Decrease In Signal-to-noise Ratio Research Articles

Optimization of improved motion-sensitized driven-equilibrium (iMSDE) blood suppression for carotid artery wall imaging.

BackgroundImproved motion-sensitized driven-equilibrium (iMSDE) preparations have been successfully used in carotid artery wall imaging to achieve blood suppression, but it causes notable signal loss, mostly due to inherent T2 decay, eddy current effects and B1+ inhomogeneity. In this study, we investigate the signal to noise ratio (SNR) and blood suppression performance of iMSDE using composite RF pulses and sinusoidal gradients. Optimized first moment (m1) values for iMSDE prepared T1- and T2- weighted (T1- and T2-w) imaging are presented.MethodsTwelve healthy volunteers and six patients with carotid artery disease underwent iMSDE and double inversion recovery (DIR) prepared T1- and T2-w fast spin echo (FSE) MRI of the carotid arteries. Modified iMSDE module using composite RF pulses and sinusoidal gradients were evaluated with a range of m1. SNR of adjacent muscle, vessel wall and the lumen were reported. The optimized iMSDE module was also tested in a 3D variable flip angle FSE (CUBE) acquisition.ResultsThe SNR of muscle was highest using sinusoidal gradients, and the relative improvement over the trapezoidal gradient increased with higher m1 (p<0.001). Optimal SNR was observed using an iMSDE preparation scheme containing two 180° composite pulses and standard 90° and -90° pulses (p=0.151). iMSDE produced better blood suppression relative to DIR preparations even with a small m1 of 487 mT*ms2/m (p<0.001). In T1-w iMSDE, there was a SNR decrease and an increased T2 weighting with increasing m1. In T2-w iMSDE, by matching the effective echo time (TE), the SNR was equivalent when m1 was <= 1518 mT*ms2/m, however, higher m1 values (2278 – 3108 mT*ms2/m) reduced the SNR. In the patient study, iMSDE improved blood suppression but reduced vessel wall CNR efficiency in both T1-w and T2-w imaging. iMSDE also effectively suppressed residual flow artifacts in the CUBE acquisition.ConclusionsiMSDE preparation achieved better blood suppression than DIR preparation with reduced vessel wall CNR efficiency in T1-w and T2-w images. The optimized m1s are 487 mT*ms2/m for T1-w imaging and 1518 mT*ms2/m for T2-w imaging. Composite 180° refocusing pulses and sinusoidal gradients improve SNR performance. iMSDE further improves the inherent blood suppression of CUBE.

Optimized dynamic contrast-enhanced cone-beam CT for target visualization during liver SBRT

The pharmacokinetic behavior of iodine contrast agents makes it difficult to achieve significant enhancement during contrast-enhanced cone-beam CT (CE-CBCT). This study modeled this dynamic behavior to optimize CE-CBCT and improve the localization of liver lesions for SBRT. We developed a model that allows for controlled study of changing iodine concentrations using static phantoms. A projection database consisting of multiple phantom images of differing iodine/scan conditions was built. To reconstruct images of dynamic hepatic concentrations, hepatic contrast enhancement data from conventional CT scans were used to re-assemble the projections to match the expected amount of contrast. In this way the effect of various parameters on image quality was isolated, and using our dynamic model we found parameters for iodine injection, CBCT scanning, and injection/scanning timing which optimize contrast enhancement. Increasing the iodine dose, iodine injection rate, and imaging dose led to significant increases in signal-to-noise ratio (SNR). Reducing the CBCT imaging time also increased SNR, as the image can be completed before the iodine exits the liver. Proper timing of image acquisition played a significant role, as a 30 second error in start time resulted in a 40% SNR decrease. The effect of IV contrast is severely degraded in CBCT, but there is promise that, with optimization of the injection and scan parameters to account for iodine pharmacokinetics, CE-CBCT which models venous-phase blood flow kinetics will be feasible for accurate localization of liver lesions.

