A non-invasive technique for 3-dimensional assessment of articular cartilage thickness based on MRI part 2: Validation using CT arthrography

Established methods for the measurement of articular cartilage thickness are invasive and cannot be sequentially applied in living subjects. In the present study, the distribution of cartilage thickness throughout entire joint surfaces was determined from MR images obtained with a fat-suppressed gradient-echo sequence at a resolution of 0.31 × 0.31 × 2.00 mm³, and compared to that derived from CT arthrography. A minimal distance algorithm was employed to produce 3D cartilage thickness maps of seven cadaveric human knee joints. The mean amount of deviation of the cartilage volumes was 5.6% (±4.6), statistical analysis showing that there was high agreement between the two methods (r = 0.995, slope = 1.037, y-intercept = -90.5 mm³). The 3D thickness maps yielded a striking agreement between the two methods, the maximum values generally yielding a deviation of none or one thickness interval of 0.5 mm. This investigation shows that accurate 3D assessment of articular cartilage thickness can be performed with MRI, this technique having the advantage that it is suitable for investigating living subjects.     

High resolution MRI relaxation measurements of water in the articular cartilage of the meniscectomized rat knee at 4.7 T

Measurements by magnetic resonance imaging (MRI) of the spin-spin (T2), spin-lattice (T1) and spin-density (M0) parameters of water protons, optimized by using the Cramér-Rao Lower Bound (CRLB) theory, were made to quantify the effect of surgically induced osteoarthritis on rat knee cartilage at 4.7 T. Partial meniscectomy was performed on the right medial condyle of four Sprague Dawley rats, leaving the left medial condyle as a control. The animals were euthanized 3 weeks after the operation; the entire limbs were removed and T2 and T1 relaxation measurements and M0 measurements of the protons of water were obtained using conventional Carr-Purcell-Meiboom-Gill (CPMG) and saturation recovery methods. M0 was normalized with respect to a water phantom, to obtain the relative spin-density M0%. Weight-bearing cartilage areas on the meniscectomized medial condyles exhibited a significant increase of T2 relaxation time (p < 0.001) and of M0% (p < 0.01) with respect to the control; T1 relaxation times did not show any statistically significant changes. CRLB-based sampling optimization offered an insight to improved measurement precision and a reduction of scanning time against conventional sampling methods methods. Quantitative MRI assessment of the meniscectomized rat knee shows that cartilage exhibits changes in T2 and M0 values 3 weeks after operation.     

Carotid plaque high-resolution MRI at 3 T: evaluation of a new imaging score for symptomatic plaque assessment

Purpose

To assess the sensitivity and specificity of intra-plaque hemorrhage (IPH), large lipid-rich necrotic core (LR-NC) and ulceration or cap rupture (UCR) for symptomatic carotid plaque characterization and to evaluate a new imaging score [Hemorrhage, Ulceration or cap rupture, Lipid-rich necrotic Core (HULC) score based on the sum of presence/absence of IPH, UCR and LR-NC; range 0–3] for assessment of recently symptomatic carotid plaques.

Material and methods

Twenty-seven recently symptomatic (< 8 weeks) and 36 asymptomatic patients with a carotid plaque thicker than 2 mm were prospectively imaged on a 3-T magnetic resonance (MR) system using high-resolution, multi-contrast MR sequences. Prior to analysis, all images were reviewed to assess image quality of each sequence. Sensitivity and specificity of IPH, LR-NC, UCR and HULC scores were calculated.

Results

Fifty-one patients were analyzed (26 symptomatic carotids and 67 asymptomatic carotids) after exclusion of studies with poor image quality. Sensitivity and specificity for symptomatic carotid plaque was, respectively, 46.1% and 97% for IPH, 84.6% and 73.1% for UCR and 80.7% and 76.1% for LR-NC. A HULC score of 2 or more showed a sensitivity of 73% and a specificity of 92.5%.

Conclusion

At 3 T, intra-plaque hemorrhage is the most specific criterion to characterize symptomatic carotid plaque. The HULC score offers the best compromise between sensitivity and specificity.     

