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Yau-Yau Wai

Publications and source records attributed to Yau-Yau Wai.

11 recordsLinked to original sources
PubMed2006

The effects of single-trial averaging on the temporal resolution of functional MRI.

Computer simulations and event-related functional MRI (ER-fMRI) experiments were performed to investigate the effects of single-trial averaging and the corresponding contrast-to-noise ratio (CNR) on the minimal resolvable hemodynamic timing difference between brain areas. Three ER-fMRI sessions with temporally delayed (250, 500 and 1,000 ms) visual stimulations between two hemifields, each with 70 repeated single trials, were examined on two subjects. From the computer simulation, the temporal resolution improved as the CNR increased, which reached 500 and 100 ms for CNRs of 1.55 and 6.44, respectively. In the ER-fMRI experiments, the measured CNR increased as more single trials were averaged. The detectability of temporal differences was positively correlated (P<.05) with the CNR in all sessions for one subject but only in the 1,000-ms session for the other subject. Temporal resolution of 1,000 ms was achieved when more than 10 trials were averaged. The 500- and 250-ms delays might be differentiable when more than 20 trials were averaged, but the results were subject-dependent. This study demonstrated that the CNR could be significantly improved by single-trial averaging, which led to an improved temporal resolution of ER-fMRI. Temporal resolution in the range of hundreds of milliseconds was subject-dependent, which might be attributed to the intrinsic spatial variations in the timing of the blood oxygenation level-dependent (BOLD) response.

Adult↗
PubMed2006

Novel diffusion anisotropy indices: an evaluation.

PURPOSE: To systematically evaluate diffusion anisotropy (DA) using newly defined indices based on the diffusion deviation and mean diffusivity approach. MATERIALS AND METHODS: Measures of amplitude, area, and volume of the DA index (DAI) were measured and compared with regard to their sensitivity to changes in DA, susceptibility to noise in the original diffusion-weighted (DW) images, and contrast-to-noise ratio (CNR) in homogenous regions. Simulations were performed under different levels of noise and DA. Human DTI data were acquired from eight normal volunteers. RESULTS: Indices of area and volume measures provided improved resolution for characterizing the DA compared to the eigenvalue ratio. The amplitude measure showed consistent performances with good CNR and less susceptibility to noise in the original data. CONCLUSION: These indices are rotationally invariant without the requirement of eigenvalue sorting. At low anisotropy, all indices have a similar CNR. For larger DA, the first index (the deviation tensor divided by the DT) shows improved sensitivity, contrast-to-noise ratio (CNR), and noise immunity compared to the other indices.

Adult↗
PubMed2006

Congenital myotonic dystrophy: variability in muscle involvement and histopathological process.

In order to understand the involvement of specific muscles in congenital myotonic dystrophy type 1 (DM1), we studied the clinical manifestations, and the genetic effects on various tissues in 2 siblings with congenital DM1. The distal leg muscles were more severely involved than the thigh muscles, as seen in the skeletal muscle magnetic resonance imaging. Molecular genetic analysis of the myotonic dystrophy type 1 protein kinase showed an elongation of the CTG triplet repeats between 850 and 1400 in the leukocytes, skin, fat, tendon, and muscles. Muscle biopsies showed a significant difference in the fiber type distribution between these two congenital DM1 patients. One revealed a prominent involvement of the tibialis anterior muscle with a predominance of type 1 fibers, similar to those muscle fiber distributions in older congenital or classic DM1 patients, suggesting a neurotrophic influence during muscle development. Another revealed a predominance of type 2 fibers in all muscle specimens, and dystrophic changes were observed in the peroneus longus muscle indicating a delayed differentiation or maturation of muscle fibers. We conclude that despite nearly the same number of CTG repeats in the leukocytes, highly individual variability of muscle differentiation may occur at teenagers of congenital DM1 in addition to different pathological findings in various skeletal muscles of patients with congenital DM1.

Adolescent↗
PubMed2005

Contrast-enhanced carotid magnetic resonance angiography: comparison of single-dose and double-dose of gadolinium using the randomly segmented central k-space ordering technique.

