Real-time MRI: recent advances using radial FLASH

Self-gated cine phase-contrast balanced SSFP flow quantification at 0.55 T

PURPOSE

To implement cine phase-contrast balanced SSFP (PC-bSSFP) for low-field 0.55T cardiac MRI by exploiting the intrinsic flow sensitivity of the bSSFP slice-select gradient and the in-plane phase-cancelation properties of radial trajectories, enabling self-gated and referenceless PC-bSSFP flow quantification at 0.55 T.

METHODS

A free-running, tiny golden-angle radial PC-bSSFP approach was implemented on 0.55T and 1.5T systems. Cardiac and respiratory self-gating was incorporated to enable electrocardiogram-free scanning during breath-hold and free-breathing. By exploiting the intrinsic in-plane phase-cancelation properties of radial acquisitions and background phase fitting, referenceless single-point PC-bSSFP was realized. In vivo data were acquired in the ascending aorta of healthy subjects at 0.55 T and 1.5 T during breath-hold and free-breathing. Flow data, SNR, and velocity-to-noise ratio were compared relative to data obtained with phase-contrast spoiled gradient-echo variants.

RESULTS

Velocities acquired with PC-bSSFP compared well with data from phase-contrast spoiled gradient-echo (RMSEv  = 5.8 cm/s). PC-bSSFP at 0.55 T resulted in high-quality cine magnitude images and phase maps with sufficient SNR and velocity-to-noise ratio. Breath-hold and free-breathing PC-bSSFP performed very similarly, with comparable flow quantification (RMSEv  = 5.7 cm/s). Referenceless single-point PC-bSSFP results agreed well with two-point PC-bSSFP (-1.8 ± 5.2 cm/s) while reducing scan times 2-fold.

CONCLUSION

PC-bSSFP is feasible on low-field 0.55T systems, producing high-quality cine images while permitting simultaneous aortic flow measurements during breath-hold and free-breathing and without the need for electrocardiogram gating.

Adaptive model-based Magnetic Resonance

PURPOSE

Conventional sequences are static in nature, fixing measurement parameters in advance in anticipation of a wide range of expected tissue parameter values. We set out to design and benchmark a new, personalized approach-termed adaptive MR-in which incoming subject data is used to update and fine-tune the pulse sequence parameters in real time.

METHODS

We implemented an adaptive, real-time multi-echo (MTE) experiment for estimating T₂ s. Our approach combined a Bayesian framework with model-based reconstruction. It maintained and continuously updated a prior distribution of the desired tissue parameters, including T₂ , which was used to guide the selection of sequence parameters in real time.

RESULTS

Computer simulations predicted accelerations between 1.7- and 3.3-fold for adaptive multi-echo sequences relative to static ones. These predictions were corroborated in phantom experiments. In healthy volunteers, our adaptive framework accelerated the measurement of T₂ for n-acetyl-aspartate by a factor of 2.5.

CONCLUSION

Adaptive pulse sequences that alter their excitations in real time could provide substantial reductions in acquisition times. Given the generality of our proposed framework, our results motivate further research into other adaptive model-based approaches to MRI and MRS.

MRI of Temporomandibular Joint Disorders: Recent Advances and Future Directions

Temporomandibular joint disorders (TMDs) are a prevalent disease covering pain and dysfunction of temporomandibular joints and masticatory muscles, which can be detrimental to quality of life. Magnetic resonance imaging (MRI) is a powerful and noninvasive tool for the imaging and understanding of TMD. With the recent technical development of dynamic and quantitative MRI techniques, including diffusion-weighted imaging, T₂ mapping, and ultrashort/zero echo time, it is now feasible in TMD imaging and has been preliminarily investigated with promising results. In this review we will discuss the recent advances of MRI techniques in TMD and its future directions, and hope to highlight the scientific potential and clinical value of novel MRI techniques in diagnosing and treating TMD. LEVEL OF EVIDENCE: 4 TECHNICAL EFFICACY STAGE: 3.

