MRI Radiofrequency Research Articles

Different Sodium Concentrations of Noncancerous and Cancerous Prostate Tissue Seen on MRI Using an External Coil

Abstract Background Sodium (23Na) MRI of prostate cancer (PCa) is a novel but under-documented technique conventionally acquired using an endorectal coil. These endorectal coils are associated with challenges (e.g., a non-uniform sensitivity profile, limited prostate coverage, patient discomfort) that could be mitigated with an external 23Na MRI coil. Purpose To quantify tissue sodium concentration (TSC) differences within the prostate of participants with PCa and healthy volunteers using an external 23Na MRI radiofrequency coil at 3 Tesla. Materials and Methods A prospective study was conducted from January 2022 to June 2024 in healthy volunteers and participants with biopsy-proven PCa. Prostate 23Na MRI was acquired on a 3 Tesla PET/MRI scanner using a custom-built two-loop (diameter, 18 cm) butterfly surface coil tuned for the 23Na frequency (32.6 MHz). The percent difference in TSC (ΔTSC) between prostate cancer lesions and surrounding noncancerous prostate tissue of the peripheral zone (PZ) and transition zone (TZ) was evaluated using a one-sample t-test. TSC was compared to apparent diffusion coefficient (ADC) measurements as a clinical reference. Results 6 healthy volunteers (mean age, 54.5 years ± 12.7) and 20 participants with PCa (mean age, 70.7 years ± 8.3) were evaluated. A total of 31 lesions were detected (21 PZ, 10 TZ) across PCa participants. Compared to noncancerous prostate tissue, prostate cancer lesions had significantly lower TSC (ΔTSC, -14.1% ± 18.2, p = 0.0002) and ADC (ΔADC, -26.6% ± 18.7, p < 0.0001). Conclusion We used an external 23Na MRI coil for whole-gland comparison of TSC in PCa and noncancerous prostate tissue at 3 Tesla. PCa lesions presented with lower TSC compared to surrounding noncancerous PZ and TZ tissue. These findings demonstrate the feasibility of an external 23Na MRI coil to quantify TSC in the prostate and offer a promising, non-invasive approach to prostate cancer diagnosis and management.

An in silico testbed for fast and accurate MR labeling of orthopedic implants

One limitation on the ability to monitor health in older adults using magnetic resonance (MR) imaging is the presence of implants, where the prevalence of implantable devices (orthopedic, cardiac, neuromodulation) increases in the population, as does the pervasiveness of conditions requiring MRI studies for diagnosis (musculoskeletal diseases, infections, or cancer). The present study describes a novel multiphysics implant modeling testbed using the following approaches with two examples: (1) an in silico human model based on the widely available Visible Human Project (VHP) cryo-section dataset; (2) a finite element method (FEM) modeling software workbench from Ansys (Electronics Desktop/Mechanical) to model MR radio frequency (RF) coils and the temperature rise modeling in heterogeneous media. The in silico VHP-Female model (250 parts with an additional 40 components specifically characterizing embedded implants and resultant surrounding tissues) corresponds to a 60-year-old female with a body mass index of 36. The testbed includes the FEM-compatible in silico human model, an implant embedding procedure, a generic parameterizable MRI RF birdcage two-port coil model, a workflow for computing heat sources on the implant surface and in adjacent tissues, and a thermal FEM solver directly linked to the MR coil simulator to determine implant heating based on an MR imaging study protocol. The primary target is MR labeling of large orthopedic implants. The testbed has very recently been approved by the US Food and Drug Administration (FDA) as a medical device development tool for 1.5 T orthopedic implant examinations.

Evaluation of Low-Loss Polymer Switches for Multinuclear MRI/S.

Implementation of multinuclear MRI/S as a diagnostic tool in clinical settings faces many challenges. One of those challenges is the development of highly sensitive multinuclear RF coils. Current multi-tuning techniques incorporate lossy components that impact the highest achievable SNR for at least one of the coil frequencies. As a result, optimization of multinuclear coil designs continues to be a priority for RF hardware engineers. To address this challenge, a new frequency switching technology that incorporates stimuli-responsive polymer materials was explored. Q measurements were used as a comparison metric between single-tuned, a standard switching network, and the proposed switching technology. The Q losses measured in the new switching method remained below 38% when compared to single-tuned coils. These results are consistent with low loss values reported using traditional switching networks. Furthermore, preliminary testing indicates that there is potential for improvement. These results establish the new technology as a promising alternative to traditional switching techniques.Clinical Relevance- A low loss multi-tuning technique for MRI radiofrequency coils has the potential of improving the study and diagnosis of disease.

