BME 560 Medical Imaging: X-ray, CT, and Nuclear Methods X-ray Instrumentation Part 1 Today • X-ray Systems • The X-ray tube – Principle – Components • X-ray Spectra • Filtering Discovery of X-rays • Wilhelm Conrad Roentgen (Röntgen) (1845-1923) • Observed X-ray fluorescence from a vacuum tube in 1895. • Coined the term “X-ray” • First Nobel prize in Physics (1901) X-ray System Source Collimator Filter Subject Anti-scatter Detector X-ray System Produces X-rays from electrical energy Tailors X-ray spectrum Converts X-rays to light and records Source Restrictor (Collimator) Determines size and shape of beam Filter Subject Anti-scatter Selectively removes scattered photons Detector X-ray Sources • Generate energetic electrons (particulate radiation) – Generate electrons – Accelerate electrons (kinetic energy) toward target – Focus electrons on target • Energetic electrons on target generate X-rays (EM radiation) – Bremsstrahlung – Characteristic radiation X-ray Tube This basic design dates to a 1913 tube developed by William Coolidge (GE) Cathode: generates electrons (hot cathode) Anode (or target): converts electrons to X-rays electrons Rotor: Rotates anode to dissipate heat Glass enclosure: Maintains vacuum X-rays X-ray Tube operation 1. Heat the cathode filament to “boil” electrons off. 2. Set up a high electric field (with high-voltage source) from cathode to anode. 3. Electrons removed from the filament travel from cathode to anode and have uniform energy equal to the potential difference. 100 kVp potential results in 100 keV electrons hitting anode X-ray Tube Operation • Why do you think this process must be under vacuum? Cathode Operation • The hot cathode works by thermionic emission – The process of using thermal energy to overcome electron binding energy and free electrons (also called Edison effect) – The filament is generally made of tungsten because of its high melting point and other features. • There are also cold-cathode devices using other methods to generate free electrons. Cathode Operation The focusing cup helps to direct the electron flow to a particular spot on the anode. Focal spot: The region on the anode struck by the electron beam. When might a small focal spot be good? When might a large focal spot be good? The focal track is the region on the rotating anode impacted by the electron beam. It wears out first. Anode Operation • The anode has two functions: 1. Convert energetic electrons to useful X-rays 2. Step 1 generates a lot of heat – must dissipate heat Material choice is important High Z High melting point Good thermal properties Characteristic X-rays Clean vacuum Bremsstrahlung production Efficiency = 9 x 10-10 Z (atomic number)V(voltage) (This is an approximation.) Efficiency: the ratio of the Bremsstrahlung x-ray energy to the incident electron energy. The remaining portion of the electron energy (1 - Efficiency) is converted into heat in the x-ray target. Anode heating is a major issue in x-ray tubes. Exercise: Calculate the efficiency for x-ray production for 100keV electron beams on tungsten (Z = 74). < 1% efficiency! Anode Operation • Common anode surface materials – Tungsten (Z = 74): most common for routine usage; sometimes alloyed with other materials (Rhenium) – Molybdenum (Z = 42): has nice characteristic Xrays at 17.5 ad 19.6 keV, good for mammography – Rhodium (Z = 45): similar to Mo, expensive Anode Operation • Anode base material is only for structure and thermal properties: – Molybdenum – graphite Surface Base X-ray Housing • The tube is encased in a housing for physical integrity. • The housing also provides shielding in all directions except the window. • The housing provides thermal properties and may enclose an oil envelope or other coolant. Window: blocks visible light, but permits Xrays to exit – beryllium is good. Heating and Cooling • Heating is most pronounced at the focal spot. • The anode base has to carry heat away from the focal spot as it rotates. • Ultimately, heat is dissipated in the housing. Anode damaged by local overheating From Sprawls Anode Angle Anode angle determines apparent focal spot size and focal track size e- X Typical anode angles range from 7 to 20 degrees e- X Heel Effect The effect of beam intensity decreasing severely toward the anode side of the beam The X-rays nearly parallel to the face of the anode are attenuated more because of the penetration depth of the electron beam. How does anode angle affect this? X-ray Tube Operation Be able to name all the parts and explain their function (except maybe B) Other Technologies • Transmission Target (Thin target) e-beam X-ray • Field emission (cold cathode) High local electric field removes electrons by tunneling Carbon nanotube-based X-ray devices X-ray Operation • Key parameters – Voltage (kVp): Determines the highest energy of X-ray produced (penetration) – Current (mA): Determines the X-ray flux (photons/area/time) produced – Time (s) • Often refer to mAs – 100 mA for 1 second is same number of photons as 50 mA for 2 seconds – Heat loading is different, though. Spectral Effects Current Photons per mAs per mm^2 at 750 mm 12000000 10000000 8000000 6000000 10 mA s 20 mA s 4000000 50 mA s 2000000 0 0 10 20 30 40 50 Energy (keV) 60 70 80 90 100 Spectral Effects Voltage Photons per mAs per mm^2 at 750 mm 800000 700000 600000 500000 400000 50 kVp 100 kVp 300000 150 kVp 200000 100000 0 0 10 20 30 40 50 Energy (keV) Spectral Effects Target material Photons per mAs per mm^2 at 750 mm 10000000 1000000 100000 10000 Mo (Z = 42) 1000 Rh (Z = 45) W (Z = 74) 100 10 1 0 2.5 5 7.5 10 12.5 15 17.5 Energy (keV) 20 22.5 25 27.5 30 Spectral Effects Anode Angle Photons per mAs per mAs per mm^2 at 750 mm 250000 200000 150000 6 degree 15 degree 100000 22 degree 50000 0 0 10 20 30 40 50 Energy (keV) 60 70 80 90 100 X-ray System Produces X-rays from electrical energy Tailors X-ray spectrum Converts X-rays to light and records Source Restrictor (Collimator) Determines size and shape of beam Filter Subject Anti-scatter Selectively removes scattered photons Detector Restriction • Absorb the beam except in desired direction – Reduce dose – Reduce scatter • Plates – preformed shapes • Collimators - adjustable Filtration • Low-energy X-rays do not penetrate soft tissue well – Contribute to dose – Not imaged • Use a material with high attenuation of the lowenergy, but relatively low attenuation of highenergy X-rays. No Kedges! – Aluminum is the standard. Filtration • Refer to NIST database – Aluminum – Copper – Cerium • Total filtration is referenced to equivalent thickness of Al (Al-eq) Filtration All parts provide some filtering Filtration Effective Energy • The effective energy is the equivalent monoenergetic beam with same HVL as the polychromatic beam • A weighted “average” energy • “Quality”, “penetration” Compensation Filters • Filters of nonuniform thickness may be used to compensate for beam nonuniformity or subject thickness.