Wideband SSFP: alternating repetition time balanced steady state free precession with increased band spacing.

Balanced steady-state free precession (SSFP) imaging is limited by off-resonance banding artifacts, which occur with periodicity 1/TR in the frequency spectrum. A novel balanced SSFP technique for widening the band spacing in the frequency response is described. This method, called wideband SSFP, utilizes two alternating repetition times with alternating RF phase, and maintains high SNR and T(2)/T(1) contrast. For a fixed band spacing, this method can enable improvements in spatial resolution compared to conventional SSFP. Alternatively, for a fixed readout duration this method can widen the band spacing, and potentially avoid the banding artifacts in conventional SSFP. The method is analyzed using simulations and phantom experiments, and is applied to the reduction of banding artifacts in cine cardiac imaging and high-resolution knee imaging at 3T.     

Principles and applications of balanced SSFP techniques

During the past 5 years balanced steady-state free precession (SSFP) has become increasingly important for diagnostic and functional imaging. Balanced SSFP is characterized by two unique features: it offers a very high signal-to noise ratio and a T2/T1-weighted image contrast. This article focuses on the physical principles, on the signal formation, and on the resulting properties of balanced SSFP. Mechanisms for contrast modification, recent clinical application, and potential extensions of this technique are discussed.     

Accurate phosphorus metabolite images of the human heart by 3D acquisition-weighted CSI.

Fourier imaging modalities suffer from significant signal contamination between adjacent voxels, especially when the spatial resolution is comparable to the size of the anatomical structures. This contamination can be positive or negative, depending on the spatial response function and the geometry of the object. Such a situation arises in human cardiac (31)P chemical shift imaging (CSI). Acquisition-weighted CSI reduces this contamination substantially, which is demonstrated by comparing conventional CSI to Hanning-weighted 3D (31)P-CSI experiments in 13 healthy volunteers at 2 T. The nominal spatial resolution and the total number of scans were identical for both experiments. The improved spatial response function of the acquisition-weighted experiment led to a significantly (P < 0.0001) higher myocardial PCr/ATP ratio (2.05 +/- 0.31, mean +/- SD, N = 33, corrected for saturation and blood contribution) compared to the conventional CSI experiment (1.60 +/- 0.46). This is explained by the absence of negative contamination from skeletal muscle, which also resulted in an increase of the observed SNR (from 5.4 +/- 1.4 to 7.2 +/- 1.4 for ATP). With acquisition-weighted CSI, metabolic images with a nominal resolution of 16 ml could be obtained in a measurement time of 30 min. After correction for the inhomogeneous B(1) field of the surface coil, these images show uniform ATP distribution in the entire myocardium, including the posterior wall.     

Analysis of multiple-acquisition SSFP.

Refocused steady-state free precession (SSFP) is limited by its high sensitivity to local field variation, particularly at high field strengths or the long repetition times (TRs) necessary for high resolution. Several methods have been proposed to reduce SSFP banding artifact by combining multiple phase-cycled SSFP acquisitions, each differing in how individual signal magnitudes and phases are combined. These include maximum-intensity SSFP (MI-SSFP) and complex-sum SSFP (CS-SSFP). The reduction in SSFP banding is accompanied by a loss in signal-to-noise ratio (SNR) efficiency. In this work a general framework for analyzing banding artifact reduction, contrast, and SNR of any multiple-acquisition SSFP combination method is presented. A new sum-of-squares method is proposed, and a comparison is performed between each of the combination schemes. The sum-of-squares SSFP technique (SOS-SSFP) delivers both robust banding artifact reduction and higher SNR efficiency than other multiple-acquisition techniques, while preserving SSFP contrast.     

Practical approaches to the evaluation of signal-to-noise ratio performance with parallel imaging: application with cardiac imaging and a 32-channel cardiac coil.

