Progressive Magnetic Resonance Image Reconstruction Based on Iterative Solution of a Sparse Linear System

Image reconstruction from nonuniformly sampled spatial frequency domain data is an important problem that arises in computed imaging. Current reconstruction techniques suffer from limitations in their model and implementation. In this paper, we present a new reconstruction method that is based on solving a system of linear equations using an efficient iterative approach. Image pixel intensities are related to the measured frequency domain data through a set of linear equations. Although the system matrix is too dense and large to solve by direct inversion in practice, a simple orthogonal transformation to the rows of this matrix is applied to convert the matrix into a sparse one up to a certain chosen level of energy preservation. The transformed system is subsequently solved using the conjugate gradient method. This method is applied to reconstruct images of a numerical phantom as well as magnetic resonance images from experimental spiral imaging data. The results support the theory and demonstrate that the computational load of this method is similar to that of standard gridding, illustrating its practical utility

Pulsed arterial spin labelling at ultra-high field with a B1 +-optimised adiabatic labelling pulse

Arterial spin labelling (ASL) techniques benefit from the increased signal-to-noise ratio and the longer T 1 relaxation times available at ultra-high field. Previous pulsed ASL studies at 7\ua0T concentrated on the superior regions of the brain because of the larger transmit radiofrequency inhomogeneity experienced at ultra-high field that hinders an adequate inversion of the blood bolus when labelling in the neck. Recently, researchers have proposed to overcome this problem with either the use of dielectric pads, through dedicated transmit labelling coils, or special adiabatic inversion pulses.We investigate the performance of an optimised time-resampled frequency-offset corrected inversion (TR-FOCI) pulse designed to cause inversion at much lower peak B 1 (+) . In combination with a PICORE labelling, the perfusion signal obtained with this pulse is compared against that obtained with a FOCI pulse, with and without dielectric pads.Mean grey matter perfusion with the TR-FOCI was 52.5\ua0±\ua010.3\ua0mL/100\ua0g/min, being significantly higher than the 34.6\ua0±\ua02.6\ua0mL/100\ua0g/min obtained with the FOCI pulse. No significant effect of the dielectric pads was observed.The usage of the B 1 (+) -optimised TR-FOCI pulse results in a significantly higher perfusion signal. PICORE-ASL is feasible at ultra-high field with no changes to operating conditions

Diffusion imaging in humans at 7T using readout-segmented EPI and GRAPPA

Anatomical MRI studies at 7T have demonstrated the ability to provide high‐quality images of human tissue in vivo. However, diffusion‐weighted imaging at 7T is limited by the increased level of artifact associated with standard, single‐shot, echo‐planar imaging, even when parallel imaging techniques such as generalized autocalibrating partially parallel acquisitions (GRAPPA) are used to reduce the effective echo spacing. Readout‐segmented echo‐planar imaging in conjunction with parallel imaging has the potential to reduce these artifacts by allowing a further reduction in effective echo spacing during the echo‐planar imaging readout. This study demonstrates that this approach does indeed provide a substantial improvement in image quality by reducing image blurring and susceptibility‐based distortions, as well as by allowing the acquisition of diffusion‐weighted images with a high spatial resolution. A preliminary application of the technique to high‐resolution diffusion tensor imaging provided a high level of neuroanatomical detail, which should prove valuable in a wide range of applications

In vivo estimation of gamma‐aminobutyric acid levels in the neonatal brain

Gamma‐aminobutyric acid (GABA) is the major inhibitory neurotransmitter in the brain, and plays a key role in brain development. However, the in vivo levels of brain GABA in early life are unknown. Using edited MRS, in vivo GABA can be detected as GABA+ signal with contamination of macromolecule signals. GABA+ is evaluated as the peak ratio of GABA+/reference compound, for which creatine (Cr) or water is typically used. However, the concentrations and T1 and T2 relaxation times of these references change during development. Thus, the peak ratio comparison between neonates and children may be inaccurate. The aim of this study was to measure in vivo neonatal brain GABA+ levels, and to investigate the dependency of GABA levels on brain region and age. The basal ganglia and cerebellum of 38 neonates and 12 children were measured usingGABA‐edited MRS. Two different approaches were used to obtain GABA+ levels: (i) multiplying the GABA/water ratio by the water concentration; and (ii) multiplying the GABA+/Cr by the Cr concentration. Neonates exhibited significantly lower GABA+ levels compared with children in both regions, regardless of the approach employed, consistent with previous ex vivo data. A similar finding of lower GABA+/water and GABA+/Cr in neonates compared with children was observed, except for GABA+/Cr in the cerebellum. This contrasting finding resulted from significantly lower Cr concentrations in the neonate cerebellum, which were approximately 52% of those of children. In conclusion, care should be taken to consider Cr concentrations when comparing GABA+/Cr levels between different‐aged subjects

