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.     

Breath-hold 3D steady-state free precession coronary MRA compared with conventional X-ray coronary angiography

PURPOSE: To evaluate the use of breath-hold three-dimensional (3D) steady-state free precession (SSFP) coronary magnetic resonance angiography (MRA) in patients with coronary artery disease (CAD) in comparison with conventional coronary x-ray angiography (XRA). MATERIALS AND METHODS: Twenty-eight patients with suspected CAD were examined with the use of a breath-hold 3D-SSFP-MRA sequence and conventional XRA. To assess the accuracy of MRA, two clinicians who were blinded to patient information independently reviewed the MRA and XRA data, which were presented in a randomized order. To identify discrepancies between MRA and XRA, and assess features of coronary lesions on MRA, two additional clinicians examined MRA and XRA data that were presented side by side, divided into proximal, mid, and distal segments, and compared them segment by segment. RESULTS: The sensitivity and specificity for diagnosing significant coronary stenoses (> 50% diameter narrowing) were 64% and 94%, respectively. At sites of coronary lesions identified on XRA, bright signals and enlarged vessel profiles, in addition to the characteristic narrow lumen, were frequently observed on MRA. CONCLUSION: Breath-hold SSFP coronary MRA has good specificity but inconclusive sensitivity in diagnosing significant coronary stenoses, and provides important image features for depicting coronary lesions.     

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.     

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.     

Magnetic resonance portography using contrast-enhanced fat-saturated three-dimensional steady-state free precession imaging

PURPOSE: To assess the feasibility of contrast-enhanced fat-saturated three-dimensional steady-state free precession (FIESTA) imaging for contrast-enhanced magnetic resonance (MR) portography. MATERIALS AND METHODS: Contrast-enhanced fat-saturated three-dimensional fast spoiled gradient-echo (SPGR) and FIESTA were performed as MR portography. In 10 cases, fat-saturated three-dimensional FIESTA was first performed and followed by fast SPGR, and the order of post-contrast imaging was reversed in the other 10 cases. Signal-to-noise ratios (SNRs) and contrast-to-noise ratios (CNRs) were estimated for portal and visceral veins on the source images. The visualization of portal vein was scored on three-dimensional MR portography. Portal venous system disorders were assessed using three-dimensional MR portography. RESULTS: The SNRs, CNRs, and visual assessment of portal and visceral veins were significantly higher in contrast-enhanced fat-saturated three-dimensional FIESTA than contrast-enhanced fat-saturated three-dimensional fast SPGR (P < 0.05). The contrast-enhanced fat-saturated three-dimensional FIESTA provided high venous signals even at 8 minutes after gadolinium injection. The abnormalities of portal venous system were well visualized with MR portography using contrast-enhanced fat-saturated three-dimensional FIESTA. CONCLUSION: Contrast-enhanced fat-saturated three-dimensional FIESTA was valuable for MR portography, with flexible time window and high vascular signals. This imaging may allow for other post-contrast imaging options before portography and release patients from consecutive breath-holds.     

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).     

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.     

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.     

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.     

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.     

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.     

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).     

Cardiac fat navigator-gated steady-state free precession 3D magnetic resonance angiography of coronary arteries.

Motion artifacts and the lack of accurate detection of cardiac motion present a major challenge for high-resolution cardiac MRI. Recently a multidimensional cardiac fat navigator was proposed to provide a fast and direct measurement of bulk cardiac motion. The objective of this study was to demonstrate the feasibility of employing the cardiac fat navigator in balanced steady-state free precession (SSFP) free-breathing 3D coronary MRA (CMRA). The cardiac fat navigator echo is optimized to provide both motion monitoring and epicardial fat suppression. Steady-state magnetization preparation, which is needed for SSFP CMRA, is optimized by comparing three preparation schemes: alpha/2, linear ramp with 20 RF pulses (20LR), and Kaiser ramp with six RF pulses (6KR). The present preliminary human study shows that the 6KR preparation provides better image quality than both the alpha/2 (P<0.0025) and the 20LR preparations (P<0.025) for free-breathing SSFP 3D CMRA (N=11).     

Free-breathing 3D steady-state free precession coronary magnetic resonance angiography: comparison of diaphragm and cardiac fat navigators

PURPOSE: To compare the performance of the conventional diaphragm navigator (DNAV) and the recently developed cardiac fat navigator (FatNAV) in suppressing respiration-induced cardiac motion in free-breathing 3D balanced steady-state free precession coronary MRA (SSFP CMRA). MATERIALS AND METHODS: In 16 healthy volunteers the right coronary artery (RCA) was imaged at 1.5T using a navigator-gated 3D SSFP CMRA sequence. DNAV and FatNAV gating were performed in random order. Image quality difference was scored by three experienced readers blinded to the gating technique. Blood signal-to-noise ratio (SNR), blood-to-myocardium contrast-to-noise ratio (CNR), and navigator efficiency were calculated. RESULTS: Diagnostically interpretable CMRA was obtained successfully in all 16 subjects with FatNAV gating (0% failure rate) and only 14 subjects with DNAV gating (12% failure rate). Compared to DNAV gating, FatNAV gating provided similar SNR and CNR, better image quality (P < 0.01), and 28% improvement in navigator efficiency (P = 0.002). CONCLUSION: FatNAV gating provides more effective motion suppression and better image quality than DNAV gating for free-breathing 3D SSFP CMRA of the RCA in healthy subjects.     

