Journal of the Korean Physical Society, Vol. 61, No. 1, July 2012, pp. 141∼146
A Study Evaluating the Dependence of the Patient Dose on the CT Dose Change in a SPECT/CT Scan Woo-Hyun Kim Department of Nuclear Medicine, Asan Medical Center, Seoul 138-736, Korea
Ho-Sung Kim Department of Nuclear Medicine, Asan Medical Center, Seoul 138-736, Korea and Department of Nuclear Engineering, Chosun University, Gwangju 501-759, Korea
Kyung-Rae Dong Department of Radiological Technology, Gwangju Health College University, Gwangju 501-701, Korea and Department of Nuclear Engineering, Chosun University, Gwangju 501-759, Korea
Woon-Kwan Chung∗ Department of Nuclear Engineering, Chosun University, Gwangju 501-759, Korea
Jae-Hwan Cho Department of Radiological Science, Gyeongesan University College, Gyeongesan 712-718, Korea and Department of Computer Science, Soonchunhyang University, Asan 336-745, Korea
Jae-Woo Shin Department of Biomaterials, Chonbuk National University School of Dentistry, Jeonju 570-752, Korea and Department of Dental Laboratory Technology, Gwangju Health College University, Gwangju 501-701, Korea (Received 7 May 2012, in final form 15 May 2012) This study assessed ways of reducing the patient dose by examining the dependence of the patient dose on the CT (computed tomography) dose in a SPECT (single-photon emission computed tomography)/CT scan. To measure the patient dose, we used Precedence 16 SPECT/CT along with a phantom for the CT dose measurement (CT dose phantom kit for adult’s head and body, Model 76-414-4150), a 100-mm ionization chamber (CT Ion Chamber) and an X-ray detector (Victoreen Model 4000M+). In addition, the patient dose was evaluated under conditions similar to those for an actual examination using an ImPACT (imaging performance assessment of CT scanners) dosimetry calculator in the Monte Carlo simulation method. The experimental method involved the use of a CT dose phantom to measure the patient dose under different CT conditions (kVp and mAs) to determine the CTDI (CT dose index) under each condition. An ImPACT dosimetry calculator was also used to measure CTDIw (CT dose indexwater ), CTDIv (CT dose indexvolume ), DLP (dose-length product), and effective dose. According to the patient dose measurements using the CT dose phantom, the CTDI showed an approximately 54 fold difference between when the maximum (140 kVp and 250 mAs) and the minimum dose (90 kVp and 25 mAs) was used. The CTDI showed a 4.2 fold difference between the conditions (120 kVp and 200 mAs) used mainly in a common CT scan and the conditions (120 kVp and 50 mAs) used mainly in a SPECT/CT scan. According to the measurement results using the dosimetry calculator, the effective dose showed an approximately 35 fold difference between the conditions for the maximum and the minimum doses, as in the case with the CT dose phantom. The effective dose showed a 4.1 fold difference between the conditions used mainly in a common CT scan and those used mainly in a SPECT/CT scan. This study examined the patient dose by reducing the CT dose in a SPECT/CT scan. As various examinations can be conducted due to the development of equipment, the patient faces increasing medical exposure. At this juncture, radiation workers and equipment manufacturers are required to make efforts to obtain as much medical information as possible while using the minimum radiation dose. PACS numbers: 87.58.-b, 87.58.Ce, 87.58.Sp, 87.58.Vr
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Journal of the Korean Physical Society, Vol. 61, No. 1, July 2012 Keywords: SPECT/CT, CTDIw, CTDIv, DLP, Effective dose DOI: 10.3938/jkps.61.141
I. INTRODUCTION
Medical equipment has developed rapidly with advances in science and technology. The dependence on medical imaging has increased because medical imaging is critical for evaluating anatomical and structural variations in and the physiological and biochemical status of the human body in a precise manner and to determine the diagnosis of and the treatment plan for a disease. Consequently, the development of medical equipment has enabled anatomical and functional imaging simultaneously with a single set of equipment. In recent years, it has been possible to visualize the functional and morphological parts of the human body simultaneously. This has increased the usefulness of positron emission tomography/computed tomography (PET/CT), which is conducive to an accurate diagnosis. As a result, increasing attention has been paid to single photon emission computed tomography/computed tomography (SPECT/ CT), which has encouraged further development of this type of equipment. CT imaging has the strong point that it represents changes in the size of an organ and the density of tissue and is a landmark for precise space occupation and local area. On the other hand, it is not sufficient for displaying the metabolic status of a disease. In contrast, a SPECT image can define the dynamic physiological and pathophysiological processes, but cannot display an accurate