Nastaren Abad¹, Afis Ajala¹, Yihe Hua¹, and Tom K.F Foo^1
1General Electric Global Research, Niskayuna, NY, United States

Impulse response function (IRF) harvesting is a powerful technique for gradient characterization. In this study a field monitoring system was utilized to characterize a head-only, high-performance gradient coil (MAGNUS), enabling expansion of the spatial response pattern into higher order spherical harmonics. The predictive power of IRF-estimated waveforms and field evolution is demonstrated by comparing to nominal and active field monitored approaches for arbitrary gradient waveforms. The calculated modulation function indicated good correspondence for arbitrary waveforms and k-space trajectories varying with slew rate, amplitudes and number of interleaves.

Rosie Goodburn¹, Tom Bruijnen², Wajiha Bano¹, Uwe Oelfke¹, and Andreas Wetscherek^1
1Radiotherapy and Imaging, The Institute of Cancer Research, London, United Kingdom, ²Department of Radiotherapy, University Medical Center Utrecht, Utrecht, Netherlands

Synthetic CT (sCT) is a key component of adaptive MR-guided radiotherapy. Generation of thoracic sCT is challenging and benefits from ultrashort echo-time (UTE) imaging that provides contrast in short-T2* tissues such as bone and lung. However, UTE imaging is susceptible to artifacts caused by eddy currents and gradient delays. In this work, we measured the gradient impulse response function (GIRF) of an MR-Linac with standard scanner hardware and used it to retrospectively correct k-space trajectories. Images reconstructed with GIRF and gradient-delay corrected trajectories exhibited higher image quality, including reduced halo artifacts, blurring, destructive phase artifacts, and improved uniformity and contrast.

Hannah Scholten¹, Manuel Stich², and Herbert Köstler^1
1Department of Diagnostic and Interventional Radiology, University of Würzburg, Würzburg, Germany, ²Siemens Healthcare, Erlangen, Germany

The gradient system transfer function (GSTF) constitutes a comprehensive tool for correcting k-space trajectory distortions due to gradient infidelities. We developed a new phantom-based measurement approach that allows determining the GSTF with a high frequency resolution without relying on long readout durations. Our first results include self- and B₀ cross-terms with high SNR and resolutions below 10 Hz, resolving the true width of several resonances. The high resolution enables us to capture long-lasting eddy current effects, which is especially promising for the application of GSTF-based correction methods in, for example, diffusion-weighted imaging.

Thomas K.F. Foo¹, Louis Frigo², Myung-Ho In³, Nastaren Abad¹, Vincent B Ho⁴, and Matt A Bernstein^3
1GE Research, Niskayuna, NY, United States, ²GE Healthcare, Waukesha, WI, United States, ³Mayo Clinic, Rochester, MN, United States, ⁴Uniformed Services University of the Health Sciences, Bethesda, MD, United States

A phase contrast pulse sequence was modified to use a single-sided bipolar encoding to calibrate the transverse gradient offsets for asymmetric gradient coils. The phase difference approach cancels the concomitant gradient field effects from all gradient waveforms except for the bipolar gradient waveforms. By fitting the measured phase offset in the phase difference images to the applied bipolar gradient amplitudes, the gradient offset value can be calculated. This was used for pre-emphasis compensation for the zeroth and first order concomitant gradient fields.

Hannes Dillinger¹, Sebastian Kozerke¹, and Christian Guenthner^1
1Institute for Biomedical Engineering, University and ETH Zurich, Zurich, Switzerland

In this work, gradient fidelity is compared between high-field (3T) and lower-field (0.75T) configurations on the same MRI system by measuring the Gradient Modulation Transfer Function (GMTF) and audio noise spectra. Results demonstrate that at lower field mechanical resonances are reduced, leading to improved gradient fidelity and reduction of audio noise levels of up to 70%. Furthermore, it is shown that spectral peaks in the audio spectrum can be related to resonances in the GMTF, which suggests mechanical resonances can be identified using the audio spectrum.

