Imaging periodic currents using alternating balanced steady-state free precession.

Existing functional brain MR imaging methods detect neuronal activity only indirectly via a surrogate signal such as deoxyhemoglobin concentration in the vascular bed of cerebral parenchyma. It has been recently proposed that neuronal currents may be measurable directly using MRI (ncMRI). However, limited success has been reported in neuronal current detection studies that used standard gradient or spin echo pulse sequences. The balanced steady-state free precession (bSSFP) pulse sequence is unique in that it can afford the highest known SNR efficiency and is exquisitely sensitive to perturbations in free precession phase. It is reported herein that when a spin phase-perturbing periodic current is locked to an RF pulse train, phase perturbations are accumulated across multiple RF excitations and the spin magnetization reaches an alternating balanced steady state (ABSS) that effectively amplifies the phase perturbations due to the current. The alternation of the ABSS signal therefore is highly sensitive to weak periodic currents. Current phantom experiments employing ABSS imaging resulted in detection of magnetic field variations as small as 0.15nT in scans lasting for 36 sec, which is more sensitive than using gradient-recalled echo imaging.     

Efficient implementation of hardware‐optimized gradient sequences for real‐time imaging

This work improves the performance of interactive real‐time imaging with balanced steady‐state free precession. The method employs hardware‐optimized gradient pulses, together with a novel phase‐encoding strategy that simplifies the design and implementation of the optimized gradient waveforms. In particular, the waveforms for intermediate phase‐encode steps are obtained by simple linear combination, rather than separate optimized waveform calculations. Gradient waveforms are redesigned in real time as the scan plane is manipulated, and the resulting sequence operates at the specified limits of the MRI gradient subsystem for each new scan‐plane orientation. The implementation provides 14‐25% improvement in the sequence pulse repetition time over the vendor‐supplied interactive real‐time imaging sequence for similar scan parameters on our MRI scanner. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Small‐tip fast recovery imaging using non‐slice‐selective tailored tip‐up pulses and radiofrequency‐spoiling

Small‐tip fast recovery (STFR) imaging is a new steady‐state imaging sequence that is a potential alternative to balanced steady‐state free precession. Under ideal imaging conditions, STFR may provide comparable signal‐to‐noise ratio and image contrast as balanced steady‐state free precession, but without signal variations due to resonance offset. STFR relies on a tailored “tip‐up,” or “fast recovery,” radiofrequency pulse to align the spins with the longitudinal axis after each data readout segment. The design of the tip‐up pulse is based on the acquisition of a separate off‐resonance (B0) map. Unfortunately, the design of fast (a few ms) slice‐ or slab‐selective radiofrequency pulses that accurately tailor the excitation pattern to the local B0 inhomogeneity over the entire imaging volume remains a challenging and unsolved problem. We introduce a novel implementation of STFR imaging based on “non‐slice‐selective” tip‐up pulses, which simplifies the radiofrequency pulse design problem significantly. Out‐of‐slice magnetization pathways are suppressed using radiofrequency‐spoiling. Brain images obtained with this technique show excellent gray/white matter contrast, and point to the possibility of rapid steady‐state T₂/T₁‐weighted imaging with intrinsic suppression of cerebrospinal fluid, through‐plane vessel signal, and off‐resonance artifacts. In the future, we expect STFR imaging to benefit significantly from parallel excitation hardware and high‐order gradient shim systems. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Positive contrast with alternating repetition time SSFP (PARTS): A fast imaging technique for SPIO‐labeled cells

There has been recent interest in positive‐contrast MRI methods for noninvasive tracking of cells labeled with superparamagnetic iron‐oxide nanoparticles. Low‐tip‐angle balanced steady‐state free precession sequences have been used for fast, high‐resolution, and flow‐insensitive positive‐contrast imaging; however, the contrast can be compromised by the limited suppression of the on‐resonant and fat signals. In this work, a new technique that produces positive contrast with alternating repetition time steady‐state free precession is proposed to achieve robust background suppression for a broad range of tissue parameters. In vitro and in vivo experiments demonstrate the reliability of the generated positive contrast. The results indicate that the proposed method can enhance the suppression level by up to 18 dB compared with conventional balanced steady‐state free precession. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Real-time MRI of joint movement with trueFISP

PURPOSE: To develop a technique for dynamic magnetic resonance imaging (MRI) of joint motion based on a combination of real-time TrueFISP (fast imaging with steady state precession) imaging with surface radiofrequency (RF) coils.MATERIALS AND METHODS: The metacarpal, elbow, tarsal, and knee joint of five volunteers and the knees of four patients were examined with a real-time TrueFISP sequence during movement of the joints.RESULTS: All examined joints could be assessed under dynamic conditions with high image contrast and high temporal resolution.CONCLUSION: Dynamic MRI of joints with TrueFISP is feasible and can provide information supplemental to static joint examinations.     

