Balanced phase-contrast steady-state free precession (PC-SSFP): a novel technique for velocity encoding by gradient inversion.

A technique for measuring velocity is presented that combines cine phase contrast (PC) MRI and balanced steady-state free precession (SSFP) imaging, and is thus termed PC-SSFP. Flow encoding was performed without the introduction of additional velocity encoding gradients in order to keep the repetition time (TR) as short as in typical SSFP imaging sequences. Sensitivity to through-plane velocities was instead established by inverting (i.e., negating) all gradients along the slice-select direction. Velocity sensitivity (VENC) could be adjusted by altering the first moments of the slice-select gradients. Disturbances of the SSFP steady state were avoided by acquiring different flow echoes in consecutively (i.e., sequentially) executed scans, each over several cardiac cycles, using separate steady-state preparation periods. A comparison of phantom measurements with those from established 2D-cine-PC MRI demonstrated excellent correlation between both modalities. In examinations of volunteers, PC-SSFP exhibited a higher intrinsic signal-to-noise ratio (SNR) and consequently low phase noise in measured velocities compared to conventional PC scans. An additional benefit of PC-SSFP is that it relies less on in-flow-dependent signal enhancement, and thus yields more uniform SNRs and better depictions of vessel geometry throughout the whole cardiac cycle in structures with slow and/or pulsatile flow.     

Referenceless phase velocity mapping using balanced SSFP

Phase contrast MRI (PC‐MRI) is an established technique for measuring blood flow velocities in vivo. Although spoiled gradient recalled echo (GRE) PC‐MRI is the most widely used pulse sequence today, balanced steady state free precession (SSFP) PC‐MRI has been shown to produce accurate velocity estimates with superior SNR efficiency. We propose a referenceless approach to flow imaging that exploits the intrinsic refocusing property of balanced SSFP, and achieves up to a 50% reduction in total scan time. With the echo time set to exactly one half of the sequence repetition time (TE = TR/2), we show that non‐flow‐related image phase tends to vary smoothly across the field‐of‐view, and can be estimated from static tissue regions to produce a phase reference for nearby voxels containing flowing blood. This approach produces accurate in vivo one‐dimensional velocity estimates in half the scan time compared with conventional balanced SSFP phase‐contrast methods. We also demonstrate the feasibility of referenceless time‐resolved 3D flow imaging (called “7D” flow) in the carotid bifurcation from just three acquisitions. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Aortic pulse wave velocity measurements with undersampled 4D flow‐sensitive MRI: comparison with 2D and algorithm determination

Purpose:

To compare pulse wave velocity (PWV) measurements obtained from radially undersampled 4D phase‐contrast magnetic resonance imaging (PC‐MRI) with 2D PC measurements and to evaluate four PWV algorithms.

Materials and Methods:

PWV was computed from radially undersampled 3D, 3‐directionally velocity‐encoded PC‐MRI (4D) acquisitions performed on a 3T MR scanner in 18 volunteers. High temporal resolution 2D PC scans serving as a reference standard were available in 14 volunteers. Four PWV algorithms were tested: time‐to‐upstroke (TTU), time‐to‐peak (TTP), time‐to‐foot (TTF), and cross‐correlation (XCorr). Bland–Altman analysis was used to determine inter‐ and intraobserver reproducibility and to compare differences between algorithms. Differences in age and PWV measurements were analyzed with Student's t‐tests. The variability of age‐corrected data was assessed with a Brown‐Forsythe analysis of variance (ANOVA) test.

Results:

2D (4.6–5.3 m/s) and 4D (3.8–4.8 m/s) PWV results were in agreement with previously reported values in healthy subjects. Of the four PWV algorithms, the TTU, TTF, and XCorr algorithms gave similar and reliable results. Average biases of +0.30 m/s and ?0.01 m/s were determined for intra‐ and interobserver variability, respectively. The Brown‐Forsythe test revealed that no differences in variability could be found between 2D and 4D PWV measurements.

Conclusion:

4D PC‐MRI with radial undersampling provides reliable and reproducible measurements of PWV. TTU, TTF, and XCorr were the preferred PWV algorithms. J. Magn. Reson. Imaging 2013;37:853–859. © 2012 Wiley Periodicals, Inc.

