Radiofrequency Pulses Research Articles

Better neural images by combining ultrahigh field strength MRI with innovative MRI sequences

Better MRI scanning technologies and protocols can provide insights into neurological disorders. In this review, we describe the basic concepts of MRI and, in the process, we convey to the reader the relevance of MRI as a high-resolution imaging modality of tissue structure and metabolism. We outline the main parameters for improving MRI resolution and sensitivity for the ultimate goal of optimizing the diagnosis of neurological diseases. A key to obtaining high-resolution images by MRI is the strength of the magnet’s external field strength (Bo). The higher the field strength, the better the signal-to-noise (SNR) of acquired signals. Hence, this results in improved sensitivity and resolution of the reconstructed images. This article recapitulates the advancement of MRI technology towards Ultra-High-Field Strength (UHF) apparatus and the consequent benefits in SNR. Other keys towards improving MRI images include deftly modifying the parameters of longitudinal magnetization relaxation time (T1), transverse magnetization relaxation time (T2), repetition times between radiofrequency (RF) pulses (TR), and the time of reading post-pulse (TE). Such parameters can be controlled through acquisition software associated with the MRI machines. The review profiles the cumulative efforts by researchers to complement UHF-MRI hardware with innovative RF pulse sequences protocols such as Diffusion Weighted Imaging (DWI), Pulse Gradient Spin Echo (PGSE), Oscillating Gradient Spin Echo (OGSE), Blood Oxygen Level Dependent (BOLD)-MRI and Arterial Spin Label (ASL)-MRI. Collectively, these advances in both MRI hardware and software have pushed the field to attain a mesoscopic level of resolution. Further enhancements in analyzing MRI images through Artificial Intelligence (AI) algorithms may advance resolutions beyond the mesoscopic stage and perhaps even toward the microscopic resolution of living tissue.

Outcomes of catheter ablation for typical atrioventricular nodal reentrant tachycardia with low power energy

Abstract Background Atrioventricular nodal reentrant tachycardia (AVNRT) is most common supraventricular tachycardia with acceptable outcome of catheter ablation. However, the serious complication of atrioventricular block (AVB) can occur and affects 0.2% to 2.3% of patients during or after procedures. Purpose We hypothesized that slow pathway ablation with 20 watt (W) energy would facilitate acceptable clinical outcomes and low procedure related AVB needed permanent pacemaker. Methods Prospective single center trial was performed. Slow pathway ablation was performed in 212 patients (mean age 56.5±18.4 years, 131 women) from January 2017 to May 2020. Twenty watt energy was delivered at the posterior region of the triangle of Koch until accelerated junctional rhythm was obtained. Results A total of 212 patients who underwent ablation for typical AVNRT were included. The acute success rate, the mean radiofrequency pulses applied per patient were 98.1% and 8.6±6.6, respectively. Transient AVB occurred in 5 patients (2.4%) and no permanent AVB requiring permanent pacemaker implantation occurred. Those who failed 20 W ablation were younger (success group age vs failed group age 58.2±17.4 vs 31.8±17.9 years, p=0.005) and had a faster heart rate just before slow pathway ablation (success group heart rate vs failed group heart rate 69.2±11.3 vs 76.8±5.6 bpm, p=0.59). The recurrence rate of 20 W ablation was 2.6% and the follow-up period was 18.3±4.1 months. Conclusion The low power approach for slow pathway ablation is highly effective and safe. If the heart rate just before ablation is lesser than 70 bpm, slow pathway ablation with 20 W energy could be considered. Further well-designed large-randomized studies are needed to define the role and effect of catheter ablation for typical AVNRT with low power energy. Funding Acknowledgement Type of funding sources: None.

Assessment of Cerebrovascular Reserve in the Setting of Moyamoya Disease Using Arterial Spin-Labeling and Acetazolamide Challenge

Arterial spin-labeling is a safe, noninvasive, contrast-free MR imaging technique that can be used to assess CBF. In this review, we will briefly discuss the advantages and disadvantages of arterial spin-labeling juxtaposed to previously established modalities of cerebral perfusion imaging, summarize the materials and methods required for arterial spin-labeling, and provide clinical case examples in which application of arterial spin-labeling combined with an acetazolamide challenge was used safely to improve patient outcomes in the setting of Moyamoya disease. Arterial spin-labeling takes advantage of the directionality of arterial blood flow by labeling the patient's own endogenous water within the neck via radiofrequency pulse and imaging of the downstream brain after an appropriate delay. This tool allows repeatable intra- and intersession estimation of CBF to better assess postinterventional outcomes in patients with cerebrovascular dysfunction. One specific example of the utility of arterial spin-labeling is its application in patients with Moyamoya disease by selection and stratification of those most likely to benefit from surgical intervention. The combination of arterial spin-labeling with acetazolamide challenge is an effective way to assess cerebrovascular reserve in Moyamoya disease and is easily adaptable on a clinical 3T MR imaging scanner using a commercially available arterial spin-labeling pulse sequence to generate augmentation and steal maps. The entire study can be completed in only 20 minutes in a routine clinical setting.Learning Objective: To describe the implementation and interpretation of arterial spin-labeling combined with an acetazolamide challenge