Comparison of PD BLADE with fat saturation (FS), PD FS and T2 3D DESS with water excitation (WE) in detecting articular knee cartilage defects

The purpose of this study, is to compare the sequences: 1) proton density (PD) BLADE (BLADE is a PROPELLER-equivalent implementation of the Siemens Medical System) with fat saturation (FS) coronal (COR), 2) PD FS COR, 3) multi-planar reconstruction (MPR) with 3 mm slice thickness and 4) multi-planar reconstruction (MPR) with 1.5 mm slice thickness, both from the T2 3D-double-echo steady state (DESS) with water excitation (WE) sagittal (SAG), regarding their abilities to identify changes in the femorotibial condyle cartilage in knee MRI examinations. Thirty three consecutive patients with osteoarthritis (18 females, 15 males; mean age 56 years, range 37–71 years), who had been routinely scanned for knee examination using the previously mentioned image acquisition techniques, participated in the study. A quantitative analysis was performed based on the relative contrast (ReCON) measurements, which were taken both on normal tissues as well as on pathologies. Additionally, a qualitative analysis was performed by two radiologists. Motion and pulsatile flow artifacts were evaluated. The PD BLADE FS COR sequence produced images of higher contrast between Menisci and Cartilage, Fluid and Cartilage, Pathologies and Cartilage as well as of the Conspicuousness Superficial Cartilage and it was found to be superior to the other sequences (p < 0.001). The sequences T2 3D DESS 1.5 mm and T2 3D DESS 3 mm were significantly superior to the PD BLADE FS COR and the PD FS COR sequences in the visualization of Bone and Cartilage and the Conspicuousness Deep Surface Cartilage. This pattern of results is also confirmed by the quantitative analysis. PD FS BLADE sequences are ideal for the depiction of the cartilage pathologies compared to the conventional PD FS and T2 3D DESS sequences.     

Feasibility of free-breathing T1-weighted 3D radial VIBE for fetal MRI in various anomalies

Rationale and objectivesIn magnetic resonance (MR) fetal imaging, the image quality acquired by the traditional Cartesian-sampled breath-hold T1-weighted (T1W) sequence may be degraded by motion artifacts arising from both mother and fetus. The radial VIBE sequence is reported to be a viable alternative to conventional Cartesian acquisition for both pediatric and adult MR, yielding better image quality. This study evaluated the role of radial VIBE in fetal MR imaging and compared its image quality and motion artifacts with those of the Cartesian T1W sequence.Materials and methodsWe included 246 pregnant women with 50 lesions on 1.5-T MR imaging. Image quality and lesion conspicuity were evaluated by two radiologists, blinded to the acquisition schemes used, using a five-point scale, where a higher score indicated a better trajectory method. Mixed-model analysis of variance and interobserver variability assessment were performed.ResultsThe radial VIBE sequence showed a significantly better performance than conventional T1W imaging in the head and neck, fetal body, and placenta region: 3.92 ± 0.88 vs 3 ± 0.74, p < 0.001, 3.8 ± 0.94 vs 3.15 ± 0.87, p < 0.001, and 4.17 ± 0.63 vs 3.12 ± 0.72, p < 0.001, respectively. Additionally, fewer motion artifacts were observed in all regions with the radial VIBE sequence (p < 0.01). Of 50 lesions, 49 presented better lesion conspicuity on radial VIBE images than on T1W images (4.34 ± 0.91 vs 3.48 ± 1.46, p < 0.001).ConclusionFor fetal imaging, the radial VIBE sequences yielded better image quality and lesion conspicuity, with fewer motion artifacts, than conventional breath-hold Cartesian-sampled T1W sequences.     

Mri measurements of T1, T2 and D for gels undergoing volume phase transition

Gels consist of crosslinked polymer network swollen in solvent. The network of flexible long-chain molecules traps the liquid medium they are immersed in. Some gels undergo abrupt volume change, a phase transition process, by swelling-shrinking in response to external stimuli changes in solvent composition, temperature, pH, electric field, etc. We report that during volume phase transition changes of NMR longitudinal relaxation time T(1), NMR transverse relaxation time T(2), and diffusion coefficient D of the PMMA gel, and D of the NIPA gel. We describe how the gels were synthesized and the reason of using the snapshot FLASH imaging sequence to measure T(1), T(2), and D. Since T(1), T(2) and D maps have identical field of view and data are extracted from identical areas from their respective maps, these values can be correlated quantitatively on a pixel-by-pixel basis. Thus a complete set of NMR parameters is measured in-situ: the gels are in their natural state, immersed in the liquid, during the phase transition. The results of spectroscopic method agree with that of snapshot FLASH imaging method. For the PMMA gel T(1), T(2) and D decrease when gels undergo volume phase transition between deuterated acetone concentration of 30% and 40%. At its contracted state, T(1) is reduced to a little less than one order of magnitude, T(2) over two orders of magnitude, and D over one order of magnitude, smaller from values of PMMA gel at the swollen state. At an elevated temperature of 54 degrees C the thermosensitive NIPA gel is at a contracted state, with its D reduced to almost one order of magnitude smaller from that of the swollen NIPA at room temperature.     

