Artifact reduction in true-FISP imaging of the coronary arteries by adjusting imaging frequency.

The presence of resonance frequency offsets often causes artifacts in images acquired with true fast imaging with steady-state precession (true-FISP). One source of resonance offsets is a suboptimal setting of the synthesizer frequency. The goal of this work was to demonstrate that shifting the synthesizer frequency could minimize the off-resonance related image artifacts in true-FISP. A simple scouting method was developed to estimate the optimal synthesizer frequency for the volume of interest (VOI). To improve fat suppression, a similar scouting method was also developed to determine the optimal frequency offset for the fat saturation pulse. Coronary artery imaging was performed in healthy subjects using a 3D true-FISP sequence to validate the effectiveness of the frequency corrections. Substantial reduction in image artifacts and improvement in fat suppression were observed by using the water and fat frequencies estimated by the scouting scans. Frequency shifting is a useful and practical method for improving coronary artery imaging using true-FISP.     

Contrast-enhanced coronary artery imaging using 3D trueFISP.

An ECG-triggered, segmented, magnetization-prepared, 3D, trueFISP sequence was recently developed for coronary artery imaging. Fat saturation was achieved by a chemically selective fat saturation pulse, which is susceptible to field inhomogeneities. In addition, the blood-myocardial contrast was compromised because data were acquired during signal transience to steady state. The goals of this work were to investigate the potential benefits of T(1)-shortening agents in improving blood-myocardial contrast, and to develop a technique to make fat suppression robust to resonance offsets for coronary artery imaging using trueFISP. A magnetization-preparation scheme using saturation and inversion pulses was developed for simultaneous suppression of tissues over a wide range of T(1)'s, including myocardium and fat. An additional advantage of this method is that it is insensitive to heart rate variations. Computer simulations were used to design the magnetization preparation, and volunteer studies were performed to compare precontrast imaging to contrast-enhanced (CE) imaging. Results showed consistent fat suppression and a 78% increase in the blood-myocardial contrast-to-noise ratio (CNR) for postcontrast imaging over precontrast imaging. In conclusion, contrast agents are useful for trueFISP coronary artery imaging.     

3D magnetization-prepared true-FISP: a new technique for imaging coronary arteries.

The purpose of this work was to develop an ECG-triggered, segmented 3D true-FISP (fast imaging with steady-state precession) technique to improve the signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) of breath-hold coronary artery imaging. The major task was to optimize an appropriate magnetization preparation scheme to permit saturation of the epicardial fat signal. An alpha/2 preparation pulse was used to speed up the approach to steady-state following a frequency-selective fat-saturation pulse in each heartbeat. The application of dummy cycles was found to reduce the oscillation of the magnetization during data acquisition. The fat saturation and magnetization preparation scheme was validated with simulations and phantom studies. Volunteer studies demonstrated substantially increased SNR (55%) and CNR (178%) for coronary arteries compared to FLASH (fast low-angle shot) with the same imaging time. In conclusion, true-FISP is a promising technique for coronary artery imaging.     

Coronary MRA with 3D undersampled projection reconstruction TrueFISP.

Undersampled projection reconstruction (PR) offers improved imaging efficiency allowing a relative tradeoff between signal-to-noise ratio (SNR) or streak artifact and the number of acquired k-space views rather than the tradeoff between resolution or aliasing artifact and the number of acquired k-space lines inherent to Fourier imaging techniques. TrueFISP (true fast imaging with steady state precession) is ideally suited for undersampled PR imaging because of its inherently high SNR. The purpose of this work was to investigate the feasibility of using undersampled three-dimensional (3D) PR TrueFISP for breathhold coronary artery imaging. Phantom studies and a preliminary in vivo comparison demonstrated the improved spatial resolution of PR over Fourier TrueFISP with the same imaging time. In a 24-heartbeat coronary imaging scheme, segmented 3D PR TrueFISP provided a 1.0 x 1.0 mm(2) isotropic in-plane voxel size while acquiring between 93 and 153 views per partition. Streak artifacts were present in some images but were not found to impede coronary artery delineation. In conclusion, 3D undersampled PR TrueFISP provides isotropic in-plane voxel size within a single breathhold and is a promising technique for coronary artery imaging.     

