X ray physics. Two types of x-ray imaging,- and two lectures

Transcription

1 X ray physics Lectures DTU Mikael Jensen oct Two types of x-ray imaging,- and two lectures Planar x-ray CT 1

2 Why use x-rays? Non invasive, high resolution Quick, widespread Konrad Røntgen, 1896 X-rays give rapid, high resolution anatomical information (many photons, good S/N) Rapid introduction, simple technology 2

3 Electromagnetic spectrum X-ray 1 nm gamma-rays 1 fm ν λ = c E = h ν Planar x-ray imaging Direct projection ( image) : 2D shadow! Black is low attenuation: Air, lung White is high attenuation : Bone. Geometry factors: FilmFocusDistance, FilmObjectDistance, FocusObjectDistance Geometrical enlargement 3

4 X ray image is shadow image Skull Hand x-ray 4

5 Chest And possibility of tomography Much can be gained from hgh S/N 5

6 Generation of X-rays X-ray emission from Wolfram Anode X-ray tube K α Kβ 1200 Wolfram Relative intensity at fixed electron current mm Al filter cut-off V=50kV V=90kV V=130kV Photon energy (kev) Specify kv and mas! 6

7 Spectrum simulation: Parameters: Anode, kv, mas, filter Radiation interaction Ionization Direct kinetic energy transfer Atomic and molecular exitation Radiative processes Nuclear reactions 7

8 Ionising radiation Releases energy through Ionisation But also Recombination And through Secondary radiation Allways ending as heat And perhaps chemical change A biological relevant measure for energy transfer LET = Linear Energy Transfer. Measured in kev/µm LET=-dE/dx Dose = Energy deposited per unit mass Measured in Gray (Gy)= J/ Kg D=dE/dM x M 8

9 Macroscopic Description Range I(x)= { Io for x<r 0 for x>r Not valid for X-rays α, β Extinction I(x)= Io exp(-µx) γ Two measures of thickness Linear x measured in meter (µm, mm, cm) Area weight= x ρ Area weight in gram/m 2 (g/cm 2, mg/cm 2 ) 9

10 Two types of interaction A single event removes the particle (the wawe).constant probabiblity of removal per unit length γ The single event retards the ionising particle slightly. Results in a well defined enrgy loss per unit length. E= 24 MeV Al folie 3 mg/cm 2 α E=23 MeV Exponential decrease γ No x dx N N N-dN x dn= - µ N dx N(x)= No exp(- µx) N(x) is the number of photons per unit area, intensity, flux 10

11 Microscopic interaction Probability measured in barns (10-24 cm 2 ) m and millibarn (mb, cm 2 )? m Number of Reactions P N target number N beam number per area P=σN target N beam N target number per area N beam number 11

12 Cave: electrons are normally not monoenergetic Energy loss betaparticles (electrons) R= 407 E 1,38 mg/cm 2 0,15 MeV<E<0,8 MeV Range? decceleration Bremsstrahlung R= (542 E-133) mg/cm 2 0,8 MeV<E<3 MeV Glendenin formulas Interaction of gamma and X-rays 3 important ways of interaction Photoelectric effect Compton scattering Pair production (Eγ >1022 kev) σ tot =σ foto +σ compton +σ pair! 12

13 Photoelectric effect σ = foto konstant Z 5 (-3, 5) Eγ E γ E=Eγ-Eb Einstein Compton effect Compton σ = compton konstant Z E 13

14 Compton scattering Klein-Nishina formula for Compton scattering θ Is the angle of deflection for the photon α Is the energy of the primary gamma ray relative to 511keV (m e c 2 ) 14

15 Angles Compton angular description 15

16 Lead water 16

17 Why pay interest in these interactions? Some of the photons should pass the sample Some should be stopped in the sample. The radiation should be stopped in the detector (film, plate, screen.) The radiation should be be collimated Mass attenuation coefficient µ ρ 50 kev 100 kev 200 kev Air 0,208 0,154 0,122 Water 0,227 0,171 0,137 Fat 0,212 0,169 0,136 Musle 0,226 0,169 0,136 Bone 0,424 0,186 0,131 Lead 8,041 5,549 0,999 Data from 17