Time domain principal component analysis for rapid, real-time 2D MRI reconstruction from undersampled data

PURPOSE

A rapid real-time 2D accelerated method was developed for magnetic resonance imaging (MRI) using principal component analysis (PCA) in the temporal domain. This method employs a moving window of previous dynamic frames to reconstruct the current, real-time frame within this window. This technique could be particularly useful in real-time tracking applications such as in MR-guided radiotherapy, where low latency real-time reconstructions are essential.

METHODS

The method was tested retrospectively on 15 fully-sampled data sets of lung patient data acquired on a 3T Philips Achieva system. High frequency data are incoherently undersampled, while the central low-frequency data are always acquired to characterize the temporal fluctuations through PCA. The undersampling pattern is derived in such a way that all of k-space is acquired within a pre-determined number of frames. The missing data in the current frame are then filled in by fitting the temporal characterizations to the acquired undersampled data, using a pre-determined number of PCs. A subset of six patients was used to test the contour ability of the images. Various accelerations between 3x and 8x were tested along with the optimal number of PCs for fitting. A comparison was also performed with previous work from our group proposed by Dietz et al. as well as with a standard low resolution acquisition. In order to determine how the method would perform at lower signal to noise ratio (SNR), noise levels of 2×, 4×, and 6× were added to the 3T data. Metrics such as normalised mean square error and Dice coefficient were used to measure the reconstruction image quality and contour ability.

RESULTS

The proposed method demonstrated good temporal robustness as consistent metrics were detected for the duration of the imaging session. It was found that the optimal number of PCs for temporal fitting was dependent on the acceleration rate. For the data tested, five PCs were found to be optimal at the acceleration rates of 3× and 4×. This number decreases to three at accelerations of 5× and 6× and further decreases to two at an acceleration rate of 8×, likely due to greater instability with fewer acquired data points. The use of too many PCs for fitting increased the chances of noisy reconstruction which affected contourability.

CONCLUSIONS

The proposed 2D real-time MR acceleration method demonstrated greater robustness in the metrics over time when compared with previous real-time PCA methods using metrics such as normalised mean squared error, peak SNR and structural similarity up to an acceleration of 8x. Improved temporal robustness of image structure contourability and accurate definition was also demonstrated using several metrics including the Dice coefficient. Reconstruction of raw acquired data can be performed at approximately 50 ms per frame using an Intel core i5 CPU. The method has the advantage of being very flexible in terms of hardware requirements as it can operate successfully on a single coil channel and does not require specialized computing power to implement in real-time.

Passive needle guide tracking with radial acquisition and phase-only cross-correlation

PURPOSE

Acceleration of a passive tracking sequence based on phase-only cross-correlation (POCC) using radial undersampling.

METHODS

The phase-only cross-correlation (POCC) algorithm allows passive tracking of interventional instruments in real-time. In a POCC sequence, two cross-sectional images of a needle guide with a positive MR contrast are continuously acquired from which the instrument trajectory is calculated. Conventional Cartesian imaging for tracking is very time consuming; here, a higher temporal resolution is achieved using a highly undersampled radial acquisition together with a modified POCC algorithm that incorporates the point-spread-function. Targeting and needle insertion is performed in two phantom experiments with 16 fiducial targets, each using 4 and 16 radial projections for passive tracking. Additionally, targeting of eight deep lying basivertebral veins in the lumbar spines is performed for in vivo proof-of-application with four radial projections for needle guide tracking.

RESULTS

The radially undersampled POCC sequence yielded in the phantom experiments a lateral targeting accuracy of 1.1 ± 0.4 mm and 1.0 ± 0.5 mm for 16 and 4 radial projections, respectively, without any statistically significant difference. In the in vivo application, a mean targeting duration of 62 ± 13 s was measured.

CONCLUSION

Radial undersampling can drastically reduce the acquisition time for passive tracking in a POCC sequences for MR-guided needle interventions without compromising the targeting accuracy.

