Imaging Coil Research Articles

247.5: Magnetic Resonance Imaging of Renal Oxygen Metabolism by Means of 17-O Administration During Ex Vivo Organ Perfusion

Background: Renal normothermic machine perfusion (NMP) is a method studied for pre-transplant evaluation of kidney graft quality. This technique creates an isolated near-physiological environment by circulating a perfusion solution at body temperature with oxygen and nutrients. However, reliable ex vivo organ viability markers are still lacking. Magnetic resonance imaging (MRI) has recently been applied as a new rapid non-invasive technique to study renal viability ex vivo. Oxygen-17 (17O) offers a novel sensitive assessment of oxidative metabolism. In our MRI compatible perfusion setup, we aimed to study oxygen metabolism during NMP. Methods: Three viable porcine kidneys were retrieved at a slaughterhouse and preserved by oxygenated hypothermic machine perfusion. In addition, one discarded human kidney was shipped to our center in static cold storage. Subsequently each kidney was perfused for 4 hours at 37°C inside a clinical 3T MRI scanner. Blood gas, glucose, and electrolytes were periodically monitored. Anatomical T2-weighted images and T2* maps were obtained using a 1H 18Ch-bodyflex coil and dynamic radial 17O MR images were acquired using a 17O Tx/Rx loop coil and a 17O 3D-UTE sequence. The 17O perfusion protocol consisted of 4 minutes of deoxygenation with nitrogen gas, 8 minutes of 70%-enriched 17O2 gas infusion at 0.1L/min, and 20 minutes of subsequent carbogen (95% O2/ 5% CO2) oxygenation. 17O-magnitude data was analyzed dynamically over time. To study the 17O-related global signal change, a region of interest was defined in the anatomical T2-weighted images and co-registered to the dynamic 17O-magnitude data. Results: Porcine kidneys showed homogenous perfusion with increased H217O-concentration in the medulla and decreased concentration in the cortex. The discarded human kidney demonstrated a focused concentration in the medial side of the lower pole with little to no signal intensity in the upper pole and the majority of the cortex, which was consistent with its reason for discard (wide cist in the lateral pole). Six minutes after administration of 17O2-gas, a rapid H217O signal increase was observed until minute 20 of the dynamic acquisition, where saturation was reached and the signal plateaued. Overall, an enhancement of 16% between the first and last data point was measured. Conclusion: In these pilot experiments, we were able to quantify the regional production of H217O in time, both in porcine and human kidneys. The slope of occurrence of H217O may be an interesting ex vivo organ viability biomarker, immediately related to the rate of oxidative metabolism in the organ and hence organ quality. As the kidney is in an ex vivo setting, any increase in H217O-concentration must be the direct result of metabolism within the organ. The current setup has great potential for investigating renal oxygen metabolism ex vivo. NUKEM Isotopes GmbH, Alzenau, Germany. Department of Radiology, University Medical Center Freiburg, Freiburg, Germany.

A uniformity correction method to reduce scan time for 7T sodium imaging of brain tumors.

Sodium imaging shows great potential for the characterization of brain tumors. Intensity correction is required but the additional scan time is costly. Recent developments can halve the time but were optimized in normal brains and may not be applicable in brain tumor imaging. We aim to develop an individualized uniformity correction for sodium imaging optimized for brain tumor patients that reduces scan time but provides high-resolution images for clinical practice. Two-, 4-, and 6-mm iso-cubic voxel resolution birdcage coil images were used to calculate the 2-mm iso-cubic voxel individual sensitivity maps in healthy subjects (n=3). Cut profiles were compared to determine the optimal approach. In addition, a 3-dimensional phantom was developed to test a generalized uniformity correction approach in both healthy subjects (n=3) and tumor patients (n=3). The cut profiles showed that the average correlation coefficient between 2- and 4-mm birdcage image correction results was r=.9937, and r=.9876 for 2- and 6-mm birdcage images. The correlation result between individual map correction and phantom map correction was r=.9817. The 4mm birdcage coil image provided the optimal approach for both as a compromise between the time-savings effect and image quality. This method allows for a 2-mm iso-cubic voxel resolution clinical sodium scan within 12minutes. We also presented prescanned phantom sensitivity map results, which were designed to cover all patient head sizes. This approach provides an alternative solution in more time-sensitive cases.

