Evaluation of phantoms used in image quality performance testing of dental cone beam computed tomography systems

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1 The University of Toledo The University of Toledo Digital Repository Theses and Dissertations 2015 Evaluation of phantoms used in image quality performance testing of dental cone beam computed tomography systems Haitham N. Alahmad University of Toledo Follow this and additional works at: Recommended Citation Alahmad, Haitham N., "Evaluation of phantoms used in image quality performance testing of dental cone beam computed tomography systems" (2015). Theses and Dissertations This Thesis is brought to you for free and open access by The University of Toledo Digital Repository. It has been accepted for inclusion in Theses and Dissertations by an authorized administrator of The University of Toledo Digital Repository. For more information, please see the repository's About page.

2 A Thesis entitled Evaluation of Phantoms Used in Image Quality Performance Testing of Dental Cone Beam Computed Tomography Systems by Haitham N Alahmad Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Master of Science in Biomedical Science Degree in Medical Physics E. Ishmael Parsai, Ph.D, Committee Chair Kerry Krugh, Ph.D, Committee Member Diana Shvydka, Ph.D, Committee Member Patricia Komuniecki, Ph.D, Dean College of Graduate Studies The University of Toledo August 2015

3 Copyright 2015, Haitham N Alahmad This document is copyrighted material. Under copyright law, no parts of this document may be reproduced without the expressed permission of the author.

4 An Abstract of Evaluation of Phantoms Used in Image Quality Performance Testing of Dental Cone Beam Computed Tomography Systems by Haitham N Alahmad Submitted to the Graduate Faculty as partial fulfillment of the requirements for the Master of Science in Biomedical Science Degree in Medical Physics The University of Toledo August 2015 Cone beam computed tomography (CBCT) units dedicated for dental and maxillofacial imaging have gained widespread use in recent years. So far there are no standardized testing methods to evaluate the image quality or the dose on these units. Although the vendors commonly provide an image quality phantom with the machine, the procedures, image quality parameters evaluated, and performance criteria are limited and specific to the vendors. The goal of this study is to evaluate the available phantoms as a testing tool for image quality assessment of dental CBCT units. In addition, the optimal diameter and features of a QA phantom for dental CBCT testing was assessed. Two commercially available phantoms were evaluated to assess the adequacy of each for use in the standardized testing procedures. These included ACR CT phantom (Gammex 464) and CATPHAN (Phantom Laboratory). Additionally, a prototype dental CT phantom (CIRS Inc.) was evaluated. Scans were made on three different machines; i- CAT Classic (Imaging Sciences International), ILUMA (IMTEC/3M) and GALILEOS Comfort (Sirona). A CT scan of the three phantoms were also performed with a conventional CT scanner (Toshiba Aquilion 16) to verify the pixel values. The image

5 quality parameters that were evaluated included: image noise, image uniformity, pixel value accuracy, pixel value linearity, contrast scale, and high contrast spatial resolution. There is not one phantom evaluated that provided superior results for all image quality tests. The prototype phantom was adequate for all tests with the exception of the high contrast resolution test whereas the ACR phantom and the CATPHAN due to larger diameter relative to the prototype demonstrated higher noise and non-uniformity, pixel value inaccuracy, and lower contrast scale because of more beam hardening artifact occurring with the large diameter. The optimal diameter for a phantom specialized in dental CBCT testing was found to be between 16 and 17 cm in diameter.

6 To my parents, Nasser and Norah: I hope that you will always be proud of me.

7 Acknowledgements I would like to express my gratitude to Dr. Kerry Krugh. This thesis would not have been possible without your support and mentoring. Thank you for introducing the topic to me and thank you for all the remarks and comments that were very helpful. I would like also to send a special thank you for Dr. Parsai and Dr. Shvydka. You have been nothing but supportive of me since I started the program. I would like to thank Whittaker Family Dental (Defiance, OH), Dr. Matthew Lark dental (Toledo, OH), and Harbor Light Oral & Maxillofacial Surgeons (Toledo, OH) for allowing us to work on their CBCT machines. Also I would like to thank Vladimir Varchena of CIRS Inc. (Norfolk, VA) for lending us the prototype phantom. At last but not the least, this project would also not have been possible without the love and support from my Dad, Mom, my wife Ayesha, and my sister Ghada. Thank you guys for your support and I love you all so much. v

8 Table of Contents Abstract...iii Acknowledgements...v Table of Contents....vi List of Tables....viii List of Figures.ix List of Abbreviations.. xi 1. Introduction Radiographic imaging in dentistry Computed tomography Dental Cone Beam Computed Tomography (CBCT) Technology Quality assurance in dental CBCT Radiation dosimetry Image quality testing in dental CBCT Existing phantoms designed for dental CBCT Determination of water-equivalent diameter for human dental and maxillofacial region Objectives of the Study Materials and Methods.25 vi

9 2.1. Dental CBCT units Protocols Set up and Positioning Phantoms Image Quality Analysis Calculation of water-equivalent diameter (Dw) for human dental and maxillofacial region Results Discussion Conclusion...55 References..57 vii

10 List of Tables 1.1 Quantitative image quality testing of dental CBCT systems suggested by the SEDENTEXCT project Technical differences between the i-cat, ILUMA, and GALILEOS Scan protocols used in scanning the phantoms Noise levels measured in uniformity sections in the phantoms Normalization factors for noise Normalized noise levels Average noise for each phantom on each machine Average difference between the average mean pixel value of the peripheral ROIs and the central ROI Pixel values of the ACR phantoms materials Pixel values of the CATPHAN materials Pixel values of the CIRS prototype materials Values of y-intercept in the three phantoms Contrast scale of the three phantoms Calculation of the water-equivalent diameter of the dental and maxillofacial region.. 51 viii

