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Biomedical subjects

W S Ng

Publications and source records attributed to W S Ng.

At least 19 recordsLinked to original sources
PubMed2001

Target site modifications and efflux phenotype in clinical isolates of Streptococcus pneumoniae from Hong Kong with reduced susceptibility to fluoroquinolones.

Ciprofloxacin-susceptible (n = 7) and -resistant (MIC >or=4 mg/L) (n = 15) clinical isolates of Streptococcus pneumoniae from diverse sources in Hong Kong were studied for target site modifications and efflux phenotype. Reserpine-inhibited efflux of ciprofloxacin and/or levofloxacin was common in both susceptible and non-susceptible isolates. The ParC substitutions K137N and/or S79F or Y were associated with increased ciprofloxacin MICS. The GyrA substitution S81F was only found in isolates with full resistance to ciprofloxacin (MIC >or=16 mg/L) and levofloxacin (MIC >or=8 mg/L). Among clinical isolates of S. pneumoniae, accumulation of target site mutations in strains with an efflux mechanism was associated with increasing MICs of fluoroquinolones.

Amino Acid Substitution↗
PubMed2001

Computer-assisted transurethral laser resection of the prostate (CALRP): theoretical and experimental motion plan.

Computer-Assisted transurethral laser resection of the prostate (CALRP) is a treatment modality that was designed and developed based on an integrated system of computer, robotics and laser technology in association with a minimally invasive surgery known as laser transurethral resection of the prostate (LRP). CALRP possesses complementary capabilities that could remedy many of the problems faced by surgeons in conventional LRP by delivering a treatment with repeatability and reliability. The work deals primarily in determining the feasibility study of the computer-assisted lasing motion plan (planned motion sequence controlled by a program) for LRP. A theoretical motion plan that analyzes numerically the lasing motion of the fiber was designed by calculating the profile removal rate and in vitro experiments conducted on human cadaveric prostate to verify and validate the designed motion plan. The novel motion plan, which was executed experimentally using the LaserTrode lightguide, accomplished the objective of resecting the enlarged prostate with the aid of computer and robotics technology.

Cadaver↗
PubMed2001

Experimental study of transurethral robotic laser resection of the prostate using the LaserTrode lightguide.

A longer operating time and steeper learning curve in mastering the techniques for transurethral laser resection of the prostate are the main problems faced by surgeons in addition to the existing ones in standard transurethral resection of the prostate (TURP). However, these disadvantages can be alleviated with the introduction of a treatment procedure designed and developed based on an integrated system of computer, robotics and laser technology. In vitro experiments were carried out to determine variables affecting the vaporization and coagulation lesions, in order to study the effectiveness and feasibility of robotics for this procedure. Human cadaveric prostates and fresh tauted chicken breast tissues were irradiated with different parameters using the LaserTrode lightguide in contact with the tissue. The effects of irrigant flow rate, fiber/tissue angle of inclination, number of passes, direction, speed and power of lase on the volume of tissue vaporized and coagulated, were assessed. The final phase of the experiments includes executing the robotic motion plan for the laser resection procedure on the human cadaveric prostate tissue embedded in an anatomically alike prostate phantom. It was concluded from our study that power and speed of lase are the most significant parameters influencing the volume of the vaporized and coagulated lesion. Comparison of removal rate using the new treatment procedure of robotic laser resection of the prostate with TURP and HoLRP evinced equivalent results.

Animals↗
PubMed2001

Magnetic tracker calibration for an augmented reality system for therapy.

Magnetic trackers are widely used position/orientation measurement devices in interactive computer systems. These trackers output six-degree-of-freedom position and orientation of their receiver coordinates with respect to their transmitter coordinates. It is a potential solution for real-time position/orientation measurement in an augmented reality system for therapy (ART). However, an obstruction to be overcome is that a magnetic tracker is vulnerable to measurement inaccuracy caused by magnetic field distortions (including ferric and non-ferric distortions) when it is used in metallic environments. The purpose of this paper is to discuss calibration methods used to compensate distortions produced by metallic objects which are moving during the measurement procedure, but fixed with a receiver. Past work on the calibration of magnetic trackers has concentrated on static distortions, which are produced by stationary metallic objects in the environment. Compared with static distortions, in this paper a relatively dynamic situation is considered where the distortion is not constant with time. However, the location of the metallic objects is reflected in the outputs of the receiver so that the distortion can be compensated. This type of distortions, which is called 'quasi-static distortion' in this paper, is common in an ART. Compensation of quasi-static distortion is more complicated than that of static distortion and has not been mentioned in past publications. Some new compensation methods for quasi-static distortion will be introduced here. The results demonstrate that the methods are capable of significantly reducing quasi-static distortions.