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.     

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.     

Comparison of wideband steady‐state free precession and T2‐weighted fast spin echo in spine disorder assessment at 1.5 and 3 T

Wideband steady‐state free precession (WB‐SSFP) is a modification of balanced steady‐state free precession utilizing alternating repetition times to reduce susceptibility‐induced balanced steady‐state free precession limitations, allowing its use for high‐resolution myelographic‐contrast spinal imaging. Intertissue contrast and spatial resolution of complete‐spine‐coverage 3D WB‐SSFP were compared with those of 2D T₂‐weighted fast spin echo, currently the standard for spine T₂‐imaging. Six normal subjects were imaged at 1.5 and 3 T. The signal‐to‐noise ratio efficiency (SNR per unit‐time and unit‐volume) of several tissues was measured, along with four intertissue contrast‐to‐noise ratios; nerve‐ganglia:fat, intradural‐nerves:cerebrospinal fluid, nerve‐ganglia:muscle, and muscle:fat. Patients with degenerative and traumatic spine disorders were imaged at both MRI fields to demonstrate WB‐SSFP clinical advantages and disadvantages. At 3 T, WB‐SSFP provided spinal contrast‐to‐noise ratios 3.7–5.2 times that of fast spin echo. At 1.5 T, WB‐SSFP contrast‐to‐noise ratio was 3–3.5 times that of fast spin echo, excluding a 1.7 ratio for intradural‐nerves:cerebrospinal fluid. WB‐SSFP signal‐to‐noise ratio efficiency was also higher. Three‐dimensional WB‐SSFP disadvantages relative to 2D fast spin echo are reduced edema hyperintensity, reduced muscle signal, and higher motion sensitivity. WB‐SSFP's high resolution and contrast‐to‐noise ratio improved visualization of intradural nerve bundles, foraminal nerve roots, and extradural nerve bundles, improving detection of nerve compression in radiculopathy and spinal‐stenosis. WB‐SSFP's high resolution permitted reformatting into orthogonal planes, providing distinct advantages in gauging fine spine pathology. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.     

Fast multislice pH‐weighted chemical exchange saturation transfer (CEST) MRI with Unevenly segmented RF irradiation

Chemical exchange saturation transfer (CEST) MRI is a versatile imaging technique for measuring microenvironment properties via dilute CEST labile groups. Conventionally, CEST MRI is implemented with a long radiofrequency irradiation module, followed by fast image acquisition to obtain the steady state CEST contrast. Nevertheless, the sensitivity, scan time, and spatial coverage of the conventional CEST MRI method may not be optimal. Our study proposed a segmented radiofrequency labeling scheme that includes a long primary radiofrequency irradiation module to generate the steady state CEST contrast and repetitive short secondary radiofrequency irradiation module immediately after the image acquisition so as to maintain the steady state CEST contrast for multislice acquisition and signal averaging. The proposed CEST MRI method was validated experimentally with a tissue‐like pH phantom and optimized for the maximal contrast‐to‐noise ratio. In addition, the proposed sequence was evaluated for imaging ischemic acidosis via pH‐weighted endogenous amide proton transfer MRI, which showed similar contrast as conventional amide proton transfer MRI. In sum, a fast multislice relaxation self‐compensated CEST MRI sequence was developed, with significantly improved sensitivity and suitable for in vivo applications. Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.     

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.     

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.     

