Comparison of RF body coils for MRI at 3 T: a simulation study using parallel transmission on various anatomical targets

The performance of multichannel transmit coil layouts and parallel transmission (pTx) RF pulse design was evaluated with respect to transmit B₁ (B₁ ⁺) homogeneity and specific absorption rate (SAR) at 3 T for a whole body coil. Five specific coils were modeled and compared: a 32‐rung birdcage body coil (driven either in a fixed quadrature mode or a two‐channel transmit mode), two single‐ring stripline arrays (with either 8 or 16 elements), and two multi‐ring stripline arrays (with two or three identical rings, stacked in the z axis and each comprising eight azimuthally distributed elements). Three anatomical targets were considered, each defined by a 3D volume representative of a meaningful region of interest (ROI) in routine clinical applications. For a given anatomical target, global or local SAR controlled pTx pulses were designed to homogenize RF excitation within the ROI. At the B₁ ⁺ homogeneity achieved by the quadrature driven birdcage design, pTx pulses with multichannel transmit coils achieved up to about eightfold reduction in local and global SAR. When used for imaging head and cervical spine or imaging thoracic spine, the double‐ring array outperformed all coils, including the single‐ring arrays. While the advantage of the double‐ring array became much less pronounced for pelvic imaging, with a substantially larger ROI, the pTx approach still provided significant gains over the quadrature birdcage coil. For all design scenarios, using the three‐ring array did not necessarily improve the RF performance. Our results suggest that pTx pulses with multichannel transmit coils can reduce local and global SAR substantially for body coils while attaining improved B₁ ⁺ homogeneity, particularly for a “z‐stacked” double‐ring design with coil elements arranged on two transaxial rings. Copyright © 2015 John Wiley & Sons, Ltd.     

Increased sensitivity of 31P MRSI using direct detection integrated with multi‐echo polarization transfer (DIMEPT)

Here, we show that the sensitivity of ³¹P MRSI of ³¹P spins J‐coupled to protons can be increased by almost a factor of three when compared with an optimal direct detection free induction decay. By direct detection integrated with multi‐echo polarization transfer (DIMEPT), multiple signals from polarization transfer and direct detection can be acquired in one repetition time, with minimal mutual interference, provided that the number of refocusing pulses in the multi‐echo polarization transfer part is even. The DIMEPT sequence was implemented on a 7‐T body scanner and tested on a phantom and on the breasts of five healthy volunteers. The in vivo signal‐to‐noise ratio (SNR) enhancement for the J‐coupled phosphomonoesters was 270% when compared with an Ernst angle pulse‐acquire sequence. However, the phosphodiester signals, presumably mainly mobile phospholipids, had T₂ values that were too short to be enhanced. Uncoupled ³¹P spins, with sufficiently long T₂ values, such as inorganic phosphate, were SNR enhanced by a factor of 1.9 relative to an Ernst‐angle excitation pulse‐acquire sequence by multi‐echo direct detection. Copyright © 2014 John Wiley & Sons, Ltd.     

Whole‐body radiofrequency coil for 31P MRSI at 7 T

Widespread use of ultrahigh‐field ³¹P MRSI in clinical studies is hindered by the limited field of view and non‐uniform radiofrequency (RF) field obtained from surface transceivers. The non‐uniform RF field necessitates the use of high specific absorption rate (SAR)‐demanding adiabatic RF pulses, limiting the signal‐to‐noise ratio (SNR) per unit of time. Here, we demonstrate the feasibility of using a body‐sized volume RF coil at 7 T, which enables uniform excitation and ultrafast power calibration by pick‐up probes. The performance of the body coil is examined by bench tests, and phantom and in vivo measurements in a 7‐T MRI scanner. The accuracy of power calibration with pick‐up probes is analyzed at a clinical 3‐T MR system with a close to identical ¹H body coil integrated at the MR system. Finally, we demonstrate high‐quality three‐dimensional ³¹P MRSI of the human body at 7 T within 5 min of data acquisition that includes RF power calibration. Copyright © 2016 John Wiley & Sons, Ltd.     

