MRI-guided Interventions Research Articles

Vacuum-based and body-mounted robotic-patient interface with an integrated metasurface for MRI-guided interventions

Abstract The increased clinical relevance of image-guided procedures, particularly of interventional magnetic resonance imaging (iMRI), highlights the need for advanced devices and robotics to optimize those procedures. To enable a realistic integration of robotics into the clinical workflow, robotic-patient interfaces (RPI) are required to ensure both functionality and usability. However, current concepts have issues with patient accessibility and safety, user handling, or performing interventions on versatile body regions. This work presents a novel silicone-based RPI to flexibly mount the ’Micropositioning Robotics for Image-Guided Surgery’ (μRIGS) system on differently shaped body regions through vacuum, regulated with an electronic miniature pump. Maximum holding forces of 60N at −0.1 bar and 66N at −0.2 bar relative vacuum pressure were reached depending on the applied human body area and tensile force angles. MRI with the integrated metasurface indicated up to 200% signal enhancement, enabling improved tissue contrast within the first 20mm in depth. The multifunctional design supported the incorporation of the sterile iMRI workflow concept. This RPI enables a realistic integration of future technologies such as miniature robots, tracking markers, and instrument holders into complex interventional workflows. Further long-term studies are needed to evaluate the effects of vacuum application lasting for 1-2 hours on different skin types.

MR-based navigation for robot-assisted endovascular procedures

AbstractThere is increasing interests in robotic and computer technologies to accurately perform endovascular intervention. One major limitation of current endovascular intervention—either manual or robot-assisted is the surgical navigation which still relies on 2D fluoroscopy. Recent research efforts are towards MRI-guided interventions to reduce ionizing radiation exposure, and to improve diagnosis, planning, navigation, and execution of endovascular interventions. We propose an MR-based navigation framework for robot-assisted endovascular procedures. The framework allows the acquisition of real-time MR images; segmentation of the vasculature and tracking of vascular instruments; and generation of MR-based guidance, both visual and haptic. The instrument tracking accuracy—a key aspect of the navigation framework—was assessed via 4 dedicated experiments with different acquisition settings, framerate, and time. The experiments showed clinically acceptable tracking accuracy in the range of 1.30–3.80 mm RMSE. We believe that this work represents a valuable first step towards MR-guided robot-assisted intervention.

Magnetic Resonance-guided Procedures: Consensus on Rationale, Techniques, and Outcomes

Magnetic resonance (MR) image guidance has demonstrated significant potential in the field of interventional radiology in several applications. This article covers the main points of MR-guided hepatic tumor ablation as a representative of MR-guided procedures. Patient selection and appropriate equipment utilization are essential for successful MR-guided tumor ablation. Intra-procedural planning imaging enables the visualization of the tumor and surrounding anatomical structures in most cases without application of contrast agent, ensuring optimal planning of the applicator tract. MRI enables real-time, multiplanar imaging, thus simultaneous observation of the applicator and target tumor is possible during targeting with adaptable slice angulations in case of challenging tumor positions. Typical ablation zone appearance during therapy monitoring with MRI enables save assessment of the therapy result, resulting in a high primary efficacy rate. Recent advancements in ablation probes have shortened treatment times, while technical strategies address applicator visibility issues. MR-imaging immediately after the procedure is used to rule out complications and to assess technical success. Especially in smaller neoplasms, MRI-guided liver ablation demonstrates positive outcomes in terms of technical success rates, as well as promising survival and recurrence rates. Additionally, percutaneous biopsy under MR guidance offers an alternative to classic guidance modalities, providing high soft tissue contrast and thereby increasing the reliability of lesion detection, particularly in cases involving smaller lesions. Despite these advantages, use of MR-guidance in clinical routine is still limited to few indications and centers, due to by high costs, extended duration, and the need for specialized expertise. In conclusion, MRI-guided interventions could benefit from ongoing advancements in hardware, software and devices. Such progress has the potential to expand diagnostic and treatment options in the field of interventional radiology.

Cardiac MRI at Low Field Strengths.

