Presentation is loading. Please wait.

Presentation is loading. Please wait.

BME 560 Medical Imaging: X-ray, CT, and Nuclear Methods

Similar presentations


Presentation on theme: "BME 560 Medical Imaging: X-ray, CT, and Nuclear Methods"— Presentation transcript:

1 BME 560 Medical Imaging: X-ray, CT, and Nuclear Methods
Radiation Biology Part 1

2 Today Energy Transfer to Tissues Cell Effects Units and Terms

3 Energy Transfer to Tissues
The impact of ionizing radiation on tissue is all about energy transfer Radiation Energy Ionization of molecules Chemical change Biological change Interaction of EM radiation with matter Production of free radicals Changes to DNA

4 Energy Transfer to Tissues
Remember the four ways EM radiation interacts with matter Rayleigh scatter Photoelectric effect (frees energetic electron) Compton scatter (frees energetic electron) Pair production (consumes electron) Changes to electron state may result in ionized particles

5 Energy Transfer to Tissues
Particulate radiation (proton, electron, alpha) also transfers energy to tissues Collisional transfer: Frees electrons Radiative transfer: Produces X-rays, which may in turn free electrons Changes to electron state may result in ionized particles

6 Energy Transfer to Tissues
Types of ionization action Direct: Free electron directly impacts target Indirect: Free electron produces chemical change in another molecule that reacts with target

7 Energy Transfer to Tissues
Free radicals: Highly reactive ions, atoms, or molecules Reactive oxygen species (ROS) do a majority (2/3) of the damage produced by ionizing radiation Hydroxyl radical OH0 Hydrogen peroxide H2O2 H2O H2O+ + e- H+ + OH0 ionization Very reactive! OH0 + OH0 H2O2

8 Energy Transfer to Tissues
Linear Energy Transfer (LET) Energy transferred to a medium per unit of path length Related to linear attenuation coefficient Units: keV / micrometer Radiation LET (keV/mm) Cobalt-60 x rays 0.2 250 kV x rays 2.0 10 MeV protons 4.7 150 MeV protons 0.5 2.5 MeV a particles 166 2 GeV Fe ions 1000

9 Cell Effects DNA is believed to be the main pathway for biological impact of radiation Other parts of a cell may be damaged by ionization, but Cells can self-repair many forms of damage, Or, if necessary, self-kill (apoptosis)

10 Cell Effects Ionization products result in breakage of the DNA helix
Most DNA breaks can be self-repaired

11 Types of DNA damage Estimation of number of changes resulting from 1Gy low LET radiation Initial Physical Damage Number of Events Ionization in cell nucleus ~1M Ionization directly in DNA ~2000 Excitation directly in DNA Damaged Bases DNA single-strand breaks 1000 Damaged sugars 1200 Double strand breaks 40 DNA-protein cross links 150 Chromosome aberrations ~1 Lethal events ~

12 Cell Effects Incorrect rejoining of breaks in chromosomes results in aberrations. Most aberrations are lethal to the daughter cell

13 Cell Effects Surviving daughters have DNA mutations.
Most mutations do not cause a significant functional change. Many mutations with functional change will result in the cell killing itself or being killed by its neighbors, But some will produce a survivable, reproducible cell with altered function. The mutated gene is an oncogene.

14 Cell Effects Surviving daughters have DNA mutations.
Most mutations do not cause a significant functional change. Many mutations with functional change will result in the cell killing itself or being killed by its neighbors, But some will produce a survivable, reproducible cell with altered function. The mutated gene is an oncogene. A mutated cell mush retain its ability to divide indefinitely in order to produce a colony sufficient to maintain cancerous growth.

15 Cell Effects To be dangerous, the cell must:
Suffer an ionization event Suffer DNA damage Be unable to repair DNA damage Suffer a nonlethal chromosomal aberration Divide successfully (and maintain this ability) Suffer a functional change that does not trigger apoptosis or killing by neighbors

16 Cell Effects Cell survival curve Usually modeled as 𝑆=exp⁡(−𝛼𝐷−β 𝐷 2 )
Sublethal range Cell survival curve Usually modeled as “Shoulder” 𝑆=exp⁡(−𝛼𝐷−β 𝐷 2 ) Fraction of cells surviving Dose A: high LET B: low LET

17 Cell Effects Cell damage may be lethal or sublethal
Sublethal damage is repaired Fractioning the dose (applying the same dose over multiple exposures or longer time) causes less damage because cells can repair: Dose-rate effect Take advantage of this in radiotherapy: Cancer cells take longer to repair than normal tissue

18 Cell Effects Cell survival is a function of cell type and radiation type Non- or slowly proliferating cells (nerve, muscle, secretory) are less susceptible to radiation damage. Highly-proliferating cells (epithelial, stem cells) are more susceptible. This leads to the concept of radiation “quality” How effective is the radiation type at causing biological damage?

