Biomedical subjects

J Hsieh

Publications and source records attributed to J Hsieh.

At least 37 records · Page 2Linked to original sources

PubMed1999

Three-dimensional artifact induced by projection weighting and misalignment.

In recent years, the use of computer graphic techniques to produce three-dimensional (3-D) and reformatted images from a set of axial computed tomography (CT) images has gained significant interest. In most cases, the CT images are generated with the projection data set weighted prior to reconstruction, to combat motion artifacts, data inconsistency, or redundant data samples. In this paper, we investigate the potential bias introduced to the reconstruction as a result of the interaction of the projection weights and the isocenter misalignment (ISM). We demonstrate that when the weights applied to the conjugate rays are significantly different, bias will result which favors the sample with a higher weight. Although the error is not easily detected in axial CT images, it can be quite visible in 3-D or multiplanar reformatted (MPR) images. In this paper, we first present a theoretical framework to analyze and predict the bias. The theoretical prediction is validated by both computer simulations and phantom experiments. Several schemes to combat this artifact are subsequently presented; and their effectiveness is demonstrated.

Artifacts↗

PubMed1999

Cervical positional effects on snoring and apneas.

We examined the effects of cervical position on the Obstructive Sleep Apnea Syndrome (OSAS) through the use of a custom-designed cervical pillow which promoted neck extension. Twelve subjects with OSAS were recruited from a tertiary sleep disorder clinic population. Of the twelve subjects, three had mild cases of OSAS, four had moderate cases, and the remaining five had severe cases. The subjects used their usual pillows during two consecutive recorded baseline nights in our laboratory. The subjects then used the cervical pillow for five days at home, and returned for two consecutive recorded nights at our laboratory while using the cervical pillow. During the nights in our laboratory, the subjects completed questionnaires, were videotaped to record head and body position, and had their breathing parameters recorded during sleep. Subjects with mild OSAS cases had a non-significant improvement in the severity of their snoring and a significant improvement in their respiratory disturbance index with the cervical pillow, while subjects with moderate OSAS cases showed no improvement in these parameters. Subjects with severe OSAS cases showed slight improvement in some measures of their abnormal respiratory events during the experimental period.

Adult↗

PubMed1998

Kinetic mechanism for the sequential binding of two single-stranded oligodeoxynucleotides to the Escherichia coli Rep helicase dimer.

Escherichia coli Rep helicase is a DNA motor protein that unwinds duplex DNA as a dimeric enzyme. Using fluorescence probes positioned asymmetrically within a series of single-stranded (ss) oligodeoxynucleotides, we show that ss-DNA binds with a defined polarity to Rep monomers and to individual subunits of the Rep dimer. Using fluorescence resonance energy transfer and stopped-flow techniques, we have examined the mechanism of ss-oligodeoxynucleotide binding to preformed Rep dimers in which one binding site is occupied by a single-stranded oligodeoxynucleotide, while the other site is free (P2S dimer). We show that ss-DNA binding to the P2S Rep dimer to form the doubly ligated P2S2 dimer occurs by a multistep process with the initial binding step occurring relatively rapidly with a bimolecular rate constant of k1 = approximately 2 x 10(6) M-1 s-1 [20 mM Tris (pH 7.5), 6 mM NaCl, 5 mM MgCl2, 5 mM 2-mercaptoethanol, and 10% (v/v) glycerol, 4 degrees C]. A minimal kinetic mechanism is proposed which suggests that the two strands of ss-DNA bound to the Rep homodimer are kinetically distinct even within the P2S2 Rep dimer, indicating that this dimer is functionally asymmetric. The implications of these results for the mechanisms of DNA unwinding and translocation by the functional Rep dimer are discussed.

Adenosine Triphosphatases↗

PubMed1998

Adaptive streak artifact reduction in computed tomography resulting from excessive x-ray photon noise.

The quality of a computed tomography (CT) image is often degraded by streaking artifacts resulting from excessive x-ray quantum noise. Often, a patient has to be rescanned at a higher technique or at a larger slice thickness in order to obtain an acceptable image for diagnosis. This results in a higher dose to the patient, a degraded cross plane resolution, or a reduced patient throughput. In this paper, we propose an adaptive filtering approach in Radon space based on the local statistical properties of the CT projections. We first model the noise characteristics of a projection sample undergoing important preprocessing steps. A filter is then designed such that its parameters are dynamically adjusted to adapt to the local noise characteristics. Because of the adaptive nature of the filter, a proper balance between streak artifact suppression and spatial resolution preservation is achieved. Phantom and clinical studies have been conducted to evaluate the robustness of our approach. Results demonstrate that the adaptive filtering approach is effective in reducing or eliminating quantum noise induced artifacts in CT. At the same time, the impact on the spatial resolution is kept at a low level.

