B(1) mapping with selective pulses

Knowledge of B distribution is crucial for many applications, such as quantitative MRI. A novel method has been developed to improve the accuracy of the conventionally applied double‐angle method for B mapping. It solves the remaining issues raised by the use of selective pulses for slice selection to accelerate the acquisition process. A general approach for reconstructing B maps is presented first. It takes B‐induced slice profile distortions over off‐resonance frequencies into account. It is then shown how the ratio between the prescribed flip angles can be adjusted to reach a compromise between the level of noise propagated onto B maps and the width of the range in which the field can be mapped. Lastly, several solutions are proposed for reducing the B‐dependent pollution of regions distal to the image slice which participates significantly in the inaccuracy of B mapping. These methods were experimentally tested by comparison with gold standard B maps obtained on a phantom using a non‐selective and thus much slower technique. As they are independent and lead to significant improvements, these solutions can be combined to achieve high precision and fast B mapping using spin‐echo DAM. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.     

A single‐scan T mapping method based on two gradient‐echo images with compensation for macroscopic field inhomogeneity

T‐weighted imaging (TWI) and quantitative T mapping with conventional gradient‐echo acquisition are often hindered by severe signal loss induced by macroscopic field inhomogeneity. Various z‐shimming approaches have been developed for TWI/T mapping in which the effects of macroscopic field inhomogeneity are suppressed while the sensitivity of T‐related signal intensity to alterations in the microscopic susceptibility is maintained. However, this is often done at the cost of significantly increased imaging time. In this work, a fast T mapping method with compensation for macroscopic field inhomogeneity was developed. A proton density‐weighted image and a composite T‐weighted image, both of which were essentially free from macroscopic field inhomogeneity‐induced signal loss, were used for the T calculation. The composite T‐weighted image was reconstructed from a number of gradient‐echo images acquired with successively incremented z‐shimming compensation. Because acquisition of the two images and z‐shimming compensation were realized in a single scan, the total acquisition time for obtaining a T map with the proposed method is the same as the time taken for a conventional multiecho gradient‐echo imaging sequence without compensation. The performance and efficiency of the proposed method were demonstrated and evaluated at 4.7 T. Magn Reson Med 60:1388–1395, 2008. © 2008 Wiley‐Liss, Inc.     

B interferometry for the calibration of RF transmitter arrays

Multiple‐channel RF transmission holds great promise for MRI, especially for human applications at high fields. For calibration it requires mapping the effective RF magnetic fields, B, of the transmitter array. This is challenging to do accurately and fast due to the large dynamic range of B and tight SAR constraints. In the present work, this problem is revisited and solved by a novel mapping approach relying on an interference principle. The B fields of individual transmitter elements are measured indirectly by observing their interference with a SAR‐efficient baseline RF field. In this fashion even small RF fields can be observed in the B ‐sensitive large‐flip‐angle regime. Based on a set of such experiments B maps of the individual transmitter channels are obtained by solving a linear inverse problem. Confounding relaxation and off‐resonance effects are addressed by an extended signal model and nonlinear fitting. Using the novel approach, 2D mapping of an 8‐channel transmitter array was accomplished in less than a minute. For validation it is demonstrated that mapping results do not vary with T₁ or parameters of the mapping sequence. In RF shimming experiments it is shown that the measured B maps accurately reflect the linearity of RF superposition. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Dynamically applied B1+ shimming solutions for non‐contrast enhanced renal angiography at 7.0 tesla