Edwin Versteeg¹, Dennis W.J. Klomp¹, and Jeroen C.W. Siero^1,2
1Radiology, University Medical Center Utrecht, Utrecht, Netherlands, ²Spinoza centre for neuroimaging Amsterdam, Amsterdam, Netherlands

A silent gradient axis driven at 20 kHz can reduce sound in MR-sequences. This silent gradient axis consists of a separate coil that is positioned on the patient table and operated in synergy with the whole-body gradients. The positioning of this coil with respect to the whole-body gradients is prone to operator errors which leads to image artifacts. In this work, we show that this misalignment can be characterized and corrected by measuring the point spread function (PSF) of the silent gradient axis. Using this method, a significant reduction in ghosting artifacts was observed in phantom experiments.

Jennifer Nussbaum¹, Maria Engel¹, and Klaas Paul Pruessmann^1
1Institute for Biomedical Engineering, ETH Zurich and University of Zurich, Zurich, Switzerland

Concurrent acquisition of dynamic fields has become a prominent way to correct for field imperfections. However, when using field probes in close proximity to a head coil, the field measurement can be corrupted by eddy currents on the head coil that are induced by the gradient fields. In this work, we provide a one-time calibration solution to correct for this issue.

Soheil Taraghinia¹, Volkan Acikel², Reza Babaloo^1,3, and Ergin Atalar^1,3
1UMRAM, Bilkent University, Ankara, Turkey, ²Aselsan A.S., Ankara, Turkey, ³Electrical and Electronics Engineering, Bilkent University, Ankara, Turkey

In this work, a switching H-bridge gradient power amplifier utilizing eGaN devices is implemented. A single-stage 150 V/ 50 A full-bridge GPA for an insert gradient array system at 1 MHz effective switching frequency is fabricated and tested. Pulse width modulation signals are generated digitally using a Virtex 7 family FPGA board. A single-stage LC low pass filter is designed to attenuate the ripple current.

Reza Babaloo^1,2, Soheil Taraghinia², and Ergin Atalar^1,2
1Department of Electrical and Electronics Engineering, Bilkent University, Ankara, Turkey, ²National Magnetic Resonance Research Center (UMRAM), Ankara, Turkey

Providing accurate gradient currents is challenging due to the nonlinearity of the gradient system arising from gradient power amplifiers and power supply stages which causes droop in the output currents. This work introduces a nonlinear model for the gradient array system by using the state-space averaging technique and the nonlinear inversion of the model compensates for droop. The feasibility of the method is depicted by simulation and experimental results. The proposed method can provide desired currents by lower voltages which results in minimizing the needed power. It is also possible to use small capacitors to lower the cost of system.

Sarah M Jacobs¹, Edwin Versteeg¹, Leonie NC Visser², Anja G van der Kolk^1,3, Dennis WJ Klomp¹, and Jeroen CW Siero^1,4
1Department of Radiology and Nuclear Medicine, University Medical Center Utrecht, Utrecht, Netherlands, ²Alzheimer Center Amsterdam, Department of Neurology, Amsterdam Neuroscience, Vrije Universiteit Amsterdam, Amsterdam UMC, Amsterdam, Netherlands, ³Department of Radiology, the Netherlands Cancer Institute, Amsterdam, Netherlands, ⁴Spinoza Centre for Neuroimaging Amsterdam, Amsterdam, Netherlands

Acoustic noise can negatively impact patients from anxiety and communication problems to transient and permanent hearing loss. In this pilot study we used a silent gradient axis that is switched at the inaudible frequency of 20 kHz with a silent readout module implemented in an MPRAGE sequence, and investigated subject experience and image quality of fast and quiet anatomical brain imaging at 7T. Here we show preliminary evidence that our silent gradient axis with silent readout module incorporated into a T₁-weighted MPRAGE sequence is perceived more quiet and positive and delivers images of largely acceptable quality.