Evaluation of a novel 8-channel RX coil for speech production MRI at 0.55T.

Speech production MRI benefits from lower magnetic fields due to reduced off-resonance effects at air-tissue interfaces and from the use of dedicated receiver coils due to higher SNR and parallel imaging capability. Here we present a custom designed upper airway coil for 1H imaging at 0.55 Tesla and evaluate its performance in comparison with a vendor-provided prototype 16-channel head/neck coil. Four adult volunteers were scanned with both custom speech and prototype head-neck coils. We evaluated SNR gains of each of the coils over eleven upper airway volumes-of-interest measured relative to the integrated body coil. We evaluated parallel imaging performance of both coils by computing g-factors for SENSE reconstruction of uniform and variable density Cartesian sampling schemes with R = 2, 3, and 4. The dedicated coil shows approximately 3.5-fold SNR efficiency compared to the head-neck coil. For R = 2 and 3, both uniform and variable density samplings have g-factor values below 1.1 in the upper airway region. For R = 4, g-factor values are higher for both trajectories. The dedicated coil configuration allows for a significant SNR gain over the head-neck coil in the articulators. This, along with favorable g values, makes the coil useful in speech production MRI.

Volume and surface coil simultaneous reception (VSSR) method for intensity inhomogeneity correction in MRI.

Addressing intensity inhomogeneity is critical in magnetic resonance imaging (MRI) because associated errors can adversely affect post-processing and quantitative analysis of images (i.e., segmentation, registration, etc.), as well as the accuracy of clinical diagnosis. Although several prior methods have been proposed to eliminate or correct intensity inhomogeneity, some significant disadvantages have remained, including alteration of tissue contrast, poor reliability and robustness of algorithms, and prolonged acquisition time. In this study, we propose an intensity inhomogeneity correction method based on volume and surface coils simultaneous reception (VSSR). The VSSR method comprises of two major steps: 1) simultaneous image acquisition from both volume and surface coils and 2) denoising of volume coil images and polynomial surface fitting of bias field. Extensive in vivo experiments were performed considering various anatomical structures, acquisition sequences, imaging resolutions, and orientations. In terms of correction performance, the proposed VSSR method was comparatively evaluated against several popular methods, including multiplicative intrinsic component optimization and improved nonparametric nonuniform intensity normalization bias correction methods. Experimental results show that VSSR is more robust and reliable and does not require prolonged acquisition time with the volume coil. The VSSR may be considered suitable for general implementation.

Observation on the Effect of MRI Image Scanning on Knee Pain in Football Injury.

To study the effect of football injury on knee pain based on MRI image scanning, in this paper, a total of 31 knee injuries of 29 male professional football players from December 2012 to April 2015 were used as the experimental group. The players were 23.6 ± 3.5 years old and received professional football training time 15.3 ± 3.6 years; 31 outpatients of the same age group with acute knee joint acute injury were randomly selected as the control group; both groups were imaged with a 1.5 TMR scanner and knee joint standard array coil imaging, and 2 senior radiation surgeons evaluate knee cartilage, meniscus, ligaments, tendons, bone marrow, infrapatellar fat pad, and joint effusions. Pearson's chi-squared test and nonparametric test for two independent samples were used for statistical testing of the evaluation results. The experimental results showed that there were significant differences in the incidence of articular cartilage, lateral collateral ligament, tendon or ligament injury, multiligament or tendon injury, and bone marrow edema between the two groups (P < 0.05). There was no significant difference in the incidence of medial collateral ligament injury, infrapatellar fat pad edema, and joint effusion. MRI shows that knee injuries in male professional football players often involve ligaments or tendons, mostly multiligament or tendon injuries. The lesions of articular cartilage and meniscus are more common and serious, and bone marrow edema is also more common in football injuries. MRI has high diagnostic accuracy for various clinical knee injuries, and it belongs to a noninvasive examination method. It can not only reflect the pathological changes and changes of the knee joints of patients but also provide information for the formulation of clinical programs and the judgment of prognosis, for timely, accurate, and comprehensive imaging reference.