The purpose of this study was to detail a strategy for performing non‐contrast enhanced renal magnetic resonance angiography studies at 7.0 T. It is demonstrated that with proper B management, these studies can be successfully performed at ultrahigh field within local specific absorption rate constraints. An inversion prepared gradient echo acquisition, standard for non‐contrast renal magnetic resonance angiography studies, required radiofrequency pulse specific B shimming solutions to be dynamically applied to address the field dependent increases in both B₀ and B inhomogeneity as well as to accommodate limitation in available power. By using more efficient B shimming solutions for the inversion preparation and more homogeneous solutions for the excitation, high quality images of the renal arteries were obtained without venous and background signal artifacts while working within hardware and safety constraints. Finite difference time domain simulations confirmed in vivo measurements with respect to B distributions and homogeneity for the range of shimming strategies used and allowed the calculation of peak local specific absorption rate values normalized by input power and B. Increasing B homogeneity was accompanied by decreasing local specific absorption rate per Watt and increasing maximum local specific absorption rate per [B]², which must be considered, along with body size and respiratory rate, when finalizing acquisition parameters for a given individual. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Myocardial T measurements in iron‐overloaded thalassemia: An in vivo study to investigate optimal methods of quantification

Reproducible and accurate myocardial T measurements are required for the quantification of iron in heart tissue in transfused thalassemia. The aim of this study was to determine the best method to measure the myocardial T from multi‐gradient‐echo data acquired both with and without black‐blood preparation. Sixteen thalassemia patients from six centers were scanned twice locally, within 1 week, using an optimized bright‐blood T sequence and then subsequently scanned at the standardization center in London within 4 weeks, using a T sequence both with and without black‐blood preparation. Different curve‐fitting models (monoexponential, truncation, and offset) were applied to the data and the results were compared by means of reproducibility. T measurements obtained using the bright‐ and black‐blood techniques. The black‐blood data were well fitted by the monoexponential model, which suggests that a more accurate measure of T can be obtained by removing the main source of errors in the bright‐blood data. For bright‐blood data, the offset model appeared to underestimate T values substantially and was less reproducible. The truncation model gave rise to more reproducible T measurements, which were also closer to the values obtained from the black‐blood data. Magn Reson Med 60:1082–1089, 2008. © 2008 Wiley‐Liss, Inc.     

Broadband slab selection with B mitigation at 7T via parallel spectral‐spatial excitation

Chemical shift imaging benefits from signal‐to‐noise ratio (SNR) and chemical shift dispersion increases at stronger main field such as 7 Tesla, but the associated shorter radiofrequency (RF) wavelengths encountered require B mitigation over both the spatial field of view (FOV) and a specified spectral bandwidth. The bandwidth constraint presents a challenge for previously proposed spatially tailored B mitigation methods, which are based on a type of echovolumnar trajectory referred to as “spokes” or “fast‐k_z”. Although such pulses, in conjunction with parallel excitation methodology, can efficiently mitigate large B inhomogeneities and achieve relatively short pulse durations with slice‐selective excitations, they exhibit a narrow‐band off‐resonance response and may not be suitable for applications that require B mitigation over a large spectral bandwidth. This work outlines a design method for a general parallel spectral‐spatial excitation that achieves a target‐error minimization simultaneously over a bandwidth of frequencies and a specified spatial‐domain. The technique is demonstrated for slab‐selective excitation with in‐plane B mitigation over a 600‐Hz bandwidth. The pulse design method is validated in a water phantom at 7T using an eight‐channel transmit array system. The results show significant increases in the pulse's spectral bandwidth, with no additional pulse duration penalty and only a minor tradeoff in spatial B mitigation compared to the standard spoke‐based parallel RF design. Magn Reson Med 61:493–500, 2009. © 2009 Wiley‐Liss, Inc.     

Ultrashort T relaxometry for quantitation of highly concentrated superparamagnetic iron oxide (SPIO) nanoparticle labeled cells

A new method was developed to measure ultrashort T relaxation in tissues containing a focal area of superparamagnetic iron oxide (SPIO) nanoparticle‐labeled cells in which the T decay is too short to be accurately measured using regular gradient echo T mapping. The proposed method utilizes the relatively long T₂ relaxation of SPIO‐labeled cells and acquires a series of spin echo images with the readout echo shifted to sample the T decay curve. MRI experiments in phantoms and rats with SPIO‐labeled tumors demonstrated that it can detect ultrashort T down to 1 ms or less. The measured T values were about 10% higher than those from the ultrashort TE (UTE) technique. The shorter the TE, the less the measurements deviated from the UTE T mapping. Combined with the regular T mapping, this technique is expected to provide quantitation of highly concentrated iron‐labeled cells from direct cell transplantation. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