Feasibility of real-time cardiac MRI in mice using tiny golden angle radial sparse

Cardiovascular magnetic resonance imaging has proven valuable for the assessment of structural and functional cardiac abnormalities. Even although it is an established imaging method in small animals, the long acquisition times of gated or self-gated techniques still limit its widespread application. In this study, the application of tiny golden angle radial sparse MRI (tyGRASP) for real-time cardiac imaging was tested in 12 constitutive nexilin (Nexn) knock-out (KO) mice, both heterozygous (Het, N = 6) and wild-type (WT, N = 6), and the resulting functional parameters were compared with a well-established self-gating approach. Real-time images were reconstructed for different temporal resolutions of between 16.8 and 79.8 ms per image. The suggested approach was additionally tested for dobutamine stress and qualitative first-pass perfusion imaging. Measurements were repeated twice within 2 weeks for reproducibility assessment. In direct comparison with the high-quality, self-gated technique, the real-time approach did not show any significant differences in global function parameters for acquisition times below 50 ms (rest) and 31.5 ms (stress). Compared with WT, the end-diastolic volume (EDV) and end-systolic volume (ESV) were markedly higher (P < 0.05) and the ejection fraction (EF) was significantly lower in the Het Nexn-KO mice at rest (P < 0.001). For the stress investigation, a clear decrease of EDV and ESV, and an increase in EF, but maintained stroke volume, could be observed in both groups. Combined with ECG-triggering, tyGRASP provided first-pass perfusion data with a temporal resolution of one image per heartbeat, allowing the quantitative assessment of upslope curves in the blood-pool and myocardium. Excellent inter-study reproducibility was achieved in all the functional parameters. The tyGRASP is a valuable real-time MRI technique for mice, which significantly reduces the scan time in preclinical cardiac functional imaging, providing sufficient image quality for deriving accurate functional parameters, and has the potential to investigate real-time and beat-to-beat changes.

Dynamic MRI for articulating joint evaluation on 1.5 T and 3.0 T scanners: setup, protocols, and real-time sequences

Dynamic magnetic resonance imaging (MRI) is a non-invasive method that can be used to increase the understanding of the pathomechanics of joints. Various types of real-time gradient echo sequences used for dynamic MRI acquisition of joints include balanced steady-state free precession sequence, radiofrequency-spoiled sequence, and ultra-fast gradient echo sequence. Due to their short repetition time and echo time, these sequences provide high temporal resolution, a good signal-to-noise ratio and spatial resolution, and soft tissue contrast. The prerequisites of the evaluation of joints with dynamic MRI include suitable patient installation and optimal positioning of the joint in the coil to allow joint movement, sometimes with dedicated coil support. There are currently few recommendations in the literature regarding appropriate protocol, sequence standardizations, and diagnostic criteria for the use of real-time dynamic MRI to evaluate joints. This article summarizes the technical parameters of these sequences from various manufacturers on 1.5 T and 3.0 T MRI scanners. We have reviewed pertinent details of the patient and coil positioning for dynamic MRI of various joints. The indications and limitations of dynamic MRI of joints are discussed.

Real-time three-dimensional MRI for the assessment of dynamic carpal instability

Background

Carpal instability is defined as a condition where wrist motion and/or loading creates mechanical dysfunction, resulting in weakness, pain and decreased function. When conventional methods do not identify the instability patterns, yet clinical signs of instability exist, the diagnosis of dynamic instability is often suggested to describe carpal derangement manifested only during the wrist’s active motion or stress. We addressed the question: can advanced MRI techniques provide quantitative means to evaluate dynamic carpal instability and supplement standard static MRI acquisition? Our objectives were to (i) develop a real-time, three-dimensional MRI method to image the carpal joints during their active, uninterrupted motion; and (ii) demonstrate feasibility of the method for assessing metrics relevant to dynamic carpal instability, thus overcoming limitations of standard MRI.

Methods

Twenty wrists (bilateral wrists of ten healthy participants) were scanned during radial-ulnar deviation and clenched-fist maneuvers. Images resulting from two real-time MRI pulse sequences, four sparse data-acquisition schemes, and three constrained image reconstruction techniques were compared. Image quality was assessed via blinded scoring by three radiologists and quantitative imaging metrics.