11 List of Figures 1.1 Principle of computed tomography Dental cone beam computed tomography Difference between conventional CT and CBCT Phantom proposed by SEDENTEXCT project by Leeds Test Objects Inc QUART DVT AP phantom designed Phantom suggested by Torgersen et al Set up and positioning of the phantoms in dental CBCT units Modules of CT ACR accreditation phantom Modules of CATPHAN CIRS prototype phantom for dental CBCT units Sections of the CIRS prototype phantom Measuring noise in the uniformity section in each phantom on image acquired by i-cat Measuring the difference in pixel values of the periphery from the center in the uniformity sections in each phantom on images acquired by ILUMA Measuring the mean pixel values of different materials in each phantoms on images acquired by i-cat.. 35 ix

12 2.9 The high contrast spatial resolution section in each phantom on images acquired by i-cat Measuring the mean CT number from head CT scan Noise levels measured in the three phantoms Average noise levels for each phantom in each machine Average difference between the periphery and the center in the uniformity section in each phantom Pixel values of different materials in the phantoms (i-cat vs. CT) Pixel values of different materials in the phantoms (ILUMA vs. CT) Pixel values of different materials in the phantoms (GALILEOS vs. CT) Contrast scale of the three phantoms for each machine The high contrast resolution of the three phantoms x

13 List of Abbreviations 3D... AAPM ACR... AEC... CBCT. CCD... CT CTDI... CS... DAP DLP FOV FPD Gy... HU... ICRP II. kvp. ma.. mas. MDCT. MSCT. NCRP. QA.. QC.. ROI.. Sv... TFT..... TLD.... Three Dimensional American Association of Physicists in Medicine American College of Radiology Automatic Exposure Control Cone Beam Computed Tomography Charged Coupled Device Computed Tomography CT dose Index Contrast Scale Dose Area Product Dose Length Product Field of View Flat Panel Detector Gray Hounsfield Unit International Commission on Radiation Protection Image Intensifier killovoltage Peak milliamprege milliamprege - second.multidetector CT Multislice CT National Commission on Radiation Protection & Measurments Quality Assurance Quality Control Region of Interest Sievert Thin Film Transistor Thermolumenence Detector xi

14 Chapter 1 Introduction 1.1. Radiographic Imaging in Dentistry X-rays has been always used in dentistry to help doctors assess patient condition and plan treatments. Intraoral radiography is the technique that has been commonly used where a small dental film or digital sensor is placed in the mouth and an external x-ray source is used to expose the image receptor. However, there was always the need for a more comprehensive imaging technique; a method that enables the imaging of both of the jaws and adjacent structures on one image. Therefore, panoramic imaging was introduced and first made commercially available in the 1960s. Panoramic imaging was considered the most comprehensive test used in dental practice. Panoramic imaging uses the principle of tomography to produce a two-dimensional image of a curved structure. The x-ray tube and the image receptor rotate simultaneously around an imaginary fulcrum trough located in the patient. The structures that are in the focal trough (the jaws and teeth) will appear in the image while the structures in the other layers will be blurred out. 1

15 Panoramic images, however, suffer from the shortcomings that all plain film radiography techniques suffer from; magnification, distortion, superimposition, and misrepresentation of structures (Scarfe & Farman, 2008). Analog tomography was also used in dental applications. There are even some panoramic units that have the capabilities of producing tomograms. This mode of imaging had the advantage of removing the superimposition of structures that is present in the panoramic images. However, imaging by this mode of scanning is limited to only one slice, and there was no capability for three-dimensional (3D) reconstruction or multiplanar viewing. Therefore, the idea of utilizing 3D imaging techniques in the dental practice came up to the surface Computed Tomography In 1972, Hounsfield and Cormack revolutionized medicine when their work lead to the invention of computed tomography. Computed tomography (CT) is an imaging modality in which the x-ray tube produces a thin fan beam of x-rays while rotating around the patient. An array of detectors on the opposite side rotates with the tube in synchrony and collects the beam transmitted through the patient. These transmission measurements can then be reconstructed to give a cross-sectional image of the scanned object. Most of the current commercially available CT scanners use multidetector systems where multiple rows of detectors (e.g. 8, 16, or 64) are used to scan multiple slices per one tube rotation. With the aid of slip ring technology, the tube can be continuously rotated in one direction. The gantry rotations can be progressed through the 2

16 longitudinal axis of the patient in a step-and-shoot fashion which is known as axial scanning mode, or the gantry can be continuously rotated during the scan while moving the table in the longitudinal axis to cover the area of interest. This simultaneous motion of the gantry and the table results in a spiral or helical trajectory of the tube rotation around the patient. This technique helps to cover more anatomical area in less time (Figure 1-1). A set of reconstructed CT images can be post-processed to result in axial, sagittal and/or coronal images. 3D images of the area of interest can be also formed by stacking the reconstructed 2D images. CT is one of the most common imaging procedures requested by physicians in the United States. About 67 million CT scans were performed in the year of 2006 in the United States (NRCP report No. 160, 2009). One of the reasons for such a higher number of CT exams is the many clinical applications in which CT can be utilized. There is no doubt that CT is a strong diagnostic tool that was made possible by the advancements in computers. In addition to the well-known diagnostic role of CT to detect injuries and abnormalities, it can be also utilized to give information about the stage of cancer, plan radiation treatments and surgeries, guide interventional procedures, and monitor the effectiveness of radiation treatment plans. CT has several advantages over general radiography. It removes the superimposition of structures that plain radiography suffers from. It also produces images with high contrast sensitivity; structures in the same slice that differ in their density by only 0.6% can be 3 Figure 1-1: Principle of Computed Tomography. (Taken from

