Phased array ghost elimination (PAGE) for segmented SSFP imaging with interrupted steady-state.

Steady-state free precession (SSFP) has recently proven to be valuable for cardiac imaging due to its high signal-to-noise ratio and blood-myocardium contrast. Data acquired using ECG-triggered, segmented sequences during the approach to steady-state, or return to steady-state after interruption, may have ghost artifacts due to periodic k-space distortion. Schemes involving several preparatory RF pulses have been proposed to restore steady-state, but these consume imaging time during early systole. Alternatively, the phased-array ghost elimination (PAGE) method may be used to remove ghost artifacts from the first several frames. PAGE was demonstrated for cardiac cine SSFP imaging with interrupted steady-state using a simple alpha/2 magnetization preparation and storage scheme and a spatial tagging preparation.     

Ghost artifact cancellation using phased array processing.

In this article, a method for phased array combining is formulated which may be used to cancel ghosts caused by a variety of distortion mechanisms, including space variant distortions such as local flow or off-resonance. This method is based on a constrained optimization, which optimizes SNR subject to the constraint of nulling ghost artifacts at known locations. The resultant technique is similar to the method known as sensitivity encoding (SENSE) used for accelerated imaging; however, in this formulation it is applied to full field-of-view (FOV) images. The method is applied to multishot EPI with noninterleaved phase encode acquisition. A number of benefits, as compared to the conventional interleaved approach, are reduced distortion due to off-resonance, in-plane flow, and EPI delay misalignment, as well as eliminating the need for echo-shifting. Experimental results demonstrate the cancellation for both phantom as well as cardiac imaging examples.     

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.     

Characterization and reduction of the transient response in steady-state MR imaging.

Refocused steady-state free precession (SSFP) imaging sequences have recently regained popularity as faster gradient hardware has allowed shorter repetition times, thereby reducing SSFP's sensitivity to off-resonance effects. Although these sequences offer fast scanning with good signal-to-noise efficiency, the "transient response," or time taken to reach a steady-state, can be long compared with the total imaging time, particularly when using 2D sequences. This results in lost imaging time and has made SSFP difficult to use for real-time and cardiac-gated applications. A linear-systems analysis of the steady-state and transient response for general periodic sequences is shown. The analysis is applied to refocused-SSFP sequences to generate a two-stage method of "catalyzing," or speeding up the progression to steady-state by first scaling, then directing the magnetization. This catalyzing method is compared with previous methods in simulations and experimentally. Although the second stage of the method exhibits some sensitivity to B(1) variations, our results show that the transient time can be significantly reduced, allowing imaging in a shorter total scan time. Magn Reson Med 46:149-158, 2001.     

Reduction of transient signal oscillations in true-FISP using a linear flip angle series magnetization preparation.

An electrocardiogram (ECG)-triggered, magnetization-prepared, segmented, 3D true fast imaging with steady-state precession (true-FISP) sequence with fat saturation was recently proposed for coronary artery imaging. A magnetization preparation scheme consisting of an alpha/2 radiofrequency (RF) pulse followed by 20 constant flip angle dummy RF cycles was used to reduce signal oscillations in the approach to steady state. However, if large resonance offsets on the order of 70-100 Hz are present, significant magnetization oscillations will still occur during data acquisition, which will result in image ghosting and blurring. The goal of this work was to validate that a linear flip angle (LFA) series can be used during magnetization preparation to reduce these image artifacts. Computer simulations, phantom studies, and coronary artery imaging in healthy volunteers were performed to compare this magnetization preparation scheme with that of an alpha/2 pulse followed by constant flip angle dummy RF cycles. The results demonstrated substantial reduction in the apparent image artifacts when using linearly increasing flip angles during magnetization preparation.     

Balanced steady-state free precession vs. segmented fast low-angle shot for the evaluation of ventricular volumes, mass, and function at 3 Tesla

PURPOSE: To compare balanced steady-state free precession (SSFP) and segmented fast low angle shot (FLASH) for quantification of left and right ventricular volumes and function and for left ventricular mass at high field (3 Tesla). MATERIALS AND METHODS: A total of 33 patients (19 male, mean age 54 years) with various forms of heart disease underwent ventricular function studies using cine SSFP and FLASH sequences with identical slice orientations. RESULTS: Using SSFP, left ventricular end-diastolic (+10 mL [4.7%], P < 0.001) and end-systolic volumes (+9 mL [6.1%], P < 0.001) measured larger whereas mass was considerably smaller (-23 g [-12.9%], P < 0.001) and ejection fraction (-1% [-3.2%], P < 0.01) marginally smaller. Right ventricular end-diastolic (+4 mL [2.6%], P = 0.001) and end-systolic volumes (+4 mL [5.1%], P < 0.01) were also larger, but no significant difference for right ventricular ejection fraction (P = 0.05) was found. CONCLUSION: Similar to previous results at 1.5 Tesla, at high magnetic field the cine SSFP technique led to discrete but significantly higher ventricular volume measurements and to a significantly smaller measurement of left ventricular mass in patients. The effect on left and right ventricular ejection fraction was minor, although the difference remained significant for the left ventricle.     

