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Kuan-Hung Cho

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6 papers

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YNIMG Journal 2025 Journal Article

On the use of multi-echo NODDI MRI with released intrinsic diffusivity for the assessment of tissue diffusion and relaxation properties in experimental ischaemic stroke

Ezequiel Farrher
Kuan-Hung Cho
Chia-Wen Chiang
Ming-Jye Chen
Sheng-Min Huang
Li-Wei Kuo
Chang-Hoon Choi
N. Jon Shah

The multi-echo neurite orientation dispersion and density imaging (MTE-NODDI) model has been proposed to overcome one of the shortcomings of conventional NODDI, namely the echo time (TE) dependence of the compartmental signal fractions, which stems from the intrinsic differences in the compartmental transverse relaxation times (T2). However, the model continues to be constrained by the limitation of having a fixed, brain-wide intrinsic diffusivity, d. The primary aim of this work is to assess the benefits and shortcomings of using MTE-NODDI to investigate the diffusion and T2 properties of ischaemic stroke tissue following middle cerebral artery occlusion (MCAo) in rat models. Given the known alterations in the diffusion properties in ischaemic tissue, a secondary aim is to assess an estimation approach for MTE-NODDI parameters that enables d to be released while also mitigating the consequent model degeneracy. Using the MTE-NODDI parameters, the spatiotemporal evolution of diffusion and T2 properties in ischaemic tissue was characterised from day one to day 23 post-MCAo. The proposed approach enables access to several unique tissue features that would otherwise be obscured by the conventional approach. Importantly, a marked reduction in d was observed, leading to significant changes in other MTE-NODDI parameters compared to the model employing a fixed d. The isotropic signal fraction displayed a significant increase in ischemic tissue, which appears in contradiction with previous works. Regarding the intra- and extra-neurite T2 values, T 2, in and T 2, en, a significant increment was observed at the ischaemic tissue, while the condition T 2, in ≥ T 2, en displayed a tendency to hold in both tissue types. More generally, some parameters, such as the isotropic signal fraction, the intrinsic diffusivity and both compartmental T2 values, display unique, heterogeneous spatiotemporal evolution, where the core and border zones of the ischaemic tissue show different behaviours. Overall, the newly estimated parameters show greater consistency with analogous estimates reported by published models, and are anticipated to significantly enhance the understanding of tissue properties following ischaemic stroke.

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YNIMG Journal 2021 Journal Article

Spatiotemporal characterisation of ischaemic lesions in transient stroke animal models using diffusion free water elimination and mapping MRI with echo time dependence

Ezequiel Farrher
Chia-Wen Chiang
Kuan-Hung Cho
Farida Grinberg
Richard P. Buschbeck
Ming-Jye Chen
Kuo-Jen Wu
Yun Wang

BACKGROUND AND PURPOSE: The excess fluid as a result of vasogenic oedema and the subsequent tissue cavitation obscure the microstructural characterisation of ischaemic tissue by conventional diffusion and relaxometry MRI. They lead to a pseudo-normalisation of the water diffusivity and transverse relaxation time maps in the subacute and chronic phases of stroke. Within the context of diffusion MRI, the free water elimination and mapping method (FWE) with echo time dependence has been proposed as a promising approach to measure the amount of free fluid in brain tissue robustly and to eliminate its biasing effect on other biomarkers. In this longitudinal study of transient middle cerebral artery occlusion (MCAo) in the rat brain, we investigated the use of FWE MRI with echo time dependence for the characterisation of the tissue microstructure and explored the potential of the free water fraction as a novel biomarker of ischaemic tissue condition. METHODS: Adult rats received a transient MCAo. Diffusion- and transverse relaxation-weighted MRI experiments were performed longitudinally, pre-occlusion and on days 1, 3, 4, 5, 6, 7 and 10 after MCAo on four rats. Histology was performed for non-stroke and 1, 3 and 10 days after MCAo on three different rats at each time point. RESULTS: The free water fraction was homogeneously increased in the ischaemic cortex one day after stroke. Between three and ten days after stroke, the core of the ischaemic tissue showed a progressive normalisation in the amount of free water, whereas the inner and outer border zones of the ischaemic cortex depicted a large, monotonous increase with time. The specific lesions in brain sections were verified by H&E and immunostaining. The tissue-specific diffusion and relaxometry MRI metrics in the ischaemic cortex were significantly different compared to their conventional counterpart. CONCLUSIONS: Our results demonstrate that the free water fraction in FWE MRI with echo time dependence is a valuable biomarker, sensitive to the progressive degeneration in ischaemic tissue. We showed that part of the heterogeneity previously observed in conventional parameter maps can be accounted for by a heterogeneous distribution of free water in the tissue. Our results suggest that the temporal evolution of the free fluid fraction map at the core and inner border zone can be associated with the pathological changes linked to the evolution of vasogenic oedema. Namely, the homogeneous increase in free water one day after stroke and its tendency to normalise in the core of the ischaemic cortex starting three days after stroke, followed by a progressive increase in free water at the inner border zone from three to ten days after stroke. Finally, the monotonous increase in free fluid in the outer border zone of the cortex reflects the formation of fluid-filled cysts.

