Rapid Fourier imaging using steady-state free precession

Reversal of the read gradient in a SSFP imaging experiment allows a full spin echo to be collected in the interval tau between successive rf pulses. Orthogonal gradient pulses are used to dephase and subsequently rephase the transverse magnetization each tau enabling 2D or 3D Fourier techniques. The minimum data collection time per slice in the 3D technique is 3.1 s (128 X 256). For a 2D data collection, an oscillating bipolar sawtooth gradient is used to select the slice. Each phase-encode value must be averaged over an equivalent portion of the oscillating slice-selection gradient and this condition gives a minimum of 25 s for 2D data collection. Excellent slice selection is achieved with less than 5% of the signal lying outside the slice profile central lobe. Images at 0.14 T show tissue contrast may be manipulated by changing the rf pulse angle, an example of which is the presence or absence of gray/white matter contrast at rf pulse angles of 30 and 90 degrees, respectively. The pulse angle theta dependence of five samples with different values of T2/T1 was measured and numerically calculated with good agreement between theory and experiment for theta less than or equal to 90 degrees.     

Cardiac CINE imaging with IDEAL water-fat separation and steady-state free precession

PURPOSE: To decompose multicoil CINE steady-state free precession (SSFP) cardiac images acquired at short echo time (TE) increments into separate water and fat images, using an iterative least-squares "Dixon" (IDEAL) method. MATERIALS AND METHODS: Multicoil CINE IDEAL-SSFP cardiac imaging was performed in three volunteers and 15 patients at 1.5 T. RESULTS: Measurements of signal-to-noise ratio (SNR) matched theoretical expectations and were used to optimize acquisition parameters. TE increments of 0.9-1.0 msec permitted the use of repetition times (TRs) of 5 msec or less, and provided good SNR performance of the water-fat decomposition, while maintaining good image quality with a minimum of banding artifacts. Images from all studies were evaluated for fat separation and image quality by two experienced radiologists. Uniform fat separation and diagnostic image quality was achieved in all images from all studies. Examples from volunteers and patients are shown. CONCLUSION: Multicoil IDEAL-SSFP imaging can produce high quality CINE cardiac images with uniform water-fat separation, insensitive to Bo inhomogeneities. This approach provides a new method for reliable fat-suppression in cardiac imaging.     

MR imaging of brain surface using steady-state free precession.

We present an imaging technique that affords direct and noninvasive visualization of brain surface structure. This technique utilizes the signal before the rf pulse in steady-state free precession. This signal highly reflects the spin-spin relaxation time T2 as was studied in our laboratory (Matsui et al. J. Magn. Reson. 62, 12, 1985). Therefore the cerebrospinal fluid (CSF), having a long T2, is depicted as high intensity. The CSF permeates cerebral sulci and fissures. The imaging time with this technique is less than 1 min.     

Evaluation of the articular cartilage of the knee joint with vastly undersampled isotropic projection reconstruction steady-state free precession imaging

PURPOSE: To determine the feasibility of the vastly undersampled isotropic projection reconstruction steady-state free precession (VIPR-SSFP) sequence for evaluating the articular cartilage of the knee joint. MATERIALS AND METHODS: A magnetic resonance (MR) examination of the knee was performed on 33 subjects using a GE 1.5T scanner and a phased-array extremity coil. VIPR-SSFP, proton density-weighted fast spin-echo (PD-FSE), fat-suppressed T2-weighted fast spin-echo (T2-FSE), and three-dimensional fat-suppressed spoiled gradient recall-echo (SPGR) sequences were performed on three asymptomatic volunteers and 10 patients with osteoarthritis of the knee joint. Signal-to-noise efficiency, and contrast-to-noise ratio (CNR) measurements were calculated for all sequences and compared with the use of paired t-tests. The VIPR-SSFP sequence was then performed on 20 consecutive patients who were undergoing a routine MR examination of the knee. RESULTS: The cartilage signal-to-noise efficiency of the VIPR-SSFP sequence was not significantly different from that of the PD-FSE and SPGR sequences. The cartilage signal-to-noise efficiency of the VIPR-SSFP sequence was significantly higher (P < 0.05) than that of the T2-FSE sequence. The VIPR-SSFP sequence produced images with significantly higher (P < 0.05) CNR between cartilage and synovial fluid than the PD-FSE and SPGR sequences, and significantly higher (P < 0.05) CNR between cartilage and subchondral bone than the T2-FSE sequence. The VIPR-SSFP sequence allowed excellent visualization of the articular cartilage of the knee joint in all subjects. All articular cartilage defects identified on the PD-FSE, T2-FSE, and SPGR images were well visualized on the VIPR-SSFP images. CONCLUSION: VIPR-SSFP images had high cartilage signal-to-noise efficiency and high CNR between cartilage and adjacent synovial fluid and subchondral bone; therefore, the sequence is well suited for evaluating the articular cartilage of the knee joint.     

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.     

