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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.
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Ablação por Cateter , Imageamento por Ressonância Magnética , Animais , Arritmias Cardíacas , Ablação por Cateter/métodos , Desenho de Equipamento , Átrios do Coração , Imageamento por Ressonância Magnética/métodos , SuínosRESUMO
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.
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Técnicas de Imagem por Elasticidade , Aceleração , Imagem Ecoplanar , Humanos , Imageamento por Ressonância Magnética , Reprodutibilidade dos TestesRESUMO
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.
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Meios de Contraste , Angiografia por Ressonância Magnética , Angiografia Digital , Humanos , Sensibilidade e EspecificidadeRESUMO
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.
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Processamento de Imagem Assistida por Computador/métodos , Imageamento por Ressonância Magnética/métodos , Processamento de Sinais Assistido por Computador , Adolescente , Adulto , Criança , Pré-Escolar , Meios de Contraste , Fibrose Cística/diagnóstico por imagem , Feminino , Humanos , Pulmão/diagnóstico por imagem , Masculino , Imagens de Fantasmas , Isótopos de XenônioRESUMO
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.
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Imagem Ecoplanar/métodos , Técnicas de Imagem por Elasticidade/métodos , Hepatopatias/diagnóstico por imagem , Fígado/diagnóstico por imagem , Adulto , Artefatos , Estudos de Viabilidade , Feminino , Humanos , Processamento de Imagem Assistida por Computador/métodos , Masculino , Pessoa de Meia-Idade , Movimento (Física) , Estudos Prospectivos , Reprodutibilidade dos Testes , Respiração , Adulto JovemRESUMO
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.
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Doenças do Recém-Nascido/diagnóstico por imagem , Imageamento por Ressonância Magnética/instrumentação , Desenho de Equipamento , Feminino , Humanos , Recém-Nascido , Unidades de Terapia Intensiva Neonatal , MasculinoRESUMO
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.
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Eletrocardiografia , Processamento de Imagem Assistida por Computador/métodos , Imageamento por Ressonância Magnética , Adulto , Algoritmos , Artefatos , Técnicas de Imagem de Sincronização Cardíaca , Eletrodos , Voluntários Saudáveis , Humanos , Modelos Lineares , Masculino , Pessoa de Meia-Idade , Modelos Estatísticos , Reprodutibilidade dos TestesRESUMO
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 â¼5 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.
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Artefatos , Braquiterapia/instrumentação , Braquiterapia/métodos , Marcadores Fiduciais , Neoplasias dos Genitais Femininos/radioterapia , Processamento de Imagem Assistida por Computador/métodos , Imageamento Tridimensional/instrumentação , Imageamento Tridimensional/métodos , Imageamento por Ressonância Magnética/instrumentação , Imageamento por Ressonância Magnética/métodos , Metais , Radioterapia Assistida por Computador/instrumentação , Radioterapia Assistida por Computador/métodos , Radioterapia Guiada por Imagem/instrumentação , Radioterapia Guiada por Imagem/métodos , Animais , Galinhas , Gráficos por Computador , Simulação por Computador , Campos Eletromagnéticos , Desenho de Equipamento , Feminino , Aumento da Imagem/instrumentação , Aumento da Imagem/métodos , Imagens de Fantasmas , SoftwareRESUMO
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.
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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.
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Fibrilação Atrial/cirurgia , Técnicas de Imagem de Sincronização Cardíaca/instrumentação , Eletrocardiografia/instrumentação , Imagem por Ressonância Magnética Intervencionista/instrumentação , Cirurgia Assistida por Computador/instrumentação , Idoso , Animais , Fibrilação Atrial/diagnóstico , Procedimentos Cirúrgicos Cardiovasculares/instrumentação , Eletrodos , Desenho de Equipamento , Análise de Falha de Equipamento , Feminino , Humanos , Masculino , Pessoa de Meia-Idade , Reprodutibilidade dos Testes , Sensibilidade e Especificidade , Suínos , Resultado do TratamentoRESUMO
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.
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Imageamento por Ressonância Magnética/instrumentação , Magnetismo/instrumentação , Pletismografia de Impedância/instrumentação , Próteses e Implantes , Tecnologia sem Fio/instrumentação , Animais , Desenho de Equipamento , Análise de Falha de Equipamento , Projetos Piloto , SuínosRESUMO
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.
