Original contributionDevelopment of a high-precision image-processing automatic measurement system for MRI visceral fat images acquired using a binomial RF-excitation pulse
Introduction
The increased number of cases of metabolic syndrome is a cause of concern [1], [2], [3]. Metabolic syndrome is a condition in which insulin action is reduced due to visceral fat accumulation and risk factors for arteriosclerosis, including abnormal glucose metabolism (abnormal glucose tolerance, diabetes), abnormal lipid metabolism (hypertriglyceridemia, hypo-high-density lipoprotein cholesterolemia), and hypertension. Each factor independently increases the risk, and the incidence of arteriosclerosis is increased synergistically when multiple factors are present concomitantly. Visceral fat accumulation is an essential requirement for diagnosis of metabolic syndrome, along with the presence of two conditions among hyperglycemia, hypertension and hyperlipidemia [4], [5], [6], [7].
Since visceral fat accumulation is present in metabolic syndrome, accurate measurement of the level of visceral fat is important as a basis for diagnosis of the disorder. In current measurements (or estimations) of the amount of visceral fat, waist circumference is measured as a simple method [8], [9], and X-ray computed tomography (CT) and magnetic resonance imaging (MRI) are used for precise examination [10], [11], [12]. In X-ray CT, only a single cross-sectional view at the navel level is acquired to reduce X-ray exposure, but accurate, reproducible fat measurement is difficult using imaging of only a single slice because the visceral organs move while the patient is lying on a bed. MRI for abdominal imaging is advantageous since there is no X-ray exposure, but magnetic field homogeneity is likely to be disturbed by respiratory movement and air in the intestine [13], [14], [15]. Fat-selective MRI is commonly performed using the FLAIR (fluid-attenuated inversion recovery), Dixon, Chess (chemical shift selective saturation), and Binominal methods [16]. The scanning time is prolonged in FLAIR due to use of an inversion recovery (IR) pulse and in the Dixon method due to the requirement for fusion of multiple images. The Dixon method uses echo times or echo time shifts such that the phase difference of water and fat is at either 0 or a multiple of π at the echo times. The water suppression effect of the Dixon method is decreased in the abdominal region where a change of magnetic susceptibility is large and a magnetic field is inhomogeneous. In the Chess method, fat suppression decreases in regions in which the magnetic field is relatively heterogeneous or the magnetic susceptibility changes markedly, which is a major problem in abdominal imaging.
In this study, we optimized a sequence using a 1-2-1 binomial radiofrequency (RF) excitation pulse, in which the acquisition time is short and motion artifact is less influential, and measured changes in the amount of visceral fat after wearing of clothes with a fat-reducing effect during walking (M2). We acquired images of several cross-sections over the abdomen, rather than a single slice, and investigated the reproducibility of measurements by calculating the fat volume, rather than the area, from several slices (M1). We also developed software for high-speed high-precision visceral fat image-processing automatic measurement, thereby reducing the influence of coil sensitivity heterogeneity and increasing its applicability in clinical practice (M3).
Section snippets
Materials and methods
The subjects were 20 healthy adult volunteers. The study was fully explained to the subjects and informed consent was obtained. The study was also approved by the Ethics Committee of our institute. Images centering on the navel were acquired in the supine position using a Magnetom Sonata 1.5 T (maximum amplitude 40 mT/m, slew rate 200 mT/m per millisecond). MRI instrument (Siemens) with a Phased Array Coil and a Spine Coil. MR images were taken in the axial plane using the optimized T1-weighted
(M1) Number of slices and reproducibility
Images of the same cross-section at the navel level obtained in the first and second acquisitions in one subject are shown in Fig. 3. The amount of fat in the section changed due to visceral organ movement. The relationship between the number of acquired slices and the relative error of the fat volume is shown in Fig. 4. The relative error of the subcutaneous fat volume in a single slice was 1.68±0.58% and remained at a similar level as the number of slices increased. The relative error
Discussion
Abundant information with various contrasts can be obtained by MRI depending on the sequence and acquisition conditions. MRI is superior for imaging soft tissues such as fat, but image processing is difficult because of a lack of clear criteria. CT is advantageous in this respect due to establishment of the Hounsfield number [24], [25], but has the disadvantage of X-ray exposure, with an increased risk when many slices are imaged. The poor reproducibility of single-slice imaging due to visceral
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