Radiation Dose and Dose Modulation in MDCT

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Radiation Dose and Dose Modulation in MDCT Powered By Docstoc
					Radiation Dose Optimization Techniques
 in MDCT Era: From Basics To Practice

                    Chang Hyun Lee, MD
                           JM Goo, MD, HJ Lee, MD
                         EJ Chun, MD, CM Park, MD
                                        JG Im, MD

       Seoul National University Hospital

1. To discuss the importance of
   radiation dose modulation in MDCT
2. To review the characteristics of
   radiation dose modulation
   techniques in different MDCT
3. To explain how to use and apply this
   techniques to the patients during
   MDCT scan
1.   Basic concepts in CT dose index
     -   Fundamental dose parameters
     -   CT dose index
     -   Parameters affecting CTDI
2.   Radiation dose increase in MDCT scanner
     -   Radiation dose in MDCT
     -   Radiation risk
3.   Effective dose in various CT examination
     -   Effective dose
     -   Calculation and estimates of effective dose
     -   Radiation dose summary
4.   Radiation dose modulation techniques in
     different MDCT scanners – Reference mAs –
     Reference noise index – it’s advantages and
5.   Practical tips for optimizing radiation dose in
Fundamental Dose Parameters

                        BACK TO
Fundamental Dose Parameters

                    10 mSv = 1 rem
  Fundamental Dose Parameters
• Exposure
   – Roentgens (C/kg) or air kerma (J/kg – mGy)
   – Ionization in air per unit mass or amount of energy
     imparted per unit mass
   – Related to intensity of radiation at point of measurement
   – Irradiated area, penetrating power, tissue sensitivity에 대
     한 고려가 없음.
   Fundamental Dose Parameters
• Absorbed dose
   – Joules/kg or mGray (mGy)
   – Energy absorbed by material per unit mass
   – Depends on radiation type, energy & material irradiated
   – Quoted locally or averaged over area e.g. organ
• Absorbed dose – amount of radiation energy deposited in
  the patient‟s body as a result of exposure
• Radiation exposure: radiation source-related term
• Radiation dose: body-related term

             D                   D                   ~ 3D
  Fundamental Dose Parameters
• Effective dose
   – mSv
   – Measure of radiation risk to patient
   – Attempts to reflect equivalent whole body dose that
     results in same stochastic risk
   – Applies organ sensitivity factors
   – Enables risk comparison between different procedures
     and modalities
Fundamental Dose Parameters
•   Absorbed dose, however, does not account for
    differing sensitivities of the organs to radiation
•   Equivalent dose in a tissue is a product of the
    tissue type and the radiation weighting factor
•   Equivalent dose has the same numerical value as
    absorbed dose and is measured in sievert or rem.
•   Effective dose is computed by summing the
    absorbed doses in the organs weighted by their
    radiation sensitivity.
•   Effective dose estimates the whole-body dose that
    would be required to produce the same risk as
    partial-body dose delivered by a localized
    radiological procedure - useful in evaluating
    potential biological risk of a specific radiologic
CT Dose Index

                BACK TO
          CT dose index

• CTDI was developed originally by
  Shope et al. back in 1981.
• The basic radiation dose parameter in
  CT is the CT dose index (CTDI).
• Represents absorbed radiation dose
  in a CT dose phantom, measured in
  the gray or rad.
       CT dose index: CTDI
• CTDI has been defined for use with single-detector
  row CT scanners.
• CTDI is the total energy absorbed within a dose
  profile deposited within one nominal collimation.
          CT dose index

• Three derivatives of the CTDI
• CTDI100: radiation exposure measured
  by means of an ionization chamber
  with a length of 100 mm.
• CTDI100w: weighted radiation dose in
  axial (nonhelical) CT scan
• CTDIvol: volume CTDI = CTDIw/pitch
CT dose index: CTDI

         (Multiple-scan average dose)
Measurement equipment: CTDI
 • Ionization chamber
 • Thermoluminescent Dosimeters (TLD)
 • X-ray film
Ionization chamber: CTDI
Ionization chamber: CTDI
Average dose in scan plane: CTDIw