RF pulse optimization for Bloch‐Siegert B mapping

The Bloch–Siegert (B–S) method of B mapping has been shown to be fast and accurate, yet has high SAR and moderately long TE. These limitations can lengthen scan times and incur signal loss due to B₀ inhomogeneity, particularly at high field. The B–S method relies on applying a band‐limited off‐resonant B–S radiofrequency pulse to induce a B‐dependent frequency‐shift for resonant spins. A method for optimizing the B–S radiofrequency pulse is presented here, which maximizes B–S B measurement sensitivity for a given SAR and T₂. A 4‐ms optimized pulse is shown to have 35% less SAR compared with the conventional 6‐ms Fermi pulse while still improving B map angle‐to‐noise ratio by 22%. The optimized pulse performance is validated both in phantom and in vivo brain imaging at 7 T. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Fast CPMG‐based Bloch‐Siegert B1+ mapping

A novel method for B mapping based on the Bloch‐Siegert (BS) shift was recently presented. This method applies off‐resonant pulses before signal acquisition to encode B₁ information into the signal phase. BS‐based methods possess significant advantages in measurement time and accuracy compared to magnitude‐based B methods. This study extends the idea of BS B mapping to Carr, Purcell, Meiboom, Gill (CPMG)‐based multi‐spin‐echo (BS‐CPMG‐MSE) and turbo‐spin‐echo (BS‐CPMG‐TSE) imaging. Compared to BS‐based spin echo imaging (BS‐SE), faster acquisition of the B information was possible using the BS‐CPMG‐TSE sequence. Furthermore, signal loss by T₂* effects could be minimized using these spin echo‐based techniques. These effects are critical for gradient echo‐based BS methods at high field strengths. However, multi‐spin‐echo‐based BS B₁ methods inherently possess high specific absorption rates. Thus, the relative specific absorption rate of BS‐CPMG‐TSE sequences was estimated and compared with the specific absorption rate produced by BS‐SE sequences. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Design of a radiative surface coil array element at 7 T: The single‐side adapted dipole antenna

Ultra high field MR imaging (≥7 T) of deeply located targets in the body is facing some radiofrequency‐field related challenges: interference patterns, reduced penetration depth, and higher Specific Absorbtion Ratio (SAR) levels. These can be alleviated by redesigning the elements of the transmit or transceive array. This is because at these high excitation field (B₁) frequencies, conventional array element designs may have become suboptimal. In this work, an alternative design approach is presented, regarding coil array elements as antennas. Following this approach, the Poynting vector of the element should be oriented towards the imaging target region. The single‐side adapted dipole antenna is a novel design that fulfills this requirement. The performance of this design as a transmit coil array element has been characterized by comparison with three other, more conventional designs using finite difference time domain (FDTD) simulations and B measurements on a phantom. Results show that the B level at the deeper regions is higher while maintaining relatively low SAR levels. Also, the B field distribution is more symmetrical and more uniform, promising better image homogeneity. Eight radiative antennas have been combined into a belt‐like surface array for prostate imaging. T₁‐weighted (T1W) and T₂‐weighted (T2W) volunteer images are presented along with B measurements to demonstrate the improved efficiency. Magn Reson Med, 2011. © 2011 Wiley Periodicals, Inc.     

