Biomedical subjects

C X Yu

Publications and source records attributed to C X Yu.

At least 19 recordsLinked to original sources

PubMed2000

The analgesic effects of peripheral and central administration of melatonin in rats.

To explore the site and mechanism of the analgesic action of melatonin, the present study was designed to evaluate the analgesic effects of intraperitoneal (i.p.) and intracerebroventricular (i.c.v. ) administration of melatonin, and to investigate the effect of i.c. v. naloxone on the analgesic effect induced by i.p. melatonin in rats. Antinociception was determined by tail-flick latency to hot water at 50 degrees C. On i.p. administration, melatonin (30, 60 and 120 mg/kg) produced the antinociceptive effect in a dose-dependent manner, with an A(50) of 72.8 mg/kg. Administered i.c.v., melatonin (0.25, 0.5 and 1 mg/kg) also resulted in dose-dependent antinociception, with an A(50) of only 0.693 mg/kg. Injected i.c.v. to rats, 10 microg of naloxone antagonized significantly the antinociceptive effect induced by i.p. melatonin. It is concluded that melatonin has an analgesic effect in rats and the central nervous system (CNS) may be the primary site for melatonin to elicit the response, and the effect of melatonin is related to the central opioid system.

Analgesics↗

PubMed2000

Selective MT(2) melatonin receptor antagonist blocks melatonin-induced antinociception in rats.

The present study was undertaken to assess the effects of intracerebroventricular (i.c.v.) luzindole (a selective MT(2) melatonin receptor antagonist) and prazosin (a selective MT(3) melatonin receptor antagonist) on melatonin-induced antinociception, so as to clarify which of melatonin receptor subtypes within the central nervous system (CNS) was mediating antinociception. The pain threshold of rats was measured by the hot water (50 degrees C) tail-flick test. It was found that intraperitoneal (i.p.) melatonin (30, 60, 120 mg/kg) resulted in a dose-dependent antinociception. Luzindole (50, 100 microgram) administered intracerebroventricularly antagonized significantly the antinociceptive effect induced by i.p. melatonin (120 mg/kg), whereas prazosin (50 microgram) did not. Neither luzindole (100 microgram, i.c.v.) nor prazosin (50 microgram, i.c.v.) affected the nociceptive threshold when given alone. The results suggest that melatonin-induced antinociception is mediated through the MT(2) melatonin receptor subtype within the CNS.

Analgesics↗

PubMed2000

An efficient method of measuring the 4 mm helmet output factor for the Gamma knife.

It is essential to have accurate measurements of the 4 mm helmet output factor in the treatment of trigeminal neuralgia patients using the Gamma Knife. Because of the small collimator size and the sharp dose gradient at the beam focus, this measurement is generally tedious and difficult. We have developed an efficient method of measuring the 4 mm helmet output factor using regular radiographic films. The helmet output factor was measured by exposing a single Kodak XV film in the standard Leksell spherical phantom using the 18 mm helmet with 30-40 of its plug collimators replaced by the 4 mm plug collimators. The 4 mm helmet output factor was measured to be 0.876 +/- 0.009. This is in excellent agreement with our EGS4 Monte Carlo simulated value of 0.876 +/- 0.005. This helmet output factor value also agrees with more tedious TLD, diode and radiochromic film measurements that were each obtained using two separate measurements with the 18 mm helmet and the 4 mm helmet respectively. The 4 mm helmet output factor measured by the diode was 0.884 +/- 0.016, and the TLD measurement was 0.890 +/- 0.020. The radiochromic film measured value was 0.870 +/- 0.018. Because a single-exposure measurement was performed instead of a double exposure measurement, most of the systematic errors that appeared in the double-exposure measurements due to experimental setup variations were cancelled out. Consequently, the 4 mm helmet output factor is more precisely determined by the single-exposure approach. Therefore, routine measurement and quality assurance of the 4 mm helmet output factor of the Gamma Knife could be efficiently carried out using the proposed single-exposure technique.

Film Dosimetry↗

PubMed2000

Evaluation of EGS4/PRESTA multiple-scattering algorithms for 90Sr/90Y intravascular brachytherapy dosimetry.

