Chapter 2 The X-ray Beam

X-ray production requires three things Lots of electrons Give electrons kinetic energy Convert electron kinetic energy into electromagnetic energy (x-rays)

X-ray machine X-ray machine is an electrical / mechanical device which converts energy from one form into another

X-ray Production The production of x-rays requires a rapidly moving stream of electrons that are suddenly decelerated or stopped. The negative electrode (cathode) is heated, and electrons are emitted (thermionic emission). The electrons are attracted to the anode, move rapidly towards the positive electrode, and are stopped or decelerated.

tube housing

X-ray Tube Housing Metal or glass envelope Negatively charged electrode Positively charged electrode

Cathode Filament Source of electrons Coiled tungsten wire Focusing cup Filament current Thermionic emission Coiled tungsten wire Large and small Focusing cup Space charge effect

Anode Rotating anode Target Requires a stator and rotor to rotate Tungsten metal High melting point Efficient x-ray production Target Decelerates and stops electrons Energy converted to heat and x-rays Bremsstrahlung and characteristic interactions

Target Interactions Bremsstrahlung interactions Braking or slowing down radiation 85% of x-ray beam Characteristic interactions Projectile electron energy at least 69.5 keV Inner shell electron ionized 15% of x-ray beam X-ray properties the same

Target Interactions (cont.)

X-ray Exposure

X-ray Exposure (cont.)

X-ray Emission Spectrum The range and intensity of x-rays emitted changes with different exposure technique settings on the control panel.

X-ray Beam Quantity and Quality Quantity: number of x-ray photons in primary beam Quality: penetrating power of x-ray beam Kilovoltage Milliamperage Exposure time

Kilovoltage Creates potential difference Determines the speed of the electrons in tube current Greater speed results in greater quantity and quality of primary beam Increasing electron speed will increase x-ray beam penetrability

Milliamperage Unit to measure tube current or number of electrons flowing per unit time mA directly proportional to quantity of x-rays produced Double the mA will double the number of x-ray photons produced

Milliamperage and Time Exposure time determines the length of time x-rays are produced. Increasing time will increase the total number of x-rays produced. Exposure time and x-ray quantity are directly proportional.

Milliamperage and Time (cont.) Milliamperage × time = mAs 200 mA × 0.25 s = 50 mAs 400 mA × 0.25 s = 100 mAs 200 mA × 0.50 s = 100 mAs Changing the mA or time will change the quantity of x-rays produced.

Three things to have usable x-rays

X-rays must come from as close to a point source as possible X-rays must travel in straight lines X-rays must create contrast

Why can’t x-rays come from a point source??????

Line Focus Principle Relationship between actual and effective focal spot Actual focal spot: area on anode target exposed to tube current electrons Effective focal spot: focal spot size as measured directly under anode target The smaller the anode angle the smaller the effective focal spot size

Anode Heel Effect X-rays are more intense on the cathode side of the x-ray tube. The intensity of the x-rays decrease toward the anode. Place the thicker anatomic area under the cathode end for more even exposure to the image receptor.

filtration

Heat Units During x-ray production, most of the electron kinetic energy is converted to heat and can damage the x-ray tube. Heat units (HU) = mA × time × kVp × generator factor

Extending X-ray Tube Life If applicable, warm up tube. Avoid excessive heat generation. Do not hold down rotor button without making exposure. Use lower mA and longer exposure time. Don’t move tube while energized. Recognize unusual noises, and report.