Parametric dependence of myocardial blood oxygen level dependent,balanced steady‐state free‐precession imaging at 1.5 T: Theory and experiments

Myocardial blood oxygen level dependent, balanced steady‐state free precession (bSSFP) imaging is a relatively new technique for evaluating myocardial oxygenation changes in the presence of coronary artery stenosis. However, the dependence of myocardial bSSFP blood oxygen level dependent signal on imaging parameters has not been well studied. In this work, modeling capillaries as cylinders that act as magnetic perturbers, the Monte Carlo method was used to simulate spin relaxation via diffusion in a field variation inside and outside blood vessels. bSSFP signal changes at various levels of capillary blood oxygen saturation, for a range of pulse repetition times, flip angle, capillary blood volume fraction, vessel wall permeability, water diffusion coefficient, vessel angle to static magnetic field, and the impact of bulk frequency shifts were studied. The theoretical dependence of bSSFP blood oxygen level dependent contrast on pulse repetition times and flip angle was confirmed by experiments in an animal model with controllable coronary stenosis. Results showed that, with the standard bSSFP acquisition, optimum bSSFP blood oxygen level dependent contrast could be obtained at pulse repetition times = 6.0 ms and flip angle = 70°. Additional technical improvements that preserve the image quality may be necessary to further increase the myocardial bSSFP blood oxygen level dependent sensitivity at 1.5 T through even longer pulse repetition times. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

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.     

Improved coronary MR angiography using wideband steady state free precession at 3 tesla with sub‐millimeter resolution

Purpose:

To suppress off‐resonance artifacts in coronary artery imaging at 3 Tesla (T), and therefore improve spatial resolution.

Materials and Methods:

Wideband steady state free precession (SSFP) sequences use an oscillating steady state to reduce banding artifacts. Coronary artery images were obtained at 3T using three‐dimensional navigated gradient echo, balanced SSFP, and wideband SSFP sequences.

Results:

The highest in‐plane resolution of left coronary artery images was 0.68 mm in the frequency‐encoding direction. Wideband SSFP produced an average SNR efficiency of 70% relative to conventional balanced SSFP and suppressed off‐resonance artifacts.

Conclusion:

Wideband SSFP was found to be a promising approach for obtaining noncontrast, high‐resolution coronary artery images at 3 Tesla with reliable image quality. J. Magn. Reson. Imaging 2010;31:1224–1229. © 2010 Wiley‐Liss, Inc.     

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.     

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.     

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.     

Assessment of regional myocardial oxygenation changes in the presence of coronary artery stenosis with balanced SSFP imaging at 3.0 T: theory and experimental evaluation in canines

PURPOSE: To examine the dependence of steady-state free-precession (SSFP) -based myocardial blood-oxygen-level-dependent (BOLD) contrast on field strength using theoretical and experimental models. MATERIALS AND METHODS: Numerical simulations using a two-pool exchange model and a surgically prepared dog model were used to assess the SSFP-based myocardial BOLD signal changes at 1.5T and 3.0T. Experimental studies were performed in eight canines with pharmacological vasodilation under various levels of left circumflex coronary artery stenosis. Experimentally obtained BOLD signal changes were correlated against microsphere-based true flow changes. RESULTS: Theoretical results showed that, at 3.0T, relative to 1.5T, a threefold increase in oxygen sensitivity can be expected. Experimental studies in canines showed near similar results-a 2.5 +/- 0.2-fold increase in BOLD sensitivity at 3.0T relative to 1.5T (P < 0.05). Based on the scatter gram of BOLD data and microsphere data, it was found that the minimum regional flow difference that can be detected with SSFP-based myocardial BOLD imaging at 1.5T and 3.0T were 2.9 and 1.6, respectively (P < 0.05). CONCLUSION: This study demonstrated that SSFP-based myocardial BOLD sensitivity is substantially greater at 3.0T compared with 1.5T. The findings here suggest that SSFP-based myocardial BOLD imaging at 3.0T may have the necessary sensitivity to detect the clinically required minimum flow difference of 2.0.     

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.     

Fast three‐dimensional proton spectroscopic imaging of the human brain at 3 T by combining spectroscopic missing pulse steady‐state free precession and echo planar spectroscopic imaging

The combination of the principles of two fast spectroscopic imaging (SI) methods, spectroscopic missing pulse steady‐state free precession and echo planar SI (EPSI) is described as an approach toward fast 3D SI. This method, termed missing pulse steady‐state free precession echo planar SI, exhibits a considerably reduced minimum total measurement time T_min, allowing a higher temporal resolution, a larger spatial matrix size, and the use of k‐space weighted averaging and phase cycling, while maintaining all advantages of the original spectroscopic missing pulse steady‐state free precession sequence. The minor signal‐to‐noise ratio loss caused by using oscillating read gradients can be compensated by applying k‐space weighted averaging. The missing pulse steady‐state free precession echo planar SI sequence was implemented on a 3 T head scanner, tested on phantoms and applied to healthy volunteers. Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.     

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.     

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.     

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.     

Single‐shot steady‐state free precession can detect myocardial edema in patients: A feasibility study

Purpose

To demonstrate the ability of single‐shot, T₂/T₁ weighted steady‐state free precession (SSFP) to detect myocardial edema in patients with an acute myocardial infarction.

Materials and Methods

This study was performed in a series of patients (n = 10) referred for the assessment of acute myocardial infarcts (AMI). Localizers were used to obtain true short axis views of the left ventricle (LV). These views were used to plan and obtain T₂‐weighted STIR (short TI inversion recovery) images of the LV. These slices were then acquired using single‐shot dark blood‐prepared SSFP with a large (31) number of dummy pulses. Lastly, Contrast agent was injected, and late enhancement (LE) images were acquired. Images were analyzed using a multi‐segment model of the heart. SSFP images were compared with STIR images, with STIR images used as the standard of truth for the presence of edema. LE images were used to identify segments which were positive for microvascular obstruction.

Results

All techniques were successful in all patients. A total of 312 segments were analyzed. Excluding segments positive for microvascular obstruction, SSFP had a sensitivity/specificity of 80%/89%. Including segments positive for microvascular obstruction, sensitivity/specificity was 71%/88%. On a patient‐based analysis, no AMI was missed using SSFP (sensitivity = 100%).

Conclusion

Using single‐shot SSFP to detect myocardial edema in patients with AMI is feasible with a moderate sensitivity and high specificity. J. Magn. Reson. Imaging 2009. © 2009 Wiley‐Liss, Inc.