A characterization of ABL‐101 as a potential tracer for clinical fluorine‐19 MRI

The two main challenges that prevent the translation of fluorine‐19 (¹⁹F) MRI for inflammation monitoring or cell tracking into clinical practice are (i) the relatively low signal‐to‐noise ratio generated by the injected perfluorocarbon (PFC), which necessitates long scan times, and (ii) the need for regulatory approval and a high biocompatibility of PFCs that are also suitable for MRI. ABL‐101, an emulsion of perfluoro(t‐butylcyclohexane), is a third‐generation PFC that is already used in clinical trials, but has not yet been used for ¹⁹F MRI. The objective of this study was therefore to assess the performance of ABL‐101 as a ¹⁹F MRI tracer. At magnetic field strengths of 3, 9.4 and 14.1 T, the CF₃ groups of ABL‐101 generated a large well‐separated singlet with T₂/T₁ ratios of >0.27, >0.14 and > 0.05, respectively. All relaxation times decreased with the increase in magnetic field strength. The detection limit of ABL‐101 in a 0.25 mm³ voxel at 3 T, 37°C and with a 3‐minute acquisition time was 7.21mM. After intravenous injection, the clearance half‐lives of the ABL‐101 ¹⁹F MR signal in mouse (n = 3) spleen and liver were 6.85 ± 0.45 and 3.20 ± 0.35 days, respectively. These results demonstrate that ABL‐101 has ¹⁹F MR characteristics that are similar to those of PFCs developed specifically for MRI, while it has clearance half‐lives similar to PFCs that have previously been used in large doses in non‐MRI clinical trials. Overall, ABL‐101 is thus a very promising candidate tracer for future clinical trials that use ¹⁹F MRI for cell tracking or the monitoring of inflammation.     

Probing different perfluorocarbons for in vivo inflammation imaging by 19F MRI: image reconstruction,biological half‐lives and sensitivity

Inflammatory processes can reliably be assessed by ¹⁹F MRI using perfluorocarbons (PFCs), which is primarily based on the efficient uptake of emulsified PFCs by circulating cells of the monocyte–macrophage system and subsequent infiltration of the ¹⁹F‐labeled cells into affected tissue. An ideal candidate for the sensitive detection of fluorine‐loaded cells is the biochemically inert perfluoro‐15‐crown‐5 ether (PFCE), as it contains 20 magnetically equivalent ¹⁹F atoms. However, the biological half‐life of PFCE in the liver and spleen is extremely long, and so this substance is not suitable for future clinical applications. In the present study, we investigated alternative, nontoxic PFCs with predicted short biological half‐lives and high fluorine content: perfluorooctyl bromide (PFOB), perfluorodecalin (PFD) and trans‐bis‐perfluorobutyl ethylene (F‐44E). Despite the complex spectra of these compounds, we obtained artifact‐free images using sine‐squared acquisition‐weighted three‐dimensional chemical shift imaging and dedicated reconstruction accomplished with in‐house‐developed software. The signal‐to‐noise ratio of the images was maximized using a Nutall window with only moderate localization error. Using this approach, the retention times of the different PFCs in murine liver and spleen were determined at 9.4 T. The biological half‐lives were estimated to be 9 days (PFD), 12 days (PFOB) and 28 days (F‐44E), compared with more than 250 days for PFCE. In vivo sensitivity for inflammation imaging was assessed using an ear clip injury model. The alternative PFCs PFOB and F‐44E provided 37% and 43%, respectively, of the PFCE intensities, whereas PFD did not show any signal in the ear model. Thus, for in vivo monitoring of inflammatory processes, PFOB emerges as the most promising candidate for possible future translation of ¹⁹F MR inflammation imaging to human applications. Copyright © 2013 John Wiley & Sons, Ltd.     

