Inverse field-based approach for simultaneous B1 mapping at high fields – A phantom based study

Jin, Jin; F, Liu; Zuo, Zhentao; Xue, Rong; Li, Mingyan; Liu, Yu; Weber, Ewald; Crozier, Stuart

doi:10.1016/j.jmr.2012.02.004

Cited by 9 publications

(9 citation statements)

References 40 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…An EM source (EMS) model was inversely reconstructed to fit the phantom‐measured data. The EMS model was previously used in the study of gradient field‐related safety and radiofrequency field maps . In this work, the EMS model was expressed with the current density distribution; that is, the current flowing on the surface of a sphere outside the DSV region was assumed to generate magnetic gradient fields in accordance with the phantom‐measured fields.…”

Section: Methodsmentioning

confidence: 99%

“…The EMS model was previously used in the study of gradient fieldrelated safety 23 and radiofrequency field maps. 24,25 In this work, the EMS model was expressed with the current density distribution; that is, the current flowing on the surface of a sphere outside the DSV region was assumed to generate magnetic gradient fields in accordance with the phantom-measured fields. A magnetic energy minimization strategy was applied to build the EMS model, which has been shown to be effective in gradient coil design.…”

Section: B Reconstitution Of the Current Density Distribution Withmentioning

confidence: 99%

“…In this study, we combined the phantom measurement and a designated inverse electromagnetic (EM) method to characterize the gradient field both inside and outside of the DSV for the Australian MRI‐Linac system. A grid phantom was first designed to measure a small number of gradient field samples by using the three‐dimensional (3D) Prewitt operator .…”

Section: Introductionmentioning

confidence: 99%

“…19,20 In the Australian MRI-Linac system, as the scanner has a smaller DSV (30-cm DSV, AE4.05 ppm) than commercial whole-body MRI scanners 21 (50-cm DSV, 1 ppm) because of the split structure, the GNL-induced distortions for the target anatomy at the periphery or outside the DSV need to be resolved. 22 In this study, we combined the phantom measurement 17 and a designated inverse electromagnetic (EM) method [23][24][25] to characterize the gradient field both inside and outside of the DSV for the Australian MRI-Linac system. A grid phantom was first designed to measure a small number of gradient field samples by using the three-dimensional (3D) Prewitt operator.…”

Section: Introductionmentioning

confidence: 99%

See 3 more Smart Citations

Geometric distortion characterization and correction for the 1.0 T Australian MRI‐linac system using an inverse electromagnetic method

et al. 2020

Self Cite

View full text Add to dashboard Cite

Purpose The magnetic resonance imaging (MRI)‐Linac system combines a MRI scanner and a linear accelerator (Linac) to realize real‐time localization and adaptive radiotherapy for tumors. Given that the Australian MRI‐Linac system has a 30‐cm diameter of spherical volume (DSV) with a shimmed homogeneity of ±4.05 parts per million (ppm), a gradient nonlinearity (GNL) of <5% can only be assured within 15 cm from the system's isocenter. GNL increases from the isocenter and escalates close to and outside of the edge of the DSV. Gradient nonlinearity can cause large geometric distortions, which may provide inaccurate tumor localization and potentially degrade the radiotherapy treatment. In this study, we aimed to characterize and correct the geometric distortions both inside and outside of the DSV. Methods On the basis of phantom measurements, an inverse electromagnetic (EM) method was developed to reconstitute the virtual current density distribution that could generate gradient fields. The obtained virtual EM source was capable of characterizing the GNL field both inside and outside of the DSV. With the use of this GNL field information, our recently developed “GNL‐encoding” reconstruction method was applied to correct the distortions implemented in the k‐space domain. Results Both phantom and in vivo human images were used to validate the proposed method. The results showed that the maximal displacements within an imaging volume of 30 cm × 30 cm × 30 cm after using the fifth‐order spherical harmonic (SH) method and the proposed method were 6.1 ± 0.6 mm and 1.8 ± 0.6 mm, respectively. Compared with the fifth‐order SH‐based method, the new solution decreased the percentage of markers (within an imaging volume of 30 cm × 30 cm × 30 cm) with ≥1.5‐mm distortions from 6.3% to 1.3%, indicating substantially improved geometric accuracy. Conclusions The experimental results indicated that the proposed method could provide substantially improved geometric accuracy for the region outside of the DSV, when comparing with the fifth‐order SH‐based method.

