Free-breathing Cardiac Imaging Research Articles

Spatially Resolved MR-Compatible Doppler Ultrasound: Proof of Concept for Triggering of Diagnostic Quality Cardiovascular MRI for Function and Flow Quantification at 3T.

We demonstrate the use of a magnetic-resonance (MR)-compatible ultrasound (US) imaging probe using spatially resolved Doppler for diagnostic quality cardiovascular MR imaging (MRI) as an initial step toward hybrid US/MR fetal imaging. A newly developed technology for a dedicated MR-compatible phased array ultrasound-imaging probe acquired pulsed color Doppler carotid images, which were converted in near-real time to a trigger signal for cardiac cine and flow quantification MRI. Ultrasound and MR data acquired simultaneously were interference free. Conventional electrocardiogram (ECG) and the proposed spatially resolved Doppler triggering were compared in 10 healthy volunteers. A synthetic "false-triggered" image was retrospectively processed using metric optimized gating (MOG). Images were scored by expert readers, and sharpness, cardiac function and aortic flow were quantified. Four-dimensional (4-D) flow (two volunteers) showed feasibility of Doppler triggering over a long acquisition time. Imaging modalities were compatible. US probe positioning was stable and comfortable. Image quality scores and quantified sharpness were statistically equal for Doppler- and ECG-triggering (p ). ECG-, Doppler-triggered, and MOG ejection fractions were equivalent (p ), with false-triggered values significantly lower (p < 0.0005). Aortic flow showed no difference between ECG- and Doppler-triggered and MOG (p > 0.05). 4-D flow quantification gave consistent results between ECG and Doppler triggering. We report interference-free pulsed color Doppler ultrasound during MR data acquisition. Cardiovascular MRI of diagnostic quality was successfully obtained with pulsed color Doppler triggering. The hardware platform could further enable advanced free-breathing cardiac imaging. Doppler ultrasound triggering is applicable where ECG is compromised due to pathology or interference at higher magnetic fields, and where direct ECG is impossible, i.e., fetal imaging.

Real-time free-breathing cardiac imaging with self-calibrated through-time radial GRAPPA.

Real-time free-breathing cardiac imaging with highly undersampled radial trajectories has previously been successfully demonstrated using calibrated radial generalized autocalibrating partially parallel acquisition (rGRAPPA). A self-calibrated approach for rGRAPPA is proposed that removes the need for the calibration prescan. To investigate the effect of various parameters on image quality, a comprehensive imaging study on one normal swine was performed. Root mean squared errors (RMSEs) were computed with respect to gold standard acquisitions, and several acquisition/reconstruction strategies were compared. Additionally, the method was tested on 13 human subjects, and RMSEs relative to standard through-time radial GRAPPA were computed. Real-time images with high spatiotemporal resolution were obtained. Image quality was comparable to calibrated through-time rGRAPPA with endocardial and epicardial borders clearly delineated. In the swine, the average RMSE between self-calibrated and gold-standard calibrated images was 5.18 ± 0.84%. In normal human subjects, the average RMSE between self-calibrated and calibrated through-time rGRAPPA was 3.79 ± 0.64%. For lower accelerations rates (R = 6-8) image quality was similar to comparable calibrated scans though RMSE increased for higher degrees of undersampling (R = 12-16). Highly accelerated real-time imaging with undersampled radial trajectories without additional calibration data is feasible. Image quality was acceptable for real-time cardiac MRI applications demanding high speed. Magn Reson Med 77:250-264, 2017. © 2016 Wiley Periodicals, Inc.

A fully automated binning method for improved SHARP reconstruction of free-breathing cardiac images

A fully automated binning method for improved SHARP reconstruction of free-breathing cardiac images

Self-navigation with compressed sensing for 2D translational motion correction in free-breathing coronary MRI: a feasibility study.

