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A 6.3kg Single-Sided Magnet for 3D, Point-of-Care Brain Imaging
Patrick C McDaniel1,2, Clarissa Z Cooley2, Jason P Stockmann2, and Lawrence L Wald2,3

1Massachusetts Institute of Technology, Cambridge, MA, United States, 2Athinoula A Martinos Center for Biomedical Imaging, Charlestown, MA, United States, 3Harvard Medical School, Boston, MA, United States

Synopsis

MRI, as currently used, requires transporting the patient to the scanner. A truly point-of-care MRI device, possibly even hand-held, could increase the utility of MRI extending its reach and enabling new applications, such as continuous bedside monitoring. In this work, we design and construct a light-weight (6.3kg), single-sided permanent magnet designed to image the cortical region it is positioned over (~8cm x 8cm x 3cm ROI). We describe the magnet optimization and compare the predicted and measured B0 field pattern and validate its imaging potential by acquiring 1D depth profiles in a phantom.

Introduction

Size, expense and siting issues prohibit conventional MRI systems from point-of-care use in almost all clinical settings. Although effort is building toward inexpensive, easily-sited systems for rural, developing-world, or even bedside settings1,2 and considerable work has been put into single-sided MR devices for rock and materials characterization3–7 only a few studies8 have focused on highly portable, or hand-held devices for medical applications. In this work, we assess the feasibility of a lightweight single-sided device capable of imaging a few centimeters into the human brain. We optimize a rare-earth permanent magnet array for this purpose in a “cap-like” configuration, and show that a ~8x8x3cm3 imaging region can be achieved with reasonable field gradient in depth and a mean $$$\bar{B}_{0}$$$ of about $$$64mT$$$, proton freq. = 2.7MHz. We validate the magnet performance with 1D depth profile images from a multiple-disc phantom.

Methods

The starting point for the cap-shaped magnet was an equatorial portion of a “Halbach Sphere”9 . This magnetization pattern was discretized into 37 blocks, whose compositions and positions were then optimized using a genetic optimization in Matlab (Mathworks, Natick MA). Blocks were allowed to use one of 7 easily procurable material/size combinations varying from empty/non-magnetic through an N52 NdFeB material, 1”x1”x1.375” block. The genetic algorithm also had the ability to shift all blocks along $$$\hat{x}$$$ by $$$\pm 1cm$$$, and to shift 6 blocks along $$$\hat{y}$$$ by $$$\pm 1cm$$$. The cost function employed the percent variation in magnetic field over an ROI; mean $$$|\bar{B}_{0}|$$$ was constrained to be at least $$$50mT$$$; and magnet symmetry about the x-y and x-z planes was imposed. A hemi-ellipsoidal ROI with $$$4cm$$$ major radii and a $$$3cm$$$ minor radius was used. This ROI penetrates $$$3cm$$$ into cerebral cortex (Figure 1) and roughly matches the excitation region of a loop Tx coil. $$$|\bar{B}_{0}|$$$ maps were analyzed using COMSOL (COMSOL Inc, Burlington MA). After the optimal design was chosen, a former to hold the magnets was constructed with 3D printing (Formlabs Form2, Somerville MA). The prescribed NdFeB magnets (Applied Magnets, Plano TX), were then epoxied into the former. A $$$|\bar{B}_{0}|$$$ map was acquired using a 3-axis Hall-effect magnetometer (Metrolab, Geneva, Switzerland) moved by a stepper robot. A , 5-turn Tx/Rx coil was tuned to $$${f}_{c}=2.685 MHz$$$ and matched ($$$BW (3dB)=150kHz$$$). The coil fit closely about a phantom containing three $$$D=10mm$$$, $$$h=5mm$$$ discs of 0.09% Gd-DPTA solution spaced $$$5mm$$$ apart (Figure 5A). The unshielded phantom and coil were placed in the sensitive ROI of the magnet, and data were acquired using a hard pulse TSE sequence ($$${f}_{c}=2.69MHz$$$; echo train length = 6; $$${N}_{ave}=64$$$; 128 samples; $$$BW=1221 \frac{Hz}{Px}$$$; $$$TR=923ms$$$; pulse lengths $$${t}_{90}$$$/$$${t}_{180}$$$=$$$2\mu s$$$/$$$4\mu s$$$).

