Randall Kroeker

Optimal k-Space Sampling for Dynamic Contrast-Enhanced MRI with an Application to MR Renography

For time‐resolved acquisitions with k‐space undersampling, a simulation method was developed for selecting imaging parameters based on minimization of errors in signal intensity versus time and physiologic parameters derived from tracer kinetic analysis. Optimization was performed for time‐resolved angiography with stochastic trajectories (TWIST) algorithm applied to contrast‐enhanced MR renography. A realistic 4D phantom comprised of aorta and two kidneys, one healthy and one diseased, was created with ideal tissue time‐enhancement pattern generated using a three‐compartment model with fixed parameters, including glomerular filtration rate (GFR) and renal plasma flow (RPF). TWIST acquisitions with different combinations of sampled central and peripheral k‐space portions were applied to this phantom. Acquisition performance was assessed by the difference between simulated signal intensity (SI) and calculated GFR and RPF and their ideal values. Sampling of the 20% of the center and 1/5 of the periphery of k‐space in phase‐encoding plane and data‐sharing of the remaining 4/5 minimized the errors in SI (<5%), RPF, and GFR (both <10% for both healthy and diseased kidneys). High‐quality dynamic human images were acquired with optimal TWIST parameters and 2.4 sec temporal resolution. The proposed method can be generalized to other dynamic contrast‐enhanced MRI applications, e.g., MR angiography or cancer imaging. Magn Reson Med, 2009.

CAIPIRINHA-Dixon-TWIST (CDT)-volume-interpolated breath-hold examination (VIBE): a new technique for fast time-resolved dynamic 3-dimensional imaging of the abdomen with high spatial resolution.

PurposeThe purpose of this study was to assess the feasibility and image quality of a novel, highly accelerated T1-weighted sequence for time-resolved imaging of the abdomen during the first pass of contrast media transit using controlled aliasing in parallel imaging results in higher acceleration (CAIPIRINHA) under sampling, view-sharing techniques, and Dixon water-fat separation (CAIPRINHA–Dixon–time-resolved imaging with interleaved stochastic trajectories–volumetric interpolated breath-hold examination [CDT-VIBE]). Materials and MethodsIn this retrospective, institutional review board–approved study, 47 patients (median age, 62 years; 25 men, 22 women) scanned on a 3.0-T magnetic resonance system (Skyra; Siemens) were included. The CDT-VIBE (repetition time/echo time1/echo time2, 4.1/1.33/2.56 milliseconds; acquisition time, 29 seconds) was used in place of the standard arterial phase acquisition and started 15 seconds after the injection of 0.1 mmol/kg Gd-DOTA (Dotarem, Guerbet). Within 29 seconds, 14 high spatial resolution (1.2 × 1.2 × 3 mm3) 3-dimensional data sets were acquired and reconstructed using view sharing (temporal resolution, 2.1 seconds). The CDT-VIBE images were evaluated independently by 2 blinded, experienced radiologists with regard to image quality and the number of hepatic arterial–dominant phases present on an ordinal 5-point scale (5, excellent; 1, nondiagnostic). Added diagnostic information with CDT-VIBE relative to portal venous phase VIBE was assessed. ResultsIn all patients, CDT-VIBE measurements were successfully acquired. The image quality was diagnostic in 46 of the 47 patients. Both readers assessed the highest image quality present in the data sets with a median score of 4 (range, 3–5 for both readers; &kgr;, 0.789) and the worst image quality with a median score of 3 (range, 1–4 for both readers; &kgr;, 0.689). With a range between 1 and 8 (median, 5), hepatic arterial–dominant data sets (of the 14 acquired) were obtained in each case. There was an added diagnostic value with CDT-VIBE in 10 of the 47 patients (21%). ConclusionsThe CDT-VIBE is a robust approach allowing, for the first time, dynamic imaging of the upper abdomen with high temporal resolution and preservation of high spatial resolution.

Development and evaluation of TWIST Dixon for dynamic contrast-enhanced (DCE) MRI with improved acquisition efficiency and fat suppression

To develop a new pulse sequence called time‐resolved angiography with stochastic trajectories (TWIST) Dixon for dynamic contrast enhanced magnetic resonance imaging (DCE‐MRI).

Coronary artery imaging: 3D segmented k‐space data acquisition with multiple breath‐holds and real‐time slab following

The purpose of this work was to develop a multiple‐breath‐hold (BH) imaging method for coronary arteries in which a segment of k‐space is acquired in each BH. The goal was to increase the resolution, or the signal‐to‐noise ratio (SNR) and coverage, of three‐dimensional‐(3D)‐BH volume‐targeted scanning (VCATS). To correct for slab position differences, a real‐time slab following technique using navigator echoes for motion detection was used. Sixteen normal volunteers were imaged to compare the method with a single‐BH scan. Results showed that higher resolution, or larger coverage and higher SNR, were achieved by the multiple‐BH method without respiratory motion artifacts. In conclusion, 3D segmented k‐space data acquisition with multiple‐BHs and real‐time slab following is a promising approach for extending the capabilities of VCATS further. J. Magn. Reson. Imaging 2001;13:301–307.

