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Biomedical subjects

J M Kofler

Publications and source records attributed to J M Kofler.

15 recordsLinked to original sources
PubMed2005

The calibration of experimental self-developing Gafchromic HXR film for the measurement of radiation dose in computed tomography.

A prototype, self-developing Gafchromic HXR film has sensitivity an order of magnitude larger than that of the commercially available Gafchromic XR film used in interventional radiological applications. The higher sensitivity of the HXR film allows the possibility of acquisition of high-resolution calibrated dose profiles within the diagnostic range of exposure levels, below 10 R (87.7 mGy). We employed a commercially available, optical flatbed scanner for digitization of the film and image analysis software to determine the response of the HXR films to ionizing radiation. Spatial uniformity and temporal repeatability of the flatbed scanner were determined and used in optimization of the digitization protocol. The HXR film postexposure density growth and sensitivity to ambient light were determined using multiple scans of two simultaneously exposed sheets, one stored in light-tight conditions and the other continuously illuminated with white light. A calibrated step wedge of the HXR film was obtained by simultaneous irradiation of a portion of a film strip and a calibrated ionization chamber using a radiographic x-ray tube with beam characteristics matched to a typical CT scanner (8 mm Al HVL, 120 kVp). Repeated digitization of the calibration film was used to determine the precision of the film response measurements. The precision, as measured by the standard deviation of multiple measurements, was better than 1% over the full dynamic range of film response. This precision was measured using exposures ranging from 0.5 to 12 R (4.4 to 105.3 mGy). This exposure range is highly relevant to x-ray computed tomography. Preliminary radiation dose profiles demonstrate the utility of this technique.

Abdomen↗
PubMed2001

Improved method for determining resolution zones in ultrasound phantoms with spherical simulated lesions.

Quantification of ultrasound (US) imager performance simulating human observers is addressed using size-dependent lesion signal-to-noise ratio (LSNR) analysis of images of spherical simulated lesions in phantoms. LSNR values obtained over a broad range of image depths can be used with a single detectability threshold to determine the depth range over which lesions of a given size and contrast are deemed to be detectable, yielding a performance metric. Optimal LSNR analysis requires a priori knowledge of lesion locations in the image so that LSNR values relate to the center of each lesion. Phantoms having a regular array of accurately positioned spherical simulated lesions are described, along with easily employed and robust software that accurately determines lesion locations in images, even when only a few are detectable by visual inspection. The software accounts for image spatial calibration inaccuracies and accommodates sector, curvilinear, etc., formats. The minimum number of equivalent lesions to yield an acceptable mean (counteracting speckle variations) also is addressed.

Image Processing, Computer-Assisted↗
PubMed2001

Update on patient radiation doses at a large tertiary care medical center.

Radiation procedures in diagnostic radiology and nuclear medicine examinations, especially at referral centers, contribute a significant proportion to population dose; hence, there is a presumed detriment. Knowledge of the magnitude of dose from each type of exam is helpful in determining where to implement dose reduction efforts. Additionally, new records of dose data facilitate comparisons with past measurements. In this paper, updated patient exam dose data (frequency and effective dose equivalent) are provided for a large, comprehensive, tertiary care medical center that served more than 340,000 patients in 1997. Patient billing code data were used to study 31 different types of diagnostic exams encompassed in five major categories (angiography, fluoroscopy, radiography, nuclear medicine, and computerized tomography). Organ doses for each radiographic and nuclear medicine exam were estimated using published Monte Carlo conversion factors and appropriate exposure values. Estimates of organ doses were utilized to compute effective dose equivalent (EDE) per ICRP 26 and collective effective dose equivalent. Mean effective dose equivalent was also calculated for each exam category. Total collective effective dose equivalent had decreased from 1988 (2,030 person-Sv) to 1997 (1,817 person-Sv). The largest contributors to collective effective dose equivalent were angiography (768 person-Sv), computerized tomography (447 person-Sv), and nuclear medicine (355 person-Sv). Radiography (150 person-Sv) and fluoroscopy (97 person-Sv) contributed the least to collective effective dose equivalent. Mean effective dose equivalent contributions remained the same, with angiography accounting for the highest component, followed by nuclear medicine, computerized tomography, fluoroscopy, and radiography, respectively. Effective dose equivalent, collective effective dose equivalent, and mean effective dose equivalent values were calculated and tabulated in five major categories. These data provide updated information as to trends in exam and; collective dose from 31 common types of radiologic exams performed at a large medical center, which can be used as an up to date baseline for analyses of trends in U.S. radiation doses due to medical imaging procedures. Although minor changes were observed in comparing the mean effective dose equivalent data to those of a previous study, substantial differences were evident in the collective effective dose equivalent data. This was due primarily to variations in the number of patients examined and changes in technology and practice.

