Adiabatic RF pulse design for Bloch‐Siegert B mapping

The Bloch–Siegert (B–S) mapping method has been shown to be fast and accurate, yet it suffers from high Specific Absorption Rate (SAR) and moderately long echo time. An adiabatic RF pulse design is introduced here for optimizing the off‐resonant B–S RF pulse to achieve more B–S measurement sensitivity for a given pulse width. The extra sensitivity can be used for higher angle‐to‐noise ratio maps or traded off for faster scans. Using numerical simulations and phantom experiments, it is shown that a numerically optimized 2‐ms adiabatic B‐S pulse is 2.5 times more efficient than a conventional 6‐ms Fermi‐shaped B–S pulse. The adiabatic B–S pulse performance is validated in a phantom, and in vivo brain mapping at 3T and 7T are shown. Magn Reson Med 70:829–835, 2013. © 2012 Wiley Periodicals, Inc.     

Myocardial T mapping free of distortion using susceptibility‐weighted fast spin‐echo imaging: A feasibility study at 1.5 T and 3.0 T

This study demonstrates the feasibility of applying free‐breathing, cardiac‐gated, susceptibility‐weighted fast spin‐echo imaging together with black blood preparation and navigator‐gated respiratory motion compensation for anatomically accurate T mapping of the heart. First, T maps are presented for oil phantoms without and with respiratory motion emulation (T = (22.1 ± 1.7) ms at 1.5 T and T = (22.65 ± 0.89) ms at 3.0 T). T relaxometry of a ferrofluid revealed relaxivities of R = (477.9 ± 17) mM^?1s^?1 and R = (449.6 ± 13) mM^?1s^?1 for UFLARE and multiecho gradient‐echo imaging at 1.5 T. For inferoseptal myocardial regions mean T values of 29.9 ± 6.6 ms (1.5 T) and 22.3 ± 4.8 ms (3.0 T) were estimated. For posterior myocardial areas close to the vena cava T‐values of 24.0 ± 6.4 ms (1.5 T) and 15.4 ± 1.8 ms (3.0 T) were observed. The merits and limitations of the proposed approach are discussed and its implications for cardiac and vascular T‐mapping are considered. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.     

Coaxial waveguide for travelling wave MRI at ultrahigh fields

At high magnetic fields the performance of a volume‐type body coil inside a human sized MR‐scanner is influenced by the waveguide action of the scanner's bore. This can result in undesirable strong radio frequency fields ) outside the coil's target volume. A radio frequency (RF) transmit system, exploiting this waveguide action of the bore, is proposed in this work. A coaxial waveguide section is introduced between the antenna and the imaging region. It is shown that the coaxial waveguide has several advantages over the initially proposed travelling wave setup based on the cylindrical waveguide. First, a novel radio frequency matching principle (based on the transmission line impedance matching) is feasible with the coaxial waveguide achieving better radio frequency transmission characteristics, such as homogeneity and power efficiency of field. In case of body torso imaging, the coaxial waveguide prevents unwanted specific absorptive rate (SAR) deposition outside the target region and thus, effectively decreases local peak SAR values by factor of 5. A 3‐fold gain in the prostate can be achieved with the coaxial waveguide in comparison with the initially proposed travelling wave setup. Magn Reson Med 70:875–884, 2013. © 2012 Wiley Periodicals, Inc.     

Conditions for achieving artifact-free T preparation in MRI

A theoretical analysis of the stimulated gradient-echo method for achieving T-preparation in magnetic resonance imaging is performed. The results show that T-weighting without the “shading effect” due to magnetic field variation can only be achieved if the dephasing requirement of the stimulated echo is satisfied. It is also shown that the effective echo-time for T-weighting depends on the polarities of the dephasing and the rephasing gradient pulses that generate the stimulated echo. The results from the theoretical analysis are experimentally validated.     

Differential effect of isoflurane, medetomidine, and urethane on BOLD responses to acute levo-tetrahydropalmatine in the rat

In this study, a new approach to measure local electrical conductivity in tissue is presented, which is based on the propagating B phase and the homogeneous Helmholtz equation. This new MRI technique might open future opportunities for tumor and lesion characterization based on conductivity differences, while it may also find application in radio frequency safety assessment. Prerequisites for conductivity mapping using only the B phase (instead of the complex B field) are addressed. Furthermore it was found that the B phase can be derived directly from the measurable transceive phase arg(B B) in the head. Validation for a human head excited by a 7 T‐birdcage coil using simulations and measurements showed that it is possible to measure in vivo conductivity patterns in the brain using B phase information only. Conductivity contrast between different brain tissues is clearly observed. The measured mean values for white matter, gray matter and cerebrospinal fluid differed 54%, 26%, and ?13% respectively from literature values. The proposed method for B phase measurements is very suited for in vivo applications, as the measurement is short (less than a minute per imaged slice) and exposes the patient to low RF power, contrary to earlier proposed approaches. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Improved RF performance of travelling wave MR with a high permittivity dielectric lining of the bore

