Skin sodium measured with 23Na MRI at 7.0 T

Skin sodium (Na⁺) storage, as a physiologically important regulatory mechanism for blood pressure, volume regulation and, indeed, survival, has recently been rediscovered. This has prompted the development of MRI methods to assess Na⁺ storage in humans (²³Na MRI) at 3.0 T. This work examines the feasibility of high in‐plane spatial resolution ²³Na MRI in skin at 7.0 T. A two‐channel transceiver radiofrequency (RF) coil array tailored for skin MRI at 7.0 T (f = 78.5 MHz) is proposed. Specific absorption rate (SAR) simulations and a thorough assessment of RF power deposition were performed to meet the safety requirements. Human skin was examined in an in vivo feasibility study using two‐dimensional gradient echo imaging. Normal male adult volunteers (n = 17; mean ± standard deviation, 46 ± 18 years; range, 20–79 years) were investigated. Transverse slices of the calf were imaged with ²³Na MRI using a high in‐plane resolution of 0.9 × 0.9 mm². Skin Na⁺ content was determined using external agarose standards covering a physiological range of Na⁺ concentrations. To assess the intra‐subject reproducibility, each volunteer was examined three to five times with each session including a 5‐min walk and repositioning/preparation of the subject. The age dependence of skin Na⁺ content was investigated. The ²³Na RF coil provides improved sensitivity within a range of 1 cm from its surface versus a volume RF coil which facilitates high in‐plane spatial resolution imaging of human skin. Intra‐subject variability of human skin Na⁺ content in the volunteer population was <10.3%. An age‐dependent increase in skin Na⁺ content was observed (r = 0.78). The assignment of Na⁺ stores with ²³Na MRI techniques could be improved at 7.0 T compared with current 3.0 T technology. The benefits of such improvements may have the potential to aid basic research and clinical applications designed to unlock questions regarding the Na⁺ balance and Na⁺ storage function of skin. Copyright © 2014 John Wiley & Sons, Ltd.     

Comprehensive MRI for the detection of subtle alterations in diastolic cardiac function in apoE/LDLR–/– mice with advanced atherosclerosis

ApoE/LDLR^–/– mice represent a reliable model of atherosclerosis. However, it is not clear whether cardiac performance is impaired in this murine model of atherosclerosis. Here, we used MRI to characterize cardiac performance in vivo in apoE/LDLR^–/– mice with advanced atherosclerosis. Six‐month‐old apoE/LDLR^–/– mice and age‐matched C57BL/6J mice (control) were examined using highly time‐resolved cine‐MRI [whole‐chamber left ventricle (LV) imaging] and MR tagging (three slices: basal, mid‐cavity and apical). Global and regional measures of cardiac function included LV volumes, kinetics, time‐dependent parameters, strains and rotations. Histological analysis was performed using OMSB (orceine with Martius, Scarlet and Blue) and ORO (oil red‐O) staining to demonstrate the presence of advanced coronary atherosclerosis. MR‐tagging‐based strain analysis in apoE/LDLR^–/– mice revealed an increased frequency of radial and circumferential systolic stretch (25% and 50% of segments, respectively, p ≤ 0.012), increased radial post‐systolic strain index (45% of segments, p = 0.009) and decreased LV untwisting rate (?30.3° (11.6°)/cycle, p = 0.004) when compared with control mice. Maximal strains and LV twist were unchanged. Most of the cine‐MRI‐based LV functional and anatomical parameters also remained unchanged in apoE/LDLR^–/–mice, with only a lower filling rate, longer filling time, shorter isovolumetric contraction time and slower heart rate observed in comparison with control mice. The coronary arteries displayed severe atherosclerosis, as evidenced by histological analysis. Using comprehensive MRI methods, we have demonstrated that, despite severe coronary atherosclerosis in six‐month‐old apoE/LDLR^–/– mice, cardiac performance including global parameters, twist and strains, was well preserved. Only subtle diastolic alterations, possibly of ischemic background, were uncovered. Copyright © 2016 John Wiley & Sons, Ltd.     