17 easily differentiated. Also the CT technology enables viewing images in different planes (axial, coronal, and sagittal) by using the multiplaner reformation tool (MPR). However, due to radiation dose concerns, the use of conventional CT in dentistry has been limited. Other reasons for this limited role are high acquisition and operating cost and large space requirement. Thus cone beam computed tomography (CBCT) units dedicated for dental imaging were introduced Dental Cone Beam Computed Tomography (CBCT) Technology The dental (maxillofacial) CBCT technology was made commercially available in early 2000s. It produces cross-sectional images and reconstructed 3D images of the area of interest. The CBCT technology was initially developed for angiographic applications. Now this technology is being used in many clinical applications such as radiation therapy, surgery, and fluoroscopy. It is also used in many dental applications such as implant planning, endodontics, maxillofacial surgery, and orthodontics (Pauwels et al., 2015). There are two types of dental CBCT equipment arrangement. The first is the upright CBCT where the patient sits on a chair or stands up while the scanning occurs (Figure 1-2). The second configuration is where the patient lies in the supine position during the scan. The latter looks like a conventional CT scan but smaller Figure 1-2: patient sitting in chair while scanning in dental CBCT unit in size. The upright configuration is more common for dental CBCT units. 4

18 CBCT is considered a new generation of computed tomography. The major difference between the CBCT and the conventional CT is the geometry of the x-ray beam. Figure 1-3 shows the difference in the beam geometry; in conventional CT, a collimator restricts the x-ray beam into a fan-beam geometry, while CBCT uses conical or pyramidal shaped x-ray beams. Unlike conventional CT, an area detector is used in CBCT (rectangular mostly or circular in shape). In CBCT the gantry rotates around a fulcrum point in the patient only one time. A complete volumetric data set can be collected during this single rotation. The rotation might be a full 360 rotation or a partial rotation (180 plus the fan angle) taking in the range of 10 to 40 seconds. The size of the rotation angle can be fixed or variable depending on the manufacturer. For most manufacturers smaller rotation angles usually are preset with lower mas which means less dose and nosier images. During the single rotation of the gantry, about 150 to 600 planar projections, also called basis images, are collected. The basis images (projections) look like a series of radiographic images, each captured from a slightly different angle than the next. Figure 1-3: Conventional CT beam geometry (left), CBCT beam geometry (right). (Taken from Journal of Canadian Dental Association) 5

19 The number of projections usually depends on the scan time, frame rate, angle of gantry rotation and the speed of rotation. The number of projections can be fixed or variable. When more projections are collected, better image quality can be obtained at the expense of higher radiation dose and longer reconstruction times. Also more projections lead to an increase in the signal-to-noise and reduction of metallic artifacts (Scarfe and Farman, 2008). The tube potential used in dental CBCT typically ranges from 60 to 120 kvp and it is fixed for most vendors. Tubes are usually filtered with aluminum of thickness ranging from 2.5 to 10 mm. The tube current applied typically ranges from 1 to 10 ma which, in some models, can be manually varied by the operator depending on the patient size and desired image quality. Some vendors apply automatic exposure control (AEC) in which a suitable ma value can be obtained from the scout image. Other vendors apply non-patient specific ma modulation in which certain ma values are preset for each scan angle. The x-ray generation in CBCT might be continuous or pulsed. The continuous production of x-ray contributes to a higher dose to the patient. The x-ray beam can be made pulsed to correspond with the sampling of the detector. By this way the dose to the patient is dramatically reduced. In the pulsed mode, the real exposure time is less than the scanning time. For example, the exposure time for a 20-second scan on the i-cat, a dental CBCT unit by Imaging Sciences International (Hatfield, PA), is only 3.6 seconds. That means that the x-ray turns on for ms for each of the 306 projections (Imaging Sciences International Inc., 2006). Pulsed systems may show improved image quality due to the reduced motion effect from the gantry rotation (Pauwels et al., 2015). 6

20 When CBCT systems were first developed, the detector used, and still for some models, was an image intensifier (II) paired with charged coupled device (CCD). This type has some technical problems. For example, its circular field of view (FOV) suffers from truncation artifacts at the periphery. Also because of the gantry rotation, the sensitivity of the image intensifier can be easily affected by the magnetic field of the earth (Scarfe & Farman, 2008). Now, the flat panel detector (FPD) is the most commonly used in CBCT systems. The FPD is an amorphous silicon panel which consists of a single scintillation phosphor (typically cesium iodide doped with thallium) coupled with a thin film transistor (TFT) array. It detects radiation by indirect capture; the scintillator converts the incident radiation into visible light and the light photons are converted into electric charge by the photoconductor layer (amorphous silicon). The signal is then registered by the TFT panel. The FPD produces image quality superior to the images by the II/CCD detector type. The reconstructed CBCT images obtained using FPD have less noise than the images obtained using II/CCD detector type (Baba, Ueda and Okabe, 2004). In addition, the FPD is less bulky and it has higher resolution, greater dynamic range and shows less peripheral artifacts in the FOV (Scarfe & Farman, 2008). More recently, complementary metal-oxide-semiconductor (CMOS) detectors are being incorporated in dental CBCT (Pauwels et al., 2015). The reconstruction technique used in conventional CT is called the filtered back projection. In this technique each axial slice is reconstructed separately from the 2D projection data. For CBCT, the most common reconstruction technique used is the modified Feldkamp algorithm (or the FDK method) which is a variation of the filtered 7