Analytical Solutions to Minimum-Norm Problems

For G∈Rm×n and g∈Rm, the minimization min∥Gψ−g∥2, with ψ∈Rn, is known as the Tykhonov regularization. We transport the Tykhonov regularization to an infinite-dimensional setting, that is min∥T(h)−k∥, where T:H→K is a continuous linear operator between Hilbert spaces H,K and h∈H,k∈K. In order to avoid an unbounded set of solutions for the Tykhonov regularization, we transform the infinite-dimensional Tykhonov regularization into a multiobjective optimization problem: min∥T(h)−k∥andmin∥h∥. We call it bounded Tykhonov regularization. A Pareto-optimal solution of the bounded Tykhonov regularization is found. Finally, the bounded Tykhonov regularization is modified to introduce the precise Tykhonov regularization: min∥T(h)−k∥with∥h∥=α. The precise Tykhonov regularization is also optimally solved. All of these mathematical solutions are optimal for the design of Magnetic Resonance Imaging (MRI) coils.

Electron Streaming Effect Associated With the Elekta Unity Anterior Imaging Coil

The presence of the static field in magnetic resonance guided radiation therapy (MRgRT) systems, such as the Elekta Unity MR-Linac (MRL), influences charged particle motion due to the Lorentz force in and around patients. Consequently, for the Unity, changes to out-of-field dose (OFD) relative to conventional linacs occur due to the electron streaming effect (ESE) and spiraling contaminant electrons (SCE). This work investigates OFD associated with irradiation of the anterior MR imaging coil, which is in situ for all treatments on the Unity. Film measurements and Monaco simulations were performed to quantify the magnitude of OFD at the superior and inferior ends of the coil as a function of coil tilt relative to the beam direction. The dependence of OFD on field size and the relative electron density (RED) assigned to the coil and surrounding air are reported. The doses at both coil ends were clinically significant, with nearly 23.0% of the Dmax dose to water being recorded for the largest field (8.0 × 22.0 cm2) and 6.8% for the smallest field (3.0 × 3.0 cm2). Monaco simulations of OFD agreed with film within 5.0%, when appropriate calculation conditions were set. OFD decreases as coil tilt is reduced, and there is no evidence of ESE when the coil is horizontal. Clinically, the potential magnitude of cranio–caudal streaming dose from a tilted coil necessitates the use of appropriate shielding. A clinical case involving coil-induced ESE during treatment of a lesion in the right angle is presented. The planning-based investigation revealed that ESE doses associated with the coil and an immobilization vacuum bag can be clinically significant.

Accelerated MRI at 9.4 T with electronically modulated time-varying receive sensitivities.

To investigate how electronically modulated time-varying receive sensitivities can improve parallel imaging reconstruction at ultra-high field. Receive sensitivity modulation was achieved by introducing PIN diodes in the receive loops, which allow rapid switching of capacitances in both arms of each loop coil and by that alter B1- profiles, resulting in two distinct receive sensitivity configurations. A prototype 8-channel reconfigurable receive coil for human head imaging at 9.4T was built, and MR measurements were performed in both phantom and human subject. A modified SENSE reconstruction for time-varying sensitivities was formulated, and g-factor calculations were performed to investigate how modulation of receive sensitivity profiles during image encoding can improve parallel imaging reconstruction. The optimized modulation pattern was realized experimentally, and reconstructions with the time-varying sensitivities were compared with conventional static SENSE reconstructions. The g-factor calculations showed that fast modulation of receive sensitivities in the order of the ADC dwell time during k-space acquisition can improve parallel imaging performance, as this effectively makes spatial information of both configurations simultaneously available for image encoding. This was confirmed by in vivo measurements, for which lower reconstruction errors (SSIM=0.81 for acceleration R= 4) and g-factors (max g=2.4; R= 4) were observed for the case of rapidly switched sensitivities compared to conventional reconstruction with static sensitivities (SSIM=0.74 and max g=3.2; R= 4). As the method relies on the short RF wavelength at ultra-high field, it does not yield significant benefits at 3T and below. Time-varying receive sensitivities can be achieved by inserting PIN diodes in the receive loop coils, which allow modulation of B1- patterns. This offers an additional degree of freedom for image encoding, with the potential for improved parallel imaging performance at ultra-high field.

Evaluation of MU2net as an online secondary dose check for MR guided radiation therapy with the Elekta unity MR linac.

