Download Basic CT Physics - Society for Pediatric Radiology

Basic CT Physics
Christina L. Sammet, Ph.D., DABR
Medical Physicist, RSO/LSO
Research Assistant Professor,
Northwestern University
Disclosures
• I am a member of the Bayer HealthCare Informatics
Global Advisory Board.
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Outline
1.) Current technology for cardiac CT imaging
2.) What influences radiation dose in cardiac imaging?
3.) What do these radiation units mean?
4.) How is radiation dose calculated in CT imaging?
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Outline
1.) Current technology for cardiac CT imaging
2.) What influences radiation dose in cardiac imaging?
3.) What do these radiation units mean?
4.) How is radiation dose calculated in CT imaging?
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Current technology for Cardiac
CT imaging
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Axial vs. Helical Scanning
Bushberg JT, et al. The Essential Physics of Medical Imaging, Wolters Kluwer, Philadelphia, 3rd Edition, 2012
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Axial vs. Helical Scanning
Bushberg JT, et al. The Essential Physics of Medical Imaging, Wolters Kluwer, Philadelphia, 3rd Edition, 2012
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Dual Source CT Scanner
Bushberg JT, et al. The Essential Physics of Medical Imaging, Wolters Kluwer, Philadelphia, 3rd Edition, 2012
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Volume Scanners
Bushberg JT, et al. The Essential Physics of Medical Imaging, Wolters Kluwer, Philadelphia, 3rd Edition, 2012
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Outline
1.) Current technology for cardiac CT imaging
2.) What influences radiation dose in cardiac imaging?
3.) What do these radiation units mean?
4.) How is radiation dose calculated in CT imaging?
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What influences radiation dose
in cardiac imaging?
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Z-axis dose modulation
Bushberg JT, et al. The Essential Physics of Medical Imaging, Wolters Kluwer, Philadelphia, 3rd Edition, 2012
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Z-axis dose modulation
Bushberg JT, et al. The Essential Physics of Medical Imaging, Wolters Kluwer, Philadelphia, 3rd Edition, 2012
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“On the fly” Dose Modulation
Vendor
GE
Philips
Siemens
Toshiba
Longitudinal Tube
Current
Modulation
Auto mA
Z-modulation (ZDOM)
CareDose
N/A
Longitudinal +
Angular Tube
Current Modulation
Smart mA
3D Modulation (DDOM)
CareDose 4D
Sure Exposure 3D
Bushberg JT, et al. The Essential Physics of Medical Imaging, Wolters Kluwer, Philadelphia, 3rd Edition, 2012
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Prospective vs. Retrospective
Cardiac Gating
Bushberg JT, et al. The Essential Physics of Medical Imaging, Wolters Kluwer, Philadelphia, 3rd Edition, 2012
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Beam Shaping Filter
Bushberg JT, et al. The Essential Physics of Medical Imaging, Wolters Kluwer, Philadelphia, 3rd Edition, 2012
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Beam Shaping Filter
Bushberg JT, et al. The Essential Physics of Medical Imaging, Wolters Kluwer, Philadelphia, 3rd Edition, 2012
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Outline
1.) Current technology for cardiac CT imaging
2.) What influences radiation dose in cardiac imaging?
3.) What do these radiation units mean?
4.) How is radiation dose calculated in CT imaging?
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What do these radiation units
mean?
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Absorbed Dose and the “Gray”
• Gray = Joule/kilogram = Energy/unit mass
• Physical quantity, and does not take into
account any biological context
• Diagnostic imaging procedures are in the
mGy range
• mGy is unit used for organ doses, CTDI
and SSDE
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Effective Dose and the “Sievert”
• Derived unit of radiation that is a measure
of the health effect of low levels of
ionizing radiation on the human body.
• The Sievert represents the probability of
radiation induced carcinogenisis and
genetic damage
• Diagnostic imaging procedures are in the
mSv range
• mSv is used exclusively for effective doses
• Dependent on the organs exposed and the
age of patient
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Radiobiology
https://www.med-ed.virginia.edu/courses/rad/radbiol/02bio/bio-04-01.html
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Radiobiology
Bushberg JT, et al. The Essential Physics of Medical Imaging, Wolters Kluwer, Philadelphia, 3rd Edition, 2012.
