Denoising parameter dependence of coronary artery depictability in compressed sensing magnetic resonance angiography.

Using numerical indices and visual evaluation, we evaluated the dependence of coronary-artery depictability on the denoising parameter in compressed sensing magnetic resonance angiography (CS-MRA). This study was conducted to clarify the acceleration factor (AF) and denoising factor (DF) dependence of CS-MRA image quality. Vascular phantoms and clinical images were acquired using three-dimensional CS-MRA on a clinical 1.5 T system. For the phantom measurements, we compared the full width at half maximum (FWHM), sharpness, and contrast ratio of the vascular profile curves for various AFs and DFs. In the clinical cases, the FWHM, sharpness, contrast ratio, signal-to-noise ratio, noise level values, and visual evaluation results were compared for various DFs. Phantom image analyses demonstrated that the respective measurements of the FWHM, sharpness, and contrast ratios did not significantly change with an increase in AF. The FWHM and sharpness measurements slightly changed with the DF level. However, the contrast ratio tended to increase with an increase in the DF level. In the clinical cases, the FWHM and sharpness showed no significant differences, even when the DF level was changed. However, the contrast ratio tended to decrease as the DF level increased. When the DF levels of the clinical cases increased, the background signals of the myocardium, fat, and noise levels decreased. We investigated the dependence of the coronary-artery depictability on AF and DF using CS-MRA. Analysis of the coronary-artery profile curves indicated that a better image quality was achieved with a stronger DF on coronary CS-MRA.

Comparison of spin-echo echo planar imaging and gradient-recalled echo sequences in magnetic resonance elastography of liver at 1.5T same MRI scanner.

PURPOSE: Magnetic resonance elastography (MRE) is used to measure liver stiffness with gradient-recalled echo (GRE)-based and spin-echo echo planar imaging (SE-EPI)-based sequences. We compared the liver stiffness (LS) values of the two sequences on a 1.5-T MR imaging scanner. METHODS: This is a retrospective study. An MRE imaging section was obtained from a horizontal section of the liver. Region of interest was drawn on the elastogram, and the mean LS and pixel values were measured and compared. The correlations between proton density fat fraction, R2* values, and biochemical data from electronic medical records were confirmed, and multivariate analysis was performed. RESULTS: The mean LS values were 3.01 ± 1.78 kPa for GRE and 3.13 ± 1.57 kPa for SE-EPI, showing excellent agreement and a strong correlation between the two sequences (correlation coefficient r = 0.96). The mean pixel values were 369.5 ± 142.7 pixels for GRE and 490.1 ± 197.9 pixels for SE-EPI, showing a significant difference by the Wilcoxon rank sum test (p < 0.01). There were no LS unmeasurable cases in SE-EPI, but seven (2.5%) were unmeasurable in GRE, and multivariate analysis showed a significant difference with p < 0.001 in R2* values (mean, 92.7 Hz) for the GRE method. CONCLUSION: The GRE and SE-EPI methods were comparable for LS measurements in 1.5-T liver MRE, indicating that SE-EPI MRE is more useful because GRE MRE may not measure cases with high R2* values and the region of interest tends to be smaller.

Noise power spectrum in compressed sensing magnetic resonance imaging.

Compressed sensing magnetic resonance imaging (CS-MRI) uses random undersampling and nonlinear iterative reconstruction. This study was conducted to clarify the noise power spectrum (NPS) characteristics of CS-MRI. We measured two-dimensional (2D) NPS of CS-MRI with various acceleration factors (AF) and denoising factors (DF) and compared their appearance to those of conventional parallel MR images. Results showed that the 2D NPS of CS-MRI exhibited the following characteristics: (1) local decrease in the low-frequency region, (2) gradual decrease in the high-frequency region, and (3) a stripe pattern aligned at unequal intervals in the phase-encoding direction. Specifically, the 2D NPS of CS-MRI reflects the random undersampling pattern of k-space data. Additionally, 2D NPS allowed visualization of AF-dependent noise characteristics of CS-MRI. Furthermore, 1D NPS graph shapes clarified the CS-MRI noise characteristic dependence on AF and DF.

Influence of Gd-EOB-DTPA on T1 dependence of the proton density fat fraction using magnetic resonance spectroscopy.

