A mixed-integer optimization approach for homogeneous magnet design

The magnet system is an important part in magnetic resonance imaging (MRI) equipment, the performance of which directly affects the quality of the imaging. Improving the homogeneity of the magnetic field is an effective means to increase the signal-to-noise ratio (SNR). A high SNR ensures a good imaging. This paper mainly investigates the optimal design and homogenizing field analysis of main magnet for ferrite yoke flat-board type in superconducting MRI using the finite element method (FEM). In order to get a high homogeneous magnetic field, the pole piece is added on the magnetic pole. We use electric pure iron as the material of the pole piece which has low coercive force, high permeability, and high saturation flux density. In order to increase the calculation speed, three-dimensional analysis is replaced by two-dimensional analysis while keeping the precision adequate. Through the optimizing algorithm, we can obtain an ideal pole piece dimension of the magnet. The distribution of the magnetic field was simulated and tested experimentally. By comparing the numerical calculation results with the experimental results, we find that a superconducting magnet MRI having homogeneity in volume of interest less than 600 ppm can be designed by optimizing the dimension of the pole piece using the proposed method without shimming.

Large intracranial vessel occlusive vasculopathy after radiation therapy in children: Clinical features and usefulness of magnetic resonance imaging

To prospectively differentiate, at magnetic resonance (MR) imaging, patients with locally advanced nonmucinous rectal cancer who will respond to long-course chemotherapy and radiation therapy (CRT) from those who will not respond, with histopathologic results as the reference standard. Institutional review board approval for this study was obtained, and all patients provided written informed consent. High-spatial-resolution T2-weighted MR images were acquired before and 6-8 weeks after CRT in 53 patients (33 men, 20 women; mean age, 63 years; age range, 42-79 years). Patients were categorized as responders to CRT (patients with T3 cancer that converted to T2 or a lower stage, patients with T4 cancer that converted to T3 or a lower stage) or as nonresponders (patients with stable or progressive disease). At the posttreatment MR imaging examination, a decrease in signal intensity was considered to represent a morphologic response with fibrosis. Before CRT and surgery, tumor volume was calculated at MR imaging by multiplying cross-sectional area by section thickness. Tumor length was measured at MR imaging and in the histopathologic specimen. Nodal downstaging was evaluated. The relationship between pathologic response, morphologic MR imaging response, and percentage volume reduction was evaluated with the Mann-Whitney-Wilcoxon two-sample test. Morphologic response assessment with MR imaging achieved a positive predictive value (PPV) of 84.2% (32 of 38) and a negative predictive value (NPV) of 66.7% (10 of 15). Volume reduction extent (> or = 70%) was significantly different between patients in whom disease was downstaged and those in whom it was not downstaged (P = .000005) and showed additional diagnostic value, with an overall accuracy of 86.8% (46 of 53). Presurgical MR imaging and histopathologic tumor length did not show a significant difference. MR imaging accuracy for lymph node (N) stage was 86.8% (46 of 53) on the basis of morphologic criteria. After CRT, morphologic and volumetric evaluation at MR imaging had a high PPV and a low NPV for response assessment. The detection of small clusters of residual tumor cells within fibrosis remains a problem. http://radiology.rsnajnls.org/cgi/content/full/250/3/730/DC1.

Background: Magnetic resonance (MR) image guided radiation therapy system, enables real time MR guided radiotherapy (RT) without additional radiation exposure to patients during treatment. However, MR image lacks electron density information required for dose calculation. Image fusion algorithm with deformable registration between MR and computed tomography (CT) was developed to solve this issue. However, delivered dose may be different due to volumetric changes during image registration process. In this respect, synthetic CT generated from the MR image would provide more accurate information required for the real time RT. Materials and Methods: We analyzed 1,209 MR images from 16 patients who underwent MR guided RT. Structures were divided into five tissue types, air, lung, fat, soft tissue and bone, according to the Hounsfield unit of deformed CT. Using the deep learning model (U-NET model), synthetic CT images were generated from the MR images acquired during RT. This synthetic CT images were compared to deformed CT generated using the deformable registration. Pixelto- pixel match was conducted to compare the synthetic and deformed CT images. Results and Discussion: In two test image sets, average pixel match rate per section was more than 70% (67.9 to 80.3% and 60.1 to 79%; synthetic CT pixel/deformed planning CT pixel) and the average pixel match rate in the entire patient image set was 69.8%. Conclusion: The synthetic CT generated from the MR images were comparable to deformed CT, suggesting possible use for real time RT. Deep learning model may further improve match rate of synthetic CT with larger MR imaging data.

