Magnetization Transfer Contrast Research Articles

Relayed nuclear Overhauser enhancement imaging with magnetization transfer contrast suppression at 3 T.

To develop a pulsed CEST magnetization-transfer method for rapidly acquiring relayed nuclear Overhauser enhancement (rNOE)-weighted images with magnetic transfer contrast (MTC) suppression at clinical field strength (3 T). Using a pulsed CEST magnetization-transfer method with low saturation powers (B1 ) and long mixing time (tmix ) to suppress contributions due to strong MTC from solid-like macromolecules, a low B1 also minimized direct water saturation. These MTC contributions were further reduced by subtracting the Z-spectral signals at two or three offsets by assuming that the residual MTC is a linear function between -3.5 ppm and -12.5 ppm. Phantom studies of a lactic acid (Lac) solution mixed with cross-linked bovine serum albumin show that strong MTC interference has a significant impact on the optimum B1 for detecting rNOEs, due to lactate binding. The MTC could be effectively suppressed using a pulse train with a B1 of 0.8 μT, a pulse duration (tp ) of 40 ms, a tmix of 60 ms, and a pulse number (N) of 30, while rNOE signal was well maintained. As a proof of concept, we applied the method in mouse brain with injected hydrogel and a cell-hydrogel phantom. Results showed that rNOE-weighted images could provide good contrast between brain/cell and hydrogel. The developed pulsed CEST magnetization-transfer method can achieve MTC suppression while preserving most of the rNOE signal at 3 T, which indicates the potential for translation of this technique to clinical applications related to mobile proteins/lipids change.

A deep learning approach for magnetization transfer contrast MR fingerprinting and chemical exchange saturation transfer imaging

Semisolid magnetization transfer contrast (MTC) and chemical exchange saturation transfer (CEST) MRI based on MT phenomenon have shown potential to evaluate brain development, neurological, psychiatric, and neurodegenerative diseases. However, a qualitative MT ratio (MTR) metric commonly used in conventional MTC imaging is limited in the assessment of quantitative semisolid macromolecular proton exchange rates and concentrations. In addition, CEST signals measured by MTR asymmetry analysis are unavoidably contaminated by upfield nuclear Overhauser enhancement (NOE) signals of mobile and semisolid macromolecules. To address these issues, we developed an MTC-MR fingerprinting (MTC-MRF) technique to quantify tissue parameters, which further allows an estimation of accurate MTC signals at a certain CEST frequency offset. A pseudorandomized RF saturation scheme was used to generate unique MTC signal evolutions for different tissues and a supervised deep neural network was designed to extract tissue properties from measured MTC-MRF signals. Through detailed Bloch equation-based digital phantom and in vivo studies, we demonstrated that the MTC-MRF can quantify MTC characteristics with high accuracy and computational efficiency, compared to a conventional Bloch equation fitting approach, and provide baseline reference signals for CEST and NOE imaging. For validation, MTC-MRF images were synthesized using the tissue parameters estimated from the deep-learning method and compared with experimentally acquired MTC-MRF images as the reference standard. The proposed MTC-MRF framework can provide quantitative 3D MTC, CEST, and NOE imaging of the human brain within a clinically acceptable scan time.

Three-dimensional fast single-point macromolecular proton fraction mapping of the human brain at 0.5 Tesla.

Fast single-point macromolecular proton fraction (MPF) mapping is a recent magnetic resonance imaging (MRI) method enabling quantitative assessment of myelin content in neural tissues. To date, the reported technical implementations of MPF mapping utilized high-field MRI equipment (1.5 T or higher), while low-field applications might pose challenges due to signal-to-noise ratio (SNR) limitations and short T1 . This study aimed to evaluate the feasibility of MPF mapping of the human brain at 0.5 T. The three-dimensional MPF mapping protocol was implemented according to the single-point synthetic-reference method, which includes three spoiled gradient-echo sequences providing proton density, T1 , and magnetization transfer contrast weightings. Whole-brain MPF maps were obtained from three healthy volunteers with spatial resolution of 1.5×1.5×2 mm3 and the total scan time of 19 minutes. MPF values were measured in a series of white and gray matter structures and compared with literature data for 3 T magnetic field. MPF maps enabled high contrast between white and gray matter with notable insensitivity to paramagnetic effects in iron-rich structures, such as globus pallidus, substantia nigra, and dentate nucleus. MPF values at 0.5 T appeared in close agreement with those at 3 T. This study demonstrates the feasibility of fast MPF mapping with low-field MRI equipment and the independence of brain MPF values of magnetic field. The presented results confirm the utility of MPF as an absolute scale for MRI-based myelin content measurements across a wide range of magnetic field strengths and extend the applicability of fast MPF mapping to inexpensive low-field MRI hardware.

