High-resolution cine MRI with TGRAPPA for fast assessment of left ventricular function at 3 Tesla

Simultaneous multi-parametric acquisition and reconstruction techniques in cardiac magnetic resonance imaging: Basic concepts and status of clinical development

Simultaneous multi-parametric acquisition and reconstruction techniques (SMART) are gaining attention for their potential to overcome some of cardiovascular magnetic resonance imaging's (CMR) clinical limitations. The major advantages of SMART lie within their ability to simultaneously capture multiple "features" such as cardiac motion, respiratory motion, T1/T2 relaxation. This review aims to summarize the overarching theory of SMART, describing key concepts that many of these techniques share to produce co-registered, high quality CMR images in less time and with less requirements for specialized personnel. Further, this review provides an overview of the recent developments in the field of SMART by describing how they work, the parameters they can acquire, their status of clinical testing and validation, and by providing examples for how their use can improve the current state of clinical CMR workflows. Many of the SMART are in early phases of development and testing, thus larger scale, controlled trials are needed to evaluate their use in clinical setting and with different cardiac pathologies.

Single-Breath-Hold Evaluation of Cardiac Function with Use of Time-Resolved Parallel Cardiac Magnetic Resonance

Using cardiac magnetic resonance, we tested whether a single-breath-hold approach to cardiac functional evaluation was equivalent to the established multiple-breath-hold method. We examined 39 healthy volunteers (mean age, 31.9 ± 11.4 yr; 22 men) by using 1.5 T with multiple breath-holds and our proposed single breath-hold. Left ventricular and right ventricular ejection fractions (LVEF and RVEF), LV and RV end-diastolic volumes (LVEDV and RVEDV), and LV myocardial mass (LVMM) were compared by using Bland-Altman plots; LVEF and RVEF were tested for equivalence by inclusion of 95% confidence intervals (CIs). Equivalence of the methods was assumed within the range of -5% to 5%. In the multiple- versus the single-breath-hold method, LVEF was 0.62 ± 0.05 versus 0.62 ± 0.04, and RVEF was 0.59 ± 0.06 versus 0.59 ± 0.07. The mean difference in both methods was -0.2% (95% CI, -1 to 0.6) for LVEF and 0.3% (95% CI, -0.8 to 1.5) for RVEF. The mean differences between methods fit within the predetermined range of equivalence, including the 95% CI. The mean relative differences between the methods were 3.8% for LVEDV, 4.5% for RVEDV, and 1.6% for LVMM. Results of our single-breath-hold method to evaluate LVEF and RVEF were equivalent to those of the multiple-breath-hold technique. In addition, LVEDV, RVEDV, and LVMM showed low bias between methods.

Improved k-t PCA Algorithm Using Artificial Sparsity in Dynamic MRI

The k-t principal component analysis (k-t PCA) is an effective approach for high spatiotemporal resolution dynamic magnetic resonance (MR) imaging. However, it suffers from larger residual aliasing artifacts and noise amplification when the reduction factor goes higher. To further enhance the performance of this technique, we propose a new method called sparse k-t PCA that combines the k-t PCA algorithm with an artificial sparsity constraint. It is a self-calibrated procedure that is based on the traditional k-t PCA method by further eliminating the reconstruction error derived from complex subtraction of the sampled k-t space from the original reconstructed k-t space. The proposed method is tested through both simulations and in vivo datasets with different reduction factors. Compared to the standard k-t PCA algorithm, the sparse k-t PCA can improve the normalized root-mean-square error performance and the accuracy of temporal resolution. It is thus useful for rapid dynamic MR imaging.

