Quantification of trabecular structure in the distal femur using magnetic resonance phase imaging

A new approach for quantifying trabecular bone tissue using the phase images of a simple gradient-echo sequence is presented. The proposed method is based on the hypothesis that the differences in susceptibility between bone and bone marrow cause magnetic field (i.e., precession phase) variations between the image voxels. Phase images of the distal femur were obtained in vivo and characterised with the use of the phase variance. Computer simulations and experimental results indicate that the distribution of the phases varies with echo time and image resolution, as expected. Keeping these fixed, however, the phase variance is found to strongly reflect variations in trabecular structure.     

Ictal imaging using functional magnetic resonance

Magnetic resonance imaging (MRI) can now provide maps of human brain function with high spatial and temporal resolution. This noninvasive technique can also map the coritical activation that occurs during focal seizures, as demonstrated here by the results obtained using a conventional 1.5 T clinical MRI system for the investigation of a 4-year-old boy suffering from frequent partial motor seizures of his right side. FLASH images (TE = 60 ms) were acquired every 10 s over a period of 25 min, and activation images derived by subtracting baseline images from images obtained during clinical seizures. Functional MRI revealed sequential activation associated with specific gyri within the left hemisphere with each of five consecutive clinical seizures, and also during a period that was not associated with a detectable clinical seizure. The activated regions included gyri that were structurally abnormal. These results demonstrate (a) that functional MRI can potentially provide new insights into the dynamic events that occur in the epileptic brain and their relationship to brain structure; and (b) that there is the possibility of obtaining similar information in the absence of clinical seizures, suggesting the potential for studies in patients with interictal electrical disturbances.     

170 nm nuclear magnetic resonance imaging using magnetic resonance force microscopy

We demonstrate one-dimensional nuclear magnetic resonance imaging of the semiconductor GaAs with 170 nm slice separation and resolve two regions of reduced nuclear spin polarization density separated by only 500 nm. This was achieved by force detection of the magnetic resonance, magnetic resonance force microscopy (MRFM), in combination with optical pumping to increase the nuclear spin polarization. Optical pumping of the GaAs created spin polarization up to 12 times larger than the thermal nuclear spin polarization at 5K and 4T. The experiment was sensitive to sample volumes of 50 microm(3) containing approximately 4 x 10(11)71 Ga/Hz. These results demonstrate the ability of force-detected magnetic resonance to apply magnetic resonance imaging to semiconductor devices and other nanostructures.     

Low-field magnetic resonance imaging using multiplicative regularization

In this paper we present a magnetic resonance imaging (MRI) technique that is based on multiplicative regularization. Instead of adding a regularizing objective function to a data fidelity term, we multiply by such a regularizing function. By following this approach, no regularization parameter needs to be determined for each new data set that is acquired. Reconstructions are obtained by iteratively updating the images using short-term conjugate gradient-type update formulas and Polak-Ribière update directions. We show that the algorithm can be used as an image reconstruction algorithm and as a denoising algorithm. We illustrate the performance of the algorithm on two-dimensional simulated low-field MR data that is corrupted by noise and on three-dimensional measured data obtained from a low-field MR scanner. Our reconstruction results show that the algorithm effectively suppresses noise and produces accurate reconstructions even for low-field MR signals with a low signal-to-noise ratio.     

Diagnosis of sacroiliac joint infection in pregnancy by magnetic resonance imaging

MR imaging was used to diagnose sacroiliac joint infection as the source of persistent back and hip pain in a pregnant woman with bacterial endocarditis and fever who refused diagnostic exposure to any ionizing radiation.     