Angiography↗
PubMed1999

Techniques for measuring radiographic repeat rates.

Radiographs that must be repeated, which are commonly referred to as "repeats," represent additional, non-billable costs due to increased film, chemistry, and equipment use as well as increased personnel time. Furthermore, patients receive additional radiation exposure from repeats and must remain on the premises until the second exam is completed. Compounding the overt negative financial impact on the department is an increased burden on the waiting room and support staff, and a decrease in patient throughput. A continuous improvement team was assigned to develop an improved technique for monitoring and reducing the number of repeated radiographs in a subset of our radiology department. This paper presents a novel method of accurately measuring the repeat rate through the use of radiographic repeat labels. The labels remove the guesswork from repeat analysis and heighten the technologists' awareness of common problems. Additionally, the labels allow for detailed analysis of the cause of repeated radiographs, which can provide insight for determining remedial actions. Repeat analysis data from our institution acquired using the labels before and after implementing remedial actions are presented.

Documentation↗
PubMed1998

Comparison of selected ultrasound performance tests with varying overall receiver gain and dynamic range, using conventional and magnified field of view.

Most ultrasound (US) scanner vendors currently offer a feature that allows a region of the ultrasound image to be magnified or zoomed. Although the methods of magnification vary among vendors, the ability to "zoom in" on a selected portion of the image has gained clinical acceptance. However, using this feature introduces additional steps in the quality assurance (QA) measurement procedures. No studies exist that demonstrate the advantage of a magnified field of view (FOV) over the conventional FOV for QA purposes. The purpose of this study was to investigate the influence of a magnified versus nonmagnified FOV on various common QA performance tests as a function of the overall receiver gain and dynamic range. Additionally, since the performance tests are subject to variations caused by scanner settings, sets of QC tests were recorded using several different scanner settings to investigate any change in the sensitivity of the QC measurements with respect to the magnified and nonmagnified fields of view. The lateral and axial resolution, slice thickness, and caliper accuracy (vertical and horizontal) as a function of varying overall receiver gain and dynamic range, were obtained using conventional (no zoom) as well as a magnified (zoom) field of view (FOV). Each measurement was performed three times by a single observer using a 4 MHz "vector" format transducer on a single diagnostic medical ultrasound scanner. The results show no statistical significance in the variability of most recorded masurements when using the conventional versus the magnified FOV. However, in some cases, such as lateral resolution, the average value measured using the magnified FOV was typically 0.5 mm lower than when using a conventional FOV.

Calibration↗
PubMed1996

Use of personal computers and digitized film angiograms to produce subtraction angiograms.

Digitization of film angiograms for subtraction purposes alleviates many of the problems associated with conventional film subtraction techniques. In this method, conventional film angiograms are digitized with a laser digitizer. An unenhanced image is subtracted from a contrast material-enhanced image with image processing software and a personal computer. Image registration and the contrast of the subtraction mask image are adjusted as necessary to obtain complete subtractions. Subtraction images produced with this method are superior to subtraction images produced with conventional film subtraction techniques; all areas and densities of a digitized subtraction image exhibit nearly perfect subtraction. Digitized film angiograms and image processing software provide a useful alternative to traditional techniques for producing subtraction angiograms. Digital film subtraction provides more control over image registration, eliminates the need for an intermediate subtraction mask image, and allows the user to control the contrast, window width, and window level of the final image.

Angiography, Digital Subtraction↗
PubMed1996

Technical exhibits.

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Exhibitions as Topic↗
PubMed1995

Quantitative evaluation of low-cost frame-grabber boards for personal computers.

Nine moderately priced frame-grabber boards for both Macintosh (Apple Computers, Cupertino, CA) and IBM-compatible computers were evaluated using a Society of Motion Pictures and Television Engineers (SMPTE) pattern and a video signal generator for dynamic range, gray-scale reproducibility, and spatial integrity of the captured image. The degradation of the video information ranged from minor to severe. Some boards are of reasonable quality for applications in diagnostic imaging and education. However, price and quality are not necessarily directly related.

Computers↗
PubMed1995

Blood flow velocity measurements: a comparison of 25 clinical ultrasonographic units.