Artifacts↗

PubMed1998

Future directions in CT technology.

Progress in helical CT acquisition and processing over the last 5 years has lead to new applications and new requirements. Recent progress to meet these requirements is discussed in this article. The potential of future acquisition developments and their potential to further import application are explored.

Data Display↗

PubMed1997

Major domain swiveling revealed by the crystal structures of complexes of E. coli Rep helicase bound to single-stranded DNA and ADP.

Crystal structures of binary and ternary complexes of the E. coli Rep helicase bound to single-stranded (ss) DNA or ssDNA and ADP were determined to a resolution of 3.0 A and 3.2 A, respectively. The asymmetric unit in the crystals contains two Rep monomers differing from each other by a large reorientation of one of the domains, corresponding to a swiveling of 130 degrees about a hinge region. Such domain movements are sufficiently large to suggest that these may be coupled to translocation of the Rep dimer along DNA. The ssDNA binding site involves the helicase motifs Ia, III, and V, whereas the ADP binding site involves helicase motifs I and IV. Residues in motifs II and VI may function to transduce the allosteric effects of nucleotides on DNA binding. These structures represent the first view of a DNA helicase bound to DNA.

Adenosine Diphosphate↗

PubMed1997

Analysis of the temporal response of computed tomography fluoroscopy.

Computed tomography fluoroscopy (CTF) has found many applications in interventional procedures. The advantage of CTF is its ability to remove overlapping structures in the human anatomy and, thereby, provide accurate spatial information. Another advantage is its ability to reconstruct images in a short period of time so that "real-time" information can be provided. In this paper, we perform theoretical analysis and computer simulations to evaluate the temporal response of the CTF. We investigate the impact of various reconstruction algorithms on CTF and demonstrate the inherent limitations of the CTF in terms of the time lag and delay. The analysis provides a useful guidance to the CTF operators to avoid potential pitfalls.

Algorithms↗

PubMed1997

Nonstationary noise characteristics of the helical scan and its impact on image quality and artifacts.

Helical computed tomography has replaced the conventional step-and-shoot CT in many clinical applications. Because of its clinical importance, a number of comparative studies have been performed to evaluate its performance parameters, such as the noise and the slice sensitivity profile. In this paper, the nonstationary noise characteristics of helical images are carefully examined. Their impact on the low contrast detectability and three-dimensional (3-D) image artifacts is assessed. An analytical equation is derived to relate the interactions between various helical reconstruction schemes and the fan-beam filtered backprojection algorithm. We demonstrate that for many popular helical weights, the noise variation within the field of view of the reconstruction is close to a factor of 3. Several approaches to overcome the undesired characteristics are proposed.

Algorithms↗

PubMed1996

ATPase activity of Escherichia coli Rep helicase is dramatically dependent on DNA ligation and protein oligomeric states.

The Escherichia coli Rep helicase catalyzes the unwinding of duplex DNA using the energy derived from ATP binding and hydrolysis. Rep functions as a dimer but assembles to its active dimeric form only on binding DNA. Each promoter of a dimer contains a DNA binding site that can bind either single-stranded (S) or duplex (D) DNA. The dimer can bind up to two oligodeoxynucleotides in five DNA-ligation states: two half-ligated states, P2S and P2D, and three fully-ligated states, P2S2, P2D2, and P2SD. We have previously shown that the relative stabilities of these ligation states are allosterically regulated by the binding and hydrolysis of ATP and have proposed an "active rolling" model for DNA unwinding where the enzyme cycles through a series of these ligation states in a process that is coupled to the catalytic cycle of ATP hydrolysis [Wong, I., & Lohman, T.M., (1992), Science 256, 350-355]. THe basal ATPase activity of Rep protein is stimulated by ss DNA binding and by protein dimerization. We have measured the steady-state ATPase activities of Rep bound to dT(pT)15 in each distinct ss DNA ligation state (PS, P2S, and P2S2) to compare with our previous measurements with unligated Rep monomer (P) [Moore, K.J.M., & Lohman, T.M. (1994) Biochemistry 33, 14550]. We find the ATPase activity of Rep is influenced dramatically by both dimerization and ss DNA ligation state, with the following kcat values for ATP hydrolysis increasing by over 4 orders of magnitude: 2.1 x 10(-3) s(-1) for P, 2.17 +/- 0.04 s(-1) for PS, 16.5 +/- 0.2 s(-1) for P2S, and 71 +/- 2.5 s(-1) for P2S2 (20 mM Tris-HCl, pH 7.5, 6mM NaCl, 5 mM MgCl2, 10% glycerol, 4 degrees C). The apparent KM's for ATP hydrolysis are 2.05 +/- 0.1 microM for PS and 2.7 +/- 0.2 microM for P2S. These widely different ATPase activities reflect the allosteric effects of DNA ligation and demonstrate that cooperative communication occurs between the ATP and DNA site of both subunits of the Rep dimer. These results further emphasize the need to explicitly consider the population distribution of oligomerization and DNA ligation states of the helicase when attempting to infer information about elementary processes such as helicase translocation based solely on macroscopic steady-state ATPase measurements.

