Steady state free precession magnetization transfer imaging

The formerly proposed concept for magnetization transfer imaging (MTI) using balanced steady‐state free precession (SSFP) image acquisitions is in this work extended to nonbalanced protocols. This allows SSFP‐based MTI of targets with high susceptibility variation (such as the musculoskeletal system), or at ultra‐high magnetic fields (where balanced SSFP suffers from considerable off‐resonance related image degradations). In the first part, SSFP‐based MTI in human brain is analyzed based on magnetization transfer ratio (MTR) histograms. High correlations are observed among all different SSFP MTI protocols and thereby ensure proper conceptual extension to nonbalanced SSFP. The second part demonstrates SSFP‐based MTI allowing fast acquisition of high resolution volumetric MTR data from human brain and cartilage at low (1.5T) to ultra‐high (7.0T) magnetic fields. Magn Reson Med 60:1261–1266, 2008. © 2008 Wiley‐Liss, Inc.     

Optimized balanced steady-state free precession magnetization transfer imaging.

Balanced steady-state free precession (bSSFP) suffers from a considerable signal loss in tissues. This apparent signal reduction originates from magnetization transfer (MT) and may be reduced by an increase in repetition time or by a reduction in flip angle. In this work, MT effects in bSSFP are modulated by a modification of the bSSFP sequence scheme. Strong signal attenuations are achieved with short radio frequency (RF) pulses in combination with short repetition times, whereas near full, i.e., MT-free, bSSFP signal is obtained by a considerable prolongation of the RF pulse duration. Similar to standard methods, the MT ratio (MTR) in bSSFP depends on several sequence parameters. Optimized bSSFP protocol settings are derived that can be applied to various tissues yielding maximal sensitivity to MT while minimizing contribution from other impurities, such as off-resonances. Evaluation of MT in human brain using such optimized bSSFP protocols shows high correlation with MTR values from commonly used gradient echo (GRE) sequences. In summary, a novel method to generate MTR maps using bSSFP image acquisitions is presented and factors that optimize and influence this contrast are discussed.     

Practical applications of balanced steady‐state free‐precession (bSSFP) imaging in the abdomen and pelvis

Balanced steady‐state free‐precession (bSSFP) is an important pulse sequence that may be underutilized in abdominal and pelvic magnetic resonance imaging (MRI). bSSFP offers several advantages for abdominal and pelvic MRI that include: bright blood effects, a relative insensitivity to the dephasing effects which occur in structures with linear movement, low specific absorption rate (SAR), high signal‐to‐noise ratio (SNR), high spatial resolution, and rapid acquisition times. Bright blood effects can be exploited to diagnose or confirm vascular pathologies when gadolinium‐enhanced imaging cannot be performed, is indeterminate, or is degraded by artifact. The relative insensitivity to dephasing artifact in areas of linear movement is useful when imaging the biliary, urinary, and gastrointestinal tracts where dephasing artifacts may mimic filling defects such as calculi or polyps. Low SAR imaging is important in pediatric and pregnant patients and may be useful in patients with medical devices that restrict SAR levels. Rapid acquisition times and high SNR are extremely valuable assets in abdominal and pelvic MRI and bSSFP (which can be performed as static or cine acquisitions) and can be added to most existing abdominal and pelvic protocols when deemed suitable without significantly prolonging examination times. This article reviews the fundamentals of bSSFP imaging, presents vascular and nonvascular applications of bSSFP in abdominal and pelvic MRI, and discusses potential limitations (including imaging artifacts) of bSSFP. Level of Evidence: 5 J. Magn. Reson. Imaging 2017;45:11–20.     

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.     

Superbalanced steady state free precession

Steady state free precession (SSFP) signal theory is commonly derived in the limit of quasi-instantaneous radiofrequency (RF) excitation. SSFP imaging protocols, however, are frequently set up with minimal pulse repetition times and RF pulses can thus constitute a considerable amount to the actual pulse repetition time. As a result, finite RF pulse effects can lead to 10-20% signal deviation from common SSFP theory in the transient and in the steady state which may impair the accuracy of SSFP-based quantitative imaging techniques. In this article, a new and generic approach for intrinsic compensation of finite RF pulse effects is introduced. Compensation is based on balancing relaxation effects during finite RF excitation, similar to flow or motion compensation of gradient moments. RF pulse balancing, in addition to the refocusing of gradient moments with balanced SSFP, results in a superbalanced SSFP sequence free of finite RF pulse effects in the transient and in the steady state; irrespective of the RF pulse duration, flip angles, relaxation times, or off-resonances. Superbalancing of SSFP sequences can be used with all quantitative SSFP techniques where finite RF pulse effects are expected or where elongated RF pulses are used.     

Flip angle profile correction for T1 and T2 quantification with look‐locker inversion recovery 2D steady‐state free precession imaging

Fast methods using balanced steady‐state free precession have been developed to reduce the scan time of T₁ and T₂ mapping. However, flip angle (FA) profiles created by the short radiofrequency pulses used in steady‐state free precession deviate substantially from the ideal rectangular profile, causing T₁ and T₂ mapping errors. The purpose of this study was to develop a FA profile correction for T₁ and T₂ mapping with Look‐Locker 2D inversion recovery steady‐state free precession and to validate this method using 2D spin echo as a reference standard. Phantom studies showed consistent improvement in T₁ and T₂ accuracy using profile correction at multiple FAs. Over six human calves, profile correction provided muscle T₁ estimates with mean error ranging from excellent (?0.6%) at repetition time/FA = 18 ms/60° to acceptable (6.8%) at repetition time/FA = 4.9 ms/30°, while muscle T₂ estimates were less accurate with mean errors of 31.2% and 47.9%, respectively. Magn Reson Med, 2012. © 2012 Wiley Periodicals, 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.     

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.