Design and characterization of biocompatible perfluorocarbon nanodroplets for theragnostic application

We have developed stable emulsions (≥ 3 months) made of nanodroplets (nD) of perfluorocarbon (PFC) dispersed in water to serve as theragnostic agent. nD are stabilized by in house fluorinated surfactants, named FTAC, which chemical structure can be modified to tune their properties. We have characterized nD size distributions (mean diameters from 200 to 600 nm), density, adiabatic compressibility, interfacial tension. US properties of the emulsions have been investigated such as attenuation. Lastly, ultrasonic signals backscattered by nanoemulsions were studied and compared with water to extract signal to noise ratio (SNR), by emitting single negative pulses at ≈40 MHz. At similar mean/mode diameters, we showed a strong dependence in SNR values (i) with size distribution, altered by the nature of the surfactant or by a centrifugation/filtration process, (ii) with core nature, and (iii) with nD volume fraction. Besides, hydrophobic drugs such as a thalidomide derivative (with anti-angiogenic properties), have been encapsulated by addition of 10% of triacetin in the nD core. So as to study drug release from nD, a dedicated setup of US cavitation was designed. Cavitation was generated in controlled conditions using a 1 MHz focused transducer (US bursts: 12-22 MPa peak) in a thin wall container immersed in water which temperature and degassed level were kept constant. The generation of cavitation in nD solutions resulted in a strong and reproducible SNR decrease.

The effect of solar radio bursts on the GNSS radio occultation signals

AbstractSolar radio burst (SRB) is the radio wave emission after a solar flare, covering a broad frequency range, originated from the Sun's atmosphere. During the SRB occurrence, some specific frequency radio wave could interfere with the Global Navigation Satellite System (GNSS) signals and therefore disturb the received signals. In this study, the low Earth orbit‐ (LEO‐) based high‐resolution GNSS radio occultation (RO) signals from multiple satellites (COSMIC, CHAMP, GRACE, SAC‐C, Metop‐A, and TerraSAR‐X) processed in University Corporation for Atmospheric Research (UCAR) were first used to evaluate the effect of SRB on the RO technique. The radio solar telescope network (RSTN) observed radio flux was used to represent SRB occurrence. An extreme case during 6 December 2006 and statistical analysis during April 2006 to September 2012 were studied. The LEO RO signals show frequent loss of lock (LOL), simultaneous decrease on L1 and L2 signal‐to‐noise ratio (SNR) globally during daytime, small‐scale perturbations of SNR, and decreased successful retrieval percentage (SRP) for both ionospheric and atmospheric occultations during SRB occurrence. A potential harmonic band interference was identified. Either decreased data volume or data quality will influence weather prediction, climate study, and space weather monitoring by using RO data during SRB time. Statistically, the SRP of ionospheric and atmospheric occultation retrieval shows ~4% and ~13% decrease, respectively, while the SNR of L1 and L2 show ~5.7% and ~11.7% decrease, respectively. A threshold value of ~1807 SFU of 1415 MHz frequency, which can result in observable GNSS SNR decrease, was derived based on our statistical analysis.

Radiation dose reduction in soft tissue neck CT using adaptive statistical iterative reconstruction (ASIR)

PurposeTo compare objective and subjective image quality in neck CT images acquired at different tube current–time products (275mAs and 340mAs) and reconstructed with filtered-back-projection (FBP) and adaptive statistical iterative reconstruction (ASIR). Materials and methodsHIPAA-compliant study with IRB approval and waiver of informed consent. 66 consecutive patients were randomly assigned to undergo contrast-enhanced neck CT at a standard tube-current–time-product (340mAs; n=33) or reduced tube-current–time-product (275mAs, n=33). Data sets were reconstructed with FBP and 2 levels (30%, 40%) of ASIR-FBP blending at 340mAs and 275mAs. Two neuroradiologists assessed subjective image quality in a blinded and randomized manner. Volume CT dose index (CTDIvol), dose-length-product (DLP), effective dose, and objective image noise were recorded. Signal-to-noise ratio (SNR) was computed as mean attenuation in a region of interest in the sternocleidomastoid muscle divided by image noise. ResultsCompared with FBP, ASIR resulted in a reduction of image noise at both 340mAs and 275mAs. Reduction of tube current from 340mAs to 275mAs resulted in an increase in mean objective image noise (p=0.02) and a decrease in SNR (p=0.03) when images were reconstructed with FBP. However, when the 275mAs images were reconstructed using ASIR, the mean objective image noise and SNR were similar to those of the standard 340mAs CT images reconstructed with FBP (p>0.05). Subjective image noise was ranked by both raters as either average or less-than-average irrespective of the tube current and iterative reconstruction technique. ConclusionAdapting ASIR into neck CT protocols reduced effective dose by 17% without compromising image quality.