A blood-mimicking flow phantom was used to evaluate the precision of velocity measurements acquired using 25 pulsed Doppler ultrasonographic units from four vendors. Measurements were made at four constant flow rates (12 to 50 cm/s peak velocity). The average standard deviation values of the peak and time-averaged velocities among all units and all flow rates were found to be 7 and 9% of the mean, respectively, while the corresponding values for a subgroup of 20 identical units were 5 and 8%. Considered in conjunction with other published data, this suggests that units should be calibrated to an institutional standard at the time of acceptance testing.

Blood Flow Velocity↗
PubMed1994

Spatial and temporal response characteristics of ionization chambers used in diagnostic radiology for exposure measurements and quality control.

The temporal and spatial (partial volume) response characteristics of ionization chambers used for measuring radiation exposures in x-ray quality control (QC) programs were evaluated. Five ionization chambers were evaluated using a pencil beam scanning x-ray source and a conventional radiographic system. The spatial response was determined by recording the exposure during scanning of the pencil beam or in increments using a slitted lead sheet on a conventional x-ray system. The temporal response was determined by recording x-ray wave forms using the different ionization chambers. The effects of partial volume irradiation of the chambers makes them unsuitable for use under these conditions, except those designed for exposure measurements in computed tomography. The temporal response of many chambers resembles that of a typical resistive-capacitive circuit, making them unsuitable for exposure time measurements or x-ray wave form evaluation. The appropriate ionization chamber must be selected for exposure measurements and quality control in diagnostic radiology.

Equipment Design↗
PubMed1994

Technical exhibits.

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Diagnostic Imaging↗
PubMed1991

Sensitometric responses of selected medical radiographic films.

Radiographic films produce different densities and contrast when processor changes occur, and the magnitude and rate of change vary with film type. The ability to detect and interpret the clinical importance of film density changes may depend on the method of sensitometry used. The characteristics of several medical radiographic films and various sensitometers were examined under three sensitometric variations and five processing variations. Of all variations used, only exposure with a single-versus a double-sided sensitometer caused a film type to have a marked different response. The results indicate that mismatching the sensitometer spectral output with the spectral sensitivity of the film in most cases does not affect the density changes of the film. The fact that a few films may be sensitive to differences in spectral content of the exposing light and dual- versus single-sided exposure and that only a limited number of film types were tested, however, leads to the prudent conclusion that the exposure conditions for quality control purposes should match clinical exposure conditions as closely as possible.

Technology, Radiologic↗
PubMedDate not supplied

Performance testing of a dedicated magnetic resonance peripheral blood flow scanner.

This work presents a novel investigation of an NMR device to measure blood flow. Continuous-wave magnetic resonance scanners are now available for noninvasive peripheral blood flow measurements. The appearance of dedicated peripheral blood flow scanners in the clinical setting requires analysis of system performance. No reports were found that explore methods or results of clinical MR blood flow scanner performance evaluation. This work was undertaken to identify the testing procedures and characteristics of a clinical, dedicated NMR blood flow scanner. The dependence of flow measurement on tubing size, material, offset from isocenter, and angulation was examined. In addition, the precision and accuracy of the scanner were investigated. The results demonstrate a strong dependence between the measured blood flow and both the tube size and flow rate. The tube offset results exhibited a region of weak dependence in the center of the bore, which becomes stronger near the edge. The measured flow rate was found to be relatively insensitive to tube angles less than 30 degrees to 55 degrees. The repeatability was typically better than 5% and the table positioning was found to be highly accurate.

Humans↗
PubMedDate not supplied

Comparison of tissue harmonic imaging with conventional US in abdominal disease.

Harmonic waves are generated from nonlinear distortion of an acoustic signal as an ultrasound wave insonates tissues in the body. These beams are integer multiples of a fundamental transmitted frequency. Potential advantages of harmonic imaging include improved axial resolution due to higher frequencies and better lateral resolution due to narrower beams. Decreased noise from side lobes improves signal-to-noise ratios and reduces artifacts. Deleterious effects of the body wall are also reduced. The authors prospectively studied ultrasonographic (US) findings in 100 adult patients with 202 abdominal lesions by comparing harmonic US images with conventional US images. The results were subjected to statistical analysis. Harmonic imaging was superior to conventional US in regard to lesion visibility and diagnostic confidence. Harmonic imaging was particularly useful for depicting cystic lesions and those containing echogenic tissues such as fat, calcium, or air. In patients with a body mass index of 30 or more, harmonic imaging was clearly better for lesion visibility and confidence of diagnosis. The authors recommend routine use of harmonic imaging for abdominal US studies in all adult patients.

Abdomen↗