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Biomedical subjects

R T Ho

Publications and source records attributed to R T Ho.

At least 19 recordsLinked to original sources
PubMed2001

Dose distributions at extreme irradiation depths of gamma knife radiosurgery: EGS4 Monte Carlo calculations.

The accuracy of the dose planning system (Leksell GammaPlan), used in Gamma Knife (type B) radiosurgery at extreme irradiation depths, was verified using the Monte Carlo technique. EGS4 Monte Carlo calculations were employed to calculate the dose distribution along the x, y and z axes for an irradiation relatively shallow in a spherical bony cavity water phantom. Two different sizes of the collimator helmets, 8 and 18 mm, of the Leksell Gamma Knife Unit were studied. The results of GammaPlan showed good consistency with the Monte Carlo results. Furthermore, small dose enhancements were observed in the skull bone where accurate dose measurements are difficult due to the presence of the air-phantom interface. Therefore, the results of this project can promote confidence to all Gamma Knife centres in the world when using the Leksell GammaPlan.

Humans↗
PubMed2001

Unipolar defibrillator?

An unusual case of "unipolar" pacing and myopotential over-sensing leading to an inappropriate ICD shock in a patient with an implanted defibrillator is reported. The reasons for unipolar behavior in a system using a committed bipolar device are discussed.

Algorithms↗
PubMed2000

Stereotactic dose planning system used in Leksell Gamma Knife model-B: EGS4 Monte Carlo versus GafChromic films MD-55.

The accuracy of the dose planning system (Leksell GammaPlan) used in Gamma Knife (model-B) radiosurgery was verified using both the GafChromic films MD-55 (improved) and the EGS4 Monte Carlo technique. The Monte Carlo technique was employed to calculate the dose distribution along the x-, y- and z-axes when a single shot with opening of all 201 sources was delivered at the centre of a simulated water phantom with a diameter of 160 mm. Collimator helmets with different size of the Gamma Knife unit were verified. Good consistency (typical discrepancy less than 2%) was obtained between the results of Monte Carlo and GammaPlan. Small discrepancies, however, were obtained by GafChromic films. Discrepancies, as great as 10% when using the 4 mm collimator helmet, at the low percentage isodose curve along the z-axis of the measurement results were probably due to the small energy dependency of the GafChromic films. Significant discrepancies were not observed along x- and y-axes because such small discrepancies were easily over-washed by other gamma beams coming from the x- and y-directions. Similar results showing the discrepancies between the GafChromic films and GammaPlan were obtained when using the 8, 14 and 18 mm collimator helmets. However, the discrepancies along the z-axis became smaller as scattering effect increased when using larger collimator sizes. We suggest that the Monte Carlo technique should also be applied in stereotactic dose planning system verification as it is an ideal and reliable computational technique.

Film Dosimetry↗
PubMed2000

EKG abnormalities during partial seizures in refractory epilepsy.

PURPOSE: This study assessed the frequency and character of ictal cardiac rhythm and conduction abnormalities in intractable epilepsy. Sudden unexpected death in epilepsy (SUDEP) is a major cause of excess mortality in people with refractory epilepsy, and cardiac arrhythmias during seizures may be responsible. The frequency of cardiac abnormalities during seizures in patients with refractory epilepsy must be determined. METHODS: Fifty-one seizures in 43 patients with intractable partial epilepsy were analyzed prospectively from CCTV-EEG monitoring with one ECG channel. Arrhythmias, repolarization abnormalities, and PR and QTc intervals were determined for preictal (3 min), ictal, and postictal (3 min) periods for one or more seizures per patient. Parametric statistics were used for continuous variables, and nonparametric statistics were used for categoric variables. RESULTS: Of the patients, 39% had one or more abnormalities of rhythm and/or repolarization during or immediately after seizures. Abnormalities included asystole (one), atrial fibrillation (one), marked or moderate sinus arrhythmia (six), supraventricular tachycardia (one), atrial premature depolarizations (APDs; eight), ventricular premature depolarizations (VPDs; two), and bundle-branch block (three). Mean seizure duration was longer in patients with abnormalities than in those without (204 vs. 71 s; p < 0.001). Generalized tonic-clonic seizures were also associated with increased occurrence of ictal ECG abnormalities (p = 0.006) as compared with complex partial seizures. There were no clinically significant differences in mean preictal and ictal/postictal PR and QTc intervals. CONCLUSIONS: Cardiac rhythm and conduction abnormalities are common during seizures, particularly if they are prolonged or generalized, in intractable epilepsy. These abnormalities may contribute to SUDEP.

