Riederer, Stephen J.

PurposeThe purpose of this work is to derive and demonstrate constrained‐phase dual‐echo Dixon imaging within a maximum likelihood framework solved with a regularized graph‐cuts‐guided optimization.Theory and MethodsDual‐echo Dixon reconstruction is fundamentally underdetermined; however, adopting a constrained‐phase signal model reduces the number of unknowns and the nonlinear problem can be solved under a maximum likelihood framework. Period shifts in the field map (manifesting as fat/water signal swaps) must also be corrected. Here, a regularized cost function promotes a smooth field map and is solved with a graph‐cuts‐guided greedy binary optimization. The reconstruction shown here is compared to two other prevalent Dixon reconstructions in experimental phantom and human studies.ResultsReconstructed images of the water and fat signal are shown for a phantom study, and in vivo studies of foot/ankle, pelvis, and CE‐MRA of the thighs. The method shown here compared favorably with the other two methods. Large field inhomogeneities on the order of 20 ppm were resolved, thereby avoiding the fat and water signal swaps present in images reconstructed with the other methods.ConclusionConstrained‐phase dual‐echo Dixon imaging solved with a regularized graph‐cuts‐guided optimization has been derived and demonstrated to successfully separate water and fat images in the presence of large magnetic field inhomogeneities. Magn Reson Med 78:2203–2215, 2017. © 2017 International Society for Magnetic Resonance in Medicine.

We sought to obtain high resolution 3D contrast-enhanced MR angiograms of the carotid arteries, vertebral arteries and aortic arch with venous signal suppression utilizing novel timing techniques. A secondary purpose was to compare contrast-enhanced MR angiograms with 2D time-of-flight MR angiograms of the carotid arteries.

PurposeThe purpose of this work is to compare the behavior of the signal‐to‐noise ratio (SNR) and contrast‐to‐noise ratio (CNR) in contrast‐enhanced MR angiography with background suppression performed by either a Dixon‐type or subtraction‐type method.Theory and MethodsTheoretical expressions for the SNR and CNR for both background suppression techniques were derived. The theoretical Dixon:subtraction SNR and CNR ratios were compared to empirical ratios measured from phantom and in vivo studies for Dixon techniques utilizing one, two, and three echoes. Specifically, the SNR and CNR ratios were compared as the concentration of contrast material in the blood changed.ResultsEmpirical measurements of the SNR and CNR ratios compared favorably with the ratios predicted by theory. As the contrast concentration was reduced, the SNR advantage of the Dixon techniques increased asymptotically. In the ideal case, the SNR improvement over subtraction contrast‐enhanced MR angiography was at least twofold for one‐ and two‐echo Dixon techniques and at least a factor of for the three‐echo Dixon technique.ConclusionExpressions showing a contrast concentration‐dependent SNR and CNR improvement of at least a factor of two when Dixon‐type contrast‐enhanced MR angiography is used in place of subtraction‐type contrast‐enhanced MR angiography were derived and validated with phantom and in vivo experiments. Magn Reson Med 74:81–92, 2015. © 2014 Wiley Periodicals, Inc.

PurposeTo develop and validate a method for choosing the optimal two‐dimensional CAIPIRINHA kernel for subtraction contrast‐enhanced MR angiography (CE‐MRA) and estimate the degree of image quality improvement versus that of some reference acceleration parameter set at R ≥ 8.MethodsA metric based on patient‐specific coil calibration information was defined for evaluating optimality of CAIPIRINHA kernels as applied to subtraction CE‐MRA. Evaluation in retrospective studies using archived coil calibration data from abdomen, calf, foot, and hand CE‐MRA exams was accomplished with an evaluation metric comparing the geometry factor (g‐factor) histograms. Prospective calf, foot, and hand CE‐MRA studies were evaluated with vessel signal‐to‐noise ratio (SNR).ResultsRetrospective studies show g‐factor improvement for the selected CAIPIRINHA kernels was significant in the feet, moderate in the abdomen, and modest in the calves and hands. Prospective CE‐MRA studies using optimal CAIPIRINHA show reduced noise amplification with identical acquisition time in studies of the feet, with minor improvements in the hands and calves.ConclusionA method for selection of the optimal CAIPIRINHA kernel for high (R ≥ 8) acceleration CE‐MRA exams given a specific patient and receiver array was demonstrated. CAIPIRINHA optimization appears valuable in accelerated CE‐MRA of the feet and to a lesser extent in the abdomen. Magn Reson Med 73:2234–2242, 2015. © 2014 Wiley Periodicals, Inc.

Gastric emptying, accommodation, and motility can be quantified with magnetic resonance imaging (MRI). The first step in image analysis entails segmenting the stomach from surrounding structures, usually by a time-consuming manual process. We have developed a semiautomated process to segment and measure gastric volumes with MRI. Gastric images were acquired with a three-dimensional gradient echo MRI sequence at 5, 10, 20, and 30 min after ingestion of a liquid nutrient (Ensure, 296 ml) labeled with gadolinium in 20 healthy volunteers and 29 patients with dyspeptic symptoms. The agreement between gastric volumes measured by manual segmentation and our new semiautomated algorithm was assessed with Lin's concordance correlation coefficient (CCC) and the Bland Altman test. At 5 min after a meal, food volumes measured by manual (352 ± 4 ml) and semiautomated (346 ± 4 ml) techniques were correlated {CCC[95% confidence interval (CI)] 0.70 (0.52, 0.81)}; air volumes measured by manual (88 ± 6 ml) and semiautomated (84 ± 6 ml) techniques were also correlated [CCC (95% CI) 0.89 (0.82, 0.94)]. Findings were similar at subsequent time points. The Bland Altman test was not significant. The time required for semiautomated segmentation ranged from an average of 204 s for the 5-min images to 233 s for the 20-min images. These times were appreciably smaller than the typical times of many tens of minutes, even hours, required for manual segmentation. To conclude, a semiautomated process can measure gastric food and air volume using MRI with comparable accuracy and far better efficiency than a manual process.

PurposeIn 2D SENSE‐accelerated 3D Cartesian acquisition, the net acceleration factor R is the product of the two individual accelerations, R = RY × RZ. Acceleration Apportionment tailors acceleration parameters (RY, RZ) to improve parallel imaging performance on a patient‐ and coil‐specific basis and is demonstrated in contrast‐enhanced MR angiography.MethodsA performance metric is defined based on coil sensitivity information which identifies the (RY, RZ) pair that optimally trades off image quality with scan time reduction on a patient‐specific basis. Acceleration Apportionment is evaluated using retrospective analysis of contrast‐enhanced MR angiography studies, and prospective studies in which optimally apportioned parameters are compared with standard acceleration parameters.ResultsThe retrospective studies show strong variability in optimal acceleration parameters between anatomic regions and between patients. Prospective application of apportionment to foot contrast‐enhanced MR angiography with an 8‐channel receiver array provides a 20% increase in net acceleration with improved contrast‐to‐noise ratio. Application to 16‐channel contrast‐enhanced MR angiography of the feet and calves suggests 10% acceleration increase to R > 13 and no contrast‐to‐noise ratio loss. The specific implementation allows the optimum (RY, RZ) pair to be determined within one minute.ConclusionOptimum 2D SENSE acceleration parameters can be automatically chosen on a per‐exam basis to allow improved performance without disrupting the clinical workflow. Magn Reson Med 71:672–680, 2014. © 2013 Wiley Periodicals, Inc.