[ieee 2013 pan american health care exchanges (pahce) - medellin, colombia (2013.04.29-2013.05.4)]...

Abstract –– One of the major challenges with the use of computedtomography (CT) is to establish the proper tradeoff between radiation dose and image quality. Imaging phantoms are a primary tool that can be used to optimize acquisition protocols without exposing the patients to any unnecessary risk. The purpose of this work was to develop a flexible and low cost imaging phantom to be used for CT optimization. A phantom container made of acrylic and with an elliptical shape was built. A scaffold to insert up to 9 samples with materials of radiological relevance was included within the phantom. Samples of iodinated contrast agent and bone-like material (calcium chloride) were inserted to the phantom. For experimentation two 64-slice systems were used: a dual-source and a conventional single-source CT systems. Imaging protocols with tube potential ranging from 80 to 140 kVp were used. Image noise, contrast, contrast-to-noise ratio (CNR) and figure of merit were calculated in order to assess the performance of the systems at the various acquisition schemes. A strong linear relationship (R2>0.95 in all cases) was found between iodine and calcium materials at different concentrations relative to the CT numbers. This was the case at all tube potentials evaluated and independent of the CT system. At lower tube potential values image contrast increased but image noise also increased. When evaluating a dose-weighted CNR, it was found that the lower kVp provided the most CNR per dose. In conclusion, the development of a low cost imaging phantom which can be used for CT protocols was demonstrated. Experimental evaluations of the phantom across CT protocols with several acquisition parameters and different vendors were performed. It was also demonstrated that this phantom allows to evaluate these systems (and corresponding protocol) performance by relating both achievable CNR and scanner radiation output.

Keywords –– Phantoms, Tomography, Image Quality, Biomedical Imaging, Noise measurement, Radiation Dosage.

I. INTRODUCTION

Computed Tomography (CT) is one of the leading clinical imaging modalities used for diagnosis since its introduction in the 1970s [1], and is regarded as one of the most important inventions of the twentieth century. Current CT scanners allow fast isotropic imaging of the human body, permitting to differentiate the structures and organs by means of differences in linear attenuation coefficients of those tissues when irradiated with x-rays [2]. However, a major ongoing challenge with the use of CT is that it involves ionizing radiation. It is accepted that typical CT examinations involve relatively low exposures of radiation and the benefits typically outweighs the small risk [3]. Yet, the exponential growth in the use of CT examinations has lead to concerns of CT, increasing the lifetime attributable risk of cancer with special concern in pediatrics [4].

In x-ray based examinations, such as CT, the amount of radiation used is directly related to the image quality obtained: the higher the x-ray exposure the better the image quality. Hence, there is a need to balance the radiation exposure and the image quality to get the most benefit from CT using the lowest possible radiation needed. The guiding principle in CT (and other ionizing radiation examinations) is to use ALARA: As Low as Reasonably Achievable dose to the task at hand [5].

In CT, image acquisition parameters such as tube potential (kVp), tube current (mA), pitch and exposure time have a direct impact on radiation dose and image quality [6, 7]. Therefore adjustments made to the CT scanner for any specific protocol have to be established carefully, so that the patient´s safety is not put into unnecessary risk [8, 9]. On the other hand besides the parameters mentioned above, factors such as patient size, age, gender, presence of prosthesis or contrast agents and the nature of the study have to be considered before establishing the scanning parameters [10]. Image quality is one of the most important features of the image due that slight anatomical details are of significant relevance when diagnosing.

Imaging phantoms are one of the most important tools used to optimize and balance radiation dose and image quality in CT examinations. Imaging phantoms are inert objects that can be repetitively scanned. Phantoms used in CT, typically have defined geometric forms that are guided by typical anthropomorphic shapes of adults or children. Likewise, they are composed of materials that should mimic patient attenuation (eg soft tissues, bone) [11, 12]. In addition, phantoms should allow quantitative and qualitative measures of image quality such as contrast, spatial resolution and noise [13, 14]. Specific phantoms for different applications are also possible. Furthermore, there are numerous commercially available types of phantoms varying on composition, shape, price, and several special features in order to measure specific variables [15].