This phantom study assessed the effect of Gd-EOB-DTPA on T1 bias (difference in T1 between water and fat) of the proton density fat fraction when using magnetic resonance spectroscopy. Phantoms containing varying fat percentages, without and with Gd-EOB-DTPA (precontrast and postcontrast, respectively), were scanned with repetition times ranging from 1000 to 5000 ms. The relationship between the proton density fat fraction at a reference repetition time of 5000 ms and that using different repetition times, was evaluated in the precontrast and postcontrast phantoms using linear regression and Bland-Altman analyses. In the precontrast phantom, as the repetition time increased, the slope tended to approach one. In the postcontrast phantom, the slope and intercept were near one and zero, respectively. The mean difference was smaller in the postcontrast phantom (range - 0.24 to - 0.01%) than in the precontrast phantom (range 0.12 to 3.52%). We conclude that I1 bias is minimized by Gd-EOB-DTPA.

Correction to: Influence of Gd-EOB-DTPA on proton density fat fraction using the six-echo Dixon method in 3 Tesla magnetic resonance imaging.

A diameter of glass bottles in phantoms in the above article (2 cm) was incorrect. The correct diameter is 4.5 cm.

Multicenter, multivendor phantom study to validate proton density fat fraction and T2* values calculated using vendor-provided 6-point DIXON methods.

PURPOSE: To evaluate the reproducibility of proton density fat fraction (PDFF) and T2* in a fat-water phantom on three different 3 T MRI systems using 6-point DIXON methods. METHODS: A phantom which included varying fat volume percentages (true fat fraction [FF]) was scanned by three 3 T MR machines, and PDFF and T2* were measured. RESULTS: The mean difference between true FF and PDFF was small in all vendors (-2.11% to 0.41%). However, the difference ratio for T2* values was large among vendors (1.79 to 3.36). CONCLUSIONS: True FF and PDFF were consistent across vendors; however, T2* varied greatly.

Influence of Gd-EOB-DTPA on proton density fat fraction using the six-echo Dixon method in 3 Tesla magnetic resonance imaging.

The purpose of this study was to evaluate whether disodium gadoxetate (Gd-EOB-DTPA) affects proton density fat fractions (PDFFs) during use of the multiecho Dixon (meDixon) method in phantom and simulation magnetic resonance imaging (MRI) studies at 3 T. Fat-water phantoms comprising vegetable fat-water emulsions with varying fat volume percentages (0, 5, 10, 15, 20, 30, 40, and 50) and Gd-EOB-DTPA concentrations (0 and 0.4 mM) were prepared. Phantoms without Gd-EOB-DTPA were defined as precontrast, and those with Gd-EOB-DTPA were defined as postcontrast. All phantoms were scanned with a 3 T MRI system using the meDixon method, and precontrast and postcontrast PDFFs were calculated. Simulated pre and postcontrast PDFFs in the liver were calculated using a theoretical formula. The relationship between PDFFs measured in the pre and postcontrast phantoms was evaluated using linear regression and Bland-Altman analysis. The regression analysis comparing the pre and postcontrast PDFFs yielded a slope of 0.77 (P < 0.001). The PDFFs on the postcontrast phantom were smaller than those on the precontrast phantom. The mean difference between the PDFFs on the pre and postcontrast phantoms was 6.12% (95% confidence interval 3.13 to 9.10%; limits of agreement -0.88 to 13.11%). The simulated PDFF on the postcontrast phantom was smaller than that on the precontrast phantom. We demonstrated that the PDFF measured using the meDixon was smaller on postcontrast than on precontrast at 3 T, if a low flip angle was used. This tendency was also seen in the simulation study results.

Evaluation of six-point modified dixon and magnetic resonance spectroscopy for fat quantification: a fat-water-iron phantom study.

This study aimed to evaluate (1) the agreement between the true fat fraction (FF) and proton density FF (PDFF) measured using a six-echo modified Dixon (6mDixon) and magnetic resonance spectroscopy (MRS) and (2) the influence of fat on T2* values. The study was performed using phantoms of varying fat and iron content. Point-resolved spectroscopy (PRESS) and stimulated echo acquisition mode (STEAM) with single-echo (S) and multiecho (M) (PRESS-S, PRESS-M, STEAM-S, and STEAM-M) were used for MRS. In phantoms without iron, the agreement between the true FF and measured PDFF was tested using Bland-Altman analysis. The influence of iron on PDFF was evaluated in phantoms with iron. The relationship between the true FF and T2* value was assessed in phantoms without iron, wherein the mean differences (limits of agreement) for each method were as follows: 6mDixon 2.9% (-2.4 to 8.1%); STEAM-S 3.2% (-9.5 to 16.0%); STEAM-M -0.7% (-6.9 to 5.5%); PRESS-S 8.9% (-14.5 to 32.4%); and PRESS-M -5.8% (-18.3 to 6.7%). In the 20% fat phantoms with iron, as iron increased, PDFFs with STEAM-S, PRESS-S, and PRESS-M were considerably overestimated, while, PDFF with STEAM-M was stable at 0.04-0.2 mM iron concentrations (17.2 and 21.4%, respectively), and PDFF with 6mDixon was reliable at even 0.4 mM iron concentration (24.8%). The T2* value showed a negative correlation with the true FF (r = -0.942, P = 0.005). STEAM-M and 6mDixon were reliable methods of fat quantification in the absence of iron, and the T2* value was shortened by fat.

