Stable and efficient retrospective 4D-MRI using non-uniformly distributed quasi-random numbers.

The purpose of this work is the development of a robust and reliable three-dimensional (3D) Cartesian imaging technique for fast and flexible retrospective 4D abdominal MRI during free breathing. To this end, a non-uniform quasi random (NU-QR) reordering of the phase encoding (k y -k z ) lines was incorporated into 3D Cartesian acquisition. The proposed sampling scheme allocates more phase encoding points near the k-space origin while reducing the sampling density in the outer part of the k-space. Respiratory self-gating in combination with SPIRiT-reconstruction is used for the reconstruction of abdominal data sets in different respiratory phases (4D-MRI). Six volunteers and three patients were examined at 1.5 T during free breathing. Additionally, data sets with conventional two-dimensional (2D) linear and 2D quasi random phase encoding order were acquired for the volunteers for comparison. A quantitative evaluation of image quality versus scan times (from 70 s to 626 s) for the given sampling schemes was obtained by calculating the normalized mutual information (NMI) for all volunteers. Motion estimation was accomplished by calculating the maximum derivative of a signal intensity profile of a transition (e.g. tumor or diaphragm). The 2D non-uniform quasi-random distribution of phase encoding lines in Cartesian 3D MRI yields more efficient undersampling patterns for parallel imaging compared to conventional uniform quasi-random and linear sampling. Median NMI values of NU-QR sampling are the highest for all scan times. Therefore, within the same scan time 4D imaging could be performed with improved image quality. The proposed method allows for the reconstruction of motion artifact reduced 4D data sets with isotropic spatial resolution of 2.1 × 2.1 × 2.1 mm3 in a short scan time, e.g. 10 respiratory phases in only 3 min. Cranio-caudal tumor displacements between 23 and 46 mm could be observed. NU-QR sampling enables for stable 4D-MRI with high temporal and spatial resolution within short scan time for visualization of organ or tumor motion during free breathing. Further studies, e.g. the application of the method for radiotherapy planning are needed to investigate the clinical applicability and diagnostic value of the approach.

Desynchronization of Cartesian k-space sampling and periodic motion for improved retrospectively self-gated 3D lung MRI using quasi-random numbers.

PURPOSE: To demonstrate that desynchronization between Cartesian k-space sampling and periodic motion in free-breathing lung MRI improves the robustness and efficiency of retrospective respiratory self-gating. METHODS: Desynchronization was accomplished by reordering the phase (ky ) and partition (kz ) encoding of a three-dimensional FLASH sequence according to two-dimensional, quasi-random (QR) numbers. For retrospective respiratory self-gating, the k-space center signal (DC signal) was acquired separately after each encoded k-space line. QR sampling results in a uniform distribution of k-space lines after gating. Missing lines resulting from the gating process were reconstructed using iterative GRAPPA. Volunteer measurements were performed to compare quasi-random with conventional sampling. Patient measurements were performed to demonstrate the feasibility of QR sampling in a clinical setting. RESULTS: The uniformly sampled k-space after retrospective gating allows for a more stable iterative GRAPPA reconstruction and improved ghost artifact reduction compared with conventional sampling. It is shown that this stability can either be used to reduce the total scan time or to reconstruct artifact-free data sets in different respiratory phases, both resulting in an improved efficiency of retrospective respiratory self-gating. CONCLUSION: QR sampling leads to desynchronization between repeated data acquisition and periodic respiratory motion. This results in an improved motion artifact reduction in shorter scan time. Magn Reson Med 77:787-793, 2017. © 2016 International Society for Magnetic Resonance in Medicine.

In vivo imaging of the spectral line broadening of the human lung in a single breathhold.

