Biomedical subjects

T Q Li

Publications and source records attributed to T Q Li.

At least 37 records · Page 2Linked to original sources

PubMed1999

Functional MRI of human brain during breath holding by BOLD and FAIR techniques.

BOLD (blood oxygenation level-dependent) and FAIR (flow-sensitive alternating inversion recovery) imaging techniques were used to investigate the oxygenation and hemodynamic responses of human brain during repeated challenges of breath holding and prolonged single breath holding. The effects of different breathing techniques on BOLD and FAIR image contrasts were carefully examined. With a periodic breath-holding paradigm of 30 s, global changes in gray matter were observable both in T*2-weighted and FAIR images. T*2-weighted images showed 1-4% relative signal intensity increases, while FAIR images demonstrated relative cerebral blood flow (CBF) increase up to 30-70%. The activated pixels depicted in FAIR images were about three times less than those seen in T*2-weighted images. With prolonged breath holding, it was observed that signal intensities in T*2-weighted and FAIR images were dependent on the breathing techniques used. Breath holding after expiration gave rise to immediate signal intensity increases in T*2-weighted and FAIR images, whereas breath holding performed after deep inspiration signals showed a biphasic change both in flow and T*2-weighted. T*2-weighted and FAIR signals showed a transient decrease before rising above the baseline level.

Adult↗

PubMed1999

A FAIR study of motor cortex activation under normo- and hypercapnia induced by breath challenge.

In this study an arterial spin-tagging technique based on flow-sensitive alternating inversion recovery (FAIR) with single-shot spiral data acquisition was used to study how the basal cerebral blood flow (CBF) elevated by breath holding affects the regional cerebral blood flow (rCBF) response to focal brain activation in the motor cortex. Six subjects were examined using three types of activation studies. These were (a) bilateral finger tapping paced at 4 Hz under normal breathing, (b) repeated expiration breath holding of 30 s, and (c) simultaneous breath holding and finger tapping. It was found that in five of six subjects the prevailing CBF level adjusted by breath challenge and the increase in rCBF in motor cortex associated with bilateral finger tapping were completely additive. This finding from FAIR-based functional magnetic resonance imaging is in accordance with that reported from published positron emission tomography studies. The results indicate that in the majority of the subjects examined the regulatory mechanisms for vasodilatory reaction to CO(2) and rCBF response to neural activation in motor cortex region are independent.

Adult↗

PubMed1999

Orientation of trabecular bone in human vertebrae assessed by MRI.

The aim of the study was to assess the relative number of bone trabeculae in different orientations by using magnetic inhomogeneity measurements by MR imaging. Twelve defatted human vertebrae (L2-L4) were studied by MR imaging and CT. In the MR measurements the reversible transaxial decay rate, R2', was determined using the GESFIDE sequence. The relative contribution to R2' of bone trabeculae oriented along the x (R2'x), y (R2'y) and z (R2'y) axes was assessed, by rotation of the specimen in the magnetic field. The results were validated by CT measurements of trabecular structure at a resolution of 0.2 x 0.2 x 1 mm. using custom-made software. R2' ranged from 4.9 to 32 s-1. After separation, theoretical R2'x ranged from 2.3 to 10.7 s-1, R2'y ranged from 2.6 to 14.0 s-1 and R2'z ranged from 4.7 to 17.9 s-1. The number of bone trabeculae per millimeter identified in axial CT images ranged from 0.15 to 0.38 and from 0.039 to 0.22 per millimeter in sagittal images. The difference between axial and sagittal CT images was statistically significant and due to anisotropic voxels. The correlation between R2' and the number of bone trabeculae per millimeter was statistically significant (r = 0.83, p < 0.001 for x orientation: r = 0.63 for y orientation, and r = 0.59 for z orientation, p < 0.05 for both). The relative amount of bone trabeculae in different orientations can be assessed in vitro using R2' measurements.

Aged↗

PubMed1999

Increased extracellular brain water after coronary artery bypass grafting is avoided by off-pump surgery.