21 back projection to keep track of the beam divergence in both cone and fan angles. In this method the 3D images are reconstructed from the 2D projection data that is collected during a single rotation around the patient. Further image processing can be performed to produce 3D images by stacking the reconstructed 2D images. Each element in this 3D volumetric data is called a volume element or a voxel. The minimum size of the voxel that can be reconstructed is determined by the size of the pixelated elements of the detector. Each voxel is assigned a value which represents the attenuation properties of the material in that voxel. Also 2D cross-sectional images can be viewed using the multiplanar reformation tool. CBCT reconstruction time depends on the voxel size chosen, the FOV, and the number of projections images collected. Typical reconstruction time is less than 2 minutes. The FOV size in dental CBCT differs according to the manufacturers. Some units have an option for extended view which is used when trying to scan areas that are larger than the detector size. There are two ways for extended view imaging. The first way is by obtaining two separate scans and then fusing the two data sets to result in volumetric data for the large area of interest. This method has the disadvantage of overexposing the overlapping area. The second way is to shift the position of the detector laterally and collimate the beam asymmetrically (Scarfe and Farman, 2008). 8

22 1.4. Quality assurance for dental CBCT A typical quality assurance program for evaluating the performance of an imaging system that utilizes ionization radiation for image formation involves two components; image quality and radiation dosimetry Radiation dosimetry Radiation dose assessment for any radiation emitting devoice is necessary for the safety of patients. It provides a method of monitoring and estimation of the radiation dose delivered to the patients. In conventional CT, the standardized method of choice to characterize the dose is the CT Dose Index (CTDI). However, because of the nature of the geometry of the beam in CBCT and the amount of scatter, the CTDI cannot be properly adapted in CBCT (Pauwels et al., 2012). The pencil ionization chamber that is used in measuring the dose in conventional CT is 10 cm long which may be useful for CBCT machines with height FOV up to 6 cm. However, it is not ideal for machines with FOV larger than 8 cm because the scatter produced would not be measured which would lead to an underestimation of the dose (Araki et al., 2013). Also, a range of FOV sizes can be chosen which can exceed the size of the pencil chamber. Large FOV can affect the dose distribution (Pauwels et al., 2012). In addition, the iso-center of the scan can be changed from central to peripheral in some scans, which in turns produces an asymmetric dose distribution (Pauwels et al., 2012). Similarly, choosing a full arc rotation of the tube versus a partial arc will affect the dose distribution. (Pauwels et al., 2012). 9

23 The issues discussed in the prior paragraph lead to an inaccurate estimation of the dose when using the concept of CTDI in dental CBCT. Therefore there is not yet a standardized method for evaluation of the radiation dose in dental CBCT and the most accurate way to characterize the dose is by point-by-point measurement using TLDs in an anthropomorphic phantom (e.g., the RANDO phantom) which is not practical for routine QA radiation dosimetry measurement. Effective dose, measured in Sievert (Sv), allows for the assessment of patient risk and also allows for comparing different types of medical imaging modalities. Dental CBCT offers relatively lower effective dose than conventional CT but higher effective dose when it is compared to panoral radiography. A study done by Dr. Ludlow et al compared the effective dose for 3 commercially available CBCT units: CB Mercury, NewTom 3G, and i-cat. Twenty-four TLDs were placed throughout the layers of a head and neck RANDO phantom. After taking the measurements, they calculated the total body effective dose by summing the doses for individual organs calculated using both of the 1990 and 2005 ICRP tissue weighting factors. The results showed that the effective doses produced by these CBCT scanners were 4 to 42 times greater than equivalent panoramic images (Ludlow et al., 2006). Based on the results, a 12-inch FOV scan on the i-cat is equivalent to a dose of 21 single panoramic exposures. Also the results showed lower doses when using smaller FOV and reduced values of kv and ma. They concluded that CBCT dose varies substantially depending on the device, FOV and selected technique factors. Effective dose detriment is several to many times higher than conventional panoramic imaging and 10

24 an order of magnitude or more less than reported doses for conventional CT (Ludlow et al., 2006). Another study that supports the findings of Dr. Ludlow was done byj A Roberts et al (2009). They studied the effective dose delivered to patients during a CBCT examination placing TLDs in a RANDO phantom and scanning it using the i-cat dental CBCT. They calculated the effective dose using both the 1990 and the 2007 ICRP tissueweighting factors. They concluded that doses from CBCT are low compared with conventional CT but significantly higher than conventional dental radiography techniques. The SEDENTEXCT project established by the European Atomic Energy Community (Euratom) under the European Commission on Radiation Protection proposed a new method that enables a more accurate estimation of the dose using a specially designed phantom. The dose indices suggested are dose index 1 (DI1), dose index2 (DI2) and dose area product (DAP). However, this method yet to be accepted amongst the medical physics community as the standard method for assessing the dose in dental CBCT Image quality testing in dental CBCT The second component of routine QA evaluation of dental CBCT is an assessment of image quality. The image quality parameters that are important for assessment are image noise and low contrast detectability, image uniformity, pixel value accuracy and linearity, contrast scale, high contrast spatial resolution, and image artifacts. 11

25 Image noise and low contrast detectability Noise is defined as a random or stochastic variation in the signal that, in ideal situations, follows Poisson distribution. According to Poisson statistics the quantum noise per pixel in an image is given by N where N is the number of photons. Thus, quantum noise is determined by those factors impacting the amount of photons used per pixel for image formation (kvp, mas, filtration, voxel size, and number of projections). In addition, there are a variety of other sources of image noise, most notably in CT, the image reconstruction kernel. Noise strongly impacts low contrast detectability which is defined as the ability to evaluate structures that have attenuation properties that are slightly different than that of the background. Due to the low mas and small voxel size that are typically used in dental CBCT systems, images contain relatively high noise levels. In addition, the nature of the beam in CBCT produces a large amount of scatter radiation. Both the relatively high noise levels and increased scatter radiation result in a reduced contrast detectability of dental CBCT images in comparison with conventional CT Image uniformity Uniformity across an image is an important parameter to evaluate and quantify. Ideally, tissues that have the same attenuation properties should have same pixel value in any location in the image. Due to the divergence of the x-ray beam used in CBCT, there 12