During the adaptive workflow associated with MRgRT, a secondary dose calculation is required and MU2net (DOSIsoft, France) is one commercial option. The suitability of MU2net to be used in conjunction with the online Monaco treatment planning system of the Elekta Unity (Elekta AB, Stockholm, Sweden), is evaluated in this work. Monaco and MU2net point doses are compared for various fields on and off axis and at different SSDs. To investigate the comparative effects of attenuation due to the cryostat, couch and posterior coil, measured, MU2net and Monaco dose outputs at the isocentre, as a function of gantry angle, were compared. Point doses for the beams of nine step and shoot IMRT (SSIMRT) test plans (courtesy Elekta) were calculated with Monaco v5.4 and compared to corresponding doses computed with MU2net. In addition, Monaco v5.4 and MU2net point doses were compared for 1552 beams treated on the Unity at our facility. For the on-axis fields investigated the agreement between MU2net and measured data is acceptable. MU2net and Monaco point doses for the Elekta SSIMRT test plans were within ± 5.0% and ± 6.4% for beams delivered from gantry zero and at planned beam angles, respectively. For the 1552 beams delivered approximately 80.0% of MU2net and Monaco point doses agree within ± 5.0%, therefore it is recommended to correlate MU2net Dose Reference Points (DRPs) with pre and post treatment dosimetry verification. Computational accuracy of MU2net could be enhanced with improved modelling of attenuation due to the couch, cryostat and posterior MR imaging coil.

Revisiting the minimum-norm problem

The design of optimal Magnetic Resonance Imaging (MRI) coils is modeled as a minimum-norm problem (MNP), that is, as an optimization problem of the form min_{xinmathcal{R}}|x|, where mathcal{R} is a closed and convex subset of a normed space X. This manuscript is aimed at revisiting MNPs from the perspective of Functional Analysis, Operator Theory, and Banach Space Geometry in order to provide an analytic solution to the following MRI problem: min_{psiinmathcal{R}}|psi|_{2}, where mathcal{R}:={psiin mathbb{R}^{n}:frac{|Apsi-b|_{infty}}{|b|_{infty}} leq D}, with Ainmathcal{M}_{mtimes n}(mathbb{R}), D>0, and binmathbb{R}^{m}setminus{0}.

Calibrationless multi-slice Cartesian MRI via orthogonally alternating phase encoding direction and joint low-rank tensor completion.

We propose a multi-slice acquisition with orthogonally alternating phase encoding (PE) direction and subsequent joint calibrationless reconstruction for accelerated multiple individual 2D slices or multi-slice 2D Cartesian MRI. Specifically, multi-slice multi-channel data are first acquired with random or uniform PE undersampling while orthogonally alternating PE direction between adjacent slices. They are then jointly reconstructed through a recently developed low-rank multi-slice Hankel tensor completion (MS-HTC) approach. The proposed acquisition and reconstruction strategy was evaluated with human brain MR data. It effectively suppressed aliasing artifacts even at high acceleration factor, outperforming the existing MS-HTC approach, where PE direction is the same between adjacent slices. More importantly, the new strategy worked robustly with uniform undersampling or random undersampling without any consecutive central k-space lines. In summary, our proposed multi-slice MRI strategy exploits both coil sensitivity and image content similarities across adjacent slices. Orthogonally alternating PE direction among slices substantially facilitates the low-rank completion process and improves image reconstruction quality. This new strategy is applicable to uniform and random PE undersampling. It can be easily implemented in practice for Cartesian parallel imaging of multiple individual 2D slices without any coil sensitivity calibration.

Dual-Band Transceiver High Impedance Coil Array for Ultrahigh Field Magnetic Resonance Imaging

A novel approach for designing an element of an arrayed radio frequency coil for ultrahigh field magnetic resonance imaging (UHF MRI) is presented. The purpose of this approach is to achieve the dual-band transceiver regime in a compact array of magnetic antennas. Our work qualitatively develops the concept of the so-called high-impedance coil (HIC), combined with the adequate interfacing circuitry, which offers the decoupling of the HICs in the array due to the very high self-impedance compared to the mutual impedance. This concept has not been previously thought applicable to dual-band transceiver arrays. We show that, by slightly modifying only the cable shield, we achieve a dual-band transceiver high-impedance coil (dual-band TRHIC). The needed modification implies two asymmetric gaps granting two useful eigenmodes to the cable loop. The resonant excitation of these modes will allow the magnetic resonant scanning of both hydrogen and Phosphorus in the 7 T dc magnetic field. To verify our ideas, we simulated and measured a single TRHIC in both transmitting and receiving regimes and similarly studied two linear arrays made of two and three proposed TRHICs. The method of asymmetric gaps, in our opinion, allows one to engineer the targeted multifrequency operation of arrayed TRHICs decoupled at these frequencies.

