Wireless, customizable coaxially shielded coils for magnetic resonance imaging.

Anatomy-specific radio frequency receive coil arrays routinely adopted in magnetic resonance imaging (MRI) for signal acquisition are commonly burdened by their bulky, fixed, and rigid configurations, which may impose patient discomfort, bothersome positioning, and suboptimal sensitivity in certain situations. Herein, leveraging coaxial cables' inherent flexibility and electric field confining property, we present wireless, ultralightweight, coaxially shielded, passive detuning MRI coils achieving a signal-to-noise ratio comparable to or surpassing that of commercially available cutting-edge receive coil arrays with the potential for improved patient comfort, ease of implementation, and substantially reduced costs. The proposed coils demonstrate versatility by functioning both independently in form-fitting configurations, closely adapting to relatively small anatomical sites, and collectively by inductively coupling together as metamaterials, allowing for extension of the field of view of their coverage to encompass larger anatomical regions without compromising coil sensitivity. The wireless, coaxially shielded MRI coils reported herein pave the way toward next-generation MRI coils.

Wearable Coaxially-Shielded Metamaterial for Magnetic Resonance Imaging.

Recent advancements in metamaterials have yielded the possibility of a wireless solution to improve signal-to-noise ratio (SNR) in magnetic resonance imaging (MRI). Unlike traditional closely packed local coil arrays with rigid designs and numerous components, these lightweight, cost-effective metamaterials eliminate the need for radio frequency cabling, baluns, adapters, and interfaces. However, their clinical adoption is limited by their low sensitivity, bulky physical footprint, and limited, specific use cases. Herein, a wearable metamaterial developed using commercially available coaxial cable, designed for a 3.0 T MRI system is introduced. This metamaterial inherits the coaxially-shielded structure of its constituent cable, confining the electric field within and mitigating coupling to its surroundings. This ensures safer clinical adoption, lower signal loss, and resistance to frequency shifts. Weighing only 50 g, the metamaterial maximizes its sensitivity by conforming to the anatomical region of interest. MRI images acquired using this metamaterial with various pulse sequences achieve an SNR comparable or even surpass that of a state-of-the-art 16-channel knee coil. This work introduces a novel paradigm for constructing metamaterials in the MRI environment, paving the way for the development of next-generation wireless MRI technology.

Computational-Design Enabled Wearable and Tunable Metamaterials via Freeform Auxetics for Magnetic Resonance Imaging.

Metamaterials hold significant promise for enhancing the imaging capabilities of magnetic resonance imaging (MRI) machines as an additive technology, due to their unique ability to enhance local magnetic fields. However, despite their potential, the metamaterials reported in the context of MRI applications have often been impractical. This impracticality arises from their predominantly flat configurations and their susceptibility to shifts in resonance frequencies, preventing them from realizing their optimal performance. Here, a computational method for designing wearable and tunable metamaterials via freeform auxetics is introduced. The proposed computational-design tools yield an approach to solving the complex circle packing problems in an interactive and efficient manner, thus facilitating the development of deployable metamaterials configured in freeform shapes. With such tools, the developed metamaterials may readily conform to a patient's knee, ankle, head, or any part of the body in need of imaging, and while ensuring an optimal resonance frequency, thereby paving the way for the widespread adoption of metamaterials in clinical MRI applications.

Computational-design Enabled Wearable and Tunable Metamaterials via Freeform Auxetics for Magnetic Resonance Imaging.

Metamaterials hold significant promise for enhancing the imaging capabilities of MRI machines as an additive technology, due to their unique ability to enhance local magnetic fields. However, despite their potential, the metamaterials reported in the context of MRI applications have often been impractical. This impracticality arises from their predominantly flat configurations and their susceptibility to shifts in resonance frequencies, preventing them from realizing their optimal performance. Here, we introduce a computational method for designing wearable and tunable metamaterials via freeform auxetics. The proposed computational-design tools yield an approach to solving the complex circle packing problems in an interactive and efficient manner, thus facilitating the development of deployable metamaterials configured in freeform shapes. With such tools, the developed metamaterials may readily conform to a patient's kneecap, ankle, head, or any part of the body in need of imaging, and while ensuring an optimal resonance frequency, thereby paving the way for the widespread adoption of metamaterials in clinical MRI applications.

