Intracardiac MR imaging (ICMRI) guiding-sheath with amplified expandable-tip imaging and MR-tracking for navigation and arrythmia ablation monitoring: Swine testing at 1.5 and 3T.

PURPOSE: Develop a deflectable intracardiac MR imaging (ICMRI) guiding-sheath to accelerate imaging during MR-guided electrophysiological (EP) interventions for radiofrequency (500 kHz) ablation (RFA) of arrythmia. Requirements include imaging at three to five times surface-coil SNR in cardiac chambers, vascular insertion, steerable-active-navigation into cardiac chambers, operation with ablation catheters, and safe levels of MR-induced heating. METHODS: ICMRI's 6 mm outer-diameter (OD) metallic-braided shaft had a 2.6 mm OD internal lumen for ablation-catheter insertion. Miniature-Baluns (MBaluns) on ICMRI's 1 m shaft reduced body-coil-induced heating. Distal section was a folded "star"-shaped imaging-coil mounted on an expandable frame, with an integrated miniature low-noise-amplifier overcoming cable losses. A handle-activated movable-shaft expanded imaging-coil to 35 mm OD for imaging within cardiac-chambers. Four MR-tracking micro-coils enabled navigation and motion-compensation, assuming a tetrahedron-shape when expanded. A second handle-lever enabled distal-tip deflection. ICMRI with a protruding deflectable EP catheter were used for MR-tracked navigation and RFA using a dedicated 3D-slicer user-interface. ICMRI was tested at 3T and 1.5T in swine to evaluate (a) heating, (b) cardiac-chamber access, (c) imaging field-of-view and SNR, and (d) intraprocedural RFA lesion monitoring. RESULTS: The 3T and 1.5T imaging SNR demonstrated >400% SNR boost over a 4 × 4 × 4 cm3 FOV in the heart, relative to body and spine arrays. ICMRI with MBaluns met ASTM/IEC heating limits during navigation. Tip-deflection allowed navigating ICMRI and EP catheter into atria and ventricles. Acute-lesion long-inversion-time-T1-weighted 3D-imaging (TWILITE) ablation-monitoring using ICMRI required 5:30 min, half the time needed with surface arrays alone. CONCLUSION: ICMRI assisted EP-catheter navigation to difficult targets and accelerated RFA monitoring.

Fusing acceleration and saturation techniques with wave amplitude labeling of time-shifted zeniths MR elastography.

PURPOSE: To design a new 2D gradient recalled echo MR elastography (MRE) pulse sequence with inflow saturation for measuring liver stiffness in half the breath-hold time compared to standard of care (SC) 2D GRE MRE sequences. METHODS: FASTWALTZ (fusing acceleration and saturation techniques with wave amplitude labeling of time-shifted zeniths) MRE employs an interleaved dual TR strategy with wave amplitude labeling and compressed SENSE undersampling to reduce breath-hold time while incorporating inflow saturation to suppress flow artifacts. The sequence was implemented and compared with SC MRE both in phantoms and in vivo in 5 asymptomatic volunteers. Stiffness values, region of interest size, and breath-hold times were compared between sequences. RESULTS: Stiffness values were comparable between FASTWALTZ and SC MRE for both phantoms and in-vivo data. In volunteers, the group mean stiffness values at 60 Hz and region of interest size were 1.96 ± 0.30 kilopascals and 2279 ± 516 mm2 for SC MRE, and 1.95 ± 0.29 kilopascals and 2061 ± 464 mm2 for FASTWALTZ. Breath-hold duration for FASTWALTZ was 6.3 s compared to 13.3 s for SC MRE. CONCLUSION: FASTWALTZ provides comparable stiffness values in half the breath-hold time compared to SC MRE and may have clinical benefits in patients with limited breath-holding capacity.

MR Angiography Series: Fundamentals of Contrast-enhanced MR Angiography.

The Society for Magnetic Resonance Angiography (SMRA) is a group of researchers and clinicians who are passionate about the benefits of MR angiography (MRA) but understand its challenges. Their mission is to study MRA, continually improve and innovate for the benefit of patients, and most important, educate the medical community so they can take full advantage of the benefits of MRA and overcome its challenges. In support of that mission, the authors have created a series of self-learning modules on behalf of the SMRA to demystify MRA protocols and help the reader perform patient-friendly high-quality MRA on a routine basis in clinical practice. The full digital presentation is available online. ©RSNA, 2021.