Information-Guided Dynamic Nuclear Polarization

A spin ensemble in thermal equilibrium continuously undergoes random fluctuations analogous to those observed in Brownian motion, a process known as spin noise. Here, we investigate the dynamics of a system comprising a spin-1 paramagnetic center and a hyperfine-coupled spin-1/2 nucleus in the vicinity of a level crossing. We theoretically show that nuclear spins polarize efficiently under the combined action of thermal fluctuations and a closed-loop feedback protocol. The latter articulates periodic observations of the electronic magnetization and radio-frequency pulses connecting hybrid states with opposite nuclear spin alignment. Since nuclear polarization emerges from electronic spin fluctuations, not spin order, this microwave-free technique generically benefits from warmer, not colder, operation temperatures. Furthermore, because the spin dynamics at play near a level crossing is rather insensitive to the absolute value of the magnetic field, our work promises opportunities for high-field dynamic nuclear polarization, which is difficult to attain through present methods.

A Three-Level RF-PWM Method Based on Phase-Shift Control and MPWM for ADTx

A radio frequency pulse width modulation (RF-PWM) scheme based on phase-shift control and mapping PWM (MPWM) is described. Pulse coding is the key to improving the coding efficiency, flexibility, and configurability of the all-digital transmitter (ADTx). To solve the problem that the real-time performance of the system is limited by the time resolution, the phase-shift control principle is adopted to constrain the output pulse state. It decomposes the original signal into two phase-modulated constant envelope signals, and directly converts the two-level pulse waveform by MPWM. Finally, the fast generation of the three-level digital RF modulated signal is completed by vector synthesis. In this way, the rear power amplifier can be directly driven, and the difficulty of physical implementation is greatly reduced. Different from the traditional mapping strategy of traversal search, the proposed scheme does not require complicated error calculation and comparison. Simulation and offline experiments show that the proposed scheme has better comprehensive performance than other mapping schemes. For 16QAM modulated signals at a 300 MHz carrier, the proposed scheme can achieve nearly 70% coding efficiency (CE), less than −50 dBc, and 1% adjacent channel power ratio (ACPR) and error vector magnitude (EVM).

Surface Accessibility of an Intrinsically Disordered Protein Probed by 2D Time-Resolved Laser-Assisted NMR Spectroscopy.

Probing the protein surface accessibility of different residues is a powerful way of characterizing the overall conformation of intrinsically disordered proteins (IDPs). We present a two-dimensional (2D) time-resolved photo-CIDNP (TR-CIDNP) experiment suitable for IDP analysis. Pulse stretching of high-power laser pulses, band-selective decoupling of 13Cα, and simultaneous application of radiofrequency and laser pulses were implemented to quantitatively analyze the IDP surface at ultrahigh resolution. Comparative analysis with other methods that measure protein surface accessibility validated the newly developed method and emphasized the importance of dye charge in photo-CIDNP. Using the neutral riboflavin dye, surface accessibilities were measured to be nearly identical for the four Tyr residues of α-synuclein (α-Syn), whose 1Hα-13Cα correlations were well-resolved in the 2D TR-CIDNP spectrum. Having confirmed the similarity between the time-resolved and steady-state photo-CIDNP results for α-Syn, we used the more sensitive latter method to show that divalent cations induce compaction of the C-terminal region and release of the N-terminal region of α-Syn. The photo-CIDNP method presented herein can be used as an orthogonal and independent method for investigating important biological processes associated with changes in the overall IDP conformation.

Sheared two-dimensional radiofrequency excitation for off-resonance robustness and fat suppression in reduced field-of-view imaging.

