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.     

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.     

Optimization of on‐resonant magnetization transfer contrast in coronary vein MRI

Magnetization transfer contrast has been used commonly for endogenous tissue contrast improvements in angiography, brain, body, and cardiac imaging. Both off‐resonant and on‐resonant RF pulses can be used to generate magnetization transfer based contrast. In this study, on‐resonant magnetization transfer preparation using binomial pulses were optimized and compared with off‐resonant magnetization transfer for imaging of coronary veins. Three parameters were studied with simulations and in vivo measurements: flip angle, pulse repetitions, and binomial pulse order. Subsequently, first or second order binomial on‐resonant magnetization transfer pulses with eight repetitions of 720° and 240° flip angle were used for coronary vein MRI. Flip angles of 720° yielded contrast enhancement of 115% (P < 0.0006) for first order on‐resonant and 95% (P < 0.0006) for off‐resonant magnetization transfer. There was no statistically significance difference between off‐resonant and on‐resonant first order binomial Magnetization transfer at 720°. However, for off‐resonance pulses, much more preparation time is needed when compared with the binomials but with considerably reduced specific absorption rate. 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.     

Contrast‐enhanced whole‐heart coronary MRI with bolus infusion of gadobenate dimeglumine at 1.5 T

We sought to investigate the T₁ kinetics of blood and myocardium after three infusion schemes of gadobenate dimeglumine (Gd‐BOPTA) and subsequently compared contrast‐enhanced whole‐heart coronary MRI after a bolus Gd‐BOPTA infusion with nonenhanced coronary MRI at 1.5 T. Blood and myocardium T₁ was measured in seven healthy adults, after each underwent three Gd‐BOPTA infusion schemes (bolus: 0.2 mmol/kg at 2 mL/sec, hybrid: 0.1 mmol/kg at 2 mL/sec followed by 0.1 mmol/kg at 0.1 mL/sec, and slow: 0.2 mmol/kg at 0.3 mL/sec). Fourteen additional subjects underwent contrast‐enhanced coronary MRI with an inversion‐recovery steady‐state free precession sequence after bolus Gd‐BOPTA infusion. Images were compared with nonenhanced T₂‐prepared steady‐state free precision whole‐heart coronary MRI in signal‐to‐noise ratio, contrast‐to‐noise ratio, depicted vessel length, vessel sharpness, and subjective image quality. Bolus and slow infusion schemes resulted in similar T₁ during coronary MRI, whereas the hybrid infusion method yielded higher T₁ values. A bolus infusion of Gd‐BOPTA significantly improved signal‐to‐noise ratio, contrast‐to‐noise ratio, depicted coronary artery length, and subjective image quality, when all segments were collectively compared but not when compared segment by segment. In conclusion, whole‐heart steady‐state free precision coronary MRI at 1.5 T can benefit from a bolus infusion of 0.2 mmol/kg Gd‐BOPTA. Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.     

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.     

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.     

3D magnetization‐prepared imaging using a stack‐of‐rings trajectory

Efficient acquisition strategies for magnetization‐prepared imaging based on the three‐dimensional (3D) stack‐of‐rings k‐space trajectory are presented in this work. The 3D stack‐of‐rings can be acquired with centric ordering in all three dimensions for greater efficiency in capturing the desired contrast. In addition, the 3D stack‐of‐rings naturally supports spherical coverage in k‐space for shorter scan times while achieving isotropic spatial resolution. While non‐Cartesian trajectories generally suffer from greater sensitivity to system imperfections, the 3D stack‐of‐rings can enhance magnetization‐prepared imaging with a high degree of robustness to timing delays and off‐resonance effects. As demonstrated with phantom scans, timing errors and gradient delays only cause a bulk rotation of the 3D stack‐of‐rings reconstruction. Furthermore, each ring can be acquired with a time‐efficient retracing design to resolve field inhomogeneities and enable fat/water separation. To demonstrate its effectiveness, the 3D stack‐of‐rings are considered for the case of inversion‐recovery‐prepared structural brain imaging. Experimental results show that the 3D stack‐of‐rings can achieve higher signal‐to‐noise ratio and higher contrast‐to‐noise ratio within a shorter scan time when compared to the standard inversion‐recovery‐prepared sequence based on 3D Cartesian encoding. The design principles used for this specific case of inversion‐recovery‐prepared brain imaging can be applied to other magnetization‐prepared imaging applications. Magn Reson Med 63:1210–1218, 2010. © 2010 Wiley‐Liss, Inc.     