Application of travelling wave MR to human body imaging is restricted by the limited peak power of the available RF amplifiers. Nevertheless, travelling wave MR advantages like a large field of view excitation and distant location of transmit elements would be desirable for whole body MRI. In this work, improvement of the efficiency of travelling wave MR is demonstrated. High permittivity dielectric lining placed next to the scanner bore wall effectively reduces attenuation of the travelling wave in the longitudinal direction and at the same time directs the radial power flow toward the load. First, this is shown with an analytical model of a metallic cylindrical waveguide with the dielectric lining next to the wall and loaded with a cylindrical phantom. Simulations and experiments also reveal an increase of efficiency in the center of the bore for travelling wave MR with a dielectric lining. Phantom experiments show up to a 2‐fold gain in with the dielectric lining. This corresponds to a 4‐fold increase in power efficiency of travelling wave MR. In vivo experiments demonstrate an 8‐fold signal‐to‐noise ratio gain with the dielectric lining. Overall, it is shown that dielectric lining is a constructive method to improve efficacy of travelling wave MR. Magn Reson Med 70:885–894, 2013. © 2012 Wiley Periodicals, Inc.     

Noncontrast‐enhanced renal angiography using multiple inversion recovery and alternating TR balanced steady‐state free precession

Noncontrast‐enhanced renal angiography techniques based on balanced steady‐state free precession avoid external contrast agents, take advantage of high inherent blood signal from the contrast mechanism, and have short steady‐state free precession acquisition times. However, background suppression is limited; inflow times are inflexible; labeling region is difficult to define when tagging arterial flow; and scan times are long. To overcome these limitations, we propose the use of multiple inversion recovery preparatory pulses combined with alternating pulse repetition time balanced steady‐state free precession to produce renal angiograms. Multiple inversion recovery uses selective spatial saturation followed by four nonselective inversion recovery pulses to concurrently null a wide range of background species while allowing for adjustable inflow times; alternating pulse repetition time steady‐state free precession maintains vessel contrast and provides added fat suppression. The high level of suppression enables imaging in three‐dimensional as well as projective two‐dimensional formats, the latter of which has a scan time as short as one heartbeat. In vivo studies at 1.5 T demonstrate the superior vessel contrast of this technique. Magn Reson Med 70:527–536, 2013. © 2012 Wiley Periodicals, Inc.     

Cerebral blood oxygenation in rat brain during hypoxic hypoxia. Quantitative MRI of effective transverse relaxation rates

Oxygenation-sensitive MRI of respiratory challenges in the brain of experimental animals will considerably benefit from a quantitative relationship between cerebral blood oxygenation and MRI parameters. Here, a multi-echo gradient-echo MRI technique was used to determine effective transverse relaxation rates R = 1/T of rat brain in vivo during short periods of hypoxia and interleaved normoxic phases. The differential contribution ΔR observed during hypoxia was found to increase linearly with arterial blood deoxygenation for mild to moderate conditions. Severe deoxygenation resulted in a plateau most likely due to enhanced cerebral blood flow.     

Field strength dependence of R1 and R relaxivities of human whole blood to prohance,vasovist, and deoxyhemoglobin

This study has measured the longitudinal and transverse (T) relaxivity curves for ProHance (Gadoteridol), Vasovist (Gadofosveset) and deoxyhemoglobin at 1.5, 3.0, and 7.0 Tesla. The plots of R₁ versus both contrast agent and deoxyhemoglobin concentration were linear. The plots of R versus deoxyhemoglobin concentration showed a quadratic dependence. R versus contrast agent concentration showed a parabolic dependence with a minimum occurring at contrast agent concentrations of approximately 1.5 mM, corresponding to an accessible concentration in vivo. Monte Carlo simulations were performed to support the hypothesis that the minimum results from the susceptibility of the red blood cells being matched to the susceptibility of the plasma. Relaxivity values (s^?1mM^?1) for R and R₁ for all agents and all three field strengths are given. Magn Reson Med 60:1313–1320, 2008. © 2008 Wiley‐Liss, Inc.     