PurposeRespiratory motion correction remains a challenge in coronary magnetic resonance imaging (MRI) and current techniques, such as navigator gating, suffer from sub-optimal scan efficiency and ease-of-use. To overcome these limitations, an image-based self-navigation technique is proposed that uses “sub-images” and compressed sensing (CS) to obtain translational motion correction in 2D. The method was preliminarily implemented as a 2D technique and tested for feasibility for targeted coronary imaging.MethodsDuring a 2D segmented radial k-space data acquisition, heavily undersampled sub-images were reconstructed from the readouts collected during each cardiac cycle. These sub-images may then be used for respiratory self-navigation. Alternatively, a CS reconstruction may be used to create these sub-images, so as to partially compensate for the heavy undersampling. Both approaches were quantitatively assessed using simulations and in vivo studies, and the resulting self-navigation strategies were then compared to conventional navigator gating.ResultsSub-images reconstructed using CS showed a lower artifact level than sub-images reconstructed without CS. As a result, the final image quality was significantly better when using CS-assisted self-navigation as opposed to the non-CS approach. Moreover, while both self-navigation techniques led to a 69% scan time reduction (as compared to navigator gating), there was no significant difference in image quality between the CS-assisted self-navigation technique and conventional navigator gating, despite the significant decrease in scan time.ConclusionsCS-assisted self-navigation using 2D translational motion correction demonstrated feasibility of producing coronary MRA data with image quality comparable to that obtained with conventional navigator gating, and does so without the use of additional acquisitions or motion modeling, while still allowing for 100% scan efficiency and an improved ease-of-use. In conclusion, compressed sensing may become a critical adjunct for 2D translational motion correction in free-breathing cardiac imaging with high spatial resolution. An expansion to modern 3D approaches is now warranted.

Sub-30 ms real-time, free-breathing cardiac imaging with SPIRiT

Background Combined with Variable density Incoherent SpatioTemporal Acquisition (VISTA, [1]), SPIRiT [2] reconstruction can provide an avenue for highly accelerated real-time, free-breathing cardiac imaging. Yet the high computational cost of iterative algorithms limits practical clinical implementation. The Fast Iterative Shrinkage Thresholding Algorithm (FISTA) [3] can potentially reduce the computational cost of SPIRiT [4]. In this work, we combine FISTA optimization with VISTA sampling and spatiotemporally regularized SPIRiT reconstruction to demonstrate the feasibility of real-time cine accelerated to rate 15, while providing a pathway to practical clinical application.

Self-calibrating through-time spiral GRAPPA for real-time CMR

Background Through-Time non-Cartesian GRAPPA, a novel parallel imaging method for non-Cartesian trajectories, has recently been shown to provide real-time, free-breathing cardiac images with temporal resolutions of less than 35 ms per frame [Seiberlich N, et al. MRM 2011 Dec;66 (6):1682-8]. The drawback to this method is the need for several fully-sampled datasets for calibration stemming from the non-Cartesian nature of the data, which leads to a longer overall scan time. By acquiring interleaved spiral datasets and combining them to form fully-sampled datasets for the generation of the GRAPPA weights, as in TGRAPPA [Breuer FA, et al. MRM 2005 Apr;53(4):9815.], there is no need for additional calibration data. However, this interleaved calibration method poses the risk of increased artifacts if the temporal footprint of the calibration data is too long. The goal of this study is to test the extent to which self-calibrating through-time spiral GRAPPA can be used for real-time free-breathing CMR.

Artifact-reduced simultaneous MRI of multiple rats with liver cancer using PROPELLER

To explore simultaneous magnetic resonance imaging (MRI) for multiple hepatoma-bearing rats in a single session suppressing motion- and flow-related artifacts to conduct preclinical cancer research efficiently. Our institutional Animal Experimental Committee approved this study. We acquired PROPELLER (periodically rotated overlapping parallel lines with enhanced reconstruction) T2 - and diffusion-weighted images of the liver in one healthy and 11 N1-S1 hepatoma-bearing rats in three sessions using a 3-T clinical scanner and dedicated multiarray coil. We compared tumor volumes on MR images and those on specimens, evaluated apparent diffusion coefficients (ADC) of the tumor, and compared them to previously reported values. Each MRI session took 39-50 minutes from anesthesia induction to the end of scans for four rats (10-13 minutes per rat). PROPELLER provided artifact-reduced T2 - and diffusion-weighted images of the rat livers. Tumor volumes on MR images ranged from 0.04-1.81 cm(3) and were highly correlated with those on specimens. The ADC was 1.57 ± 0.37 × 10(-3) mm(2) /s (average ± SD), comparable to previously reported values. PROPELLER allowed simultaneous acquisition of artifact-reduced T2 - and diffusion-weighted images of multiple hepatoma-bearing rats. This technique can promote high-throughput preclinical MR research for liver cancer.