Three-dimensional contrast-enhanced steady-state free precession for improved catheter-directed coronary magnetic resonance angiography

PURPOSE: To demonstrate the feasibility of three-dimensional thick-partition, contrast-enhanced, catheter-directed coronary artery magnetic resonance angiography (MRA) and test the hypothesis that three-dimensional imaging improves coronary artery background contrast-to-noise ratio (CNR) compared to two-dimensional imaging. MATERIALS AND METHODS: Catheters were advanced into the coronary arteries of swine (N = 6) under MR guidance. Three-dimensional coronary MRA was performed after intracoronary injection of a small dose of contrast media using magnetization-prepared steady-state free precession (SSFP) with two thick partitions. For comparison, two magnetization-prepared two-dimensional SSFP scans were also performed, one with no signal averaging and one with two signal averages. All sequences had the same coverage and in-plane spatial resolution. RESULTS: The coronary artery was successfully catheterized in all (6/6) animals. CNR for three-dimensional imaging was 11.1 +/- 1.2 for proximal arterial segments and 4.3 +/- 0.4 for distal segments. Without averaging, two-dimensional imaging CNRs for proximal and distal segments were 5.0 +/- 0.7 and 1.2 +/- 0.2, respectively. With averaging, two-dimensional imaging CNRs for proximal and distal segments were 9.4 +/- 1.5 and 2.9 +/- 0.4, respectively. Three-dimensional imaging showed a statistically significant increase in CNR over all two-dimensional imaging for both proximal and distal segments (P < 0.05). CONCLUSION: Three-dimensional thick-partition, contrast-enhanced, catheter-directed coronary MRA is feasible and improves CNR over two-dimensional projection imaging.     

Improved magnetization preparation for navigator steady-state free precession 3D coronary MR angiography.

The purpose of this work was to investigate a new magnetization preparation scheme for navigator steady-state free precession (SSFP) 3D coronary MR angiography (MRA) that executes the navigator and fat saturation pulses in steady state after the dummy RFs in order to minimize the delay between the magnetization preparation and the image echoes. Compared to the previous preparation scheme that executes the navigator and fat saturation pulses before the dummy RFs, the new scheme was found to provide more effective motion suppression, significantly improved blood-to-myocardium contrast-to-noise ratio (46%, P < 0.001) at slightly but insignificantly decreased blood signal-to-noise ratio (SNR) (2%, P = 0.73), significantly reduced fat SNR (32%, P < 0.001), and better overall image quality (P = 0.05; Wilcoxon paired sample signed rank test).     

Feasibility and performance of breath-hold 3D true-FISP coronary MRA using self-calibrating parallel acquisition.

Spatial resolution in 3D breath-hold coronary MR angiography (MRA) is limited by imaging time. The purpose of this work was to investigate the feasibility of improving the spatial resolution of coronary MRA using generalized autocalibrating partially parallel acquisition (GRAPPA) and fast imaging with steady state precession (True-FISP) data acquisition. Coronary data were acquired in 10 healthy volunteers. In five volunteers, the data were fully acquired in k-space and decimated for GRAPPA with an outer reduction factor (ORF) of 2. The coil calibration in GRAPPA was improved by segmented least-squares fitting along the frequency-encoding direction. More than 5% of the total k-space lines were required for the calibration to achieve acceptable artifact suppression despite slightly lower signal-to-noise ratio (SNR). In another five volunteers, coronary data were obtained with both conventional and accelerated data acquisitions in the same imaging time. GRAPPA allowed a submillimeter in-plane resolution, and improved coronary artery definition with an acceptable loss of SNR. In conclusion, 3D breath-hold coronary MRA by GRAPPA and True-FISP is highly feasible.     

Breath-hold three-dimensional true-FISP imaging of coronary arteries using asymmetric sampling

PURPOSE: To evaluate the feasibility of using asymmetric sampling in a three-dimensional, magnetization-prepared, segmented true-FISP (fast imaging with steady-state precession) sequence in order to reduce the sensitivity to resonance offsets, while simultaneously improving imaging speed. MATERIALS AND METHODS: Asymmetric sampling reduces the repetition time, leading to reduced resonance offset effects and improved resolution in a fixed imaging time. However, it introduces additional phase terms due to blood flow, which can cause image artifacts. Computer simulations were performed to study the off-resonance and flow effects of asymmetric sampling in true-FISP. Coronary artery imaging was performed in healthy volunteers. RESULTS: Simulations and volunteer studies show that image artifacts due to flow-induced phase variations may be acceptable at low velocities. Volunteer studies demonstrate that relatively high-resolution coronary artery images can then be acquired within a single breath-hold with segmented three-dimensional true-FISP imaging using data asymmetry in the readout direction. CONCLUSION: Asymmetric sampling is a useful modification to true-FISP for reducing the off-resonance artifacts and improving imaging speed when the flow velocities are small.     