Optimization of alternating TR‐SSFP for fat‐suppression in abdominal images at 3T

Magnetic resonance imaging is widely used in the work‐up and monitoring of patients with Crohn's disease. Balanced steady‐state free precession sequences are an important part of the imaging protocol and until now primarily 1.5T scanners have been used in daily clinical practice. This is largely because running balanced steady‐state free precession sequences in 3T magnets has technical problems related to increased B₀ inhomogeneity and specific absorption rate (SAR) deposition. A modified form of alternating repetition time steady‐state free precession sequence is presented to acquire 3D‐isotropic abdominal images with fat‐suppression at 3T within a breath‐hold. The modifications include an adjusted radiofrequency pulse shape, suitable phase‐cycling scheme and TR₁/TR₂ ratio. Results show that the proposed sequence is successful in obtaining high contrast 3D‐isotropic abdominal images within a breath‐hold. Furthermore, the proposed methodology is easy to implement in a clinical setting and does not require any postprocessing steps. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Power Independent of Number of Slices (PINS) radiofrequency pulses for low-power simultaneous multislice excitation

This communication describes radiofrequency pulses capable of performing spatially periodic excitation, inversion, and refocusing. The generation of such pulses either by multiplication of existing radiofrequency pulses by a Dirac comb function or by means of Fourier series expansion is described. Practical schemes for the implementation of such pulses are given, and strategies for optimizing the pulse profile at fixed pulse duration are outlined. The pulses are implemented using a spin-echo sequence. The power deposition is independent of the number of slices acquired, and hence the power deposition per slice is considerably reduced compared to multislice imaging. Excellent image quality is obtained both in phantoms and in images of the human head. These pulses should find widespread application for multiplexed imaging, in particular at high static magnetic field strengths and for pulse sequences that have a high radiofrequency power deposition and could lead to dramatic increases in scanning efficiency.     

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.     

Selective missing pulse steady state free precession (MP-SSFP): inner volume and chemical shift selective imaging in a steady state sequence

PURPOSE: To examine new sequences that restrict acquisition of spins to those excited by both of the RF pulses in missing pulse steady state free precession (MP-SSFP) MRI. MATERIALS AND METHODS: New MP-SSFP sequences were created by replacing one of the slice selective pulses (SSPs) with an orthogonal SSP for inner volume imaging, and with a chemical shift selective (CHESS) pulse for chemical shift imaging. The inner volume sequence was applied to a reduced field of view at the center of a resolution phantom; resulting images were evaluated for differences in the aliased signal. The CHESS sequence was applied to volunteers, as well as to and fat, water, and acetic acid phantoms. Results were evaluated with SNR measurements. RESULTS: The inner volume sequence eliminated the aliased signal, while nonselected fat and water levels were suppressed to that of noise by the CHESS sequence. CONCLUSION: Results suggest a novel steady state technique for rapid inner volume or chemical shift imaging.     

CEST‐FISP: A novel technique for rapid chemical exchange saturation transfer MRI at 7 T

Chemical exchange saturation transfer (CEST) and magnetization transfer techniques provide unique and potentially quantitative contrast mechanisms in multiple MRI applications. However, the in vivo implementation of these techniques has been limited by the relatively slow MRI acquisition techniques, especially on high‐field MRI scanners. A new, rapid CEST‐fast imaging with steady‐state free precession technique was developed to provide sensitive CEST contrast in ～20 sec. In this study at 7 T with in vitro bovine glycogen samples and initial in vivo results in a rat liver, the CEST‐fast imaging with steady‐state free precession technique was shown to provide equivalent CEST sensitivity in comparison to a conventional CEST‐spin echo acquisition with a 50‐fold reduction in acquisition time. The sensitivity of the CEST‐fast imaging with steady‐state free precession technique was also shown to be dependent on k‐space encoding with centric k‐space encoding providing a 30–40% increase in CEST sensitivity relative to linear encoding for 256 or more k‐space lines. Overall, the CEST‐fast imaging with steady‐state free precession acquisition technique provides a rapid and sensitive imaging platform with the potential to provide quantitative CEST and magnetization transfer imaging data. Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.     

A segment-interleaved motion-compensated acquisition in the steady state (SIMCAST) technique for high resolution imaging of the inner ear

MRI, with ever-increasing spatial resolution, recently has depicted progressively more anatomic details of the inner ear and is playing an important role in the diagnostic evaluation of patients with sensorineural hearing loss. We present a three-dimensional (3D) segment-interleaved, motion-compensated acquisition in steady state (SIMCAST) sequence that allows further increase in spatial resolution in reasonable scan times minimizing artifacts due to susceptibility and motion. The sequence uses gradient moment nulling over TR and segmented interleaved acquisition of multiple data sets with different radiofrequency (RF) phase-cycling schemes. Combination of data from multiple acquisitions by averaging and maximum intensity projection were compared. Images of phantoms and in vivo inner ears were obtained with both full and fractional echoes and compared with other high resolution techniques such as three-dimensional gradient-echo and two-dimensional (2D) and three-dimensional fast spin-echo (FSE) sequences. The new sequence achieves improved signal-to-noise ratio (SNR) and spatial resolution resulting in improved depiction of inner ear structures.     

