Segmented K-space Acquisition Research Articles

Line scan-based rapid magnetic resonance imaging of repetitive motion

Two-dimensional (2D) line scan-based dynamic magnetic resonance imaging (MRI) is examined as a means to capture the interior of objects under repetitive motion with high spatiotemporal resolutions. The method was demonstrated in a 9.4-T animal MRI scanner where line-by-line segmented k-space acquisition enabled recording movements of an agarose phantom and quail eggs in different conditions—raw and cooked. A custom MR-compatible actuator which utilized the Lorentz force on its wire loops in the scanner’s main magnetic field effectively induced the required periodic movements of the objects inside the magnet. The line-by-line k-space segmentation was achieved by acquiring a single k-space line for every frame in a motion period before acquisition of another line with a different phase-encode gradient in the succeeding motion period. The reconstructed time-course images accurately represented the objects’ displacements with temporal resolutions up to 5.5 ms. The proposed method can drastically increase the temporal resolution of MRI for imaging rapid periodic motion of objects while preserving adequate spatial resolution for internal details when their movements are driven by a reliable motion-inducing mechanism.

Realistic Dynamic Numerical Phantom for MRI of the Upper Vocal Tract.

Dynamic and real-time MRI (rtMRI) of human speech is an active field of research, with interest from both the linguistics and clinical communities. At present, different research groups are investigating a range of rtMRI acquisition and reconstruction approaches to visualise the speech organs. Similar to other moving organs, it is difficult to create a physical phantom of the speech organs to optimise these approaches; therefore, the optimisation requires extensive scanner access and imaging of volunteers. As previously demonstrated in cardiac imaging, realistic numerical phantoms can be useful tools for optimising rtMRI approaches and reduce reliance on scanner access and imaging volunteers. However, currently, no such speech rtMRI phantom exists. In this work, a numerical phantom for optimising speech rtMRI approaches was developed and tested on different reconstruction schemes. The novel phantom comprised a dynamic image series and corresponding k-space data of a single mid-sagittal slice with a temporal resolution of 30 frames per second (fps). The phantom was developed based on images of a volunteer acquired at a frame rate of 10 fps. The creation of the numerical phantom involved the following steps: image acquisition, image enhancement, segmentation, mask optimisation, through-time and spatial interpolation and finally the derived k-space phantom. The phantom was used to: (1) test different k-space sampling schemes (Cartesian, radial and spiral); (2) create lower frame rate acquisitions by simulating segmented k-space acquisitions; (3) simulate parallel imaging reconstructions (SENSE and GRAPPA). This demonstrated how such a numerical phantom could be used to optimise images and test multiple sampling strategies without extensive scanner access.

Rapid volumetric T1 mapping of the abdomen using three-dimensional through-time spiral GRAPPA.

To develop an ultrafast T1 mapping method for high-resolution, volumetric T1 measurements in the abdomen. The Look-Locker method was combined with a stack-of-spirals acquisition accelerated using three-dimensional (3D) through-time spiral GRAPPA reconstruction for fast data acquisition. A segmented k-space acquisition scheme was proposed and the time delay between segments for the recovery of longitudinal magnetization was optimized using Bloch equation simulations. The accuracy of this method was validated in a phantom experiment and in vivo T1 measurements were performed with 35 asymptomatic subjects on both 1.5 Tesla (T) and 3T MRI systems. Phantom experiments yielded close agreement between the proposed method and gold standard measurements for a large range of T1 values (200 to 1600 ms). The in vivo results further demonstrate that high-resolution T1 maps (2 × 2 × 4 mm(3)) for 32 slices can be achieved in a single clinically feasible breath-hold of approximately 20 s. The T1 values for multiple organs and tissues in the abdomen are in agreement with the published literature. A high-resolution 3D abdominal T1 mapping technique was developed, which allows fast and accurate T1 mapping of multiple abdominal organs and tissues in a single breath-hold.

Optimized three‐dimensional sodium imaging of the human heart on a clinical 3T scanner

Optimization of sequence and sequence parameters to allow three-dimensional (3D) sodium imaging of the entire human heart in vivo in a clinically reasonable time. A stack of spirals pulse sequence was optimized for cardiac imaging by considering factors such as spoiling, nutation angles, repetition time, echo time, T1/T2 relaxation, off-resonance, data acquisition window, motion, and segmented k-space acquisition. Simulations based on Bloch equations as well as the exact trajectory used for data acquisition provided the basis for choice of parameter combinations for sodium imaging. Sodium phantom scanning was used to validate the choice of parameters and for corroboration with simulations. In vivo cardiac imaging in six volunteers was also performed with an optimized sequence. Phantom studies showed good correlation with simulation results. Images obtained from human volunteers showed that the heart can be imaged with a nominal resolution of 5 × 5 × 10 mm(3) and with a signal-to-noise ratio >15 (in the septum) in about 6-10 minutes. Long axis views of the reformatted human heart show true 3D imaging capability. Optimization of the sequence and its parameters allowed in vivo 3D sodium imaging of the entire human heart in a clinically reasonable time.