Mapping and correcting hyperpolarized magnetization decay with radial keyhole imaging.

PURPOSE: Hyperpolarized (HP) media enable biomedical imaging applications that cannot be achieved with conventional MRI contrast agents. Unfortunately, quantifying HP images is challenging, because relaxation and radio-frequency pulsing generate spatially varying signal decay during acquisition. We demonstrate that, by combining center-out k-space sampling with postacquisition keyhole reconstruction, voxel-by-voxel maps of regional HP magnetization decay can be generated with no additional data collection. THEORY AND METHODS: Digital phantom, HP 129 Xe phantom, and in vivo 129 Xe human (N = 4 healthy; N = 2 with cystic fibrosis) imaging was performed using radial sampling. Datasets were reconstructed using a postacquisition keyhole approach in which 2 temporally resolved images were created and used to generate maps of regional magnetization decay following a simple analytical model. RESULTS: Mean, keyhole-derived decay terms showed excellent agreement with the decay used in simulations (R2 = 0.996) and with global attenuation terms in HP 129 Xe phantom imaging (R2 > 0.97). Mean regional decay from in vivo imaging agreed well with global decay values and displayed spatial heterogeneity that matched expected variations in flip angle and oxygen partial pressure. Moreover, these maps could be used to correct variable signal decay across the image volume. CONCLUSIONS: We have demonstrated that center-out trajectories combined with keyhole reconstruction can be used to map regional HP signal decay and to quantitatively correct images. This approach may be used to improve the accuracy of quantitative measures obtained from hyperpolarized media. Although validated with gaseous HP 129 Xe in this work, this technique can be generalized to any hyperpolarized agent.

Respiratory-triggered spin-echo echo-planar imaging-based mr elastography for evaluating liver stiffness.

BACKGROUND: Magnetic resonance elastography (MRE) has proven to be useful for assessing chronic liver disease. However, MRE images are acquired with breath-holding (BH) to limit respiratory motion artifacts, which may be difficult in some patients. PURPOSE: To implement a respiratory-triggered (RT) spin-echo echo-planar imaging (SE-EPI) MRE technique and to validate its performance through comparison to a BH SE-EPI MRE technique. STUDY TYPE: Prospective feasibility study. SUBJECTS: Twenty-three adult volunteers (18 without and 5 with liver disease). FIELD STRENGTH/SEQUENCES: 1.5 T Philips Ingenia MR scanner; RT and BH SE-EPI MRE sequences. ASSESSMENT: Four axial images were obtained through the middle of the liver with each technique. Liver stiffness measurements (in kPa) were made from elastograms, with 95% confidence maps overlaid, for both MRE sequences. STATISTICAL TESTS: Liver stiffness measurements were compared using the paired t-test (two-sided). Absolute agreement between the two techniques was evaluated using Lin's concordance coefficient (rc ). Bland-Altman analysis was used to assess the mean bias between the techniques and 95% limits of agreement, using BH MRE as the reference standard. RESULTS: There was excellent agreement (rc = 0.98; 95% confidence interval: 0.96-0.99) between RT and BH SE-EPI MRE. Mean (±SD) stiffness values from BH and RT SE-EPI MRE techniques were 2.40 ± 1.15 kPa and 2.37 ± 1.06 kPa, respectively, with no significant difference (P = 0.54) and no significant bias (mean bias of +0.03 kPa; 95% limits of agreement: -0.39 to 0.45 kPa). Measurable regions of interest in the liver were slightly smaller with the RT technique (mean difference of 1.91 cm2 ; P = 0.04). DATA CONCLUSION: RT SE-EPI MRE is feasible and yields comparable results to BH SE-EPI MRE. LEVEL OF EVIDENCE: 1 Technical Efficacy: Stage 2 J. Magn. Reson. Imaging 2019;50:391-396.

Assessment of Gold Nanoparticle-Mediated-Enhanced Hyperthermia Using MR-Guided High-Intensity Focused Ultrasound Ablation Procedure.

