effect of patient gowns in digital radiograph x-ray ... · effect of patient gowns in digital...

Effect of Patient Gowns in Digital Radiograph X-ray Imaging: An Evaluation of a Linear Artifact and Lung Field Histogram

1Jong-Woong Lee, 2Eun-Soo Kim, 3Myeong Seong Kim, 4Jung Keon Jeon,

5Dae Cheol Kweon 1, First Author Department of Radiology, Kyung Hee University Hospital at Gang-dong, Seoul

134-727, Republic of Korea,[email protected] 2, Department of Electronic Engineering, Kwangwoon University, Seoul 139-701

Republic of Korea, [email protected] 3, Department of Radiology, The Korean National Cancer Center, Goyang 410-769,

Republic of Korea, [email protected] 4, Department of Radiology, Kyung Hee University Hospital at Gang-dong, Seoul 134-727,

Republic of Korea, [email protected] *5 ,Corresponding Author Department of Radiologic Science, Shinhan University, Uijeongbu 480-701,

Republic of Korea, [email protected]

Abstract The objective of this paper was to assess the artifacts in chest digital radiography of patients as

well as those of chest X-ray phantom (Fluke model) and anthropomorphic chest phantom phantom. Through investigative research, it was discovered that patient gowns are suitable when used for film-screen system X-ray examinations, but not for digital radiography system X-ray examinations. This study investigated the types of artifacts fabricated by a patient gown through normal high tube voltage exposure procedures performed in university hospitals, and then analyzed the lung field histogram, which included the artifact. The value of the human lung field histogram varied from ‘985’ to ‘3,351’, which suggests an inclusion of the patient’s gown, as the patient’s gown presented values that ranged between ‘1,595’ and ‘2,013’. Thus, it can be deduced that new materials that contain low radiolucency levels should be used for patient gowns in order to ensure the absence of overlapping pixel values in key areas of diagnoses

Keywords: Digital radiography, Histogram, Dynamic range, Pixel value, Patient gown

1. Introduction

Unlike 3-dimensional diagnostic medical modality imaging such as magnetic resonance imaging and computed tomography, conventional X-ray projection radiography remains a largely superimposed image dependent on the attenuation and composition of the body [1-2]. When an X-ray passes through a material in a straight line, its degree of reduction due to the absorption of radiation scatters and exponentially declines, in accordance with Lambert’s law [3]. In other words, the X-ray beam penetrating the subject forms an image by the attenuation coefficient of the material by radiation and the attenuation difference of the X-ray energy according to the thickness of the materials. In a recent study using data from the department of diagnostic radiation, it was found that a digital radiography (DR) system stores each pixel value to memory by matching the attenuation information of the X-ray passing through the subject to the bit depth of pixels and also by separating the integral value according to time as a number [4]. The wide dynamic range of the DR has the advantage of displaying large differences in physical contrast such as bone and soft issue on one screen [5] when compared to analogs (film/screen type) in routine chest radiography [6-8]. For the 12-bit image typically applied for DR, a 212 gray scale is used. In other words, the image of a wide area is represented through the variation of light and shade in 4,096 steps. Chest digital radiography and digital mammography, both of which use 12-bit images, did not show significant differences in the quality of images created compared to the film/screen type [9-11]. The human eye can distinguish a dynamic range of 25 or 32 steps from white to black. Among the areas invisible to the naked eye, the identifiable part from the entire dynamic range can be visualized by the proper post-processing adjustment of images. This is possible by regulating the window width/level, which is an advantage of digital imaging [12]. There

Effect of Patient Gowns in Digital Radiograph X-ray Imaging: An Evaluation of a Linear Artifact and Lung Field Histogram Jong-Woong Lee, Eun-Soo Kim, Myeong Seong Kim, Jung Keon Jeon, Dae Cheol Kweon

Journal of Convergence Information Technology (JCIT) Volume 10, Number 3, May 2015

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are many different types of patient-related artifacts prevalent in X-rays. These artifacts include hair, gowns or other foreign objects that lay over the chest during examination. Artifacts in the X-ray imaging system are largely generated by image acquisition artifacts and image processing artifacts [13]. The artifact created by the test condition was significantly reduced by the wide dynamic range of the DR system compared to the analog radiography system. However, while foreign body artifacts caused by test gowns rarely occur in analog radiography, they do occur in DR due to its wide dynamic range [Fig. 1].