Role of the Extracellular Matrix in Loss of Muscle Force With Age and Unloading Using Magnetic Resonance Imaging, Biochemical Analysis, and Computational Models.

The focus of this review is the application of advanced MRI to study the effect of aging and disuse related remodeling of the extracellular matrix (ECM) on force transmission in the human musculoskeletal system. Structural MRI includes (i) ultra-low echo times (UTE) maps to visualize and quantify the connective tissue, (ii) diffusion tensor imaging (DTI) modeling to estimate changes in muscle and ECM microstructure, and (iii) magnetization transfer contrast imaging to quantify the macromolecular fraction in muscle. Functional MRI includes dynamic acquisitions during contraction cycles enabling computation of the strain tensor to monitor muscle deformation. Further, shear strain extracted from the strain tensor may be a potential surrogate marker of lateral transmission of force. Biochemical and histological analysis of muscle biopsy samples can provide “gold-standard” validation of some of the MR findings. The review summarizes biochemical studies of ECM adaptations with age and with disuse. A brief summary of animal models is included as they provide experimental confirmation of longitudinal and lateral force transmission pathways. Computational muscle models enable exploration of force generation and force pathways and elucidate the link between structural adaptations and functional consequences. MR image findings integrated in a computational model can explain and predict subject specific functional changes to structural adaptations. Future work includes development and validation of MRI biomarkers using biochemical analysis of muscle tissue as a reference standard and potential translation of the imaging markers to the clinic to noninvasively monitor musculoskeletal disease conditions and changes consequent to rehabilitative interventions.

Optimizing neuromelanin contrast in the substantia nigra and locus coeruleus using a magnetization transfer contrast prepared 3D gradient recalled echo sequence

Neuromelanin (NM) loss in the substantia nigra (SN) and locus coeruleus (LC) is being investigated as an imaging biomarker for Parkinson’s disease (PD) using magnetization transfer contrast (MTC) magnetic resonance imaging. The MTC pulse operates in a way to suppress tissue with high macromolecular content thereby highlighting the presence of NM in the LC and the SN. The MTC pulse also leads to a reduction in the effective T1 of the tissue. In the past, a 3D gradient echo (GRE) sequence has usually been run with a single flip angle (FA) generally to highlight the T1 shortening effect when trying to visualize NM. We contend that the NM will be best seen with a low FA (relative to the Ernst angle) because the NM has high water content relative to the surrounding tissues. Therefore, the goal of this paper was to optimize the NM contrast in the SN and LC as a function of flip angle using a 3D GRE MTC strategically acquired gradient echo (STAGE) imaging approach. In order to accomplish this, short repeat time (62 ​ms), 3D GRE imaging data were collected for 7 different flip angles ranging from 5° to 40° for 14 healthy volunteers (age range 24–43 years, mean ​± ​SD ​= ​34.8 ​± ​6.0 years, 6 males). By measuring the contrast-to-noise ratio between these structures and the surrounding tissues, we found that the FA showing the best NM contrast was 15° - 20° for the SN and 20° - 25° for the LC. Using STAGE imaging with just two flip angles (15° and 30°) made it possible to quantify not only tissue properties such as T1 and proton density but also to generate synthetic MTC images at an arbitrary FA. These synthetic images make it possible to optimize the contrast for any changes in tissue property that might occur in the LC or SN as a function of age or disease. In conclusion, practically, two scans could be collected in roughly 7 ​min each for both FAs in a standard clinical imaging setting to evaluate the signal intensity and volume of the NM in the LC and SN.

