- Research article
- Open Access
Benefits of high-dielectric pad for neuroimaging study in 7-Tesla MRI
Journal of Analytical Science and Technology volume 14, Article number: 17 (2023)
This study aimed to evaluate whether the use of a high-dielectric pad is effective in increasing transmit and receive sensitivity in areas of low signal intensity in the human brain at high magnetic fields and assess its usefulness in neuroimaging studies. The novelty of this study lies in the first reported use of diffusion tensor imaging (DTI) results to evaluate the effect of the pad on neuroimaging. Six volunteers underwent MR scanning using a 7 T MR system. T1-weighted images (T1w) and diffusion-weighted images (DWI) were acquired to demonstrate the benefits of a high-dielectric pad made of barium titanate (BaTiO3). For all imaging experiments, two datasets were acquired per person, one with and one without a high-dielectric pad. Enhancement of signal sensitivity in neuroimaging has been analyzed by DTI study. Higher signal intensities and spatial contrast were demonstrated in the in T1w images acquired using high-dielectric pad than in those acquired without high-dielectric pad. Especially in DTI studies, increased quantitative anisotropy (QA) signals were observed in the corticospinal tract (CST), frontopontine tract (FPT), splenium of corpus callosum (SCC), fornix (FX), inferior fronto-occipital fasciculus (IFOF), cerebellum (CB), middle cerebellar peduncle (MCP), and body of corpus callosum (BCC) (FDR < 0.05). The signal differences accounted for an overall 20% increase. A high-dielectric pad is effective in enhancing signal intensity in human brain images acquired using 7 T MRI. Our results show that the use of such pad can increase the spatial resolution, tissue contrast, and signal intensity in neuroimaging studies. These findings suggest that high-dielectric pads may provide a relatively simple and low-cost method for spatiotemporal brain imaging studies.
Theoretically, ultra-high field magnetic resonance imaging (MRI) (≥ 7 T) has a higher signal-to-noise ratio (SNR), which leads to higher spatiotemporal resolution and enhanced tissue contrast, compared with lower-field MRI (≤ 3 T) (Vaughan et al. 2001). However, ultra-high field imaging faces technical challenges due to radiofrequency (RF) inhomogeneity, increased tissue heating, and reduced wavelength of the RF field (B1). To compensate for B1 interference with the human body, RF shimming is commonly used; it is a technique that uses a transmit array (active shimming) and high-dielectric material (passive shimming) (Steensma et al. 2019). Compared to the active shimming required for advanced coil designs and additional hardware, passive shimming, such as the use of high-dielectric pads placed between the coils and the human body, is a more cost effective and straightforward approach to significantly improve image quality. The high-dielectric pad is composed of high-permittivity materials (e.g., barium titanate [BaTiO3] and calcium titanate [CaTiO3]), which are effective in altering the spatial distribution of electromagnetic fields and enlarging the shortened wavelength of B1 for high-field MRI applications of the brain or body (Teeuwisse et al. 2012a). This high-dielectric pad could be useful for neuroimaging studies targeting subcortical structures in the brain using ultra-high field MRI. To date, most previous studies have demonstrated the effectiveness of high-dielectric pads in neuroimaging by comparing simulations and/or images, such as B0, B1, specific absorption rate (SAR), echo planar imaging (EPI), and T1 and T2 images (O’Reilly et al. 2016; Teeuwisse et al. 2012b; Van Gemert et al. 2019). Although these methods are useful for estimating image quality, data post-processing are needed for a more accurate and specific neuroimaging evaluation.
In this study, we evaluated the effect of a high-dielectric pad in a neuroimaging study using a 7 T MRI scanner, and the pad was placed around the subject’s inner birdcage head coil. T1-weighted images (T1w) and diffusion weighted images (DWI) were acquired to evaluate feasibility of high-dielectric pad to enhance anatomical and neuronal imaging.
Two pads of dimensions 440 (width) × 220 (length) × 2 (height) mm3 were formed. Ba-TiO3 (Alfa Aesar, Ward Hill, MA) mixed in deuterated water (35%) was prepared and heat-sealed within a polypropylene pad after removing as much entrapped air as possible. This high-dielectric pad was placed to cover the subjects’ heads (Fig. 1). The permittivity of the high-dielectric pad was approximately 215. Aqueous suspension of barium titanate (εr ~ 2000) in deionized water (εr ~ 80) was prepared at room temperature. The barium titanate was mixed with distilled, deionized water in volume/volume ratios to 40%. Complex permittivity measurements were performed using a dielectric assessment kit (DAK-12, SPEAG, Zurich, Switzerland). The relative permittivity with a saturated suspension of 35% (v/v) is approximately 215.
For all imaging experiments, two datasets were acquired per person, one with and one without a high-dielectric pad. Six volunteers (age range = 25–52 years, mean age = 38.17 ± 11.89 years, male/female = 5/1) were included in the study, with approval from the local medical ethics committee.
