Effect of automatic kV selection on dose and ... - BIR Publications

T he British Journal of Radiology, 70 (1997 ), 1036–1042 © 1997 The British Institute of Radiology

Effect of automatic kV selection on dose and contrast for a mammographic X-ray system 1K C YOUNG, PhD, 1M L RAMSDALE, MSc, 1A RUST, MSc and 2J COOKE, MB BS, MRCP, FRCR 1National Co-ordinating Centre for the Physics of Mammography, Department of Medical Physics, Royal Surrey County Hospital, Guildford GU2 5XX, and 2Jarvis Breast Screening Centre, Stoughton Road, Guildford GU1 1LJ, UK Abstract. The e
ect of automatic tube potential ( kV) selection on breast dose and contrast has been assessed using a Philips MammoDiagnost 3000 mammography X-ray set. The performance of the X-ray set using automatic kV selection has been compared with that found using a fixed kV of 28. The AUTOKV mode selected 25 kV for breasts with thickness up to about 50 mm, which increased the contrast by 5–10%, and increased the mean glandular dose (MGD) per film by, on average, 30–40%. For large breasts with a compressed thickness of 70 mm and above, kVs up to 30 were selected so that the average MGD per film was reduced by 19% from 3.62 to 2.94 mGy, with an estimated loss in contrast of about 4–8%. For all breasts the mean MGD per film was 1.85±0.05 mGy where AUTOKV was used, and 1.74±0.08 mGy per film when 28 kV was used. The overall image quality of the mammograms was found to be higher when AUTOKV was used. Overall, the AUTOKV facility on this X-ray set generally worked well and resulted in slightly higher contrast and slightly better image quality at the price of a small increase in the average dose for this patient group when compared with the usual UK procedure of using a fixed 28 kV.

Introduction Each year up to one and a half million women undergo mammographic examinations in the National Health Service Breast Screening Programme (NHSBSP). It is particularly important in the screening of predominantly healthy women, that the use of radiation is optimized and fully justified. Therefore, the need to obtain high quality images to maximize cancer detection has to be balanced against the radiation dose to the breast and the consequent risk of cancer induction. One of the main factors a
ecting both image quality and radiation dose is the X-ray beam quality which is primarily determined by the X-ray tube anode target material, X-ray beam filter and applied tube potential ( kV). In recent years the conventional procedure in the UK has been to use a molybdenum anode X-ray tube, molybdenum filter and a kilovoltage of 28 kV (28 kV Mo/Mo). However, it is well known from theoretical considerations that the optimal energy is di
erent for breasts of di
erent size and composition [1, 2] and therefore that 28 kV Mo/Mo may not provide the optimal beam quality for all breasts [3, 4]. Although the ideal technique would be to use a kV/target/filter combination appropriate for each individual breast, it has not been practical to do this manually with the standard mammographic X-ray set. In some centres a higher kV is used Received 21 February 1997 and in revised form 30 May 1997, accepted 9 June 1997. 1036

for large or dense breasts, especially where long exposure times may be a problem. However, many X-ray manufacturers are now supplying mammographic X-ray systems with the ability to select automatically a beam quality tailored to each patient with the potential for reducing dose and increasing contrast for di
erent breasts. Some manufacturers have developed mammography X-ray sets which automatically select kV, anode material and filtration and a previous paper has evaluated such a system [5]. Other machines automatically select the kV with no change in anode or filter and the objective of this paper is to evaluate the e
ect of automatic kV selection with reference to one such system, the Philips MammoDiagnost 3000. Radiation dose and contrast for this system have been investigated for di
erent breast thicknesses and compared with those found using conventional techniques. The methodology used in this investigation is similar to that employed previously [5 ]. The two main methods recognized in the UK for the assessment of patient dose in mammography have been used [6 ]. In the first method, dose is calculated for a standard breast model which comprises a central region with a 50550 mixture by weight of adipose and glandular tissue and a superficial region of adipose tissue 5 mm thick. In the second method the mean glandular doses (MGDs) for actual breasts were estimated by using the postexposure mAs to estimate incident air kerma and using the conversion factors in IPSM Report 59/2 T he British Journal of Radiology, October 1997

E
ect of automatic kV selection in mammography

[ 6]. In both methods it is assumed that breast composition is a 50550 mixture of adipose and glandular tissue. Some assumption about composition is necessary as one does not know the actual composition of the breasts. Although this may introduce an error for the dose estimates for individual women, it should not greatly a
ect the dose comparisons between the di
erent beam qualities as these are compared using the same assumptions and averaged over a number of breasts. The major e
ect on overall image quality of changing beam quality is to alter the radiographic contrast. In this study, radiographic contrast has been assessed using plastic materials to simulate breast tissue. It has previously been shown that higher contrast, measured in this way, correlates with higher image quality scores as measured with a mammographic test object [ 5]. However, the significance of radiographic contrast in a mammogram may be di
erent from that seen with a test object and excess overall contrast could lead to a loss in latitude which might adversely a
ect clinical images. Therefore, in this investigation image quality was also judged subjectively by an experienced radiologist who assessed a series of mammograms produced using conventional and automatic kV selection.