Cardiac MR imaging is well established for assessment of cardiovascular structure and function, myocardial scar, quantitative flow, parametric mapping, and myocardial perfusion. Despite the clear evidence supporting the use of cardiac MRI for a wide range of indications, it is underutilized clinically. Recent developments in low-field MRI technology, including modern data acquisition and image reconstruction methods, are enabling high-quality low-field imaging that may improve the cost-benefit ratio for cardiac MRI. Studies to-date confirm that low-field MRI offers high measurement concordance and consistent interpretation with clinical imaging for several routine sequences. Moreover, low-field MRI may enable specific new clinical opportunities for cardiac imaging such as imaging near metal implants, MRI-guided interventions, combined cardiopulmonary assessment, and imaging of patients with severe obesity. In this review, we discuss the recent progress in low-field cardiac MRI with a focus on technical developments and early clinical validation studies. EVIDENCE LEVEL: 5 TECHNICAL EFFICACY: Stage 1.

Toward Robust Partial-Image Based Template Matching Techniques for MRI-Guided Interventions.

We have developed an MRI-safe needle guidance toolkit for MRI-guided interventions intended to enable accurate positioning for needle-based procedures. The toolkit allows intuitive and accurate needle angulation and entry point positioning according to an MRI-based plan, using a flexible, patterned silicone 2D grid. The toolkit automatically matches the grid on MRI planning images with a physical silicon grid placed conformally on the patient's skin and provides the Interventional Radiologist an easy-to-use guide showing the needle entry point on the silicon grid as well as needle angle information. The radiologist can use this guide along with a 2-degree-of-freedom (rotation and angulation relative to the entry point) hand-held needle guide to place the needle into the anatomy of interest. The initial application that we are considering for this toolkit is arthrography, a diagnostic procedure to evaluate the joint space condition. However, this toolkit could be used for any needle-based and percutaneous procedures such as MRI-guided biopsy and facet joint injection. For matching the images, we adopt a transformation parameter estimation technique using the phase-only correlation method in the frequency domain. We investigated the robustness of this method against rotation, displacement, and Rician noise. The algorithm was able to successfully match all the dataset images. We also investigated the accuracy of identifying the entry point from registered template images as a prerequisite for a future targeting study. Application of the template matching algorithm to locate the needle entry points within the MRI dataset resulted in an average entry point location estimation accuracy of 0.12 ±0.2 mm. This promising result motivates a more detailed assessment of this algorithm in the future including a targeting study on a silicon phantom with embedded plastic targets to investigate the end-to-end accuracy of this automatic template matching algorithm in the interventional MRI room.

Susceptibility artifacts evaluation for non-metallic biopsy needles in a biological-engineered 3D tumor model

Abstract Nowadays, needle artifacts are an essential restriction for MRI-guided interventions, as they influence the visually perceived needle size and needle destination. Standard MRI needles made of Nickel-Titanium (NiTi) alloys still produce massive artifacts in MRI due to materials’ interactions with the magnetic environment. The use of non-metallic materials can reduce these artifacts. Therefore, we propose a nonmetallic concept of a coaxial needle design concept with a fiber-enforced inner core and an outer polymeric hollow sheet. This work aims to evaluate the artifact performance of the proposed needles in a custom-made three-dimensional (3D) tumor model with a relevant size and thickness for medical interventions under MRI guidance using a 3T field strength with a T1-weighted gradient-echo sequence. Three coaxial MRcompatible needles were inserted in the custom-made 3D tissue phantom, one standard needle from NiTi, and two proposed non-metallic needles. Artifact’s width and length were measured for each needle. Overall, the non-metallic needles showed significantly lower artifacts than the standard NiTi needle inside the 3D tumor model.

Hybrid polymer vessel phantoms for feasibility studies and clinical training of MRI-guided interventions

Article Hybrid polymer vessel phantoms for feasibility studies and clinical training of MRI-guided interventions was published on August 1, 2022 in the journal Current Directions in Biomedical Engineering (volume 8, issue 2).

Challenges in MRI-Guided Breast Biopsy and Some Suggested Strategies: Case Based Review.

With the increasing use of MRI in clinical practice, the need for MRI-guided intervention is also increasing. Indeterminate lesions identified on MRI without mammographic or sonographic correlates will need to be approached under MRI guidance. MRI-guided biopsy is a skill that can be acquired with proper training and guidance. These procedures have their own set of challenges and issues; some of them are specific to the patient habitus in this region. Adequate knowledge and understanding of the challenges can help the radiologist to be better equipped to face these issues and solve them promptly during the procedure, thus increasing the overall success rate of the procedure. Not much local data from Asian countries is available on this front. This paper aims to share common challenges one may face while performing MRI-guided biopsy and share some tips and tricks to address these problems. Hopefully, this will help the readers achieve a higher success rate for MRI-guided interventions in their clinical practice.