Single-shot morpho-functional and structural characterization of the left-ventricle in a mouse model of acute ischemia-reperfusion injury with an optimized 3D IntraGate cine FLASH sequence at 7T MR

Preclinical cardiac MR is challenging and time-consuming. A fast and comprehensive acquisition protocol and standardized image post-processing may improve preclinical research, reducing acquisition time, costs and variability of results. In the present study, we evaluated the feasibility of a contrast-enhanced 3D IntraGate steady-state cine sequence (ce-3D-IG-cine) with short acquisition time (11 min) for a single-shot combined characterization of left ventricle (LV) remodeling and infarct size (IS) in a mouse model of acute ischemia-reperfusion injury.Sixteen male C57BL/6N mice underwent 7T cardiac MR (Bruker, BioSpec 70/30) including optimized ce-3D-IG-cine (total scan time 11 min) at day 1, 5 and 28 after surgery. LV end-diastolic volume (EDV_MR) and ejection fraction (EF_MR) extracted from MR were compared to ones from short-axis (SA-EDV_echo, SA-EF_echo) and parasternal long-axis (LA-EDV_echo, LA-EF_echo) echocardiography. IS was manually and semiautomatically segmented from ce-3D-IG-cine using different standard deviation (SD +2, +3, +4, +5, +6 in respect to a reference tissue).Mice were sacrificed at day 28, immediately after imaging. IS at day 28 was compared to injury burden at histology.MR and echocardiographic morpho-functional parameters were compared, as IS from MR and histology. Bland-Altman plots were used to assess the agreement in ischemic burden segmentation.Volumetric and functional parameters measured on ce-3D-IG-cine correlated to the correspondent echocardiographic parameter (EDV_MR vs SA-EDV_echo: ρ = 0.813; EDV_MR vs LA-EDV_echo: ρ = 0.845; EF_MR vs SA-EF_echo ρ = 0.612; EF_MR vs LA-EF_echo ρ = 0.791; p < 0.001 in all cases).Manually segmented IS strongly correlated with the scar at histology (ρ = 0.904, p < 0.001). A threshold of +3SD showed the highest performance for semiautomatic assessment of IS compared to manual segmentation (ρ = 0.965, p < 0.001), with an overall reproducibility of 73%, and a peak reproducibility of 80% at day 1.The ce-3D-IG-cine sequence, manually or semiautomatically segmented using 3SD threshold, allows fast and comprehensive LV morpho-functional and structural characterization in myocardial ischemia-reperfusion injury model.     

Reproducibility of the quantitative assessment of cartilage morphology and trabecular bone structure with magnetic resonance imaging at 7 T

To assess the reproducibility of quantitative measurements of cartilage morphology and trabecular bone structure of the knee at 7 T, high-resolution sagittal spoiled gradient-echo images and high-resolution axial fully refocused steady-state free-precession (SSFP) images from six healthy volunteers were acquired with a 7-T scanner. The subjects were repositioned between repeated scans to test the reproducibility of the measurements. The reproducibility of each measurement was evaluated using the coefficient(s) of variation (CV). The computed CV were 1.13% and 1.55% for cartilage thickness and cartilage volume, respectively, and were 2.86%, 1.07%, 2.27% and 3.30% for apparent bone volume over total volume fraction (app.BV/TV), apparent trabecular number (app.Tb.N), apparent trabecular separation (app.Tb.Sp) and apparent trabecular thickness (app.Tb.Th), respectively. The results demonstrate that quantitative assessment of cartilage morphology and trabecular bone structure is reproducible at 7 T and motivates future musculoskeletal applications seeking the high-field strength's superior signal-to-noise ratio.     