Adolescent↗
PubMed1999

Monte Carlo calculated output factors of a Leksell Gamma Knife unit.

The Leksell Gamma Knife is a standard radiosurgical tool for treating brain lesions by directing beams of gamma radiation to a specific region. The diameter of the gamma beams is confined by collimator systems and available collimator sizes are 4, 8, 14 and 18 mm. The reduction in dose rate for each collimator helmet is called the output factor (OPF). Experimental determination of OPFs is difficult due to the extremely narrow beams for which the dose is determined. In the present work, the PRESTA version of the EGS4 Monte Carlo code was used to obtain relative OPFs for the Leksell Gamma Knife for collimator sizes of 14, 8 and 4 mm (relative to that of the 18 mm collimator). A spherical probe with a radius of 1 mm was utilized in this computer experiment. Our Monte Carlo results gave OPFs of 0.974, 0.951 and 0.872 for the 14 mm, 8 mm and 4 mm collimators respectively, relative to the 18 mm collimator. Our calculated OPF for the 4 mm collimator helmet was more than 8% higher than the value currently used, but in good agreement with the average of experimental values obtained by various Gamma Knife centres throughout the world and with the value now recommended by the manufacturer, Elekta (Elekta Instrument AB, Skeppargatan 8, S-114 52 Stockholm, Sweden).

Brain Diseases↗
PubMed1999

Monte Carlo calculations and GafChromic film measurements for plugged collimator helmets of Leksell Gamma Knife unit.

The Monte Carlo technique and GafChromic films were employed to verify the accuracy of the dose planning system (Leksell GammaPlan) used in Gamma Knife (type B) radiosurgery when plugged collimator helmets were used. The EGS4 Monte Carlo code was used to calculate the dose distribution along the x, y, and z axes when a single shot was delivered at the center point (unit center point: x = 100, y = 100, z = 100) of a spherical polystyrene phantom, with gamma angle of 90 degrees. Two different sizes of the plugged collimator helmets, 4 and 18 mm, were studied. Two typical plugged patterns, 51 plugs and 99 plugs along the y direction, were examined. The results of our Monte Carlo trials showed good consistency with GammaPlan calculations and GafChromic film measurements. Furthermore, the Monte Carlo results showed that radiation leakage from the plugs was too small to affect the overall isodose curve distribution even when the heavily plugged pattern of up to 99 plugs was employed. The results of this project provide confidence to all Gamma Knife centers using the Leksell GammaPlan treatment planning system.

Computer Simulation↗
PubMed1998

Monte Carlo calculation of single-beam dose profiles used in a gamma knife treatment planning system.

The accuracy of single-beam dose profiles used in the algorithm of the Gamma Knife treatment planning system (Leksell GammaPlan) is verified. EGS4 Monte Carlo calculation was employed to calculate the dose distributions of single-beams in a spherical water phantom with diameter 160 mm. The beams were directed to the center of the phantom. Collimators of 4, 8, 14, and 18 mm sizes were studied. The single-beam dose profiles provided by Elekta (Manufacturer of Leksell Gamma Knife) were excellently consistent with the results of Monte Carlo for the 4, 14, and 18 mm collimators. The maximum discrepancy was less than 3% at all radial distances. For the 8 mm collimator, the maximum discrepancy was 8% in the relative dose in the radial distance range from 4.3 mm to 5.2 mm. Excellent agreement in dose profiles along x, y, and z axes for all collimator helmets by summing over all 201 sources was observed between the cases using the default single-beam dose profiles and the calculated Monte Carlo results, except for the 8 mm collimator helmet along z axis. Such difference may however be too small to give a clinical significance.

