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H H Liu

Publications and source records attributed to H H Liu.

At least 19 recordsLinked to original sources
PubMed1998

Calculating dose distributions and wedge factors for photon treatment fields with dynamic wedges based on a convolution/superposition method.

A convolution/superposition based method was developed to calculate dose distributions and wedge factors in photon treatment fields generated by dynamic wedges. This algorithm used a dual source photon beam model that accounted for both primary photons from the target and secondary photons scattered from the machine head. The segmented treatment tables (STT) were used to calculate realistic photon fluence distributions in the wedged fields. The inclusion of the extra-focal photons resulted in more accurate dose calculation in high dose gradient regions, particularly in the beam penumbra. The wedge factors calculated using the convolution method were also compared to the measured data and showed good agreement within 0.5%. The wedge factor varied significantly with the field width along the moving jaw direction, but not along the static jaw or the depth direction. This variation was found to be determined by the ending position of the moving jaw, or the STT of the dynamic wedge. In conclusion, the convolution method proposed in this work can be used to accurately compute dose for a dynamic or an intensity modulated treatment based on the fluence modulation in the treatment field.

Humans↗
PubMed1997

Calculating dose and output factors for wedged photon radiotherapy fields using a convolution/superposition method.

We have developed a convolution/superposition method to calculate dose distributions in photon treatment fields with beam modifiers such as physical wedges. The dose component due to wedge generated radiation was accounted for by using an extended phantom model, which integrated a wedge, an air gap, and a patient phantom as the calculation phantom. The inhomogeneities in the extended phantom and the effect of beam hardening by the wedge were both corrected for in the convolution dose calculation. The calculated dose was verified by Monte Carlo simulation of the same extended phantom. A new dual photon source model was also used in the convolution method to account for both primary photons from the target and extra-focal photons from the primary collimator and flattening filter. Thus, realistic photon energy fluence distributions in the extended phantom were used for the dose calculation. The calculated dose distributions and the wedge factors agreed with the measured data within 2% for a variety of treatment fields including asymmetric fields. Our results showed that the wedge-generated radiation could contribute a significant fraction of the total dose in patients. This dose component depends on a specific field configuration, thus wedge factor changes with photon energy, wedge angle, field size, depth, and patient phantom SSD. The variation of the wedge factor can be predicted accurately by our convolution approach with the extended phantom model, which allows for more accurate dose or monitor unit computation for photon fields with beam modifiers.

Algorithms↗
PubMed1997

Correcting kernel tilting and hardening in convolution/superposition dose calculations for clinical divergent and polychromatic photon beams.

To account for clinical divergent and polychromatic photon beams, we have developed kernel tilting and kernel hardening correction methods for convolution dose calculation algorithms. The new correction methods were validated by Monte Carlo simulation. The accuracy and computation time of the our kernel tilting and kernel hardening correction methods were also compared to the existing approaches including terma divergence correction, dose divergence correction methods, and the effective mean kernel method with no kernel hardening correction. Treatment fields of 10 x 10-40 x 40 cm2 (field size at source to axis distance (SAD)) with source to source distances (SSDs) of 60, 80, and 100 cm, and photon energies of 6, 10, and 18 MV have been studied. Our results showed that based on the relative dose errors at a depth of 15 cm along the central axis, the terma divergence correction may be used for fields smaller than 10 x 10 cm2 with a SSD larger than 80 cm; the dose divergence correction with an additional kernel hardening correction can reduce dose error and may be more applicable than the terma divergence correction. For both these methods, the dose error increased linearly with the depth in the phantom; the 90% isodose lines at the depth of 15 cm were shifted by about 2%-5% of the field width due to significant underestimation of the penumbra dose. The kernel hardening effect was less prominent than the kernel tilting effect for clinical photon beams. The dose error by using nonhardening corrected kernel is less than 2.0% at a depth of 15 cm along the central axis, yet it increased with a smaller field size and lower photon energy. The kernel hardening correction could be more important to compute dose in the fields with beam modifiers such as wedges when beam hardening is more significant. The kernel tilting correction and kernel hardening correction increased computation time by about 3 times, and 0.5-1 times, respectively. This can be justified by more accurate dose calculations for the majority of clinical treatments.