39K and 23Na relaxation times and MRI of rat head at 21.1 T

At ultrahigh magnetic field strengths (B₀ ≥ 7.0 T), potassium (³⁹K) MRI might evolve into an interesting tool for biomedical research. However, ³⁹K MRI is still challenging because of the low NMR sensitivity and short relaxation times. In this work, we demonstrated the feasibility of ³⁹K MRI at 21.1 T, determined in vivo relaxation times of the rat head at 21.1 T, and compared ³⁹K and sodium (²³Na) relaxation times of model solutions containing different agarose gel concentrations at 7.0 and 21.1 T. ³⁹K relaxation times were markedly shorter than those of ²³Na. Compared with the lower field strength, ³⁹K relaxation times were up to 1.9‐ (T₁), 1.4‐ (T_2S) and 1.9‐fold (T_2L) longer at 21.1 T. The increase in the ²³Na relaxation times was less pronounced (up to 1.2‐fold). Mono‐exponential fits of the ³⁹K longitudinal relaxation time at 21.1 T revealed T₁ = 14.2 ± 0.1 ms for the healthy rat head. The ³⁹K transverse relaxation times were 1.8 ± 0.2 ms and 14.3 ± 0.3 ms for the short (T_2S) and long (T_2L) components, respectively. ²³Na relaxation times were markedly longer (T₁ = 41.6 ± 0.4 ms; T_2S = 4.9 ± 0.2 ms; T_2L = 33.2 ± 0.2 ms). ³⁹K MRI of the healthy rat head could be performed with a nominal spatial resolution of 1 × 1 × 1 mm³ within an acquisition time of 75 min. The increase in the relaxation times with magnetic field strength is beneficial for ²³Na and ³⁹K MRI at ultrahigh magnetic field strength. Our results demonstrate that ³⁹K MRI at 21.1 T enables acceptable image quality for preclinical research. Copyright © 2016 John Wiley & Sons, Ltd.     

Evaluation of transmit efficiency and SAR for a tight fit transceiver human head phased array at 9.4 T

Ultra‐high field (UHF, ≥7 T) tight fit transceiver phased arrays improve transmit (Tx) efficiency (B₁⁺/√P) in comparison with Tx‐only arrays, which are usually larger to fit receive (Rx)‐only arrays inside. One of the major problems limiting applications of tight fit arrays at UHFs is the anticipated increase of local tissue heating, which is commonly evaluated by the local specific absorption rate (SAR). To investigate the tradeoff between Tx efficiency and SAR when a tight fit UHF human head transceiver phased array is used instead of a Tx‐only/Rx‐only RF system, a single‐row eight‐element prototype of a 400 MHz transceiver head phased array was constructed. The Tx efficiency and SAR of the array were evaluated and compared with that of a larger Tx‐only array, which could also be used in combination with an 18‐channel Rx‐only array. Data were acquired on the Siemens Magnetom whole body 9.4 T human MRI system. Depending on the head size, positioning and the RF shim strategy, the smaller array provides from 11 to 23% higher Tx efficiency. In general, the Tx performance, evaluated as B₁⁺/√SAR, i.e. the safety excitation efficiency (SEE), is also not compromised. The two arrays provide very similar SEEs evaluated over 1000 random RF shim sets. We demonstrated that, in general, the tight fit transceiver array improves Tx performance without compromising SEE. However, in specific cases, the SEE value may vary, favoring one of the arrays, and therefore must be carefully evaluated.     

Eight‐channel transceiver RF coil array tailored for 1H/19F MR of the human knee and fluorinated drugs at 7.0 T