Results

Real-time MRI data-acquisition employing sparse radial sampling with a gradient-recalled-echo acquisition and constrained iterative reconstruction appeared to provide a practical tradeoff between imaging speed (temporal resolution up to 135 ms per slice) and image quality. The method effectively reduced streaking artifacts arising from data undersampling and enabled the derivation of quantitative measures pertinent to evaluating dynamic carpal instability.

Conclusion

This study demonstrates that real-time, three-dimensional MRI of the moving wrist is feasible and may be useful for the evaluation of dynamic carpal instability.

Dynamic water/fat separation and B 0 inhomogeneity mapping-joint estimation using undersampled triple-echo multi-spoke radial FLASH

PURPOSE

To achieve dynamic water/fat separation and B 0 field inhomogeneity mapping via model-based reconstructions of undersampled triple-echo multi-spoke radial FLASH acquisitions.

METHODS

This work introduces an undersampled triple-echo multi-spoke radial FLASH sequence, which uses (i) complementary radial spokes per echo train for faster spatial encoding, (ii) asymmetric echoes for flexible and nonuniform echo spacing, and (iii) a golden angle increment across frames for optimal k-space coverage. Joint estimation of water, fat, B 0 inhomogeneity, and coil sensitivity maps from undersampled triple-echo data poses a nonlinear and non-convex inverse problem which is solved by a model-based reconstruction with suitable regularization. The developed methods are validated using phantom experiments with different degrees of undersampling. Real-time MRI studies of the knee, liver, and heart are conducted without prospective gating or retrospective data sorting at temporal resolutions of 70, 158, and 40 ms, respectively.

RESULTS

Up to 18-fold undersampling is achieved in this work. Even in the presence of rapid physiological motion, large B 0 field inhomogeneities, and phase wrapping, the model-based reconstruction yields reliably separated water/fat maps in conjunction with spatially smooth inhomogeneity maps.

CONCLUSIONS

The combination of a triple-echo acquisition and joint reconstruction technique provides a practical solution to time-resolved and motion robust water/fat separation at high spatial and temporal resolution.

Fabrication and Characterization of a 20-MHz Microlinear Phased-Array Transducer for Intervention Guidance

This paper describes the design and fabrication of a miniature ultrasonic phased-array transducer used for intervention guidance. Currently, ultrasound probes are often placed at the body surface of the patients, leading to several drawbacks including the limitation of penetration and image quality. In order to improve the reliability of the guiding process, we propose a miniature phased-array transducer that can be placed adjacent to the intervention device during the interventional procedure. In this paper, we report the work that has been carried out on the development of this miniature phased-array transducer. It comprised 48 elements housed in a 3-mm-diameter needle. A specially designed flexible circuit was used for accommodating the transducer array in the long, thin needle housing. The center frequency and the fractional bandwidth were approximately 20 MHz and 42%, respectively, with an average crosstalk lower than -30 dB. The axial and azimuth resolutions were approximately 80 and [Formula: see text], respectively. The imaging capability of the transducer was further evaluated by acquiring the B-mode images of a needle in a cow liver. The performance of the proposed phased-array transducer demonstrates the feasibility of such an approach for interventional guidance.

A Review of Disintegration Mechanisms and Measurement Techniques

Pharmaceutical solid dosage forms (tablets or capsules) are the predominant form to administer active pharmaceutical ingredients (APIs) to the patient. Tablets are typically powder compacts consisting of several different excipients in addition to the API. Excipients are added to a formulation in order to achieve the desired fill weight of a dosage form, to improve the processability or to affect the drug release behaviour in the body. These complex porous systems undergo different mechanisms when they come in contact with physiological fluids. The performance of a drug is primarily influenced by the disintegration and dissolution behaviour of the powder compact. The disintegration process is specifically critical for immediate-release dosage forms. Its mechanisms and the factors impacting disintegration are discussed and methods used to study the disintegration in-situ are presented. This review further summarises mathematical models used to simulate disintegration phenomena and to predict drug release kinetics.

