Objective To simulate the magnetic and electric fields produced by RF coil geometries commonly used at low field. Based on these simulations, the specific absorption rate (SAR) efficiency can be... Show moreObjective To simulate the magnetic and electric fields produced by RF coil geometries commonly used at low field. Based on these simulations, the specific absorption rate (SAR) efficiency can be derived to ensure safe operation even when using short RF pulses and high duty cycles.Methods Electromagnetic simulations were performed at four different field strengths between 0.05 and 0.1 T, corresponding to the lower and upper limits of current point-of-care (POC) neuroimaging systems. Transmit magnetic and electric fields, as well as transmit efficiency and SAR efficiency were simulated. The effects of a close-fitting shield on the EM fields were also assessed. SAR calculations were performed as a function of RF pulse length in turbo-spin echo (TSE) sequences.Results Simulations of RF coil characteristics and B-1(+) transmit efficiencies agreed well with corresponding experimentally determined parameters. Overall, the SAR efficiency was, as expected, higher at the lower frequencies studied, and many orders of magnitude greater than at conventional clinical field strengths. The tight-fitting transmit coil results in the highest SAR in the nose and skull, which are not thermally sensitive tissues. The calculated SAR efficiencies showed that only when 180 degrees refocusing pulses of duration similar to 10 ms are used for TSE sequences does SAR need to be carefully considered.Conclusion This work presents a comprehensive overview of the transmit and SAR efficiencies for RF coils used for POC MRI neuroimaging. While SAR is not a problem for conventional sequences, the values derived here should be useful for RF intensive sequences such as T-1 rho, and also demonstrate that if very short RF pulses are required then SAR calculations should be performed. Show less
Koolstra, K.; O'Reilly, T.; Bornert, P.; Webb, A. 2021
Objective To correct for image distortions produced by standard Fourier reconstruction techniques on low field permanent magnet MRI systems with strong B-0 inhomogeneity and gradient field... Show moreObjective To correct for image distortions produced by standard Fourier reconstruction techniques on low field permanent magnet MRI systems with strong B-0 inhomogeneity and gradient field nonlinearities. Materials and methods Conventional image distortion correction algorithms require accurate Delta B-0 maps which are not possible to acquire directly when the B-0 inhomogeneities also produce significant image distortions. Here we use a readout gradient time-shift in a TSE sequence to encode the B-0 field inhomogeneities in the k-space signals. Using a non-shifted and a shifted acquisition as input, Delta B-0 maps and images were reconstructed in an iterative manner. In each iteration, Delta B-0 maps were reconstructed from the phase difference using Tikhonov regularization, while images were reconstructed using either conjugate phase reconstruction (CPR) or model-based (MB) image reconstruction, taking the reconstructed field map into account. MB reconstructions were, furthermore, combined with compressed sensing (CS) to show the flexibility of this approach towards undersampling. These methods were compared to the standard fast Fourier transform (FFT) image reconstruction approach in simulations and measurements. Distortions due to gradient nonlinearities were corrected in CPR and MB using simulated gradient maps. Results Simulation results show that for moderate field inhomogeneities and gradient nonlinearities, Delta B-0 maps and images reconstructed using iterative CPR result in comparable quality to that for iterative MB reconstructions. However, for stronger inhomogeneities, iterative MB reconstruction outperforms iterative CPR in terms of signal intensity correction. Combining MB with CS, similar image and Delta B-0 map quality can be obtained without a scan time penalty. These findings were confirmed by experimental results. Discussion In case of B-0 inhomogeneities in the order of kHz, iterative MB reconstructions can help to improve both image quality and Delta B-0 map estimation. Show less
As low-field MRI technology is being disseminated into clinical settings around the world, it is important to assess the image quality required to properly diagnose and treat a given disease and... Show moreAs low-field MRI technology is being disseminated into clinical settings around the world, it is important to assess the image quality required to properly diagnose and treat a given disease and evaluate the role of machine learning algorithms, such as deep learning, in the enhancement of lower quality images. In this post hoc analysis of an ongoing randomized clinical trial, we assessed the diagnostic utility of reduced-quality and deep learning enhanced images for hydrocephalus treatment planning. CT images of post-infectious infant hydrocephalus were degraded in terms of spatial