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CT 自動曝露控制宣導及使用說明 黎靜盈 APPLICATION SPECILIST ROTARY TRADING CO.,LTD. TOSHIBA MEDICAL EQUIPMENT EXCLUSIVE DISTRIBUTOR OF TAIWAN Contents z Biologic effects of ionizing radiation z CT usage and dose contribution z CT imaging techniques (parameters) and dose z Technology to reduce CT dose --- Tube current modulation (Sure Exposure) z Pediatric issue z Practical tips and strategies Contents z Biologic effects of ionizing radiation z CT usage and dose contribution z CT imaging techniques (parameters) and dose z Technology to reduce CT dose --- Tube current modulation (Sure Exposure) z Pediatric issue z Practical tips and strategies Biologic effects of ionizing radiation 人體吸收輻射能量時,細胞和水 分子會首先被游離或激發,造成 DNA雙鏈全斷或只斷單鏈的傷 害(直接傷害)。因為水分佔了 人體約 70 % 的重量,而水分子 被游離後會產生有害的OH自由 基,這些自由基接續產生一連串 化學反應,使得細胞分子受到損 傷(間接傷害)。所幸細胞有自 行修復的能力,大部分的細胞會 恢復正常。 假若細胞嚴重受損 而無法修復或修復有錯誤時,則 其將顯現出健康受損的症狀。 Linear non-threshold theory 輻射對人體的健康效應,通常分為機率效應和確定效應兩大類。當人體 在短時間內接受劑量超過某一程度以上時,因為許多細胞死亡或已無法 修復,而產生疲倦、噁心、嘔吐、皮膚紅斑、脫髮、血液中白血球及淋 巴球顯著減少等症狀。當接受劑量更高時,症狀的嚴重程度加大,甚至 死亡,這種情況稱為確定效應。通常確定效應必須在接受劑量超過一定 程度以上才會發生。 從日本核爆生存者長期調查顯示,接受低劑量(約250毫西弗以 下)者,並無任何臨床症狀,白血病或其他實體癌的發生率都和一般人 相同。但是為了輻射安全的緣故,國際放射防護委員會( ICRP )做了一 個很保守又很重要的假設:人體只要接受到輻射,不管劑量是多少,都 有引發癌症和不良遺傳的機率存在,沒有低限劑量值,而且致癌或不良 遺傳的機率與接受劑量成正比(直線關係),劑量愈高,罹患的機率也 愈大,這種情況稱為機率效應。 Linear non-threshold theory 從臨床的觀點來區分輻射的健康效 應:健康效應發生在受照射本人身上 的,稱為軀體效應,若發生在受照射 者的後代子孫身上的,稱為遺傳效 應。從發生效應的快慢而言,則分為 急性效應和慢性效應。 PNAS November 25, 2003 vol. 100 no. 24 13765 Linear non-threshold theory The International Commission on Radiological Protection had previously estimated that a dose of 10 mSv would cause one in 2000 lifetime cancers. ( ICRP 1991 ; 21 ( 1-31 ) : 1 - 201 ) Contents z Biologic effects of ionizing radiation z CT usage and dose contribution z CT imaging techniques (parameters) and dose z Technology to reduce CT dose --- Tube current modulation (ACE) z Pediatric issue z Practical tips and strategies CT usage and dose contribution Nishizawa, Acta Radiol Jap 2004; 64:151-158 CT usage and dose contribution Cancer risks from diagnostic radiology E J HALL, DPhil, DSc, FACR, FRCR and D J BRENNER, PhD, DSc The British Journal of Radiology, 81 (2008), 362–378 Dose in radiology department http://www.radiologyinfo.org March 30, 2007 Contents z Biologic effects of ionizing radiation z CT usage and dose contribution z CT imaging techniques (parameters) and dose z Technology to reduce CT dose --- Tube current modulation (ACE) z Pediatric issue z Practical tips and strategies CT imaging techniques and dose Dose depends on: scanner geometry, filtration, kV, collimation, scan mode , mA , pitch , sec … CT imaging techniques and dose CTDI depends on: Scan mode CTDI increases as phantom/patient size decrease Pediatric CTDI can be underestimated with standard phantom CT imaging techniques and dose CTDI depends on: Collimation Collimation ↑,CTDI ↓ Collimation ↓,CTDI ↑ CT imaging techniques and dose CTDI depends on: kVp z CTDI increases with kVp z Approx ∝ kVp2 CT imaging techniques and dose CTDI depends on: Pitch CT imaging techniques and dose CTDIw (mGy) CTDIvol with scan length (mGy‧cm) EDLP (mSv mGy-1cm-1 ) mSv CT imaging techniques and dose AAPM Report NO.96 January 2008 The measurement ,reporting and Management Radiation Dose in CT. How to reduce CT dose ALARA as low as reasonably achievable How to reduce CT dose Guidelines or regulation Contents z Biologic effects of ionizing radiation z CT usage and dose contribution z CT imaging techniques (parameters) and dose z Technology to reduce CT dose --- Tube current modulation (AEC) z Pediatric issue z Practical tips and strategies Tube current modulation (ACE) z Unlike screen-film radiographs, images acquired with computed tomography (CT) never look overexposed Noise The Thehigher higherthe thedose, dose, the thelower lowerthe thenoise. noise. high 1 noise dose low dose low high Tube current modulation (ACE) DoseX X/2 X/4 X/8 X/16 X/64 Tube current modulation (ACE) DoseX X/2 X/4 X/8 X/16 X/64 z z z z Patients size AEC Z-axis AEC Rotational AEC Combination Tube current modulation (ACE) z z A) patient size AEC: higher mA is