Optimized 31P MRS in the human brain at 7 T with a dedicated RF coil setup

The design and construction of a dedicated RF coil setup for human brain imaging (¹H) and spectroscopy (³¹P) at ultra‐high magnetic field strength (7 T) is presented. The setup is optimized for signal handling at the resonance frequencies for ¹H (297.2 MHz) and ³¹P (120.3 MHz). It consists of an eight‐channel ¹H transmit–receive head coil with multi‐transmit capabilities, and an insertable, actively detunable ³¹P birdcage (transmit–receive and transmit only), which can be combined with a seven‐channel receive‐only ³¹P array. The setup enables anatomical imaging and ³¹P studies without removal of the coil or the patient. By separating transmit and receive channels and by optimized addition of array signals with whitened singular value decomposition we can obtain a sevenfold increase in SNR of ³¹P signals in the occipital lobe of the human brain compared with the birdcage alone. These signals can be further enhanced by 30 ± 9% using the nuclear Overhauser effect by B₁‐shimmed low‐power irradiation of water protons. Together, these features enable acquisition of ³¹P MRSI at high spatial resolutions (3.0 cm³ voxel) in the occipital lobe of the human brain in clinically acceptable scan times (~15 min). © 2015 The Authors. NMR in Biomedicine published by John Wiley & Sons Ltd.     

Adiabatic multi‐echo 31P spectroscopic imaging (AMESING) at 7 T for the measurement of transverse relaxation times and regaining of sensitivity in tissues with short T2* values

An adiabatic multi‐echo spectroscopic imaging (AMESING) sequence, used for ³¹P MRSI, with spherical k‐space sampling and compensated phase‐encoding gradients, was implemented on a whole‐body 7‐T MR system. One free induction decay (FID) and up to five symmetric echoes can be acquired with this sequence. In tissues with low T₂* and high T₂, this can theoretically lead to a potential maximum signal‐to‐noise ratio (SNR) increase of almost a factor of three, compared with a conventional FID acquisition with Ernst‐angle excitation. However, with T₂ values being, in practice, ≤400 ms, a maximum enhancement of approximately two compared with low flip Ernst‐angle excitation should be feasible. The multi‐echo sequence enables the determination of localized T₂ values, and was validated with ³¹P three‐dimensional MRSI on the calf muscle and breast of a healthy volunteer, and subsequently applied in a patient with breast cancer. The T₂ values of phosphocreatine, phosphodiesters (PDE) and inorganic phosphate in calf muscle were 193 ± 5 ms, 375 ± 44 ms and 96 ± 10 ms, respectively, and the apparent T₂ value of γ‐ATP was 25 ± 6 ms. A T₂ value of 136 ± 15 ms for inorganic phosphate was measured in glandular breast tissue of a healthy volunteer. The T₂ values of phosphomonoesters (PME) and PDE in breast cancer tissue (ductulolobular carcinoma) ranged between 170 and 210 ms, and the PME to PDE ratios were calculated to be phosphoethanolamine/glycerophosphoethanolamine = 2.7, phosphocholine/glycerophosphocholine = 1.8 and PME/PDE = 2.3. Considering the relatively short T₂* values of the metabolites in breast tissue at 7 T, the echo spacing can be short without compromising spectral resolution, whilst maximizing the sensitivity. Copyright © 2013 John Wiley & Sons, Ltd.     

Slice‐selective FID acquisition,localized by outer volume suppression (FIDLOVS) for 1H‐MRSI of the human brain at 7 T with minimal signal loss

