Medical Linac. Block diagram. Electron source. Bending magnet. Accelerating structure. Klystron or magnetron. Pulse modulator.

Transcription

1 Block diagram Medical Linac Electron source Bending magnet Accelerating structure Pulse modulator Klystron or magnetron Treatment head 1

2 Medical Linac 2

3 Treatment Head 3

4 Important Accessories Wedges Dynamic wedges Blocks Multileaf Collimator (MLC) Electronic Portal Imaging (EPID) 4

5 Electron Accelerators angle Wedges 3 or more fixed wedges auto-wedge dynamic wedge Modify dose distribution 5

6 Multileaf Collimator (MLC) Used to define any field shape for radiation beams Several variations to the theme: different leaf widths (1cm to 0.4cm) replaces collimators or additional to normal collimators 6

7 Intensity MLC pattern 1 Modulation MLC pattern 2 Achieved using a Multi Leaf Collimator (MLC) The field shape is altered step-by-step or dynamically while dose is delivered MLC pattern 3 Intensity map 7

8 IMRT Multiple individual fields, each of them intensity modulated in two dimensions Linac based IMRT 8

9 IMRT Continuous rotation of a one dimensional fan beam which consists of many beamlets which can be turned on or off Tomotherapy 9

10 Components of Helical Tomotherapy Binary MLC Ring detector at exit side Helical Scanning 10

11 Cyclotron The first circular accelerator was the cyclotron Developed by Lawrence in 1931 (for $25) Grad student Livingston built it for his thesis About 4 inches in diameter 11

12 Cyclotron Principle of operation Particle acceleration is achieved using an RF field between dees with a constant magnetic field to guide the particles 12

13 Cyclotron Principle of operation qvb the particle is accelerated f v = 2πρ for mv p Bρ = = e e Note that the frequency remains constant as = 2 mv = ρ Limited by relativity since v in velocity and momentum won' t cancel v v eb 2π mv as << c = eb 2πm v approaches c 13

14 Cyclotron Why don t the particles hit the pole pieces? The fringe field (gradient) provides vertical and (less obviously) horizontal focusing 14

15 Cyclotron TRIUMF in Canada has the world s largest cyclotron 15

16 Cyclotron TRIUMF 16

17 Cyclotron NSCL cyclotron at Michigan State 17

18 Cyclotron 18

19 Betatron Since electrons quickly become relativistic they could not be accelerated in cyclotrons Kerst and Serber invented the betatron for this purpose (1940) Principle of operation Electrons are accelerated with induced electric fields produced by changing magnetic fields (Faraday s law) The magnetic field also served to guide the particles and its gradients provided focusing 19

20 Betatron Principle of operation Steel ρ Coil <B> B 0 B guide = 1/2 B average Vacuum chamber 20

21 Betatron Principle of operation A requirement for the B field of B Borbit = 2 dϕ db Emf = = A dt dt 2 db E2πR = πr dt R db E = 2 dt The force on the electron is F p dp er db = = dt 2 dt erb = = erb 2 orbit then the betatron is 21

22 Synchrotron The next idea was to constrain the particles to a constant ρ and accelerate them with RF fields Both the B field and the frequency (velocity) will increase Oliphant (Australia) first developed the idea but it was classified McMillan scooped the idea, named it the synchrotron and proposed to build one Later Oliphant tried to build one but ran out of funds and graduate students In the US, Berkeley (Bevatron) and Brookhaven (Cosmotron) raced to build one BNL won 22

23 Bending Recall from our study of making momentum measurements ( Tm) = 0. p( GeV ) Bρ 334 The LHC circumference is ~27 km Packing fraction of ~64% gives ρ~2.8 km Thus B needed for p=7 TeV is ~8.3 T The use of superconducting magnets using superfluid He at 1.8K are needed to reach this field Final magnet current is A Bending achieved by m dipoles 23

24 Bending LHC dipoles 24

25 Bending 25

26 Longitudinal Motion Phase stability is what keeps the beam together longitudinally 26

27 Longitudinal Motion The non-synchronous particle will oscillate about the synchronous one The longitudinal phase space looks like ΔE φ 27

28 Longitudinal Motion In a synchrotron, the particle s momentum must be incremented on each turn by a precise voltage that will keep pace with the increasing magnetic field V = V The frequency is just 1/period 0 sin φ s f = β 2 π c R 28

29 Longitudinal Motion A synchronous particle is one that always arrives at the desired phase lag φ s on the flank of the rising RF wave (particle A) For this to occur the accelerating RF frequency must be an integer multiple of f f a = hf h is called the harmonic number Chosen to make RF high in a convenient band for the cavity and electronics h for the LHC is 35460, RF = 400 MHz The accelerator has buckets in which a particle could be located and arrive synchronously 29

30 Transverse Motion Beam enters the synchrotron as a bundle of trajectories spread about an ideal orbit x () s, z() s dx x =, ds z = dz ds Unless corrected, the beam particles would naturally leave the beampipe A restoring field is used that causes the beam to oscillate about the ideal orbit 30

