The impact of SID and collimation on backscatter in radiography

Transcription

1 The impact of SID and collimation on backscatter in radiography Poster No.: C-2931 Congress: ECR 2010 Type: Scientific Exhibit Topic: Physics in Radiology Authors: M. Joyce 1, J. T. Ryan 2, M. F. Mc Entee 1, P. C. Brennan 2 ; 1 Dublin/ IE, 2 Lidcombe/AU Keywords: Source to image distance, Collimation, Attenuator thickness DOI: /ecr2010/C-2931 Any information contained in this pdf file is automatically generated from digital material submitted to EPOS by third parties in the form of scientific presentations. References to any names, marks, products, or services of third parties or hypertext links to thirdparty sites or information are provided solely as a convenience to you and do not in any way constitute or imply ECR's endorsement, sponsorship or recommendation of the third party, information, product or service. ECR is not responsible for the content of these pages and does not make any representations regarding the content or accuracy of material in this file. As per copyright regulations, any unauthorised use of the material or parts thereof as well as commercial reproduction or multiple distribution by any traditional or electronically based reproduction/publication method ist strictly prohibited. You agree to defend, indemnify, and hold ECR harmless from and against any and all claims, damages, costs, and expenses, including attorneys' fees, arising from or related to your use of these pages. Please note: Links to movies, ppt slideshows and any other multimedia files are not available in the pdf version of presentations. Page 1 of 18

2 Purpose INTRODUCTION: When considering optimisation of medical exposures, it is crucial that the radiation dose to the patient is kept as low as reasonably achievable (ALARA) whilst ensuring that image quality is sufficient for diagnostic purposes (1-3). To continue to reduce dose it is essential to regularly investigate optimisation strategies and develop an evidence based radiographic practice (4). One such optimisation technique which is commonly reported yet poorly implemented in the clinical environment is increasing the source to imagereceptor distance (SID) for X-ray examinations. Increasing the SID has the advantage of being low cost, maintaining patient comfort and due to improvements in the geometric properties should not result in a degradation of image sharpness. However, whilst the concepts of increasing SIDs, and the associated inverse square law, are well recognised by most radiologists and radiographers, there is disagreement between authors as to its level of effectiveness. Previous studies investigating the effect of increasing SID for specific radiographic examinations have noted substantial reductions in effective dose ranging from 33% to 44% and reductions of between 13.4% and 70% in entrance surface dose (ESD) in both clinical and phantom based studies (5-8). Poletti and McLean (9) employed the Monte Carlo simulation computer package PCXMC (10,11) as a means of determining the effective doses for various projections when both the collimation and SID were altered. The study showed that increasing the distance from the image receptor to anywhere up to 200cm or so is the most effective use of altering the SID and also argues that it is best radiographic practice to collimate to the region of interest as opposed to collimation to the entrance surface or receptor plate. However, in the clinical setting this practice would be challenged as collimation is dependent upon body habitus, patient anatomy and size. The backscatter factor (BSF) is known to be closely related to entrance surface dose and is generally regarded as the ratio between a dose quantity measured at the outer surface of an object along the central axis of the primary X-ray beam and the equivalent dose quantity at the same position free in air (12). The use of lead shielding in this study was to mimic the free in air position so that the two dose quantities could be measured simultaneously from one exposure. Page 2 of 18

3 The current work was devised to investigate the mechanisms through which SID reduces dose by using a homogenous attenuator methodology to remove anatomical attenuation, heterogeneous scatter and anatomical variations from the equation. It aims to examine what effect increasing the SID has on the ratio between the dose at the image-receptor and the dose in front of the X-ray entrance point for a homogeneous attenuator and examine the combined effect of this with collimation. As the disparity in patient size is an important factor in X-ray attenuation, a range of attenuator thicknesses will be examined to simulate a variety of patient sizes and different anatomical structures. By eliminating the variable nature of heterogeneous attenuation it is anticipated that the mechanism behind SID optimisation will become more apparent and provide a platform for further clinical and phantom based studies. MAIN AIMS: To investigate the relationship between increasing SID and the backscatter factor To investigate what impact altering the collimation field size has on this relationship Methods and Materials This study assessed the optimisation capabilities of three SIDs: 100cm; 130cm and 150cm respectively. All exposures were performed using a direct digital radiography GE MAXIRay 100 tube assembly at 90kVp with a focal spot size of 0.6mm 2, total filtration of 3.5mm of aluminium (Al) equivalent thickness and an X-ray tube target angle of 17. For each SID the mas was manually set to maintain a constant X-ray tube potential at the image-receptor by applying an adjustment factor (AF) to the mas of the original SID (8). Estimated and corresponding experimental values can be seen in Table 1. SID AF Estimated mas Experimental mas 100cm Page 3 of 18

