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Biomedical subjects

K L Lam

Publications and source records attributed to K L Lam.

At least 19 recordsLinked to original sources
PubMed1997

Computerized classification of malignant and benign microcalcifications on mammograms: texture analysis using an artificial neural network.

We investigated the feasibility of using texture features extracted from mammograms to predict whether the presence of microcalcifications is associated with malignant or benign pathology. Eighty-six mammograms from 54 cases (26 benign and 28 malignant) were used as case samples. All lesions had been recommended for surgical biopsy by specialists in breast imaging. A region of interest (ROI) containing the microcalcifications was first corrected for the low-frequency background density variation. Spatial grey level dependence (SGLD) matrices at ten different pixel distances in both the axial and diagonal directions were constructed from the background-corrected ROI. Thirteen texture measures were extracted from each SGLD matrix. Using a stepwise feature selection technique, which maximized the separation of the two class distributions, subsets of texture features were selected from the multi-dimensional feature space. A backpropagation artificial neural network (ANN) classifier was trained and tested with a leave-one-case-out method to recognize the malignant or benign microcalcification clusters. The performance of the ANN was analysed with receiver operating characteristic (ROC) methodology. It was found that a subset of six texture features provided the highest classification accuracy among the feature sets studied. The ANN classifier achieved an area under the ROC curve of 0.88. By setting an appropriate decision threshold, 11 of the 28 benign cases were correctly identified (39% specificity) without missing any malignant cases (100% sensitivity) for patients who had undergone biopsy. This preliminary result indicates that computerized texture analysis can extract mammographic information that is not apparent by visual inspection. The computer-extracted texture information may be used to assist in mammographic interpretation, with the potential to reduce biopsies of benign cases and improve the positive predictive value of mammography.

Breast Diseases↗
PubMed1996

Uncertainties in CT-based radiation therapy treatment planning associated with patient breathing.

PURPOSE: To evaluate uncertainties associated with treatment-planning computed tomography (CT) data obtained with the patient breathing freely. METHODS AND MATERIALS: Patients with thoracic or abdominal tumors underwent a standard treatment-planning CT study while breathing quietly and freely, followed by CT scans while holding their breath at normal inhalation and normal exhalation. Identical treatment plans on all three CT data sets for each patient pointed out differences in: (a) radiation path lengths; (b) positions of the organs; (c) physical volumes of the lung, liver, and kidneys; (d) the interpretation of plan evaluation tools such as dose-volume histograms and normal tissue complication probability (NTCP) models; and (e) how well the planning CT data set represented the average of the inhalation and exhalation studies. RESULTS: Inhalation and exhalation data differ in terms of radiation path length (nearly one quarter of the cases had path-length differences > 1 cm), although the free breathing and average path lengths do not exhibit large differences (0-9 mm). Liver and kidney movements averaged 2 cm, whereas differences between the free breathing and average positions averaged 0.6 cm. The physical volume of the liver between the free breathing and static studies varied by as much as 12%. The NTCP calculations on exhale and inhale studies varied from 3 to 43% for doses that resulted in a 15% NTCP on the free-breathing studies. CONCLUSION: Free-breathing CT studies may improperly estimate the position and volume of critical structures, and thus may mislead evaluation of plans based on such volume dependent criteria such as dose-volume histograms and NTCP calculations.

Abdominal Neoplasms↗
PubMed1996

Measurement of patient setup errors using port films and a computer-aided graphical alignment tool.

Patient orientations were measured for 49 patients treated in the abdomen, chest, and pelvic regions over the course of 20 months. Setup errors were determined using a curve-matching graphical interface to compare digitized port films to digitized simulation films. Data representing both "initial patient setup" and "patient setup at treatment" are presented and compared. Data were sorted by anatomic area and analyzed both at the population level and on a patient-by-patient basis. For each population, setup errors were observed to be primarily random, with population standard deviations of 5-6 mm for each of three translations and 2-3 degrees for each of two rotations. Rotations about the patients' inferior-superior axes were not measured. For each site, correlations between translations and/or rotations were small. The results are consistent with those from previous studies. The data set is among the largest collected to date.

Abdominal Neoplasms↗
PubMed1996

In phantom determination of collimator scatter factor.

The collimator scatter factor Sc has generally been measured in air using an ionization chamber inside a buildup cap or mini-phantom. Here, Sc was measured in phantom at 10 cm depth for 6 and 15 MV photons with square collimator settings of 2.5-40.0 cm. The results were consistent with in air measurements with a mini-phantom to within 0.4%. In the measurements, a series of Cerrobend field shaping blocks were used to define the field size in the phantom while the collimator settings were varied from the field size in the phantom to twice that value. Corrections of up to 2% for scattered radiation from the added Cerrobend field shaping blocks were necessary. Since a buildup cap or mini-phantom is not used, the smallest field size that can be measured is limited only by the size of the detector and the measurement is performed with full scatter resembling the treatment condition of a patient.