All subjects underwent MRI using a 7 T MR system (Magnetom Terra, Siemens Healthcare, Erlangen, Germany) with a 32-channel phased-array head coil (Nova Medical, Wilmington, MA, USA). Three-dimensional (3D) anatomical brain scans were acquired using magnetization prepared rapid acquisition gradient echo (MP-RAGE) sequence-induced T1w images with the following settings: repetition time (TR) = 4500 ms, echo time (TE) = 1.48 ms, flip angle (FA) = 6°, slice thickness = 0.8 mm, in-plane resolution = 0.8 × 0.8 mm2, matrix size = 304 × 304, number of slices = 192, total scan time = 8 min 8 s. Diffusion weighted images (DWI) were acquired using a multiband spin-echo EPI sequence with the following setting: TR/TE = 6800/72 ms, slice thickness = 1.2 mm, in-plane resolution = 1.2 × 1.2 mm2, matrix size = 180 × 180, number of slices = 109, b-value = 2500 s/mm2, 64 diffusion weighting directions, total scan time = 17 min 56 s.
DTI-based assessment of magnetic susceptibility effects
Group connectometry analysis was performed using DSI-studio software (https://dsi-studio.labsolver.org) to study the effect of the high-dielectric pad. Diffusion data were reconstructed in the MNI space using q-space diffeomorphic reconstruction (QSDR) (Yeh et al. 2016) to obtain the spin distribution function (SDF) (with sample length ratio = 1.25), which was sampled by local fiber directions from the common atlas to estimate the local connectome. A t-score threshold of 2.5 was assigned and tracked using a deterministic fiber tracking algorithm (Yeh et al. 2016) to obtain correlational tractography. The seeding region was placed throughout the brain. The tracks were filtered using topology informed pruning (Yeh et al. 2016) with four iterations. A length threshold of 20 voxels was used to select tracks. To estimate the false discovery rate (FDR), 4000 randomized permutations were applied to the group label to obtain the null distribution of track length. The paired t test was used to analyze differences between with and without high-dielectric pad. The fiber track increase rate was calculated as follows:
Anatomical image results
Figure 2 shows three representative slices of T1w images with and without a high-dielectric pad. There was no apparent signal dropout and distortion in images with the addition of the high-dielectric pad. The mean signal intensities between with and without pad were 82.23 ± 109.13 (maximum intensity: 1168) and 82.15 ± 110.14 (maximum intensity: 1321), respectively. It seems that the rough signal due to the B1 field inhomogeneity appears to have been compensated for by passive shimming.
DTI image results
Compared to the images acquired without high-dielectric pad, those acquired with high-dielectric pad showed a significantly higher quantitative anisotropy (QA) signal in the corticospinal tract (CST), frontopontine tract (FPT), splenium of the corpus callosum (SCC), fornix (FX), inferior fronto-occipital fasciculus (IFOF), cerebellum (CB), middle cerebellar peduncle (MCP), and body of the corpus callosum (BCC) (p < 0.003). Out of them, the FX (p < 0.035) and IFOF (p < 0.044) regions were significantly observed in each sub-region (false discovery rate; FDR < 0.05) (Figs. 3, 4, 5; Table 1). The fiber track increase rate differences accounted for an overall 20% increase (fiber track increase rate observed in whole brain). In the case of the sub-regions, the fiber track increase rate differences were observed as follows: 12% increase in CST, 17% increase in FPT, 27% increase in FX, 20% increase in IFOF, 8% increase in CB, 14% increase in MCP, and 4% increase in BCC. Figure 6 shows increased number of fiber tracks by using high-dielectric pad.
In this study, we applied high-dielectric pads around the human brain using 7 T MRI system equipment that showed enhanced spatial resolution and tissue contrast in anatomical images, and increased signal intensity in DTI results. Among DTI values, QA was used. Neuroimaging studies commonly use fractional anisotropy (FA) values rather than QA values in DTI study, however there are differences between them. FA is defined for each voxel and associated with axonal integrity. On the other hand, QA is defined for each fiber orientation and associated with axonal density (Yeh et al. 2019). Both QA and FA values decrease when there is axonal loss in the brain. However, although QA and FA have similar meanings, we wanted to focus on axonal density rather than axonal integrity. The T1w images in Fig. 2 show improved image SNR and tissue contrast without magnetic susceptibility induced artifacts when acquired using the high-dielectric pad. It seems that high permittivity materials increase the strength of the transmit (B1 +) and receive (B1 −) magnetic fields in areas that have intrinsically low sensitivity owing to both geometry and high operating frequency (Haines et al. 2010; Snaar et al. 2011; Yang et al. 2006). Our results are consistent with those reported by a previous study (Teeuwisse et al. 2012a), suggesting that high-dielectric pads have enhanced SNR with no detrimental effect of magnetic susceptibility on the brain image at 7 T MRI. On the other hand, the brain image using high-dielectric pad (Fig. 2b) was blurrier than image of without pad (Fig. 2a). It seems effects of low-pass filtering. Low-pass filters are commonly used in MRI to reduce the impact of high-frequency noise in the imaging data. By using a high-dielectric material, the filter effect can be greatly enhanced, allowing for the effective reduction in high-frequency noise and the improvement of image quality. Also, the dielectric constant of a material, combined with its physical properties, can greatly impact the performance of the low-pass filter and the overall quality of the MRI images produced. However, additional simulation and experimental results are required to demonstrate the spectral distribution in the electromagnetic field in further studies. One of the challenges of EPI are artifacts which associated with signal loss and imaging blurring leading to non-diagnostic quality derived from B1 field inhomogeneity (Afacan et al. 2019). In this case, high-resolution EPI images come to overcome, providing high-quality, high-resolution images (Afacan et al. 2019). However, this approach has practical limitations such as time consumption and/or SAR increase. These issues pose challenges to most of neuroimaging studies. Here, we propose possibility for the solution with high-dielectric pad.