Longitudinal and noninvasive assessment of emphysema evolution in a murine model using proton MRI

Ultrashort echo time (550 μs) MR imaging was implemented to track the emphysema development in mice lung challenged with elastase. Two parameters, namely, signal intensity and T, were used to monitor the disease evolution. Nine mice were imaged before and at 24 h as well as at 3 and 8 weeks after elastase instillation. Five mice instilled with saline served as controls. At week 8, the mean normalized signal intensity ± SD was 0.89 ± 0.20 for healthy controls and 0.64 ± 0.10 for animals with emphysema. Similarly, a reduced value of T (1.27 ± 0.35 ms vs 0.96 ± 0.18 ms) was found in the emphysema group. The mean signal intensity drop and the reduction of T were prominent at 3 weeks following elastase instillation and stabilized between 3 and 8 weeks. The results indicated an excellent agreement between MR findings and histological morphometry (signal intensity, r = ?0.78, P = 0.004; T, r = ?0.78, P = 0.001). This result shows that proton MRI allows structural changes at alveolar level to be monitored longitudinally. This technique, applied routinely in preclinical trials will represent a valuable tool for assessment of drug therapy efficacy. Magn Reson Med, 2012. © 2011 Wiley Periodicals, Inc.     

Boosting 19F MRI—SNR efficient detection of paramagnetic contrast agents using ultrafast sequences

¹⁹F MRI offers high specificity but usually low sensitivity. Here, paramagnetic relaxation enhancement is assessed as a method to improve SNR efficiency in ¹⁹F MRI. Compounds with short relaxation times are used that combine fluorine and a paramagnetic ion within the same molecule. Different molecular designs provide T₁ values in the range of 1.4–15 ms and /T₁ ratios from 0.3 to 1. Gradient echo, as well as ultrafast radial MR sequences, is optimized to achieve highest SNR efficiency. Compared to nonparamagnetic compounds, ultrafast sequences can yield a gain of up to a factor 27 in sensitivity, whereas the gain with gradient echo is only factor 11. Comparison among the paramagnetic compounds shows that /T₁ close to unity is a prerequisite for highest SNR efficiency gain and that best results are obtained for compounds with T₁ in the range of 1–5 ms. Magn Reson Med 69:1056–1062, 2013. © 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(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.     

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.     

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.     

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.     

Magnetic poly(lactide‐co‐glycolide) and cellulose particles for MRI‐based cell tracking

Biodegradable, superparamagnetic microparticles and nanoparticles of poly(lactide‐co‐glycolide) (PLGA) and cellulose were designed, fabricated, and characterized for magnetic cell labeling. Monodisperse nanocrystals of magnetite were incorporated into microparticles and nanoparticles of PLGA and cellulose with high efficiency using an oil‐in‐water single emulsion technique. Superparamagnetic cores had high magnetization (72.1 emu/g). The resulting polymeric particles had smooth surface morphology and high magnetite content (43.3 wt % for PLGA and 69.6 wt % for cellulose). While PLGA and cellulose nanoparticles displayed highest r values per millimole of iron (399 sec^?1 mM^?1 for cellulose and 505 sec^?1 mM^?1 for PLGA), micron‐sized PLGA particles had a much higher r per particle than either. After incubation for a month in citrate buffer (pH 5.5), magnetic PLGA particles lost close to 50% of their initial r molar relaxivity, while magnetic cellulose particles remained intact, preserving over 85% of their initial r molar relaxivity. Lastly, mesenchymal stem cells and human breast adenocarcinoma cells were magnetically labeled using these particles with no detectable cytotoxicity. These particles are ideally suited for noninvasive cell tracking in vivo via MRI and due to their vastly different degradation properties, offer unique potential for dedicated use for either short (PLGA‐based particles) or long‐term (cellulose‐based particles) experiments. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.     

High‐resolution cerebral blood volume imaging in humans using the blood pool contrast agent ferumoxytol

Cerebral blood volume maps are usually acquired using dynamic susceptibility contrast imaging which inherently limits the spatial resolution and signal to noise ratio of the images. In this study, we used ferumoxytol (AMAG Pharmaceuticals, Inc., Cambridge, MA), an FDA‐approved compound, to obtain high‐resolution cerebral blood volume maps with a steady‐state approach in seven healthy volunteers. maps (0.8 × 0.8 × 1 mm³) were acquired before and after injection of ferumoxytol and an intraindividual normalization protocol was used to obtain quantitative values. The results show excellent contrast between white and gray matter as well as fine highly detailed vascular structures. An average blood volume of 4% was found in the brain of all volunteers, consistent with prior literature values. A linear relationship was found between ferumoxytol dose (mg/kg) and (1/s) in gray (R² = 0.98) and white matter (R² = 0.98). A quadratic relationship was found in the sagittal sinus (R² = 0.98). The cerebral blood volume maps compare well with lower resolution dynamic susceptibility contrast‐MRI and their use should improve the evaluation of small and heterogeneous lesions and facilitate intrapatient and interpatient comparisons. Magn Reson Med 70:705–710, 2013. © 2012 Wiley Periodicals, 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.     

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.