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Purpose Variable flip angle (VFA) imaging is widely used for whole‐brain T1 quantification. Because of the requirement to acquire at least two sets of MR images at different flip angles, VFA relaxometry is relatively slow. Here, whole‐brain VFA T1 mapping at 1.5 T is accelerated by using efficient spiral non‐Cartesian imaging Methods To quantify T1 in the human brain, radiofrequency spoiled gradient‐echo imaging is performed at two optimized flip angles using an interleaved 2D multislice sequence with high spoiling efficiency. The acquisitions are accelerated by using a spiral trajectory with 10 interleaves combined with a dedicated magnetization preparation to ensure steady‐state conditions in minimal time. Results The investigated MR scan protocol allowed the acquisition of whole‐brain T1 maps at a clinically relevant resolution in only 40 s (0.7 s per slice) with high reproducibility. White and gray matter T1 peaks clearly could be delineated by calculation of whole‐brain T1 histograms, and the delivered T1 values showed good agreement with the reference method in selected regions of interest. Conclusion Due to the use of a fast spiral k‐space trajectory, whole‐brain VFA T1 mapping could be accelerated by an order of magnitude compared to conventional Cartesian sampling.

The goal of this work was to analyze the clinical and technical usability of recently introduced steady state free precession T2 mapping technique. The TESS-T2 values were compared to conventionally used multi-echo spin-echo sequence (usually referred to as CPMG). The results showed that although T2 values determined by TESS are lower than those of CPMG, they are highly correlated between two methods. Both, clinical and experimental studies might benefit from the speed of the 3D-TESS. The results of this study showed the clinical usability of a 3D-TESS sequence for T2 mapping of human articular knee cartilage. 3D-TESS provides highly correlated T2-values to conventional CMPG sequence with some additional benefits, such as dramatic shortening of acquisition time and fair insensitivity to B1 and B0 variations. Also, T2 values acquired with 3D-TESS can differentiate between reference and impaired cartilage.

A magnetic resonance imaging (MRI) method for the quantification of the longitudinal (T1) and/or transverse (T2) relaxation times in a sample. According to the MRI method, a sample is subjected to an unbalanced steady state free precession (SSFP) sequence comprising a series of consecutive radiofrequency (RF) pulses. By means of the unbalanced SSFP sequence, a first order SSFP FID signal (F1), a lowest order SSFP FID signal (F0), and a lowest order SSFP Echo signal (F−1) are acquired. Based on the F0-signal, the F1-signal and the F−1-signal, the longitudinal (T1) and/or transverse (T2) relaxation times of the sample are determined.

Purpose Conventional T1 mapping using three‐dimensional (3D) radiofrequency (RF) spoiled gradient echo (SPGR) imaging with short repetition times (TR) is adversely affected by incomplete spoiling (i.e. residual T2 dependency). In this work, an optimized interleaved 2D multislice SPGR sequence scheme and an adapted postprocessing procedure are evaluated for highly T2‐insensitive T1 quantification of human brain tissues. Methods An efficient 2D multislice SPGR protocol including a relatively long TR of 200 ms is investigated with careful consideration of cross talk and magnetization transfer effects. Based on the derived scan protocol, T1 is quantified from the signal ratio of two SPGR datasets acquired at different flip angles. The effect of nonideal RF excitation profiles is incorporated into the SPGR signal model by performing Bloch simulations. Results Simulations showed that the parameters of the SPGR protocol (such as TR and the spoiler gradient moments) guarantee virtually complete spoiling. This result was confirmed by T1 measurements both in vitro using a 2% agar probe doped with 0.1 mM Gd (Gadovist) and in vivo in the human brain. Conclusion The derived 2D multislice SPGR protocol offers efficient, highly reproducible, and in particular T2‐insensitive T1 quantification of human brain tissues.

