Lecture (5)....._ CT Instrumentation .pdf

Computed tomography instrumentation Dr. Nahla Atallah 1. CT Tubes  X-ray tube for CT is similar in design to the conventional radiography tube, but is specially designed to handle and dissipate excessive heat units – much higher heat loading  Ceramic targ...

Computed tomography instrumentation Dr. Nahla Atallah 1. CT Tubes  X-ray tube for CT is similar in design to the conventional radiography tube, but is specially designed to handle and dissipate excessive heat units – much higher heat loading  Ceramic target backing Decreases tube weight X-Ray Tube  X-ray production just as in radiography: Boil electrons off cathode, aim at anode; produce bremsstrahlung and characteristics X-rays.  Tube Continuously On - So Tube needs Heat Capacity and significant cooling system  Rotate Tube around patient to create views. X-RAY TUBE AND X-RAY PRODUCTION THERMIONIC EMISSION: emission the electron from the surface of the metal after heating kVp  Potential difference between cathode and anode (Volts) Tube voltage (kVp) CHANGE INTENSITY - kVp ENERGY – kVp IN CT  80-140  TOO LOW – NOISE (NOT ENOUGH PENETRATION OF THE PATIENT ) TOO LOW OF kVp: NOISE !!!! mA – tube current  The number of electrons flowing from cathode to anode S –time of exposure mAs tube current for certain length of time TUBE CURRENT CHANGE INTENSITY CURRENT ENERGY – NO CHANGE 2 * mA = 2 * number of photons 4 * mA = 4 * number of photons REDUCTION OF HEAT UNITS – TECHNIQUE COMPENSATION  kVp  mA  Time INCREASED NOISE  An X-ray tubes heat capacity is expressed in heat units.  Modern CT systems utilize X-ray tubes that have a heat capacity of approximately 3.5 to 5 million heat unit (MHU). A CT X-ray tube must possess a high heat dissipation rate.  Many CT X-ray tubes utilize a combination of oil and air cooling systems to eliminate heat and maintain continuous operational capabilities X-RAY Production results in a lot of heat and very little x-rays being generated HEAT UNITS CALCULATION HU= kVp X mA x time MOST CT TUBES HEAT CAPACITY 3-5 MILLION HU X-Ray Tube Conventional Siemens Straton Tube Radiography (Grounded Anode, Balanced Shaft, Tube Radiant Cooling and Active Cooling) ANODE +++++ MADE OF TUNGSTEN AND MOLYBDENUM TUNGSTEN TARGET TARGET MADE OF TUNGSTEN AND RHENIUM FOCAL SPOT- CT UTILIZES DIFFERENT FOCAL SPOTS  THE FILAMENT SIZE – LENGTH – FOCAL SPOT SMALLER FOCAL SPOT - Low mA SMALLER FOCAL SPOT – sharper image CATHODE -------- MADE OF TUNGSTEN IN CT – STILL SMALL AND LARGE 2-Filtration:  Radiation from x-ray tubes consists of long and short wave-lengths.  In CT, filtration serves 2 purpose: 1- Remove long wavelength x-rays because they do not play a role in CT image formation, but instead contribute to patient dose.  2- also filtration shapes the energy distribution across the radiation beam to produce uniform beam hardening when the x-rays pass through the filter and the object.  The filter is positioned between the x-ray tube and patient, and they shape the beam to produce more uniformity at the detectors. FILTRATION MATERIAL  ALUMINIUM ( SPECIAL FILTER IN CT) TO MAKE THE BEAM HARDER AND MORE MONO ENERGETIC FILTRATION CHANGE INTENSITY FILTRATION ENERGY –  Detectors X-Ray Beam Patient Cross-Section Bow-Tie or Equalizing Filter X-Ray Tube 3. HIGH VOLTAGE GENERATOR –(HVG)  Generates high voltage potential between cathode and anode of an x-ray tube Detector : *The detectors capture the x-ray photons and convert them into electrical signals. *The detector must has several characteristics essential for CT image production:- *CT detectors must possess high capture efficiency, absorption efficiency, and conversion efficiency.  Capture efficiency: refers to Capture the transmitted photon s from the patient; the size of the detector area facing the beam and distance between two detectors determine capture efficiency.  Absorption efficiency: refers to the number of photons absorbed by the detector and depends on the atomic number, physical density, size, and thickness of the detector face.  conversion efficiency: refers to the ability of detector to convert photons transmitted from patient to electric signal. CT DETECTORS Detectors  Function: as image receptors(Used to record photon activity ) for remnant radiation, then converts the measurement into an electrical signal proportional to the radiation intensity.  Two basic detector types are used:  1.Scintillation (solid state) detector 2. Ionization (xenon gas) detector.  Materials include: cadmium tungstate, cesium iodide, gadolinium or yttrium Types of Detector:  Scintillation Detector:- Convert x-ray to light, and then the light is converted into electrical energy  Gas Ionization Detector:- Convert x-ray energy directly to electrical energy.  Ref.  Radiologic Science for Technologists: Physics, Biology, and Protection, 11th edition