OBJECTIVE: To determine if coronary artery bypass graft (CABG) surgery without cardiopulmonary bypass (CPB) avoids the brain swelling known to occur after CPB, to quantify these brain water compartment changes, and to identify the water shifts as due to intracellular or extracellular water. DESIGN: Prospective, controlled, and blinded. SETTING: Cardiac surgical unit in a university teaching hospital. SUBJECTS: Patients scheduled for CABG who were assigned to conventional (n = 10) or off-pump (n = 7) surgery according to their coronary anatomy. INTERVENTIONS: Magnetic resonance imaging (MRI) examinations were performed 1 day before surgery and 1 hour and 1 week after CABG surgery. MAIN OUTCOME MEASURES: Extracellular and intracellular water homeostasis was described quantitatively by calculating the averaged apparent diffusion coefficient of brain water using diffusion-weighted MRI. Blinded visual ordering of the images from the three examinations was performed according to brain size using conventional MRI. RESULTS: The average diffusion coefficient of brain water increased 4.7%+/-1.5% immediately after CABG with CPB and normalized after 1 week but did not change after CABG without CPB. No focal ischemic changes were seen in either group, and no gross neurologic deficits were observed. Visual analysis showed consistent brain swelling after CPB and variable changes in those operated without CPB. CONCLUSION: Changes consistent with increased extracellular brain water seen after CABG with CPB were not observed in patients undergoing CABG without CPB. The clinical significance of brain water changes and increased brain water content after surgery with CPB remains undefined.

Brain↗

PubMed1999

Measures of spatial structure in samples of genotypes for multiallelic loci.

Various spatial autocorrelation statistics have been widely used both in theoretical population genetics and to study the spatial distribution of diploid genotypes in many plant and animal populations. However, previous simulation studies have considered only diallelic loci. In this paper, we use a large number of space-time simulations to characterize for the first time the parametric and statistical values of Moran's I-statistics for converted individual genotypes as well as for join-count statistics. A wide range of levels of dispersal and numbers of alleles and allele frequencies are modelled and the results reveal the different general effects of each of these factors on these statistics. We also examine the range of appropriate sampling designs and sizes for which predicted values can be interpolated for specific sampling schemes for any given population genetic field survey. Numbers of alleles and allele frequencies each affect some statistics but not others. The results indicate generally low standard deviations. The results also develop precise and efficient methods of estimating gene dispersal, based on the various autocorrelation measures of standing spatial patterns of genetic variation within populations. The results also extend these methods to loci with multiple alleles, typical of those studied through modern molecular methods.

Alleles↗

PubMed1999

Gender differences in cerebral blood flow and oxygenation response during focal physiologic neural activity.

Using functional magnetic resonance imaging techniques CBF and oxygenation changes were measured during sustained checkerboard stimulation in 38 right-handed healthy volunteers (18 men and 20 women). The average blood oxygenation level dependent (BOLD) contrast technique signal intensity change was 1.67 +/- 0.6% in the group of male volunteers and 2.15 +/- 0.6% in the group of female volunteers (P < .05). Baseline regional CBF (rCBF) values in activated gray matter areas within the visual cortex were 57 +/- 10 mL x 100 g(-1) x min(-1) in women and 50 +/- 12 mL x 100 g(-1) x min(-1) in men, respectively (P = .09). Despite a broad overlap between both groups the rCBF increase was significantly higher in women compared to men (33 +/- 5 mL x 100 g(-1) x min(-1) versus 28 +/- 4 mL x 100 g(-1) x min(-1), P < .01). The increase of rCBF was not correlated with the baseline rCBF (mL x 100 g(-1) x min(-1)) (r(s) = 0.01, P = .9). Moreover, changes of rCBF were not correlated with changes in BOLD signal intensities (r(s) = 0.1, P = .7). Enhanced rCBF response in women during visual stimulation could be related to gender differences in visual physiology or may reflect gender differences in the vascular response to focal neuronal activation. Gender differences must be considered when interpreting the results of functional magnetic resonance imaging studies.

Adult↗

PubMed1999

Cerebral blood flow-related signal changes during breath-holding.