Dynamic visualization of arachnoid adhesions in a patient with idiopathic syringomyelia using high‐resolution cine magnetic resonance imaging at 3T

A 39‐year‐old female patient with thoracic syringomyelia underwent routine magnetic resonance imaging (MRI) and 3 T MRI to investigate the value of retrospectively cardiac‐gated cine steady‐state free precession (SSFP) MRI in the preoperative and postoperative diagnosis of arachnoid membranes in the spinal subarachnoid space. Therefore, 3T MRI included sagittal and transverse retrospectively cardiac‐gated cine balanced fast‐field echo (balanced‐FFE) sequences both preoperatively and after microsurgical lysis of arachnoid adhesions and expansive duraplasty. Arachnoid membranes were detected and this result was correlated with intraoperative findings and the results of routine cardiac‐gated phase‐contrast cerebrospinal fluid (CSF) flow MRI. Retrospectively cardiac‐gated cine SSFP MRI enabled imaging of arachnoid membranes with high spatial resolution and sufficient contrast to delineate them from hyperintense CSF preoperatively and postoperatively. The images were largely unaffected by artifacts. Surgery confirmed the presence of arachnoid adhesions in the upper thoracic spine. Not all arachnoid membranes that were seen on cine balanced‐FFE sequences caused significant spinal CSF flow blockages in cardiac‐gated phase‐contrast CSF flow studies. In conclusion, retrospectively cardiac‐gated cine SSFP MRI may become a valuable tool for the preoperative detection of arachnoid adhesions and the postoperative evaluation of microsurgical adhesiolysis in patients with idiopathic syringomyelia. J. Magn. Reson. Imaging 2010. © 2010 Wiley‐Liss, Inc.     

Perfusion imaging.

Measurement of tissue perfusion is important for the functional assessment of organs in vivo. Here we report the use of 1H NMR imaging to generate perfusion maps in the rat brain at 4.7 T. Blood water flowing to the brain is saturated in the neck region with a slice-selective saturation imaging sequence, creating an endogenous tracer in the form of proximally saturated spins. Because proton T1 times are relatively long, particularly at high field strengths, saturated spins exchange with bulk water in the brain and a steady state is created where the regional concentration of saturated spins is determined by the regional blood flow and regional T1. Distal saturation applied equidistantly outside the brain serves as a control for effects of the saturation pulses. Average cerebral blood flow in normocapnic rat brain under halothane anesthesia was determined to be 105 +/- 16 cc.100 g-1.min-1 (mean +/- SEM, n = 3), in good agreement with values reported in the literature, and was sensitive to increases in arterial pCO2. This technique allows regional perfusion maps to be measured noninvasively, with the resolution of 1H MRI, and should be readily applicable to human studies.     

Finite RF pulse correction on DESPOT2

Magnetization transfer and finite radiofrequency (RF) pulses affect the steady state of balanced steady state free precession. As quantification of transverse relaxation (T₂) with driven equilibrium single pulse observation of T₂ is based on two balanced steady state free precession acquisitions, both effects can influence the outcome of this method: a short RF pulse per repetition time (T_RF/TR ? 1) leads to considerable magnetization transfer effects, whereas prolonged RF pulses (T_RF/TR > 0.2) minimize magnetization transfer effects, but lead to increased finite pulse effects. A correction for finite pulse effects is thus implemented in the driven equilibrium single pulse observation of T₂ theory to compensate for reduced transverse relaxation effects during excitation. It is shown that the correction successfully removes the driven equilibrium single pulse observation of T₂ dependency on the RF pulse duration. A reduction of the variation in obtained T₂ from over 50% to less than 10% is achieved. We hereby provide a means of acquiring magnetization transfer‐free balanced steady state free precession images to yield accurate T₂ values using elongated RF pulses. Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.