Portable magnetic resonance imaging of patients indoors, outdoors and at home.

Mobile medical imaging devices are invaluable for clinical diagnostic purposes both in and outside healthcare institutions. Among the various imaging modalities, only a few are readily portable. Magnetic resonance imaging (MRI), the gold standard for numerous healthcare conditions, does not traditionally belong to this group. Recently, low-field MRI technology companies have demonstrated the first decisive steps towards portability within medical facilities and vehicles. However, these scanners' weight and dimensions are incompatible with more demanding use cases such as in remote and developing regions, sports facilities and events, medical and military camps, or home healthcare. Here we present in vivo images taken with a light, small footprint, low-field extremity MRI scanner outside the controlled environment provided by medical facilities. To demonstrate the true portability of the system and benchmark its performance in various relevant scenarios, we have acquired images of a volunteer's knee in: (i) an MRI physics laboratory; (ii) an office room; (iii) outside a campus building, connected to a nearby power outlet; (iv) in open air, powered from a small fuel-based generator; and (v) at the volunteer's home. All images have been acquired within clinically viable times, and signal-to-noise ratios and tissue contrast suffice for 2D and 3D reconstructions with diagnostic value. Furthermore, the volunteer carries a fixation metallic implant screwed to the femur, which leads to strong artifacts in standard clinical systems but appears sharp in our low-field acquisitions. Altogether, this work opens a path towards highly accessible MRI under circumstances previously unrealistic.

Prepolarized MRI of hard tissues and solid-state matter.

Prepolarized MRI (PMRI) is a long-established technique conceived to counteract the loss in signal-to-noise ratio (SNR) inherent to low-field MRI systems. When it comes to hard biological tissues and solid-state matter, PMRI is severely restricted by their ultra-short characteristic relaxation times. Here we demonstrate that efficient hard-tissue prepolarization is within reach with a special-purpose 0.26 T scanner designed for ex vivo dental MRI and equipped with suitable high-power electronics. We have characterized the performance of a 0.5 T prepolarizer module, which can be switched on and off in 200 µs. To this end, we have used resin, dental and bone samples, all with T1 times of the order of 20 ms at our field strength. The measured SNR enhancement is in good agreement with a simple theoretical model, and deviations in extreme regimes can be attributed to mechanical vibrations due to the magnetic interaction between the prepolarization and main magnets.

Magneto-stimulation limits in medical imaging applications with rapid field dynamics.

Objective.The goal of this work is to extend previous peripheral nerve stimulation (PNS) studies to scenarios relevant to magnetic particle imaging (MPI) and low-field magnetic resonance imaging (MRI), where field dynamics can evolve at kilo-hertz frequencies.Approach.We have constructed an apparatus for PNS threshold determination on a subject's limb, capable of narrow and broad-band magnetic stimulation with pulse characteristic times down to 40µs.Main result.From a first set of measurements on 51 volunteers, we conclude that the PNS dependence on pulse frequency/rise-time is compatible with traditional stimulation models where nervous responses are characterized by a rheobase and a chronaxie. Additionally, we have extended pulse length studies to these fast timescales and confirm thresholds increase significantly as trains transition from tens to a few pulses. We also look at the influence of field spatial distribution on PNS effects, and find that thresholds are higher in an approximately linearly inhomogeneous field (relevant to MRI) than in a rather homogeneous distribution (as in MPI).Significance.PNS constrains the clinical performance of MRI and MPI systems. Extensive magneto-stimulation studies have been carried out recently in the field of MPI, where typical operation frequencies range from single to tens of kilo-hertz. However, PNS literature is scarce for MRI in this fast regime, relevant to small (low inductance) dedicated MRI setups, and where the resonant character of MPI coils prevents studies of broad-band stimulation pulses. This work advances in this direction.

Simultaneous imaging of hard and soft biological tissues in a low-field dental MRI scanner.

Magnetic Resonance Imaging (MRI) of hard biological tissues is challenging due to the fleeting lifetime and low strength of their response to resonant stimuli, especially at low magnetic fields. Consequently, the impact of MRI on some medical applications, such as dentistry, continues to be limited. Here, we present three-dimensional reconstructions of ex-vivo human teeth, as well as a rabbit head and part of a cow femur, all obtained at a field strength of 260 mT. These images are the first featuring soft and hard tissues simultaneously at sub-Tesla fields, and they have been acquired in a home-made, special-purpose, pre-medical MRI scanner designed with the goal of demonstrating dental imaging at low field settings. We encode spatial information with two pulse sequences: Pointwise-Encoding Time reduction with Radial Acquisition and a new sequence we have called Double Radial Non-Stop Spin Echo, which we find to perform better than the former. For image reconstruction we employ Algebraic Reconstruction Techniques (ART) as well as standard Fourier methods. An analysis of the resulting images shows that ART reconstructions exhibit a higher signal-to-noise ratio with a more homogeneous noise distribution.