19 Units and Terms Linear Energy Transfer (LET)
Unit: keV per micrometer (energy per length) A function of radiation type, energy, medium (generally water or water-equivalent for tissues) Survival curve depends on LET. Equal doses of different types of radiation do not give equal biological effect – they have different LET.

20 Units and Terms Relative Biologic Effectiveness (RBE) Unitless
Expresses the relative effect as compared to X-ray RBE = 1 for gamma-ray and X-ray; >1 for particles Dose of X-ray for certain biological effect 𝑅𝐵𝐸 𝑟 = 𝐷 𝑋𝑟𝑎𝑦 𝐷 𝑟 Dose of radiation r for same effect

21 Units and Terms Radiation Exposure
Amount of radiation producing a specified amount of ionized particles in air Unit: coulomb/kg (producing 1 C of charge in 1 KG of air) Old unit: Roentgen (1 C/kg = 3876 R) Only applies to EM radiation Does not correlate with biological effects

22 Units and Terms Absorbed Dose Energy deposited per unit mass of medium
Unit: Gray (Gy) = 1 joule per kilogram Old unit: Rad = .01 Gy Depends on the radiation type and the absorbing medium

23 Units and Terms Relate Absorbed Dose to Exposure
Exposure is defined in air Absorbed dose can be any medium If medium is air, 1 R ≈ .87 rad; 1 Gy ≈ C/kg For other materials, 𝐷𝑜𝑠𝑒 𝐺𝑦 ≈ 𝜇 𝜌 𝑚𝑎𝑡𝑒𝑟𝑖𝑎𝑙 𝜇 𝜌 𝑎𝑖𝑟 ×𝐸𝑥𝑝𝑜𝑠𝑢𝑟𝑒( 𝐶 𝑘𝑔 )

24 Units and Terms Kerma Kinetic Energy Released in Material
Unit: Gray (Gy) = 1 joule per kilogram Accounts for secondary effects of energetic electrons (interactions elsewhere or Bremsstrahlung) where absorbed dose does not

25 Units and Terms Radioactivity Disintegrations per unit time
Unit: Becquerel (Bq) = 1 decays per second Unit: Curie (Ci) = 3.7 x 1010 decays per second Applies to radioisotopes

26 Units and Terms Equivalent Dose Dose accounting for biological effect
Unit: Sievert (Sv) = 1 Gy (energy per unit mass) Old unit: Rem Apply a weighting factor for the type of radiation Single type of radiation: 𝐻= 𝑊 𝑅 𝐷 𝑅 Multiple types: 𝐻= 𝑅 𝑊 𝑅 𝐷 𝑅 WR = weighting factor for type of radiation DR = absorbed dose in Gy

27 Units and Terms Equivalent Dose – weighting factors

28 Units and Terms Effective Dose
Measure of whole-body cancer risk due to non-uniform exposure Unit: Sievert (Sv) = 1 Gy (energy per unit mass) Apply a weighting factor for each tissue type exposed 𝐸= 𝑇 𝑊 𝑇 𝐻 𝑇 = 𝑇 𝑊 𝑇 𝑅 𝑊 𝑅 𝐷 𝑅𝑇 WT = weighting factor for tissue type T WR = weighting factor for type of radiation DRT = absorbed dose in Gy for tissue T

29 Units and Terms Effective Dose – tissue weights Organs
Tissue weighting factors ICRP30(I36) 1979 ICRP60(I3) 1990 ICRP103(I6) 2007 Gonads 0.25 0.20 0.08 Red Bone Marrow 0.12 Colon - Lung Stomach Breasts 0.15 0.05 Bladder 0.04 Liver Oesophagus Thyroid 0.03 Skin 0.01 Bone surface Salivary glands Brain Remainder of body 0.30 Total 1.00

30 Units and Terms Contrived Units for Biological Effect Physical Units
LET keV/mm related RBE Radioactivity Bq or Ci Equivalent dose Sv Absorbed dose Gy related weight for radiation type Radiation exposure C/kg weight for tissue type related Kerma Gy Effective dose Sv

31 Units and Terms Example problem
A hand X-ray is taken at 20 keV. Let Wbone=Wmuscle= Find the effective dose for an exposure of 2.5 x 10-4 C/kg.

32 Reference Hall, E. J., Center for Radiological Research, Columbia University, “Web-Rad-Train,”


Download ppt "BME 560 Medical Imaging: X-ray, CT, and Nuclear Methods"

Similar presentations


Ads by Google