Adenosine Triphosphatases↗

PubMed1996

Thalidomide inhibits TNF response and increases survival following endotoxin injection in rats.

Sepsis is a leading cause of death following major trauma and complicated abdominal surgery. Tumor necrosis factor-alpha (TNF) is believed to be a central mediator in the inflammatory response syndrome. Numerous methods of blunting the TNF response in sepsis have been attempted with suggestion of increased survival and decreased organ injury. Thalidomide, shown in vitro to selectively inhibit TNF production, has been used clinically in states of chronic TNF elevation with encouraging results. In this study, we examined the effect of thalidomide administration in a rat model of acute septic shock. Femoral artery cannulation was performed and baseline TNF measured. Dose response was determined by giving varying doses of thalidomide by gavage. Rats were injected intraarterially with endotoxin and serial samples drawn. TNF was measured by ELISA. For survival, thalidomide was given by gavage and endotoxin injected intraperitoneally. Serum TNF elevation occurred after endotoxin injection with peak levels at 90 min. Thalidomide treated rats had lower TNF levels at all time points (P = <0.01 at 90 and 120 min), with the inhibition being dose dependent. Survival in treated rats exceeded that of untreated rats (53% vs 19%, P = <0.05) at 48 and 72 hr. In conclusion, we found that thalidomide administration leads to increased survival following acute endotoxemia, which may be due to the observed TNF inhibition.

Analysis of Variance↗

PubMed1996

A general approach to the reconstruction of x-ray helical computed tomography.

Helical Computed Tomography (HCT) has become the method of choice for many routine clinical studies. The advantages of HCT include the capability of scanning a complete anatomical volume in a single breath hold, the capability of generating images at any desired location, and the improved patient throughput. However, these advantages come at the expense of some image quality compromises. This is mainly caused by the fact that the projection set is inherently incomplete and inconsistent, due to the constant patient translation during the data acquisition process. In this paper, we will briefly review the research work performed in this area and present a more general approach to the problem. We give two specific examples of the general approach and compare the performance of one of the examples with one of the best methods available today.

Algorithms↗

PubMed1994

Determinants of plasma vitamins and lipids: the Working Well Study.

This study was conducted to assess the determinants of plasma concentrations of alpha-tocopherol, beta-carotene, retinol, and cholesterol fractions in a randomly selected subset of 203 workers participating in a worksite-based health intervention trial. Workers were from four companies in eastern and central Massachusetts, and all completed an 84-item semiquantitative food frequency questionnaire as part of baseline (preintervention) self-assessment instruments. At the time of fasting blood sampling, each participant also completed a short screening questionnaire for assessment of changes in dietary habits and tobacco exposure and for collection of data on use of vitamins and nutritional supplements. On the basis of the self-reported data, the authors found that they could explain 35% of the variability in plasma beta-carotene, 73% of the variability in alpha-tocopherol, 36% of the variability in retinol, and 19% of the variability in cholesterol. Plasma beta-carotene levels appeared to be affected by the use of supplements that did not contain carotene, indicating a beta-carotene sparing capability of other agents contained in these preparations. Plasma alpha-tocopherol levels were not similarly affected. These results compare favorably with those from studies that used more intensive dietary assessment techniques as the comparison criterion. Results are discussed in terms of implications for use of self-reported data in epidemiologic study analyses.

Adult↗

PubMed1994

Predictive respiratory gating: a new method to reduce motion artifacts on CT scans.