Magnets are a keystone of the ESRF upgrade programme. The specifications of the magnets of the ESRF II lattice are stringent: high gradients, extended Good Field Region (GFR) and vertical gaps large enough for the X-ray beam ports. The magnet design approach is presented here. Shape optimization of the magnet poles is systematically used. The magnet design is treated as an ill-posed, non linear, constrained problem. Iterative algorithms have been developed; the algorithms converge in less than 10 iterations, leading to very short computation time. This design method has been applied to high gradient quadrupole magnets. The shape optimization leads to original pole profiles.

Fabrication of precision meso-scale diameter ZrO2 ceramic bars using new magnetic pole designs in ultra-precision magnetic abrasive finishing

To assess the feasibility of magnetic resonance (MR) imaging-guided focal cryoablation in patients with locally recurrent prostate cancer after radiation therapy. This was a prospective study, and informed consent was obtained from all patients. Ten consecutive patients with histopathologically proved recurrent prostate cancer after radiation therapy, without evidence of distant metastases, were treated while under general anesthesia in a 1.5-T MR unit. A urethral warmer was inserted. Cryoneedles were transperineally inserted under real-time MR imaging. Then, a rectal warmer was inserted. Ice ball growth was continuously monitored under MR imaging guidance. Two freeze-thaw cycles were performed. Follow-up consisted of a visit to the urologist, measurement of prostate-specific antigen level, and multiparametric MR imaging at 3, 6, and 12 months. Potential complications were recorded. All patients were successfully treated. In one patient, the urethral warmer could not be inserted and the procedure was cancelled. Two months later, the procedure was successfully repeated. Another patient had urinary retention. Follow-up data were available for all patients. A local recurrence or remnant tumor was found in two patients after 6 months and in another patient after 12 months. These three patients underwent successful retreatment with MR imaging-guided focal cryoablation. MR imaging-guided focal cryoablation of recurrent prostate cancer after radiation therapy is feasible and safe. Initial results are promising; however, longer follow-up is needed and more patients must be studied.

To compare diagnostic accuracy of T2-weighted magnetic resonance (MR) imaging with that of multiparametric (MP) MR imaging combining T2-weighted imaging with diffusion-weighted (DW) MR imaging, dynamic contrast material-enhanced (DCE) MR imaging, or both in the detection of locally recurrent prostate cancer (PCa) after radiation therapy (RT). This retrospective HIPAA-compliant study was approved by the institutional review board; informed consent was waived. Fifty-three men (median age, 70 years) suspected of having post-RT recurrence of PCa underwent MP MR imaging, including DW and DCE sequences, within 6 months after biopsy. Two readers independently evaluated the likelihood of PCa with a five-point scale for T2-weighted imaging alone, T2-weighted imaging with DW imaging, T2-weighted imaging with DCE imaging, and T2-weighted imaging with DW and DCE imaging, with at least a 4-week interval between evaluations. Areas under the receiver operating characteristic curve (AUC) were calculated. Interreader agreement was assessed, and quantitative parameters (apparent diffusion coefficient [ADC], volume transfer constant [K(trans)], and rate constant [k(ep)]) were assessed at sextant- and patient-based levels with generalized estimating equations and the Wilcoxon rank sum test, respectively. At biopsy, recurrence was present in 35 (66%) of 53 patients. In detection of recurrent PCa, T2-weighted imaging with DW imaging yielded higher AUCs (reader 1, 0.79-0.86; reader 2, 0.75-0.81) than T2-weighted imaging alone (reader 1, 0.63-0.67; reader 2, 0.46-0.49 [P ≤ .014 for all]). DCE sequences did not contribute significant incremental value to T2-weighted imaging with DW imaging (reader 1, P > .99; reader 2, P = .35). Interreader agreement was higher for combinations of MP MR imaging than for T2-weighted imaging alone (κ = 0.34-0.63 vs κ = 0.17-0.20). Medians of quantitative parameters differed significantly (P < .0001 to P = .0233) between benign tissue and PCa (ADC, 1.64 × 10(-3) mm(2)/sec vs 1.13 × 10(-3) mm(2)/sec; K(trans), 0.16 min(-1) vs 0.33 min(-1); k(ep), 0.36 min(-1) vs 0.62 min(-1)). MP MR imaging has greater accuracy in the detection of recurrent PCa after RT than T2-weighted imaging alone, with no additional benefit if DCE is added to T2-weighted imaging and DW imaging.