High-intensity focused ultrasound (HIFU) has gained increasing popularity as a noninvasive therapeutic procedure to treat solid tumors. However, collateral damage due to the use of high acoustic powers during HIFU procedures remains a challenge. The objective of this study is to assess the utility of using gold nanoparticles (gNPs) during HIFU procedures to locally enhance heating at low powers, thereby reducing the likelihood of collateral damage. Phantoms containing tissue-mimicking material (TMM) and physiologically relevant concentrations (0%, 0.0625%, and 0.125%) of gNPs were fabricated. Sonications at acoustic powers of 10, 15, and 20 W were performed for a duration of 16 s using an MR-HIFU system. Temperature rises and lesion volumes were calculated and compared for phantoms with and without gNPs. For an acoustic power of 10 W, the maximum temperature rise increased by 32% and 43% for gNPs concentrations of 0.0625% and 0.125%, respectively, when compared to the 0% gNPs concentration. For the power of 15 W, a lesion volume of 0, 44.5 ± 7, and 63.4 ± 32 mm3 was calculated for the gNPs concentration of 0%, 0.0625%, and 0.125%, respectively. For a power of 20 W, it was found that the lesion volume doubled and tripled for concentrations of 0.0625% and 0.125% gNPs, respectively, when compared to the concentration of 0% gNPs. We conclude that gNPs have the potential to locally enhance the heating and reduce damage to healthy tissue during tumor ablation using HIFU.

Neonatal imaging using an on-site small footprint MR scanner.

With its soft-tissue definition, multiplanar capabilities and advanced imaging techniques, magnetic resonance imaging (MRI) for neonatal care can provide better understanding of pathology, allowing for improved care and counseling to families. However, MR imaging in neonates is often difficult due to patient instability and the complex support necessary for survival. In our institution, we have installed a small footprint magnet in the neonatal intensive care unit (NICU) to minimize patient risks and provide the ability to perform MR imaging safely in this population. With this system, we have been able to provide more information with regard to central nervous system disorders, abdominal pathology, and pulmonary and airway abnormalities, and have performed postmortem imaging as an alternative or supplement to pathological autopsy. In our experience, an MR scanner situated within the NICU has allowed for safer and more expedited imaging of this vulnerable population.

Initial investigation of a novel noninvasive weight loss therapy using MRI-Guided high intensity focused ultrasound (MR-HIFU) of visceral fat.

PURPOSE: MRI-guided high intensity focused ultrasound (MR-HIFU) allows noninvasive heating of deep tissues. Specifically targeting visceral fat deposits with MR-HIFU could offer an effective therapy for reversing the development of obesity, diabetes, and metabolic syndrome. METHODS: Overweight rats received either MR-HIFU of visceral fat, sham treatment, no treatment, or ex vivo temperature calibration. Conventional MR thermometry methods are not effective in fat tissue. Therefore, the T2 of fat was used to estimate heating in adipose tissue. RESULTS: HIFU treated rats lost 7.5% of their body weight 10 days after HIFU, compared with 1.9% weight loss in sham animals (P = 0.008) and 1.3% weight increase in untreated animals (P = 0.004). Additionally, the abdominal fat volume in treated animals decreased by 8.2 mL 7 days after treatment (P = 0.002). The T2 of fat at 1.5 Tesla increased by 3.3 ms per °C. The fat T2 was 103.3 ms before HIFU, but increased to 128.7 ms (P = 0.0005) after HIFU at 70 watts for 16 s and to 131.9 ms (P = 0.0005) after HIFU at 100 watts for 16 s. CONCLUSION: These experiments demonstrate that MR-HIFU of visceral fat could provide a safe, effective, and noninvasive weight loss therapy for combating obesity and the subsequent medical complications. Magn Reson Med 76:282-289, 2016. © 2015 Wiley Periodicals, Inc.

Gradient-induced voltages on 12-lead ECGs during high duty-cycle MRI sequences and a method for their removal considering linear and concomitant gradient terms.

