Keywords: Relaxometry, Quantitative Imaging, Relaxometry, MR-STAT, Clinical

Motivation:  This is the first work assessing quantitative values (T₁ and T₂) from Magnetic Resonance Spin TomogrAphy in Time-domain (MR-STAT) as a fast relaxometry technique in clinical setting. 

Goal(s): To assess MR-STAT as viable option for fast relaxometry in the clinic. 

Approach: We applied MR-STAT to investigate the quantitative T₁ and T₂ values of brain tissue at 3T in a heterogeneous cohort of 50 subjects (10 healthy volunteers, mixed-pathology 40 patients). 

Results: Quantitative values in normal appearing brain tissue were comparable to earlier literature. Furthermore, individual case examples (glioma, multiple sclerosis) confirmed the ability to discern pathological tissue in T₁ and T₂ values. 

Impact: Voxel values in clinical MRI are subjective. Magnetic Resonance Spin TomogrAphy in Time-domain (MR-STAT) quickly quantifies MR tissue properties and gives voxels quantitative values. This work demonstrates MR-STAT as fast relaxometry technique in a clinical population and assesses quantitative values.  

Keywords: Relaxometry, Modelling, Microstructure, Nervous System

Motivation: T₁ estimations using the MP2RAGE methodology are biased by Magnetization Transfer (MT) effects. A quantitative MT-based solution was previously proposed in which the inversion efficiency of the preparation pulse (Q) was fixed; we sought to alleviate this strong hypothesis.

Goal(s): To better understand the influence of free water T₂ (T_2,f) on Q in the scope of MP2RAGE-T₁ brain mapping.

Approach: A better modeling of the MP2RAGE involving a discretized preparation pulse is employed, and tested for fixed and free T_2,f values.

Results: The T₁ bias is highly dependent on the T_2,f values, calling for a better and accurate estimation of this parameter.

Impact: Quantitative MT-derived T₁ estimation in the MP2RAGE methodology remains dependent on the estimated free water T₂ values because of its impact on the inversion efficiency pulse.

Keywords: Relaxometry, Quantitative Imaging, Toxicity; Radiotherapy; Biomarker; Head and Neck

Motivation: Due to limited options for early monitoring and interventions, radiation therapy (RT) for head and neck cancer patients often leads to dysphagia following treatment due to fibrosis development in swallowing muscles.

Goal(s): The goal of this work is to report findings from a retrospective cohort who received RT treatment and longitudinal T1/T2 map imaging. 

Approach: Evaluating at pre-, mid-, post-, and late- RT allows for statistical analysis of changes in median T1/T2 values.

Results: Our hypothesis was confirmed that kinetics are discernable for T1 only, but limited correlation to dose and objective measures of muscle damage was seen in various structures.

Impact: At present, there exists no option for the early monitoring of injury from radiation therapy of swallowing muscles related to dysphagia. The aim of this work is to investigate Quantitative MR Relaxometry as a method to monitor for iatrogenic injury.

Keywords: Multi-Contrast, Quantitative Imaging, bSSFP, brain, T1, T2, Relaxometry, field inhomogeneity, proton density

Motivation: Addressing the need for simplified, time-efficient, and unbiased quantitative imaging in human brain.

Goal(s): Evaluating novel analytical method, ORACLE, to simultaneously quantify T1, T2, proton density (PD), and magnetic field inhomogeneity (B0) in human brain based on balanced steady-state free precession (bSSFP) profiles.

Approach: Acquiring bSSFP data for simultaneous multi-parameter quantification and reference multi-echo spin-echo, MP2RAGE and dual-echo gradient-echo data in 4 human subjects.

Results: Quantifications using bSSFP profiles and ORACLE was consistent with reference methods, although magnetization transfer effects led to a 15% consistent underestimation of T1. Proposed method has faster acquisition and parametric estimation comparing to other methods combined.

Impact: This novel analytical time-efficient method extracts a wide range of quantitative parameters from bSSFP profiles, which can be a valuable alternative to existing reference methods, multi-echo spin-echo, MP2RAGE and dual-echo gradient-echo, that quantify one parameter at a time.

Keywords: Relaxometry, Body

Motivation: The acquisition time of volumetric relaxometry techniques is constrained by the sampling efficiency and confounding factors such as fat or the B1 inhomogeneity.

Goal(s): To develop a method for volumetric T1- and T2-mapping that improves efficiency while accounting for confounding factors.

Approach: A Cartesian acquisition with spiral profile ordering is employed to increase sampling efficiency. A subspace reconstruction is proposed to perform B1- and water-specific T1- and T2-mapping considering the exact k-space sampling pattern.

Results: Simulation, phantom and in-vivo experiments were performed to evaluate the proposed method with regard to its quantification performance and its B1 sensitivity at 3T.

Impact: Subspace reconstruction in combination with a CASPR trajectory can improve the scan's efficiency and sampling flexibility while correcting for confounders such as fat or B1. The technique may be valuable to develop volumetric relaxometry in clinically acceptable scan times.

