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
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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
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Contents
 Components of CT Scan
 Generations of CT
 Overview of Scan Parameters
 Dose Assessment in CT
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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.
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Generations of CT
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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
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Overview of Scan Parameters
 Localizer radiograph
 Helical/axial
 Pitch
 kV
 mA, mAs, effective mA
 Tube Current Modulation
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Overview of Scan Parameters
Localizer radiograph
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Overview of Scan Parameters
Axial/Helical
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Overview of Scan Parameters
Pitch
 Pitch = Table feed per rotation
 Influences:
•
Total scan time (e.g. breath hold)
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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)
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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
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Overview of Scan Parameters
Tube Current Modulation
Z-axis AEC
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Overview of Scan Parameters
Tube Current Modulation
Rotation AEC
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Overview of Scan Parameters
Tube Current Modulation
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Dose Assessment in CT - CTDI
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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
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Dose Assessment in CT
Existing Software Packages
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Dose Assessment in CT
Existing Software Packages
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Thanks for your attention.
Source : NASA
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