Flow‐independent T2‐prepared inversion recovery black‐blood MR imaging

Purpose:

To develop a magnetization preparation method to achieve robust, flow‐independent blood suppression for cardiac and vascular magnetic resonance imaging (MRI).

Materials and Methods:

T₂Prep‐IR sequence consists of a T₂ preparation followed by a nonselective adiabatic inversion pulse. T₂Prep separates the initial longitudinal magnetization of arterial wall from lumen blood. After the inversion recovery pulse the imaging acquisition is then delayed for a period that allows the blood signal to approach the zero‐crossing point. Compared to the conventional double inversion recovery (DIR) preparation, T₂Prep‐IR prepares all the spins regardless of their velocity and direction. T₂Prep‐IR was incorporated into the fast spin echo and fast gradient echo acquisition sequences and images in various planes were acquired in the carotid arteries, thoracic aorta, and heart of normal volunteers. Blood suppression and image quality were compared qualitatively between two different preparations.

Results:

For in‐plane flow carotid images, persistent flow‐related artifacts on the DIR images were removed with T₂Prep‐IR. For cardiac applications, T₂Prep‐IR provided robust blood suppression regardless of the flow direction and velocity, including the cardiac long‐axis views and the aorta that are often problematic with DIR.

Conclusion:

T₂Prep‐IR may overcome the flow dependence of DIR by providing robust flow‐independent black‐blood images. J. Magn. Reson. Imaging 2010;31:248–254. © 2009 Wiley‐Liss, Inc     

Radiofrequency pulses for simultaneous short T2 excitation and long T2 suppression

Magnetic resonance imaging of short T₂ musculoskeletal tissues such as ligaments, tendon, and cortical bone often requires specialized pulse sequences to detect sufficiently high levels of signal, as well as additional techniques to suppress unwanted long T₂ signals. We describe a specialized radiofrequency technique for imaging short T₂ tissues based on applying hard 180° radiofrequency excitation pulses to achieve simultaneous short T₂ tissue excitation and long T₂ tissue signal suppression for three‐dimensional ultrashort echo time applications. A criterion for the pulse duration of the 180° radiofrequency pulses is derived that allows simultaneous water and fat suppression. This opens up possibilities for direct imaging of short T₂ tissues, without the need for additional suppression techniques. Bloch simulations and experimental studies on short T₂ phantoms and specimen were used to test the sequence performance. Magn Reson Med, 2011. © 2010 Wiley‐Liss, Inc.     

Fast high‐resolution T1 mapping using inversion‐recovery look‐locker echo‐planar imaging at steady state: Optimization for accuracy and reliability

A fast T₁ measurement sequence using inversion recovery Look‐Locker echo‐planar imaging at steady state (IR LL‐EPI SS) is presented. Delay time for a full magnetization recovery is not required in the sequence, saving acquisition time significantly for high‐resolution T₁ mapping. Imaging parameters of the IR LL‐EPI SS sequence were optimized to minimize the bias from the excitation pulses imperfection and to maximize the accuracy and reliability of T₁ measurements, which are critical for its applications. Compared with the conventional inversion recovery Look‐Locker echo‐planar imaging (IR LL‐EPI) sequence, IR LL‐EPI SS method preserves similar accuracy and reliability, while saving 20% in acquisition time. Optimized IR LL‐EPI SS provided quantitative T₁ mapping with 1 × 1 × 4 mm³ resolution and whole‐brain coverage (28 slices) in approximately 4 min. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Transverse relaxometry with reduced echo train lengths via stimulated echo compensation