Hematocrit and lactate trends help predict outcomes in trauma independent of CT and other clinical parameters.

Background: Hematocrit and lactate have an established role in trauma as indicators of bleeding and cell death, respectively. The wide availability of CT imaging and clinical data poses the question of how these can be used in combination to predict outcomes. Purpose: To assess the utility of hematocrit or lactate trends in predicting intensive care unit (ICU) admission and hospital length of stay (LOS) in patients with torso trauma combined with clinical parameters and injury findings on CT. Materials and Methods: This was a single-center retrospective study of adults with torso trauma in one year. Trends were defined as a unit change per hour. CT findings and clinical parameters were explanatory variables. Outcomes were ICU admission and hospital LOS. Multivariate logistic and negative binomial regression models were used to calculate the odds ratio (OR) and incident rate ratio (IRR). Results: Among 840 patients, 561 (72% males, age 39 ± 18) were included, and 168 patients (30%) were admitted to the ICU. Decreasing hematocrit trend [OR 2.54 (1.41-4.58), p = 0.002] and increasing lactate trend [OR 3.85 (1.35-11.01), p = 0.012] were associated with increased odds of ICU admission. LOS median was 2 (IQR: 1-5) days. Decreasing hematocrit trend [IRR 1.37 (1.13-1.66), p = 0.002] and increasing lactate trend [2.02 (1.43-2.85), p < 0.001] were associated with longer hospital LOS. Conclusion: Hematocrit and lactate trends may be helpful in predicting ICU admission and LOS in torso trauma independent of organ injuries on CT, age, or admission clinical parameters.

Utilization of a two-material decomposition from a single-source, dual-energy CT in acute traumatic vertebral fractures.

Purpose: The purpose of this study is to utilize a two-material decomposition to quantify bone marrow edema on a dual-energy computed tomography (DECT) scanner at the cervical, thoracic, and lumbar spine acute fractures in correlation with short tau inversion recovery (STIR) hyperintensity on magnetic resonance imaging (MRI) in comparison with the normal bone marrow. Materials and methods: This retrospective institutional review board-approved study gathered patients over 18 years old who had acute cervical, thoracic, or lumbar spinal fractures scanned on a DECT scanner. Those who had a spinal MRI done with bone marrow STIR hyperintensity within 3 weeks of the DECT were included. The water (calcium) and fat (calcium) density (mg/cm3) measurements of the region of interest of the bone marrow were obtained at a normal anatomic equivalent site and at the fracture site where STIR hyperintensity was noted on MRI. A statistical analysis was performed using the paired t-test and Wilcoxon signed rank test (p > 0.05). Results: A total of 20 patients met the inclusion criteria (males n = 17 males, females n = 3). A total of 32 fractures were analyzed: 19 cervical and 13 thoracolumbar. There were statistically significant differences in the water (43 ± 24 mg/cm3) and fat (36 ± 31 mg/cm3) density (mg/cm3) at the acute thoracic and lumbar spine fractures in correlation with edema on STIR images (both paired t-test <0.001, both Wilcoxon signed ranked test p < 0.01). There were no significant differences in the water (-10 ± 46 mg/cm3) or fat (+7 ± 50 mg/cm3) density (mg/cm3) at the cervical spine fractures. Conclusion: The DECT two-material decomposition using water (calcium) and fat (calcium) analyses has the ability to quantify a bone marrow edema at the acute fracture site in the thoracic and lumbar spine.

Bayesian reconstruction of magnetic resonance images using Gaussian processes.