Auto-SENSE perfusion imaging of the whole human heart

PURPOSE: To show the application of auto-sensitivity encoding (SENSE)-a self-calibrating parallel imaging technique-to first pass perfusion imaging of the whole human heart. MATERIALS AND METHODS: The self-calibrating parallel imaging method auto-SENSE was implemented for a saturation recovery turbo-fast low-angle shot (FLASH) sequence on a 1.5-T scanner using a standard four-element body phased array coil. By reducing the acquisition time per slice by a factor of two compared to conventional turbo FLASH imaging, the number of imaged slices could be doubled to six to ten with an unchanged temporal resolution of one image per heartbeat. This technique has been tested in eight healthy volunteers for contrast-enhanced heart perfusion imaging. RESULTS: Auto-SENSE heart perfusion imaging with improved coverage of the human heart could be performed successfully in all volunteers. A first quantitative comparison of perfusion values between the auto-SENSE and the non-SENSE techniques shows good agreement. CONCLUSION: Auto-SENSE allows perfusion imaging of the whole human heart without gaps.     

Flow artifacts in steady-state free precession cine imaging.

Steady-state free precession (SSFP) cardiac cine images are frequently corrupted by dark flow artifacts, which can usually be eliminated by reshimming and retuning the scanner. A theoretical explanation for these artifacts is provided in terms of spins moving through an off-resonant point in the magnetic field, and the theory is validated using phantom experiments. The artifacts can be reproduced in vivo by detuning the center frequency by an amount in the range of half the inverse repetition time (TR). Since this offset is similar in magnitude to the frequency difference between the water and lipid peaks, a likely cause of the artifacts in vivo is that the center frequency is tuned incorrectly to the lipid peak rather than the water peak.     

Free-breathing radial acquisitions of the heart.

There is considerable interest in performing free-breathing acquisitions of the heart in order to obtain high-quality images without the need for multiple, long breathholds. In this article a 3D motion-correction method is described that is based on image registration of in-plane data and through-plane slice tracking. A number of fast radial undersampled images are acquired, each of which is free of motion artifacts. Initially, in-plane translational and rotational motion between each image was corrected before combining the data to give a fully sampled image. At the next stage, correction of in-plane deformation, in addition to translations and rotations, was performed in the image domain. Through-plane translational motion was compensated using a navigator echo to move the acquisition plane. Using this method, information on the motion of the heart was captured at the same time as acquiring the image data. No motion model, assumptions about the motion, or training data are required. The method is demonstrated on phantom data and cardiac images acquired on free-breathing volunteers.     

Rapid fat-suppressed isotropic steady-state free precession imaging using true 3D multiple-half-echo projection reconstruction.

Three-dimensional projection reconstruction (3D PR)-based techniques are advantageous for steady-state free precession (SSFP) imaging for several reasons, including the capability to achieve short repetition times (TRs). In this paper, a multi-half-echo technique is presented that dramatically improves the data-sampling efficiency of 3D PR sequences while it retains this short-TR capability. The k-space trajectory deviations are measured quickly and corrected on a per-sample point basis. A two-pass RF cycling technique is then applied to the dual-half-echo implementation to generate fat/water-separated images. The resultant improvement in the signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) was demonstrated in volunteer studies. Volumetric images with excellent spatial resolution, coverage, and contrast were obtained with high speed. The non-contrast-enhanced SSFP studies show that this technique has promising potential for MR angiography (MRA).     

Addressing efficiency and residual magnetization cross talk in multi-slice 2D steady-state free precession imaging of the heart.

This work presents an efficient method for achieving steady state in multi-slice 2D balanced steady-state free precession (SSFP) imaging of cardiac function. With current techniques, data acquisition for each slice is preceded by one or two heartbeats of dummy excitations. Depending on the number of heartbeats required for data acquisition, these dummy heartbeats can represent a large fraction of the total imaging time. As described here, FIESTA-SP (FIESTA with steady-state preparation) increases the imaging efficiency to nearly 100% by eliminating dummy heartbeats. Steady state for each slice is achieved using a linear flip angle series of excitations during the first cardiac phase of the first heartbeat for each slice. Because imaging proceeds immediately from one slice to the next, a heretofore-unseen issue arises where residual magnetization from each slice contaminates subsequent acquisitions. Accelerating the approach to steady state for each slice and eliminating slice cross talk are important for both multi-slice and interactive real-time imaging.     