SU-D-103-06: Bismuth Shield Usage in MDCT Thoracic Scans: Organ Dose Vs. Image Quality

Purpose: To study Bismuth (Bi) shielding for the breast, organ dose and image quality were compared under the following conditions: (1) tube current modulation (TCM), (2) TCM with a Bi shield placed after topogram, and (3) manually reduced tube current (RTC) with no Bi. Methods: All measurements were performed with a 64‐slice scanner at 120 kVp. Organ dose was measured with MOSFETs using an adult male anthropomorphic phantom with supine breast attachments. The reference exposure and reduced exposure (with 4‐ply Bi shield) was measured with an ion chamber located at the level of the breast. The mA was reduced by normalizing the reference mA to the ratio of the reduced exposure to the reference exposure. Image quality was measured using a high contrast insert placed in the lung. Regions of Interest (ROIs) were drawn in thoracic organs to measure signal‐to‐noise ratio (SNR), percent contrast (%Contrast), noise, and HU values. Results: Organ doses (mGy) for the three scans (TCM, TCM with Bi, and RTC) were 11.1, 6.89, and 6.04 to the breast; 8.83, 8.01, and 7.62 to the lung; and 8.20, 7.36 and 8.40 to the heart, respectively. HU increase was greatest in the TCM with Bi scan for organs closest to the shield. The SNRs were 37.6, 34.1, and 43.3 and the %Contrast values were 349.2, 326.3, and 354.3 with TCM, TCM with Bi, and RTC, respectively. Conclusion: For thoracic CT, this RTC method provides a dose reduction to the breast similar to that of the TCM with Bi. Decrease in SNR and %Contrast in the TCM with Bi scan was expected due to decrease in photons reaching the detectors and beam hardening from the shield, respectively. Increased SNR in RTC scan was due to the increased mA compared to TCM scan at the level of measurement. U.S. NRC Health Physics Fellowship Grant No. NRC‐HQ‐12‐G‐38‐0022

A Diffusion-Based EM Algorithm for Distributed Estimation in Unreliable Sensor Networks

We address the problem of distributed estimation of a parameter from a set of noisy observations collected by a sensor network, assuming that some sensors may be subject to data failures and report only noise. In such scenario, simple schemes such as the Best Linear Unbiased Estimator result in an error floor in moderate and high signal-to-noise ratio (SNR), whereas previously proposed methods based on hard decisions on data failure events degrade as the SNR decreases. Aiming at optimal performance within the whole range of SNRs, we adopt a Maximum Likelihood framework based on the Expectation-Maximization (EM) algorithm. The statistical model and the iterative nature of the EM method allow for a diffusion-based distributed implementation, whereby the information propagation is embedded in the iterative update of the parameters. Numerical examples show that the proposed algorithm practically attains the Cramer–Rao Lower Bound at all SNR values and compares favorably with other approaches.