18 Soft tissue contrast Mainly due to the Compton effect - Because Z is tpically low - And - E xray is high The image is really electron density The film / detector Should stop the x-rays! ( Silver is good ) but old- fashioned 18

19 Late 1990 s A new approach to imaging appeared DR or DDR or Direct Capture imaging Too early to tell which system will prevail 19

20 IMAGE CAPTURE CR PSP photostimulable phosphor plate REPLACES FILM IN THE CASSETTE DR NO CASSETTE PHOTONS CAPTURED DIRECTLY ONTO A TRANSISTOR SENT DIRECTLY TO A MONITOR CR vs DR CR imaging plate processed in a Digital Reader Signal sent to computer Viewed on a monitor DR transistor receiver (like bucky) directly into digital signal seen immediately on monitor 20

21 Active Matrix Array (AMA) Pixels are read sequentially, one at a time Each TFT and detector represents a pixel DEL = charge collecting detector element DEL collects e- 21

22 DDR only using amorphous selenium (a-se) The exit x-ray photon interact with the a-si (detector element/del). Photon energy is trapped on detector (signal) The TFT stores the signal until readout, one pixel at a time Active matrix array of silicon photodiodes 22

23 Image Resolution Signal Sampling Frequency Good sampling under sampling 23

24 DR Initial expense high very low dose to pt image quality of 100s using a 400s technique Therfore ¼ the dose needed to make the image Flat Panel TFT Detectors Have to be very careful with terminology One vendor claims: Detector has sharpness of 100 speed screen May be true: TFT detectors can have very sharp edges due to DEL alignment But! Spatial resolution is not as good as 100 speed screen. TFT detector = 3.4 lp/mm 100 speed screen = 8 10 lp/mm 24

25 TFT Array Detectors Detector is refreshed after exposure If no exposures are produced... detector refreshed every sec Built in AEC, An ion chamber between grid and detector Patient Dose Important factors that affect patient dose DQE: when using CsI systems Both systems fill factor The percentage of the pixel face that contains the x-ray detector. Fill factor is approximately 80% 25

26 DDR has all the advantages of CR imaging techniques Post processing & PACS Questions? DR, original and processed 26

27 Damage due to ionisation proportional to energy loss Biological damage from ionisation 4 steps: 1. Ionisation 2. Free radicals 3. DNA change H- OH- 4. Lack of repair 27

28 Consequence of DNA damage Single events: Most likely DNA repair No repair? Cell death No repair, cell survives? Small chance it is chenged to a cancer cell. Cells and tissue under rapid cell division most radiation sensitive Stochastical effect Damage risk proportional to dose Damage No known lower limit Effect can show up late Dose 28

29 Deterministic effect Threshold value Rapid onset Often local Cell death LD50 humans: 5 Gy 1 Gy Dosis Stocastic Risk ICRP says: Hiroshima Nagasaki 4-5% / Sievert (Sv) total risc fatal cancer Dose 29

30 A biological relevant measure for energy transfer LET = Linear Energy Transfer. Measured in kev/µm LET=-dE/dx Dose = Energy deposited per unit mass Measured in Gray (Gy)= J/ Kg D=dE/dM x M Gray ( J/kg) Unit of dose With important biological weight factors linked to Sievert (Sv) (still J/kg) 30

31 X ray doses Single exposure, small area, short path -limb, teeth, chest few micro Sievert Multiple exposures whole body, low energy to enhance contrast (CT.) several milli Sievert X ray doses Note that this is given as effective dose - In Sievert! - -Why? 31

32 Radiation sources (a) X-ray (diagnostic) Dose rate is high! Dose depends on: Distance Area of collimation HV mas Filtering Surface dose is always higher than depth dose. Doses µgy per image Justified? Some procedures are not justified Acceptability Outcome Dose 32