A central goal of modern magnetic resonance imaging (MRI) is to reduce the time required to produce high-quality images. Efforts have included hardware and software innovations such as parallel imaging, compressed sensing, and deep learning-based reconstruction. Here, we propose and demonstrate a Bayesian method to build statistical libraries of magnetic resonance (MR) images in k-space and use these libraries to identify optimal subsampling paths and reconstruction processes. Specifically, we compute a multivariate normal distribution based upon Gaussian processes using a publicly available library of T1-weighted images of healthy brains. We combine this library with physics-informed envelope functions to only retain meaningful correlations in k-space. This covariance function is then used to select a series of ring-shaped subsampling paths using Bayesian optimization such that they optimally explore space while remaining practically realizable in commercial MRI systems. Combining optimized subsampling paths found for a range of images, we compute a generalized sampling path that, when used for novel images, produces superlative structural similarity and error in comparison to previously reported reconstruction processes (i.e. 96.3% structural similarity and < 0.003 normalized mean squared error from sampling only 12.5% of the k-space data). Finally, we use this reconstruction process on pathological data without retraining to show that reconstructed images are clinically useful for stroke identification. Since the model trained on images of healthy brains could be directly used for predictions in pathological brains without retraining, it shows the inherent transferability of this approach and opens doors to its widespread use.

Quantification of spinal bone marrow fat fraction using three-material decomposition technique on dual-energy CT: A phantom study.

BACKGROUND: Two-material decomposition is insufficient to quantify the fat fraction of spinal bone marrow, which is comprised of a mixture of bone minerals, water, and yellow marrow (fat). PURPOSE: To develop an accurate three-material decomposition-based bone marrow fat fraction ( F F 3 M D $F{F_{3MD}}$ ) quantification technique for dual-energy CT. METHODS: Bone marrow edema phantoms containing trabecular bone minerals, water, and fat were constructed using fat fractions and bone mineral density values matching those expected in healthy and edematous bone, and scanned on a commercial dual-energy CT. Fat quantified by F F 3 M D $F{F_{3MD}}$ were compared to MRI-based fat fraction ( F F M R I $F{F_{MRI}}$ ) and conventional two-material-decomposition-based fat fraction ( F F 2 M D $F{F_{2MD}}$ ) to evaluate its accuracy and dependency on various bone mineral densities. RESULTS: F F 3 M D $F{F_{3MD}}$ demonstrated an excellent correlation with F F M R I $F{F_{MRI}}\;$ (r = 0.97, R2  = 0.96) in the phantom, significantly more accurate than FF2MD when confounding bone minerals are present (50 mg/cm3 : r = 1.02, R2  = 0.95 vs. r = 0.65, R2  = 0.79 (p < 0.01); 100 mg/cm3 : r = 0.81, R2  = 0.47 vs. r = 0.21, R2  = 0.21 (p < 0.05)). CONCLUSIONS: F F 3 M D $F{F_{3MD}}$ accurately quantified bone marrow fat fraction, when compared with F F M R I $F{F_{MRI}}$ , in the specially constructed bone marrow phantom.

Gadolinium Deposition in the Rat Brain Measured with Quantitative MRI versus Elemental Mass Spectrometry.

Background T1-weighted MRI and quantitative longitudinal relaxation rate (R1) mapping have been used to evaluate gadolinium retention in the brain after gadolinium-based contrast agent (GBCA) administration. Whether MRI measures accurately reflect gadolinium regional distribution and concentration in the brain remains unclear. Purpose To compare gadolinium retention in rat forebrain measured with in vivo quantitative MRI R1 and ex vivo laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) mapping after gadobenate, gadopentetate, gadodiamide, or gadobutrol administration. Materials and Methods Adult female Sprague-Dawley rats were randomly assigned to one of five groups (eight per group) and administered gadobenate, gadopentetate, gadodiamide, gadobutrol (2.4 mmol/kg per week for 5 weeks), or saline (4.8 mL/kg per week for 5 weeks). MRI R1 mapping was performed at baseline and 1 week after the final injection to determine R1 and ΔR1. Postmortem brains from the same rats were analyzed with LA-ICP-MS elemental mapping to determine regional gadolinium concentrations. Student t tests were performed to compare results between GBCA and saline groups. Results Rats that were administered gadobenate showed gadolinium-related MRI ΔR1 in 39.5% of brain volume (ΔR1 = 0.087 second-1 ± 0.051); gadopentetate, 20.6% (ΔR1 = 0.069 second-1 ± 0.018); gadodiamide, 5.4% (ΔR1 = 0.055 second-1 ± 0.019); and gadobutrol, 2.2% (ΔR1 = 0.052 second-1 ± 0.041). Agent-specific gadolinium-related ΔR1 was detected in multiple forebrain regions (neocortex, hippocampus, dentate gyrus, thalamus, and caudate-putamen) in rats treated with gadobenate or gadopentetate, whereas rats treated with gadodiamide showed gadolinium-related ΔR1 in caudate-putamen. By contrast, LA-ICP-MS elemental mapping showed a similar regional distribution pattern of heterogeneous retained gadolinium in the forebrain of rats treated with gadobenate, gadopentetate, or gadodiamide, with the average gadolinium concentration of 0.45 µg · g-1 ± 0.07, 0.50 µg · g-1 ± 0.10, and 0.60 µg · g-1 ± 0.11, respectively. Low levels (0.01 µg · g-1 ± 0.00) of retained gadolinium were detected in the forebrain of gadobutrol-treated rats. Conclusion Differences in in vivo MRI longitudinal relaxation rate versus ex vivo elemental mass spectrometry measures of retained gadolinium in rat forebrains suggest that some forms of retained gadolinium may escape detection with MRI. © RSNA, 2022 Online supplemental material is available for this article.