Algorithms↗
PubMed1997

Measuring dose distributions for enhanced dynamic wedges using a multichamber detector array.

This paper investigates measuring dose distributions for enhanced dynamic wedges (EDWs) using a commercial multichamber detector array. The technical aspects of using the chamber array, including chamber calibration, selection of measurement parameters, and use of the reference chamber, have been fully investigated. The measurement results from the chamber array were also confirmed by those from the single chamber and radiographic film measurements. The results reported here showed that proper operation of the chamber array is essential to measure dose accurately for the EDW fields; the chamber detector array can be used more efficiently than a single chamber without compromising the dose measurement accuracy.

Biophysical Phenomena↗
PubMed1997

A dual source photon beam model used in convolution/superposition dose calculations for clinical megavoltage x-ray beams.

A realistic model of photon beams generated by clinical linear accelerators has been incorporated in a convolution/superposition method to compute dose distributions in photon treatment fields. In this beam model, a primary photon source represents photons directly from the target, and an extra-focal photon source represents scattered photons from the primary collimator and the flattening filter. Monte Carlo simulation was used to study clinical linear accelerators producing photon beams. From the output of the Monte Carlo simulation, the fluence and spectral distributions of each photon component, as well as the geometrical characteristics of each photon source with respect to its distance to the isocenter and its source distribution, were analyzed. These quantities were used to reproduce realistic photon distributions in treatment fields, and thus to compute dose distributions using the convolution method. Our results showed that compared to the primary photon fluence, the extra-focal photon fluence from the primary collimator and the flattening filter was 11%-16% at the isocenter, among which 70% was contributed by the flattening filter. The variation of extra-focal photons in different treatment fields was predicted accurately by accounting for the finite size of the extra-focal source. Compared to measurements, dose distributions in photon treatment fields, including those of asymmetric jaw settings and at different SSDs were calculated accurately, particularly in the penumbral region, by using the convolution method with the new dual source photon beam model.

Computer Simulation↗
PubMed1997

Calculating output factors for photon beam radiotherapy using a convolution/superposition method based on a dual source photon beam model.

A realistic photon beam model based on Monte Carlo simulation of clinical linear accelerators was implemented in a convolution/superposition dose calculation algorithm. A primary and an extra-focal sources were used in this beam model to represent the direct photons from the target and the scattered photons from other head structures, respectively. The effect of the finite size of the extra-focal source was modeled by a convolution of the source fluence distribution with the collimator aperture function. Relative photon output in air (Sc) and in phantom (Scp) were computed using the convolution method with this new photon beam model. Our results showed that in a 10 MV photon beam, the Sc, Sp (phantom scatter factor), and Scp factors increased by 11%, 10%, and 22%, respectively, as the field size changed from 3 x 3 cm2 to 40 x 40 cm2. The variation of the Sc factor was contributed mostly by an increase of the extra-focal radiation with field size. The radiation backscattered into the monitor chamber inside the accelerator head affected the Sc by about 2% in the same field range. The output factors in elongated fields, asymmetric fields, and blocked fields were also investigated in this study. Our results showed that if the effect of the backscattered radiation was taken into account, output factors in these treatment fields can be predicted accurately by our convolution algorithm using the dual source photon beam model.

Computer Simulation↗
PubMed1997

Involvement of interleukin-1 receptor mechanisms in development of arterial hypotension in rat heatstroke.