Diagnosis of Gastroesophageal Reflux Disease Using Real-time Magnetic Resonance Imaging

A small angle (His angle) between the oesophagus and the fundus of the stomach is considered to act as flap valve and anti-reflux barrier. A wide angle results in dysfunction of the oesophagogastric junction and subsequently in gastroesophageal reflux disease (GERD). Here, we used real-time magnetic resonance imaging (MRI) at 50 ms resolution (20 frames per second) in 12 volunteers and 12 patients with GERD to assess transport of pineapple juice through the oesophagogastric junction and reflux during Valsalva. We found that the intra-abdominal part of the oesophagus was bended towards the left side resulting in an angle of 75.3 ± 17.4, which was significantly larger during Valsava (P = 0.017). Reflux and several underlying pathologies were detected in 11 out of 12 patients. Our data visualize oesophagogastric junction physiology and disprove the flap valve hypothesis. Further, non-invasive real-time MRI has considerable potential for the diagnosis of causative pathologies leading to GERD.

Single-shot T1 mapping of the corpus callosum: a rapid characterization of fiber bundle anatomy

Using diffusion-tensor magnetic resonance imaging and fiber tractography the topographic organization of the human corpus callosum (CC) has been described to comprise five segments with fibers projecting into prefrontal (I), premotor and supplementary motor (II), primary motor (III), and primary sensory areas (IV), as well as into parietal, temporal, and occipital cortical areas (V). In order to more rapidly characterize the underlying anatomy of these segments, this study used a novel single-shot T1 mapping method to quantitatively determine T1 relaxation times in the human CC. A region-of-interest analysis revealed a tendency for the lowest T1 relaxation times in the genu and the highest T1 relaxation times in the somatomotor region of the CC. This observation separates regions dominated by myelinated fibers with large diameters (somatomotor area) from densely packed smaller axonal bundles (genu) with less myelin. The results indicate that characteristic T1 relaxation times in callosal profiles provide an additional means to monitor differences in fiber anatomy, fiber density, and gray matter in respective neocortical areas. In conclusion, rapid T1 mapping allows for a characterization of the axonal architecture in an individual CC in less than 10 s. The approach emerges as a valuable means for studying neocortical brain anatomy with possible implications for the diagnosis of neurodegenerative processes.

High-resolution dynamic speech imaging with joint low-rank and sparsity constraints

PURPOSE

To enable dynamic speech imaging with high spatiotemporal resolution and full-vocal-tract spatial coverage, leveraging recent advances in sparse sampling.

METHODS

An imaging method is developed to enable high-speed dynamic speech imaging exploiting low-rank and sparsity of the dynamic images of articulatory motion during speech. The proposed method includes: (a) a novel data acquisition strategy that collects spiral navigators with high temporal frame rate and (b) an image reconstruction method that derives temporal subspaces from navigators and reconstructs high-resolution images from sparsely sampled data with joint low-rank and sparsity constraints.

RESULTS

The proposed method has been systematically evaluated and validated through several dynamic speech experiments. A nominal imaging speed of 102 frames per second (fps) was achieved for a single-slice imaging protocol with a spatial resolution of 2.2 × 2.2 × 6.5 mm(3) . An eight-slice imaging protocol covering the entire vocal tract achieved a nominal imaging speed of 12.8 fps with the identical spatial resolution. The effectiveness of the proposed method and its practical utility was also demonstrated in a phonetic investigation.

CONCLUSION

High spatiotemporal resolution with full-vocal-tract spatial coverage can be achieved for dynamic speech imaging experiments with low-rank and sparsity constraints.

Parallel Magnetic Resonance Imaging as Approximation in a Reproducing Kernel Hilbert Space

In Magnetic Resonance Imaging (MRI) data samples are collected in the spatial frequency domain (k-space), typically by time-consuming line-by-line scanning on a Cartesian grid. Scans can be accelerated by simultaneous acquisition of data using multiple receivers (parallel imaging), and by using more efficient non-Cartesian sampling schemes. To understand and design k-space sampling patterns, a theoretical framework is needed to analyze how well arbitrary sampling patterns reconstruct unsampled k-space using receive coil information. As shown here, reconstruction from samples at arbitrary locations can be understood as approximation of vector-valued functions from the acquired samples and formulated using a Reproducing Kernel Hilbert Space (RKHS) with a matrix-valued kernel defined by the spatial sensitivities of the receive coils. This establishes a formal connection between approximation theory and parallel imaging. Theoretical tools from approximation theory can then be used to understand reconstruction in k-space and to extend the analysis of the effects of samples selection beyond the traditional image-domain g-factor noise analysis to both noise amplification and approximation errors in k-space. This is demonstrated with numerical examples.