The purpose of this study was to evaluate the feasibility of an eight‐channel dual‐tuned transceiver surface RF coil array for combined ¹H/¹⁹F MR of the human knee at 7.0 T following application of ¹⁹F‐containing drugs. The ¹H/¹⁹F RF coil array includes a posterior module with two ¹H loop elements and two anterior modules, each consisting of one ¹H and two ¹⁹F elements. The decoupling of neighbor elements is achieved by a shared capacitor. Electromagnetic field simulations were performed to afford uniform transmission fields and to be in accordance with RF safety guidelines. Localized ¹⁹F MRS was conducted with 47 and 101 mmol/L of flufenamic acid (FA) – a ¹⁹F‐containing non‐steroidal anti‐inflammatory drug – to determine T₁ and T₂ and to study the ¹⁹F signal‐to‐dose relationship. The suitability of the proposed approach for ¹H/¹⁹F MR was examined in healthy subjects. Reflection coefficients of each channel were less than ?17 dB and coupling between channels was less than ?11 dB. Q_L/Q_U was less than 0.5 for all elements. MRS results demonstrated signal stability with 1% variation. T₁ and T₂ relaxation times changed with concentration of FA: T₁/T₂ = 673/31 ms at 101 mmol/L and T₁/T₂ = 616/26 ms at 47 mmol/L. A uniform signal and contrast across the patella could be observed in proton imaging. The sensitivity of the RF coil enabled localization of FA ointment administrated to the knee with an in‐plane spatial resolution of (1.5 × 1.5) mm² achieved in a total scan time of approximately three minutes, which is well suited for translational human studies. This study shows the feasibility of combined ¹H/¹⁹F MRI of the knee at 7.0 T and proposes T₁ and T₂ mapping methods for quantifying fluorinated drugs in vivo. Further technological developments are necessary to promote real‐time bioavailability studies and quantification of ¹⁹F‐containing medicinal compounds in vivo. Copyright © 2015 John Wiley & Sons, Ltd.     

Phase imaging with multiple phase‐cycled balanced steady‐state free precession at 9.4 T

While phase imaging with a gradient echo (GRE) sequence is popular, phase imaging with balanced steady‐state free precession (bSSFP) has been underexplored. The purpose of this study was to investigate anatomical and functional phase imaging with multiple phase‐cycled bSSFP, in expectation of increasing spatial coverage of steep phase‐change regions of bSSFP. Eight different dynamic 2D pass‐band bSSFP studies at four phase‐cycling (PC) angles and two T_E/T_R values were performed on rat brains at 9.4 T with electrical forepaw stimulation, in comparison with dynamic 2D GRE. Anatomical and functional phase images were obtained by averaging the dynamic phase images and mapping correlation between the dynamic images and the stimulation paradigm, and were compared with their corresponding magnitude images. Phase imaging with 3D pass‐band and 3D transition‐band bSSFP was also performed for comparison with 3D GRE phase imaging. Two strategies of combining the multiple phase‐cycled bSSFP phase images were also proposed. Contrast between white matter and gray matter in bSSFP phase images significantly varied with PC angle and became twice as high as that of GRE phase images at a specific PC angle. With the same total scan time, the combined bSSFP phase images provided stronger phase contrast and visualized neuronal fiber‐like structures more clearly than the GRE phase images. The combined phase images of both 3D pass‐band and 3D transition‐band bSSFP showed phase contrasts stronger than those of the GRE phase images in overall brain regions, even at a longer T_E of 20 ms. In contrast, phase functional MRI (fMRI) signals were weak overall and mostly located in draining veins for both bSSFP and GRE. Multiple phase‐cycled bSSFP phase imaging is a promising anatomical imaging technique, while its usage as fMRI does not seem desirable with the current approach.     

An approach to evaluation of the point-spread function for 23Na magnetic resonance imaging

Despite the technical challenges that require lengthy acquisitions to overcome poor signal-to-noise ratio (SNR), sodium (²³Na) magnetic resonance imaging (MRI) is an intriguing area of research due to its essential role in human metabolism. Low SNR images can impact the measurement of the point-spread function (PSF) by adding uncertainty into the resulting quantities. Here, we present methods to calculate the PSF by using the modulation transfer function (MTF), and a 3D-printed line-pair phantom in the context of ²³Na MRI. A simulation study investigated the effect of noise on the resulting MTF curves, which were derived by direct modulation (DM) and a method utilizing Fourier harmonics (FHs). Experimental data utilized a line-pair phantom with nine spatial frequencies, filled with different concentrations (15, 30, and 60 mM) of sodium in 3% agar. MTF curves were calculated using both methods from data acquired from density-adapted 3D radial projections (DA-3DRP) and Fermat looped orthogonally encoded trajectories (FLORET). Simulations indicated that the DM method increased variability in the MTF curves at all tested noise levels over the FH method. For the experimental data, the FH method resulted in PSFs with a narrower full width half maximum with reduced variability, although the improvement in variability was not as pronounced as predicted by simulations. The DA-3DRP data indicated an improvement in the PSF over FLORET. It was concluded that a 3D-printed line-pair phantom represents a convenient method to measure the PSF experimentally. The MTFs from the noisy images in ²³Na MRI have reduced variability from a FH method over DM.     