Application of Highly Flexible Adaptive Image Receive Coil for Lung MR Imaging Using Zero TE Sequence: Comparison with Conventional Anterior Array Coil.

(1) Background: Highly flexible adaptive image receive (AIR) coil has become available for clinical use. The present study aimed to evaluate the performance of AIR anterior array coil in lung MR imaging using a zero echo time (ZTE) sequence compared with conventional anterior array (CAA) coil. (2) Methods: Sixty-six patients who underwent lung MR imaging using both AIR coil (ZTE-AIR) and CAA coil (ZTE-CAA) were enrolled. Image quality of ZTE-AIR and ZTE-CAA was quantified by calculating blur metric value, signal-to-noise ratio (SNR), and contrast-to-noise ratio (CNR) of lung parenchyma. Image quality was qualitatively assessed by two independent radiologists. Lesion detection capabilities for lung nodules and emphysema and/or lung cysts were evaluated. Patients’ comfort levels during examinations were assessed. (3) Results: SNR and CNR of lung parenchyma were higher (both p < 0.001) in ZTE-AIR than in ZTE-CAA. Image sharpness was superior in ZTE-AIR (p < 0.001). Subjective image quality assessed by two independent readers was superior (all p < 0.05) in ZTE-AIR. AIR coil was preferred by 64 of 66 patients. ZTE-AIR showed higher (all p < 0.05) sensitivity for sub-centimeter nodules than ZTE-CAA by both readers. ZTE-AIR showed higher (all p < 0.05) sensitivity and accuracy for detecting emphysema and/or cysts than ZTE-CAA by both readers. (4) Conclusions: The use of highly flexible AIR coil in ZTE lung MR imaging can improve image quality and patient comfort. Application of AIR coil in parenchymal imaging has potential for improving delineation of low-density parenchymal lesions and tiny nodules.

Super-resolution reconstruction of fiber optic coil image based on lightweight network

Super-resolution reconstruction of fiber optic coil image based on lightweight network

Electromagnetic acoustic imaging methods: resolution, signal-to-noise, and image contrast in phantoms.

Purpose: Electromagnetic acoustic imaging (EMAI) is a hybrid imaging technique using radio-frequency irradiation to induce ultrasound (US), providing an US image in which spatial conductivity differences provide image contrast. The method is potentially clinically important in that the added diagnostic parameter has been shown to be useful in cancer detection and vascular space delineation. Approach: We report the development of coil configurations and imaging processing techniques designed to address the low signal-to-noise of EMAI and demonstrate achievable resolution and contrast in phantoms along with EMAI signals in excised animal tissue. Experiment results are compared with theoretical calculations. Results: EMAI signal intensities depend on the square of the ampere-turns in the coil radio frequency coil as predicted theoretically. Resolution is shown to be comparable to conventional US imaging with contrast and signal intensity depending on source conductivity. Optimizing signal-to-noise depends on coil design, orientation of the electromagnetic fields, and coherent processing. Conclusions: Two-dimensional EMAI images are shown to have the expected resolution of conventional US with image contrast dependent on conductivity. Achievable signal-to-noise is sufficient to form potentially clinically useful images.

Selective responses to faces, scenes, and bodies in the ventral visual pathway of infants

Three of the most robust functional landmarks in the human brain are the selective responses to faces in the fusiform face area (FFA), scenes in the parahippocampal place area (PPA), and bodies in the extrastriate body area (EBA). Are the selective responses of these regions present early in development or do they require many years to develop? Prior evidence leaves this question unresolved. We designed a new 32-channel infant magnetic resonance imaging (MRI) coil and collected high-quality functional MRI (fMRI) data from infants (2-9months of age) while they viewed stimuli from four conditions-faces, bodies, objects, and scenes. We find that infants have face-, scene-, and body-selective responses in the location of the adult FFA, PPA, and EBA, respectively, powerfully constraining accounts of cortical development.

High-resolution intravascular magnetic resonance imaging of the coronary artery wall at 3.0 Tesla: toward evaluation of atherosclerotic plaque vulnerability.

