Biomed Imaging Interv J 2006; 2(3):e35
© 2006 Biomedical Imaging and
Evaluation of radiation dose and image quality following changes to tube potential (kVp) in conventional paediatric chest radiography
S Ramanaidu1,*, MSc, TDCR,
RB Sta Maria2, DCR(R),
KH Ng1, PhD, MIPEM, DABMP,
J George1, MBBS, FRCR,
G Kumar1, MBBS, FRCR
1 Department of Biomedical Imaging, University
of Malaya Medical Centre, Kuala Lumpur, Malaysia
2 Department of Diagnostic Imaging, Paediatric Institute, Kuala
Lumpur Hospital, Kuala Lumpur, Malaysia
Purpose: A study
of radiation dose and image quality following changes to the tube potential
(kVp) in paediatric chest radiography.
Materials and Method:
A total of 109 patients ranging from 1 month to 15 years were included in two
phases of the study. Phase 1 investigated the range of entrance surface air
kerma (ESAK) values received from patients exposed to the existing exposure
factors. In the second phase, new exposure factors using recommended values of
tube potential (kVp) with reduced mAs were used. ESAK values were measured
using thermoluminescent dosemeters (TLDs). Image quality in both phases was
evaluated using image quality criteria proposed by the Council of the European
Communities (CEC). Results of both techniques were analysed for any
overall mean ESAK before the changes was 0.22 mGy (range: 0.05-0.43) Following
changes in tube potential, the overall mean reduced to 0.15 mGy (range:
0.03-0.38), a significant reduction by 34%. The interquartile range was reduced
from 45% to 40%. However, doses to those below a year in age still remained
high. Assessment of image quality was found to have no significant differences
as far as the two techniques used were concerned. However, higher image scores
were achieved using higher kVps.
Significant dose reduction was achieved through appropriate changes in tube
potential and reduction of mAs without any loss in image quality. � 2006
Biomedical Imaging and Intervention Journal. All rights reserved.
paediatric, ESAK, image quality, chest x-ray
In recent years concern has been raised over the hazards of
exposure to small doses of ionising radiation. The probability of a fatal cancer
being induced in an individual patient from a single x-ray examination,
although small, is dependent on the age of the patient and the type of
examination. Exposure during childhood results in a likely two- to three-fold
increase in lifetime risk for certain detrimental cancers compared with adults
. It is important that the radiation dose to children arising from
diagnostic medical exposure is minimised.
Despite rapid development in medical imaging, including the
advent of computed radiography and digital imaging, conventional chest
radiography remains the most frequent radiological examination among children
in major Malaysian hospitals. There are no studies in Malaysia
to evaluate the range of radiation dose received by paediatric patients while
existing studies on patient radiation doses are mainly done on adult patients
The Commission of European Communities (CEC) has recognised
the need for the special treatment of children and has published guidelines
suggesting examples of good radiographic practice and present useful image
criteria with the aim of producing high quality images at the lowest possible
dose to the patient . Good radiographic technique includes the use of
optimum kVp. Lower kilovoltages should be avoided in paediatric chest
examinations. The CEC recommends the use of 60 - 80 kVp for children between
0-15 years of age. Reduction in patient doses can be achieved through changes
in tube potential and the advantage lies in absence of cost implications .
Using a higher tube potential has shown a 16-36% reduction in entrance surface
doses in neo-natal radiography without any impairment of the diagnostic image
The objective of the present study is to identify the level
of radiation dose and image quality during paediatric chest radiography.
Comparison in dose and image quality will be made following changes to the tube
potential being used. Thus far, no studies on the assessment of radiation doses
to paediatric patients exist in Malaysia.
The present study will provide a useful baseline data to determine doses to
paediatric patients. This will allow imaging departments in Malaysia
to compare their performances and to undertake necessary remedial actions so
that radiation doses to children are minimal.
General survey of chest radiographic technique
A general questionnaire was distributed to 7 major hospitals
in the country to identify the current exposure factors being used in chest
radiography of paediatric patients. Superintendent radiographers were required
to record the range of kilovoltages (kVp), milliamperes-second (mAs), time and
film-screen combination currently used in their respective departments.
Phases of the study
As the objective of this study was to compare the radiation
dose and image quality following changes in tube potential during paediatric
chest radiography, the study was conducted in 2 phases. Phase One of the study
included survey of radiation dose and an assessment of image quality of the
current technique (Technique A) using the existing exposure factors in the
radiographic examination of the postero-anterior (PA) and antero-posterior (AP)
chest of children. In the second phase, the study was repeated using the
recommended technique (Technique B) with tube potential in the range of 60-80 kVp
. Both set of results were compared for any differences.
