Devavrat Likhite

Technical Note: Evaluation of pre‐reconstruction interpolation methods for iterative reconstruction of radial k‐space data

Purpose: To evaluate the use of three different pre‐reconstruction interpolation methods to convert non‐Cartesian k‐space data to Cartesian samples such that iterative reconstructions can be performed more simply and more rapidly. Methods: Phantom as well as cardiac perfusion radial datasets were reconstructed by four different methods. Three of the methods used pre‐reconstruction interpolation once followed by a fast Fourier transform (FFT) at each iteration. The methods were: bilinear interpolation of nearest‐neighbor points (BINN), 3‐point interpolation, and a multi‐coil interpolator called GRAPPA Operator Gridding (GROG). The fourth method performed a full non‐Uniform FFT (NUFFT) at each iteration. An iterative reconstruction with spatiotemporal total variation constraints was used with each method. Differences in the images were quantified and compared. Results: The GROG multicoil interpolation, the 3‐point interpolation, and the NUFFT‐at‐each‐iteration approaches produced high quality images compared to BINN, with the GROG‐derived images having the fewest streaks among the three preinterpolation approaches. However, all reconstruction methods produced approximately equal results when applied to perfusion quantitation tasks. Pre‐reconstruction interpolation gave approximately an 83% reduction in reconstruction time. Conclusion: Image quality suffers little from using a pre‐reconstruction interpolation approach compared to the more accurate NUFFT‐based approach. GROG‐based pre‐reconstruction interpolation appears to offer the best compromise by using multicoil information to perform the interpolation to Cartesian sample points prior to image reconstruction. Speed gains depend on the implementation and relatively standard optimizations on a MATLAB platform result in preinterpolation speedups of ˜ 6 compared to using NUFFT at every iteration, reducing the reconstruction time from around 42 min to 7 min.

Interstudy repeatability of self-gated quantitative myocardial perfusion MRI.

To evaluate the interstudy repeatability of multislice quantitative cardiovascular magnetic resonance myocardial blood flow (MBF), myocardial perfusion reserve (MPR), and extracellular volume (ECV). A unique saturation recovery self‐gated acquisition was used for the perfusion scans.

Split Bregman multicoil accelerated reconstruction technique: A new framework for rapid reconstruction of cardiac perfusion MRI

PURPOSE Rapid reconstruction of undersampled multicoil MRI data with iterative constrained reconstruction method is a challenge. The authors sought to develop a new substitution based variable splitting algorithm for faster reconstruction of multicoil cardiac perfusion MRI data. METHODS The new method, split Bregman multicoil accelerated reconstruction technique (SMART), uses a combination of split Bregman based variable splitting and iterative reweighting techniques to achieve fast convergence. Total variation constraints are used along the spatial and temporal dimensions. The method is tested on nine ECG-gated dog perfusion datasets, acquired with a 30-ray golden ratio radial sampling pattern and ten ungated human perfusion datasets, acquired with a 24-ray golden ratio radial sampling pattern. Image quality and reconstruction speed are evaluated and compared to a gradient descent (GD) implementation and to multicoil k-t SLR, a reconstruction technique that uses a combination of sparsity and low rank constraints. RESULTS Comparisons based on blur metric and visual inspection showed that SMART images had lower blur and better texture as compared to the GD implementation. On average, the GD based images had an ∼18% higher blur metric as compared to SMART images. Reconstruction of dynamic contrast enhanced (DCE) cardiac perfusion images using the SMART method was ∼6 times faster than standard gradient descent methods. k-t SLR and SMART produced images with comparable image quality, though SMART was ∼6.8 times faster than k-t SLR. CONCLUSIONS The SMART method is a promising approach to reconstruct good quality multicoil images from undersampled DCE cardiac perfusion data rapidly.

Deformable and Rigid Model-Based Image Registration for Quantitative Cardiac Perfusion

Background: Inter-frame image registration is a major hurdle in accurate quantification of myocardial perfusion using MRI. The registration is not standard, in that changing contrast between frames makes it difficult to register the images automatically.