Algorithms↗
PubMed1998

'Tweaked' GammaPlan for target volume measurement in non-fiducial based images: a simple routine for follow-up assessment.

The GammaPlan provides accurate estimation of target volume. However, a stereotactic frame and fiducials are required. At follow-up MR/CT, fiducials are no longer available. Surgeons rely on visual impression, 2-D measurements, or other methods to estimate the volume of the treated targets. These methods are not objective and may give rise to misleading conclusions. By modifying the image header files of GammaPlan version 3, it is possible to scale the images which are not acquired with stereotactic fiducials. The target is then mapped and its volume measured as usual. The target volumes in 7 patients were measured by the tweaked GammaPlan and compared with volume measurements using the standard version. The mean error was less than 2%. In a separate study, phantom syringes with known volumes of water were used for MR imaging. The tweaked version again gave accurate volume estimation of the phantom syringes with a less than 5% error for most cases. This method has subsequently been used in all Gamma Knife follow-up cases. Significant volume changes have been detected where conventional assessment showed no apparent change. Moreover, such volume changes correlated with clinical improvement or deterioration. It is recommended that all Gamma Knife users report tumor response by volume change. Our method is simple, reliable and does not require additional cost.

Follow-Up Studies↗
PubMed1996

Detection of hepatitis B virus DNA and RNA in kidneys of HBV related glomerulonephritis.

Glomerular deposition of hepatitis B virus (HBV) antigens are observed in chronic HBsAg carriers with different glomerulonephritides yet the etiologic role of HBV remains uncertain. We examined the paraffin section of kidney biopsies from 40 chronic HBsAg carriers with membranous nephropathy (MGN), mesangiocapillary glomerulonephritis (MCGN) or IgA nephropathy (IgAN) for HBV DNA and HBV RNA using in situ hybridization (ISH). Glomerular HBV antigens were present in all biopsies by immunofluorescence. HBsAg or HBcAg mRNA was also studied in RNA extracted from frozen renal tissue using a two-step polymerase chain reaction (PCR) following reverse transcription (RT). HBcAg DNA was not easily detected with ISH alone, but was readily found in 31 biopsies (78%) following PCR. HBV DNA was detected mainly in the cytoplasm of proximal tubular epithelia but not in glomerular cells. HBsAg and/or HBcAg mRNA were detected by RT-PCR in extracted RNA from 13 biopsies (33%). The PCR findings were further confirmed by (a) Southern blot hybridization using a cloned HBV probe and (b) absence of PCR product following treating RNA with RNase or omitting the RT. It is plausible that HBV DNA in renal tubules represents endocytosis of HBV DNA in the urinary filtrate and the HBV RNA extracted from kidney biopsies could derive from infiltrating cells bearing HBV RNA. Hence, ISH with specific HBV core gene RNA probe was performed subsequently. HBcAg RNA, localized in the nuclei and cytoplasm of glomerular and tubular cells, was detected in 56%, 20%, and 36% of renal biopsies in chronic HBsAg carriers with MGN, MCGN, and IgAN, respectively. Our findings indicate the presence of viral transcription in glomerular cells and renal tubular epithelia, supporting an etiological role of HBV in some chronic HBsAg carriers who develop coexisting glomerulonephritides.

Adolescent↗
PubMed1996

Transforming growth factor-beta mRNA expression in CD4+ T cells from patients with primary glomerulonephritides.