In both phases of the study, radiographic technique was
carried out without the use of a secondary radiation grid, consistent with
established guidelines on the non-usage of grids during paediatric chest
radiography . Exposures were also made without the use of automatic exposure
control as the x-ray machine used in this study did not have this facility.
The patients participating in both phases of this study were
randomly chosen from the group of patients attending a major Paediatric
Radiology Department in Kuala Lumpur, Malaysia.
Approximately 18,000 radiographic examinations are performed every year. 51% of
these examinations are chest. The department has two x-ray units but only one
unit is exclusively used for chest examinations. Exposures were carried out
with a 3 phase, 12 pulse generator (Phillips CP 50, Holland)
and a Phillips x-ray tube with 17� anode angle. The tube has a total filtration of 2.7 mm
Aluminium equivalent. No attempt was made to include any additional filtration.
Variation in kilovoltage was within � 5 kV while the variation
in tube output did not exceed � 3%. The tube output was found to vary linearly with the
timer setting to within 3%. Lanex regular intensifying screens and Agfa-gevaert
films were used with Kodak cassettes giving a nominal speed class of 400. Films
were processed using a Kodak M4 automatic processor.
Phase one of the study
Data was collected of patients aged between 1 month and 15
years undergoing AP/PA chest radiography. The initial set of exposure factors,
especially selection of kV and mAs or time, was determined by the radiographer
using the usual practice of selection of these factors suitable for the
patient. The radiographic technique employed depended on the co-operation of
the patient. Children who were co-operative and able to stand were done in the
erect PA position otherwise the AP supine position was chosen. An FFD of 180 cm
was used for erect patients while a FFD of 100 cm was used for supine patients.
No attempt was made to identify any differences in dose between these two
projections. The x-ray tube was centred to the 4th thoracic vertebrae and
collimated to reduce the irradiated area to the minimum. The antero-posterior
(AP) thickness of the chest was measured using callipers at the level of the
At the time of the examination patient data: sex, age,
height, weight and AP thickness were noted. Exposure parameters recorded were
tube voltage (kVp), tube current (mA), time or mAs and FFD. According to the recommendations
from the CEC , results are represented for separate age groups. Group 0-1
year includes children between one month and 1 year. A child is 1 year old
until its 2nd birthday etc. The other groups were 1-5 years, 6-10 years and
11-15 years. Grouping of patients into the recommended age classifications,
unfortunately, reduced the number of patients within each sub-group. Comparison
of doses were carried out giving due consideration to the variations that might
occur due to the small sample sizes. However, the results are expected to
indicate general trends in doses hence creating greater awareness of the doses
received by paediatric patients.
Entrance surface air kerma (ESAK) measurements were made by
attaching a sachet containing 3 thermoluminiscent dosemeters (TLDs) to the
patients� skin on the central axis of the x-ray beam. The lithium fluoride TLD
chips (TL 100, Harshaw) were later read using a TLD reader (Harshaw QS 3500).
The TLD system used in this survey was calibrated by the National Radiation
Laboratory, New Zealand
and found to be performing within recommended levels of precision and accuracy.
The calibration of the TLDs was traceable to the National Protocol recommended
by the NRPB . The overall uncertainty at the 95% confidence level was � 20%.
The ESAK for each patient was calculated by averaging the readings from the
three TLDs in each sachet.
Assessment of image quality
A checklist based on a modified version of the CEC image quality
criteria  formed the basis of image quality assessment in the present study,
as shown in Table 1. Two radiologists blinded to the study were invited to
evaluate the radiographs. This review centred on the visibility of specific
anatomical structures. Each viewer was asked to assess the visibility of the
structures on a graded scoring system in the range of 1 to 4 from the criterion
being cannot be assessed to the structure can be visualised very well. In this
scoring scheme the score for each observer would be in the range of 1 to 4 for
each criterion and an overall score of 4 to 16 for each radiograph. Higher
scores indicate better image quality. The summation of all the statements in
the questionnaire led to the assessment of radiographic quality of the image
and represented features that were dependent on the radiographic technique used
for the examination, particularly the tube voltage.
Table 1 Criteria used to evaluate image
Phase two of the study
Once approval was obtained from the senior radiologist of
the department to introduce the �recommended technique� (Technique B), which
included raising the tube voltage following that recommended by the CEC, a new
set of exposure factors were then derived, with the minimum set at 60 kVp for
all age range. Choice of kVp was based on the antero-posterior thickness.
