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기술논문 흉부 CT 촬영에서저선량프로토콜의선량과화질 : 표준선량프로토콜과비교 이원정 *, 안봉선, 박영선 * 근로복지공단직업성폐질환연구소, 대전보건대학교방사선과 2012 년 3 월 16 일접수 / 2012 년 5 월 22 일 1 차수정 / 2012 년 6 월 3 일 2 차수정 / 2012 년 6 월 5 일채택 임상에서사용하고있는흉부 CT 촬영의저선량프로토콜과표준선량프로토콜간의선량과화질을비교분석하였다. 흉부저선량프로토콜 (120 kvp, 30 mas) 과표준선량프로토콜 (120 kvp, 180 mas) 로촬영 (Brilliance TM CT 16slice, PHILIPS) 한 61 명의조영제를사용하지않은영상에서기관분기부위치의종격동영상을본연구를위해사용하였다. 상행대동맥과가시아래근에서 CT number 와잡음을측정하였고, Back-ground 잡음을측정하여신호대잡음비 (signal-to-noise ratio. SNR) 와대조도잡음비 (contrast-to-noise ratio, CNR) 를구하였다. 두부아크릴팬텀을이용하여선량을측정하였고, 워터팬텀으로얻은영상에서 CT number 와잡음을측정하였다. 모든측정은 3 회실시하여평균값을 SPSS 프로그램 (version 14.0) 으로분석하였고, 그래프는시그마플롯프로그램 (version10.0) 을사용하였다. 결과 : 상행대동맥과가시아래근에서저선량프로토콜영상이표준선량프로토콜영상보다유의하게높은잡음을보였고, SNR 과 CNR 은유의하게낮았다. 두영상에서비만지수에대한잡음은양의관련성을보였지만, SNR 과 CNR 은음의관련성을보였다. 팬텀결과에서저선량프로토콜의선량이표준선량프로토콜보다유의하게낮았지만 (0.35 mgy vs. 1.95 mgy, p=0.008), 잡음은저선량프로토콜에서유의하게높았다 (p=0.029). 저선량프로토콜이표준선량프로토콜보다유의하게낮은선량을보였지만, 화질평가도유의하게낮은결과를보임으로서임상에서사용하는저선량프로토콜의노출선량은화질을고려하여상향조정할필요가있다. 중심어 : 흉부저선량, 방사선선량, 화질, 신호대잡음비, 대조도잡음비 1. 서론 1) 영상의학분야에서컴퓨터단층촬영 (computed tomography, CT) 은정밀검사방법으로사용빈도가점점증가하고있지만 [1], 다른검사방법에비해높은환자피폭선량을수반한다 [2]. 그동안계속적인 CT 장치의기술개발로수년전부터도입되고있는나선형다중검출기 CT (spiral multidetector CT) 의연속적인주사 (continuous scan) 는검사시간단축과얇은절편 (thin slice) 검사로고식적인 CT (conventional CT) 에서해결되지못했던공간분해능과시간분해능의향상을가져와해상력 (high resolution) 이높은영상을얻을수있고, 다양한영상재구성 (multiplanar reformated image) 이가능하여진단적가치는더욱향상되었으나 [3] 환자에대한방사선피폭선량은오히려증가시킬수있다 [4]. CT 촬영에서방사선에의한환자피폭을감소시킬수있는방법으로는관전압 (tube voltage, kvp) 과관전류 책임저자 : 이원정, atomlwj@yahoo.co.kr (426-858) 경기도안산시상록구구룡로 87 번지근로복지공단직업성폐질환연구소임상연구부 (tube current, ma), 피치 (pitch) 등의방법들이소개되었고 [2, 5-7], 비교적일찍부터관심을가져온흉부 CT 촬영 [8] 에서는노출선량 (mas, ma sec) 을감소시키는방법이주로사용되어왔다 [9-12]. 흉부 CT 에서화질 (image quality) 은여러가지요인들과관련이있는데그중에서도선량은화질과유의한양의관련성을보이는것으로알려져있다 [6, 7]. 하지만, 질병 (diseases) 진단의민감도를높이기위한선량증가는환자의피폭도증가시키기때문에화질을고려해서노출선량이적정하게이루어질필요가있다. 또한, 현재임상에서사용되고있는표준선량프로토콜 (standard-dose protocol) 은표준화되어있지않고, 환자피폭선량감소를위한저선량프로토콜 (low-dose protocol) 의최적의선량에대해서도연구자마다이견이있다 [8,9,13,14]. 이와같은배경하에, 본연구에서는임상에서흉부진단에선량감소를위해사용하고있는저선량프로토콜과표준선량프로토콜의노출조건에서선량을측정하고, 객관적으로평가한화질결과를비교함으로서민감도를고려한적정한선량감소가이루어지고있는지에대해알아보았다. JOURNAL OF RADIATION PROTECTION, VOL.37 NO.2 JUNE 2012 84

