Abstract
After the fabrication of magnetic resonance superconducting magnets, the magnetic field inhomogeneity needs to be accurately measured for subsequent shimming. However, conventional measurement methods are susceptible to magnetic fields, have poor compatibility, and are difficult to adapt to various types of magnets. This paper proposes a new field measuring system based on a three-axis movable platform. The system utilizes non-magnetic materials and an innovative hand-wheel lifting design that can be adapted to various aperture magnets, thus obviating the necessity for electrically driven equipment and addressing safety concerns in strong magnetic fields. In addition, the measurement system offers high accuracy up to 1 mm and a wide measurable range. The fields of 3 T and 7 T magnets were mapped using the designed system with diameter of spherical volume (DSV) of 160 mm and 130 mm, respectively. Experimental results demonstrate that the magnetic field measurement system has strong compatibility and can accurately map the magnetic field at arbitrary positions, which is critical for shimming studies.
Keywords: Magnetic field homogeneity, Magnetic field mapping, MRI
Graphical abstract
1. Introduction
At present, various types of magnetic resonance imaging (MRI), especially high-field MRI systems, are increasingly used in medical diagnosis and research [[1], [2], [3], [4]]. The quality of the MRI magnetic field significantly impacts the signal-to-noise ratio and image resolution [5,6]. Compared to conventional MRI equipment, the development and application of high-field MRI systems impose higher requirements on the homogenization of the magnetic field [7,8]. A prerequisite for this task is the realization of precise field measurements [9]. Considering the differences in the apertures of MRI main magnets for different applications, it is crucial to develop a compatible measurement system that can be applied to a wide range of magnets.
To accurately map the spatial profile of the magnetic field, measurements need to be taken at a series of points over the imaging sphere surface. Conventional methods achieve this using an array of probes to record the fields simultaneously. This detector, commonly referred to as a magnetic field camera in MRI applications [10], is typically used in medical scanners with large apertures. Some studies use multiple independent probes to form an array [11,12], or customize special probe molds for fixation and movement during field mapping [13,14].
However, these methods have limitations, especially for magnets with small bore sizes. Probe arrays can only measure spheres with a fixed diameter of spherical volume (DSV) and require a specific holder structure. This makes it impossible to measure the magnetic field for animal MRI magnets with small bore sizes due to the volume of the probe array itself, causing serious inconvenience for subsequent shimming work. The measurement methods in the literature [11] use customized probe molds according to the aperture of the magnets, which are less compatible, costly, and do not facilitate the fixation and movement of the probes. In addition, as shown in Fig. 1, both methods, due to the probe's sector configuration, can only measure the sphere region. For magnets with more specific spatial shapes, such as cylinders, these methods fail to meet the measurement requirements.
Fig. 1.
Rotating mapping method using a combination of probe array and a rotatable support frame.
To address these problems, this study proposes a magnetic field measurement system using a custom-built three-axis movable platform and a magnetic resonance magnetometer. The system offers precise positioning and high compatibility, allowing it to adapt flexibly to various magnet types. It can perform magnetic field measurements at any point within the imaging area, regardless of shape, to evaluate the magnetic field uniformity. It is also noted that the system does not require a specific support structure inside the magnet, and the measurement process remains unaffected by the magnetic field, making it suitable for magnets with different field strengths.
2. Experiments
2.1. Experimental setup
The core component of this study is a custom-built three-axis movable platform. Its main structure includes three movable platforms along the X, Y, and Z axes, with movement controlled by a threaded rod. A front-mounted outstretched arm secures the measuring probe. The bottom of the platform is equipped with a seated scale with a minimum resolution of 1 mm. The probe's spatial position is adjusted using three axial hand crank wheels, with a travel range of 360° and a minimal indication unit of 0.02°. The fine screw movement allows precise sampling with a 600 mm × 700 mm × 700 mm (X, Y, Z) space, meeting the requirements for a maximum 600 mm DSV measurement.