PET/MRI in the presence of metal implants: Completion of the attenuation map from PET emission data

We present a novel technique for accurate whole-body attenuation correction in the presence of metallic endoprosthesis, on integrated non-time-of-flight (non-TOF) PET/MRI scanners. The proposed implant PET-based attenuationmap completion (IPAC) method performs a joint reconstruction of radioactivity and attenuation from the emission data to determine the position, shape, and linear attenuation coefficient (LAC) of metallic implants. Methods: The initial estimate of the attenuation map was obtained using the MR Dixon method currently available on the Siemens Biograph mMR scanner. The attenuation coefficients in the area of the MR image subjected to metal susceptibility artifacts are then reconstructed fromthe PET emission data using the IPAC algorithm. The method was tested on 11 subjects presenting 13 different metallic implants, who underwent CT and PET/MR scans. Relative mean LACs and Dice similarity coefficients were calculated to determine the accuracy of the reconstructed attenuation values and the shape of the metal implant, respectively. The reconstructed PET images were compared with those obtained using the reference CT-based approach and the Dixon-based method. Absolute relative change (aRC) images were generated in each case, and voxel-based analyses were performed. Results: The error in implant LAC estimation, using the proposed IPAC algorithm, was 15.7%± 7.8%, which was significantly smaller than the Dixon- (100%) and CT-(39%) derived values. A mean Dice similarity coefficient of 73% ± 9% was obtained when comparing the IPAC- with the CT-derived implant shape. The voxel-based analysis of the reconstructed PET images revealed quantification errors (aRC) of 13.2% ± 22.1% for the IPACwith respect to CT-corrected images. The Dixon-based method performed substantially worse, with a mean aRC of 23.1% ± 38.4%. Conclusion: We have presented a non-TOF emission-based approach for estimating the attenuation map in the presence of metallic implants, to be used for whole-body attenuation correction in integrated PET/MR scanners. The Graphics Processing Unit implementation of the algorithm will be included in the open-source reconstruction toolbox Occiput.io

Edited MRS 法を用いたin vivo 新生児脳のGABA 信号の定量化

【目的】γ - アミノ酪酸（GABA）は脳内において主要な神経抑制性物質である[1]。近年、1H-MRSでのスペクトル差分法（edited-MRS）を用いたin vivo ヒト脳内GABA レベル測定が行われているが、新生児脳内GABA の報告はなされていない。Edited MRS 法でのGABA 信号（3ppm）は、高分子由来の信号寄与があるためGABA+ とされ、また通常はクレアチン（Cr）や水（water）などを指標としたピーク面積比（GABA+/Cr or GABA+/water）により評価が行われている[2]。しかしながら、Crやwater の濃度やT1・T2 値は成長に伴い変化するため、ピーク面積比による新生児と他の年齢群との比較では正確な情報が得られない可能性がある。本研究は、臨床用3T MR 装置（Siemens）を用いたedited-MRS 法（MEGA-PRESS: TE/TR = 69/1500 ms[3]）で、新生児脳内GABA レベルを調べることを目的とした。【方法】対象は新生児38 名とし、対照児童群12 名（10.2 ± 3.6 歳（平均± SD））と比較した。データ処理にはGannet [4] およびin house ソフトウエアを用いた。【結果】新生児群は児童群と比較し、基底核・小脳共に有意に低いGABA+ レベルを示し、これはex vivo GABA（検体、脳脊髄液[5,6]）の報告と傾向が一致していた。新生児小脳のCr 濃度は児童群の約半分であった。【考察】新生児脳のGABA+/Cr 比の有意に高い値は、有意に低いCr 濃度によることが考えられた。そのため異なる年齢間での比較にCr を指標として用いる場合は注意が必要であることが示唆された。【結論】新生児の脳内GABA+ レベルは、児童群よりも有意に低いことが示された。臨床MR脳機能研究