Wireless coils based on resonant and nonresonant coupled‐wire structure for small animal multinuclear imaging

Earlier work on RF metasurfaces for preclinical MRI has targeted applications such as whole‐body imaging and dual‐frequency coils. In these studies, a nonresonant loop was used to induce currents into a metasurface that was operated as a passive inductively powered resonator. However, as we show in this study, the strategy of using a resonant metasurface reduces the impact of the loop on the global performance of the assembled coil. To mitigate this deficiency, we developed a new approach that relies on the combination of a commercial surface coil and a coupled‐wire structure operated away from its resonance. This strategy enables the extension of the sensitive volume of the surface coil while maintaining its local high sensitivity without any hardware modification. A wireless coil based on a two parallel coupled‐wire structure was designed and electromagnetic field simulations were carried out with different levels of matching and coupling between both components of the coil. For experimental characterization, a prototype was built and tested at two frequencies, 300 MHz for ¹H and 282.6 MHz for ¹⁹F at 7 T. Phantom and in vivo MRI experiments were conducted in different configurations to study signal and noise figures of the structure. The results showed that the proposed strategy improves the overall sensitive volume while simultaneously maintaining a high signal‐to‐noise ratio (SNR). Metasurfaces based on coupled wires are therefore shown here as promising and versatile elements in the MRI RF chain, as they allow customized adjustment of the sensitive volume as a function of SNR yield. In addition, they can be easily adapted to different Larmor frequencies without loss of performance.     

Density‐weighted concentric rings k‐space trajectory for 1H magnetic resonance spectroscopic imaging at 7 T

It has been shown that density‐weighted (DW) k‐space sampling with spiral and conventional phase encoding trajectories reduces spatial side lobes in magnetic resonance spectroscopic imaging (MRSI). In this study, we propose a new concentric ring trajectory (CRT) for DW‐MRSI that samples k‐space with a density that is proportional to a spatial, isotropic Hanning window. The properties of two different DW‐CRTs were compared against a radially equidistant (RE) CRT and an echo‐planar spectroscopic imaging (EPSI) trajectory in simulations, phantoms and in vivo experiments. These experiments, conducted at 7 T with a fixed nominal voxel size and matched acquisition times, revealed that the two DW‐CRT designs improved the shape of the spatial response function by suppressing side lobes, also resulting in improved signal‐to‐noise ratio (SNR). High‐quality spectra were acquired for all trajectories from a specific region of interest in the motor cortex with an in‐plane resolution of 7.5 × 7.5 mm² in 8 min 3 s. Due to hardware limitations, high‐spatial‐resolution spectra with an in‐plane resolution of 5 × 5 mm² and an acquisition time of 12 min 48 s were acquired only for the RE and one of the DW‐CRT trajectories and not for EPSI. For all phantom and in vivo experiments, DW‐CRTs resulted in the highest SNR. The achieved in vivo spectral quality of the DW‐CRT method allowed for reliable metabolic mapping of eight metabolites including N‐acetylaspartylglutamate, γ‐aminobutyric acid and glutathione with Cramér‐Rao lower bounds below 50%, using an LCModel analysis. Finally, high‐quality metabolic mapping of a whole brain slice using DW‐CRT was achieved with a high in‐plane resolution of 5 × 5 mm² in a healthy subject. These findings demonstrate that our DW‐CRT MRSI technique can perform robustly on MRI systems and within a clinically feasible acquisition time.     

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.     

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.     

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.     

19 F MRSI of capecitabine in the liver at 7 T using broadband transmit–receive antennas and dual‐band RF pulses

Capecitabine (Cap) is an often prescribed chemotherapeutic agent, successfully used to cure some patients from cancer or reduce tumor burden for palliative care. However, the efficacy of the drug is limited, it is not known in advance who will respond to the drug and it can come with severe toxicity. ¹⁹ F Magnetic Resonance Spectroscopy (MRS) and Magnetic Resonance Spectroscopic Imaging (MRSI) have been used to non‐invasively study Cap metabolism in vivo to find a marker for personalized treatment. In vivo detection, however, is hampered by low concentrations and the use of radiofrequency (RF) surface coils limiting spatial coverage. In this work, the use of a 7T MR system with radiative multi‐channel transmit–receive antennas was investigated with the aim of maximizing the sensitivity and spatial coverage of ¹⁹ F detection protocols. The antennas were broadband optimized to facilitate both the ¹H (298 MHz) and ¹⁹ F (280 MHz) frequencies for accurate shimming, imaging and signal combination. B₁⁺ simulations, phantom and noise measurements showed that more than 90% of the theoretical maximum sensitivity could be obtained when using B₁⁺ and B₁^? information provided at the ¹H frequency for the optimization of B₁⁺ and B₁^? at the ¹⁹ F frequency. Furthermore, to overcome the limits in maximum available RF power, whilst ensuring simultaneous excitation of all detectable conversion products of Cap, a dual‐band RF pulse was designed and evaluated. Finally, ¹⁹ F MRS(I) measurements were performed to detect ¹⁹ F metabolites in vitro and in vivo. In two patients, at 10 h (patient 1) and 1 h (patient 2) after Cap intake, ¹⁹ F metabolites were detected in the liver and the surrounding organs, illustrating the potential of the set‐up for in vivo detection of metabolic rates and drug distribution in the body. Copyright © 2015 John Wiley & Sons, Ltd.     