In comparison to 1.5 and 3 T, MR spectroscopic imaging at 7 T benefits from signal‐to‐noise ratio (SNR) gain and increased spectral resolution and should enable mapping of a large number of metabolites at high spatial resolutions. However, to take full advantage of the ultra‐high field strength, severe technical challenges, e.g. related to very short T₂ relaxation times and strict limitations on the maximum achievable B₁ field strength, have to be resolved. The latter results in a considerable decrease in bandwidth for conventional amplitude modulated radio frequency pulses (RF‐pulses) and thus to an undesirably large chemical‐shift displacement artefact. Frequency‐modulated RF‐pulses can overcome this problem; but to achieve a sufficient bandwidth, long pulse durations are required that lead to undesirably long echo‐times in the presence of short T₂ relaxation times. In this work, a new magnetic resonance spectroscopic imaging (MRSI) localization scheme (free induction decay acquisition localized by outer volume suppression, FIDLOVS) is introduced that enables MRSI data acquisition with minimal SNR loss due to T₂ relaxation and thus for the first time mapping of an extended neurochemical profile in the human brain at 7 T. To overcome the contradictory problems of short T₂ relaxation times and long pulse durations, the free induction decay (FID) is directly acquired after slice‐selective excitation. Localization in the second and third dimension and skull lipid suppression are based on a T₁‐ and B₁‐insensitive outer volume suppression (OVS) sequence. Broadband frequency‐modulated excitation and saturation pulses enable a minimization of the chemical‐shift displacement artefact in the presence of strict limits on the maximum B₁ field strength. The variable power RF pulses with optimized relaxation delays (VAPOR) water suppression scheme, which is interleaved with OVS pulses, eliminates modulation side bands and strong baseline distortions. Third order shimming is based on the accelerated projection‐based automatic shimming routine (FASTERMAP) algorithm. The striking SNR and spectral resolution enable unambiguous quantification and mapping of 12 metabolites including glutamate (Glu), glutamine (Gln), N‐acetyl‐aspartatyl‐glutamate (NAAG), γ‐aminobutyric acid (GABA) and glutathione (GSH). The high SNR is also the basis for highly spatially resolved metabolite mapping. Copyright © 2009 John Wiley & Sons, Ltd.     

ECLIPSE utilizing gradient‐modulated offset‐independent adiabaticity (GOIA) pulses for highly selective human brain proton MRSI

A multitude of extracranial lipid suppression methods exist for proton MRSI acquisitions. Popular and emerging lipid suppression methods each have their inherent set of advantages and disadvantages related to the achievable level of lipid suppression, RF power deposition, insensitivity to B₁⁺ field and lipid T₁ heterogeneity, brain coverage, spatial selectivity, chemical shift displacement (CSD) errors and the reliability of spectroscopic data spanning the observed 0.9‐4.7 ppm band. The utility of elliptical localization with pulsed second order fields (ECLIPSE) was previously demonstrated with a greater than 100‐fold in extracranial lipid suppression and low power requirements utilizing 3 kHz bandwidth AFP pulses. Like all gradient‐based localization methods, ECLIPSE is sensitive to CSD errors, resulting in a modified metabolic profile in edge‐of‐ROI voxels. In this work, ECLIPSE is extended with 15 kHz bandwidth second order gradient‐modulated RF pulses based on the gradient offset‐independent adiabaticity (GOIA) algorithm to greatly reduce CSD and improve spatial selectivity. An adiabatic double spin‐echo ECLIPSE inner volume selection (TE = 45 ms) MRSI method and an ECLIPSE outer volume suppression (TE = 3.2 ms) FID‐MRSI method were implemented. Both GOIA‐ECLIPSE MRSI sequences provided artifact‐free metabolite spectra in vivo, with a greater than 100‐fold in lipid suppression and less than 2.6 mm in‐plane CSD and less than 3.3 mm transition width for edge‐of‐ROI voxels, representing an ~5‐fold improvement compared with the parent, nongradient‐modulated method. Despite the 5‐fold larger bandwidth, GOIA‐ECLIPSE only required a 1.9‐fold increase in RF power. The highly robust lipid suppression combined with low CSD and sharp ROI edge transitions make GOIA‐ECLIPSE an attractive alternative to commonly employed lipid suppression methods. Furthermore, the low RF power deposition demonstrates that GOIA‐ECLIPSE is very well suited for high field (≥3 T) MRSI applications.     