Design and Development of an MRI Compatible Circuit Suitable for Pre-Clinical Physiological Monitoring in a 4.7T Magnetic Resonance Imaging

As monitoring preclinical MRI is essential, it is important that the equipment used must be MRI-compatible. This is because interaction with an MRI radiofrequency (RF) transmitting field can cause RF burns due to inappropriate equipment. And non-compatible MRI monitoring equipment can give inaccurate readings in the MRI system if the equipment has ferromagnetic components. Preclinical MRI is designed for animals, the model used to study diseases related to humans. Animal models are imaged using a smaller scanner with high magnetic fields like the 4.7T. The main parameters of interest are the respiratory rate, the temperature, the pressure transducer and the electrocardiogram (ECG), this is because the animal model is used as a laboratory experiment for human disease. An MRI-compatible lead Ⅱ ECG simulator, with approximately 0.5 mV output amplitude was attached to the input of the ECG preamplifier to represent a small animal’s signal. The transmitted signal is the ECG, the pressure sensor, the temperature, and the battery (voltage), these are all linked to different channels (outside, near and inside magnet) and were acquired with the pressure sensor pad taped on a male adult volunteer’s thumb to record the human pulse of 70 bpm, which is similar to the respiration rate of an anaesthetized mouse or rat. The use of a signal gating generator should be explored in future work, this could make the monitoring parameters to be independent of the RF pulse influence. Finally, fibre optics should be included in the design process.

Basic Principles of and Practical Guide to Clinical MRI Radiofrequency Coils.

Radiofrequency (RF) coils are an essential MRI component used for transmission of the RF field to excite nuclear spins and for reception of the MRI signal. They play an important role in image quality in terms of signal-to-noise ratio, signal uniformity, and image resolution. However, they are also associated with potential image artifacts and RF heating that may lead to patient burns. Knowledge of the basic principles of RF coils-including coil designs commonly used in clinical MRI and the anatomy of RF receive coils-facilitates understanding of the use and safety issues of RF coils. Selection of suitable RF coils for individual applications and proper use of RF coils in particular MRI techniques such as parallel imaging are needed to achieve optimal image quality, prevent image artifacts, and reduce the risk of RF burns. The ability to correctly identify RF coil problems and distinguish them from other problems with image artifacts resembling those of RF coil problems allows effective handling of the problems and efficient clinical MRI operation. Quality control of RF coils is required to ensure consistent image quality for clinical MRI and avoid coil problems that may affect image diagnostic evaluation or interrupt patient imaging. There are different phantom test methods for RF coil quality control; the appropriate one to use depends on the coil design and MRI system. An invited commentary by Ohliger is available online. Online supplemental material is available for this article. ©RSNA, 2022.

Invited Commentary: MRI Radiofrequency Coils-Current Uses and Future Innovation.

Invited Commentary: MRI Radiofrequency Coils-Current Uses and Future Innovation.

Metamaterial Liner for MRI Excitation—Part 2: Design and Performance at 4.7T

The theoretical foundations of a metamaterial (MM) liner for the MRI bore that facilitates the propagation of reduced-cutoff cylindrical waveguide modes are presented in the Part 1 companion paper to this work. Here, in Part 2, the practical design and modelling of the novel MM liner is applied to a body MRI radio-frequency (RF) transmitter for 4.7T. An equivalent network mesh model is developed to reduce the distributed structure of the MM to one that uses lumped discrete tuning elements. The close match between full-wave simulation and the network model is demonstrated by comparison of the longitudinal propagation dispersion curves and input impedance. The application of the methods developed for analysis of the MM liner behavior are demonstrated in the design of a practical MM liner as an effective MRI RF transmitter. The liner’s simulated performance was evaluated using three key metrics of MR radio-frequency (RF) coil performance: transmit efficiency, the variation of the transmit field (a measure of homogeneity), and the 10g averaged local specific absorption rate (SAR). These metrics are compared to those of the commonly used birdcage (BC) coil. The main advantage of the MM liner is the 29.6% reduction in maximum SAR normalized to transmit field relative to the BC coil, with comparable transmit efficiency and homogeneity. Thus, the liner can potentially replace the BC coil as an RF transmitter and provide an enhanced safety margin, or allow the use of faster, more SAR-aggressive imaging sequences.