High-field magnetic resonance imaging using solenoid radiofrequency coils

High-resolution magnetic resonance imaging using dedicated high-field radiofrequency micro-coils at 16.4 T (700 MHz) was investigated. Specific solenoid coils primarily using silver and copper as conductors with enamel and polyurethane coatings were built to establish which coil configuration produces the best image. Image quality was quantified using signal-to-noise ratio and signal variation over regions of interest. Benchmarking was conducted using 5-mm diameter coils, as this size is comparable to an established coil of the same size. Our 1.4-mm-diameter coils were compared directly to each other, from which we deduce performance as a function of conductor material and coating. A variety of materials and conductor coatings allowed us to choose an optimal design, which we used to image a kidney section at 10-micron resolution. We applied zero-fill extrapolation to achieve 5-micron resolution.     

Mapping of cerebrovascular reactivity using BOLD magnetic resonance imaging

Blood oxygen level-dependent (BOLD) contrast MRI is a simple non-invasive method of estimating "perfusion," and combined with a vasodilatory stimulus, may allow estimation of cerebral vascular reserve. We compared BOLD carbon dioxide (CO2) reactivity in the middle cerebral artery (MCA) perfusion territory to MCA flow velocity reactivity determined using transcranial Doppler ultrasound (TCD) in 16 patients with unilateral carotid artery stenosis or occlusion. Both BOLD and TCD reactivities were calculated from measurements acquired when the subjects were breathing air, and again when breathing a 6% CO2/air mixture, and were normalized by dividing by the difference in end tidal (ET) CO2. There was a significant correlation between interhemispheric MCA reactivity difference (contralateral-ipsilateral to the stenosis or occlusion) determined by BOLD MRI and TCD (r = 0.75, p < 0.001). In contrast, treating each hemisphere individually, there was no correlation between the absolute BOLD and TCD MCA CO2 reactivities (r = 0.08, p = 0.670). This appeared to be due to a variable BOLD signal change in the non-stenosed hemisphere between subjects, with little change in the normal hemisphere of a few subjects. In one patient, focal regions of reduced reactivity were seen in non-infarcted regions of the stenosed hemisphere, in the borderzones between arterial territories. BOLD reactivity maps provide information on the whole MCA territory reactivity, and may identify small regions of impaired reactivity which are not detected using TCD. However, BOLD reactivity maps only appear to provide semi-quantitative rather than quantitative data.     

Parallel magnetic resonance imaging using coils with localized sensitivities

The purpose of this study was to present clinical examples and illustrate the inefficiencies of a conventional reconstruction using a commercially available phased array coil with localized sensitivities. Five patients were imaged at 1.5 T using a cardiac-synchronized gadolinium-enhanced acquisition and a commercially available four-element phased array coil. Four unique sets of images were reconstructed from the acquired k-space data: (a) sum-of-squares image using four elements of the coil; localized sum-of-squares images from the (b) anterior coils and (c) posterior coils and a (c) local reconstruction. Images were analyzed for artifacts and usable field-of-view. Conventional image reconstruction produced images with fold-over artifacts in all cases spanning a portion of the image (mean 90 mm; range 36-126 mm). The local reconstruction removed fold-over artifacts and resulted in an effective increase in the field-of-view (mean 50%; range 20-70%). Commercially available phased array coils do not always have overlapping sensitivities. Fold-over artifacts can be removed using an alternate reconstruction method. When assessing the advantages of parallel imaging techniques, gains achieved using techniques such as SENSE and SMASH should be gauged against the acquisition time of the localized method rather than the conventional sum-of-squares method.     