Validation of fast MR thermometry at 1.5 T with gradient-echo echo planar imaging sequences: phantom and clinical feasibility studies

The purpose of this work was to validate in phantom studies and demonstrate the clinical feasibility of MR proton resonance frequency thermometry at 1.5 T with segmented gradient-echo echo planar imaging (GRE-EPI) sequences during liver tumour radiofrequency (RF) ablation. Classical GRE acquisitions and segmented GRE-EPI acquisitions were performed at 1.5 T during simultaneous RF heating with an MR-compatible RF electrode placed in an agar gel phantom. Temperature increments were calculated and compared with four optical temperature probe measurements using Bland- Altman analysis. In a preliminary clinical feasibility study, the rapid GRE-EPI sequence (echo train length = 13) was used for MR temperature monitoring of RF ablation of liver tumours in three patient procedures. For phantom experiments, the Bland-Altman mean of differences between MR and optical probe temperature measurements was <0.4 degrees C, and the 95% limits of agreement value was <1.4 degrees C. For the in vivo studies, respiratory-triggered GRE-EPI acquisitions yielded a temperature accuracy of 1.3 +/- 0.4 degrees C (acquisition time = 0.6 s/image, spatial coverage of three slices/respiratory cycle). MR proton resonance frequency thermometry at 1.5 T yields precise and accurate measurements of temperature increment with both classical GRE and rapid GRE-EPI sequences. Rapid GRE-EPI sequences minimize intra-scan motion effects and can be used for MR thermometry during RF ablation in moving organs. Copyright (c) 2008 John Wiley & Sons, Ltd.     

Micro MRI of the mouse brain using a novel 400 MHz cryogenic quadrature RF probe

The increasing number of mouse models of human disease used in biomedical research applications has led to an enhanced interest in non‐invasive imaging of mice, e.g. using MRI for phenotyping. However, MRI of small rodents puts high demands on the sensitivity of data acquisition. This requirement can be addressed by using cryogenic radio‐frequency (RF) detection devices. The aim of this work was to investigate the in vivo performance of a 400 MHz cryogenic transmit/receive RF probe (CryoProbe) designed for MRI of the mouse brain. To characterize this novel probe, MR data sets were acquired with both the CryoProbe and a matched conventional receive‐only surface coil operating at room temperature (RT) using conventional acquisition protocols (gradient and spin echo) with identical parameter settings. Quantitative comparisons in phantom and in vivo experiments revealed gains in the signal‐to‐noise ratio (SNR) of 2.4 and 2.5, respectively. The increased sensitivity of the CryoProbe was invested to enhance the image quality of high resolution structural images acquired in scan times compatible with routine operation (< 45 min). In high resolution (30 × 30 × 300 µm³) structural images of the mouse cerebellum, anatomical details such as Purkinje cell and molecular layers could be identified. Similarly, isotropic (60 × 60 × 60 µm³) imaging of mouse cortical and subcortical areas revealed anatomical structures smaller than 100 µm. Finally, 3D MR angiography (52 × 80 × 80 µm³) of the brain vasculature enabled the detailed reconstruction of intracranial vessels (anterior and middle cerebral artery). In conclusion, this low temperature detection device represents an attractive option to increase the performance of small animal MR systems operating at 9.4 Tesla. Copyright © 2009 John Wiley & Sons, Ltd.     