PURPOSE: To present a technique, which allows for the in vivo quantification of the spectral line broadening of the human lung in a single breathhold. The line broadening is an interesting parameter of the lung because it can provide information about important lung properties, namely: inflation and oxygen uptake. The proposed technique integrates the asymmetric spin-echo (ASE) approach, which is commonly used to quantify the line broadening, with a single shot turbo spin-echo pulse sequence with half-Fourier acquisition (HASTE), to reduce the acquisition times. MATERIALS AND METHODS: Imaging experiments were performed at 1.5 Tesla on 14 healthy volunteers, using a ASE-prepared HASTE sequence. The line broadening was quantified using a two-points method. Data were acquired at different breathing states: functional residual capacity (FRC) and total lung capacity (TLC), and with different breathing gases: room-air and pure-oxygen. Image acquisition was accomplished within a single breathhold of approximately 15 s duration. The violation of the Carr-Purcell-Meiboom-Gill conditions, deriving from inhomogeneities of the static magnetic field, was overcome by means of radiofrequency-phase cycling and generalized autocalibrating partially parallel acquisitions (GRAPPA) reconstruction. RESULTS: Significant increase of the line broadening was observed with both lung inflation and oxygen concentration (P < 0.0001). Values of the line broadening obtained within the lung parenchyma at different breathing states (1.48 ± 0.29 ppm at FRC and 1.95 ± 0.43 ppm at TLC) are in agreement with previous reports and show excellent reproducibility, with a coefficient of variation <0.03. The mean relative difference observed with oxygen-enhancement was approximately 14%. CONCLUSION: The presented technique offers a robust way to quantify the spectral line broadening of the human lung in vivo. Image acquisition can be accomplished in a single breathhold, which could be suitable for clinical applications on patients with lung diseases. J. Magn. Reson. Imaging 2016;44:745-757.

Self-navigation under non-steady-state conditions: Cardiac and respiratory self-gating of inversion recovery snapshot FLASH acquisitions in mice.

PURPOSE: An algorithm is presented to enable cardiac and respiratory self-gating in combination with Inversion Recovery Look-Locker read-outs. METHODS: A radial inversion recovery snapshot FLASH sequence was adapted for retrospective cardiac T1 measurements in mice. Cardiac and respiratory data were extracted from the k-space center of radial projections and an adapted method for retrospective cardiac synchronization is introduced. Electrocardiogram (ECG) data was acquired concurrently for validation of the proposed self-gating technique. T1 maps generated by the proposed technique were compared with maps reconstructed with the ECG reference. RESULTS: Respiratory gating and cardiac trigger points could be obtained for the whole time course of the relaxation dynamic and correlate very well to the ECG signal. T1 maps reconstructed with the self-gating technique are in very good agreement with maps reconstructed with the external reference. CONCLUSION: The proposed method extends "wireless" cardiac MRI to non-steady-state inversion recovery measurements. T1 maps were generated with a quality comparable to ECG based reconstructions. As the method does not rely on an ECG trigger signal it provides easier animal handling. Magn Reson Med 76:1887-1894, 2016. © 2016 International Society for Magnetic Resonance in Medicine.

Quantification of perfusion in murine myocardium: A retrospectively triggered T1 -based ASL method using model-based reconstruction.

PURPOSE: A method for the quantification of perfusion in murine myocardium is demonstrated. The method allows for the reconstruction of perfusion maps on arbitrary time points in the heart cycle while addressing problems that arise due to the irregular heart beat of mice. METHODS: A flow-sensitive alternating inversion recovery arterial spin labeling method using an untriggered FLASH-read out with random sampling is used. Look-Locker conditions are strictly maintained. No dummy pulses or mechanism to reduce deviation from Look-Locker conditions are needed. Electrocardiogram and respiratory data are recorded for retrospective gating and triggering. A model-based technique is used to reconstruct missing k-space data to cope with the undersampling inherent in retrospectively gated methods. Acquisition and reconstruction were validated numerically and in phantom measurements before in vivo experimentation. RESULTS: Quantitative perfusion maps were acquired within a single slice measurement time of 11 min. Perfusion values are in good accordance to literature values. Myocardial infarction could be clearly visualized and results were confirmed with histological results. CONCLUSION: The proposed method is capable of producing quantitative perfusion maps on arbitrary positions in the heart cycle within a short measurement time. The method is robust against irregular breathing patterns and heart rate changes and can be implemented on all scanners.

Blood volume fraction imaging of the human lung using intravoxel incoherent motion.