Results of the AP thickness in the first phase showed the minimum thickness was
approximately 8 cm which formed the lower limit of the scale. Each increase in
1 cm followed an increase of 2 kV.
Increase in kVp necessitates a decrease in mAs to maintain
similar image density. Reduction in mAs was made using the common radiography
practice of an increase of 15% in kVp requires a decrease of 50% in the mAs
value . The radiographer selects the appropriate kVp after measuring the AP
thickness. The appropriate reduction in mAs was then calculated and the new
exposure was made. To maintain the consistency of the film density in both
phases of the study, readings of the optical density at selected anatomical
positions were recorded and compared.
Radiation dosimetry, image quality assessment, recording of
patient data and exposure factors were similar to those carried out in the
first phase. Radiographs of patients in the second phase were evaluated using
the same protocols and by the same two radiologists in the first phase of the
Data obtained in the survey was analysed using the SPSS
statistical package. Large samples (n>30) which were normally distributed
were analysed using parametric statistical tests (e.g. t-test, Pearsons r).
Sub-groups with smaller numbers were analysed using non-parametric tests (e.g.
Mann-Whitney, Kruskal-Wallis) .
Radiographic technique in major hospitals
The preliminary survey to identify the range of kVps used in
seven major hospitals showed most of the centres were using tube potentials
below 60 kVp during paediatric chest radiography, particularly in children
below 5 years old (Figure 1). Only one centre was using tube potentials above
60 kVp. Due to time constraints no ESAK measurements were done in this initial
survey. Evaluation of other parameters, e.g. type of equipment, tube
filtration, film-screen combinations and exposure time were excluded due to
insufficient respondent information.
Figure 1 Range of tube potential used
by various hospitals during paediatric chest radiography.
A total of 109 patients between the ages of 1 month to 15
years were included in the study. Table 2 summaries the patient parameters
accrued in the survey. 43 patients were examined in the first phase of the
study using the existing exposure factors, while 66 patients were examined
following the use of new exposure factors, i.e. with the change in tube
potential following the values recommended by the CEC. The overall mean age was
4.5 years (range: 1 month to 12 years).
Table 2 Patient data [mean ±
standard deviation (range)] from both techniques
Applied tube potential and ESAK
A summary of the applied tube potential, mAs and ESAK values
obtained before and after the change in applied tube potential during both
phases (techniques) is shown in Table 3.
Table 3 Comparison of applied kVp, mAs
and Entrance Surface Air Kerma (ESAK) between Technique A
and Technique B
The existing range of kVps used in the first phase of the
study (Technique A) showed kVps higher than 70 were used in older children
(above 10 years) while lower kVps below 60 kVp were predominant among younger
children aged below 5 years. In the second phase of the study, the minimum kVp
was set at 60 kVp. The mean applied kVp for the all the groups increased from
56.3 to 67.8, an increase of 20%, which was statistically significant (t-test,
Effect of change of tube potential on entrance surface
The mean ESAK before the changes for the all the patients
was 0.22 mGy (range 0.05-0.39). Following the increases in kVp the mean ESAK
was 0.15 mGy (range 0.03-0.34) which was significantly lower that the ESAK
obtained during Technique A (t-test, p<0.05). Overall reduction of 34% in
the ESAK was achieved following an increase of 20 % in kVp and a reduction of
57% in mAs values
Variations in ESAK within subgroups are shown in Figure 2 in
the form of a box-whisker plot. There was a significant reduction of doses
received by the 0-1 year old group during Technique B. Observations of the
doses received in Technique B showed a trend in lower doses in all the
sub-groups. The highest reduction was achieved in the 5-10 years age group i.e.
an increase of 14% in kVp produced a reduction of 50% in ESAK.
Figure 2 Range of Entrance Surface Air
Kerma (ESAK) within age sub-groups from both techniques.
Plotting the values of ESAK against kVp for both techniques
produced a scatter gram as shown in Figure 3. The variation of ESAK with
applied kVp displayed the trend towards lower ESAKs with increased kVp.
Figure 3 Relationship between Entrance
Surface Air Kerma (ESAK) and applied tube potential.
The exposure given to a particular patient, hence the dose
received, is closely related to the antero-posterior (AP) thickness of the
patient�s chest. Figure 4 shows the relationship between ESAK and the AP
thickness of the patients. A negative correlation was seen indicating lower
doses were achieved with increasing body thickness.
Figure 4 Relationship between Entrance
Surface Air Kerma (ESAK) and AP thickness.
