A method is presented for rapid simultaneous quantification of the longitudinal T 1 relaxation, the transverse T 2 relaxation, the proton density (PD), and the amplitude of the local radio frequency B 1 field. All four parameters are measured in one single scan by means of a multislice, multiecho, and multidelay acquisition. It is based on a previously reported method, which was substantially improved for routine clinical usage. The improvements comprise of the use of a multislice spin-echo technique, a background phase correction, and a spin system simulation to compensate for the slice-selective RF pulse profile effects. The aim of the optimization was to achieve the optimal result for the quantification of magnetic resonance parameters within a clinically acceptable time. One benchmark was high-resolution coverage of the brain within 5 min. In this scan time the measured intersubject standard deviation (SD) in a group of volunteers was 2% to 8%, depending on the tissue (voxel size ؍ 0.8 ؋ 0.8 ؋ 5 mm). As an example, the method was applied to a patient with multiple sclerosis in whom the diseased tissue could clearly be distinguished from healthy reference values. Additionally it was shown that, using the approach of synthetic MRI, both accurate conventional contrast images as well as quantification maps can be generated based on the same scan. Tissues in the human body can be distinguished with magnetic resonance imaging (MRI) depending on their MR parameters, such as the longitudinal T 1 relaxation, the transverse T 2 relaxation, and the proton density (PD). In clinical routine, the MR scanner settings, such as echo time (T E ), repetition time (T R ), and flip angle (␣), are most often chosen to highlight, or saturate, the image intensity of tissues, resulting in T 1 -weighting or T 2 -weighting in a contrast image. These procedures are well-established and relatively quick. A major disadvantage of using such contrast images is that the absolute intensity has no direct meaning and diagnosis relies on comparison with surrounding tissues in the image. In many cases it is therefore necessary to perform several different contrast scans. A more direct approach is the absolute quantification of the tissue parameters T 1 , T 2 , and PD. In this case, pathology can be examined on a pixel basis to establish the absolute deviation compared to the normal values. Automatic segmentation of such tissue images would be straightforward and the progress of the disease could then be expressed in absolute numbers. An excellent overview of the use of absolute quantification on neurodegenerative diseases is provided in Ref. 1.Although the advantages of absolute quantification are obvious, its clinical use is still limited. At least two major hurdles need to be addressed to stimulate widespread clinical usage. For many methods, the excessive scan time associated with the measurement of the three parameters has so far prohibited its clinical application. However, in recent years there has been substantial progress (see, e.g., R...
An imaging method called "quantification of relaxation times and proton density by twin-echo saturation-recovery turbofield echo" (QRAPTEST) is presented as a means of quickly determining the longitudinal T 1 and transverse T* 2 relaxation time and proton density (PD) within a single sequence. The method also includes an estimation of the B 1 field inhomogeneity. High-resolution images covering large volumes can be achieved within clinically acceptable times of 5-10 min. Constant progress in the time efficiency and accuracy of magnetic resonance imaging (MRI) has increased interest in quantifying, rather than qualifying, tissue parameters in large volumes of interest. Quick measurement of the longitudinal T 1 relaxation time (1-7), as well as the transverse T 2 and T* 2 relaxation time (6 -13), has been the subject of active investigation over the recent years. New fast imaging methods for measuring the proton density (PD) have also been published, although less frequently (6,8). The effective scan times have diminished, allowing the methods to enter the clinical arena. The quantification of MR relaxation times and water concentration may improve the detection and staging of various diseases. Examples include the use of T 1 relaxation time for diseases such as Parkinson's (14), Alzheimer's, and multiple sclerosis (15,16), and T* 2 relaxation time to assess iron deposition in thalassemia (17,18). One can expect that in many circumstances the combined measurement of several MR parameters would lead to a better diagnostic accuracy. For example, characterization of atherosclerotic plaques in the main vessels is improved when quantification of T 1 , T 2 , and PD replaces qualitative assessment of the vessel walls (19). In another example, the rapid quantification of metabolites in spectroscopic imaging is critically dependent on the accurate determination of water concentration (20,21).Beyond these immediate clinical benefits, rapid quantification could profoundly alter the way MRI is performed. Once the relaxation times and PD are measured for a region of interest (ROI), in principle any contrast image with a certain combination of echo time (TE) and repetition time (TR) or prepulses can be reconstructed in postprocessing. This would make the application of MR more similar to the CT approach (i.e., acquisition of a single quantification scan with subsequent generation of images with the desired contrast and orientation). This may significantly reduce patient scanning and planning time for MRI because instead of a performing a survey and reference scan, and acquiring all of the contrast images at different orientations, one only has to perform a single quantification scan. Unfortunately, most existing methods for measuring relaxation times and PD are limited by scan times that are clinically unacceptable. Several scans are generally required to measure both T 1 and T 2 , or T* 2 and the parameters might even depend on each other. Multiple images increase the chance of misregistration. Moreover, fast methods often hav...
Background: Conventional magnetic resonance imaging (MRI) has relatively long scan
• A method for segmenting the brain and estimating tissue volume is presented • This method measures white matter, grey matter, cerebrospinal fluid and remaining tissue • The method calculates tissue fractions in voxel, thus accounting for partial volume • Repeatability was 2.2% for total brain volume with imaging resolution <2.0 mm.
In Multiple Sclerosis (MS) the relationship between disease process in normal-appearing white matter (NAWM) and the development of white matter lesions is not well understood. In this study we used single voxel proton ‘Quantitative Magnetic Resonance Spectroscopy’ (qMRS) to characterize the NAWM and thalamus both in atypical ‘Clinically Definite MS’ (CDMS) patients, MRIneg (N = 15) with very few lesions (two or fewer lesions), and in typical CDMS patients, MRIpos (N = 20) with lesions, in comparison with healthy control subjects (N = 20). In addition, the metabolite concentrations were also correlated with extent of brain atrophy measured using Brain Parenchymal Fraction (BPF) and severity of the disease measured using ‘Multiple Sclerosis Severity Score’ (MSSS). Elevated concentrations of glutamate and glutamine (Glx) were observed in both MS groups (MRIneg 8.12 mM, p<0.001 and MRIpos 7.96 mM p<0.001) compared to controls, 6.76 mM. Linear regressions of Glx and total creatine (tCr) with MSSS were 0.16±0.06 mM/MSSS (p = 0.02) for Glx and 0.06±0.03 mM/MSSS (p = 0.04) for tCr, respectively. Moreover, linear regressions of tCr and myo-Inositol (mIns) with BPF were −6.22±1.63 mM/BPF (p<0.001) for tCr and −7.71±2.43 mM/BPF (p = 0.003) for mIns. Furthermore, the MRIpos patients had lower N-acetylaspartate and N-acetylaspartate-glutamate (tNA) and elevated mIns concentrations in NAWM compared to both controls (tNA: p = 0.04 mIns p<0.001) and MRIneg (tNA: p = 0.03 , mIns: p = 0.002). The results suggest that Glx may be an important marker for pathology in non-lesional white matter in MS. Moreover, Glx is related to the severity of MS independent of number of lesions in the patient. In contrast, increased glial density indicated by increased mIns and decreased neuronal density indicated by the decreased tNA, were only observed in NAWM of typical CDMS patients with white matter lesions.
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