Compare The Grid And Air Gap Techniques Information Technology Essay

The research design for this study is of a comparative quantitative, quasi-experimental nature. The rationale for this comparative study is to see what effect two different anti-scatter techniques have on image quality. The properties that make this a quasi-experimental study are manipulation of specific variables and control in experimentation and testing (Parahoo, 2006). True experimental research is characterised by three properties: manipulation, control and randomisation (Parahoo, 2006). As the researcher will not randomise any variables, this study is considered as quasi-experimental and not a true experimental study.

In this research manipulation was achieved since a standard anti-scatter technique was compared to an air gap technique adapted for the projection of the hip in the lateral position. Exposure factor variables were also manipulated using an automatic exposure control (AEC). Control was maintained by testing the two techniques under the same conditions using the same research tools. Furthermore, the researcher kept control of the study by testing only the anti-scatter techniques on one specific projection. Therefore the results achieved by this study are specific to the lateral hip projection. However, the principle could be applied to other projections in order to discover which technique works better in providing good image quality in that specific projection.

3.3 Research Method

3.3.1 Background of the study

The following methodology was adapted from a study carried out in the United Kingdom by Goulding (2006) who looked at the air gap and the grid technique used to image the hip laterally in the University Hospital she trained in. The study was conducted with the help of reporting radiographers in the Accident and Emergency (A&E) department where they performed both grid and air gap technique as routine projections on patients. Goulding (2006) looked at image quality by attaining the hip radiographs performed with both anti scatter techniques separately. Goulding (2006) collected her data by asking reporting radiographers to comment on these radiographs. In Goulding’s (2006) study the radiographs on which she based her findings and results were conducted on patients of different size and this may have lacked reliability due to different exposure factors used for each examination, different patient dose depending on patient size as well as image quality.

Using a similar methodology in this study the researcher assessed image quality using a quality control phantom and an anthropomorphic phantom. In doing so the researcher will made sure that tests done on both anti-scatter techniques to assess for image quality were more precise. The methodology for this research and the tools used to measure image quality in both grid and air gap technique are explained in the following sub-sections.

3.3.2 Research tools

In this study the tools discussed in this section were used to gather the data. They were used to test the anti-scatter techniques being compared and investigated in this study which will be explained further on in this chapter.

Since this research looks at image quality in two anti-scatter techniques, a lead quality control phantom (PTW Normi 13) was a very important tool used to collect the data. According to Carlton & Adler (2006), spatial resolution and contrast resolution are the most important properties upon which devices and techniques can be tested. The lead quality control phantom (Appendix B) is designed to perform constancy and acceptance tests on plain digital x-ray systems and is able to test image receptors for their homogeneity, spatial resolution and contrast resolution (PTW-Freiburg, 2005). However, in this research, spatial resolution and contrast resolution were the two relevant key tests for image quality. Spatial resolution is measured by counting the largest amount of line pairs per millimetre (Lp/mm) while contrast resolution is measured by the low contrast steps seen on the resultant image. The areas on the phantom that are used to measure spatial and contrast resolution are shown in Appendix B.

In collecting the data, the researcher made use of an anthropomorphic pixy phantom AR10A (Appendix B) to image the hip laterally using a horizontal beam. This phantom was used so that the exposures of both grid and air gap technique performed on the quality control phantom could be done to image a hip that resembles that of a human. As the anthropomorphic phantom used had the same attenuation coefficient of a human body, it stops the radiation passing through it in the same way that a human body would.

Although this study evaluates image quality in two anti-scatter techniques, the radiation given to the subject/object at each exposure using the air gap and grid technique was also recorded and compared.  The amount of radiation exposed by the tube at each exposure was also measured using a dose area product (DAP) metre. This was important in order to see how much radiation was being used at each exposure to produce an image using the grid and air gap technique.

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All the exposures (in this experimental testing) were made using an automatic exposure control (AEC) which is incorporated in the erect bucky in the digital x-ray system used. This device determined how much mAs was used in each exposure so that the right amount of x-ray photons irradiated the image receptor to produce an image with adequate quality. This device was used since the mAs that is used in an exposure determines how good the image quality is as well as the patient dose. Therefore when the readings using the tools mentioned were gathered from all exposures, the researcher could compare these results and identify the ideal technique and exposure that should be used in imaging the hip laterally. This technique and exposure should ideally produce a good quality image with as low a dose as possible.

3.3.3 Method

The following two subsections will explain in detail how the data was collected during the experimentation on the anti scatter techniques. The researcher made sure that the tools used in the testing were kept the same to test both techniques. The same digital x-ray system was also used throughout the entire experimentation.

