MR Compatible Research Articles

A 3D printed pulmonary mock loop for hemodynamic studies in congenital heart disease

Background. With the increasing survival of the congenital heart disease population, there is a growing need for in-depth understanding of blood circulation in these patients. Mock loops provide the opportunity for comprehensive hemodynamic studies without burden and risks for patients. This study aimed to evaluate the ability of the presented mock loop to mimic the hemodynamics of the pulmonary circulation with and without stenosis and the MR compatibility of the system. Methods. A pulsatile pump with two chambers, separated by a flexible membrane, was designed and 3D printed. A cough assist device applied an alternating positive and negative pressure on the membrane. One adult, and three pediatric pulmonary bifurcations were 3D printed and incorporated in the setup. Two pediatric models had a 50% stenosis of the left branch. Bilateral compliance chambers allowed for individual compliance tuning. A reservoir determined the diastolic pressure. Two carbon heart valves guaranteed unidirectional flow. The positive pressure on the cough assist device was tuned until an adequate stroke volume was reached with a frequency of 60 bpm. Flow and pressure measurements were performed on the main pulmonary artery and the two branches. The MR compatibility of the setup was evaluated. Results. A stroke volume with a cardiac index of 2 l min−1 m−2 was achieved in all models. Physiological pressure curves were generated in both normal and stenotic models. The mock loop was MR compatible. Conclusion. This MR compatible mock loop, closely resembles the pulmonary circulation thereby providing a controllable environment for hemodynamic studies.

Perspectives in Wireless Radio Frequency Coil Development for Magnetic Resonance Imaging

This paper addresses the scientific and technological challenges related to the development of wireless radiofrequency (RF) coils for magnetic resonance imaging (MRI) based on published literature together with the authors’ interpretation and further considerations. Key requirements and possible strategies for the wireless implementation of three important subsystems, namely the MR receive signal chain, control signaling and on-coil power supply, are presented and discussed. For RF signals of modern MRI setups (e.g. 3 T, 64 RF receive channels), with on-coil digitization and advanced methods for dynamic range (≥ 16 bit) and data rate compression, still data rates > 500 Mbps will be required. For wireless high-speed MR data transmission, 60 GHz WiGig and optical wireless communication appear to be suitable strategies, however, on-coil functionality during MRI scans remains to be verified. Besides RF signals, control signals for on-coil components, e.g. active detuning, synchronization to the MR system and B0 shimming, have to be managed. Wireless power supply becomes an important issue, especially with a large amount of additional on-coil components. Wireless power transfer systems (> 10 W) seem to be an attractive solution compared to bulky MR compatible batteries and energy harvesting with low power output. In our opinion, completely wireless RF coils will ultimately become feasible in the future by combining efficient available strategies from recent scientific advances and novel research. Besides ongoing improvement of all three subsystems, innovations are specifically required regarding wireless technologies, MR compatibility, and wireless power supply.

An ultra-high field strength MR image-guided robotic needle delivery system for in-bore small animal interventions.

The purpose of this study was to develop and validate an image-guided robotic needle delivery system for accurate and repeatable needle targeting procedures in mouse brains inside the 12 cm inner diameter gradient coil insert of a 9.4 T MR scanner. Many preclinical research techniques require the use of accurate needle deliveries to soft tissues, including brain tissue. Soft tissues are optimally visualized in MR images, which offer high-soft tissue contrast, as well as a range of unique imaging techniques, including functional, spectroscopy and thermal imaging, however, there are currently no solutions for delivering needles to small animal brains inside the bore of an ultra-high field MR scanner. This paper describes the mechatronic design, evaluation of MR compatibility, registration technique, mechanical calibration, the quantitative validation of the in-bore image-guided needle targeting accuracy and repeatability, and demonstrated the system's ability to deliver needles in situ. Our six degree-of-freedom, MR compatible, mechatronic system was designed to fit inside the bore of a 9.4 T MR scanner and is actuated using a combination of piezoelectric and hydraulic mechanisms. The MR compatibility and targeting accuracy of the needle delivery system are evaluated to ensure that the system is precisely calibrated to perform the needle targeting procedures. A semi-automated image registration is performed to link the robot coordinates to the MR coordinate system. Soft tissue targets can be accurately localized in MR images, followed by automatic alignment of the needle trajectory to the target. Intra-procedure visualization of the needle target location and the needle were confirmed through MR images after needle insertion. The effects of geometric distortions and signal noise were found to be below threshold that would have an impact on the accuracy of the system. The system was found to have negligible effect on the MR image signal noise and geometric distortion. The system was mechanically calibrated and the mean image-guided needle targeting and needle trajectory accuracies were quantified in an image-guided tissue mimicking phantom experiment to be 178 ± 54 μm and 0.27 ± 0.65°, respectively. An MR image-guided system for in-bore needle deliveries to soft tissue targets in small animal models has been developed. The results of the needle targeting accuracy experiments in phantoms indicate that this system has the potential to deliver needles to the smallest soft tissue structures relevant in preclinical studies, at a wide variety of needle trajectories. Future work in the form of a fully-automated needle driver with precise depth control would benefit this system in terms of its applicability to a wider range of animal models and organ targets.