Exposure Optimization Trial for Patients With Medical Implants During MRI Exposure: Balance Between the Completeness and Efficiency.

Elongated conductors, such as pacemaker leads, can couple to the MRI radio-frequency (RF) field during MRI scan and cause dangerous tissue heating. By selecting proper RF exposure conditions, the RF-induced power deposition can be suppressed. As the RF-induced power deposition is a complex function of multiple clinical factors, the problem remains how to perform the exposure selection in a comprehensive and efficient way. The purpose of this work is to demonstrate an exposure optimization trail that allows a comprehensive optimization in an efficient and traceable manner. The proposed workflow is demonstrated with a generic 40 cm long cardio pacemaker, major components of the clinical factors are decoupled from the redundant data set using principle component analysis, the optimized exposure condition can not only reduce the in vivo power deposition but also maintain good image quality.

Analytical Approach for MRI RF Array Coils Decoupling by Using Counter-Coupled Passive Resonators

We introduce an analytical approach to design decoupling filters for MRI radiofrequency array elements, adopting counter-coupled passive resonators as unit-cells. Specifically, our method is based on a magneto-static hypothesis, thus a deep comprehension of the physical interactions between all the elements in the system and design guidelines can be achieved. In particular, the couplings between adjacent and next-nearest neighbors coils pairs are both modeled, hence addressing the requirements for MRI arrays. The analytically-obtained filter solution is subsequently refined resorting to targeted full-wave simulations, reducing the computational effort. To prove the validity of the proposed approach, we conceived a test-case consisting of three planar RF coils, tuned at the 7T proton Larmor frequency. We demonstrated through full-wave simulations that the analytical design method is accurate and effective. Moreover, we fabricated a prototype and we performed benchtop measurements, both in unloaded conditions and in the presence of a biological phantom, resulting in excellent agreement with simulations. The developed analytical framework can be useful to model and control the mutual interactions between the various elements of an RF MRI system. In addition, the possibility to print the decoupling elements and the RF coils on the same dielectric substrate leads to a mechanically robust prototype.

Evaluation of the Utility of Self Produced MRI Radiofrequency Shielding Material

Evaluation of the Utility of Self Produced MRI Radiofrequency Shielding Material

The Relationship between MRI Radiofrequency Energy and Function of Nonconditional Implanted Cardiac Devices: A Prospective Evaluation.

Background The risks associated with MRI in individuals who have implanted cardiac devices are thought to arise from the interaction between the implanted device and static, gradient, and radiofrequency magnetic fields. Purpose To determine the relationship between the peak whole-body averaged specific absorption rate (SAR) and change in magnetic field per unit time (dB/dt), maximum specific energy dose, imaging region, and implanted cardiac device characteristics and their function in patients undergoing MRI. Materials and Methods This prospective observational cohort study was conducted from October 16, 2003, to January 22, 2015 (https://ClinicalTrials.gov, NCT01130896). Any individual with an implanted cardiac device who was referred for MRI was included. Clinical MRI protocols without SAR restriction were used. Exclusion criteria were newly implanted leads, abandoned or epicardial leads, and dependence on a pacemaker with an implantable cardioverter defibrillator without asynchronous pacing capability. For each MRI pulse sequence, the calculated whole-body values for SAR, dB/dt, and scan duration were collected. Atrial and ventricular sensing, lead impedance, and capture threshold were evaluated before and immediately after (within 10 minutes) completion of each MRI examination. Generalized estimating equations with Gaussian family, identity link, and an exchangeable working correlation matrix were used for statistical analysis. Results A total of 2028 MRI examinations were performed in 1464 study participants with 2755 device leads (mean age, 67 years ± 15 [standard deviation]; 930 men [64%]). There was no evidence of an association between radiofrequency energy deposition, dB/dt, or scan duration and changes in device parameters. Thoracic MRI was associated with decreased battery voltage immediately after MRI (β = -0.008 V, P < .001). Additionally, right ventricular (RV) lead length was associated with decreased RV sensing (β = -0.012 mV, P = .05) and reduced RV capture threshold (β = -0.002 V, P < .01) immediately after MRI. Conclusion There was no evidence of an association between MRI parameters that characterize patient exposure to radiofrequency energy and changes in device and lead parameters immediately after MRI. Nevertheless, device interrogation before and after MRI remains mandatory due to the potential for device reset and changes in lead or generator parameters. © RSNA, 2020 See also the editorial by Shellock in this issue.