Optimized dose regimen for whole-body FDG-PET imaging

BackgroundThe European Association of Nuclear Medicine procedure guidelines for whole-body fluorodeoxyglucose positron-emission tomography (FDG-PET) scanning prescribe a dose proportional to the patient’s body mass. However, clinical practice shows degraded image quality in obese patients indicating that using an FDG dose proportional to body mass does not overcome size-related degradation of the image quality. The aim of this study was to optimize the administered FDG dose as a function of the patient’s body mass or a different patient-dependent parameter, providing whole-body FDG-PET images of a more constant quality.MethodsUsing a linear relation between administered dose and body mass, FDG-PET imaging was performed on two PET/computed tomography scanners (Biograph TruePoint and Biograph mCT, Siemens). Image quality was assessed by the signal-to-noise ratio (SNR) in the liver in 102 patients with a body mass of 46 to 130 kg. Moreover, the best correlating patient-dependent parameter was derived, and an optimized FDG dose regimen was determined. This optimized dose regimen was validated on the Biograph TruePoint system in 42 new patients. Furthermore, this relation was verified by a simulation study, in which patients with different body masses were simulated with cylindrical phantoms.ResultsAs expected, both PET systems showed a significant decrease in SNR with increasing patient’s body mass when using a linear dosage. When image quality was fitted to the patient-dependent parameters, the fit with the patient’s body mass had the highest R2. The optimized dose regimen was found to be Anew= c/t × m2, where m is the body mass, t is the acquisition time per bed position and c is a constant (depending on scanner type). Using this relation, SNR no longer varied with the patient’s body mass. This quadratic relation between dose and body mass was confirmed by the simulation study.ConclusionA quadratic relation between FDG dose and the patient’s body mass is recommended. Both simulations and clinical observations confirm that image quality remains constant across patients when this quadratic dose regimen is used.

Lateral Resolution and Signal to Noise Ratio in Electrostatic Force Detection Based on Scanning Probe Microscopy

The lateral resolution (LR) and signal-to-noise ratio (SNR) are the essential factors in the applications of scanning probe microscopy in quantitative measurement of surface charge distribution, potential profile, and dielectric properties. We use a model system to comprise Au nanoparticles (NPs) embedded in a polystyrene (PS) matrix to study the effects of various experimental parameters, such as modulation bias voltage, tip-sample distance, and actual tip shape, on the electrostatic interactions between the tips and samples. The results show that LR and SNR decrease when the tip-sample distance increases, while SNR increases with tip modulation voltage. LR is less sensitive to tip modulation voltage, but shows complex dependence on the sample geometric structure. In combination with a numerical simulation based on the integral capacitance model, the electrostatic force interaction between tip and sample was quantitatively analyzed.

Quantification and visualization of flow in the Circle of Willis: Time‐resolved three‐dimensional phase contrast MRI at 7 T compared with 3 T

The assessment of both geometry and hemodynamics of the intracranial arteries has important diagnostic value in internal carotid occlusion, sickle cell disease, and aneurysm development. Provided that signal to noise ratio (SNR) and resolution are high, these factors can be measured with time-resolved three-dimensional phase contrast MRI. However, within a given scan time duration, an increase in resolution causes a decrease in SNR and vice versa, hampering flow quantification and visualization. To study the benefits of higher SNR at 7 T, three-dimensional phase contrast MRI in the Circle of Willis was performed at 3 T and 7 T in five volunteers. Results showed that the SNR at 7 T was roughly 2.6 times higher than at 3 T. Therefore, segmentation of small vessels such as the anterior and posterior communicating arteries succeeded more frequently at 7 T. Direction of flow and smoothness of streamlines in the anterior and posterior communicating arteries were more pronounced at 7 T. Mean velocity magnitude values in the vessels of the Circle of Willis were higher at 3 T due to noise compared to 7 T. Likewise, areas of the vessels were lower at 3 T. In conclusion, the gain in SNR at 7 T compared to 3 T allows for improved flow visualization and quantification in intracranial arteries.

On Denial of Service Attacks for Wireless Sensor Networks

denial-of-service (DoS) attacks for wireless sensor networks (WSNs), we investigated the security aspects of the physical layer. We conducted the simulative performance analysis of jamming attacks for signal-to-noise ratio (SNR), bit error rate (BER), network throughput and packet delivery ratio (PDR) using IEEE 802.15.4 based OPNET simulative model for WSN under constant and varying intensity of jamming attacks. Under constant jamming attack, simulations revealed that average sink node PDR degrades from 79.01% in a normal scenario, to 59.22% in jammed scenario. Also, normal scenario shows maximum PDR of 89.68% and minimum PDR of 70.02% while jammed scenario shows a maximum PDR of 64.93% and minimum PDR of 49.90%. Under varying intensity of jamming attack, simulations revealed that average sink node PDR decreases, from 79.01% in a normal scenario, by 5.54%, 4.53%, 6.36% and 3.35% with the introduction of one, two, three and four jammers respectively. Further, the average SNR decreases, from 73.59%, in a normal scenario, by 5.43%, 5.63%, 10.44% and 20.39% with the introduction of one, two, three and four jammers respectively. Keywordsireless Sensor Network (WSN), ZigBee, Security, Denial of Service (DoS), Jamming Attack.