Estimating extraction fraction and blood flow by combining firstpass myocardial perfusion and T1 mapping results

Background Quantifying myocardial perfusion is complicated by the complexity of pharmacokinetic model being used and the reliability of perfusion parameter estimates. More complex modeling provides more information about the underlying physiology, but too many parameters in complex models introduce a new problem of reliable estimation. To overcome the problem of multiple parameters, we have developed a technique that combines knowledge from two different cardiac magnetic resonance (MR) imaging techniques: dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) and T1 mapping. Using extracellular volume (ECV) estimates from T1 mapping may allow more robust model parameter estimates. Methods Simulations and human scans were performed. The myocardial perfusion scans used an ungated saturation recovery prepared TurboFLASH pulse sequence. Four short-axis (SA) slices were acquired after a single saturation pulse with a saturation recovery time of ~25 ms before the first slice. Gadoteridol was injected and ~240 frames were acquired over a minute with shallow breathing and no electrocardiograph (ECG) gating. This was followed 20±5 minutes later by an injection of regadenoson to induce hyperemia. The data were acquired using an under-sampled golden angle radial acquisition. Modified look-locker inversion recovery (MOLLI) T1 mapping was performed in 3 slices pre- and post-contrast. The pre- and post-contrast T1 maps were used for ECV estimation. Quantification of perfusion was done using a 4-parameter model with additional information about ECV supplied during model fitting. Phase contrast scans of the coronary sinus (CS) were acquired at rest and immediately after the stress perfusion acquisition to estimate global flow. Results Without ECV information, the 5-parameter model fails to converge to a unique solution and often gives incorrect estimates for the perfusion parameters. The myocardial blood flow (MBF) estimates during rest and stress were 0.9±0.1 and 2.3±0.6 mL/min/g, respectively. The extraction fraction estimates were 0.49±0.04 and 0.34±0.05 during rest and stress, respectively. Conclusions These results show that it is possible to successfully fit a dynamic perfusion model with an extraction fraction parameter by using information from T1 mapping scans. This hybrid approach is especially important when the 5-parameter model alone fails to converge on a unique solution. This work is a good example of exploiting information overlaps between various cardiac MR imaging techniques.

Inter-study repeatability of self-gated CMR perfusion: A comparison of Fermi and compartment models

Background Recent developments in cardiovascular magnetic resonance (CMR) perfusion have made it possible to rapidly acquire multiple slices continuously without the need for any ECG-triggering. Promising results have been shown for visual assessment and quantification of perfusion using self-gated techniques [1-3]. This work compares the repeatability of a free breathing ungated acquisition using the Fermi model and a compartment model.

Initial results of a new very rapid rest/regadenoson stress myocardial perfusion protocol in patients with atrial fibrillation

Background Cardiovascular magnetic resonance (CMR) myocardial perfusion is a well established method for detection of significant obstructive coronary artery disease (CAD). In patients with arrhythmias, standard methods using ECGgating can result in poor image quality. Additionally, with typical stress/rest protocols, a true rest state may not be achieved after administration of regadenoson. However, rest-first may present issues with peri-infarct ischemia and so here we give little time for late enhancement by keeping rest and stress perfusion scans close in time. Given these issues, the two-fold aim of this study is to evaluate the accuracy of a rapid rest-first protocol using an ungated myocardial image pulse sequence.

Regadenoson blood flow in type 1 diabetes (RABIT1D Study)

Background Cardiovascular disease is the leading cause of mortality in type 1 diabetics (T1D). T1D patients have increased coronary artery calcification (CAC) compared to nondiabetics. We hypothesize that myocardial blood flow (MBF) reserve can be measured in long-standing T1D patients using regadenoson stress cardiac magnetic resonance (CMR) perfusion imaging and is a marker of extensive atherosclerotic disease associated with CAC.