Respiratory Motion of the Heart: Kinematics and the Implications for the Spatial Resolution in Coronary Imaging

Respiratory motion is a major limiting factor in improving image resolution and signal-to-noise ratio in MR coronary imaging. In this work the effects of respiration on the cardiac position were studied quantitively by imaging the heart during diastole at various positions of tidal respiration with a breath-hold segmented fast gradient echo technique. It was found that during tidal breathing the movement of the heart due to respiration is dominated by superior-inferior (SI) motion, which is linearly related to the SI motion of the diaphragm. The motion of the heart due to respiration is approximately a global translation. These results provide motivation for employing adaptive motion correction techniques to reduce image blurring in nonbreath-hold coronary MR imaging.     

In vivo measurement of water diffusion in the human heart

Existing magnetic resonance methods for diffusion imaging, including echo planar, are ineffective in the beating heart due to motion-induced signal attenuation. To overcome this problem, we used a diffusion-weighted stimulated echo-echo planar magnetic resonance imaging sequence. The two lobes of the diffusion-sensitizing gradient were synchronized to the same point in successive cardiac cycles in order to fix the cardiac position and avoid bulk motion effects. The apparent diffusion coefficients (ADCs) of the interventricular septum in 12 healthy subjects for diffusion gradients along the x-, y-, and z-directions were 1.40 ± 0.27, 1.48 ± 0.35, and 1.78 ± 0.27 × 10^?3 mm²/s. The ADCs of the interventricular septum in a second group of 15 healthy subjects for diffusion gradients along the short axis, horizontal and vertical long axes were 0.92 ± 0.15. 1.50 ± 0.15, and 1.10 ± 0.24 × mm²/s. Because the ADCs were less than the measured values for skeletal muscle and their standard deviations were low, it seems unlikely that bulk motion effects made the dominant contribution to the measured myocardial ADC for the interventricular septum, although motion and/or susceptibility artifacts frequently degraded measurements in the free wall of the left ventricle. Additional evidence that ADC was not predominantly determined by wall motion was obtained in a third group of patients with various cardiac abnormalities, in whom there was only a weak correlation between ADC and ejection fraction. Although further study is needed to better understand the factors contributing to the myocardial ADC, we hypothesize that the measured diffusional anisotropy in the septum might be explained largely on the basis of myofiber orientation.     

Image-based EPI ghost correction using an algorithm based on projection onto convex sets (POCS).

This work describes the use of a method, based on the projection onto convex sets (POCS) algorithm, for reduction of the N/2 ghost in echo-planar imaging (EPI). In this method, ghosts outside the parent image are set to zero and a model k-space is obtained from the Fourier transform (FT) of the resulting image. The zeroth- and first-order phase corrections for each line of the original k-space are estimated by comparison with the corresponding line in the model k-space. To overcome problems of phase wrapping, the first-order phase corrections for the lines of the original k-space are estimated by registration with the corresponding lines in the model k-space. It is shown that applying these corrections will result in a reduction of the ghost, and that iterating the process will result in a convergence towards an image in which the ghost is minimized. The method is tested on spin-echo EPI data. The results show that the method is robust and remarkably effective, reducing the N/2 ghost to a level nearly comparable to that achieved with reference scans.     

A method to improve the BO homogeneity of the heart in vivo

A homogeneous static (B_o) magnetic field is required for many NMR experiments such as echo planar imaging, localized spectroscopy, and spiral scan imaging. Although semi-automated techniques have been described to improve the B_o field homogeneity, none has been applied to the in vivo heart. The acquisition of cardiac field maps is complicated by motion, blood flow, and chemical shift artifact from epicardial fat. To overcome these problems, an ungated three-dimensional (3D) chemical shift image (CSI) was collected to generate a time and motion-averaged B_o field map. B_o heterogeneity in the heart was minimized by using a previous algorithm that solves for the optimal shim coil currents for an input field map, using up to third-order current-bounded shims (1). The method improved the B_o homogeneity of the heart in all 11 normal volunteers studied. After application of the algorithm to the unshimmed cardiac field maps, the standard deviation of proton frequency decreased by 43%, the magnitude ¹H spectral linewidth decreased by 24%, and the peak-peak gradient decreased by 35%. Simulations of the high-order (second- and third-order) shims in B_o field correction of the heart show that high order shims are important, resulting for nearly half of the improvement in homogeneity for several subjects. The T₂* of the left ventricular anterior wall before and after field correction was determined at 4.0 Tesla. Finally, results show that cardiac shimming is of benefit in cardiac ³¹P NMR spectroscopy and cardiac echo planar imaging.     