     

Time‐resolved absolute velocity quantification with projections

Quantitative information on time‐resolved blood velocity along the femoral/popliteal artery can provide clinical information on peripheral arterial disease and complement MR angiography as not all stenoses are hemodynamically significant. The key disadvantages of the most widely used approach to time‐resolve pulsatile blood flow by cardiac‐gated velocity‐encoded gradient‐echo imaging are gating errors and long acquisition time. Here, we demonstrate a rapid nontriggered method that quantifies absolute velocity on the basis of phase difference between successive velocity‐encoded projections after selectively removing the background static tissue signal via a reference image. The tissue signal from the reference image's center k‐space line is isolated by masking out the vessels in the image domain. The performance of the technique, in terms of reproducibility and agreement with results obtained with conventional phase contrast‐MRI was evaluated at 3 T field strength with a variable‐flow rate phantom and in vivo of the triphasic velocity waveforms at several segments along the femoral and popliteal arteries. Additionally, time‐resolved flow velocity was quantified in five healthy subjects and compared against gated phase contrast‐MRI results. To illustrate clinical feasibility, the proposed method was shown to be able to identify hemodynamic abnormalities and impaired reactivity in a diseased femoral artery. For both phantom and in vivo studies, velocity measurements were within 1.5 cm/s, and the coefficient of variation was less than 5% in an in vivo reproducibility study. In five healthy subjects, the average differences in mean peak velocities and their temporal locations were within 1 cm/s and 10 ms compared to gated phase contrast‐MRI. In conclusion, the proposed method provides temporally resolved arterial velocity with a temporal resolution of 20 ms with minimal post processing. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Whole heart magnetization‐prepared steady‐state free precession coronary vein MRI

Purpose

To compare two coronary vein imaging techniques using whole‐heart balanced steady‐state free precession (SSFP) and a targeted double‐oblique spoiled gradient‐echo (GRE) sequences in combination with magnetization transfer (MT) preparation sequence for tissue contrast improvement.

Materials and Methods

Nine healthy subjects were imaged with the proposed technique. The results are compared with optimized targeted MT prepared GRE acquisitions. Both quantitative and qualitative analyses were performed to evaluate each imaging method.

Results

Whole‐heart images were successfully acquired with no visible image artifact in the vicinity of the coronary veins. The anatomical features and visual grading of both techniques were comparable. However, the targeted small slab acquisition of the left ventricular lateral wall was superior to whole‐heart acquisition for visualization of relevant information for cardiac resynchronization therapy (CRT) lead implantation.

Conclusion

We demonstrated the feasibility of whole‐heart coronary vein MRI using a 3D MT‐SSFP imaging sequence. A targeted acquisition along the lateral left ventricular wall is preferred for visualization of branches commonly used in CRT lead implantation. J. Magn. Reson. Imaging 2009;29:1293–1299. © 2009 Wiley‐Liss, Inc.     

Single scan PC‐MRI by alternating the velocity encoding gradient polarity between phase encoding steps

The purpose of this study was to develop a faster approach to phase contrast magnetic resonance imaging. This article proposes a phase contrast imaging scheme called single scan phase contrast in which the polarity of the velocity‐encoding gradient is alternated between phase encoding steps. In single scan phase contrast, ghost images due to moving spins form. The signal intensity of the ghost images is modulated by the sine of the motion‐induce phase shift. Prior to image acquisition, the region of interest containing moving spins is identified, and the field of view is configured so to avoid overlap between the object in the image and the ghost image(s) due to motion in the region of interest. The image values of the region of interest and the ghost image are used to quantify velocity. At best, single scan phase contrast reduces the total acquisition time by a factor of two when compared to phase contrast. In this study, single scan phase contrast is validated against phase contrast in phantom and in vivo. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Optimized parallel imaging for dynamic PC‐MRI with multidirectional velocity encoding