2. 대상및방법 2.1 영상획득과화질평가임상에서사용하는흉부저선량프로토콜 (120 kvp, 30 mas, 2-mm thickness) 과표준선량프로토콜 (120 kvp, 180 mas, 1.2-mm thickness) 로촬영 (Brilliance TM CT 16slice, PHILIPS, Netherand) 한 61 명의조영제를사용하지않은영상 (non-contrast image) 에서기관분기부위치 (level of carina bifurcation) 의종격동영상 (mediastinal image) 을본연구를위해사용하였다. 본연구에서는자체연구과제수행으로얻은영상 ( 기관연구윤리심의위원회에서승인과대상자로부터검사동의서받았음 ) 을후향적으로분석하였다. 연구대상자의평균나이는 65.9±8.6 세 ( 범위, 49-81 세 ) 였고, 평균비만지수 (body mass index) 는 23.8±3.1 kg/m 2 ( 범위, 16.4-29.1) 이었다. 비만지수는다시 2 개그룹 (25 kg/m 2 미만, 25 kg/m 2 이상 ) 으로분류하였고, 각그룹의대상자수는 39 명 (63.9%) 과 22 명 (36.1%) 이었다. 저선량프로토콜과표준선량프로토콜로얻은영상의상행대동맥 (ascending aorta) 과가시아래근 (inpraspinatus muscle) 에서 CT number (HU, hounsfield unit) 와잡음 (noise) 을측정하였고 (Fig. 1), 신호대잡음비 (signal-tonoise ratio. SNR) 와대조도잡음비 (contrast-to-noise ratio. CNR) 를구하기위해서 back-ground 잡음을측정 ( 영상앞쪽 1 cm 위치공기중 ) 하였다. 측정에서관심영역 (region of interest) 은모두 100~110 cm2범위였고, 3 회측정하여평균값을구하였다. 관심영역내의잡음은 CT number 의표준편차 (HU, standard deviation) 로정의하였다. 두프로토콜로얻은영상을 2 개의판독용모니터 (5M) 에각각나타낸후표준선량프로토콜영상에서먼저위치를확인및측정하였고, 이어서저선량프로토콜영상을측정하였다. SNR 과 CNR 은관련문헌을참고 [6] 로다음과같은공식으로계산하였다 : SNR= 상행대동맥 ( 또는가시아래근 ) CT number/back-ground noise. CNR=( 가시아래근 CT number- 상행대동맥 CT number)/background noise. 2.2 팬텀을이용한선량측정및화질평가영상획득과동일한장치와노출조건에서한국의료영상품질관리원가이드라인을참조 [15] 하여, 선량과화질평가용팬텀을 sequence 모드에서자동노출을사용하지않고스캔하였다. 선량측정은 CT 장치테이블위에직경 16 cm 의두부아크릴팬텀 (10S5-3CT, RADCAL, USA) 을올려놓고겐트리중심부에서 10cm 길이의연필형이온챔버 (ion-chamber, 3 cc) 를선량측정계 (Unfors Mult- O-Meter, RADCAL, USA) 에연결한후, 중심부와가장자리구멍 ( 총 5 군데 ) 에번갈아위치시킨후 3 회반복측정하였다. 화질평가를위해서는선량측정과같은방법으로워터팬텀 (AAPM CT Performance Phantom, 76-410-4130, USA) 을스캔하여얻은영상의 4 군데를동일한 면적 (1062.1 mm2 ) 으로하여 3 회반복하여 CT number 와잡음을측정하였다. 2.3 통계분석팬텀을이용한두프로토콜간에선량, CT number 및잡음비교는 Mann-Whitney U test 를실시하여통계적인유의성을검정하였다. 두프로토콜로얻은모든연구대상자각각영상에서측정한 CT number 및잡음으로부터 SNR 과 CNR 을구하였고, 평균을 Pared t-test 로비교하였다. 연구대상자의비만지수그룹간에 CT number 및잡음, SNR 과 CNR 비교는 Student t-test 로분석하였고, 비만지수에대한상행대동맥의잡음및 SNR, CNR 의관련성은 Pearson 상관분석을실시하였다. 모든통계분석은 SPSS (version 19.0) 를이용하였고, 그래프는시그마플롯프로그램 (version 10.0) 을사용하였다. 3. 결과 3.1 저선량프로토콜과표준선량프로토콜영상간의화질비교상행대동맥과가시아래근에서잡음은저선량프로토콜영상이표준선량프로토콜영상보다유의하게높았고, 두영상모두상행대동맥보다는가시아래근에서높은잡음을보였다 (Table 1). 저선량프로토콜영상과표준선량프로토콜영상의잡음차이는상행대동맥과가시아래근에서비슷하였다. 상행대동맥과가시아래근에서의 SNR 은저선량프로토콜영상보다표준선량프로토콜영상이유의하게높았고, CNR 도표준선량프로토콜영상이유의하게높았다 (Fig 1). 두영상모두상행대동맥보다가시아래근에서높은 SNR 을보였다. Table 1. Comparison of Parameters Related to Image Quality between Low-dose and Standard-dose Protocol. AA Parameter Low-dose Standard-dose P-value CT number (HU) 37.31(6.01) 43.22(4.66) <0.001 Noise (HU) 42.32(7.83) 25.25(4.31) <0.001 SNR 1.80(0.51) 3.17(0.92) <0.001 CT number (HU) 47.28(4.77) 53.75(4.47) <0.001 IM Noise (HU) 46.38(9.83) 28.35(5.83) <0.001 SNR 2.30(0.63) 3.92(1.07) <0.001 Back-ground noise 22.69(9.58) 16.23(10.79) <0.001 CNR 0.49(0.40) 0.74(0.46) <0.001 Data are expressed as the mean with standard deviation. Noise was defiened as standard deviation of CT number. Statistical analysis was performed by Paired t-test. AA= Ascending aorta. IM=Infraspinatus muscle. HU=hounsfield unit. SNR=Signal-to-noise ratio. CNR=Contrast-to-noise ratio. 