Keywords: Relaxometry, Relaxometry, Ultra-high field; 7T; R2*; high resolution; deep gray mattern; subcortical

Motivation: While conventional 7T systems provide increased SNR, their gradient systems lack the performance to acquire high-resolution GRE data with very fast sampling/many echoes.

Goal(s): To leverage a 7T with high-performance gradients to acquire more echoes than on conventional 7T systems and assess the potential for high-resolution R2* mapping.

Approach: Multi-echo GRE data with varying number of echoes was acquired at a 7T Terra.X Impulse Edition and 7T Plus. Conventional logarithmic-linear and advanced stretched exponential fits were performed.

Results: Sampling more echoes enables fitting advanced models which yield lower R2* standard deviation and root-mean-squared-errors.

Impact: Combining 7T with high-performance gradients enables high spatial and temporal resolution multi-echo GRE data. Hence, R2* mapping can be performed with novel multi-parametric fits, enabling new avenues in biophysical modeling as well as signal denoising and decomposition algorithms.

Keywords: Relaxometry, Multi-Contrast, Myelin Water Imaging

Motivation: Multi-echo gradient-echo (mGRE) MRI enabled non-invasive quantification of myelin water fraction (MWF) of the human brain.

Goal(s): The MWF may depend on field strength that changes the T₂^* decay and the results need to be verified by histological staining.
 

Approach: We performed mGRE-based MWF on in-vivo and ex-vivo human brain at high resolution and revealed the accuracy of the measurements using histological staining at both 3T and 7T.

Results: The MWF-derived from 7T was systematically higher than those from 3T and the in-vivo and ex-vivo measurements showed good agreement. The MWF at 3T and 7T both demonstrated good correlations with myelin basic protein.

Impact: These findings indicated the MWF mapping could reliably depict the myelin content in the human brain, although the measurement were field-strength dependent.

Keywords: Relaxometry, Contrast Mechanisms, Spin Lock

Motivation: Composite metric R2-R1rho (1/T2–1/T1rho) could be utilized for probing chemical exchange.. However, a persistent challenge lies in the confounding influences of magnetization transfer (MT) effects on R2, which was not fully discussed in literatures.

Goal(s): In this study, we reported our investigation on this effect and proposed an acquisition strategy to improve the specificity of the composite metric (R2-R1rho) to thethe chemical exchange effect.

Approach: We demonstrated our findings and the proposed method using simulation, phantom, and in vivo experiments.

Results: From our preliminary results, composite metric (R2-R1rho) to detect chemical exchange without the influence from magnetization transfer effect.

Impact: Using improved composite metric (R2-R1rho) to detect chemical exchange without the influence from magnetization transfer effect.

Keywords: Relaxometry, Relaxometry, T2 Mapping, T2* Mapping

Motivation:  T₂ and T₂* mapping can quantitatively characterize tissue pathologies, improving diagnosis and treatment, but their clinical applications are hindered by relatively long scan times.

Goal(s): To develop an ultrafast method for simultaneously obtaining T₂ and T₂* mapping.

Approach: A biaxial spatiotemporally encoded (SPEN) sequence with multi-spin-echo trains and spiral out-in-out-in trajectory was developed to obtain multiple images with different echo times within a single shot. The acquired signal was fitted to yield simultaneous T₂ and T₂* mapping.

Results: Numerical simulations and in vivo rat brain and kidney experiments were conducted to validate the proposed method.

Impact: We developed an ultrafast technique to simultaneously obtain T₂ and T₂* mapping in 150 ms, potentially facilitating the use of T₂ and T₂* mapping in scenarios requiring high time resolution.

Keywords: Relaxometry, MR Fingerprinting

Motivation: T₁ and T₂ accuracy in the brain is difficult to assess, since there is no ground truth available.

Goal(s): To investigate how well relaxometry methods agree.

Approach: We compare Magnetic Resonance Fingerprinting (MRF) T₁ and T₂ mapping with Variable Flip Angle (VFA) T₁ mapping and Multi-Echo Spin Echo (T₂) mapping in 11 anatomical brain regions for 10 healthy volunteers, and in the relevant spheres of the NIST phantom.

Results: MRF underestimates T₁ and T₂ in comparison with T₁ VFA and T₂ MESE in the human brain, especially in myelin-dense areas. Less T₁ and no T₂ bias is present in the NIST phantom.

Impact: Quantitative T₁ and T₂ relaxometry techniques are more consistent in the NIST phantom than the human brain. Deviations could be caused by magnetisation transfer, whose impact on T₁ and T₂ relaxation mechanisms needs further investigation. 

Keywords: Relaxometry, Validation, Histology, Myelin, Iron, In vivo vs ex vivo, Fixation

Motivation: Estimating myelin and iron concentration maps from in-vivo MRI yields biased estimates because MR-to-microstructure linear mappings are derived from fixed-postmortem human brain tissue.

