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Purpose: A dosimetric artifact has been observed with Varian EPIDs in the presence of beam interrupts. This work determines the root cause and significance of this artifact. Methods: Integrated mode EPIDimages were acquired both with and without a manual beam interrupt for rectangular, sliding gap IMRT fields. Simultaneously, the individual frames were captured on a separate computer using a frame‐grabber system. Synchronization of the individual frames with the integrated images allowed the determination of precisely how the EPID behaved during regular operation as well as when a beam interrupt was triggered. The ability of the EPID to reliably monitor a treatment in the presence of beam interrupts was tested by comparing the difference between the interrupt and non‐interrupt images.Results: The interrupted images acquired in integrated acquisition mode displayed unanticipated behaviour in the region of the image where the leaves were located when the beam interrupt was triggered. Differences greater than 5% were observed as a result of the interrupt in some cases, with the discrepancies occurring in a non‐uniform manner across the imager. The differences measured were not repeatable from one measurement to another. Examination of the individual frames showed that the EPID was consistently losing a small amount of dose at the termination of every exposure. Inclusion of one additional frame in every image rectified the unexpected behaviour, reducing the differences to 1% or less. Conclusions: Although integrated EPIDimages nominally capture the entire dose delivered during an exposure, a small amount of dose is consistently being lost at the end of every exposure. The amount of missing dose is random, depending on the exact beam termination time within a frame. Inclusion of an extra frame at the end of each exposure effectively rectifies the problem, making the EPID more suitable for clinical dosimetry applications. The authors received support from Varian Medical Systems in the form of software and equipment loans as well as technical support.

Two new methods for measuring x-ray coherent scattering form factors using an image plate were developed based on a matrix representation of the problem. The methods were tested experimentally using tube potentials of 50, 70 and 92 kV and different filtrations. Water and fat samples were measured and compared to data in the literature. For water, average absolute relative differences between 0.105 and 0.217 were measured when compared to the literature. Literature data for fat show a wide range of values. Our measured values were in the middle of the literature data range with average absolute relative differences between 0.126 and 0.528. The accuracy of the matrix methods was limited by the ill-conditioning of the problem and also experimentally by the limitations of our equipment. The matrix methods developed are shown to be relatively low resolution in momentum transfer parameter and limited to measurements on amorphous materials such as tissues where the attenuation of the x-ray beam is small. These methods, however, can be used to measure form factors with relatively inexpensive, clinical equipment.

Background: The 5-HT 3 receptor is a serotonin receptor believed to reside on enteric neurons. However, several studies belie an exclusive neural localization. Our hypothesis is that the 5-HT 3 receptor agonist, 2-methyl-5-HT (2Me5HT), induces chloride secretion despite neural blockade, which can be blocked by a 5-HT 3 receptor antagonist. Methods: Rat distal colon was stripped of its muscularis, mounted as mucosal sheets in Ussing chambers, and short-circuited. Adjacent sheets were pretreated with 1 μmol/L of the neurotoxin, tetrodotoxin, and incubated with 2Me5HT (50 μmol/L) alone or with a 5-HT 3 (0.3 μmol/L ondansetron or 0.3 μmol/L tropisetron) or a 5-HT 4 (0.3 μmol/L GR11808) receptor antagonist. Short-circuit current (I sc ) was measured continuously. Results: 2Me5HT caused an increase in I sc , which was significantly (P

A key requirement for the development of the field of medical x-ray scatter imaging is accurate characterization of the differential scattering cross sections of tissues and phantom materials. The coherent x-ray scattering form factors of five tissues (fat, muscle, liver, kidney, and bone) obtained from butcher shops, four plastics (polyethylene, polystyrene, lexan (polycarbonate), nylon), and water have been measured using an energy-dispersive technique. The energy-dispersive technique has several improvements over traditional diffractometer measurements. Most notably, the form factor is measured on an absolute scale with no need for scaling factors. Form factors are reported in terms of the quantity x = λ(-1)sin (θ/2) over the range 0.363-9.25 nm(-1). The coherent form factors of muscle, liver, and kidney resemble those of water, while fat has a narrower peak at lower x, and bone is more structured. The linear attenuation coefficients of the ten materials have also been measured over the range 30-110 keV and parameterized using the dual-material approach with the basis functions being the linear attenuation coefficients of polymethylmethacrylate and aluminum.

