American Registry of Radiologic Technologists (ARRT) Practice Exam

Which measurement indicates the effectiveness of energy transfer from ionizing radiation to soft tissue?

1. Roentgen

2. Rad

3. LET

4. Gray

The correct answer is: LET

The effectiveness of energy transfer from ionizing radiation to soft tissue is best indicated by the concept of Linear Energy Transfer (LET). LET is a measure of the amount of energy that is deposited by ionizing radiation as it travels through a medium, which in this case is soft tissue. It reflects how densely the energy is transferred in a given length of tissue, influencing the biological effects of the radiation. Higher LET values suggest a greater effectiveness in causing ionization and, consequently, potentially more biological damage, which makes this measure crucial in radiation therapy and safety assessments. While other options like roentgen, rad, and gray are units of measurement related to ionizing radiation, they do not specifically address the efficacy of energy transfer to tissue. Roentgen measures exposure to ionizing radiation in air, rad is used to quantify absorbed dose (energy absorbed per unit mass), and gray is the SI unit equivalent to rad but focuses on absorbed dose as well. However, none of these units detail the effectiveness of the energy transfer process itself to biological tissues, which is distinctly captured by LET.