Abstract. A survey of radiographic technique and estimated entrance surface dose has been carried out for 364 chest radiographs performed with mobile X-ray ...
T he British Journal of Radiology, 71 (1998), 640–645
© 1998 The British Institute of Radiology
A study of chest radiography with mobile X-ray units 1P D SIMPSON, BSc, MSc, 2C J MARTIN, PhD, FIPEM, 3C L DARRAGH, BSc, Dip Bio-Med Eng and 4R ABEL, HDCR 1Wessex Regional Medical Physics Department, Royal United Hospital, Combe Park, Bath, Avon BA1 3NG, 2Health Physics, Department of Clinical Physics and Bio-Engineering, West Glasgow Hospitals University NHS Trust, 22 Western Court, Glasgow G12 8SQ, Departments of 3Bio-Medical Physics and Bio-Engineering and 4Radiology, Aberdeen Royal Hospitals NHS Trust, Foresterhill, Aberdeen AB25 2ZD, UK Abstract. A survey of radiographic technique and estimated entrance surface dose has been carried out for 364 chest radiographs performed with mobile X-ray equipment in the Intensive Therapy Unit (ITU) and 30 wards at Aberdeen Royal Infirmary. Data for these two types of location were compared, as were those for two film/screen systems used on the wards. Image quality assessments were made on sets of radiographs for two patients. Entrance skin doses for chest radiographs performed in the ITU were 50% greater than on the wards with the same film/screen system. The main technique difference was the use of shorter focus-to-skin distances (FSDs) in ITU. Doses with the Kodak Insight system were 20% higher than those using Du Pont Quanta III in similar locations. No correlation was found between image quality and entrance surface dose (ESD). Results from the survey were used to recommend exposure factors for shorter FSDs. A follow-up study revealed a 35–45% reduction in ESD for Kodak Insight and a 20% reduction for Quanta III.
Mobile X-ray units are used for radiography on patients who cannot be moved from their hospital beds. Such examinations are routinely performed in intensive therapy units, and frequently carried out in other wards. They may make up half of all chest radiographs taken in large hospitals [1]. Standard configurations are more difficult to use when examining seriously ill patients and differences in techniques may lead to lack of comparability in the appearance of the radiograph, which can affect diagnosis, and may give more variation in the radiation doses received by patients. Recent studies have demonstrated substantial dose reductions through optimization of radiographic technique for chest examinations in fixed installations [2, 3]. There is a similar need for optimization of technique for mobile radiography. A survey of mobile chest radiographs has been carried out in Aberdeen Royal Infirmary (ARI), where two types of film/screen combination are used. The results have been analysed to determine the pattern of exposure. Changes in technique have been implemented to standardize film quality, based on the results of the survey, and doses reassessed.
Methods Patient dose survey A survey of radiographic technique and entrance skin dose was carried out for all mobile radioReceived 8 September 1997 and in revised form 9 January 1998, accepted 4 February 1998. 640
graphs performed over a period of 5 weeks in the Intensive Therapy Unit (ITU) and 30 wards in ARI, 1 year after introduction of a Kodak Insight film/screen system. The wards included in the survey were those dealing with thoracic, cardiac, vascular and general surgery, chest medicine and oncology. Radiographers were asked to use the techniques and exposure factors that would normally have been chosen, so that results would be representative of standard practice. Forms were provided for each mobile X-ray unit and data entered for 364 radiographs taken during this period. The exposure factors (tube potential (kV) and mAs), film/screen combination used, focus-toskin distance (FSD) (measured for every radiograph using a tape) and angle of the patient’s torso to the vertical, were all recorded. Whether or not each patient is able to sit up in bed is an important factor in determining both the FSD and the diagnostic information that can be obtained from the examination. The angle of inclination between the patient’s torso and the vertical was measured using a large protractor with 10 numbered segments representing ranges of angles between the vertical and horizontal. The height and weight of each patient, the radiographer’s initials and any comments on the radiograph produced were also included, together with the patient identification number and the time and date of the examination so that radiographs could be recalled for inspection. Following analysis of results from the first survey, changes were made in technique and data collected for a further 124 patients over a period of 2 weeks. T he British Journal of Radiology, June 1998
Chest radiography using mobile X-ray units
Two film/screen combinations were routinely employed for ward radiography. The Du Pont Quanta III system was used with tube potentials of 65–75 kV and the Kodak Insight system with tube potentials of 90–120 kV and cassettes incorporating anti-scatter grids. Kodak Insight is an asymmetric system with differing emulsions on either side of the film base, which are designed to match differing screens included in the cassette [4]. The aim is to enable the visibility of both parenchymal lung detail and mediastinal detail to be optimized on a single film. The use of two emulsions with high contrast in different exposure ranges results in a broader latitude system [5]. The mobile X-ray units were all AMX-4 (GE Medical Systems, Milwaukee, USA) apart from one Dean mobile (GEC) which was used on one block of wards. Patient entrance surface doses (ESDs) were derived from calculations based on the exposure factors used and measurements of output. The filtration of each unit was determined and outputs measured for exposure settings used