Quantitative rapid imaging has been an area of focus in the field of MRI. Quantitative MRI can provide data that can be used as imaging biomarkers for better characterization of tissue pathology, follow-up, patient-specific management, prognosis, and proper therapy. Conventional approaches are relatively time-consuming and typically measure only one tissue property at a time. Parallel imaging, compressed sensing, synthetic imaging, and MR fingerprinting (MRF) are current solutions to accelerate MRI data acquisition. Recent innovations and research aspects of rapid and quantitative MRI in musculoskeletal imaging will be presented with the aim of understanding synthetic and MRF imaging.
Quantitative Imaging
Synthetic MRI and MR Fingerprinting: Research Perspective
Magnetic resonance (MR) imaging is important in the diagnosis of musculoskeletal disorders owing to its excellent tissue contrast. In the last few years, quantitative and rapid imaging has been an area of focus in the field of magnetic resonance imaging (MRI). Quantitative MR imaging can provide data that can be used as imaging biomarkers for better characterization of tissue pathology, follow-up, patient-specific management, prognosis prediction, and proper therapy [1]. MR imaging allows the measurement of various tissue properties such as T1 longitudinal relaxation time, T2 transverse relaxation time, proton density (M0), diffusion, and perfusion [2]. Conventional T1 and T2 mapping methods measure tissue properties by measuring signal changes obtained by varying a single acquisition parameter, keeping all others constant. These approaches are relatively time-consuming and typically measure only one tissue property at a time [3]. Even with recent advancements in MR scanners, MRI is still a slow imaging: uncomfortable for patients, potential for motion-related artifacts, limits clinical indication and availability, and increases healthcare cost. Rapid MR imaging can improve the patients’ experience and can make MRI more efficient by optimizing the radiologic workflow [4, 5]. This, in turn, could lead to an increased emphasis on patient-centered care, while decreasing reimbursements and improving MR technology. Rapid MR imaging data acquisition is essential in musculoskeletal imaging because patients with musculoskeletal disorder-related pain tend to move, causing motion artifacts, especially when the MR scan time is long. Furthermore, long MRI scan times increase the costs and limit the number of patients for whom MRI is necessary. From the viewpoint of patient comfort, reducing the scan time is also helpful. Patients can experience boredom or discomfort during scans, and the scanned images may contain motion artifacts [5]. Parallel imaging, compressed sensing, synthetic imaging, and MR fingerprinting (MRF) are current solutions to accelerate MRI data acquisition. Synthetic MR imaging is a promising and feasible acceleration MR imaging technique. With synthetic MRI, radiologists can review multi-contrast images in a single scan. Synthetic MR imaging is a novel method that generates T1-weighted, T2-weighted, PD-weighted, and inversion recovery images based on MR quantification (relaxation times and PD) within a single scan [6-8]. The settings of echo time (TE), repetition time (TR), and inversion time (TI) can be modified by the reader. This enables the generation of T1-weighted, T2-weighted, PD-weighted, and inversion recovery images in a single scan, resulting in different tissue contrast with one acquisition, and may also reduce the entire MR scan time [4, 5]. Recent research has shown the feasibility of conventional MR images [4, 5, 8] as well as quantitative T2 mappings of cartilage [9]. A novel approach of MRF was recently introduced [10]. In this approach, the MRF uses an acquisition in which imaging unit settings are allowed to vary greatly in a seemingly random manner to generate incoherent magnetization signals. This technique allows simultaneous and efficient measurements of multiple tissue properties with one acquisition [2, 10]. While the original MRF description focused on measuring T1, T2, static magnetic field (B0) inhomogeneity, or off-resonance frequency and proton density M0 [10], recent works on the MRF have shown the feasibility to measure other properties such as field inhomogeneity in the radio frequency transmit (B1) [11], T1 and T2*[12], and perfusion [13] and microvascular properties [14]. In this lecture, recent innovations and research aspects of rapid and quantitative MR imaging in musculoskeletal imaging will be presented with the aim of understanding synthetic and MRF imaging. These techniques could be used in various combinations and at various magnetic field strengths in clinical and research settings to improve musculoskeletal imaging. Therefore, optimization of acquisition parameters and clinical applications of MRF is a topic of high interest for research.1. O'Connor JP, Aboagye EO, Adams JE, Aerts HJ, Barrington SF, Beer AJ, et al. Imaging biomarker roadmap for cancer studies. Nat Rev Clin Oncol. 2017;14(3):169-86.
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