IRM cardiaque : technologie actuelle, applications cliniques et perspectives futures

The field of cardiovascular MRI has evolved rapidly over the past decade, feeding new applications across a broad spectrum of clinical and research areas. Advances in magnet hardware technology, and key developments such as segmented k-space acquisitions, advanced motion encoding techniques, ultra-rapid perfusion imaging and delayed myocardial enhancement imaging have all contributed to a revolution in how patients with ischemic and non-ischemic heart disease are diagnosed and treated. Actually, cardiac MRI is a widely accepted method as the “gold standard” for detection and characterization of many forms of cardiac diseases. The aim of this review is to present an overview of cardiac MRI technology, advances in clinical applications, and future directions.

3D saturation recovery imaging for free breathing myocardial T1 mapping

Background Longitudinal magnetization relaxation time (T1) mapping can overcome limitations of late gadolinium enhancement for the detection of diffuse fibrosis in the myocardium. Several T1 mapping methods have been proposed in recent years; however most of them are limited to 2D breath-hold acquisitions with associated limitations in signal-to-noise ratio (SNR) and spatial resolution. A MOdified Look-Locker Inversion recovery (MOLLI) approach has been widely used, however systematic T1 errors have been reported which can be overcome by using a Saturation recovery single SHot Acquisition (SASHA). In this work, we extend the 2D SASHA to 3D using a 1D diaphragmatic navigator for respiratory motion correction and segmented k-space acquisitions. The proposed free breathing 3D SASHA method was compared to breathhold 2D SASHA and 2D MOLLI in healthy volunteers.

Cardiac arterial spin labeling using segmented ECG‐gated Look‐Locker FAIR: Variability and repeatability in preclinical studies

MRI is important for the assessment of cardiac structure and function in preclinical studies of cardiac disease. Arterial spin labeling techniques can be used to measure perfusion noninvasively. In this study, an electrocardiogram-gated Look-Locker sequence with segmented k-space acquisition has been implemented to acquire single slice arterial spin labeling data sets in 15 min in the mouse heart. A data logger was introduced to improve data quality by: (1) allowing automated rejection of respiration-corrupted images, (2) providing additional prospective gating to improve consistency of acquisition timing, and (3) allowing the recombination of uncorrupted k-space lines from consecutive data sets to reduce respiration corruption. Finally, variability and repeatability of perfusion estimation within-session, between-session, between-animal, and between image rejection criteria were assessed in mice. The criterion used to reject images from the T(1) fit was shown to affect the perfusion estimation. These data showed that the between-animal coefficient of variability (24%) was greater than the between-session variability (17%) and within-session variability (11%). Furthermore, the magnitude of change in perfusion required to detect differences was 30% (within-session) and 55% (between-session) according to Bland-Altman repeatability analysis. These technique developments and repeatability statistics will provide a platform for future preclinical studies applying cardiac arterial spin labeling.

Real-time and single shot CMR increases throughput and improves reliability

Summary The purpose of this study is to compare exam time and quality of images obtained using standard segmented kspace acquisitions versus single-shot and real time techniques that require no breath-holds. Background The typical CMR exam utilizes segmented k-space acquisitions that require repeated breath-holds, regular cardiac rhythm, and total exam times on the order of forty-five minutes. Widespread utilization of CMR has been hampered by these prolonged exam times and limited reliability in patients with irregular rhythm and/or inability to breath-hold. Methods Cardiac MRI studies performed at our lab over a twenty-five day period were retrospectively reviewed and 16 consecutive studies identified that did not involve stress imaging or MR angiography; the majority included cine, late gadolinium enhancement (LGE), and velocity mapping. Indications for CMR were: cardiomyopathy 11, cardiac mass 2, valvular 1, viability 1, pericardial 1. Studies were separated into two groups: eight were performed using standard segmented k-space acquisitions and eight in which cardiac arrhythmia or inability to breath-hold led to the use of real-time and single-shot methods exclusively. All images were qualitatively assessed for image quality, ability to estimate function.