The purpose of this work is to evaluate the EGS4/PRESTA electron multiple-scattering (MS) algorithms for dose calculation in intravascular brachytherapy (IVBT) using a 90Sr/90Y source. The small source size and the small volume of interest in IVBT require very fine spatial resolution, which may break down the constraints of Molière's MS theory as implemented in EGS4. The theory is accurate only when the electron step sizes are large enough to allow the number of collisions omega0 to be much greater than e = 2.7183. When step sizes are too small to allow at least 2.7183 collisions, as may be necessitated by the fine geometry, the algorithm may switch off MS, producing dosimetric artefacts. This study showed that switching off MS could produce a dose deviation of up to 6% when the half-thickness (d/2) of the dose scoring region is comparable with the Moliere minimum step size (t(min) = 2.7183). The effect of switching off MS is negligible if d/2 >> t(min) For the case of omega0 > e, if the electron step sizes are chosen to allow five to 40 collisions, with increasing step size, the doses surrounding the source increase and the error decreases. On the other hand, when larger step sizes are chosen, the dose calculation voxel size must also be increased in order for the calculations to converge. A good compromise between accuracy and applicability for IVBT simulation can be made, if the thickness of the scoring region is 0.1 mm and the electron step sizes are in the range allowing 10 to 30 collisions.

Algorithms↗

PubMed1999

Intensity-modulated radiation therapy with dynamic multileaf collimators.

Intensity-modulated radiotherapy (IMRT) has been considered as a means of providing dose distributions that conform to concave target volumes. For computer-controlled multileaf collimators (MLCs) to be used to modulate x-ray beams, some procedure must be used to determine the sequence of leaf positions used to produce the desired modulation. This article derives and compares four leaf-sequencing algorithms. MLC leaf sequencing can be accomplished by representing the areal intensity modulation of a beam with a series of beam profiles. A velocity-modulation equation for computing the modulation required for a one-dimensional profile, described originally using more extensive algebra, is derived using a graphic approach. The velocity-modulation approach is compared with an equal incremental step-and-shoot approach derived by Bortfeld and Boyer. An areal step-and-shoot technique derived by Xia and Verhey is introduced and compared with the profile-by-profile methods. Finally, an approach is considered using multiple repeated arcs developed by Yu. This wide variety of methods can yield an approach to IMRT that conforms to the engineering constraints imposed by the design of a particular linear accelerator.

Algorithms↗

PubMed1998

The effects of intra-fraction organ motion on the delivery of dynamic intensity modulation.

Computer-optimized treatment plans, aimed to enhance tumour control and reduce normal tissue complication, generally require non-uniform beam intensities. One of the techniques for delivering intensity-modulated beams is the use of dynamic multileaf collimation, where the beam aperture moves and the field shape changes during irradiation. When intensity-modulated beams are delivered with dynamic collimation, the problem of intra-fraction organ motion can cause distortions to the desired beam intensities. Unlike static field treatments, where intra-fraction organ motion only affects the boundaries creating broad dose penumbra, the interplay of the movement of the beam aperture and the movement of the patient anatomy can create 'hot' and 'cold' spots throughout the field. The mechanism for creating these effects is not well understood. This paper provides a simple analytical model which illustrates the fundamental mechanism for creating the dosimetric variations in the target when both the beam aperture and the target move during irradiation. Numerical analysis was carried out which calculates the cumulative primary photon fluence, or beam intensity, received by each point in the target, for a given pattern of motion. The results show that, for clinically realistic parameters, the magnitude of intensity variations in the target can be greater than 100% of the desired beam intensity. The magnitude of the photon intensity variations is strongly dependent on the speed of the beam aperture relative to the speed of the target motion, and the width of the scanning beam relative to the amplitude of target motion. The effects of fractionation as well as methods of minimizing and eliminating the dosimetric effects of intra-fraction organ motion are discussed.

Biophysical Phenomena↗

PubMed1998

Design considerations for the sides of multileaf collimator leaves.

The use of multileaf collimators (MLCs) in radiation therapy is becoming increasingly widespread as more commercial vendors offer them as options on medical linear accelerators. In most of the commercially available multileaf collimators, the sides of the leaves have a 'tongue and groove' (TG) feature, which is necessary for blocking the straight through leakage when the leaves are staggered together. The dosimetric effects of the TG design have not been fully studied for applications in block replacement or in dynamic intensity modulation. This article analytically explains the TG effect by calculating a deficit in photon flux when the side of a leaf is exposed. It is shown that the maximum photon deficit occurs when the leaf thickness is shared equally by the tongue and groove of adjacent leaves. New designs are proposed based on the theoretical analysis and improvements of the new design in leakage and the TG effect are verified with Monte-Carlo simulations. Measurements of the TG effect on an MLC from one manufacturer are also conducted. The results of the measurements confirm the analytical explanations of the TG effect.