Goal(s): We assessed whether taking into account the changes of relaxation rates from in-vivo to hydrated fixed ex-vivo specimens would allow the use of current MR-to-microstructure linear mappings for in-vivo MRI.

Approach: We introduced a pipeline that accounts for the major relaxation-rate changes during fixation and hydration, and compared the estimated MRI-based myelin parameters to their counterparts from light microscopy in the human corpus callosum.

Results: We found that including these changes significantly improved the accuracy of the myelin estimates.

Impact: We proposed a new method that significantly improves the MRI-based myelin and iron maps estimation from in-vivo longitudinal and effective transverse relaxation rates.

Keywords: Relaxometry, White Matter

Motivation: There remains a gap in knowledge surrounding what microstructural features impact similar myelin qMRI metrics.

Goal(s): The goal of this work is to evaluate the relationship between a novel axon diameter surrogate and myelin qMRI metrics.

Approach: Ex vivo ferret spinal cords were imaged at 15T using multiple spin echo, selective inversion recovery and diffusion tensor imaging sequences.

Results: BPF was found to have a negative correlation with the axon diameter surrogate measure. MET₂ derived metrics were found to have no correlation with the axon diameter surrogate measure. 

Impact: The diffusion time dependence of the radial diffusivity is thought to report on axon diameter towards detecting microstructural changes as a result of pathology. This work provides further evaluation of this metric and other relaxation parameters of white matter. 

Keywords: Relaxometry, Relaxometry, MR-STAT, Multiple sclerosis, Machine Learning

Motivation: Radiology workflow around multiple sclerosis (MS) patients is time-consuming. 

Goal(s): To automatically count and measure individual white matter anomalies in MS patients from a five-minute Magnetic Resonance Spin TomogrAphy in Time-domain (MR-STAT) scan. 

Approach: We imaged ten healthy volunteers (HV) and six MS patients using a five-minute MR-STAT protocol. Resulting quantitative data from seven HVs was fit to a multivariate Gaussian probabilistic model. The model was tested on three HVs and six MS patients. 

Results: Automatic anomaly detection was moderately accurate in MS patients. No anomalies were found in HVs. These results underline the potential for a shorter acquisition with automatic outlier detection. 

Impact: MRI protocols for MS patients are lengthy and the assessing the images is a time-consuming task for the radiologist. We combine a fast (five-minute) MR-STAT relaxometry scan with a data-driven, automatic outlier detection strategy to potentially accelerate the clinical workflow.

Keywords: Relaxometry, Spinal Cord

Motivation: In spinal cord T₂* relaxometry, only mono-exponential models have previously been used, neglecting the potential presence of multiple compartments.   

Goal(s): Determine if a mono- or bi-exponential model is better for T₂* relaxometry in the cervical spinal cord white matter (WM) and grey matter (GM).

Approach: Mask the ventral half of the GM and WM in a 7-echoes GRE acquisition. Fit a mono-/bi-exponential model to the average signal in the masks for each slice. Compute R², F-tests and p-values.

Results: A bi-exponential model gave statistically better results in WM. No relevant difference between the models was measured in GM for most slices.

Impact: A bi-exponential model is statistically better than a mono-exponential one for T₂* relaxometry in spinal cord white matter, but not grey matter. The model can be used for contrast optimization, e.g. to improve lesion detection in demyelinating diseases.

Keywords: Relaxometry, Spectroscopy, Downfield

Motivation: NAD⁺ is a key metabolite in aging and disease, but its absolute in vivo tissue quantification requires correction for T₁ and T₂ relaxation effects.

Goal(s): Our goal was to determine T₁ and T₂ of NAD⁺ in human brain in vivo at 7T.

Approach: We utilized spectrally-selective downfield spectroscopy with slice localization.

Results: We measured an average T₁ of 164.6±28.1ms and an average T₂ of 33.5±10.3ms across three NAD⁺ downfield resonances.

Impact: Our measurements of NAD⁺ T₁ and T₂ in human brain can be used as correction factors to quantify absolute concentration of NAD⁺, a potential biomarker to study metabolic derangements in many diseases.

Keywords: Relaxometry, Relaxometry, Low-Field MRI

Motivation: Quantitative magnetic resonance imaging can provide novel insights into the brain’s tissue microstructure, however, such methods have limited availability at ultra-low field. 

Goal(s): To develop DESPOT1 approach at ultra-low field.

Approach: We acquired spoiled gradient recalled echo images across multiple flip angles at low field (64 mT) and fit the signal to estimate T1 using the DESPOT1 framework.

Results: While challenged by limited SNR at low field, our results demonstrate the feasibility to measure T1 at low field from multiple flip angle images. Through additional optimization, such methods may allow low field systems to provide sensitive measures of brain tissue microstructure.  

Impact: Our results demonstrate the ability to perform T1 relaxation time mapping via DESPOT1 at ultra-low field for the first time. Improvements in the described approach could enable  sensitive measurements of brain microstructure at ultra-low field.