Compressed sensing with signal averaging for improved sensitivity and motion artifact reduction in fluorine‐19 MRI

Fluorine‐19 (¹⁹F) MRI of injected perfluorocarbon emulsions (PFCs) allows for the non‐invasive quantification of inflammation and cell tracking, but suffers from a low signal‐to‐noise ratio and extended scan time. To address this limitation, we tested the hypotheses that a ¹⁹F MRI pulse sequence that combines a specific undersampling regime with signal averaging has both increased sensitivity and robustness against motion artifacts compared with a non‐averaged fully sampled pulse sequence, when both datasets are reconstructed with compressed sensing. As a proof of principle, numerical simulations and phantom experiments were performed on selected variable ranges to characterize the point spread function of undersampling patterns, as well as the vulnerability to noise of undersampling and reconstruction parameters with paired numbers of x signal averages and acceleration factor x (NAx ‐AFx ). The numerical simulations demonstrated that a probability density function that uses 25% of the samples to fully sample the k‐space central area allowed for an optimal balance between limited blurring and artifact incoherence. At all investigated noise levels, the Dice similarity coefficient (DSC) strongly depended on the regularization parameters and acceleration factor. In phantoms, the motion robustness of an NA8‐AF8 undersampling pattern versus NA1‐AF1 was evaluated with simulated and real motion patterns. Differences were assessed with the DSC, which was consistently higher for the NA8‐AF8 compared with the NA1‐AF1 strategy, for both simulated and real cyclic motion patterns (P < 0.001). Both strategies were validated in vivo in mice (n = 2) injected with perfluoropolyether. Here, the images displayed a sharper delineation of the liver with the NA8‐AF8 strategy than with the NA1‐AF1 strategy. In conclusion, we validated the hypotheses that in ¹⁹F MRI the combination of undersampling and averaging improves both the sensitivity and the robustness against motion artifacts.     

Optimized truncation to integrate multi‐channel MRS data using rank‐R singular value decomposition

Multi‐channel phased receive arrays have been widely adopted for magnetic resonance imaging (MRI) and spectroscopy (MRS). An important step in the use of receive arrays for MRS is the combination of spectra collected from individual coil channels. The goal of this work was to implement an improved strategy termed OpTIMUS (i.e., op timized t runcation to i ntegrate m ulti‐channel MRS data u sing rank‐R s ingular value decomposition) for combining data from individual channels. OpTIMUS relies on spectral windowing coupled with a rank‐R decomposition to calculate the optimal coil channel weights. MRS data acquired from a brain spectroscopy phantom and 11 healthy volunteers were first processed using a whitening transformation to remove correlated noise. Whitened spectra were then iteratively windowed or truncated, followed by a rank‐R singular value decomposition (SVD) to empirically determine the coil channel weights. Spectra combined using the vendor‐supplied method, signal/noise² weighting, previously reported whitened SVD (rank‐1), and OpTIMUS were evaluated using the signal‐to‐noise ratio (SNR). Significant increases in SNR ranging from 6% to 33% (P ≤ 0.05) were observed for brain MRS data combined with OpTIMUS compared with the three other combination algorithms. The assumption that a rank‐1 SVD maximizes SNR was tested empirically, and a higher rank‐R decomposition, combined with spectral windowing prior to SVD, resulted in increased SNR.     

Knee MRI under varying flexion angles utilizing a flexible flat cable antenna

The aim of this study is to fabricate and test a novel flexible flat cable antenna (FFCA) for MRI of the knee at different flexion angles. The FFCA was made of a flat cable, a tuning/matching circuit and a signal transmission line. To test its feasibility and validity, in vitro and in vivo experiments were carried out on a 3.0 T MR scanner. The in vitro experiment suggested that the proposed FFCA could achieve a high signal‐to‐noise ratio (SNR) of 336, while the SNR of an eight‐channel knee coil was 291, and phantom images from the FFCA are homogeneously distributed. In the in vivo experiment, the FFCA had a higher SNR of 169 in the region of interest and more than 48.5 cm of longitudinal coverage, while the corresponding values for the commercial coil were 153 and 22.5 cm. Finally, five sagittal knee images at different flexion angles were acquired. The FFCA could acquire satisfactory knee images at different flexion angles, with the advantages of simplicity, low cost, large field of view and high SNR. It may therefore be further used to improve MR image quality of the knee joint. Copyright © 2015 John Wiley & Sons, Ltd.     