Real-time phase-contrast flow MRI of the ascending aorta and superior vena cava as a function of intrathoracic pressure (Valsalva manoeuvre)

OBJECTIVE

Real-time phase-contrast flow MRI at high spatiotemporal resolution was applied to simultaneously evaluate haemodynamic functions in the ascending aorta (AA) and superior vena cava (SVC) during elevated intrathoracic pressure (Valsalva manoeuvre).

METHODS

Real-time phase-contrast flow MRI at 3 T was based on highly undersampled radial gradient-echo acquisitions and phase-sensitive image reconstructions by regularized non-linear inversion. Dynamic alterations of flow parameters were obtained for 19 subjects at 40-ms temporal resolution, 1.33-mm in-plane resolution and 6-mm section thickness. Real-time measurements were performed during normal breathing (10 s), increased intrathoracic pressure (10 s) and recovery (20 s).

RESULTS

Real-time measurements were technically successful in all volunteers. During the Valsalva manoeuvre (late strain) and relative to values during normal breathing, the mean peak flow velocity and flow volume decreased significantly in both vessels (p < 0.001) followed by a return to normal parameters within the first 10 s of recovery in the AA. By contrast, flow in the SVC presented with a brief (1-2 heartbeats) but strong overshoot of both the peak velocity and blood volume immediately after pressure release followed by rapid normalization.

CONCLUSION

Real-time phase-contrast flow MRI may assess cardiac haemodynamics non-invasively, in multiple vessels, across the entire luminal area and at high temporal and spatial resolution.

ADVANCES IN KNOWLEDGE

Future clinical applications of this technique promise new insights into haemodynamic alterations associated with pre-clinical congestive heart failure or diastolic dysfunction, especially in cases where echocardiography is technically compromised.

Real-time phase-contrast flow MRI of haemodynamic changes in the ascending aorta and superior vena cava during Mueller manoeuvre

AIM

To evaluate the potential of real-time phase-contrast flow magnetic resonance imaging (MRI) at 40 ms resolution for the simultaneous determination of blood flow in the ascending aorta (AA) and superior vena cava (SVC) in response to reduced intrathoracic pressure (Mueller manoeuvre).

MATERIALS AND METHODS

Through-plane flow was assessed in 20 healthy young subjects using real-time phase-contrast MRI based on highly undersampled radial fast low-angle shot (FLASH) with image reconstruction by regularized non-linear inversion. Haemodynamic alterations (three repetitions per subject = 60 events) were evaluated during normal breathing (10 s), inhalation with nearly closed epiglottis (10 s), and recovery (20 s).

RESULTS

Relative to normal breathing and despite interindividual differences, reduced intrathoracic pressure by at least 30 mmHg significantly decreased the initial peak mean velocity (averaged across the lumen) in the AA by -24 ± 9% and increased the velocity in the SVC by +28 ± 25% (p < 0.0001, n = 23 successful events). Respective changes in flow volume per heartbeat were -25 ± 9% in the AA and +49 ± 44% in the SVC (p < 0.0001, n = 23). Flow parameters returned to baseline during sustained pressure reduction, while the heart rate was elevated by 10% (p < 0.0001) after the start (n = 24) and end (n = 17) of the manoeuvre.

CONCLUSIONS

Real-time flow MRI during low intrathoracic pressure non-invasively revealed quantitative haemodynamic adjustments in both the AA and SVC.