Regional variations of T2* in healthy and pathologic achilles tendon in vivo at 7 Tesla: Preliminary results

The aim of this study was to investigate T in the Achilles tendon (AT), in vivo, using a three‐dimensional ultrashort time echo (3D‐UTE) sequence, to compare field strength differences (3 and 7 T) and to evaluate a regional variation of T in healthy and pathologic tendon. Ten volunteers with no history of pain in the AT and five patients with chronic Achilles tendinopathy were recruited. 3D‐UTE images were measured with the following echo times, at echo time = [0.07, 0.2, 0.33, 0.46, 0.59, 0.74, 1.0, 1.5, 2.0, 4.0, 6.0, and 9.0 ms]. T values in the AT were calculated by fitting the signal decay to biexponential function. Comparing volunteers between 3 and 7 T, short component T was 0.71 ± 0.17 ms and 0.34 ± 0.09 ms (P < 0.05); bulk long component T was 12.85 ± 1.87 ms and 10.28 ± 2.28 ms (P < 0.05). In patients at 7 T, bulk T was 0.53 ± 0.17 ms (P = 0.045, compared to volunteers), T was 11.49 ± 4.28 ms (P = 0.99, compared to volunteers). The results of this study suggest that the regional variability of AT can be quantified by T in in vivo conditions. Advanced quantitative imaging of the human AT using a 3D‐UTE sequence may provide additional information to standard clinical imaging. Finally, as the preliminary patient data suggest, T may be a promising marker for the diagnosis of pathological changes in the AT. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.     

Performance of sampling density‐weighted and postfiltered density‐adapted projection reconstruction in sodium magnetic resonance imaging

Sampling density‐weighted apodization projection reconstruction sequences are evaluated for three‐dimensional radial imaging. The readout gradients of the sampling density‐weighted apodization sequence are designed such that the locally averaged sampling density matches a Hamming filter function. This technique is compared with density‐adapted projection reconstruction with nonfiltered and postfiltered image reconstruction. Sampling density‐weighted apodization theoretically allows for a 1.28‐fold higher signal‐to‐noise ratio compared with postfiltered density‐adapted projection reconstruction sequences, if T decay is negligible compared with the readout duration T_RO. Simulations of the point‐spread functions are performed for monoexponential and biexponential decay to investigate the effects of T decay on the performance of the different sequences. Postfiltered density‐adapted projection reconstruction performs superior to sampling density‐weighted apodization for large T_RO/T ratios [>1.36 (monoexponential decay); >0.35 (biexponential decay with T/T = 10)], if signal‐to‐noise ratio of point‐like objects is considered. In conclusion, it depends on the readout parameters, the T relaxation times, and the dimensions of the subject which of both sequences is most suitable. Magn Reson Med, 2013. © 2012 Wiley Periodicals, Inc.     

Myocardial T measurement in iron‐overloaded thalassemia: An ex vivo study to investigate optimal methods of quantification

Myocardial T measurement has been increasingly used for iron quantification to assess the risk of cardiac complications in thalassemia patients. In this study the noise effects were evaluated along with different curve‐fitting models on an iron overloaded ex vivo heart in order to determine the optimal method of T measurement and to help understand issues affecting reproducibility and accuracy. Gradient multiecho short axis images were acquired with differing numbers of excitations to generate varying signal‐to‐noise ratio (SNR) images. A noise correction method was implemented; linear and nonlinear curve‐fitting algorithms were compared and different curve‐fitting models (monoexponential, truncation, baseline subtraction, and offset) were evaluated. This study suggests that the T decay curve in an ex vivo heart can be fitted by a monoexponential model and accurate T measurements can be obtained with proper noise correction. With MRI noise, T is generally overestimated by including late low SNR data points, but underestimated by the offset or baseline subtraction models, which are in fact equivalent. In this situation the truncation model proves to be reproducible and more accurate than the other models. The study also shows that the nonlinear algorithm is preferred in T curve fitting. Magn Reson Med 60:350–356, 2008. © 2008 Wiley‐Liss, Inc.     