anatomical location for a diagnosis and evaluation of a specific disease. SPECT/CT combines the strong points of SPECT with thereof CT. SPECT/CT provides functional information from SPECT and anatomical information from CT in a single examination. The CT information can be used to quickly acquire an optimized and attenuationcorrected SPECT image. The above features increase the sensitivity and the specificity for the abnormal part (lesion) and for the physiological uptake of a radioactive isotope (part of normal uptake), which helps localize the lesion in a precise manner and which is conducive to an accurate diagnosis [1,2]. On the other hand, despite the recent increase in the use of SPECT/CT, the patient inevitably will face an increased dose. Recently, concerns have been raised about radiation exposure in the wake of the nuclear disaster in Fukushima, Japan. At the global level, efforts to reduce radiation exposure have been made by international organizations, such as the International Commission on Radiological Protection (ICRP), the International Atomic Energy Agency (IAEA) and agencies of countries around the world. As a result, increasing concern has also been paid to medical exposure in exami∗ E-mail:
[email protected]; Fax: +82-62-232-9218
nations conducted in hospitals. The agencies concerned have cooperated and supported ways to help radiation workers of medical institutions reduce the patient dose by establishing a recommended standard for the patient dose in medical exposure [3]. In particular, the ICRP published ICRP 73 in 1996, which was a report on radiation protection and safety in medical treatment. The report recommended that countries should comply with the diagnostic reference level (DRL) for patients [4]. As explained thus far, the increase in the use of SPECT/CT is an issue that should not be overlooked when increasing attention has been paid to radiation exposure, because the patient dose tends to increase as the patient dose from CT is added to the existing patient dose from SPECT. In a SPECT/CT scan, CT is often used for attenuation correction and localization of the space occupation. Consequently, low-dose radiation is frequently used compared to that of a CT scan that is conducted in the radiology department [5]. On the other hand, many patients who undergo a SPECT/CT scan receive a CT scan simultaneously in the radiology department. Therefore, the patients’ dose tends to increase when the CT dose is increased in a SEPCT/CT scan for the purpose of diagnosis [6]. Efforts should be made to reduce the patient dose in a SPECT/CT scan. In addition, improving the awareness of the patient dose among workers in medical institutions ia important. Against this backdrop, this study assessed ways of reducing the patient dose by examining the variation in the patient dose with changing CT dose in a SPECT/CT scan.
II. MATERIALS AND METHODS 1. Experimental Equipment
The SPECT/CT in this hospital was used to obtain a SPECT image of the phantom, where a radioactive isotope (99m TcO4 − ) was injected before measuring the difference in patient dose with changing CT dose. Precedence 16 (Phillips, USA) was used as the SPECT/CT equipment. In addition, a phantom for the CT dose measurement (CT dose phantom kit for an adult’s head and body, Model 76-414-4150, USA), a 100-mm ionization chamber (CT Ion Chamber) and an X-ray detector (Victoreen Model 4000M+, USA) were used to measure the patient dose (Fig. 1). In addition, the patient dose was evaluated under conditions similar to those for an actual examination by using an ImPACT dosimetry calculator in a Monte Carlo simulation.
A Study Evaluating the Dependence of the Patient Dose · · · – Woo-Hyun Kim et al.
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Fig. 1. (Color online) CT dose phantom, X-ray detector and ionization chamber.
2. Image Acquisition Method
The measurement was conducted at tube voltages of 90, 120, and 140 kVp and tube currents of 25, 30, 50, 100, 150, 200, and 250 mAs. The measurement method was to examine seven tube currents (mAs) at each tube voltage (kVp), obtain a phantom image and measure the patient dose. First, the CT dose phantom (body part) was used to measure the computed tomography dose index (CTDI) for each CT dose used in image acquisition. In this case, the image acquisition conditions were identical to the aforementioned CT factors, except that the scan length was changed to 4.83 cm for the measurement. Information on the CT equipment was entered and updated to the ImPACT dosimetry calculator, which intended to set up the factors suitable for the CT equipment. Subsequently, the dose used in the experiment was used to measure the patients’ dose. A SPECT image was obtained only once. CT images were obtained when the tube current was changed to seven different values for each tube voltage, which had three different values, which means that the measurement was conducted 21 times in total. Finally, the patient dose was measured for 21 different CT doses.