Purpose Variable flip angle (VFA) T1 quantification using three‐dimensional (3D) radiofrequency (RF) spoiled gradient echo imaging offers the acquisition of whole‐brain T1 maps in clinically acceptable times. However, conventional VFA T1 relaxometry is biased by incomplete spoiling (i.e., residual T2 dependency). A new postprocessing approach is proposed to overcome this T2‐related bias. Methods T1 is quantified from the signal ratio of two spoiled gradient echo (SPGR) images acquired at different flip angles using an analytical solution for the RF‐spoiled steady‐state signal in combination with a global T2 guess. T1 accuracy is evaluated from simulations and in vivo 3D SPGR imaging of the human brain at 3 Tesla. Results The simulations demonstrated that the sensitivity of VFA T1 mapping to T2 can considerably be reduced using a global T2 guess. The method proved to deliver reliable and accurate T1 values in vivo for white and gray matter in good agreement with inversion recovery reference measurements. Conclusion Based on a global T2 estimate, the accuracy of VFA T1 relaxometry in the human brain can substantially be improved compared with conventional approaches which rely on the generally wrong assumption of ideal spoiling.

In general MR image contrast is expected to be resolution independent but a pronounced loss of contrast is observed between fluids and tissues with contemporary musculoskeletal protocols (typical inplane resolution << 1 mm) using nonbalanced steady state free precession such as double echo steady state. For nonbalanced steady state free precession diffusion sensitivity increases with increasing spoiler moments which increase with decreasing voxel size suggesting diffusion damping as the major cause for the observed contrast variation. This is confirmed by simulations and measurements indicating that for fluids diffusion effects become apparent already for resolutions ?x < 1 mm whereas tissues typically require ?x < 200 µm. Gradient spoiling however is generically not minimized but frequently applied along the readout direction. For anisotropic steady state free precession scans the loss of contrast between fluids and tissues from diffusion can thus be minimized by simply moving the spoiler gradients to the lowest resolution direction. Copyright © 2012 Wiley Periodicals Inc.

Purpose Respiratory organ motion is still the major challenge of various image-guided treatments in the abdomen. Dynamic organ motion tracking, necessary for the treatment control, can be performed with volumetric time-resolved MRI that sequentially acquires one image and one navigator slice. Here, a novel imaging method is proposed for truly simultaneous high temporal resolution acquisition. Methods A standard balanced steady state free precession sequence was modified to simultaneously acquire two superimposed slices with different phase cycles, namely an image and a navigator slice. Instead of multiband RF pulses, two separate RF pulses were used for the excitation. Images were reconstructed using offline CAIPIRINHA reconstruction. Phantom and in vivo measurements of healthy volunteers were performed and evaluated. Results Phantom and in vivo measurements showed good image quality with high signal-to-noise ratio (SNR) and no reconstruction issues. Conclusion We present a novel imaging method for truly simultaneous acquisition of image and navigator slices for four-dimensional (4D) MRI of organ motion. In this method, the time lag between the sequential acquisitions is eliminated, leading to an improved accuracy of organ motion models, while CAIPIRINHA reconstruction results in an improved SNR compared with an existing 4D MRI approach.

Purpose Magnetic resonance-guided high-intensity focused ultrasound is considered to be a promising treatment for localized cancer in abdominal organs such as liver, pancreas, or kidney. Abdominal motion, anatomical arrangement, and required sustained sonication are the main challenges. Methods MR acquisition consisted of thermometry performed with segmented gradient-recalled echo echo-planar imaging, and a segment-based one-dimensional MR navigator parallel to the main axis of motion to track the organ motion. This tracking information was used in real-time for: (i) prospective motion correction of MR thermometry and (ii) HIFU focal point position lock-on target. Ex vivo experiments were performed on a sheep liver and a turkey pectoral muscle using a motion demonstrator, while in vivo experiments were conducted on two sheep liver. Results Prospective motion correction of MR thermometry yielded good signal-to-noise ratio (range, 25 to 35) and low geometric distortion due to the use of segmented EPI. HIFU focal point lock-on target yielded isotropic in-plane thermal build-up. The feasibility of in vivo intercostal liver treatment was demonstrated in sheep. Conclusion The presented method demonstrated in moving phantoms and breathing sheep accurate motion-compensated MR thermometry and precise HIFU focal point lock-on target using only real-time pencil-beam navigator tracking information, making it applicable without any pretreatment data acquisition or organ motion modeling.