Real-Time MRI-Guided Prostate Interventions.

Simple SummaryMagnetic resonance imaging has shown to be a reliable imaging method for detecting clinically significant prostate cancer and directly targeting lesions during biopsy. As newer treatment methods emerge, the role of MRI in minimally-invasive (focal) treatment of prostate cancer is also increasing. Here, we review the real-time MRI-guided prostate interventions for prostate cancer diagnosis and treatment, focusing on the technical aspects of each modality.Prostate cancer (PCa) is the second most common cause of cancer death in males. Targeting MRI-visible lesions has led to an overall increase in the detection of clinically significant PCa compared to the prior practice of random ultrasound-guided biopsy of the prostate. Additionally, advances in MRI-guided minimally invasive focal treatments are providing new options for patients with PCa. This review summarizes the currently utilized real-time MRI-guided interventions for PCa diagnosis and treatment.

Optimised passive marker device visibility and automatic marker detection for 3-T MRI-guided endovascular interventions: a pulsatile flow phantom study

BackgroundPassive paramagnetic markers on magnetic resonance imaging (MRI)-compatible endovascular devices induce susceptibility artifacts, enabling MRI-visibility and real-time MRI-guidance. Optimised visibility is crucial for automatic detection and device tracking but depends on MRI technical parameters and marker characteristics. We assessed marker visibility and automatic detection robustness for varying MRI parameters and marker characteristics in a pulsatile flow phantom.MethodsGuidewires with varying iron(II,III) oxide nanoparticle (IONP) concentration markers were imaged using gradient-echo (GRE) and balanced steady-state free precession (bSSFP) sequences at 3 T. Furthermore, echo time (TE), slice thickness (ST) and phase encoding direction (PED) were varied. Artifact width was measured and contrast-to-noise ratios were calculated. Marker visibility and image quality were scored by two MRI interventional radiologists. Additionally, a deep learning model for automatic marker detection was trained and the effects of the parameters on detection performance were evaluated. Two-tailed Wilcoxon signed-rank tests were used (significance level, p < 0.05).ResultsMedan artifact width (IQR) was larger in bSSFP compared to GRE images (12.7 mm (11.0–15.2) versus 8.4 mm (6.5–11.0)) (p < 0.001) and showed a positive relation with TE and IONP concentration. Switching PED and doubling ST had limited effect on artifact width. Image quality assessment scores were higher for GRE compared to bSSFP images. The deep learning model automatically detected the markers. However, the model performance was reduced after adjusting PED, TE, and IONP concentration.ConclusionMarker visibility was sufficient and a large range of artifact sizes was generated by adjusting TE and IONP concentration. Deep learning-based marker detection was feasible but performance decreased for altered MR parameters. These factors should be considered to optimise device visibility and ensure reliable automatic marker detectability in MRI-guided endovascular interventions.

12-year review of MRI-detected breast cancers. How often was a diagnosis of malignancy established using targeted ultrasound and standard 14-gauge core biopsy?

This retrospective study aimed to determine the percentage of MRI-detected breast cancers diagnosed using targeted ultrasound and standard 14-gauge (14g) biopsy in the setting of an Australian breast MRI service. This is of clinical relevance because malignancies not identifiable on mammography or by ultrasound may then require more invasive, technically demanding and costly MRI-guided interventional procedures, usually by large-core vacuum-assisted biopsy (VAB) or hook-wire localisation with open surgical biopsy. On review of the 12-year period 2006-2018, we identified 67 new breast cancer events in 64 women where the diagnosis was made on the basis of the MRI scan findings either alone (n = 60) or in combination with a concurrent mammogram (n = 7), with no recorded clinical abnormality. The percentage in which malignancy was confirmed on histopathology using targeted ultrasound in combination with 14g biopsy was determined versus other biopsy methods, for both invasive cancer and insitu disease. Ultrasound-guided 14g biopsy was successful in establishing the presence of malignancy in 42/46 (91%) of events with a final diagnosis of invasive cancer, with 2 more proven by MRI-guided interventional procedures (1 VAB and 1 hook-wire) and 1 by open surgical biopsy. In the final case, a 5 mm focus on MRI with no sonographic correlate at the initial presentation was only identified and biopsied by ultrasound at 12-month follow-up. For events with a final diagnosis of DCIS/pLCIS (pleomorphic LCIS, n = 2), US-guided 14g biopsy was successful in 10/21 (48%), while 4/7 events with corresponding mammographic microcalcifications were proven by x-ray stereotactic interventions. A further 5 events had MRI-guided interventions (3 VAB and 2 hook-wires) and 1 an open surgical biopsy to confirm malignancy. In the final case (a woman with a 30 × 20 mm focal area of non-mass enhancement with corresponding microcalcifications consistent with DCIS), a pathologic diagnosis was not made until the patient presented 5 years later with invasive disease. There were also 3 instances of upgrades to invasion on final surgical pathology, one from pLCIS to microinvasion and 2 others from DCIS to IDC. Among the DCIS/pLCIS events, semi-random 14g core biopsy (sampling at the expected location of the MRI abnormality without a specific sonographic correlate) proved to be successful in 3 women. Ultrasound-guided 14g core biopsy established a malignant diagnosis in 91% of invasive cancers and in 48% of DCIS/pLCIS cases. This relatively non-invasive, technically easy to perform and low-cost biopsy procedure can be used immediately when targeted ultrasound shows a correlate for a suspicious MRI scan finding. Careful imaging-pathologic correlation is required after 14g biopsy, and a discordant result will usually prompt recourse to an MRI-guided VAB or hook-wire localisation.