85 JOURNAL OF RADIATION PROTECTION, VOL.37 NO.2 JUNE 2012

A B Fig. 1. Non-contrast mediastinal images of low-dose (A, 120 kvp, 30 mas, 2-mm thickness) and standard-dose protocol (B, 120 kvp, 180 mas, 1.2-mm thickness) on a subject with 66-year-old man (Body mass index=29.1 kg/m 2 ). CT number and noise were measured each at infraspinatus muscle (*) and ascending aorta ( ). (A) Signal-to-noise ratio (SNR) at ascending aorta and contrast-to-noise ratio (CNR) were 1.80, 0.04, respectively. (B) SNR at ascending aorta and CNR were 3.15, 0.30, respectively. A B C Fig. 2. Scatter plots of body mass index on subject versus parameters related to image quality (A, noise; B, signal-to-noise ratio; C, contrast-to-noise ratio) in low-dose and standard-dose protocol. Noise, signal-to-noise ratio and contrast-to-noise ratio were significantly correlated with body mass index of subjects. Noise and signal-to-noise ratio were measured at ascending aorta. 3.2 연구대상자의비만지수에대한잡음및 SNR, CNR 의관련성비만지수를두그룹으로나누어상행대동맥에서의 CT number 와잡음, SNR 과 CNR 을비교한결과를 Table 2 에서보여주고있다. 두영상에서 CT number 는유의한차이를보이지않았다. 두영상모두비만지수가 25 이상그룹이 25 미만그룹보다잡음이통계적으로유의하게높았고, SNR 과 CNR 은유의하게낮았다. 비만지수두그룹간에잡음의차이는저선량프로토콜영상이표준선량프로토콜영상보다컸다. 또한, 두그룹간에 SNR 은표준선량프로토콜영상에서크게나타났지만, CNR 은비슷한차이를보였다. 연구대상자의비만지수에대한상행대동맥에서측정 한잡음과 SNR 과 CNR 의상관관계를분석한결과 (Fig 2), 저선량프로토콜영상과표준선량프로토콜영상에서비만지수가증가함에따라잡음도증가하는의미있는양의관련성을보였지만, SNR 과 CNR 은두영상모두에서비만지수가증가함에따라감소하는유의한음의관련성을보였다. 잡음은비만지수가증가할수록두영상간에상관성차이가컸지만, SNR 은비만지수가증가할수록두영상간에차이가적어졌다. 비만지수에대한잡음과 CNR 의상관성은저선량프로토콜영상이표준선량프로토콜영상보다더크게나타났지만 SNR 은표준선량프로토콜영상에서더큰것으로나타났다. JOURNAL OF RADIATION PROTECTION, VOL.37 NO.2 JUNE 2012 86

Table 2. Comparison of Parameters Related to Image Quality between Groups Stratified by Body Mass Index in Low-dose and Standard-dose Protocol. Lowdose Standarddose Protocol Body Mass Index (kg/m2) less than 25 (39, 63.9%) 25 or greater (22, 36.1%) P-value CT number (HU) 36.90(6.97) 38.05(3.80) 0.404 Noise (HU) 38.96(6.68) 48.28(6.03) <0.001 SNR 1.97(0.49) 1.51(0.41) <0.001 Back-ground noise 19.72(6.51) 27.95(11.85) 0.005 CNR 0.60(0.43) 0.30(0.23) 0.001 CT number (HU) 43.41(4.90) 42.88(4.29) 0.670 