Seminar 8. Radiology S8 1

Transcription

1 Seminar 8 Radiology Medical imaging. X-ray image formation. Energizing and controlling the X-ray tube. Image detectors. The acquisition of analog and digital images. Digital image processing. Selected radiological methods. S8 1

2 X-ray image formation X-ray image reflects the 2-dimensional distribution (projection of 3-dimensional object on a plane) of the product of the absorption coefficient and the thickness of the patient body S8 2

3 Biophysical background Microscopic description Two effects are mostly responsible for the attenuation of the X-ray beam interaction with atomic electrons 1) Photoelectric effect (A) 2) Compton (incoherent) scattering (C and D) Remark: There are also rays (B, E) that penetrate human body without interaction Remark: The rays recorded by the detector (B, C, E) responsible for image formation S8 3

4 Macroscopic description Attenuation of the X-ray radiation in the investigated object human body the law of absorption I(x) = I(0)*exp( -µ*x) µ linear absorption coefficient x thickness Linear absorption coefficient depends on the X-ray energy and chemical composition (effective atomic number Zeff) of the object µ = const(x-ray energy)*ne NE number of electrons/cm 3 = ρ N A v Z e A f f Material Zeff Density (g/cm 3 ) Relative µ (H2O = 1) Water (blood) Muscle Fat Air Bone Iodine Differentiation Water(blood)/muscle impossible Air/muscle possible Bone/muscle possible Remark: There is possibility to optimize X-ray energy S8 4

5 X-ray tube X-ray tube housing 1) Absorbs radiation, except for the radiation that passes through the window as X-ray beam 2) Cooling HT high voltage S8 5

6 X-ray tube = energy converter electrical energy X- ray radiation and heat Airtight envelope (glass, ceramic, metal) support and electrical insulation maintain a vacuum in the tube electric current flows as the electron beam Cathode emits electrons and focuses them at the anode filament like in a light bulb Electric DC potential (U) between anode and cathode 40 kv 160 kv Electron energy = e*u Anode is the component in which the X-rays and heat are produced X-rays are produced when moving electrons are suddenly stopped Focal spot X-rays are produced in a very small area on the surface of the anode known as the focal spot rectangular(circular) shape (0.1 mm 2.0 mm) S8 6

7 View of a modern X-ray tube S8 7

8 X-ray production 2 processes 1) Bremsstrahlung production electron passes close to the atom electron is deflected and slowed down by the electrostatic force from the anode atoms the energy lost by the electrons appears in the form of X-ray photons (EM radiation) 2) Characteristic X-rays production orbital electron ejected form the target atom (photoelectric effect) a vacancy is created energy will be released when the vacancy is filled photon energy is characteristic for the target (anode) atom X-ray tube energy spectrum S8 8

9 Maximum energy energy of the electron beam (kvp p peak) Minimum energy applied filter (Al, Cu thickness ~mm) S8 9

10 Power supply DC generator (high voltage generator) Three parameters are adjusted 1) Voltage (kvp) 2) Current (ma) 3) Time of operation exposure time (s) Total intensity of the X-ray beam (total number of photons) as well as the amount of produced heat is proportional to the product of the tube current and the exposure time to describe the X-ray tube operation the product of current and time is commonly used (so called charge mas) Two parameters (kvp and mas) are used to characterize X- ray tube operation in the medical examination S8 10

11 Scattered radiation and contrast Contrast general definition C = I 1 I 1 I 2 S8 11

12 Contrast reduction scattered radiation Remark: Rays C and D (Fig. p. 3) are responsible for production of scattered radiation S8 12

13 Radiographic detectors 1) Conventional radiography a film to create the image 2) Computed radiography (CR) an image plate (IP) made of photo-stimulable phosphor is used to create the image 3) Direct radiography (DR) captures the image directly onto a flat panel detector (FPD) A scintillator made from cesium iodide (CsI) combined with silicon photo-diode or charge coupled device CCD 4) Fluoroscopy a continuous beam of radiation and the images appear on the screen like on a TV (real time imaging) S8 13

14 1) Conventional radiographic detector X-ray film Film structure Base clear polyester material Emulsion silver bromide crystals (grains) suspended in gelatine (G) each grain contains ~10 9 silver atoms The photographic process Formation of a film image two-step process 1) Exposure of the film to radiation which forms an invisible latent image 2) Chemical process (development) that converts the latent image into a visible image S8 14

15 Single grain A) Unexposed film AgBr grain contains Ag + and Br - ions and a place, so called sensitive speck (crystal structure imperfection) B) Exposure absorption of light photons by Br - frees the extra electrons the free electrons move to the sensitive speck, causing it to become negatively charged silver ion reaches the speck and is neutralized an atom of black metallic silver is deposited C) The process is repeated several times (depends on the number of photons that reach grain) and finally a grain of metallic silver is produced latent image (Fig. C) D) Development (automatic processor) a few steps to fix the metallic silver grains and to remove all unexposed grains E) Visible image metallic silver S8 15

16 How to describe quantitatively the visible image optical density I 0 Optical density at point 1 D1 = log( ) I 1 I 0 Optical density at point 2 D2 = log( ) I 2 S8 16

