Richard L. Morin

A practical method to measure the MTF of CT scanners

A method has been devised which enables the modulation transfer function(MTF) of computerized tomographic (CT) scanners to be simply and rapidly determined. The method relies upon the measurement of the standard deviation of the pixel values within the image of cyclic bar patterns. The method is shown to produce results which are indistinguishable from those derived by less practical conventional methods.

Monte Carlo evaluation of the Sievert integral for brachytherapy dosimetry

The Sievert integral, widely used to compute dose distributions about filtered line sources, assumes that the emitted energy fluence is exponentially attenuated by the filter thickness traversed by the primary photons. To evaluate this approximation, a Monte Carlo simulation was performed realistically modelling the diffusion and energy degradation of primary photons due to coherent, incoherent and photoelectric interactions in the source filter. Estimates of the exposure rate at points near the source were obtained using analytical averaging. Comparison of the two models shows that for 226Ra and 192Ir sources, the Sievert algorithm consistently overestimates the exposure rate per unit activity. However, such errors may be significantly reduced if source intensity is expressed in terms of exposure rate. Computed exposure rate distributions based on exposure rate calibrations are also less sensitive to uncertainties in available spectroscopic data.

Dose calibrator response to brachytherapy sources: A Monte Carlo and analytic evaluation

It is well known that the dose calibrator response/unit exposure rate depends significantly upon source energy. However, investigation of 137Cs, 192Ir, and 226Ra brachytherapy sources by empirical, analytical, and Monte Carlo techniques shows that source filtration significantly affects the calibrator reading to exposure rate conversion factor. The results demonstrate that for each clinically used filtration thickness an exposure calibrated standard source is required to establish the response of the well chamber. An interesting consequence of this analysis is that the Sievert point dose algorithm for clinical sources overestimates the dose on the order of 3% at distances of approximately 3.5 cm from the source.

The significance of electron binding corrections in Monte Carlo photon transport calculations

Many Monte Carlo simulations ignore coherent scattering events and utilise the Klein-Nishina free electron distribution, rather than the incoherent differential cross-section, for choosing the trajectories of incoherently scattered photons. We assess the accuracy of this model by comparing its results with those of the complete bound electron model (form factor approach), which simulates coherent scattering events, and uses the appropriate bound electron angular scattering distributions. Both analytic and Monte Carlo calculations demonstrate that use of the free electron scattering distributions significantly underestimates the angular distribution of scattered photon energy resulting from low and medium energy photons incident upon carbon, iron, and platinum barriers. In using the free electron approximations to calculate barrier transmission, significant errors occur only for primary photon energies below 100 keV. Implementation of the complete bound electron model reduces the computational efficiency of our Monte Carlo code by only 10-25%.

Removal of streaking artifact in computed tomography

In transmission computed tomography, relative X-ray attenuation measurements are made at various angles around a patients body. These input projection data are reconstructed to yield a cross-sectional view of internal structure. If the body section contains material that severely attenuates the X-ray beam (e.g., surgical clips, lead fragments), high-density streaks that obliterate internal structure will be produced in the process of image reconstruction. This loss of diagnostic information renders the scan useless. A technique has been developed that removes this imaging artifact. The approach views the affected projection data as misinformation. These data are assigned new values, and image reconstruction is performed without changing existing computer hardware or software. Projection data for a head section containing a lead fragment were obtained by Monte Carlo simulation. Three methods of obtaining replacement data were examined. A nearest-neighbor pattern recognition technique yielded excellent results.

Routine quality control tests for film-screen mammographic systems with automatic exposure control.

Quality control of the contrast and density of mammograms is of extreme importance not only because of patient dose considerations but also because of the need to monitor changes in the breast over extended periods of time. A phantom and test technique has been developed and used at two institutions for monitoring the ability of mammographic generators and phototiming systems to provide consistent contrast and density. The phantom consists of a solid acrylic block and an embedded aluminum step wedge designed specially for low kVp use. Optical densities of various portions of the phantom are used to determine constancy of density and contrast. By minimizing fluctuations due to processing and film handling, normal variations were reduced enough to determine changes in contrast and density due to generator and phototimer changes equivalent to those monitored in processor quality control programs. The data have been correlated with changes in processor function. Changes in density and contrast values have also been related to phototimer malfunction and reduced image quality.

Hydrophilic-Lipophilic Balance as Predictor of Mucus Coating with Barium Sulfate

In order to improve barium coating of the mucus layer covering the alimentary tract, various surface active substances are added to commercially available contrast media. The action of additives can be expressed in part by their hydrophilic-lipophilic balance (HLB). In an experimental study using human colonic mucus and barium sulfate USP, surfactants of varying HLB (range, 3–11) were used to study the range of HLB resulting in optimal coating. The best coating occurred with an HLB of 7. Quality of coating may be predicted based on knowledge of the HLB of various surface active additives.

The Effect Of CT Reconstruction Time Upon Patient Throughput In A University Setting

This study was undertaken to assess the effect of reconstruction time upon computed tomography (CT) patient throughput. Time studies were performed to analyze patient throughput and the distribution of examination time on a Pfizer 0450 and a Siemens DR3 CT scanners. Analyses were performed for both head and body examinations. An overall reduction in examination time of about 32% was observed for the DR3, owing primarily to the nearly instant image reconstruction available on this scanner. Our initial time study on the 0450 scanner proved to be useful in predicting the increase in patient throughput on the DR3. Hence the time study approach was felt to be valuable in both its predictive use as well as a tool to analyze current operating characteristics.

Evaluation Of A New Video Based Medical Image Processing System

A prototype video based medical image processing system is evaluated. The system is an adaptation of the Measuronics LMS II technology which is currently used for remote sensing applications. Initial investigations have identified many potential applications of the system in medical image processing. In addition to enhancement of a single radiograph, this system is capable of image subtraction (temporal and energy) and image superposition. This new system may provide an economical means to increase diagnostic information using conventional radiographic techniques and equipment.