Quantification of bone marrow edema using dual-energy CT at fracture sites in trauma.

PURPOSE: The purpose of our study was to analyze the change in water and fat density within the bone marrow using the GE Revolution dual-energy computed tomography (DECT) platform using two-material decomposition analyses at extremity, spine, and pelvic fracture sites compared to normal bone marrow at equivalent anatomic sites in adult patients who sustained blunt trauma. METHODS: This retrospective study included 26 consecutive adults who sustained blunt torso trauma and an acute fracture of the thoracolumbar vertebral body, pelvis, or upper and lower extremities with a total of 32 fractures evaluated. Two-material decomposition images were analyzed for quantitative analysis. Statistical analysis was performed using the paired t-test and Shapiro-Wilk test for normality. RESULTS: There were statistically significant differences in the water and fat densities in the bone marrow at the site of an extremity, vertebral body, or pelvic fracture when compared to the normal anatomic equivalent (p < 0.01). CONCLUSION: In this preliminary study, DECT basis material images, using water (calcium) and fat (calcium) decomposition illustrated significant differences in water and fat content between fracture sites and normal bone in a variety of anatomical sites.

Auxetics-Inspired Tunable Metamaterials for Magnetic Resonance Imaging.

Auxetics refers to structures or materials with a negative Poisson's ratio, thereby capable of exhibiting counterintuitive behaviors. Herein, auxetic structures are exploited to design mechanically tunable metamaterials in both planar and hemispherical configurations operating at megahertz (MHz) frequencies, optimized for their application to magnetic resonance imaging (MRI). Specially, the reported tunable metamaterials are composed of arrays of interjointed unit cells featuring metallic helices, enabling auxetic patterns with a negative Poisson's ratio. The deployable deformation of the metamaterials yields an added degree of freedom with respect to frequency tunability through the resultant modification of the electromagnetic interactions between unit cells. The metamaterials are fabricated using 3D printing technology and an ≈20 MHz frequency shift of the resonance mode is enabled during deformation. Experimental validation is performed in a clinical (3.0 T) MRI system, demonstrating that the metamaterials enable a marked boost in radiofrequency field strength under resonance-matched conditions, ultimately yielding a dramatic increase in the signal-to-noise ratio (≈4.5×) of MRI. The tunable metamaterials presented herein offer a novel pathway toward the practical utilization of metamaterials in MRI, as well as a range of other emerging applications.

Fat Fraction Measurements Using a Three-Material Decomposition Dual-Energy CT Technique Accounting for Bone Minerals: Evaluation in a Bone Marrow Phantom Using MRI as Reference.