Unlike the electrically-driven field measurement device [15], this device's three displacement platforms and other components are made of non-magnetic materials. It is designed with hand-cranked wheel lifting, meeting the measurement needs of various types of strong magnetic field equipment. The front end of the displacement platform is equipped with a probe fixing device, generally using high-strength, non-bending carbon fiber rods. This allows the probe to be securely fixed to the magnet for field mapping. Its structure and detailed view are shown in Fig. 2. The study uses a probe based on the principle of nuclear magnetic resonance, with an integrated Hall sensor that increases the measurement speed while maintaining an accuracy of up to 1 ppm.
Fig. 2.
Structure of the three-axis movable platform and detailed view.
2.2. Magnetic field measurement
Before measurement, the shape and size of the measured magnet DSV and the number of sampling points should be determined. Next, the three-axis movable platform needs to be adjusted, and its point mapping method is shown in Fig. 3. The measurement point of the probe is placed in the center of the magnet, and the readings of the X, Y, and Z three-axis upward scales of the moving table are recorded as the origin of the measurement coordinates. After the origin is determined, the measured DSV area is divided into several layers with equal spacing, and a series of points is taken as the measurement points at equal angles on each layer. For example, in a spherical region, a few rounded surfaces on the sphere can be selected as measurement points, with points on these surface planes determined based on equal angular spacing. Except for the first and last poles, all other points on each plane are taken at equal angles. The coordinates of the required measurement points can be calculated according to the origin determined in the previous section. Subsequently, the probe is repositioned using the three-axis movable platform, allowing the magnetic field strength at each point to be measured sequentially. The equipment used during the measurements was a Metrolab PT2026 main unit with probe model 1326, PT2026 is a reliable and high-precision magnetic field measurement instrument, and its accuracy is widely recognized in the industry.
Fig. 3.
Three-axis movable platform and point mapping method.
Using the above method, we measured the uniformity of the magnetic field of two 300-mm aperture animal MRI magnets developed by the Institute of Electrical Engineering, Chinese Academy of Sciences. Different DSV sizes are considered to verify the compatibility of the device, as shown in Fig. 4. The field strengths of the two magnets were 7 T and 3 T, and their DSVs were chosen to be 130 mm and 160 mm, respectively.
Fig. 4.
Two magnetic resonance magnets for animal imaging, designed by the Institute of Electrical Engineering, Chinese Academy of Sciences (IEE, CAS): (a) 7 T/ϕ300 mm magnetic resonance magnet, (b) 3 T/ϕ300 mm magnetic resonance magnet.
3. Results and discussion
For the 7 T system, a total of 54 sampling points were acquired within a 130 mm DSV, distributed every 18.5 mm along each slice in the Z-axial direction and every 40° around each circumferential circle. After point-by-point measurement, the magnetic field uniformity of the magnet was calculated to be 96.32 ppm. The measurement results are shown in Fig. 5. Detailed magnetic field measurement results are presented in Table 1 in appendix. Although the measurement sampling points shown in Fig. 5 are not very dense due to a rapid shimming consideration, our method allows for accurate sampling in an arbitrary closed space. The total number of measurement points for the 130 mm spherical surface is 54, supporting more than 7-order harmonics decomposition, which have adequate main harmonic components to conduct the shimming operation.
Fig. 5.
Measurement results in 130 mm DSV: (a) magnetic field spatial distribution, (b) harmonic components of each order.
For the 3 T system, a total of 134 sampling points were acquired within a 180 mm DSV, distributed every 15 mm along each slice in the Z-axial direction and every 30° around each circumferential circle. After point-by-point measurement, the magnetic field uniformity of the magnet was calculated to be 1300 ppm. The measurement results are shown in Fig. 6. After dismantling the machine, it was found that the winding error of the internal coil resulted in a higher degree of inhomogeneity in the magnetic field. Table 2 in appendix shows the specific magnetic field measurement results.
Fig. 6.
