RADIOLOGY AN DIAGNOSTIC IMAGING

Transcription

1 Day 2 p. 1 RADIOLOGY AN DIAGNOSTIC IMAGING Dr hab. Zbigniew Serafin, MD, PhD serafin@cm.umk.pl

2 and Radiation Protection mainly based on: C. Scott Pease, MD, Allen R. Goode, MS, J. Kevin McGraw, MD, Don Baker, PhD, John Jackson, MA, Spencer B. Gay, MD: Basic Radiobiology.

3 The core of an atom exists precariously: massive repulsive electromagnetic forces between closely-assembled protons in the nucleus must be counterbalanced. "Stability" thus reflects the balance of power between strong nuclear force, weak nuclear force, and electromagnetic force. The nuclear binding energy quantifies the energy necessary to maintain coherence. Isotopes are atoms with the same atomic number (proton count) but different atomic masses (number of neutrons). Heavier elements are more likely to have binding energies insufficient to maintain a stable nuclear configuration. Such radioisotopes may undergo decay by emission of energetic quanta.

4 Alpha particles large and positively charged tend to cause ionizations and lose energy over a very short distance are composed of two protons and two neutrons (i.e. a naked helium nucleus) large size & relatively high charge prevent deep penetration of matter (blocked by dead skin or paper) chronic exposure to inhaled alpha particles is a lung cancer risk are important in the uranium decay series, of which radon is a product

5 Neutrons uncharged particles may carry significant kinetic energy ("Fast Neutrons") may collide with a nuclear proton, causing its ejection produce biologically-important ionizations and excitations due to such collisions are often produced as part of fussion reactions

6 Beta Particles are smaller and less energetic than alpha particles have a negative charge are created when a neutron transmutates into a proton are emitted during decay of iodine-131, phosphorus-32, carbon-14, and strontium-90

7 Gamma rays and X-rays (photons) represent pure electromagnetic energy progress at the speed of light having no mass or charge, are neither attracted to nor repulsed by charged particles gamma-rays originate from the nucleus, usually carries higher energies than X-rays X-rays originate from electron clouds

8 NOTE: regardless of the type of energy carrier or the specific type of energy-matter interaction, biologic hazard ultimately results from: i. atomic ionizations (loss of one or more electrons positivelycharged ion) ii. excitations induced by electromagnetic radiation from many sources, including radiology Photons interact with subatomic structures in one of the following three ways: Photoelectric absorption Compton Scatter Pair production The particular type of interaction reflects probability statistics based on both the energy of the photon and the atomic number of the traversed atom. For most tissues of the body, average atomic number does not vary greatly though cortical bone has the highest effective atomic

9 Linear Energy Transfer the amount of energy transferred to the matter in the form of ionizations and excitations. LET indicates the potential for biologically important damage from radiation. LET can be thought of in two ways: an average energy for a given path length traveled or an average path length for a given deposited energy. The standard unit of measure is kev/um.

10 Ionizations lead to chemical changes: Free radical production Broken bonds, importantly double-strand DNA breaks Since the intracellular environment is essentially aqueous, water is the most likely molecule encountered by radioactive energies. Radiolysis of water may produce H, OH. Damage caused by such free radicals represents the INDIRECT action of ionizing radiation. Most biological effects of low LET radiation can be attributed to free radicals. Less commonly, nucleoproteins or DNA may be ionized directly by charged particles, but not electromagnetic radiations.

11 ionizing radiations free radicals (indirect effect) ionizations (direct effect) changes in configuration of DNA macromolecules interference with DNA structure or replication

12 Cell death is operationally defined as loss of function, such as reproductive capacity for stem cells or synthesis of some specific product (enzyme, hormone). Apoptosis is the process of programmed cell death biochemical pathways within a cell leading to its own organized dismantling. When DNA is damaged and not successfully repaired, the cell may die cell death may occur immediately (interphase death) or during its attempt to divide (mitotic death) or after a few cell divisions (abortive colonies).