Performance and safety assessment of an integrated transmit array for body imaging at 7 T under consideration of specific absorption rate,tissue temperature,and thermal dose

In this study, the performance of an integrated body-imaging array for 7 T with 32 radiofrequency (RF) channels under consideration of local specific absorption rate (SAR), tissue temperature, and thermal dose limits was evaluated and the imaging performance was compared with a clinical 3 T body coil. Thirty-two transmit elements were placed in three rings between the bore liner and RF shield of the gradient coil. Slice-selective RF pulse optimizations for B₁ shimming and spokes were performed for differently oriented slices in the body under consideration of realistic constraints for power and local SAR. To improve the B₁⁺ homogeneity, safety assessments based on temperature and thermal dose were performed to possibly allow for higher input power for the pulse optimization than permissible with SAR limits. The results showed that using two spokes, the 7 T array outperformed the 3 T birdcage in all the considered regions of interest. However, a significantly higher SAR or lower duty cycle at 7 T is necessary in some cases to achieve similar B₁⁺ homogeneity as at 3 T. The homogeneity in up to 50 cm-long coronal slices can particularly benefit from the high RF shim performance provided by the 32 RF channels. The thermal dose approach increases the allowable input power and the corresponding local SAR, in one example up to 100 W/kg, without limiting the exposure time necessary for an MR examination. In conclusion, the integrated antenna array at 7 T enables a clinical workflow for body imaging and comparable imaging performance to a conventional 3 T clinical body coil.     

Ultra‐high‐field RF coil development for evaluating upper extremity imaging applications

The purpose of this study is to develop and evaluate a custom‐designed 7  T MRI coil and explore its use for upper extremity applications. An RF system composed of a transverse electromagnetic transmit coil and an eight‐channel receive‐only array was developed for 7  T upper extremity applications. The RF system was characterized and evaluated using scattering parameters and B₁⁺ mapping. Finite difference time domain simulations were performed to evaluate the B₁⁺ field distribution and specific absorption rate for the forearm region of the upper extremity. High‐resolution 7  T images were acquired and compared with those at 3 T. The simulation and experimental results show very good B₁⁺ field homogeneity across the forearm. High‐resolution images of musculotendinous, osseocartilaginous, and neurovascular structures in the upper extremity are presented with T₁ volumetric interpolated breath‐hold examination, T₂ double‐echo steady state, T₂* susceptibility weighted imaging (SWI), diffusion tensor imaging, and time‐of‐flight sequences. Comparison between 3  T and 7  T is shown. Intricate contextual anatomy can be delineated in synovial, fibrocartilaginous, interosseous, and intraosseous trabecular structures of the forearm, as well as palmar and digital vascular anatomy (including microvascular detail in SWI). Ultra‐high‐field 7  T imaging holds great potential in improving the sensitivity and specificity of upper extremity imaging, especially in wrist and hand pathology secondary to bone, ligament, nerve, vascular, and other soft or hard tissue etiology.     

Proton spectroscopic imaging of the human prostate at 7 T

The sensitivity of proton MR Spectroscopic Imaging (¹H‐MRSI) of the prostate can be optimized by using the high magnetic field strength of 7 T in combination with an endorectal coil. In the work described in this paper we introduce an endorectal transceiver at 7 T, validate its safety for in vivo use and apply a pulse sequence, optimized for three‐dimensional (3D) ¹H‐MRSI of the human prostate at 7 T. A transmit/receive endorectal RF coil was adapted from a commercially available 3 T endorectal receive‐only coil and validated to remain within safety guidelines for radiofrequency (RF) power deposition using numerical models, MR thermometry of phantoms, and in vivo temperature measurements. The ¹H‐MRSI pulse sequence used adiabatic slice selective refocusing pulses and frequency‐selective water and lipid suppression to selectively obtain the relevant metabolite signals from the prostate. Quantum mechanical simulations were used to adjust the inter‐pulse timing for optimal detection of the strongly coupled spin system of citrate resulting in an echo time of 56 ms. Using this endorectal transceiver and pulse sequence with slice selective adiabatic refocusing pulses, 3D ¹H‐MRSI of the human prostate is feasible at 7 T with a repetition time of 2 s. The optimized inter‐pulse timing enables the absorptive detection of resonances of spins from spermine and citrate in phase with creatine and choline. These potential tumor markers may improve the in vivo detection, localization, and assessment of prostate cancer. Copyright © 2009 John Wiley & Sons, Ltd.     