PURPOSE: To present a technique for non-contrast-enhanced in vivo imaging of the blood volume fraction of the human lung. The technique is based on the intravoxel incoherent motion (IVIM) approach. However, a substantial novelty is introduced here: the need for external diffusion sensitizing gradients is eliminated by exploiting the internal magnetic field gradients typical of the lung tissue, due to magnetic susceptibility differences at air/tissue interfaces. MATERIALS AND METHODS: A single shot turbo spin-echo sequence with stimulated-echo preparation and electrocardiograph synchronization was used for acquisition. Two images were acquired in a single breath-hold of 10 seconds duration: one reference image and one blood-suppressed image. The blood volume fraction was quantified using a two-compartment signal decay model, as given by the IVIM theory. Experiments were performed at 1.5T in eight healthy volunteers. RESULTS: Values of the blood volume fraction obtained within the lung parenchyma (36 ± 16%) are in good agreement with previous reports, obtained using contrast-enhanced magnetic resonance angiography (33%), and show relatively good reproducibility. CONCLUSION: The presented technique offers a robust way to quantify the blood volume fraction of the human lung parenchyma without using contrast agents. Image acquisition can be accomplished in a single breath-hold and could be suitable for clinical applications on patients with lung diseases. J. Magn. Reson. Imaging 2015;41:1454-1464. © 2014 Wiley Periodicals, Inc.

Myocardial perfusion quantification using the T1 -based FAIR-ASL method: the influence of heart anatomy, cardiopulmonary blood flow and look-locker readout.

PURPOSE: The quantification of myocardial perfusion using a Look-Locker flow-sensitive alternating inversion recovery- arterial spin labeling experiment is considered. Due to the anatomy of the heart, a substantial but unintended partial inversion of the inflowing blood occurs during the slice-selective inversion. Both, the partial inversion as well as the Look-Locker pulse train, influence the myocardial perfusion quantification and are addressed in this work. METHODS: The mean relaxation time approximation is used to calculate the monoexponential relaxation time of the signal in perfused tissue under Look-Locker readout. The left ventricular blood serves as an approximation of the inflowing blood in the description of FAIR-ASL measurements with global and slice-selective inversion to correctly quantify the myocardial perfusion. RESULTS: The analysis shows that the myocardial perfusion can be overestimated if the T1 -based quantification method is not adapted respecting the Look-Locker pulse train explicitly. Additionally, it turns out that without correction for the partial inversion of the blood pool during the slice-selective inversion the myocardial perfusion is underestimated. CONCLUSION: It is shown that the Look-Locker readout as well as the nonideal slice-selective inversion experiment have a considerable influence and have to be included properly to correctly quantify myocardial perfusion.

Fast retrospectively triggered local pulse-wave velocity measurements in mice with CMR-microscopy using a radial trajectory.

BACKGROUND: The aortic pulse-wave velocity (PWV) is an important indicator of cardiovascular risk. In recent studies MRI methods have been developed to measure this parameter noninvasively in mice. Present techniques require additional hardware for cardiac and respiratory gating. In this work a robust self-gated measurement of the local PWV in mice without the need of triggering probes is proposed. METHODS: The local PWV of 6-months-old wild-type C57BL/6J mice (n=6) was measured in the abdominal aorta with a retrospectively triggered radial Phase Contrast (PC) MR sequence using the flow-area (QA) method. A navigator signal was extracted from the CMR data of highly asymmetric radial projections with short repetition time (TR=3 ms) and post-processed with high-pass and low-pass filters for retrospective cardiac and respiratory gating. The self-gating signal was used for a reconstruction of high-resolution Cine frames of the aortic motion. To assess the local PWV the volume flow Q and the cross-sectional area A of the aorta were determined. The results were compared with the values measured with a triggered Cartesian and an undersampled triggered radial PC-Cine sequence. RESULTS: In all examined animals a self-gating signal could be extracted and used for retrospective breath-gating and PC-Cine reconstruction. With the non-triggered measurement PWV values of 2.3±0.2 m/s were determined. These values are in agreement with those measured with the triggered Cartesian (2.4±0.2 m/s) and the triggered radial (2.3±0.2 m/s) measurement. Due to the strong robustness of the radial trajectory against undersampling an acceleration of more than two relative to the prospectively triggered Cartesian sampling could be achieved with the retrospective method. CONCLUSION: With the radial flow-encoding sequence the extraction of a self-gating signal is feasible. The retrospective method enables a robust and fast measurement of the local PWV without the need of additional trigger hardware.