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) ( ) Observed X-ray fluorescence from a vacuum tube in Coined the term “X-ray” First Nobel prize in Physics (1901)

X-ray System Source Collimator Filter Subject Anti-scatterDetector

X-ray System Source Restrictor (Collimator) Filter Subject Anti-scatterDetector Produces X-rays from electrical energy Determines size and shape of beam Tailors X-ray spectrum Selectively removes scattered photons Converts X-rays to light and records

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 electrons X-rays Cathode: generates electrons (hot cathode) Anode (or target): converts electrons to X-rays Rotor: Rotates anode to dissipate heat Glass enclosure: Maintains vacuum This basic design dates to a 1913 tube developed by William Coolidge (GE)

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 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 X- rays 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 Base Surface

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 X- rays 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- Typical anode angles range from 7 to 20 degrees XX

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) Field emission (cold cathode) e-beam X-ray 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

Spectral Effects Voltage

Spectral Effects Target material

Spectral Effects Anode Angle

X-ray System Source Restrictor (Collimator) Filter Subject Anti-scatterDetector Produces X-rays from electrical energy Determines size and shape of beam Tailors X-ray spectrum Selectively removes scattered photons Converts X-rays to light and records

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 low- energy, but relatively low attenuation of high- energy X-rays. No K- edges! –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.