13 interference with DNA structure and function chromosome breakage gene mutation cell cycle influence effect on cell multiplication division delay tissue effects (reduced growth, abortive colonies, degeneration) interphase death

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15 Dq quasi-threshold dose or sub-lethal dose (SLD) most radiosensitive phases: G2-phase and mitosis (M-phase) least radiosensitive phase: latter part of S-phase (synthesis of DNA)

16 Law of Bergonie and Tribondeau The radiosensitivity of cell is directly proportional to their reproductive activity and inversely proportional to their degree of differentiation. Cells most active in reproducing themselves and cells not fully mature will be most harmed by radiation. The more mature and specialized in performing functions as cell is, the less sensitive it is to radiation.

17 Radioresistant cells Radiosensitive cells

18 Radioresistant cells Radiosensitive cells bone germinal cells liver lymphoid tissues kidney basal cells cartilage hematopoietic tissues muscle epithelium of the GI tract nervous tissue

19 children could be expected to be more radiosensitive than adults fetuses more radiosensitive than children and embryos especially in the first weeks of pregnancy when organs are forming Radiosensitivit y Low Intermediate High Cell type muscle cells, nerve cells osteoblasts, endothelial cells, fibroblasts, spermatids spermatogonia, lymphocytes, stem cells, intestinal mucosa cells and erythroblast

20 Deterministic effects of radiation: are predictable, are occurring with dose-dependent severity, generally do not occur below a certain threshold value, are generally associated with intermediate to high radiation exposure (orders of magnitude above most doses used in diagnostic radiology) examples: cataracts (single dose of 2-6 Gy) transient erythema (2-6 Gy) desquamation (> 10 Gy) epilation (3-7 Gy) sterility (> 6 Gy in males and 4-6 Gy in females)

21 Whole body irradiation human LD50 is estimated at 3.25 Gy

22 Whole body irradiation prodromal syndrome associated with exposures as low as 1 Gy, nearly universal above 2 Gy mechanism increased tissue and cell permeability, allowing substances like serotonin and histamine to enter chemosensitive cells of the GI tract and activate neural pathways to the vomiting center in the medulla has a latent period of 2-6 hours Sx/Si: sense of fatigue, headache, confusion, depression, vomiting, diarrhea at higher doses recovery after 2-3 days (may be shorter for very mild cases) more common in women than men, children and elderly at higher risk.

23 Whole body irradiation hematopoietic syndrome associated with exposures of at least 3 Gy mechanism loss of pluripotent stem cells from hematopoietic tissues has a latency period 2-4 weeks Si/Sx: pancytopenia, leading to infection and hemorrhage survival: 50% spontaneous recovery at exposure of 3.5 Gy. 180 days required to regain maximum function death 1-2 months post-exposure from infection; anemia is not a cause of death

24 Whole body irradiation GI syndrome associated with exposures of at least 7 to10 Gy mechanism loss of stem cells from intestinal crypts, leading to eventual loss of GI mucosa has a latency period 3-5 days Si/Sx: diarrhea and vomiting leading to profound dehydration survival: none; death in 1-2 weeks post-exposure

25 Whole body irradiation cerebrovascular syndrome associated with catastrophically high acute exposures 100 Gy mechanism severe damage to CNS, cardiovascular and respiratory systems latency period of minutes to hours Si/Sx: ataxia, disorientation, hypotension, shock and respiratory distress survival: none; dath within one day

26 Stochastic effects of radiation probability that an effect will occur is related to exposed dose severity of effect is unrelated to exposed dose all or nothing involve a degree of randomness usually do not recognize a threshold dose hereditary / genetic effects carcinogenesis

27 Stochastic effects of radiation Genetically Significant Dose (GSD). the gonadal dose equivalent received by persons of reproductive potential also taking into account the expected number of children for that population the 1991 estimated GSD in the United States is approximately 0.3 msv from man-made radiation (medical and dental X-rays, radiopharmaceuticals, commercial nuclear power, miscellaneous occupational exposure, weapons-testing fallout, consumer products, air travel) WHAT CT DOSE IS SAFE?

28 Stochastic effects of radiation Carcinogenesis most analyses utilize the cohort of Japanese atomic bombing survivors for extrapolating low-dose exposure risk statistical noise prevents direct assessment of human risk for exposures below 50 msv most common neopalsms: thyroid cancer (Hiroshima, Chernobyl) breast cancer (Hiroshima, Nova Scotia, mammography) leukemia (Hiroshima) lung cancer (Hiroshima, uranium miners) bone cancer (radiotherapy) skin cancer (early radiology)

29 (D) is it possible?