Measurement results in 180 mm DSV: (a) magnetic field spatial distribution, (b) harmonic components of each order.
The proposed method has the following advantages: it does not require a complex support system and offers flexibility to adapt to magnets with different apertures. It is magnetically compatible and can be applied to various systems, including high-field MRI. In addition, it can be used to map magnetic fields at any point within a large volume.
However, the system also has some limitations. To maximize overall system performance and ensure magnetic compatibility, a hand-crank wheel with a screw is used for movement control. The measurement process relies on scale reading to determine the probe position, which inevitably introduces reading errors. As a result, the actual measurement points may not fully match the calculated points, leading to some degree of error in the measured magnetic field.
4. Conclusion
This study presents a new design for field measurement in MRI applications. The proposed measurement system is based on a custom-built three-axis movable platform, allowing for point-by-point field mapping in the imaging area. The system is designed with non-magnetic materials, achieving millimeter-accurate positioning within the spatial range. Experimental measurements on two animal magnets demonstrate the system's compatibility with MRI magnets of different field strengths and aperture sizes.
CRediT authorship contribution statement
Yiqing Yin: Writing – original draft, Investigation, Data curation. Wenchen Wang: Investigation, Data curation. Shihe Zhao: Resources, Conceptualization. Hui Wang: Validation, Data curation. Junsheng Cheng: Resources, Investigation. Shunzhong Chen: Methodology, Data curation. Wenhui Yang: Formal analysis, Data curation. Feng Liu: Writing – review & editing. Yaohui Wang: Supervision, Project administration, Methodology.
Declaration of competing interest
The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Shihe Zhao is an employee of Beijing Guangrui Tongda Instrument Co., Ltd., China.
Acknowledgement
This work is supported by the National Science Foundation of China (Grant No. 52293423 and Grant No.52277031).
Biographies
Yiqing Yin received his B.S. degree in electrical engineering and automation from Hohai University, Nanjing, China, in 2023 and is currently pursuing the M.S. degree at the Institute of Electrical Engineering, Chinese Academy of Sciences (IEE, CAS), with research interests in radio frequency and magnetic field measurements of magnetic resonance imaging systems.
Yaohui Wang received the B.S. and M.S. degrees from Northwestern Polytechnical University, Xi'an, China, in 2012 and 2015, respectively, and completed the Ph.D. degree from the University of Queensland, Brisbane, Australia, in 2017. His research interests include magnetic resonance imaging system, superconducting magnet and electromagnetic instrument. Due to his contributions on MRI hardware, he was elected as Junior Fellow of International Society of Magnetic Resonance in Medicine (ISMRM) in 2020.
Footnotes
Peer review under the responsibility of Innovation Academy for Precision Measurement Science and Technology (APM), CAS.
Appendix.
Table 1.
Measured magnetic field values (unit: T) over a 130 mm spherical surface.
|
Φ |
0° | 40° | 80° | 120° | 160° | 200° | 240° | 280° | 320° |
|---|---|---|---|---|---|---|---|---|---|
| Z/m | |||||||||
| −0.045 | 7.02577187 | 7.02577966 | 7.02577649 | 7.02577447 | 7.02578003 | 7.02577024 | 7.02577477 | 7.02576607 | 7.02577877 |
| −0.025 | 7.02575403 | 7.02575674 | 7.02575793 | 7.02575714 | 7.02576289 | 7.02577011 | 7.02577304 | 7.02576237 | 7.02575907 |
| −0.005 | 7.02577227 | 7.02576842 | 7.02577537 | 7.02577618 | 7.02577439 | 7.02577235 | 7.02577579 | 7.02577895 | 7.02576249 |
| 0.005 | 7.02577881 | 7.02577454 | 7.02577648 | 7.02577273 | 7.02576583 | 7.02576308 | 7.02577492 | 7.02578488 | 7.02576551 |
| 0.025 | 7.02577181 | 7.02575904 | 7.02576373 | 7.02576818 | 7.02576357 | 7.02577343 | 7.02576657 | 7.02577929 | 7.02576451 |
| 0.045 | 7.02576527 | 7.02575605 | 7.02576149 | 7.02576586 | 7.02576232 | 7.02577613 | 7.02576493 | 7.02576829 | 7.02575715 |
Table 2.