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Radiobiology
Bushberg JT, et al. The Essential Physics of Medical Imaging, Wolters Kluwer, Philadelphia, 3rd Edition, 2012.
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Pediatric Radiosensitivity
•
According to ICRP publication 103 (2007) the
excess relative risk of cancer for the adult
population is 4.1 - 4.8% per Sv.
•
Children have a higher relative risk when compared
to adults due to their increased growth rate and
ongoing cellular differentiation.
•
Depending on the age of exposure, children have a
lifetime risk of radiation-induced carcinogenesis 2-3
times higher than adults.
Pediatric Radiosensitivity
• Reference: Donald J. Peck and Ehsan Samei, Image Wisely “How to Understand
and Communicate Radiation Risk”, Image Wisely, ACR (2010).
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Outline
1.) Current technology for cardiac CT imaging
2.) What influences radiation dose in cardiac imaging?
3.) What do these radiation units mean?
4.) How is radiation dose calculated in CT imaging?
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How is radiation dose
calculated in CT imaging?
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Dose in CT
• The relatively simple geometry and
method of calculating dose in radiography
is not applicable to CT where there are
thousands of projections.
• A new method of dosimetry is needed
and is currently under development by
the medical physics community
• Though not an ideal method of patient
dosimetry, currently CT Dose reports
include the parameters Computed
Tomography Dose Index (CTDIvol) and
the Dose-Length-Product (DLP).
Bushberg, Jerrold, et al. The Essential Physics of Medical Imaging. Philadelphia, 2012.
CT Dose Report Examples
CTDIvol
CTDIvol is the dose delivered by the selected
protocol to a SINGLE SLICE of a plastic cylinder.
Bushberg, Jerrold, et al. The Essential Physics of Medical Imaging. Philadelphia, 2012.
CTDIvol
CTDIvol does not factor in ANY information about the patient.
Bushberg, Jerrold, et al. The Essential Physics of Medical Imaging. Philadelphia, 2012.
CTDIvol
Very important point about CTDIvol in Pediatric Imaging!
• The phantom sizes used to estimate the CTDIvol may be
inappropriate for pediatric sized patient.
• For Instance, a body protocol that references a 32cm
CTDI phantom will grossly underestimate the dose for a
small ped (usually assumed to be about a factor of two).
•
Most methods that convert Dose Length Product to
Effective Dose attempt to correct for patient size.
SSDE
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CTDIvol
Other Important things to remember about CTDIvol
• The CTDIvol displayed on the scanner before the Topogram (ie.
such as when you are building a protocol) will be only dependent
on the technique and the phantom used (16cm or 32cm).
Therefore, it will not accurately reflect the patient dose.
• If you have a scanner with dose modulation that uses the
topogram to estimate patient thickness, you will notice that the
CTDIvol will adjust to reflect the anticipated decreased (or
increased dose).
• You will get yet another CTDIvol number in the dose report. This
will reflect the dose modulation based on the topogram (what you
saw right before you scanned) and the “on-the-fly” dose
modulation.
DLP
• Dose Length Product is just the product of CTDIvol and the length
of the scan in the z-axis.
• DLP has units of mGy*length
• DLP is takes into account the length of the anatomy that was
exposed and is used for effective dose calculations
What can be done with DLP?
•
•
•
•
•
“Shrimpton” Method
“Deak” Method
Alessio Online Calculator
ImPact Calculator
“Huda” Method
AJR:199, August 2012.
Example: The Deak Method
Radiology: Volume 257: Number 1—October 2010.
Example: The Deak Method
Conversion of Effective Dose
to Cancer Risk?
. Bushberg, Jerrold, et al. The Essential Physics of Medical Imaging. Philadelphia, 2012
Conversion of Effective Dose to
Individual Patient’s Cancer Risk?
Big No-No!!!
Risk estimates apply to a
population that would be exposed
to that effective dose, not the
individual!
When communicating
with patients it is better
to convert effective
doses to background
radiation or another
familiar source of
radiation.
Thank you!
Christina L. Sammet, Ph.D., DABR
Email: [email protected]
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