Non‐contrast‐enhanced MR angiography for selective visualization of the hepatic vein and inferior vena cava with true steady‐state free‐precession sequence and time‐spatial labeling inversion pulses: Preliminary results

Purpose

To selectively visualize the hepatic vein and inferior vena cava (IVC) using three‐dimensional (3D) true steady‐state free‐precession (SSFP) MR angiography with time‐spatial labeling inversion pulse (T‐SLIP), and to optimize the acquisition protocol.

Materials and Methods

Respiratory‐gated 3D true SSFP scans were conducted in 23 subjects in combination with two different T‐SLIPs (one placed in the thorax to suppress the arterial signal and the other in the abdomen to suppress the portal venous signal). One of the most important factors was the inversion time (TI) of abdominal T‐SLIP, and the image quality was evaluated at four different TIs of 800, 1200, 1600, and 2000 msec in terms of relative signal‐to‐noise ratio (SNR), contrast‐to‐noise ratio (CNR), and mean visualization scores.

Results

No significant difference was observed in SNR and CNR between each TI. However, IVC visualization scores were better at TIs of 1600 and 2000 msec, and overall image quality was better at TIs of 1200 and 1600 msec. Therefore, the TI of 1600 msec was considered to provide the optimal balance between IVC visualization and signal suppression of the portal vein in our protocol.

Conclusion

True SSFP scan with T‐SLIPs enabled selective visualization of the hepatic vein and IVC without an exogenous contrast agent. J. Magn. Reson. Imaging 2009;29:474–479. © 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.     

Automated estimation of aortic strain from steady‐state free‐precession and phase contrast MR images

The strain values extracted from steady‐state free‐precession (SSFP) and phase contrast (PC) images acquired with a 1.5T scanner on a compliant flow phantom and within the thoracic aorta of 52 healthy subjects were compared. Aortic data were acquired perpendicular to the aorta at the level of the pulmonary artery bifurcation. Cross sectional areas were obtained by using an automatic and robust segmentation method. While a good correlation (r = 0.99) was found between the aortic areas extracted from SSFP and PC sequences, a lower correlation (r = 0.71) was found between the corresponding aortic strain values. Strain values estimated using SSFP and PC sequences were equally correlated with age. Interobserver reproducibility was better for SSFP than for PC. Strain values in the ascending and descending aorta were better correlated for SSFP (r = 0.8) than for PC (r = 0.65) and fitted with the expectation of a larger strain in the ascending aorta when using SSFP. The spatial and temporal resolutions of the acquisitions had a minor influence upon the estimated strain values. Thus, if PC acquisitions can be used to estimate both pulse wave velocity and aortic strain, an additional SSFP sequence may be useful to improve the accuracy in estimating the aortic strain. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Flow‐induced disturbances in balanced steady‐state free precession images: Means to reduce or exploit them

In this work computer simulations and phantom measurements are presented that show the effect of flow on in‐plane balanced steady‐state free precession images. The images were studied for various flow velocities, excitation regions, relaxation times, RF‐pulse angles, and off‐resonance frequencies. The work shows that flow‐induced disturbances are present in the images, but can be reduced by the application of inhomogeneous excitation regions. Also, a velocity quantification method that utilizes the disturbances was developed and proved to quantify flow velocities accurately. The work concluded that the flow‐induced disturbances can be reduced to improve image quality, but can also be exploited to quantify the flow velocity. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.