On the physiology of normal swallowing as revealed by magnetic resonance imaging in real time

The aim of this study was to assess the physiology of normal swallowing using recent advances in real-time magnetic resonance imaging (MRI). Therefore ten young healthy subjects underwent real-time MRI and flexible endoscopic evaluations of swallowing (FEES) with thickened pineapple juice as oral contrast bolus. MRI movies were recorded in sagittal, coronal, and axial orientations during successive swallows at about 25 frames per second. Intermeasurement variation was analyzed and comparisons between real-time MRI and FEES were performed. Twelve distinct swallowing events could be quantified by real-time MRI (start time, end time, and duration). These included five valve functions: oro-velar opening, velo-pharyngeal closure, glottal closure, epiglottic retroflexion, and esophageal opening; three bolus transports: oro-velar transit, pharyngeal delay, pharyngeal transit; and four additional events: laryngeal ascent, laryngeal descent, vallecular, and piriform sinus filling and pharyngeal constriction. Repetitive measurements confirmed the general reliability of the MRI method with only two significant differences for the start times of the velo-pharyngeal closure (t(8) = −2.4, P ≤ 0.046) and laryngeal ascent (t(8) = −2.6, P ≤ 0.031). The duration of the velo-pharyngeal closure was significantly longer in real-time MRI compared to FEES (t(8) = −3.3, P ≤ 0.011). Real-time MRI emerges as a simple, robust, and reliable tool for obtaining comprehensive functional and anatomical information about the swallowing process.

Aggregated motion estimation for real-time MRI reconstruction

PURPOSE

In real-time MRI serial images are generally reconstructed from highly undersampled datasets as the iterative solutions of an inverse problem. While practical realizations based on regularized nonlinear inversion (NLINV) have hitherto been surprisingly successful, strong assumptions about the continuity of image features may affect the temporal fidelity of the estimated reconstructions.

THEORY AND METHODS

The proposed method for real-time image reconstruction integrates the deformations between nearby frames into the data consistency term of the inverse problem. The aggregated motion estimation (AME) is not required to be affine or rigid and does not need additional measurements. Moreover, it handles multi-channel MRI data by simultaneously determining the image and its coil sensitivity profiles in a nonlinear formulation which also adapts to non-Cartesian (e.g., radial) sampling schemes. The new method was evaluated for real-time MRI studies using highly undersampled radial gradient-echo sequences.

RESULTS

AME reconstructions for a motion phantom with controlled speed as well as for measurements of human heart and tongue movements demonstrate improved temporal fidelity and reduced residual undersampling artifacts when compared with NLINV reconstructions without motion estimation.

CONCLUSION

Nonlinear inverse reconstructions with aggregated motion estimation offer improved image quality and temporal acuity for visualizing rapid dynamic processes by real-time MRI.

Real-time cardiovascular magnetic resonance at 1.5 T using balanced SSFP and 40 ms resolution

BACKGROUND

While cardiovascular magnetic resonance (CMR) commonly employs ECG-synchronized cine acquisitions with balanced steady-state free precession (SSFP) contrast at 1.5 T, recent developments at 3 T demonstrate significant potential for T1-weighted real-time imaging at high spatiotemporal resolution using undersampled radial FLASH. The purpose of this work was to combine both ideas and to evaluate a corresponding real-time CMR method at 1.5 T with SSFP contrast.

METHODS

Radial gradient-echo sequences with fully balanced gradients and at least 15-fold undersampling were implemented on two CMR systems with different gradient performance. Image reconstruction by regularized nonlinear inversion (NLINV) was performed offline and resulted in real-time SSFP CMR images at a nominal resolution of 1.8 mm and with acquisition times of 40 ms.

RESULTS

Studies of healthy subjects demonstrated technical feasibility in terms of robustness and general image quality. Clinical applicability with access to quantitative evaluations (e.g., ejection fraction) was confirmed by preliminary applications to 27 patients with typical indications for CMR including arrhythmias and abnormal wall motion. Real-time image quality was slightly lower than for cine SSFP recordings, but considered diagnostic in all cases.

CONCLUSIONS

Extending conventional cine approaches, real-time radial SSFP CMR with NLINV reconstruction provides access to individual cardiac cycles and allows for studies of patients with irregular heartbeat.