Improved radial GRAPPA calibration for real‐time free‐breathing cardiac imaging

To generate real-time, nongated, free-breathing cardiac images, the undersampled radial trajectory combined with parallel imaging in the form of radial GRAPPA has shown promise. However, this method starts to fail at high undersampling factors due to the assumptions that must be made for the purposes of calibrating the GRAPPA weight sets. In this manuscript, a novel through-time radial GRAPPA calibration scheme is proposed which greatly improves image quality for the high acceleration factors required for real-time cardiac imaging. This through-time calibration method offers better image quality than standard radial GRAPPA, but it requires many additional calibration frames to be acquired. By combining the through-time calibration method proposed here with the standard through-k-space radial GRAPPA calibration method, images with high acceleration factors can be reconstructed using few calibration frames. Both the through-time and the hybrid through-time/through-k-space methods are investigated to determine the most advantageous calibration parameters for an R = 6 in vivo short-axis cardiac image. Once the calibration parameters have been established, they are then used to reconstruct several in vivo real-time, free-breathing cardiac datasets with temporal resolutions better than 45 msec, including one with a temporal resolution of 35 msec and an in-plane resolution of 1.56 mm(2) .

144 Prospective self-gating for simultaneous compensation of cardiac and respiratory motion

Introduction: In cardiac imaging, self-gating principles may be applied for either cardiac triggering [1] or respiratory gating [2]. These methods simplify patient setup and scan planning as placement of physiological sensors and dedicated respiratory navigation methods become unnecessary. Also, since motion data are extracted from the imaging data themselves, motion information and actual object motion are potentially more consistent. So far, self-gating methods have been retrospective involving extended temporal coverage in acquisition to account for varying heart rates and to ensure a sufficient number of heart cycles acquired in the same respiratory state. Therefore, relatively low scan efficiencies are obtained relative to conventionally triggered techniques. In this work a prospective self-gating approach for time-efficient free breathing cardiac imaging is presented neither requiring an electrocardiogram (ECG) nor respiratory navigation. The motion data needed for synchronization are extracted and processed in real-time from repeatedly acquired data at k-space center. Image quality obtained with the proposed method was found to be comparable to the ECG triggered breathheld acquisitions while the scan efficiency was significantly increased. Methods: To repeatedly measure the k-space center a modified SSFP sequence [3] was implemented, providing a signal which contains both cardiac and respiratory synchronous variations (Figure 1). Cardiac variations arise from changes of the blood volume in the heart during the cardiac cycle. Respiratory-related variations stem from moving structures such as the liver or abdomen which shift in and out of the sensitive imaging volume during the respiratory cycle. Due to the local sensitivities of the coil elements it was found that some elements exhibit stronger cardiac variations while others provided better respiratory signal variations. This property was used to select the best elements for either cardiac triggering or respiratory gating. The best triggering signal was chosen by measuring the standard deviation between trigger times and selecting the coil with the lowest trigger variability. The best gating signal was chosen as the one having the most frequent occurrence of values above the predetermined threshold. All these calculations were performed prior to the scan in a short preparation period. Since cardiac and respiratory signals are superimposed, real-time separation is necessary in order to use motion information prospectively. To this end, two efficient filters were designed complying with real-time requirements. A Butterworth band-pass filter extracts the cardiac related signal component followed by a peak-finding algorithm providing the cardiac trigger. For extracting the respiratory motion variation an adaptive averaging filter was implemented which uses the detected triggerpoints to average over one cardiac cycle. In addition an adaptive threshold was implemented for definition of the respiratory gating window. To test the feasibility of the approach a cardiac four-chamber view of a healthy volunteer was acquired (TR = 4.5ms, TE = 2.6ms, flip angle = 60°, scan matrix = 192x186, FOV = 320x320mm, slice thickness = 8mm, 11 lines/segment, 30 cardiac phases) using a five channel cardiac coil array. To asses the quality of the extracted self-gating motion curves, signals from an ECG and the respiratory belt were simultaneously acquired and stored. As reference a retrospective self-gating acquisition described in [4] was performed. In the retrospective approach the data are evaluated after the acquisition, therefore requiring temporal oversampling of the data to ensure that every profile is acquired at least once in an acceptable motion state. Additionally, a standard ECG triggered breathheld acquisition was acquired as second reference. Results: Cardiac and respiratory signal variations could be extracted in real-time and were comparable to the wired ECG and respiratory belt signals (Figure 2). The standard deviation between the wired ECG signal and the self-gating trigger was less than 11 msec. Reconstructed images were of comparable quality relative to the retrospective self-gated approach. At the same time, scan efficiency of the prospective method was significantly increased relative to the retrospective approach (prospective: 64 sec + 10 sec preparation period; retrospective: 160 sec). Both, the prospective and the retrospective methods compared well with the standard ECG, breathheld acquisition (Figure 3). Discussion: It has been shown that cardiac and respiratory variations can be accurately detected prospectively using signals originating from the k-space center and thereby eliminating the need for external cardiac and respiratory signal detection. Compared to the retrospective self-gating approach total scan time could be reduced while image quality was well maintained. Further studies are warranted to test the robustness in larger cohorts of volunteers and patients.