In this work, two practical methods for the measurement of signal-to-noise-ratio (SNR) performance in parallel imaging are described. Phantoms and human studies were performed with a 32-channel cardiac coil in the context of ultrafast cardiac CINE imaging at 1.5 T using steady-state free precession (SSFP) and TSENSE. SNR and g-factor phantom measurements using a "multiple acquisition" method were compared to measurements from a "difference method". Excellent agreement was seen between the two methods, and the g-factor shows qualitative agreement with theoretical predictions from the literature. Examples of high temporal (42.6 ms) and spatial (2.1x2.1x8 mm3) resolution cardiac CINE SSFP images acquired from human volunteers using TSENSE are shown for acceleration factors up to 7. Image quality agrees qualitatively with phantom SNR measurements, suggesting an optimum acceleration of 4. With this acceleration, a cardiac function study consisting of 6 image planes (3 short-axis views, 3 long-axis views) was obtained in an 18-heartbeat breath-hold.     

4D radial coronary artery imaging within a single breath-hold: cine angiography with phase-sensitive fat suppression (CAPS).

Coronary artery data acquisition with steady-state free precession (SSFP) is typically performed in a single frame in mid-diastole with a spectrally selective pulse to suppress epicardial fat signal. Data are acquired while the signal approaches steady state, which may lead to artifacts from the SSFP transient response. To avoid sensitivity to cardiac motion, an accurate trigger delay and data acquisition window must be determined. Cine data acquisition is an alternative approach for resolving these limitations. However, it is challenging to use conventional fat saturation with cine imaging because it interrupts the steady-state condition. The purpose of this study was to develop a 4D coronary artery imaging technique, termed "cine angiography with phase-sensitive fat suppression" (CAPS), that would result in high temporal and spatial resolution simultaneously. A 3D radial stacked k-space was acquired over the entire cardiac cycle and then interleaved with a sliding window. Sensitivity-encoded (SENSE) reconstruction with rescaling was developed to reduce streak artifact and noise. Phase-sensitive SSFP was employed for fat suppression using phase detection. Experimental studies were performed on volunteers. The proposed technique provides high-resolution coronary artery imaging for all cardiac phases, and allows multiple images at mid-diastole to be averaged, thus enhancing the signal-to-noise ratio (SNR) and vessel delineation.     

Spiral balanced steady-state free precession cardiac imaging.

Balanced steady-state free precession (SSFP) sequences are useful in cardiac imaging because they achieve high signal efficiency and excellent blood-myocardium contrast. Spiral imaging enables the efficient acquisition of cardiac images with reduced flow and motion artifacts. Balanced SSFP has been combined with spiral imaging for real-time interactive cardiac MRI. New features of this method to enable scanning in a clinical setting include short, first-moment nulled spiral trajectories and interactive control over the spatial location of banding artifacts (SSFP-specific signal variations). The feasibility of spiral balanced SSFP cardiac imaging at 1.5 T is demonstrated. In observations from over 40 volunteer and patient studies, spiral balanced SSFP imaging shows significantly improved contrast compared to spiral gradient-spoiled imaging, producing better visualization of cardiac function, improved localization, and reduced flow artifacts from blood.     

Ultrashort TE chemical shift imaging (UTE-CSI).

A fundamental modification to the conventional chemical shift imaging (CSI) method is described that improves the imaging of species with short T2's (i.e., less than approximately 2 ms). This approach minimizes the delay before each k-space point is collected. This results in different time delays, T(d), for different free induction decay (FID) acquisitions in k-space. On a clinical 1.5 T system this yields an effective delay due to transmit/receive switching of 70 micros and an echo time (TE) from the center of the excitation pulse to the center of k-space of 170 micros, as compared with 1-2 ms for conventional CSI techniques. Using this method, the signal decay before acquisition is greatly reduced, thus enabling imaging of species with very short T2)(e.g., 200 micros) and increasing the signal-to-noise ratio (SNR) of species with intermediate T2. Increases in the SNR of the short T2 components of 23Na in the heart, and 31P acquisitions of bone are about 27% and 400%, respectively, compared to an optimized conventional CSI approach. The imperfections of this approach are also described, and the magnitude of the resultant image artifacts is quantified for different practical imaging situations. These artifacts were not found to be significant in the described applications. This new method allows us to obtain information on short T2 components without degrading the image quality from long T2 components.     