Arjama A Halder^1,2, Eric J Lessard^1,2, William B Handler¹, and Blaine A Chronik^1,2
1xMR Labs, Physics and Astronomy, London, ON, Canada, ²Medical Biophysics, Western University, London, ON, Canada

This abstract aims to investigate the electromagnetic environment within a high-performance shoulder cut-out gradient coil designed to be used as a part of an all-in-one platform for head and neck imaging applications. The results achieved in this abstract and future measurements that will be performed with this coil will allow the quantification for gradient induced electric field effects experienced by patients and ensure that the coil is within the expected PNS limits.

Sebastian Littin¹, Tristan A. Kuder², Feng Jia¹, Arthur Magill², Philipp Amrein¹, Frederik B. Laun³, Sebastian Bicklehaupt⁴, Mathias Davids^5,6,7, Valerie Klein^5,7, Mark E. Ladd², and Maxim Zaitsev^1,8
1Department of Radiology, Medical Physics, University Freiburg, Faculty of Medicin, Freiburg, Germany, ²Medical Physics in Radiology, German Cancer Research Center, Heidelberg, Germany, ³Department of Radiology, MR Physics, University Medical Center Erlangen, Erlangen, Germany, ⁴Department of Radiology, University Medical Center Erlangen, Erlangen, Germany, ⁵Department of Radiology, A. A. Martinos Center for Biomedical Imaging, Charlestown, MA, United States, ⁶Harvard Medical School, Boston, MA, United States, ⁷Computer Assisted Clinical Medicine, Heidelberg University, Medical Faculty Mannheim, Heidelberg, Germany, ⁸High Field MR Center Center for Medical Physics and Biomedical Engineering, Medical University of Vienna, Vienna, Austria

Local gradient coils allow for enhanced performance in terms of both gradient amplitude and switching rate. We recently presented the concept of a single channel non-linear breast gradient coil. Here we show results of the characterization, safety assessment and initial diffusion encoded images from phantom experiments of the prototype coil. Is an order of magnitude increase of gradient strength for diffusion weighting feasible using local gradient coils?

Haile Baye Kassahun¹, Sadeq S Alsharafi¹, Ahmed M Badawi¹, and AbdEl-Monem M El-Sharkawy^1
1Systems and Biomedical Engineering, Cairo University, Cairo, Egypt

A two-channel biplanar Z-gradient coil design is introduced by dividing the conventional coil surface into two sections radially. The corresponding lower and upper sections are driven by the same current strength but in opposite directions. Coils were designed using the Fourier series expansion (target field) method. For the same target field, the dissipated power of the two-channel biplanar Z-gradient coil was lower than that of the conventional coil (with corresponding dimensions) by at least 25% thereby reducing the ohmic losses. In the future, the benefit of using multi-channel bi-planar gradient coils may be further investigated.

Congcong Liu^1,2, Shi Su¹, Ye Li^1,2, Xin Liu^1,2, Hairong Zheng^1,2, Dong Liang^1,2,3, and Haifeng Wang^1,2
1Paul C. Lauterbur Research Center for Biomedical Imaging, Shenzhen Institutes of Advanced Technology, Chinese Academy of Sciences, Shenzhen, China, ²Shenzhen College of Advanced Technology, University of Chinese Academy of Sciences, Shenzhen, China, ³Research Center for Medical AI, Shenzhen Institute of Advanced Technology,Chinese Academy of Sciences, Shenzhen, China

In this work, a novel biplanar matrix Z2 nonlinear gradient coil with an open structure has been designed, which could be employed in open spatial low-field MRI system, and an efficient encoding strategy for O-Space imaging was proposed in combination with the proposed biplanar matrix nonlinear gradient coil, which has the potential to shift the center placements (CP’s) in nonlinear gradient imaging without an external linear gradient field. The biplanar Z2 encoding field generated by a nonlinear gradient element consisting of curve segments was optimized and implemented to show the feasibility of this proposed biplanar matrix coil and encoding approach.

