Novel method for rapid, simultaneous T₁, T^*₂, and proton density quantification

Abstract

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...

Evaluation of Automatic Measurement of the Intracranial Volume Based on Quantitative MR Imaging

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“…11 Recently, a novel MR imaging quantification sequence, QRAPMASTER, was developed for rapid and simultaneous sampling of the longitudinal relaxation time (T1), the transversal relaxation time (T2), and PD. 13,14 Included in this measurement is the correction for the B1 field inhomogeneity. With dedicated software, SyMRI Brain Studio (SyntheticMR, Linköping, Sweden), the QRAPMASTER quantification maps can be processed to automatically segment the ICV, as well as CSF, gray matter, and white matter, a process called "synthetic tissue mapping."…”

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