In-plane velocity encoding with coherent steady-state imaging

Self-gated cine phase-contrast balanced SSFP flow quantification at 0.55 T

PURPOSE

To implement cine phase-contrast balanced SSFP (PC-bSSFP) for low-field 0.55T cardiac MRI by exploiting the intrinsic flow sensitivity of the bSSFP slice-select gradient and the in-plane phase-cancelation properties of radial trajectories, enabling self-gated and referenceless PC-bSSFP flow quantification at 0.55 T.

METHODS

A free-running, tiny golden-angle radial PC-bSSFP approach was implemented on 0.55T and 1.5T systems. Cardiac and respiratory self-gating was incorporated to enable electrocardiogram-free scanning during breath-hold and free-breathing. By exploiting the intrinsic in-plane phase-cancelation properties of radial acquisitions and background phase fitting, referenceless single-point PC-bSSFP was realized. In vivo data were acquired in the ascending aorta of healthy subjects at 0.55 T and 1.5 T during breath-hold and free-breathing. Flow data, SNR, and velocity-to-noise ratio were compared relative to data obtained with phase-contrast spoiled gradient-echo variants.

RESULTS

Velocities acquired with PC-bSSFP compared well with data from phase-contrast spoiled gradient-echo (RMSEv  = 5.8 cm/s). PC-bSSFP at 0.55 T resulted in high-quality cine magnitude images and phase maps with sufficient SNR and velocity-to-noise ratio. Breath-hold and free-breathing PC-bSSFP performed very similarly, with comparable flow quantification (RMSEv  = 5.7 cm/s). Referenceless single-point PC-bSSFP results agreed well with two-point PC-bSSFP (-1.8 ± 5.2 cm/s) while reducing scan times 2-fold.

CONCLUSION

PC-bSSFP is feasible on low-field 0.55T systems, producing high-quality cine images while permitting simultaneous aortic flow measurements during breath-hold and free-breathing and without the need for electrocardiogram gating.

Improving contrast to noise ratio of resonance frequency contrast images (phase images) using balanced steady-state free precession

Recent MRI studies have exploited subtle magnetic susceptibility differences between brain tissues to improve anatomical contrast and resolution. These susceptibility differences lead to resonance frequency shifts which can be visualized by reconstructing the signal phase in conventional gradient echo (GRE) acquisition techniques. In this work, a method is proposed to improve the contrast to noise ratio per unit time (CNR efficiency) of anatomical MRI based on resonance frequency contrast. The method, based on the balanced steady-state free precession (bSSFP) MRI acquisition technique, was evaluated in its ability to generate contrast between gray and white matter in human brain at 3T and 7T. The results show substantially improved CNR efficiency of bSSFP phase images (2.85±0.21 times at 3 T and 1.71±0.11 times at 7 T) compared to the GRE data in a limited spatial area. This limited spatial coverage is attributed to the sensitivity of bSSFP to macroscopic B(0) inhomogeneities. With this CNR improvement, high resolution bSSFP phase images (resolution=0.3×0.3×2 mm(3), acquisition time=10min) acquired at 3T had sufficient CNR to allow the visualization of cortical laminar structures in invivo human primary visual cortex. Practical application of the proposed method may require improvement of B(0) homogeneity and stability by additional preparatory scans and/or compensation schemes such as respiration and drift compensation. Without these additions, the CNR benefits of the method may be limited to studies at low field or limited regions of interest.

Interleaved balanced SSFP imaging: artifact reduction using gradient waveform grouping

PURPOSE

To analyze steady-state signal distortions in interleaved balanced steady-state free precession (bSSFP) caused by slightly unbalanced eddy-current fields and develop a general strategy for mitigating these artifacts.

MATERIALS AND METHODS

We considered bSSFP sequences in which two gradient waveforms are interleaved in a "groupwise" fashion, ie, each waveform is executed consecutively two or more times before switching to the other waveform (we let "N" count the number of times each waveform is executed consecutively). The steady-state signal profile over the bSSFP passband was calculated using numerical Bloch simulations and measured experimentally in a uniform phantom. The proposed "grouped" interleaved bSSFP strategy was applied to cardiac velocity mapping using interleaved phase-contrast imaging with N=2 and N=6 in one healthy volunteer.

RESULTS

Simulation and phantom measurements show that signal distortions are systematically reduced with increasing grouping number N. For most tissues, significant suppression was achieved with N=4, and increasing N beyond this value produced only marginal gains. However, signal distortions for blood remain relatively high even for N>4. In vivo cardiac velocity mapping using interleaved phase-contrast imaging with N=6 demonstrated reduced image artifact levels compared to the N=2 acquisition.

