Computed Tomography Dose Index

Computed Tomography Dose Index, Includes various CTDI parameters and the way of calculating effective dose from various Computed Tomography procedures along with their conversion factor.

1. Computed Tomography Dose Index Anjan Dangal B.S Medical Imaging National Academy Of Medical Sciences Bir Hospital , Kathmandu, Nepal November 2020.

2. Dose represents the amount of energy deposited in tissue from radiation per mass of tissue, and is measured in J/kg = Gray (Gy ). Unfortunately, this energy damages the tissues it hits. Consequence: Deterministic effect:Erythema, Ulceration, necrosis Stochastic Effects: cancer

3. D eterministic effects relate directly to the amount of radiation a single cell receives; these require a large dose to become apparent . Stochastic effects can occur (randomly) with very small doses, and even one cell can turn cancerous. Generally speaking, in CT we are mostly interested in the stochastic effects.

4. Exposure Absorbed Dose Equivalent Dose Energy/gm (Energy/gm) X Q.F # ion/ cc of air Roentgen rem rads Quality factor :Cell killing ability of radiation X ray , Gamma : 1 Proton, Neutron, Alpha Particle: >1 ( 2-20) Gray: 1Gy = 100rad Sievert 1Sv = 100rem

5. How is radiation dose measured in CT? CT dose is not measured directly on patient. CT dose is measured using standard phantoms . Measurements are then used to estimate patient dose.

6. Dose Gradient: Radiograph vs CT

7. How is Radiation dose measured? CTDI 100 CTDI W CTDI VOL DLP Effective dose Dose Distribution Pitch Scan Length Computed Tomography Dose Index (CTDI) Measurement done at the centre of phantom of a single slice

8. CTDI Obtained by making measurement in acryllic cylinder phantom. Holes in centre and periphery Placement of pensil shaped ionization chamber.

9. CTDI100 – Dose Distribution In CT: the highest dose delivered is at the periphery Dose is uniform on the surface but decreases towards centre.

10. CTDI VOL :15mGy Independent of scan length

11. Computed Tomography Dose Index (CTDI): Measurement done at the centre of phantom of a single slice: cannot accommodate the anatomy covered because we do just confine to single slice.

12. Volume CTDI in mGy (CTDI VOL): X ray tube voltage (KV) X ray tube current (mA) X ray tube rotation time (s) CT Pitch (P) Phantom size (S or L)

13. CTDI is the rate at which you the energy is put on the patient. DLP is total amount of the energy that you are putting the patient. CTDI = Radiation Intensity DLP = Total radiation used to perform a CT Scan.

14. CTDI Phantoms 16cm (S) (Head Phantom) 32cm (L) (Body Phantom) Large CTDI is = 2 X Small CTDI

15. 58.1/15.7 = 3.7 Abdomen CTDI VOL = 15.7 X 2 = 31.4 mGy (S) 58.1/31.4= 1.85

16. Axial CT CTDI VOL is directly proportional to : Tube current (mA) Tube rotation time (s) mAs

17. Helical CT CTDI VOL is directly proportional to: mA,s (mAs) Inversely proportional to pitch:1/Pitch CTDI VOL is directly proportional to : Effective mAs: mAs/Pitch

18. CTDI VOL = kV ( 2.6) 80 140 kV CTDI = > 400%

19. CTDI VOL “Universal Parameter” CTDI VOL for Head = 60mGy on Small Phantom KV X ray filter: Head, Body or pediatric Model: Type of CT Scanner Vendor: Philips/ Toshiba/ GE CTDI vol >>> mAs !

20. CTDI VOL/ DLP & Patient Dose

21. CTDI Vol is the radiation incident on patient, not the patient dose. CTDI VOL = 20mGy: CTDI Vol is not the indicator of any kind of patient dose.

22. Patient Dose Total CTDI VOL = Organ Dose Total DLP = Effective Dose

23. O rgan dose can be calculated. It can help to predict the corresponding organ dose. Monte Carlo Simulation: The most complete computational method for estimating organ and tissue doses is based on Monte Carlo simulations. The simulations account for many scanner and technique specifics,including scanner geometry, bow-tie filtration , beam collimation, tube potential, and current as well as the CT dose index (CTDI).

24. Jones and Shrimpton used a simulated hermaphroditic patient (MIRD-5 phantom) having mathematically modeled organs and tissues . The mathematic phantom was divided from head to mid thigh into 208 axial slabs of 5 mm thickness . Then, accounting for tube voltage and using CT scanner–specific data for geometry and beam shaping, they simulated a CT scan and calculated absorbed doses to all organs of the body for the irradiation of each axial slab. Summing contributions from all slabs exposed during a particular CT examination yielded the total organ doses.

25. Effective Dose Effective dose is a parameter meant to reflect the relative risk from exposure to ionizing radiation . The effective dose (E) is a measure of the risk of cancer induction in the patient from the effects of the radiation. It takes into account the total amount of absorbed dose received and averages it to give a whole body effective dose .

26. Method 1: Using Organ dose estimates and ICRP 26, 60, 103 Tissue-weighting factors are meant to represent the relative radiation sensitivity of each type of body tissue as determined from population averages over age and sex and are derived primarily from the atomic bomb survivors cohort. For partial-body irradiation, effective dose is the weighted summation of the absorbed dose to each specified organ and tissue multiplied by the ICRP-defined tissue-weighting factor for that same organ or tissue.

27. R evisions were intended to reflect advances in knowledge about the radiation sensitivity of various organs and tissues . Tissue-weighting factors are meant to represent the relative radiation sensitivity of each type of body tissue as determined from population averages over age and sex and are derived primarily from the atomic bomb survivors cohort

28. E = Effective Dose T = all ICRP specified tissue and organ W t = ICRP specified tissue weighting factor H t = Dose to particular organ or tissue E T = overall tisue Ez = Overall irradiated Slabs

29. Using DLP and K Coefficients from the European Guideline E = k × DLP , where k coefficient is specific only to the anatomic region scanned.

30. Measuring Effective Dose Effective Dose in NCCT Head: (1100X 0.0021 ) + 4.4 X 0.0021 = 2.31 msv Effective Dose in CT IVU: ((771.7 X 3) + 4.7 ) X 0.015 = 34.79 msv

31. Body Region KvP Effective mAs Scan Length(cm) CTDI VOL(mGy) DLP (mGy-cm) K factor Effective Dose(mSv) Head 120 390 17 60 (S) 1000 0.002 2 Chest 120 110 30 7.4 (L) 220 0.018 4.5 Abdomen 120 210 25 14 (L) 350 0.016 5.7 Pelvis 120 210 30 14 420 0.014 6

32. CT Dosimetry CTDI 100 Effective dose DLP CTDI VOL CTDI W Measurement done on a standard phantom using 100 cm chamber. Taking into account the distribution variation based on large and small patient. When the scan involves pitch. Taking account of the whole length exposure Risk estimation to the body

33. Automatic Tube Current Modulation mA is varied: Around the patient, Along the patient(scan length), & between patient

34. Quality Reference mAs Quality Reference mAs = Effective mAs Effective mAs = mAs/pitch Once set in protocol and is different for different body region. Can convert Effective mAs in CTDI Vol i.e the Universal parameter. Modulation: Five Strength

35. Thankyou

36. Refrences Estimating Effective Dose forCT Using Dose–Length Product Compared With Using Organ Doses : Consequences of Adopting International Commission on Radiological Protection Publication 103 or Dual-Energy Scanning, Jodi A Christner, Mayo Clinic, Available from www.ajronline.org , DOI:10.2214/AJR.09.3462 Videos from Walter Huda, available on Youtube.