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Doenças do Recém-Nascido/diagnóstico , Unidades de Terapia Intensiva Neonatal , Imageamento por Ressonância Magnética/métodos , Feminino , Humanos , Recém-Nascido , Imageamento por Ressonância Magnética/instrumentação , MasculinoRESUMO
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.
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Unidades de Terapia Intensiva Neonatal , Imageamento por Ressonância Magnética/instrumentação , Ruído , Acústica , Desenho de EquipamentoRESUMO
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.
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Artefatos , Aumento da Imagem/instrumentação , Interpretação de Imagem Assistida por Computador/instrumentação , Imageamento Tridimensional/instrumentação , Imageamento por Ressonância Magnética/instrumentação , Magnetismo/instrumentação , Humanos , Movimento (Física) , Reprodutibilidade dos Testes , Sensibilidade e EspecificidadeRESUMO
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.
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Terapia com Prótons , Prótons , Terapia com Prótons/métodos , Acústica , Som , Radiação Ionizante , Imagens de Fantasmas , Método de Monte CarloRESUMO
BACKGROUND: Transporting premature infants from a neonatal intensive care unit (NICU) to a radiology department for MRI has medical risks and logistical challenges. OBJECTIVE: To develop a small 1.5-T MRI system for neonatal imaging that can be easily installed in the NICU and to evaluate its performance using a sheep model of human prematurity. MATERIALS AND METHODS: A 1.5-T MRI system designed for orthopedic use was adapted for neonatal imaging. The system was used for MRI examinations of the brain, chest and abdomen in 12 premature lambs during the first hours of life. Spin-echo, fast spin-echo and gradient-echo MR images were evaluated by two pediatric radiologists. RESULTS: All animals remained physiologically stable throughout the imaging sessions. Animals were imaged at two or three time points. Seven brain MRI examinations were performed in seven different animals, 23 chest examinations in 12 animals and 19 abdominal examinations in 11 animals. At each anatomical location, high-quality images demonstrating good spatial resolution, signal-to-noise ratio and tissue contrast were routinely obtained within 30 min using standard clinical protocols. CONCLUSION: Our preliminary experience demonstrates the feasibility and potential of the neonatal MRI system to provide state-of-the-art MRI capabilities within the NICU. Advantages include overall reduced cost and site demands, lower acoustic noise, improved ease of access and reduced medical risk to the neonate.
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Unidades de Terapia Intensiva Neonatal , Terapia Intensiva Neonatal/métodos , Imageamento por Ressonância Magnética/instrumentação , Animais , Desenho de Equipamento , Análise de Falha de Equipamento , Estudos de Viabilidade , Feminino , Humanos , Recém-Nascido , Imageamento por Ressonância Magnética/métodos , Masculino , Projetos Piloto , Reprodutibilidade dos Testes , Sensibilidade e Especificidade , OvinosRESUMO
BACKGROUND: A device for moving the head during MR imaging, called a Weighted Head Accelerator Mechanism (WHAM), rotates the head of a supine subject within programmable rotation limits and acceleration profiles. The WHAM can be used with custom MRI sequences to visualize the deformation and recoil of in vivo brain parenchyma with high temporal resolution, allowing element-wise calculation of strain and shear forces in the brain. Unlike previous devices, the WHAM can be configured to provide a wide range of motion and acceleration profiles. NEW METHOD: The WHAM was calibrated using a high-speed camera on a laboratory bench and in 1.5 Tesla and 3.0 Tesla MRI scanners using gel phantoms and human subjects. The MR imaging studies employed a spatial spin-saturation tagging sub-sequence, followed by serial image acquisition. In these studies, 256 images were acquired with a temporal resolution of 2.56 ms. Deformation of the brain was quantified by following the spatial tags in the images. RESULTS: MR imaging showed that the WHAM drove quantifiable brain motions using g forces less than those typically observed in day-to-day activities, with peak accelerations of â¼250 rad/sec2. COMPARISON WITH EXISTING METHODS: The peak pre-contact accelerations and velocities achieved by the WHAM device in this study are both higher than devices used in previous studies, while also allowing for modification of these factors. CONCLUSIONS: MR imaging performed with the WHAM provides a direct method to visualize and quantify "brain slosh" in response to rotational acceleration. Consequently, this approach might find utility in evaluating strategies to protect the brain from mild traumatic brain injury (mTBI).