Assessment of intracoronary stent location and extension in intravascular ultrasound sequences.

PURPOSE: An intraluminal coronary stent is a metal scaffold deployed in a stenotic artery during percutaneous coronary intervention (PCI). In order to have an effective deployment, a stent should be optimally placed with regard to anatomical structures such as bifurcations and stenoses. Intravascular ultrasound (IVUS) is a catheter-based imaging technique generally used for PCI guiding and assessing the correct placement of the stent. A novel approach that automatically detects the boundaries and the position of the stent along the IVUS pullback is presented. Such a technique aims at optimizing the stent deployment. METHODS: The method requires the identification of the stable frames of the sequence and the reliable detection of stent struts. Using these data, a measure of likelihood for a frame to contain a stent is computed. Then, a robust binary representation of the presence of the stent in the pullback is obtained applying an iterative and multiscale quantization of the signal to symbols using the Symbolic Aggregate approXimation algorithm. RESULTS: The technique was extensively validated on a set of 103 IVUS of sequences of in vivo coronary arteries containing metallic and bioabsorbable stents acquired through an international multicentric collaboration across five clinical centers. The method was able to detect the stent position with an overall F-measure of 86.4%, a Jaccard index score of 75% and a mean distance of 2.5 mm from manually annotated stent boundaries, and in bioabsorbable stents with an overall F-measure of 88.6%, a Jaccard score of 77.7 and a mean distance of 1.5 mm from manually annotated stent boundaries. Additionally, a map indicating the distance between the lumen and the stent along the pullback is created in order to show the angular sectors of the sequence in which the malapposition is present. CONCLUSIONS: Results obtained comparing the automatic results vs the manual annotation of two observers shows that the method approaches the interobserver variability. Similar performances are obtained on both metallic and bioabsorbable stents, showing the flexibility and robustness of the method.

Computer-aided detection of intracoronary stent in intravascular ultrasound sequences.

PURPOSE: An intraluminal coronary stent is a metal mesh tube deployed in a stenotic artery during percutaneous coronary intervention (PCI), in order to prevent acute vessel occlusion. The identification of struts location and the definition of the stent shape is relevant for PCI planning and for patient follow-up. The authors present a fully automatic framework for computer-aided detection (CAD) of intracoronary stents in intravascular ultrasound (IVUS) image sequences. The CAD system is able to detect stent struts and estimate the stent shape. METHODS: The proposed CAD uses machine learning to provide a comprehensive interpretation of the local structure of the vessel by means of semantic classification. The output of the classification stage is then used to detect struts and to estimate the stent shape. The proposed approach is validated using a multicentric data-set of 1,015 images from 107 IVUS sequences containing both metallic and bioabsorbable stents. RESULTS: The method was able to detect struts in both metallic stents with an overall F-measure of 77.7% and a mean distance of 0.15 mm from manually annotated struts, and in bioabsorbable stents with an overall F-measure of 77.4% and a mean distance of 0.09 mm from manually annotated struts. CONCLUSIONS: The results are close to the interobserver variability and suggest that the system has the potential of being used as a method for aiding percutaneous interventions.

Bioresorbable everolimus-eluting vascular scaffold for the treatment of chronic total occlusions: CTO-ABSORB pilot study.