Transverse relaxation (T₂) mapping has many applications, including imaging of iron accumulation in grey matter. Using the typical multiecho spin‐echo sequence with long echo trains, stimulated echo compensation can enable T₂ fitting under conditions of variable radio frequency homogeneity arising from slice profile and in‐plane radio frequency variation. Substantial reduction in the number of refocusing pulses could enable use at high magnetic fields where specific absorption rate is a major limitation, and enable multislice use with reduced incidental magnetization transfer at all field strengths. We examine the effect of reduced echo train lengths and multislice imaging on T₂ fitting using stimulated echo compensation applied to iron‐rich subcortical grey matter in human brain at 4.7 T. Our findings indicate that reducing from 20 echoes to as few as four echoes can maintain consistent T₂ values when using stimulated echo compensation in grey and white matter, but not for cerebrospinal fluid. All territories produce marginal results when using standard exponential fitting. Savings from reduced echoes can be used to substantially increase slice coverage. In multislice mode, the resulting incidental magnetization transfer decreased brain signal but had minimal effect on measured T₂ values. Magn Reson Med 70:1340–1346, 2013. © 2013 Wiley Periodicals, Inc.     

Imaging with positive T(1) -contrast using superstimulated echoes

This article presents the basic principles of the superstimulated echo mechanism and shows preliminary results of its application to T₁‐weighted imaging with positive T₁‐contrast. A superstimulated echo scheme uses a preparation of square‐wave modulated, periodically inverted z‐magnetization, which after signal evolution during the mixing time TM is fully converted into transverse magnetization. This avoids the 50% signal loss of a conventional stimulated echo. Furthermore, its implementation as a preparation module for standard turbo spin echo (TSE) imaging allows producing images with positive T₁‐contrast. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Phase‐sensitive,dual‐acquisition,single‐slab, 3D,turbo‐spin‐echo pulse sequence for simultaneous T2‐weighted and fluid‐attenuated whole‐brain imaging

Conventional T₂‐weighted turbo/fast spin echo imaging is clinically accepted as the most sensitive method to detect brain lesions but generates a high signal intensity of cerebrospinal fluid (CSF), yielding diagnostic ambiguity for lesions close to CSF. Fluid‐attenuated inversion recovery can be an alternative, selectively eliminating CSF signals. However, a long time of inversion, which is required for CSF suppression, increases imaging time substantially and thereby limits spatial resolution. The purpose of this work is to develop a phase‐sensitive, dual‐acquisition, single‐slab, three‐dimensional, turbo/fast spin echo imaging, simultaneously achieving both conventional T₂‐weighted and fluid‐attenuated inversion recovery–like high‐resolution whole‐brain images in a single pulse sequence, without an apparent increase of imaging time. Dual acquisition in each time of repetition is performed, wherein an in phase between CSF and brain tissues is achieved in the first acquisition, while an opposed phase, which is established by a sequence of a long refocusing pulse train with variable flip angles, a composite flip‐down restore pulse train, and a short time of delay, is attained in the second acquisition. A CSF‐suppressed image is then reconstructed by weighted averaging the in‐ and opposed‐phase images. Numerical simulations and in vivo experiments are performed, demonstrating that this single pulse sequence may replace both conventional T₂‐weighted imaging and fluid‐attenuated inversion recovery. Magn Reson Med 63:1422–1430, 2010. © 2010 Wiley‐Liss, Inc.     

Reduction of B1 sensitivity in selective single‐slab 3d turbo spin echo imaging with very long echo trains

Single‐slab 3D turbo/fast spin echo (SE) imaging with very long echo trains was recently introduced with slab selection using a highly selective excitation pulse and short, nonselective refocusing pulses with variable flip angles for high imaging efficiency. This technique, however, is vulnerable to image degradation in the presence of spatially varying B₁ amplitudes. In this work we develop a B₁ inhomogeneity‐reduced version of single‐slab 3D turbo/fast SE imaging based on the hypothesis that it is critical to achieve spatially uniform excitation. Slab selection was performed using composite adiabatic selective excitation wherein magnetization is tipped into the transverse plane by a nonselective adiabatic‐half‐passage pulse and then slab is selected by a pair of selective adiabatic‐full‐passage pulses. Simulations and experiments were performed to evaluate the proposed technique and demonstrated that this approach is a simple and efficient way to reduce B₁ sensitivity in single‐slab 3D turbo/fast SE imaging with very long echo trains. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Optimization of RF excitation to maximize signal and T2 contrast of tissues with rapid transverse relaxation