Clinical evaluation of CAIPIRINHA: Comparison against a GRAPPA standard

To evaluate image quality when using a CAIPIRINHA sampling pattern in comparison to a standard GRAPPA sampling pattern in patients undergoing a routine three‐dimensional (3D) breathheld liver exam. CAIPIRINHA uses an optimized phase encoding sampling strategy to alter aliasing artifacts in 3D acquisitions to improve parallel imaging reconstruction.

Time-resolved MR angiography with generalized autocalibrating partially parallel acquisition and time-resolved echo-sharing angiographic technique for hemodialysis arteriovenous fistulas and grafts.

PURPOSE To evaluate the imaging of hemodialysis arteriovenous (AV) fistulas and grafts with use of magnetic resonance (MR) angiography with generalized autocalibrating partially parallel acquisition (GRAPPA) and time-resolved echo-sharing angiographic technique (TREAT) and compare the findings with those of digital subtraction angiography (DSA). MATERIALS AND METHODS The vascular tree directly related to AV fistulas and grafts was divided into nine segments. Images of each segment obtained on GRAPPA MR angiography were evaluated for the presence of stenosis, occlusion, and any other disease (eg, pseudoaneurysm) by two independent observers and compared with a consensus reading of the same segments on DSA imaging. Sensitivity and specificity were calculated with use of DSA as the gold standard modality, and each image on MR angiography and DSA was rated for quality. Linear-weighted kappa scores were calculated as a measure of interobserver variability in the detection of pathologic processes. RESULTS A total of 80 segments were evaluated by each observer. For both observers, sensitivity rates for the detection of stenosis, occlusion, and any disease were 100% (95% CI, 52%-100%), 100% (95% CI, 20%-100%), and 100% (95% CI, 60%-100%), respectively. For observer 1, specificity rates for the detection of stenosis, occlusion, and any disease were 96% (95% CI, 88%-99%), 100% (95% CI, 94%-100%), and 96% (95% CI, 88%-99%), respectively. For observer 2, the specificity rates for the detection of stenosis, occlusion, and any disease were 93% (95% CI, 84%-98%), 100% (95% CI, 94%-100%), and 93% (95% CI, 84%-97%), respectively. Linear-weighted kappa values for MR angiography and DSA were 0.78+/-0.084 and 0.62+/-0.152, respectively. CONCLUSION Time-resolved MR angiography with GRAPPA and TREAT offers excellent image quality and provides an accurate and reliable modality for the detection of pathologic processes in hemodialysis AV fistulas and grafts.

Application of time‐resolved angiography with stochastic trajectories (twist)‐dixon in dynamic contrast‐enhanced (dce) breast mri

To evaluate a magnetic resonance imaging (MRI) technique that integrates time‐resolved angiography with stochastic trajectories (TWIST) view sharing and Dixon for a breast dynamic contrast‐enhanced (DCE)‐MRI application.

Automatic view planning for cardiac MRI acquisition

Conventional cardiac MRI acquisition involves a multi-step approach, requiring a few double-oblique localizers in order to locate the heart and prescribe long- and short-axis views of the heart. This approach is operator-dependent and time-consuming. We propose a new approach to automating and accelerating the acquisition process to improve the clinical workflow. We capture a highly accelerated static 3D full-chest volume through parallel imaging within one breath-hold. The left ventricle is localized and segmented, including left ventricle outflow tract. A number of cardiac landmarks are then detected to anchor the cardiac chambers and calculate standard 2-, 3-, and 4-chamber long-axis views along with a short-axis stack. Learning-based algorithms are applied to anatomy segmentation and anchor detection. The proposed algorithm is evaluated on 173 localizer acquisitions. The entire view planning is fully automatic and takes less than 10 seconds in our experiments.

Steady‐state first‐pass perfusion (SSFPP): A new approach to 3D first‐pass myocardial perfusion imaging

To describe and characterize a new approach to first‐pass myocardial perfusion utilizing balanced steady‐state free precession acquisition without the use of saturation recovery or other magnetization preparation.

Automatic LV localization and view planning for cardiac MRI acquisition

BackgroundLocalization of the heart is typically performed using amulti-step approach involving the acquisition of double-oblique localizer images. Based on the localizers thestandard heart views are planned. This approach isoperator-dependent and time consuming.ObjectiveTo demonstrate feasibility of a fully automatic and fastapproach to heart localization and slice prescriptionfrom a highly-accelerated, single breath-hold 3D acquisi-tion through a machine learning method.MethodsA 3D full-chest MR scan is obtained through parallelimaging within a single breath-hold. A single volume isacquired at mid-diastole using an ECG gated segmentedacquisition with T2-prepared SSFP readout with chemi-cal shift fat suppression. Typical protocol parametersare: 400x400x220 mm