Off-resonance effects in the transient response of SSFP sequences.

A previously developed eigenvector formalism is adapted to off-resonance in the transient response of quasiperiodic steady-state free precession (SSFP) sequences, including TrueFISP as a special case. The effective relaxation rates for essentially parallel and perpendicular deviations from the steady state are determined analytically in leading order perturbation theory. The latter are a known cause of oscillatory artifacts and therefore constitute the main target of a variety of preparation techniques. In addition, the former also play a dominating role in applications such as inversion recovery (IR) TrueFISP, which intentionally measure far away from the equilibrium. For both components, the approach toward equilibrium turns out to depend sensitively on field inhomogeneities, especially for smaller ratios of T2/T1. For the perpendicular deviations, the calculations show that--except very close to banding artifacts, where the steady-state signal is almost zero--field inhomogeneities additionally increase their effective relaxation rate almost as much as in the free induction decay (FID). The analytical results are tested against numerical simulation and MR measurements.     

Coronary artery imaging using three-dimensional breath-hold steady-state free precession with two-dimensional iterative partial fourier reconstruction

PURPOSE: To assess the feasibility of using a two-dimensional partial Fourier (PF) reconstruction scheme to reduce the acquisition time of magnetic resonance imaging (MRI) of coronary arteries. MATERIALS AND METHODS: Symmetric k-space data sets of coronary arteries were collected in seven volunteers using a three-dimensional breath-hold steady-state free precession (SSFP) sequence. Partial, asymmetric k-space data sets were generated by removing 25% of the data in the readout direction and 25% of the data in the phase encoding direction. The missing data were then estimated using a two-dimensional projection-onto-convex-sets (POCS) algorithm or filled with zeroes. Images were reconstructed from the full data set, the PF data set, and the zero-filled (ZF) data set, respectively. Coronary artery sharpness was evaluated quantitatively and qualitatively. RESULTS: Coronary artery sharpness in PF images was comparable to that in full k-space images and significantly better than that in ZF images. CONCLUSION: Two-dimensional POCS PF reconstruction is a potentially useful technique for reducing acquisition time or improving spatial resolution for breath-hold coronary MR angiography.     

Free-breathing, three-dimensional coronary artery magnetic resonance angiography: comparison of sequences

PURPOSE: To compare six free-breathing, three-dimensional, magnetization-prepared coronary magnetic resonance angiography (MRA) sequences. MATERIALS AND METHODS: Six bright-blood sequences were evaluated: Cartesian segmented gradient echo (C-SGE), radial SGE (R-SGE), spiral SGE (S-SGE), spiral gradient echo (S-GE), Cartesian steady-state free precession (C-SSFP), and radial SSFP (R-SSFP). The right coronary artery (RCA) was imaged in 10 healthy volunteers using all six sequences in randomized order. Images were evaluated by two observers with respect to signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR), visible vessel length, vessel edge sharpness, and vessel diameter. RESULTS: C-SSFP depicted RCA over the longest distance with high vessel sharpness, good SNR, and excellent background suppression. S-GE provided best SNR and CNR in proximal segments, but more vessel blurring and poorer background suppression, resulting in poor visualization of distal segments. R-SSFP images showed good background suppression and best vessel sharpness, but only moderate SNR. C-SGE provided good SNR and reasonable CNR, but lowest vessel sharpness. S-SGE and R-SGE visualized the RCA over the smallest distance, mostly due to vessel blurring and low SNR, respectively. CONCLUSION: Overall, Cartesian SSFP provided the best image quality with excellent vessel sharpness, visualization of long vessel segments, and good SNR and CNR.     