Calibration↗
PubMed2000

Automatic prostate boundary recognition in sonographic images using feature model and genetic algorithm.

This paper describes the development of a model-based boundary recognition system for transrectal prostate ultrasonographic images. It consists of two techniques: boundary modeling and boundary searching with model constraints. To achieve higher specificity of the model, a method called feature modeling is derived from the existing point distribution modeling method. To improve the robustness of the searching technique, the genetic algorithm is used. Incremental genetic algorithm with crowding replacement and binary string chromosome type was found experimentally to give good search results. It was shown that the system could recognize the boundary with considerable accuracy and consistency within a few minutes in transrectal ultrasonographic images taken from approximate middle position of the prostate.

Algorithms↗
PubMed1999

An evaluation of susceptibility testing methods for ampicillin-sulbactam using a panel of beta-lactamase-producing bacteria.

Bacteria possessing TEM-1-like beta-lactamases are generally regarded as susceptible to ampicillin-sulbactam (SAM), while those harboring OXA-1 enzymes are considered resistant. The current study was undertaken to compare susceptibility testing using various combinations of ampicillin and sulbactam to improve clinical correlation. Members of the Enterobacteriaceae family harboring TEM-1, SHV-1 or OXA-1-like beta-lactamases were tested using the agar dilution method. A substantial proportion of strains harboring OXA-1-like beta-lactamases showed false susceptibility to SAM at the 1:1 ratio or fixed sulbactam concentration of 8 microg/ml. At a fixed sulbactam concentration of 4 microg/ml, the activity of ampicillin-sulbactam appeared to be reduced, with large numbers of TEM-1 producers becoming frankly resistant. Results obtained with the 2:1 ratio exhibited the closest correlation with that obtained by the currently recommended disk diffusion test. However, very major errors were still found between the disk diffusion test and agar dilution test, suggesting the necessity for consideration of a change in criteria for interpretation of disk diffusion test results. In conclusion, SAM susceptibility testing by agar dilution using other than a 2:1 ratio is not recommended and results should be interpreted with caution.

Agar↗
PubMed1998

Outlining the prostate boundary using the harmonics method.

In a computerised ultrasound image guidance for automated prostatectomy system, it is necessary to identify a smooth, continuous contour for the prostate (boundary) from the ultrasound image. The radial bas-relief (RBR) method, which has been reported previously, can extract a skeletonised image from an ultrasound image automatically. After this process the prostate boundary is clearly revealed. However, analysis of the image is far from complete, as there are many spurious branches that create too much ambiguity for the system to define the actual boundary. There are also sections missing from the prostate boundary. Therefore further post-processing is required to describe and define the prostate boundary. In the paper, the harmonics method is used to describe the prostate boundary. The harmonics method uses Fourier information for noise removal and encodes a smooth boundary. The results of using the harmonics method after application of the RBR method on ultrasound images are presented. Factors that affect the performance are also highlighted and discussed.

Fourier Analysis↗
PubMed1998

Pre-oxygenation using face mask or mouthpiece with and without nose clip: patient preferences and efficacy.

We assessed the effectiveness of mouthpieces for oxygenation before induction of anaesthesia. In part 1 of the study we asked 52 day case patients whether they would prefer mask, mouthpiece, or mouthpiece with nose clip, and which they would be willing to use. In part 2 we pre-oxygenated 18 volunteers using each of the three methods in a crossover study. We measured time to 90% end tidal oxygen concentration (FE'O2), or maximum FE'O2 after 3 min if less than 90%. Of the patients surveyed, 20 (39%) preferred mask, 23 (44%) mouthpiece, one mouthpiece with nose clip and nine (17%) expressed no preference. Only 88% would be willing to use the face mask, but all would accept either the face mask or the mouthpiece with nose clip. In part 2 of the study, the mouthpiece alone proved significantly worse than the mask or mouthpiece with nose clip. The latter two did not differ significantly. Offering the alternative of mouthpiece with nose clip would increase patient acceptance of effective pre-oxygenation.

Adolescent↗
PubMed1998

Comparison of a novel, inhibitor-potentiated disc-diffusion test with other methods for the detection of extended-spectrum beta-lactamases in Escherichia coli and Klebsiella pneumoniae.

