Testing random number generators for Monte Carlo applications.

Central to any system for modelling radiation transport phenomena using Monte Carlo techniques is the method by which pseudo random numbers are generated. This method is commonly referred to as the Random Number Generator (RNG). It is usually a computer implemented mathematical algorithm which produces a series of numbers uniformly distributed on the interval [0,1). If this series satisfies certain statistical tests for randomness, then for practical purposes the pseudo random numbers in the series can be considered to be random. Tests of this nature are important not only for new RNGs but also to test the implementation of known RNG algorithms in different computer environments. Six RNGs have been tested using six statistical tests and one visual test. The statistical tests are the moments, frequency (digit and number), serial, gap, and poker tests. The visual test is a simple two dimensional ordered pair display. In addition the RNGs have been tested in a specific Monte Carlo application. This type of test is often overlooked, however it is important that in addition to satisfactory performance in statistical tests, the RNG be able to perform effectively in the applications of interest. The RNGs tested here are based on a variety of algorithms, including multiplicative and linear congruential, lagged Fibonacci, and combination arithmetic and lagged Fibonacci. The effect of the Bays-Durham shuffling algorithm on the output of a known "bad" RNG has also been investigated.

Radiographic measurement of bone mineral: reviewing dual energy x-ray absorptiometry.

Features of the three major brands of DEXA equipment available in Australasia have been summarised. Radiation hazard is small with patient effective doses of the order of a few microSieverts. In vivo measurement precision of the order of 1% is achievable for PA scans of the lumbar spine. Lateral scans can achieve measurement precision of the order of 4%. Recent technological developments using X-ray fan beams and multi element detector arrays on C-arm devices have resulted in faster scan times, higher resolution images, and an ability to perform PA and lateral scanning without the need to reposition the patient. Accuracy of DEXA is dependent upon specific instrumentation and data reduction algorithms, but results generally correlate well with ashed bone measurements. Major sources of inaccuracy include inhomogeneous distributions of fat, and machine specific factors such as edge detection algorithms. Lack of absolute inter unit comparability may cause difficulties in clinical practice.

A Monte Carlo model for bone mineral measurement using dual energy X-ray absorptiometry.

A detailed Monte Carlo model designed to simulate bone mineral measurements using a typical K-edge DEXA technique (LUNAR DPX-L bone densitometer) has been developed. Factors considered in the model include measurement geometry, K-edge filtration of the X-ray spectrum, photon statistics, count rate corrections and detector resolution. Two semi infinite elliptical phantoms simulating the lumbar spine and the femoral neck respectively have been used. Each phantom consists of four tissue components-fat, lean soft tissue, cortical bone and trabecular bone. Model results display similar levels of accuracy and precision to those observed in clinical practice. The Monte Carlo model of bone mineral measurements provides a valid tool for study of the LUNAR DPX-L DEXA technique.

ATR-FTIR studies on ion interaction of lithium perchlorate in polyacrylate/poly(ethylene oxide) blends.

The interaction behaviours between components of polyacrylate (PAc)/poly(ethylene oxide) (PEO) and lithium perchlorate (LiClO(4)) were investigated in detail by Attenuated Total Reflectance (ATR)-Fourier Transformed Infrared (FTIR) spectroscopy. Solution cast films of the PAc/PEO and PAc/PEO/LiClO(4) were examined. No obvious shifting of the characteristic ether and ester group stretching modes of PEO and PAc was observed, indicating incompatibility of the binary PAc/PEO blend. The spectroscopic studies on the PAc/PEO/LiClO(4) blends reveal that Li(+) ions coordinate individually to the polymer components at the ether oxygen of PEO and the C-O of the ester group of PAc. Frequency changes observed on the nu(C-O-C) and omega(CH(2)) of PEO confirm the coordination between PEO and Li(+) ions resulting in crystallinity suppression of PEO. The absence of experimental evidence on the formation of PEO-Li(+)-PAc complexes suggests that LiClO(4) does not enhance the compatibility of PAc/PEO blend.

Radiation doses to the lens of the eye during computerised tomography of the orbit; a comparison of four modern computerised tomography units.

A previous study has looked at the estimated radiation dose to the lens of the eye during CT scanning of the orbit, pituitary fossa and brain with a recently installed GE 9800 Quick CT using a phantom simulating the lens of the eye. In this study the same phantom was used to compare the estimated radiation dose delivered to the lens of the eye by three other newly installed CT units of different manufacture. There was significant variation between the doses measured from the four machines when operated with the parameters commonly used in clinical practice. However, in all cases scanning of the phantom using routine techniques delivered a dose to the simulated lens which was much lower than the threshold dose for cataracts. There is, never-the-less, a potential for delivery of much higher doses.

Corrosion of lead shielding in a radiology department.

Corrosion of lead sheet used for structural radiation protection in the Princess Alexandra Hospital radiology department has been identified. The corrosion is thought to have been caused by organic acid vapours released from oregon timber wall panelling. Non-destructive testing (NDT) and X-ray transmission measurements were used to define the extent and severity of damage, and subsequently to provide the data necessary for estimation of staff and public radiation doses. Although radiation dose limits have not been exceeded, corrective actions including structural modifications and staff information sessions have been undertaken.