Improved 3‐Tesla cardiac cine imaging using wideband

Cine balanced steady‐state free precession (SSFP) is the most widely used sequence for assessing cardiac ventricular function at 1.5 T because it provides high signal‐to‐noise ratio efficiency and strong contrast between myocardium and blood. At 3 T, the use of SSFP is limited by susceptibility‐induced off‐resonance, resulting in either banding artifacts or the need to use a short‐sequence pulse repetition time that limits the readout duration and hence the achievable spatial resolution. In this work, we apply wideband SSFP, a variant of SSFP that uses two alternating pulse repetition times to establish a steady state with wider band spacing in its frequency response and overcome the key limitations of SSFP. Prospectively gated cine two‐dimensional imaging with wideband SSFP is evaluated in healthy volunteers and compared to conventional balanced SSFP, using quantitative metrics and qualitative interpretation by experienced clinicians. We demonstrate that by trading off temporal resolution and signal‐to‐noise ratio efficiency, wideband SSFP mitigates banding artifacts and enables imaging with approximately 30% higher spatial resolution compared to conventional SSFP with the same effective band spacing. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Multiple repetition time balanced steady‐state free precession imaging

Although balanced steady‐state free precession (bSSFP) imaging yields high signal‐to‐noise ratio (SNR) efficiency, the bright lipid signal is often undesirable. The bSSFP spectrum can be shaped to suppress the fat signal with scan‐efficient alternating repetition time (ATR) bSSFP. However, the level of suppression is limited, and the pass‐band is narrow due to its nonuniform shape. A multiple repetition time (TR) bSSFP scheme is proposed that creates a broad stop‐band with a scan efficiency comparable with ATR‐SSFP. Furthermore, the pass‐band signal uniformity is improved, resulting in fewer shading/banding artifacts. When data acquisition occurs in more than a single TR within the multiple‐TR period, the echoes can be combined to significantly improve the level of suppression. The signal characteristics of the proposed technique were compared with bSSFP and ATR‐SSFP. The multiple‐TR method generates identical contrast to bSSFP, and achieves up to an order of magnitude higher stop‐band suppression than ATR‐SSFP. In vivo studies at 1.5 T and 3 T demonstrate the superior fat‐suppression performance of multiple‐TR bSSFP. Magn Reson Med 62:193–204, 2009. © 2009 Wiley‐Liss, Inc.     

Whole heart magnetization‐prepared steady‐state free precession coronary vein MRI

Purpose

To compare two coronary vein imaging techniques using whole‐heart balanced steady‐state free precession (SSFP) and a targeted double‐oblique spoiled gradient‐echo (GRE) sequences in combination with magnetization transfer (MT) preparation sequence for tissue contrast improvement.

Materials and Methods

Nine healthy subjects were imaged with the proposed technique. The results are compared with optimized targeted MT prepared GRE acquisitions. Both quantitative and qualitative analyses were performed to evaluate each imaging method.

Results

Whole‐heart images were successfully acquired with no visible image artifact in the vicinity of the coronary veins. The anatomical features and visual grading of both techniques were comparable. However, the targeted small slab acquisition of the left ventricular lateral wall was superior to whole‐heart acquisition for visualization of relevant information for cardiac resynchronization therapy (CRT) lead implantation.

Conclusion

We demonstrated the feasibility of whole‐heart coronary vein MRI using a 3D MT‐SSFP imaging sequence. A targeted acquisition along the lateral left ventricular wall is preferred for visualization of branches commonly used in CRT lead implantation. J. Magn. Reson. Imaging 2009;29:1293–1299. © 2009 Wiley‐Liss, Inc.     

Artifact‐reduced two‐dimensional cine steady state free precession for myocardial blood‐ oxygen‐level‐dependent imaging

Purpose:

To minimize image artifacts in long TR cardiac phase‐resolved steady state free precession (SSFP) based blood‐oxygen‐level‐dependent (BOLD) imaging.

Materials and Methods:

Nine healthy dogs (four male, five female, 20–25 kg) were studied in a clinical 1.5 Tesla MRI scanner to investigate the effect of temporal resolution, readout bandwidth, and motion compensation on long repetition time (TR) SSFP images. Breath‐held 2D SSFP cine sequences with various temporal resolutions (10–204 ms), bandwidths (239–930 Hz/pixel), with and without first‐order motion compensation were prescribed in the basal, mid‐ventricular, and apical along the short axis. Preliminary myocardial BOLD studies in dogs with controllable coronary stenosis were performed to assess the benefits of artifact‐reduction strategies.

Results:

Shortening the readout time by means of increasing readout bandwidth had no observable reduction in image artifacts. However, increasing the temporal resolution in the presence of first‐order motion compensation led to significant reduction in image artifacts. Preliminary studies demonstrated that BOLD signal changes can be reliably detected throughout the cardiac cycle.