This study investigated the type of artifact caused by test gowns in the normal exposure of high tube voltage performed at the university hospital and then analyzed the lung field histogram of these artifacts. In addition, this investigative research study conducted a comparative analysis of the histograms according to the thickness of the fabric by designing a fabric step wedge of the same material as that of the test gown and researched artifact reduction methods.

a b

Figure 1. Frontal chest radiography shows an artifact caused by a patient gown. a, Right upper quadrant linear artifact (arrow) caused by patient gown. b, The same artifact (arrow)

viewed in the left upper quadrant region.

2. Experimental Methods

Philips Digital Diagnost VR (Philips, Netherlands), an amorphous silicon flat-panel detector of the indirect method widely used in clinical practice, was used in this study. A detector from the Trixell Company was used with the Digital Diagnost VR equipment, and X-ray conversion was performed using a cesium iodide scintillator. The active panel size measured 43 cm by 43 cm and the pixel matrix measured 3001 by 3001 with 143 μm-sized pixels and a limiting resolution of 3.5 lp/mm. The kV ranged from 40 to 150 kV, the mA from 500 to 1,100 mA and the exposure time from 1 to 4 msec. A chest X-ray phantom (FLUKE Biomedical Model 76-211, USA) was used to reproduce the linear artifact of the test gown. The chest X-ray phantom consisted of four 25 × 25 × 2.54 cm acrylic plates, an air-gap of 5.08 cm, and an aluminum plate sized 25 × 25 × 0.2 cm. The fabric (100% cotton) was designed as a step wedge of five steps 3 cm in width × 5 cm in height × 0.5 mm in thickness [Fig. 2].

Figure 2. Fabric step wedge


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In the real experimental condition, the anthropomorphic chest phantom was used for chest radiography of patients in test gowns, which created artifacts in the right upper quadrant of the chest region [Fig. 3].

a b c

Figure 3. Chest radiography of the artifact used for the Rando phantom. a, Patient gown. b, Patient gown wears of the Rando phantom. c, Chest radiography artifact (arrow)

viewed in the right upper quadrant region.

In order to examine the linear artifact caused by a patient gown at the time of the chest posterior-anterior (PA) test, a retrospective analysis of chest PA tests was conducted in this hospital under the chest PA X-ray exposure conditions described in Table 1.

Table 1. Digital radiography exposure conditions Parameter Value

Tube voltage (kV) 125 Tube current (mAs) 2

Permanent filter 2.5 mmAl Added filter 0.1 mmCu + 1.0 mmAl

X-ray field size 43 × 43 cm Focal spots 2 mm

Anode angle 13° Grid In

Automatic exposure control On

A total 2,038 retrospective analyses of the linear artifacts in chest PAs were acquired from the

university hospital. Computed radiography (CR) systems of the image plate (IP) type were excluded from the survey, and only images obtained via chest PA tests in patients over the age of 18 wearing gowns were included. In order to measure the histogram of a standard chest PA radiography image where the linear artifact was not generated by a contaminated patient gown, a reference and test image was obtained by measuring the pixel intensity (grayscale value) histogram of the lung sections using the free line region of interest (ROI) tool of the DICOM image processing software (ImageJ version 1.43u; National Institutes of Health, Bethesda, MD, USA) [Fig. 4].

Figure 4. Frontal chest radiograph showing the free line ROI of the lung field.