Loss Path Influence on the MRI Radio Frequency Pulse Sequence: A Theoretical Evidence.

The RF pulse is initiated from either the loop or loopless MRI antenna. It has shown an increased advancement in recent times. Somehow, the concept has proven successful in the MR imaging procedure. Using the fundamental theories of the MRI concept, mathematical experimentation was carried out analytically to investigate the Loss Path Concept (LPC). The LPC was proposed to be one of the defects responsible for poor/blurred medical imaging of certain parts of the body. The LPC results obtain in this mathematical experimentation was found to be -56 dB and 6.7 dB. Theoretically, the LPC can be resolved mathematically by incorporating the molecular boundaries of the tissues. Practically, LPC can be resolved by introducing a detachable RF strip detector to synchronise-particles across different molecular boundaries and prevent patients from excess exposure to RF radiation.

Design, Implementation, and Evaluation of a Head and Neck MRI RF Array Integrated with a 511 keV Transmission Source for Attenuation Correction in PET/MR.

The goal of this work is to further improve positron emission tomography (PET) attenuation correction and magnetic resonance (MR) sensitivity for head and neck applications of PET/MR. A dedicated 24-channel receive-only array, fully-integrated with a hydraulic system to move a transmission source helically around the patient and radiofrequency (RF) coil array, is designed, implemented, and evaluated. The device enables the calculation of attenuation coefficients from PET measurements at 511 keV including the RF coil and the particular patient. The RF coil design is PET-optimized by minimizing photon attenuation from coil components and housing. The functionality of the presented device is successfully demonstrated by calculating the attenuation map of a water bottle based on PET transmission measurements; results are in excellent agreement with reference values. It is shown that the device itself has marginal influence on the static magnetic field B0 and the radiofrequency transmit field B1 of the 3T PET/MR system. Furthermore, the developed RF array is shown to outperform a standard commercial 16-channel head and neck coil in terms of signal-to-noise ratio (SNR) and parallel imaging performance. In conclusion, the presented hardware enables accurate calculation of attenuation maps for PET/MR systems while improving the SNR of corresponding MR images in a single device without degrading the B0 and B1 homogeneity of the scanner.

RF coils: A practical guide for nonphysicists.

Radiofrequency (RF) coils are an essential MRI hardware component. They directly impact the spatial and temporal resolution, sensitivity, and uniformity in MRI. Advances in RF hardware have resulted in a variety of designs optimized for specific clinical applications. RF coils are the “antennas” of the MRI system and have two functions: first, to excite the magnetization by broadcasting the RF power (Tx‐Coil) and second to receive the signal from the excited spins (Rx‐Coil). Transmit RF Coils emit magnetic field pulses ( B1+) to rotate the net magnetization away from its alignment with the main magnetic field (B0), resulting in a transverse precessing magnetization. Due to the precession around the static main magnetic field, the magnetic flux in the receive RF Coil ( B1−) changes, which generates a current I. This signal is “picked‐up” by an antenna and preamplified, usually mixed down to a lower frequency, digitized, and processed by a computer to finally reconstruct an image or a spectrum. Transmit and receive functionality can be combined in one RF Coil (Tx/Rx Coils). This review looks at the fundamental principles of an MRI RF coil from the perspective of clinicians and MR technicians and summarizes the current advances and developments in technology. Level of Evidence: 1 Technical Efficacy: Stage 6 J. Magn. Reson. Imaging 2018;48:590–604.