3. Patient Dose Evaluation Method
The CT dose phantom was examined for a CT dose that was identical to the dose when the image was obtained before recording the CTDI value displayed on the X-ray detector. The evaluation was performed for computed tomography dose indexwater (CTDIw), computed tomography dose indexvolume (CTDIv), doselength product (DLP) and effective dose, which were calculated using the ImPACT dosimetry calculator (Fig. 2). In phantom radiography, the scan length was set to the shortest value (4.8 cm) because the phantom length
Fig. 2. (Color online) Calculation of the patient dose by using an ImPACT dosimetry calculator.
was shorter than the scan range for an actual patient. On the other hand, for dose evaluation in the ImPACT dosimetry calculator, the scan length was set to a value (40 cm) similar to that when the patient was examined. The CTDIw and CTDIv were calculated as follows: First, the CTDI was calculated by dividing the integral value of the dose profile in the Z-axis direction, which was measured in air in a single slice scan or in a phantom for the CT dose measurement, by the slice thickness. The CTDI is defined as +∞ 1 D1 (z)Dz , (1) CTDI(mGy) = nT −∞ where D1 (z) is the single-scan dose profile in the z-axis direction, and T is the slice thickness. The weighted CTDI (CTDIw), which is the CTDI when the phantom is used, was calculated based on the CTDI100 acquired in the middle of the phantom and the CTDI100 acquired at the edge of the phantom (1 cm from the surface): CTDIW =
2 1 center + periphery. (2) 3CTDI100 3CTDI100
The CTDIvolume (CTDIv) is the CTDI on the axis of the scan that is introduced and used to evaluate the patient dose in a more accurate manner. This is the value when the change in exposure on the z axis is considered: CTDIv = CTDIw × NT/I, CTDIv = CTDIw /Pitch, (3)
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Table 1. Measurement of the dependence of the patient dose on the CT dose change using a CT dose phantom. kVp
90
120
140
mAs 25 30 50 100 150 200 250 25 30 50 100 150 200 250 25 30 50 100 150 200 250
CTDI (mGy) 0.02 0.03 0.05 0.10 0.16 0.22 0.28 0.06 0.08 0.14 0.28 0.42 0.57 0.71 0.10 0.13 0.20 0.43 0.65 0.87 1.09
where I is the distance where the table moves for each rotation in a spiral CT. The dose-length product (DLP) is the measurement value of the total dose for all images and was calculated by multiplying the CTDIv by the scan length: DLP(mGy × cm) = CTDIv × scan length.
(4)
The effective dose was introduced to evaluate the radiation damage caused by local radiation exposure, which can be expressed, according to the definition in ICRP Publication 60 in 1990, as (5) E= WT × WR × DT,R , where E represents the effective dose, WT is the tissueweighting factor, WR is the radiation-weighting factor (radiation-weighting factor of X-rays is 1), and DT R is the average absorption dose of tissue.
III. RESULTS 1. Measurement Using the CT Dose Phantom
A CT dose phantom and X-ray detector were used to measure the CTDI at the same dose as that when an image was obtained. The measurement results confirmed
Table 2. Measurement of the dependence of the patient dose on the CT dose change using an ImPACT dosimetry calculator. kVp
90
120
140
mAs 25 30 50 100 150 200 250 25 30 50 100 150 200 250 25 30 50 100 150 200 250
CTDIw (mGy) 0.5 0.6 1 1.9 2.9 3.8 4.8 1.1 1.3 2.2 4.3 6.5 8.7 10.8 1.5 1.9 3.1 6.2 9.4 12.5 15.6
CTDIv (mGy) 0.8 0.9 1.5 3.1 4.6 6.2 7.7 1.7 2.1 3.4 6.9 10.4 13.9 17.3 2.5 3 4.9 10 15 20 24.9
DLP (mGy) 31 37 61 123 185 246 307 69 84 138 278 418 555 693 99 121 198 399 600 798 996
Effective Dose (mSv) 0.54 0.67 1.1 2.2 3.3 4.4 5.5 1.3 1.6 2.7 5.4 8 11 13 1.9 2.3 3.8 7.7 12 15 19
that the patient dose increased linearly with increasing CT dose. The results revealed an approximately 54 fold difference between the maximum dose (140 kVp and 250 mAs) and the minimum dose (90 kVp and 25 mAs). The difference between the conditions of 120 kVp and 200 mAs, which are used mainly in the radiology department, and the conditions of 120 kVp and 50 mAs, which are used mainly in SPECT/CT scans, was found to be 4.2 fold (Table 1).