Two-dimensional (2D) echo-planar radiofrequency (RF) pulses are widely used for reduced field-of-view (FOV) imaging in applications such as diffusion-weighted imaging. However, long pulse durations render the 2D RF pulses sensitive to off-resonance effects, causing local signal losses in reduced-FOV images. This work aims to achieve off-resonance robustness for 2D RF pulses via a sheared trajectory design. A sheared 2D RF pulse design is proposed to reduce pulse durations while covering identical excitation k-space extent as a standard 2D RF pulse. For a given shear angle, the number of sheared trajectory lines is minimized to obtain the shortest pulse duration, such that the excitation replicas are repositioned outside the slice stack to guarantee unlimited slice coverage. A target fat/water signal ratio of 5% is chosen to achieve robust fat suppression. Simulations, imaging experiments on a custom head and neck phantom, and in vivo imaging experiments in the spinal cord at 3 T demonstrate that the sheared 2D RF design provides significant improvement in image quality while preserving profile sharpnesses. In regions with high off-resonance effects, the sheared 2D RF pulse improves the signal by more than 50% when compared to the standard 2D RF pulse. The proposed sheared 2D RF design successfully reduces pulse durations, exhibiting significantly improved through-plane off-resonance robustness, while providing unlimited slice coverage and high fidelity fat suppression. This method will be especially beneficial in regions suffering from a variety of off-resonance effects, such as spinal cord and breast.

Demonstration of sub- GV/m accelerating field in a photoemission electron gun powered by nanosecond X -band radio-frequency pulses

Radiofrequency (RF) electron guns operating at high accelerating gradients offer a pathway to producing bright electron bunches. Such beams are expected to revolutionize many areas of science: they could form the backbone of next-generation compact x-ray free-electron lasers or provide coherent ultrafast quantum electron probes. We report on the experimental demonstration of an RF photoemission electron source supporting an accelerating field close to 400~MV/m at the photocathode surface. The gun was operated in an RF transient mode driven by short $\sim 9$~ns X-band (\SI{11.7}{\giga\hertz}) RF pulses. We did not observe any major RF breakdown or significant dark current over a three-week experimental run at high accelerating fields. The demonstrated paradigm provides a viable path to forming relativistic electron beams with unprecedented brightness.

Phase-based fast 3D high-resolution quantitative T2 MRI in 7 T human brain imaging

Magnetic resonance imaging (MRI) is a powerful and versatile technique that offers a range of physiological, diagnostic, structural, and functional measurements. One of the most widely used basic contrasts in MRI diagnostics is transverse relaxation time (T2)-weighted imaging, but it provides only qualitative information. Realizing quantitative high-resolution T2 mapping is imperative for the development of personalized medicine, as it can enable the characterization of diseases progression. While ultra-high-field (≥ 7 T) MRI offers the means to gain new insights by increasing the spatial resolution, implementing fast quantitative T2 mapping cannot be achieved without overcoming the increased power deposition and radio frequency (RF) field inhomogeneity at ultra-high-fields. A recent study has demonstrated a new phase-based T2 mapping approach based on fast steady-state acquisitions. We extend this new approach to ultra-high field MRI, achieving quantitative high-resolution 3D T2 mapping at 7 T while addressing RF field inhomogeneity and utilizing low flip angle pulses; overcoming two main ultra-high field challenges. The method is based on controlling the coherent transverse magnetization in a steady-state gradient echo acquisition; achieved by utilizing low flip angles, a specific phase increment for the RF pulses, and short repetition times. This approach simultaneously extracts both T2 and RF field maps from the phase of the signal. Prior to in vivo experiments, the method was assessed using a 3D head-shaped phantom that was designed to model the RF field distribution in the brain. Our approach delivers fast 3D whole brain images with submillimeter resolution without requiring special hardware, such as multi-channel transmit coil, thus promoting high usability of the ultra-high field MRI in clinical practice.

LnDOTA-d8 , a versatile chemical-shift thermometer for 2 H solid-state NMR.

2 H solid-state nuclear magnetic resonance (NMR) is a method for examining the mobility and orientation of molecules in the field of biophysics. In studies on lipid bilayer membranes, 2 H NMR is often adopted to detect a phase transition from the gel to the liquid-crystal phase, which is observed as a change in spectral shape, and to evaluate the ordering of lipid alkyl chains using quadrupole coupling values. Because the mobility of membrane lipids is highly temperature dependent, precise temperature control is a prerequisite for evaluating the physical properties of membranes. Generally, NMR instruments monitor the temperature of the variable temperature (VT) gas. The temperature inside the sample tube and the VT gas match only when the heat generated by the radio frequency (rf) pulse emitted from the coil or magic angle spinning is significantly lower than the cooling capacity of the VT gas. In other words, the sample temperature inside the tube depends on the measurement method. Therefore, in this study, we took advantage of temperature-dependent changes in the chemical shift of a paramagnetic metal-ligand complex. We designed and synthesized a deuterated ligand complex and evaluated its temperature dependence as a thermometer for 2 H solid-state NMR spectroscopy. We chose Tb, Dy, Ho, and Er as the paramagnetic central metals. We then measured the 2 H NMR spectrum of each metal complex and confirmed the 2 H chemical shift to be temperature dependent. Furthermore, with the use of the thermometer molecule with Er, we succeeded in accurately evaluating the segmental melting of an alkyl chain in lipid bilayers with 0.1°C accuracy.

