 3 T close bore MRI systems to aid needle-guided breast biopsy. Fiducial based passive tracking and electromagnetic based active tracking were used in 3 T and 0.5 T, respectively, to establish the location and orientation of the US probe inside the magnet bore. Our results indicate that simultaneous US and MR imaging are feasible with properly-designed shielding, resulting in negligible broadband noise and minimal periodic RF noise in both modalities. US can be used for real time display of the needle trajectory, while MRI can be used to confirm needle placement.     

MRI artifact cancellation due to rigid motion in the imaging plane

A post-processing technique has been developed to suppress the magnetic resonance imaging (MRI) artifact arising from object planar rigid motion. In two-dimensional Fourier transform (2-DFT) MRI, rotational and translational motions of the target during magnetic resonance magnetic resonance (MR) scan respectively impose nonuniform sampling and a phase error an the collected MRI signal. The artifact correction method introduced considers the following three conditions: (1) for planar rigid motion with known parameters, a reconstruction algorithm based on bilinear interpolation and the super-position method is employed to remove the MRI artifact, (2) for planar rigid motion with known rotation angle and unknown translational motion (including an unknown rotation center), first, a super-position bilinear interpolation algorithm is used to eliminate artifact due to rotation about the center of the imaging plane, following which a phase correction algorithm is applied to reduce the remaining phase error of the MRI signal, and (3) to estimate unknown parameters of a rigid motion, a minimum energy method is proposed which utilizes the fact that planar rigid motion increases the measured energy of an ideal MR image outside the boundary of the imaging object; by using this property all unknown parameters of a typical rigid motion are accurately estimated in the presence of noise. To confirm the feasibility of employing the proposed method in a clinical setting, the technique was used to reduce unknown rigid motion artifact arising from the head movements of two volunteers.     

Time‐Dependent T1–T2 Switchable Magnetic Resonance Imaging Realized by c(RGDyK) Modified Ultrasmall Fe3O4 Nanoprobes

To achieve the accurate diagnosis of tumor with the magnetic resonance imaging (MRI), nanomaterials‐based contrast agents are developed rapidly. Here, a tumor targeting nanoprobe of c(RGDyK) modified ultrasmall sized iron oxide is reported with high saturation magnetization and high T₁‐weighted imaging capability, attributed to a large number of paramagnetic centers on the surface of nanoprobes and rapid water proton exchange rate (inner sphere model), as well as strong superparamagnetism (outer sphere model). These nanoprobes could actively target and gradually accumulate at the tumor site with a time‐dependent T₁–T₂ contrast enhancement imaging effect. In in vivo MRI experiments, the nanoprobes exhibit the best T₁ contrast enhancement at 30 min after intravenous administration, followed by gradually vanishing and generating T₂ contrast enhancement with increasing time at tumor site. This is likely due to time‐dependent nanoprobes aggregation in tumor, in good agreement with in vitro experiment where aggregated nanoprobes display larger r₂/r₁ value (19.1) than that of the dispersed nanoprobes (2.8). This dynamic property is completely different from other T₁‐T₂ dual‐modal nanoprobes which commonly exhibit the T₁‐ and T₂‐weighted enhancement effect at the same time. To sum up, these c(RGDyK) modified ultrasmall Fe₃O₄ nanoprobes have significant potential to improve the diagnostic accuracy and sensitivity in MRI.     

A new pulse sequence for "fast recovery" fast-scan NMR imaging

This paper describes the "fast recovery" (FR) method for fast NMR imaging. The FR method combines a sequence of four RF pulses-alternating selective 90 degrees nutation pulses and nonselective 180 degrees pulses-with a gradient field pulse sequence which includes "spoiler" pulses to destroy the coherence between successive sequence cycles. We use the 2-D backprojection method of image reconstruction, but other imaging methods could be applied. The paper analyzes the behavior of the macroscopic magnetization-compares the FR method with other methods and proposes "figure of merit" expressions for relative signal-to-noise (S/N) ratios, scan time reduction ratios, and image contrast-and presents experimental results, including backprojection image reconstruction 2-D images and computed T1 and T2 images. For the FR method, in theory and practice, we find that, after each scan sequence cycle, magnetization is restored to equilibrium quickly and exactly; scan time can consequently be less than a tenth that for the saturation recovery method without any penalty in signal-to-noise ratio. Image contrast is even higher than that of the SR method, and compromise "optimum" sequence (interpulse timing) parameters give high image contrast for a wide range of tissue T1 and T2 (spin-lattice and spin-spin relaxation time) values.     