Ultrashort echo time MRI requires specialized pulse sequences to overcome the short T₂ of the MR signal encountered in tissues such as ligaments, tendon, or cortical bone. Theoretical work is presented, supported by simulations and experimental data on optimizing the radiofrequency excitation to maximize signal‐to‐noise ratio and contrast‐to‐noise ratio. The theoretical calculations and simulations are based on the classic Bloch equations and lead to a closed form expression for the optimal radiofrequency pulse parameters to maximize the MR signal in the presence of rapid T₂ decay. In the steady state, the spoiled gradient recalled echo signal amplitude in response to the radiofrequency excitation pulses is not maximized by the classic Ernst angle but by a more general criterion we call “generalized Ernst angle.” Finally, it is shown that T₂ contrast is maximized by flipping the magnetization at the Ernst angle with a radiofrequency pulse duration proportional to the targeted T₂. Experimental studies on short T₂ phantoms confirm these optimization criteria for both signal‐to‐noise ratio and contrast‐to‐noise ratio. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Improved delayed enhanced myocardial imaging with T2-Prep inversion recovery magnetization preparation

Purpose

To develop a magnetization preparation method that improves the differentiation of enhancing subendocardial infarction (MI) from ventricular blood for myocardial delayed‐enhancement (DE) magnetic resonance imaging (MRI).

Materials and Methods

T₂Prep‐IR is a magnetization preparation pulse that consists of a T₂ preparation (T₂Prep) followed immediately by a nonselective inversion recovery (IR) pulse. The first imaging excitation is then delayed an inversion time (TI) to allow nulling of normal myocardium in DE study. The amount of T₂ contrast is determined by the effective echo time of the T₂Prep pulse, TE_eff. TE_eff is selected to differentiate MI and blood that share similar T₁ values but have different T₂ values. The T₂Prep‐IR preparation was incorporated into a fast gradient echo sequence to produce an image with both T₁ and T₂ weighting. Simulations predict that this method will generate improved contrast between MI and chamber blood compared to conventional IR methods.

Results

Comparisons between images acquired using conventional IR and T₂Prep‐IR in patients with MI indicate that this new approach significantly improves the blood‐MI contrast (122 ± 32% higher than that of IR with P < 0.05).

Conclusion

Our preliminary patient studies confirm that this preparation is helpful for improved delineation of subendocardial infarction. J. Magn. Reson. Imaging 2008;28:1280–1286. © 2008 Wiley‐Liss, Inc.     

Effect of blood flow on double inversion recovery vessel wall MRI of the peripheral arteries: Quantitation with T2 mapping and comparison with flow‐insensitive T2‐prepared inversion recovery imaging

Blood suppression in the lower extremities using flow‐reliant methods such as double inversion recovery may be problematic due to slow blood flow. T₂ mapping using fast spin echo (FSE) acquisition was utilized to quantitate the effectiveness of double inversion recovery blood suppression in 13 subjects and showed that 25 ± 12% of perceived vessel wall pixels in the popliteal arteries contained artifactual blood signal. To overcome this problem, a flow‐insensitive T₂‐prepared inversion recovery sequence was implemented and optimal timing parameters were calculated for FSE acquisition. Black blood vessel wall imaging of the popliteal and femoral arteries was performed using two‐dimensional T₂‐prepared inversion recovery‐FSE in the same 13 subjects. Comparison with two‐dimensional double inversion recovery‐FSE showed that T₂‐prepared inversion recovery‐FSE reduced wall‐mimicking blood artifacts that inflated double inversion recovery‐FSE vessel wall area measurements in the popliteal artery. Magn Reson Med 63:736–744, 2010. © 2010 Wiley‐Liss, Inc.     