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YNIMG Journal 2010 Journal Article

Diffusion orientation transform revisited

Erick Jorge Canales-Rodríguez
Ching-Po Lin
Yasser Iturria-Medina
Chun-Hung Yeh
Kuan-Hung Cho
Lester Melie-García

Diffusion orientation transform (DOT) is a powerful imaging technique that allows the reconstruction of the microgeometry of fibrous tissues based on diffusion MRI data. The three main error sources involving this methodology are the finite sampling of the q-space, the practical truncation of the series of spherical harmonics and the use of a mono-exponential model for the attenuation of the measured signal. In this work, a detailed mathematical description that provides an extension to the DOT methodology is presented. In particular, the limitations implied by the use of measurements with a finite support in q-space are investigated and clarified as well as the impact of the harmonic series truncation. Near- and far-field analytical patterns for the diffusion propagator are examined. The near-field pattern makes available the direct computation of the probability of return to the origin. The far-field pattern allows probing the limitations of the mono-exponential model, which suggests the existence of a limit of validity for DOT. In the regimen from moderate to large displacement lengths the isosurfaces of the diffusion propagator reveal aberrations in form of artifactual peaks. Finally, the major contribution of this work is the derivation of analytical equations that facilitate the accurate reconstruction of some orientational distribution functions (ODFs) and skewness ODFs that are relatively immune to these artifacts. The new formalism was tested using synthetic and real data from a phantom of intersecting capillaries. The results support the hypothesis that the revisited DOT methodology could enhance the estimation of the microgeometry of fiber tissues.

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YNIMG Journal 2010 Journal Article

The effect of finite diffusion gradient pulse duration on fibre orientation estimation in diffusion MRI

Chun-Hung Yeh
J-Donald Tournier
Kuan-Hung Cho
Ching-Po Lin
Fernando Calamante
Alan Connelly

An essential step for fibre-tracking is the accurate estimation of neuronal fibre orientations within each imaging voxel, and a number of methods have been proposed to reconstruct the orientation distribution function based on sampling three-dimensional q-space. In the q-space formalism, very short (infinitesimal) gradient pulses are the basic requirement to obtain the true spin displacement probability density function. On current clinical MR systems however, the diffusion gradient pulse duration (δ) is inevitably finite due to the limit on the achievable gradient intensity. The failure to satisfy the short gradient pulse (SGP) requirement has been a recurrent criticism for fibre orientation estimation based on the q-space approach. In this study, the influence of a finite δ on the DW signal measured as a function of gradient direction is described theoretically and demonstrated through simulations and experimental models. Our results suggest that the current practice of using long δ for DW imaging on human clinical MR scanners, which is enforced by hardware limitations, might in fact be beneficial for estimating fibre orientations. For a given b-value, the prolongation of δ is advantageous for estimating fibre orientations for two reasons: first, it leads to a boost in DW signal in the transverse plane of the fibre. Second, it stretches out the shape of the measured diffusion profile, which improves the contrast between DW orientations. This is especially beneficial for resolving crossing fibres, as this contrast is essential to discriminate between different fibre directions.