Quantitative assessment of ventricular function using three-dimensional SSFP magnetic resonance angiography

PURPOSE: To evaluate three-dimensional (3D), free-breathing, steady-state free precession (SSFP) magnetic resonance angiography (MRA) for volumetric assessment of ventricular function. MATERIALS AND METHODS: In 18 subjects (mean age = 21.5 years) 3D datasets of the heart and great vessels were acquired using an ECG-triggered, free-breathing SSFP technique with a T2-preparation prepulse. Data were acquired during end-systole (ES) and end-diastole (ED) for assessment of stroke volumes (SVs). Through-plane flow measurements of the great arteries were performed as well as 2D-cine SSFP imaging for comparison. For image analysis of the 3D SSFP datasets a simplex mesh model was used. Papillary muscles were excluded from ventricular volumes using thresholds. Intra- and interobserver variability (Bland-Altman analysis) and correlations (Pearson's coefficient) between volumetric and flow measurements were assessed. RESULTS: ES and ED datasets were acquired successfully in all subjects. The best correlation was observed between flow vs. 3D SSFP SV for the LV (r = 0.85, mean difference = -1.0 mL) and the RV (r = 0.89, mean difference = -2.2 mL) with high intra- (LV: r = 0.93; RV: r = 0.94) and interobserver (LV: r = 0.91; RV: r = 0.93) reproducibility. CONCLUSION: 3D SSFP datasets combined with semiautomatic segmentation algorithms allow highly accurate and reproducible assessment of left (LV) and right ventricular (RV) SVs in free-breathing subjects.     

Balanced alternating steady-state elastography.

A conventional balanced steady-state free precession (b-SSFP) sequence scheme was modified such that the dynamic equilibrium becomes very sensitive to small cyclic displacements, generating two distinct and alternating steady states. This novel technique is proposed for the visualization of propagating transverse acoustic shear waves, as used in MR elastography (MRE) to determine the mechanical properties of materials or in vivo soft tissue. Experiments with tissue-like agarose gel phantoms and simulations demonstrate that the novel sequence offers an increase in phase sensitivity by about one order in magnitude compared to standard motion-encoding methods. In addition, the new method benefits from the very short acquisition times achieved by b-SSFP protocols.     

Variable flip angle schedules in bSSFP imaging of hyperpolarized noble gases

Balanced steady-state free precession imaging sequences provide signal-to-noise ratio benefits for MRI of hyperpolarized nuclei. Hyperpolarized magnetization decays during the imaging sequence to thermal equilibrium, effectively necessitating imaging in a transient state characterized by nonconstant transverse magnetization and k-space filtering when using constant flip angles. This work presents an analytical method for calculation of variable flip angle schedules which maintain constant transverse magnetization in balanced steady-state free precession imaging of hyperpolarized nuclei. The approach is based on direct inversion of the Bloch equations and does not require any numerical optimization. Input parameters are pulse sequence timings and effective relaxation times, which take diffusion of hyperpolarized gas in imaging gradients into account. Provision of constant transverse magnetization is demonstrated in phantom experiments and human lung imaging using hyperpolarized (3) He. The benefit of a flat k-space filter is demonstrated by reduced blurring in (3) He and digital phantom data, and high quality (3) He ventilation images from human lungs are obtained.     

Fat attenuation using a dual steady-state balanced-SSFP sequence with periodically variable flip angles.

A refocused-SSFP sequence based on balanced-FFE (TrueFisp, Fiesta) that attenuates fat signal is presented. The sequence uses periodically variable flip angles and produces a dual steady state of the signal, which is obtained after a dual transient phase if an appropriate preparation is used. The off-resonance profile of the steady-state signal exhibits large stopbands that can be employed for fat suppression. Numerical simulations were performed to investigate the signal behavior and the off-resonance properties of the sequence. Experimental results obtained with a Philips Gyroscan Intera 1.5T MR scanner demonstrated fat attenuation in phantoms and abdominal images in volunteers.     

Single‐shot steady‐state free precession can detect myocardial edema in patients: A feasibility study

Purpose

To demonstrate the ability of single‐shot, T₂/T₁ weighted steady‐state free precession (SSFP) to detect myocardial edema in patients with an acute myocardial infarction.