Biometry↗
PubMed1996

Flattening-filter-based empirical methods to parametrize the head scatter factor.

Parametrizing the collimator scatter factor, Sc (or head scatter factor), of a linear accelerator by the side of the equivalent square of the collimator setting at the isocenter does not accurately predict the change in Sc when the width and length of a rectangular field are exchanged. We have studied two methods based on measurements of square fields to predict Sc's of rectangular fields more accurately. The first method parametrizes Sc by the side of the equivalent square of the flattening filter region visible from the point of calculation. The Sc's of rectangular fields were predicted to an accuracy of 1% from measurements with square fields. The second method computes Sc of rectangular configurations by integrating radiation that can reach the point of calculation from a point source at the target and a second extended source at the flattering filter. The radial distribution of the extended source at the level of the flattering filter is computed from Sc of square fields measured at the isocenter. Effects of extended distance are modeled by separately performing inverse square law corrections for the two sources. This method also predicted the measured values to within 1% accuracy.

Biometry↗
PubMed1996

Image compression in digital mammography: effects on computerized detection of subtle microcalcifications.

Our previous receiver operating characteristic (ROC) study indicated that the detection accuracy of microcalcifications by radiologists is significantly reduced if mammograms are digitized at 0.1 mm x 0.1 mm. Our recent study also showed that detection accuracy by computer decreases as the pixel size increases from 0.035 mm x 0.035 mm. It is evident that very large matrix sizes have to be used for digitizing mammograms in order to preserve the information in the image. Efficient compression techniques will be needed to facilitate communication and archiving of digital mammograms. In this study, we evaluated two compression techniques: full frame discrete cosine transform (DCT) with entropy coding and Laplacian pyramid hierarchical coding (LPHC). The dependence of their efficiency on the compression parameters was investigated. The techniques were compared in terms of the trade-off between the bit rate and the detection accuracy of subtle microcalcifications by an automated detection algorithm. The mean-square errors in the reconstructed images were determined and the visual quality of the error images was examined. It was found that with the LPHC method, the highest compression ratio achieved without a significant degradation in the detectability was 3.6:1. The full frame DCT method with entropy coding provided a higher compression efficiency of 9.6:1 at comparable detection accuracy. The mean-square errors did not correlate with the detection accuracy of the microcalcifications. This study demonstrated the importance of determining the quality of the decompressed images by the specific requirements of the task for which the decompressed images are to be used. Further investigation is needed for selection of optimal compression technique for digital mammograms.

Breast Diseases↗
PubMed1995

Automated localization of the prostate at the time of treatment using implanted radiopaque markers: technical feasibility.

PURPOSE: Prostate movement is a major consideration in the formation of target volumes for conformal radiation therapy of prostate cancer. The goal of this study was to determine the technical feasibility of using implanted radiopaque markers and digital imaging to localize the prostate at the time of treatment, thus allowing for reduction of the margin required for uncertainty in target position. METHODS AND MATERIALS: Radiopaque markers implanted around the prostate prior to treatment are visible on electronic radiographs generated with a portal imager or diagnostic imaging device. The locations of the images of these markers on the digital radiographs were automatically determined by a template-matching algorithm. The coordinates of the markers were found by projecting rays through the marker locations on orthogonal radiographs using a three-dimensional (3D) point-matching algorithm. Prostate and/or patient movement was inferred from the marker displacements. Images generated from known movements of a phantom with implanted markers were tested with this algorithm. Locations of markers from daily images of patients with implanted markers were determined by both manual and automatic techniques to determine the efficacy of automated localization on typical clinical images. RESULTS: Prostate movements can be automatically detected in a phantom using low-energy photons within 30 s after image acquisition and with a precision of better than 1 mm in translation and 1 degree in rotation (indistinguishable from the uncertainty in measuring precision). CONCLUSION: The studies show that on-line repositioning of the patient based on localization of the markers at the time of treatment is feasible, and may reduce the uncertainty in prostate location when combined with practical on-line repositioning techniques.

Feasibility Studies↗
PubMed1995

Computer-aided detection of mammographic microcalcifications: pattern recognition with an artificial neural network.