PURPOSE: To restore 12-lead electrocardiographic (ECG) signal fidelity inside MRI by removing magnetic field gradient-induced voltages during high gradient duty cycle sequences. THEORY AND METHODS: A theoretical equation was derived to provide first- and second-order electrical fields induced at individual ECG electrodes as a function of gradient fields. Experiments were performed at 3T on healthy volunteers using a customized acquisition system that captured the full amplitude and frequency response of ECGs, or a commercial recording system. The 19 equation coefficients were derived via linear regression of data from accelerated sequences and were used to compute induced voltages in real-time during full resolution sequences to remove ECG artifacts. Restored traces were evaluated relative to ones acquired without imaging. RESULTS: Measured induced voltages were 0.7 V peak-to-peak during balanced steady state free precession (bSSFP) with the heart at the isocenter. Applying the equation during gradient echo sequencing, three-dimensional fast spin echo, and multislice bSSFP imaging restored nonsaturated traces and second-order concomitant terms showed larger contributions in electrodes further from the magnet isocenter. Equation coefficients are evaluated with high repeatability (ρ = 0.996) and are dependent on subject, sequence, and slice orientation. CONCLUSION: Close agreement between theoretical and measured gradient-induced voltages allowed for real-time removal. Prospective estimation of sequence periods in which large induced voltages occur may allow hardware removal of these signals.

Wavelet-space correlation imaging for high-speed MRI without motion monitoring or data segmentation.

PURPOSE: This study aims to (i) develop a new high-speed MRI approach by implementing correlation imaging in wavelet-space, and (ii) demonstrate the ability of wavelet-space correlation imaging to image human anatomy with involuntary or physiological motion. METHODS: Correlation imaging is a high-speed MRI framework in which image reconstruction relies on quantification of data correlation. The presented work integrates correlation imaging with a wavelet transform technique developed originally in the field of signal and image processing. This provides a new high-speed MRI approach to motion-free data collection without motion monitoring or data segmentation. The new approach, called "wavelet-space correlation imaging", is investigated in brain imaging with involuntary motion and chest imaging with free-breathing. RESULTS: Wavelet-space correlation imaging can exceed the speed limit of conventional parallel imaging methods. Using this approach with high acceleration factors (6 for brain MRI, 16 for cardiac MRI, and 8 for lung MRI), motion-free images can be generated in static brain MRI with involuntary motion and nonsegmented dynamic cardiac/lung MRI with free-breathing. CONCLUSION: Wavelet-space correlation imaging enables high-speed MRI in the presence of involuntary motion or physiological dynamics without motion monitoring or data segmentation.

Real-time active MR-tracking of metallic stylets in MR-guided radiation therapy.

PURPOSE: To develop an active MR-tracking system to guide placement of metallic devices for radiation therapy. METHODS: An actively tracked metallic stylet for brachytherapy was constructed by adding printed-circuit micro-coils to a commercial stylet. The coil design was optimized by electromagnetic simulation, and has a radio-frequency lobe pattern extending ∼5 mm beyond the strong B0 inhomogeneity region near the metal surface. An MR-tracking sequence with phase-field dithering was used to overcome residual effects of B0 and B1 inhomogeneities caused by the metal, as well as from inductive coupling to surrounding metallic stylets. The tracking system was integrated with a graphical workstation for real-time visualization. The 3 Tesla MRI catheter-insertion procedures were tested in phantoms and ex vivo animal tissue, and then performed in three patients during interstitial brachytherapy. RESULTS: The tracking system provided high-resolution (0.6 × 0.6 × 0.6 mm(3) ) and rapid (16 to 40 frames per second, with three to one phase-field dithering directions) catheter localization in phantoms, animals, and three gynecologic cancer patients. CONCLUSION: This is the first demonstration of active tracking of the shaft of metallic stylet in MR-guided brachytherapy. It holds the promise of assisting physicians to achieve better targeting and improving outcomes in interstitial brachytherapy.

A novel acoustically quiet coil for neonatal MRI system.

MRI acoustic exposure has the potential to elicit physiological distress and impact development in preterm and term infants. To mitigate this risk, a novel acoustically quiet coil was developed to reduce the sound pressure level experienced by neonates during MR procedures. The new coil has a conventional high-pass birdcage RF design, but is built on a framework of sound abating material. We evaluated the acoustic and MR imaging performance of the quiet coil and a conventional body coil on two small footprint NICU MRI systems. Sound pressure level and frequency response measurements were made for six standard clinical MR imaging protocols. The average sound pressure level, reported for all six imaging pulse sequences, was 82.2 dBA for the acoustically quiet coil, and 91.1 dBA for the conventional body coil. The sound pressure level values measured for the acoustically quiet coil were consistently lower, 9 dBA (range 6-10 dBA) quieter on average. The acoustic frequency response of the two coils showed a similar harmonic profile for all imaging sequences. However, the amplitude was lower for the quiet coil, by as much as 20 dBA.