 • Weighted average CTDI represents
   the average dose in scan plane of
   Perspex phantom
 CTDIw = [2/3 CTDI100 (periphery) +
                1/3 CTDI100 (center)] x 33.7
Average dose in scanned volume:

• Axial: CTDIvol=CTDIw x (slice width/couch
• Helical: CTDIvol = CTDIw / Pitch
        Noncontiguous exposure along z-axis
• Advantages of CTDI as a dose descriptor

• Disadvantages of CTDI
Parameters affecting CTDI

                        BACK TO
 Effect of scan parameters on
• mA and scan time (mAs per rotation)
• CTDIvol increase linearly with mA and
  scan time
• E.g 2 x mAs = 2 x CTDIvol
 Variation of CTDIvol with kVp

• CTDIvol increases with kVp
• Approx ∝kVp2

                               ImPact 2005
     CTDI and slice width

• CTDI increases if irradiated width does
  not match nominal width
           CTDI = Area / nT

            T1         T2     T3
Variation of CTDIvol with no. of slices

 • Number of slices
 • CTDIvol is independent of number of
   – Absorbed dose: energy absorbed per unit
Effect of pitch on CTDIvol

              • CTDIvol is inversely
                proportional to pitch
              • E.g. doubling pitch
                halves the CTDIvol
              • … but only if mA
                remains constant
              • On some systems
                mA automatically
                adjusted for pitch so
                CTDIvol is constant
       Effect of patient size
• For same scan parameters (mAs, kV) the dose
  increases as phantom/patient size decreases
• For pediatrics CTDI can underestimate dose by ~ x 2
  if measured in standard sized phantoms
Radiation dose in MDCT

                         BACK TO
   Radiation dose in MDCT

• ~ 60 million CTs per year – doubled
  in 5 years
• > 60,000 CT examinations at
  University of Alabama at Birmingham,
• ~ 67% total radiation exposure is
  from CT
Radiation Dose in MDCT

          Y Imanishi et al.
          Eur Radiol (2005) 15:41-46
CT examination is increasing in Japan


                  No. of image (x10,000)
     80,000      年検査件数(x1,000)
                  No. of exam/year (x1,000)
                  No. of CT




              1979         1989               2000

                     Nishizawa, Acta Radiol Jap 2004; 64:151-158
Scan area is also increasing

          Nishizawa, Acta Radiol Jap 2004; 64:151-158
 Does multi-slice CT impart more
     or less radiation dose?

• An increase by 10-30% may occur
   with multi-slice detector array

 ICRP (International Commission on Radiological Protection) Publication 87
                Doses from Chest CT
                       Helical CT         MDCT (4)       LDCT (single)

kV                     120                120            120
mA100-210              300-350            25
Rotation time (s/r)    1-1.5              0.5            2

Table feed (mm/s)      10                 7-11           20/2s/r
FOV (cm)               27                 35             30

Organ dose (mGy)
   Bone marrow         5.91               7.19           2.51
   Lung                20.94              19.59          3.09
   Stomach             8.59               19.83          1.41
   Breast              18.24              20.20          2.41
   Liver               0.43               19.62          1.64
   Esophagus           18.12              18.16          2.90
   Thyroid             8.23               23.70          2.41

Effective dose (mSv)   7.62               11.0           1.40

                            Nishizawa, Acta Radiol Jap 2004; 64:suppl
Radiation Risk

                 BACK TO
Audience Response Question

• What is the conventional estimate of
  the long term risk of death from
  cancer for 10 mSv whole body
  exposure for an average person?
     A. Negligible
     B. 1:20,000
     C. 1:2,000
     D. 1:200
Audience Response Question

• What is the conventional estimate of
  the long term risk of death from
  cancer for 10 mSv whole body
  exposure for an average person?
     A. Negligible
     B. 1:20,000
     C. 1:2,000
     D. 1:200
Audience Response Question

• The radiation dose from a chest
  radiograph is approximately what
  fraction of the dose from yearly
  natural background radiation?
     A. 1:100
     B. 1:10
     C. 1:1
     D. 10:1
Audience Response Question

• The radiation dose from a chest
  radiograph is approximately what
  fraction of the dose from yearly
  natural background radiation?
     A. 1:100
     B. 1:10
     C. 1:1
     D. 10:1
Audience Response Question