High-fidelity control of spin ensemble dynamics via artificial intelligence: from quantum computing to NMR spectroscopy and imaging.

High-fidelity control of spin ensemble dynamics is essential for many research areas, spanning from quantum computing and radio-frequency (RF) engineering to NMR spectroscopy and imaging. However, attaining robust and high-fidelity spin operations remains an unmet challenge. Using an evolutionary algorithm and artificial intelligence (AI), we designed new RF pulses with customizable spatial or temporal field inhomogeneity compensation. Compared with the standard RF shapes, the new AI-generated pulses show superior performance for bandwidth, robustness, and tolerance to field imperfections. As a benchmark, we constructed a spin entanglement operator for the weakly coupled two-spin-1/2 system of 13CHCl3, achieving high-fidelity transformations under multiple inhomogeneity sources. We then generated band-selective and ultra-broadband RF pulses typical of biomolecular NMR spectroscopy. When implemented in multipulse NMR experiments, the AI-generated pulses significantly increased the sensitivity of medium-size and large protein spectra relative to standard pulse sequences. Finally, we applied the new pulses to typical imaging experiments, showing a remarkable tolerance to changes in the RF field. These AI-generated RF pulses can be directly implemented in quantum information, NMR spectroscopy of biomolecules, magnetic resonance imaging techniques for in vivo and materials sciences.

New advances in medical imaging technology for the evaluation of anthracycline-induced cardiotoxicity.

New advances in medical imaging technology for the evaluation of anthracycline-induced cardiotoxicity.

An optimization model for equitable accessibility to magnetic resonance imaging technology in developing countries

Magnetic Resonance Imaging (MRI) is a sophisticated and costly technology that provides highly accurate diagnoses of various medical conditions using a powerful magnetic field, radiofrequency pulses, and a computer to produce detailed pictures of internal body parts and organs. The dissemination of MRI use at medium and high-complexity healthcare facilities increases the cost of healthcare systems. It imposes accessibility challenges concerning the equitable availability of essential healthcare technology in developing countries. Despite the importance of this technology, very few studies approach this problem from multiple location–allocation perspectives. We propose an optimization model for equitable accessibility to MRI technology. We study this problem for the Brazilian National Health System at the municipality level, and recommend alternative locations, and acquire the new devices and technologies equitably throughout the country and health system. We show that while some municipalities have an oversupply, several regions in the country have no access to MRI technology. The models propose the number of new MRIs and their locations for needing municipalities considering equity principles. The results show, for instance, that the acquisition of 210 MRIs is enough to satisfy 95% of the demand for such service, with patients traveling 44 km on average in northern Brazil. We report accessibility gains from adopting the location–allocation plans developed using the optimization model proposed in this study.

On the use of cross polarization in solid-state NMR: 1H spin-lock versus adiabatic demagnetization in the rotating frame

Cross polarization (CP) is a widely used solid-state nuclear magnetic resonance (NMR) technique for enhancing the polarization of dilute S spins from much larger polarization of abundant I spins such as 1H. To achieve such a polarization transfer, the I spin should either be spin-locked or be converted to the dipolar ordered state through adiabatic demagnetization in the rotating frame. In this work, we analyze the spin dynamics of the Hartmann-Hahn CP (HHCP) utilizing the 1H spin-locking, and the dipolar-order CP (DOCP) having the 1H adiabatic demagnetization. We further propose an adiabatic demagnetization CP (ADCP) where a constant radio-frequency pulse is applied on the S spin while 1H is adiabatically demagnetized. Our analyses indicate that ADCP utilizes the adiabatic passage to effectively achieve the polarization transfer from the 1H to S spins. In addition, the dipolar ordered state generated during the 1H demagnetization process could also be converted into the observable S polarization through DOCP, further enhancing the polarized signals. It is shown by both static and magic-angle-spinning (MAS) NMR experiments that ADCP has dramatically broadened the CP matching condition over the other CP schemes. Various samples have been used to demonstrate the polarization transfer efficiency of this newly proposed ADCP scheme.