Immunoreactivity to transforming growth factor-beta (TGF-beta) was detected in glomerular cells from patients with chronic glomerulonephritis, yet it remains uncertain whether the glomerular TGF-beta deposits reflect TGF-beta produced by the glomerular cells alone or from additional extrarenal origin. We examined the TGF-beta messanger RNA (mRNA) expression in circulating CD4+ T cells from 45 patients with different primary glomerulonephritides. By semiquantitative polymerase chain reaction, the TGF-beta mRNA expression was significantly raised in histopathologic entities characterized by increased mesangial proliferation or segmental sclerosis but not in histopathologic entities with little mesangial proliferation or sclerosis. These findings suggest that, other than TGF-beta produced de novo, TGF-beta from systemic source can also contribute to mesangial proliferation by binding to the glomerular cells after being filtered by the glomeruli. Increased TGF-beta gene expression in CD4+ T cells in these patients may potentially enhance glomerular injury.

CD4-Positive T-Lymphocytes↗
PubMed1994

CD4-positive cells from patients with IgA nephropathy demonstrate increased mRNA of cytokines that induce the IgA switch and differentiation.

IgA nephropathy (IgAN) is characterized by raised serum IgA1 and mesangial IgA1 deposits. We have previously shown increased T-cell activation in IgAN. Recently, transforming growth factor-beta (TGF-beta) has been shown to induce IgA isotype switch at a clonal level and interleukin 5 (IL5) promotes differentiation into IgA-bearing B cells. In the present study we have examined the TGF-beta and IL5 mRNA expression by mitogen-activated CD4-positive T cells from patients with IgAN (n = 25), patients with other primary nephritides (CGN) (n = 24), and healthy control subjects (n = 25). The cytokine genes were analysed by reverse transcription (RT)-polymerase chain reaction (PCR) and were semi-quantitated by normalizing the differences occurring during RT and PCR using a housekeeping gene, beta-actin. CD4-positive T cells from IgA nephritic patients expressed a higher level of IL5 mRNA than healthy controls (P < 0.01) and patients with CGN (P < 0.005). CD4-positive T cells from IgA nephritic patients expressed a higher level of TGF-beta mRNA than healthy controls (P < 0.01) but no difference was demonstrated on comparison with CGN patients. Elevated TGF-beta mRNA expression in patients with CGN probably reflects its other important function as a 'sclerogenic' factor involved in the glomerulosclerosis found in these nephritides. Our data suggest that there is increased expression of cytokine genes which induce the IgA isotype switch and differentiation; these immunological abnormalities may be important in the pathogenesis of IgAN.

Adult↗
PubMed1994

Increased mRNA encoding for transforming factor-beta in CD4+ cells from patients with IgA nephropathy.

IgA nephropathy (IgAN) is a mesangial proliferative glomerulonephritis characterized by predominant mesangial IgA deposits. Recently, transforming growth factor-beta (TGF-beta) is shown to exert widespread effects on extracellular matrix by enhancing its accumulation. In an experimental model of acute mesangial glomerulonephritis TGF-beta appeared to be involved in the process of glomerulosclerosis, and treatment with antagonists of TGF-beta prevented the development of glomerulosclerosis. We examined the TGF-beta mRNA expression by mitogen activated CD4+ T cells from 31 patients with IgAN, 25 healthy controls and 10 patients with minimal change nephropathy (MCN) or focal glomerulonephritis (FGN) who were comparable in age and sex. The cytokine gene was analyzed with reverse transcription followed by polymerase chain reaction and was semiquantitated by normalizing the differences occurring during reverse transcription and polymerase chain reaction using a housekeeping gene, beta-actin. CD4+ T cells from IgA nephritic patients expressed a higher level of TGF-beta mRNA than that of healthy controls or that of MCN/FGN [TGF-beta/actin ratio 1.11 (median), range 0.24 to 3.87 vs. 0.88, range 0.2 to 3.83, P = 0.0157 and 0.36 range 0.09 to 1.6, P = 0.006]. When the biopsies were classified into three grades according to the severity of glomerular and interstitial pathology, there were highly significant differences between the TGF-beta mRNA in CD4+ T cells from the three groups of IgA nephritic patients (grade 1, 0.52, range 0.24 to 0.79; grade 2, 1.2, range 0.5 to 3.33; grade 3, 2.17, range 1.45 to 3.87].(ABSTRACT TRUNCATED AT 250 WORDS)

Adult↗
PubMed1994

Increase of both circulating Th1 and Th2 T lymphocyte subsets in IgA nephropathy.