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YNIMG Journal 2008 Journal Article

Evaluation of the accuracy and angular resolution of q-ball imaging

Kuan-Hung Cho
Chun-Hung Yeh
Jacques-Donald Tournier
Yi-Ping Chao
Jyh-Horng Chen
Ching-Po Lin

Q-ball imaging (QBI) has been proposed for the mapping of multiple intravoxel fiber structures using the Funk–Radon transform on high angular resolution diffusion images (HARDI). However, the accuracy and the angular resolution of QBI to define fiber orientations and its dependence on diffusion imaging parameters remain unclear. The phantom models, made up of sheets of parallel capillaries filled with water, were designed to evaluate the accuracy and the angular resolution of QBI at different |q| values. With an inner diameter of 20 μm and an outer diameter of 90 μm, the capillaries afforded a restrictive environment compared with the diffusion measurement scale. Further, the angular resolutions of QBI at various |q| value were also quantified on the corpus callosum in the human brain. The full width at half maximum (FWHM) of the main lobe of normalized orientation distribution function (nODF) was calculated and adopted to quantify the angular resolution of QBI. With the phantom model, a higher |q| value resulted in worse accuracy but better angular resolution for QBI. The same trend where a higher |q| value yielded a better angular resolution was also observed in the human study. Upon comparison of QBI with T2WI, QBI with |q|=277 cm−1 (b =3000 s/mm2) was found to be insufficient to differentiate capillaries crossing at 45°. However, when encoding with |q|=320, 358, and 392 cm−1 (b =4000, 5000, and 6000 s/mm2), the deviation angles between the primary ODF and the 45° phantoms were −4. 91°±2. 72°, −1. 37°±2. 32°, and −0. 69°±1. 54° with adequate signal-to-noise ratio (SNR). These results were consistent with the FWHM-nODF, which showed that a |q| value of 320 cm−1 was the threshold to resolve capillaries intersecting at 45°. Additionally, it was demonstrated in both the phantom model and the human brain that QBI encoding with lower |q| values may result in underestimation of the orientations of the crossing fibers. In conclusion, QBI was found to accurately resolve crossing fiber orientations and was highly dependent on the selected |q| value.

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YNIMG Journal 2008 Journal Article

Resolving crossing fibres using constrained spherical deconvolution: Validation using diffusion-weighted imaging phantom data

J.-Donald Tournier
Chun-Hung Yeh
Fernando Calamante
Kuan-Hung Cho
Alan Connelly
Ching-Po Lin

Diffusion-weighted imaging can potentially be used to infer the connectivity of the human brain in vivo using fibre-tracking techniques, and is therefore of great interest to neuroscientists and clinicians. A key requirement for fibre tracking is the accurate estimation of white matter fibre orientations within each imaging voxel. The diffusion tensor model, which is widely used for this purpose, has been shown to be inadequate in crossing fibre regions. A number of approaches have recently been proposed to address this issue, based on high angular resolution diffusion-weighted imaging (HARDI) data. In this study, an experimental model of crossing fibres, consisting of water-filled plastic capillaries, is used to thoroughly assess three such techniques: constrained spherical deconvolution (CSD), super-resolved CSD (super-CSD) and Q-ball imaging (QBI). HARDI data were acquired over a range of crossing angles and b-values, from which fibre orientations were computed using each technique. All techniques were capable of resolving the two fibre populations down to a crossing angle of 45°, and down to 30° for super-CSD. A bias was observed in the fibre orientations estimated by QBI for crossing angles other than 90°, consistent with previous simulation results. Finally, for a 45° crossing, the minimum b-value required to resolve the fibre orientations was 4000s/mm2 for QBI, 2000s/mm2 for CSD, and 1000s/mm2 for super-CSD. The quality of estimation of fibre orientations may profoundly affect fibre tracking attempts, and the results presented provide important additional information regarding performance characteristics of well-known methods.

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