Artifact reduction of coaxial needles in magnetic resonance imaging-guided abdominal interventions at 1.5 T: a phantom study

Needle artifacts pose a major limitation for MRI-guided interventions, as they impact the visually perceived needle size and needle-to-target-distance. The objective of this agar liver phantom study was to establish an experimental basis to understand and reduce needle artifact formation during MRI-guided abdominal interventions. Using a vendor-specific prototype fluoroscopic T1-weighted gradient echo sequence with real-time multiplanar acquisition at 1.5 T, the influence of 6 parameters (flip angle, bandwidth, matrix, slice thickness, read-out direction, intervention angle relative to B0) on artifact formation of 4 different coaxial MR-compatible coaxial needles (Nitinol, 16G–22G) was investigated. As one parameter was modified, the others remained constant. For each individual parameter variation, 2 independent and blinded readers rated artifact diameters at 2 predefined positions (15 mm distance from the perceived needle tip and at 50% of the needle length). Differences between the experimental subgroups were assessed by Bonferroni-corrected non-parametric tests. Correlations between continuous variables were expressed by the Bravais–Pearson coefficient and interrater reliability was quantified using the intraclass classification coefficient. Needle artifact size increased gradually with increasing flip angles (p = 0.002) as well as increasing intervention angles (p < 0.001). Artifact diameters differed significantly between the chosen matrix sizes (p = 0.002) while modifying bandwidth, readout direction, and slice thickness showed no significant differences. Interrater reliability was high (intraclass correlation coefficient 0.776–0.910). To minimize needle artifacts in MRI-guided abdominal interventions while maintaining optimal visibility of the coaxial needle, we suggest medium-range flip angles and low intervention angles relative to B0.

An anthropomorphic pelvis phantom for MR-guided prostate interventions.

To design and manufacture a pelvis phantom for magnetic resonance (MR)-guided prostate interventions, such as MRGB (MR-guided biopsy) or brachytherapy seed placement. The phantom was designed to mimic the human pelvis incorporating bones, bladder, prostate with four lesions, urethra, arteries, veins, and six lymph nodes embedded in ballistic gelatin. A hollow rectum enables transrectal access to the prostate. To demonstrate the feasibility of the phantom for minimal invasive MRI-guided interventions, a targeted inbore MRGB was performed. The needle probe was rectally inserted and guided using an MRI-compatible remote controlled manipulator (RCM). The presented pelvis phantom has realistic imaging properties for MR imaging (MRI), computed tomography (CT) and ultrasound (US). In the targeted inbore MRGB, a prostate lesion was successfully hit with an accuracy of 3.5mm. The experiment demonstrates that the limited size of the rectum represents a realistic impairment for needle placements. The phantom provides a valuable platform for evaluating the performance of MRGB systems. Interventionalists can use the phantom to learn how to deal with challenging situations, without risking harm to patients.