The diffusion sensitivity of fast steady-state free precession imaging

Steady-state free precession (SSFP) imaging with an added field gradient pulse is strongly sensitive to self-diffusion and other motions of water. In an earlier theoretical analysis of diffusion attenuation due to a single gradient pulse Wu and Buxton (J. Magn. Reson. 90, 243, 1990) concluded that the diffusion sensitivity would be increased with smaller flip angles. In this paper a partial partition analysis of the different echo pathways contributing to the signal is used to illustrate the contribution of stimulated echo pathways with long diffusion times as the source of the enhanced diffusion sensitivity with low flip angles. Experimental imaging studies in phantoms and the brain of a human subject demonstrate substantially greater signal attenuation with small flip angles (<30°). The theoretical equation of Wu and Buxton provides a reasonable fit to the experimental data, accounting for the flip angle and TR dependence, but the estimated diffusion coefficients are larger than expected from previous studies. The large attenuation observed in the human studies, particularly in cerebrospinal fluid, is most likely due to other tissue motions. Both the theoretical calculations and the experimental data show that for the same gradient strength the diffusion sensitivity of SSFP is much greater than the diffusion sensitivity of conventional spin-echo methods.     

Intrinsic fat suppression in TIDE balanced steady-state free precession imaging.

A novel fat-suppressed balanced steady-state free precession (b-SSFP) imaging method based on the transition into driven equilibrium (TIDE) sequence with variable flip angles is presented. The new method, called fat-saturated (FS)-TIDE, exploits the special behavior of TIDE signals from off-resonance spins during the flip angle ramp. As shown by simulations and experimental data, the TIDE signal evolution for off-resonant isochromats during the transition from turbo spin-echo (TSE)-like behavior to the true fast imaging with steady precession (TrueFISP) mode undergoes a zero crossing. The resulting signal notch for off-resonant spins is then used for fat suppression. The efficiency of FS-TIDE is demonstrated in phantoms and healthy volunteers on a 1.5T system. The resulting images are compared with standard TrueFISP data with and without fat suppression. It is demonstrated that FS-TIDE provides a fast and stable means for homogenous fat suppression in abdominal imaging while maintaining balanced SSFP-like image contrast and signal-to-noise ratio (SNR). The scan time of FS-TIDE is not increased compared to normal TrueFISP imaging without fat suppression and identical k-space trajectories. Because of the intrinsic fat suppression, no additional preparation is needed. Possible repetition times (TRs) are not firmly limited to special values and are nearly arbitrary.     

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.     

Unenhanced steady state free precession versus traditional MR imaging for congenital heart disease

Purpose

To assess potential benefits of three dimensional (3D) steady state free precession (SSFP) magnetic resonance sequence for congenital heart disease (CHD).

Materials and methods

Twenty consecutive patients with CHD (male:female ratio,14:6, mean age, 27.5 ± 8.5 years) underwent both 3D SSFP and traditional MR imaging (TMRI) [including two dimensional (2D) SSFP and contrast enhanced magnetic resonance angiography (CEMRA)]. Image quality and diagnosis were compared, and Bland–Altman analysis was used to evaluate consistency of 3D SSFP and CEMRA for diameter measurements.

Results

A total of 35 intra and 81 extra cardiac anomalies were identified in all patients. The image quality of 3D SSFP and TMRI for either intra or extra cardiac anomalies of all patients scored ≥3, which allowed an establishment of diagnosis for all cases. The diagnostic sensitivity, specificity, and accuracy of 3D SSFP for the detection of intra cardiac anomalies were all 100%, whereas for extra cardiac anomalies they were 93.8%, 93.8%, 100%, respectively. Mean differences (3D SSFP minus CEMRA) for aorta and pulmonary arteries were 0.5 ± 1.2 mm and 0.0 ± 1.7 mm, respectively, showing good consistency of 3D SSFP and CEMRA for diameter measurements.

Conclusion

3D SSFP MRI can be an alternative image modality to TMRI for patients with congenital heart disease, especially for those who have renal insufficiency, breath-hold difficulty or who are allergic to contrast agent. It can also provide powerful complementary information for patients who undergo TMRI, especially at ventriculoarterial connection site.     

Fat-suppressed steady-state free precession imaging using phase detection.

Fully refocused steady-state free precession (SSFP) is a rapid, efficient imaging sequence that can provide diagnostically useful image contrast. In SSFP, the signal is refocused midway between excitation pulses, much like in a spin-echo experiment. However, in SSFP, the phase of the refocused spins alternates for each resonant frequency interval equal to the reciprocal of the sequence repetition time (TR). Appropriate selection of the TR results in a 180 degrees phase difference between lipid and water signals. This phase difference can be used for fat-water separation in SSFP without any increase in scan time. The technique is shown to produce excellent non-contrast-enhanced, flow-independent angiograms of the peripheral vasculature.