2. Measurement Using the ImPACT Dosimetry Calculator
An ImPACT dosimetry calculator was used to measure the effective dose as an actual SPECT/CT scan range was applied. According to the measurement results, there was a 35 fold difference between the maximum and the minimum doses, which was different result from the case with the CT dose phantom. On the other hand, the difference in dose was 4.1 times under the conditions of 120 kVp, 200 mAs and 120 kVp, 50 mAs, which shows that the patient dose changed in a manner similar to that with the CT dose phantom (Table 2) (Fig. 3).
A Study Evaluating the Dependence of the Patient Dose · · · – Woo-Hyun Kim et al.
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Fig. 3. (Color online) Measurement of the patient dose by using an ImPACT dosimetry calculator and measurement of the patient dose by using a CT dose phantom.
IV. DISCUSSION As the patient dose has increased due to an increase in the use of SPECT/CT, it has become necessary to conduct a range of studies and to collect information for the purpose of dose reduction. Under the recognition that the role of CT in a SPECT/CT scan is attenuation correction and localization, this study intended to reduce the patient dose by decreasing the CT dose. On the other hand, an unconditional decrease in the CT dose might reduce the image quality, which would prevent an accurate diagnosis. As a result, for a reasonable decrease in dose, a phantom was used to evaluate the image quality before examining the subsequent variation of the patient dose. The investigation was conducted on methods to reduce the patient dose in a CT scan that was conducted for a diagnosis as part of an effort to reduce the CT dose. The ICRP recommended a range of methods to obtain a minimum radiation dose in a proper range based on the principles of radiation protection in medical exposure, such as justification, optimization and dose limit. The methods can be generally narrowed down to two aspects [7]. The first aspect is related to radiation workers, such as doctors, radiologists and radiation medical specialists. First, restricting the unnecessary patient dose by setting the minimum scan range within a range that is sufficient for examination purposes is important. Second, checking whether or not the image can be skipped before using a contrast medium in an examination focusing on contrast enhancement is essential. Third, a protocol for preset factors, such as the tube voltage, scan length, slice thick-
ness, pitch and applied voltage, are needed to ensure that the examination can be conducted according to the body part and physique of the patient. Fourth, if the patient is a child and organs such as the thyroid, crystalline lens of the eyes, breasts, or genital glands (testicles, ovaries and uterus) are included in the scan range, shielding of such organs, even if they are not a concern, is important. Fifth, minimizing cases where a CT scan needs to be conducted again due to a mistake made by the radiation worker in the process of scanning is important. The second aspect is related to the equipment manufacturer. First, reducing the effects from scattered rays by increasing the preliminary filtration of the radiation spectrum is important. Second, the accuracy of automatic exposure control equipment needs to be improved. Third, an image reconstruction method that can reduce the image noise generated due to the low dose is required. As explained thus far, a range of methods have been used to reduce the patient dose in a CT scan conducted in the radiology department. Since the image is obtained for diagnostic purposes, dose reduction is performed to the point where the image quality is not deteriorated. On the other hand, considering that the purpose of a CT scan in SPECT/CT is not a diagnosis but attenuation correction and localization, the use of radiation in medical practice can be justified when the damage that medical exposure causes to the patient can be reduced, even though image quality is reduced somewhat due to the increased noise in dose reduction [8–11]. In this context, radiation workers need to recognize that they are obliged to have a firm justification for using radiation in medical practice by optimizing methods to minimize the
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patient’s medical exposure [12,13]. This study had some limitations. Confirming if the experimental method and analysis are appropriate was difficult because there were few studies that could be used for comparison. In addition, determining a suitable phantom for the purpose of the experiment is not important. Moreover, conducting the experiment for an actual patient was impossible. Therefore, using a phantom was inevitable. Because the size of the phantom used in the experiment was different from the body size of a patient, conducting an accurate comparison of the experimental results was difficult. A future study should find ways to overcome the aforementioned limitations and examine the CT dose reduction under conditions similar to those for an actual human body.
V. CONCLUSION This study examined the patient dose by reducing the CT dose in a SPECT/CT scan. As various examinations can be conducted due to the development of equipment, the patient faces increasing medical exposure. At this juncture, radiation workers and equipment manufacturers are required to make efforts to obtain as much medical information as possible while using the minimum radiation dose. In addition, it is important to use radiation for medical practice with a focus not on the persons concerned but on patients.
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