1895, W. C. Roentgen Discovery of x-ray The first x-ray image: Fig.16.1 Radiology Diagnostic radiology (radiologist) Radiation therapy (therapeutic radiologist) Nuclear medicine 1. Production of X-Ray Beams High speed electron striking an atom x-ray X-ray unit: Fig. 16.3 Cathode or filament: source of electrons, T determines the number of electrons Vacuum tube: evacuated space for electron acceleration High positive voltage source in kvp (kilovolt peak) Mammography: 25 ~ 50 kvp Chest: ~350 kvp Anode or target: rotating Material with higher atomic number for efficiency High melting point Tungsten: Z = 74, melting point = 3400 C Power Electron current: 100 ~ 500 or 1000 ma 1 A and 100 kv 100 kw 99% appears as heat: damaged anodes in Fig. 16.5 100 kw boil a cup of cold water in < 1 s Line-focus principle in Fig. 16.4 10 ~ 20 Rotating anode: usually 3600 rpm, 10000 rpm for high-speed anode High speed rotation requires a good balance to prevent any vibration Tube loading chart in Fig. 16.6 Bremsstrahlung (Fig.16.7a) - 1 - KHU, EI 468
High energy electrons deflected by target atom nucleus emits bremsstrahlung x-ray photon Braking radiation White radiation: broad spectrum with dominant component Z of the target and kvp determines the amount of x-ray Characteristic radiation (Fig. 16.7b) High energy electron ejects inner shell electron creating a hole outer shell electron fills hole emitting radiation with energy characteristic of the energy level spacings K α or K β x-ray (Table 16.1) High energy x-ray Used in mammography X-ray spectrum in Fig. 16.8 2. How X-Rays Are Absorbed Heavy elements such as calcium (bone) are better absorbers Light elements such as carbon, oxygen, hydrogen, air (fat, muscle, tumor) are poor absorbers X-ray image: Fig. 16.9 Attenuation of x-ray Reduction due to absorption and scattering Measurements of attenuation in Fig. 16.10 and 11 Soft x-ray (lower energy): more absorption Hard x-ray (high energy): less absorption, greater penetration For monoenergetic x-ray, I Ie µ x = o where µ is the linear attenuation coefficient µ depends on the energy of x-ray: smaller µ for harder beam HVL (half value layer) = 0.693 : HVL = 2.5 mm for Al µ HVL = 0.1 mm for lead good shielding material for x-ray: 1.5 mm lead plate reduces x-ray energy by a factor of 2 15 = 30000 µ Mass attenuation coefficient, µ m = where ρ is density: Fig. 16.12 ρ - 2 - KHU, EI 468
( ) m x I = Ie µ ρ where ρx is area density in grams per cm 2 o Interaction of x-ray with matter (Fig. 16.14) Photoelectric effect (PE): Fig. 16.13a Energy of x-ray excite electron photoelectron generation ionizing Compton effect (CE): Fig. 16.13b X-ray photon collides with outer bound electron electron escapes (ionizing) and x-ray photon scatters (different direction) Pair production (PP): Fig. 16.13c Occur for very high energy x-ray Of no use for diagnostic x-ray X-ray images: Fig. 16. 15, 16, 17, and 18 Contrast media: high Z material, iodine, barium compound Contrast media: low absorption material, air Subtraction technique for contrast enhancement (DSA, digital subtraction angiography) Compton effect (scattering) degrades x-ray images 3. Making an X-ray Image Roentgenogram: x-ray image Setup: x-ray tube subject film Factors governing image quality Tube setting kvp: lowest kvp with enough exposure highest contrast ma-s: tube current times exposure time, exposure adjustment, tube heating and patient motion limit ma-s Geometrical factors Small spot size reduces blurring (Fig. 16.19) Penumbra and patient movement limit sharpness (Fig. 16.20) Scatter Scatter produces fog, which degrades image contrast Grid reduces the amount of scattered x-ray that reaches the film (Fig. 16.23) Grid requires higher beam intensity which increases patient exposure - 3 - KHU, EI 468
Patient movement Holding breath reduces blurring Heart movement cannot be hold blurring X-ray film Film-screen cassette in Fig. 16.24 High speed film: less exposure, more sensitive, less details Low speed film Film is processed using chemicals 4. Radiation to Patients from X-rays Radiation exposure Unit: roentgen (R) A measure of the amount of electric charge produced by ionization in air 1 R = 2.58 10-4 C/kg of air Typical exposure in Table 16.2 EAP (exposure-area product) Unit: rap = R-cm 2, roentgen-area product 1 rap = 100 R-cm 2 High kvp high energy x-ray more penetration and less absorption but more scattering Reducing patient exposure Filtration or beam hardening (Fig. 16.27 and 28) remove low-energy x-ray which does not contribute forming an image due to absorption Collimation Unnecessary exposure (Fig. 16.29) Collimation: lead slabs confine the beam to the region of interest 5. Producing Live X-ray Images - Fluoroscopy X-ray image viewed on a sheet coated with a fluorescent material or fluorescent screen fluoroscopy (Fig. 16.30 and 31) Motion can be observed in real-time Modern fluoroscopy (Fig. 16.33) - 4 - KHU, EI 468
Use minimal exposure Use image amplifier or image intensifier tube in Fig. 16.32 Use video recorder and monitor 6. X-ray Slices of the Body X-ray from many directions: projections or scans Computerized tomography (CT) or computerized axial tomography (CAT): Fig. 16.37 and 38 In 1972, Hounsfield Cut view Narrow x-ray beams with about 140 kvp Pixel value: -500 (air) ~ +500 (bone), 0 maps to the density of water (Fig. 16.39) Cross-sectional images: Fig. 16.40 and 41) < 1% difference in density can be imaged Requires image reconstruction algorithms 7. Radiographs Taken without Film DR (digital radiography) - 5 - KHU, EI 468