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    • br Introduction Magnetic resonance imaging MRI

    2019-04-25


    Introduction Magnetic resonance imaging (MRI) has great utility in evaluating musculoskeletal tumors. However, some lesions have an appearance on MRI that make it challenging to differentiate benign from malignant lesions. Even intravenous (IV) biotin-LC-LC-tyramide sometimes does not allow differentiation of benign from malignant lesions: hemangiomas, Schmorl\'s nodes, and degenerative vertebral endplate changes all enhance with IV contrast. Opposed-phase MRI (also termed chemical shift imaging, or in-and-out of phase imaging) has shown value in this setting based on its ability to detect small amounts of fat, suggestive of a benign process. In these opposed-phase sequences, signal characteristics consistent with normal fat tissue are not found in malignant, marrow-replacing bone lesions [1,2]. This imaging technique exploits the difference in signal seen on in-phase (IP) and out-of-phase (OP) sequences: this difference is based on the phenomenon that hydrogen atoms attached to water and lipid precess at different frequencies. On the in-phase imaging, fat and water signals are additive when these tissues are in the same voxel. On OP images, the two vectors are opposite, resulting in the two signals canceling. When lipid and water exist simultaneously in a benign lesion, the result is a drop in signal on OP images when compared to IP images of the same lesion (see Fig. 1a–d). IP and OP sequences are easily acquired during a standard musculoskeletal protocol, using a dual gradient-echo technique with T1 weighting. Protocol for IP and OP MRI differs depending on the strength of the magnetic field: with a 1.5 T magnet, the interval on TE (time to echo) between IP and OP images is 2.3ms. The images for both opposed and in-phase-imaging can be taken in about 20–30s, or a single breath hold. Many benign bone lesions contain variable amounts of fat, while malignant bone lesions replace or destroy the fatty bone marrow. A drop in signal on OP images indicates at least some fat content in the lesion, suggestive of a benign process (see Fig. 2a–d). Imaging software uses a region of interest (ROI) to measure the drop in signal, thus yielding a quantitative result. Previous studies have determined a drop in signal of 20% or more to be suggestive of a benign lesion [3]. A drop of 20% on OPI was found to capture all malignant lesions in the study, although this cut-off did allow for some benign lesions being included in the malignant category [3]. This 20% threshold seems to maximize the sensitivity and specificity of the test. A malignant lesion tends to contain little or no fat, resulting in little drop in signal from in-phase to out-of-phase sequences (see Fig. 3a–d). Previous research has shown that this correlates to the histology of the lesions [3]. The hypothesis of our study is that these sequences may play an important role in the imaging of musculoskeletal tumors, enhancing the ability to differentiate benign from malignant disease states and in some cases obviating the need for biopsy. Our goal is to investigate and quantify the extent of this capability, using data collected from blinded radiologists regarding their concern for malignancy, accuracy in diagnosis, and confidence in reading these imaging studies.
    Materials and methods All MR imaging was performed by using a 1.5T unit (various vendors) and a phased-array surface coil. The following pulse sequences were used for all patients as part of our standard institutional protocol: T1-weighted spin-echo (400–700/8–16 [repetition time ms/echo time ms]) and fat-suppressed T2-weighted fast spin-echo (2000–5000/80–100, with an echo train length of 8) sequences, followed by in-phase (100–165/4.6; flip angle, 30°) and out-of-phase (100–165/2.3; flip angle, 30°) fast multiplanar spoiled gradient-echo sequences acquired using Dixon technique. Apart from TE, all other parameters were held constant for acquisition of in-phase and out-of-phase images. The field of view for exams varied depending on coverage required for the body area being scanned; the acquisition matrix was typically 256×192, and the section thickness and gap were typically 4.0mm, with a skip of 1.0mm.