Noise (HU) 23.30(3.00) 28.70(4.15) <0.001 SNR 3.47(0.75) 2.65(0.97) 0.001 Back-ground noise 13.68(7.12) 20.76(14.40) 0.040 CNR 0.87(0.44) 0.52(0.40) 0.004 Data are expressed as the mean with standard deviation. Subjects were devided into two groups according to the body mass index. CT number, noise and SNR were measured at ascending aorta. Noise was defiened as standard deviation of CT number. Statistical analysis was performed by Student t-test. HU=Hounsfield units, SNR= signal-to-noise ratio, CNR=contrast-to-noise ratio. 3.3 팬텀을이용한저선량프로토콜과표준선량프로토콜간의선량및화질비교저선량프로토콜과표준선량프로토콜의노출조건에서팬텀을이용한선량과화질평가에관련된 CT number 와잡음을비교한결과 (Table 3) 에서선량은저선량프로토콜이표준선량프로토콜보다유의하게낮았지만 (0.35 mgy vs. 1.95 mgy, p=0.008), 잡음은오히려저선량프로토콜영상에서유의하게높은결과를보였다 (p=0.029). Table 3. Comparison of Radiation Dose, CT Number and Noise between Low-dose and Standard-dose Protocol using Phantom. Parametor Low-dose Standard-dose P-value Radiation dose (mgy) 0.35(0.1) 1.95(0.5) 0.008 CT number (HU) 0.4(0.94) -0.9(0.26) 0.029 Noise (HU) 99.6(10.3) 40.1(4.3) 0.029 Data are expressed as the mean with standard deviation. Radiation dose was measured at 5-point using acrylicphantom. CT number and noise were measured at 4- point using water-phantom. Noise was defined as standard deviation of CT number. Statistical analysis was performed by Mann-Whitney U test. 4. 고찰및결론 흉부 CT 영상이폐질환진단에영상의학분야검사에서 가장민감도가높은것은이미알려져있다. 하지만, 다른검사에비해환자에게조사되는선량은매우높아선량감소를위한방법들이연구되어왔다. 흉부 CT 촬영에서선량을감소시키기위한방법으로는해부학적으로관전류의적응 (anatomy-adapted tube current modulation) [16,17], 관전압 [6,18] 과관전류를감소시키는방법 [8-12] 등이사용되고있다. 환자피폭선량감소를위한낮은노출조건 ( 관전류와관전압 ) 으로얻은영상은잡음을증가시켜화질을떨어뜨릴수있기때문에진단의민감도에영향이적은영상을얻기위해서는적정한노출조건설정이필요하다 [6,18]. 임상에서흉부 CT 촬영시환자피폭선량감소를위해노출선량조절에의한저선량프로토콜사용이증가하고있지만표준선량프로토콜에비해낮은화질로인해민감도를떨어뜨릴수있어논란의여지가있다 [10,13,19]. 흉부 CT 영상의화질평가를위한객관적평가와주관적평가는높은일치도를보이지만 [6,18], 주관적평가는평가자간에차이를보일수있어 [12,20] CT number 와잡음을측정하여구한 SNR 과 CNR 의객관적인평가가더신뢰성을보장할수있다 [4,18,19]. 따라서, 임상에서흉부저선량프로토콜을위해사용되는노출선량으로부터얻은영상의객관적평가를실시하여노출선량의적정성여부를알아볼필요가있었다. 흉부 CT 영상에서객관적인화질평가를위해 CT number 나잡음은기관분기부위치의상행또는하행대동맥에서주로측정되는데 [11-13,17], 이러한이유는다른부위에비해측정부위가넓으면서혈액으로만구성되어있어비교적균일한측정결과를얻을수있기때문이다. 우리연구에서도저선량프로토콜과표준선량프로토콜에서기관분기부위치의상행대동맥과가시아래근에서동일한면적으로 CT number 와잡음을측정한후 SNR 과 CNR 을구하였고, 연구대상자의영상에서저선량프로토콜영상이표준선량프로토콜영상보다높은잡음을보인결과는팬텀영상의결과와도일치하였다. 실제임상에서는저선량프로토콜과표준선량프로토콜의노출선량은다양하게사용하고있고 [8,9,13,14,20], Prasad 등 [20] 의연구에서표준선량 (220-280 mas) 을 50% 로감소 (110-140 mas) 하고도흉부영상의주관적인해부학적구조물평가에서유의한차이를보이지않았다. 또다른연구에서는변조전달함수 (modulation transfer function) 를이용한객관적평가와주관적평가에서관전압을 120 kvp 로설정했을때적어도노출선량은 40 mas 이상은되어야했고 [9], 관전압을일정 ( 보통 120 kvp) 하게유지하고노출선량을 30~50 mas 정도로낮추어촬영하면잡음의증가로민감도에영향을줄수있다는연구결과 [13] 를참고로할때, 우리연구에서 30 mas 를사용한저선량프로토콜이표준선량프로토콜보다 82% 의높은선량감소를보였지만, 팬텀영상과연구대상자영상에서두프로토콜간에유의한화질차이를보여실제임상에서는민감도를고려하지않은너무낮은선량이사용되고있음을알수있었다. 흉부는다른부위에비해뼈, 공기등의방사선감약 87 JOURNAL OF RADIATION PROTECTION, VOL.37 NO.2 JUNE 2012