Noninvasive Measurement of Physiological Signals on a Modified Home Bathroom Scale

A commercial bathroom scale with both handlebar and footpad electrodes was modified to enable measurement of four physiological signals: the ballistocardiogram (BCG), electrocardiogram (ECG), lower body impedance plethysmogram (IPG), and lower body electromyogram (EMG). The BCG, which describes the reaction of the body to cardiac ejection of blood, was measured using the strain gauges in the scale. The ECG was detected using handlebar electrodes with a two-electrode amplifier. For the lower body IPG, the two electrodes under the subject's toes were driven with an ac current stimulus, and the resulting differential voltage across the heels was measured and demodulated synchronously with the source. The voltage signal from the same two footpad electrodes under the heels was passed through a passive low-pass filter network into another amplifier, and the output was the lower body EMG signal. The signals were measured from nine healthy subjects, and the average signal-to-noise ratio (SNR) while the subjects were standing still was estimated for the four signals as follows: BCG, 7.6 dB; ECG, 15.8 dB; IPG, 10.7 dB. During periods of motion, the decrease in SNR for the BCG signal was found to be correlated to the increase in rms power for the lower body EMG (r = 0.89, p <; 0.01). The EMG could, thus, be used to flag noise-corrupted segments of the BCG, increasing the measurement robustness. This setup could be used for monitoring the cardiovascular health of patients at home.

対向アレイコイルによる手部MRIの感度領域の拡張

It is difficult for Rheumatoid Arthritis (RA) patients to remain in a strenuous position for a long time during examinations. The field of view (FOV): 250 mm is needed for hand examinations from the wrist to the finger. Two channel phased array coils are effective to use when examinations of the 'off center' are taken for the upper and lower extremities. The area of the array coils' sensitivity can be expanded by shifting both coil elements 40-60% in the opposite direction of the element's diameter. This method is given credibility due to the increased signal-to-noise ratio (SNR) in the peripheral regions (shifted directions), but loses value in the central area, as indicated by the decrease in SNR. This was confirmed in the image of the hand using visual assessment including the fat suppression technique. It was verified that the sensitivity area was expanded using Scheffe's method of paired comparison (Ura's modified method). An application at the other regions of the body can be expected to be used in the case of using parallel positioned coils during clinical situation.

The Impact of Reduced Injected Radioactivity on Image Quality of Molecular Breast Imaging Tomosynthesis.

This study's objective is to compare image quality in 3-D molecular breast imaging tomosynthesis (MBIT) with that in planar molecular breast imaging (MBI) over a range of breast radioactivity concentrations. Using gelatin and point source phantoms lesion contrast, lesion signal-to-noise ratio (SNR) and spatial resolution were compared for a range of lesion sizes and depths. For both MBI and MBIT, lesion contrast is essentially constant with changing activity while SNR decreases by a factor of 1.5 - 2 between 100% and 25% activity levels. For nearly all lesion sizes and locations contrast and SNR are significantly higher for MBIT than MBI, potentially permitting greater reductions in injected dose. Spatial resolution in MBI is dependent on lesion depth but independent of lesion location with MBIT. Reconstructed MBIT spatial resolution is substantially better than that in the projection images, suggesting future use of higher sensitivity collimators for even further reductions in injected activity.