Measured magnetic field values (unit: T) over a 180 mm spherical surface.
|
Φ |
0° | 30° | 60° | 90° | 120° | 150° | 180° | 210° | 240° | 270° | 300° | 330° |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Z/m | ||||||||||||
| −0.075 | 3.0052578 | 3.0054602 | 3.0056423 | 3.0057498 | 3.0057569 | 3.0056771 | 3.0055593 | 3.0053405 | 3.0052527 | 3.0052907 | 3.0051600 | 3.0051377 |
| −0.06 | 3.0049228 | 3.0052327 | 3.0055986 | 3.0057843 | 3.0057981 | 3.0056827 | 3.0055355 | 3.0053472 | 3.0052200 | 3.0050762 | 3.0048918 | 3.0047649 |
| −0.045 | 3.0042986 | 3.0048639 | 3.0054489 | 3.0057353 | 3.0057664 | 3.0056715 | 3.0054814 | 3.0053169 | 3.0051869 | 3.0049939 | 3.0046983 | 3.0043783 |
| −0.03 | 3.0038446 | 3.0045880 | 3.0053761 | 3.0057139 | 3.0057729 | 3.0056914 | 3.0055150 | 3.0053361 | 3.0052032 | 3.0049755 | 3.0045737 | 3.0040378 |
| −0.015 | 3.0036565 | 3.0044846 | 3.0053217 | 3.0056980 | 3.0057923 | 3.0057378 | 3.0055655 | 3.0054105 | 3.0052568 | 3.0049929 | 3.0045115 | 3.0038503 |
| 0 | 3.0037216 | 3.0045820 | 3.0053780 | 3.0057079 | 3.0058432 | 3.0058126 | 3.0056676 | 3.0055339 | 3.0053645 | 3.0050151 | 3.0044833 | 3.0038407 |
| 0.015 | 3.0042437 | 3.0050029 | 3.0055546 | 3.0058957 | 3.0060290 | 3.0060166 | 3.0058058 | 3.0057729 | 3.0055805 | 3.0051686 | 3.0046122 | 3.0041606 |
| 0.03 | 3.0048697 | 3.0054796 | 3.0059690 | 3.0062638 | 3.0063886 | 3.0063944 | 3.0062800 | 3.0061480 | 3.0059300 | 3.0054796 | 3.0049852 | 3.0045649 |
| 0.045 | 3.0057549 | 3.0061885 | 3.0065178 | 3.0067690 | 3.0068843 | 3.0068775 | 3.0067566 | 3.0066109 | 3.0063846 | 3.0059949 | 3.0056579 | 3.0054880 |
| 0.06 | 3.0065704 | 3.0068332 | 3.0070594 | 3.0072637 | 3.0073652 | 3.0073456 | 3.0072254 | 3.0070543 | 3.0068378 | 3.0065461 | 3.0063221 | 3.0063338 |
| 0.075 | 3.0070437 | 3.0072080 | 3.0073477 | 3.0074891 | 3.0075659 | 3.0075606 | 3.0074489 | 3.0073034 | 3.0071424 | 3.0069681 | 3.0068627 | 3.0069065 |
References
- 1.Wang Q., Liu J., Zheng J., Qin J., Ma Y., Xu Q.…Liu F. Progress of ultra-high-field superconducting magnets in China. Supercond. Sci. Technol. 2021;35(2) [Google Scholar]
- 2.Abduljalil A.M., Schmalbrock P., Novak V., Chakeres D.W. Enhanced gray and white matter contrast of phase susceptibility-weighted images in ultra-high-field Journal of Magnetic Resonance Imaging. Off. J. Int. Soc. Magn. Reson. Med. 2003;18(3):284–290. doi: 10.1002/jmri.10362. [DOI] [PubMed] [Google Scholar]
- 3.Deistung A., Rauscher A., Sedlacik J., Stadler J., Witoszynskyj S., Reichenbach J.R. Susceptibility weighted imaging at ultra high magnetic field strengths: theoretical considerations and experimental results. Magn. Reson. Med.: Off. J. Int. Soc. Magn. Reson. Med. 2008;60(5):1155–1168. doi: 10.1002/mrm.21754. [DOI] [PubMed] [Google Scholar]
