Vasiliki Mallikourti¹, James Ross¹, Oliver Maier², Markus Bödenler³, Rudolf Stollberger ⁴, Lionel Broche¹, and Mary-Joan MacLeod^5
1Aberdeen Biomedical Imaging Centre, University of Aberdeen, Aberdeen, United Kingdom, ²Institute of Medical Engineering, Graz University of Technology, Graz, Austria, ³Institute of eHealth, University of Applied Sciences FH JOANNEUM, Graz, Austria, ⁴BioTechMed-Graz, Graz, Austria, ⁵Institute of Medical Sciences, University of Aberdeen, Aberdeen, United Kingdom

Field-Cycling imaging (FCI) is a new imaging technique able to image over a range of low magnetic field levels by rapid switching, to explore the variations in T1 relaxation time as a function of the magnetic field strength, known as T1 dispersion. Here we measured the T1 dispersion contrast from patients with ischaemic stroke. The infarct region in T1 maps corresponded to clinical images. T1 dispersions had different profiles between stroke areas and healthy brain areas, showing that ischaemic strokes can be imaged below 200 mT without contrast agents.

Bart de Vos¹, Rob F. Remis², and Andrew G. Webb^1,2
1Leiden University Medical Center, C.J. Gorter Center for High Field MRI, Leiden, Netherlands,, Leiden, Netherlands, ²Delft University of Technology, Circuits and Systems, Delft, Netherlands, Delft, Netherlands

In this work a low field point-of-care system design framework is created using target field methods for all of the hardware components. A new target field method for Halbach-based magnet optimization with variable ring diameter and spacing is derived. Magnet, gradient and RF are combined into a single framework which includes a feedback loop for dealing with the component interdependencies. The result is a pipeline which with a few user inputs can create an optimal magnet, gradient and RF design in minutes.

Irene Kuang¹, Jason Stockmann^2,3, Elfar Adalsteinsson^1,4, and Jacob White^1
1Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, MA, United States, ²A. A. Martinos Center for Biomedical Imaging, Massachusetts General Hospital, Charlestown, MA, United States, ³Harvard Medical School, Boston, MA, United States, ⁴Institute for Medical Engineering and Science, Massachusetts Institute of Technology, Cambridge, MA, United States

We demonstrate feasibility of 2D imaging without gradient coils in a spokes-and-hub permanent magnet for a low-cost hand-held educational MR scanner. With the ease of physically assembling the imaging system in mind, the magnet was designed asymmetrically, to introduce a built-in z-gradient, and with an axis of rotation for the top magnet, so it can be tilted by either linear or rotational actuators, and generate negative or positive x-gradients. Results from simulation, Hall-effect sensor measurements, and spin echo data show that the permanent-z plus x-gradient can be used to generate orthogonal fields, and disambiguate signals from a two-tube phantom.

Jonathan B Martin¹, Sai Abitha Srinivas¹, Christopher Vaughn¹, Heng Sun¹, Mark Griswold^2,3, and William Grissom^1,4
1Biomedical Engineering, Vanderbilt University, Nashville, TN, United States, ²Department of Radiology, Case Western Reserve University, Cleveland, OH, United States, ³Biomedical Engineering, Case Western Reserve University, Cleveland, OH, United States, ⁴Electrical Engineering, Vanderbilt University, Nashville, TN, United States

A class of selective RF pulses which use the Bloch-Siegert shift to localize selective excitation without B₀  gradients has recently been proposed. Here we report an improved design method for both the frequency-gradient inducing component pulse and the frequency-selective component pulse, which reduces ripple in the magnetization profile and produces flatter passbands across the excited B₁⁺ range.  The pulses’ production of multiphoton resonances across B₁⁺ and their suitability for fast gradient-free CPMG imaging are investigated. They are deployed for the first time on a low-field scanner to produce slice-selective selection.

Ye Tian¹, Yongwan Lim¹, and Krishna S. Nayak^1
1Ming Hsieh Department of Electrical and Computer Engineering, University of Southern California, Los Angeles, CA, United States

Real-time MRI is a powerful tool to study organ function in a dynamic environment. Many of these regions of interest have both water and fat present, such as the heart and joints. Water/fat separation usually requires multi-echo sequences that utilize chemical shift to distinguish water and fat signals. At 0.55T, the chemical shifts are smaller, making it possible to use long readouts (e.g. spirals) to more efficiently acquire images. Here, we demonstrate spiral out-in-out-in bSSFP imaging combined with region-growing field map estimation for water/fat separation. We demonstrate high quality water/fat separated real-time movies of cardiac function and wrist motion.

Ruoxun Zi¹, Hersh Chandarana¹, and Kai Tobias Block^1
1Center for Biomedical Imaging, Department of Radiology, NYU School of Medicine, New York, NY, United States

Low-field systems have recently gained interest because of the potential to make MRI more accessible, but the field reduction comes at expense of low SNR and challenges with fat suppression. This work describes a new concept for fat separation at low field, based on radial acquisition during frequency-sweep RF saturation and combined with frequency-resolved XD-GRASP reconstruction. The approach is demonstrated for free-breathing abdominal imaging using a radial stack-of-stars 3D bSSFP sequence at 0.55T, showing that fat and water can be reliably distinguished based on the response to saturation at different frequencies. Fat-suppressed composite images can be calculated as final step.

Zhixing Wang¹, Xue Feng¹, Rajiv Ramasawmy², Adrienne E. Campbell-Washburn², John P. Mugler III³, and Craig H. Meyer^1,3
1Biomedical Engineering, University of Virginia, Charlottesville, VA, United States, ²Cardiovascular Branch, Division of Intramural Research, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, MD, United States, ³Radiology & Medical Imaging, University of Virginia, Charlottesville, VA, United States

Concomitant (Maxwell) fields are problematic for TSE imaging when the readout waveforms vary along the echo train, leading to severe signal dropouts and image blurring. These effects are exaggerated when using prolonged, high-gradient amplitude readouts at lower magnetic-field strengths. The purpose of this work was to implement 2D TSE imaging using annular spiral rings with compensation of self-squared concomitant-gradient terms at the echo time and across echo spacings via gradient waveform modifications, and with compensation of the residual Maxwell- and B₀-field induced phase accruals along the readout via image reconstruction. This method reduced artifacts at both 0.55T and 1.5T.