Magnetic Resonance Imaging Setup Research Articles

Shaping the RF Transmit Field in 7T MRI Using a Nonuniform Metasurface Constructed of Short Conducting Strips.

The ability of metamaterial structures to offer unique properties and new solutions has opened new avenues in a wide range of applications, including super-resolution in optics and efficient antennas in radiofrequency (RF) engineering. In magnetic resonance imaging (MRI), metamaterials hold the promise of increasing the RF magnetic field intensity while minimizing power deposition. Here, we propose a metasurface based on a two-dimensional (2D) array of short conducting strips combined with a high dielectric substrate, which was tuned to operate at ultrahigh field 7T human MRI. While studied in optics and electromagnetics in the GHz-to-THz range, this study is the first to design such a metasurface for proton imaging at 7T MRI. We performed electromagnetic (EM) simulation of the brain MRI setup with the new metasurface placed in the proximity to the temporal lobe, which showed 2.2-fold local increase in the RF transmit efficiency, with superior performance than an array of electric dipoles. In this study, we also investigate the effect of the spatial distribution of the subunits to control the target RF field's distribution. While the common design is based on a uniform distribution of the subunits, nonuniform distribution, such as a denser center (convex) or more condensed edges (concave), provides an extra dimension to tailor both the magnetic and electric fields. The concave distribution achieved 1.5-1.8-fold reduction in the power deposition compared to the uniform distribution in the brain MRI setups examined.

Tibial and femoral articular cartilage exhibit opposite outcomes for T1ρ and T2* relaxation times in response to acute compressive loading in healthy knees

Abnormal loading is thought to play a key role in the disease progression of cartilage, but our understanding of how cartilage compositional measurements respond to acute compressive loading in-vivo is limited. Ten healthy subjects were scanned at two timepoints (7 ± 3 days apart) with a 3 T magnetic resonance imaging (MRI) scanner. Scanning sessions included T1ρ and T2* acquisitions of each knee in two conditions: unloaded (traditional MRI setup) and loaded in compression at 40 % bodyweight as applied by an MRI-compatible loading device. T1ρ and T2* parameters were quantified for contacting cartilage (tibial and femoral) and non-contacting cartilage (posterior femoral condyle) regions. Significant effects of load were found in contacting regions for both T1ρ and T2*. The effect of load (loaded minus unloaded) in femoral contacting regions ranged from 4.1 to 6.9 ms for T1ρ, and 3.5 to 13.7 ms for T2*, whereas tibial contacting regions ranged from −5.6 to −1.7 ms for T1ρ, and −2.1 to 0.7 ms for T2*. Notably, the responses to load in the femoral and tibial cartilage revealed opposite effects. No significant differences were found in response to load between the two visits. This is the first study that analyzed the effects of acute loading on T1ρ and T2* measurements in human femoral and tibial cartilage separately. The results suggest the effect of acute compressive loading on T1ρ and T2* was: 1) opposite in the femoral and tibial cartilage; 2) larger in contacting regions than in non-contacting regions of the femoral cartilage; and 3) not different visit-to-visit.

Harnessing Surface Plasmons for Magnetic Resonance Imaging Applications

Metamaterials have great potential to control near-field electromagnetic response at will. Lately metamaterial-based devices have been used to manipulate the rf magnetic field in magnetic resonance imaging (MRI) via Fabry-Perot resonances or subdiffraction lenses. This work shows that a negative-permeability metamaterial incorporated into an MRI setup supports magnetic surface plasmons, allowing local boosting of the magnetic field and an improved signal-to-noise ratio. These results could pave the way for high-performance MRI systems.

Magnetic Heating Stimulated Cargo Release with Dose Control using Multifunctional MR and Thermosensitive Liposome.