Conventional two-material dual-energy CT (DECT) decomposition is insufficient to model bone marrow, which contains three materials: bone minerals, red marrow (water), and yellow marrow (fat). We explore an image-domain three-material decomposition DECT technique accounting for bone minerals in a bone-water-fat phantom. Three-material decomposition fat fraction (FF3MD) exhibited stronger correlation than two-material decomposition fat fraction (FF2MD) with MRI-based fat fraction (r = 0.95 vs r = 0.69). With increasing bone minerals, correlation of FF3MD remained stable (r = 0.81-1.02), whereas correlation of FF2MD decreased (r = 0.21-0.65).

Acute cholecystitis: diagnostic value of dual-energy CT-derived iodine map and low-keV virtual monoenergetic images.

PURPOSE: To compare conventional and dual-energy CT (DECT) for the diagnosis of acute cholecystitis and gangrene. METHODS: Fifty-seven consecutive adult patients with abdominal pain who underwent IV contrast-enhanced abdominal DECT on a dual-layer (dlDECT) or rapid-switching (rsDECT) scanner from September, 2018 to April, 2021 with cholecystectomy and pathology-confirmed cholecystitis were retrospectively reviewed, and compared with 57 consecutive adult patients without cholecystitis from the same interval scanned with DECT. Images were reviewed independently by two abdominal radiologists with 12 and 16 years of experience in two sessions 4 weeks apart, blinded to clinical data. Initially, only blended reconstructions (simulating conventional single-energy CT images) were reviewed (CT). Subsequently, CT and DECT reconstructions including low-keV virtual monoenergetic images and iodine maps were reviewed. Gallbladder fossa hyperemia, pericholecystic fluid, subjective presence of gangrene, heterogeneous wall enhancement, sloughed membranes, intramural air, abscess, overall impression of the presence of acute cholecystitis, and intramural iodine density were assessed. RESULTS: Gallbladder fossa hyperemia was detected with increased sensitivity on DECT (R1, 61.4%; R2, 75.4%) vs. CT (R1, 22.8%; R2, 15.8%). DECT showed increased sensitivity for gangrene (R1, 24.6%; R2, 38.6%) vs. CT (R1, 5.3%; R2, 14%), heterogeneous wall enhancement (DECT: R1, 33.3%; R2, 63.2% vs. CT: R1, 7%; R2, 31.6%), and cholecystitis (DECT: R1, 86%; R2, 89.5% vs. CT: R1, 77.2%; R2, 70.2%). In addition, DECT was more sensitive for the detection of acute cholecystitis (R1, 86%; R2, 89.5%) vs. CT (R1, 77.2%; R2, 70.2%). Iodine density threshold of 1.2 mg/ml, 0.8 mg/mL, and 0.5 mg/mL showed specificity for gangrenous cholecystitis of 78.26%, 86.96%, and 95.65%, respectively, using the rsDECT platform. CONCLUSION: DECT showed improved sensitivity compared to conventional CT for detection of acute cholecystitis. Iodine density measurements may be helpful to diagnose gangrene.

Machine learning combining CT findings and clinical parameters improves prediction of length of stay and ICU admission in torso trauma.

OBJECTIVE: To develop machine learning (ML) models capable of predicting ICU admission and extended length of stay (LOS) after torso (chest, abdomen, or pelvis) trauma, by using clinical and/or imaging data. MATERIALS AND METHODS: This was a retrospective study of 840 adult patients admitted to a level 1 trauma center after injury to the torso over the course of 1 year. Clinical parameters included age, sex, vital signs, clinical scores, and laboratory values. Imaging data consisted of any injury present on CT. The two outcomes of interest were ICU admission and extended LOS, defined as more than the median LOS in the dataset. We developed and tested artificial neural network (ANN) and support vector machine (SVM) models, and predictive performance was evaluated by area under the receiver operating characteristic (ROC) curve (AUC). RESULTS: The AUCs of SVM and ANN models to predict ICU admission were up to 0.87 ± 0.03 and 0.78 ± 0.02, respectively. The AUCs of SVM and ANN models to predict extended LOS were up to 0.80 ± 0.04 and 0.81 ± 0.05, respectively. Predictions based on imaging alone or imaging with clinical parameters were consistently more accurate than those based solely on clinical parameters. CONCLUSIONS: The best performing models incorporated imaging findings and outperformed those with clinical findings alone. ML models have the potential to help predict outcomes in trauma by integrating clinical and imaging findings, although further research may be needed to optimize their performance. KEY POINTS: • Artificial neural network and support vector machine-based models were used to predict the intensive care unit admission and extended length of stay after trauma to the torso. • Our input data consisted of clinical parameters and CT imaging findings derived from radiology reports, and we found that combining the two significantly enhanced the prediction of both outcomes with either model. • The highest accuracy (83%) and highest area under the receiver operating characteristic curve (0.87) were obtained for artificial neural networks and support vector machines, respectively, by combining clinical and imaging features in the prediction of intensive care unit admission.