Recent advances in parallel imaging for MRI

Magnetic Resonance Imaging (MRI) is an essential technology in modern medicine. However, one of its main drawbacks is the long scan time needed to localize the MR signal in space to generate an image. This review article summarizes some basic principles and recent developments in parallel imaging, a class of image reconstruction techniques for shortening scan time. First, the fundamentals of MRI data acquisition are covered, including the concepts of k-space, undersampling, and aliasing. It is demonstrated that scan time can be reduced by sampling a smaller number of phase encoding lines in k-space; however, without further processing, the resulting images will be degraded by aliasing artifacts. Nearly all modern clinical scanners acquire data from multiple independent receiver coil arrays. Parallel imaging methods exploit properties of these coil arrays to separate aliased pixels in the image domain or to estimate missing k-space data using knowledge of nearby acquired k-space points. Three parallel imaging methods-SENSE, GRAPPA, and SPIRiT-are described in detail, since they are employed clinically and form the foundation for more advanced methods. These techniques can be extended to non-Cartesian sampling patterns, where the collected k-space points do not fall on a rectangular grid. Non-Cartesian acquisitions have several beneficial properties, the most important being the appearance of incoherent aliasing artifacts. Recent advances in simultaneous multi-slice imaging are presented next, which use parallel imaging to disentangle images of several slices that have been acquired at once. Parallel imaging can also be employed to accelerate 3D MRI, in which a contiguous volume is scanned rather than sequential slices. Another class of phase-constrained parallel imaging methods takes advantage of both image magnitude and phase to achieve better reconstruction performance. Finally, some applications are presented of parallel imaging being used to accelerate MR Spectroscopic Imaging.

Cardiovascular magnetic resonance evaluation of left ventricular peak filling rate using steady-state free precession and phase contrast sequences

BACKGROUND

We investigated a practical method to measure peak filling rate (PFR) as an indicator of diastolic function of the left ventricle. Ten adult volunteers underwent cine MR imaging using steady-state free precession (SSFP) and phase contrast (PC) sequences to measure PFR. Two PC image sets were acquired at the mitral valve orifice, and PFR was determined from the set with high true temporal resolution (temporal PC method) or with high spatial resolution (spatial PC method). SSFP images covering the left ventricle were acquired, and a time-volume curve was generated around the peak filling phase. PFR was determined using parabolic curve fitting on the first-derivative curve of the LV time-volume curve.

FINDINGS

PFR values estimated by the PC methods correlated well with those estimated by the SSFP method, despite apparent underestimation. The underestimation was smaller for the temporal PC method (12 %) than for the spatial PC method (28 %). Intra- and inter-observer repeatabilities were better for the PC methods than for the SSFP method.

CONCLUSIONS

PFR measurement by PC imaging with high true temporal resolution is convenient and offers excellent repeatability and acceptable accuracy, indicating suitability for clinical use.

Reliability and reproducibility of quantitative assessment of left ventricular function and volumes with 3-slice segmentation of cine steady-state free precession short axis images

OBJECTIVES

Quantitative assessment of left ventricular (LV) functional parameters in cardiac MR requires time-consuming contour tracing across multiple short axis images. This study assesses global LV functional parameters using 3-slice segmentation on steady state free precision (SSFP) cine short axis images and compares the results with conventional multi-slice segmentation of LV.

METHODS

Data were collected from 61 patients who underwent cardiac MRI for various clinical indications. Semi-automated cardiac MR software was used to trace LV contours both at multiple slices from base to apex as well as just 3 slices (base, mid, and apical) by two readers. Left ventricular ejection fraction (LVEF), LV volumes, and LV mass were calculated using both methods.

RESULTS

Bland-Altman plot revealed narrow limits of agreement (-4.4% to 5.1%) between LVEF obtained by the two methods. Bland-Altman analysis showed slightly wider limits of agreement between end-diastolic volumes (-5.0 to 12.0%; -3.9 to 8.5 ml/m(2)), end-systolic volumes (-10.9 to 14.7%; -4.1 to 6.5 ml/m(2)), and LV mass (-5.2 to 12.7%; -4.8 to 10.2g/m(2)) obtained by the two methods. There was a small mean difference between LV volumes and LV mass obtained using multi-slice and 3-slice segmentation. No statistically significant difference existed between the LV parameters obtained by the two readers using 3-slice segmentation (p>0.05). Multi-slice assessment required approximately 15 min per study while 3-slice assessment required less than 5 min.