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Encéfalo , Imageamento por Ressonância Magnética , Humanos , Imageamento por Ressonância Magnética/métodos , Encéfalo/diagnóstico por imagem , Encéfalo/fisiologia , Cabeça , Aceleração , ReologiaRESUMO
Computational fluid dynamics (CFD) simulations of respiratory airflow have the potential to change the clinical assessment of regional airway function in health and disease, in pulmonary medicine and otolaryngology. For example, in diseases where multiple sites of airway obstruction occur, such as obstructive sleep apnea (OSA), CFD simulations can identify which sites of obstruction contribute most to airway resistance and may therefore be candidate sites for airway surgery. The main barrier to clinical uptake of respiratory CFD to date has been the difficulty in validating CFD results against a clinical gold standard. Invasive instrumentation of the upper airway to measure respiratory airflow velocity or pressure can disrupt the airflow and alter the subject's natural breathing patterns. Therefore, in this study, we instead propose phase contrast (PC) velocimetry magnetic resonance imaging (MRI) of inhaled hyperpolarized 129Xe gas as a non-invasive reference to which airflow velocities calculated via CFD can be compared. To that end, we performed subject-specific CFD simulations in airway models derived from 1H MRI, and using respiratory flowrate measurements acquired synchronously with MRI. Airflow velocity vectors calculated by CFD simulations were then qualitatively and quantitatively compared to velocity maps derived from PC velocimetry MRI of inhaled hyperpolarized 129Xe gas. The results show both techniques produce similar spatial distributions of high velocity regions in the anterior-posterior and foot-head directions, indicating good qualitative agreement. Statistically significant correlations and low Bland-Altman bias between the local velocity values produced by the two techniques indicates quantitative agreement. This preliminary in vivo comparison of respiratory airway CFD and PC MRI of hyperpolarized 129Xe gas demonstrates the feasibility of PC MRI as a technique to validate respiratory CFD and forms the basis for further comprehensive validation studies. This study is therefore a first step in the pathway towards clinical adoption of respiratory CFD.
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Isótopos de Xenônio , Humanos , Hidrodinâmica , Imageamento por Ressonância Magnética , TraqueiaRESUMO
A strategy to increase the robustness of active MR tracking of microcoils in low signal-to-noise ratio conditions was developed and tested. The method employs dephasing magnetic field gradient pulses that are applied orthogonal to the frequency-encoding gradient pulse used in conventional point-source MR tracking. In subsequent acquisitions, the orthogonal dephasing gradient pulse is rotated while maintaining a perpendicular orientation with respect to the frequency-encoding gradient. The effect of the dephasing gradient is to apply a spatially dependent phase shift in directions perpendicular to the frequency-encoding gradient. Since the desired MR signal for robust MR tracking comes from the small volume of nuclear spins near the small detection coil, the desired signal is not dramatically altered by the dephasing gradient. Undesired MR signals arising from larger volumes (e.g., due to coupling with the body coil or surface coils), on the other hand, are dephased and reduced in signal intensity. Since the approach requires no a priori knowledge of the microcoil orientation with respect to the main magnetic field, data from several orthogonal dephasing gradients are acquired and analyzed in real time. One of several selection algorithms is employed to identify the "best" data for use in the coil localization algorithm. This approach was tested in flow phantoms and animal models, with several multiplexing schemes, including the Hadamard and zero-phase reference approaches. It was found to provide improved MR tracking of untuned microcoils. It also dramatically improved MR tracking robustness in low signal-to-noise-ratio conditions and permitted tracking of microcoils that were inductively coupled to the body coil.
Assuntos
Algoritmos , Aorta/anatomia & histologia , Aorta/cirurgia , Cateterismo Cardíaco/métodos , Imagem por Ressonância Magnética Intervencionista/métodos , Imageamento por Ressonância Magnética/métodos , Animais , Aumento da Imagem/métodos , Reprodutibilidade dos Testes , Sensibilidade e Especificidade , SuínosRESUMO
The radiology department at a midwestern U.S. children's hospital has created a scorecard that is presented quarterly to the institutional leadership and is available to all radiology employees on the institutional intranet. The scorecard currently has 33 measures in six areas: clinical services (safety, quality, timeliness); education; research; professionalism, communication, and user satisfaction; finances and administration; and staffing. For each measure, the goal, current value of the measure, interval at which the measure is updated, date of last update, and previous value of the measure are listed. Each measure was reviewed over time to determine those measures for which target goals were met. Results indicate that a visible and transparent department scorecard is one of the more powerful tools available to the radiology leadership to call attention to and improve performance in specific areas. The use of such a scorecard can help develop a departmental culture of quality improvement, focus healthcare providers on specific quality improvement projects, and drive departmental performance.