CCVD Synthesis of Carbon‐Encapsulated Cobalt Nanoparticles for Biomedical Applications

Carbon‐encapsulated ferromagnetic Cobalt nanoparticles (Co@C) have been synthesized by catalytic chemical vapour deposition (CCVD). The nanoparticles, mainly ranging between 10 and 15 nm, are tightly encapsulated by 2–3 concentric graphitic carbon shells and protected from oxidation. Because of their magnetic properties (saturation magnetization of 106 emu/g and a coercivity HC of 250 Oe), Co@C nanoparticles have been investigated for hyperthermia application. Although the observed values of the specific absorption rate (28.7 W/gCo@C at 30 kA/m and 215.4 W/gCo@C at 70 kA/m) are slightly lower than required in actual hyperthermia therapies, the observed strong heating effect provides a very promising starting point for future clinical application. It is also demonstrated that these nanoparticles can at the same time be used for magnetic resonance imaging (MRI) with an efficiency comparable to commercially available T2 contrast agents.     

Information-Bearing Noncoherently Modulated Pilots for MIMO Training

In this correspondence, we deal with noncoherent communications over multiple-input-multiple-output (MIMO) wireless links. For a Rayleigh flat block-fading channel with M transmit- and N receive-antennas and a channel coherence interval of length T, it is well known that for TGtM, or, at high signal-to-noise-ratio (SNR) rhoGt1 and Mlesmin{N,lfloorT/2rfloor}, unitary space-time modulation (USTM) is capacity-achieving, but incurs exponential demodulation complexity in T. On the other hand, conventional training-based schemes that rely on known pilot symbols for channel estimation simplify the receiver design, but they induce certain SNR loss. To achieve desirable tradeoffs between performance and complexity, we propose a novel training approach where USTM symbols over a short length T_tau(tau is a small fraction of T, and recovers part of the SNR loss experienced by the conventional training-based schemes. When rhorarrinfin and TgesT_tau ges2M=2Nrarrinfin, but the ratios alpha=M/T, alpha₁ =T_tau/T are fixed, we obtain analytical expressions of the asymptotic SNR loss for both the conventional and new training-based approaches, serving as a guideline for practical designs     

Simultaneous Activation of Short‐Wave Infrared (SWIR) Light and Paramagnetism by a Functionalized Shell for High Penetration and Spatial Resolution Theranostics

Nanoparticle emitting short‐wave infrared (SWIR) light has received increased attention in the molecular imaging field due to its deeper tissue penetration, fast imaging, high sensitivity, and resolution. The simultaneously activated SWIR excited directly by an 808 nm laser and T₁‐weighted magnetic resonance imaging (MRI) signal are found in one single‐shell nanoparticle NaErF₄@NaGdF₄ (Er@Gd), which is used as a dual‐modality imaging contrast agent in vivo to accurately determine the position of tumors. The conjugated cypate is then aggregated on the surface of Er@Gd@SiO₂‐Cy/bovine serum albumin. With the guidance of dual modality imaging, photothermal therapy is effectively used to ablate tumors in a mouse model. The design of single‐shell nanomaterial activation of SWIR imaging and MRI signals is expected to provide a new strategy for high penetration and spatial resolution cancer theranostics.     

Four-dimensional spectral-spatial imaging using projection reconstruction

An n-dimensional (n-D) filtered backprojection image reconstruction algorithm has been developed and used in the reconstruction of 4-D spectral-spatial magnetic resonance imaging (MRI) data. The algorithm uses n-1 successive stages of 2-D filtered backprojection to reconstruct an n-D image. This approach results in a reduction in computational time on the order of N(n-2) relative to the single-stage technique, where N(n) is the number of elements in an n-D image. The authors describe implementation of the algorithm, including digital filtering and sampling requirements. Images obtained from simulated data are presented to illustrate the accuracy and potential utility of the technique.     

