Improved waveform fidelity using local HYPR reconstruction (HYPR LR)

Abstract

The recently introduced HYPR (HighlY constrained backPRojection) method allows reconstruction of serial images from highly undersampled data. In HYPR, individual timeframes are obtained via unfiltered backprojections of normalized sinograms using anatomical constraints provided by a composite image. Here we develop the idea of constraining the backprojected data further to a series of local regions of interest in order to decrease the corruption of local information by distant signals. HYPR LR (local reconstruction) permits the use of a longer temporal window in the formation of the composite image, resulting in increased signal-to-noise ratio and quantitative reconstruction accuracy. Unlike HYPR, the new HYPR LR method can be applied to images acquired with arbitrary k-space trajectories. It is suitable for a broad range of medical imaging applications involving serial changes in image sequence, offering exciting new opportunities in the future.

FIG. 1

Schematic comparison of backprojection used in HYPR and HYPR LR. a: Radon projection, inherent to the original HYPR, adds mixed temporal information together. b: HYPR weighting images are reconstructed used unfiltered backprojection which spreads information across the whole image. c: When the Radon projection is done locally by calculating the line integral over a local region, it contains information only about the local region. d: A localized unfiltered backprojection, which deposits the local projection information into this local region, effectively avoids the crosstalk between objects.

FIG. 2

Schematic of HYPR LR for radial sampling with convolution-based filtering reconstruction.

FIG. 3

Comparison of image quality and waveform accuracy using 20 angles per frame and a composite length of 40 frames. Timeframe 12 is shown with the corresponding waveforms of both objects for each method. Original HYPR (hyprO) has the lowest artifact level, but significantly distorts the waveforms. When the weighting image resolution is varied from 1/9th (hyprLR9) to 1/27th (hyprLR27) of the original, artifact levels are reduced at the cost of decreased waveform accuracy.

FIG. 4

Comparison of original HYPR and HYPR LR using 20 angles per frame with 5 frame (hypr5,hyprLR5) and 11 frame (hypr5,hyprLR5) sliding window composites. Original HYPR waveforms are improved using a narrow sliding window while HYPR LR images are accurate regardless of the window width. HYPR and HYPR LR images (timeframe 12 shown) are of similar quality, with artifact levels reduced using a wider sliding window.

FIG. 5

Comparison of the waveforms, SNR, and image quality for both HYPR methods for a reasonable choice of operating parameters: 20 projections per frame, a 7-frame sliding window composite for original HYPR (hyprO-sw7), and a factor of 13 reduction in weighting image resolution with full composite for HYPR LR (hyprLR13).

FIG. 6

Comparisons of temporal waveforms, A/V ratios, image quality, and profiles along x for both original HYPR (hyprO) and HYPR LR (hyprLR) with reasonable reconstruction parameters. The ROI averaged signal intensity and A/V ratios of HYPR LR are better correlated with the truth than those of the original HYPR. Images and profiles show the HYPR LR produces distortions at the boundary between the two vessels, while original HYPR shows distortions over a wider area.

FIG. 7

Effects of changing the order of operations in HYPR LR. Filtering then dividing (F/D, Eq. [2]) results in sharper edges than dividing then filtering (D/F, Eq. [5]) with no significant difference regarding streak artifacts.