Effect of change in kVp on image quality
Image Criteria Score
From the 109 patients a total of 48 radiographs were
retrieved for image evaluation and analysis. 24 radiographs were acquired from
the first phase and the second phase of the study. The poor retrieval rate was
due to problems of tracing the films back from the wards and clinics as at the
time of the study the filing system in the Paediatric Institute was being
Image quality of the chest radiographs was assessed in order
to investigate if the radiographs produced with higher kVp were of inferior
quality. The overall average radiologists� scores for both sets of radiographs
from the two techniques showed no significant statistical differences although
the overall scores of the images obtained during Technique B were slightly
higher. Analysis of the individual scores for each criterion showed some
variations as shown in Figure 5. There was an increase in values for individual
criteria scores within Technique B except for the visualisation of the
retro-cardiac lung and mediastinum (Criterion D). Wide variations were noted in
the visualisation of Criterion B (trachea and the proximal bronchi).
Figure 5 Comparison of image scores
for each criterion.
Optical density measurements on selected areas on all
Film density on selected areas on all radiographs was
measured to compare the influence of altering kVp on the image density. A
summary of the densities measured are shown in Table 4. While the results
indicate slightly higher densities recorded under the new technique, there were
no significant differences between both sets of density measurements.
Table 4 Comparison of optical density
measurements between Technique A and Technique B
This survey indicated that there was a common practice of
selecting lower kVps during paediatric chest radiography in major Malaysia
hospitals. This finding is not surprising since many radiographers and
radiologists believe that the small size of children particularly the younger
age group, necessitates use of lower kVp to enhance radiographic contrast. The
range of kVps used in these hospitals was similar to the range used in the
present study before recommending any change in tube potential.
The overall range of kVps used for all age groups in the
first phase of the study was between 45-76 kVp. Following changes the range
increased to between 60-82, a mean increase of 20%. There was substantial decrease
in the mAs (52%), a major contributor to patient dose. Tube potential below 60
kVp, the lower bound of the level recommended by CEC was predominant for
children below 5 years of age, where there were occasions of kVps below 50 kVp.
In the second phase of the study, the mean increase for the below 1 year and
11-15 years age group increased by 28% and 21% respectively. The middle two
groups had smaller increases of 14% as the existing exposure factors were
already in the above 60 kVp range hence it did not necessitate major increases.
The overall mean ESAK before the changes was 0.22 (range
0.05-0.43). Higher doses were predominant among the younger age group while
lower ESAKs were found in older children. The magnitude of the spread is of
concern since the examinations were done in one room over a period of few
weeks. This is indication of the variations of patient size and preference of
individual radiographers for selection of different exposure doses. Doses
seemed to decrease with age and the doses were much higher in children below 1
year. Doses were much higher that the achievable ESAK of 0.07 mGy recommended
by the CEC for a 10 month old child . The slightly higher doses in the
younger children could be due to the common practice among radiographers to use
lower kVps and higher mAs to enhance radiographic contrast. Following changes
in the tube potential in the second phase of the study, the overall mean ESAK
was 0.15 mGy, a significant reduction of 34%. The interquartile range was
reduced from 40% to 45% indicating that the spread of doses had been greatly
reduced. However, there still existed a wider range of doses particularly in
the under 1-year-olds.
The prevalence of higher doses among younger children could
also be due to the influence of technical parameters, namely additional
filtration. The total tube filtration of the x-ray tube in the present study
was 2.7 mm aluminium equivalent without the inclusion of any additional
filtration. The CEC recommends the use of additional filtration of up to 1mm
aluminium plus 0.1-0.2 mm copper. For standard diagnostic radiographic
voltages, every 0.1 mm copper equals about 3mm aluminium . Reduction in
patient ESAK of over 50% has been achieved without any loss in image quality
with the installation of additional filtration of 3 mm aluminium . As a
step towards optimisation of radiographic practice it would be useful if follow
up studies are conducted to evaluate reductions in ESAK following the use of
This study included patients of varying ages, though of
different body sizes. To compare patient studies, results need to be presented
in a comparative way. It has been suggested that equating ESAK with equivalent
patient diameter (EPD) provided a means of comparing patient doses, which takes
into account the different body sizes, irrespective of age . The author�s
own experience has shown that most radiographers do not favour calculating EPDs
manually before each x-ray examination. Dividing the children into age groups
is not ideal but it is easy and practical to be employed by radiographers
[3,12]. However, with current development in x-ray equipment which incorporates
automatic exposure charts based on radiographic technique, patient�s weight and
height, exposure factors appropriate for the x-ray examination can be
automatically selected. Interestingly, the results showed that older children
with larger body thicknesses received lesser doses than younger children. It is
common for radiographers to use higher kVps with reduced mA for older children,
which substantially reduced the patient dose.
