Synopsis

A new framework based on deep complex network is introduced for fast reconstruction of undersampled 3D CMR data. In this work, the complex nature of the acquired data was maintained throughout the network to achieve optimal performance. High resolution 3D LGE dataset of 217 subjects was acquired at prospective acceleration rates ranging from 3 to 5 and used to train and validate (7:3) the proposed methods. Further retrospective undersampling was performed to test the capacity of the proposed methods to perform at higher acceleration rates. LGE scar quantification showed high correlation with the ground-truth.

Introduction

Methods

The proposed URUS reconstruction algorithm utilizes complex deep network to create artifact-free MRI images from the acquired undersampled complex k-space data (Fig. 1). The acquired 3D multi-coil k-space data was zero-filled, transformed to image space using FFT, combined using B1-weighted combination and 1 iteration of total variation minimization was performed to reduce the noise level in the input images [4]. This 3D dataset was sliced in the z-direction to multiple 2D-slices of size (N×N×2) and successively fed into 2D complex convolutional network of U-net architecture. Each 2D input image was destructed into basic components using two complex convolutional layers, each of 64 (3x3x2) filters. The complex convolution operations were performed using real-valued arithmetic convolution, where the complex convolution of an image, w: C^NxN, and filter, h: C^KxK can be represented as:
Real(w * h) = w_{r *} h_r - w_{i *} h_i
Imag(w * h) = w_{r *} h_i + w_{i *} h_r

Where * and _* stand for complex and real-valued convolution operation, respectively. w_r and h_r are the real parts of the image and filter, and similarly w_i and h_i are imaginary parts of image and filter, respectively. Each convolution layer is followed by radial batch normalization layer that standardize the complex data, so that its magnitude has standard distribution of both mean and standard deviation equal 1, and unchanged phase. In addition, Complex ReLU was applied for both real and imaginary parts of the data. A contracting path with three stages of downsampling were performed to increase the receptive field of the network so that artifacts removal on larger scale is enabled. Two convolutional layers followed each downsampling stage to extract higher order features on larger scale. Similarly, an expansive path was utilized to upsample the produced feature maps to reach the original image size using bilinear interpolation. The output maps were reconstructed using two convolution layers to form the output image. In training, mean-squared error function was minimized, so the network learn a mapping function between the aliased complex input and Low-dimensional-structure self-learning and thresholding compressed-sensing (LOST) reconstructed magnitude images [5]. To evaluate the performance of the proposed reconstruction technique, 3D LGE images were acquired in 217 patients using a randomly prospective undersampled k-space with acceleration factors: 3 (n=130), 4 (n=24), and 5(n=63) [3]. 3D LGE images were acquired with spatial resolution of 1.0-1.5 mm3, with 100-120 slices. The dataset was randomly divided into training (70%) and testing (30%) subsets. To quantify the scare volume, the left ventricular myocardium was manually extracted from each 2D slices in all patients with scar in the testing dataset for URUS and LOST reconstructed images. Full-width half-maximum and 6 standard deviations-based methods were used to automatically quantify the scar volume within the extracted myocardium. To investigate higher undersampling rates, each acquired k-space data was further undersampled with 2 higher factors, so that acceleration factors ranging from 5 to 7 were achieved (i.e. all data acquired with rate 3 was under sampled to rate 5, and similarly rates 4 and 5 were undersampled to rates 6 and 7, respectively). This model was implemented using PyTorch and tested on PC equipped with 12GB NVIDIA Xp GPU.

Results

Conclusion

Acknowledgements

References

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