• The radiation dose for One CT scan
  versus One chest radiograph?
     A. CT < CR
     B. CT > CR, CT < 10 CR
     C. CT > CR, 10 CR < CT < 100 CR
     D. CT > CR, CT = 100 ~ 250 CR
     E. CT > CR, CT > 500 CR
Audience Response Question

• The radiation dose for One CT scan
  versus One chest radiograph?
     A. CT < CR
     B. CT > CR, CT < 10 CR
     C. CT > CR, 10 CR < CT < 100 CR
     D. CT > CR, CT = 100 ~ 250 CR
     E. CT > CR, CT > 500 CR
          Relative Dose Beliefs

Diagnostic CT Scans: Assessment of Patient, Physician, and
Radiologist Awareness of Radiation Dose and Possible Risks. Lee
CL et al. Radiology 2004
            Cancer Risk Beliefs

Diagnostic CT Scans: Assessment of Patient, Physician, and
Radiologist Awareness of Radiation Dose and Possible Risks. Lee
CL et al. Radiology 2004
Radiation Cancer Risk Conventional
 • 5% excess cancer deaths per Sv (100
 • 1:2,000 (0.05%) excess cancer
   deaths per 10 mSv (1 rem)

                         Brenner et al.
                         Estimated Radiation
                         Risks Potentially
                         Associated with
                         Full-Body CT
                         Radiology 2004
Age-Dependent Cancer Risk

Brenner et al. Estimated Radiation Risks Potentially
Associated with Full-Body CT Screening. Radiology 2004
     Radiation Risk in Context

• Baseline risk of cancer in life time 20-
• Younger patients at higher risk
• Late middle aged adult getting average
  CT has life time risk of cancer increased
  from ~20% to ~20.01%
  – Not measurable
       Radiation – Common
        Doses and Risks
• Chest radiograph (adult): 0.02 mSv
   0.0001 – 0.000002% deaths
• UGI/BE: 1-7 mSv
   0.005-0.035% deaths
• Mammography 2.6 mGy, but
  – Effective Dose 0.13 mSv (so cancer risk much
• Natural background: 3 mSv/yr
• Medical population average 0.3-0.6
Comparable Non-Radiation Risks

• Assume 10 mSv (1 rem) CT scan
  – Smoking 140 cigarettes in a lifetime
    (lung cancer)
  – Spending 7 months in New York City (air
    pollution – lung cancer)
  – Driving 4,000 miles in a car (accident)
  – Flying 250,000 miles in a jet (accident)
Audience Response Question

•    A single CT scan has a lifetime risk
     of death from cancer similar to:

A.   Smoking 7 cigarettes
B.   Driving 40,000 miles in a car
C.   Having a standard IVU
D.   Flying into low earth orbit
Audience Response Question

•    A single CT scan has a lifetime risk
     of death from cancer similar to:

A.   Smoking 7 cigarettes
B.   Driving 40,000 miles in a car
C.   Having a standard IVU
D.   Flying into low earth orbit
Effective dose

                 BACK TO
           Effective dose
• Effective dose
  – Estimate of stochastic radiation risk
• Dose Length Product (DLP)
  – Related to stochastic radiation risk
                                      Genetic mutation
 Calculation of Effective dose
• Direct approach impractical
• Usual approach
  – Mathematical anthropomorphic phantom
  – Computer simulated irradiation using Monte
    Carlo techniques
     • Statistical calculations of photon interactions
 Estimates of Effective Dose
• Effective dose = DLP x Conversion factor (mSv)
 CTDI  radiation risk

Diagram shows algorithm for the estimation
    of radiation exposure risk from CT
European DRLs for CT
Radiation dose - Summary

                 KT Bae. JMRI 2004
 Radiation dose - Summary

• CTDI100 is the fundamental CT dose
• CTDIw characterises a CT scanner in terms
  of dose
• CTDIvol is used to represent average
  absorbed dose to irradiated area
• DLP for a given examination type is roughly
  proportional to stochastic radiation risk
• Effective dose is used for more accurate
  risk estimates
How ?