MR angiography using steady-state free precession.

Contrast-enhanced MR angiography (CE-MRA) using steady-state free precession (SSFP) pulse sequences is described. Using SSFP, vascular structures can be visualized with high signal-to-noise ratio (SNR) at a substantial (delay) time after the initial arterial pass of contrast media. The peak blood SSFP signal was diminished by <20% 30 min after the initial administration of 0.2 mmol/kg of Gd-chelate. The proposed method allows a second opportunity to study arterial or venous structures with high image SNR and high spatial resolution. A mask subtraction scheme using spin echo SSFP-S(-) acquisition is also described to reduce stationary background signal from the delayed SSFP angiography images.     

Fast proton spectroscopic imaging using steady-state free precession methods.

Various pulse sequences for fast proton spectroscopic imaging (SI) using the steady-state free precession (SSFP) condition are proposed. The sequences use either only the FID-like signal S(1), only the echo-like signal S(2), or both signals in separate but adjacent acquisition windows. As in SSFP imaging, S(1) and S(2) are separated by spoiler gradients. RF excitation is performed by slice-selective or chemical shift-selective pulses. The signals are detected in absence of a B(0) gradient. Spatial localization is achieved by phase-encoding gradients which are applied prior to and rewound after each signal acquisition. Measurements with 2D or 3D spatial resolution were performed at 4.7 T on phantoms and healthy rat brain in vivo allowing the detection of uncoupled and J-coupled spins. The main advantages of SSFP based SI are the short minimum total measurement time (T(min)) and the high signal-to-noise ratio per unit measurement time (SNR(t)). The methods are of particular interest at higher magnetic field strength B(0), as TR can be reduced with increasing B(0) leading to a reduced T(min) and an increased SNR(t). Drawbacks consist of the limited spectral resolution, particularly at lower B(0), and the dependence of the signal intensities on T(1) and T(2). Further improvements are discussed including optimized data processing and signal detection under oscillating B(0) gradients leading to a further reduction in T(min).     

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.     

Enhanced spectral shaping in steady-state free precession imaging.

Balanced steady-state free precession (SSFP) is hindered by the inherent off-resonance sensitivity and unwanted bright fat signal. Multiple-acquisition SSFP combination methods, where multiple datasets with different fixed RF phase increments are acquired, have been used for shaping the SSFP spectrum to solve both problems. We present a new combination method (weighted-combination SSFP or WC-SSFP) that preserves SSFP contrast and enables banding-reduction and fat-water separation. Methods addressing the banding artifact have focused on either getting robust banding-reduction (complex-sum SSFP) or improved SNR efficiency (sum-of-squares SSFP). The proposed method achieves both robust banding-reduction and an SNR efficiency close to that of the sum-of-squares method. A drawback of fat suppression methods that create a broad stop-band around the fat resonance is the wedge shape of the stop-band leading to imperfect suppression. WC-SSFP improves the suppression of the stop-band without affecting the pass-band performance, and prevents fat signal from obscuring the tissues of interest in the presence of considerable resonant frequency variations. The method further facilitates the use of SSFP imaging by providing a control parameter to adjust the level of banding-reduction or fat suppression to application-specific needs.     

Signal-to-noise ratio behavior of steady-state free precession.