Rationale: Magnetic resonance imaging (MRI) is one of the most widely used diagnostic tools in the clinic. In this setting, real-time monitoring of therapy and tumor site would give the clinicians a handle to observe therapeutic response and to quantify drug amount to optimize the treatment. In this work, we developed a liposome-based cargo (cancer drugs) delivery strategy that could simultaneously monitor the real-time alternating magnetic field-induced cargo release from the change in MRI relaxation parameter R1 and the location and condition of liposome from the change in R2. The tumor site can then be monitored during the cargo release because liposomes would passively target the tumor site through the enhanced permeability and retention (EPR) effect. Physical insights from the experimental results and corresponding Monte Carlo spin dynamics simulations were also discussed.Methods: Superparamagnetic iron oxide (SPIO) nanoparticles, diethylenetriaminepentaacetic acid gadolinium(III) (Gd(III)-DTPA), and a model cancer drug (fluorescein) were co-loaded in PEGylated thermosensitive liposomes. The liposomes were characterized by transmission electron cryo-microscopy (cryoTEM), dynamic light scattering (DLS), and inductively coupled plasma optical emission spectrometry (ICP-OES). Alternating magnetic field (AMF) was used to create controlled mild hyperthermia (39-42°C) and facilitate controlled cargo (fluorescein) release from the thermosensitive liposomes. MRI relaxation parameters, R1 and R2, were measured at room temperature. The temporal variation in R1 was used to obtain the temporal profile of cargo release. Due to their similar sizes, both the gadolinium and cargo (model cancer drug fluorescein) would come out of the liposomes together as a result of heating. The temporal variation in R2 was used to monitor SPIO nanoparticles to enhance the tumor contrast. Monte Carlo spin dynamics simulations were performed by solving the Bloch equations and modeling SPIO nanoparticles as magnetized impenetrable spheres.Results: TEM images and DLS measurements showed the diameter of the liposome nanoparticle ~ 200 nm. AMF heating showed effective release of the model drug. It was found that R1 increased linearly by about 70% and then saturated as the cargo release process was completed, while R2 remained approximately constant with an initial 7%-drop and then recovered. The linear increase in R1 is consistent with the expected linear cargo release with time upon AMF heating. Monte Carlo spin dynamics simulations suggest that the initial temporal fluctuation of R2 is due to the plausible changes of SPIO aggregation and the slow non-recoverable degradation of liposomal membrane that increases water permeability with time by the heating process. The simulations show an order of magnitude increase in R2 at higher water permeability.Conclusion: We have performed MR parameter study of the release of a cargo (model cancer drug, fluorescein) by magnetic heating from thermosensitive multifunctional liposomes loaded with dual contrast agents. The size of the liposome nanoparticles loaded with model cancer drug (fluorescein), gadolinium chelate, and SPIO nanoparticles was appropriate for a variety of cancer therapies. A careful and detailed analysis with theoretical explanation and simulation was carried out to investigate the correlation between MRI relaxation parameters, R1 and R2, and different cargo release fractions. We have quantified the cargo release using R1, which shows a linear relation between each other. This result provides a strong basis for the dosage control of drug delivered. On the other hand, the fairly stable R2 with almost constant value suggests that it could be used to monitor the position and condition of the liposomal site, as SPIO nanoparticles mostly remained in the aqueous core of the liposome. Because our synthesized SPIO-encapsulated liposomes could be targeted to tumor site passively by the EPR effect, or actively through magnetofection, this study provides a solid ground for developing MR cancer theranostics in combination of this nanostructure and AMF heating strategy. Furthermore, our simulation results predict a sharp increase in R2 during the AMF heating, which opens up the exciting possibility of high-resolution, high-contrast real-time imaging of the liposomal site during the drug release process, provided AMF heating could be incorporated into an MRI setup. Our use of the clinically approved materials, along with confirmation by theoretical simulations, make this technique a promising candidate for translational MR cancer theranostics.

Multiparametric Whole-body MRI with Diffusion-weighted Imaging and ADC Mapping for the Identification of Visceral and Osseous Metastases From Solid Tumors

The purpose of this study was to investigate the use of multiparametric, whole-body, diffusion-weighted imaging (WB-DWI) and apparent diffusion coefficient (ADC) maps with T2-weighted magnetic resonance imaging (MRI) at 3T for the detection and monitoring of metastatic disease in patients. Fifty-four participants (32 healthy subjects and 22 patients) were scanned with WB-DWI methods using a 3T MRI scanner. Axial, sagittal, or coronal fat-suppressed T2-weighted (T2WI), T1-weighted (T1WI), and DWI images were acquired. Total MRI acquisition and set-up time was approximately 45 minutes. Metastatic disease on MRI was confirmed based on T2WI characteristics. The number of lesions was established on computed tomography (CT) or positron emission tomography (PET-CT). Whole-body ADC maps and T2WI were constructed, and region-of-interests were drawn in normal and abnormal-appearing tissue for quantitative analysis. Statistical analysis was performed using a paired t tests and P < .05 was considered statistically significant. There were 91 metastatic lesions detected from the CT or PET-CT with a missed recurrent lesion in the prostate. Multiparametric WB-MRI had excellent sensitivity (96%) for detection of metastatic lesions compared to CT. ADC map values and the ADC ratio in metastatic bone lesions were significantly increased (P < .05) compared to normal bone. In soft tissue, ADC map values and ratios in metastatic lesions were decreased compared to normal soft tissue. We have demonstrated that multiparametric WB-MRI is feasible for oncologic staging to identify bony and visceral metastasis in breast, prostate, pancreatic, and colorectal cancers. WB-MRI can be tailored to fit the patient, such that an "individualized patient sequence" can be developed for a comprehensive evaluation for staging and response during treatment.