Image‐guided radio‐frequency gain calibration for high‐field MRI

High‐field (≥ 3T) MRI provides a means to increase the signal‐to‐noise ratio, due to its higher tissue magnetization compared with 1.5T. However, both the static magnetic field (B₀) and the transmit radio‐frequency (RF) field (B) inhomogeneities are comparatively higher at higher field strengths than those at 1.5T. These challenging factors at high‐field strengths make it more difficult to accurately calibrate the transmit RF gain using standard RF calibration procedures. An image‐based RF calibration procedure was therefore developed, in order to accurately calibrate the transmit RF gain within a specific region‐of‐interest (ROI). Using a turbo fast low‐angle shot (TurboFLASH) pulse sequence with centric k‐space reordering, a series of ‘saturation‐no‐recovery’ images was acquired by varying the flip angle of the preconditioning pulse. In the resulting images, the signal null occurs in regions where the flip angle of the preconditioning pulse is 90°. For a given ROI, the mean signal can be plotted as a function of the nominal flip angle, and the resulting curve can be used to quantitatively identify the signal null. This image‐guided RF calibration procedure was evaluated through phantom and volunteer imaging experiments at 3T and 7T. The image‐guided RF calibration results in vitro were consistent with standard B₀ and B maps. The standard automated RF calibration procedure produced approximately 20% and 15–30% relative error in the transmit RF gain in the left kidney at 3T and brain at 7T, respectively. For initial application, a T₂ mapping pulse sequence was applied at 7T. The T₂ measurements in the thalamus at 7T were 60.6 ms and 48.2 ms using the standard and image‐guided RF calibration procedures, respectively. This rapid, image‐guided RF calibration procedure can be used to optimally calibrate the flip angle for a given ROI and thus minimize measurement errors for quantitative MRI and MR spectroscopy. Copyright © 2009 John Wiley & Sons, Ltd.     

Multislice T1‐prepared 2D single‐shot EPI: analysis of a clinical T1 mapping method unbiased by B0 or B1 inhomogeneity

Quantitative MR imaging is as sensitive in detecting lesions as qualitative imaging, but it is potentially more specific in differentiating disease. T₁ mapping in particular might help to assess acute ischemic stroke, multiple sclerosis, epilepsy and Alzheimer's disease better. Thus, a rapid and robust clinical technique is vital. In 1990, Ordidge and colleagues developed the multislice T₁ ‐prepared two‐dimensional (2D) single‐shot echo planar imaging technique. Subsequent studies demonstrated its clinical viability, but none performed an in‐depth analysis of the strengths and advantages of this T₁ mapping method. Herein, theoretical and experimental evidence shows that the technique accounts for 2D slice profile effects and is unbiased by B₀ or B₁ inhomogeneity. This is verified explicitly by varying the linear shims, the T₁ preparation flip angle and the excitation flip angle. Furthermore, it is shown that the repetition time (and hence scan time) can be reduced without a loss of T₁ accuracy. Copyright © 2016 John Wiley & Sons, Ltd.     

High-resolution mapping of human brain metabolites by free induction decay (1)H MRSI at 7 T