Materials and Methods

This study was performed in a series of patients (n = 10) referred for the assessment of acute myocardial infarcts (AMI). Localizers were used to obtain true short axis views of the left ventricle (LV). These views were used to plan and obtain T₂‐weighted STIR (short TI inversion recovery) images of the LV. These slices were then acquired using single‐shot dark blood‐prepared SSFP with a large (31) number of dummy pulses. Lastly, Contrast agent was injected, and late enhancement (LE) images were acquired. Images were analyzed using a multi‐segment model of the heart. SSFP images were compared with STIR images, with STIR images used as the standard of truth for the presence of edema. LE images were used to identify segments which were positive for microvascular obstruction.

Results

All techniques were successful in all patients. A total of 312 segments were analyzed. Excluding segments positive for microvascular obstruction, SSFP had a sensitivity/specificity of 80%/89%. Including segments positive for microvascular obstruction, sensitivity/specificity was 71%/88%. On a patient‐based analysis, no AMI was missed using SSFP (sensitivity = 100%).

Conclusion

Using single‐shot SSFP to detect myocardial edema in patients with AMI is feasible with a moderate sensitivity and high specificity. J. Magn. Reson. Imaging 2009. © 2009 Wiley‐Liss, Inc.     

Optimization of signal behavior in the transition to driven equilibrium in steady-state free precession sequences.

A new technique to avoid the initial signal fluctuations in steady-state free precession (SSFP)-sequences, such as trueFISP, FIESTA, and refocused FFE, is presented. The "transition into driven equilibrium" (TIDE) sequence uses modified flip angles over the initialization phase of a SSFP experiment, which not only avoids image artifacts but also improves the signal-to-noise ratio (SNR) and contrast behavior compared to conventional approaches. TIDE is demonstrated to be robust against variations of T(1) and T(2), and leads to a monotonous signal evolution for off-resonance spins. The basic principles can also be applied repetitively to optimize continuous 3D acquisitions.     

Oscillating dual-equilibrium steady-state angiography.

A novel technique of generating noncontrast angiograms is presented. This method, called oscillating dual-equilibrium steady-state angiography (ODESSA), utilizes a modified steady-state free precession (SSFP) pulse sequence. The SSFP sequence is modified such that flowing material reaches a steady state which oscillates between two equilibrium values, while stationary material attains a single, nonoscillatory steady state. Subtraction of adjacent echoes results in large, uniform signal from all flowing spins and zero signal from stationary spins. Venous signal can be suppressed based on its reduced T2. ODESSA arterial signal is more than three times larger than that of traditional phase-contrast angiography (PCA) in the same scan time, and also compares favorably with other techniques of MR angiography (MRA). Pulse sequences are implemented in 2D, 3D, and volumetric-projection modes. Angiograms of the lower leg, generated in as few as 5 s, show high arterial signal-to-noise ratio (SNR) and full suppression of other tissues.     

Whole-heart steady-state free precession coronary artery magnetic resonance angiography.

Current implementations of coronary artery magnetic resonance angiography (MRA) suffer from limited coverage of the coronary arterial system. Whole-heart coronary MRA was implemented based on a free-breathing steady-state free-precession (SSFP) technique with magnetization preparation. The technique was compared to a similar implementation of conventional, thin-slab coronary MRA in 12 normal volunteers. Three thin-slab volumes were prescribed: 1) a transverse slab, covering the left main (LM) artery and proximal segments of the left anterior ascending (LAD) and left circumflex (LCX) coronary arteries; 2) a double-oblique slab covering the right coronary artery (RCA); and 3) a double-oblique slab covering the proximal and distal segments of the LCX. The whole-heart data set was reformatted in identical orientations. Visible vessel length, vessel sharpness, and vessel diameter were determined and compared separately for each vessel. Whole-heart coronary MRA visualized LM/LAD (11.7 +/- 3.4 cm) and LCX (6.9 +/- 3.6 cm) over a significantly longer distance than the transverse volume (LM/LAD, 6.1 +/- 1.1 cm, P < 0.001; LCX, 4.2 +/- 1.2 cm, P < 0.05). Improvements in visible vessel length for RCA and LCX in the whole-heart approach vs. their respective targeted volumes were not significant. It is concluded that the whole-heart coronary MRA technique improves visible vessel length and facilitates high-quality coronary MRA of the complete coronary artery tree in a single measurement.