TU-AB-201-07: Image Guided Endorectal HDR Brachytherapy Using a Compliant Balloon Applicator

Purpose: High dose rate endorectal brachytherapy is an option to deliver a focal, high-dose radiotherapy to rectal tumors for patients undergoing non-operative management. We investigate a new multichannel, MR compatible applicator with a novel balloon-based design to provide improved treatment geometry. We report on the initial clinical experience using this applicator. Methods: Patients were enrolled on an IRB-approved, dose-escalation protocol evaluating the use of the anorectal (AR-1) applicator (Ancer Medical, Hialeah, FL), a multichannel applicator with two concentric balloons. The inner balloon supports 8 source lumens; the compliant outer balloon expands to separate the normal rectal wall and the source lumens, yet deforms around a firm, exophytic rectal mass, leading to dose escalation to tumor while sparing normal rectum. Under general anesthesia, gold fiducial markers were inserted above and below the tumor, and the AR applicator was placed in the rectum. MRI-based treatment plans were prepared to deliver 15 Gy in 3 weekly fractions to the target volume while sparing healthy rectal tissue, bladder, bowel and anal muscles. Prior to each treatment, CBCT/Fluoroscopy were used to place the applicator in the treatment position and confirm the treatment geometry using rigid registration of the CBCT and planning MRI. After registration of the applicator images, positioning was evaluated based on the match of the gold markers. Results: Highly conformal treatment plans were achieved. MR compatibility of the applicator enabled good tumor visualization. In spite of the non-rigid nature of the applicators and the fact that a new applicator was used at each treatment session, treatment geometry was reproducible to within 2.5 mm. Conclusions: This is the first report on using the AR applicator in patients. Highly conformal plans, confidence in MRI target delineation, in combination with reproducible treatment geometry provide encouraging feedback for continuation with dose escalation in these patients.

MR compatibility aspects of a silicon photomultiplier-based PET/RF insert with integrated digitisation

The combination of Positron Emission Tomography (PET) and Magnetic Resonance Imaging (MRI) into a single device is being considered a promising tool for molecular imaging as it combines the high sensitivity of PET with the functional and anatomical images of MRI. For highest performance, a scalable, MR compatible detector architecture with a small form factor is needed, targeting at excellent PET signal-to-noise ratios and time-of-flight information. Therefore it is desirable to use silicon photo multipliers and to digitize their signals directly in the detector modules inside the MRI bore.A preclinical PET/RF insert for clinical MRI scanner was built to demonstrate a new architecture and to study the interactions between the two modalities.The disturbance of the MRI’s static magnetic field stays below 2 ppm peak-to-peak within a diameter of 56 mm (90 mm using standard automatic volume shimming). MRI SNR is decreased by 14%, RF artefacts (dotted lines) are only visible in sequences with very low SNR. Ghosting artefacts are visible to the eye in about 26% of the EPI images, severe ghosting only in 7.6%.Eddy-current related heating effects during long EPI sequences are noticeable but with low influence of 2% on the coincidences count rate. The time resolution of 2.5 ns, the energy resolution of 29.7% and the volumetric spatial resolution of 1.8 mm3 in the PET isocentre stay unaffected during MRI operation.Phantom studies show no signs of other artefacts or distortion in both modalities. A living rat was simultaneously imaged after the injection with 18F-Fluorodeoxyglucose (FDG) proving the in vivo capabilities of the system.

Development of PET insert for simultaneous PET/MR imaging of human brain.

