X-ray Tube and Generator Basic principles and construction

Transcription

1 X-ray Tube and Generator Basic principles and construction Dr Slavik Tabakov - Production of X-rays and Patient Dose OBJECTIVES - X-ray tube construction - Anode - types, efficiency - Classical X-ray generator (block diagram) - Medium frequency X-ray generator (block diagram) - Principle of radiographic contrast formation - X-ray film and film/screen combination -Various radiographic contrasts (definitions) - Image Intensifier construction - Fluoroscopic image and dose 1

2 . Data for mid-1980 NRPB, 1989 Estimated annual collective dose to UK population from Diagnostic Radiology for 1990 is approx. 20,000 mansv. On the basis of risk estimate this could be responsible for up to 700 cancer deaths/year! Safety in Diagnostic Radiology, IPEM, 1995 Approximately 90% of the total collective dose to UK population from man-made radiation sources arises from Diagnostic Radiology Diagnostic Radiology, IPEM, 1995 Safety in In most industrialised countries there are between 300 and 900 X-ray examinations for every 1000 inhabitants every year. Over half of these are chest examinations (these figures does not include dental X-ray examinations or mass screening programs). Doses varies widely from hospital to hospital, even in the same country, sometimes by a factor of 100. Radiation and You, EU, Luxembourg

3 Distribution of X-ray dose from the Tube through the Patient to the X-ray film Exposure ~ 80 kv, 30 1m 100% 2% 1% 0.25% ~100 kv High temp. ; Electron cloud vacuum Production of X-rays and Bremsstrahlung (stopping radiation) thermal electron emission in vacuum (10-6 mbar) and target bombardment White X-ray spectrum (gamma quanta with all energies) and its final view (after tube filtration) 3

4 Imaginary model Electron radius Nucleus radius Atom radius Real (approximate) m m m Scaled-up approx. model (linear) 1 mm 10 mm mm (100 m) Volume ratio: e vs A ~ Inter-atom dist in crystal m 100 m Space charge effect - X-ray tube function characteristics PRE-Heating of Cathode High temp. ; Electron cloud 4

5 Cathode W wire filament (~10x0.2 mm) Anode W plate (melting at 3370 o C) Construction: stationary and rotation Anode stem (Cu) with radiator Cathode assembly (inside broken Tube) Melted tungsten at anode target Stationary anode angle determines focal spot less power Anode angle EF = sin α. AF Rotational increased thermal focus more power Effective focus ; Thermal (Actual) focus 5

6 Anode heat - storage and dissipation (cooling) P max ~ f 3/2.D 1/2.n 1/2 / sin α The maximal power of the rotating anode(p max ) depends from the effective focal spot size (f); the diameter of the target track (D); the angle of the anode (α); and the speed of rotation (n - r.p.m.): Intensity of X-ray radiation : W ~ I.U 2.Z Anode efficiency η ~ k.u.z (Z-anode atom. No.) (intensity per energy unit - η = W/I.U ) X-ray Intensity distribution: -In all directions inside the Tube housing (only a fraction of X-rays used output dose) -The overall output intensity decreases with ageing of Tube - Decreased intensity at Anode site (Heel effect) it is more obvious with old Tubes 6

7 X-ray Tube Housing Insulating Oil; Output window; Pb lining; Leakage radiation Tube and Housing cooling and T o protection Fine focus and Large focus effects X-ray image resolution depends on the size of the X-ray tube focal spot (effective focus) Fine (~ 0.5mm) or Broad (~1mm) The BF smears the contours of the imaged objects (this increases with the increase of object-to-film distance) Focus Object Film 7

8 X-ray Generator Classified by: -Power -Rectification -Pulses or frequency -Circuits kvp and Dose pulses (waveforms) from various X-ray generators 8

9 kv control circuit (including auto-transf., HV Transformer, rectification) + - Filament circuit Basic diagram of Classical X-ray Generator with the Tube Contemporary medium-frequency X-ray Generator (smaller HV transformer; frequency varies the kv) 9

10 (smaller HV transformer; frequency varies the kv) U / f ~ A. n voltage U with frequency f A - cross section of the transform core; n - number of transformer windings (transformer ratio); Block-diagram of modern computer-controlled X-ray Generator 10

11 SUMMARY X-ray tube: - Focal spot (spatial resolution; power) - Total filtration at tube output (pat. dose) - Tube housing (leakage radiation) X-ray Generator: - kv control (image contrast, pat. dose 2 ) - ma control (image brightness, pat.dose) -Time (msec) control (image brightness and patient dose) X-ray output spectrum Radiographic Contrast and Film + Film-Screen Detector 11

12 The X-ray source radiation I o passes through the object (the body) and is modulated by the body tissues (µ.d) on its way. This modulated radiation beam I x interacts with the detector, where the modulated radiation is transformed into modulated light the X-ray image. The contrast of the image depends on the energy of the X-ray beam. I x = I o. e -(µ.d) 12