Noninvasive Assessment of Advanced Fibrosis Based on Hepatic Volume in Patients with Nonalcoholic Fatty Liver Disease.

BACKGROUND/AIMS: Noninvasive liver fibrosis evaluation was performed in patients with nonalcoholic fatty liver disease (NAFLD). We used a quantitative method based on the hepatic volume acquired from gadoxetate disodium-enhanced (Gd-EOB-DTPA-enhanced) magnetic resonance imaging (MRI) for diagnosing advanced fibrosis in patients with NAFLD. METHODS: A total of 130 patients who were diagnosed with NAFLD and underwent Gd-EOB-DTPA-enhanced MRI were retrospectively included. Histological data were available for 118 patients. Hepatic volumetric parameters, including the left hepatic lobe to right hepatic lobe volume ratio (L/R ratio), were measured. The usefulness of the L/R ratio for diagnosing fibrosis ≥F3-4 and F4 was assessed using the area under the receiver operating characteristic (AUROC) curve. Multiple regression analysis was performed to identify variables (age, body mass index, serum fibrosis markers, and histological features) that were associated with the L/R ratio. RESULTS: The L/R ratio demonstrated good performance in differentiating advanced fibrosis (AUROC, 0.80; 95% confidence interval, 0.72 to 0.88) from cirrhosis (AUROC, 0.87; 95% confidence interval, 0.75 to 0.99). Multiple regression analysis showed that only fibrosis was significantly associated with the L/R ratio (coefficient, 0.121; p<0.0001). CONCLUSIONS: The L/R ratio, which is not influenced by pathological parameters other than fibrosis, is useful for diagnosing cirrhosis in patients with NAFLD.

Influence of Gadoxetate Disodium on Oxygen Saturation and Heart Rate during Dynamic Contrast-enhanced MR Imaging.

PURPOSE: To investigate whether gadoxetate disodium affects peripheral capillary oxygen saturation (SpO2) and/or heart rate (HR) during dynamic contrast material-enhanced (DCE) magnetic resonance (MR) imaging in patients with liver diseases. MATERIALS AND METHODS: This retrospective study was approved by the institutional review board, who waived the requirement for informed consent. Four hundred fifty-eight patients (171 women [mean age, 66.5 years; range, 23-87 years] and 287 men [mean age, 61.1 years; range, 25-89 years]) who underwent liver DCE MR imaging with gadoxetate disodium (0.025 mmol per kilogram of body weight) from October 28, 2013, to June 24, 2014, were included in this study. They were monitored for SpO2 and HR during DCE MR imaging. Motion artifact severity was graded by using a five-point scale, and transient severe motion (TSM) was defined by a score of at least 4. The association between TSM and baseline predictors was assessed, and HR and SpO2 at each postcontrast phase were compared with those at the precontrast phase in the TSM and non-TSM groups. RESULTS: Four hundred thirty-six patients were included in the non-TSM group, and 22 were included in the TSM group. Although the motion score was the worst at the arterial phase, the observed mean differences in SpO2 and HR between the precontrast phase and the arterial phase were less than 1% and 5 beats per minute, respectively (mean SpO2 ± standard deviation for the non-TSM group, 96.7% ± 1.8 vs 96.9% ± 1.8 [P = .11]; SpO2 for the TSM group, 96.4% ± 1.6 vs 96.1% ± 1.6 [P > .99]) (HR for the non-TSM group, 68.9 beats per minute ± 12.4 vs 70.9 beats per minute ± 12.1 [P < .0001]; HR for the TSM group, 75.0 beats per minute ± 11.8 vs 79.9 beats per minute ± 10.2 [P < .0001]). CONCLUSION: Intravenous gadoxetate disodium (a weight-based dose) does not cause changes in SpO2 and HR that lead to image quality degradation.

Diffusion analysis with triexponential function in hepatic steatosis.