Comparison of quantitative imaging of cartilage for osteoarthritis: T2, T1ρ, dGEMRIC and contrast-enhanced computed tomography

Evaluation of glycosaminoglycan (GAG) concentration in articular cartilage is of particular interest to the study of degenerative joint diseases such as osteoarthritis (OA). Noninvasive imaging techniques such as magnetic resonance imaging (MRI) and computed tomography (CT) have demonstrated the potential to assess biochemical markers of cartilage integrity such as GAG content; however, many imaging techniques are available and the optimization of particular techniques in the diagnosis of joint disease remains an active area of research. In order to highlight the differences between these various approaches, this work compares MRI (T1, T2 and T1ρ) and contrast-enhanced CT in human articular cartilage, in both the presence and absence of gadolinium-based contrast agent. Pre- and postcontrast T2 values were found to be similar on a regional level and correlated with each other. As expected, T1 values were shortened significantly on both a global and a spatial basis in the presence of gadolinium (Gd); similar results were found for T1ρ. T2 values were found to correlate mildly with postcontrast T1, T1(Gd) and with precontrast T1ρ values. In addition, contrast-enhanced CT values correlated with both precontrast T1ρ and T1(Gd) more strongly than with precontrast T2. Finally, T1(Gd) and precontrast T1ρ were found to be moderately correlated with CT data. However, T1(Gd) and precontrast T1ρ were found to be almost completely uncorrelated. Together, these results indicate that T1ρ, T2 and contrast-enhanced techniques may provide complementary information about the molecular environment in cartilage during the evolution of OA.     

Transition to 1D conduction with decreasing thickness of the crystals of TaS3 and NbSe3 quasi-1D conductors

It is found that, with decreasing thickness of the crystals of TaS₃ and NbSe₃ quasi-1D conductors, the dependences of the conductivity of these crystals on temperature and electric field change from the form typical of bulk samples to a nearly power law behavior typical of 1D electron systems.     

Quantitative MRI of the liver: Evaluation of extracellular volume fraction and other quantitative parameters in comparison to MR elastography for the assessment of hepatopathy

BackgroundChronic liver diseases pose a major health problem worldwide, while common tests for diagnosis and monitoring of diffuse hepatopathy have considerable limitations. Preliminary data on the quantification of hepatic extracellular volume fraction (ECV) with magnetic resonance imaging (MRI) for non-invasive assessment of liver fibrosis are encouraging, with ECV having the potential to overcome several of these constraints.PurposeTo clinically evaluate ECV provided by quantitative MRI for assessing the severity of liver disease.Materials and methodsIn this prospective study, multiparametric liver MRI, including T1 mapping and magnetic resonance elastography (MRE), was performed in subjects with and without hepatopathy between November 2018 and October 2019. T1, T2, T2*, proton density fat fraction and stiffness were extracted from parametric maps by regions of interest and ECV was calculated from T1 relaxometries. Serum markers of liver disease were obtained by clinical database research. For correlation analysis, Spearman rank correlation was used. ROC analysis of serum markers and quantitative MRI data for discrimination of liver cirrhosis was performed with MRE as reference standard.Results109 participants were enrolled (50.7 ± 16.1 years, 61 men). ECV, T1 and MRE correlated significantly with almost all serum markers of liver disease, with ECV showing the strongest associations (up to r = 0.67 with MELD, p < 0.01). ECV and T1 correlated with MRE (0.75 and 0.73, p < 0.01 each). ECV (AUC 0.89, cutoff 32.2%, sensitivity 85%, specificity 87%) and T1 mapping (AUC 0.85, cutoff 592.5 ms, sensitivity 83%, specificity 75%) featured good performances in detection of liver cirrhosis with only ECV performing significantly superior to model of end stage liver disease (MELD), AST/ALT ratio and international normalized ratio (p < 0.01, respectively).ConclusionQuantification of hepatic extracellular volume fraction with MRI is suitable for estimating the severity of liver disease when using MRE as the standard of reference. It represents a promising tool for non-invasive assessment of liver fibrosis and cirrhosis.     

Fluorescence ratio of the two D sodium lines in flames for D1 and D2 excitation

The ratio of the fluorescence intensities from the two lines of the yellow sodium doublet has been investigated in several hydrogen-based and acetylene-based flames and with various types of lasers as excitation sources. The systematic deviation from 2 for the ratio D₂/D₁ when either D₁ or D₂ are used as exciting transitions can be simply explained by the relative difference in the values of the collisional mixing constants between the excited P-levels and the collisional quenching constants transferring atoms from both P-levels to the ground S-level. Results obtained in flames characterized by different values of the quantum efficiency follow the behavior predicted by the fluorescence expressions obtained by a simple, steady-state theoretical approach.     

Electron-impact excitation of the 3 P o and 3 D o levels of a nickel atom

The excitation cross sections of transitions from the ³ P ^o and ³ D ^o levels of a nickel atom in electron-atom collisions at an exciting-electron energy of 50 eV have been analyzed by the method of extended crossing beams with detection of the optical signal from excited atoms. Eleven optical excitation functions have been recorded in the electron-energy range of 0?C200 eV. The direct excitation cross sections of the energy levels and the contribution of the cascade transitions were calculated using the data obtained.     