Optimization and validation of methods for mapping of the radiofrequency transmit field at 3T

MRI techniques such as quantitative imaging and parallel transmit require precise knowledge of the radio‐frequency transmit field (B). Three published methods were optimized for robust B mapping at 3T in the human brain: three‐dimensional (3D) actual flip angle imaging (AFI), 3D echo‐planar imaging (EPI), and two‐dimensional (2D) stimulated echo acquisition mode (STEAM). We performed a comprehensive comparison of the methods, focusing on artifacts, reproducibility, and accuracy compared to a reference 2D double angle method. For the 3D AFI method, the addition of flow‐compensated gradients for diffusion damping reduced the level of physiological artifacts and improved spoiling of transverse coherences. Correction of susceptibility‐induced artifacts alleviated image distortions and improved the accuracy of the 3D EPI imaging method. For the 2D STEAM method, averaging over multiple acquisitions reduced the impact of physiological noise and a new calibration method enhanced the accuracy of the B maps. After optimization, all methods yielded low noise B maps (below 2 percentage units), of the nominal flip angle value (p.u.) with a systematic bias less than 5 p.u. units. Full brain coverage was obtained in less than 5 min. The 3D AFI method required minimal postprocessing and showed little sensitivity to off‐resonance and physiological effects. The 3D EPI method showed the highest level of reproducibility. The 2D STEAM method was the most time‐efficient technique. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.     

Increasing signal homogeneity and image quality in abdominal imaging at 3 T with very high permittivity materials

The appearance of severe signal drop‐outs in abdominal imaging at 3 T arises primarily from areas of very low B transmit field in the body, and is problematic in both obese as well as very thin subjects. In this study, we show how thin patient‐friendly pads containing new high permittivity materials can be designed and optimized, and when placed around the subject increase substantially the B uniformity and the image quality. Results from nine healthy volunteers show that inclusion of these dielectric pads results in statistically significant decreases in the coefficient of variance of the B field, with stronger and more uniform fields being produced. In addition there are statistically significant decreases in time‐averaged power required for scanning. These differences are present in both quadrature‐mode operation (coefficient of variance decrease, P < 0.0001, mean 25.4 ± 10%: power decrease, P = 0.005, mean 14 ± 14%) and also for the RF‐shimmed case (coefficient of variance decrease, P = 0.01, mean 16 ± 13%: power decrease, P = 0.005, mean 22 ± 11%) of a dual‐transmit system. Magn Reson Med, 2012. © 2012 Wiley Periodicals, Inc.     

Toward understanding transverse relaxation in human brain through its field dependence

Apparent transverse‐relaxation rate constants (R = 1/T) were measured in various regions of the healthy human brain using a multiecho adiabatic spin‐echo sequence at five different magnetic fields, 1.5, 1.9, 3, 4.7, and 7 T. The R values showed a clear dependence on magnetic field strength (B₀). The regional distribution of the R was well explained by the sum of three components: (1) regional nonhemin iron concentration ([Fe]), (2) regional macromolecular mass fraction (f_M), and (3) a region‐independent factor. Accordingly, R = α[Fe] + βf_M + γ, where coefficients α, β, and γ were experimentally determined at each magnetic field by a least square fitting method using multiple regression analysis. Although the coefficient α linearly increased with B₀, β showed a quadratic dependence on top of a field‐independent component. The coefficient γ also increased slightly with B₀ on top of a field‐independent component. The linear dependence of α on B₀ was consistent with that observed for the transverse‐relaxation rate of water protons in ferritin solutions as found previously by others. The quadratic dependence of β on B₀ was accounted for by isochronous and anisochronous exchange mechanisms using intrinsic‐relaxation parameters obtained from the literature. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

RF shimming for spectroscopic localization in the human brain at 7 T

Spectroscopic imaging of the human head at short echo times (≤15 ms) typically requires suppression of signals from extracerebral tissues. However, at 7 T, decreasing efficiency in B generation (hertz/watt) and increasing spectral bandwidth result in dramatic increases in power deposition and increased chemical shift registration artifacts for conventional gradient‐based in‐plane localization. In this work, we describe a novel method using radiofrequency shimming and an eight‐element transceiver array to generate a B field distribution that excites a ring about the periphery of the head and leaves central brain regions largely unaffected. We have used this novel B distribution to provide in‐plane outer volume suppression (>98% suppression of extracerebral lipids) without the use of gradients. This novel B distribution is used in conjunction with a double inversion recovery method to provide suppression of extracerebral resonances with T₁s greater than 400 ms, while having negligible effect on metabolite ratios of cerebral resonances with T₁s > 1000 ms. Despite the use of two adiabatic pulses, the high efficiency of the ring distribution allows radiofrequency power deposition to be limited to 3‐4 W for a pulse repetition time of 1.5 sec. The short echo time enabled the acquisition of images of the human brain, displaying glutamate, glutamine, macromolecules, and other major cerebral metabolites. Magn Reson Med, 2010. © 2009 Wiley‐Liss, Inc.     