Autonomous magnetic resonance imaging

Access to Magnetic Resonance Imaging (MRI) across developing countries ranges from being prohibitive to scarcely available. For example, eleven countries in Africa have no scanners. One critical limitation is the absence of skilled manpower required for MRI usage. Some of these challenges can be mitigated using autonomous MRI (AMRI) operation. In this work, we demonstrate AMRI to simplify MRI workflow by separating the required intelligence and user interaction from the acquisition hardware. AMRI consists of three components: user node, cloud and scanner. The user node voice interacts with the user and presents the image reconstructions at the end of the AMRI exam. The cloud generates pulse sequences and performs image reconstructions while the scanner acquires the raw data. An AMRI exam is a custom brain screen protocol comprising of one T₁-, T₂- and T₂*-weighted exams. A neural network is trained to incorporate Intelligent Slice Planning (ISP) at the start of the AMRI exam. A Look Up Table was designed to perform intelligent protocolling by optimizing for contrast value while satisfying signal to noise ratio and acquisition time constraints. Data were acquired from four healthy volunteers for three experiments with different acquisition time constraints to demonstrate standard and self-administered AMRI. The source code is available online. AMRI achieved an average SNR of 22.86 ± 0.89 dB across all experiments with similar contrast. Experiment #3 (33.66% shorter table time than experiment #1) yielded a SNR of 21.84 ± 6.36 dB compared to 23.48 ± 7.95 dB for experiment #1. AMRI can potentially enable multiple scenarios to facilitate rapid prototyping and research and streamline radiological workflow. We believe we have demonstrated the first Autonomous MRI of the brain.     

Contrast-enhanced high-resolution magnetic resonance imaging of autologous cartilage implants of the knee joint

Over the past decade, a number of interventions for durable cartilage repair have emerged. Magnetic resonance (MR) tomography is an excellent noninvasive method for monitoring cartilage repair tissues throughout the postsurgical period. However, evaluating cartilage morphology after matrix-based autologous cartilage implantation (ACI) with MR imaging (MRI) still remains a challenge. In this study, we combined a high-resolution cartilage-sensitive fast-spin echo (FSE) sequence with intravenous application of a contrast agent for enhancing synovial fluid. Two independent musculoskeletal radiologists interpreted the pictures for the thickness, length and surface of the cartilage implants. A multivariate two-way analysis of variance with two repeated measures was performed and showed that evaluation of cartilage implant morphology was significantly improved after the application of gadodiamide on proton density FSE images. Contrast-enhanced MRI of articular cartilages is a promising technique in the postoperative follow-up of patients after ACI.     

Porosity measurements in natural porous rocks using magnetic resonance imaging

A magnetic resonance imaging (MRI) method is presented to measure localized porosity values inside natural porous rocks for the purpose of obtaining frequency distributions of the porosity (porosity distributions). The method is applied to study six different cores, including three Berea sandstone samples, Casper sandstone, Indiana limestone, and San Andres dolomite. An image of the porosity is shown for a transverse and a longitudinal slice in order to show qualitative variations of the porosity within each core sample. The porosity distribution for the entire core has been acquired, and it is shown with a Gaussian fit to the data. In addition, for cores known to have a layered structure, a bimodal distribution is fit to the data, and the fit is used to estimate the value of the porosity for two characteristic types of layers within the core sample.     

Improved simultaneous multislice magnetic resonance imaging using total variation regularization

Controlled aliasing in parallel imaging results in higher acceleration(CAIPIRINHA) for simultaneous multislice imaging has been proposed recently, which combines multiband excitation and phase cycling techniques to reduce scan time and improve subsequent imaging reconstruction. In this work, the total variation(TV) regularization method is used to further improve CAIPIRINHA. The TV regularization uses an edge-preserving prior, which establishes a relationship between neighboring pixels for image reconstruction. It reduces artifacts and suppresses noise amplification simultaneously.The results are presented with a standard eight-channel head coil with an acceleration factor of 4, where the TV-regularized CAIPIRINHA generates an improved reconstruction as compared with a typical nonregularized CAIPIRINHA.     

Towards T 1-limited magnetic resonance imaging using Rabi beats

Two proof-of-principle experiments toward T ₁-limited magnetic resonance imaging with NV centers in diamond are demonstrated. First, a large number of Rabi oscillations is measured and it is demonstrated that the hyperfine interaction due to the NV??s ¹⁴N can be extracted from the beating oscillations. Second, the Rabi beats under V-type microwave excitation of the three hyperfine manifolds is studied experimentally and described theoretically.