MR image reconstruction using deep density priors

Algorithms for Magnetic Resonance (MR) image reconstruction from undersampled measurements exploit prior information to compensate for missing k-space data. Deep learning (DL) provides a powerful framework for extracting such information from existing image datasets, through learning, and then using it for reconstruction. Leveraging this, recent methods employed DL to learn mappings from undersampled to fully sampled images using paired datasets, including undersampled and corresponding fully sampled images, integrating prior knowledge implicitly. In this article, we propose an alternative approach that learns the probability distribution of fully sampled MR images using unsupervised DL, specifically Variational Autoencoders (VAE), and use this as an explicit prior term in reconstruction, completely decoupling the encoding operation from the prior. The resulting reconstruction algorithm enjoys a powerful image prior to compensate for missing k-space data without requiring paired datasets for training nor being prone to associated sensitivities, such as deviations in undersampling patterns used in training and test time or coil settings. We evaluated the proposed method with T1 weighted images from a publicly available dataset, multi-coil complex images acquired from healthy volunteers (N=8) and images with white matter lesions. The proposed algorithm, using the VAE prior, produced visually high quality reconstructions and achieved low RMSE values, outperforming most of the alternative methods on the same dataset. On multi-coil complex data, the algorithm yielded accurate magnitude and phase reconstruction results. In the experiments on images with white matter lesions, the method faithfully reconstructed the lesions. Keywords: Reconstruction, MRI, prior probability, machine learning, deep learning, unsupervised learning, density estimationComment: Published in IEEE TMI. Main text and supplementary material, 19 pages tota

MRI-based inverse finite element approach for the mechanical assessment of patellar articular cartilage from static compression test

The mechanical property of articular cartilage determines to a great extent the functionality of diarthrodial joints. Consequently, the early detection of mechanical and, thus, functional changes of cartilage is crucial for preventive measures to maintain the mobility and the quality of life of individuals. An alternative to conventional mechanical testing is the inverse finite element approach, enabling non-destructive testing of the tissue. We evaluated a method for the assessment of the equilibrium material properties of the patellar cartilage based on magnetic resonance imaging during patellofemoral compression. We performed ex vivo testing of two equine patellas with healthy cartilage, one with superficial defects, and one with synthetically degenerated cartilage to simulate a pre-osteoarthritic stage. Static compression with 400N for 2h resulted in morphological changes comparable to physiological in vivo deformations in humans. We observed a decrease of the equilibrium Young's modulus of the degenerated cartilage by -59%, which was in the range of the results from indentation (-74%) and confined compression tests (-58%). With the reported accuracy of magnetic resonance imaging and its reproducibility, the results indicate the potential to measure differences in Young's modulus with regard to cartilage degeneration and consequently to distinguish between healthy and pre-osteoarthritic cartilag

Reducing the Interval Between Volume Acquisitions Improves "Sparse” Scanning Protocols in Event-related Auditory fMRI

Sparse and clustered-sparse temporal sampling fMRI protocols have been devised to reduce the influence of auditory scanner noise in the context of auditory fMRI studies. Here, we report an improvement of the previously established clustered-sparse acquisition scheme. The standard procedure currently used by many researchers in the field is a scanning protocol that includes relatively long silent pauses between image acquisitions (and therefore, a relatively long repetition time or cluster-onset asynchrony); it is during these pauses that stimuli are presented. This approach makes it unlikely that stimulus-induced BOLD response is obscured by scanner-noise-induced BOLD response. It also allows the BOLD response to drop near baseline; thus, avoiding saturation of BOLD signal and theoretically increasing effect size. A possible drawback of this approach is the limited number of stimulus presentations and image acquisitions that are possible in a given period of time, which could result in an inaccurate estimation of effect size (higher standard error). Since this line of reasoning has not yet been empirically tested, we decided to vary the cluster-onset asynchrony (7.5, 10, 12.5, and 15s) in the context of a clustered-sparse protocol. In this study sixteen healthy participants listened to spoken sentences. We performed whole-brain fMRI group statistics and region of interest analysis with anatomically defined regions of interest (auditory core and association areas). We discovered that the protocol, which included a short cluster-onset asynchrony (7.5s), yielded more advantageous results than the other protocols, which involved longer cluster-onset asynchrony. The short cluster-onset asynchrony protocol exhibited a larger number of activated voxels and larger mean effect sizes with lower standard errors. Our findings suggest that, contrary to prior experience, a short cluster-onset asynchrony is advantageous because more stimuli can be delivered within any given period of time. Alternatively, a given number of stimuli can be presented in less time, and this broadens the spectrum of possible fMRI application