3.3.3.1 The Grid Technique

Testing for this technique was divided in two stages. In the first stage the researcher made use of the quality control phantom (PTW Normi 13). The phantom was placed on a custom made table in contact with the erect imaging receptor. A stationary parallel grid was placed between the phantom and the receptor since this is the type of grid used in a lateral hip shoot through projection. In this technique, the object to image distance (OID) was that of 0cm since the phantom was in contact with the grid and image receptor. The source to image distance (SID) used was that of one metre (100 cm) since this is the standard SID used in such a projection in the radiology department of the local hospital. The kV used was kept constant at 75 kV and the phantom was centred to the central AEC. The light beam diaphragm was set around the contours of the quality control phantom. A further exposure was made using the same grid technique setting. However, this time the grid was removed. This was done in order to find out whether the grid was working effectively in absorbing scatter radiation, which in turn could affect image quality. The DAP metre was recorded so that the researcher could have an approximate idea of the dose given to the phantom.

The second stage in testing the grid technique was done by using the anthropomorphic phantom. The researcher set up the pixy phantom AR10A with the hip in contact with the grid and receptor. The hip was centred with the central AEC and exposed. The kV and the SID were the same as the ones used in testing the quality control phantom 75kV and 100cm SID. The set-ups used to test the grid techniques using both phantoms can be found in Appendix B.

3.3.3.2 The Air Gap Technique

To test for the air gap technique the researcher also divided the tests into two stages. The same quality control phantom used previously in the grid technique was also utilised in this test/experiment. The PTW Normi 13 was placed on a custom-made table. However, in this technique, an air gap between the phantom and the image receptor was applied. There were a total of six air gaps applied, varying from 10cm to 60cm. This was done in order to see which air gap was more effective in reducing scatter radiation reaching the receptor. To achieve this aim the object to image distance (OID) was increased by 10 cm after each exposure to a maximum of 60 cm. The source to object distance (SOD) was kept at 100 cm to reduce object magnification as much as possible since this may create a loss in image sharpness. The source to image distance (SID) depended on what OID was used. Therefore when an OID of 20cm was applied, the SID was that of 120cm. This was done to ensure that the distance of the source to the object remained at 100cm. In each exposure the phantom was centred to the central AEC and the light beam diaphragm was set around the contours of the quality control phantom. The researcher also made use of the DAP metre to see which air gap produced a good quality image with a reasonably low dose. This was done so that the air gap exposures could be compared with the standard grid technique.

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In the second stage of testing for the air gap technique the researcher also used the same anthropomorphic phantom. The setting of the technique to image the hip laterally was adapted from Goulding’s (2006) study by using the same patient positioning that the author used in her study. This setting involved applying an air gap between the phantom’s hip and the receptor, keeping the SOD at 100cm. A total of six exposures were also performed on the pixy phantom AR10A with the same OIDs and SIDs used to image the quality control phantom. The researcher made sure that the phantom’s hip was centred with the central AEC of the erect image receptor. Both settings used to perform testing on the air gap technique can be found in Appendix B.

3.4 Data Collection

The data was collected during February 2010. The data record sheets used to record the data can be found in Appendix A.

·         Exposure Factors

The exposure factors used to produce the images in the grid and air gap technique were recorded. The kV was a constant factor while the mAs changed according to the technique used and its setting. The mAs was manipulated by means of the AED. This was done so that the amount of x-ray photons needed to produce the image and the length of the exposure was recorded depending on the technique used.

·         Object to Image Distance (OID)

The OID used in testing the grid and air gap technique was recorded. This was important, particularly in the application of the air gap technique. This is because the OID in the air gap technique determined the magnitude of the air gap that should be used to achieve a good quality image while keeping the radiation dose as low as possible. Therefore the researcher could see and analyse the effect on the image quality each time a specific OID was used in relation to exposure factors. In the air gap technique the SID depended on what OID was used. The researcher kept the SOD at 100cm to reduce as much as possible magnification of the resultant image.

·         Dose Area Product (DAP)

The DAP metre was recorded at each exposure for both grid and air gap techniques. Although this metre does not measure the radiation dose given to the phantoms at each exposure, it gives an indication of whether the dose would be low or high. A high DAP reading would mean that more radiation was used in the exposure and therefore the resultant patient dose may be higher. The readings from this metre for both techniques were compared in relation to image quality of the radiographs.