Liver trauma: hepatic vascular injury on computed tomography as a predictor of patient outcome.

OBJECTIVES: To evaluate hepatic vascular injury (HVI) on CT in blunt and penetrating trauma and assess its relationship to patient management and outcome. METHOD AND MATERIALS: This retrospective study was IRB approved and HIPAA compliant. Informed consent was waived. Included were patients ≥ 16 years old who sustained blunt or penetrating trauma with liver laceration seen on a CT performed at our institution within 24 h of presentation over the course of 10 years and 6 months (August 2007-February 2018). During this interval, 171 patients met inclusion criteria (123 males, 48 females; mean age 34; age range 17-80 years old). Presence of HVI was evaluated and liver injury was graded in a blinded fashion by two radiologists using the 1994 and 2018 American Association for the Surgery of Trauma (AAST) liver injury scales. Hospital length of stay and treatment (angioembolization or operative) were recorded from the electronic medical record. Multivariate linear regressions were used to determine our variables' impact on the length of stay, and logistic regressions were used for categorical outcomes. RESULTS: Of the included liver trauma patients, 25% had HVI. Patients with HVI had a 3.2-day longer length of hospital stay on average and had a 40.3-fold greater odds of getting angioembolization compared to those without. Patients with high-grade liver injury (AAST grades IV-V, 2018 criteria) had a 3.2-fold greater odds of failing non-operative management and a 14.3-fold greater odds of angioembolization compared to those without. CONCLUSION: HVI in liver trauma is common and is predictive of patient outcome and management. KEY POINTS: • Hepatic vascular injury occurs commonly (25%) with liver trauma. • Hepatic vascular injury is associated with increased length of hospital stay and angioembolization. • High-grade liver injury is associated with failure of non-operative management and with angioembolization.

Polarization insensitive, metamaterial absorber-enhanced long-wave infrared detector.

Detecting low energy photons, such as photons in the long-wave infrared range, is a technically challenging proposition using naturally occurring materials. In order to address this challenge, we herein demonstrate a micro-bolometer featuring an integrated metamaterial absorber (MA), which takes advantage of the resonant absorption and frequency selective properties of the MA. Importantly, our micro-bolometer exhibits polarization insensitivity and high absorption due to a novel metal-insulator-metal (MIM) absorber design, operating at 8-12 µm wavelength. The metamaterial structures we report herein feature an interconnected design, optimized towards their application to micro-bolometer-based, long-wave infrared detection. The micro-bolometers were fabricated using a combination of conventional photolithography and electron beam lithography (EBL), the latter owing to the small feature sizes within the design. The absorption response was designed using the coupled mode theory (CMT) and the finite integration technique, with the fabricated devices characterized using Fourier-transform infrared spectroscopy (FTIR). The metamaterial-based micro-bolometer exhibits a responsivity of approximately 198 V/W over the 8-12 µm wavelength regime, detectivity of ∼ 0.6 × 109 Jones, thermal response time of ∼ 3.3 ms, and a noise equivalent temperature difference (NETD) of ∼33 mK under 1mA biasing current at room-temperature and atmosphere pressure. The ultimate detectivity and NETD are limited by Johnson noise and heat loss with thermal convection through air; however, further optimization could be achieved by reducing the thermal conductivity via vacuum packaging. Under vacuum conditions, the detectivity may be increased in excess of two-fold, to ∼ 1.5 × 109 Jones. Finally, an infrared image of a soldering iron was generated using a single-pixel imaging process, serving as proof-of-concept of this detection platform. The results presented in this work pave the road towards high-efficiency and frequency-selective detection in the long-wave infrared range through the integration of infrared MAs with micro-bolometers.