PURPOSE: To evaluate a gating system, called predictive respiratory gating (PRG), that reduces motion-induced artifacts on computed tomographic (CT) scans of patients who cannot suspend respiration. MATERIALS AND METHODS: PRG uses a respiration monitor and a new algorithm to predict when a motionless period is about to occur. It automatically starts scanning so the scan is temporally centered around the motionless period at end inspiration or end expiration. To demonstrate PRG, CT was performed on a motion phantom and a quietly breathing volunteer with and without gating. RESULTS: Scans of the phantom obtained with PRG contained less motion-induced streaking and blurring than did scans acquired without PRG. Scans of the volunteer gated at end expiration contained significantly less artifact than nongated scans (P < .03). CONCLUSION: PRG reduced motion artifact on scans of a spontaneously breathing volunteer. PRG may be able to reduce motion artifacts on scans of patients unable to suspend respiration.

Adult↗

PubMed1994

Effects of cigarette smoking and its cessation on lipid metabolism and energy expenditure in heavy smokers.

The relationship between thermogenic and potentially atherogenic effects of cigarette smoking (CS) and its cessation was investigated. Heavy smokers (n = 7, serum cotinine > 200 ng/ml, > 20 cigarettes/d) were maintained on isoenergetic, constant diets for 2 wk, 1 wk with and 1 wk without CS. Stable isotope infusions with indirect calorimetry were performed on day 7 of each phase, after an overnight fast. CS after overnight abstention increased resting energy expenditure by 5% (not significant vs. non-CS phase; P = 0.18). CS increased the flux of FFA by 77%, flux of glycerol by 82%, and serum FFA concentrations by 73% (P < 0.02 for each), but did not significantly affect fat oxidation. Hepatic reesterification of FFA increased more than threefold (P < 0.03) and adipocyte recycling increased nonsignificantly (P = 0.10). CS-induced lipid substrate cycles represented only 15% (estimated 11 kcal/d) of observed changes in energy expenditure. De novo hepatic lipogenesis was low (< 1-2 g/d) and unaffected by either acute CS or its chronic cessation. Hepatic glucose production was not affected by CS, despite increased serum glycerol and FFA fluxes. Cessation of CS caused no rebound effects on basal metabolic fluxes. In conclusion, a metabolic mechanism for the atherogenic effects of CS on serum lipids (increased hepatic reesterification of FFA) has been documented. Increased entry of FFA accounts for CS-induced increases in serum FFA concentrations. The thermogenic effect of CS is small or absent in heavy smokers while the potentially atherogenic effect is maintained, and cessation of CS does not induce a rebound lipogenic milieu that specifically favors accrual of body fat in the absence of increased food intake.

Adipocytes↗

PubMed1993

Effects of FK506 and cyclosporine on dynamic insulin secretion from isolated dog pancreatic islets.

Pancreatic islet transplantation may be the most ideal treatment for patients with insulin-dependent diabetes mellitus. However, immunosuppressive agents such as cyclosporine A(CsA) and FK506, used for these transplanted patients have been reported to cause glucose intolerance. In the present study, we have compared the effects of CsA and FK506 on glucose-stimulated insulin release from the isolated dog pancreatic islets, which have been maintained in culture for 3 days after isolation. The isolated dog pancreatic islets, pretreated for 24 hr with either CsA or FK506 (1, 10, and 100 nM), were perifused with 16.7 mM glucose. Pretreatment with both drugs suppressed glucose-stimulated insulin secretion in a dose-dependent fashion. CsA (100 nM), which is a therapeutically relevant concentration, significantly suppressed both the first and second phases of glucose-stimulated insulin release compared with 100 nM FK506. These findings suggest that, with a therapeutically relevant concentration, FK506 may be less toxic than CsA against pancreatic islets in patients with organ or cell transplantation.

Animals↗

PubMed1992

Reinforcement of subliminal flexion reflexes by transcranial magnetic stimulation of motor cortex in subjects with spinal cord injury.

