Efficacy of the projection onto convex sets (POCS) algorithm at Gd-EOB-DTPA-enhanced hepatobiliary-phase hepatic MRI.

Shinichi Nakamura,Kazuo Awai,Ichiro Yoshinaka,Kazunori Harada,Masafumi Kidoh,Yasuyuki Yamashita,Takeshi Nakaura,Tomohiro Namimoto

doi:10.1186/s40064-016-2968-9

Shinichi Nakamura, Kazuo Awai + Show 6 more

Open Access

https://doi.org/10.1186/s40064-016-2968-9

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Abstract

PurposeTo investigate the efficacy of the projection onto convex sets (POCS) algorithm at Gd-EOB-DTPA-enhanced hepatobiliary-phase MRI.MethodsIn phantom study, we scanned a phantom and obtained images by conventional means (P1 images), by partial-Fourier image reconstruction (PF, P2 images) and by PF with the POCS algorithm (P3 images). Then we acquired and compared subtraction images (P2–P1 images and P3–P1 images). In clinical study, 55 consecutive patients underwent Gd-EOB-DTPA (EOB)-enhanced 3D hepatobiliary-phase MRI on a 1.5T scanner. Images were obtained using conventional method (C1 images), PF (C2 images), and PF with POCS (C3 images). The acquisition time was 17-, 14-, and 14 s for protocols C1, C2 and C3, respectively. Two radiologists assigned grades for hepatic vessel sharpness and we compared the visual grading among the 3 protocols. And one radiologist compared signal-to-noise-ratio (SNR) of the hepatic parenchyma.ResultsIn phantom study, there was no difference in signal intensity on a peripheral phantom column on P3–P1 images. In clinical study, there was no significant difference between C1 and C3 images (2.62 ± 0.49 vs. 2.58 ± 0.49, p = 0.70) in the score assigned for vessel sharpness nor in SNR (13.3 ± 2.67 vs. 13.1 ± 2.51, p = 0.18).ConclusionThe POCS algorithm makes it possible to reduce the scan time of hepatobiliary phase (from 17 to 14 s) without reducing SNR and without increasing artifacts.

Highlights

Conventional magnetic resonance imaging (MRI) techniques reconstruct three-dimensional (3D) images from completely sampled k-space data directly by inverse Fourier transform (Kumar et al 1975)
The signal observed on the P2–P1 image in the peripheral phantom column is indicative of a difference in signal intensity (SI) on P1 and P2 images and of blurring on the P2 image
Our phantom- and clinical studies suggest that the projection onto convex sets (POCS) algorithm is useful for reducing the acquisition time without deterioration of the image quality on 3D volumetric interpolated gradient-echo sequences used in hepatic MRI

Summary

Introduction

Conventional magnetic resonance imaging (MRI) techniques reconstruct three-dimensional (3D) images from completely sampled k-space data (i.e. a fully sampled, bandlimited, spatial frequency space) directly by inverse Fourier transform (Kumar et al 1975). Fast MRI techniques use strategies to reduce the total scan time (Kruger et al 2002; Sabati et al 2003). Nakamura et al SpringerPlus (2016) 5:1311 of the k-space in either the phase-encoding direction (to reduce Nphase) (Feinberg et al 1986) or the frequency-encoding direction (to reduce repetition time; TR) (Haacke et al 1990), or both. The disadvantage of this technique is image blurring (Chen et al 2008). The peripheral portion of k-space provides information on only the details and edges of objects in the image domain (Moratal et al 2008). Blurring of the edge occurs when the phase constraint is insufficient to recover the missing k-space information (Xu and Haacke 2001) and decreases image quality

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