Computer Simulation↗

PubMed1998

Verification of the omni wedge technique.

The optimal field shape achieved using a multileaf collimator (MLC) often requires collimator rotation to minimize the adverse effects of the scalloped dose distribution the leaf steps produce. However, treatment machines are designed to deliver wedged fields parallel or perpendicular to the direction of the leaves. An analysis of cases from our clinic showed that for 25% of the wedged fields used to treat brain and lung tumors, the wedge direction and optimal MLC orientation differed by 20 degrees or more. The recently published omni wedge technique provides the capability of producing a wedged field with orientation independent of the orientation of the collimator. This paper presents a comparison of the three-dimensional (3D) dose distributions of the omni wedged field with distributions of wedged fields produced using both the universal and dynamic wedge techniques. All measurements were performed using film dosimetry techniques. The omni wedge generated fields closely matched the conventional wedged fields. Throughout 95% of the irradiated volume (excluding the penubra), the dose distribution of the omni wedged field ranged from +5.5 to -3.5 +/- 1.5% of that of the conventionally wedged fields. Calculation of the omni wedged field is as accurate as conventional wedged field calculation when using a 3D treatment planning systems. For two-dimensional treatment planning systems, where one must assume that the omni wedged field is identical to a conventional field, the calculated field and the delivered field differs by a small amount.

Brain Neoplasms↗

PubMed1995

A multileaf collimator field prescription preparation system for conventional radiotherapy.

PURPOSE: The purpose of this work is to develop a prescription preparation system for efficient field shaping using a multileaf collimator that can be used in community settings as well as research institutions. The efficiency advantage of the computer-controlled multileaf collimator, over cerrobend blocks, to shape radiation fields has been shown in conformal treatments, which typically require complete volumetric computerized tomographic data for three-dimensional radiation treatment planning--a utility not readily available to the general community. As a result, most patients today are treated with conventional radiation therapy. Therefore, we believe that it is very important to fully use the same efficiency advantage of multileaf collimator as a block replacement in conventional practice. METHODS AND MATERIAL: The multileaf collimator prescription preparation system developed by us acquires prescription images from different sources, including film scanner and radiation treatment planning systems. The multileaf collimator angle and leaf positions are set from the desired field contour defined on the prescription image, by minimizing the area discrepancies. Interactive graphical tools include manual adjustment of collimator angle and leaf positions, and definition of portions of the field edges that require maximal conformation. Data files of the final leaf positions are transferred to the multileaf collimator controller via a dedicated communication link. RESULTS: We have implemented the field prescription preparation system and a network model for integrating the multileaf collimator and other radiotherapy modalities for routine treatments. For routine plan evaluation, isodose contours measured with film in solid water phantom at prescription depth are overlaid on the prescription image. Preliminary study indicates that the efficiency advantage of the MLC over cerrobend blocks in conformal therapy also holds true for conventional treatments. CONCLUSION: Our model of computer-controlled prescription, evaluation, and treatment using multileaf collimators can be effectively implemented in both community settings and research institutions. The resultant increase in treatment efficiency and accuracy is now available for conventional radiotherapy.

Equipment Design↗

PubMed1995

Optimization of leaf positions when shaping a radiation field with a multileaf collimator.

Computer-controlled multileaf collimators are receiving increasing acceptance for clinical use in radiation therapy due to their efficiency in shaping radiation fields. Because of the physical leaf width of the multileaf collimator, a smooth field contour prescribed by a physician is approximated by a staircase-shaped field boundary. The point of intersection with the prescribed field contour determines the error of the approximation and may affect the outcome of the treatment. This article provides analytically optimized solutions for leaf positioning based on two different geometric object functions.

Humans↗

PubMed1995

A method for implementing dynamic photon beam intensity modulation using independent jaws and a multileaf collimator.