Detection superiority of 7T MRI protocol in patients with epilepsy and suspected focal cortical dysplasia.

In 11 adult patients with suspicion of Focal cortical dysplasia (FCD) on 1.5 T (n = 1) or 3 T (n = 10) magnetic resonance imaging (MRI), 7 T MRI was performed. Visibility, extent, morphological features and delineation were independently rated and subsequently discussed by three observers. Additionally, head-to-head comparisons with corresponding 3 T images were made in the eight patients with a previous 3 T MRI and sustained suspicion of FCD. Comparison with histopathology was done in the five patients that underwent surgery. All lesions, seen at 1.5 and 3 T, were also recognized on 7 T. At 7 T FLAIR highlighted the FCD-like lesions best, whereas T2 and T2* were deemed better suited to review structure and extent of the lesion. Image quality with the used 7 T MRI setup was higher than the quality with the used 3 T MRI setup. In 2 out of 11 patients diagnosis changed, in one after re-evaluation of the images, and in the other based on histopathology. With the used 7 T MRI setup, FCD-like lesions can be detected with more confidence and detail as compared to lower field strength. However, concordance between radiologic diagnosis and final diagnosis seems to be lower than expected.Electronic supplementary materialThe online version of this article (doi:10.1007/s13760-016-0662-x) contains supplementary material, which is available to authorized users.

Upright Magnetic Resonance Imaging Tasks in the Knee Osteoarthritis Population: Relationships Between Knee Flexion Angle, Self-Reported Pain, and Performance

ObjectiveTo characterize the ability of patients with symptomatic knee osteoarthritis (OA) to perform a weight-bearing activity compatible with upright magnetic resonance imaging (MRI) scanning and how this ability is affected by knee pain symptoms and flexion angles. DesignCross-sectional observational study assessing effects of knee flexion angle, pain level, and study sequence on accuracy and duration of performing a task used in weight-bearing MRI evaluation. Visual feedback of knee position from an MRI compatible sensor was provided. Pain levels were self-reported on a standardized scale. SettingSimulated MRI setup in a research laboratory. ParticipantsConvenience sample of individuals (N=14; 9 women, 5 men; mean, 69±14y) with symptomatic knee OA. InterventionsNot applicable. Main Outcome MeasuresAveraged absolute and signed angle error from target knee flexion for each minute of trial and duration tolerance (the duration that subjects maintained position within a prescribed error threshold). ResultsAbsolute targeting error increased at longer trial durations (P<.001). Duration tolerance decreased with increasing pain (mean ± SE, no pain: 3min 19s±11s; severe pain: 1min 49s±23s; P=.008). Study sequence affected duration tolerance (first knee: 3min 5s±9.1s; second knee: 2min 19s±9.7s; P=.015). ConclusionsThe study provided evidence that weight-bearing MRI evaluations based on imaging protocols in the range of 2 to 3 minutes are compatible with patients reporting mild to moderate knee OA-related pain.

Prospective observer and software-based assessment of magnetic resonance imaging quality in head and neck cancer: Should standard positioning and immobilization be required for radiation therapy applications?