IgA nephropathy (IgAN), characterized by glomerular deposition of IgA and frequently elevated plasma IgA levels, has increased T helper cell activity. In vitro measurement of cytokines in supernatant of cultured peripheral lymphocytes revealed conflicting findings. We examined the profile of cytokine mRNA expressed in purified CD4+ cells in patients with IgAN in order to study their pattern of Th1 (releases IL-2 and interferon-gamma (IFN-gamma)) and Th2 (releases IL-4 and IL-5) T cell response. We assessed the circulating CD4+ T cells in patients and normal controls by the expression of messenger RNA (mRNA) for IL-2, IL-4, IL-5 and IFN-gamma. The cytokine mRNAs were analysed with reverse transcription-polymerase chain reaction and were measured semiquantitatively by using a housekeeping gene, beta-actin. Compared with the control subjects, CD4+ T lymphocytes from patients with IgAN expressed a higher level of IL-2 mRNA (P = 0.007), IFN-gamma mRNA (P = 0.04), IL-4 mRNA (P = 0.048), and IL-5 mRNA (P = 0.016). Within these patients with IgAN, a good correlation was demonstrated between the gene expression of cytokines in Th1 or Th2 cells. The IL-2 mRNA levels in Th1 cells from these patients with IgAN also correlated significantly with the IL-4 or IL-5 mRNA levels in their Th2 cells. Our study revealed IgAN is associated with activation in circulating lymphocytes of the IL-2, IFN-gamma, IL-4 and IL-5 gene cluster, a pattern compatible with activation of both the Th1- and Th2-like T lymphocyte population. The increased transcription of these cytokines genes may be contributory to the immunopathologic findings in IgAN.

Base Sequence↗
PubMed1994

Outcome of surgery and anesthesia in patients 80 years of age and older.

We studied retrospectively the outcome of surgery and anesthesia in 294 patients aged 80 and older who underwent 362 surgical procedures. Of these, 52 cases (14.4%) were admitted to intensive care unit (ICU) post-operatively. The 48-hour, 48-hour-to-30-day and overall mortality rates were 0.8%, 3.8% and 5.2% respectively. ICU. admission and mortality rate were closely associated with the ASA physical status classification. The overall mortality rates were 1.3%, 5.7% and 27% in class II, III and IV respectively. The respective ICU admission rates were 3%, 20% and 56%. ICU admission rate and mortality rate were 42.5% and 13.8% in emergency surgery vs. 5.8% and 2.5% in elective surgery. Patients with 3 or more co-existing diseases had the mortality rate of 23% and the rates of ICU admission and mortality for those with 3 or more peri-operative complications were 22% and 16% respectively vs. 11% and 4% for those with 2 or less peri-operative complications. General surgery was associated with the highest mortality rate (15%) than other type of procedures. 22 of 74 cases who underwent intra-abdominal surgery required controlled ventilation for more than 24 hours post-operatively. 59% (13 cases) of such cases could not weaned successfully from ventilators and died subsequently. Only 2.5% (7 cases) of cases who underwent extra-pleural and extra-peritoneal procedures required controlled ventilation for more than 24 hours post-operatively. Of these, 2 died subsequently. For comparison, the study were artificially divided into two phases, each of three years long: they were the period from 1986-1988 and the period from 1989-1992.(ABSTRACT TRUNCATED AT 250 WORDS)

Aged↗