Noise correlations and SNR in phased‐array MRS

The acquisition of magnetic resonance spectroscopy (MRS) signals by multiple receiver coils can improve the signal‐to‐noise ratio (SNR) or alternatively can reduce the scan time maintaining a reliable SNR. However, using phased array coils in MRS studies requires efficient data processing and data combination techniques in order to exploit the sensitivity improvement of the phased array coil acquisition method. This paper describes a novel method for the combination of MRS signals acquired by phased array coils, even in presence of correlated noise between the acquisition channels. In fact, although it has been shown that electric and magnetic coupling mechanisms produce correlated noise in the coils, previous algorithms developed for MRS data combination have ignored this effect. The proposed approach takes advantage of a noise decorrelation stage to maximize the SNR of the combined spectra. In particular Principal Component Analysis (PCA) was exploited to project the acquired spectra in a subspace where the noise vectors are orthogonal. In this subspace the SNR weighting method will provide the optimal overall SNR. Performance evaluation of the proposed method is carried out on simulated ¹H‐MRS signals and experimental results are obtained on phantom ¹H‐MR spectra using a commercially available 8‐element phased array coil. Noise correlations between elements were generally low due to the optimal coil design, leading to a fair SNR gain (about 0.5%) in the center of the field of view (FOV). A greater SNR improvement was found in the peripheral FOV regions. Copyright © 2009 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.     

Quantification issues of in vivo 1H NMR spectroscopy of the rat brain investigated at 16.4 T

The accuracy and precision of the quantification of metabolite concentrations in in vivo ¹H NMR spectroscopy are affected by linewidth and signal‐to‐noise ratio (SNR). To study the effect of both factors in in vivo ¹H NMR spectra acquired at ultrahigh field, a reference spectrum was generated by summing nine in vivo ¹H NMR spectra obtained in rat brain with a STEAM sequence at 16.4 T. By progressive deterioration of linewidth and SNR, 6400 single spectra were generated. In an accuracy study, the variation in the mean concentrations of five metabolites was mainly dependent on SNR, whereas 11 metabolites were predominantly susceptible to the linewidth. However, the standard deviations of the concentrations obtained were dependent almost exclusively on the SNR. An insignificant correlation was found between most of the heavily overlapping metabolite peaks, indicating independent and reliable quantification. Two different approaches for the consideration of macromolecular signals were evaluated. The use of prior knowledge derived by parameterization of a metabolite‐nulled spectrum demonstrated improved fitting quality, with reduced Cramér–Rao lower bounds, compared to the calculation of a regularized spline baseline. Copyright © 2012 John Wiley & Sons, Ltd.     

Brain temperature by Biosensor Imaging of Redundant Deviation in Shifts (BIRDS): comparison between TmDOTP5−and TmDOTMA−

Chemical shifts of complexes between paramagnetic lanthanide ions and macrocyclic chelates are sensitive to physiological variations (of temperature and/or pH). Here we demonstrate utility of a complex between thulium ion (Tm³⁺) and the macrocyclic chelate 1,4,7,10‐tetramethyl 1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetraacetate (or DOTMA^4?) for absolute temperature mapping in rat brain. The feasibility of TmDOTMA^? is compared with that of another Tm³⁺‐containing biosensor which is based on the macrocyclic chelate 1,4,7,10‐tetraazacyclododecane‐ 1,4,7,10‐tetrakis(methylene phosphonate) (or DOTP^8?). In general, the in vitro and in vivo results suggest that Biosensor Imaging of Redundant Deviation in Shifts (BIRDS) which originate from these agents (but exclude water) can provide temperature maps with good accuracy. While TmDOTP^5? emanates three major distinct proton resonances which are differentially sensitive to temperature and pH, TmDOTMA^? has a dominant pH‐insensitive proton resonance from a ? CH₃ group to allow higher signal‐to‐noise ratio (SNR) temperature assessment. Temperature (and pH) sensitivities of these resonances are practically identical at low (4.0T) and high (11.7T) magnetic fields and at nominal repetition times only marginal SNR loss is expected at the lower field. Since these resonances have extremely short relaxation times, high‐speed chemical shift imaging (CSI) is needed to detect them. Repeated in vivo CSI scans with BIRDS demonstrate excellent measurement stability. Overall, results with TmDOTP^5? and TmDOTMA^? suggest that BIRDS can be reliably applied, either at low or high magnetic fields, for functional studies in rodents. Copyright © 2009 John Wiley & Sons, Ltd.     