A novel, inhibitor-potentiated disc-diffusion test for detecting extended-spectrum beta-lactamases (ESBLs) in bacteria was evaluated. This test uses the principle of augmentation (by > or = 10 mm) of inhibition zones produced by ceftazidime, cefotaxime, ceftriaxone or aztreonam discs on Mueller-Hinton agar supplemented with clavulanate (4 mg/L). The test was initially compared with the double-disc synergy test, Kirby-Bauer disc-diffusion test and Etest ESBL screen with a panel of 45 reference strains with known resistance profiles. This panel consisted of 27 ESBL-positive Escherichia coli strains expressing 14 Bush group 2be enzymes and 18 other E. coli and Klebsiella pneumoniae strains (14 non-ESBL beta-lactamase producers and four non-beta-lactamase producers). The Kirby-Bauer disc-diffusion test was the least sensitive method: 11-44% of the ESBL-positive control strains were misclassified as susceptible to ceftazidime, cefotaxime, ceftriaxone or aztreonam when interpreted by National Committee for Clinical Laboratory Standards (NCCLS) criteria. The sensitivities of the inhibitor-potentiated disc-diffusion test, the double-disc synergy test (when discs were 25 or 30 mm apart) and the Etest ESBL screen (with a breakpoint of > 4-fold reduction in ceftazidime MIC in the presence of clavulanate) were 100%, 96% and 89-96%, respectively. The inhibitor-potentiated disc-diffusion test was further evaluated with 81 E. coli and K. pneumoniae clinical isolates, which were identified as putative ESBL-producers by the double-disc synergy test. For these isolates, the sensitivity of both the inhibitor-potentiated disc-diffusion test and the Etest ESBL screen was 100%. In conclusion, the inhibitor-potentiated disc-diffusion test is a sensitive, convenient and inexpensive method of screening for ESBLs in E. coli and K. pneumoniae isolates, with potential for incorporation into routine clinical laboratory service.

Anti-Bacterial Agents↗
PubMed1998

Correlation of in vitro susceptibility testing results for amoxicillin-clavulanate and ampicillin-sulbactam using a panel of beta-lactamase-producing Enterobacteriaceae.

Correlation between in vitro susceptibility results for amoxicillin-clavulanate (AMC) and ampicillin-sulbactam (SAM) was studied using 136 clinical and control strains of Enterobacteriaceae harboring TEM-1, SHV-1 or OXA-1-like beta-lactamases. Determination of minimal inhibitory concentration of antibiotics was performed by agar dilution. The beta-lactamases were initially characterized using isoelectric focusing. Further identification was done by DNA hybridization with or without prior PCR amplification. All strains sensitive to SAM were found to be sensitive also to AMC. In contrast, among those susceptible to AMC, only 50% were sensitive to SAM while 36% gave intermediate results and 14% were resistant. Major differences were found solely among SHV-producers while minor differences occurred mostly among TEM-producers. This phenomenon is probably related to the differential activities of clavulanate and sulbactam against various beta-lactamases. In conclusion, testing of Enterobacteriaceae isolates for susceptibility to AMC and SAM should be performed and reported individually to avoid erroneous designation of susceptibility.

Amoxicillin-Potassium Clavulanate Combination↗
PubMed1997

Computerised prostate boundary estimation of ultrasound images using radial bas-relief method.

A new method is presented for automatic prostate boundary detection in ultrasound images taken transurethrally or transrectally. This is one of the stages in the implementation of a robotic procedure for prostate surgery performed by a robot known as the robot for urology (UROBOT). Unlike most edge detection methods, which detect object edges by means of either a spatial filter (such as Sobel, Laplacian or something of that nature) or a texture descriptor (local signal-to-noise ratio, joint probability density function etc.), this new approach employs a technique called radial bas-relief (RBR) to outline the prostate boundary area automatically. The results show that the RBR method works well in the detection of the prostate boundary in ultrasound images. It can also be useful for boundary detection problems in medical images where the object boundary is hard to detect using traditional edge detection algorithms, such as ultrasound of the uterus and kidney.

Humans↗
PubMed1997

Blood loss during first trimester termination of pregnancy: comparison of two anaesthetic techniques.

We have compared the effects of two anaesthetic techniques on blood loss during suction termination of pregnancy. Forty-eight ASA grade I-II patients were allocated randomly to one of two groups: group 1 received propofol induction followed by a standard propofol infusion; group 2 received propofol induction followed by maintenance with 1% isoflurane. Both groups received bolus doses of either propofol (group 1) or isoflurane (group 2) if anaesthesia was too light. All patients were given fentanyl 1 micrograms kg-1 on induction. The products of conception were evacuated into a closed suction bottle. The products were diluted with 500 ml of heparinized saline, filtered and left for 20 min. Estimation of blood loss was performed by atomic absorption spectrometry. Mean blood losses were 40.4 ml for the isoflurane group and 18.8 ml for the propofol group. This difference was statistically significant (P = 0.0011), although actual volumes of blood loss were small and not clinically significant.

Abortion, Induced↗
PubMed1993

The use of ultrasound in automated prostatectomy.