for chest radiography. A Keithley 35050A dosimeter with a 15 cc ionization chamber and a calibration traceable to national standards was employed to measure outputs at a focus-tochamber distance of 1 m. Measurements were recorded over a range of tube potentials from 50 to 110 kV in steps of 10 kV, at 5 mAs intervals taking extra measurements at 95 kV, 105 kV and other tube potentials used for chest radiography. Measurements were made for all lower mAs settings used, as the linearity of the output with respect to mAs at shorter exposure times could not be guaranteed. Second order polynomial equations linking the output to the set tube potential for each X-ray unit were established from this data, which enabled outputs for tube potentials not tested to be calculated where necessary. ESDs were calculated from the equation: ESD=d (1/FSD)2 BSF A where d is the absorbed dose to air at a distance A of 1 m from the focus of the X-ray set at the tube potential and mAs used for the examination, FSD is the focus-to-skin distance for each radiograph and BSF is the back scatter factor for an anteroposterior (AP) chest radiograph performed with the specified conditions obtained from a National Radiological Protection Board (NRPB) report [6]. The uncertainty in ESD measurements was estimated to be ±10%. An effective patient diameter (D (m)), equal to the diameter of a cylinder of the e same weight (W ( kg)) and height (H (m)), was derived to study variations in ESD with patient size from the equation [7]: W D =2√ e 1000.p.H T he British Journal of Radiology, June 1998
Image quality assessment Characteristic curves for the two film/screen combinations were determined from two series of exposures. The beam was filtered by 16 mm of aluminium for which the spectrum of transmitted X-rays is similar to that for 20 cm of water [8]. The filter was attached to the light beam diaphragm and a focus-to-film distance of 150 cm used. Measurements were made with tube potentials similar to those employed clinically; 70 kVp for Quanta III and 100 kVp for Insight. A grid cassette was used for the measurements on the Insight film/ screen combination. Assessments of radiographic image quality based on the visualization of clinical features for films taken of different patients were affected by the underlying pathologies and could not therefore provide useful comparisons. However, during the study a number of patients had several chest radiographs while their condition was monitored. Image quality assessments were made of multiple radiographs from two such patients, one treated in the ITU and the other in one of the wards. It was hoped that, through assessment of films from individual patients, the effect of interpatient variation resulting from different pathologies could be minimized. Five films were selected for each, giving a range of ESDs and these were evaluated by four radiologists and 20 radiographers, using clinical image quality criteria dependent on the imaging system and not the radiographic technique (Table 1). These represented an extension of the Commission of the European Communities (CEC) Quality Criteria For Diagnostic Radiographic T echniques [9]. In addition, optical densities were measured in particular areas for each of the films from these patients with a Parry 1305DT densitometer, to determine how densities varied with Table 1. Criteria used for evaluation of image quality of clinical radiographs Imaging of vertebrae
Intervertebral disc spaces visible behind cardiac shadow ( low kV) Good detail in vertebral bodies, including pedicles ( high kV)
Imaging of vascular pattern in lung
Hilum Apices Lung lateral border Lung bases Behind diaphragm (high kV)
Visually sharp reproduction of
Bifurcation of trachea Proximal bronchi Borders of heart Diaphragm Costophrenic angles
Visualization of
Retrocardiac lung Mediastinum
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P D Simpson, C J Martin, C L Darragh and R Abel
ESD. Nine films taken of the ITU patient were included for this assessment.
Results Patient dose results Mean exposure factors and ESDs for the dose survey are given in Table 2. Results for the ITU and the wards are presented separately, as for patients in the ITU, the film density within the lung tissue is commonly lower because of lung pathology and the patients are less able to cooperate in optimizing examination conditions. Comparing results for the two locations with the Kodak Insight film/screen system, ESDs for examinations performed in the ITU were 50% greater than for ones carried out in the wards. ESDs for patients on the wards, examined using Kodak
Insight were 20% greater than for those where the Du Pont Quanta III film/screen system was used. Mean ESDs for ward areas using the Quanta III system were just below the UK reference level for chest radiography. However, those performed with the Insight system were above the reference level and those in ITU were 70% greater. Plots of ESD against equivalent patient diameter showed that although average dose increased with patient size, there was a range of 2.5 in doses for patients of similar size (Figure 1). The spread was greater for examinations performed with Kodak Insight than for those with Quanta III. The range of FSDs used on the wards was 100– 150 cm while that in the ITU was 85–150 cm. The difference in the mean FSD appeared to be the major factor linked to higher doses in ITU (Table 2). Plots of ESD against FSD show that
Table 2. Data on surveys of mobile chest radiographs ITU, ARI
Wards, ARI
Wards, ARI
Kodak Insight
Kodak Insight
Du Pont Quanta III
Main survey of practice No. of examinations Mean kVp Mean mAs Mean FSD (cm) Mean ESD (mGy)
156 99.2 4.1 102.3 0.51±0.14
152 99.0 4.2 122.9 0.34±0.11
56 68.4 8.6 123.5 0.29±0.08
Follow-up survey No. of examinations Mean kVp Mean mAs Mean FSD (cm) Mean ESD (mGy)
43 100.2 2.86 105 0.33±0.11
57 100.5 2.81 135.3 0.18±0.09
24 70 9.3 140.9 0.23±0.05