In developing countries in regions such as Latin America, there is limited access to image quality evaluation in medical imaging devices such as CT, to the exception of typical routines performed by the manufacturers. As a result, there is also a lack of availability of phantoms to evaluate

Development of a computed tomography phantom to evaluate radiation dose and image quality

Daniel Gómez Cardona, BS1, Edgar Santiago Estrada León, BS1,Alejandro Zuluaga Santamaría, MD2, Luis Benítez, MD3, Juan Carlos Ramírez Giraldo, PhD4

1Biomedical Engineering Program. Escuela de Ingeniería de Antioquia – CES Univeristy2Centro de Diagnóstico Médico CEDIMED, 3Centro Médico Clínica de Occidente, 4Siemens Medical Solutions USA

2013 PAN AMERICAN HEALTH CARE EXCHANGES (PAHCE). CONFERENCE, WORKSHOPS, AND EXHIBITS. COOPERATION / LINKAGESINTERCAMBIOS DE CUIDADO MÉDICO PANAMERICANOS. CONFERENCIA, TALLERES Y EXHIBICIONES. COOPERACIÓN / ENLACES

APRIL 29 – MAY 4, 2013, MEDELLIN, COLOMBIA ISBN: 978-1-4673-6257-3 IEEE Catalog Number: CFP1318G-ART

978-1-4673-6257-3/13/$31.00 ©2013 IEEE

performance and optimize protocols, beyond what manufacturers provide for their regular maintenance.

The purpose of this study is to present the development of a low cost imaging phantom that can be used for CT optimization in routine CT examinations.

II. METHODOLOGY

A. Phantom Design

Three main factors were considered for phantom design: geometry, dimensions and materials.

Geometry and dimensions: An elliptic shape for the container was selected to simulate a standard 70 kg adult patient torso. This shape is similar to others reported in scientific publications or commercial phantoms [15]. The dimensions chosen were 300 mm width, 200 mm height and 160 mm depth.

A scaffold was built and located near the isocenter of the phantom. The scaffold allowed the insertion of materials of radiological relevance. The container and scaffold (Figure 1) were designed in the software Solid Edge ST2 (Siemens PLM software, Plano, Texas, USA)

Materials: For the container and scaffold the material selected was acrylic (PMMA). Acrylic offers easy handling and it has been reported as a tissue equivalent material in diagnostic radiology dosimetry [16, 17]. Soft tissue, bone and a CT contrast agent were simulated as well. The soft tissue was simulated by water [17], bone by calcium chloride [18] and iodine using a commercially available solution called Iopamiron 370 mg/ml (Bracco Imaging s.p.a, Italy), which is clinically used as a CT contrast agent.

The calcium chloride and iodine were dissolved in distilledwater to mimic concentrations expected to be observed during an actual examination. Calcium chloride concentrations were 593, 451, 343 and 212 mg/ml; and iodine concentrations were 30, 15, 7.5, 3.75 and 1.8 mg/ml for iodine.

Figure 1 Design of Container and Scaffold using Solid Edge®

The water, calcium and iodine samples were stored in assay tubes of 100 mm length and 15 mm diameter. These tubes were located inside the scaffold: 4 calcium samples, 4 iodine samples, and 1 tube containing water, which was located in the center of the phantom.

B. Experiments

CT scans were performed using a single source CT scanner (Aquilion 64, Toshiba Medical Systems, Tokyo, Japan), and a dual-source CT scanner (SOMATOM Definition, Siemens Medical Solutions, Forchheim, Germany). The acquisition parameters for each scanner are summarized in table 1 and 2.

Table 1 Acquisition parameters dual-source CT scanner

Protocol dual-source CT scannerParameters ValuekVp 80 120 140Quality reference mAs 350 210 80

Pitch 0,75 1 0,75

Table 2 Acquisition parameters single-source CT scanner

Protocol single-source CT scannerParameters ValuekVp 80 120 135mAs 189 176 53Pitch 1,328 1,25 1,328Standard Deviation 25 12,5 25

C. Image Quality Metrics:

The variables used to determine the efficiency of the phantom were: noise, contrast, contrast-to-noise ratio (CNR), scanner radiation output (volume CT dose index CTDIvol) and a figure of merit (FOM) relating CNR and dose.