resolution, noise, and contrast between brain and CSF and enhanced using deep learning algorithms. Both degraded and enhanced images were presented to three experienced pediatric neurosurgeons accustomed to working in low-to middle-income countries (LMIC) for assessment of clinical utility in treatment planning for hydrocephalus. In addition, enhanced images were presented alongside their ground truth CT counterparts in order to assess whether reconstruction errors caused by the deep learning enhancement routine were acceptable to the evaluators. Results indicate that image resolution and contrast-to-noise ratio between brain and CSF predict the likelihood of an image being characterized as useful for hydrocephalus treatment planning. Deep learning enhancement substantially increases contrast-to-noise ratio improving the apparent likelihood of the image being useful; however, deep learning enhancement introduces structural errors which create a substantial risk of misleading clinical interpretation. We find that images with lower quality than is customarily acceptable can be useful for hydrocephalus treatment planning. Moreover, low quality images may be preferable to images enhanced with deep learning, since they do not introduce the risk of misleading information which could misguide treatment decisions. These findings advocate for new standards in assessing acceptable image quality for clinical use. Show less
Ronen, I.; O'Reilly, T.; Froeling, M.; Webb, A.G. 2020
We examined approaches for obtaining H-1 NMR spectra of brain metabolites on a low-field (B-0 = 0.05 T) portable MRI scanner, which was developed in our laboratory with the aim of bringing cost... Show moreWe examined approaches for obtaining H-1 NMR spectra of brain metabolites on a low-field (B-0 = 0.05 T) portable MRI scanner, which was developed in our laboratory with the aim of bringing cost-effective radiological services to populations in underserved, remote regions. The low static magnetic field B-0 dictates low signal to noise ratio for metabolites in the mM concentration range, and results in an overall spectral region for the H-1 resonances of these metabolites narrower than the linewidth obtainable in our scanner. The narrow spectral range also precludes the possibility of suppressing the large contribution of the water resonance at the acquisition stage.We used a spectroscopic Carr-Purcell-Meiboom-Gill (CPMG) sequence to acquire multiecho data from solutions of J-coupled brain metabolites, focusing on lactic acid, a metabolite whose concentration is negligible in the healthy brain and increases significantly in several disease conditions. The J spectra we obtained for lactate from the Fourier transformation of the multiecho data are spectrally well-resolved for a range of echo spacing values. We show that the J spectra at different echo spacings fit well with simulations of the evolution of echo train signal of the lactate under the same conditions. Applying a Jrefocused variant of the CPMG sequence, the J modulation of the echo decay is removed, providing a way for subtracting the large contribution of the non-modulated component in the J spectrum in conditions where notching it using post-processing methods is impossible. We also demonstrate by means of experimental data and simulations that in our experimental conditions, J-spectra of other prominent brain metabolites, such as the neurotransmitter glutamate, do not yield discernible peaks and only contribute to a broad peak at zero frequency. (C) 2020 The Author(s). Published by Elsevier Inc. Show less
Modern clinical MRI systems utilise very high magnetic fields strengths to produce high resolution images of the human body. The high up-front and maintenance cost of these systems means that much... Show moreModern clinical MRI systems utilise very high magnetic fields strengths to produce high resolution images of the human body. The high up-front and maintenance cost of these systems means that much of the world lacks access to this technology. In this paper we propose a low cost, head-only, homogenous Halbach magnet array with the potential for paediatric neuroimaging in low-resource settings. The homogeneity of the Halbach array is improved by allowing the diameter of the Halbach array to vary along its length, and also adding smaller internal shim magnets. The constructed magnet has a bore diameter of 27 cm, mean B o field strength of 50.4 mT and a homogeneity of 2400 ppm over a 20 cm diameter spherical volume. The level of homogeneity of the system means that coil-based gradients can be used for spatial encoding which greatly increases the flexibility in image acquisition. 3D images of a "brain phantom" were acquired over a 22 x 22 x 22 cm field of view with a 3.5 mm isotropic resolution using a spinecho sequence. Future development of a low-cost gradient amplifier and an open-source spectrometer has the potential of offering a fully open-source, low-cost MRI system for paediatric neuroimaging in low-resource settings. (C) 2019 Elsevier Inc. All rights reserved. Show less