Motion-insensitive, steady-state free precession imaging

Steady-state free precession (SSFP) pulse sequences employing gradient reversal echoes and short repetition time (TR) between successive rf excitation pulses offer high signal-to-noise ratio per unit time. However, SSFP sequences are very sensitive to motion. A new SSFP method is presented which avoids the image artifacts and loss of signal intensity due to motion. The pulse sequence is designed so that the time integral of each of the three gradients is zero over each TR time interval. The signal then consists of numerous echoes which are superimposed. These echoes are isolated by combining the data from N different scans. In each scan a specific phase shift is added during every TR interval. Each of these N isolated echoes produces a motion-insensitive, artifact-free image. Because all the echoes are sampled simultaneously, the signal-to-noise ratio per unit time in this SSFP method is higher than in existing SSFP techniques which sample only one echo at a time. The new method was implemented and used to produce both two- and three-dimensional images of the head and cervical spin of a human patient. In these images the high signal intensity of cerebrospinal fluid is preserved regardless of its motion. Further work is required to evaluate the imaging parameters (TR, TE, rf tip angle) so as to give optimal tissue contrast for the various echoes.     

Frequency-modulated steady-state free precession imaging.

Exploration of the possibilities of steady-state free precession (SSFP) excitation has led to the discovery that it is tolerant of slow variations in spectral offset frequency. The effect has been used to eliminate banding artifacts from images obtained with the fully balanced SSFP imaging sequence.     

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.     

Assessment of ventricular function with single breath-hold real-time steady-state free precession cine MR imaging

OBJECTIVE: The aim of our study was to evaluate whether a recently developed real-time steady-state free precession (trueFISP) cine sequence could be used to assess left ventricular function in a single breath-hold. CONCLUSION: Using real-time trueFISP permits one to assess left ventricular function in a single breath-hold. The dramatic reduction in data acquisition time does require some compromises. The temporal and spatial resolutions of images obtained with real-time trueFISP were considerably lower than those achieved with segmented trueFISP. Further reduction of the TR or the use of sensitivity encoding could improve temporal resolution and eliminate other limitations of real-time trueFISP.     

Quantitative in vivo diffusion imaging of cartilage using double echo steady-state free precession

Single-shot echo-planar imaging techniques are commonly used for diffusion-weighted imaging (DWI) but offer rather poor spatial resolution and field-of-view coverage for species with short T(2) . In contrast, steady-state free precession (SSFP) has shown promising results for DWI of the musculoskeletal system, but quantification is generally hampered by its prominent sensitivity on relaxation times. In this work, a new and truly diffusion-weighted (that is relaxation time independent) SSFP DWI technique is introduced using a double-echo steady-state approach. Within this framework (and this is in contrast to common SSFP DWI techniques using SSFP-Echo) both primary echo paths of nonbalanced SSFP are acquired, namely the FID and the Echo. Simulations and in vitro measurements reveal that the ratio of the Echo/FID signal ratios of two double-echo steady-state scans acquired with and without diffusion sensitizing dephasing moments provides a highly relaxation independent quantity for diffusion quantification. As a result, relaxation-independent high-resolution (0.4 × 0.4 - 0.6 × 0.6 mm(2) in-plane resolution) quantitative in vivo SSFP DWI is demonstrated for human articular cartilage using diffusion-weighted double-echo steady-state scans in the knee and ankle joint at 3.0 T. The derived diffusion coefficients for cartilage (D ～ 1.0-1.5 μm(2) /ms) and synovial fluid (D ～ 2.6 μm(2) /ms) are in agreement with previous work.     

MR imaging of articular cartilage

With the advent of new treatments for articular cartilage disorders, accurate noninvasive assessment of articular cartilage, particularly with MR imaging, has become important. Understanding the MR imaging features of articular cartilage has led to the development of two types of routinely available MR imaging techniques which have demonstrated clinical accuracy and interobserver reliability. Received: 25 January 2000 Revision requested: 21 March 2000 Revision received: 31 March 2000 Accepted: 3 April 2000     

Dark flow artifacts with steady-state free precession cine MR technique: causes and implications for cardiac MR imaging

Steady-state free precession cine images from cardiac magnetic resonance imaging studies of 24 patients were reviewed retrospectively to identify dark flow artifacts. The cause and features of the artifacts were studied in flow phantom experiments. Dark flow artifacts were recognized in eight of the 24 cases and were characterized by low or inhomogeneous signal intensity in blood pools with little change in adjacent tissues. The artifacts could be mimicked in flow phantom experiments by deliberately deshimming the gradients and appeared periodically during imaging with off-centered frequencies. These artifacts appeared to be caused by spins moving within an inhomogeneous magnetic field.     