Respiration based steering for high intensity focused ultrasound liver ablation.

PURPOSE: Respiratory motion makes hepatic ablation using high intensity focused ultrasound (HIFO) challenging. Previous HIFU liver treatment had required apnea induced during general anesthesia. We describe and test a system that allows treatment of the liver in the presence of breathing motion. METHODS: Mapping a signal from an external respiratory bellow to treatment locations within the liver allows the ultrasound transducer to be steered in real time to the target location. Using a moving phantom, three metrics were used to compare static, steered, and unsteered sonications: the area of sonications once a temperature rise of 15°C was achieved, the energy deposition required to reach that temperature, and the average rate of temperature rise during the first 10 s of sonication. Steered HIFU in vivo ablations of the porcine liver were also performed and compared to breath-hold ablations. RESULTS: For the last phantom metric, all groups were found to be statistically significantly different (P ≤ 0.003). However, in the other two metrics, the static and unsteered sonications were not statistically different (P > 0.9999). Steered in vivo HIFU ablations were not statistically significantly different from ablations during breath-holding. CONCLUSIONS: A system for performing HIFU steering during ablation of the liver with breathing motion is presented and shown to achieve results equivalent to ablation performed with breath-holding.

A 1.5T MRI-conditional 12-lead electrocardiogram for MRI and intra-MR intervention.

PURPOSE: High-fidelity 12-lead electrocardiogram (ECG) is important for physiological monitoring of patients during MR-guided intervention and cardiac MRI. Issues in obtaining noncorrupted ECGs inside MRI include a superimposed magneto-hydro-dynamic voltage, gradient switching-induced voltages, and radiofrequency heating. These problems increase with magnetic field. The aim of this study is to develop and clinically validate a 1.5T MRI-conditional 12-lead ECG system. METHODS: The system was constructed with transmission lines to reduce radiofrequency induction and switching circuits to remove induced voltages. Adaptive filters, trained by 12-lead measurements outside MRI and in two orientations inside MRI, were used to remove the magneto-hydro-dynamic voltage. The system was tested on 10 (one exercising) volunteers and four arrhythmia patients. RESULTS: Switching circuits removed most imaging-induced voltages (residual noise <3% of the R-wave). Magneto-hydro-dynamic voltage removal provided intra-MRI ECGs that varied by <3.8% from those outside the MRI, preserving the true S-wave to T-wave segment. In premature ventricular contraction (PVC) patients, clean ECGs separated premature ventricular contraction and sinus rhythm beats. Measured heating was <1.5°C. The system reliably acquired multiphase (steady-state free precession) wall-motion-cine and phase-contrast-cine scans, including subjects in whom 4-lead gating failed. The system required a minimum repetition time of 4 ms to allow robust ECG processing. CONCLUSION: High-fidelity intra-MRI 12-lead ECG is possible.

Voltage-based device tracking in a 1.5 Tesla MRI during imaging: initial validation in swine models.

PURPOSE: Voltage-based device-tracking (VDT) systems are commonly used for tracking invasive devices in electrophysiological cardiac-arrhythmia therapy. During electrophysiological procedures, electro-anatomic mapping workstations provide guidance by integrating VDT location and intracardiac electrocardiogram information with X-ray, computerized tomography, ultrasound, and MR images. MR assists navigation, mapping, and radiofrequency ablation. Multimodality interventions require multiple patient transfers between an MRI and the X-ray/ultrasound electrophysiological suite, increasing the likelihood of patient-motion and image misregistration. An MRI-compatible VDT system may increase efficiency, as there is currently no single method to track devices both inside and outside the MRI scanner. METHODS: An MRI-compatible VDT system was constructed by modifying a commercial system. Hardware was added to reduce MRI gradient-ramp and radiofrequency unblanking pulse interference. VDT patches and cables were modified to reduce heating. Five swine cardiac VDT electro-anatomic mapping interventions were performed, navigating inside and thereafter outside the MRI. RESULTS: Three-catheter VDT interventions were performed at >12 frames per second both inside and outside the MRI scanner with <3 mm error. Catheters were followed on VDT- and MRI-derived maps. Simultaneous VDT and imaging was possible in repetition time >32 ms sequences with <0.5 mm errors, and <5% MRI signal-to-noise ratio (SNR) loss. At shorter repetition times, only intracardiac electrocardiogram was reliable. Radiofrequency heating was <1.5°C. CONCLUSION: An MRI-compatible VDT system is feasible.