BACKGROUND AND PURPOSE: In the past, functional MR imaging techniques have been used successfully to determine cerebrovascular reactivity (CVR) to various stimuli, complementing the arsenal of functional brain investigations feasible with MR imaging. While previous studies have focused on blood oxygenation changes under vasodilatory stress, the aim of this study was to assess regional cerebral blood flow (rCBF) changes during breath-holding by using a flow-sensitive alternating inversion recovery (FAIR) imaging technique. METHODS: In six healthy volunteers, FAIR images were acquired during alternating periods of breath-holding and breathing at 40-second intervals after inspiration and at 30-second intervals after expiration, for a total dynamic scanning time of 10 minutes. To quantify the rCBF changes, we obtained 2.5-minute baseline samples during normal breathing. RESULTS: Repeated challenges of breath-holding induced an overall rise in rCBF. In general, rCBF changes were greatest in gray matter and were insignificant in white matter. Using the mean values of the baseline images collected before breath-holding to calculate the rCBF changes, we found that quantitative analysis yielded an rCBF increase of 47% to 87% after breath-holding. The rCBF changes clearly depended on the breath-holding duration and technique; however, for one given breath-holding paradigm the results showed relatively small interindividual variability. CONCLUSION: rCBF changes during a simple vascular challenge can be detected and quantified by means of functional MR imaging at 1.5 T. Noninvasive assessment of CVR could become a useful clinical tool to identify persons with impaired CVR.

Adult↗

PubMed1998

Diffusion-weighted MR imaging of acute cerebral ischemia.

Diffusion-weighted MR imaging has been used in studies on experimental animal models and on patients with acute cerebral ischemia. Compared with CT and conventional MR techniques, diffusion-weighted imaging can provide earlier and more precise detection of the location and the extent of an ischemic lesion during the critical first few hours after the onset of stroke. Quantitative apparent diffusion coefficient (ADC) mapping of the brain water can also be carried out by recording a series of diffusion-weighted images with different amplitudes of the displacement encoding gradients. ADC maps can provide important information about the extra- and intracellular water homeostasis. ADC reduction of the tissue water is one of the early signals of the pathophysiological cascade resulting from ischemic tissue injury. Diffusion MR imaging has become a valuable tool in stroke research. It may also prove a valuable tool in monitoring the efficiency of therapeutic effects in stroke patients. It is our intention to provide an overview of the recent development in this area with emphasis on the diffusion-weighted MR techniques, and to discuss the possible underlying biophysical mechanisms responsible for the contrast of diffusion-weighted imaging.

Acute Disease↗

PubMed1998

Functional magnetic resonance imaging of regional cerebral blood oxygenation changes during breath holding.

BACKGROUND AND PURPOSE: Recently, noninvasive MRI methods have been developed that are now capable of detecting and mapping regional hemodynamic responses to various stress tests, which involve the use of vasoactive substances such as acetazolamide or inhalation of carbon dioxide. The aim of this study was to assess regional cerebral blood oxygenation changes during breath holding at 1.5 T. METHODS: In 6 healthy volunteers, T2*-weighted gradient echo images were acquired for a total dynamic scanning time of 10 minutes during alternating periods of breath holding and normal breathing at 40-second intervals after inspiration, at 30-second intervals after expiration, and at 18 seconds after expiration. To quantify the relative signal changes, 2.5-minute baseline image sampling with normal breathing was carried out. RESULTS: Repeated challenges of breath holding of various durations induced an overall rise in blood oxygen level-dependent (BOLD) signal intensities. In general, BOLD signal intensity increases were greatest in gray matter and nonsignificant in white matter. Depending on the breath-holding duration and techniques, BOLD signal intensity increases of all activated pixels varied from 0.8% to 3.5%. CONCLUSIONS: The present study demonstrates that cerebral blood oxygenation changes during breath holding can be detected by means of fMRI at 1.5 T. The breath-holding test, a short and noninvasive method to study cerebral hemodynamics with fMRI, could become a useful alternative to the acetazolamide or CO2 test.

Adult↗

PubMed1994

Turbulent pipe flow studied by time-averaged NMR imaging: measurements of velocity profile and turbulent intensity.

A time-averaged method to obtain quantitative measurements in turbulent flow by phase flow encoding NMR imaging is introduced. With this method time-averaged velocity profiles and turbulence intensities can be determined. Time-averaged velocity profiles for pipe flow of water driven by a constant pressure gradient at Reynolds numbers from 1200 to 9400 were visualized. A precise correlation between the pixel intensity of the time-averaged NMR flow image and the local turbulence intensity of the flow is derived. The measured turbulence intensities are in agreement with published data obtained using other experimental methods.