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The histograms elucidated the number of counted pixels distributed on the X-ray image for each level (gray value) from darkest (0) to brightest (3200). The y-axis (count) of the histogram represents the total number of pixels corresponding to the intensity level (x-axis). The fabric step wedge was placed on the surface of the wall detector to reproduce the linear artifact caused by the dynamic range, and test images were acquired by exposure under the experimental conditions listed in Table 1 after raising the FLUKE 76-211 phantom over the wedge [Fig. 5].

a b c

Figure 5. Fabric step wedge (a), phantom FLUKE model 76-211 (b) and rectangular ROI of the fabric step wedge X-ray image (c)

The mean histogram value of the fabric step wedge was measured from the images acquired using

the rectangular ROI of ImageJ [Fig. 5c]. In order to investigate the relationship between the kV and the linear artifacts, the experiment was carried out with the automatic exposure control (AEC; adjust automatic current-exposure time product) mode off in the experimental conditions detailed in Table 1, while reducing the kV per stage to 125, 117, 109, 96 and 90 kV. The change in the histogram within the same area was measured by setting up the ROI in the same area and coordinating using the ROI manager [Fig. 6].

a b c d e f

Figure 6. Rectangular ROI of the fabric step wedge X-ray image by kV change. a: 90 kV, b: 96 kV, c: 102 kV, d: 109 kV, e: 117 kV, f: 125 kV

This study was conducted via a retrospective review of clinical specimens obtained from existing

data and pathology/diagnostic tests such as charts, test results, records, literature and pathological or diagnostic specimens. Such sources have been disclosed, and were not vulnerable to forgery or invasion of privacy. In addition, this study was approved by the institutional review board (IRB) of the Kyung Hee University Hospital. Therefore, the subjects were thoroughly protected and remained anonymous, and the individual results obtained are confidential. 3. Results

Results of linear artifacts caused by patient gowns were confirmed in 11 cases in 2,038 retrospective analyses. The subjects in these cases were six women and five men, and they were all of thin body type by comparison with other subjects. Images were analyzed from the two right upper quadrants (RUQs) and the three left upper quadrants (LUQs) of the chest in men. In women, images were analyzed from the two RUQs and the four LUQs of the chest. In the experimental condition shown in Table 1, the lung field pixel values of the standard chest PA radiography images were measured for patients who did not show linear artifacts due to gowns. The pixel values of the lung field were measured using free line ROI manager of ImageJ program. The minimum value indicated was 985, while the maximum value was 3,351. The mean value was 1,844 and the SD was 377 [Table 2].


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Table 2. Free line ROI property of lung field Size information Color level information

Area (mm2) 17012.40 Min 985 Length (mm) 607.94 Max 3351

X spacing (mm) 0.143 Mean 1844.81 Y spacing (mm) 0.143 Sum 927320

Number of points 33 SD 377.90

The fabric step wedge was placed on the surface of the wall detector to reproduce linear artifacts

due to the dynamic range, a FLUKE Model 76-211 chest phantom was placed in front of the wedge and then the pixel values were measured using ImageJ after exposing it, as shown in Table 3. The pixel value of the rectangle ROI indicated that the minimum value was 1,595, the maximum value was 2,013, the mean value was 1,788 and the SD was 57. The pixel values were narrowly distributed in comparison with the histogram of the standard chest PA radiography. In the results of the comparison of the pixel values of the two histograms, it was confirmed that the pixel value of the fabric step wedge was entirely contained within the lung field pixel value of the standard chest PA radiography.

Table 3. Rectangle ROI property of fabric step wedge Size information Color level information

Area (mm2) 2890.20 Min 1595

Length (mm) 216.79 Max 2013

X spacing (mm) 0.143 Mean 1788.33

Y spacing (mm) 0.143 Sum 252757760

SD 57.28

In order to investigate the relationship between the kV and the linear artifacts, the tube voltage was

increased per stage with the AEC mode off under the experimental conditions shown in Table 1, and the histogram of the fabric step wedge according to the kV was increased per stage and measured using the rectangular ROI of ImageJ [Table 4].