Two-Dimensional Beamforming with Nonsinusoidal Signals

A two-dimensional beamforming technique is presented for two-dimensional arrays of (N × N) sensors receiving plane wavefronts with nonsinusoidal time variations in the form of a single rectangular pulse of duration T or in the form of a coded sequence of rectangular pulses with nominal time duration T. The three-dimensional energy pattern has a main beam for small angles of incidence and a number of sidelobes for large angles of incidence. For wavefronts with rectangular time variations, the maximum sidelobe has the magnitude 1/N and, for coded time variations, the maximum sidelobe exceeds 1/N. The magnitude of the sidelobes of the energy pattern can be decreased by increasing the number of sensors in the array. The resolution angle can be decreased by the ratio (N1/N2) when the number of sensors is increased from (N1 × N1) to (N2 × N2) and it can be decreased by the ratio (T1/T2) when the nominal time duration is decreased from T2 to T1. Waveform distortion results in a degradation of the resolution angle.     

Downlink transmission of broadband OFCDM Systems-part I: hybrid detection

The broadband orthogonal frequency and code division multiplexing (OFCDM) system with two-dimensional spreading (time and frequency domain spreading) is becoming a very attractive technique for high-rate data transmission in future wireless communication systems. In this paper, a quasianalytical study is presented on the downlink performance of the OFCDM system with hybrid multi-code interference (MCI) cancellation and minimum mean square error (MMSE) detection. The weights of MMSE are derived and updated stage by stage of MCI cancellation. The effects of channel estimation errors and sub-carrier correlation are also studied. It is shown that the hybrid detection scheme performs much better than pure MMSE when good channel estimation is guaranteed. The power ratio between the pilot channel and all data channels should be set to 0.25, which is a near optimum value for the two-dimensional spreading system with time domain spreading factor (N/sub T/) of 4 and 8. On the other hand, in a slow fading channel, a large value of the channel estimation window size /spl gamma/N/sub T/, where /spl gamma/ is an odd integer, is expected. However, /spl gamma/=3 is large enough for the system with N/sub T/=8 while /spl gamma/=5 is more desirable for the system with N/sub T/=4. Although performance of the hybrid detection degrades in the presence of the sub-carrier correlation, the hybrid detection still works well even the correlation coefficient is as high as 0.7. Finally, given N/sub T/, although performance improves when the frequency domain spreading factor (N/sub F/) increases, the frequency diversity gain is almost saturated for a large value of N/sub F/ (i.e., N/sub F/ /spl ges/ 32).     

Rb(5PJ)+(Ne、N2)碰撞能量转移

在气体样品池条件下,研究了Rb(5PJ) (Ne、N2)碰撞能量转移过程.对于5PJ与Ne的碰撞,电子态能量仅能转移为Ne原子的平动能.在与N2的碰撞中,向分子振转态的转移是重要的.调频半导体激光器稍微调离共振线,激发Rb原子至Rb(5P3/2)态,在不同的Ne或N2气压下,测量了5P1/2→5S1/2与5P3/2→5S1/2荧光强度比.利用速率方程分析,可以得到碰撞转移速率系数,对于Ne,5P3/2→5P1/2转移速率系数为1.53×10-12cm3s-1.对于N2,由5PJ Ne和5PJ N2二种情况下5P1/2与5P3/2荧光的相对强度比,利用最小二乘法确定5P3/2→5P1/2.转移速率系数为8.83×1011cm3s-1,5PJ态猝灭速率系数为1.25×10-10cm3s-1.对实验结果进行了定性的讨论.     