AIMS: We sought to assess the safety and performance of the Absorb everolimus-eluting bioresorbable vascular scaffold (BVS) in percutaneous chronic total occlusion (CTO) revascularisation guided by intracoronary imaging. The feasibility of using the BVS in CTO lesions is unknown. METHODS AND RESULTS: Thirty-five consecutive true CTO lesions (EuroCTO Club definition) were included in this prospective registry. After mandatory predilatation and IVUS analysis, all target lesions were treated with BVS and no other stents were deployed. Optical coherence tomography (OCT) was performed after BVS implantation. Multislice computed tomography (MSCT) was performed at baseline and at six to eight months. The mean age was 60.7±9.7 years; 80% were male; 20% were diabetic; 37% had a previous PCI. The most frequently treated vessel was the RCA (46%). According to the Japanese-CTO (J-CTO) complexity score, most lesions were classified as intermediate (49%) or difficult-very difficult (26%); 34% were moderate-severely calcified. Most cases (86%) were treated with an anterograde strategy, 60% by radial or biradial approach. In 71% a cutting balloon was used. The total scaffold length implanted per lesion was 52.5±22.9 mm. All scaffolds were successfully delivered and deployed. Post-dilatation was undertaken in 63%. By OCT, final minimum scaffold area and lumen stenosis were 7.1±1.5 mm2 and 11.7±6.6%, without areas of significant strut malapposition. At complete six-month follow-up, no major adverse events were observed. MSCT identified two cases of scaffold reocclusion. CONCLUSIONS: BVS for CTO recanalisation demonstrates excellent feasibility and safety as well as midterm efficacy. Appropriate lesion preparation is key to aiding adequate expansion of these scaffolds in this setting.

Relation between plaque type, plaque thickness, blood shear stress, and plaque stress in coronary arteries assessed by X-ray angiography and intravascular ultrasound.

PURPOSE: Atheromatic plaque progression is affected, among others phenomena, by biomechanical, biochemical, and physiological factors. In this paper, the authors introduce a novel framework able to provide both morphological (vessel radius, plaque thickness, and type) and biomechanical (wall shear stress and Von Mises stress) indices of coronary arteries. METHODS: First, the approach reconstructs the three-dimensional morphology of the vessel from intravascular ultrasound (IVUS) and Angiographic sequences, requiring minimal user interaction. Then, a computational pipeline allows to automatically assess fluid-dynamic and mechanical indices. Ten coronary arteries are analyzed illustrating the capabilities of the tool and confirming previous technical and clinical observations. RESULTS: The relations between the arterial indices obtained by IVUS measurement and simulations have been quantitatively analyzed along the whole surface of the artery, extending the analysis of the coronary arteries shown in previous state of the art studies. Additionally, for the first time in the literature, the framework allows the computation of the membrane stresses using a simplified mechanical model of the arterial wall. CONCLUSIONS: Circumferentially (within a given frame), statistical analysis shows an inverse relation between the wall shear stress and the plaque thickness. At the global level (comparing a frame within the entire vessel), it is observed that heavy plaque accumulations are in general calcified and are located in the areas of the vessel having high wall shear stress. Finally, in their experiments the inverse proportionality between fluid and structural stresses is observed.

Estudo iWONDER (Imaging WhOle vessel coroNary tree with intravascular ultrasounD and iMap in patiEnts with acute miocaRdial infarction): racional e desenho do estudo / iWONDER (imaging whOle vessel coroNary tree with intravascular ultrasounD and iMap in patiEnts with acute miocaRdial infarction)study: rationale and study design

Introdução: O ultrassom intracoronário (USIC) é um método capaz de fornecer medida sensível e reprodutível das dimensões da artéria coronária, da placa aterosclerótica e do lúmen arterial. Avanços em sua tecnologia permitem agora a caracterização da composição e da morfologia das placasateroscleróticas. Embora estudos prévios tenham reportado dados utilizando USIC com análise de radiofrequência, o uso de uma nova modalidade (iMap®, Boston Scientific, Santa Clara, Estados Unidos) de caracterização da placa ateroscleróticaé muito pouco conhecida. Nosso objetivo será analisar as características morfológicas, teciduais e fenotípicas das placas ateroscleróticas consideradas angiograficamente “culpadas” e“não-culpadas” em pacientes submetidos a angiografia coronáriadecorrente de infarto agudo do miocárdio (IAM). Métodos: Estudo prospectivo, transversal, em único centro (Hospital São Paulo – Escola Paulista de Medicina – UniversidadeFederal de São Paulo/UNIFESP, São Paulo, SP, Brasil). Serão selecionados50 pacientes para análise ultrassonográfica, de acordo com os seguintes critérios de inclusão: idade < 75 anos, IAM sem supradesnivelamento do segmento ST ou IAM com supradesnivelamento do segmento ST recente, com ou sem uso de fibrinolítico prévio. Conclusões: O presente estudoobjetivará a caracterização morfológica, tecidual e fenotípica da placa aterosclerótica utilizando uma nova modalidade de imagem ainda pouco estudada em pacientes com IAM.