Magnetization transfer ratio quantifies polyneuropathy in hereditary transthyretin amyloidosis.

ObjectiveTo quantify peripheral nerve lesions in symptomatic and asymptomatic hereditary transthyretin amyloidosis with polyneuropathy (ATTRv‐PNP) by analyzing the magnetization transfer ratio (MTR) of the sciatic nerve, and to test its potential as a novel biomarker for macromolecular changes.MethodsTwenty‐five patients with symptomatic ATTRv‐PNP, 30 asymptomatic carriers of the mutant transthyretin gene (mutTTR), and 20 age‐/sex‐matched healthy controls prospectively underwent magnetization transfer contrast imaging at 3 Tesla. Two axial three‐dimensional gradient echo sequences with and without an off‐resonance saturation rapid frequency pulse were conducted at the right distal thigh. Sciatic nerve regions of interest were manually drawn on 10 consecutive axial slices in the images without off‐resonance saturation, and then transferred to the corresponding slices that were generated by the sequence with the off‐resonance saturation pulse. Subsequently, the MTR and cross‐sectional area (CSA) of the sciatic nerve were evaluated. Detailed neurologic and electrophysiologic examinations were conducted in all ATTRv‐PNP patients and mutTTR‐carriers.ResultsSciatic nerve MTR and CSA reliably differentiated between ATTRv‐PNP, mutTTR‐carriers, and controls. MTR was lower in ATTRv‐PNP (26.4 ± 0.7; P < 0.0001) and in mutTTR‐carriers (32.6 ± 0.8; P = 0.0005) versus controls (39.4 ± 2.1), and was also lower in ATTRv‐PNP versus mutTTR‐carriers (P = 0.0009). MTR correlated negatively with the NIS‐LL and positively with CMAPs and SNAPs. CSA was higher in ATTRv‐PNP (34.3 ± 1.7 mm3) versus mutTTR‐carriers (26.0 ± 1.1 mm3; P = 0.0005) and versus controls (20.4 ± 1.2 mm3; P < 0.0001). CSA was also higher in mutTTR‐carriers versus controls.InterpretationMTR is a novel imaging marker that can quantify macromolecular changes in ATTRv‐PNP and differentiate between symptomatic ATTRv‐PNP and asymptomatic mutTTR‐carriers and correlates with electrophysiology.

The Evaluation of Optic Nerves Using 7 Tesla "Silent" Zero Echo Time Imaging in Patients with Leber's Hereditary Optic Neuropathy with or without Idebenone Treatment.

Magnetic Resonance Imaging (MRI) of the Optic Nerve is difficult due to the fine extended nature of the structure, strong local magnetic field distortions induced by anatomy, and large motion artefacts associated with eye movement. To address these problems we used a Zero Echo Time (ZTE) MRI sequence with an Adiabatic SPectral Inversion Recovery (ASPIR) fat suppression pulse which also imbues the images with Magnetisation Transfer contrast. We investigated an application of the sequence for imaging the optic nerve in subjects with Leber’s hereditary optic neuropathy (LHON). Of particular note is the sequence’s near-silent operation, which can enhance image quality of the optic nerve by reducing the occurrence of involuntary saccades induced during Magnetic Resonance (MR) scanning.

Quick MR Neuromelanin Imaging Using a Chemical Shift Selective Pulse.

Not only magnetization transfer contrast (MTC) pulse, but also chemical shift selective (CHESS) pulse would be a useful additional one for shortening the scan time of neuromelanin imaging. We compared three sequences among turbo-spin echo (TSE) images with CHESS, MTC, and without an additional pulse in the same short time, 3 min 20 s. The TSE with CHESS image was the most useful for the diagnosis of neuromelanin within the limited time.

