Chapter 16 Physics of Diagnostic X-Rays

Size: px

Start display at page:

Download "Chapter 16 Physics of Diagnostic X-Rays"

Eleanore Paul
7 years ago
Views:

1 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 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 kw boil a cup of cold water in < 1 s Line-focus principle in Fig ~ 20 Rotating anode: usually 3600 rpm, rpm for high-speed anode High speed rotation requires a good balance to prevent any vibration Tube loading chart in Fig Bremsstrahlung (Fig.16.7a) KHU, EI 468

2 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 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 Attenuation of x-ray Reduction due to absorption and scattering Measurements of attenuation in Fig 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) = : 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 = µ Mass attenuation coefficient, µ m = where ρ is density: Fig ρ KHU, EI 468

3 ( ) m x I = Ie µ ρ where ρx is area density in grams per cm 2 o Interaction of x-ray with matter (Fig ) Photoelectric effect (PE): Fig a Energy of x-ray excite electron photoelectron generation ionizing Compton effect (CE): Fig b X-ray photon collides with outer bound electron electron escapes (ionizing) and x-ray photon scatters (different direction) Pair production (PP): Fig c Occur for very high energy x-ray Of no use for diagnostic x-ray X-ray images: Fig , 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 ) Penumbra and patient movement limit sharpness (Fig ) Scatter Scatter produces fog, which degrades image contrast Grid reduces the amount of scattered x-ray that reaches the film (Fig ) Grid requires higher beam intensity which increases patient exposure KHU, EI 468

4 Patient movement Holding breath reduces blurring Heart movement cannot be hold blurring X-ray film Film-screen cassette in Fig 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 = 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 and 28) remove low-energy x-ray which does not contribute forming an image due to absorption Collimation Unnecessary exposure (Fig ) 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 and 31) Motion can be observed in real-time Modern fluoroscopy (Fig ) KHU, EI 468

5 Use minimal exposure Use image amplifier or image intensifier tube in Fig 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 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 ) Cross-sectional images: Fig and 41) < 1% difference in density can be imaged Requires image reconstruction algorithms 7. Radiographs Taken without Film DR (digital radiography) KHU, EI 468

Production of X-rays and Interactions of X-rays with Matter

Production of X-rays and Interactions of X-rays with Matter Goaz and Pharoah. Pages 11-20. Neill Serman Electrons traveling from the filament ( cathode) to the target (anode) convert a small percentage