MRI in the neonatal ICU: initial experience using a small-footprint 1.5-T system.

OBJECTIVE: The objective of our study was to develop a small 1.5-T MRI system for neonatal imaging that can be installed in the neonatal ICU (NICU) and to evaluate its performance in 15 neonates. SUBJECTS AND METHODS: A 1.5-T MR system designed for orthopedic use was adapted for neonatal imaging. Modifications included raising and leveling the magnet, construction of a patient table, and integration of imaging electronics from a high-performance adult-sized scanner. The system was used to perform MR examinations of the brain, abdomen, and chest in 15 medically stable neonates using standard clinical protocols. The scanning time was limited to 60 minutes. The MR examinations were performed without administering sedation to the patients. ECG, heart rate, oxygen saturation, and temperature were monitored continuously throughout the examination. The images were evaluated by two pediatric radiologists for overall study quality, motion artifact, spatial resolution, signal-to-noise ratio, and contrast. RESULTS: All 15 neonates were successfully imaged without sedation. No adverse MRI-related events were noted. In total, 19 brain and seven abdominal examinations were performed. Six chest and two cardiac examinations were also obtained. Gross (versus physiologic) subject motion proved to be the most influential factor in determining overall study and image quality. High-quality diagnostic images were obtained at each anatomic location. CONCLUSION: The customized neonatal MRI system provides state-of-the-art MRI capabilities in the NICU.

New techniques for determining the longitudinal effects of local hemodynamics on the intima-media thickness in arteriovenous fistulae in an animal model.

Remodeling in the arteriovenous fistulas (AVFs) is believed to be a hemodynamic-driven process, which results in extreme changes in the diameter and intima-media thickening (IMT) of vessels over time. This study aims to describe the successful development of techniques that enabled correlation of changes in local and longitudinal wall shear stress (WSS) with the temporal variations of the diameter and IMT in the venous segment of AVFs. An AVF was created between the femoral artery and vein of a 50-kg pig. We have previously shown the successful use of CT-scan and ultrasound techniques for anatomical and flow measurements in AVFs, respectively. In this study, we developed new techniques involving markers (both in vivo and ex vivo), casting (ex vivo), and micro-MRI (ex vivo; 7 Tesla). A radiopaque marker (ROM) was sutured to the AVF at the day of surgery, which was visible in the CT-scan images, micro-MRI, and histology sections. Therefore, ROM served as a fixed local reference for both in vivo and ex vivo states of AVFs. Immediately after sacrificing the pig, a procedure was developed to create a cast from the AVF and thus, maintaining the in vivo state of the AVF during the histology process. Then, micro-MRI and histology techniques were conducted on the AVF to measure IMT in the vein. Along the ROM, the local changes in WSS levels for two cross-sections were tracked at 2D (D: days) and 28D post surgery. WSS levels reduced from 2D to 28D for both cross-sections. Also, the recirculation zones, which formed at 2D for both sections, became smaller in size at 28D. These hemodynamic changes were then mapped onto the corresponding IMT measurements from histology and micro-MRI. It was observed that the recirculation zones at 2D and 28D corresponded to the largest IMT in the two sections. In summary, the new methodologies allowed us to define a fixed local reference at all time points in the AVF, which enabled accurate tracking of local changes in hemodynamics (WSS), configuration (diameter), and structure (IMT) of the venous segment over time. This also empowered study of the interactions between these parameters, which could improve our understanding about the hemodynamic-driven remodeling in AVFs. From a clinical point of view, this information could be translated into local and early therapeutic interventions for dialysis patients.

Characterization of acoustic noise in a neonatal intensive care unit MRI system.