Rats, under urethan anesthesia, were exposed to a high ambient temperature (42 degrees C) to induce heatstroke and to assess the hemodynamic changes associated with heatstroke. Compared with normothermic controls, rats with heatstroke showed higher values of colonic temperature, heart rate, and plasma levels of interleukin (IL)-1 but lower values of R wave amplitude, P-R and Q-T intervals, systolic wave amplitude, diastolic and dicrotic wave duration, mean arterial pressure, stroke volume, and cardiac output. Animals injected intravenously with an IL-1-receptor antagonist at the time of heatstroke induction were protected from some of the cardiovascular effects of heatstroke, such as depressed ventricular depolarization, decreased stroke volume, decreased cardiac output, and arterial hypotension. The hemodynamic changes associated with heatstroke could be mimicked by IL-1beta administration. Other cardiovascular parameters such as total peripheral vascular resistance were unaffected by heatstroke induction or IL-1beta treatment. The results indicate that a selective decline in stroke volume or ventricular depolarization resulting from increased plasma levels of IL-1 may be an important mechanism signaling arterial hypotension or circulatory failure in rat heatstroke.

Animals↗
PubMed1996

[Expression of cDNA of human chorionic gonadotropin beta-subunit (beta-hCG) cDNA in insect cells and effect of expressed product on mouse lymphocytes in vitro].

Expression vector pVL 1393-hCG beta containing beta-hCG cDNA has been constructed using an unfused protein nuclear polyhedrosis virus (AcNPV) expression vector. The insect cells (Sf 9) were cotransfected by the expression vector and nuclear polyhedrosis virus genomic DNA, and recombinant virus AcNPV-hCG beta was screened out, beta-hCG cDNA was expressed in insect cells infected by recombinant virus and recombinant beta-hCG (r beta-hCG) was secreted into medium. The purity of r beta-hCG, purified by immuno-affinity chromatography, was about 90% and the molecular weight of r beta-hCG was 22,500 Da. Like hCG, r beta-hCG suppressed significantly proliferation of induced lymphocytes, as well as production of IL-2 to some extent, on a parallel with suppression of lymphoproliferation.

Animals↗
PubMed1995

Improved gene expression by a modified bicistronic retroviral vector.

We have previously described the construction of a bicistronic retroviral vector using the picornavirus internal ribosome entry site (IRES), which allows two genes expression simultaneously from a single transcript. This vector transcribes RNA efficiently; however, in some cases the levels of protein production are low. In this report, we further modified the bicistronic vector by abolishing the functional viral gag initiation codon that is retained in the vector at 5' to the first initiation codon of transduced gene. Five different genes, human interleukin 2 (hIL-2), human interleukin 4 (hIL-4), human granulocyte macrophage stimulating factor (hGM-CSF), herpes simplex virus thymidine kinase (HSV-tk) gene, and hepatitis C virus (HCV) core gene (C190), were tested on this modified vector for gene transfer and expression. Our results demonstrated that the new bicistronic vector greatly increased the protein levels when compared with the original one. As the RNA levels and splicing patterns from these two vectors remained similar, the improvement was most likely resulted from the increased translational efficiency.

Animals↗
PubMed1995

Breast dose during electron-beam CT: measurement with film dosimetry.

PURPOSE: To measure the breast dose and dose distribution from examinations performed with electron-beam computed tomography (CT). MATERIALS AND METHODS: Radiographic film was placed bilaterally at three depths in the breasts of an anthropomorphic phantom and exposed with clinically relevant protocols. The average breast dose and the dose distribution were obtained from the digitized and calibrated film and compared with those obtained with conventional CT. RESULTS: Breast doses from electron-beam CT chest, coronary artery calcium, and cardiac function examinations were 21.9, 2.87, and 1.55 mGy, respectively. Within the breast, the dose decreased linearly by a factor of three from the axilla to the sternum. The breast dose from a conventional CT chest examination ranged from 18 to 33 mGy and did not vary substantially across the breast. CONCLUSION: For chest examinations, the average breast dose from an electron-beam CT scanner was comparable to that from conventional CT scanners, despite the differences in dose distribution.

Breast↗
PubMed1995

[Cell-specific expression of AFP gene is dependent on some nuclear proteins].

