Weavers, Paul T.

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.

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.