1. Radiation Dose in CT Scanning: A Team Approach
John R. Mayo, MD
Director of Advanced Cardiac Imaging
Professor of Radiology and Cardiology
University of British Columbia
This lecture deals with the important subject of patient dose from CT exams.

2. Overview Background In my practice: Diagnostic utility
Radiation exposure
Evidence for harm from CT radiation exposure
Subject effect
In my practice:
Appropriate indications
Appropriate dose: Diagnostic reference values
Scanner dose modulation
Relationship between image quality and dose

3. CT Diagnostic Utility
Provides anatomic information equivalent to gross pathology, diagnostic if pathology causes anatomic changes (e.g. lung cancer)
Contrast media, physiologic maneuvers add information (e.g. PE, airways disease)
Fast, available, relatively cheap
Compatible with devices and sensors
Summary: superb imaging modality

4. Limitation: Radiation Exposure
Relative to plain radiography, CT is a high radiation exposure examination
Low scatter fraction, therefore noise visible
Combined with current high utilization, CT accounts for >60% of population medical radiation exposure
Radiation surveys* show wide variation in radiation exposure for identical exams
*Aldrich J, Bilawich A, Mayo J. CARJ 2006;57:79-85

5. Increasing CT Dose at VGH
CT Exams 4%
CT Dose 20% CT
1991
Dose from
all other exams
All other exams
CT Exams 26%
CT Dose 60%
In the early 1990s I carried out a study of the patient dose from all radiological procedures for the whole of Canada. For BC CT studies accounted for 4% of all x-ray exams but accounted for 20% of the patient dose. I do not have the same figures for the whole of BC, but at VGH in 2002 CT studies accounted for 26% of the studies and for 74% of the patient dose. CT
2002 CT

6. Dose Variation in BC: Chest CT

7. Is CT Radiation Exposure Harmful?
Debate about harm from exposures <20 mSv
Limitations in Atomic Bomb data:
Limited dosimetry
Particulate as well as x-ray, gamma exposure
Environmental toxins
Mono-ethnic exposure (Japanese)
War time stressed population
Advantages of Atomic Bomb data
Population cross section with point exposure

8. New Low Level Radiation Study
15 country study* of low level radiation exposure
Outcome: cancer incidence, fatal cancer rate
Cohort: 407,000 atomic industry workers, >5 million person years follow-up, 90% males, average exposure 19 mSv, excellent dosimetry
Results: Increased risk of cancer, dose response effect, risk not dominated by high level exposure
Conclusion: supports concept of small but measurable cancer risk at low exposure (<20 mSv)
*Cardis E et al. Radiation Research 2007;167:

9. New Low Level Radiation Study
Limitation:
No data on children
Limited data on females, 98% of the dose was received by males
The study loses significance if lung cancer in Canadian workers is excluded
This may indicate inadequate control for smoking in Canadian cohort

10. CT scanner dose modifiers
The cancer inducing effect of low dose radiation varies according to:
The subject (age, sex)
The exposed region’s radiation sensitivity
The CT scanner radiation distribution:
Position in scanner
Bow tie filtration
Dose modulation
Position of arms, subcutaneous fat, radio-opaque objects
CT has highly asymmetric dose, (high surface, low central) which causes problems in dosimetry

11. Age versus Cancer Mortality Risk
Lifetime mortality risk (%/Sv) 10 70 60 40 50 30 20 15 5 25
Age at time of exposure
female
male
ICRP 60 average
1996 Re-analysis Hiroshima data
Higher risk conversion factor
Lower risk conversion factor
Young persons, especially females, are much more radiosensitive than the 30 yo male for which the standard risk is taken. Conversely all persons over 50 are less radiosensitive.

12. Estimation of risk: Equivalent Dose
For the reference subject, a 30 year old hermaphrodite, the risk is 50 excess fatal cancers per million exposed to 1 mSv
This compares to the natural cancer risk of 250,000 cancer deaths per million
Risk is strongly influenced by age (lower in older patients (mobile fossils)), and gender (higher in females)

13. Summary
Since there appears to be a risk, we should only perform “indicated” exams
Unfortunately, the evidence base for “indicated” imaging investigations is minimal
Radiologists should minimize CT radiation exposure without compromising diagnostic accuracy
Special consideration: children, young adults

14. Appropriate Use
All CT examinations must have appropriate indications with supportive:
History
Clinical findings
Laboratory findings
When evidence based, appropriate clinical decision making support (e.g. Well’s criteria for PE)
Screening exams (e.g. lung cancer screening) should be evidence based
Follow up exams should affect management