The noise was measured by selecting 4 rectangular regions of interest (ROI) containing water at 3 ,6, 9 and 12 o’clock positions in the phantom. Noise was then quantified by the resulting standard deviation of the CT numbers within such ROI. Contrast was measured by using the rectangular ROIs placed in the iodine and calcium samples, and subtracting the mean CT number obtained with the water mean CT number:

The CNR was calculated as:



The radiation exposure was quantified by the CTDIvol. A FOM was used to relate the radiation dose and the resulting image quality for the different acquisitions.

The FOM was calculated as follows:

Linear regression was used to confirm the linear relationship between the concentration of the materials and the corresponding CT numbers. The Pearson correlation coefficient was computed and used to identify the materials behavior. Matlab® R2010a (Mathworks, Natwick, MA, USA) and Microsoft Office Excel® were used for calculations and data processing.

III. RESULTS

A. Developed Phantom

The container and scaffold (Figure 2) are reflected as specified in the design; shape, dimensions and composition based on literature and design criteria. The dimensions were 300 mm width, 200 mm height and 160 mm depth. The total weight of the phantom filled with water was of about 18 kg. Total cost of the phantom container including materials and machining was approximately $ 200 US dollars.

Figure 2 Image of the developed phantom container (left),and samples of radiologic relevance (right)

B. Linear behavior between material concentration and CT attenuation

As expected, a linear relationship between material concentration and its corresponding CT number was confirmed with the experiments. In all cases Pearson correlation coefficients R2>0.95 were obtained.

For example, for iodine R2 = 0.957 and 0.965 for 80 and 140 kVp, respectively (Figure 3). For calcium chloride R2=0.998 and 0.999 for 80 and 140 kVp, respectively (Figure 4). These values correspond to the study performed on the dual source system.

Figure 3 Linear Regressions between iodine concentration and corresponding CT attenuation at 80 kVp (A) and 140 kVp (B).

Figure 4. Linear Regressions between calcium chloride concentration and corresponding CT attenuation at 80 kVp (A) and 140 kVp (B).

y = 28.99x + 55.09R² = 0.957

0100200300400500600

0 5 10 15 20

CT

att

enua

tion

(Hu)

Iodine Concentration(mg/ml)A

y = 14.63x + 25.38R² = 0.965

0100200300400500600

0 5 10 15 20CT

att

enua

tion

(Hu)

Iodine concentration (mg/ml)B

y = 1.715x + 109.94R² = 0.998

0

500

1000

1500

0 200 400 600 800CT

att

enua

tion

(Hu)

Calcium Concentration (mg/ml)A

y = 0.987x + 79.37R² = 0.999

0

500

1000

1500

0 200 400 600 800

CT

att

enua

tion

(Hu)

Calcium concentration(mg/ml)B



C. Image Quality Metrics

Sample images of the phantom at different energies and at the two evaluated systems showed consistent behavior of the phantom (Figure 5).

Figure 5 Sample images of the phantom at different acquisition settings. Siemens 80 kVp* (a). Siemens 140 kVp (b). Toshiba 80 kVp (c). Toshiba 135 kVp (d). *Note that the image corresponding to 80kVp in the Siemens system was acquired with a limited field of view of 26 cm.

As expected image noise, contrast, and corresponding CNR, CTDIvol and FOM were affected by acquisition parameters (kVp, mAs). A full summary of these results is shown in Table 3. With lower kVp, contrast increased but image noise also increased. Because in these experiments the dose (CTDIvol) was not matched, there was not a marked trend for the CNR. Neither it is possible to comment directly on the radiation dose.

However, the FOM allows one to compare how much ‘CNR’ one can gain per unit dose. This study found that in the dual source system, the FOM is improved with the use of lower kVp (80 kVp) for both calcium and iodine (Figure 6). FOM was improved for the single source system at 80 and 120 kVp relative to the 135 kVp, but it was relatively similar between 80 and 120 kVp (Figure 6). FOM were also higher for both iodine and calcium chloride in the dual source system relative to the single source system.