Steady-state free precession (SSFP) is a rapid gradient-echo imaging technique that has recently gained popularity and is used in a variety of applications, including cardiac and real-time imaging, because of its high signal and favorable contrast between blood and myocardium. The purpose of this work was to examine the signal-to-noise ratio (SNR) behavior of images acquired with SSFP, and the dependence of SNR on imaging parameters such as TR, bandwidth, and image resolution, and the use of multi-echo sequences. In this work it is shown that the SNR of SSFP sequences is dependent only on pulse sequence efficiency, voxel dimensions, and relaxation parameters (T1 and T2). Notably, SNR is insensitive to bandwidth unless increases in bandwidth significantly decrease efficiency. Finally, we examined the relationship between pulse sequence performance (TR and efficiency) and gradient performance (maximum gradient strength and slew rate) for several imaging scenarios, including multi-echo sequences, to determine the optimum matching of maximum gradient strength and slew rate for gradient hardware designs. For standard modern gradient hardware (40 mT/m and 150 mT/m/ms), we found that the maximum gradient strength is more than adequate for the imaging resolution that is commonly encountered with rapid scouting (3 mm x 4 mm x 10 mm voxel). It is well matched for typical CINE and real-time cardiac imaging applications (1.5 mm x 2 mm x 6 mm voxel), and is inadequate for optimal matching with slew rate for high-resolution applications such as musculoskeletal imaging (0.5 x 0.8 x 3 mm voxel). For the lower-resolution methods, efficiency could be improved with higher slew rates; this provokes interest in designing methods for limiting dB/dt peripherally while achieving high switching rates in the imaging field of view. The use of multi-echo SSFP acquisitions leads to substantial improvements in sequence performance (i.e., increased efficiency and shorter TR).     

Myocardial tagging with SSFP.

This work presents the first implementation of myocardial tagging with refocused steady-state free precession (SSFP) and magnetization preparation. The combination of myocardial tagging (a noninvasive method for quantitative measurement of regional and global cardiac function) with the high tissue signal-to-noise ratio (SNR) obtained with SSFP is shown to yield improvements in terms of the myocardium-tag contrast-to-noise ratio (CNR) and tag persistence when compared to the current standard fast gradient-echo (FGRE) tagging protocol. Myocardium-tag CNR and tag persistence were studied using numerical simulations as well as phantom and human experiments. Both quantities were found to decrease with increasing imaging flip angle (alpha) due to an increased tag decay rate and a decrease in myocardial steady-state signal. However, higher alpha yielded better blood-myocardium contrast, indicating that optimal alpha is dependent on the application: higher alpha for better blood-myocardium boundary visualization, and lower alpha for better tag persistence. SSFP tagging provided the same myocardium-tag CNR as FGRE tagging when acquired at four times the bandwidth and better tag- and blood-myocardium CNRs than FGRE tagging when acquired at equal or twice the receiver bandwidth (RBW). The increased acquisition efficiency of SSFP allowed decreases in breath-hold duration, or increases in temporal resolution, as compared to FGRE.     

Quantitative diffusion imaging with steady-state free precession.

The addition of a single, unbalanced diffusion gradient to the steady-state free precession (SSFP) imaging sequence sensitizes the resulting signal to free diffusion. Unfortunately, the confounding influence of both longitudinal (T1) and transverse (T2) relaxation on the diffusion-weighted SSFP (dwSSFP) signal has made it difficult to quantitatively determine the apparent diffusion coefficient (ADC). Here, a multistep method in which the T1, T2, and spin density (Mo) constants are first determined using a rapid mapping technique described previously is presented. Quantitative ADC can then be determined through a novel inversion of the appropriate signal model. The accuracy and precision of our proposed method (which we term DESPOD) was determined by comparing resulting ADC values from phantoms to those calculated from traditional diffusion-weighted echo planar imaging (dwEPI) images. Error within the DESPOD-derived ADC maps was found to be less than 3%, with good precision over a biologically relevant range of ADC values.     

Cardiac SSFP imaging at 3 Tesla.

Balanced steady-state free precession (SSFP) techniques provide excellent contrast between myocardium and blood at a high signal-to-noise ratio (SNR). Hence, SSFP imaging has become the method of choice for assessing cardiac function at 1.5T. The expected improvement in SNR at higher field strength prompted us to implement SSFP at 3.0T. In this work, an optimized sequence protocol for cardiac SSFP imaging at 3.0T is derived, taking into account several partly adverse effects at higher field, such as increased field inhomogeneities, longer T(1), and power deposition limitations. SSFP contrast is established by optimizing the maximum amplitude of the radiofrequency (RF) field strength for shortest TR, as well as by localized linear or second-order shimming and local optimization of the resonance frequency. Given the increased SNR, sensitivity encoding (SENSE) can be employed to shorten breath-hold times. Short-axis, long-axis, and four-chamber cine views obtained in healthy adult subjects are presented, and three different types of artifacts are discussed along with potential methods for reducing them.     