17 Film contrast characteristic The characteristic curve (H-D curve Hurter & Driffield) Three characteristic regions 1) Low exposure (toe) white image no contrast 2) Linear region it is desirable to expose film within this region the highest contrast 3) High exposure (shoulder) dark image no contrast S8 17

18 In radiology charge is used to characterize exposure since both quantities are linearly related Charge = 3 mas Charge = 12 mas S8 Charge = 48 mas 18

19 Example of the X-ray examination mammography S8 19

20 Left breast micro-calcification cancer Lcc Lb S8 20

21 Right breast Rcc Rb S8 21

22 2) Computed radiography (CR) The image plate (IP) is used instead of the X-ray film and the x-ray exposure is made. Next, the IP is run through a special laser scanner that reads (photo-stimulated luminescence - PSL) and digitizes the image. The digital image can then be treated using conventional digital image-processing software. Principle of IP operation A foil (thickness = ~0.2 mm) composed of Ba compounds (BaFX, X = Cl, Br, I) with small amount of Eu 2+ ions (BaFCl:Eu 2+ ) The mechanism of the PSL is illustrated by the energy level diagram of BaFBr:Eu 2+ S8 22

23 Laser scanner (He-Ne 633 nm) reads and digitizes the image Laser scanner operation S8 23

24 Advantages of image plate in comparison with X-ray film 1) Characteristic curve a wider dynamic range 2) One IP may be used many times 3) Digital image computed radiography (CR) S8 24

25 Analog and digital signals Digital image Digital image 3-dimensional matrix 2 dimensions are used to describe position + 3 rd dimension is used to give the values of the parameter Quantization Pixel = picture element total number of pixels = 512*512, 1024*1024, 2048*2048 (HR) Digitalization Remark: Analog image may be considered as the digital image characterize by the pixel size 0 and the qualitative value of the parameter Remark: Scanner is used to transforms analog to digital image S8 25

26 Remark: In the digital imaging the binary numerical system is commonly used. The binary (base 2) numerical system has 2 possible values, often represented as 0 or 1, for each place-value. In contrast, the decimal (base 10) numeral system has 10 possible values (0,1,2,3,4,5,6,7,8, or 9) for each place-value A bit is the smallest possible piece of information. A bit can have only two possible states: 1 or 0. We may find it useful to think of this in terms of "yes or no", "true or false", "on or off". By gathering groups of bits together, we can make much more complicated information. A byte is simply a group of 8 bits. 8 bits (1 byte) is equivalent to 2 8 = 256 possibilities. Intensity within each pixel = (8 14) bits Photon counting Poisson distribution Mean value = N N Standard Deviation = Final result of photon counting N ± N S8 26

27 The transformation between white and black can be realized with different number of intermediate grey scale levels. Practically it is difficult to distinguish between the first three bars despite the fact that the number of used grey level differs considerably (the numbers describe numbers of used grey levels) S8 27

28 3) Direct radiography DR typically captures the image directly onto a flat panel detector. Image processing or enhancement can be applied on DR images as well as CR images due to the digital format of each. A scintillator in the detector s outer layer, which is made from cesium iodide (CsI), converts X-ray to light S8 28

29 The light is then channeled through the photodiode layer where it is converted to a digital output signal. Another possibility, the light is read out by optical fibers connected with charged couple device (CCD). S8 29

30 Charge Coupled Device Silicon wafer (thickness = ~0.3 mm, maximum dimension ~(5*5) cm 2 ). Maximum ~(5000*5000) electrodes are evaporated on the silicon surface each electrode is polarized and forms an electric potential well S8 30

31 Direct radiology CCD detector (24*36 mm 2 ) (30*40 mm 2 ) S8 31

32 S8 32

33 Comparison of conventional (a) and direct (b) radiology S8 33

34 4) Fluoroscopic imaging system Disadvantage of the conventional (film as detector) radiography time consuming procedure to produce an instantaneous and continuous image (real time image) fluoroscopic imaging system C-arm system Modern fluorosope uses a device called an image intensifier to enhance the analog output of the real time x-ray image, where it is picked up by either a video or CCD camera. S8 34

35 Mechanism of action of an intensifier tube Four steps: 1) X-ray photons (~50 kev) many light photons (~2.5 ev) 2) Light photons emission of electrons (WOCs, WOBa) 3) DC voltage (~20 kv) accelerates towards the output screen 4) Output screen converts electron energy into bright visible image Net result of operation a much brighter image at the output screen that at the input phosphor gain of the intensifier tube ~10000 Remark: An increase of the number of light photons is achieved by the increase of the energy of electrons S8 35

36 Clinical application contrast radiology Contrast radiology physical background ds The law of absorption I I(x) = I0exp(-µx) ln( 0 ) = µx I Before contrast injection M = µsds + µbdb + µbldn After contrast injection L = µsds + µbdb + µjdn S8 36

37 Two possibilities: 1. µjdn >> µsds + µbdb routine contrast radiology 2. µjdn µsds + µbdb DSA (digital subtraction angiography) Routine contrast radiology coronarography Catheter femoral artery contrast medium (I Z = 53, Ba Z = 56 compounds) image of contrast distribution S8 37

38 S06_01 Remark: White-black reverse in comparison with X-ray film intensifier tube operation S06_14 S8 38

39 Digital subtraction angiography DSA O = L - M = (µj - µbl)dn µjdn S8 39