Conclusion:

Artifact‐reduction methods used in this study provide significant improvement in image quality compared with conventional long TR SSFP BOLD MRI. It is envisioned that the methods proposed here may enable reliable detection of myocardial oxygenation changes throughout the cardiac cycle with long TR SSFP‐based myocardial BOLD MRI. J. Magn. Reson. Imaging 2010;31:863–871. ©2010 Wiley‐Liss, Inc.     

S5FP: spectrally selective suppression with steady state free precession.

A method is presented that employs the inherent spectral selectivity of the Steady-State Free Precession (SSFP) pulse sequence to provide a spectral band of suppression. At TE = TR/2, SSFP partitions the magnetization into two phase-opposed spectral components. Z-storing one of these components simultaneously further excites the other, which is then suppressed by gradient crushing and RF spoiling. The Spectrally Selective Suppression with SSFP (S(5)FP) method is shown to provide significant attenuation of fat signals, while the water signals are essentially unaffected and provide the normal SSFP contrast. Fat suppression is achieved with relatively little temporal overhead (less than 10% reduction in temporal resolution). S(5)FP was validated using simulations, phantoms, and human studies.     

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).     

Intrascanner and interscanner variability of magnetization transfer‐sensitized balanced steady‐state free precession imaging

Recently, a new and fast three‐dimensional imaging technique for magnetization transfer ratio (MTR) imaging has been proposed based on a balanced steady‐state free precession protocol with modified radiofrequency pulses. In this study, optimal balanced steady‐state free precession MTR protocol parameters were derived for maximum stability and reproducibility. Variability between scans was assessed within white and gray matter for nine healthy volunteers using two different 1.5 T clinical systems at six different sites. Intrascanner and interscanner MTR measurements were well reproducible (coefficient of variation: c_v < 0.012 and c_v < 0.015, respectively) and results indicate a high stability across sites (c_v < 0.017) for optimal flip angle settings. This study demonstrates that balanced steady‐state free precession MTR not only benefits from short acquisition time and high signal‐to‐noise ratio but also offers excellent reproducibility and low variability, and it is thus proposed for clinical MTR scans at individual sites as well as for multicenter studies. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Improving non‐contrast‐enhanced steady‐state free precession angiography with compressed sensing

Flow‐independent angiography offers the ability to produce vessel images without contrast agents. Angiograms are acquired with magnetization‐prepared three‐dimensional balanced steady‐state free precession sequences, where the phase encodes are interleaved and the preparation is repeated before each interleaf. The frequent repetition of the preparation significantly decreases the scan efficiency. The number of excitations can instead be reduced with compressed sensing by exploiting the compressibility of the angiograms. Hence, the phase encodes can be undersampled to save scan time without significantly degrading image quality. These savings can be allotted for preparing the magnetization more often, or alternatively, improving resolution. The enhanced resolution and contrast achieved with the proposed method are demonstrated with lower leg angiograms. Depiction of the vasculature is significantly improved with the increased resolution in the phase‐encode plane and higher blood‐to‐background contrast. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

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.     

High-resolution steady-state free precession coronary magnetic resonance angiography within a breath-hold: parallel imaging with extended cardiac data acquisition.

Coronary artery imaging data are conventionally acquired in a single imaging frame during mid-diastole. The data acquisition window must be sufficiently short to avoid cardiac motion artifacts. A short data acquisition window results in decreased imaging efficiency and limited spatial resolution. Parallel imaging may lessen these limitations, but requires highly accurate coil sensitivity. The purpose of this work was to increase the imaging efficiency and spatial resolution in coronary artery imaging using parallel imaging with an extended acquisition window. External coil calibration data were acquired before and after a short mid-diastolic period of accelerated imaging data acquisition. It was assumed that residual cardiac motion in the extended acquisition window would not impede accurate estimation of coil sensitivity since only low spatial frequency signals were acquired for coil calibration. Experimental studies were performed in five healthy volunteers at 3 T using steady-state free precession sequence. Statistical comparison was made between the proposed method and conventional data acquisition for visual quality of image and vessel sharpness. The proposed technique demonstrated higher visual grading and improved vessel sharpness. The proposed method is a new approach to enhance the imaging efficiency and spatial resolution in coronary artery imaging.     

3D breath-held cardiac function with projection reconstruction in steady state free precession validated using 2D cine MRI

PURPOSE: To develop and validate a three-dimensional (3D) single breath-hold, projection reconstruction (PR), balanced steady state free precession (SSFP) method for cardiac function evaluation against a two-dimensional (2D) multislice Fourier (Cartesian) transform (FT) SSFP method. MATERIALS AND METHODS: The 3D PR SSFP sequence used projections in the x-y plane and partitions in z, providing 70-80 msec temporal resolution and 1.7 x 1.7 x 8-10 mm in a 24-heartbeat breath hold. A total of 10 volunteers were imaged with both methods, and the measurements of global cardiac function were compared. RESULTS: Mean signal-to-noise ratios (SNRs) for the blood and myocardium were 114 and 42 (2D) and 59 and 21 (3D). Bland-Altman analysis comparing the 2D and 3D ejection fraction (EF), left ventricular end diastolic volume (LVEDV) and end systolic volume (LVESV), and end diastolic myocardial mass (LVEDM) provided values of bias +/-2 SD of 0.6% +/- 7.7 % for LVEF, 5.9 mL +/- 20 mL for LVEDV, -2.8 mL +/- 12 mL for LVESV, and -0.61 g +/- 13 g for LVEDM. 3D interobserver variability was greater than 2D for LVEDM and LVESV. CONCLUSION: In a single breath hold, the 3D PR method provides comparable information to the standard 2D FT method, which employs 10-12 breath holds.     