Retrospective Respiratory Motion Correction for Navigated Cine Velocity Mapping

In general, high spatial and temporal resolutions in cine cardiac imaging require long scan times, making breath-hold acquisition impossible in many cases. To enable free-breathing cardiac imaging, methods such as navigator gating were developed to reduce image artifacts due to respiratory motion. Nevertheless, residual image blurring is seen in images acquired late in the cardiac cycle. Image blurring itself hampers accurate blood flow quantification, especially in vessels exhibiting high flows during diastole. In the present work, the navigator gating and slice tracking method was extended by using navigator information to correct for in-slice motion components throughout the cardiac cycle. For this purpose, a standard two-dimensional (2D) cine phase contrast sequence with navigator gating and slice position correction was used, and navigator information was recorded along with the raw k-space data. In postprocessing, in-plane motion components arising from respiration during the actual data acquisition were estimated and corrected according to the Fourier shift theorem. In phantom experiments, the performance of the correction algorithm for different slice angulations with respect to the navigator orientation was validated. In vivo, coronary flow measurements were performed in 9 healthy volunteers. The correction algorithm led to considerably improved vessel sharpness throughout the cardiac cycle in all measured subjects [increase in vessel sharpness: 16+/-11% (mean+/-SD)]. Furthermore, these improvements resulted in increased volume flow rates [16+/-13% (mean+/-SD)] after retrospective correction indicating the impact of the method. It is concluded that retrospective respiratory motion corrections for navigated cine two-dimensional (2D) velocity mapping can correct for in-plane motion components, providing better image quality for phases acquired late in the cardiac cycle. Therefore, this method holds promise in particular for free-breathing coronary flow quantification.

Motion correction with PROPELLER MRI: application to head motion and free-breathing cardiac imaging.

A method for motion correction, involving both data collection and reconstruction, is presented. The PROPELLER MRI method collects data in concentric rectangular strips rotated about the k-space origin. The central region of k-space is sampled for every strip, which (a) allows one to correct spatial inconsistencies in position, rotation, and phase between strips, (b) allows one to reject data based on a correlation measure indicating through-plane motion, and (c) further decreases motion artifacts through an averaging effect for low spatial frequencies. Results are shown in which PROPELLER MRI is used to correct for bulk motion in head images and respiratory motion in nongated cardiac images. Magn Reson Med 42:963-969, 1999.