Transverse relaxometry with stimulated echo compensation

Presented is a fitting model for transverse relaxometry data acquired with the multiple‐refocused spin‐echo sequence. The proposed model, requiring no additional data input or pulse sequence modifications, compensates for imperfections in the transmit field and radiofrequency (RF) profiles. Exploiting oscillatory echo behavior to estimate alternate coherence pathways, the model compensates for prolonged signal decay from stimulated echo pathways yielding precise monoexponential T₂ quantification. Verified numerically and experimentally at 4.7 T in phantoms and the human brain, over 95% accuracy is readily attainable in realistic imaging situations without sacrificing multislice capabilities or requiring composite or adiabatic RF pulses. The proposed model allows T₂ quantitation in heterogeneous transmit fields and permits thin refocusing widths for efficient multislice imaging. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

3D flow‐independent peripheral vessel wall imaging using T2‐prepared phase‐sensitive inversion‐recovery steady‐state free precession

Purpose:

To develop a 3D flow‐independent peripheral vessel wall imaging method using T₂‐prepared phase‐sensitive inversion‐recovery (T₂PSIR) steady‐state free precession (SSFP).

Materials and Methods:

A 3D T₂‐prepared and nonselective inversion‐recovery SSFP sequence was designed to achieve flow‐independent blood suppression for vessel wall imaging based on T₁ and T₂ properties of the vessel wall and blood. To maximize image contrast and reduce its dependence on the inversion time (TI), phase‐sensitive reconstruction was used to restore the true signal difference between vessel wall and blood. The feasibility of this technique for peripheral artery wall imaging was tested in 13 healthy subjects. Image signal‐to‐noise ratio (SNR), wall/lumen contrast‐to‐noise ratio (CNR), and scan efficiency were compared between this technique and conventional 2D double inversion recovery – turbo spin echo (DIR‐TSE) in eight subjects.

Results:

3D T₂PSIR SSFP provided more efficient data acquisition (32 slices and 64 mm in 4 minutes, 7.5 seconds per slice) than 2D DIR‐TSE (2–3 minutes per slice). SNR of the vessel wall and CNR between vessel wall and lumen were significantly increased as compared to those of DIR‐TSE (P < 0.001). Vessel wall and lumen areas of the two techniques are strongly correlated (intraclass correlation coefficients: 0.975 and 0.937, respectively; P < 0.001 for both). The lumen area of T₂PSIR SSFP is slightly larger than that of DIR‐TSE (P = 0.008). The difference in vessel wall area between the two techniques is not statistically significant.

Conclusion:

T₂PSIR SSFP is a promising technique for peripheral vessel wall imaging. It provides excellent blood signal suppression and vessel wall/lumen contrast. It can cover a 3D volume efficiently and is flow‐ and TI‐independent. J. Magn. Reson. Imaging 2010;32:399–408. © 2010 Wiley‐Liss, Inc.     

Motion and flow insensitive adiabatic T2‐preparation module for cardiac MR imaging at 3 tesla

A versatile method for generating T₂‐weighting is a T₂‐preparation module, which has been used successfully for cardiac imaging at 1.5T. Although it has been applied at 3T, higher fields (B₀ ≥ 3T) can degrade B₀ and B₁ homogeneity and result in nonuniform magnetization preparation. For cardiac imaging, blood flow and cardiac motion may further impair magnetization preparation. In this study, a novel T₂‐preparation module containing multiple adiabatic B₁‐insensitive refocusing pulses is introduced and compared with three previously described modules [(a) composite MLEV4, (b) modified BIR‐4 (mBIR‐4), and (c) Silver‐Hoult–pair]. In the static phantom, the proposed module provided similar or better B₀ and B₁ insensitivity than the other modules. In human subjects (n = 21), quantitative measurement of image signal coefficient of variation, reflecting overall image inhomogeneity, was lower for the proposed module (0.10) than for MLEV4 (0.15, P < 0.0001), mBIR‐4 (0.27, P < 0.0001), and Silver‐Hoult–pair (0.14, P = 0.001) modules. Similarly, qualitative analysis revealed that the proposed module had the best image quality scores and ranking (both, P < 0.0001). In conclusion, we present a new T₂‐preparation module, which is shown to be robust for cardiac imaging at 3T in comparison with existing methods. Magn Reson Med 70:1360–1368, 2013. © 2012 Wiley Periodicals, Inc.     