Evaluation of Improvement of Tissue Contrast and Reduction of Imaging Time by Shortening TR in Brain T&lt;sub&gt;2&lt;/sub&gt; FLAIR Using MTC Pulse

T2 fluid-attenuated inversion recovery (FLAIR) using inversion recovery pulse to suppress cerebrospinal fluid signal needs adequate T1 recovery time after data acquisition, otherwise, the T2-weighted contrast in brain tissue will get lower. Over 10000 ms of repetition time (TR) is recommended for the 1.5 T MR scanner, so it is difficult to shorten the imaging time. We verified whether T2 FLAIR combined with the magnetization transfer contrast (MTC) pulse shows better gray-to-white matter (GM/WM) and lesion-to-normal tissue contrasts even when the TR is shortened compared to the conventional T2 FLAIR. Optimal parameters of the MTC pulse were determined with a self-produced phantom, which modeled on cerebral cortical gray and white matters. GM/WM contrasts of the phantom were measured in T2 FLAIR with the MTC pulse while decreasing TR gradually from 10000 ms to 6500 ms. Although GM/WM contrast of the phantom in T2 FLAIR with the MTC pulse gradually decreased as the TR got shortened, the T2 FLAIR with the MTC pulse of 6500 ms of TR still showed 27% higher contrast than the conventional T2 FLAIR (TR 10000 ms). GM/WM contrast in T2 FLAIR with the MTC pulse was improved also in healthy volunteers, but improvement in thalamo-medullary contrast was less than that of cerebral cortico-medullary and putamino-medullary contrasts. It seems to be because thalamus, which is a deep gray matter, shows a higher MTC effect than other gray matters. Thus, it is necessary to note that the tissue contrast might differ between T2 FLAIR with the MTC pulse and the conventional T2 FLAIR. Because general lesions with an elongated T2 value show lower MTC effect compared to the normal brain tissue, a clinical case with thalamic lesion showed that the lesion-to-normal tissue contrast improved in T2 FLAIR with the MTC pulse of 6500 ms of TR. Although it is necessary to note the difference in contrast between some tissues, T2 FLAIR with the MTC pulse improves GM/WM and lesion-to-normal tissue contrasts even when the TR is shortened compared to the conventional T2 FLAIR, and it enables to shorten the imaging time.

D-glucose weighted chemical exchange saturation transfer (glucoCEST)-based dynamic glucose enhanced (DGE) MRI at 3T: early experience in healthy volunteers and brain tumor patients.

Dynamic glucose enhanced (DGE) MRI has shown potential for imaging glucose delivery and blood-brain barrier permeability at fields of 7T and higher. Here, we evaluated issues involved with translating d-glucose weighted chemical exchange saturation transfer (glucoCEST) experiments to the clinical field strength of 3T. Exchange rates of the different hydroxyl proton pools and the field-dependent T2 relaxivity of water in d-glucose solution were used to simulate the water saturation spectra (Z-spectra) and DGE signal differences as a function of static field strength B0 , radiofrequency field strength B1 , and saturation time tsat . Multislice DGE experiments were performed at 3T on 5 healthy volunteers and 3 glioma patients. Simulations showed that DGE signal decreases with B0 , because of decreased contributions of glucoCEST and transverse relaxivity, as well as coalescence of the hydroxyl and water proton signals in the Z-spectrum. At 3T, because of this coalescence and increased interference of direct water saturation and magnetization transfer contrast, the DGE effect can be assessed over a broad range of saturation frequencies. Multislice DGE experiments were performed in vivo using a B1 of 1.6 µT and a tsat of 1 second, leading to a small glucoCEST DGE effect at an offset frequency of 2 ppm from the water resonance. Motion correction was essential to detect DGE effects reliably. Multislice glucoCEST-based DGE experiments can be performed at 3T with sufficient temporal resolution. However, the effects are small and prone to motion influence. Therefore, motion correction should be used when performing DGE experiments at clinical field strengths.

Fractal Analysis of Cardiac Fibrosis Patterns Derived From Magnetization Transfer Contrast MRI in Patients With End-Stage Renal Disease.