Timing of Planning Magnetic Resonance Imaging and Patient Selection for Adaptive Radiation Therapy in Newly Diagnosed High-Grade Glioma.

To evaluate magnetic resonance (MR) imaging in the planning of radiation therapy for patients with cervical cancer. MR imaging was performed in 15 patients with predominantly advanced cervical carcinomas. Skin markings of the radiation ports were labeled with MR-detectable tubing and beads to assess the adequacy of port placement. Volume and extent of tumor at MR imaging were compared with those at physical and computed tomographic (CT) examinations. Tumor involvement of normal structures (parametrium, vagina, endometrium) was more extensive at MR imaging than at clinical examination in six patients (40%) and at CT in five patients (33%), resulting in modification of the radiation fields in eight patients (53%). Tumor size was larger on MR images than at clinical examination in 11 patients (73%). MR imaging can depict tumor volume and better delineate the extent of involvement than physical and CT examination. This can be useful for placement of radiation ports, especially when small boost fields are used.

This paper presents an innovative design for magnetic poles used in active magnetic bearings (AMBs). The proposed hybrid sandwiched magnetic pole design not only practically reduces the bearing size, but also dramatically diminishes its cost. The magnetic force to counterbalance the gravity of the rotor is provided by the embedded permanent magnets (PMs), while the electromagnetic bearing is merely used to regulate the position deviation of the rotor. Therefore, the required electricity consumption to activate the coil wound on the poles is fairly limited. This implies that the required ampere-turns and size of the AMB can both be reduced efficiently. The numbers of associated amplifiers, sensing circuits, and magnetic poles are thus reduced as well. The coupling effect of magnetic forces is taken into account such that either poles for electromagnets (EMs) or the embedded PMs are located 90° apart. The merits of this type of design by orthogonal pole allocation include: simpler control synthesis and decoupled sensing signals for rotor position deviation. Since the variation of coil current is a function of flux density change between PMs and the silicon steel strips attached on the rotor, the PMs can also be used to provide necessary magnetic flux source for sensing rotor position deviation. More importantly, two perpendicular PMs can prevent the rotor from getting extremely close to either PM, such that the magnetic attraction force becomes too strong to be counterbalanced. On the other hand, the concave-like pole profile design enhances the average flux density by 11% and the required ampere-turns are further reduced by 18%, compared with the traditional rectangular pole design. One of the induced benefits of reduced size for AMB is to diversify applications where the operating space is limited. The proposed design has been assessed using intensive computer simulations and experiments.

BackgroundRadiation-induced parotid damage is one of the most common complications in patients with nasopharyngeal carcinoma (NPC) undergoing radiotherapy (RT). Intravoxel incoherent motion (IVIM) magnetic resonance (MR) imaging has been reported for evaluating irradiated parotid damage. However, the changes of IVIM perfusion-related parameters in irradiated parotid glands have not been confirmed by conventional perfusion measurements obtained from dynamic contrast-enhanced (DCE) MR imaging. The purposes of this study were to monitor radiation-induced parotid damage using IVIM and DCE MR imaging and to investigate the correlations between changes of these MR parameters.MethodsEighteen NPC patients underwent bilateral parotid T1-weighted, IVIM and DCE MR imaging pre-RT (2 weeks before RT) and post-RT (4 weeks after RT). Parotid volume; IVIM MR parameters, including apparent diffusion coefficient (ADC), pure diffusion coefficient (D), pseudo-diffusion coefficient (D*), and perfusion fraction (f); and DCE MR parameters, including maximum relative enhancement (MRE), time to peak (TTP), Wash in Rate, and the degree of xerostomia were recorded. Correlations of parotid MR parameters with mean radiation dose, atrophy rate and xerostomia degree, as well as the relationships between IVIM and DCE MR parameters, were investigated.ResultsFrom pre-RT to post-RT, all of the IVIM and DCE MR parameters increased significantly (p < 0.001 for ADC, D, f, MRE, Wash in Rate; p = 0.024 for D*; p = 0.037 for TTP). Change rates of ADC, f and MRE were negatively correlated with atrophy rate significantly (all p < 0.05). Significant correlations were observed between the change rates of D* and MRE (r = 0.371, p = 0.026) and between the change rates of D* and TTP (r = 0.396, p = 0.017). The intra- and interobserver reproducibility of IVIM and DCE MR parameters was good to excellent (intraclass correlation coefficient, 0.633–0.983).ConclusionsEarly radiation-induced changes of parotid glands could be evaluated by IVIM and DCE MR imaging. Certain IVIM and DCE MR parameters were correlated significantly.