Steady-state free precession MR imaging: improved myocardial tag persistence and signal-to-noise ratio for analysis of myocardial motion

Tagging with balanced steady-state free-precession (SSFP) magnetic resonance (MR) imaging by using a steady-state storage scheme for myocardial motion analysis was evaluated. Signal-to-noise ratio (SNR), blood-tissue contrast, and tag persistence in volunteers and phantoms showed improved performance of SSFP imaging with tagging compared with that of radiofrequency spoiled gradient-echo (SPGR) MR imaging with tagging. Choice of flip angle with SSFP imaging involved a trade-off among SNR, blood-tissue contrast, and tag persistence. Increased SNR and tag persistence can be achieved simultaneously with SSFP imaging compared with SPGR tagging methods. As a result, the proposed technique may be useful for analysis of diastolic ventricular function.     

Application of refocused steady-state free-precession methods at 1.5 and 3 T to in vivo high-resolution MRI of trabecular bone: simulations and experiments

PURPOSE: To evaluate the potential of fully-balanced steady-state free-precession (SSFP) sequences in in vivo high-resolution (HR) MRI of trabecular bone at field strengths of 1.5 and 3 T by simulation and experimental methods. MATERIALS AND METHODS: Using simulation studies, refocused SSFP acquisition was optimized for our imaging purposes with a focus on signal-to-noise ratio (SNR) and SNR efficiency. The signal behavior in trabecular bone was estimated using a magnetostatic model of the trabecular bone and marrow. Eight normal volunteers were imaged at the proximal femur, calcaneus, and the distal tibia on a GE Signa scanner at 1.5 and at 3 T with an optimized single-acquisition SSFP sequence (three-dimensional FIESTA) and an optimized multiple-acquisition SSFP sequence (three-dimensional FIESTA-c). Images were also acquired with a fast gradient echo (FGRE) sequence for evaluation of the SNR performance of SSFP methods. RESULTS: Refocused SSFP images outperformed FGRE acquisitions in both SNR and SNR efficiency at both field strengths. At 3 T, susceptibility effects were visible in FIESTA and FGRE images and much reduced in FIESTA-c images. The magnitude of SNR boost at 3 T was closely predicted by simulations. CONCLUSION: Single-acquisition SSFP (at 1.5 T) and multiple-acquisition SSFP (at 3 T) hold great potential for HR-MRI of trabecular bone.     

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.     

Navigator-gated coronary magnetic resonance angiography using steady-state-free-precession: comparison to standard T2-prepared gradient-echo and spiral imaging

RATIONALE AND OBJECTIVES: Recent developments of magnetic resonance imaging enabled free-breathing coronary MRA (cMRA) using steady-state-free-precession (SSFP) for endogenous contrast. The purpose of this study was a systematic comparison of SSFP cMRA with standard T2-prepared gradient-echo and spiral cMRA. METHODS: Navigator-gated free-breathing T2-prepared SSFP-, T2-prepared gradient-echo- and T2-prepared spiral cMRA was performed in 18 healthy swine (45-68 kg body-weight). Image quality was investigated subjectively and signal-to-noise ratio (SNR), contrast-to-noise ratio (CNR) and vessel sharpness were compared. RESULTS: SSFP cMRA allowed for high quality cMRA during free breathing with substantial improvements in SNR, CNR and vessel sharpness when compared with standard T2-prepared gradient-echo imaging. Spiral imaging demonstrated the highest SNR while image quality score and vessel definition was best for SSFP imaging. CONCLUSION: Navigator-gated free-breathing T2-prepared SSFP cMRA is a promising new imaging approach for high signal and high contrast imaging of the coronary arteries with improved vessel border definition.