VII. Fundamental Principles of Exposure

Question 1

Selection of exposure factors

1. Maintain consistent specific receptor exposure

Answer 1

1. Maintain Consistent Specific Receptor Exposure

The goal is to produce diagnostically useful images while minimizing unnecessary dose.
Exposure factors (kVp, mAs, SID, etc.) should be adjusted to achieve consistent image receptor exposure regardless of patient size or anatomy.
Digital systems can compensate for some exposure variation, but consistency ensures optimal image quality and patient safety.

Question 2

2. Control scatter

Answer 2

Scatter radiation degrades image contrast and increases patient dose.
Use of collimation, grids, and optimal kVp selection helps reduce scatter production and improve image quality.
Minimizing scatter enhances the signal-to-noise ratio (SNR) and overall image clarity.

Question 3

3. Adjust for differences in:
   a. Structure Composition:
   b. Source-to-image receptor distance (SID):

Answer 3

a. Structure Composition:
Denser tissues (e.g., bone) require higher exposure; softer tissues (e.g., lungs) require less.
Consider the atomic number and density of the area being imaged.
Use compensating filters when necessary to balance exposure.
b. Source-to-Image Receptor Distance (SID):
According to the inverse square law, increasing SID reduces beam intensity.
Adjust mAs to compensate for longer SID (e.g., chest x-rays at 72” require more exposure than abdomen at 40”).
Maintaining correct SID also affects image magnification and sharpness.

Question 4

Control Patient Exposure

Higher kVp levels

Answer 4

Reduce patient dose but may lower contrast.

Question 5

Control Patient Exposure

Additional filtration

Answer 5

Removes low-energy photons that contribute to unnecessary dose.

Question 6

Control Patient Exposure

Interfacing with automatic exposure control (AEC) systems

Answer 6

Ensures consistent exposure levels.

Question 7

Control Patient Exposure

ALARA principles

Answer 7

Minimize radiation exposure to As Low As Reasonably Achievable.

Question 8

Control Patient Exposure

Monitor patient exposure

Answer 8

Use tools like dose area product (DAP).

Question 9

Control Patient Exposure

Dose area product (DAP)

Answer 9

DAP is a quantity that reflects not only the patient radiation dose but also the amount of tissue irradiated; therefore it may be a better indicator of risk than dose. DAP is expressed in units of mGya-cm2.
DAP increases with increasing radiation field size even if the dose remains unchanged. Smaller field size results in lower DAP and less risk because less tissue is exposed.

Question 10

*Control Patient Exposure-Vendor-specific values *

Vendor-specific values: Relationship to patient exposure

Answer 10

Must understand how each vendor’s system translates exposure into dose.
Exposure indicators may reflect image receptor dose, not actual patient dose.

Question 11

*Control Patient Exposure-Vendor-specific values *

Vendor-specific values: Reader calibration

Answer 11

Calibration ensures the system accurately displays dose indicators.
Regular checks and phantom testing are essential.

Question 12

*Control Patient Exposure-Vendor-specific values *

Vendor-specific values: Centering and beam collimation

Answer 12

Poor centering or collimation can affect AEC readings and dose estimates.
Always verify correct positioning and beam alignment.

Question 13

*Control Patient Exposure-Vendor-specific values *

Vendor-specific values: Optimal value ranges

Answer 13

Vendors provide reference ranges for acceptable exposure indicators.
Technologists must aim to stay within these ranges to ensure image quality and patient safety.