The effects of noise in cardiac diffusion tensor imaging and the benefits of averaging complex data

There is growing interest in cardiac diffusion tensor imaging (cDTI), but, unlike other diffusion MRI applications, there has been little investigation of the effects of noise on the parameters typically derived. One method of mitigating noise floor effects when there are multiple image averages, as in cDTI, is to average the complex rather than the magnitude data, but the phase contains contributions from bulk motion, which must be removed first. The effects of noise on the mean diffusivity (MD), fractional anisotropy (FA), helical angle (HA) and absolute secondary eigenvector angle (E2A) were simulated with various diffusion weightings (b values). The effect of averaging complex versus magnitude images was investigated. In vivo cDTI was performed in 10 healthy subjects with b = 500, 1000, 1500 and 2000 s/mm². A technique for removing the motion‐induced component of the image phase present in vivo was implemented by subtracting a low‐resolution copy of the phase from the original images before averaging the complex images. MD, FA, E2A and the transmural gradient in HA were compared for un‐averaged, magnitude‐ and complex‐averaged reconstructions. Simulations demonstrated an over‐estimation of FA and MD at low b values and an under‐estimation at high b values. The transition is relatively signal‐to‐noise ratio (SNR) independent and occurs at a higher b value for FA (b = 1000–1250 s/mm²) than MD (b ≈ 250 s/mm²). E2A is under‐estimated at low and high b values with a transition at b ≈ 1000 s/mm², whereas the bias in HA is comparatively small. The under‐estimation of FA and MD at high b values is caused by noise floor effects, which can be mitigated by averaging the complex data. Understanding the parameters of interest and the effects of noise informs the selection of the optimal b values. When complex data are available, they should be used to maximise the benefit from the acquisition of multiple averages. The combination of complex data is also a valuable step towards segmented acquisitions. Copyright © 2016 John Wiley & Sons, Ltd.     

Boosting the SNR by adding a receive‐only endorectal monopole to an external antenna array for high‐resolution,T2‐weighted imaging of early‐stage cervical cancer with 7‐T MRI

The aim of this study was to investigate the signal‐to‐noise ratio (SNR) gain in early‐stage cervical cancer at ultrahigh‐field MRI (e.g. 7 T) using a combination of multiple external antennas and a single endorectal antenna. In particular, we used an endorectal monopole antenna to increase the SNR in cervical magnetic resonance imaging (MRI). This should allow high‐resolution, T₂‐weighted imaging and magnetic resonance spectroscopy (MRS) for metabolic staging, which could facilitate the local tumor status assessment. In a prospective feasibility study, five healthy female volunteers and six patients with histologically proven stage IB1–IIB cervical cancer were scanned at 7 T. We used seven external fractionated dipole antennas for transmit–receive (transceive) and an endorectally placed monopole antenna for reception only. A region of interest, containing both normal cervix and tumor tissue, was selected for the SNR measurement. Separated signal and noise measurements were obtained in the region of the cervix for each element and in the near field of the monopole antenna (radius < 30 mm) to calculate the SNR gain of the endorectal antenna in each patient. We obtained high‐resolution, T₂‐weighted images with a voxel size of 0.7 × 0.8 × 3.0 mm³. In four cases with optimal placement of the endorectal antenna (verified on the T₂‐weighted images), a mean gain of 2.2 in SNR was obtained at the overall cervix and tumor tissue area. Within a radius of 30 mm from the monopole antenna, a mean SNR gain of 3.7 was achieved in the four optimal cases. Overlap between the two different regions of the SNR calculations was around 24%. We have demonstrated that the use of an endorectal monopole antenna substantially increases the SNR of 7‐T MRI at the cervical anatomy. Combined with the intrinsically high SNR of ultrahigh‐field MRI, this gain may be employed to obtain metabolic information using MRS and to enhance spatial resolutions to assess tumor invasion.     