Self‐gated cardiac Cine MRI of the rat on a clinical 3 T MRI system

The ability to perform small animal functional cardiac imaging on clinical MRI scanners may be of particular value in cases in which the availability of a dedicated high field animal MRI scanner is limited. Here, we propose radial MR cardiac imaging in the rat on a whole‐body clinical 3 T scanner in combination with interspersed projection navigators for self‐gating without any additional external triggering requirements for electrocardiogram (ECG) and respiration. Single navigator readouts were interspersed using the same TR and a high navigator frequency of 54 Hz into a radial golden‐angle acquisition. The extracted navigator function was thresholded to exclude data for reconstruction from inhalation phases during the breathing cycle, enabling free breathing acquisition. To minimize flow artifacts in the dynamic cine images a center‐out half echo radial acquisition scheme with ramp sampling was used. Navigator functions were derived from the corresponding projection navigator data from which both respiration and cardiac cycles were extracted. Self‐gated cine acquisition resulted in high‐quality cardiac images which were free of major artifacts with spatial resolution of up to 0.21 × 0.21 × 1.00 mm³ and a contrast‐to‐noise ratio (CNR) of 21 ± 3 between the myocardium and left ventricle. Self‐gated golden ratio based radial acquisition successfully acquired cine images of the rat heart on a clinical MRI system without the need for dedicated animal ECG equipment. Copyright © 2014 John Wiley & Sons, Ltd.     

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.     

MRS: a noninvasive window into cardiac metabolism

A well‐functioning heart requires a constant supply of a balanced mixture of nutrients to be used for the production of adequate amounts of adenosine triphosphate, which is the main energy source for most cellular functions. Defects in cardiac energy metabolism are linked to several myocardial disorders. MRS can be used to study in vivo changes in cardiac metabolism noninvasively. MR techniques allow repeated measurements, so that disease progression and the response to treatment or to a lifestyle intervention can be monitored. It has also been shown that MRS can predict clinical heart failure and death. This article focuses on in vivo MRS to assess cardiac metabolism in humans and experimental animals, as experimental animals are often used to investigate the mechanisms underlying the development of metabolic diseases. Various MR techniques, such as cardiac ³¹P‐MRS, ¹H‐MRS, hyperpolarized ¹³C‐MRS and Dixon MRI, are described. A short overview of current and emerging applications is given. Cardiac MRS is a promising technique for the investigation of the relationship between cardiac metabolism and cardiac disease. However, further optimization of scan time and signal‐to‐noise ratio is required before broad clinical application. In this respect, the ongoing development of advanced shimming algorithms, radiofrequency pulses, pulse sequences, (multichannel) detection coils, the use of hyperpolarized nuclei and scanning at higher magnetic field strengths offer future perspective for clinical applications of MRS. Copyright © 2015 John Wiley & Sons, Ltd.     

Assessing muscle-specific potassium concentrations in human lower leg using potassium magnetic resonance imaging

Noninvasively assessing tissue potassium concentrations (TPCs) using potassium magnetic resonance imaging (³⁹K MRI) could give valuable information on physiological processes connected to various pathologies. However, because of inherently low ³⁹K MR image resolution and strong signal blurring, a reliable measurement of the TPC is challenging. The aim of this work was to investigate the feasibility of a muscle-specific TPC determination with a focus on the influence of a varying residual quadrupolar interaction in human lower leg muscles. The quantification accuracy of a muscle-specific TPC determination was first assessed using simulated ³⁹K MRI data. In vivo ³⁹K and corresponding sodium (²³Na) MRI data of healthy lower leg muscles (n = 14, seven females) were acquired on a 7-T MR system using a double-resonant ²³Na/³⁹K birdcage Tx/Rx RF coil. Additional ¹H MR images were acquired on a 3-T MR system and used for tissue segmentation. Quantification of TPC was performed after a region-based partial volume correction (PVC) using five external reference phantoms. Simulations not only underlined the importance of PVC for correctly assessing muscle-specific TPC values, but also revealed the strong impact of a varying residual quadrupolar interaction between different muscle regions on the measured TPC. Using ³⁹K T₂^* decay curves, we found significantly higher residual quadrupolar interaction in tibialis anterior muscle (TA; ω_q = 194 ± 28 Hz) compared with gastrocnemius muscle (medial/lateral head, GM/GL; ω_q = 151 ± 25 Hz) and soleus muscle (SOL; ω_q = 102 ± 32 Hz). If considered in the PVC, TPC in individual muscles was similar (TPC = 98 ± 11/96 ± 14/99 ± 8/100 ± 12 mM in GM/GL/SOL/TA). Comparison with tissue sodium concentrations suggested that residual quadrupolar interactions might also influence the ²³Na MRI signal of lower leg muscles. A TPC determination of individual lower leg muscles is feasible and can therefore be applied in future studies. Considering a varying residual quadrupolar interaction for PVC of ³⁹K MRI data is essential to reliably assess potassium concentrations in individual muscles.     