Recently, there has been great interest on the development of combined PET/MR, which is a useful tool for both functional and anatomic imaging. The purpose of this study was to develop a MR compatible PET insert for simultaneous PET and MR imaging of human brain and to evaluate the performance of the hybrid PET-MRI. The PET insert consisted of 18 detector blocks arranged in a ring of 390 mm diameter with 60 mm axial FOV. Each detector block was composed of 4 × 4 matrix of detector modules, each of which consisted of a 4 × 4 array LYSO coupled to a 4 × 4 GAPD array. The PET gantry was shielded with gold-plated conductive fabric tapes. The charge signals of PET detector transferred via 4 m long flat cables were fed into the position decoder circuits (PDCs) and then transferred to FPGA-embedded DAQ modules. The PDCs and DAQ modules were enclosed in an aluminum box and located at the rear of the MR bore inside MRI room. 3-T human MRIs of two different vendors were used to evaluate the MR compatibility of developed PET insert. No significant changes of the PET performance and the homogeneity of MR images caused by the non-compatibility of PET-MRI were observed with the 2 different MRIs. The signal intensities of MR images were slightly degraded (<3.6%) with the both MRI systems. The difference between independently and simultaneously acquired PET images of brain phantom was negligibly small (<4.3%). High quality simultaneous brain PET and MRI of 3 normal volunteers were successfully acquired. Experimental results indicate that the high performance compact and lightweight PET insert for hybrid PET/MRI, which could be utilized with the MRI from various manufactures, can be developed using GAPD arrays and charge signal transmission method proposed in this study.

Assessing the MR compatibility of dental retainer wires at 7 Tesla

To determine the MR compatibility of common dental retainer wires at 7 Tesla in terms of potential RF heating and magnetic susceptibility effects. Electromagnetic simulations and experimental results were compared for dental retainer wires placed in tissue-mimicking phantoms. Simulations were then performed for a human model with wire in place. Finally, image quality was assessed for different scanning protocols and wires. Simulations and experimental data in phantoms agreed well, with the length of the wire correlating to maximum heating in phantoms being approximately 47 mm. Even in this case, no substantial heating occurs when scanning within the specific absorption rate (SAR) guidelines for the head. Image distortions from the most ferromagnetic dental wire were not significant for any brain region. Dental retainer wires appear to be MR compatible at 7 Tesla.

Sci-Fri PM: Delivery - 09: Real-Time MR Tumour Tracking Using a Linac-MR System

Objective To investigate the feasibility of direct (i.e. no surrogates), real-time MR tumor tracking using a linac-MR system. Methods This study is primarily focused on lung tumour tracking due to its extreme range of motion. Lung tumour motions were simulated using two in-house built MR compatible motion phantoms. Algorithms have been developed to auto-contour and to track tumor motion in real-time from 4D MR images. A real-time MLC controlling system was developed for tumor tracking. Processing time delays between the completion of MR imaging and the beginning of beam delivery in our tracking system have also been investigated. Results and Discussion Both phantoms showed successful MR compatibility, and they were scanned with 3T MRI while in motion. The auto-contouring algorithm showed promising tumor contouring capabilities. The algorithm was applied to (1) In-vivo MR images of lung tumor; (2) MR images of healthy lung (coronal, sagittal, transverse) with 10 different tumor shapes inserted; and (3) MR images of motion phantoms. By combining the auto-contouring algorithm with an MLC controlling system, real-time MR tumor tracking was demonstrated in principle. Processing time delays of 50∼400 ms were determined for our tracking system. Conclusions A potential real-time MR tumor tracking using a linac-MR system was studied. We have demonstrated the feasibility of (1) real-time tumor auto-contouring; (2) real-time beam conformity to the Beam's Eye view shape of the tumor using 4D MR imaging. Processing time delays were investigated.

Performance Evaluation of an MRI-Compatible Pre-Clinical PET System Using Long Optical Fibers

We have designed and constructed an MR-compatible PET system for fully simultaneous PET/MR studies of small animals. The scanner uses long optical fibers to distance the magnetic field sensitive PET PMTs from the high magnetic field at the center of an MR scanner. It is a single slice system with an inner diameter of 7 cm. A full evaluation of the performance of the PET system and the results of an MR compatibility assessment in a Philips Achieva whole body 3 T MRI scanner are presented. The reconstructed resolution of the PET scanner is 1.5 mm at the center falling to 2.5 mm at the edge of the field of view; the system sensitivity is 0.95%; the count rate is linear up to an activity of 6 MBq (~4 kcps) and the scatter fraction is 42% which can be reduced to 26% using MR-compatible gamma shields. Simultaneous PET/MR images of phantoms and a mouse have been acquired. The system is highly MR compatible, as demonstrated here, showing no degradation in performance of either the MR or PET system in the presence of the other modality. The system will be used to demonstrate novel pre-clinical applications of simultaneous PET/MR.