A mathematical model is derived for digitally controlled linear accelerators to deliver a desired photon intensity distribution by combining collimator motion and machine dose rate variations. It shows that, at any instant, the quotient of the machine dose rate and the speed of collimator motion is proportional to the gradient of the desired in-air photon fluence distribution. The model is applicable for both independently controlled collimator jaws and multileaf collimators and can be implemented by controlling different parameters to accommodate linear accelerators from different manufactures. For independent jaws, each pair of jaws creates photon fluence variations along the direction of the jaw movement. For multileaf collimators, where each leaf is independently controlled, any two-dimensional (2D) photon fluence distribution can be delivered. The model has been implemented for wedged isodose distributions using independent jaws, and 2D intensity modulation using a multileaf collimator. One-dimensional (1D) wedged isodose distributions are created by moving an independent jaw at constant speed while varying machine dose rate. 2D intensity modulation has been implemented using a 'dynamic stepping' scheme, which controls the leaf progression during irradiation at constant machine dose rate. With this automated delivery scheme, the beam delivery time for dynamic intensity modulation, which depends on the complexity of the desired intensity distribution, approaches that of conventional beam modifiers. This paper shows the derivation of the model, its application, and our delivery scheme. Examples of 1D dynamic wedges and 2D intensity modulations will be given to illustrate the versatility of the model, the simplicity of its application, and the efficiency of beam delivery. These features make this approach practical for delivering conformal therapy treatments.

Biophysical Phenomena↗

PubMed1995

Intensity-modulated arc therapy with dynamic multileaf collimation: an alternative to tomotherapy.

The desire to improve local tumour control and cure more cancer patients, coupled with advances in computer technology and linear accelerator design, has spurred the developments of three-dimensional conformal radiotherapy techniques. Optimized treatment plans, aiming to deliver high dose to the target while minimizing dose to the surrounding tissues, can be delivered with multiple fields each with spatially modulated beam intensities or with multiple-slice treatments. This paper introduces a new method, intensity-modulated arc therapy (IMAT), for delivering optimized treatment plans to improve the therapeutic ratio. It utilizes continuous gantry motion as in conventional arc therapy. Unlike conventional arc therapy, the field shape, which is conformed with the multileaf collimator, changes during gantry rotation. Arbitrary two-dimensional beam intensify distributions at different beam angles are delivered with multiple superimposing arcs. A system capable of delivering the IMAT has been implemented. An example is given that illustrates the feasibility of this new method. Advantages of this new technique over tomotherapy and other slice-based delivery schemes are also discussed.

Algorithms↗

PubMed1995

Photon dose calculation incorporating explicit electron transport.

Significant advances have been made in recent years to improve photon dose calculation. However, accurate prediction of dose perturbation effects near the interfaces of different media, where charged particle equilibrium is not established, remain unsolved. Furthermore, changes in atomic number, which affect the multiple Coulomb scattering of the secondary electrons, are not accounted for by current photon dose calculation algorithms. As local interface effects are mainly due to the perturbation of secondary electrons, a photon-electron cascade model is proposed which incorporates explicit electron transport in the calculation of the primary photon dose component in heterogeneous media. The primary photon beam is treated as the source of many electron pencil beams. The latter are transported using the Fermi-Eyges theory. The scattered photon dose contribution is calculated with the dose spread array [T.R. Mackie, J.W. Scrimger, and J.J. Battista, Med. Phys. 12, 188-196 (1985)] approach. Comparisons of the calculation with Monte Carlo simulation and TLD measurements show good agreement for positions near the polystyrene-aluminum interfaces.

Algorithms↗

PubMed1994

A multileaf collimator field prescription preparation system for conventional radiotherapy.

PURPOSE: The purpose of this work is to develop a prescription preparation system for efficient field shaping using a multileaf collimator that can be used in community settings as well as research institutions. The efficiency advantage of the computer-controlled multileaf collimator, over cerrobend blocks, to shape radiation fields has been shown in conformal treatments, which typically require complete volumetric computerized tomographic data for three-dimensional radiation treatment planning--a utility not readily available to the general community. As a result, most patients today are treated with conventional radiation therapy. Therefore, we believe that it is very important to fully use the same efficiency advantage of multileaf collimator as a block replacement in conventional practice. METHODS AND MATERIAL: The multileaf collimator prescription preparation systems developed by us acquires prescription images from different sources, including film scanner, and radiation treatment planning systems. The multileaf collimator angle and leaf positions are set from the desired field contour defined on the prescription image, by minimizing the area discrepancies. Interactive graphical tools include manual adjustment of collimator angle and leaf positions, and definition of portions of the field edges that require maximal conformation. Data files of the final leaf positions are transferred to the multileaf collimator controller via a dedicated communication link. RESULTS: We have implemented the field prescription preparation system and a network model for integrating the multileaf collimator and other radiotherapy modalities for routine treatments. For routine plan evaluation, isodose contours measured with film in solid water phantom at prescription depth are overlaid on the prescription image. Preliminary study indicates that the efficiency advantage of the MLC over cerrobend blocks in conformal therapy also holds true for conventional treatments. CONCLUSION: Our model of computer-controlled prescription, evaluation, and treatment using multileaf collimators can be effectively implemented in both community settings and research institutions. The resultant increase in treatment efficiency and accuracy is now available for conventional radiotherapy.