·         Signal to noise ratio (SNR)

The signal to noise ratio (SNR) consists of the un-attenuated photons that have penetrated the subject without interaction (signal) and the Compton scatter and other factors that degrade image quality (noise). The SNR was used to determine how much contrast resolution an image had after each exposure. The higher the SNR the better the contrast resolution of an image (Dendy & Heaton, 2006). However a high SNR also means high mAs and consequently a high patient dose. The SNR was calculated by dividing the mean pixel value by the standard deviation of the signal of each exposure. The mean pixel value and standard deviation of the signal were recorded after each exposure provided by the digital x-ray system. Therefore the equation used was:

Signal to Noise ratio = mean pixel value/standard deviation (reference)

·         Spatial Resolution and Contrast Resolution

The spatial and contrast resolution readings were recorded by the researcher from the radiographs achieved using the quality control phantom in the grid and air gap technique. The line pairs per millimetre (Lp/mm) were measured to test for spatial resolution, while for contrast resolution the low contrast steps were counted. The data recorded was tabulated in tables 2a and 2b respectively in the data record sheet. This recorded data enabled the researcher to compare the image quality in both techniques.

Unlike Goulding (2006) in this study two independent radiologists that were chosen randomly from the researcher were asked to report on image quality on all the radiographs performed on the anthropomorphic pixy phantom AR10A. Radiologists were chosen in this study since in Malta there are no reporting radiographers that report on the appendicular skeleton. The radiologists were asked to report on the images by answering a likert scale (1=very poor and 5=very good) to assess image quality. The results were tabulated in table 3 of the data record sheet.

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3.5 Validity and Reliability

Validity refers to the degree the research instrument used in the study measures what it is intended to measure. Therefore:

“Validity reflects the accuracy with which the findings reflect the phenomenon being studied” (Parahoo, 2006, p.80)

In this study, the researcher consulted with the medical physicist at the local hospital who was asked to assess the content validity of the research tools used to collect the data. The medical physicist considered the research tools valid since the same tools are used in the medical imaging department to test for image quality on the digital x-ray systems. As the research tools were deemed to be totally valid, the data collected to measure image quality in the grid and air gap technique can also be said to be valid.

Reliability refers to how consistent an instrument is in measuring what it is intended to measure (Parahoo, 2006). To maintain the equivalence reliability of the lead quality control phantom used for assessment of image quality, two independent observers were asked to measure both spatial and contrast resolution of the two images achieved using the same exposure factors, OID and SID. The researcher tested for the reliability of the automatic exposure device used. This was done by exposing the lead phantom twice without manipulating the setting or exposure factors and the results were recorded. The spatial resolution, contrast resolution and DAP metre readings were the same in both images and so the AEC was considered reliable enough to use in the testing and data collection.

3.6 Ethical Considerations

Ethics is defined by Polit & Beck (2006) as a system of moral values that are designed to protect the participant from the research procedures as the researcher has professional, legal and social obligations towards the participants involved in the study. However, in this research, no human subjects were involved in the experimentation and collection of data, so there were no ethical issues regarding the exposures done on the PTW NORMI 13 phantom and the anthropomorphic phantom pixy AR10A. Permission was sought for the use of the x-ray equipment from Medical Imaging Department at the local hospital. Experimentation was performed under supervision and precautions were taken to ensure that radiation would not harm any other members of the staff or public where the study was performed.

3.7 Limitations of the study

Limitations were encountered by the researcher throughout this study. The study was conducted using a quality control phantom and an anthropomorphic phantom. Although both phantoms are manufactured to mimic and represent a patient as well as to produce equivalent scatter radiation, patient size was a variable that could not be added to the study. The DAP metre was used in this study so that the researcher could have an idea of the dose being attenuated by the phantoms used. Ideally the actual patient dose should be measured but this could not be done since no human subjects were used. Expansion of this study would lead to a better understanding of the dose given to patients while comparing the air gap and grid technique for the lateral hip shoot through.

This study was carried out using a digital x-ray system in the radiology department at the local hospital. Tube output and technique setup may be different when using other systems. In the radiology department, computed radiography is used to perform a lateral hip shoot through examination rather than a digital system which is what the researcher used in this study. In data analysis the readings from the quality control phantoms were interpreted by the researcher himself and not by a number of people. If more than one person interpreted the results, the results may have varied. Although these limitations are valid, they had no effect on the data collected and the results achieved.

3.8 Conclusion

This chapter described the methodology and the research design of this study. The next chapter consists of presentation, analysis and discussion of the data.

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