The material-dependent x-ray scattering properties of amorphous substances such as tissues and phantom materials used in imaging are determined by their scattering form factors, measured as a function of the momentum transfer argument, x. Incoherent scattering form factors, F(inc), are calculable for all values of x while coherent scattering form factors, F(coh), cannot be calculated except at large x because of their dependence on long-range order. As a result, measuring F(coh) is very important to the developing field of x-ray scatter imaging. Previous measurements of F(coh), based on crystallographic techniques, have shown significant variability, as these techniques are not optimal for amorphous materials. We have developed an energy-dispersive technique that uses a polychromatic x-ray beam and an energy-sensitive detector. We show that F(coh) can be measured directly, with no scaling parameters, by computing the ratio of two spectra: the first, measured at a given scattering angle and the second, the direct transmission spectrum with no scattering. Experiments have been constructed on this principle and used to measure F(coh) for water and polyethylene to explore the reliability of the technique. A 121 kVp x-ray spectrum and seven different scattering angles between 1.67 and 15.09 degrees were used, resulting in a measurable range of x between 0.5 and 9.5 nm(-1). These are the first measurements of F(coh) made without the need for a scaling factor. Resolution in x varies between 10% for small scattering angles and 2% for large scattering angles. Accuracy in F(coh) is shown to be strongly dependent on the precision of the experimental geometry and varies between 5% and 15%. Comparison with previous published measurements for water shows values of the average absolute relative difference between 8% and 14%.

A case is presented of a 33-yr-old parturient with Harrington fusion of her spine who received spinal anaesthesia with 15 mg hyperbaric bupivacaine for Caesarean delivery. Multiple attempts of needle insertion in both midline and paramedian at the L3-4 interspace were unsuccessful, whereas the procedure was performed uneventfully at the midline of the L5S1 interspace. The anatomical considerations and difficulties in achieving reliable epidural anaesthesia after Harrington fusion are reviewed. Spinal anaesthesia performed at the L5S1 interspace may provide less technical difficulty and a more reliable result in such patients.

Purpose: To develop a frame-by-frame correction for the energy response of amorphous silicon electronic portal imaging devices (a-Si EPIDs) to radiation that has transmitted through the multileaf collimator (MLC) and to integrate this correction into the backscatter shielded EPID (BSS-EPID) dose-to-water conversion model. Methods: Individual EPID frames were acquired using a Varian frame grabber and iTools acquisition software then processed using in-house software developed inMATLAB. For each EPID image frame, the region below the MLC leaves was identified and all pixels in this region were multiplied by a factor of 1.3 to correct for the under-response of the imager to MLC transmitted radiation. The corrected frames were then summed to form a corrected integrated EPID image. This correction was implemented as an initial step in the BSS-EPID dose-to-water conversion model which was then used to compute dose planes in a water phantom for 35 IMRT fields. The calculated dose planes, with and without the proposed MLC transmission correction, were compared to measurements in solid water using a two-dimensional diode array. Results: It was observed that the integration of the MLC transmission correction into the BSS-EPID dose model improved agreement between modeled and measured dose planes. In particular, the MLC correctionmore » produced higher pass rates for almost all Head and Neck fields tested, yielding an average pass rate of 99.8% for 2%/2 mm criteria. A two-sample independentt-test and fisher F-test were used to show that the MLC transmission correction resulted in a statistically significant reduction in the mean and the standard deviation of the gamma values, respectively, to give a more accurate and consistent dose-to-water conversion. Conclusions: The frame-by-frame MLC transmission response correction was shown to improve the accuracy and reduce the variability of the BSS-EPID dose-to-water conversion model. The correction may be applied as a preprocessing step in any pretreatment portal dosimetry calculation and has been shown to be beneficial for highly modulated IMRT fields.« less