Manouchehr Takrimi¹ and Ergin Atalar^1,2
1UMRAM, Bilkent University, Ankara, Turkey, ²Department of Electrical and Electronics Engineering, Bilkent University, Ankara, Turkey

A novel hybrid gradient coil consisting of two conventional actively shielded x/y gradient coils and a programmable active-shield z-gradient array coil is introduced and simulated.  The proposed hybrid gradient coil can dynamically provide the required magnetic profile based on diverse applications in MRI. This is achieved by using a set of independent power amplifiers that feed the conventional coils and the array elements. To show the flexibility, three magnetic profiles are simulated: (a) a conventional profile; (b) a profile with twice z-gradient intensity and a shifted FOV; (c) a profile with quadruple z-gradient intensity when the active-shield is reverse fed.

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

As recently demonstrated by Cooley et al., for Halbach-array based permanent-magnet MR imagers, time-varying encoding fields can be generated mechanically, by rotating the entire array. In this paper we show that in spokes-and-hub based permanent magnet imagers, time-varying encoding fields can also be generated mechanically, but with much smaller motions. In particular, we show that a quarter-degree magnet tilt, along an easily precessed axis, can produce a spatially-discriminating field gradient.  We demonstrate the spatial discrimation using both simulation and measurements from our recently-presented prototype imager. 

Matt A Bernstein¹ and Seung-Kyun Lee^2
1Radiology, Mayo Clinic, Rochester, MN, United States, ²Biomedical Engineering, Sungkyunkwan University, Suwon, Korea, Republic of

Applying systematic dimensional analysis based linear algebraic methods, we derive the important scaling relation for MR gradient design: gradient coil inductance L scales as the square gradient efficiency (measured in T/m/A) and the fifth power of radius.  The inclusion of "turns" as a unit in the dimensional matrix produces the result uniquely, greatly simplifying the analysis.

Ting-Ou Liang¹, Yi-Dan Chen², Shao Ying Huang², and Erping Li^1
1Zhejiang University, Hangzhou, China, ²Singapore University of Technology and Design, Singapore, Singapore

Two current-model-based approaches are presented to accelerate the calculation of the magnetic field of permanent-magnet-array(PMA) that consists of magnet blocks. It enables a speedy convergence of optimization of a PMA design that is a key component of the popular body-part-dedicated portable MRI in the recent years. This method can be extended to magnet of other shapes. Both Halbach(60blocks) and sparse Inward-Outward ring-pair-PMA(1,200 blocks) are used as examples to demonstrate the speed and accuracy of the algorithm. The best acceleration is 78.92% and 77.94%calculation-time reduction compared to the conventional nested for-loops, and 99.81% and 99.91%reduction compared to FEM-based commercial software.

Yenal Gokpek¹ and Ozkan Doganay^1
1Institute of Health Sciences, Ege University, IZMIR, Turkey

In this study, the design and construction of a hybrid Helmholtz coil system and numerical modelling were investigated for production of homogeneous magnetic field over a volume of interest that consist of cylindrical geometry (h=700mm, r=90mm). The 4‑coil system was constructed and tested by two commercially available low power DC power supplies. The magnetic field created by the 4‑coil system was found to be B=2.5182±0.0035mT over the volume of interest. We found that the simulated and measured magnetic fields are in good agreement (R=0.9759, p<0.05).

Ziyi Pan¹, Jieying Zhang¹, Hai Luo², Weiqian Wang², Xiao Chen², Ziyue Wu², and Hua Guo^1
1Center for Biomedical Imaging Research, Department of Biomedical Engineering, School of Medicine, Tsinghua University, Beijing, China, ²Marvel Stone Healthcare, Wuxi, Jiangsu, China

Two-dimension (2D) diffusion-relaxation (D-T₂) correlation measured by a low-cost, portable single-sided NMR system can separate fat and water in biological tissues (for example for liver steatosis detection). Diffusion-editing CPMG sequence is commonly used for such measurement. Recently, a novel approach is proposed to measure the D-T₂ correlation using the simplest CPMG-only sequence. The CPMG-only sequence is less sensitive to motion, and can extend to systems with more imperfect magnetic and radiofrequency fields. This work compares the proposed CPMG-only sequence with the traditional diffusion-editing SE-CPMG sequence. Results show the SE-CPMG and CPMG-only sequences can achieve comparable performance on water-fat separation.