OBJECTIVE. The purpose of this study is to use fractal analysis to characterize diffusely fibrotic, focally fibrotic, and healthy myocardium in patients with end-stage renal disease (ESRD) on the basis of previously published magnetization transfer (MT) contrast images. MATERIALS AND METHODS. The MT ratio values of patients with ESRD (n = 34) and healthy control subjects (n = 19) were used to construct anatomically faithful 3D left ventricular reconstructions. Established MT ratio threshold values were used to define healthy, diffusely enhanced, focally enhanced, and total enhanced tissue domains. The fractal dimension (FD) for reach domain was calculated using a 3D box-counting algorithm. RESULTS. Patients with ESRD showed a higher FD across all fibrotic domains compared with control subjects, in whom diffusely and focally enhanced myocardium showed largely planar distributions (mean [± SD] FD, 2.12 ± 0.02 and 1.92 ± 0.09, respectively), whereas the combined domain was fractal in 3D (mean FD, 2.41 ± 0.04). The FD and volume of fibrotic tissue were logarithmically correlated in the population with ESRD. Fractal analysis of MT-weighted cardiac MRI data revealed that the geometric characteristics of cardiac scar in patients with ESRD transition from fractal in 2D to planar in 2D to fractal in 3D as scar volume increases. CONCLUSION. Fatal arrhythmias in individuals with ESRD are increasingly attributed to cardiac fibrosis. Histologic analysis reveals that fibrosis progresses via a fractal expansion pattern. The method presented in this study can be applied to characterize the irregular space-filling morphometry of any pathologic substrate identified by contrast enhancement across noninvasive imaging modalities.

Voxel-wise Optimization of Pseudo Voigt Profile (VOPVP) for Z-spectra fitting in chemical exchange saturation transfer (CEST) MRI.

Chemical exchange saturation transfer (CEST) MRI is a promising approach for detecting biochemical alterations in cancers and neurological diseases, but the quantification can be challenging. Among numerous quantification methods, Lorentzian difference (LD) is relatively simple and widely used, which employs Lorentzian line-shape as a reference to describe the direct saturation (DS) of water and takes account of difference against experimental CEST spectra data. However, LD often overestimates CEST and nuclear overhauser enhancement (NOE) effects. Specifically, for fast-exchanging CEST species require higher saturation power (B1_sat) or in the presence of strong magnetization transfer (MT) contrast, Z-spectrum appears more like a Gaussian line-shape rather than a Lorentzian line-shape. To improve the conventional LD analysis, the present study developed and validated a novel fitting algorithm through a linear combination of Gaussian and Lorentzian function as the reference spectra, namely, Voxel-wise Optimization of Pseudo Voigt Profile (VOPVP). The experimental Z-spectra were pre-fitted with Gaussian and Lorentzian method independently, in order to determine Lorentzian proportionality coefficient (a). To further compensate for the line-shape changes under different B1_sat's, a B1-dependent adjustment was applied to the experimental Z-spectra (Z_exp) according to the prior knowledge learned from 5-pool Bloch equation-based simulations at a range of B1_sat's. Then, the obtained Z-spectra (Z_B1adj) was fitted by the previously defined VOPVP function. Considering the asymmetric component of MT, the positive- and negative-side of Z-spectra were fitted separately, while the middle part (-0.6 to 0.6 ppm, consisted primarily of DS) was fitted using Lorentzian function. Finally, the difference between Z_VOPVP and Z_exp was defined as the CEST and NOE contrast. To validate our VOPVP method, an extensive simulation of CEST Z-spectra was performed using 5-pool model and 6-pool model with greater MT component. In comparison with LD approach, VOPVP exhibited lower sum of squares due to error (SSE) and higher goodness of fit (R-square) for the experimental Z-spectra at all B1_sat. Moreover, the results indicated that VOPVP fitting improved the overestimated contributions from amide proton transfer (APT) and NOE through LD at all B1_sat. Despite that the relationship for B1-dependent adjustment was pre-determined using a single 5-pool model, the VOPVP fittings obtained accurate quantification for multiple 6-pool models with a range of T1w's and T2w's. The robustness of VOPVP fitting was also proved by simulations using 3T parameters. Furthermore, we assessed VOPVP in vivo in a glioblastoma-bearing mouse. Compared to LD maps, VOPVP quantification maps displayed higher contrast-to-noise ratio between tumor and normal contralateral tissue for APT, glutamate and nuclear overhauser effect (NOE), when B1_sat >1 µT. As an improvement of LD method, VOPVP fitting can serve as a simple, robust and more accurate approach for quantifying CEST and NOE contrast.