30 In utero exposure Radiation risks to the fetus: Fetal demise Congenital malformation CNS/cognitive effects Carcinogenesis Intrauterine growth retardation risk = first trimester > second > third dose of 0.1 Gy during the period of major organogenesis gives significant risk of congenital malformation

31 attenuation the removal of photons from a beam of x- rays as it passes through matter is caused by both absorption and scattering of the primary photons Linear Attenuation Coefficient fraction of photons removed from a monoenergetic beam of x-rays per unit thickness of material (cm -1 ) however, as the thickness increases, the relationship is not linear LAC normalized to unit density is called the Mass Attenuation Coefficient 31

32 attenuation C.F. Wolbarst. Physics of Radiology, pp. 108,

33 radiation units KERMA Absorbed Dose Exposure Dose Equivalent Dose Effective Dose

34 KERMA = Kinetic Energy Released in MAtter = kinetic energy transferred to charged particles by indirectly ionizing radiation, per mass matter SI units are 1 Gy = 1 J/kg (traditional 1 rad = 0.01 Gy) Absorbed Dose = amount of energy deposited by ionizing radiation per unit mass of material SI units are 1 Gy = 1 J/kg (traditional 1 rad = 0.01 Gy) used to calculate organ dose

35 Exposure = amount of electrical charge (ionization) produced by ionizing radiation per mass of air SI units are C/kg (traditional R = 2.58x10-4 C/kg) used to compare assessment of equipment performance

36 Dose = Exposure conversion factor SI units are C/kg Exposure is nearly proportional to dose in soft tissue over the diagnostic radiology range for bone, the conversion factor approaches 4 C.F. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed., p

37 Equivalent Dose = Dose w R ; weighs the quality of radiation SI units are Sv (traditional rem 1 rem = 10 msv) in general, high LET (Linear Energy Transfer) radiation (e.g., alpha particles and protons) are much more damaging than low LET radiation, which include electrons and ionizing radiation such as x- rays and gamma rays and thus are given different radiation weighting factors (w R ) X-rays/gamma rays/electrons: LET 2 kev/μm; w R = 1 protons (< 2MeV): LET 20 kev/μm; w R = 5-10 neutrons (E dep.): LET 4-20 kev/μm; w R = 5-20 alpha Particle: LET 40 kev/μm; w R = 20

38 Effective Dose = a measure of radiation- and organ-specific damage in humans takes into account different radiosensitiveness of tissues (tissue weighting factors w T ) SI units are Sv (traditional rem 1 rem = 10 msv) first calculate the equivalent dose to each organ: (H T ) [Sv] Effective Dose (E) = w T H T C.F. Bushberg, et al. The Essential Physics of Medical Imaging, 2 nd ed., p

39 EXERCISE indentify the sources of background radiation, and describe the magnitude of each source indentify the sources of medical radiation, and describe the magnitude of each source what is estimated average annual total exposure to radiation (msv)? 39

40 EXERCISE c.f. NCRP Press Report. Medical Radiation Exposures of the U.S. Population Greatly Increased Since the Early 1980s. 3 March

41 EXERCISE 0.28 msv/yr 0.40 msv/yr 0.39 msv/yr 0.27 msv/yr 0.14 msv/yr 0.07 msv/yr <0.01 msv/yr 2.00 msv/yr 3.00 msv/yr c.f. NCRP Report # msv/yr 41

42 EXERCISE c.f. NCRP Press Report. Medical Radiation Exposures of the U.S. Population Greatly Increased Since the Early 1980s. 3 March

43 EXERCISE discuss ALARA rule (As Low As Reasonably Achievable) and its application to radiation protection 1) indications for imaging 2) choice of imaging method 3) imaging parameters 4) radiation shielding 5) documentation 43

44 EXERCISE discuss ALARA rule (As Low As Reasonably Achievable) and its application to radiation protection 1) indications for imaging 2) choice of imaging method 3) imaging parameters 4) radiation shielding 5) documentation 44