계수차이가큰물질로구성되어있고, 폐구역 (upper, middle, lower) 마다병변을발견하기위해요구되는선량이다르기때문에 [14] 환자의두께나조직에따라선량노출이자동조절되는방법 (auto tube current modulation) 이피폭선량감소효과가크다 [2,5,16]. Prakash 등 [12] 의연구결과에서연구대상자의체중을고려한선량자동노출은주관적화질평가에서유의한차이없이 17-43% 선량감소를보였다. 연구대상자의비만지수는잡음을증가시켜화질을떨어뜨리는요인으로알려져있는데 [11], 환자상태 ( 두께등 ) 에관계없이관전류가일정한 (fixed ma) 프로토콜에서는비만지수가증가함에따라영상잡음도증가하는양의상관관계를보임으로써작은환자일수록과노출 (over-exposure) 되고, 큰환자에게는노출이부족 (underexposure) 되는현상을보였다. 환자의상태에따라관전류가조절되는 (adaptive ma) 프로토콜에서는비만지수가변화하더라도영상잡음은평균값에서크게벗어나지않아비만지수가노이즈에영향을미치지않았다. 또한, 환자상태에따라관전류가조절되는프로토콜이관전류가일정한프로토콜보다측정된잡음은통계적으로유의하게높았지만주관적인화질평가결과에는영향을미치지않았고, 30% 의선량감소효과를가져왔다 [17]. 우리연구는저선량프로토콜과표준선량프로토콜모두관전류가일정한프로토콜을사용하여비만지수에대한잡음, SNR 과 CNR 이유의한상관성을보였는데, 비만지수에따른화질의영향을피하고일정한영상위해서는자동노출을사용할필요가있었다. 또한, 비만지수가낮아질수록두영상간에잡음차이가줄어들면서 SNR 은더커지는결과를보임으로서비만지수가낮아질수록선량이화질에미치는영향이크게작용했다. 본연구는두프로토콜간에선량차이에따른영상의화질차이를비교하고자하였지만후향적으로설계된연구로영상의두께차이가화질에미친영향을배제할수없었다. 즉영상의두께가증가할수록잡음이감소하여더높은 SNR 과 CNR 을얻을수있기때문에동일한두께의영상을비교했다면저선량프로토콜영상과표준선량프로토콜영상간에는더큰차이를보였을것이다. 흉부 CT 촬영에서높은방사선노출로인한환자의위해성을감소시킬목적으로시행하는저선량프로토콜과표준선량프로토콜간의선량과화질을비교한본연구결과에서저선량프로토콜의선량이유의하게낮았지만, 잡음증가로객관적인화질평가결과는표준선량프로토콜영상보다유의하게낮았다. 따라서, 질병진단의민감도를고려한국제방사선방호위원회 (International Commission on Radiological Protection, ICRP) 에서제시한정당화 (justification) 와최적화 (optimization) 에근거 [2] 하여현재임상에서사용하는저선량프로토콜의노출선량은상향조정할필요가있다. 또한, 선량과화질을고려한최적의노출선량선정을위해서는단계적인노출선량을설정한전향적인팬텀연구가필요하다. 감사의글본연구에협조해주신근로복지공단안산산재병원영상의학팀 CT 실근무자분들께감사드립니다. 참고문헌 01. 건강보험심사평가원. CT 촬영비용및장비설치현황. http://stat.kosis.kr/nsieu/index.jsp?horg=354. 02. International Commission on Radiological Protection. Managing patient dose in computed tomography. ICRPPublication 87. Oxford; Pegamon Press. 2000. 03. Rydberg J, Buckwalter KA, Caldemeyer KS, Phillips MD, Conces DJ Jr, Aisen AM, Persohn SA, Kopecky KK. Multisection CT: scanning techniques and clinical applications. Radiographics. 2000; 20(6):1787-1806. 04. Tzedakis A, Damilakis J, Perisinakis K, Stratakis J, Gourtsoyiannis N. The effect of z overscanning on patient effective dose from multidetector helical computed tomography examinations. Med. Phys. 2005;32(6):1621-1629. 05. McCollough CH, Bruesewitz MR, Kofler(Jr) JM. CT dose reduction and dose management tools: Overview of available options. Radiographics. 