15. Appropriate dose
Diagnostic reference levels have been developed using the Dose Length Product (DLP), the CTDIw times the scan length
DLP is a measure of CT radiation exposure
Radiologists should be familiar with DLP values for clinical scans in their institution
Average DLP values should be at or below the reference level, indicating good practice

16. EC Diagnostic Reference Levels for CT

17. Monitoring Appropriate dose

18. Convert DLP to Effective Dose
Region of Body
E/DLP Conversion Factor
mSv.mGy-1.cm-1
Head
0.0023
Neck
0.0054
Chest
0.017
Abdomen
0.015
Pelvis
0.019
Using the DLP value it is possible to estimate patient dose.

19. Example: Cardiac CT dose
743 times equals 12.6 mSv
Above the EC reference value for chest CT
Reason for the high dose is the helical retrospective acquisition protocol of cardiac CT
Chest CT dose in our hospital
Standard dose, DLP 200 to 400, mSv
Low dose, DLP 100, 1.7 mSv (follow up, screening studies, young patients)

20. CT Scanner Dose Modulation
Manufacturers have devised systems to adjust radiation dose based on:
Body part size
ECG tube current modulation (Cardiac)
Z axis overscan** (Collimator shutter action)
* Tzedakis A, Damilakis J et al. The effect of z overscanning on patient effective dose
from multidetector helical computed tomography examinations.
Medical Physics 2005; 32:

21. Tube Current Modulation – x,y
mA is constantly changed as the tube rotates depending on patient thickness
lat AP
lat AP
lat
40 cm
100% mAs
50% mAs
20 cm
0 cm
0% mAs

22. Tube Current Modulation – z
Attenuation is measured every 180o rotation and corrected the following rotation. This type of adaptive modulation can reduce dose by %.
shoulders
hip
liver mA
lung
legs
neck
Z axis

23. Combined tube current modulation (x,y,z)
Provides acceptable noise and diagnostic information
Substantial reduction in radiation dose, up to 42%*
Should be used in most patients
* Rizzo S et al, AJR 2006; 186:

24. Technologist training
Radiologists must ensure that technologists are minimizing exposure by:
Ensuring the minimum patient volume is scanned
Ensuring all dose modulation techniques are consistently used
Ensuring patients are centered in the gantry

25. *Mayo JR et al. Radiology 1997;202:453-457
Image quality and dose
Radiologists consistently correlate increased image quality with increased dose
However, little research into the impact of radiation dose on diagnostic accuracy
Computer simulated dose reduction is a useful experimental technique for this research*
*Mayo JR et al. Radiology 1997;202:

26. Real 100 mA scan

27. Simulated 100 mA scan

28. Computer simulated reduced dose scans
In a side by side trial of real versus simulated reduced dose scans the real scan was correctly identified 50.1% of the time
Advantages
No additional radiation dose
Reduced dose scans have identical location, level of inspiration and artifact

29. Investigated simulated dose reduction on chest CT findings
Repeated measures experimental design
150 clinical chest CT scans (200 – 350 mA)
Computer simulated 100 and 40 mA scans
4 chest radiologists interpreted the complete chest CT scans in random order assessing 14 mediastinal structures and lung findings
Mayo, J. R. et al. Radiology 2004;232:

30. 200 mAs Mayo, J. R. et al. Radiology 2004;232:749-756
Copyright ©Radiological Society of North America, 2004

31. 100 mAs Mayo, J. R. et al. Radiology 2004;232:749-756
Copyright ©Radiological Society of North America, 2004

32. 40 mAs Mayo, J. R. et al. Radiology 2004;232:749-756
Copyright ©Radiological Society of North America, 2004

33. 200 mAs Mayo, J. R. et al. Radiology 2004;232:749-756
Copyright ©Radiological Society of North America, 2004

34. 100 mAs Mayo, J. R. et al. Radiology 2004;232:749-756
Copyright ©Radiological Society of North America, 2004

35. Results Significant (p<0.05) decrease in subjective image quality
Significantly more disagreements on image findings at reduced dose
Concluded that reduced dose affects reader evaluation of CT findings
But, no evaluation of diagnostic accuracy

36. Summary Do the obvious first:
Create a team approach to radiation dose minimization and quality optimization between radiologists, technologists, technical support and medical physicists
Eliminate unnecessary examinations
Monitor the actual dose delivered in practice and compare to reference dose levels
Utilize all available dose reduction tools
Encourage further research into the relationship between CT dose, image quality and diagnostic accuracy

37. Thank you