(as low as possible reasonably achievable)
Radiation Dose Modulation Techniques

                              BACK TO
Automatic Exposure Control (AEC)

 • Automatic exposure control systems
   are now available in all MDCT
 • Each system operates on different
   basis, using a range of control
 • Tube current (mA) adjusted relative to
   patient attenuation
Automatic Exposure Control (AEC)

  • Patients size
  • Z-axis AEC
  • Rotational
  • Combination
Automatic Exposure Control (AEC)
 • Patient size AEC: adjust the tube current
   based upon the overall size of the patient.
   The same mA is used for an entire
   examination or scan.
 • Z-axis AEC: tube current is adjusted for
   each rotation of the x-ray tube. The mA
   would be low through the thorax and
   higher through the abdomen.
 • Rotational AEC: The tube current is
   decreased and increased rapidly
   (modulated) during the course of each
   rotation. The amplitude of mA modulation
   during rotational AEC reflect the patient
Automatic Exposure Control (AEC)

 • Three levels of automatic exposure control. A) patient size
   AEC: higher mA is used for larger patient, b) z-axis AEC:
   higher mA used at more attenuating z-axis positions, c)
   rotational AEC: the degree of modulation depends on
   asymmetry at each z-axis position, d) combined effects of
   using all three levels of AEC.
           Benefits of AEC

• Consistent image quality
  – Depending on its capabilities, there is less
    image noise variation between patients and
    within a single scan series. With rotational AEC,
    there is also a slight reduction in the variation
    of noise across the field of view.
• Reduction in photon starvation artifact
  - Tube current is varied during the course of
    rotation, it can be increased for the most
    attenuating scan angles (e.g. laterally through
    the shoulders)
          Benefits of AEC

• Potential for dose reduction through
  exposure optimization
  – AEC systems by themselves will not
    automatically lead to a reduction in
    patient dose. However, when used
    correctly, their introduction should
    generally tend to result in reduced dose.
          Benefits of AEC

• Reduced tube loading (extended
  scan runs)
  – MDCT generally have fewer problems
    with tube cooling, but in general, if the
    radiation exposure to the patient is
    reduced by lowering tube currents, the
    heating of the x-ray tube is also
            Methods for AEC

•   Standard deviation (SD) based AEC
    - by specifying image quality in terms of SD
      of pixel values.
    - High SD values  noisy image
    - Low SD values  low noise image
    - set the tube current to achieve the
      requested SD on an image by image basis
          Methods for AEC

• Standard deviation (SD) based AEC
  – Advantage: image quality resulting from
    protocols form different scanners can be
    compared more easily.
  – Disadvantage: easily to enter an SD which is
    lower than would be needed, resulting in higher
    patient doses than the ones achieved without
  – Image noise is inversely proportional to the
    square of the tube current, so halving the SD
    results in an increase in the mA, and therefore
    the patient dose, by a factor of 4.
         Methods for AEC

• Reference mAs AEC control
  – Uses the familiar concept of setting an
    mA (or mAs) related value
  – Assesses the size of the patient cross-
    section beig scanned, and adjusts the
    tube current relative to the reference
         Methods for AEC

• Reference mAs AEC control
  – Advantages: permits more flexible
    adjustment of tube current according to
    patient size than with SD AEC control.
    With SD based systems, the AEC
    response to different patient sizes is
  – Can vary their response depending on
    the image quality requirements.
  – Users are familiar with typical mAs
    values for their scanners
          Methods for AEC
• Reference Image AEC control
  – Uses reference image that has previously been
    scanned and judged to be of appropriate
    quality, and then attempts to adjusts the tube
    current to match the noise in the reference
  – Advantage: the required image quality is
    expressed using an existing clinical image rather
    than an abstract value of SD
  – Disadvantage: temptation to pick a „pretty‟
    image. This lead to use higher doses than are
    necessary. It is also difficult to compare scan
    protocols, as there is no value associated with
    the image quality in the reference image.
AEC systems on MDCT scanners