In vivo free induction decay based 3D multivoxel longitudinal hadamard spectroscopic imaging in the human brain at 3 T

We propose and demonstrate a full 3D longitudinal Hadamard spectroscopic imaging scheme for obtaining chemical shift maps, using adiabatic inversion pulses to encode the spins' positions. The approach offers several advantages over conventional Fourier‐based encoding methods, including a localized point spread function; no aliasing, allowing for volumes of interest smaller than the object being imaged; an option for acquiring noncontiguous voxels; and inherent outer volume rejection. The latter allows for doing away with conventional outer volume suppression schemes, such as point resolved spectroscopy (PRESS) and stimulated echo acquisition mode (STEAM), and acquiring non‐spin‐echo spectra with short acquisition delay times, limited only by the excitation pulse's duration. This, in turn, minimizes T₂ decay, maximizes the signal‐to‐noise ratio, and reduces J‐coupling induced signal decay. Results are presented for both a phantom and an in vivo healthy volunteer at 3 T. Magn Reson Med 69:903–911, 2013. © 2012 Wiley Periodicals, Inc.     

Diffusion-weighted whole-body MRI with background body signal suppression: technical improvements at 3.0 T

Purpose:

To improve image quality of diffusion‐weighted body magnetic resonance imaging (MRI) with background body signal suppression (DWIBS) at 3.0 T.

Materials and Methods:

In 30 patients and eight volunteers, a diffusion‐weighted spin‐echo echo‐planar imaging sequence with short TI inversion recovery (STIR) fat suppression was applied and repeated using slice‐selective gradient reversal (SSGR) and/or dual‐source parallel radiofrequency (RF) transmission (TX). The quality of diffusion‐weighted images and gray scale inverted maximum intensity projections (MIP) were visually assessed by intraindividual comparison with respect to the level of fat suppression and signal homogeneity. Moreover, the contrast between lesions/lymph nodes and background (C_lb) was analyzed in the MIP reconstructions.

Results:

By combining STIR with SSGR, fat suppression was significantly improved (P < 0.001) and C_lb was increased two times. The use of TX allowed the reduction of acquisition time and improved image quality with regard to signal homogeneity (P < 0.001) and fat suppression (P = 0.005).

Conclusion:

DWIBS at 3.0 T can be improved by using SSGR and TX. J. Magn. Reson. Imaging 2012;456‐461. © 2011 Wiley Periodicals, Inc.     

FASCINATE: A pulse sequence for simultaneous acquisition of T2‐weighted and fluid‐attenuated images

A pulse sequence that enables simultaneous acquisition of T₂‐weighted and fluid‐attenuated images is presented. This sequence is referred to as FASCINATE (Fluid‐Attenuated Scan Combined with Interleaved Non‐ATtEnuation). In this new technique, the inversion pulse of conventional fast fluid‐attenuated inversion recovery (FLAIR) is replaced with a fast spin echo (FSE) acquisition that has an additional 180(y)–90(x) pulse train for driven inversion. By using appropriate scan parameters, the first part of the sequence provides T₂‐weighted images and the second part provides fluid‐attenuated images, thus allowing simultaneous acquisition in a single scan time comparable to that of fast FLAIR. FASCINATE was compared with conventional scanning techniques using a normal volunteer and a patient. A signal simulation was also conducted. In the human study, both T₂‐weighted and fluid‐attenuated images from FASCINATE showed the same image quality as conventional images, suggesting the potential for this technique to replace the combination of fast FLAIR and T₂‐weighted FSE for scan time reduction. Magn Reson Med 51:205–211, 2004. © 2003 Wiley‐Liss, Inc.     