We are developing a computer program for automated detection of clustered microcalcifications on mammograms. In this study, we investigated the effectiveness of a signal classifier based on a convolution neural network (CNN) approach for improvement of the accuracy of the detection program. Fifty-two mammograms with clustered microcalcifications were selected from patient files. The clusters on the mammograms were ranked by experienced mammographers and divided into an obvious group, an average group, and a subtle group. The average and subtle groups were combined and randomly divided into two sets, each of which was used as training or test set alternately. The obvious group served as an additional independent test set. Regions of interest (ROIs) containing potential individual microcalcifications were first located on each mammogram by the automated detection program. The ROIs from one set of the mammograms were used to train CNNs of different configurations with a back-propagation method. The generalization capability of the trained CNNs was then examined by their accuracy of classifying the ROIs from the other set and from the obvious group. The classification accuracy of the CNNs for the ROIs was evaluated by receiver operating characteristic (ROC) analysis. It was found that CNNs of many different configurations can reach approximately the same performance level, with the area under the ROC curve (Az) of 0.9. We incorporated a trained CNN into the detection program and evaluated the improvement of the detection accuracy by the CNN using free response ROC analysis. Our results indicated that, over a wide range of true-positive (TP) cluster detection rate, the CNN classifier could reduce the number of false-positive (FP) clusters per image by more than 70%. For the obvious cases, at a TP rate of 100%, the FP rate reduced from 0.35 cluster per image to 0.1 cluster per image. For the average and subtle cases, the detection accuracy improved from a TP rate of 87% at an FP rate of four clusters per image to a TP rate of 90% at an FP rate of 1.5 clusters per image.

Automation↗
PubMed1995

Mechanical and dosimetric quality control for computer controlled radiotherapy treatment equipment.

Modern computer controlled radiotherapy treatment equipment offers the possibility of delivering complex, multiple field treatments with minimal operator intervention, thus making multiple field conformal therapy practical. Conventional quality control programs are inadequate for this new technology, so new quality control procedures are needed. A reasonably fast, sensitive, and complete daily quality control program has been developed in our clinic that includes nearly automated mechanical as well as dosimetric tests. Automated delivery of these quality control fields is performed by the control system of the MM50 racetrack microtron, directed by the CCRS sequence processor [D. L. McShan and B. A. Fraass, Proceedings of the XIth International Conference on the use of computers in Radiation Therapy, 20-24 March 1994, Manchester, U.K. (North Western Medical Physics Department, Manchester, U.K., 1994), pp. 210-211], which controls the treatment process. The mechanical tests involve multiple irradiations of a single film to check the accuracy and reproducibility of the computer controlled setup of gantry and collimator angles, table orientation, collimator jaws, and multileaf collimator shape. The dosimetric tests, which involve multiple irradiations of an array of ionization chambers in a commercial dose detector (Keithly model 90100 Tracker System) rigidly attached to the head of the treatment gantry, check the output and symmetry of the treatment unit as a function of gantry and collimator angle and other parameters. For each of the dosimetric tests, readings from the five ionization chambers are automatically read out, stored, and analyzed by the computer, along with the geometric parameters of the treatment unit for that beam.(ABSTRACT TRUNCATED AT 250 WORDS)

Humans↗
PubMed1994

Digitization requirements in mammography: effects on computer-aided detection of microcalcifications.

We have developed a computerized method for detection of microcalcifications on digitized mammograms. The program has achieved an accuracy that can detect subtle microcalcifications which may potentially be missed by radiologists. In this study, we evaluated the dependence of the detection accuracy on the pixel size and pixel depth of the digitized mammograms. The mammograms were digitized with a laser film scanner at a pixel size of 0.035 mm x0.035 mm and 12-bit gray levels. Digitization with larger pixel sizes or fewer number of bits was simulated by averaging adjacent pixels or by eliminating the least significant bits, respectively. The SNR enhancement filter and the signal-extraction criteria in the computer program were adjusted to maximize the accuracy of signal detection for each pixel size. The overall detection accuracy was compared using the free response receiver operating characteristic curves. The results indicate that the detection accuracy decreases significantly as the pixel size increases from 0.035 mm x 0.035 mm to 0.07 mm x 0.07 mm (P < 0.007) and from 0.07 mm x 0.07 mm to 0.105 mm x 0.105 mm (P < 0.002). The detection accuracy is essentially independent of pixel depth from 12 to 9 bits and decreases significantly (P < 0.003) from 9 to 8 bits; a rapid decrease is observed as the pixel depth decreases further from 8 to 7 bits (P < 0.03) or from 7 to 6 bits (P < 0.02).(ABSTRACT TRUNCATED AT 250 WORDS)

Biophysical Phenomena↗
PubMed1990

Dynamic digital subtraction evaluation of regional pulmonary ventilation with nonradioactive xenon.