Our purpose was to assess the influence of liver steatosis on diffusion by triexponential analysis. Thirty-three patients underwent diffusion-weighted magnetic resonance imaging with multiple b values for perfusion-related diffusion, fast free diffusion, and slow restricted diffusion coefficients (D p, D f, D s) and fractions (F p, F f, F s). They also underwent dual-echo gradient-echo imaging for measurement of the hepatic fat fraction (HFF). Of these, 13 patients were included in the control group and 20 in the fatty liver group with HFF >5 %. The parameters of the two groups were compared by use of the Mann-Whitney U test. The relationships between diffusion coefficients and HFFs were assessed by use of the Pearson correlation. D p and D f were reduced significantly in the steatotic liver group compared with those in the control group (D p = 27.72 ± 6.61 × 10(-3) vs. 33.33 ± 6.47 × 10(-3) mm(2)/s, P = 0.0072; D f = 1.70 ± 0.53 × 10(-3) vs. 2.06 ± 0.40 × 10(-3) mm(2)/s, P = 0.0224). There were no significant differences in the other parameters between the two groups. Furthermore, D p and D f were correlated with HFF (P < 0.0001, r = -0.64 and P = 0.0008, r = -0.56, respectively). Decreased liver perfusion in steatosis caused the reduction in D p, and extracellular fat accumulation and intracellular fat droplets in steatosis led to the reduction in D f. Thus, the influence of hepatic steatosis should be taken into consideration when triexponential function analysis is used for assessment of diffuse liver disease.

Diffusion analysis with triexponential function in liver cirrhosis.

PURPOSE: To acquire more detailed information noninvasively through on diffusion and perfusion in normal and cirrhotic livers, we analyzed three diffusion components using triexponential function. MATERIALS AND METHODS: Thirty-nine subjects (10 with noncirrhotic liver, 29 with cirrhosis) were assessed using diffusion-weighted magnetic resonance imaging (DWI) with multiple b-values. We derived perfusion-related diffusion, fast free diffusion, and slow restricted diffusion coefficients (Dp , Df , Ds ) and fractions (Fp , Ff , Fs ) calculated from triexponential function using DWI data. Moreover, the triexponential analysis was compared with biexponential and monoexponential analyses. All derived diffusion coefficients were correlated with relative enhancement ratio (RER) using dynamic contrast-enhanced MRI. RESULTS: In triexponential analysis, Fp , Dp , and Ds were significantly reduced in cirrhosis, whereas Ff was significantly increased in cirrhosis. There was no correlation between each diffusion coefficient obtained with the triexponential analysis in both groups, i.e., Dp , Df , and Ds , did not necessarily provide the same kind of information, but there was a positive correlation between each diffusion coefficient with the biexponential analysis in cirrhosis. A positive correlation was found between Dp and RER in the portal phase. CONCLUSION: Triexponential analysis makes it possible to noninvasively obtain more detailed tissue diffusion and perfusion information and to assist in the diagnosis of liver cirrhosis.

Three-dimensional magnetic resonance imaging for stringent diagnosis of advanced fibrosis associated with nonalcoholic steatohepatitis.

BACKGROUND: The definitive diagnosis of nonalcoholic steatohepatitis (NASH) is currently based on histopathological assessment. This study aimed to elucidate the utility of a novel noninvasive method, three-dimensional magnetic resonance imaging (3D-MRI), for diagnosing advanced fibrosis in patients with NASH, using histopathological diagnosis as the reference standard. METHODS: This retrospective study included 30 consecutive patients who had been diagnosed with NASH by histopathology and had undergone 3D-MRI before biopsy. 3D-MRI provided a three-dimensional reconstruction of the liver from contrast-enhanced hepatobiliary phase MR images. In the present study, histopathological advanced fibrosis was defined as stage 3 and 4 NASH. Advanced fibrosis, diagnosed by 3D-MRI, was considered to be diffuse irregularity of the entire surface of the liver. The diagnostic features of 3D-MRI and the noninvasive evaluation systems (APRI, FIB-4 index, and BARD score) for identifying advanced and nonadvanced fibrosis of NASH were determined and compared. RESULTS: Nine (30 %) of the 30 study patients were diagnosed histopathologically with advanced fibrosis, and 11 (37 %) of 30 patients were diagnosed with advanced fibrosis using 3D-MRI. The sensitivity, specificity, positive predictive value (PPV), and negative predictive value (NPV) of 3D-MRI for diagnosing advanced fibrosis were 100, 90, 82, and 100 %, respectively. The sensitivities of APRI, the FIB-4 index, and BARD score ranged from 78 to 89 %, the specificities from 71 to 90 %, the PPVs from 54 to 78 %, and the NPVs from 88 to 94 %. CONCLUSION: Compared with the common noninvasive methods for diagnosing advanced fibrosis associated with NASH, 3D-MRI was more accurate.