SU‐GG‐J‐38: Real‐Time MR Tumor Tracking Using a Linac‐MR System

Purpose: To investigate the feasibility of direct (i.e. no surrogates), real‐time MR tumor tracking using a linac‐MR system. Method and Materials: This study is focused primarily on investigating the requirements for lung tumor tracking due to its extreme range of motion. Two MR compatible motion phantoms have been built to simulate tumor motions. Various motion patterns of lung tumors were generated and used to drive our phantoms. Algorithms have been developed to auto‐contour and to track tumor motion in real‐time from 4D MR phantom images and simulated tumor images. Tumor tracking is achieved by controlling an MLC in real‐time. Processing time delays between the completion of MR imaging and the beginning of beam delivery in our tracking system have also been investigated. Results: Both phantoms showed successful MR compatibility. The phantoms were scanned with 3T MRI while in motion. The auto‐contouring algorithm showed successful tumor contouring capabilities. The algorithm was applied to (1) In‐vivo MR images of lung tumor; (2) MR images of healthy lung (coronal, sagittal, transverse) with 10 different tumor shapes inserted; and (3) MR images of motion phantoms. By combining the auto‐contouring algorithm with an MLC controlling system, real‐time MR tumor tracking was demonstrated in principle. Processing time delays of 50∼400 ms were determined for our tracking system. Conclusion: A potential real‐time MR tumor tracking system was studied. Lung tumor motions were simulated using two in‐house built motion phantoms with various tumor motion patterns. Our auto‐contouring algorithm was integrated within a MLC controlling system, and its real‐time tracking capability and processing time delays was evaluated. We have demonstrated the feasibility of (1) real‐time tumor auto‐contouring; (2) real‐time beam conformity to the Beam's Eye view shape of the tumor using 4D MR imaging.

Development of MR Compatible Manipulandum for Finger Movements

Magnetically compatible robots are required as haptic interfaces for neuroscience studies. The purpose of this paper is to develop an MR compatible robot using ultrasonic motor, which is able to work within an MRI/fMRI scanner and to acquire MRI images continuously during finger motions. In addition, the force field control method of ultrasonic motor is discussed. The characteristic of voltage phase and motor torque is investigated experimentally, and these results are used for the force field control of ultrasonic motors. The position dependent force fields and velocity dependent force fields are evaluated with a force sensor. Also, MR compatibility is verified experimentally. Finally, some brain activities are investigated with fMRI during finger movements.

MG312 MRI対応手・腕運動用マニピュランダムの開発(MG31 医療福祉ロボット,あたり前のことを知る)

Magnetically compatible robots are required as haptic interfaces for enhancing neuroscience studies. The purpose of this paper is to develop an MR compatible robot using ultrasonic motors, which is able to work within an MRI/fMRI scanner and to acquire MRI images continuously during fingers and Arm motions. We propose a Manipulandum for the fingers and Arm movements. Our Manipulandum has one feature. Finally, MR compatibility of our Manipulandum is verified experimentally.

NeuroArm: an MR compatible robot for microsurgery

Introduction: Technologies such as microscopy have pushed surgeons to the limits of their dexterity and endurance. Robotic advances such as tremor filtration and motion scaling permit maximal use of magnification, and enable precise, tremor-free tool manipulation during microsurgery. To our knowledge, however, no such device exists for image-guided, ambidextrous microneurosurgery, prompting us to develop our own system. Methods: We approached a company with exceptional experience in space robotics to design and construct an appropriate system. A systematic and structured approached was followed for the design of neuroArm. Engineers studied the operating room environment with the assistance of surgeons and nursing staff. A preliminary design was developed, and subsequently evaluated by surgical staff. Selected materials were tested at 3.0 T to ensure MR compatibility and performance characteristics of the robot actuators and encoders were evaluated. Results: We have developed an MR compatible ambidextrous robot capable of both microneurosurgery and stereotaxy. The design is based on a SCARA configuration and has 8-DOF (including tool actuation). The end-effector is designed to interface with standard neurosurgical tools and is equipped with a 3-DOF optical force sensor for haptic feedback. Comprehensive testing of materials was conducted in a 3-T magnet to ensure compatibility. Breadboard testing results suggest a tool tip resolution of 30 μm. Discussion: A systematic approach has been applied to the development of a unique and dexterous neurosurgical robot. The system promises to enhance surgical performance, reduce fatigue, and improve surgical outcomes. This seamless integration of robotics with iMRI will further revolutionize neurosurgery.