Clinical value of routine use of thin-section 3D MRI using 3D FSE sequences with a variable flip angle technique for internal derangements of the knee joint at 3T

PurposeTo determine the clinical value of routine use of thin-section 3D MRI using 3D FSE sequences with a variable flip angle technique for internal derangements of the knee joint at 3 T.Method and MaterialsThirty-four knees in 34 patients suspected of having internal derangements of the knee joint were included. Following standard 2D MRI protocol including sagittal PDWI, T1WI and T2*WI, coronal fat-suppressed PDWI, and axial fat-suppressed PDWI with 3-4 mm thicknesses, fat-suppressed and water-excitation PDWI using 3D FSE sequences with a variable flip angle technique with 0.6 mm thickness were obtained in coronal plane and the three major planes with 1 mm thickness (3D MRI) was reformatted. The standard 2D MRI protocol and reformatted 3D MRI protocol (three sagittal 2D sequence images plus 3D MRI) were independently analyzed by two radiologists concerning presence or absence of lesions in the menisci, cartilage, and ligament. Interobserver agreements in both the MRI protocols were assessed by weighted-kappa coefficients. Regarding diagnostic accuracy, areas under the receiver operating characteristic curves (Az values) of both the MRI protocols were compared.ResultsThirty-eight meniscal lesions, 39 cartilage lesions, and 20 ligamentous lesions were surgically detected. Excellent interobserver agreements (kappa = 0.91–0.98) were seen in both the MRI protocols, with a slightly better tendency in the reformatted 3D MRI protocol. Average Az values in detection of the meniscal, cartilage, and ligamentous lesions were significantly higher in the reformatted 3D MRI protocol than in the standard 2D MRI protocol (p < 0.01 or p < 0.001).ConclusionRoutine use of reformatted thin-section 3D MRI using 3D FSE sequences with a variable flip angle technique may improve diagnostic accuracy and confidence in detection of internal derangements of the knee joint.     

Implementation of a rapid inversion-prepared dual-contrast gradient echo sequence for quantitative dynamic contrast-enhanced magnetic resonance imaging of the human prostate

The first step in quantitative pharmacokinetic modeling is to determine the arterial input function (AIF) by deriving the contrast medium (CM) concentration from an appropriate imaging sequence by monitoring changes in either the amplitude or the phase signal of an accommodative artery. The bolus passage is best detected on T2- or T2*-weighted images, while extravasation is best assessed on T1-weighted images. Here, an imaging sequence is used that employs a parallel acquisition technique for the interleaved acquisition of an inversion-prepared T1-weighted image and a T1/T2*-mixed-weighted image for determination of the AIF.

The sequence was applied in six patients with prostate cancer. A method is presented for quantifying the AIF derived from the signal intensity-time courses of both the T1/T2*-mixed-weighted and the T1-weighted image. Furthermore, in some patients the signal intensity-time course of the T1-weighted image exhibits flow-induced signal modulations. To reduce the effect of this flow-related signal enhancement the corresponding phase information was used.

The sequence presented here has the potential to improve the quantification of the AIF at all time points and pharmacokinetic modeling of the CM dynamics of the prostate.     

Effect of hybrid of compressed sensing and parallel imaging on the quantitative values measured by 3D quantitative synthetic MRI: A phantom study

IntroductionRecently, three-dimensional (3D) quantitative synthetic magnetic resonance imaging (MRI), which quantifies tissue properties and creates multiple contrast-weighted images, has been enabled by 3D-quantification using an interleaved Look-Locker acquisition sequence with a T2 preparation pulse (3D-QALAS). However, the relatively long scan time has hindered its introduction into clinical practice. A hybrid of compressed sensing and parallel imaging (Compressed sensing-sensitivity encoding: CS-SENSE) can accelerate 3D-QALAS; however, whether CS-SENSE affects the quantitative values acquired by 3D-QALAS remains unexplored. Therefore, this study aimed to examine the effects of reduction factors of CS-SENSE (R_CSS) on the quantitative values derived from 3D-QALAS, by assessing the signal-to-noise ratio (SNR) of the quantitative maps, as well as accuracy (linearity and bias) and repeatability of measured quantitative values.MethodsIn this study, the ISMRM/NIST standardized phantom was scanned on a 1.5-T MRI scanner with 3D-QALAS using R_CSS in the range between 1 and 3, with intervals of 0.2, and between 3 and 10 with intervals of 0.5. The T1, T2, and proton density (PD) values were calculated from the imaging data. For each quantitative value, the SNR, the coefficient of determination (R²) of a linear regression model, the error rate, and the within-subject coefficient of variation (wCV) were calculated for each R_CSS and compared.ResultsWithin the clinically-relevant dynamic range of the brain of T1 and T2 (T1: 200–1400 ms; T2; 50–400 ms) and PD value of 15–100% calculated from 3D-QALAS, the effects of R_CSS on quantitative values was small between 1 and 2.8, with SNR ≧ 10, R² ≧ 0.9, error rate ≦ 10%, and wCV ≦ 10%, except for T2 values of 186.1 and 258.4 ms.ConclusionsCS-SENSE enabled the reduction of the scan time of 3D-QALAS by 63.5% (R_CSS = 2.8) while maintaining the SNR of quantitative maps and accuracy and repeatability of the quantitative values.     