To validate the feasibility of generating high-resolution intravascular 3.0 Tesla (T) magnetic resonance imaging of the coronary artery wall to further plaque imaging. A receive-only 0.014-inch diameter magnetic resonance imaging guidewire (MRIG) was manufactured for intravascular imaging within a phantom experiment and the coronary artery wall of the swine. For coronary artery wall imaging, both high-resolution images and conventional resolution images were acquired. A 16-channel commercial surface coil for magnetic resonance imaging was employed for the control group. For the phantom experiment, the MRIG showed a higher signal-to-noise ratio than the surface coil. The peak signal-to-noise ratio of the MRIG and the surface coil-generated imaging were 213.6 and 19.8, respectively. The signal-to-noise ratio decreased rapidly as the distance from the MRIG increased. For the coronary artery wall experiment, the vessel wall imaging by the MRIG could be identified clearly, whereas the vessel wall imaging by the surface coil was blurred. The average signal-to-noise ratio of the artery wall was 21.1±5.40 by the MRIG compared to 8.4±2.19 by the surface coil, where the resolution was set at 0.2 mm × 0.2 mm × 2 mm. As expected, the high-resolution sequence clearly showed more details than the conventional resolution sequence set at 0.7 mm × 0.7 mm × 2.0 mm. Histological examination showed no evidence of mechanical injuries in the target vessel walls. The study validated the feasibility of generating magnetic resonance imaging (MRI) at 0.2 mm × 0.2 mm × 2 mm for the coronary artery wall using a 0.014 inch MRIG.

Feasibility of Brain Imaging Using a Digital Surround Technology Body Coil: A Study Based on SRGAN-VGG Convolutional Neural Networks.

Brain imaging using conventional head coils presents several problems in routine magnetic resonance (MR) examination, such as anxiety and claustrophobic reactions during scanning with a head coil, photon attenuation caused by the MRI head coil in positron emission tomography (PET)/MRI, and coil constraints in intraoperative MRI or MRI-guided radiotherapy. In this paper, we propose a super resolution generative adversarial (SRGAN-VGG) network-based approach to enhance low-quality brain images scanned with body coils. Two types of T1 fluid-attenuated inversion recovery (FLAIR) images scanned with different coils were obtained in this study: joint images of the head-neck coil and digital surround technology body coil (H+B images) and body coil images (B images). The deep learning (DL) model was trained using images acquired from 36 subjects and tested in 4 subjects. Both quantitative and qualitative image quality assessment methods were performed during evaluation. Wilcoxon signed-rank tests were used for statistical analysis. Quantitative image quality assessment showed an improved structural similarity index (SSIM) and peak signal-to-noise ratio (PSNR) in gray matter and cerebrospinal fluid (CSF) tissues for DL images compared with B images (P <.01), while the mean square error (MSE) was significantly decreased (P <.05). The analysis also showed that the natural image quality evaluator (NIQE) and blind image quality index (BIQI) were significantly lower for DL images than for B images (P <.0001). Qualitative scoring results indicated that DL images showed an improved SNR, image contrast and sharpness (P<.0001). The outcomes of this study preliminarily indicate that body coils can be used in brain imaging, making it possible to expand the application of MR-based brain imaging.

A 28-channel receive-only imaging coil array for concurrent multichannel TMS and fMRI acquisition at 3T

A 28-channel receive-only imaging coil array for concurrent multichannel TMS and fMRI acquisition at 3T

The misunderstood meander: Redesigning MRI meander-line surface coils to reduce noise, increase uniformity, and eliminate image artifacts

Meander-line, or zig-zag, MRI surface coils theoretically promise spatially uniform fields with optimal field localization close to the coil. In reality, they suffer poorer than expected field localizations and acquired images are often highly inhomogeneous, plagued by repeating stripe-like signal-loss artifacts. We show that both these detrimental effects arise from coil design based on the same invalid approximation in the underlying theory. Here, the conventional approximation is corrected, yielding a modified coil design that validates the new theory by rectifying the above problems. Specifically, an easily implementable coil correction, which amounts to the addition of a single extra turn of wire, is introduced and shown to increase signal uniformity by an order of magnitude, eliminate image artifacts, and reduce unwanted signal interference from deeper within the sample by tightening the coil field localization to close to the coil, as intended for zig-zag designs. With independent optimization of coil size and imaging depth possible, such corrected meander-lines surface coils may be well suited for large area, near-surface imaging and spectroscopy applications.