PurposeThe purpose of this study was to investigate the potential of a head and neck magnetic resonance simulation and immobilization protocol on reducing motion-induced artifacts and improving positional variance for radiation therapy applications. Methods and materialsTwo groups (group 1, 17 patients; group 2, 14 patients) of patients with head and neck cancer were included under a prospective, institutional review board–approved protocol and signed informed consent. A 3.0-T magnetic resonance imaging (MRI) scanner was used for anatomic and dynamic contrast-enhanced acquisitions with standard diagnostic MRI setup for group 1 and radiation therapy immobilization devices for group 2 patients. The impact of magnetic resonance simulation/immobilization was evaluated qualitatively by 2 observers in terms of motion artifacts and positional reproducibility and quantitatively using 3-dimensional deformable registration to track intrascan maximum motion displacement of voxels inside 7 manually segmented regions of interest. ResultsThe image quality of group 2 (29 examinations) was significantly better than that of group 1 (50 examinations) as rated by both observers in terms of motion minimization and imaging reproducibility (P < .0001). The greatest average maximum displacement was at the region of the larynx in the posterior direction for patients in group 1 (17 mm; standard deviation, 8.6 mm), whereas the smallest average maximum displacement was at the region of the posterior fossa in the superior direction for patients in group 2 (0.4 mm; standard deviation, 0.18 mm). Compared with group 1, maximum regional motion was reduced in group 2 patients in the oral cavity, floor of mouth, oropharynx, and larynx regions; however, the motion reduction reached statistical significance only in the regions of the oral cavity and floor of mouth (P < .0001). ConclusionsThe image quality of head and neck MRI in terms of motion-related artifacts and positional reproducibility was greatly improved by use of radiation therapy immobilization devices. Consequently, immobilization with external and intraoral fixation in MRI examinations is required for radiation therapy application.

Multi-scale characterization of pasta during cooking using microscopy and real-time magnetic resonance imaging

Macroscopic properties of pasta, such as the texture, are formed during cooking by a complex interplay of water and heat with the structuring agents starch and gluten. The impact of the starch-to-gluten ratio on microstructure and water distribution in pasta was analyzed by a multi-scale approach combining magnetic resonance imaging (MRI) and light microscopy. The cooking process and thus the water distribution was monitored non-invasively using 1H MRI in real-time with a temporal resolution of 45s. Our MRI set-up allowed following the water ingress by imaging the reduction of the uncooked core. The water ingress rate was neither dependent on pasta composition nor on the presence of salt in the cooking media (0.7% NaCl). Starch-rich samples showed a more homogeneous water distribution in the gelatinized zone, which was mirrored in a more homogeneous microstructure. In contrast, gluten-rich samples showed both a heterogeneous water distribution and microstructure. Thus, the gluten content affected local water content in the gelatinized zone but not the water ingress.

Low field MRI with magnetoresistive mixed sensors

Low-field magnetic resonance imaging (MRI) requires very sensitive magnetometers for detection of weak magnetic signals in femtoTesla range. Magnetoresistive mixed sensors could be used as the sensitive magnetometers [1]. These sensors are the combination of a Giant Magnetoresistive (GMR) field sensor and a superconducting loop used as a flux-to-field transformer. A MRI set-up based on mixed sensor has been mounted. This system operates in static field up to 10mT without shielded room. We will present the LF-MRI setup with the implementation of mixed sensors and the first MRI images obtained at low field without prepolarization.

SU‐E‐J‐78: Initial Experience with MR Simulation in Radiation Therapy Using An Open‐Platform MR Scanner: Considerations for Routine Clinical Use

Purpose: To evaluate the considerations associated with use of magnetic resonance imaging (MRI) as a simulation tool in radiation therapy. Methods: A 1.0 Tesla(1T) Open‐platform MRI scanner (Panorama, Philips Healthcare) was used to acquire whole pelvis T1, T2 and 3D bTFE images on 3 prostate cancer patients to define extra capsular involvement. For the MRI setup, a flat table top insert (Civco) and our standard immobilization devices, were used to reproduce patient positioning during treatment. MR images were used in conjunction with CT simulation datasets to define anatomical structures and validate typical treatment planning clinical flow with the goal of using MRI as a stand‐alone modality. Contouring was done on T2 images and validated against T1 and 3D bTFE datasets. Dose calculation on MRI was performed by assigning bulk densities to structures based on average CT intensities (HUs) and compared with CT calculations using heterogeneity correction. Atlas‐based contouring has been investigated to improve on organ‐specific HU representation. Results: Significant differences in organ contours were noted between MRI and CT. For the 3 patients, with MR, reduction in prostate and penile bulb volumes was on average, 25% (max. 36%) and 22% (max. 31%), respectively. Seminal vesicle volumes varied on average by 71% (max. of 202%). MR was found to be especially useful in defining boundaries of the prostate and penile bulb; the latter structure was indiscernible on CT. MRI‐based calculations with assigned bulk densities agreed within 1% of CT‐based planning calculations with heterogeneity correction for all cases. Conclusions: This study confirms the feasibility of using MRI as a sole imaging modality for treatment planning simulation and dose calculations. Significant volume reduction in the PTV based on the MRI images may be beneficial for dose escalation studies, and improved structure delineation affords possible additional dose sparing to surrounding critical organs.