Purpose: To develop a method for removing the support arm backscatter from images acquired using current Varian electronic portal imaging devices (EPIDs).Methods: The effect of arm backscatter on EPID images was modeled using a kernel convolution method. The parameters of the model were optimized by comparing on-arm images to off-arm images. The model was used to develop a method to remove the effect of backscatter from measured EPID images. The performance of the backscatter removal method was tested by comparing backscatter corrected on-arm images to measured off-arm images for 17 rectangular fields of different sizes and locations on the imager. The method was also tested using on- and off-arm images from 42 intensity modulated radiotherapy (IMRT) fields.Results: Images generated by the backscatter removal method gave consistently better agreement with off-arm images than images without backscatter correction. For the 17 rectangular fields studied, the root mean square difference of in-plane profiles compared to off-arm profiles was reduced from 1.19% (standard deviation 0.59%) on average without backscatter removal to 0.38% (standard deviation 0.18%) when using the backscatter removal method. When comparing to the off-arm images from the 42 IMRT fields, the mean {gamma} and percentage of pixels with {gamma} < 1 weremore » improved by the backscatter removal method in all but one of the images studied. The mean {gamma} value (1%, 1 mm) for the IMRT fields studied was reduced from 0.80 to 0.57 by using the backscatter removal method, while the mean {gamma} pass rate was increased from 72.2% to 84.6%.Conclusions: A backscatter removal method has been developed to estimate the image acquired by the EPID without any arm backscatter from an image acquired in the presence of arm backscatter. The method has been shown to produce consistently reliable results for a wide range of field sizes and jaw configurations.« less

Purpose: To develop a method for removing the effect of support arm backscatter from Varian electronic portal imaging devices(EPIDs), improving the dosimetric abilities of the imager.Methods: A physical, kernel based model of the backscatter signal produced during an exposure was developed. The model parameters were determined through an optimization process, comparing measured images without arm backscatter(EPID removed from arm) to measured images that include arm backscatter. The backscatter model was used to develop a backscatter correction process that removes the support arm backscatter from measured EPIDimages. The correction process was tested by applying the method to measured images of 17 rectangular asymmetric fields and comparing the Result to off‐arm images. The same process was repeated with 42 IMRT fields. Results: The backscatter removal process was able to effectively remove the arm backscatter from all of the measured images and accurately predict the measured off‐arm images. Comparing the corrected images to the measured off‐arm images, the mean absolute difference at the centre of each rectangular field was 0.29% (standard deviation 0.18%). This is an improvement over the uncorrected images which gave a mean difference of 1.01% (standard deviation 0.73%). The largest discrepancy observed with the corrected images was 0.6%, compared to 2.8% for the uncorrected images. Comparing the corrected IMRTimages to the measured off‐arm images, an overall mean gamma value of 0.28 (standard deviation 0.04) was found using 2%, 2mm criteria. Comparison of the uncorrected images to the measured off‐arm images resulted in an overall mean gamma of 0.40 (standard deviation 0.10). Conclusions: A method for accurately and reliably removing the effect of support arm backscatter from EPIDimages has been developed and extensively tested. The method can be applied to any measured EPIDimage and does not require any additional information about the exposure.

The electrophysiological effects of encainide (MJ9067) on canine subendocardial Purkinje fibers surviving infarction were examined 22-24 h after coronary artery ligation, using standard microelectrode techniques at pH 7.4. Encainide (5 and 10 microM) shortened APD50, decreased the action potential amplitude and overshoot, caused a 2-3 mV depolarization of the cell, and decreased the maximum rate of phase 0 depolarization. All these effects were dose dependent. APD90 was slightly, but not significantly, shortened. Encainide had a greater effect on APD50 and APD90 at longer (1,000 ms) stimulation cycle lengths than at shorter (400 ms) cycle lengths. There was a significant interaction between the effects of stimulation rate and encainide concentration on APD90, APD50, amplitude, and overshoot. The same parameters measured at a lower pH (7.1) gave similar results. There was a significant correlation (r = -0.540, p less than 0.001) between the control APD90 and the degree and direction of change of APD90 after 10 microM encainide. Automaticity of the infarcted preparations was eliminated or slowed in a dose-dependent fashion by 5 and 10 microM encainide. The curve relating membrane potential and the maximum rate of phase 0 depolarization was shifted down and toward higher potentials by 5 microM encainide. The results show that encainide produces similar changes in the action potentials of Purkinje fibers surviving infarction as in normal Purkinje fibers, and is effective in lowering the rate of spontaneous depolarization in infarcted tissue. Furthermore, encainide appears to be the only antiarrhythmic drug which has been shown to increase and decrease APD90, depending on the initial APD90 of the cell.