26 is a variation in the intensity of the incident x-rays which causes a non-uniform absorption and detection. In addition, dental CBCT images suffer from non-uniformities whenever there is a mass outside the FOV, often called exo-mass. The exo-mass causes beam hardening which reduces the image intensities in parts of the image (Bryant, Drage and Richmond, 2008) Pixel value accuracy and linearity In conventional CT, each voxel in the image is assigned a certain value. This value is calculated using the measured attenuation coefficient of the tissues in that voxel normalized to the attenuation coefficient of water and is called CT number (expressed in Hounsfield units). The following equation used to calculate the CT number of a voxel of tissues: Where: CT# = μ tissue μ water μ water x1000 (1-1) CT#: the CT number of a given material in Hounsfield units µtissue: linear attenuation coefficient of a given material µwater: linear attenuation coefficient of water Any material that possesses absorption properties that are higher than water takes a positive value. Similarly any material that has absorption properties that are lower than 13

27 water takes a negative value. For example, the CT number of air is -1000, while the CT number of dense bone is approximately The pixel intensity values measured in dental CBCT images may not accurately represent the true Hounsfield unit values (Scarfe et al., 2012). The reasons of such discrepancy in the pixel values between conventional CT and CBCT are the high amount of scatter, the applied reconstruction algorithm utilized in CBCT, and the reduced technique factors that are usually used. In addition, great variability of the pixel values can be seen in CBCT images from different machines manufacturers (Pauwels et al., 2015). In conventional CT, the CT number values across the image possess a linear relationship with respect to the linear attenuation coefficient of the materials in the image. In dental CBCT, the pixel values should also have a linear relationship with the beam attenuation. One method to assess this relationship is to compare the measured dental CBCT pixel values of a variety of materials with known HU values as determined from conventional CT scan. The linearity of the relationship can be measured by means of the correlation coefficient from linear regression analysis. 14

28 Contrast Scale (CS) The contrast scale (CS) in conventional CT is defined as the change in linear attenuation coefficient relative to the change in CT number and is expressed in the following equation: CS = μ 1(E) μ 2 (E) CT 1 (E) CT 2 (E) (1-2) Where, µ1: linear attenuation coefficient of the first material for a specific beam energy µ2: linear attenuation coefficient of the second material for a specific beam energy CT1: the CT number of the first material for a specific beam energy CT2: the CT number of the second material for a specific beam energy In this project, however, the CS is defined as the change in the pixel value between certain materials in the dental CBCT image relative to the change in the HU measured in a conventional CT image for the same materials and is expressed in the following equation: CS = PV 1(E) PV 2 (E) CT 1 (E) CT 2 (E) (1-3) where PV1: the pixel value of the first material in the dental CBCT image PV2: the pixel value of the second material in the dental CBCT image CT1: the HU of the first material in the conventional CT image CT2: the HU of the second material in the conventional CT image 15

29 Ideally, the contrast scale (i.e., slope) should be equal to unity which means any change in the pixel values measured in the CBCT image corresponds to the same amount of change in the HU measured in the conventional CT image High contrast spatial resolution The spatial resolution is defined as the ability of an image system to distinctly depict two objects as they become smaller and closer together (Bushberg, 2002). The smallest object that can be resolved is known as the limiting spatial resolution. The spatial resolution in dental CBCT is mostly dependent on the reconstructed voxel size, the physical size of the pixelated detector, and the reconstruction kernel. CBCT is capable of reconstructing isotropic voxels meaning that voxels have equivalent dimensions in the x,y, and z directions. Typical voxel size that can be reconstructed in dental CBCT varies between 0.08 mm to 0.4 mm. The limiting spatial resolution can be evaluated visually by using a line pair pattern which consists of an alternating strip and space arrangement of different widths. In literature, the line pair per distance values that have been reported for dental CBCT images range between 0.6 and 2.8 lp/mm (Brüllmann and Schulze, 2015). In addition, the spatial resolution of an imaging system can be quantified by the modulation transfer function (MTF). The MTF can be calculated by applying a Fourier Transform (FT) on a point spread function or a line spread function obtained by the imaging system. The limiting spatial resolution of the system would be empirically 16

30 considered at the 10% MTF. The 10% MTF for dental CBCT found in literature is in the range from 0.5 to 2.3 cycles/mm (Brüllmann and Schulze, 2015) Artifacts Dental CBCT images may suffer from cone beam artifacts. The cone beam artifacts result from undersampling the cone angle direction. This affects the image quality in the upper and lower edges of the reconstructed volume. The image quality improves gradually towards the center (Schulze et al., 2011). In addition, the beam divergence causes other artifacts such as aliasing and high amounts of scatter. 17

31 1.5. Existing phantoms designed for dental CBCT During the past few years some efforts were made to the purpose of designing a comprehensive quality assurance program for dental CBCT. The Safety and Efficacy of a New and Emerging Dental X-ray CT scanners, or the SEDENTEXCT, project established by the European Atomic Energy Community (Euratom) under the European Commission on Radiation Protection was one of the firsts who addressed this issue. The objective of the SEDENTEXCT project was to develop evidence-based guidelines dealing with justification, optimization and referral criteria for users of dental CBCT. They published provisional guidelines in report 172 in 2008 and published an updated version in The report included a chapter about quality standards and quality assurance and recommended that a QA program should include six topics; x-ray tube generator performance, quantitative assessment of image quality, display screen performance, patient dose assessment, clinical image quality assessment and clinical audit. They proposed an image quality program using the SEDENTEXCT phantom designed by Leeds Test Objects Inc. This phantom is a PMMA cylinder of 16 cm in diameter and a height of 16.2 cm (Figure 1-4). Nine image quality characteristics can be evaluated using this phantom and these parameters are noise, uniformity, pixel value accuracy contrast resolution, spatial resolution, geometrical accuracy, and metal artifacts. Table (1-1) summarizes the test and their frequency. Since there are no action or tolerance 18 Figure1-4: sedentexctiq phantom by Leeds Test Objects Inc.