RASER MRI: Magnetic resonance images formed spontaneously exploiting cooperative nonlinear interaction.

The spatial resolution of magnetic resonance imaging (MRI) is limited by the width of Lorentzian point spread functions associated with the transverse relaxation rate 1/T2*. Here, we show a different contrast mechanism in MRI by establishing RASER (radio-frequency amplification by stimulated emission of radiation) in imaged media. RASER imaging bursts emerge out of noise and without applying radio-frequency pulses when placing spins with sufficient population inversion in a weak magnetic field gradient. Small local differences in initial population inversion density can create stronger image contrast than conventional MRI. This different contrast mechanism is based on the cooperative nonlinear interaction between all slices. On the other hand, the cooperative nonlinear interaction gives rise to imaging artifacts, such as amplitude distortions and side lobes outside of the imaging domain. Contrast mechanism and artifacts are explored experimentally and predicted by simulations on the basis of a proposed RASER MRI theory.

Friend or Foe: How to Suppress and Measure Fat During Abdominal Resonance Imaging?

The suppression of fat signals in abdominal magnetic resonance imaging has become a basic and routine practice in the diagnosis of pathologic conditions of abdominal organs in clinical settings. Many fat-suppression techniques have been developed in the past several decades, with fat-quantification methods introduced in response in more recent years. Fat-suppression techniques can be divided into two categories. Chemical shift–based techniques include chemical shift selective (CHESS), water excitation, and the Dixon method. CHESS is the most commonly used fat-suppression method, nulling the fat signal using a fat-selective radiofrequency pulse with a spoiler gradient. Water excitation employs a binomial pulse that excites only the water protons. Finally, the Dixon method involves using the in-phase/out-of-phase cycling of fat and water. An inversionbased technique, known as short tau inversion recovery (STIR), uses a pre-excitation inversion pulse that inverts the spin of all tissues. By selecting the appropriate MRI inversion time such that the longitudinal magnetization of fat is zero, fat protons will not contribute to the MRI signal. Also, spectral attenuated inversion recovery (SPAIR) is a hybrid technique that combines the characteristics of both CHESS and STIR. The most precise fat-quantification technique known to date is a complex-based multipoint Dixon method, with which the protondensity fat fraction (PDFF) can be obtained. Multiple confounding factors must be well-corrected for accurate fat quantification. Radiologists should be familiar with the various fat suppression and measurement methods during MRI and be able to apply them to enhance patient care.

Dynamic Features of Spectra of Single and Quasi-Periodic Measuring Signals

Solving the problems of spectral processing of single and quasi-periodic signals in measurement and diagnostic systems is directly related to their isolation against the background of external interference or noise. The purpose of this work was to study single and quasi-periodic signals, i. e. signals limited in time, presented as separate components; development of a mathematical apparatus that connects the individual components of the original, time-limited signal, with the spectral characteristics of the periodic signal, which is obtained from the original by its periodization.The paper analyzes the spectrum of a quasi-periodic signal, which is presented from spectral density regions separated by spectral components with zero amplitude. The process of signal periodization is considered on the example of unipolar rectangular pulses. The representation of the analyzed complex single signal in the form of a linear combination of given functions, limited in time by the duration of the considered signal, was chosen, and it was determined that it is most logical and efficient to use radio-frequency pulses. The spectral density of the signal under consideration is presented as the sum of the spectral densities of radio-frequency pulses of the same width with a varying carrier frequency. The original signal is presented as the sum of the constituent components (radio-frequency pulses), which form a time-limited frequency spectrum – a quastr. As a result, the correlation of the considered quasi-periodic signal with the parameters of the periodic signal (amplitude, period, and initial phase) is shown.A format for representing time-limited signals in the form of components related to the spectral characteristics of a periodic signal, obtained from the original signal by periodization, has been developed. The formed mathematical apparatus allows simplifying the algorithmic support of measuring systems by eliminating the correlation signal processing.