In the past few years, radiotherapy (RT) has experienced a major technological innovation with the development of hybrid machines combining magnetic resonance (MR) imaging and linear accelerators. This new technology for MR-guided cancer treatment has the potential to revolutionize the field of adaptive RT due to the opportunity to provide high-resolution, real-time MR imaging before and during treatment application. However, from a technical point of view, several challenges remain which need to be tackled to ensure safe and robust real-time adaptive MR-guided RT delivery. In this manuscript, several technical challenges to MR-guided RT are discussed. Starting with magnetic field strength tradeoffs, the potential and limitations for purely MR-based RT workflows are discussed. Furthermore, the current status of real-time 3D MR imaging and its potential for real-time RT are summarized. Finally, the potential of quantitative MR imaging for future biological RT adaptation is highlighted.

To determine if magnetic resonance (MR) imaging findings of soft-tissue sarcoma involving bone or neurovascular structures allow prediction of local recurrence, distant metastasis, or disease-specific survival. Preoperative MR images of 46 patients with soft-tissue sarcoma were reviewed for tumor involving bone or major vessels or nerves. MR imaging findings were correlated with local recurrence, distant metastasis, and disease-specific survival after surgery and chemotherapy and/or radiation therapy. Primary tumors were predominantly in the lower extremity (n = 35 [76%]), deep (n = 44 [96%]), and high-grade (n = 35 [76%]). On MR images, bone invasion occurred in 12 patients (26%), major-vessel encasement in five patients (11%), and major-nerve encasement in seven patients (15%). In patients with (n = 12) and those without (n = 34) bone invasion, frequencies of disease-related death (in nine [75%] and 12 [35%] patients, respectively) were significantly different (P = .02); frequencies of local recurrence or distant metastasis were not significantly different. In patients with and those without major-vessel or major-nerve encasement, there were no significant differences between frequencies of local recurrence, distant metastasis, or disease-specific survival. In soft-tissue sarcoma, bone invasion on MR images was predictive of decreased disease-specific survival. MR imaging findings of bone or neurovascular involvement otherwise appear to be more important for tailoring surgery than for predicting local recurrence, distant metastasis, or survival.

This paper describes a hybrid numerical method with a combination of linear programming (LP) and nonlinear programming to design an asymmetric magnetic resonance imaging (MRI) system for head imaging. The method was successfully employed to a 0.7 T open-style MRI. Recently, we have updated the LP mathematical model in the hybrid optimization strategy by adding maximum axial and radial magnetic field strength limitation to make the magnet safe. The whole magnet consists of eight coaxial coils asymmetry about the z-axis. The six innermost coils are the main coils with positive current direction and the two outermost coils are shielding coils with negative current direction. All coils contribute their efforts to produce a central magnetic field strength of 3.0 T over a 20 cm diameter of spherical volume, and the peak to peak homogeneity is 12 ppm. The temperature bore for the magnet is formed with two different diameters cylindrical bores which makes head and shoulder access to the imaging volume easier. A 5 Gauss footprint area is restricted larger than an elliptical region with the sizes of 3.5 m in radial and 4.0 m in axial direction. The results show that the methodology is very flexible and efficient for asymmetric MRI magnet design.