Nonreciprocal Magnetic Coupling Using Nonlinear Meta-Atoms.

Breaking Lorentz reciprocity is fundamental to an array of functional radiofrequency (RF) and optical devices, such as isolators and circulators. The application of external excitation, such as magnetic fields and spatial-temporal modulation, has been employed to achieve nonreciprocal responses. Alternatively, nonlinear effects may also be employed to break reciprocity in a completely passive fashion. Herein, a coupled system comprised of linear and nonlinear meta-atoms that achieves nonreciprocity based on the coupling and frequency detuning of its constituent meta-atoms is presented. An analytical model is developed based on the coupled mode theory (CMT) in order to design and optimize the nonreciprocal meta-atoms in this coupled system. Experimental demonstration of an RF isolator is performed, and the contrast between forward and backward propagation approximates 20 dB. Importantly, the use of the CMT model developed herein enables a generalizable capacity to predict the limitations of nonlinearity-based nonreciprocity, thereby facilitating the development of novel approaches to breaking Lorentz reciprocity. The CMT model and implementation scheme presented in this work may be deployed in a wide range of applications, including integrated photonic circuits, optical metamaterials, and metasurfaces, among others.

Accuracy of Dual-Energy CT Virtual Unenhanced and Material-Specific Images: A Phantom Study.

OBJECTIVE. The purpose of this study was to determine the quantification accuracy of virtual unenhanced images and establish the lower limit of iodine quantification as a function of dose. MATERIALS AND METHODS. A large elliptical and cylindric phantom mimicking the patient abdomen was scanned on two commercial dual-energy CT scanners, an IQon Spectral CT (Philips Healthcare) and a Revolution CT with Gemstone Spectral Imaging Xtream suite (GE Healthcare). The phantom contained simulated soft tissue, blood, and bone with known elemental composition. It also contained simulated iodine concentrations (0.2-15.0 mg/mL) and iodine-enhanced blood (0.5-5.0 mg/mL). The mean absolute error in CT value for virtual unenhanced images and mean absolute percent error in iodine, calcium, and fat-specific images were measured. RESULTS. For virtual unenhanced images, when excluding the simulated bone, the mean absolute error in CT value was 8.0 ± 5.0 (SD) HU and 9.0 ± 6.2 HU for the IQon and the Revolution CT, respectively (p = 0.61). The mean error in CT value of the simulated bone was -90.5 ± 111.6 HU and -98.5 ± 117.8 HU on the IQon and the Revolution CT, respectively (p = 0.08). For iodine-specific images, the mean absolute percent error was 13.7% and 8.3% for the IQon and the Revolution CT, respectively, above 0.5 mg/mL iodine concentration, and 150% and 100% at less than 0.5 mg/mL iodine concentration. The mean absolute percent error increased from 16.2% at 100% radiation dose to 18.9% and 24% at 75% and 50% dose, respectively, on the IQon; and from 8.8% at 100% dose to 11.1% and 17.8% at 75% and 50%, respectively, on the Revolution CT. CONCLUSION. Virtual unenhanced images are reasonably accurate for simulated soft tissues and contrast materials, except for simulated bone. The lower limit of iodine quantification is radiation-dose dependent. For typical dose levels, 0.5 mg/mL iodine concentration is the lower threshold for iodine detection accuracy.

Time to conventional angiography in gastrointestinal bleeding: CT angiography compared to tagged RBC scan.