BACKGROUND: The aim of this study was to evaluate whether the single dose of gadolinium is sufficient for contrast-enhanced magnetic resonance angiography (MRA) of carotid arteries with the use of newly developed randomly segmented central k-space ordering technique called contrast-enhanced timing-robust angiography (CENTRA). METHODS: A total of 44 patients underwent carotid MRA in a 1.5T MR scanner using a fluoroscopically monitored, manually triggered, CENTRA pulse sequence. Patients were randomly assigned into two groups according to the dose of contrast medium (gadolinium chelate) administered: group 1 referred to those who received double doses (0.2 mmol/kg) and group 2 received single doses (0.1 mmol/kg). The contrast-to-noise (CNR) ratios of the seven regions of interest were calculated. The delineation of nine vascular regions and the degree of venous overlay were evaluated by two blinded readers on a five-point scale. RESULTS: For quantitative evaluation, the CNRs at the brachiocephalic artery were greater in patients in group 1 than that in group 2 (p = 0.015), while the differences did not differ between the two groups for the remaining regions of interest (p > 0.05). For qualitative evaluation, there were no significant differences between the two groups in delineation of nine vascular regions and venous overlaying (p > 0.05). CONCLUSION: With the use of the CENTRA technique, carotid MRA may be performed using a single dose of gadolinium and the image quality is comparable to that of the standard double dose protocol.

Adult↗
PubMed2004

Quality assurance of clinical MRI scanners using ACR MRI phantom: preliminary results.

Clinical magnetic resonance imaging (MRI) scanners play an important role in the diagnosis of diseases and management of patient treatment. Quality assurance (QA) of the clinical MRI scanners is mandatory to obtain optimal images in a modern hospital. In this report, the phantom test for the American College of Radiology (ACR) MRI accreditation is used as the essential part of the MRI QA protocols. Seven important assessments of MR image quality are included as follows: geometric accuracy, high-contrast resolution, slice thickness accuracy, slice position accuracy, image intensity uniformity, percent signal ghosting, and low-contrast object detectability. In addition, signal-to-noise ratio and central frequency are monitored as well. The MRI QA procedures were applied to four clinical MRI scanners in our institute twice within 3 months. According to the QA results, the service engineers were more efficient in solving scanners problems when the ACR phantom test was run.

Humans↗
PubMed2004

Temporal resolving power of perfusion- and BOLD-based event-related functional MRI.

Functional magnetic resonance imaging (fMRI) based on both perfusion and blood oxygenation level-dependent (BOLD) contrasts has been widely applied in spatiotemporal mapping of the human brain function. Temporal resolving power of fMRI is limited by the smoothed hemodynamic response function dispersed from the neuronal activity. In this study, temporal modulation transfer functions were utilized to quantify the resolving powers of perfusion and BOLD fMR signals in time domain. The impulse response function was determined using brief visual stimulations and event-related image acquisition schemes. An important feature of arterial spin labeling techniques is that quantitative perfusion and BOLD signals could be simultaneously acquired. This simultaneous BOLD response may arise from signals that are more proximal to capillary beds, and its temporal resolution may be different from that of the typical BOLD response. Therefore, we assessed and compared the temporal resolving capabilities of perfusion, simultaneous BOLD, and the typical BOLD response obtained from the gradient echo EPI pulse sequence. Full-width-at-half-maximums of perfusion and simultaneous BOLD measurements were significantly smaller than that of BOLD ones (4.3+/- 0.6 s vs 5.5 +/- 0.9 s, p<0.02 and 4.7 +/- 1.3 s vs 5.5 +/- 0.9 s, p<0.01, respectively). The corresponding temporal resolving powers of perfusion and simultaneous BOLD signals were statistically better than that of BOLD signals (0.23 +/- 0.03 Hz vs 0.17 +/- 0.02 Hz, p<0.01 and 0.21 +/- 0.04 Hz vs 0.17 +/- 0.02 Hz, p<0.01, respectively). Our results showed that the typical BOLD response was significantly smoothed from the perfusion response, thus resulting in a degraded temporal resolving power. However, results from the simultaneous BOLD and perfusion measurements were not significantly different. Biophysical implications of the experimental outcomes were further investigated using a computer simulation based on the Balloon model. By fitting the measured data into the model, an apparently longer transit time was obtained for the typical BOLD signal (1.7 s), comparing to that for the simultaneous BOLD one (1.2 s). Therefore, the simultaneous BOLD signal was regarded as less susceptible to the variations from local draining veins. Combining the simulation result with the significantly discrepant resolving powers between the two BOLD signals, we speculated that the blurred effects from large vessels played a predominant role that further reduced the temporal resolution of the BOLD-based fMRI from the perfusion response.

Cerebrovascular Circulation↗
PubMed2004

Auxological, clinical and MRI findings in Taiwanese children with growth hormone deficiency.