Metal-induced artifacts in computed tomography and magnetic resonance imaging: comparison of a biodegradable magnesium alloy versus titanium and stainless steel controls

OBJECTIVE: To evaluate metal artifacts induced by biodegradable magnesium-a new class of degradable biomaterial that is beginning to enter the orthopedic routine-on CT and MRI compared to standard titanium and steel controls. METHODS: Different pins made of titanium, stainless steel, and biodegradable magnesium alloys were scanned using a second-generation dual-energy multidetector CT and a 1.5-T MR scanner. In CT, quantitative assessment of artifacts was performed by two independent readers by measuring the noise in standardized regions of interest close to the pins. In MRI, the artifact diameter was measured. Interobserver agreement was evaluated using intraclass correlation coefficients. Artifacts were compared using Mann Whitney U tests. RESULTS: In comparison to stainless steel, biodegradable magnesium alloys induced significantly fewer artifacts in both 1.5-T MRI (p = 0.019-0.021) and CT (p = 0.003-0.006). Compared to titanium, magnesium induced significantly less artifact-related noise in CT (p = 0.003-0.008). Although artifacts were less on MRI for biodegradable magnesium compared to titanium, this result was not statistically significant. CONCLUSION: Biodegradable magnesium alloys induce substantially fewer artifacts in CT compared to standard titanium and stainless steel, and fewer artifacts in MRI for the comparison with stainless steel

Safety and reliability of Radio Frequency Identification Devices in Magnetic Resonance Imaging and Computed Tomography

BACKGROUND: Radio Frequency Identification (RFID) devices are becoming more and more essential for patient safety in hospitals. The purpose of this study was to determine patient safety, data reliability and signal loss wearing on skin RFID devices during magnetic resonance imaging (MRI) and computed tomography (CT) scanning. METHODS: Sixty RFID tags of the type I-Code SLI, 13.56 MHz, ISO 18000-3.1 were tested: Thirty type 1, an RFID tag with a 76 x 45 mm aluminum-etched antenna and 30 type 2, a tag with a 31 x 14 mm copper-etched antenna. The signal loss, material movement and heat tests were performed in a 1.5 T and a 3 T MR system. For data integrity, the tags were tested additionally during CT scanning. Standardized function tests were performed with all transponders before and after all imaging studies. RESULTS: There was no memory loss or data alteration in the RFID tags after MRI and CT scanning. Concerning heating (a maximum of 3.6 degrees C) and device movement (below 1 N/kg) no relevant influence was found. Concerning signal loss (artifacts 2 - 4 mm), interpretability of MR images was impaired when superficial structures such as skin, subcutaneous tissues or tendons were assessed. CONCLUSIONS: Patients wearing RFID wristbands are safe in 1.5 T and 3 T MR scanners using normal operation mode for RF-field. The findings are specific to the RFID tags that underwent testing

Safety of Active Implantable Devices during MRI Examinations: A Finite Element Analysis of an Implantable Pump

The goal of this study was to propose a general numerical analysis methodology to evaluate the magnetic resonance imaging (MRI)-safety of active implants. Numerical models based on the finite element (FE) technique were used to estimate if the normal operation of an active device was altered during MRI imaging. An active implanted pump was chosen to illustrate the method. A set of controlled experiments were proposed and performed to validate the numerical model. The calculated induced voltages in the important electronic components of the device showed dependence with the MRI field strength. For the MRI radiofrequency fields, significant induced voltages of up to 20 V were calculated for a 0.3T field-strength MRI. For the 1.5 and 3.0T MRIs, the calculated voltages were insignificant. On the other hand, induced voltages up to 11 V were calculated in the critical electronic components for the 3.0T MRI due to the gradient fields. Values obtained in this work reflect to the worst case situation which is virtually impossible to achieve in normal scanning situations. Since the calculated voltages may be removed by appropriate protection circuits, no critical problems affecting the normal operation of the pump were identified. This study showed that the proposed methodology helps the identification of the possible incompatibilities between active implants and MR imaging, and can be used to aid the design of critical electronic systems to ensure MRI-safet

Safety of magnetic resonance imaging of patients with a new Medtronic EnRhythm MRI SureScan pacing system: clinical study design

