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.     

Time‐resolved three‐dimensional (3D) phase‐contrast (PC) balanced steady‐state free precession (bSSFP)

In this study the feasibility of a time‐resolved, three‐dimensional (3D), three‐directional flow‐sensitive balanced steady‐state free precession (bSSFP) sequence is demonstrated. Due to its high signal‐to‐noise ratio (SNR) in blood and cerebrospinal fluid (CSF) this type of sequence is particularly effective for acquisition of blood and CSF flow velocities. Flow sensitivity was achieved with the phase‐contrast (PC) technique, implementing a custom algorithm for calculation of optimal gradient parameters. Techniques to avoid the most important sources of bSSFP‐related artifacts (including distortion due to eddy currents and signal voids due to flow‐related steady‐state disruption) are also presented. The technique was validated by means of a custom flow phantom, and in vivo experiments on blood and CSF were performed to demonstrate the suitability of this sequence for human studies. Accurate depiction of blood flow in the cerebral veins and of CSF flow in the cervical portion of the neck was obtained. Possible applications of this technique might include the study of CSF flow patterns, direct in vivo study of pathologies such as hydrocephalus and Chiari malformation, and validation for the existing CSF circulation model. Magn Reson Med, 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.     

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.     

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.     

An analytical description of balanced steady‐state free precession with finite radio‐frequency excitation

Conceptually, the only flaw in the standard steady‐state free precession theory is the assumption of quasi‐instantaneous radio‐frequency pulses, and 10–20% signal deviations from theory are observed for common balanced steady‐state free precession protocols. This discrepancy in the steady‐state signal can be resolved by a simple T₂ substitution taking into account reduced transverse relaxation effects during finite radio‐frequency excitation. However, finite radio‐frequency effects may also affect the transient phase of balanced steady‐state free precession, its contrast or its spin‐echo nature and thereby have an adverse effect on common steady‐state free precession magnetization preparation methods. As a result, an in‐depth understanding of finite radio‐frequency effects is not only of fundamental theoretical interest but also has direct practical implications. In this article, an analytical solution for balanced steady‐state free precession with finite radio‐frequency pulses is derived for the transient phase (under ideal conditions) and in the steady state demonstrating that balanced steady‐state free precession key features are preserved but revealing an unexpected dependency of finite radio‐frequency effects on relaxation times for the transient decay. Finally, the mathematical framework reveals that finite radio‐frequency theory can be understood as a generalization of alternating repetition time and fluctuating equilibrium steady‐state free precession sequence schemes. Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.     

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.     

Noncontrast‐enhanced renal angiography using multiple inversion recovery and alternating TR balanced steady‐state free precession

Noncontrast‐enhanced renal angiography techniques based on balanced steady‐state free precession avoid external contrast agents, take advantage of high inherent blood signal from the contrast mechanism, and have short steady‐state free precession acquisition times. However, background suppression is limited; inflow times are inflexible; labeling region is difficult to define when tagging arterial flow; and scan times are long. To overcome these limitations, we propose the use of multiple inversion recovery preparatory pulses combined with alternating pulse repetition time balanced steady‐state free precession to produce renal angiograms. Multiple inversion recovery uses selective spatial saturation followed by four nonselective inversion recovery pulses to concurrently null a wide range of background species while allowing for adjustable inflow times; alternating pulse repetition time steady‐state free precession maintains vessel contrast and provides added fat suppression. The high level of suppression enables imaging in three‐dimensional as well as projective two‐dimensional formats, the latter of which has a scan time as short as one heartbeat. In vivo studies at 1.5 T demonstrate the superior vessel contrast of this technique. Magn Reson Med 70:527–536, 2013. © 2012 Wiley Periodicals, Inc.