The DTI results shown in Figs. 3 and 4 show higher signal intensity in the high-dielectric pad than in the control without pad, especially in subcortical brain regions. The overall signal difference increases by 20%. The Fx, which is a part of the limbic system, carries afferent fibers to the hippocampus from the diencephalon and basal forebrain. This structural process is most closely associated with memory function (Kim et al. 2013). Previous studies have reported that fractional anisotropy (FA) in the Fx decreases with advanced age, correlates with age-related memory impairments (Kim et al. 2013), and is relatively decreased in mild cognitive impairment and Alzheimer's disease symptoms (Lee et al. 2019).
As a long associative bundle, the IFOF passes through the temporal lobe and insula, connecting the occipital cortex, temporo-basal areas, and superior parietal lobe to the frontal lobe (Bae et al. 2021). The IFOF has also been reported to play an important role in reading, attention, and visual processing (Benear et al. 2020; Douet and Chang 2015).
To date, there have been many attempts to define neural mechanisms and/or diagnostic biomarkers in clinical application studies, targeting subcortical regions (Schmahmann and Pandya 2007; Martino et al. 2010; Catani. 2008). Conventional MRI (1.5-3 T MRI) is reliable for detecting complete segmentation in the whole brain, but it is difficult to identify detailed subcortical regions due to poor contrast and low intensity signals. In addition, advanced coil designs and additional hardware are required to compensate for the magnetic inhomogeneity, in ultrahigh field MRI. Our results suggest that using a high-dielectric pad may provide a simple and low-cost method for more advanced neuroimaging studies.
This study has several limitations. First, the sample size was relatively small, consisting of only six volunteers. Second, there are no B1 and B0 simulations and imaging analyses for magnet and RF field homogeneity. Correlation analysis between B1 improvement and increased QA value would be a strong proof of the feasibility of high-dielectric Pad. Third, there are no SAR simulation or imaging analyses to demonstrate reduced tissue heating. Optimized experimentation is needed to obtain better results in future studies, considering these limitations.
Our results showed increased QA values in CST, MCP, CB, FPT, IFOF, SCC, FX and BCC with high-dielectric pad in DTI study in ultra-high field MRI. These brain regions were known as key regions in many neuroimaging studies. These signal increases are presumably associated with regional B1 field homogeneity although quantitative results were not obtained. This study suggests feasibility of high-dielectric pad to enhanced human brain imaging and neuroimaging study in ultra-high field MRI. In particular, it will be useful to get an enhanced data quality at deep brain regions which are vulnerable to contamination by magnet field inhomogeneity in ultra-high field MRI. Also, our study would be the basis for further specialized studies using quantitative and qualitative approach.
Availability of data and materials
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Magnetic resonance imaging
Specific absorption rate
Echo planar imaging
Diffusion weighted images
Diffusion tensor imaging
- BaTiO3 :
Magnetization prepared rapid acquisition gradient echo
False discovery rate
Q-space diffeomorphic reconstruction
Spin distribution function
Splenium of corpus callosum
Inferior fronto-occipital fasciculus
Middle cerebellar peduncle
Body of corpus callosum
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This research supported by Basic Science Research Capacity Enhancement Project through Korea Basic Science Institute (National research Facilities and Equipment Center) grant funded by the Ministry of Education (Grant no. 2021R1A6C101A432), and Bio & Medical Technology Development Program of the National Research Foundation funded by the Korea government (MSIT) (2020M3A9E4104384).
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Park, SE., Jeon, YJ. & Baek, HM. Benefits of high-dielectric pad for neuroimaging study in 7-Tesla MRI. J Anal Sci Technol 14, 17 (2023). https://doi.org/10.1186/s40543-023-00380-7
- High-dielectric pad
- Ultra-high field MRI
- Diffusion tensor imaging
- High signal intensity
- High tissue contrast