Fast diffusion‐weighted steady state free precession imaging of in vivo knee cartilage

Quantification of molecular diffusion with steady state free precession (SSFP) is complicated by the fact that diffusion effects accumulate over several repetition times (TR) leading to complex signal dependencies on transverse and longitudinal magnetization paths. This issue is commonly addressed by setting TR > T₂, yielding strong attenuation of all higher modes, except of the shortest ones. As a result, signal attenuation from diffusion becomes T₂ independent but signal‐to‐noise ratio (SNR) and sequence efficiency are remarkably poor. In this work, we present a new approach for fast in vivo steady state free precession diffusion‐weighted imaging of cartilage with TR << T₂ offering a considerable increase in signal‐to‐noise ratio and sequence efficiency. At a first glance, prominent coupling between magnetization paths seems to complicate quantification issues in this limit, however, it is observed that diffusion effects become rather T₂(ΔD ～ 1/10 ΔT₂) but not T₁ independent (ΔD ～ 1/2 ΔT₁) for low flip angles α ～ 10 ? 15°. As a result, fast high‐resolution (0.35 × 0.35 ? 0.50 × 0.50 mm² in‐plane resolution) quantitative diffusion‐weighted imaging of human articular cartilage is demonstrated at 3.0 T in a clinical setup using estimated T₁ and T₂ or a combination of measured T₁ and estimated T₂ values. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

View ordering for magnetization prepared steady state free precession acquisition: application in contrast-enhanced MR angiography.

Magnetization prepared segmented acquisition requires a view order that maximizes signal contrast during the acquisition of the central portion of k-space. Steady state free precession (SSFP) acquisition further requires a view order that minimizes changes in phase-encoding gradients from one repetition to the next in order to minimize eddy current artifacts. In this article, optimal view ordering schemes satisfying these two requirements are formulated and applied to inversion prepared 3D SSFP contrast-enhanced MR angiography (MRA). Experiments on phantoms and pigs demonstrated improved background suppression and reduced image artifacts.     

Assessment of magnetization transfer effects in myocardial tissue using balanced steady‐state free precession (bSSFP) cine MRI

Magnetization transfer imaging (MTI) by means of MRI exploits the mobility of water molecules in tissue and offers an alternative contrast mechanism beyond the more commonly used mechanisms based on relaxation times. A cardiac MTI method was implemented on a commercially available 1.5 T MR imager. It is based on the acquisition of two sets of cardiac‐triggered cine balanced steady‐state free precession (bSSFP) images with different levels of RF power deposition. Reduction of RF power was achieved by lengthening the RF excitation pulses of a cine bSSFP sequence from 0.24 ms to 1.7 ms, while keeping the flip angle constant. Normal volunteers and patients with acute myocardial infarcts were imaged in short and long axis views. Normal myocardium showed an MT ratio (MTR) of 33.0 ± 3.3%. In acute myocardial infarct, MTR was reduced to 24.5 ± 9.2% (P < 0.04), most likely caused by an increase in water content due to edema. The method thus allows detection of acute myocardial infarct without the administration of contrast agents. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Cine cardiac imaging using black‐blood steady‐state free precession (BB‐SSFP) at 3T

Purpose

To propose a new black‐blood (BB) pulse sequence that provides BB cine cardiac images with high blood‐myocardium contrast. The proposed technique is based on the conventional steady‐state free precession (SSFP) sequence.

Materials and Methods

Numerical simulations of the Bloch equation were conducted to compare the resulting signal‐to‐noise ratio (SNR) to that of conventional BB imaging, including the effects of changing the imaging flip angle and heart rates. Simulation results were verified using a gel phantom experiment and five normal volunteers were scanned using the proposed technique.

Results

The new sequence showed higher SNR and contrast‐to‐noise ratio (CNR) (≈100%) compared to the conventional BB imaging. Also, the borders of the left ventricle (LV) and right ventricle (RV) appear more distinguishable than the conventional SSFP. We were also able to cover about 80% of the cardiac cycle with short breath‐hold time (≈10 cardiac cycles) and with reasonable SNR and CNR.

Conclusion

Based on an SSFP conventional sequence, the new sequence provides BB cines that cover most of the cardiac cycle and with higher SNR and CNR than the conventional BB sequences. J. Magn. Reson. Imaging 2009;30:94–103. © 2009 Wiley‐Liss, Inc.