Noninterleaved Velocity Encodings for Improved Temporal and Spatial Resolution in Phase-Contrast Magnetic Resonance Imaging

A segmented k-space acquisition technique using noninterleaved velocity encodings is presented to reduce spatial and temporal blur in phase-contrast cardiovascular magnetic resonance imaging. A translating phantom with pulsatile flow was used to simulate imaging of coronary arteries on a 1.5-T GE Echospeed scanner, using both interleaved and noninterleaved velocity encodings. The results demonstrate that the use of noninterleaved velocity encodings reduces spatial and temporal blur by improving the temporal resolution.

Lung MRI using an MR‐compatible active breathing control (MR‐ABC)

This work introduces an MR-compatible active breathing control device (MR-ABC) that can be applied to lung imaging. An MR-ABC consists of a pneumotachograph for respiratory monitoring and an airway-sealing unit. Using an MR-ABC, the subjects were forced to suspend breathing for short time intervals, which were used in turn for data acquisition. While the breathing flow was stopped, data acquisition was triggered by ECG to achieve simultaneous cardiac and respiratory synchronization and thus avoid artifacts from blood flow or heart movement. The flow stoppage allowed a prolonged acquisition window of up to 1.5 sec. To evaluate the potential of an MR-ABC for segmented k-space acquisition, diaphragm displacement was investigated in five volunteers and compared with images acquired using breath-holding, a respiratory belt, and free breathing. Respiratory movement was comparatively low using the breath-hold approach, a respiratory belt or an MR-ABC. During free-breathing diaphragm displacement was comparatively large. To demonstrate the potential of an MR-ABC, lung MRI was performed using whole-chest 3D gradient-echo imaging, multislice turbo spin-echo (TSE) imaging, and short tau inversion recovery TSE (STIR-TSE). Cardiorespiratory synchronization was used for each sequence. None of the volunteers reported any discomfort or inconvenience when using an MR-ABC. Flow stoppage of up to 2.5 sec per breathing cycle was well tolerated, therefore allowing for a reduction of the total imaging time as compared to usage of a respiratory belt or MR navigator.

Visualization of flow patterns distal to aortic valve prostheses in humans using a fast approach for cine 3D velocity mapping.

The fluid dynamic performance of mechanical heart valves differs from normal valves and thus is considered related to late clinical complications in patients. Since flow patterns evolving around heart valves are complex in space and time, flow visualization based on time-resolved 3D velocity data might add important information regarding the performance of specific valve designs in vivo. However, previous cine 3D techniques for three-directional phase-contrast velocity mapping suffer from long scan duration and therefore might hamper assessment in patients. A hybrid 3D phase-contrast sequence combining segmented k-space acquisition with short EPI readout trains is presented with its validation in vitro. The technique was applied to study flow patterns downstream from a bileaflet aortic prosthesis in six patients. Navigator echoes were incorporated for respiratory motion compensation. Before flow visualization, spurious phase errors due to concomitant gradient fields and eddy currents were corrected. Flow visualization was based on particle paths and animated velocity vector plots. Dedicated algorithms for particle path integration were implemented to account for the considerable motion of the ascending aorta during the cardiac cycle. A distinct flow pattern reflecting the valve design was observed closest to the valve during early flow acceleration. Reverse flow occurred adjacent to high velocity jets and above the hinge housings. Later in systole, flow became confined to the central vessel area and reverse flow along the inner aortic curvature developed. Further downstream from the valve, flow patterns varied considerably among patients, indicating the impact of varying aortic anatomy in vivo. It is concluded that MR velocity mapping is a potential tool for studying 3D flow patterns evolving around heart valve prostheses in humans. J. Magn. Reson. Imaging 2001;13:690-698.

More than meets the eye: significant regional heterogeneity in human cortical T1☆