Phase contrast MRI with multidirectional velocity encoding requires multiple acquisitions of the same k‐space lines to encode the underlying velocities, which can considerably lengthen the total scan time. To reduce scan time, parallel imaging is often applied. In dynamic phase contrast MRI using standard generalized autocalibrating partially parallel acquisitions (GRAPPA), several central k‐spaces for autocalibration of the reconstruction (autocalibrating signal lines (ACS)) are typically acquired, separately for each velocity direction and each cardiac timeframe, for calculating the reconstruction weights. To further accelerate data acquisition, we developed two methods, which calculated weights with a substantially reduced number of ACSl lines. The effects on image quality and flow quantification were compared to fully sampled data, standard GRAPPA, and time‐interleaved sampling scheme in combination with generalized autocalibrating partially parallel acquisitions (TGRAPPA). The results show that the two proposed methods can clearly improve scan efficiency while maintaining image quality and accuracy of measured flow or myocardial tissue velocities. Compared to TGRAPPA, the proposed methods were more accurate in evaluating flow velocity. In conclusion, the proposed reconstruction strategies are promising for dynamic multidirectionally encoded acquisitions and can easily be implemented using the standard GRAPPA reconstruction algorithm. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Reconstruction of divergence‐free velocity fields from cine 3D phase‐contrast flow measurements

Three‐dimensional phase‐contrast velocity vector field mapping shows great potential for clinical applications; however measurement inaccuracies may limit the utility and robustness of the technique. While parts of the error in the measured velocity fields can be minimized by background phase estimation in static tissue and magnetic field monitoring, considerable inaccuracies remain. The present work introduces divergence‐reduction processing of 3D phase‐contrast flow data based on a synergistic combination of normalized convolution and divergence‐free radial basis functions. It is demonstrated that this approach effectively addresses erroneous flow for image reconstructions from both fully sampled and undersampled data. Using computer simulations and in vivo data acquired in the aorta of healthy subjects and a stenotic valve patient it is shown that divergence arising from measurement imperfections can be reduced by up to 87% resulting in improved vector field representations. Based on the results obtained it is concluded that integration of the divergence‐free condition into postprocessing of vector fields presents an efficient approach to addressing flow field inaccuracies. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Measuring aortic diameter with different MR techniques: Comparison of three‐dimensional (3D) navigated steady‐state free‐precession (SSFP), 3D contrast‐enhanced magnetic resonance angiography (CE‐MRA), 2D T2 black blood,and 2D cine SSFP

Purpose:

To compare nongated three‐dimensional (3D) contrast‐enhanced magnetic resonance angiography (CE‐MRA) with 3D‐navigated cardiac‐gated steady‐state free‐precession bright blood (3D‐nav SSFP) and noncontrast 2D techniques for ascending aorta dimension measurements.

Materials and Methods:

Twenty‐five clinical exams were reviewed to evaluate the ascending aorta at 1.5T using: breathhold cine bright blood (SSFP), cardiac‐triggered T2 black blood (T2 BB), axial 3D‐nav SSFP, and nongated 3D CE‐MRA. Three radiologists independently measured aortic size at three specified locations for each sequence. Means, SDs, interobserver correlation, and vessel edge sharpness were statistically evaluated.

Results:

Measurements were greatest for 3D‐nav SSFP and 3D CE‐MRA and smallest for T2 BB. There was no significant difference between 3D‐nav SSFP and 3D CE‐MRA (P = 0.43–0.86), but significance was observed comparing T2 BB to all sequences. Interobserver agreement was uniformly >0.9, with T2 BB best, followed closely by 3D‐nav SSFP and 2D cine SSFP, and 3D CE‐MRA being the worst. Edge sharpness was significantly poorer for 3D CE‐MRA compared to the other sequences (P < 0.001).

Conclusion:

If diameter measurements are the main clinical concern, 3D‐nav SSFP appears to be the best choice, as it has a sharp edge profile, is easy to acquire and postprocess, and shows very good interobserver correlation. J. Magn. Reson. Imaging 2010;31:177–184. © 2009 Wiley‐Liss, Inc.     

Improved MR phase‐contrast velocimetry using a novel nine‐point balanced motion‐encoding scheme with increased robustness to eddy current effects