32 levels so far, they suggested to establish baseline values during the acceptance testing and then monitor these values in the routine testing. Table (1-1 ): testing of image quality of dental CBCT as suggested by the SEDENTEXCT project. (Taken from report 172) Pauwels et al (2011) conducted a study where they evaluated the SEDENTEXCT proposed phantom and stated that it showed promising potential for technical image quality evaluation of CBCT. J Bamba et al (2013) also used this phantom to evaluate three dental CBCT systems and stated that all the essential image quality parameters can be assessed using this phantom. QUART, a German company, also designed a special phantom for testing dental CBCT systems (Figure 1-5). The QUART DVT AP phantom is cylindrical in design with a 16 cm diameter and 15 cm in height. It is designed to evaluate image Figure 1-5: Quart DVT AP phantom (taken from Quart website) uniformity, image noise, contrast resolution and CNR, MTF, limiting spatial resolution and image geometry. 19

33 The center for Evidence-Based Purchasing CEP, a part of the department of health in the United Kingdom, conducted a comparative evaluation of dental CBCT systems in 2010 where they compared the dose and image quality for several CBCT systems from different vendors. For image quality comparison they used CATPHAN 424. They were able to test the high contrast spatial resolution, uniformity, CT number accuracy, and geometric accuracy. However, low contrast detectability module of the CATPHAN was not adequately visualized on any of the systems they evaluated. Another phantom which is not commercially available and was designed for research purposes by Torgersen et al (2014) is shown in Figure 1-6. They tried to design an inexpensive phantom for simplified image quality assurance for image quality assessment of dental CBCT. All of the tests performed by this phantom were objective. They also developed software that can be used to evaluate the images of this phantom. The tests included low and high contrast resolution, uniformity, noise, and geometric linearity. The phantom has a diameter of 16 cm and a height of 70 cm. The SEDENTEXCT phantom and the QUART phantom are the only commercially available phantoms designed for dental CBCT but may not be cost effective for every diagnostic medical physicist to purchase. Figure 1-6: QA phantom designed by Torgersen et al for dental CBCT testing 20

34 1.6. Determination of water equivalent diameter (Dw) for human dental and maxillofacial region. The phantom that is ideally used in the assessment of image quality parameters of dental CBCT scanners would have the same attenuation properties as the dental and maxillofacial region of the human head. Most phantoms in radiologic testing are made of water-equivalent material. The optimal diameter of a cylindrical phantom can be determined by calculating the diameter of water that has similar photon attenuations as the dental and maxillofacial tissues. The CT number values in CT images expressed in Hounsfield units are calculated by using the linear attenuation coefficients of the materials in the image. The CT number is thus representative of the attenuation properties of a voxel of a tissue and can be used to calculate the diameter of water that would have equivalent attenuation. The methodology and calculation of the water-equivalent diameter (Dw) have been discussed in detail in AAPM task group report 220 report. The following equation was used: D w = 2 [ 1 CT(x, y) 1000 ROI + 1] A ROI π (1-4) (AAPM Task Group 220, 2014) where, Dw: water-equivalent diameter CT(x,y)ROI: the mean CT number in a chosen ROI AROI: the area of the ROI 21

35 In addition, by rearranging the previous equation and solving for area (A= π(d/2) 2 ), an attenuation equivalent diameter of materials other than water can be determined as in the following equation: where, D material = 2 { ( D w 2 )2 ( CT material ) } (1-5) Dmaterial: the attenuation equivalent diameter of material other than water CTmaterial: the HU of the material of interest Dw: the water-equivalent diameter calculated from Formula

36 1.7. Objectives of the Study When dental CBCT was first introduced there was no requirement for medical physics acceptance testing or annual quality control testing of dental CBCT. Since that time only some States have mandated that medical physicists perform testing on these units. Ohio Administrative Code rule 3701: was revised in July 2014 to include CBCT scanners and hybrid imaging systems. Paragraph D of that rule states that a QA testing should be performed annually for CBCT units. The annual testing should be performed by a radiation expert. To date there are no standardized test methods, phantoms, or test criteria for these units The American Association of physicists in medicine (AAPM) has recently formed a task group (TG261) for discussing this issue. Its main objective is to establish a standardized procedures and techniques for the quality control of dental and maxillofacial imaging systems. With this rising interest to comply with the regulations, there is a necessity to develop guidelines and recommendations regarding routine quality control testing of CBCT scanners. The following are the objectives of this study: 1. Evaluate the feasibility of using phantoms common to conventional CT testing for use in dental CBCT quality control testing. These phantoms include the ACR CT phantom and CATPHAN. Both of these phantoms are available and familiar to most diagnostic medical physicists. It would be more economical and convenient if these phantoms were found to be 23