Simultaneous assessment of both lung morphometry and gas exchange function within a single breath‐hold by hyperpolarized 129Xe MRI

During the measurement of hyperpolarized ¹²⁹Xe magnetic resonance imaging (MRI), the diffusion‐weighted imaging (DWI) technique provides valuable information for the assessment of lung morphometry at the alveolar level, whereas the chemical shift saturation recovery (CSSR) technique can evaluate the gas exchange function of the lungs. To date, the two techniques have only been performed during separate breaths. However, the request for multiple breaths increases the cost and scanning time, limiting clinical application. Moreover, acquisition during separate breath‐holds will increase the measurement error, because of the inconsistent physiological status of the lungs. Here, we present a new method, referred to as diffusion‐weighted chemical shift saturation recovery (DWCSSR), in order to perform both DWI and CSSR within a single breath‐hold. Compared with sequential single‐breath schemes (namely the ‘CSSR + DWI’ scheme and the ‘DWI + CSSR’ scheme), the DWCSSR scheme is able to significantly shorten the breath‐hold time, as well as to obtain high signal‐to‐noise ratio (SNR) signals in both DWI and CSSR data. This scheme enables comprehensive information on lung morphometry and function to be obtained within a single breath‐hold. In vivo experimental results demonstrate that DWCSSR has great potential for the evaluation and diagnosis of pulmonary diseases.     

DTI of human skeletal muscle: the effects of diffusion encoding parameters,signal‐to‐noise ratio and T2 on tensor indices and fiber tracts

In this study, we have performed simulations to address the effects of diffusion encoding parameters, signal‐to‐noise ratio (SNR) and T₂ on skeletal muscle diffusion tensor indices and fiber tracts. Where appropriate, simulations were corroborated and validated by in vivo diffusion tensor imaging (DTI) of human skeletal muscle. Specifically, we have addressed: (i) the accuracy and precision of the diffusion parameters and eigenvectors at different SNR levels; (ii) the effects of the diffusion gradient direction encoding scheme; (iii) the optimal b value for diffusion tensor estimation; (iv) the effects of changes in skeletal muscle T₂; and, finally, the influence of SNR on fiber tractography and derived (v) fiber lengths, (vi) pennation angles and (vii) fiber curvatures. We conclude that accurate DTI of skeletal muscle requires an SNR of at least 25, a b value of between 400 and 500 s/mm², and data acquired with at least 12 diffusion gradient directions homogeneously distributed on half a sphere. Furthermore, for DTI studies focusing on skeletal muscle injury or pathology, apparent changes in the diffusion parameters need to be interpreted with great care in view of the confounding effects of T₂, particularly for moderate to low SNR values. Copyright © 2013 John Wiley & Sons, Ltd.     

A referenceless Nyquist ghost correction workflow for echo planar imaging of hyperpolarized [1‐13C]pyruvate and [1‐13C]lactate

Single‐shot echo planar imaging (EPI), which allows an image to be acquired using a single excitation pulse, is used widely for imaging the metabolism of hyperpolarized ¹³C‐labelled metabolites in vivo as the technique is rapid and minimizes the depletion of the hyperpolarized signal. However, EPI suffers from Nyquist ghosting, which normally is corrected for by acquiring a reference scan. In a dynamic acquisition of a series of images, this results in the sacrifice of a time point if the reference scan involves a full readout train with no phase encoding. This time penalty is negligible if an integrated navigator echo is used, but at the cost of a lower signal‐to‐noise ratio (SNR) as a result of prolonged T₂* decay. We describe here a workflow for hyperpolarized ¹³C EPI that requires no reference scan. This involves the selection of a ghost‐containing background from a ¹³C image of a single metabolite at a single time point, the identification of phase correction coefficients that minimize signal in the selected area, and the application of these coefficients to images acquired at all time points and from all metabolites. The workflow was compared in phantom experiments with phase correction using a ¹³C reference scan, and yielded similar results in situations with a regular field of view (FOV), a restricted FOV and where there were multiple signal sources. When compared with alternative phase correction methods, the workflow showed an SNR benefit relative to integrated ¹³C reference echoes (>15%) or better ghost removal relative to a ¹H reference scan. The residual ghosting in a slightly de‐shimmed B₀ field was 1.6% using the proposed workflow and 3.8% using a ¹H reference scan. The workflow was implemented with a series of dynamically acquired hyperpolarized [1‐¹³C]pyruvate and [1‐¹³C]lactate images in vivo, resulting in images with no observable ghosting and which were quantitatively similar to images corrected using a ¹³C reference scan.