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15 σ D 2 Radiographic contrast C = [D 2 D 1 ]/D 1 D 1 I Intensity D Density E - Exposure Film contrast γ = [D 2 D 1 ]/[loge 2 loge 1 ] Visual contrast C = logi 2 logi 1 Signal-to-Noise Ratio: SNR C = [D 2 D 1 ]/ σ Subject Contrast C =I 2 I 1 Anatomical X-ray contrast >> Artificial X-ray contrast: (various contrast agents) <<< Barium-based (ex.stomach) Iodine-based >>> (ex.heart/vessels) <<< Interventional Radiology 15

16 X-ray film with 1 or 2 sensitive layers (AgBr emulsions) over transparent base The film is exposed to both X-rays and light inside the cassette Light (X-ray) photon excites a Bromine atom (and it looses an e-). These free e- are trapped into crystal defects. The (+) Silver ions are attracted into these ( ) defects, where they are neutralised and become Ag atoms (sensitised grains). The combination of areas in the film with different number of sensitised grains forms a LATENT IMAGE. During the process of film development the sensitised grains are stabilised (the exposed AgBr crystals reduced to stable Ag atoms). During the next process of film fixing the remaining un-sensitised grains (which had not been exposed to light photons) are removed and washed out. The final visible image contains areas with various darkness (depending on the concentration of Ag atoms). 16

17 X-ray film characteristics: -Exposure latitude (dynamic range); -Resolution (grain size) -Sensitivity (film speed) Cassette intensifying screen influence Development process influence Exposure Kilovolts (kvp) X-ray spectrum quality and quantity change Change of kv leads to change of X-ray energy, Anode effectiveness, Dose and spectrum Energy in a single exposure X ~ Z. kv 2. mas The X-ray anode efficiency η ~ Z. U a Photographic analogue: none 17

18 Exposure milli Ampers (ma) X-ray spectrum quantity change Change of ma leads to change of X-ray intensity (with no spectrum change) Energy in a single exposure X ~ Z. kv 2. mas Photographic analogue: -speed mas influence Approx. Linear function 70 kvp, 25 mas 70 kvp, 50 mas 70 kvp, 80 mas kvp influence Approx. Square function * Loss of Contrast 60 kvp, 50 mas 70 kvp, 50 mas 81 kvp, 50 mas 18

19 X-RAY FLUOROSCOPY IMAGING SYSTEMS Fluoroscopy delivers very high patient dose. This can be illustrated with an example: The electrical energy imparted to the anode during an exposure is A = C 1. U a. I a. T The X-ray tube anode efficiency is E = C 2. Z. U a From the two equations follows that the energy produced in a single exposure will be X = C. A. E = C. Z. (U a ) 2. I a. T = (C. Z). kv 2. mas Radiography of the lumbar spine (with parameters 80 kv, 30 mas): X = k = k. 192,000 Fluoroscopy - 3 minutes Barium meal (with parameters 80 kv, 1mA) X = k = k. 1,152,000 In this example fluoroscopy delivers approx. 6 times more X-ray energy (dose) 19

20 Luminescence: Fluorescence - emitting narrow light spectrum (very short afterglow ~nsec) - PM detectors; II input screens (CsI:Tl) Phosphorescence - emitting broad light spectrum (light continues after radiation) - monitor screens, II output screens (ZnCdS:Ag) The old fluoroscopic screens are no longer used due to high dose and low resolution Basic Components of an Image Intensifier - Input window (Ti or Al) 95% transmission - Input screen: CsI (new) or ZnS (old) phosphor - Photocathode (a layer of CsSb 3 ) - Accelerating electrodes zoom (e.g. 30/23/15 cm) - Output screen (2.5 cm) - II housing (mu-metal) - Output coupling to the TV camera 20

21 II Input screen: Columnar crystals of CsI which reduces dispertion (collimation); absorbs approx. 60% of X-rays Photocathode applied directly to CsI both light spectrum match very well II Accelerating electrodes 21

22 II Output screen: Phosphor (ZnCdS:Ag) on glass base The accelerated e - produce multiple light photons; thin Al foil prevent return of light (veiling glare) Coupling: fibre optic or tandem optic Conversion factor ~ (cd.m -2 /µgy.s -1 ) = (output phosphor light / input screen dose rate) Total gain (inp. X photons / out. light photons) Total gain (inp. X photons / out. light photons) 1 X-ray photon >> 1000 light photons (input screen) >> >>50 photo e - >> 3000 light photons (output screen) in this case the total gain is

23 TV camera types: Vidicon - gamma 0.7; slow response, some contrast loss (light integration), high dark current, but low noise - suitable for organs Plumbicon - gamma 1; quick response, small dark current, but high noise - suitable for cardiac examinations Modulation Transfer Function MTF descriptor of image quality Overall II-TV system MTF = MTF 1 x MTF 2 x x MTF n 23

24 Dynamic range of II -much larger than this of radiographic film (output luminance per dose unit) Resolution and Magnification of II - electronic zoom up to 4 times (lp/mm) Summary Fluoroscopic contrast bone is black (white=intensive radiation) Radiographic contrast bone is white (black=intensive radiation) 60 kv 70 kv II contrast with different kv (constant ma) 90 kv 100 kv 24