A design of a mobile, homogeneous and efficient electromagnet with a large field-of-view for neonatal low field MRI
Steffen Lother1, Steven Schiff2, Thomas Neuberger3, Peter M. Jakob1,4, and Florian Fidler1

1Research Center Magnetic-Resonance-Bavaria (MRB), Wuerzburg, Germany, 2Center of Neural Engineering, Departments of Engineering Science and Mechanics, Neurosurgery, and Physics, Penn State University, University Park, PA, United States, 3High Field MRI Facility, Huck Institutes of the Life Sciences, Penn State University, University Park, PA, United States, 4Department for Experimental Physics 5 (Biophysics), University of Wuerzburg, Wuerzburg, Germany

Synopsis

A mobile and effective electromagnet prototype with a large field-of-view for neonatal magnetic resonance imaging at 23 mT is presented. The efficient implementation succeeded by exploiting the use of steel plates as a housing system. This results in a design that ensures an optimum between large sample volumes, high homogeneity, high B0 field, low power consumption, light weight, without the necessity of a dedicated water cooling system. Simulations and measurements are shown, which illustrate the functionality and quality of this imaging system. There are multiple paths to clinical and medical applications for such low cost devices.

Purpose

Our goal was to build a low cost MRI1 prototype that can handle the minimum requirements of neonatal head imaging.

Materials and methods

Using structural steel (S235JR) as housing to return the magnetic flux effectively led to an enhancement of the magnetic field (B0).2 Furthermore, the stray fields were minimized and the housing can be used as a solid mounting suspension (for example similar to an iron yoke of a c-shaped permanent magnet)3. In addition, interference signals were effectively shielded. Hence, there is no need to provide an extra magnetic shielded room. The steel housing in Fig. 1 can be flexibly opened on each side of the walls. Another special feature is the compact design of the gradient system,4 which was embedded within the magnet coils and therefore did not require any additional space (see Fig. 1). The size of this desktop magnet is optimized for premature infants. We implemented a biplanar double donut design as this design is compact and enabled us to use horizontal aligned efficient rf-solenoidal coils for signal transmission and reception (Tx/Rx - coils). It is known that this is the most effective geometry for an MRI coil.5 A solenoid transmit/receive (Tx/Rx) coil for the Larmor frequency of 965 kHz was built with the inner diameter of 100 mm and a matched quality factor of 95.

Results

A standard field mapping method was used to characterize the B0 field of the magnet.6 The homogeneity in the simulation (Fig. 2) was quantitatively compared with the measurement (not shown here). The high-low values of this measurement were +/- 600 ppm on the edges of the phantom. In comparison, the simulation results in Fig. 2 showed a homogeneity of < +/- 40 ppm within the 10 mm radius. The simulation compared to the measurement resulted in a 15 times more homogeneous field. Reasons for this deviation were found in manufacturing tolerances and the variations in quality (DIN EN 10025) of the structural steel (S235JR) which were not included in the simulations. However, the existing inhomogeneity still permits a level of acceptable imaging quality (shown in Fig. 3). The 2D spin echo measurement in Fig. 3 shows the imaging capability of the presented system (magnet, gradient system, Tx/Rx Coil and control system). The imaging parameters of the sequence were: TE = 40 ms, TR = 400 ms, BW = 100 Hz, FoV = (100 x 100) mm, and 64 x 64 matrix size Due to zero filling by a factor of 2, the in-plane pixel resolution was 0.8 mm x 0.8 mm and slice thickness of 5 mm. With a phase oversampling factor of two and 32 averages the acquisition time was 29 min. The maximum gradient strength was 2.5 mT/m.

Discussion

About 80% of the world's population live in developing countries and the most common disease of childhood requiring neurosurgery is hydrocephalus. Hydrocephalus is also the easiest clinical condition to resolve with MRI that we are aware of. For accurate diagnosis of the location of fluid collections in relationship to the brain a spatial resolution of 2-3 mm isotropic in the x-y plane, and 5-10 mm in the z plane is necessary.7 These minimum specifications has been achieved with this magnet design. We believe that this work is a substantial step in low field MRI development and promises the next stage to bring the advantages of MRI diagnostics to the developing world, where the majority of people on the planet are without the benefit of access to MRI imaging.

Acknowledgements

We thank Toni Drießle for helpful discussions and permanently valuable engineering inputs.

References

1. Nascimento GC do, Souza RE de, Engelsberg M (1989) A simple, ultralow magnetic field NMR imaging system. J Phys E: Sci Instrum 22(9):774–779[s1]

2. Feynman R, Leighton R, Sands M (2006) The Feynman Lectures on Physics, Volume II. Addison-Wesley ISBN 0-8053-9047-2 (Chapter 37: Magnetic Materials

3. Wright SM, Brown DG, Porter JR, Spence DC, Esparza E, Cole DC, Huson FR (2002) A desktop magnetic resonance imaging system. Magn Reson Mater Phy 13(3):177–185

4. Lother S, Hoelscher U, Kampf T, Jakob P, Fidler F (2013) 3D gradient system for two B0 field directions in Earth´s field MRI. Magn Reson Mater Phy 26(6):565–573

5. Mispelter J, Lupu M, Briguet A (2006) NMR Probeheads for Biophysical and Biomedical Experiments. Imperial College Press, London.

6. Romeo F, Hoult DI (1984) Magnet field profiling - analysis and correcting coil design. Magn Reson Med 1(1):44–65

7. Mandell JG, Kulkarni AV, Warf BC, Schiff SJ (2015) Volumetric brain analysis in neurosurgery: Part 2. Brain and CSF volumes discriminate neurocognitive outcomes in hydrocephalus. J Neurosurg Pediatr 15(2):125–132 [s1]R1.1

Figures

Fig. 1 The built underpart of the magnet (donut geometry 690 mm wide) with the immersed biplanar gradient system (inside the ring) is shown. The arrow points to the generated B0 field direction.

Fig. 2 B0 field mapping simulation of the magnet with steel housing (cross section 750 mm x 300 mm with wall thickness of 10 mm, magnet coils in brown) and the legend numbered in mT with a delta of 2.4 mT for each color.

Fig. 3 The 2D spin-echo image of the pineapple with 5 mm slice thickness. The matrix size was 64 x 64 voxels.



Proc. Intl. Soc. Mag. Reson. Med. 24 (2016)
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