Ziv–Zakai Bounds on Time Delay Estimation in Unknown Convolutive Random Channels

Using the Ziv-Zakai bound (ZZB) methodology, we develop a Bayesian mean-square error bound on time delay estimation (TDE) in convolutive random channels, and compare it with time delay estimator performance and a Crame¿r-Rao bound. The channel is modeled as a tapped delay line, whose taps are Gaussian random variables that may be nonzero mean and correlated, a model widely adopted in many applications such as wideband fading in a multipath channel. The time delay has a uniform prior distribution. A ZZB is developed that incorporates the prior on the random time delay, as well as the convolutive random Gaussian channel, and does not assume the receiver has knowledge of the channel realization. The ZZB provides good performance prediction for maximum a posteriori (MAP) time delay estimation, tracking the low, medium, and high signal-to-noise ratio (SNR) regimes. The convolutive channel model includes important special cases, such as narrowband Gaussian channels corresponding to Rayleigh/Rician fading, wideband multipath channels with a power decay profile (such as exponential decay), and known channels. We show that the associated Crame¿r-Rao bound is tight only at high SNR, whereas the ZZB predicts threshold behavior and TDE breakdown as the SNR decreases. When compared to a ZZB that assumes knowledge of the channel realization, the ZZB developed here provides a more realistic and tighter bound, revealing the performance loss due to lack of channel knowledge. The MAP estimator incorporates knowledge of the channel statistics, and so performs much better than a maximum likelihood estimator that minimizes mean square error but does not use knowledge of the random channel statistics. Several examples illustrate the estimator and bound behaviors.

Respiratory-triggered Three-dimensional T2-weighted MR Cholangiography after Injection of Gadoxetate Disodium: Is It Still Reliable?

To test the null hypothesis that there is no quantitative or qualitative difference between respiratory-triggered three-dimensional (3D) T2-weighted magnetic resonance (MR) cholangiography performed before or after administration of gadoxetate disodium. For this retrospective HIPAA-compliant dual-center study, institutional review board approval was obtained, and a waiver of informed consent was granted. Between July and December 2008, 60 patients (age range, 18-82 years) who were referred for liver MR imaging with gadoxetate disodium underwent respiratory-triggered 3D MR cholangiography before and immediately after completion of portal venous phase contrast material-enhanced T1-weighted MR imaging. Quantitative signal-to-noise ratio (SNR) measurements were obtained in the extrahepatic biliary tract in both MR cholangiographic data sets in each patient. Qualitative assessment was performed by four readers with a four-point scale to assess the depiction of extra- and intrahepatic ducts up to the third order. Statistical analysis consisted of a one-sided Wilcoxon signed rank test, with a P value of less than .05 indicating a significant difference. There was a significant decrease in mean SNR in the MR cholangiographic data set after injection of gadoxetate disodium. SNR was 96 + or - 50 [standard deviation] and 78 + or - 47 before and after contrast media administration, respectively (P < .0001). For all readers, qualitative differences were most obvious in the depiction of the common bile duct and second- and third-order biliary branches, with the precontrast MR cholangiographic data sets being preferred (P < .0001). Precontrast data sets were also significantly preferred in the assessment of the right and left hepatic ducts by all readers. Gadoxetate disodium adversely affects respiratory-triggered 3D MR cholangiography, both qualitatively and quantitatively. We recommend that such a sequence be performed before injection of gadoxetate disodium.

MRI of the wrist at 7 tesla using an eight‐channel array coil combined with parallel imaging: Preliminary results

To determine the feasibility of performing MRI of the wrist at 7 Tesla (T) with parallel imaging and to evaluate how acceleration factors (AF) affect signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), and image quality. This study had institutional review board approval. A four-transmit eight-receive channel array coil was constructed in-house. Nine healthy subjects were scanned on a 7T whole-body MR scanner. Coronal and axial images of cartilage and trabecular bone micro-architecture (3D-Fast Low Angle Shot (FLASH) with and without fat suppression, repetition time/echo time = 20 ms/4.5 ms, flip angle = 10 degrees , 0.169-0.195 x 0.169-0.195 mm, 0.5-1 mm slice thickness) were obtained with AF 1, 2, 3, 4. T1-weighted fast spin-echo (FSE), proton density-weighted FSE, and multiple-echo data image combination (MEDIC) sequences were also performed. SNR and CNR were measured. Three musculoskeletal radiologists rated image quality. Linear correlation analysis and paired t-tests were performed. At higher AF, SNR and CNR decreased linearly for cartilage, muscle, and trabecular bone (r < -0.98). At AF 4, reductions in SNR/CNR were:52%/60% (cartilage), 72%/63% (muscle), 45%/50% (trabecular bone). Radiologists scored images with AF 1 and 2 as near-excellent, AF 3 as good-to-excellent (P = 0.075), and AF 4 as average-to-good (P = 0.11). It is feasible to perform high resolution 7T MRI of the wrist with parallel imaging. SNR and CNR decrease with higher AF, but image quality remains above-average.

