The effect of source to image-receptor distance on effective dose for some common X-ray projections.

A number of studies on the effects of source to image distance (SID) on dose to the patient have been published. These generally assume that the X-ray beam is collimated to a rectangular region of fixed size at the entrance surface of the patient at each SID. This is poor radiographic practice. In this work, effective doses have been determined using the commercially available Monte Carlo simulation package PCXMC. Three collimation cases were considered, namely to regions of clinical interest at the entrance and at the centre of the patient, and a fixed beam size at the image receptor. For cases of collimation to a central region of interest or to the image receptor, the saving in effective dose at increased SIDs is modest.

Patient doses from CT in New Zealand and a simple method for estimating effective dose.

A 1992 survey of doses to patients from CT scanning in New Zealand found that CT contributes 81 microSv per capita per annum, representing about 17% of the collective dose from medical irradiation. Because of the significance of CT, estimates of patient dose are now included in the National Radiation Laboratory (NRL) routine surveys of CT scanners. For some models of CT scanner, normalized organ dose data from Monte Carlo studies are available, enabling the calculation of the effective dose. For scanners for which there are no normalized organ dose data, it is shown that the computed tomography dose index (CTDI), determined at 1 cm depth in Perspex phantoms and normalized for mAs, may be used to estimate the effective dose. Estimates of effective dose derived from the CTDI may then be used to formulate reference dose levels, against which individual scanner doses may be compared.

Mammography compression force in New Zealand.

Maximum compression forces have been measured in New Zealand on 37 mammography machines, using a simple hydraulic device. The median measured maximum force was 145 N, and the range 58 to 230 N. Much greater attention needs to be paid to the setting of maximum force for compression devices by service personnel. Compression devices must be included in the quality assurance programme. Where indicated by the machine, the accuracy of the indicated force for some machines is poor.

Mammography in New Zealand: radiation dose and image quality.

The mean glandular doses to the breast, image quality and machine performance have been determined for all mammographic x-ray facilities in New Zealand, during 1988-89. For 30 mm and 45 mm phantoms the mean doses per film were 1.03 +/- 0.56 mGy and 1.97 +/- 1.06 mGy. These doses are within international guide-lines. Image quality (detection of simulated microcalcifications, and contrast-detail performance) was found to depend on focal spot size/FFD combination, breast thickness, and film processing. The best machines could resolve 0.2 mm aluminium oxide specks with the contact technique. The use of a grid improved image quality as did magnification. Extended cycle film processing reduced doses, but the claimed improvement in image quality was not apparent from our data. The machine calibration parameters kVp, HVL and timer accuracy were in general within accepted tolerances. Automatic exposure controls in some cases gave poor control of film density with changing breast thickness.

Performance assessment of X-ray image intensified television fluoroscopy systems in New Zealand.

The protocol for image intensifier (II) quality assurance recommended by the UK Hospital Physicists' Association (HPA) and the UK Department of Health and Social Security has been adapted for use in New Zealand for assessment of such systems. Test objects were constructed at the National Radiation Laboratory to match the specifications of those recommended by the HPA. Over 100 individual surveys have been made and, in a few cases, machines have been surveyed several times. Results are summarized in terms of key parameters: II input dose rate, low-contrast detectability, contrast-detail performance and limiting resolution. These are given as mean and standard deviation, and also as good, average or poor. Video waveforms, conversion factors and modulation transfer functions have also been determined on a number of systems. In several cases, video voltages were found to be seriously maladjusted.

Measurement of x-ray image intensifier sharpness in the x-ray department.

A method is presented for determining the modulation transfer function (MTF) of an image intensifier as it is found in the x-ray department. The image of an edge input into the image intensifier tube is photographed at the television camera port with a high quality camera and the photographic image scanned with a microdensitometer. Densitometric conversion enables the determination of the edge spread function and further calculation produces the line spread function and the MTF. A correction is made for the camera/lens/film/developer response function. Results from the use of the technique on over 25 x-ray image intensifiers in New Zealand hospital x-ray departments are presented and discussed. Most of the image intensifiers had been in use for more than 7 years. The low-frequency drop values were similar to reported values with an average value of 22%, but modulation values at 1 cycle/mm (the average value was 29%) were generally lower than values previously reported, indicating that the sharpness performance in the clinical environment is inferior to claims for new or state-of-the-art intensifiers.