Wideband Self‐Grounded Bow‐Tie Antenna for Thermal MR

The objective of this study was the design, implementation, evaluation and application of a compact wideband self‐grounded bow‐tie (SGBT) radiofrequency (RF) antenna building block that supports anatomical proton (¹H) MRI, fluorine (¹⁹F) MRI, MR thermometry and broadband thermal intervention integrated in a whole‐body 7.0 T system. Design considerations and optimizations were conducted with numerical electromagnetic field (EMF) simulations to facilitate a broadband thermal intervention frequency of the RF antenna building block. RF transmission (B₁⁺) field efficiency and specific absorption rate (SAR) were obtained in a phantom, and the thigh of human voxel models (Ella, Duke) for ¹H and ¹⁹F MRI at 7.0 T. B₁⁺ efficiency simulations were validated with actual flip‐angle imaging measurements. The feasibility of thermal intervention was examined by temperature simulations (f = 300, 400 and 500 MHz) in a phantom. The RF heating intervention (P_in = 100 W, t = 120 seconds) was validated experimentally using the proton resonance shift method and fiberoptic probes for temperature monitoring. The applicability of the SGBT RF antenna building block for in vivo ¹H and ¹⁹F MRI was demonstrated for the thigh and forearm of a healthy volunteer. The SGBT RF antenna building block facilitated ¹⁹F and ¹H MRI at 7.0 T as well as broadband thermal intervention (234‐561 MHz). For the thigh of the human voxel models, a B₁⁺ efficiency ≥11.8 μT/√kW was achieved at a depth of 50 mm. Temperature simulations and heating experiments in a phantom demonstrated a temperature increase ΔT >7 K at a depth of 10 mm. The compact SGBT antenna building block provides technology for the design of integrated high‐density RF applicators and for the study of the role of temperature in (patho‐) physiological processes by adding a thermal intervention dimension to an MRI device (Thermal MR).     

Three‐dimensional accurate detection of lung emphysema in rats using ultra‐short and zero echo time MRI

Emphysema is a life‐threatening pathology that causes irreversible destruction of alveolar walls. In vivo imaging techniques play a fundamental role in the early non‐invasive pre‐clinical and clinical detection and longitudinal follow‐up of this pathology. In the present study, we aimed to evaluate the feasibility of using high resolution radial three‐dimensional (3D) zero echo time (ZTE) and 3D ultra‐short echo time (UTE) MRI to accurately detect lung pathomorphological changes in a rodent model of emphysema.Porcine pancreas elastase (PPE) was intratracheally administered to the rats to produce the emphysematous changes. 3D ZTE MRI, low and high definition 3D UTE MRI and micro‐computed tomography images were acquired 4 weeks after the PPE challenge. Signal‐to‐noise ratios (SNRs) were measured in PPE‐treated and control rats. T₂* values were computed from low definition 3D UTE MRI. Histomorphometric measurements were made after euthanizing the animals. Both ZTE and UTE MR images showed a significant decrease in the SNR measured in PPE‐treated lungs compared with controls, due to the pathomorphological changes taking place in the challenged lungs. A significant decrease in T₂* values in PPE‐challenged animals compared with controls was measured using UTE MRI. Histomorphometric measurements showed a significant increase in the mean linear intercept in PPE‐treated lungs. UTE yielded significantly higher SNR compared with ZTE (14% and 30% higher in PPE‐treated and non‐PPE‐treated lungs, respectively).This study showed that optimized 3D radial UTE and ZTE MRI can provide lung images of excellent quality, with high isotropic spatial resolution (400 µm) and SNR in parenchymal tissue (>25) and negligible motion artifacts in freely breathing animals. These techniques were shown to be useful non‐invasive instruments to accurately and reliably detect the pathomorphological alterations taking place in emphysematous lungs, without incurring the risks of cumulative radiation exposure typical of micro‐computed tomography. Copyright © 2015 John Wiley & Sons, Ltd.     