Purpose For imaging of fibrous musculoskeletal components, ultra-short echo time methods are often combined with fat suppression. Due to the increased chemical shift, spectral excitation of water might become a favorable option at ultra-high fields. Thus, this study aims to compare and explore short binomial excitation schemes for spectrally selective imaging of fibrous tissue components with short transverse relaxation time (T2). Methods Water selective 1-1-binomial excitation is compared with nonselective imaging using a sub-millisecond spoiled gradient echo technique for in vivo imaging of fibrous tissue at 3T and 7T. Results Simulations indicate a maximum signal loss from binomial excitation of approximately 30 in the limit of very short T2 (0.1 ms), as compared to nonselective imaging; decreasing rapidly with increasing field strength and increasing T2, e.g., to 19 at 3T and 10 at 7T for T2 of 1 ms. In agreement with simulations, a binomial phase close to 90° yielded minimum signal loss: approximately 6 at 3T and close to 0 at 7T for menisci, and for ligaments 9 and 13, respectively. Conclusion Overall, for imaging of short-lived T2 components, short 1-1 binomial excitation schemes prove to offer marginal signal loss especially at ultra-high fields with overall improved scanning efficiency.

Purpose Rapid imaging techniques have attracted increased interest for relaxometry, but none are perfect: they are prone to static (B0) and transmit (B1) field heterogeneities, and commonly biased by T2/T1. The purpose of this study is the development of a rapid T1 and T2 relaxometry method that is completely (T2) or partly (T1) bias‐free. Methods A new method is introduced to simultaneously quantify T1 and T2 within one single scan based on a triple echo steady‐state (TESS) approach in combination with an iterative golden section search. TESS relaxometry is optimized and evaluated from simulations, in vitro studies, and in vivo experiments. Results It is found that relaxometry with TESS is not biased by T2/T1, insensitive to B0 heterogeneities, and, surprisingly, that TESS‐T2 is not affected by B1 field errors. Consequently, excellent correspondence between TESS and reference spin echo data is observed for T2 in vitro at 1.5 T and in vivo at 3 T. Conclusion TESS offers rapid T1 and T2 quantification within one single scan, and in particular B1‐insensitive T2 estimation. As a result, the new proposed method is of high interest for fast and reliable high‐resolution T2 mapping, especially of the musculoskeletal system at high to ultra‐high fields.

Purpose A novel inline image-mapping technique (B1-TRAP) is presented, which derives the actual flip angle from the frequency of signal oscillations, observed in the transient phase of unbalanced steady-state free precession sequences. Theory For short repetition times (TR), the angular frequency of distinct oscillations in the transient phase of steady-state free precession sequences is proven to be approximately proportional to the actual flip angle: inline image. The result is not influenced by off-resonance and it can be shown that deviations are only of second order in the small parameter inline image. Methods B1-TRAP makes use of this effect through a frequency analysis of the transient phase of a train of steady-state free precession signals. Results In terms of reliability and time efficiency, a two-dimensional multislice implementation was found to be optimal. Unlike many steady-state inline image-mapping methods, the accuracy of B1-TRAP was not impaired by imperfect slice profiles. Conclusion Simulations, phantom, and in vivo measurements showed that B1-TRAP offers a good compromise with respect to speed, robustness, and accuracy.

Purpose: The feasibility of imaging musculoskeletal fibrous tissue components, such as menisci, ligaments, and tendons, with a conventional spoiled gradient echo technique is explored in vivo at 3 T and 7 T. Methods: To this end, the echo time (TE1) of a conventional Fourier-encoded multicontrast three-dimensional SGPR sequence is minimized by using nonselective excitation pulses, highly asymmetric readouts, and a variable TE1 along the phase and slice encoding direction. In addition, a fully sampled second echo image (with TE2 >> TE1) can be used to highlight components with short transverse relaxation times in a difference image with positive contrast. Results: Fourier-encoded spoiled gradient echo sequences are able to provide sub-millisecond TE1 of about 800 μs for typical in-plane resolutions of about 0.5 x 0.5 mm2. As a result, high-resolution positive contrast images of fibrous tissues can be generated within clinically feasible scan-time of about 2–7 minutes. Conclusion: After optimization, Fourier-encoded spoiled gradient echo provides a highly robust and flexible imaging technique for high-resolution positive contrast imaging of fibrous tissue that can readily be used in the clinical routine.