This work describes a new approach for high‐spatial‐resolution ¹H MRSI of the human brain at 7 T. ¹H MRSI at 7 T using conventional approaches, such as point‐resolved spectroscopy and stimulated echo acquisition mode with volume head coils, is limited by technical difficulties, including chemical shift displacement errors, B₀/B₁ inhomogeneities, a high specific absorption rate and decreased T₂ relaxation times. The method presented here is based on free induction decay acquisition with an ultrashort acquisition delay (TE*) of 1.3 ms. This allows full signal detection with negligible T₂ decay or J‐modulation. Chemical shift displacement errors were reduced to below 5% per part per million in the in‐slice direction and were eliminated in‐plane. The B₁ sensitivity was reduced significantly and further corrected using flip angle maps. Specific absorption rate requirements were well below the limit (~20 % = 0.7 W/kg). The suppression of subcutaneous lipid signals was achieved by substantially improving the point‐spread function. High‐quality metabolic mapping of five important brain metabolites was achieved with high in‐plane resolution (64 × 64 matrix with a 3.4 × 3.4 × 12 mm³ nominal voxel size) in four healthy subjects. The ultrashort TE* increased the signal‐to‐noise ratio of J‐coupled resonances, such as glutamate and myo‐inositol, several‐fold to enable the mapping of even these metabolites with high resolution. Four measurement repetitions in one healthy volunteer provided proof of the good reproducibility of this method. The high spatial resolution allowed the visualization of several anatomical structures on metabolic maps. Free induction decay MRSI is insensitive to T₂ decay, J‐modulation, B₁ inhomogeneities and chemical shift displacement errors, and overcomes specific absorption rate restrictions at ultrahigh magnetic fields. This makes it a promising method for high‐resolution ¹H MRSI at 7 T and above. Copyright © 2011 John Wiley & Sons, Ltd.     

DYNAmic Multi‐coIl TEchnique (DYNAMITE) shimming of the rat brain at 11.7 T

The in vivo rat model is a workhorse in neuroscience research, preclinical studies and drug development. A repertoire of MR tools has been developed for its investigation; however, high levels of B₀ magnetic field homogeneity are required for meaningful results. The homogenization of magnetic fields in the rat brain, i.e. shimming, is a difficult task because of a multitude of complex, susceptibility‐induced field distortions. Conventional shimming with spherical harmonic (SH) functions is capable of compensating for shallow field distortions in limited areas, e.g. in the cortex, but performs poorly in difficult‐to‐shim subcortical structures or for the entire brain. Based on the recently introduced multi‐coil approach for magnetic field modeling, the DYNAmic Multi‐coIl TEchnique (DYNAMITE) is introduced for magnetic field shimming of the in vivo rat brain and its benefits for gradient‐echo echo‐planar imaging (EPI) are demonstrated. An integrated multi‐coil/radiofrequency (MC/RF) system comprising 48 individual localized DC coils for B₀ shimming and a surface transceive RF coil has been developed that allows MR investigations of the anesthetized rat brain in vivo. DYNAMITE shimming with this MC/RF set‐up is shown to reduce the B₀ standard deviation to a third of that achieved with current shim technology employing static first‐ through third‐order SH shapes. The EPI signal over the rat brain increased by 31%, and a 24% gain in usable EPI voxels could be realized. DYNAMITE shimming is expected to critically benefit a wide range of preclinical and neuroscientific MR research. Improved magnetic field homogeneity, together with the achievable large brain coverage of this method, will be crucial when signal pathways, cortical circuitry or the brain's default network are studied. Together with the efficiency gains of MC‐based shimming compared with SH approaches demonstrated recently, DYNAMITE shimming has the potential to replace conventional SH shim systems in small‐bore animal scanners. Copyright © 2014 John Wiley & Sons, Ltd.     

Quantitative T1 and proton density mapping with direct calculation of radiofrequency coil transmit and receive profiles from two‐point variable flip angle data

Quantitative T₁ mapping of brain tissue is frequently based on the variable flip angle (VFA) method, acquiring spoiled gradient echo (GE) datasets at different excitation angles. However, accurate T₁ calculation requires a knowledge of the sensitivity profile B₁ of the radiofrequency (RF) transmit coil. For an additional derivation of proton density (PD) maps, the receive coil sensitivity profile (RP) must also be known. Mapping of B₁ and RP increases the experiment duration, which may be critical when investigating patients. In this work, a method is presented for the direct calculation of B₁ and RP from VFA data. Thus, quantitative maps of T₁, PD, B₁ and RP can be obtained from only two spoiled GE datasets. The method is based on: (1) the exploitation of the linear relationship between 1/PD and 1/T₁ in brain tissue and (2) the assumption of smoothly varying B₁ and RP, so that a large number of data points can be fitted across small volume elements where B₁ and RP are approximately constant. The method is tested and optimized on healthy subjects. Copyright © 2016 John Wiley & Sons, Ltd.     