show abstract

Section: Methodsmentioning

confidence: 99%

Section: B Reconstitution Of the Current Density Distribution Withmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Geometric distortion characterization and correction for the 1.0 T Australian MRI‐linac system using an inverse electromagnetic method

et al. 2020

Self Cite

View full text Add to dashboard Cite

show abstract

“…The experimentally acquired FLASH image in CP mode is shown in Figure 4-4(a). After applying scaling factor W 0 and U V  obtained as [136], the simulated image SI calc in Figure 4…”

Section: Si Mapping and Comparisonmentioning

confidence: 99%

The development of rotating radiofrequency techniques for ultra-high field magnetic resonance imaging

Li¹

Self Cite

View full text Add to dashboard Cite

Ultra-high field (≥7 T) magnetic resonance imaging (MRI) offers improved image quality to facilitate clinical diagnosis. However, the wavelength of radiofrequency (RF) electromagnetic (EM) waves becomes a fraction of the size of the human body at ultra-high fields; therefore, constructive and destructive interference of RF magnetic field (B 1 field) forms in the human body, causing bright and dark regions in the output images. These artefacts seriously affect the accuracy and diagnostic value of the images. The RF power deposited in the human body is dramatically increased at ultrahigh fields and the resultant overheating could potentially damage tissues. The long scan duration is also an issue which is reported as the most unpleasant factor during an MRI scan at 7 T. It can cause patient discomfort, both psychologically and physically; consequently, inevitable motion of the patient could induce motion artefacts in the images that seriously degrade image quality for clinical use. In terms of hardware, phased array coils are considered to be the best choice to implement parallel transmission and parallel imaging for solving ultra-high field issues. Parallel transmission and parallel imaging use multiple coils to transmit and receive signals. Theoretically, increasing the number of coils in an array can improve the performance of parallel transmission and parallel imaging. However, this raises RF coil related issues, such as reduced B 1 penetration and increased mutual coupling, which undermine the efficiency of parallel transmission/imaging. Alternatively, a physical rotating RF coil is capable of emulating a large number of coils without these RF coil related issues, so it could be employed as a solution. However, with only one coil element, the rotating RF coil (RRFC) is limited in its ability to reduce the scan duration. The specific aim of this project is to develop RF techniques that strategically combine the array structure with the rotating concept, to provide better solutions for ultra-high field issues. By introducing the rotation, the coil has the capability of acquiring a large number of transmit/receive sensitivity profiles, and thus fewer coils are needed for parallel transmission and parallel imaging. This offers the opportunity of constructing a coil array with lower coupling and bigger coils that have higher efficiency and B 1 penetration. Compared to a stationary coil array, the rotating array will also introduce an extra degree of freedom in the spatial domain to facilitate B 1 field homogenising (B 1 shimming) and Specific Absorption Rate (SAR) control. iii The geometry of a 4-element rotating RF coil array (RRFCA) was optimised to achieve natural decoupling and reasonable B 1 penetration, and then the RRFCA prototype was built for imaging a head-size homogenous phantom at 7 T. In order to achieve an image reconstruction without significant 1 B inhomogeneity, a time interleaved, spatial varying B 1 inhomogeneity mitigation strategy for the RRFAC was developed. A dedicated rotating-SENSE algorith...

show abstract

Highly accelerated acquisition and homogeneous image reconstruction with rotating RF coil array at 7T—A phantom based study

Zuo

Jin

et al. 2014

Journal of Magnetic Resonance

View full text Add to dashboard Cite

Inverse field-based approach for simultaneous B1 mapping at high fields – A phantom based study

Cited by 9 publications

References 40 publications

Geometric distortion characterization and correction for the 1.0 T Australian MRI‐linac system using an inverse electromagnetic method

Geometric distortion characterization and correction for the 1.0 T Australian MRI‐linac system using an inverse electromagnetic method

The development of rotating radiofrequency techniques for ultra-high field magnetic resonance imaging

Highly accelerated acquisition and homogeneous image reconstruction with rotating RF coil array at 7T—A phantom based study

Contact Info

Product

Resources

About