Disclosed is a magnetic resonance method for the quantification of molecular diffusion. The method uses a diffusion-weighted (dw) double echo steady state sequence (DESS). In particular, the method allows direct quantification of molecular diffusion from two steady state scans with differing diffusion weighting such as one with diffusion-weighting and preferably one without diffusion weighting. Such a quantification of molecular diffusion allows for rapid and/or quantitative measurements of physiological and/or functional parameters of living tissue. Quantitative measurements are often a prerequisite for pre-clinical and clinical research as well as for clinical trials in drug research performed at different sites. Especially for the early diagnosis of subtle or diffuse pathological changes, quantitative MR promises to have a very significant impact.

Balanced steady state free precession (balanced SSFP) has become increasingly popular for research and clinical applications, offering a very high signal-to-noise ratio and a T2/T1-weighted image contrast. This review article gives an overview on the basic principles of this fast imaging technique as well as possibilities for contrast modification. The first part focuses on the fundamental principles of balanced SSFP signal formation in the transient phase and in the steady state. In the second part, balanced SSFP imaging, contrast, and basic mechanisms for contrast modification are revisited and contemporary clinical applications are discussed.

Apparatus and methods for quantification of transverse relaxation times (T2) using steady-state free precession sequences (generally known as fast imaging sequences) and their sensitivity to a quadratic increase of the RF pulse phase, also known as RF spoiling. Using at least two image acquisitions with different partial RF spoiling increments, T2 can be assessed with high precision and with short acquisition times in the limit of large excitation angles being independent on the longitudinal relaxation time (T1) and magnetization transfer effects as compared to other SSFP based quantitative T2 methods. This invention is not restricted to any kind of target and may be applied in 3D as well as in 2D.

Apparatus and methods for modification of the frequency response profile of steady-state free precession (SSFP) type of magnetic resonance imaging (MRI) sequences. Using alternating dephasing moments within succeeding radiofrequency (RF) excitation pulses, the frequency response function of SSFP sequences can be modified to different shapes such as near triangular or bell shaped. The particular response function as produced by alternating dephasing moments can be used, among others, for functional brain MRI, MR spectroscopy or spatial encoding.

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.

Fast quantitative MRI has become an important tool for biochemical characterization of tissue beyond conventional T1, T2, and T2*-weighted imaging. As a result, steady-state free precession (SSFP) techniques have attracted increased interest, and several methods have been developed for rapid quantification of relaxation times using steady-state free precession. In this work, a new and fast approach for T2 mapping is introduced based on partial RF spoiling of nonbalanced steady-state free precession. The new T2 mapping technique is evaluated and optimized from simulations, and in vivo results are presented for human brain at 1.5 T and for human articular cartilage at 3.0 T. The range of T2 for gray and white matter was from 60 msec (for the corpus callosum) to 100 msec (for cortical gray matter). For cartilage, spatial variation in T2 was observed between deep (34 msec) and superficial (48 msec) layers, as well as between tibial (33 msec), femoral, (54 msec) and patellar (43 msec) cartilage. Excellent correspondence between T2 values derived from partially spoiled SSFP scans and the ones found with a reference multicontrast spin-echo technique is observed, corroborating the accuracy of the new method for proper T2 mapping. Finally, the feasibility of a fast high-resolution quantitative partially spoiled SSFP T2 scan is demonstrated at 7.0 T for human patellar cartilage.

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: cv < 0.012 and cv < 0.015, respectively) and results indicate a high stability across sites (cv < 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.

The theoretical description of steady state free precession (SSFP) sequences is generally well accepted and unquestioned, although it is based on instantaneously acting radiofrequency (RF) pulses. In practice, however, all excitation processes are finite, thereby questioning the overall validity of the common SSFP signal description for use with finite RF pulses. In this paper, finite RF pulse effects on balanced SSFP signal formation are analyzed as a function of the RF time, the pulse repetition time, the flip angle (α) and relaxation times (T1,2). The observed signal modulations from finite RF pulses (compared to infinitesimal ones) can range from only a few percent (for RF time/pulse repetition time ≪ 1, α ≪ 90°, T2/T1 ∼ 1) to over 10 (for RF time/pulse repetition time ≪ 1, α ∼ 90°, T2/T1 ≪ 1) and may even exceed 100 in the limit of RF time/pulse repetition time → 1 (for α ∼ 90°, T2/T1 ≪ 1). As a result, a revision of SSFP signal theory is indicated not only for reasons of completeness but also seems advisable, e.g., for all quantitative SSFP methods. A simple modification for the common balanced SSFP equation is derived that provides an accurate framework for SSFP signal description over a wide variety of practical and physiologic parameters.