Rapid 3D radiofrequency field mapping using catalyzed double‐angle method

A new method is presented for rapid and accurate large volumetric radiofrequency (RF) field (B) mapping. This method is a modification of the double‐angle method (DAM), which accelerates imaging speed and applies 3D acquisition to improve B measurement accuracy. It reduces repetition time and scan time by introducing a catalyzation RF pulse chain at the end of each DAM repetition cycle. The catalyzation pulse chain ensures that, after each TR period, the longitudinal magnetizations reach the same state for both measurements at two flip angles for the DAM so that the long TR requirement (TR ≥ 5 T₁) for complete relaxation of longitudinal magnetization of DAM becomes unnecessary. A multi‐echo imaging sequence is additionally incorporated to further improve the efficiency of data acquisition. Simulations demonstrate an excellent flip angle measurement accuracy for catalyzed DAM even with TR << T₁. Phantom and in vivo volunteer studies are presented to demonstrate the catalyzation effect upon B mapping and the application of 3D catalyzed DAM for rapid and accurate large volume RF field mapping. Copyright © 2009 John Wiley & Sons, Ltd.     

Non‐water‐suppressed short‐echo‐time magnetic resonance spectroscopic imaging using a concentric ring k‐space trajectory

Water‐suppressed MRS acquisition techniques have been the standard MRS approach used in research and for clinical scanning to date. The acquisition of a non‐water‐suppressed MRS spectrum is used for artefact correction, reconstruction of phased‐array coil data and metabolite quantification. Here, a two‐scan metabolite‐cycling magnetic resonance spectroscopic imaging (MRSI) scheme that does not use water suppression is demonstrated and evaluated. Specifically, the feasibility of acquiring and quantifying short‐echo (T_E = 14 ms), two‐dimensional stimulated echo acquisition mode (STEAM) MRSI spectra in the motor cortex is demonstrated on a 3 T MRI system. The increase in measurement time from the metabolite‐cycling is counterbalanced by a time‐efficient concentric ring k‐space trajectory. To validate the technique, water‐suppressed MRSI acquisitions were also performed for comparison. The proposed non‐water‐suppressed metabolite‐cycling MRSI technique was tested for detection and correction of resonance frequency drifts due to subject motion and/or hardware instability, and the feasibility of high‐resolution metabolic mapping over a whole brain slice was assessed. Our results show that the metabolite spectra and estimated concentrations are in agreement between non‐water‐suppressed and water‐suppressed techniques. The achieved spectral quality, signal‐to‐noise ratio (SNR) > 20 and linewidth <7 Hz allowed reliable metabolic mapping of five major brain metabolites in the motor cortex with an in‐plane resolution of 10 × 10 mm² in 8 min and with a Cramér‐Rao lower bound of less than 20% using LCModel analysis. In addition, the high SNR of the water peak of the non‐water‐suppressed technique enabled voxel‐wise single‐scan frequency, phase and eddy current correction. These findings demonstrate that our non‐water‐suppressed metabolite‐cycling MRSI technique can perform robustly on 3 T MRI systems and within a clinically feasible acquisition time.     