Steady‐state imaging with inhomogeneous magnetization transfer contrast using multiband radiofrequency pulses

PurposeInhomogeneous magnetization transfer (ihMT) is an emerging form of MRI contrast that may offer high specificity for myelinated tissue. Existing ihMT and pulsed MT sequences often use separate radiofrequency pulses for saturation and signal excitation. This study investigates the use of nonselective multiband radiofrequency pulses for simultaneous off‐resonance saturation and on‐resonance excitation specifically for generation of ihMT contrast within rapid steady‐state pulse sequences.Theory and MethodsA matrix‐based signal modeling approach was developed and applied for both balanced steady state free precession and spoiled gradient echo sequences, accounting specifically for multiband pulses. Phantom experiments were performed using a combination of balanced steady state free precession and spoiled gradient echo sequences, and compared with model fits. A human brain imaging exam was performed using balanced steady state free precession sequences to demonstrate the achieved contrast.ResultsA simple signal model derived assuming instantaneous radiofrequency pulses was shown to agree well with full integration of the governing equations and provided fits to phantom data for materials with strong ihMT contrast (PL161 root mean square error = 0.9%, and hair conditioner root mean square error = 2.4%). In vivo ihMT ratio images showed the expected white matter contrast that has been seen by other ihMT investigations, and the observed ihMT ratios corresponded well with predictions.ConclusionsihMT contrast can be generated by integrating multiband radiofrequency pulses directly into both spoiled gradient echo and balanced steady state free precession sequences, and the presented signal modeling approach can be used to understand the acquired signals.

Imaging molecules.

Molecular imaging using MRI is gaining momentum. While sensitivity of MR is limited compared to other molecular imaging modalities, the molecular specificity is high in comparison. Moreover, MRI offers contrast based on multitude of processes and scales, from intramolecular relaxation pathways to water diffusion. Living tissue offers abundance of potential molecular targets of interest in biology and medicine. In this short perspective we focus on some direct and indirect methods to visualize endogenous molecules. We briefly discuss Spectroscopic Imaging (MRSI), Chemical Exchange Saturation Transfer (CEST) and Magnetization Transfer Contrast (MTC). Imaging molecules with MRI is part of the larger universe of imaging methods. Moreover, it is part of ever increasing pool of data combining imaging with other modalities, biology and patient outcomes.

High-resolution creatine mapping of mouse brain at 11.7 T using non-steady-state chemical exchange saturation transfer.

The current study aims to optimize the acquisition scheme for the creatine chemical exchange saturation transfer weighted (CrCESTw) signal on mouse brain at 11.7T, in which a strong magnetization transfer contrast (MTC) is present, and to further develop the polynomial and Lorentzian line-shape fitting (PLOF) method for quantifying CrCESTw signal with a non-steady-state (NSS) acquisition scheme. Studies on a Cr phantom with cross-linked bovine serum albumin (BSA) as well as on mouse brain demonstrated that the maximum CrCESTw signal was reached with a short saturation time determined by the rotating frame relaxation time of the MTC pool instead of the steady-state saturation. The saturation power for the maximal signal was around 1-1.5 μT for Cr with 20% cross-linked BSA and in vivo applications, but 2 μT was found to be most practical for signal stability. For the CrCEST acquisition with strong MTC interference, the optimal saturation power and length are completely different from those on Cr solution alone. This observation could be explained well using R1ρ theory by incorporating the strong MTC pool. Finally, a high-resolution Cr map was obtained on mouse brain using the PLOF method with the NSS CEST acquisition and a cryogenic coil. The Cr map obtained by CEST showed homogenous intensity across the mouse brain except for regions with cerebrospinal fluid.