Growth hormone deficiency (GHD) may be classified into partial isolated GHD (partial IGHD), severe IGHD or multiple pituitary hormone deficiency (MPHD) by the severity of GHD or associated with deficiency of one or more other anterior pituitary hormones during provocative tests. Morphological alterations on magnetic resonance imaging (MRI) in patients with GHD include pituitary hypoplasia, absence or interruption of pituitary stalk, and absence or ectopic posterior lobe. This study investigated the auxological, clinical severity, and anatomical characteristics of the pituitary hypothalamic region by MRI and correlated their relationships. We evaluated these parameters in 45 Taiwanese children with GHD (31 males and 14 females), aged from 3.13 to 17.91 years (10.5+/-2.5), who were divided into diagnostic subgroups of partial IGHD (18 patients), severe IGHD (13 patients), and MPHD (14 patients). We found that BA-CA, peak GH, IGF-I, IGF-I SDS, and height SDS were significantly different among these three groups. The partial IGHD group had significantly higher IGF-I than the MPHD group. There was no significant difference in prematurity, cesarean delivery, birth order, neonatal jaundice, and target height among these three groups. On MRI, patients with MPHD had significantly smaller pituitary height (PHt) SDS (p = 0.0012) and higher frequency of pituitary hypoplasia, pituitary stalk interruption, and ectopic posterior lobe (p = 0.026, 0.008, 0.005, respectively) than the other two groups. Furthermore, PHt SDS was correlated not only with peak GH (r = 0.40, p = 0.0058), but also with basal IGF-I SDS (r = 0.49, p = 0.0007) and body height SDS (r = 0.44, p = 0.025). In conclusion, morphological alterations on MRI of the hypothalamic-pituitary area are correlated with the severity of hypopituitarism. Meticulous evaluation of auxological, clinical and MRI findings can help evaluation of the severity of hypopituitarism and facilitate appropriate treatment in children with GHD.

Adolescent↗
PubMed2003

Functional MRI of amblyopia before and after levodopa.

Functional magnetic resonance imaging (fMRI) was applied to five older amblyopes with monocular amblyopia before and after levodopa treatment. During the experiment, images were acquired in two runs with visual stimulation delivered through the sound and the amblyopic eyes, respectively. The experiment was performed on each of the subjects, before and after their oral administration of levodopa/carbidopa (0.5/0.12 mg/kg) three times per day for 7 weeks. Our study demonstrated that there was no effect on the spatial extent of the visual cortical activation during the sound eye stimulation (P=0.17), but some improvement during the amblyopic eye stimulation (P=0.06). The volume ratio between the amblyopic and sound eye stimulation significantly increased after the treatment (P<0.05). This finding supports the previous studies of levodopa effect on amblyopia at the visual cortical level, and suggests that fMRI can be a useful tool in assessing changes of visual cortical activity after the treatment

Adolescent↗
PubMed2003

Dopamine transporter binding in Wilson's disease.

INTRODUCTION: In Wilson's disease (WD), brain magnetic resonance images (MRI) show increased signal intensity in T2 weighted images in the lenticular nuclei, thalamus and brainstem, including the substantia nigra. A poor therapeutic response to levodopa in WD suggests the mechanism of a postsynaptic abnormality. However positron emission tomography studies show an involvement of the nigrostriatal presynaptic dopaminergic pathway. CASE REPORT: We report the clinical manifestations in a case of WD with akinetic-rigid syndrome and initial hesitation. The brain MRI showed an increased signal intensity lesion in the substantia nigra region, in addition to basal ganglion and thalamic lesions. However, dopamine transporter (DAT) imaging with 99mTc-TRODAT-1 revealed a nonsignificantly increased DAT uptake, suggesting a normal presynaptic nigrostriatal dopaminergic terminal. CONCLUSION: We suggest that significant heterogeneity can be found in WD patients and a normal presynaptic dopaminergic pathway may occur in some patients, even those with typical akinetic-rigid syndrome and evidence of substantia nigra involvement in the brain on MRI.

Adult↗
PubMedDate not supplied

Implementations of clinical functional magnetic resonance imaging using character-based paradigms for the prediction of Chinese language dominance.

PURPOSE: Recently, functional MRI (fMRI) using word generation (WG) tasks has been shown to be effective for mapping the Chinese language-related brain areas. In clinical applications, however, patients' performance cannot be easily monitored during WG tasks. In this study, we evaluated the feasibility of a word choice (WC) paradigm in the clinical setting and compared the results with those from WG tasks. METHOD: Intrasubject comparisons of fMRI with both WG and WC paradigms were performed on six normal human subjects and two tumor patients. Subject responses in the WC paradigm, based on semantic judgments, were recorded. Activation strength, extent, and laterality were evaluated and compared. RESULTS: Our results showed that fMRI with the WC paradigm evoked weaker neuronal activation than that with the WG paradigm in Chinese language-related brain areas. It was sufficient to reveal language laterality for clinical use, however. In addition, it resulted in less nonlanguage-specific brain activation. CONCLUSION: Results from the patient data demonstrated strong evidence for the necessity of incorporating response monitoring during fMRI studies, which suggested that fMRI with the WC paradigm is more appropriate to be implemented for the prediction of Chinese language dominance in clinical environments.

Adult↗