1. 5 th September 2005 Paul Collins Computed Tomography Dosimetry Assessment of Effective Dose in Computed Tomography using an Anthropomorphic Phantom Paul Collins Brendan Tuohy Supervised by

2. 5 th September 2005 Paul Collins Computed Tomography Dosimetry Overview Motivation and Objectives Methodology Organ Location in Phantom and Measurement Patient ‘Effective Dose’ Calculation Results Conclusions

3. 5 th September 2005 Paul Collins Computed Tomography Dosimetry Motivation Indications are that patient dose is rising in CT Due to evolution of CT technology and subsequent changes in practise Conventional CT has now evolved to Multi-Slice CT (MSCT) which has to potential to vastly increase dose Objectives Evaluate changes in patient dose due to advancement of MSCT scanners Investigate the rise of patient dose in CT

4. 5 th September 2005 Paul Collins Computed Tomography Dosimetry What is MSCT SSCT (Single Slice CT) MSCT (Multi-Slice CT) Siemens, 2004 Evolved from development of the Detector Array 64 Slice CT :1 rotation = up to 64 Slice acquisition

5. 5 th September 2005 Paul Collins Computed Tomography Dosimetry Why MSCT Benefits –Near Isotropic –Fast Imaging.33sec Rotation Times –High Quality Images –Thinner Slices –Large Volume Acquisition Disadvantages –Increase Data Staff Workload Data Storage –Increase in Patient Dose Technology Changes Changes in practise 0.6mm Slice thickness 6sec scan time Siemens 64 Slice Scanner, 2004

6. 5 th September 2005 Paul Collins Computed Tomography Dosimetry Why Patient Dose is increasing Technology Changes –Extra volume scanned –Extra helical rotations –Interpolation for axial reconstruction –Z-axis over beaming –Penumbra region not utilised by detectors Protocol Changes –May be tendency to image more volume –Thinner Slices –Higher quality

7. 5 th September 2005 Paul Collins Computed Tomography Dosimetry Why slices thickness affects patient dose? –Image noise is random fluctuations of pixels values –Finite number of x-ray photons are transmitted (i.e. in a slice) –Thinner slices  Less x-ray photons  larger variation in pixel values  more noise –mA then has to be increased to provide useful diagnostic images –linear relationship between mA and patient dose

8. 5 th September 2005 Paul Collins Computed Tomography Dosimetry Evaluation of patient dose 1. Randoman aka ‘Séamus’ –Tissue Equivalent Humanoid Phantom –Verified for measurement of absorbed dose in CT –35 axial slices –Plugs to hold TLDs 2. Diagnostic TLDs –Measure Absorbed dose to Organs –~45 TLD were placed at specific organ locations for each protocol –Organ selection and patient dose calculation guided by ICRP publication 60 (International Commission on Radiation Protection)

9. 5 th September 2005 Paul Collins Computed Tomography Dosimetry Organ Location TLDs needed to be placed accurately within phantom to measure organ dose Methodology –Whole Body CT of Phantom –Calibrated Image using ImageJ software tool Any point to point distance know –Labelled Vertebral Column Could now relate each slice of phantom to specific vertebra –Human Slice Server used to locate organs according to vertebral column Whole Body Phantom CT Human Slice Server Sagittal view

10. 5 th September 2005 Paul Collins Computed Tomography Dosimetry Organ Location Slice 22 of phantom Human Slice Server –virtual reconstruction of human anatomy in any orientation or location –3D datasets from a human body frozen & digitised into 1 mm slices –Labelled images of organs/tissues/vertebrae etc. –Organs were then located in phantom using the vertebrae as a guide Human Slice Image ( T-11 )

11. 5 th September 2005 Paul Collins Computed Tomography Dosimetry Human Slice Server Screen Capture

12. 5 th September 2005 Paul Collins Computed Tomography Dosimetry Organ Selection Organs selected to allow for effective dose measurement –ICRP 60 Organs were located using previous method At least two TLDs placed within each organ e.g. –#2 TLDs in gonads –#6 TLDs in lung

13. 5 th September 2005 Paul Collins Computed Tomography Dosimetry Effective Dose Effective Dose Calculation –Sum of weighted equivalent doses in tissues and organs The remainder is composed of the following tissues and organs: adrenal, brain, upper large intestine, small intestine, kidney, muscle, pancreas, spleen, thymus and uterus. E : Effective Dose (Sievert) W T : Tissue weighting Factor H T : Equivalent Dose (Sievert) T : Tissue/Organ H T = Absorbed Dose x Radiation Factor Radiation factor for photons  1

14. 5 th September 2005 Paul Collins Computed Tomography Dosimetry Protocols Standard Imaging Protocols –Abdomen/Pelvis –Head –Chest –RT protocols (Radiotherapy) CT Scanners –Philips ACQSim (RT scanner) –Siemens Sensation Emotion Duo –Siemens Sensation Emotion 6 –Philips Brilliance 16

15. 5 th September 2005 Paul Collins Computed Tomography Dosimetry Abdomen/Pelvis Protocol Effective mAs = mAs/pitch  Effective mAs  Slice thickness  Scan Time

16. 5 th September 2005 Paul Collins Computed Tomography Dosimetry Organ Absorbed Dose Increases in average dose to organs located in primary radiation beam Highest absorbed doses to –Skin –Bone Marrow –Colon –Gonads –Oesophagus

17. 5 th September 2005 Paul Collins Computed Tomography Dosimetry Abdomen/Pelvis Protocol

18. 5 th September 2005 Paul Collins Computed Tomography Dosimetry Other Protocols Head Protocol Axial imaging mAs 260mAs to 350mAs Radiotherapy Protocols Skin dose – 32.80mGy Brain dose – 30.52mGy Axial imaging 3mm Slices

19. 5 th September 2005 Paul Collins Computed Tomography Dosimetry Radiation Risks Risks from Effective dose (ICRP 60) 1 mSv equates to a cancer risk of 1 in 20000 1 in 4000 risk for 5mSv 1 in 2500 risk for 8mSv 1 in 2000 risk for 10mSv Abdomen/Pelvis Protocol

20. 5 th September 2005 Paul Collins Computed Tomography Dosimetry Conclusions Dose is increasing 64 slice scanner UCHG –  0.33sec rotation  0.6mm slice thickness Effective dose of 64 Slice scanner?

21. 5 th September 2005 Paul Collins Computed Tomography Dosimetry Conclusions Image Quality Patient Dose Patient dose generally increases with better image quality Diagnosis is the goal of CT Is patient diagnosis improving with increasing patient dose? Does greater image quality result in better diagnosis? ALARA principle –Does diagnosis improve from 2 Slice to 6 Slice to 16 Slice?

22. 5 th September 2005 Paul Collins Computed Tomography Dosimetry Questions? Toshiba 256 detector 4D-CT scanner