BACKGROUND: Magnetic Resonance Imaging (MRI) of patients with implanted cardiac devices is currently considered hazardous due to potential for electromagnetic interference to the patient and pacemaker system. With approximately 60 million MRI scans performed worldwide per year, an estimated majority of pacemaker patients may develop an indication for an MRI during the lifetime of their pacemakers, suggesting that safe use of pacemakers in the MRI environment would be clinically valuable. A new pacing system (Medtronic EnRhythm MRItrade mark SureScantrade mark and CapSureFix MRItrade mark leads) has been designed and pre-clinically tested for safe use in the MRI environment. The EnRhythm MRI study is designed to confirm the safety and efficacy of this new pacing system. METHODS: The EnRhythm MRI study is a prospective, randomized controlled, unblinded clinical trial to confirm the safety and efficacy of MRI at 1.5 Tesla in patients implanted with a specifically designed pacemaker and lead system. The patients have standard indications for dual chamber pacemaker implantation. Successfully implanted patients are randomized in a 2:1 ratio to undergo MRI (MRI group) or to have no MRI scan (control group) at 9-12 weeks after pacemaker system implantation. Magnetic resonance (MR) scanning includes 14 head and lumbar scan sequences representing clinically relevant scans while maximizing the gradient slew rate up to 200 T/m/s, and/or the transmitted radiofrequency (RF) power up to SAR (specific absorption rate) levels of 2 W/kg body weight (upper limit of normal operating mode). Full interrogation of all device information and sensing and capture function are measured at device implantation, every follow-up and before and immediately after MRI in the MRI group and at the same time points in the control group. Complete pacemaker and lead evaluations are also done at one week and one month after the scan for the MRI and control group patients.The primary endpoint is safe and successful completion of the MRI scan as measured by freedom from both MRI-procedure related complications and clinically significant changes in the sensing and capture function of the leads. RESULTS: Results will be communicated after approximately 156 and 470 patients have completed 4 months of follow-up. TRIAL REGISTRATION: ClinicalTrials.gov identifier: NCT00433654

In vivo heating of pacemaker leads during magnetic resonance imaging

Aims Magnetic resonance imaging (MRI) is well established as an important diagnostic tool in medicine. However, the presence of a cardiac pacemaker is usually regarded as a contraindication for MRI due to safety reasons. In this study, heating effects at the myocardium-pacemaker lead tip interface have been investigated in a chronic animal model during MRI at 1.5 Tesla. Methods and results Pacemaker leads with additional thermocouple wires as temperature sensors were implanted in nine animals. Temperature increases of up to 20°C were measured during MRI of the heart. Significant impedance and minor stimulation threshold changes could be seen. However, pathology and histology could not clearly demonstrate heat-induced damage. Conclusions MRI may produce considerable heating at the lead tip. Changes of pacing parameters due to MRI could be seen in chronic experiments. Potential risk of tissue damage cannot be excluded even though no reproducible alterations at the histological level could be foun

The desire for healthy limb amputation: structural brain correlates and clinical features of xenomelia

Xenomelia is the oppressive feeling that one or more limbs of one's body do not belong to one's self. We present the results of a thorough examination of the characteristics of the disorder in 15 males with a strong desire for amputation of one or both legs. The feeling of estrangement had been present since early childhood and was limited to a precisely demarcated part of the leg in all individuals. Neurological status examination and neuropsychological testing were normal in all participants, and psychiatric evaluation ruled out the presence of a psychotic disorder. In 13 individuals and in 13 pair-matched control participants, magnetic resonance imaging was performed, and surface-based morphometry revealed significant group differences in cortical architecture. In the right hemisphere, participants with xenomelia showed reduced cortical thickness in the superior parietal lobule and reduced cortical surface area in the primary and secondary somatosensory cortices, in the inferior parietal lobule, as well as in the anterior insular cortex. A cluster of increased thickness was located in the central sulcus. In the left hemisphere, affected individuals evinced a larger cortical surface area in the inferior parietal lobule and secondary somatosensory cortex. Although of modest size, these structural correlates of xenomelia appear meaningful when discussed against the background of some key clinical features of the disorder. Thus, the predominantly right-sided cortical abnormalities are in line with a strong bias for left-sided limbs as the target of the amputation desire, evident both in our sample and in previously described populations with xenomelia. We also propose that the higher incidence of lower compared with upper limbs (∼80% according to previous investigations) may explain the erotic connotations typically associated with xenomelia, also in the present sample. These may have their roots in the proximity of primary somatosensory cortex for leg representation, whose surface area was reduced in the participants with xenomelia, with that of the genitals. Alternatively, the spatial adjacency of secondary somatosensory cortex for leg representation and the anterior insula, the latter known to mediate sexual arousal beyond that induced by direct tactile stimulation of the genital area, might play a role. Although the right hemisphere regions of significant neuroarchitectural correlates of xenomelia are part of a network reportedly subserving body ownership, it remains unclear whether the structural alterations are the cause or rather the consequence of the long-standing and pervasive mismatch between body and sel