A new way of applying transurethral ultrasound scanning in the common surgical procedure of the transurethral resection of the prostate is described. The scanning is incorporated as part of a robotic procedure for surgery, so that the overall time spent in an operation can be further shortened, and a safe and accurate operation can be achieved. The prostate dimensions obtained pre-operatively by the transrectal method and those obtained operatively are compared. A robotic system, which was developed specifically to remove prostatic adenoma automatically, is discussed. The system, called a motorized frame, is briefly described, together with its predecessor, a manual frame, in relation to ultrasound measurements. Sizing of the prostate pre-operatively using transrectal ultrasound methods is discussed, using both the manual and the motorized frame. The shortcomings of transrectal ultrasound for use in a robotic procedure are highlighted.

Humans↗
PubMed1992

Measurement of surgical diathermy current during trans-urethral resection of the prostate gland.

A system is described that has been developed to measure the surgical diathermy current passing through the patient during each cut of a trans-urethral resection of the prostate gland (TURP), with 1.5% glycine solution as irrigant. The immediate aim was to distinguish capsular tissue of the male prostate gland from the hyperplastic adenomatous tissue within it. The ultimate aim was to prevent resection into the surgical capsule which can easily result in excessive blood loss and absorption of irrigant. The system was used in a conventional theatre setting with a commercial diathermy system (Eschmann TD 411-S) of known output characteristics. Measurements were made in 12 patients during resection with a standard loop, involving typically 75 cuts in each patient (60 in adenoma and 15 in capsule). In 8 of these patients this was followed by roller ball placement on typically 10 sites each of capsule or residual adenoma at very low diathermy settings. The results showed great variability both between patients, and between different cuts in the same patient, with no consistent difference between tissues identified under direct vision as adenoma and surgical capsule. Physical reasons are suggested for the observed variability, and it is concluded that it is not possible to use this technique to guide tissue resection in the prostate.

Electrocoagulation↗
PubMed1991

The development of a surgeon robot for prostatectomies.

The removal of prostatic tissue through transurethral resection of the prostate (TURP) is an operation that can require considerable skill from a surgeon as well as being a lengthy procedure. The potential for using robotic techniques was investigated in a preliminary feasibility study using a standard six axis 'Puma' robot. This led to the construction of a manually operated 'safety frame' which has been shown to be effective through clinical trials on 30 patients. A special-purpose robot, based on the design of the manual frame, has now been constructed. Some of the safety issues are discussed which make this procedure an ideal candidate for a robotic device.

Equipment Design↗
PubMed1991

Plea for selective operative cholangiography.

A prospective study was carried out to evaluate the use of routine operative cholangiography in 382 patients undergoing elective cholecystectomy. Operative cholangiography was performed in 356 patients. In each patient, the presence or absence of clinical and operative indications for common duct exploration was correlated with the result of the operative cholangiography. In 114 patients (32%) there were one or more indications for exploration and only 34 patients showed positive cholangiographic findings and were explored. Ductal stones were found in 25 patients and nine patients had negative explorations. In the remaining 242 patients who had no indication for common duct exploration, normal cholangiogram was obtained in 234 patients and eight had false positive cholangiograms and were explored unnecessarily. No unsuspected common duct stones were detected in the whole series. Based on these results, a plea is made for selective operative cholangiography.

Cholangiography↗
PubMed1990

A randomized prospective trial of three different regimens of treatment of peritonitis in patients on continuous ambulatory peritoneal dialysis.

A randomized prospective study was undertaken in patients on continuous ambulatory peritoneal dialysis (CAPD) to evaluate the efficacy of three different antibiotic regimens for the treatment of peritonitis. There were 39 episodes in each treatment group. Patients were treated with intraperitoneal (IP) cephalothin (250 mg/L) and tobramycin (8 mg/L) in group 1, oral ofloxacin (400 mg loading followed by 300 mg daily) in group 2, and a combination of ofloxacin (400 mg followed by 300 mg daily) and rifampicin (300 mg daily). Treatment duration was 10 days. The average culture-positive rate was 75%. The overall cure rate was 80.6% with IP antibiotics, 78.4% with oral ofloxacin, and 81.1% with ofloxacin and rifampicin. After the exclusion of tunnel infections and episodes of peritonitis due to Pseudomonas and resistant organisms, the corresponding figures were 100%, 90.6%, and 93.7%, respectively. Side effects were minimal with IP treatment and with oral ofloxacin, but severe nausea and vomiting occurred in some cases with the combination of ofloxacin and rifampicin. It was concluded that oral ofloxacin is an acceptable first-line therapy for peritonitis in CAPD patients.

Administration, Oral↗