BACKGROUND: To eliminate the medical risks and logistical challenges of transporting infants from the neonatal intensive care unit (NICU) to the radiology department for magnetic resonance imaging, a small-footprint 1.5-T MRI scanner has been developed for neonatal imaging within the NICU. MRI is known to be noisy, and exposure to excessive acoustic noise has the potential to elicit physiological distress and impact development in the term and preterm infant. OBJECTIVE: To measure and compare the acoustic noise properties of the NICU MRI system against those of a conventional 1.5-T MRI system. MATERIALS AND METHODS: We performed sound pressure level measurements in the NICU MRI scanner and in a conventional adult-size whole-body 1.5-T MRI system. Sound pressure level measurements were made for six standard clinical MR imaging protocols. RESULTS: The average sound pressure level value, reported in unweighted (dB) and A-weighted (dBA) decibels for all six imaging pulse sequences, was 73.8 dB and 88 dBA for the NICU scanner, and 87 dB and 98.4 dBA for the conventional MRI scanner. The sound pressure level values measured on the NICU scanner for each of the six MR imaging pulse sequences were consistently and significantly (P = 0.03) lower, with an average difference of 14.2 dB (range 10-21 dB) and 11 dBA (range 5-18 dBA). The sound pressure level frequency response of the two MR systems showed a similar harmonic structure above 200 Hz for all imaging sequences. The amplitude, however, was appreciably lower for the NICU scanner, by as much as 30 dB, for frequencies below 200 Hz. CONCLUSION: The NICU MRI system is quieter than conventional MRI scanners, improving safety for the neonate and facilitating siting of the unit within the NICU.

Prospective motion correction using tracking coils.

Intracavity imaging coils provide higher signal-to-noise than surface coils and have the potential to provide higher spatial resolution in shorter acquisition times. However, images from these coils suffer from physiologically induced motion artifacts, as both the anatomy and the coils move during image acquisition. We developed prospective motion-correction techniques for intracavity imaging using an array of tracking coils. The system had <50 ms latency between tracking and imaging, so that the images from the intracavity coil were acquired in a frame of reference defined by the tracking array rather than by the system's gradient coils. Two-dimensional gradient-recalled and three-dimensional electrocardiogram-gated inversion-recovery-fast-gradient-echo sequences were tested with prospective motion correction using ex vivo hearts placed on a moving platform simulating both respiratory and cardiac motion. Human abdominal tests were subsequently conducted. The tracking array provided a positional accuracy of 0.7 ± 0.5 mm, 0.6 ± 0.4 mm, and 0.1 ± 0.1 mm along the X, Y, and Z directions at a rate of 20 frames-per-second. The ex vivo and human experiments showed significant image quality improvements for both in-plane and through-plane motion correction, which although not performed in intracavity imaging, demonstrates the feasibility of implementing such a motion-correction system in a future design of combined tracking and intracavity coil.

Reconstruction of thermoacoustic emission sources induced by proton irradiation using numerical time reversal.

Objective.Mapping of dose delivery in proton beam therapy can potentially be performed by analyzing thermoacoustic emissions measured by ultrasound arrays. Here, a method is derived and demonstrated for spatial mapping of thermoacoustic sources using numerical time reversal, simulating re-transmission of measured emissions into the medium.Approach.Spatial distributions of thermoacoustic emission sources are shown to be approximated by the analytic-signal form of the time-reversed acoustic field, evaluated at the time of the initial proton pulse. Given calibration of the array sensitivity and knowledge of tissue properties, this approach approximately reconstructs the acoustic source amplitude, equal to the product of the time derivative of the radiation dose rate, mass density, and Grüneisen parameter. This approach was implemented using two models for acoustic fields of the array elements, one modeling elements as line sources and the other as rectangular radiators. Thermoacoustic source reconstructions employed previously reported measurements of emissions from proton energy deposition in tissue-mimicking phantoms. For a phantom incorporating a bone layer, reconstructions accounted for the higher sound speed in bone. Dependence of reconstruction quality on array aperture size and signal-to-noise ratio was consistent with previous acoustic simulation studies.Main results.Thermoacoustic source distributions were successfully reconstructed from acoustic emissions measured by a linear ultrasound array. Spatial resolution of reconstructions was significantly improved in the azimuthal (array) direction by incorporation of array element diffraction. Source localization agreed well with Monte Carlo simulations of energy deposition, and was improved by incorporating effects of inhomogeneous sound speed.Significance.The presented numerical time reversal approach reconstructs thermoacoustic sources from proton beam radiation, based on straightforward processing of acoustic emissions measured by ultrasound arrays. This approach may be useful for ranging and dosimetry of clinical proton beams, if acoustic emissions of sufficient amplitude and bandwidth can be generated by therapeutic proton sources.