4 130cm cm Table 1: Estimated and experimental mas values for each SID The homogeneous phantom used was composed of 1cm thick poly methyl meth-acrylate (PMMA) segments (60cm x 50cm x 1cm) with each segment aligned perpendicular to the X-ray beam axis. The attenuator thicknesses ranged from 0cm to 30cm in 10cm increments to simulate a variety of patient sizes and different anatomical structures. Dosimetry measurements were acquired using Harshaw lithium fluoride thermoluminescent dosimeter chips (LiF TLD-100), 0.3cm x 0.3cm x 0.1cm, and a total of 304 TLD measurements were recorded over the 3 SID settings. TLDs were placed inside polyethylene radiolucent sachets and positioned at the image-receptor and at a distance 30cm away along the central axis of the primary X-ray beam. Two sachets were placed at the 30cm position, one with lead shielding (3.2cm x 2cm x 0.2cm) and one without. An illustration of the experimental design can be seen in Fig. 1 and 2. Fig.: Schematic of the experimental protocol illustrating the position of TLDs in relation to the image-receptor and attenuator thickness, which in this case is 15cm [x= dose at image receptor, y = dose at 30cm position (both with and without shielding)] References: M. Joyce; Diagnostic Imaging, University College Dublin, Dublin, IRELAND Page 4 of 18

5 As field size is a known contributor to backscatter, two of the most common collimation strategies were also considered; collimating to the outer surface and collimating to the image-receptor (9). Dose measurements were acquired for each of the increasing attenuator thicknesses (0cm, 10cm, 20cm and 30cm) at SIDs of 100cm, 130cm and 150cm respectively using field sizes corresponding to the 2 collimation strategies. All TLDs were read within 24 hours of exposure using a Harshaw model 5500 automatic reader (Fig. 3). All dose measurements acquired throughout the experimental process were assessed using a two-tailed paired student t test and a probability level of p<0.05 was considered significant for each test. Images for this section: Page 5 of 18

6 Fig. 1: Schematic of the experimental protocol illustrating the position of TLDs in relation to the image-receptor and attenuator thickness, which in this case is 15cm [x= dose at image receptor, y = dose at 30cm position (both with and without shielding)] Page 6 of 18

7 Fig. 2: Experimental set-up with 30cm of PMMA attenuation in place. TLDs were positions on the front surface of the attenuation (ie. 30cm) and on the image-receptor. Fig. 3: Harshaw model 5500 Automatic TLD reader operated using nitrogen gas Page 7 of 18

8 Results For the outer surface collimation strategy the results demonstrate a significant difference for each of the SIDs tested (p<0.001) with a trend of decreasing backscatter-factor with increasing SID. As illustrated in Fig. 1 a backscatter-factor of 1.39 (sd 0.065) was determined at 100cm using the full 30cm PMMA attenuator. This decreases to 1.36 (sd 0.028) and 1.31 (sd 0.040) at 130cm and 150cm respectively. Fig.: Outer Surface collimation strategy: Mean backscatter factors for each SID with increasing attenuator thickness References: M. Joyce; Diagnostic Imaging, University College Dublin, Dublin, IRELAND The inverse trend was observed for collimation to the image-receptor, with the backscatter-factor increasing from 1.35 (sd 0.029) at 100cm to 1.37 (sd 0.029) and 1.43 (0.061) at 130cm and 150cm respectively for the 30cm attenuator, as seen in Fig. 2. Page 8 of 18

9 Fig.: Image-receptor collimation strategy: Mean backscatter factors for each SID with increasing attenuator thickness References: M. Joyce; Diagnostic Imaging, University College Dublin, Dublin, IRELAND The results also show that as attenuator thickness was increased for both collimation strategies a significant increase in the mean BSF was recorded. Images for this section: Page 9 of 18

10 Fig. 1: Outer Surface collimation strategy: Mean backscatter factors for each SID with increasing attenuator thickness Page 10 of 18

11 Fig. 2: Image-receptor collimation strategy: Mean backscatter factors for each SID with increasing attenuator thickness Page 11 of 18