Segmented k-space acquisition of data was used to decrease the acquisition time and to increase the imaging resolution of the precise and accurate inversion recovery (PAIR) method of measuring T1. We validated the new TurboPAIR method by measuring T1 in 158 regions of interest in 12 volunteers, using both PAIR and TurboPAIR. We found a 3% difference between methods, which could be corrected by linear regression. After validation, the TurboPAIR method was used to test a hypothesis that there is significant regional heterogeneity in cortical T1. We measured cortical gray matter T1 in 11 right-handed volunteers, in 48 regions of interest scattered over frontal and parietal cortex, and in 46 ROIs along the central sulcus (CS). We found that T1 in the CS is less than T1 elsewhere in the cortex (p < 0.001), and that there is considerable hemispheric asymmetry in T1 in gray matter, but not in white matter. In central gray structures (caudate, thalamus, nucleus pulvinarus), and in the posterior CS (sensory cortex), right hemisphere T1 was significantly greater than left hemisphere T1 (p ≤ 0.004). In cortical gray matter of the frontal lobe and anterior CS (motor cortex), left hemisphere T1 was significantly greater than right hemisphere T1 (p ≤ 0.003). These findings demonstrate that there is considerable regional heterogeneity in human cortical T1 that is unexplained by differences in tissue iron content, but may be evidence of an inherent anatomic asymmetry of the brain.

Multiphase Segmented K-Space Velocity Mapping in Pulsatile Flow Waveforms

The aim of the present study was to obtain the precision of flow measurement in breath-hold segmented k-space flow sequences. The results are based on studies of pulsatile flow in a phantom tube. The ultimate purpose is to use these sequences to measure coronary flow. In abdominal and cardiothoracic magnetic resonance imaging the image quality is degraded due to respiratory motion. In the segmented k-space acquisition method, one obtains many phase-encoding steps or views per cardiac phase. This shortens imaging time in the order of phase-encoding lines and makes it possible to image in a single breath-hold, thereby eliminating respiratory artefacts and improving edge detection. With breath-hold multiframe cine flow images it is possible to evaluate flow in all abdominal and cardiothoracic areas, including the coronary arteries. Our study shows that velocity curves shift in time when the number of k-space k y-lines per segment (LPS) are varied; this shift is linear as a function of LPS. The mean velocity V mean in the center of mass of the pulsatile peak is constant (V mean = 40.1 ± 2.9 cm/s) and time t = −10.1 × LPS + 268 ( r = 0.993, p < 0.0001). Correlation between theoretical and experimental flow curves is also linear as a function of LPS: C = −0.977 ∗ LPS ( r = 0.987, p < 0.0001). It is concluded that velocity curves move with LPS and are smoothed when the breath-hold velocity mapping is used. The more LPS is gathered the more inaccurate results are. LPS 7 or more cannot be considered clinically relevant.

Effect of and correction for in-plane myocardial motion on estimates of coronary-volume flow rates.

The sensitivities of phase-difference (PD) and complex-difference (CD) processing strategies to in-plane motion were examined theoretically and experimentally. Errors in velocity and volume flow rate (VFR) estimates were attributed to (a) motion between different velocity encodings and, in the case of segmented k-space acquisition strategies, (b) motion over the segment duration. PD estimates were found to be insensitive to in-plane motion between velocity encodings, whereas CD VFR estimates were found to be sensitive to this motion. PD estimates, however, were affected by partial volume effects. A corrected CD (CD') scheme was developed that minimizes both partial-volume and in-plane motion effects. Segmented k-space acquisitions with sequential offset and sequential interleaved offset (or centric) phase-encoding schemes were studied. Images obtained using these techniques were found to include both blurring and replication artifacts. The amount of artifact generally increased with the number of views per segment (vps) and the in-plane velocity. PD, CD, and CD' VFR estimates were found to be degraded by these artifacts. The sequential offset phase-encoding scheme generally had acceptable VFR errors (at 4 vps. a CD' VFR error of 7.0%) when averaged over the physiologic range of myocardial motion (> 12 cm second-1); however, larger errors were observed outside this range. VFR estimates obtained using the sequential interleaved phase-encoding scheme at 4 vps were unacceptable. More accurate VFR measurements were obtained using a revised segmented PC strategy, which reversed the order in which the velocity and phase encodings were interleaved. The weighted average CD' VFR error obtained using the revised strategy was 24.5% (for 4 vps). Using displacement information obtained from the two velocity-encoded images, an estimate of the in-plane velocity was obtained and used to correct the acquired data. This decreased the VFR error (weighted average CD' error at 4 vps decreased from 24.5% to -6.3%); however, the implemented correction algorithm could potentially introduced other artifacts in the images.

Segmented K-space cine breath-hold cardiovascular MR imaging: Part 1. Principles and technique.