CONCLUSIONS

3-slice segmentation of the left ventricle at basal, mid, and apical levels on cine SSFP short axis images can provide rapid and reliable assessment of LVEF with good reproducibility. The 3-slice method also provides a reasonable estimate of the LV volumes and LV mass.

Feasibility of free-breathing, GRAPPA-based, real-time cardiac cine assessment of left-ventricular function in cardiovascular patients at 3T

OBJECTIVES

To determine the feasibility of free-breathing, GRAPPA-based, real-time (RT) cine 3T cardiac magnetic resonance imaging (MRI) with high acceleration factors for the assessment of left-ventricular function in a cohort of patients as compared to conventional segmented cine imaging.

MATERIALS AND METHODS

In this prospective cohort study, subjects with various cardiac conditions underwent MRI involving two RT cine sequences (high resolution and low resolution) and standard segmented cine imaging. Standard qualitative and quantitative parameters of left-ventricular function were quantified.

RESULTS

Among 25 subjects, 24 were included in the analysis (mean age: 50.5±21 years, 67% male, 25% with cardiomyopathy). RT cine derived quantitative parameters of volumes and left ventricular mass were strongly correlated with segmented cine imaging (intraclass correlation coefficient [ICC]: >0.72 for both RT cines) but correlation for peak ejection and filling rates were moderate to poor for both RT cines (ICC<0.40). Similarly, RT cines significantly underestimated peak ejection and filling rates (>103.2±178 ml/s). Among patient-related factors, heart rate was strongly predictive for deviation of measurements (p<0.05).

CONCLUSIONS

RT cine MRI at 3T is feasible for qualitative and quantitative assessment of left ventricular function for low and high-resolution sequences but results in significant underestimation of systolic function, peak ejection and filling rates.

Quantification of left ventricular functional parameter values using 3D spiral bSSFP and through-time non-Cartesian GRAPPA

BACKGROUND

The standard clinical acquisition for left ventricular functional parameter analysis with cardiovascular magnetic resonance (CMR) uses a multi-breathhold multi-slice segmented balanced SSFP sequence. Performing multiple long breathholds in quick succession for ventricular coverage in the short-axis orientation can lead to fatigue and is challenging in patients with severe cardiac or respiratory disorders. This study combines the encoding efficiency of a six-fold undersampled 3D stack of spirals balanced SSFP sequence with 3D through-time spiral GRAPPA parallel imaging reconstruction. This 3D spiral method requires only one breathhold to collect the dynamic data.

METHODS

Ten healthy volunteers were recruited for imaging at 3 T. The 3D spiral technique was compared against 2D imaging in terms of systolic left ventricular functional parameter values (Bland-Altman plots), total scan time (Welch's t-test) and qualitative image rating scores (Wilcoxon signed-rank test).

RESULTS

Systolic left ventricular functional values were not significantly different (i.e. 3D-2D) between the methods. The 95% confidence interval for ejection fraction was -0.1 ± 1.6% (mean ± 1.96*SD). The total scan time for the 3D spiral technique was 48 s, which included one breathhold with an average duration of 14 s for the dynamic scan, plus 34 s to collect the calibration data under free-breathing conditions. The 2D method required an average of 5 min 40s for the same coverage of the left ventricle. The difference between 3D and 2D image rating scores was significantly different from zero (Wilcoxon signed-rank test, p < 0.05); however, the scores were at least 3 (i.e. average) or higher for 3D spiral imaging.

CONCLUSION

The 3D through-time spiral GRAPPA method demonstrated equivalent systolic left ventricular functional parameter values, required significantly less total scan time and yielded acceptable image quality with respect to the 2D segmented multi-breathhold standard in this study. Moreover, the 3D spiral technique used just one breathhold for dynamic imaging, which is anticipated to reduce patient fatigue as part of the complete cardiac examination in future studies that include patients.