Materials and methods Equipment The Philips MammoDiagnost 3000 is fitted with a molybdenum anode X-ray tube and with molybdenum and rhodium filters. The system has a number of di
erent modes of operation. The AUTOTIME mode allows the user to select the kV manually and the mAs is controlled automatically by the AEC (the traditional mode). There is also an AUTOKV mode in which the system selects both the kV and mAs for each exposure. In AUTOKV mode the system starts the exposure at 25 kV and selects a higher kV (up to 30 in 1 kV increments) if it estimates that a predetermined mAs will be exceeded. In e
ect, the system increases kV in an attempt to limit exposure times to less than a pre-set time. The pre-set time may be selected by the user as 1.0, 1.5 or 2.0 s corresponding to mAs values of 110, 165 and 220, as the unit operates at a fixed 110 mA across the range of available kV. That is, AUTOKV 165 attempts to limit exposure times to less than 1.5 s while AUTOKV 220 limits exposure times to less than 2.0 s. In this study the two longer pre-set times were used at di
erent times during the evaluation of the equipment. The back-up timer limited exposures to 4.0 s when the bucky was used for standard views. The molybdenum or rhodium filter can only be selected manually. For all procedures Fuji T he British Journal of Radiology, October 1997

UM-MA film was used with Fuji UM-MA fine screens in Fuji EC-MA carbon fibre cassettes. Films were processed using Photosol chemicals in a Fuji FPM 3000 processor with a developer temperature of 33.5 °C and an extended processing cycle of 210 s duration (dry-to-dry).

Dose and image quality for clinical mammograms The MGD was assessed for a series of patients undergoing mammographic examination with the Philips MammoDiagnost 3000 being utilized during assessment clinics in a screening centre. The post-exposure mAs, breast thickness, kV, filter and compression force were recorded for 227 mammograms using the conventional mode with a fixed kV of 28, and similarly for another series of 277 mammograms using the AUTOKV mode. In the beginning of the study AUTOKV 220 was used, but later the system was adjusted to AUTOKV 165 to assess the e
ect of changing the setting. (In the results section some of the data are separated for the di
erent AUTOKV 165 and AUTOKV 220 modes. Where appropriate the data for the two modes have been pooled since the di
erences were too small or the numbers too low to be significant and the combined data referred to as AUTOKV.) The molybdenum filter was used throughout for all modes. The breast thickness indicated by the X-ray set was corrected for flexing of the compression plate by calculating a load dependent correction factor. Incident air kerma were estimated from the post-exposure mAs and measurements of output per mAs for each kV setting and corrected for individual breast thickness and attenuation by the compression plate. The MGD for each breast was calculated from the incident air kerma using the conversion factors in IPSM Report 59/2 [6 ] which are based on the simplifying assumption that the breasts have the same composition as the standard breast. During the period of assessment the optical density for radiographs of a 40 mm Perspex block was checked daily to ensure that it was within the target range 1.6±0.15. To compare patient doses between the di
erent modes of operation a normalized incident air kerma was used to calculate a normalized MGD for each mammogram. This normalized incident air kerma was that calculated for a gross film density of 1.60 for the 40 mm Perspex block with the radiographic speed equal to the average during the test period. Thus corrections were made for the variations in density due to changes in AEC performance and the speed of the processed film. The same matched batch of cassettes was used throughout the evaluation. The overall quality of 278 mammographic films produced using the Philips MammoDiagnost 3000 1037

K C Young, M L Ramsdale, A Rust and J Cooke

were assessed by an experienced radiologist. 145 of the films were produced using AUTOKV mode and 133 produced using AUTOTIME and a manually selected kV of 28. The films were read blind (i.e. without knowledge of whether manually selected 28 kV or AUTOKV was employed) and overall image quality assessed subjectively with each film ranked as one of five categories: inadequate, poor, satisfactory, good or excellent.

breasts the MGD and contrast were measured using the above method but with a stack of Perspex instead of the tissue equivalent material. Perspex was used here to allow comparison with a previous publication [5 ].