AFP is an oncodevelopmental protein. Its level decreases abruptly after birth and reaches almost undetectable level during normal adult life. However, reexpression of the gene can be observed during hepatocarcinogenesis. To further understand mechanism of regulating AFP expression, we checked several restriction enzyme map of 5' terminal and flanking sequences of AFP gene. There are no differences among adult rat liver, fetal liver and hepatoma cells. Using +2(-)-255 bp sequence probe of AFP gene to do southwestern blotting assay, the result showed that the gene-active cells, such as hepatoma cells, contained binding-proteins which were apparently lacking in adult rat liver, lung, spleen, heart and kidney cells. While the fractions of nuclear proteins from adult rat liver cells were devoid of any stimulatory effect on transcription, those of binding-proteins from hepatoma stimulated the transcription of AFP gene in vitro. The hepatoma binding-proteins can rescue transcription activity of fraction of nuclear proteins from adult rat liver cells. These results indicate that cell-specific expression of AFP gene is regulated by protein-factors.

Animals↗
PubMed1995

[Cloning and overexpression of rat ovary LH/hCG receptor cDNA in insect cells].

Luteinizing hormone/human chorionic gonadotropin receptor (LH/hCG receptor) is a glycosylated protein coupled with G-protein. This paper reports screening of LH/hCG receptor cDNA from rat ovary cDNA library and the overexpression of the cDNA in insect cells. Full length of LH/hCG receptor cDNA is 2403 bp encoding signal peptide and matured protein of LH/hCG receptor. The cDNA is overexpressed in insect cells using baculovirus expression vector pVL 1393. The apparent molecular weight of purified receptor using immunoaffinity chromatography is 120 Kd and 92 Kd under non-reducing and reducing conditions, respectively. Ligand binding assay and Scatchard Plot analysis indicates that Kd is 8.4 x 10(-9) mol/L which is similar to that of the purified receptor from CHO cells.

Animals↗
PubMed1995

[Expression of the extracellular domain (1-341) of rat ovarian lutropin receptor in insect cells and preliminary characterization of the expressed product].

Extracellular domain residues 1-341 (designed R 341) of luteinizing hormone/human chorionic gonadotropin receptor (LH/hCG receptor) has high binding affinity for ligand. This paper describes expression of cDNA coding for R 341 in insect cells and preliminary identification of the expressed protein. SDS-PAGE silver staining and immunoblotting analysis show expressed product appears in two bands, major band has molecular weight 38.5 Kd, and weak band, 40.0 Kd. Ligand binding immunoblotting and 125I-hCG-binding blotting analysis indicate that expressed product R 341 has specific binding affinity for ligand. Ligand binding assay and Scatchard analysis indicate that recombinant receptor R 341 has high binding affinity for hCG and Kd is 5.68 x 10(-10) mol/L.

Animals↗
PubMed1995

Cracked teeth--treatment rationale and case management: case reports.

The treatment rationale for, and successful management of, six teeth with cracks in the form of incomplete vertical fractures are reported. Further crack propagation was prevented by placement of either stainless steel orthodontic bands or aluminum or acrylic resin provisional crowns, and endodontic treatment was subsequently carried out. The teeth were restored with intraradicular amalgam cores and complete veneer crowns. The teeth were reexamined periodically for up to 3.5 years after treatment. During the period of review, all teeth remained asymptomatic.

Adult↗
PubMed1993

Infections due to penicillin-resistant pneumococci. Clinical, epidemiologic, and microbiologic features.

Pneumococcal infection remains a common cause of serious morbidity and mortality throughout the world. Until recently, clinical isolates of pneumococci that were penicillin resistant were rare. However, 4% to 5% of the clinical isolates in the United States were recently found to be either intermediately resistant or highly resistant to penicillin. Clinicians in every field of medicine must therefore be better informed regarding penicillin-resistant pneumococcal infections to minimize their attendant morbidity and mortality and increase compliance with preventive measures. We reviewed the molecular, genetic, and epidemiologic aspects of penicillin-resistant pneumococcal infections, with emphasis on their microbiologic and clinical features.

Adult↗