In vivo imaging of the nucleus of the solitary tract with Magnetization Transfer at 7 Tesla

The nucleus of the solitary tract (NTS) is a nuclei complex with, among others, a high concentration of noradrenergic neurons (including the noradrenergic subnuclei named A1 and A2) in the medulla. The NTS regulates several cognitive, neuroendocrine and autonomic functions. No method currently exists to anatomically visualize the NTS in vivo. Several noradrenergic and dopaminergic nuclei have been successfully imaged using Magnetization Transfer (MT) contrast manipulation. We therefore hypothesized that an efficient, high-resolution MT-weighted sequence at 7 T might successfully image the NTS. In this study, we found a hyperintensity, similar to hyperintensities found in other noradrenergic and dopaminergic nuclei, consistent with the expected NTS location, and specific to the MT-weighted images. The localization of the hyperintensity was found to be consistent between individuals and slices and in good correspondence to a histological atlas and a meta-analytic map of fMRI-based NTS activation. We conclude that the method may, for the first time, achieve NTS imaging in vivo and within a clinically-feasible acquisition time. To facilitate NTS research at lower field strengths, an NTS template was created and made publicly available.

Improving acute demyelinating lesion detection: which T1-weighted magnetic resonance acquisition is more sensitive to gadolinium enhancement?

Because of the need for a standardized and accurate method for detecting multiple sclerosis (MS) inflammatory activity, different magnetic resonance (MR) acquisitions should be compared in order to choose the most sensitive sequence for clinical routine. To compare the sensitivity of a T1-weighted image to a single dose of gadolinium (Gd) administration both with and without magnetization transfer to detect contrast enhancement in active demyelinating focal lesions. A sample of relapsing-remitting MS patients were prospectively examined separately by two neuroradiologists using a 1.5 Tesla scanner. The outcome parameters were focused on Gd-enhancement detection attributed to acute demyelination. All MR examinations with at least one Gd-enhancing lesion were considered positive (MR+) and each lesion was analyzed according to its size and contrast ratio. Thirty-six MR examinations were analyzed with a high inter-observer agreement for MR+ detection (k coefficient > 0.8), which was excellent for the number of Gd-enhancing lesions (0.91 T1 spin-echo (SE), 0.88 T1 magnetization transfer contrast (MTC) sequence and 0.99 magnetization-prepared rapid acquisition with gradient-echo (MPRAGE). Significantly more MR+ were reported on the T1 MTC scans, followed by the T1 SE, and MPRAGE scans. Confidently, the T1 MTC sequence demonstrated higher accuracy in the detection of Gd-enhancing lesions, followed by the T1 SE and MPRAGE sequences. Further comparisons showed that there was a statistically significant increase in the contrast ratio and area of Gd-enhancement on the T1 MTC images when compared with both the SE and MPRAGE images. Single-dose Gd T1 MTC sequence was confirmed to be the most sensitive acquisition for predicting inflammatory active lesions using a 1.5 T magnet in this sample of MS patients.

Reduced respiratory motion artefact in constant TR multi-slice MRI of the mouse

PurposeMulti-slice scanning in the abdomen and thorax of small animals is compromised by the effects of respiration unless imaging and respiration are synchronised. To avoid the signal modulations that result from respiration motion and a variable TR, blocks of fully relaxed slices are typically acquired during inter-breath periods, at the cost of scan efficiency. This paper reports a conceptually simple yet effective prospective gating acquisition mode for multi-slice scanning in free breathing small animals at any fixed TR of choice with reduced sensitivity to respiratory motion. MethodsMulti-slice scan modes have been implemented in which each slice has its own specific projection or phase encode loop index counter. When a breath is registered RF pulses continue to be applied but data are not acquired, and the corresponding counters remain fixed so that the data are acquired one TR later, providing it coincides with an inter-breath period. The approach is refined to reacquire the slice data that are acquired immediately before each breath is detected. Only the data with reduced motion artefact are used in image reconstruction. The efficacy of the method is demonstrated in the RARE scan mode which is well known to be particularly useful for tumour visualization. ResultsValidation in mice with RARE demonstrates improved stability with respect to ungated scanning where signal averaging is often used to reduce artefacts. SNR enhancement maps demonstrate the improved efficiency of the proposed method that is equivalent to at least a 2.5 fold reduction in scan time with respect to ungated signal averaging. A steady-state magnetisation transfer contrast prepared gradient echo implementation is observed to highlight tumour structure. Supplementary simulations demonstrate that only small variations in respiration rate are required to enable efficient sampling with the proposed method. ConclusionsThe proposed prospective gating acquisition scheme enables efficient multi-slice scanning in small animals at the optimum TR with reduced sensitivity to respiratory motion. The method is compatible with a wide range of complementary methods including non-Cartesian scan modes, partially parallel imaging, and compressed sensing. In particular, the proposed scheme reduces the need for continual close monitoring to effect operator intervention in response to respiratory rate changes, which is both difficult to maintain and precludes high throughput.