Combined volumetric T1, T2 and secular-T2 quantitative MRI of the brain: age-related global changes (preliminary results)

The combined T1, T2 and secular-T2 pixel frequency distributions of 24 adult human brains were studied in vivo using a technique based on the mixed-TSE pulse sequence, dual-space clustering segmentation and histogram gaussian decomposition. Pixel frequency histograms of whole brains and the four principal brain compartments were studied comparatively and as function of age. For white matter, the position of the T1 peak correlates with age (R2 =.7868) when data are fitted to a quadratic polynomial. For gray matter, a weaker age correlation is found (R2 =.3687). T2 and secular-T2 results are indicative of a weaker correlation with age. The technique and preliminary results presented herein may be useful for characterizing normal as well as abnormal aging of the brain, and also for comparison with the results obtained with alternative quantitative MRI methodologies.     

Electron-impact excitation cross-sections for the3 S 0−,3 P 0−, and3 D 0-States of a ruthenium atom

The excitation cross-sections of transitions of a ruthenium atom from the³ S ⁰−,³ P ⁰−, and³ D ⁰-levels are measured by the method of extended crossed beams. Based on the results of measurements, the excitation cross-sections of the states are calculated (ignoring cascade contribution). Moscow Power Engineering Institute, 14, Krasnokazarmennaya St., Moscow, E-250, 111250, Russia. Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 65, No. 1, pp. 9–13, January–February, 1998.     

A comparison of three SPRITE techniques for the quantitative 3D imaging of the 23Na spin density on a 4T whole-body machine

Sodium density maps acquired with three SPRITE-based methods have been compared in terms of the resulting quantitative information as well as image quality and acquisition times. Consideration of factors relevant for the clinical implementation of SPRITE shows that the Conical-SPRITE variant is preferred because of a 20-fold reduction in acquisition time, slightly improved image quality, and no loss of quantitative information. The acquisition of a 3D data set (32x32x16; FOV=256x256x160 mm) for the quantitative determination of sodium density is demonstrated. In vivo Conical-SPRITE 23Na images of the brain of a healthy volunteer were acquired in 30 min with a resolution of 7.5x7.5x7.5 mm and a signal-to-noise ratio of 23 in cerebrospinal fluid and 17 in brain tissue.     

Optimization of an 8-Channel Loop-Array Coil for a 7 T MRI System with the Guidance of a Co-Simulation Approach

Minimizing coupling between coil elements is technically challenging in designing large-sized, volume-type phased-array coils for human head imaging at ultrahigh fields, e.g., 7 T. As a widely used decoupling method, the capacitive decoupling method has shown excellent performance for loop array. However, building a multi-channel loop array with capacitive decoupling method is laborious that tuning frequency and matching of one coil element will affect adjacent elements and even next adjacent elements. In this study, we made an 8-channel loop-array transmit/receive radio-frequency coil on a 7 T magnetic resonance imaging system with the guidance of frequency domain three-dimensional electromagnetic and radio-frequency circuit co-simulation. The position of decoupling capacitors was investigated and values of all capacitors were predicted from co-simulation. The co-simulation approach cost about 2 days and the error of the predicted and practical capacitance was <5 %. To demonstrate the accuracy of simulation, we evaluated the simulated and measured S-parameter matrixes and B ₁ ⁺ profiles in a birdcage-like excitation mode on a cylindrical water phantom. In addition, B ₁ ⁺ maps and images of human head were shown with the fabricated coil. To demonstrate the parallel imaging performance of this coil array, GRE images using GRAPPA acceleration with the reduction factor R of 1, 2, 3, and 4 were acquired.