                    Impact 2005
AEC systems on MDCT scanners

 • GE
 • AutomA: set a desired image quality
   by entering a “Noise Index” (NI).
 • Aims to achieve the same level of
   noise in each image
 • SmartmA: varies the tube current
   sinusoidally during the course of
AEC systems on MDCT scanners
 • Philips
 • DoseRight ACS (automatic current selector)
   provides patients based AEC, by use of a
   reference image
 • DoseRight DOM (Dose Modulation)
   – Z-Dom: Z-axis AEC
   – D-Dom: Tube current is set so that 90% of
     images will have equal or lower noise than the
     reference image, with remaining 10% of images
     in a series having equal or higher noise than
     the reference image.
 • Use feedback from the previous rotation to
   asses the amplitude of mA modulation used
   for rotational AEC
AEC systems on MDCT scanners

 • Siemens
 • CARE Dose 4D use “image quality
   reference mAs”.
 • Adjust the tube current, setting a value that
   is higher or lower than the reference mAs
   depending upon the patient attenuation,
   relative to Siemens‟ reference patient size.
 • The degree to which the tube current is
   adjusted for patient size can be selected,
   using „weak‟, „average‟, or „strong (high
   degree of mA adjustment)‟, compensation
AEC systems on MDCT scanners

 • Toshiba
 • SureExposure in helical scanning only
 • Operated by selecting a target image SD
   form a drop down list.
 • AEC setup allows tube current to be limited
   by max. and min. values.
 • Variable length reference position selector
   to highlight a particular region. The AEC
   uses the mean attenuation within this scan
   region to calculate the tube current
   through this region.
AEC systems on MDCT scanners

          Automatic Exposure Control

  Report 06013, 32 to 64 slice CT scanner comparison report
                 version 1. Feb 2006, www.pasa.nhs.uk/cep.
Clinical Use of AEC system

• Clinical use of AEC system requires careful
• Generally lead to reduced patient doses
• However, it is possible to increase doses
  operating an AEC system
• Dose reduction generally accompanied by
  reduction of image quality
• Goal is to achieve diagnostic image quality
  without an impact upon clinical usefulness
     Protocol optimization

• The key is to use an appropriate
  image noise level, reference mAs or
  reference image in the AEC setup.
• This process is not a straightforward
• One approach is to focus image
  quality assessment upon the
  European Guidelines on Quality
  Criteria for Computed Tomography
The Effect of Protocol Modification

 • There are significant differences from one
   system to another
 • Changing kVp will not affect the tube
   current on Siemens CARE Dose, but it will
   do on other AECs.
 • Changing the reconstruction kernel will
   alter the tube current Toshiba
   SureExposure, but not by others
 • It is important that users are aware of the
   behavior of their system, and the effect
   that varying scan and reconstruction
   parameters has upon the AEC.
   Monitoring patient dose

• Use CTDIvol and DLP for monitoring
• Before and after the introduction AEC,
  radiation dose can be assessed.
• AEC exposure levels can be modified,
  although the effect of changing any
  other parameters such as beam
  collimation or kV should also be
  accounted for.
             AEC Future

• Introduction of the AEC into clinical
  practice should be approached carefully
• There is a need for education of users by
  the manufacturer
• Scanner need to have default AEC
  protocols that draw a sensible compromise
  between the demand of image quality and
  radiation dose.
• Common method for operation of the
  system would be of great help and it leads
  to the protocol uniformity and radiation
  dose optimization.
Practical Tips

                 BACK TO
Clinical Issues in Radiation with MDCT

  • Pediatric exposure
  • Breast radiation for chest MDCT
  • Cumulative dose from follow-ups in some
    applications, e.g. renal stones
  • Scanning in pregnancy
  • Screening
  • Automatic Exposure Control
  • Low kVp to reduce dose and contrast
       Pediatric Exposure

• Effective Doses in children may be
  50% higher with common techniques
• Strategy for dose reduction;
  – Be selective
  – Alternative diagnostic strategies
  – Reduce technique
Breast Radiation with Chest MDCT

 • Breast radiation for chest CT
 • Especially scans for pulmonary
   embolism in low-risk young women
 • Dose to breasts may be up to 19
   times that from a mammogram
 • Strategy for dose reduction: be
       Cumulative Dose
• Rescanning (e.g. renal stones)
• Often in young patients without
  serious conditions
• Indication “creep” (lower threshold)
• Up to 18 follow-ups recorded in one
• Strategy: low dose, educate
  clinicians, be alert and recommend
  alternatives (e.g., radiography or
   Scanning in Pregnancy