2006; 26(2):503-512. 06. Heyer CM, Mohr PS, Lemburg SP, Peters SA, Nicolas V. Image quality and radiation exposure at pulmonary CT angiography with 100- or 120-kVp protocol: Prospective raddomized study. Radiology. 2007;245(2):577-583. 07. Goo HW. CT radiation dose optimization and estimation: an update for radiologists. Korean. J. Radiol. 2012;13(1):1-11. 08. Naidich DP, Marshall CH, Gribbin C, Arams RS, McCauley DI. Low- dose CT of the lungs: preliminary observations. Radiology. 1990:175(3):729-731. 09. Johkoh T, Honda O, Yamamoto S, Tomiyama N, Koyama M, Kozuka T, Mihara N, Hamada S, Narumi Y, Nakamura H, Kudo M. Evaluation of image quality and spatial resolution of low-dose high-pitch multidetector-row helical high-resolution CT in 11 autopsy lungs and a wire phantom. Radiat. Med. 2001;19(6):279-284. 10. Fasola G, Belvedere O, Aita M, Zanin T, Follador A, Cassetti P, Meduri S, De Pangher V, Pignata G, Rosolen V, Barbone F, Grossi F. Low-dose computed tomography screening for lung cancer and pleural mesothelioma in an asbestos-exposed population: baseline results of a prospective, nonrandomized feasibility trial--an Alpe-adria Thoracic Oncology Multidisciplinary Group Study (ATOM JOURNAL OF RADIATION PROTECTION, VOL.37 NO.2 JUNE 2012 88

002). Oncologist. 2007;12(10):1215-1224. 11. Stolzmann P, Leschka S, Betschart T, Desbiolles L, Flohr TG, Marincek B, Alkadhi H. Radiation dose values for various coronary calcium scoring protocols in dual-source CT. Int. J. Cardiovasc. Imaging. 2009;25(4):443-451. 12. Prakash P, Kalra MK, Gilman MD, Shepard JAO, Digumarthy SR. Is weight-based adjustment of automatic exposure control necessary for the reduction of chest CT radiation dose? Korean. J. Radiol. 2010;11(1):46-53. 13. Kubo T, Lin PJ, Stiller W, Takahashi M, Kauczor HU, Ohno Y, Hatabu H. Radiation dose reduction in chest CT: a review. AJR. Am. J. Roentgenol. 2008;190(2):335-343. 14. Itoh S, Ikeda M, Arahata S Kodaira T, Isomura T, Kato T, Yamakawa K, Maruyama K, Ishigaki T. Lung cancer screening: minimum tube current required for helical CT. Radiology. 2000;215(1): 175-183. 15. 한국의료영상품질관리원. 전산화단층촬영장치품질관리검사. http://www.ikiami.or.kr/info/kmi417qd. aspx 16. Huda W, Ogden KM, Khorasani MR. Converting dose-length product to effective dose at CT. Radiology. 2008;248(3):995-1003. 