PET attenuation correction for flexible MRI surface coils in hybrid PET/MRI using a 3D depth camera

PET attenuation correction for flexible MRI radio frequency surface coils in hybrid PET/MRI is still a challenging task, as position and shape of these coils conform to large inter-patient variabilities. The purpose of this feasibility study is to develop a novel method for the incorporation of attenuation information about flexible surface coils in PET reconstruction using the Microsoft Kinect V2 depth camera. The depth information is used to determine a dense point cloud of the coil’s surface representing the shape of the coil. From a CT template—acquired once in advance—surface information of the coil is extracted likewise and converted into a point cloud. The two point clouds are then registered using a combination of an iterative-closest-point (ICP) method and a partially rigid registration step. Using the transformation derived through the point clouds, the CT template is warped and thereby adapted to the PET/MRI scan setup. The transformed CT template is converted into an attenuation map from Hounsfield units into linear attenuation coefficients. The resulting fitted attenuation map is then integrated into the MRI-based patient-specific DIXON-based attenuation map of the actual PET/MRI scan. A reconstruction of phantom PET data acquired with the coil present in the field-of-view (FoV), but without the corresponding coil attenuation map, shows large artifacts in regions close to the coil. The overall count loss is determined to be around 13% compared to a PET scan without the coil present in the FoV. A reconstruction using the new μ-map resulted in strongly reduced artifacts as well as increased overall PET intensities with a remaining relative difference of about 1% to a PET scan without the coil in the FoV.

Optimal control design of preparation pulses for contrast optimization in MRI

This work investigates the use of MRI radio-frequency (RF) pulses designed within the framework of optimal control theory for image contrast optimization. The magnetization evolution is modeled with Bloch equations, which defines a dynamic system that can be controlled via the application of the Pontryagin Maximum Principle (PMP). This framework allows the computation of optimal RF pulses that bring the magnetization to a given state to obtain the desired contrast after acquisition. Creating contrast through the optimal manipulation of Bloch equations is a new way of handling contrast in MRI, which can explore the theoretical limits of the system.Simulation experiments carried out on-resonance quantify the contrast improvement when compared to standard T1 or T2 weighting strategies. The use of optimal pulses is also validated for the first time in both in vitro and in vivo experiments on a small-animal 4.7T MR system. Results demonstrate their robustness to static field inhomogeneities as well as the fact that they can be embedded in standard imaging sequences without affecting standard parameters such as slice selection or echo type. In vivo results on rat and mouse brains illustrate the ability of optimal contrast pulses to create non-trivial contrasts on well-studied structures (white matter versus gray matter).

44. Medical device lead coupling and heating induced by MRI radiofrequency

Introduction MRI is nowadays an indispensable technique with its excellent resolution achieved without ionizing radiations. Active implantable medical devices exist (such as pacemakers) which are MRI-compatible and do not show any hazardous local heating when interacting with the MRI machine radiofrequency, but multiple devices, as when two leads are side by side, were studied [1] , [2] but are not know enough to allow the implanted patients in these cases to undergo an MRI scan. The work presented here focuses on the coupling effects and the resulting heating between two leads, by simulation and experiments. Material and methods An ASTM [3] phantom inside a radiofrequency “birdcage” antenna was modeled with the CST Microwave Studio software (CST GmbH, Darmstadt, Germany). Two simples metallic cables, isolated and 5 mm bare at one tip were inserted in the phantom, spaced with 2.5 mm. One of them was also isolated at the upper extremity (referred to as “capped”) while the other one had its two tips bare (referred to as “uncapped”). An electromagnetic simulation was made and its results were used to launch a thermal simulation. With the latter, the temperature variations versus time were collected at both cables’ tips. The same process was repeated for each cable alone. Corresponding experiment were made with a Signa HDx 1.5 T MRI machine (GE Healthcare Technologies, Milwaukee, WI), an ASTM phantom filled with gel and cables made to match the simulations. An FSE sequence was used, generating high radiofrequency heating measured by optical temperature probes. Results Simulations and experiments show a coupling between the two cables: it is indeed observed that the “capped” cable heating is halved in presence of the “uncapped” cable. It is also observed that alone, the “capped” cable heats more that the “uncapped” one, but once together the higher heating is at the “uncapped” cable’s tip (see the appendix for the curves). Conclusion This study highlights the existence of lead coupling, such as it could occur in a patient’s body. This coupling could be explained by a theoretical model of two antennas subjected to radiofrequency, including mutual inductance and capacitance between the cables. Moreover, experiments on implant leads such as pacemaker leads could allow concluding on the occurrence of the phenomenon in more concrete cases. In the end, understanding this kind of interactions will allow a evolution in the design of medical devices, for each patient to be allowed to undergo an MRI scan safely, whatever his or her implants. Acknowledgement The authors thank the Region Lorraine and FEDER for financial support. Download : Download high-res image (188KB) Download : Download full-size image