The ESAKs recorded in this study following changes in tube
potential were comparable with other studies [10,12,13]. A survey of ESAKs in
adult chest radiography in Malaysia found wide variation in doses . The mean
dose for an average adult was around 0.28 mGy. The range of doses recorded in
this survey was close to these values. Paediatric radiology cannot be compared
with adult radiography due to the differences in anatomy, selection of exposure
factors and co-operativeness of patients during radiographic examinations.
Nonetheless if the paediatric doses are similar to adult limits, this indicates
that unsuitable equipment or radiographic techniques are being used, indicative
of unacceptable radiographic practice. This study concentrated on the change in
tube potential while other methods of dose reduction and better optimisation of
radiographic technique should be reviewed to bring the doses to recommended
Evaluation of image quality based on acceptable
visualisation of anatomical features provided a satisfactory way of evaluating
diagnostic acceptability. No marked differences in image quality were found
when assessed by using image quality criteria. The general finding in the
survey indicated that with the use of higher kVps, images of acceptable
subjective value can be obtained with substantial decrease in ESAKs.
However analysis of the specific criteria indicated
variations in the visualisation of the anatomical structures. In the second
phase of the study, wider variations in scores were noted in the visualisation
of the trachea and proximal bronchi (Criterion B). This finding was discussed
with both the assessors and both agreed that the poor visualisation of these structures
was due to lack of film density. They also found the grading scale quite
confusing for this item especially if only one of the two proximal bronchi was
seen or if the trachea was visible without the clear visualisation of the
bronchi. Similarly there was overall reduced visualisation of the mediastinal
structures due to inadequate density. Dark images are usually accompanied by
the lack of reproduction of the peripheral vessels while light images mainly
affect the visualisation of trachea, proximal bronchi, retrocardiac lung and
mediastinum, parameters which are dependent on the optimum tube potential and
mAs . As this study was confined to increase in kVps with reductions in
mAs, it is possible that some patients would have required an increase in mAs
values to provide adequate film densities and better visualisation of these
Densitometric measurements did not show significant
differences between both techniques. Although it would have been expected that
there would have been reduced radiographic contrast during Technique B, the
mean density differences between the 3 areas of density measurements showed no
differences. Taking readings from the 3 areas of the chest posed a problem
especially in some images where the width of the intercostal space was too
small to obtain an accurate reading with the available densitometer. However
this study did indicate that images of acceptable radiographic densities can
achieved with increases in kVp and reduction in mAs.
Subjective evaluation of the images was performed to
identify specific anatomical features. However it would be useful to study
whether the good visualisation of anatomical structures correlates with the
clinical efficacy of the images and whether it improves the confidence of radiologists
and clinicians to reach a confirmative diagnosis.
This study has demonstrated a wide variation in patient
doses and radiographic technique during paediatric chest radiography.
Significant dose reduction and improvement in image quality was achieved
through appropriate increase in tube potential. It is recommended that the kVp
range should follow the recommended values of between 60 and 80 kVp for
children within the 1 to 15 years age range. Assessment of radiographic images
can be evaluated through comparisons with the recommended image criteria
providing a means of continuous monitoring of image quality in the radiology
Variations in ESAK found in this study are unacceptable and
call for better optimisation of radiographic technique. Responses from the
various departments in the country indicated that the current radiographic
technique in children is far from optimal and highlights the potential for
substantial dose reductions. Further studies will be undertaken to compare inter-departmental
variations and identify centres which require a change in their existing
radiographic practice. Interestingly, the present survey has helped to
influence the formulation of new exposure factors consistent with recommended
values of tube potential for paediatric patients in the department where the
study was done.
This study, being the first of its kind in Malaysia, will
serve to educate radiographers and radiologists so that patient doses can be
reduced substantially and while simultaneously maintaining image quality with
no additional cost implication. The findings of this study will be highlighted
to those involved in medical imaging and radiography of children so as to
formulate and ensure the effective implementation of regulations defining
acceptable standards of good radiographic practice during paediatric
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| Received 3 February 2006; received in revised form 7 June 2006; accepted 19 June 2006
Correspondence: Department of Biomedical Imaging
(Radiology), University of Malaya Medical Centre, 59100
Kuala Lumpur, Malaysia. Tel.: +603-79502069; Fax.: +603-79581973;
Please cite as: Ramanaidu S, Sta Maria RB, Ng KH, George J, Kumar G,
Evaluation of radiation dose and image quality following changes to tube potential (kVp) in conventional paediatric chest radiography, Biomed Imaging Interv J 2006; 2(3):e35
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