used for larger patient c) rotational AEC: the degree of modulation depends on asymmetry at each z-axis position z b) z-axis AEC: higher mA used at more attenuating z-axis positions z d) combined effects of using all three levels of AEC Tube current modulation (ACE) Tube current modulation (ACE) Benefit with AEC z z Consistent image quality Reduction in photon starvation artifact High attenuation LOW attenuation Constant mAs 199mAs, noise = 12.9HU Tube mA modulation 189mAs, noise = 9.4HU Tube current modulation (ACE) Benefit with AEC z z Potential for dose reduction through exposure optimization Reduced tube loading (extended scan runs) Dose saved Tube current modulation (ACE) Summary Sure Adjusts mA to achieve your desired level of Image quality. Ensures image quality for all patient sizes. Can reduce dose up to 40%. Exposure Scanogram • The first Scanogram performed is used in the calculation of Sure Exposure •The quality, i.e. adquate penetration, of the Scanogram directly affects the calculations (same KV) •Positioning within the isocentre can affect the exposure calculations Scanogram 3 Dimensional Sure Exposure tube current modulation AP scanogram will be used to calculate the AP / PA exposure (Y-axis). The Lateral scanogram will be used to calculate the lateral exposure (X-axis). Z axis modulation still applies. AP Modulation Graph mA graph display toggle switch LAT Modulation Graph Scanogram Dose • Scanogram Dose is low…..Very low. • The dose display on the image is a total mAs. Scanogram Dose Eff. mAs = mA x Slice thickness Table Speed Slice Thickness Table Speed Eff. mAs = = 2mm = 100mm/sec mA x 2 mm 100 mm/s Eff. mAs = mA x 0.02 sec Scanogram Dose Eff. mAs = mA x 0.02 sec For a 50 mA scanogram Eff. mAs = 50 x 0.02sec Eff. mAs = 1 mAs Water Equivalent Thickness The attenuation values from the scanogram are converted into a “Water equivalent thickness. SD level is set in Exam Plan Parameters of the reconstruction, are considered for the resultant SD Water equivalent thickness mA is calculated to get same image SD @ All positions. The operator predetermines the SD. Sure Exposure calculates the mA required to achieve the same noise level (SD) for each position. Standard Deviation • Standard Deviation (SD) is a measurement of image noise. •It indicates the CT value deviation around the mean value. •The higher the value the greater the noise level ROI Water Phantom Standard Deviation Low noise image High noise image ROI ROI Mean= 0HU SD= +/- 2HU Mean= 0HU SD= +/- 30HU SUREExposure z SUREExposure 3D – Preset presets now include target slice thickness and reconstruction parameter settings. SUREExposure z 3D – Preset Head Parameters apply to exam plans with Head body region selected. Head Region SUREExposure z 3D – Preset Neck Parameters apply to exam plans with Neck body region selected. Neck Region SUREExposure z 3D – Preset Body Parameters apply to exam plans with all other body regions selected. Body Region SUREExposure z 3D – Preset Set a target SD and SUREIQ parameter for EACH preset SUREExposure z 3D – Preset Set a minimum and maximum mA for EACH preset SUREExposure z 3D – Preset XY modulation can be set ON/OFF for EACH preset SUREExposure z z 3D – Preset Set a target slice thickness for EACH preset Note: Have the same target slice thickness for each body region SUREExposure z z SUREExposure 3D – Scanning settings menu does not need target SD settings. Easy quality selection is by preset name only. SUREExposure z 3D – Scanning Manual settings have been hidden from the main menu. SUREExposure z 3D – Scanning Manual settings require input of target slice thickness and SUREIQ parameters. New Default SUREExposure Settings Contents z Biologic effects of ionizing radiation z CT usage and dose contribution z CT imaging techniques (parameters) and dose z Technology to reduce CT dose --- Tube current modulation (ACE) z Pediatric issue z Practical tips and strategies Pediatric issue David J. Brenner1 Carl, D. Elliston1,Eric J. Hall1, Walter E. Berdon2 ;AJR:176, February 2001 Pediatric issue z High risk David J. Brenner1 Carl, D. Elliston1,Eric J. Hall1, Walter E. Berdon2 ;AJR:176, February 2001 Pediatric issue z The unique consideration in children • increased radiosensitivity of certain tissues, particularly in infancy • longer lifetime for radiation-related cancer to occur(慢 性效應) • The thyroid gland, breast tissue, and gonads are structures that have an increased sensitivity to radiation in growing children. Pediatric issue AAPM Report NO.96 January 2008 The measurement ,reporting and Management Radiation Dose in CT. Pediatric issue Physical Statistics Age (Y) 18 17 16 15 14 13 12 11 10 9 8 7 6 5 4 3 2 1.5 1 0.75 0.5 0.25 0 Weight (kg) 63.1 61.8 58.8 54.5 48.8 42.2 38.3 35.2 32.6 29.9 27.3 24.5 21.9 19.4 16.5 14.6 12.6 11.4 10.0 9.1 7.6 5.7 3.4 Hipwidth (cm) 28.0 27.8 27.4 26.7 25.8 24.6 23.5 22.6 22.0 21.4 20.7 19.9 19.1 18.3 16.9 15.8 14.4 13.7 13.0 12.3 11.6 10.6 8.1 Chestcircumf (cm) 83.4 82.5 80.7 78.0 74.5 70.3 67.8 65.9 63.9 61.8 59.8 57.8 56.1 54.5 53.7 52.4 50.8 49.5 48.0 46.0 43.7 40.6 33.2 Documenta Geigy Wissenschaftliche Tabellen Pediatric issue Age does not tell you all... Pediatric issue •The x-ray attenuation doubles for every 4 cm increase in diameter, thus the mAs needs to be doubled in order to keep the same image noise Diameter X-ray signal (50mAs) mAs for constant noise 0 100.00% 50 4 cm 50.00% 100 8 cm 25.00% 200 12 cm 12.50% 400 16 cm 6.25% 800 Pediatric issue This could mean... Suppose he needs 50 mAs... He might need 100 mAs or even more... 體重(公斤) Pediatric issue z Weight - dose table Tsai IC, Lee WL, Tsao CR, Chang Y, Chen MC, Lee T, Liao WC. Comprehensive evaluation of ischemic heart disease using multidetector row CT: Part 1- Comprehensive? Why and how? AJR:191, July 2008 E-mAs kVp CTDIVOL 0~3 280 80 6.0 4~6 320 80 6.9 7~10 400 80 8.6 11~15 520 80 11.1 16~20 200 120 14.0 21~25 250 120 17.5 26~30 300 120 21.0 31~35 350 120 24.5 36~40 400 120 28.0 41~45 450 120 31.5 46~50 500 120 35.0 51~55 550 120 38.5 56~60 600 120 42.0 61~65 650 120 45.5 66~70 700 120 49.0 71~75 750 120 52.5 76~80 800 120 56.0 81~85 850 120 59.5 86~90 900 120 63.0 91~100 950 120 65.8 100以上 800 140 83.3 320-row CT minimizes dose in pediatric abdominal studies Newborn with aortic hypoplasia and renal artery stenosis. Weight of 1.7 kg; singlerotation 320-detector-row scan at 0.5 sec, 80 kV, 40 mA, and dose of 0.3 mSv. All images courtesy of Dr. Patrik Rogalla. Extended Dose display (1) Result - Dose Tab. Extended Dose display (2) Result exposure record output. Contents z Biologic effects of ionizing radiation z CT usage and dose contribution z CT imaging techniques (parameters) and dose z Technology to reduce CT dose --- Tube current modulation (ACE) z Pediatric issue z Practical tips and strategies Practical tips and strategies Centering the patient Li et al, AJR 2007; 188:547-522 Practical tips and strategies Pregnancy • • • • • Identify probability of pregnancy Minimize dose, consider alternative Document dose Low exposure High exposure – informed decision, awareness of risk --- Questionnaire, consent form Practical tips and strategies Breast Radiation z z z Especially scans for pulmonary embolism in young women Dose to breasts may be up to 19 times that from a mammogram Strategy for dose reduction: be selective Practical tips and strategies Patient shielding Courtney et al, AJR:190, January 2008 Practical tips and strategies FDA recommendation z z z Optimize CT setting --- reduce tube current --- Develop and use a chart or table of tube-current settings based on patient weight or diameter and anatomical region of interest --- Increase table increment (axial scanning) or pitch (helical scanning). Reduce the number of multiple scans with contrast material Eliminate inappropriate referrals for CT References E J HALL et al, The British Journal of Radiology, 81 (2008), 362–378 Courtney Coursey et al, AJR:190, January 2008 Brenner DJ et al, N Engl J Med 2007;357:2277–84. Brenner DJ et al, Proc Natl Acad Sci USA 2003;100:13761–6. McCollough et al, Radiographics ,Volume 26 ;Number 2:503-513 ……. http://www.impactscan.org/ http://www.ctug.org.uk/index.html http://www.auntminnie.com/ http://www.arwen.se/radiologi/omnimas_en/OmnimAs-Instruction.htm