A method of evaluating pulmonary ventilation with a 57-cm image intensifier/television (II/TV) digital chest system is reported. With this method, the patient inhales a mixture of xenon and oxygen gases while dynamic imaging of the chest is done. Images of the airways and ventilated portions of the lungs are obtained by subtraction of images acquired before and after the xenon-oxygen mixture is administered. The feasibility of the method was evaluated by studies with xenon-filled tubes, an airway phantom, and a ventilation phantom. The results indicate that tubes larger than 3.2 mm in diameter are detectable at a xenon concentration of 41%, and that gas flow and flow distribution can be examined after image subtraction. If background subtraction is incomplete because of motion, the visibility of small airways is reduced greatly, although unventilated regions can still be delineated. The initial evaluation of this technique included imaging a healthy volunteer during xenon inhalation.

Contrast Media↗
PubMed1990

Improvement in radiologists' detection of clustered microcalcifications on mammograms. The potential of computer-aided diagnosis.

Relatively simple, but important, detection tasks in radiology are nearing accessibility to computer-aided diagnostic (CAD) methods. The authors have studied one such task, the detection of clustered microcalcifications on mammograms, to determine whether CAD can improve radiologists' performance under controlled but generally realistic circumstances. The results of their receiver operating characteristic (ROC) study show that CAD, as implemented by their computer code in its present state of development, does significantly improve radiologists' accuracy in detecting clustered microcalcifications under conditions that simulate the rapid interpretation of screening mammograms. The results suggest also that a reduction in the computer's false-positive rate will further improve radiologists' diagnostic accuracy, although the improvement falls short of statistical significance in this study.

Calcinosis↗
PubMed1990

The histopathological pattern of primary IgA nephropathy in a Malaysian patient population.

In the 10-year period from October 1977 to July 1987, 149 cases of primary IgA nephropathy were histologically confirmed through renal biopsies in the Department of Pathology, University Hospital, Kuala Lumpur. The ages of these patients ranged from 5 to 72 years, with the majority in the 20-30 year age group. There was no sex preponderance. The ethnic distribution showed a significant predominance of Chinese with 107 (71.8%) Chinese, 24 (16.1%) Malays, 15 (10.1%) Indians and 3 others. A wide range of renal glomerular pathology was seen, the commonest being diffuse mesangioproliferative glomerulonephritis (59.1%). Focal proliferative glomerulonephritis (14.1%) followed by minimal change glomerulonephritis (10.7%) were next in order of frequency. Immunofluorescence studies consistently demonstrated heavy and predominant IgA deposition in the mesangium. Weak deposition of C3, IgG and IgM were also observed in various combinations.

Adolescent↗
PubMed1988

Effect of continued feeding of cows' milk on asymptomatic infants with milk protein sensitive enteropathy.

The clinical response and the histological changes in the mucosa of the small bowel in response to continued feeding with cows' milk protein were assessed over a period of 2-6 weeks in 24 infants who had shown histological changes without immediate clinical symptoms after challenge with a diet containing cows' milk protein. Twenty of the 24 infants (83%) thrived well on cows' milk protein. Jejunal biopsy specimens taken six to eight weeks after the initial biopsy showed histological improvement in all 20 infants compared with biopsy specimens taken soon after the challenge, which had shown mucosal damage. The mucosa had returned to normal in 12, was mildly abnormal in seven, and moderately abnormal in one. Corresponding improvements in the activities of mucosal enzymes were seen. In four of the 24 infants (17%) symptoms developed between three and six weeks. Histological examination of the jejunal biopsy specimens showed that mucosal damage had progressed in two, and remained the same in two; moreover, the disaccharidase activities remained depressed. The present study shows that most infants with enteropathy caused by sensitivity to cows' milk protein but without clinical symptoms develop tolerance to the protein and the mucosa returns to normal despite continued feeding with cows' milk protein.

Animals↗
PubMed1988

Computer-aided detection of microcalcifications in mammograms. Methodology and preliminary clinical study.

The authors investigated the feasibility of using computer methods for automated detection of clustered microcalcifications on clinical mammograms. A new difference-image approach using a matched filter/box-rim filter combination effectively removed the structured background from the image. A locally adaptive gray-level thresholding technique was then used for extraction of the signals from the resulting difference image. Signal-extraction criteria based on the size, contrast, number, and clustering properties of microcalcifications were next imposed on the detected signals to distinguish true signals from noise or artifacts. The detection accuracy of the computer scheme was evaluated by means of a free response receiver operating characteristic (FROC) analysis. It was found that, for simulated subtle microcalcifications superimposed on normal mammograms, the difference-image approach with a matched filter/box-rim filter combination could yield a true-positive cluster detection rate of 80% at a false-positive detection rate of one cluster per image. In a study of 20 clinical images containing moderately subtle microcalcifications, the automated computer scheme obtained an 82% true-positive cluster detection rate at a false-positive detection rate of one cluster per image. These results indicate that the automated method has the potential to aid radiologists in screening mammograms for clustered microcalcifications.

Breast Diseases↗