12 Conclusion By eliminating the variable nature of heterogeneous attenuation in this study, the relationship between the dose received at the entrance and exit surfaces of the object was easier to discern. As the backscatter factor is closely related to entrance surface dose and depends on field geometry, X-ray beam collimation is crucial to the level of dose reduction recorded at varying SIDs. In clinical practice collimation for average sized patients to outer surface bony landmarks ensures that the required region of interest is included. However, when using longer SIDs and collimating to the entrance surface descriptors of an object then less tissue is irradiated resulting in the potential cut-off of important anatomical features albeit with a perceived reduction in dose at this distance (see Fig. 1(a)). Conversely, collimating to the image-receptor results in more tissue being in the path of the X-ray beam at the longer SID, irradiating more tissue than is necessary. The results from this study show that when collimation is kept constant at the image-receptor, the backscatter factor increases with increasing SID. Fig. 1(b) illustrates that an increase in irradiated area occurs at the longer SID and as such results in a greater amount of backscatter at that distance. The trend of increasing BSF with increasing SID may have resulted due to this greater amount of backscatter incident on the entrance surface TLDs at the longer SIDs. This study explores the basis of optimisation with increasing SID by investigating the relationship between geometry, field size collimation and beam attenuation. The results show that the method of collimation used is a critical consideration to dose and must be taken into account when reporting dose variations with increasing SID. This study provides a platform for further clinical and phantom based studies investigating SID and patient dose. Images for this section: Page 12 of 18

13 Fig. 1: Schematic of the outer surface and image-receptor collimation strategies where the shaded regions highlight the difference in the amount of tissue irradiated at SIDs of 100cm and 150cm with respect to collimation field size [Adapted from Poletti and McLean, 2004 (13)] Page 13 of 18

14 References [1]. Dosimetry Working Party of the Institute of Physical Sciences in Medicine. National Protocol for patient dose measurements in diagnostic radiology. Chilton: Doc. of the NRPB 1992; 1-31 [2]. European Commission (EC). European guidelines on quality criteria for diagnostic radiographic images. Luxembourg: Office for Official Publications of the European Communities. EUR 16260, 1996 [3]. International Commission of Radiological Protection. Recommendations of the ICRP, Publication 103. Annals of the ICRP 2007; 37(2-4) [4]. International Commission on Radiological Protection. Radiological Protection and Safety in Medicine. ICRP Publication 73. Annals of the ICRP 1996; 26(2), Pergamon Press, Oxford [5]. Brennan PC, Nash M. Increasing FFD: an effective dose-reducing tool for lateral lumber spine investigations. Radiography 1998; 4: [6]. Brennan PC, McDonnell S, O'Leary D. Increasing film-focus distance (FFD) reduces radiation for X-ray examinations. Radiation Protection Dosimetry 2004; 108(3): [7]. Dilger R, Egan I, Hayek R. Effects of focus film distance (FFD) variation on entrance testicular dose in lumbar-pelvic radiography. ACO 1997; 6(1):18-23 [8]. Robinson J, McLean D. Extended focal-film distance technique: an analysis of the factors in dose reduction for the AP knee radiograph. Radiography 2001; 7: [9]. Poletti JL, McLean D. The effect of source to image-receptor distance on effective dose for some common X-ray projections. Br J Radiol 2005; 78: [10]. Tapiovaara M, Lakkisto M, Servomaa A. PCXMC: A PC-based Monte Carlo program for calculating patient doses in medical x-ray examinations. Report STUKA139, 1997 (Helsinki: Finnish Centre for Radiation and Nuclear Safety) Page 14 of 18

15 [11]. Servomaa A, Tapiovaara M. Organ dose calculation in medical X ray examinations by the program PCXMC. Radiation Protection Dosimetry 1998; 80(1-3), [12]. Petoussi-Henss N, Zankl M, Drexler G, Panzer W, Regulla D. Calculation of backscatter factors for diagnostic radiology using Monte Carlo methods. Phys Med Biol 1998; 43: [13]. Poletti J, McLean D. The effect of source to image distance on scattered radiation to the image receptor. Australas Phys Eng Sci Med 2004; 27(4): Personal Information Name: Maria Joyce, BSc (hons), MSc. I am a final year PhD student working as part of the Diagnostic Imaging research team in University College Dublin. My primary area of interest is dosimetry, focussing mainly on the adaptation of the source to image-receptor distance (SID) as an optimisation tool in radiography. My research project seeks to amalgamate my background in physics and biomedical science with the clinical expertise provided to me by the multifaceted UCD team. Contact Details: Address: Diagnostic Imaging, Health Science Centre, University College Dublin, Belfield, Dublin 4, Ireland Page 15 of 18

16 Tel: Fax: Images for this section: Page 16 of 18

17 Fig. 1 Page 17 of 18

18 Page 18 of 18