PURPOSE: To compare CT angiography (CTA) and tagged red blood cell (RBC) scan as a function of time from these initial imaging studies to subsequent conventional angiography and catheter-directed embolization in patients with gastrointestinal (GI) bleeding. METHODS: An IRB-approved retrospective study was conducted of 35 consecutive patients diagnosed with GI bleeding that received angiography for planned catheter-directed embolization. Of these patients, 20 were diagnosed with bleeding using a tagged RBC scan, whereas 15 were diagnosed using CTA. The lengths of time between diagnostic study order to study completion, diagnostic study completion to angiography, and total time from diagnostic study order to angiography were calculated. The results of both groups were compared using a t test with p value of < 0.05 considered statistically significant. RESULTS: The mean time from diagnostic study order to study completion was 3 h and 4 min for the CTA group and 5 h and 1 min for the tagged RBC scan group (p value = 0.0001). There was no statistically significant difference between the time to angiography after completion of the preceding diagnostic study. The total mean time from diagnostic study order to intervention was 6 h and 8 min for the CTA group and 9 h and 29 min for the tagged RBC scan group, a statistically significant difference (p value = 0.028). CONCLUSIONS: In patients requiring conventional angiography for GI bleeding, CT angiography results in a faster time to angiography than tagged RBC scan, which appears to be due to the longer duration required to complete the tagged RBC scan. Decreasing time to angiography is vital, as GI bleeding can be fatal and earlier diagnosis and intervention has the potential to reduce morbidity and mortality, while also increasing sensitivity of angiography. These findings may assist ordering clinicians in deciding on the appropriate diagnostic study.

Nonhomogeneous Gadolinium Retention in the Cerebral Cortex after Intravenous Administration of Gadolinium-based Contrast Agent in Rats and Humans.

Background Gadolinium retention after repeated gadolinium-based contrast agent (GBCA) exposure has been reported in subcortical gray matter. However, gadolinium retention in the cerebral cortex has not been systematically investigated. Purpose To determine whether and where gadolinium is retained in rat and human cerebral cortex. Materials and Methods The cerebral cortex in Sprague-Dawley rats treated with gadopentetate dimeglumine (three doses over 4 weeks; cumulative gadolinium dose, 7.2 mmol per kilogram of body weight; n = 6) or saline (n = 6) was examined with antemortem MRI. Two human donors with repeated GBCA exposure (three and 15 doses; 1 and 5 months after exposure), including gadopentetate dimeglumine, and two GBCA-naive donors were also evaluated. Elemental brain maps (gadolinium, phosphorus, zinc, copper, iron) for rat and human brains were constructed by using laser ablation inductively coupled plasma mass spectrometry. Results Gadopentetate dimeglumine-treated rats showed region-, subregion-, and layer-specific gadolinium retention in the neocortex (anterior cingulate cortex: mean gadolinium concentration, 0.28 µg ∙ g-1 ± 0.04 [standard error of the mean]) that was comparable (P > .05) to retention in the allocortex (mean gadolinium concentration, 0.33 µg ∙ g-1 ± 0.04 in piriform cortex, 0.24 µg ∙ g-1 ± 0.04 in dentate gyrus, 0.17 µg ∙ g-1 ± 0.04 in hippocampus) and subcortical structures (0.47 µg ∙ g-1 ± 0.10 in facial nucleus, 0.39 µg ∙ g-1 ± 0.10 in choroid plexus, 0.29 µg ∙ g-1 ± 0.05 in caudate-putamen, 0.26 µg ∙ g-1 ± 0.05 in reticular nucleus of the thalamus, 0.24 µg ∙ g-1 ± 0.04 in vestibular nucleus) and significantly greater than that in the cerebellum (0.17 µg ∙ g-1 ± 0.03, P = .01) and white matter tracts (anterior commissure: 0.05 µg ∙ g-1 ± 0.01, P = .002; corpus callosum: 0.05 µg ∙ g-1 ± 0.02, P = .001; cranial nerve: 0.02 µg ∙ g-1 ± 0.01, P = .004). Retained gadolinium colocalized with parenchymal iron. T1-weighted MRI signal intensification was not observed. Gadolinium retention was detected in the cerebral cortex, pia mater, and pia-ensheathed leptomeningeal vessels in two GBCA-exposed human brains but not in two GBCA-naive human brains. Conclusion Repeated gadopentetate dimeglumine exposure is associated with gadolinium retention in specific regions, subregions, and layers of cerebral cortex that are critical for higher cognition, affect, and behavior regulation, sensorimotor coordination, and executive function. © RSNA, 2019 Online supplemental material is available for this article. See also the editorial by Kanal in this issue.