Phase‐contrast MRI (PC‐MRI) velocimetry is a noninvasive, high‐resolution motion assessment tool. However, high motion sensitivity requires strong motion‐encoding magnetic gradients, making phase‐contrast‐MRI prone to baseline shift artifacts due to the generation of eddy currents. In this study, we propose a novel nine‐point balanced velocity‐encoding strategy, designed to be more accurate in the presence of strong and rapidly changing gradients. The proposed method was validated using a rotating phantom, and its robustness and precision were explored and compared with established approaches through computer simulations and in vivo experiments. Computer simulations yielded a 39–57% improvement in velocity–noise ratio (corresponding to a 27–33% reduction in measurement error), depending on which method was used for comparison. Moreover, in vivo experiments confirmed this by demonstrating a 26–53% reduction in accumulated velocity error over the R–R interval. The nine‐point balanced phase‐contrast‐MRI‐encoding strategy is likely useful for settings where high spatial and temporal resolution and/or high motion sensitivity is required, such as in high‐resolution rodent myocardial tissue phase mapping. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Shared velocity encoding: a method to improve the temporal resolution of phase-contrast velocity measurements

Phase-contrast magnetic resonance imaging (PC-MRI) is used routinely to measure fluid and tissue velocity with a variety of clinical applications. Phase-contrast magnetic resonance imaging methods require acquisition of additional data to enable phase difference reconstruction, making real-time imaging problematic. Shared Velocity Encoding (SVE), a method devised to improve the effective temporal resolution of phase-contrast magnetic resonance imaging, was implemented in a real-time pulse sequence with segmented echo planar readout. The effect of SVE on peak velocity measurement was investigated in computer simulation, and peak velocities and total flow were measured in a flow phantom and in volunteers and compared with a conventional ECG-triggered, segmented k-space phase-contrast sequence as a reference standard. Computer simulation showed a 36% reduction in peak velocity error from 8.8 to 5.6% with SVE. A similar reduction of 40% in peak velocity error was shown in a pulsatile flow phantom. In the phantom and volunteers, volume flow did not differ significantly when measured with or without SVE. Peak velocity measurements made in the volunteers using SVE showed a higher concordance correlation (0.96) with the reference standard than non-SVE (0.87). The improvement in effective temporal resolution with SVE reconstruction has a positive impact on the precision and accuracy of real-time phase-contrast magnetic resonance imaging peak velocity measurements.     

Time‐resolved three‐dimensional (3D) phase‐contrast (PC) balanced steady‐state free precession (bSSFP)

In this study the feasibility of a time‐resolved, three‐dimensional (3D), three‐directional flow‐sensitive balanced steady‐state free precession (bSSFP) sequence is demonstrated. Due to its high signal‐to‐noise ratio (SNR) in blood and cerebrospinal fluid (CSF) this type of sequence is particularly effective for acquisition of blood and CSF flow velocities. Flow sensitivity was achieved with the phase‐contrast (PC) technique, implementing a custom algorithm for calculation of optimal gradient parameters. Techniques to avoid the most important sources of bSSFP‐related artifacts (including distortion due to eddy currents and signal voids due to flow‐related steady‐state disruption) are also presented. The technique was validated by means of a custom flow phantom, and in vivo experiments on blood and CSF were performed to demonstrate the suitability of this sequence for human studies. Accurate depiction of blood flow in the cerebral veins and of CSF flow in the cervical portion of the neck was obtained. Possible applications of this technique might include the study of CSF flow patterns, direct in vivo study of pathologies such as hydrocephalus and Chiari malformation, and validation for the existing CSF circulation model. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Spatially encoded phase‐contrast MRI—3D MRI movies of 1D and 2D structures at millisecond resolution

This work demonstrates that the principles underlying phase‐contrast MRI may be used to encode spatial rather than flow information along a perpendicular dimension, if this dimension contains an MRI‐visible object at only one spatial location. In particular, the situation applies to 3D mapping of curved 2D structures which requires only two projection images with different spatial phase‐encoding gradients. These phase‐contrast gradients define the field of view and mean spin‐density positions of the object in the perpendicular dimension by respective phase differences. When combined with highly undersampled radial fast low angle shot (FLASH) and image reconstruction by regularized nonlinear inversion, spatial phase‐contrast MRI allows for dynamic 3D mapping of 2D structures in real time. First examples include 3D MRI movies of the acting human hand at a temporal resolution of 50 ms. With an even simpler technique, 3D maps of curved 1D structures may be obtained from only three acquisitions of a frequency‐encoded MRI signal with two perpendicular phase encodings. Here, 3D MRI movies of a rapidly rotating banana were obtained at 5 ms resolution or 200 frames per second. In conclusion, spatial phase‐contrast 3D MRI of 2D or 1D structures is respective two or four orders of magnitude faster than conventional 3D MRI. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.