Breast intervention surgery robot under image navigation: A review

In recent years, breast cancer incidence has increased year by year, which makes research on treatment methods critical. Intervention surgery is a new development in breast cancer treatment. However, the operation is complicated and lasts for a long time. The doctor’s operation can no longer meet clinical needs. The breast intervention robot has the characteristics of minimal invasiveness, operability, and high flexibility so that it can compensate for the shortcomings of traditional interventional surgery. The combination of the robotic system and medical imaging technology is an important means to determine the lesion location and plan the puncture path. This article mainly reviews the breast intervention robot under image navigation. Based on existing literature, image navigation methods are divided into MRI, ultrasound, and CT for the introduction. First, we summarized the principles and technologies associated with materials, sensors, and actuators used in the MRI-guided breast intervention robot. In particular, the actuation is analyzed and compared in detail. Subsequently, the ultrasound-guided and CT-guided breast intervention robots are introduced. After that, we discuss the image-guided positioning technology. Finally, we summarize the research progress and trends in the future development of breast intervention robots under image navigation.

Fiber-Based Clock Synchronization Method for Medical Ultrasound System.

Brain ultrasound has attracted great attention recently due to its noninvasive treatment function for brain diseases. However, ultrasound is still difficult to pass through an intact skull. Phase correction is recognized as an effective method for skull compensation. Half-wavelength pitch transducer is important for the phase correction and, hence, thousands of elements array is required to cover large area human tissue. The clock synchronization between elements is crucial for the phase correction; however, the traditional clock scheme which is designed for 128- or 256-element system is not suitable for thousands of elements. In addition, the clock scheme needs to be magnetic resonance imaging (MRI) compatible since MRI-guided intervention is becoming a routine operation for the brain ultrasound. This study is the first to propose an optical fiber-based clock synchronization method for MRI-guided ultrasound array system. The optical fiber not only distributes the clock but also sets up a link to transmit the data for ultrasound beamformer. The link is full-duplex so both the clock and the data can be transmitted and received simultaneously. The precision of clock synchronization is less than 557 ps when using 50 MHz clock, and the period jitter of the clock is less than 10 ps (rms). Multiple 128- or 256-channel ultrasonic systems can be synchronized, and the error between the channels can be less than 10 ns when using 1-MHz ultrasound transducer. The system can work in an MRI scanning room and communicate with a console via only one fiber. In vivo primate animal study has been achieved, and it has been proven that the proposed clock scheme is suitable for MRI-guided large-scale ultrasound array system.

A compact stereotactic system for image-guided surgical intervention

Objective. Stereotactic technology enables fine navigation to small structures in the human body. While current stereotactic systems facilitate accurate targeting, they are mechanically cumbersome and limited in scope. Here, we hypothesized that a stereotactic system could be developed with a reduced footprint while maintaining broad targeting capabilities in order to improve versatility in frame placement location and surgical workflow. Approach. We designed a stereotactic system around the center-of-arc principle, with mechanical properties that would enable a compact design and ample targeting and trajectory maneuverability. To examine the opportunity for a low-cost rapidly-deployable system we developed two fabrication variants, one using three dimensional (3D)-printing and the other using conventional machining. Mechanical and image-guided accuracies were tested in phantom studies using magnetic resonance imaging (MRI) and computed tomography. Using human cadaver head specimens, we assessed the system’s surgical workflow and its ability to reliably and accurately implant electrodes in deep brain stimulation (DBS) surgery. Main results. We developed a small 7.7 × 5.4 cm2 device platform that rigidly mounts to curvilinear bone and supports the attachment of surgical instrumentation. Attachment of two surgical instruments, an imaging localizer and a compact targeting device, demonstrated successful MRI-guided intervention in phantom studies with a vector error of 1.79 ± 0.41 mm. Evaluation of the 3D-printed system for DBS surgery confirmed ease of device platform attachment and instrument functionality, as well as demonstrated a surgical targeting accuracy of 1.83 ± 0.15 mm. In addition, we found the surgical time to be 78.3 ± 5.4 min for bilateral electrode implantation. Significance. We developed a light and compact stereotactic system whose accuracy is on par with those used clinically. This technology is suitable for clinical translation and its flexibility in positioning will seamlessly expand the capabilities for stereotaxy to treat a wide range of conditions, both within neurosurgery and beyond.

Body-Mounted Robotics for Interventional MRI Procedures.