Three-dimensional black-blood MR imaging of carotid arteries with segmented steady-state free precession: initial experience

This HIPAA-compliant study had institutional review board approval. Informed consent was obtained. The purpose was to prospectively evaluate a segmented three-dimensional (3D) double inversion recovery (DIR)-prepared steady-state free precession (SSFP) magnetic resonance (MR) imaging sequence for fast high-spatial-resolution black-blood carotid arterial wall imaging. Carotid wall-lumen contrast-to-noise ratio (CNR) obtained with this sequence was compared with those obtained with two-dimensional (2D) single- and multisection black-blood fast spin-echo (SE) sequences. MR imaging of both carotid artery bifurcations over 3 cm of transverse coverage was performed in eight volunteers (seven men, one woman; age range, 26-56 years) with no known history of carotid artery disease. Adjusted for section thickness and imaging time per section, higher effective mean CNR was achieved with segmented 3D DIR-prepared SSFP than with single-section 2D DIR-prepared fast SE or multisection 2D saturation-band fast SE (P < .05). Segmented 3D DIR-prepared SSFP enables black-blood carotid arterial wall MR imaging with contiguous thin-section coverage and greater imaging speed and effective CNR than conventional 2D fast SE techniques.     

Three-dimensional MR of the inner ear with steady-state free precession.

PURPOSETo describe the steady-state free-precession MR sequence and its application to the study of the inner ear.METHODThe inner ear was imaged with CT and a 0.5-T MR unit in three dimension, to evaluate the various signals from the lumen of the labyrinth.RESULTSNormally, the signal from the perilymphatic and endolymphatic spaces is homogeneous. However, among our cases of neurosensory deafness, differences of signal and morphology were seen in patients with otosclerosis, ossifying labyrinthitis, and inner ear malformations.CONCLUSIONThree-dimensional MR, used together with routine two-dimensional fast spin-echo, is another diagnostic too]l that can provide new data in the evaluation of the normal and unhealthy inner ear.     

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.     

Starter sequence for steady-state free precession imaging.

The dynamic equilibrium exploited by balanced steady-state free precession imaging develops slowly because its formation is dependent on both spin-spin and spin-lattice relaxation times. Attempting to image before steady state is established results in artifacts due to transient signal oscillations. Using a starter sequence to precondition the spin system can significantly reduce the delay before imaging. An improved design for a steady-state starter sequence is presented. The new sequence has the advantage of uniformly exciting the steady-state response for all resonance offsets and can be phase cycled to suppress banding artifacts.     

Intravoxel incoherent motion imaging using steady-state free precession

IVIM MR imaging is a method which generates images of diffusion and perfusion in vivo. Until now, intravoxel incoherent motion (IVIM) images have been obtained using spin-echo sequences with extragradient pulses, resulting in long acquisition times (typically 2 x 8 min 32 s). A new method is proposed here, using steady-state free precession (SSFP), which allows IVIM images to be obtained in a couple of minutes. Phantom studies showed that the sensitivity of SSFP to IVIMs is much greater than that of spin echoes. In vivo images are shown.     

MR ventriculocisternography by using 3D balanced steady-state free precession imaging: technical note

This study investigated the effects of flip angle setting in 3D balanced steady-state free precession (SSFP) imaging on CSF-parenchyma contrast and section aliasing artifacts. Theoretical derivations indicated that the extent of section aliasing artifacts decreased as the flip angle was lowered, at the expense of a sacrifice in CSF-parenchyma contrast. Experimental data agreed closely with theoretical predictions. A flip angle of about 40 degrees is therefore recommended for 3D balanced SSFP MR ventriculocisternography.     

Magnetic resonance imaging of knee cartilage using a water selective balanced steady-state free precession sequence

PURPOSE: To compare an optimized water selective balanced steady-state free precession sequence (WS-bSSFP) with conventional magnetic resonance (MR) sequences in imaging cartilage of osteoarthritic knees. MATERIALS AND METHODS: Flip angles of sagittal and axial WS-bSSFP sequences were optimized in three volunteers. Subsequently, the knees of 10 patients with generalized osteoarthritis were imaged using sagittal and axial WS-bSSFP and conventional MR imaging techniques. We calculated contrast-to-noise ratios (CNR) between cartilage and its surrounding tissues to quantitatively analyze the various sequences. Using dedicated software we compared, in two other patients, the accuracy of cartilage volume measurements with anatomic sections of the tibial plateau. RESULTS: CNRtotal eff (CNR efficiency between cartilage and its surrounding tissue) using WS-bSSFP was maximal with a 20-25 degrees flip angle. CNRtotal eff was higher in WS-bSSFP than in conventional images: 6.1 times higher compared to T1-weighted gradient echo (GE) images, 5.1 compared to proton-density (PD) fast spin echo (FSE) images, and 4.8 compared to T2-weighted FSE images. The mean difference of cartilage volume measurement on WS-bSSFP and anatomic sections was 0.06 mL compared to 0.24 mL for T1-GE and anatomic sections. CONCLUSION: A WS-bSSFP sequence is superior to conventional MR imaging sequences in imaging cartilage of the knee in patients with osteoarthritis.