- 4.Sharma R. 2015. Superconductivity: Basics and Applications to Magnets. [Google Scholar]
- 5.Winkler S.A., Schmitt F., Landes H., de Bever J., Wade T., Alejski A., Rutt B.K. Gradient and shim technologies for ultra high field MRI. Neuroimage. 2018;168:59–70. doi: 10.1016/j.neuroimage.2016.11.033. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 6.Siebold H. A computer program for optimizing homogeneous field magnets with iron yoke. IEEE Trans. Magn. 1988;24(1):408–410. [Google Scholar]
- 7.Shang Y., Simegn G.L., Gillen K., Yang H.J., Han H. Advancements in MR hardware systems and magnetic field control: B0 shimming, RF coils, and gradient techniques for enhancing magnetic resonance imaging and spectroscopy. Psychoradiology. 2024;4 doi: 10.1093/psyrad/kkae013. [DOI] [PMC free article] [PubMed] [Google Scholar]
- 8.Regatte R.R., Schweitzer M.E. Ultra-high-field MRI of the musculoskeletal system at 7.0 T. J. Magn. Reson. Imag.: Off. J.Magn. Reson. Imag. Off. J. Int. Soc. Magn. Reson. Med. 2007;25(2):262–269. doi: 10.1002/jmri.20814. [DOI] [PubMed] [Google Scholar]
- 9.Price Ron, Allison Jerry, Clarke Geoffrey, Dennis Michael, Hendrick R., Edward & Keener, Carl & Masten, Jeff &, Ness-Avier, Moriel & Och Joe, Reeve Donna. 2015. 2015 American College of Radiology MRI Quality Control Manual. [Google Scholar]
- 10.Klyukhin V.I., Ball A., Bergsma F., Campi D., Cure B., Gaddi A.…Zimmerman J.K. Measurement of the CMS magnetic field. IEEE Trans. Appl. Supercond. 2008;18(2):395–398. [Google Scholar]
- 11.Tada H., Yoshizu F., Oyama S., Tanaka T., Sasaki K.I., Shimomura K. Development of magnetic field map∗∗ system for the MuSEUM experiment with high precision using CW-NMR probes. IEEE Trans. Appl. Supercond. 2022;32(6):1–5. [Google Scholar]
- 12.Wang Y., Hu H., Shang H., Peng T. 2022 IEEE 3rd China International Youth Conference on Electrical Engineering (CIYCEE) IEEE; 2022, November. Measuring system with a probe array for magnetic field homogeneity; pp. 1–5. [Google Scholar]
- 13.Qu H., Wang W., Wu H., Wang C., Wang Y., Wang Q., Liu F. Superconducting shim coils design for the 9.4 Tesla whole-body MRI magnet. IEEE Trans. Appl. Supercond. 2024;34(2):1–10. [Google Scholar]
- 14.Quettier L., Aubert G., Bredy P., et al. Tests of a prototype for assessing the field homogeneity of the Iseult/INUMAC 11.7 T whole body MRI magnet. IEEE Trans. Appl. Supercond. 2010;20(3):722–725. [Google Scholar]
- 15.Liu J., Wang Y., Wang M., et al. Implementing a two-stage, shim field-calibrated superconducting shimming method on a 7 T cryogen-free small animal MRI magnet. J. Magn. Reson. 2024 doi: 10.1016/j.jmr.2024.107787. [DOI] [PubMed] [Google Scholar]