37 suitable for testing dental CBCT units. Purchasing specially designed QA phantoms for dental CBCT testing may not be cost effective, especially for small dental offices or small medical physics consultancy companies. 2. Evaluate a prototype phantom designed specifically for dental CBCT QC phantom (the CIRS prototype phantom). 3. Evaluate the optimal dimensions of a QA phantom for dental CBCT based on the radiation attention properties of the dental and maxillofacial region of the human head and propose alterations and features for appropriate phantom design based on observations from objectives 1 and 2. 24

38 Chapter 2 Materials & Methods Evaluation of QA phantoms 2.1 Dental CBCT units Image quality testing was performed on three commercially available CBCT scanners: i-cat Classic (Imaging sciences International), ILUMA (IMTEC/3M), and GALILEOS Comfort (Sirona). Table 2-1 summarizes specifications for each scanner: Table ( 2-1 ): technical differences between the i-cat, ILUMA, and GALILEOS. Scanner Focal spot (mm) Detector Exposure mode kvp ma FOV DxH (cm) icat classic (Imaging Sciences) ILLUMA (3M/IMTEC) GALILEOS comfort (Sirona) 0.5 FPD Pulsed x FPD Continuous or x II pulsed x15 FPD: Flat panel detector. II: Image intensifier. FOV: Field of view. 25

39 2.1.1 i-cat Classic (Imaging Sciences International) The i-cat utilizes an x-ray tube with a 0.5 mm focal spot size and 15 anode angle. Technical factors for x-ray generation on this units are 120 kvp (fixed tube voltage) and a tube current that can be varied from 3 to 7 ma. The tube has filtration equivalent to 10 mm of aluminum at 120 kvp. The i-cat utilizes an amorphous silicon flat panel for detection with a readable area with dimensions of 24 cm by 20 cm. This detector allows for a maximum field of view of 16 cm in image diameter and 13 cm in height which can be collimated down to cover areas of smaller size ILUMA (IMTEC/3M) The ILUMA utilizes an x-ray tube with a 0.3 focal spot size and has a filtration of 1.27 mm of copper. The tube voltage is fixed to 120 kv and the tube current has two settings; 1 or 3.8 ma. The ILUMA utilizes a flat panel detector with dimensions of 20 cm by 24 cm which allows a reconstructed FOV of 17 cm in image diameter and 14 cm in height. The exposure mode in the ILUMA is continuous, which means the scan time equals the exposure time. 26

40 2.1.3 GALILEOS comfort (Sirona) This unit uses an x-ray tube with a 0.5 mm focal spot and has a 2.5 mm of aluminum thickness for filtration. The tube voltage is fixed to 85 kv and the tube current can be varied between 5 and7 ma. The GALILEOS uses an image intensifier for detection with an input window of 21.5 diameter which allow a reconstructed FOV is 15 cm in both diameter and height. The GALILEOS has two programs that can be used for scanning; VO1 and VO2. The difference between the two programs is with the image reconstruction algorithm and thus acquisition parameters are equivalent. 2.2 Protocols A total of eight protocols were chosen to scan the phantoms. The protocols chosen for scanning in this project were those that were commonly used in the clinical setting. Table (2-2) summarizes the scan protocols used. Note that in the following chapters the protocols will simply be referred to by the Reference number in Table Set up and positioning Figure 2-1: the platform and the tripod used to position the phantoms The phantoms were positioned vertically on a custom- made stand inserted on a tripod. The platform was leveled and lasers were used to center the phantoms in the FOV. 27

41 Table (2-2 ): protocols used in scanning the phantoms Protocol Reconstructed volume (diameter x height) (cm) Ref # kv ma scan time (s) voxel (mm) arc ( ) projections icat classic (Imaging Sciences) 16x13 cylindrical ILUMA (IMTEC/3M) Galileos Comfort (Sirona) 17x14 cylindrical 15 Spherical Phantoms Three phantoms were evaluated for dental CBCT testing. The phantoms evaluated were the ACR phantom and the CATPHAN which are commonly used in testing conventional CT. The third phantom evaluated is the CIRS prototype which is specially designed for testing dental CBCT phantoms. The details for the each phantom are as follows: 28

42 2.4.1 CT ACR Phantom Gammex 464 is the only phantom that is authorized by the American College of Radiology to be used in the accreditation process of conventional CT scanners. It can be used for initial QA assessment and routine monthly QA testing for CT machines. Several image quality parameters of CT can be evaluated using this phantom. The CT ACR phantom is 20 cm in diameter and has a height of 16 cm. It consists of 4 modules or sections. Each section has a thickness of 4 cm. The first module is used to evaluate positioning and alignment, CT number accuracy, and slice thickness (Figure 2-2: A). In this module there are 5 cylinders of different materials. These different materials are used to evaluate CT number accuracy. This module also includes two ramps each consists with a series of wires that can be used to assess the slice thickness. The second module is used to evaluate low contrast detectability and contrast-tonoise ratio. This module consists of multiple cylinders of different diameters (Figure 2-2:B) The third module is made of a uniform tissue-equivalent material used to assess uniformity and noise. This module also has 2 small BBs 10 cm apart which can be measured to assess the geometric accuracy (Figure 2-2:C) The fourth module is used to evaluate high contrast spatial resolution. It has 8 square line pair regions. The limiting spatial resolutions that can be measured are 4, 5, 6, 7, 8, 9, 10 and 12 lp/cm (Figure 2-2: D) 29

43 A B C D Figure 2-2: the four modules of the ACR CT phantom. Taken from the CT ACR accreditation phantom instructions CATPHAN504 The CATPHAN 504 designed by The Phantoms Laboratory Inc. and is commonly used to evaluate image quality and system performance for both CT and CBCT scanners. It is 20 cm in both diameter and height. It consists of 4 modules (Figure 2-3). Module CTP528 (Figure 2-3A) is used to test spatial resolution. It consists of high contrast line pairs ranging from 1 to 21 lp/cm. Module CTP 515 (Figure 2-3B) is used to assess low contrast detectability. 30