31 Strong Focusing Modern accelerators are possible because of strong focusing Simply a name for alternating magnetic field gradients that now are provided by rotated quadrupoles Invented by Courant, Livingston and Snyder from BNL But actually patented several years earlier by Christofilos, a Greek elevator engineer! Who went on to develop the first fusion machine at Livermore even though fusion was classified at the time 31

32 Transverse Motion 32

33 33 Strong Focusing A good analogy comes from optics Consider two lenses with focal lengths f1 and f2 0 1 for > = = + = f f d f f f f f d f f f

34 Strong Focusing In the case of quadrupoles, we define a strength k k α = 1 db Bρ dx in optics, = x f for a quadrupole of lb l Δx = θ = = Bρ by analogy 1 f = kl z the deflection angle from a lens is length ( db / dx) z Bρ l x the deflection angle is = lkx 34

35 Strong Focusing By rotating two quadrupoles through π/2 we produce a net focusing effect in the transverse direction 35

36 Transverse Motion Standard LHC lattice cell looks like 36

37 Transverse Motion The previous structure is called FODO Focus Drift space Defocus Drift space The envelope of oscillations follows a function called β(s) β(s) has units of length but the units bear no direct relation to the beam size The particles do not follow β(s) but rather oscillate within them in the form of a modified sinusoid 37

38 Transverse Motion We wrote down an expression for the angular deflection of a particle through a quadrupole In the vertical direction this is dz = kzds and strength k at a displacement z Thus z + k the equation of () s z = 0 for a quadrupole of length ds motion is give by In the horizontal plane we have 1 x + k() s 0 2 = ρ() s where the extra term acknowledges extra focusing due to the curvature of the orbit These equations are called Hill's equations 38

39 Transverse Motion In a class on accelerator physics we would proceed to solve these using matrix formalism (Twiss matrix) Nonetheless you can see that Hill s equations are reminiscent of harmonic motion except k depends on the position around the accelerator 39

40 Transverse Motion Let k be a constant (like in the constant gradient machines like the Cosmotron and Bevatron 2 d z + kz = 0 2 ds 2π z = z0 sin ks = z0 sinφ = z0 sin s λ dφ 1 if we define =, then β is the local ds β wavelength of the oscillation 40

41 Transverse Motion Again assuming k is constant ds 2πR Δφ = = β β Δφ R Q = 2π β It s important that Q not be an integer or simple fraction because otherwise the particle will repeat its path in the accelerator and see the same field imperfections The β function in an LHC cell varies between 30 and 180 m These will build up into resonances and blow-up the beam 41

42 Block diagram Medical Linac Electron source Bending magnet Accelerating structure Pulse modulator Klystron or magnetron Treatment head 42

43 Medical Linac 43

44 44

45 45

46 Electron Accelerators Modern accelerators have a lot of treatment options, for example X-rays or electrons (dual mode) Multiple energies 2 X-ray energies 5 or more electron energies 46

47 Electron Accelerators X Ray Collimators may be (1) rectangular (conventional) the transmission through the collimators should be less than 2% of the primary (open) beam 47

48 Electron Accelerators X Ray Collimators may be (2) Multi-Leaf collimators (MLC) the transmission through the collimators should be less than 2% of the primary (open) beam The transmission between the leaves should be checked to ensure that it is less than the manufacturer s specification Siemens MLC 48

49 Electron Accelerators Electron applicators, these may be open sided for modern accelerators using double scattering foils or scanned beams enclosed for older accelerators using single scattering foils both types should be checked for leakage adjacent to the open beam on the sides of the applicators Varian open sided electron cone 49

50 Components of Helical Tomotherapy Binary MLC Ring detector at exit side Helical Scanning 50

51 Electron Accelerators angle Wedges 3 or more fixed wedges auto-wedge dynamic wedge Modify dose distribution 51

52 Multileaf Collimator (MLC) Used to define any field shape for radiation beams Several variations to the theme: different leaf widths (1cm to 0.4cm) replaces collimators or additional to normal collimators 52

53 Electron Accelerators Modern accelerators have a lot of treatment options, for example X-rays or electrons (dual mode) Multiple energies 2 X-ray energies 5 or more electron energies 53

54 Electron Accelerators X Ray Collimators may be (1) rectangular (conventional) the transmission through the collimators should be less than 2% of the primary (open) beam 54

55 Electron Accelerators X Ray Collimators may be (2) Multi-Leaf collimators (MLC) the transmission through the collimators should be less than 2% of the primary (open) beam The transmission between the leaves should be checked to ensure that it is less than the manufacturer s specification Siemens MLC 55

56 Electron Accelerators Electron applicators, these may be open sided for modern accelerators using double scattering foils or scanned beams enclosed for older accelerators using single scattering foils both types should be checked for leakage adjacent to the open beam on the sides of the applicators Varian open sided electron cone 56

57 Components of Helical Tomotherapy Binary MLC Ring detector at exit side Helical Scanning 57