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ISSUES IN CT RADIATION DOSE

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					ISSUES IN CT RADIATION DOSE
         CT Radiation Dose Issues
 Concerns    about CT radiation doses
    – Increasing Number of CT Exams
    – Possible Increase in radiation dose per exam
 Interpreting and Using CTDI values
 CTDI versus Effective Dose
 Pediatric Radiation Dose
 Reduce of minimize patient dose
    – Technological: CT Auto Exposure Control
    – Adjusting mA for patient size and pitch
     Concerns about CT Radiation
“Larger doses and increased lifetime risk in children
produce a sharp increase in risk from CT. Estimated
lifetime cancer mortality risk from CT radiation in a 1-year
old is 0.18% for abdominal CT and 0.07% for head CT---
an order of magnitude higher than for adults. With
approximately 600,000 CT exams/year on children under
15, a rough estimate is that 500 of these children might
ultimately die from cancer attributable to CT radiation. “

Eric Hall Ph.D, et al: Feb 2001 AJR
        Concerns about CT Radiation
“It’s time the medical profession became aware of the epidemic
of patient exposure to diagnostic radiation.... Doctors have
limited knowledge of doses and potential consequences of the
massive increase in diagnostic medical radiation exposure. I
saw examples of overexposure where I work.....I saw it happen
to my daughter.... A abdomen or pelvis spiral CT exposes a
patient to ~10 mSv….The risk of 1 or 2 studies is negligible but,
in young patients, 5 or more studies may lead to overexposure....
balancing risks and benefits is increasingly important as CT
studies become easier and faster to do. In the US, ~60 million
CT studies were done in 2006. I have become a zealot in trying
to stem this tide.”
Dr. Steven Birnbaum, BMJ, 5/11/2007.
60000
        Growth in CT Procedures-HH
50000

40000

30000

20000

10000

   0
   1997   1998   1999   2000   2001   2002   2003   2004   2005   2006
    Radiation Doses from CT Procedures
   A CTDI is doses at a specific phantom location (eg:
  at center or at periphery 1 cm deep)
 CTDIw is weighted average of CTDIs at center and
  periphery:
      CTDIw = (2*CTDIperiphery + CTDIcenter)/3
    CTDIw is a single dose value for the exam and is
    often quoted the dose to patient for that procedure
   CTDIvol is more useful for helical scans:
          CTDIvol = CTDIw/Pitch
    i.e., if pitch >1, dose is spread out over more tissue
CTDIw: Dose for a “Procedure”
     Radiation Doses from CT Procedures
   Concern: A procedure covering a 10 cm Z-length
    yields the same CTDI as one covering 100 cm !

   Since there is logically more risk associated with
    irradiating more tissue, an alternative is to refer
    to a “Dose-Length Product” (DLP):
                  DLP = CTDIvol * L
     where L is the Z-length of the scan
              EFFECTIVE DOSES
   Limitations of CTDI and Associated Indices:
    – Cannot combination doses from multiple procedures
      of different body parts
    – Cannot compare to dose from procedure types
    – Cannot easily be compared to other risks
   For these purposes, a better method is based on
    concept of Effective Dose:
    – Defined as radiation dose to entire body that presents
      the same total risk (i.e. of cancer) as the (higher) dose
      actually given to part of anatomy exposed by exam
    – Calculated from CTDI (or other area specific metrics)
      using organ/tissue weighting factors
CTDIw: Dose for a “Procedure”
               Doses From CT
Exam                        Eff Dose (mSv)          BERT*
Chest x ray (PA film)            0.02              2.4 days
Skull x ray                      0.07              8.5 days
Lumbar spine                     1.3               158 days
I.V. urogram                     2.5               304 days
Upper G.I. exam                    3                1.0 year
Barium enema                       7               2.3 years
CT head                            2               243 days
CT abdomen                        10               3.3 years
PET FDG Exam (10 mCi)            3.9               1.3 years
NM Bone Scan (20 mCi)            1.3               158 days
* Time to receive equivalent effective dose from
 natural background radiation
Population Exposure from CT Radiation
   Assume:
    –   60 million CT exams/year (previously quoted)
    –   1/3 head exams at 2.3 mGy effective dose/exam
    –   2/3 body exams at 6.8 mGy effective dose/exam
    –   300 million US population
   Total Effective CT Dose: 318 million mGy
    – 2.3 mGy x 20 million exams = 46 million mGy
    – 6.8 mGy x 40 million exams = 272 million mGy
   Average effective dose from CT: ~ 1 mGy
    – By comparison: average background dose: ~3 mGy
„A‟ and „B‟ receive
dose from all tube
angles. If same
technique used for
smaller “patient”,
more dose given to
each point from all
angles since less
material must be
penetrated.
CT DOSIMETRY
Central CTDI vs Patient Size
    Phantom Rel CTDI

       6       1.86
       12      1.28
       16      1.00
       24      0.61
       28      0.48
       32      0.37
Adjusting Technique for Patient Size
CT Rad Dose vs Patient Size, Part II:
  Technique for other (adult) patients often
  remained unadjusted. An improvement (now
  available on most scanners) is “Auto-Exposure
  Control” which automatically modifies mA
  based on patient size:
 On slice-by-slice basis (based on one or two
  “scout views”) (Z-modulation)
    Patient Attenuation vs Z-position




From ImpaCT Report 05016, Feb 2005
CT Rad Dose vs Patient Size, Part II:
  Technique for other (adult) patients often
  remained unadjusted. An improvement (now
  available on most scanners) is “Auto-Exposure
  Control” which automatically modifies mA
  based on patient size:
 on a slice-by-slice basis (based on one or two
  “scout views”) (Z-modulation)
 and for some scanners, on an angle-by-angle
  (for each slice) as well (rotational modulation)
     Rotational Auto-mA Modulation




From Kalender et al, Med Phys, Nov 1999
Managing/Minimizing Radiation Dose

 Adjust techniques appropriately for patient size
 Use pediatric protocols, if provided
 Use auto-exposure control if available (Z-
  modulation and or rotational modulation)
 Adjust mA appropriately for helical scan pitch
  (some manufacturers do this automatically)

				
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posted:10/25/2011
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