44 A B C Figure 2-3: modules of Catphan 504 (taken from the phantom laboratory manual) It consists of three groups of supra-slice low contrast targets with nominal contrast levels of 0.3%, 0.5%, and 1.0%. Each group has 9 circles with diameters ranging from 2 to 15 mm. It also consists of three sub-slice groups each having 4 circles of diameters 3,5,7 and 9mm. Module CTP 404 (Figure 2-3C) has 8 targets of different materials (Air, PMP, LDPE, Polystyrene, Acrylic, Derlin, and Teflon) which can be used to assess the accuracy and linearity of the Hounsfield unit scale. This section also contains 4 wire ramps with 23 angulation. The wire ramps can be used to assess alignment and slice thickness. Also the section has 4 3mm holes positioned at 50mm from each other. These can be used to evaluate the measurement accuracy. In the center of this module there are 5 acrylic spheres of different diameters to evaluate the machine s ability to image volumes. 31

45 The last module is made of a uniform material (CTP486) module and is used to evaluate image uniformity, noise and artifact CIRS QA Phantom prototype This phantom was designed in 2010 by Computerized Imaging Reference systems (CIRS) Inc. which is still under investigation and subject to further development. This prototype is designed to be used in CBCT acceptance and periodic QA testing. It has 5 sections and can be used to measure seven dental CBCT performance parameters. It can also provide a range of bone mineral density references. It has a triangular shape with a height of 17 cm (Figure 2-4). The second layer of the phantom is used to evaluate uniformity and noise. It is made of a water-equivalent material and has a thickness of 20 mm (Figure 2-5). Figure 2-4 : Dental CBCT prototype QA phantom by CIRS Inc. 32

46 The third section is used to evaluate low contrast detectability. This section has a thickness of 30 mm and its background is made to be water equivalent. There are three sets in this sections. Each set has 8 cylindrical targets and 8 spherical targets. The cylinders are 30 mm long and have diameters of 10, 7, 5, 3.5, 2.5, 1.8, 1.2 and 0.9 mm. The spheres have diameters of 10, 8, 6.5, 5, 4, 3.2, 2.5, and 2 mm. Each set has a different density than the other; the first set is 5 HU, the second is 10 HU and the third is 20 HU above the background (Figure 2-5). The fourth section is used to evaluate CT number linearity, alignment and slice thickness. This layer also is 30 mm in thickness. To assess the CT number linearity, this section has 6 cylinders each with 13 mm diameter and 30 mm in length. Each cylinder has different density to mimic different material; adipose tissue, brain, muscle, bone, dense bone. To assess the slice thickness this section also has 3 stainless steel wires that are angled 30 from the axial plane (Figure 2-5). Figure 2-5: the layers of the dental CBCT prototype (taken from the manual provided by CIRS) 33

47 2.5 Image Quality analysis All image analysis was done using ImageJ software freely available from NIH (National Institute of Health). Image noise was measured as the standard deviation of a central ROI drawn in the uniformity section for each phantom used. To eliminate slice to slice variation the noise was averaged over twenty consecutive slices. a b c Figure 2-6: measuring noise in the uniformity section in each phantom on i-cat images. a) ACR phantom. b)catphan. C) CIRS prototype. For measuring the image uniformity, 5 equal circular ROIs were drawn in center, 12, 3, 6, and 9 o clock positions in the uniformity section in each phantom. The image uniformity was calculated as the mean difference in pixel values of the four periphery positions from that of the center. This again was averaged over twenty consecutive slices 34

48 a b c Figure 2-7: measuring the difference of the peripheral mean pixel value from the center in the uniformity section in each phantom on the ILUMA images. a) ACR phantom. b) CATPHAN. c)cirs prototype. The mean pixel value for all materials in each phantom was measured with an ROI. The pixel values were plotted against the true HU values obtained from a conventional CT scan. Linear regression was used to fit a line to the data. From the linear equation the Y-intercept was used as an assessment of pixel value accuracy, the slope as an assessment of contrast scale, and the correlation coefficient (R 2 ) as an assessment of pixel value linearity. a b c Figure 2-8: measuring the mean pixel values for different materials on the i-cat images of each phantom 35

49 a b c Figure 2-9: high contrast spatial;; resolution section of phantoms acquired on i-cat scanner. A)ACR phantom. b) CATPHAN. c) CIRS prototype For evaluating the high contrast spatial resolution, the smallest line pair that could be visually resolved in each phantom was recorded. 36

50 Diameter of dental CBCT QA phantom 2.6 Calculation of water-equivalent diameter (Dw) for human dental and maxillofacial region Ten conventional CT scans of the head area were chosen for this calculation. Since the goal is to measure the equivalent attenuation of the dental and maxillofacial tissues only, the images that had foreign objects, such as metallic implants or bismuth shields, were avoided. Such highly attenuating materials affect the accuracy of the Dw calculation (AAPM Task Group 220, 2014). In each head scan the slices from the inferior of the orbit to the inferior edge of the mandible were selected. An ROI was drawn to include the tissues in each slice. a b c Figure 2-10: measuring the mean CT number value in the CT scans. ROI was drawn to include all tissues in the slice. a) Orbits level. b) mid-level. c) Mandible level The ROI was drawn in such a way to include the entire cross-section. Irrelevant objects such as the patient table was not included in the ROI since it can lead to an overestimation of the attenuation of the ROI (AAPM Task Group 220, 2014). 37