Extended RF shimming: Sequence‐level parallel transmission optimization applied to steady‐state free precession MRI of the heart

Cardiac magnetic resonance imaging (MRI) at high field presents challenges because of the high specific absorption rate and significant transmit field (B₁⁺) inhomogeneities. Parallel transmission MRI offers the ability to correct for both issues at the level of individual radiofrequency (RF) pulses, but must operate within strict hardware and safety constraints. The constraints are themselves affected by sequence parameters, such as the RF pulse duration and TR, meaning that an overall optimal operating point exists for a given sequence. This work seeks to obtain optimal performance by performing a ‘sequence‐level’ optimization in which pulse sequence parameters are included as part of an RF shimming calculation. The method is applied to balanced steady‐state free precession cardiac MRI with the objective of minimizing TR, hence reducing the imaging duration. Results are demonstrated using an eight‐channel parallel transmit system operating at 3 T, with an in vivo study carried out on seven male subjects of varying body mass index (BMI). Compared with single‐channel operation, a mean‐squared‐error shimming approach leads to reduced imaging durations of 32 ± 3% with simultaneous improvement in flip angle homogeneity of 32 ± 8% within the myocardium.     

Real‐time multi‐directional flow MRI using model‐based reconstructions of undersampled radial FLASH – A feasibility study

The purpose of this work was to develop an acquisition and reconstruction technique for two‐ and three‐directional (2d and 3d) phase‐contrast flow MRI in real time. A previous real‐time MRI technique for one‐directional (1d) through‐plane flow was extended to 2d and 3d flow MRI by introducing in‐plane flow sensitivity. The method employs highly undersampled radial FLASH sequences with sequential acquisitions of two or three flow‐encoding datasets and one flow‐compensated dataset. Echo times are minimized by merging the waveforms of flow‐encoding and radial imaging gradients. For each velocity direction individually, model‐based reconstructions by regularized nonlinear inversion jointly estimate an anatomical image, a set of coil sensitivities and a phase‐contrast velocity map directly. The reconstructions take advantage of a dynamic phase reference obtained by interpolating consecutive flow‐compensated acquisitions. Validations include pulsatile flow phantoms as well as in vivo studies of the human aorta at 3 T. The proposed method offers cross‐sectional 2d and 3d flow MRI of the human aortic arch at 53 and 67 ms resolution, respectively, without ECG synchronization and during free breathing. The in‐plane resolution was 1.5 × 1.5 mm² and the slice thickness 6 mm. In conclusion, real‐time multi‐directional flow MRI offers new opportunities to study complex human blood flow without the risk of combining differential phase (i.e., velocity) information from multiple heartbeats as for ECG‐gated data. The method would benefit from a further reduction of acquisition time and accelerated computing to allow for extended clinical trials.     

Real-time phase-contrast MRI of cardiovascular blood flow using undersampled radial fast low-angle shot and nonlinear inverse reconstruction

Velocity‐encoded phase‐contrast MRI of cardiovascular blood flow commonly relies on electrocardiogram‐synchronized cine acquisitions of multiple heartbeats to quantitatively determine the flow of an averaged cardiac cycle. Here, we present a new method for real‐time phase‐contrast MRI that combines flow‐encoding gradients with highly undersampled radial fast low‐angle shot acquisitions and phase‐sensitive image reconstructions by regularized nonlinear inversion. Apart from calibration studies using steady and pulsatile flow, preliminary in vivo applications focused on through‐plane flow in the ascending aorta of healthy subjects. With bipolar velocity‐encoding gradients of alternating polarity that overlap the slice‐refocusing gradient, the method yields flow‐encoded images with an in‐plane resolution of 1.8 mm, section thickness of 6 mm and measuring time at 3 T of 24 ms (TR/TE = 3.44/2.76 ms; flip angle, 10º; seven radial spokes per image). Accordingly, phase‐contrast maps and corresponding velocity profiles achieve a temporal resolution of 48 ms. The evaluated peak velocities, stroke volumes, flow rates and respective variances over at least 20 consecutive heartbeats are in general agreement with literature data. Copyright © 2011 John Wiley & Sons, Ltd.     