Several well-established MR techniques are routinely used for evaluation of the heart and great vessels. Tl-weighted spin-echo images gated to the cardiac cycle provide black-blood images of the heart and vessels. Spin-echo images provide good anatomic detail, but imaging times are relatively long (approximately 5–7 min, depending on the heart rate). In addition, the images are static: only a single cardiac phase is typically acquired during this interval to avoid excessively long imaging times. Cine image acquisition often provides essential information about the cardiac chambers and flowing blood—information complementatary to the anatomic information provided on T1-weighted spin-echo images. A cine sequence of the heart consists of multiple static images that are rapidly displayed in a continuous loop. Most often, gradient-recalled echo pulse sequences are used to provide bright-blood cine images of the heart and vessels. Standard gradient-recalled echo sequences require 3–5 min to produce a cine acquisition of one to four image slices using either retrospective on prospective cardiac gating [1]. Segmented K-space cardiac sequences were described by Atkinson and Edelman [2] and allow 12–18 phases of the cardiac cycle (referred to later in this article as movie frames of a cine sequence) to be acquired in a single breath-hold using prospective cardiac gating. Segmented K-space acquisitions have been used extensively in research protocols—for example, in conjunction with MR tagging techniques [3]—although they have been less frequently used in clinical studies [4]. Commercial implementations of the pulse sequence are becoming more widely available. This essay reviews segmented K-space acquisition and illustrates different modes of application of this method for rapidly obtaining cardiac cine sequences.

CARDIOVASCULAR MR IMAGING OF CHILDREN

The role of MR imaging in the evaluation of congenital heart lesions is described. Emphasis is placed on those indications where MR imaging complements other imaging modalities, i.e., evaluation of the great vessels. Advantages and disadvantages of different MR imaging techniques (i.e., spin echo, segmented k-space acquisition, and gadolinium-enhanced angiography) are illustrated.

Black blood dual phase turbo FLASH MR imaging of the heart.

An MR imaging scheme for dark blood cardiac images acquired simultaneously in end diastolic and end systolic phases, in breath-hold times, is presented. The sequence consists of a magnetization preparation period followed by two segmented k-space acquisitions. Image quality was investigated with respect to different sequence parameters (optimal values are indicated in brackets): (a) echo time (TE)/repetition time (TR)/ flip angle (FA) (3/6.2 msec/20 degrees); (b) number of lines/ segments in the acquisition window (11 lines/segment); (c) location of acquisition windows and inversion time; and (d) thickness of slice "reinverted" during preparation (1.53 times the acquisition slice thickness). The image quality of the basal slices at end systole was critically dependent on the last parameter. High quality short axis views of the heart, with good blood signal suppression, were acquired with the optimized sequence on four volunteers from apex to base at two phases in 10 breath-holds.

Magnetic resonance imaging during upper Gl endoscopy: Technical considerations and clinical feasibility

SummaryObjective: To obtain high resolution magnetic resonance (MR) images of the intramural and extramural extent of lesions visualized during upper gastrointestinal endoscopy. Subjects and methods: Eight patients, four male and four female aged 50 to 77 years (mean 65.6 years), were studied using an endoscope that was specially constructed for use in an MR scanner using non-ferromagnetic components. A 5cm long saddle geometry receiver coil mounted on a DelrinTM former and equal in diameter to the endoscope was reverse threaded through the biopsy channel. The coil and endoscope were then inserted under direct vision into the oesophagus. After routine visual inspection the coil was placed adjacent to the site of pathology. T1-weighted spin-echo images and radiofrequency spoiled gradient-echo images with a segmented k-space acquisition were obtained with ECG gating. Results: In one patient with an oesophageal tumour extensive mediastinal involvement was seen. One patient with reflux oesophagitis showed som...

Ultra-rapid gradient echo imaging.

A novel ultra-rapid gradient echo (URGE) NMR imaging technique is introduced, which is capable of continuous high resolution 3D scanning while neither subject to fast gradient switching nor excessive RF power deposition. Sampling free induction decays instead of creating spin echoes enables maintaining a workable steady state magnetization. Due to segmented k-space acquisition, chemical shift, diffusion, and field inhomogeneity effects do not present major problems. We report on implementations acquiring from 32 x 64 x 64 partial-Fourier image sets in 0.72 s, allowing for single-shot magnetization-prepared 3D imaging, to 128 x 128 x 128 image sets in 13.3 s and 21.5 s on a standard MRI scanner.

Hybrid sequences for rapidT 1 imaging

This article generalizes the concept of the Look-LockerT1-measurement sequence to include both EPI-like and Snapshot FLASH-like elements and it provides a bridge between a number of previously demonstrated methods of quantitativeT1 imaging. It is shown that a segmentedk-space acquisition provides numerous advantages if sufficient time is available.