Magnetic Resonance Spectroscopy↗

PubMed1992

[Hyperamylasemia in acute exacerbation of patients with chronic respiratory failure].

Serum amylase level was determined in 129 cases (225 episodes) of chronic respiratory failure at acute exacerbation and in 59 cases (62 episodes) of pneumonia without respiratory failure as control. Cases with accompanying diseases, such as acute pancreatitis, parotiditis, ileus and renal dysfunction, which were expected to develop hyperamylasemia were excluded. The 225 episodes were divided according to the causes of acute exacerbation into 4 groups: pneumonia, bronchitis, right heart failure without infection, and others (e. g. hemoptysis). Hyperamylasemia (greater than 400 S-U) was observed in groups of pneumonia (15/40 = 35.5%) and bronchitis (12/95 = 12.6%), respectively but not in those of right heart failure without infection (0/73 = 0%) and other causes (0/17 = 0%). As a result, hyperamylasemia was found only under conditions of inflammation of lung parenchyma and bronchi with acute exacerbation of respiratory failure. On the other hand no hyperamylasemia was observed in 62 episodes of pneumonia alone without respiratory failure. It was concluded that both respiratory tract infection and acute respiratory failure are necessary factors for development of hyperamylasemia originating from lung or bronchi.

Aged↗

PubMed1989

[Evaluation of three multiple regression equations of rheogram of pulmonary impedance for pulmonary artery pressure measurement].

This paper reports the use of 3 multiple regression equations (designated as A, B, and C), established by right cardiac catheterization and rheogram of pulmonary impedance examination simultaneously, for the determination of mean pulmonary artery pressure (PAP) in 100 healthy persons and 129 cases of chronic cor pulmonale on exacerbation. By analysis and comparison of the PAP values obtained from these 3 equations, the results showed that on equation B the upper limit of the PAP in 95% normal value range was 2.439 kPa (18.34 mmHg), which correspond to the upper limit of normal value range of the PAP 2.394 kPa (18.0 mmHg) as measured by right cardiac catheterization. Also, the range 1.032-2.597 kPa (7.761-19.530 mmHg) and standard deviation 0.383 kPa (+/- 2.88 mmHg) of PAP value in healthy persons, using the equation B for measurement were less than those on equation A and C. Thus, it was illustrated that the values of PAP were more stable on equation B than those on the other equations. On the other hand, the results obtained from the PAP test with these 3 equations in 100 healthy persons and 129 cases of chronic cor pulmonale on exacerbation, indicated that the specificity on equation B for healthy persons was higher, while using equation B the sensitivity and specificity for cor pulmonale patients were higher than those equation A or C. Therefore, we suggest that equation B is more conformable to the actual circumstances and suitable for clinical application.

Adult↗

PubMed1989

[The clinical significance of serum creatine phosphokinase measurement in patients with chronic cor pulmonale].

The values obtained from the measurement of serum creatine phosphokinase (SCPK) in 70 cases of chronic cor pulmonale in exacerbation and 72 cases in remission were compared with the normal value as measured in 160 healthy adults at the same period. The results showed that the normal value of SCPK was 46.50 +/- 12.71 u/L in male, while 32.51 +/- 10.8 u/L in female. The differences of the values of SCPK in male and female are highly significant statistically (P less than 0.001). Although the values of SCPK were within normal limits (M +/- 2SD) in most patients with chronic cor pulmonale (72.38-84.10%) in this paper, the values of SCPK of some of them (13.60-22.36%) were higher than normal. But in a few of the patients in exacerbation (2.27-5.26%) the SCPK values were lower than normal. The increased activity of SCPK in patients with chronic cor pulmonale may be related to the vigorous movement of the respiratory muscles. While the level of SCPK decreased to extremely low degree may indicate that the respiratory muscles may be excessively fatigue. So we suggest that the levels of SCPK which are markedly higher or lower than normal could be served as an reference index for the excessive activity or fatigue of the respiratory muscles respectively in chronic cor pulmonale.

Adult↗

PubMed1988