Contactless Air-Filled Substrate-Integrated Waveguide (CLAF-SIW) Resonator for Wireless Passive Temperature Sensing

In this article a wireless passive temperature sensor is designed, fabricated, and tested. The sensor uses a contactless air-filled substrate-integrated waveguide (CLAF-SIW) resonator that consists of an air cavity surrounded by a low-impedance electromagnetic band gap (EBG) structure. The contactless feature of the sensor mitigates the fabrication complexity issue of air-filled SIW (AF-SIW) while it benefits from the high-quality factor of an air-filled resonator. The resonant frequency of the CLAF-SIW resonator is dependent on the permittivity of the substrate and the dimensions of the structure which are both temperature-dependent. Change in temperature results in a variation in the resonant frequency of the sensor. As such, we measure temperature by measuring the resonant frequency of the sensor. The developed CLAF-SIW sensor has a measured unloaded quality factor of 1340 that allows for long-range wireless measurements. The resonant frequency of the sensor is measured using a ringback-based wireless interrogation system. The interrogation system energizes the sensor using radio frequency (RF) pulses and determines the frequency of the ringback signals radiated by the sensor using frequency domain analysis. The sensor has a sensitivity of 300 kHz/ <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$^\circ \text{C}$ </tex-math></inline-formula> and an interrogation-to-sensor distance of 80cm.

Contribution of Electromagnetic Shielding and the Bloch–Siegert Effect to Magnetic-Resonance Sounding

Abstract —Calibration experiments of magnetic-resonance sounding (MRS) were carried out on the ice of the Ob’ Reservoir. For the first time, a comparative quantitative analysis of the contribution of electromagnetic shielding and the Bloch–Siegert effect to the MRS signal was carried out, and the asymmetry of this contribution when detuning from the resonant frequency was investigated. The results of theoretical calculations without regard to the Bloch–Siegert effect differ about three times from the experimental data at the maximum intensity of the radio frequency pulse for the experimental data. At the same time, when taking into account the interference of electromagnetic shielding and the Bloch–Siegert effect, the results of calculating the amplitude and phase of the signal best approximate the field experimental data. Based on the good agreement between the experiment and the theoretical calculations taking into account the interference of electromagnetic shielding and the Bloch–Siegert effect, it is demonstrated that this model should be used to solve the direct and inverse problems of MRS of underground aquifers. The results obtained allow us to calibrate the MRS method and use it not only as an indicator of underground water but also as a means of quantitative measurement. The necessity of taking into account the Bloch–Siegert effect in real field conditions is demonstrated by the example of MRS studies in the Ebro River valley (Spain). When taking into account only electromagnetic shielding (without regard to the Bloch–Siegert effect), the signal amplitude differs nearly twice at the maximum pulse intensity for experimental data, and the signal phase differs 12 times at the maximum pulse intensity. Therefore, in previously published works, approximation of the experimental data using the model without regard to the Bloch–Siegert effect was made by adding layers at depths close to the maximum for this type of antenna (approximately 50 m). In reality, however, these layers do not exist; they appear only as an artifact of the model without regard to the Bloch–Siegert effect. The best approximation of the experimental data can be achieved by taking into account the interference of electromagnetic shielding and the Bloch–Siegert effect.

State of the Art and Future Opportunities in MRI-Guided Robot-Assisted Surgery and Interventions.

Magnetic resonance imaging (MRI) can provide high-quality 3-D visualization of target anatomy, surrounding tissue, and instrumentation, but there are significant challenges in harnessing it for effectively guiding interventional procedures. Challenges include the strong static magnetic field, rapidly switching magnetic field gradients, high-power radio frequency pulses, sensitivity to electrical noise, and constrained space to operate within the bore of the scanner. MRI has a number of advantages over other medical imaging modalities, including no ionizing radiation, excellent soft-tissue contrast that allows for visualization of tumors and other features that are not readily visible by other modalities, true 3-D imaging capabilities, including the ability to image arbitrary scan plane geometry or perform volumetric imaging, and capability for multimodality sensing, including diffusion, dynamic contrast, blood flow, blood oxygenation, temperature, and tracking of biomarkers. The use of robotic assistants within the MRI bore, alongside the patient during imaging, enables intraoperative MR imaging (iMRI) to guide a surgical intervention in a closed-loop fashion that can include tracking of tissue deformation and target motion, localization of instrumentation, and monitoring of therapy delivery. With the ever-expanding clinical use of MRI, MRI-compatible robotic systems have been heralded as a new approach to assist interventional procedures to allow physicians to treat patients more accurately and effectively. Deploying robotic systems inside the bore synergizes the visual capability of MRI and the manipulation capability of robotic assistance, resulting in a closed-loop surgery architecture. This article details the challenges and history of robotic systems intended to operate in an MRI environment and outlines promising clinical applications and associated state-of-the-art MRI-compatible robotic systems and technology for making this possible.