Results

The chosen design utilized blocks with 4 different material/size combinations (Figure 2A), and resulted in a $$$11.3cm\times 22.5cm\times 21.8cm$$$, $$$6.3kg$$$ magnet (Figure 2B). The necessary magnetic material cost under $450 (USD). The constructed magnet could be held and moved by hand, and fit on an 85th-percentile adult male head phantom (Figure 3). Figure 4 shows simulated and measured field maps are shown in the x-y and y-z planes. The magnet’s field was $$$67.5mT$$$ at the ROI center with a field range of $$$4.77mT$$$ across the ROI in the simulations. For the constructed magnet, the corresponding center field and range were $$$63.6mT$$$ ($$$2.71MHz$$$) and $$$4.40mT$$$ ($$$187kHz$$$). The simulated built-in $$$|\bar{B}_{0}|$$$ gradient along the center axis of the magnet varied from $$$154\frac{mT}{m}$$$ to $$$198\frac{mT}{m}$$$ as one moves away from the magnet. For the constructed magnet, the corresponding limits were $$$88\frac{mT}{m}$$$ and $$$174\frac{mT}{m}$$$ Figure 5 shows the acquired spectrum of the 3-disc phantom centered $$$6cm$$$ from the bottom of the magnet “bowl”. Three lobes are visible in the projection, corresponding to three regions of water in the phantom. The local $$$|\bar{B}_{0}|$$$ gradients calculated from these data vary between $$$95\frac{mT}{m}$$$ (near the magnet) and $$$143\frac{mT}{m}$$$ (far from the magnet). For this acquisition, this corresponds to a depth resolution between $$$0.30mm$$$ and $$$0.20mm$$$. The three 1D projection lobes are of different heights, likely resulting from the phantom signal bandwidth ($$$135kHz$$$) being near the Tx/Rx coil bandwidth ($$$150kHz$$$).

Discussion

We have demonstrated a novel design of a low-cost, lightweight (<$450 (USD), 6.3kg) single-sided magnet for point-of-care 3D brain imaging. As the initial work towards 3D imaging, we have shown the ability of this magnet to perform high-resolution (0.2 to 0.3mm) depth profiling along the 2.5cm length of phantom. Our next steps will be to add single-sided gradient coils for phase encoding along the other two spatial dimensions and enable full 3D imaging.

Acknowledgements

NIH: 5T32EB1680, R01EB018976; Thomas Witzel for help with the Apollo console

References

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Figures

Figure 1: Magnet geometry, sensitive ROI, and simulated B0 map overlain on an adult brain MRI acquired on a high-field scanner. The sensitive region of the magnet extends 3cm deep into cortex from the epidural surface.

Figure 2: (A) Magnet design; colors indicate size/material combinations for magnet blocks; black arrows indicate the direction of magnetization; the shaded red-outlined region is the optimization ROI. (B) Selected parameters characterizing the size, strength, and cost of the magnet.

Figure 3: (A) The hand-held potential of the constructed magnet is shown, showing the concave region intended for the head. (B) Magnet shown on an 85th-percentile male adult head phantom, demonstrating that it fits as desired by analogy with Figure 1.

Figure 4: (A) Simulated (top) and measured (bottom) B0 maps in the x-y plane at z=0. Note the different scales for (A) and (B). The measured B0 map corresponds to the white dashed region on the simulated map. (B) Simulated (top) and measured (bottom) B0 maps in the y-z plane at x=1cm and x=2.5cm. In (A) and (B) the optimization ROI is indicated by the black dashed line. Different color scales are used to elucidate the similar morphologies of the simulated and measured field maps.

Figure 5: (A) Tx/Rx coil and phantom. The three blue regions contain a dyed water/Gd-DPTA solution. Labeled ticks are in units of cm (B) Acquired 1D projection of the phantom onto frequency.

Proc. Intl. Soc. Mag. Reson. Med. 26 (2018)
0943