A novel and fast approach for passive real-time tracking of interventional devices using paramagnetic markers, termed “projection-reconstruction imaging with echo-dephasing” (PRIDE) is presented. PRIDE is based on the acquisition of echo-dephased projections along all three physical axes. Dephasing is preferably set to 4π within each projection ensuring that background tissues do not contribute to signal formation and thus appear heavily suppressed. However, within the close vicinity of the paramagnetic marker, local gradient fields compensate for the intrinsic dephasing to form an echo. Successful localization of the paramagnetic marker with PRIDE is demonstrated in vitro and in vivo in the presence of different types of off-resonance (air/tissue interfaces, main magnetic field inhomogeneities, etc). In order to utilize the PRIDE sequence for vascular interventional applications, it was interleaved with balanced steady-state free precession (bSSFP) to provide positional updates to the imaged slice using a dedicated real-time feedback link. Active slice positioning (ASP) with PRIDE is demonstrated in vitro, requiring approximately 20 ms for the positional update to the imaging sequence, comparable to existing active tracking methods.

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.

Apparatus and methods of generating magnetization transfer contrast images in which signal to noise ratios are improved and/or in which image acquisition times are reduced. In certain embodiments, apparatus and methods which utilize sensitivity and/or non-sensitivity to magnetization transfer effects to improve the contrast of images which are generated. In certain additional embodiments, apparatus and methods for generating magnetization transfer contrast images which exhibit sensitivity to longitudinal and transverse relaxation times of bound and free proton pools, respectively.

A method of nuclear magnetic resonance (NMR) imaging is proposed for the rapid detection of oscillatory motion of spins or charged particles to generate shear waves or oscillating electrical currents to induce alternating magnetic fields to the object being imaged, subjected to a fast train of radio-frequency (RF) pulses to induce within the sample a steady-state NMR signal. A scan using an NMR imaging system is carried out with a RF repetition time (TR) matched to the externally imposed oscillatory motion. Small oscillatory displacements of spins in combination with imaging gradients or oscillating magnetic fields related to charge motion generating alternating spin phase dispersions during the rf pulse train disturb the steady-state magnetization. Depending on the amount of spin-phase dispersion, the amplitude and phase of the NMR signals are modulated, generating a brightness-modulation of the reconstructed phase and amplitude images revealing mechanical or electrical properties of the object, such as stiffness or electrical impedance.

A method of steady-state free precession MR imaging is provided that includes the steps of: (a) providing a balanced steady-state free precession imaging sequence that includes a plurality of phase encoding steps, wherein each phase encoding step comprises a phase encoding gradient and a slice selection gradient; and (b) acquiring imaging data by performing the plurality of phase encoding steps in sequence, wherein the imaging data is acquired is compensated for one or more effects due to eddy-currents, flow, or motion related artefacts due to the implementation of one or more artefact compensation strategies that consist of (i) “pairing” of consecutive phase encoding steps and of (ii) “through slice equilibration” of eddy-current and motion or flow related signal oscillations.

Signal from balanced steady-state free precession is affected by magnetization transfer. To investigate the possible effects on derived T2 values using variable nutation steady-state free precession, magnetization transfer-effects were modulated by varying the radiofrequency pulse duration only or in combination with variable pulse repetition time. Simulations reveal a clear magnetization transfer dependency of T2 when decreasing radiofrequency pulse duration, reaching maximal deviation of 34.6 underestimation with rectangular pulses of 300 μs duration. The observed T2 deviation evaluated in the frontal white matter and caudate nucleus shows a larger underestimation than expected by numerical simulations. However, this observed difference between simulation and measurement is also observed in an aqueous probe and can therefore not be attributed to magnetization transfer: it is an unexpected sensitivity of derived T2 to radiofrequency pulse modulation. As expected, the limit of sufficiently long radiofrequency pulse duration to suppress magnetization transfer-related signal modulations allows for proper T2 estimation with variable nutation steady-state free precession.