• 2nd-15th week gestation exposure
  – No concern < 50 mGy
  – Moderate concern 50-150 mGy
  – High concern > 150 mGy
• Risk overestimated by physicians
  – 5% of obstetricians, 6% of family
    physicians recommend abortion after CT
      Scanning in Pregnancy

• Principles of pregnancy policy
   Identify probability of pregnancy
   Minimize dose, consider alternative
   Document dose
   Low exposure – perform exam without
   High exposure – informed decision,
    awareness of risk
      Questionnaire, consent form
       Screening MDCT

• Whole-body screening may increase
  cancer risk about 1%
• Limit screening CT to proven
• Use low-dose technique for chest,
  colon, coronary calcification scans
Automatic Exposure Control

• System adapts to changes in patient
  thickness: x-y and z-axis
• Can reduce dose by 20-40%
• Implementation differs for each vendor
• Not yet perfected – may cause
  unsatisfactory increase in noise in some
  areas and patients
• Should use with caution in patients with
                Low kVp

• Low kVp increases image contrast for IV
• Allows lower IV contrast doses
• Low kVp increases noise at constant mAs
• Probably satisfactory for vascular studies
  and small patients (increase mAs)
• Not yet adequately clinically validated for
  many applications
Strategies For Dose Reduction

 • Be selective, consider risk (young,
 • Minimize technique ( mAs, pitch with 4 or
   fewer slice scanner)
 • Use AEC, low kVp when possible
 • Limit follow-up scans
 • Alternative diagnostic strategies, esp.
  Children and pregnancy
 • Be able to counsel for risk
            Actions for
     physician & radiologist…
• Justification: Ensure that patients are not irradi
  ated unjustifiably.
• Consider whether the required information be o
  btained by MRI, ultrasonography
• Consider value of contrast enhancement or
  omitting pre-contrast scan
• CT scanning in pregnancy may not be contrain
  dicated, particularly in emergency situations, al
  though examinations of the abdomen or pelvis
  should be carefully justified
                                    ICRP Publication 87
           Actions for
 physician & radiologist (cont’d)
• CT examination should not be repeated
  without clinical justification and should be
  limited to the area of interest
• Clinician has the responsibility to communicate
  to the radiologist about previous CT
  examination of the patient
• CT examination for research purpose that do
  not have clinical justification (immediate benefit
  to the person) should be subject to critical eval
  uation by an ethics committee
                                     ICRP Publication 87
          Actions for
physician & radiologist (cont’d)

• CT examination of chest in young girls and
  young females needs to be justified in view
  of high breast dose
• Once the examination has been justified,
  radiologist has the primary responsibility for
  ensuring that the examination is carried out
  with good technique
•   ImPACT, www.impactscan.org
•   National conference on dose reduction in CT, with an emphasis on
    pediatric patients.
    AJR Am J Roentgenol. 2003 Aug;181(2):321-9. Linton OW, Mettler FA
    Jr; National Council on Radiation Protection and Measurements.
•   A new pregnancy policy for a new era.
    AJR Am J Roentgenol. 2003 Aug;181(2):335-40. El-Khoury GY,
    Madsen MT, Blake ME, Yankowitz J.
•   Physicians' perceptions of teratogenic risk associated with radiography
    and CT during early pregnancy.
    AJR Am J Roentgenol. 2004 May;182(5):1107-9. Pole M, Einarson A,
    Pairaudeau N, Einarson T, Koren G.
•   Dose reduction in pediatric CT: a rational approach. Boone JM,
    Geraghty EM, Seibert JA, Wootton-Gorges SL.
•   Estimated radiation risks potentially associated with full-body CT
    screening. Radiology. 2004 Sep;232(3):735-8. Brenner DJ, Elliston CD.
•   ICRP, Publication 87, 2000
•   JRS, Guidelines for pediatric CT, 2005
•   Nishizawa K, Acta Radiologica Jap, 2004National Research Counsil, 20
• Risk versus Benefit
• For patients
• Responsibility

             Thank you !