Humans↗

PubMed1993

[Gonadal tumor and Y chromosome].

The histologic features of gonads occurrence of gonadal tumor, and RIA reproductive hormone levels in 50 cases containing a Y chromosome karyotype were analysed. The correlation between gonadal tumor and Y chromosome, clinical manifestation and genotype, and the principle of diagnosis and treatment are discussed. There were seven cases of gonadal tumor in these fifty cases the frequency of gonadal tumor is 14% (7/50). All of them were present in cases with gonadal dysgenesis, with the frequency rate of 27% (7/26). Five cases were bilateral, and two were unilateral, that made a total of twelve tumors in these seven cases. Gonadoblastoma was type of tumor commonly found, about 75% (9/12). The above date suggested that there is a higher frequency of gonadal tumor in cases of gonadal dysgenesis with a Y chromosome. Both hypergonadotropins and low sex steroids levels were the feature of gonadal dysgenesis, but no relative with occurrence of the tumor. The phenotype of these fifty cases were largely due to the process of abnormal sexual differentiation not only depend on the sex chromosome complement. In treatment, prophylactic bilateral gonadectomy is emphasized.

Female↗

PubMed1991

[Research on karyotypic consistency between chorionic villi and fetus].

In the last decade, chorionic villi sampling (CVS) has been widely accepted as a new technique in prenatal diagnosis. An inconsistency in karyotypes between CVS and fetal tissue was found in clinical practice, although the chorionic villi and the fetal tissues were from the same fertilized ovum. Karyotypes from direct chromosomal preparation of chorionic villi and cultured fetal tissues in 52 artificial abortion samples were compared. Results showed two cases with karyotypic inconsistency: (1) 46, XX/46, XY in the chorionic villus direct preparation and 46, XX in the fetal tissue; (2) 46, XY in the direct preparation of chorionic villi and 45,XY, t (15q21q) de novo in the fetal tissue. Our research demonstrates that karyotype from chorionic villus direct preparation does not always reflect the fetal karyotype.

Adult↗

PubMed1990

[Cardiovascular effect mediated by phencyclidine receptor in central nervous system].

In recent years, more and more attention has been paid to the studies about the cardiovascular function of phencyclidine (PCP) receptor, but very little about the cardiovascular function of central PCP receptor. In this study, we observed the effects of PCP on the blood pressure (BP) and heart rate (HR) by injecting the agonist or antagonist of PCP receptor into cerebral ventricle (icv) or spinal subarachnoid space (ith), to find out the cardiovascular function of central PCP receptor. It was found that icv PCP (250 nmol) elicited a slow and long lasting hypotension or fast and long lasting bradycardia. ith PCP elicited sharp hypotension and bradycardia immediately, with dose-dependent response. The hypotension and bradycardia of ith PCP (150 nmol) were significantly antagonized by ith 15 nmol dextrorphan. TCP, an agonist of PCP receptor, also elicited sharp hypotension and bradycardia immediately. However, subcutaneous injection (sc) of PCP (10 mg/kg) elicited hypertension and had no effect on HR. The results mentioned above indicate that central PCP receptors have the inhibitory effect on cardiovascular system. The cardiovascular effect of sc PCP may be the effect of combination of central and peripheral PCP receptors.

Animals↗

PubMed1990

[Relationship between spinal PCP receptor on cardiovascular effect and noradrenergic system].

By using receptor blockade, HPLC, destroying catecholaminergic nerve terminals by 6-OHDA, autoradiography, and other techniques, the relationship between effects of the spinal PCP receptor on cardiovascular function and noradrenergic system was studied. The main results were as following. Hypotension and bradycardia induced by ith PCP were significantly antagonized by prazosin and yohimbine; the MHPG levels in spinal CSF were significantly increased during the ith PCP induced hypotension and bradycardia; pretreatment with 6-OHDA to destroy NA terminals in the spinal cord significantly decreased the ith PCP induced hypotension and bradycardia, and the density of PCP receptors in the spinal cord. The results suggest that there are PCP receptors on the NA terminals in the spinal cord, which promoted the release of NA and/or inhibited the reuptake of NA. This may be a possible mechanism underlying the influence of spinal PCP receptors on cardiovascular function.

Adrenergic Fibers↗