Performance of QPET, a high-resolution 3D PET imaging system for small volumes

QPET is a positron imaging system developed at Queen's University for high-resolution, 3D imaging of small volumes; it includes a pair of planar gamma-ray detectors 25.4 cm square, which rotate about a central axis with a quasi-cylindrical geometry. The authors describe the performance of this system. Basic characteristics of the detectors are evaluated: a spatial sampling of 1 mm, a quantum efficiency of 9.3% (for 511 keV gamma rays with normal incidence), and a time resolution of 88 ns. Models are developed to characterize the system deadtime and the sensitivity in terms of the noise-equivalent counting rate. With an 8 cm diameter spherical source, the noise-equivalent counting rate reaches a maximum at just over 3 kcps for an activity concentration of 2 muCi/cc; the random coincidence events and the deadtime losses both contribute significantly and the scatter contribution is small. Spatial resolution and uniformity over the field of view are evaluated by imaging short and long line sources; a spatial resolution of 2.7 mm in the transverse directions and 2.0 mm in the axial direction is achieved, with excellent uniformity throughout the field of view. The detector response is amplitude invariant across a 20 cm transverse diameter and a 9 cm axial length with the acceptance angle limited to +/-25 degrees in the axial direction. As an example of the imaging capabilities of QPET, the authors show 3D images of (18)F uptake in the bones of a rat, showing the excellent spatial resolution. This system is best suited to limited-volume applications where high counting rates are not necessary, but where high spatial resolution and uniform detector response are priorities.     

Single Ho3+‐Doped Upconversion Nanoparticles for High‐Performance T2‐Weighted Brain Tumor Diagnosis and MR/UCL/CT Multimodal Imaging

Multimodal bio‐imaging has attracted great attention for early and accurate diagnosis of tumors, which, however, suffers from the intractable issues such as complicated multi‐step syntheses for composite nanostructures and interferences among different modalities like fluorescence quenching by MRI contrast agents (e.g., magnetic iron oxide NPs). Herein, the first example of T₂‐weighted MR imaging of Ho³⁺‐doped upconversion nanoparticles (UCNPs) is presented, which, very attractively, could also be simultaneously used for upconversion luminesence (UCL) and CT imaging, thus enabling high performance multi‐modal MRI/UCL/CT imagings in single UCNPs. The new finding of T₂‐MRI contrast enhancement by integrated sensitizer (Yb³⁺) and activator (Ho³⁺) in UCNPs favors accurate MR diagnosis of brain tumor and provides a new strategy for acquiring T₂‐MRI/optical imaging without fluorescence quenching. Unlike other multi‐phased composite nanostructures for multimodality imaging, this Ho³⁺‐doped UCNPs are featured with simplicity of synthesis and highly efficient multimodal MRI/UCL/CT imaging without fluorescence quenching, thus simplify nanostructure and probe preparation and enable win–win multimodality imaging.     

A Spectral-Scanning Nuclear Magnetic Resonance Imaging (MRI) Transceiver

An integrated spectral-scanning nuclear magnetic resonance imaging (MRI) transceiver is implemented in a 0.12$ mu$m SiGe BiCMOS process. The MRI transmitter and receiver circuitry is designed specifically for small-scale surface MRI diagnostics applications where creating low (below 1 T) and inhomogeneous magnetic field is more practical. The operation frequency for magnetic resonance detection and analysis is tunable from 1 kHz to 37 MHz, corresponding to 0–0.9 T magnetization for $^{1}$ H (Hydrogen). The concurrent measurement bandwidth is approximately one frequency octave. The chip can also be used for conventional narrowband nuclear magnetic resonance (NMR) spectroscopy from 1 kHz up to 250 MHz. This integrated transceiver consists of both the magnetic resonance transmitter which generates the required excitation pulses for the magnetic dipole excitation, and the receiver which recovers the responses of the dipoles.      

Optimal pulse sequences for magnetic resonance imaging-computing accurate t1, t2, and proton density images

Proton nuclear magnetic resonance images (proton MRI) are functions of not only proton density (rho), spin lattice relaxation time (T1), and spin-spin relaxation time (T2) but also MRI scan parameters, so the images differ for different pulse sequences and scan parameters. Systematic measuring errors also vary depending on the type of pulse sequence and the scan parameters. "Pure," objective, T1, T2, and proton density images can be computed from several different Proton MRI, however, systematic measuring errors can cause large deviations between "pure" images obtained using different methods. We tested several scan sequences, and investigated methods to improve the resolution and reduce the errors over the range of T1, T2, and proton density values encountered in representative human tissues. We found that, for given scan time, the combination of inversion recovery 3 spin echo (IR3SE) and saturation recovery 4 spin echo (SR4SE) sequences gave more accurate computed images than other comparable methods tested. It follows, then, that adopting such a sequence as "standard" allows meaningful comparison of clinical results obtained by different researchers.