Table 4. A comparison ROI property of tube voltage change Tube voltage (kV) Sum Min Max Mean SD

90 140400 1497 2060 1753.47 73.08 96 140400 1505 2061 1750.85 70.38

102 140400 1554 2085 1776.42 65.71 109 140400 1519 2024 1747.06 62.19 117 140400 1553 2024 1776.64 58.32 125 140400 1595 2006 1782.91 55.72

As a result of measuring the histogram of the same area using the ROI manager of ImageJ, the

minimum values according to the increased tube voltage constantly increased at 1,497, 1,505, 1,554, 1,519, 1,553 and 1595, but the maximum values varied at 2,060, 2,061, 2,085, 2,024, 2,024 and 2,006. The mean pixel value according to the increased tube voltage showed a constantly increasing trend, and the standard deviation was constantly reduced. However, the values did not exceed that of the pixel value of the lung field histogram [Fig. 8]. 4. Discussion

DR equipment has a wide dynamic range (latitude) of exposure, unlike the analog method. A wide dynamic range means a wide bit depth, and also indicates that the kV selection for the X-ray examination condition is wide in comparison with that of film [13]. The FS method is optimized to the dynamic range of about 101.5, and the speed varies from 100 to 400 levels that imply the step by which the eye can distinguish concentrations. Because the FS method has a relatively narrow dynamic range compared to that of DR, the width of the selection of the test conditions is narrower than DR, which


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has a wide exposure index of 104. Digital radiography can also improve the viewable image quality of underexposure or over-exposure [14].

a b c

d e f

Figure 8. A comparison histogram of kV change in the radiography. a: 90 kV, b: 96 kV, c: 102 kV, d: 109 kV, e: 117 kV, f: 125 kV

Due to the wide dynamic range of DR, the range of the maximum and minimum expression of X-

ray intensity was increased, and the contrast resolution was improved. The bit depth of the DR image receptor used in this study was usually 12 bits. This implies that there are 4096 (212) possible values for each pixel. Possible pixel values range from 0 to 4095. The pixel values of the image are then inverted so that areas of high x-ray exposure appear black and areas of low x-ray exposure appear white. However, the eye can only recognize 32 shades. Thus, DR can visualize the identifiable part from the whole dynamic range by appropriate adjustment through post-processing. This can be done through the adjustment of the window level and the window width. The window level allows the shape of the imaged tissue to be identified. The window depth determines contrast by the gray scale representation of the organization. If the window width is wider, the gray scale becomes longer. A narrow window width provides the highest contrast. The advantage of DR is that it can represent an image with a large difference in physical contrast. However, this can result in the appearance of linear artifacts that are not seen in the FS method, due to the wide dynamic range existent in DR [Fig. 3c]. In order to reproduce the linear artifact created by the patient test gown in the DR equipment, a fabric step wedge was produced from the same material as the patient test gown. The histogram of the patient’s lung field, which was not generated by the linear artifact of the patient gown, using ImageJ at the time of the chest test showed that the pixel value of the air portion was 985, and the pixel value of the bone component was widely distributed, with a pixel value of 3,351. In AEC mode, it was impossible to remove the artifact by adjusting the window width/level because the pixel value of the fabric step wedge was entirely contained in the pixel values of the lung field. The pixel value of the fabric step wedge was also investigated with the AEC mode off while changing the kV per step, but this did not remove it from the pixel value of the lung field. The intensity of the window level and window width, which are post-processing techniques of digital images, were controlled by linear, logarithmic, exponential and sigmoid transformations, and the images were compared to remove the linear artifact but in situations where the pixel values perfectly corresponded, significant changes were not shown. However, even in the current and past the foreign body artifacts caused by patient’s gown (clothing) had a lot to see in mammography and CR and DR examination [15-16].

The material of the current test gown is a fabric verified for use with the film/screen system. However, in the DR environment, because the pixel value of the test gown itself completely overlaps with the pixel value of the human body, this fabric is unsuitable for use as a material for test gowns. 5. Conclusions

Patient’s gown artifacts are distracting and can cause diagnostic inaccuracies. Therefore, In the DR system environment, the image quality and the dose exposure are proportional. The wide exposure


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range of digital radiography equipment can be used with a small dose without compromising image quality. However, care should be taken with the DR method due to artifacts caused by foreign materials at the time of chest testing, as the physical contrast is larger due to the wide dynamic range. As a result of this study, the production and design of a test gown suitable for DR is recommended. Developing a material for a test gown with a lower mass attenuation coefficient than the lung field pixel value of the human body is suggested.

6. Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the

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