17. Qi W, Li J, Du X. Method for automatic tube current selection for obtaining a consistent image quality and dose optimization in a cardiac multidetector CT. Korean J Radiol. 2009;10(6):568-574. 18. Park EA, Lee W, Kang JH, Yin YH, Chung JW, Park JH. The Image Quality and Radiation Dose of 100-kVp versus 120-kVp ECG-Gated 16-Slice CT Coronary Angiography. Korean. J. Radiol. 2009; 10(3): 235-243. 19. Takahashi M, Maguire WM, Ashtari M, Khan A, Papp Z, Alberico R, Campbell W, Eacobacci T, Herman PG. Low-dose spiral computed tomography of the thorax comparison with the standard-dose technique. Invest. Radiol. 1998; 33(2): 68-73. 20. Prasad SR, Wittram C, Shepard JA, McLoud T, Rhea J. Standard-dose and 50%-reduced-dose chest CT: comparing the effect on image quality. AJR. Am. J. Roentgenol. 2002;179(2):461-465. Radiation Dose and Image Quality of Low-dose Protocol in Chest CT: Comparison of Standard-dose Protocol Won-Jeong Lee *, Bong-Seon Ahn, and Young-Sun Park * Occupational Lung Diseases Institute, KCOMWEL, Department of Radiological Technology, Daejeon Health Science College The purpose of this study was to compare radiation dose and image quality between low-dose (LDP) and standard-dose protocol (SDP). LDP (120 kvp, 30 mas, 2-mm thickness) and SDP (120 kvp, 180 mas, 1.2-mm thickness) images obtained from 61 subjects were retrospectively evaluated at level of carina bifurcation, using multi-detector CT (Brilliance 16, Philips Medical Systems). Signal-to-noise ratio (SNR) and contrast-to-noise ratio (CNR) were calculated at ascending aorta and infraspinatus muscle, from CT number and back-ground noise. Radiation dose from two protocols measured at 5-point using acrylic-phantom, and CT number and noise measured at 4-point using water-phantom. All statistical analysis were performed using SPSS 19.0 program. LDP images showed significantly more noise and a significantly lower SNR and CNR than did SDP images at ascending aorta and infraspinatus muscle. Noise, SNR and CNR were significantly correlated with body mass index (p<0.001). Radiation dose, SNR and CNR from phantom were significant differences between two protocols. LDP showed a significant reduction of radiation dose with a significant change in SNR and CNR compared with SDP. Therefore, exposure dose on LDP in clinical applications needs resetting highly more considering image quality. Keywords : Chest low-dose, Contrast-to-noise ratio, Image quality, Radiation dose, Signal-to-noise ratio 89 JOURNAL OF RADIATION PROTECTION, VOL.37 NO.2 JUNE 2012