Mean glandular dose and contrast measurement for a standard breast model

The distribution of compressed breast thickness in the study population is shown in Figure 1. For mammograms using the AUTOTIME mode the thickness ranged from 20 to 81 mm and the mean was 53.1±0.8 mm. (Errors in this paper are quoted throughout as ±1 standard error of the mean.) For mammograms recorded using the AUTOKV modes the thickness ranged from 20 to 90 mm and the mean was 52.2±0.8 mm. The kV selected by the two separate AUTOKV modes during patient exposures are summarised in Table 1. The mean kV selected in the two AUTOKV modes for di
erent breast thicknesses are shown in Figure 2. The mean normalized MGD (i.e. after correction to a density of 1.6) was 1.74±0.08 mGy when 28 kV was used in AUTOTIME mode, and 1.85±0.05 mGy where AUTOKV was used. The normalized MGDs for fixed 28 kV and AUTOKV selection are compared for di
erent breast thicknesses in Figure 3. These di
erences are also analysed in Table 2 which shows the e
ect on kV and

To measure contrast a test object comprising a 10 mm diameter disc of 4.0 mm thick Perspex was placed on top of a stack of tissue equivalent material (CIRS, Norfolk, Virginia) simulating the standard breast model, i.e. with a 50% glandular content and a 5 mm thick adipose surface layer. The Perspex disc was positioned on the midline and centred at 60 mm from the chest wall edge of the film to avoid the automatic exposure control (AEC) detector. The stack was radiographed using both conventional technique and AUTOKV selection. The stack height was varied from 20 to 85 mm. Contrast was measured as the di
erence in optical density between the image of the Perspex disc and the adjacent background. The errors for contrast are quoted as ±0.01 and were largely determined by the precision of the densitometer. The method described in IPSM Report 59/2 [ 6] for the assessment of dose to the standard breast was followed except that tissue equivalent material was used to simulate the standard breast instead of Perspex. The blocks of tissue equivalent material had thicknesses of 5, 10 and 20 mm, each with an accuracy of better than ±0.05 mm. The incident air kerma (K) required to achieve an optical density of 1.60 (including base and fog) was determined for each thickness tested. The half value layer (HVL ) for each beam quality was measured as described in the same report [6]. The MGD for the di
erent thickness of the standard breast model was calculated from K and HVL using the table provided in the IPSM Report [6 ]. Variation of entrance air kerma and contrast with optical density was also measured and used to normalize the measurements to that expected where the background density was 1.60 including base and fog. This was the target optical density in use at the screening centre and corresponds to the mid-point of the recommended optical density range of 1.40 to 1.80 in the UK NHSBSP [7].

Mean glandular dose and contrast using di
erent kV and filter combinations for simulated large breasts In order to assess the e
ect of di
erent kV and filter combinations on MGD and contrast for large 1038

Results Dose and image quality for clinical mammograms

Figure 1. Compressed breast thickness for the study population. Table 1. Number of exposures and kV selected for various AEC modes for non-magnified films kV

AutokV 165

AutokV 220

Autotime

25 26 27 28 29 30 Total

62 14 9 5 2 1 93

142 14 13 13 0 1 183

215 215

T he British Journal of Radiology, October 1997

E
ect of automatic kV selection in mammography

Figure 4. Exposure times for standard films. Figure 2. Mean kV selected for di
erent thicknesses of breast using AUTOKV mode. (Error bars indicate 1 standard error of the mean.)

Figure 5. Proportion of mammograms that were judged to have various levels of overall image quality.

Figure 3. Comparison of average mean glandular dose (corrected for AEC and processor performance) using fixed 28 kV and AUTOKV modes for di
erent breast thicknesses.

dose of AUTOKV selection for breasts up to 50 mm, and for 70 mm and above. Since a kV of 25 was selected in AUTOKV mode for breasts of thickness up to 50 mm a 37% increase in dose was found in comparison to using 28 kV. Conversely for large breasts with compressed thickness of 70 mm and above the AUTOKV mode demonstrated a 19% dose reduction by choosing a kV up to 30. The average exposure time was longer at 1.04±0.03 s when using AUTOKV, compared with 0.73±0.04 s when using a fixed 28 kV setting. However, the distribution of exposure times for

each of the three modes used shown in Figure 4 illustrates that the longest exposure times also occurred when using a set 28 kV. The proportions of films assigned to each category of overall image quality are shown in Figure 5. In total, 10% (27/278) of the films were judged of poor or worse overall image quality with only two films judged inadequate. When a fixed 28 kV was used 14% of the films were judged of poor overall image quality compared with only 4% when AUTOKV was used. The di
erence in image quality for the two modes was significant at the p