電腦斷層 造影原理簡介 講者:林雍傑 長庚大學 醫學影像暨放射科學系 日期:2019/12/09 2019 年劑量評估程式應用研討會 Development of x-ray computed tomography - historical overview G.N.HOUNSFIELD LLAN M. CORMACK 1969 G.N. Hounsfield developed first clinically useful CT head scanner 1971 First clinically useful CT head scanner was installed at Atkinson-Morley Hospital (England) 1972 First paper on CT presented to British Institute of Radiology by Hounsfield and Dr. Ambrose 1974 Dr. Ledley introduced the whole body CT scanner (ACTA scanner) In 1979, G.N. Hounsfield shared the Nobel Prize in Physiology & Medicine with Allan MacLeod Cormack, Physics Professor who developed solutions to mathematical problems involved in CT https://youtu.be/RSyR8BlEz60 2 Background NCRP. Report No. 160, Ionizing Radiation Exposure of the Population of the United States. 2008 A single chest CT scan ≈ 350 standard chest X-rays 3 Contents Components of CT Scan Generations of CT Overview of Scan Parameters Dose Assessment in CT 4 Components of CT Scan X-ray tube: generates x-ray beams Filters Collimator: narrows the beams of x-rays Detector: detects the x-rays passing through the patient's body. 5 Generations of CT 6 Generations of CT FIFTH GENERATION ELECTRON-BEAM CT (EBCT) • Scan time : <50 ms • Principally applied to cardiac imaging SIXTH GENERATION OF CT SEVENTH GENERATION OF CT 7 Overview of Scan Parameters Localizer radiograph Helical/axial Pitch kV mA, mAs, effective mA Tube Current Modulation 8 Overview of Scan Parameters Localizer radiograph 9 Overview of Scan Parameters Axial/Helical 10 Overview of Scan Parameters Pitch Pitch = Table feed per rotation Influences: • Total scan time (e.g. breath hold) 11 Overview of Scan Parameters kV Often reduced in peds/smaller patients Contrast in image • Lower kV can give more contrast, especially with iodinated contrast agents (exploit k-edge) Tube output (mR/mAs) • Lower kV yields lower tube output –> noise increase • So, reducing kV often involves increasing mAs to offset noise increase Dose • CTDIvol ∝ kV2.5 • So, reducing kV from 120 to 80 (80/120)2.5= 0.36 (64% reduction) 12 Overview of Scan Parameters mA, mAs, effective mA Tube current (in mA) Tube Current time product (in mAs) Effective Tube Current Time Product • Effective mAs • mAs/Slice = mAs/pitch 13 Overview of Scan Parameters Tube Current Modulation Z-axis AEC 14 Overview of Scan Parameters Tube Current Modulation Rotation AEC 15 Overview of Scan Parameters Tube Current Modulation 16 Dose Assessment in CT - CTDI 17 Dose Assessment in CT - CTDI CTDIp CTDIc CTDIw = 100 1 2 CTDI + CTDIp c 3 3 CTDIvol = 𝐶𝑇𝐷𝐼𝑤 𝑝𝑖𝑡𝑐ℎ DLP = CTDIvol × 𝐿 (scan length) cm 𝑚𝑆𝑣 E (mSv) = DLP (mGy cm) × k (𝑚𝐺𝑦 𝑐𝑚) Shrimpton et al., 2006 AAPM TG Report No. 96., 2008 18 Dose Assessment in CT Existing Software Packages 19 Dose Assessment in CT Existing Software Packages 20 Thanks for your attention. Source : NASA 21