Table 3 Image Quality Metrics for iodine 3,75 mg/ml and calcium 211,7 mg/ml samples at 80,120,135/140 kVp.

kVp Contrast Noise CNR CTDIvol FOMIodine 3,75 mg/ml- Siemens system

140 82,93 9,33 8,89 5,28 3,87120 107,88 9,83 10,97 9,52 3,5680 190,48 13,60 14,01 4,03 6,98

Iodine 3,75 mg/ml- Toshiba system135 117,34 23,65 4,96 8,10 1,74120 124,67 10,11 12,33 18,40 2,8780 198,86 25,43 7,82 8,10 2,75

Calcium 211,7 mg/ml- Siemens system140 288,87 9,33 30,96 5,28 13,47120 332,59 9,83 33,83 9,52 10,9780 482,86 13,60 35,50 4,03 17,69

Calcium 211,7 mg/ml- Toshiba system135 362,70 23,65 15,34 8,10 5,39120 348,00 10,11 34,42 18,40 8,0280 542,99 25,43 21,35 8,10 7,50

A relation between the FOM and the different energies in each scanner for calcium and iodine was done (Figure 6).

Figure 6 FOM at each of the tube voltage evaluated for both calcium chloride (up) and iodine (down), for a single source (Toshiba) and a dual-source CT (Siemens) system from different vendors.

7,5 8,025,39

0

17,69

10,97

0

13,47

0

5

10

15

20

80 120 135 140

FOM

kVp

CALCIUM Toshiba Siemens

2,75 2,871,74

0

6,98

3,56

0

3,87

0

2

4

6

8

80 120 135 140

FOM

kVp

IODINE Toshiba Siemens



IV. DISCUSSION

In this work an imaging phantom simulating a typical human torso was developed. The phantom is compatible with CT and allows quantitative comparisons of important performance metrics such as image noise, contrast, CNR and radiation exposure. This was demonstrated in two different CT systems of two different vendors. The phantom geometry matches that of a typical patient torso of a 70 kg patient. The phantom is easy to transport due to its low weight of less than 2 kg empty.

The phantom allows flexible selection of materials to be inserted. When evaluating a clinically used iodinated contrast material, and a bone-like surrogate (calcium chloride), a remarkable linear relationship between the material concentration and CT attenuation was found, as one would expect. This result was consistent at different tube voltages (80, 120, 135 and 140 kVp) and also across two different vendors (Toshiba and Siemens).

Good quality images of the phantom were obtained. It can be observed how the different materials present a reasonable attenuation coefficient which coincides with data reported on the literature for similar materials. It is possible to see how there is a greater contrast when scanning at lower kVp energies in both systems. It is also noticeable how the fact of having higher energies, and hence higher radiation dose, has a direct positive impact on the image quality. As a curious fact, it can be seen that a smaller field of view can be seen when using the dual source scanner; this is a special feature of this system regarding its x-rays tubes, which affects directly the image obtained of the phantom in this case.

Scanning at low energies provided increased contrast but at the same time increased image noise. The evaluation of the figure of merit (FOM) suggests that the protocols used with the Siemens system were more efficient than the ones performed with the Toshiba scanner. This is because the former provided more CNR per unit dose. However, a limitation here is that in this study image kernels were not matched (eg resolution vs. image noise), and neither slice thickness. Yet, the purpose of the study was to show that with the developed phantom, experiments can be designed to carefully assess imaging performance of different protocols but also across scanners.

Regarding future work, different 3D reconstruction techniques and their 3D image quality metrics could be included; furthermore, adding motion to this phantom would be of great importance due that it would allow the simulation of several motion artifacts (e.g patient breathing).

The most efficient acquisition protocol was obtained when using low energies (80 kVp) in the dual source system. Recent publications have highlighted that the use of low

kVp can maintain image quality (eg CNR) and reduce dose, relative to conventional scanning at 120 kVp that is routinely used in most institutions [19].

V. CONCLUSION

The development of a low cost imaging phantom which can be used for CT protocol optimization in routine CT examinations was demonstrated. Phantom dimensions and shape simulated that of a typical adult of 70 kg. Phantom insertions also allow quantitative measures of noise, contrast and contrast-to-noise ratio.

Experimental evaluations of the phantom across CT protocols with different acquisition parameters and across different vendors, demonstrated that it allows to evaluate system (and protocol) performance by relating both achievable CNR and scanner radiation output.

ACKNOWLEDGMENTS

Authors thank Centro Medico Clínica de Occidente (CMC) and Centro Avanzado de Diagnóstico Médico Cedimed for assistance in phantom scanning. Authors also acknowledge assistance from technologist Guillermo Correa from CediMed for use of the single-source CT system, and from the Siemens Engineer Juan Carlos Sanabria for the use of the dual-source CT system.

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