MAGNETIC RESONANCE SIGNAL CHANGES DURING TIME IN EQUINE LIMBS REFRIGERATED AT 4°C

When ex vivo magnetic resonance (MR) imaging studies are undertaken, specimen conservation should be taken into account when interpreting MR imaging results. The purpose of this study was to assess MR changes during time in the anatomic structures of the equine digit on eight cadaver limbs stored at 4 degrees C. The digits were imaged within 12 h after death and then after 1, 2, 7, and 14 days of refrigeration. After the last examination, four feet were warmed at room temperature for 24 h and reimaged. Sequences used were turbo spin echo (TSE) T1, TSE T2, short tau inversion recovery (STIR), and double-echo steady state (DESS). Images obtained were compared subjectively side by side for image quality and signal changes. Signal-to-noise ratio (SNR) was measured and compared between examinations. There were no subjective changes in image quality. A mild size reduction of the synovial recesses was detected subjectively. No signal change was seen subjectively except for bone marrow that appeared slightly hyperintense in STIR and slightly hypointense in TSE T2 sequence after refrigeration compared with day 0. Using quantitative analysis, significant SNR changes in bone marrow of refrigerated limbs compared with day 0 were detected in STIR and TSE T2 sequences. Warming at room temperature for 24 h produced a reverse effect on SNR compared with refrigeration with a significant increase in SNR in TSE T2 images. After 14 days of refrigeration a statistically significant decrease of SNR was found in bone marrow in TSE T2 and DESS sequences. The SNR in the deep digital flexor tendon was not characterized by significant change in SNR.

Comparative Evaluation of the Geometrical Accuracy of Intravascular Magnetic Resonance Imaging: A Phantom Study

To evaluate and compare the accuracy of cross-sectional imaging using an intravascular antenna in the context of vascular morphological measurements performed during a magnetic resonance imaging (MRI)-guided vascular intervention. Cross-sectional imaging of a multimodality vascular phantom was performed using intravascular and surface MRI, multidetector computed tomography, and intravascular ultrasound (IVUS). Using a balanced steady-state free-precession sequence, 18 sequences parameters sets were investigated (12 for intravascular MRI and 6 for surface MRI). Vessel diameters for all images and modalities were computed using an automated vessel segmentation algorithm. Using IVUS as a gold standard, imaging using an intravascular antenna leads to an increase in geometrical accuracy in comparison to traditional surface MRI. This level of accuracy appears to follow a significant inverse proportionality relation in respect to vessel wall signal-to-noise ratio (SNR). Taking into account the rapid decrease in SNR as a function of the distance to the intravascular antenna, these results imply that, for a given level of geometrical accuracy, faster sequences can be used for the imaging of smaller vessels. Imaging using an intravascular antenna appears as a valuable assistance to increase the accuracy of vascular morphological measurements. This increase in geometrical accuracy would be beneficial during the realization of an MRI-guided intervention, either to perform pretreatment measurements or to assess the outcome of the procedure. Acquisition parameters should be tailored to vessel size and procedural time constraints.

Optimization of design and operating parameters of a space-based optical-electronic system with a distributed aperture

Using a gradient optimization method with objective functions formulated in terms of a signal-to-noise ratio (SNR) calculated at given values of the prescribed spatial ground resolution, optimization problems of geometrical parameters of a distributed optical system and a charge-coupled device of a space-based optical-electronic system are solved for samples of the optical systems consisting of two and three annular subapertures. The modulation transfer function (MTF) of the distributed aperture is expressed in terms of an average MTF taking residual image alignment (IA) and optical path difference (OPD) errors into account. The results show optimal solutions of the optimization problems depending on diverse variable parameters. The information on the magnitudes of the SNR can be used to determine the number of the subapertures and their sizes, while the information on the SNR decrease depending on the IA and OPD errors can be useful in design of a beam combination control system to produce the necessary requirements to its accuracy on the basis of the permissible deterioration in the image quality.