A 20 msec train (500 Hz; 0.1-0.2 msec duration) of percutaneous electrical stimulation (ES) applied to the plantar surface was used to condition muscle responses evoked in tibialis anterior (TA) by transcranial magnetic stimulation of the motor cortex in 8 subjects with traumatic spinal cord injury (SCI). The intensity of conditioning ES was adjusted to just subthreshold for evoking flexion reflexes in TA and was delivered at conditioning-test (C-T) intervals of 15-60 msec prior to cortical stimulation. Four subjects with clinically complete SCI revealed no muscle response to cortical stimulation or following combined subliminal percutaneous ES and cortical stimulation. Four subjects (3 clinically incomplete and 1 complete injury) demonstrated muscle responses with a latency of 70-80 msec time-locked to the percutaneous ES when the conditioning subliminal stimulation was delivered at C-T: 15-40 msec. These responses, resembling suprathreshold flexion reflexes, reflect the convergence of excitatory afferent and cortical inputs and provide evidence of preserved corticospinal innervation to the L4-5 segmental motoneuron or interneuron pools. In 3 of the subjects this preserved corticospinal influence was evident despite absence of motor evoked potentials (MEPs) following cortical stimulation. The effect of the combined electrical and cortical stimulation in yielding suprathreshold flexion reflexes, instead of the facilitated MEPs seen in control subjects, appears to be related to slowed central conduction, prolonged temporal dispersion of the motoneuron facilitation following cortical stimulation and segmental reflex changes associated with disrupted modulation of interneuronal pathways. The results show this conditioning paradigm to be useful in revealing preserved corticospinal innervation in some SCI subjects with absent MEPs.

Adaptation, Physiological↗

PubMed1992

Ventilatory effects of dexmedetomidine, atipamezole, and isoflurane in dogs.

Dexmedetomidine (DMED) is a novel alpha 2 adrenergic agonist that has been shown to have potent analgesic and anesthetic sparing effects. This study was designed to investigate the effects of DMED, both alone and combined with isoflurane, on resting ventilation, the hypercapnic response, and the hypoxic response in dogs. When given alone, 1 microgram/kg decreased resting ventilation by 22% but at larger doses (10, 20, and 100 micrograms/kg) resting ventilation increased, doubling at 100 micrograms/kg. Doses of 10 micrograms/kg and greater caused a maximum depression of 60% in the slope of the hypercapnic response, but no dose had a significant effect on the hypoxic ventilatory response. A dose of 3 micrograms/kg of DMED reduced isoflurane MAC from 1.3% to 0.37%, and the ventilatory effects of this 1 MAC combination were intermediate between the awake values and those of isoflurane-anesthetized (1.3%) dogs. Atipamezole is a specific centrally acting alpha 2 receptor antagonist and when given with DMED in isoflurane-anesthetized dogs prevented the ventilatory depression. However, atipamezole alone also ventilatory stimulating effects, which may indicate tonic alpha 2 adrenergic activity. The ventilatory depression caused by DMED, either alone or combined with isoflurane, at doses that significantly reduce anesthetic requirements are relatively mild.

Adrenergic alpha-Agonists↗

PubMed1992

Hemorrhagic shock inhibits lipopolysaccharide-induced myelopoiesis in both germ-free and conventional rats.

BACKGROUND: Bacterial translocation has been implicated in the alteration of the immune response after shock and trauma. This study examined the effect of bacterial translocation on myelopoiesis after hemorrhagic shock in germ-free and conventional rats. METHODS: Awake, unrestrained germ-free and conventional rats were bled to a mean arterial pressure of 30 mm Hg until the animal required infusion of 10% of the shed blood. Rats were resuscitated with shed blood and crystalloid. Sham rats were catheterized but not bled. Twenty-four hours after shock or sham, rats were administered lipopolysaccharide 100 micrograms or saline intraperitoneally. Twenty-four hours later, bone marrow cells were cultured for growth of granulocyte-macrophage colony-stimulating factor (CFU-GM). RESULTS: Lipopolysaccharide increased the number of CFU-GM/femur in sham germ-free rats (801 +/- 129 versus 455 +/- 110; p less than 0.05) and conventional rats (1458 +/- 200 versus 492 +/- 59; p less than 0.05) compared with saline-treated rats. In contrast, hemorrhagic shock inhibited lipopolysaccharide-induced CFU-GM growth in both germ-free and conventional rats. Shock, itself, was a stimulus for CFU-GM growth in germ-free but not conventional rats. Bone marrow white blood cell counts were unaffected by shock, lipopolysaccharide administration, or the germ-free state. CONCLUSIONS: Hemorrhagic shock inhibited lipopolysaccharide-induced CFU-GM proliferation independent of the germ-bearing status of the rat, and bacterial translocation exerted no influence on myelopoietic dysfunction after hemorrhagic shock.

Animals↗