FAIR true-FISP perfusion imaging of the kidneys.

Most arterial spin labeling (ASL) techniques apply echoplanar imaging (EPI) because this strategy provides relatively high SNR in short measuring times. Unfortunately, those techniques are very susceptible to static magnetic field inhomogeneities and perfusion signals from organs with fast transverse relaxation might decrease due to the exchange of water molecules in capillaries and organ tissue combined with relatively long echo times of EPI sequences. To overcome these problems a novel imaging technique, FAIR True-FISP, was developed. It combines a FAIR (flow-sensitive alternating inversion recovery) perfusion preparation and a true fast imaging with steady precession (True-FISP) data acquisition strategy. True-FISP was chosen since this sequence type does not show the mentioned disadvantages of EPI, but provides a similar SNR per measuring time. An important problem of this approach is that True-FISP sequences usually work in a steady state which is independent of a previous preparation of magnetization. For this reason a sequence structure had to be developed which keeps the advantages of True-FISP and makes the signal intensity sensitive to the FAIR preparation. Breathhold and nonbreathhold examinations of kidneys are presented and possible strategies to quantitative flow measurements are reported. It is shown that correction of spatially inhomogeneous receiver coil characteristics is easily feasible and leads to clinically valuable perfusion examinations of kidneys without application of potentially nephrotoxic contrast media.     

Robust EPI Nyquist ghost elimination via spatial and temporal encoding

Nyquist ghosts are an inherent artifact in echo planar imaging acquisitions. An approach to robustly eliminate Nyquist ghosts is presented that integrates two previous Nyquist ghost correction techniques: temporal domain encoding (phase labeling for additional coordinate encoding: PLACE and spatial domain encoding (phased array ghost elimination: PAGE). Temporal encoding modulates the echo planar imaging acquisition trajectory from frame to frame, enabling one to interleave data to remove inconsistencies that occur between sampling on positive and negative gradient readouts. With PLACE, one can coherently combine the interleaved data to cancel residual Nyquist ghosts. If the level of ghosting varies significantly from image to image, however, the signal cancellation that occurs with PLACE can adversely affect SNR‐sensitive applications such as perfusion imaging with arterial spin labeling. This work proposes integrating PLACE into a PAGE‐based reconstruction process to yield significantly better Nyquist ghost correction that is more robust than PLACE or PAGE alone. The robustness of this method is demonstrated in the presence of magnetic field drift with an in‐vivo arterial spin labeling perfusion experiment. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Multishot 3D slice-select tailored RF pulses for MRI.

A multishot 3D slice-select tailored RF pulse method is presented for the excitation of slice profiles with arbitrary resolution. This method is derived from the linearity of the small tip angle approximation, allowing for the decomposition of small tip angle tailored RF pulses into separate excitations. The final image is created by complex summation of the images acquired from the individual excitations. This technique overcomes the limitation of requiring a long pulse to excite thin slices with adequate resolution. This has implications in applications including T*(2)-weighted functional MRI in brain regions corrupted by intravoxel dephasing artifacts due to susceptibility variations. Simulations, phantom experiments, and human brain images are presented. It is demonstrated that at most four shots of 40 ms pulse length are needed to excite a 5 mm-thick slice in the brain with reduced susceptibility artifacts at 3T.     

Reduction of magnetic field inhomogeneity artifacts in echo planar imaging with SENSE and GESEPI at high field.

Geometric distortion, signal-loss, and image-blurring artifacts in echo planar imaging (EPI) are caused by frequency shifts and T(2)(*) relaxation distortion of the MR signal along the k-space trajectory due to magnetic field inhomogeneities. The EPI geometric-distortion artifact associated with frequency shift can be reduced with parallel imaging techniques such as SENSE, while the signal-loss and blurring artifacts remain. The gradient-echo slice excitation profile imaging (GESEPI) method has been shown to be successful in restoring tissue T(2)(*) relaxation characteristics and is therefore effective in reducing signal-loss and image-blurring artifacts at a cost of increased acquisition time. The SENSE and GESEPI methods are complementary in artifact reduction. Combining these two techniques produces a method capable of reducing all three types of EPI artifacts while maintaining rapid acquisition time.