This paper reports the development and initial cadaveric evaluation of a robotic framework for MRI-guided interventions using a body-mounted approach. The framework is developed based on modular design principles. The framework consists of a body-mounted needle placement manipulator, robot control software, robot controller, interventional planning workstation, and MRI scanner. The framework is modular in the sense that all components are connected independently, making it readily extensible and reconfigurable for supporting the clinical workflow of various interventional MRI procedures. Based on this framework we developed two body-mounted robots for musculoskeletal procedures. The first robot is a four-degree of freedom system called ArthroBot for shoulder arthrography in pediatric patients. The second robot is a six-degree of freedom system called PainBot for perineural injections used to treat pain in adult and pediatric patients. Body-mounted robots are designed with compact and lightweight structure so that they can be attached directly to the patient, which minimizes the effect of patient motion by allowing the robot to move with the patient. A dedicated clinical workflow is proposed for the MRI-guided musculoskeletal procedures using body-mounted robots. Initial cadaveric evaluations of both systems were performed to verify the feasibility of the systems and validate the clinical workflow.

Developing an Intuitive and Feasible Setup for In-room Control During MRI-guided Interventions

Abstract Magnetic resonance image (MRI)-guidance for minimally invasive interventions instead of CT-guidance is a promising technique to reduce radiation exposure for both patients and clinicians. Technical challenges have to be managed in order to meet safety regulations while enabling inroom system control and communication. Using nonproprietary software and hardware, several gradations for a setup for interventional MRI were elaborated. Eventually, a quickly and easily installable setup was established for technical support during MRI-guided interventions in the PET/MRI suite at the Department of Nuclear Medicine at University Hospital Leipzig. It is adaptable for most MRI suites.

Automatic needle tracking using Mask R-CNN for MRI-guided percutaneous interventions.

Accurate needle tracking provides essential information for MRI-guided percutaneous interventions. Passive needle tracking using MR images is challenged by variations of the needle-induced signal void feature in different situations. This work aimed to develop an automatic needle tracking algorithm for MRI-guided interventions based on the Mask Region Proposal-Based Convolutional Neural Network (R-CNN). Mask R-CNN was adapted and trained to segment the needle feature using 250 intra-procedural images from 85 MRI-guided prostate biopsy cases and 180 real-time images from MRI-guided needle insertion in ex vivo tissue. The segmentation masks were passed into the needle feature localization algorithm to extract the needle feature tip location and axis orientation. The proposed algorithm was tested using 208 intra-procedural images from 40 MRI-guided prostate biopsy cases, and 3 real-time MRI datasets in ex vivo tissue. The algorithm results were compared with human-annotated references. In prostate datasets, the proposed algorithm achieved needle feature tip localization error with median Euclidean distance (dxy) of 0.71mm and median difference in axis orientation angle (dθ) of 1.28°, respectively. In 3 real-time MRI datasets, the proposed algorithm achieved consistent dynamic needle feature tracking performance with processing time of 75ms/image: (a) median dxy = 0.90mm, median dθ = 1.53°; (b) median dxy = 1.31mm, median dθ = 1.9°; (c) median dxy = 1.09mm, median dθ = 0.91°. The proposed algorithm using Mask R-CNN can accurately track the needle feature tip and axis on MR images from in vivo intra-procedural prostate biopsy cases and ex vivo real-time MRI experiments with a range of different conditions. The algorithm achieved pixel-level tracking accuracy in real time and has potential to assist MRI-guided percutaneous interventions.

Design and Fabrication of Wireless Multilayer Tracking Marker for Intraoperative MRI-Guided Interventions

This article presents the design, fabrication, and evaluation of miniature magnetic resonance (MR) compatible wireless markers, which can provide three-dimensional (3-D) positional tracking under magnetic resonance imaging (MRI). To achieve the small size of such markers, rectangular spiral planar coils were stacked in multiple layers, which can be fabricated on a flexible printed circuit board. Finite-element-based simulations and analytical modeling were applied to ensure the proper adjustment of the MRI scanner resonant frequency while maintaining a high circuit quality factor. A four-layer planar tracking coil was prototyped with a size of 6.7 × 1.5 × 0.3 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , and a quality factor of 28.5. This design and fabrication approach is reportedly the first design to initiate wireless markers in such a small size, enabling straightforward integration with interventional tools. When validated under MRI, the tracking marker appeared as a very high contrast spot on the MR images. For a 48 mm distance from the isocenter, the estimated maximum errors in 3-D position was 0.48 mm. And the inherent standard deviation of marker localization was 0.12 mm. With the high MR contrast signal generated, the presented markers enable automatic and real-time tracking in 3-D but without MR image construction. In combination with the small form factor, this marker would facilitate the MRI-guided navigation of interventional tools, in particular for those assisted by teleoperated robots.