CONCLUSION

Gradient waveform "grouping" offers a simple and general strategy for mitigating steady-state eddy-current distortions in bSSFP sequences that interleave two different gradients. Blood exhibits significant distortion even with "grouping," which is a major obstacle for cardiovascular bSSFP approaches that interleave multiple gradient waveforms. The grouping concept may also benefit applications that acquire images during the transient approach to steady state.

In vivo estimation and repeatability of force-length relationship and stiffness of the human achilles tendon using phase contrast MRI

PURPOSE

To devise a method using velocity encoded phase contrast MRI and MR-compatible dynamometry, for in vivo estimation of elastic properties of the human Achilles tendon and to assess within-session and day-to-day repeatability of this technique.

MATERIALS AND METHODS

Achilles tendon force and calcaneus-movement-adjusted displacement were measured during a submaximal isometric plantarflexion in 4 healthy subjects, four repeated trials each. The measured force-length (F-L) relationship was least-squares fitted to a cubic polynomial. Typical error was calculated for tendon displacement at multiple force levels, stiffness from the "linear region," and transition point from the displacement point separating the linear and nonlinear parts of the curve.

RESULTS

Displacements of the tendon were determined up to a maximum force of 500 N, with mean stiffness of 234 +/- 53 N/mm, mean transition point of 2.70 +/- 0.23 mm and maximum tendon displacement of 3.38 mm. Variability of tendon displacement was not dependent on the force level. Overall typical errors were 0.09 mm and 0.16 mm for within-session and between days, respectively. Typical error of transition point was 0.05 mm and 0.14 mm. Stiffness had typical errors of 47.24 N/mm and 51.95 N/mm. The tendon cross-sectional area and calcaneus displacement were found to be very significant factors in minimizing the individual differences in F-L curves.

CONCLUSION

The method yielded F-L relationships, stiffness, and transition point values that showed good within and day-to-day repeatability. The technique compared well with the more conventional one using ultrasonography. Its reliability indicates potential for measuring tendon structural changes after an injury, disease, and altered loading.

SSFP and GRE phase contrast imaging using a three-echo readout

A technique for rapid in-plane phase-contrast imaging with high signal-to-noise ratio (SNR) is described. Velocity-encoding is achieved by oscillating the readout gradient, such that each 2DFT phase-encode is acquired three times following a single RF slice-selective excitation. Three images are reconstructed, from which both flow velocity and local resonance offset are calculated. This technique is compatible with both gradient-recalled echo (GRE) and balanced steady-state free precession (SSFP) imaging using a single steady-state. The proposed technique enables 1D velocity mapping with 40% higher temporal resolution and 80% higher SNR, compared to conventional PC-MRI using bipolar velocity-encoding gradient pulses.

Fractional encoding of harmonic motions in MR elastography

In MR elastography (MRE) shear waves are magnetically encoded by bipolar gradients that usually oscillate with the same frequency fv as the mechanical vibration. As a result, both the repetition time (TR) and echo time (TE) of such an MRE sequence are greater than the vibration period 1/fv. This causes long acquisition times and considerable signal dephasing in tissue with short transverse relaxation times. Here we propose a reverse concept with TR<or=1/fv which we call "fractional" MRE, i.e., only a fraction of one vibration cycle per TR, can be used for motion sensitization. The benefit of fractional MRE is twofold: 1) acquisition times in seconds can be achieved for a single-phase difference wave image, and 2) materials that combine low elasticity, high viscosity, and short T2* relaxation times show an increased phase-to-noise ratio (PNR). A twofold increase of the phase signal is predicted for liver-like materials. Volunteer studies performed in liver and biceps show the benefit of fractional MRE. Furthermore, we demonstrate the feasibility of the technique for in vivo myocardial MRE by visualizing transverse wave propagation in the interventricular septum (IVS).

Visualization and quantification of flow and velocity fields in intracranial arteriovenous malformations using phase-contrast MR angiography

OBJECTIVE

The objective of this study was to describe postprocessing tools for MR phase-contrast flow quantification images and apply those tools to cerebral arteriovenous malformations (AVMs) to visualize blood flow dynamics noninvasively.

CONCLUSION

Inflow and outflow zones were clearly depicted at different regions in the AVM. The processed images showed flow patterns including vortical flow and variations in velocity over the cardiac cycle. Particle tracking gave an impression of the overall flow state and of the venous drainage system in particular.