Gadolinium-Enhanced Susceptibility-Weighted Imaging in Multiple Sclerosis: Optimizing the Recognition of Active Plaques for Different MR Imaging Sequences.

Gadolinium SWI is MR imaging that has recently been reported to be effective in the evaluation of several neurologic disorders, including demyelinating diseases. Our aim was to analyze the accuracy of gadolinium SWI for detecting the imaging evidence of active inflammation on MS plaques when a BBB dysfunction is demonstrated by a focal gadolinium-enhanced lesion and to compare this technique with gadolinium-enhanced T1 spin-echo and T1 spin-echo with magnetization transfer contrast. MR imaging studies of 103 patients (170 examinations) were performed using a 1.5T scanner. Two neuroradiologists scrutinized signal abnormalities of the demyelinating plaques on gadolinium SWI and compared them with gadolinium T1 before and after an additional magnetization transfer pulse. Interrater agreement was evaluated among gadolinium T1 magnetization transfer contrast, gadolinium SWI, and gadolinium T1 spin-echo using the κ coefficient. The T1 magnetization transfer contrast sequence was adopted as the criterion standard in this cohort. Thus, the sensitivity, specificity, positive predictive value, and negative predictive value were calculated for gadolinium T1 spin-echo and gadolinium SWI sequences. Differences in BBB dysfunction were evident among gadolinium SWI, gadolinium T1 spin-echo, and gadolinium T1 magnetization transfer contrast. Gadolinium T1 magnetization transfer contrast demonstrated the highest number of active demyelinating plaques. Gadolinium SWI was highly correlated with gadolinium T1 magnetization transfer contrast in depicting acute demyelinating plaques (κ coefficient = 0.860; sensitivity = 0.837), and these techniques provided better performance compared with gadolinium T1 spin-echo (κ coefficient = 0.78; sensitivity = 0.645). Gadolinium SWI was able to better detect BBB dysfunction in MS plaques and had a better performance than gadolinium T1 spin-echo. Increasing SWI sequence applications in clinical practice can improve our knowledge of MS, likely allowing the addition of BBB dysfunction analysis to the striking findings of the previously reported central vein sign.

Non-contrast myocardial infarct scar assessment using a hybrid native T1 and magnetization transfer imaging sequence at 1.5T.

To develop a gadolinium-free cardiac MR technique that simultaneously exploits native T1 and magnetization transfer (MT) contrast for the imaging of myocardial infarction. A novel hybrid T one and magnetization transfer (HYTOM) method was developed based on the modified look-locker inversion recovery (MOLLI) sequence, with a train of MT-prep pulses placed before the balanced SSFP (bSSFP) readout pulses. Numerical simulations, based on Bloch-McConnell equations, were performed to investigate the effects of MT induced by (1) the bSSFP readout pulses, and (2) the MT-prep pulses, on the measured, "apparent," native T1 values. The HYTOM method was then tested on 8 healthy adult subjects, 6 patients, and a swine with prior myocardial infarction (MI). The resulting imaging contrast between normal myocardium and infarcted tissues was compared with that of MOLLI. Late gadolinium enhancement (LGE) images were also obtained for infarct assessment in patients and swine. Numerical simulation and in vivo studies in healthy volunteers demonstrated that MT effects, resulting from on-resonance bSSFP excitation pulses and off-resonance MT-prep pulses, reduce the measured T1 in both MOLLI and HTYOM. In vivo studies in patients and swine showed that the HYTOM sequence can identify locations of MI, as seen on LGE. Furthermore, the HYTOM method yields higher myocardium-to-scar contrast than MOLLI (contrast-to-noise ratio: 7.33 ± 1.67 vs. 3.77 ± 0.66, P < 0.01). The proposed HYTOM method simultaneously exploits native T1 and MT contrast and significantly boosts the imaging contrast for myocardial infarction.