Improved three‐dimensional Look–Locker acquisition scheme and angle map filtering procedure for T1 estimation

The three‐dimensional (3D) Look–Locker (LL) acquisition is a widely used fast and efficient T₁ mapping method. However, the multi‐shot approach of 3D LL acquisition can introduce reconstruction artifacts that result in intensity distortions. Traditional 3D LL acquisition generally utilizes a centric encoding scheme that is limited to a single phase‐encoding direction in k space. To optimize k‐space segmentation, an elliptical scheme with two phase‐encoding directions is implemented for the LL acquisition. This elliptical segmentation can reduce the intensity errors in the reconstructed images and improve the final T₁ estimation. One of the major sources of error in LL‐based T₁ estimation is a lack of accurate knowledge of the actual flip angle. A multi‐parameter curve‐fitting procedure can account for some of the variability in the flip angle. However, curve fitting can also introduce errors in the estimated flip angle that can result in incorrect T₁ values. A filtering procedure based on goodness of fit (GOF) is proposed to reduce the effect of false flip angle estimates. Filtering based on GOF weighting can remove probable incorrect angles that result in bad curve fitting. Simulation, phantom and in vivo studies have demonstrated that these techniques can improve the accuracy of 3D LL T₁ estimation. Copyright © 2013 John Wiley & Sons, Ltd.     

Development of a 7 T RF coil system for breast imaging

In ultrahigh‐field MRI, such as 7 T, the signal‐to‐noise ratio (SNR) increases while transmit (Tx) field (B₁⁺) can be degraded due to inhomogeneity and elevated specific absorption rate (SAR). By applying new array coil concepts to both Tx and receive (Rx) coils, the B₁⁺ homogeneity and SNR can be improved. In this study, we developed and tested in vivo a new RF coil system for 7 T breast MRI. An RF coil system composed of an eight‐channel Tx‐only array based on a tic‐tac‐toe design (can be combined to operate in single‐Tx mode) in conjunction with an eight‐channel Rx‐only insert was developed. Characterizations of the B₁⁺ field and associated SAR generated by the developed RF coil system were numerically calculated and empirically measured using an anatomically detailed breast model, phantom and human breasts. In vivo comparisons between 3 T (using standard commercial solutions) and 7 T (using the newly developed coil system) breast imaging were made. At 7 T, about 20% B₁⁺ inhomogeneity (standard deviation over the mean) was measured within the breast tissue for both the RF simulations and 7 T experiments. The addition of the Rx‐only array enhances the SNR by a factor of about three. High‐quality MR images of human breast were acquired in vivo at 7 T. For the in vivo comparisons between 3 T and 7 T, an approximately fourfold increase of SNR was measured with 7 T imaging. The B₁⁺ field distributions in the breast model, phantom and in vivo were in reasonable agreement. High‐quality 7 T in vivo breast MRI was successfully acquired at 0.6 mm isotropic resolution using the newly developed RF coil system.     

31P NMR relaxation of cortical bone mineral at multiple magnetic field strengths and levels of demineralization

Recent work has shown that solid‐state ¹H and ³¹P MRI can provide detailed insight into bone matrix and mineral properties, thereby potentially enabling differentiation of osteoporosis from osteomalacia. However, ³¹P MRI of bone mineral is hampered by unfavorable relaxation properties. Hence, accurate knowledge of these properties is critical to optimizing MRI of bone phosphorus. In this work, ³¹P MRI signal‐to‐noise ratio (SNR) was predicted on the basis of T₁ and T₂* (effective transverse relaxation time) measured in lamb bone at six field strengths (1.5–11.7 T) and subsequently verified by 3D ultra‐short echo‐time and zero echo‐time imaging. Further, T₁ was measured in deuterium‐exchanged bone and partially demineralized bone. ³¹P T₂* was found to decrease from 220.3 ± 4.3 µs to 98.0 ± 1.4 µs from 1.5 to 11.7 T, and T₁ to increase from 12.8 ± 0.5 s to 97.3 ± 6.4 s. Deuteron substitution of exchangeable water showed that 76% of the ³¹P longitudinal relaxation rate is due to ¹H–³¹P dipolar interactions. Lastly, hypomineralization was found to decrease T₁, which may have implications for ³¹P MRI based mineralization density quantification. Despite the steep decrease in the T₂*/T₁ ratio, SNR should increase with field strength as B₀^0.4 for sample‐dominated noise and as B₀^1.1 for coil‐dominated noise. This was confirmed by imaging experiments. Copyright © 2013 John Wiley & Sons, Ltd.