Quantification of MRI signal of transgenic grafts overexpressing ferritin in murine myocardial infarcts

The noninvasive detection of transplanted cells in damaged organs and the longitudinal follow‐up of cell fate and graft size are important for the evaluation of cell therapy. We have shown previously that the overexpression of the natural iron storage protein, ferritin, permits the detection of engrafted cells in mouse heart by MRI, but further imaging optimization is required. Here, we report a systematic evaluation of ferritin‐based stem cell imaging in infarcted mouse hearts in vivo using three cardiac‐gated pulse sequences in a 3‐T scanner: black‐blood proton‐density‐weighted turbo spin echo (PD TSE BB), bright‐blood T₂*‐weighted gradient echo (GRE) and black‐blood T₂*‐weighted GRE with improved motion‐sensitized‐driven equilibrium (iMSDE) preparation. Transgenic C2C12 myoblast grafts overexpressing ferritin did not change MRI contrast in the PD TSE BB images, but showed a 20% reduction in signal intensity ratio in black‐blood T₂*‐weighted iMSDE (p < 0.05) and a 30% reduction in bright‐blood T₂*‐weighted GRE (p < 0.0001). Graft size measurements by T₂* iMSDE and T₂* GRE were highly correlated with histological assessments (r = 0.79 and r = 0.89, respectively). Unlabeled wild‐type C2C12 cells transplanted to mouse heart did not change the MRI signal intensity, although endogenous hemosiderin was seen in some infarcts. These data support the use of ferritin to track the survival, growth and migration of stem cells transplanted into the injured heart. Copyright © 2012 John Wiley & Sons, Ltd.     

In vivo MRI analysis of depth‐dependent ultrastructure in human knee cartilage at 7 T

Signal intensities of T₂‐weighted magnetic resonance images depend on the local fiber arrangement in hyaline cartilage. The aims of this study were to determine whether angle‐sensitive MRI at 7 T can be used to quantify the cartilage ultrastructure of the knee in vivo and to assess potential differences with age. Ten younger (21–30) and ten older (55–76 years old) healthy volunteers were imaged with a T₂‐weighted spin‐echo sequence in a 7 T whole‐body MRI. A “fascicle” model was assumed to describe the depth‐dependent fiber arrangement of cartilage. The R/T boundary positions between radial and transitional zones were assessed from intensity profiles in small regions of interest in the femur and tibia, and normalized to cartilage thickness using logistic curve fits. The quality of our highly resolved (0.3 × 0.3 × 1.0 mm³) MR cartilage images were high enough for quantitative analysis (goodness of fit R² = 0.91 ± 0.09). Between younger and older subjects, normalized positions of the R/T boundary, with value 0 at the bone–cartilage interface and 1 at the cartilage surface, were significantly (p < 0.05) different in femoral (0.51 ± 0.12 versus 0.41 ± 0.10), but not in tibial cartilage (0.65 ± 0.11 versus 0.57 ± 0.09, p = 0.119). Within both age groups, differences between femoral and tibial R/T boundaries were significant. Using a fascicle model and angle‐sensitive MRI, the depth‐dependent anisotropic fiber arrangement of knee cartilage could be assessed in vivo from a single 7 T MR image. The derived quantitative parameter, thickness of the radial zone, may serve as an indicator of the structural integrity of cartilage. This method may potentially be suitable to detect and monitor early osteoarthritis because the progressive disintegration of the anisotropic network is also indicative of arthritic changes in cartilage. Copyright © 2013 John Wiley & Sons, Ltd.