X-ray Physics and Circuitry

Focal Spot Characteristics and Off-Focus Radiation

• Actual Focal Spot Location: The actual focal spot is positioned on the surface of the anode target.

• Directional Orientation: The effective focal spot is the one directed toward the patient, as opposed to the actual focal spot on the anode.

• Impact of Anode Angle: Decreasing the anode angle (choosing a small focal spot) results in a proportional decrease in the effective focal spot size.

• Off-Focus Radiation:

  • Definition: Radiation that is produced outside the actual focal spot area.

  • Effects: This type of radiation is responsible for causing blurred edges or ghost images on the resulting radiograph.

X-Ray Tube Loading and Heat Unit Calculations

• Tube Loading Definition: This refers to the specific amount of heat energy generated within the x-ray tube during an exposure, often quantified in heat units ($HU$).

• Heat Unit ($HU$) Formula: The formula for calculating heat units is defined as:     $HU = kVp \times mA \times \text{time (s)} \times \text{wave form factor}$

• Specific Wave Form Factors:

  • Single Phase: $1$

  • Three Phase, 6 Pulse: $1.35$

  • Average Wave Form Factor: $1.4$

  • Three Phase, 12 Pulse: $1.41$

  • High Frequency: $1.45$

Fundamental Electrical Circuit Concepts

• Series Circuits:

  • Defined by having only one path for the electricity to flow.

  • Functional implication: If a single component or part of the path breaks, the entire circuit stops functioning (e.g., traditional Christmas lights).

• Parallel Circuits:

  • Defined by having multiple paths for the electrical current.

  • Functional implication: If one branch breaks, the other branches remain operational.

• Amperes (Amps): A measure used to quantify the flow of electric current within a circuit.

• Voltage: The electrical potential difference between two distinct points in a circuit; it is the "pressure" that pushes electrons through the system.

• Resistance:

  • Measured in Ohms.

  • Definition: A measure of how much a specific material resists or slows down the movement of electrons.

The Primary Circuit: Components and Functions

• Components List: The primary circuit consists of the Main power switch, Circuit breaker, Autotransformer, $kVp$ selector, Exposure switch, Timer circuit, and the primary side of the step-up transformer.

• Circuit Breaker: Its primary purpose is to protect the circuit against electrical overloads.

• Autotransformer: This component adjusts the incoming voltage and supplies it to the step-up transformer.

• $kVp$ Selector: Provides the user with the ability to choose the desired kilovoltage for the procedure.

The Secondary Circuit: High Voltage and Rectification

• Components List: The secondary circuit includes the secondary side of the step-up transformer, the $mA$ meter, rectifiers, and the x-ray tube.

• Secondary Side of the Step-Up Transformer: This component is responsible for converting low voltage into the high voltage required for x-ray production.

• $mA$ Meter: Performs the function of measuring the current flowing between the cathode and anode during the actual exposure.

• Rectifiers: Their purpose is to convert Alternating Current ($AC$) into Direct Current ($DC$), ensuring that the current flows in only one direction.

The Filament Circuit: Electron Production

• Components List: The filament circuit contains the Step-down transformer, the Filament (Cathode), and the $mA$ selector (also known as a Rheostat).

• Step-Down Transformer: This device reduces the voltage while simultaneously increasing the current to heat the filament.

• Filament (Cathode): A heated wire that emits electrons via thermionic emission when electrical current passes through it.

• $mA$ Selector (Rheostat): This component controls the specific amount of current flowing to the filament, thereby adjusting the filament temperature and the resulting quantity of electrons produced.

Induction Principles and Operational Constants

• Incoming Line Voltage: The standard voltage entering the circuit is $220\,\text{volts AC}$.

• Sequential Order of Transformers: The autotransformer is the first transformer encountered in the circuit.

• Induction Mechanisms:

  • Autotransformer: Operates on Self induction, utilizing a single winding coil.

  • Step-up and Step-down Transformers: Operate on Mutual induction, utilizing two separate coils (one for the primary side and one for the secondary side).

• Circuit Power Types:

  • Components Working on $AC$: The autotransformer, the step-up transformer, the step-down transformer, and the incoming power supply.

  • Components Working on $DC$: The x-ray tube and the rectifiers.

Effects of Increasing Tube Current ($mA$)

• Technical Factor Correlations: When a technologist increases the $mA$ setting on the control panel, the following factors also increase:

  • Filament current

  • Thermionic emission

  • Tube current

  • Quantity of x-rays produced

  • Patient ($Pt$) dose