Rapid single‐scan T‐mapping using exponential excitation pulses and image‐based correction for linear background gradients

A method for fast quantitative T mapping based on multiple gradient‐echo (multi‐GE) imaging with correction for static magnetic field inhomogeneities is described, using an exponential excitation pulse. Field gradient maps are obtained from the phase information and modulus data are subsequently corrected, allowing for simple monoexponential T fitting. Echoes with long echo times suffering from major signal losses due to field inhomogeneities are excluded from the analysis. The acquisition time for a matrix size of 256 × 256, 1 mm in‐plane resolution, and 2 mm slice thickness amounts to 15 s per slice. An additional correction for in‐plane field gradients further improves accuracy. Phantom experiments show that the method provides accurate T values for field gradients up to 200 μT/m; for gradients up to 300 μT/m errors do not exceed 15%. In vivo T values acquired on healthy volunteers at 3T are in excellent agreement with results from the literature. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Multiecho water‐fat separation and simultaneous R estimation with multifrequency fat spectrum modeling

Multiecho chemical shift–based water‐fat separation methods are seeing increasing clinical use due to their ability to estimate and correct for field inhomogeneities. Previous chemical shift‐based water‐fat separation methods used a relatively simple signal model that assumes both water and fat have a single resonant frequency. However, it is well known that fat has several spectral peaks. This inaccuracy in the signal model results in two undesired effects. First, water and fat are incompletely separated. Second, methods designed to estimate T in the presence of fat incorrectly estimate the T decay in tissues containing fat. In this work, a more accurate multifrequency model of fat is included in the iterative decomposition of water and fat with echo asymmetry and least‐squares estimation (IDEAL) water‐fat separation and simultaneous T estimation techniques. The fat spectrum can be assumed to be constant in all subjects and measured a priori using MR spectroscopy. Alternatively, the fat spectrum can be estimated directly from the data using novel spectrum self‐calibration algorithms. The improvement in water‐fat separation and T estimation is demonstrated in a variety of in vivo applications, including knee, ankle, spine, breast, and abdominal scans. Magn Reson Med 60:1122–1134, 2008. © 2008 Wiley‐Liss, Inc.     

Rapid B1+ mapping using a preconditioning RF pulse with TurboFLASH readout

In MRI, the transmit radiofrequency field (B) inhomogeneity can lead to signal intensity variations and quantitative measurement errors. By independently mapping the local B variation, the radiofrequency‐related signal variations can be corrected for. In this study, we present a new fast B mapping method using a slice‐selective preconditioning radiofrequency pulse. Immediately after applying a slice‐selective preconditioning pulse, a turbo fast low‐angle‐shot imaging sequence with centric k‐space reordering is performed to capture the residual longitudinal magnetization left behind by the slice‐selective preconditioning pulse due to B variation. Compared to the reference double‐angle method, this method is considerably faster. Specifically, the total scan time for the double‐angle method is equal to the product of 2 (number of images), the number of phase‐encoding lines, and approximately 5T₁, whereas the slice‐selective preconditioning method takes approximately 5T₁. This method was validated in vitro and in vivo with a 3‐T whole‐body MRI system. The combined brain and pelvis B measurements showed excellent agreement and strong correlation with those by the double‐angle method (mean difference = 0.025; upper and lower 95% limits of agreement were ?0.07 and 0.12; R = 0.93; P < 0.001). This fast B mapping method can be used for a variety of applications, including body imaging where fast imaging is desirable. Magn Reson Med, 2010. © 2010 Wiley‐Liss, Inc.