Local SAR near deep brain stimulation (DBS) electrodes at 64 and 127 MHz: A simulation study of the effect of extracranial loops.

MRI may cause brain tissue around deep brain stimulation (DBS) electrodes to become excessively hot, causing lesions. The presence of extracranial loops in the DBS lead trajectory has been shown to affect the specific absorption rate (SAR) of the radiofrequency energy at the electrode tip, but experimental studies have reported controversial results. The goal of this study was to perform a systematic numerical study to provide a better understanding of the effects of extracranial loops in DBS leads on the local SAR during MRI at 64 and 127 MHz. A total of 160 numerical simulations were performed on patient-derived data, in which relevant factors including lead length and trajectory, loop location and topology, and frequency of MRI radiofrequency (RF) transmitter were assessed. Overall, the presence of extracranial loops reduced the local SAR in the tissue around the DBS tip compared with straight trajectories with the same length. SAR reduction was significantly larger at 127 MHz compared with 64 MHz. SAR reduction was significantly more sensitive to variable loop parameters (eg, topology and location) at 127 MHz compared with 64 MHz. Lead management strategies could exist that significantly reduce the risks of 3 Tesla (T) MRI for DBS patients. Magn Reson Med 78:1558-1565, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

Sodium MRI radiofrequency coils for body imaging.

The proliferation of high-field whole-body systems, advances in gradient performance and refinement of signal-to-noise ratio (SNR)-efficient short-TE sequences suitable for sodium imaging have led to a resurgence of interest in sodium imaging for body applications. With this renewed interest has come increased demand for SNR-efficient sodium coils. Efficient coils can significantly increase SNR in sodium imaging, allowing higher resolutions and/or shorter scan times. In this work, we focus on body imaging applications of sodium MRI, and review developments in MRI radiofrequency (RF) coil topologies for sodium imaging. We first provide a brief discussion of RF coil design considerations in sodium imaging. This is followed by an overview of common coil topologies, their advantages and disadvantages, and examples of each.

Assessment of computational tools for MRI RF dosimetry by comparison with measurements on a laboratory phantom

This paper presents an extended comparison between numerical simulations using the different computational tools employed nowadays in electromagnetic dosimetry and measurements of radiofrequency (RF) electromagnetic field distributions in phantoms with tissue-simulating liquids at 64 MHz, 128 MHz and 300 MHz, adopting a customized experimental setup. The aim is to quantify the overall reliability and accuracy of RF dosimetry approaches at frequencies in use in magnetic resonance imaging transmit coils.Measurements are compared against four common techniques used for electromagnetic simulations, i.e. the finite difference time domain (FDTD), the finite integration technique (FIT), the boundary element method (BEM) and the hybrid finite element method–boundary element method (FEM–BEM) approaches. It is shown that FDTD and FIT produce similar results, which generally are also in good agreement with those of FEM-BEM. On the contrary, BEM seems to perform less well than the other methods and shows numerical convergence problems in presence of metallic objects.Maximum uncertainties of about 30% (coverage factor k = 2) can be attributed to measurements regarding electric and magnetic field amplitudes. Discrepancies between simulations and experiments are found to be in the range from 10% to 30%. These values confirm other previously published results of experimental validations performed on a limited set of data and define the accuracy of our measurement setup.