Incidental magnetization transfer contrast by fat saturation preparation pulses in multislice look‐locker echo planar imaging

In this study, it is demonstrated that fat saturation (FS) preparation (prep) pulses generate incidental magnetization transfer contrast (MTC) in multislice Look‐Locker (LL) imaging. It is shown that frequency‐selective FS prep pulses can invoke MTC through the exchange between free and motion‐restricted protons. Simulation reveals that the fractional signal loss by these MTC effects is more severe for smaller flip angles (FAs), shorter repetition times (TRs), and greater number of slices (SN). These incidental MTC effects result in a signal attenuation at a steady state (up to 30%) and a T₁ measurement bias (up to 20%) when using inversion recovery (IR) LL echo‐planar imaging (EPI) sequences. Furthermore, it is shown that water‐selective MRI using binomial pulses has the potential to minimize the signal attenuation and provide unbiased T₁ measurement without fat artifacts in MR images. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Clinically constrained optimization of flexTPI acquisition parameters for the tissue sodium concentration bioscale

The rapid transverse relaxation of the sodium magnetic resonance signal during spatial encoding causes a loss of image resolution, an effect known as T₂‐blurring. Conventional wisdom suggests that spatial resolution is maximized by keeping the readout duration as short as possible to minimize T₂‐blurring. Flexible twisted projection imaging performed with an ultrashort echo time, relative to T₂, and a long repetition time, relative to T₁, has been shown to be effective for quantitative sodium magnetic resonance imaging. A minimized readout duration requires a very large number of projections and, consequentially, results in an impractically long total acquisition time to meet these conditions. When the total acquisition time is limited to a clinically practical duration (e.g., 10 min), the optimal parameters for maximal spatial resolution of a flexible twisted projection imaging acquisition do not correspond to the shortest possible readout. Simulation and experimental results for resolution optimized acquisition parameters of quantitative sodium flexible twisted projection imaging of parenchyma and cerebrospinal fluid are presented for the human brain at 9.4 and 3.0T. The effect of signal loss during data collection on sodium quantification bias and image signal‐to‐noise ratio are discussed. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

Magnetic resonance separation imaging using a divided inversion recovery technique (DIRT)

The divided inversion recovery technique is an MRI separation method based on tissue T₁ relaxation differences. When tissue T₁ relaxation times are longer than the time between inversion pulses in a segmented inversion recovery pulse sequence, longitudinal magnetization does not pass through the null point. Prior to additional inversion pulses, longitudinal magnetization may have an opposite polarity. Spatial displacement of tissues in inversion recovery balanced steady‐state free‐precession imaging has been shown to be due to this magnetization phase change resulting from incomplete magnetization recovery. In this paper, it is shown how this phase change can be used to provide image separation. A pulse sequence parameter, the time between inversion pulses (T180), can be adjusted to provide water‐fat or fluid separation. Example water‐fat and fluid separation images of the head, heart, and abdomen are presented. The water‐fat separation performance was investigated by comparing image intensities in short‐axis divided inversion recovery technique images of the heart. Fat, blood, and fluid signal was suppressed to the background noise level. Additionally, the separation performance was not affected by main magnetic field inhomogeneities. Magn Reson Med 63:1007–1014, 2010. © 2010 Wiley‐Liss, Inc.     

Comparison of optimized soft-tissue suppression schemes for ultrashort echo time MRI

Ultrashort echo time (UTE) imaging with soft-tissue suppression reveals short-T(2) components (typically hundreds of microseconds to milliseconds) ordinarily not captured or obscured by long-T(2) tissue signals on the order of tens of milliseconds or longer. Therefore, the technique enables visualization and quantification of short-T(2) proton signals such as those in highly collagenated connective tissues. This work compares the performance of the three most commonly used long-T(2) suppression UTE sequences, i.e., echo subtraction (dual-echo UTE), saturation via dual-band saturation pulses (dual-band UTE), and inversion by adiabatic inversion pulses (IR-UTE) at 3 T, via Bloch simulations and experimentally in vivo in the lower extremities of test subjects. For unbiased performance comparison, the acquisition parameters are optimized individually for each sequence to maximize short-T(2) signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) between short- and long-T(2) components. Results show excellent short-T(2) contrast which is achieved with these optimized sequences. A combination of dual-band UTE with dual-echo UTE provides good short-T(2) SNR and CNR with less sensitivity to B(1) homogeneity. IR-UTE has the lowest short-T(2) SNR efficiency but provides highly uniform short-T(2) contrast and is well suited for imaging short-T(2) species with relatively short T(1) such as bone water.