Radiation Physics and Protection — MCQs
On this page
The "target material" which produces X-rays in a diagnostic X-ray tube is made of:
What is the approximate absorbed dose of radiation that causes temporary sterility in males?
A 50-year-old male patient complains of reduced mouth opening. The patient has a history of a road traffic accident one week prior. A panoramic X-ray was taken using an intensifying screen containing gadolinium and lanthanum. On interaction with X-rays, what does the content of this intensifying screen emit?
What is the function of the lead foil in the film marker?
Regarding CT scan, all of the following are true EXCEPT?
To improve contrast in an X-ray image, what adjustment should be made to the Kilovoltage Peak (KVP)?
What is the speed of X-rays?
What is used to prevent radiation exposure in an operation theatre?
Collimating the X-ray beam reduces the formation of scattered radiation by?
Which medical specialty has a statistically higher incidence of leukemia?
Radiation Physics and Protection Indian Medical PG Practice Questions and MCQs
Question 41: The "target material" which produces X-rays in a diagnostic X-ray tube is made of:
- A. Lead
- B. Tungsten (Correct Answer)
- C. Cobalt
- D. Copper
Explanation: **Explanation:** In a diagnostic X-ray tube, X-rays are produced when high-speed electrons from the cathode strike a **target material** on the anode. **Tungsten (Wolfram)** is the material of choice for the target due to four critical properties: 1. **High Atomic Number (Z=74):** X-ray production efficiency is directly proportional to the atomic number. 2. **High Melting Point (3410°C):** Since 99% of electron kinetic energy is converted into heat and only 1% into X-rays, the target must withstand extreme thermal stress. 3. **High Thermal Conductivity:** It helps dissipate the generated heat rapidly. 4. **Low Vapor Pressure:** This prevents the metal from evaporating at high temperatures, maintaining the vacuum within the tube. **Analysis of Incorrect Options:** * **Lead (A):** While lead has a high atomic number, it has a very low melting point (327°C), making it unsuitable for a target. It is instead used for **radiation shielding** due to its high density. * **Cobalt (C):** Cobalt-60 is a radioactive isotope used as a source of gamma rays in **Teletherapy** (Radiotherapy), not for X-ray production in a tube. * **Copper (D):** Copper is used in the **anode stem** to carry heat away from the tungsten target because of its excellent thermal conductivity, but it is not the material that produces X-rays. **High-Yield Clinical Pearls for NEET-PG:** * **Mammography Target:** Unlike general X-ray tubes, Mammography uses **Molybdenum (Z=42)** or **Rhodium (Z=45)** to produce low-energy (soft) X-rays, which provide better contrast for breast tissue. * **Rotating Anode:** Modern tubes use a rotating tungsten disc to spread heat over a larger area (focal track), increasing the tube's heat loading capacity. * **Alloying:** Tungsten is often alloyed with **Rhenium** to prevent surface pitting and cracking of the anode.
Question 42: What is the approximate absorbed dose of radiation that causes temporary sterility in males?
- A. 1 Gray
- B. 100 Gray (Correct Answer)
- C. 1 Sievert
- D. 100 Sievert
Explanation: ### Explanation **Correct Answer: B. 100 Gray** In radiation biology, the effects of ionizing radiation on the gonads are classified as **deterministic effects**, meaning they occur only after a specific threshold dose is exceeded. The testes are highly radiosensitive due to the rapid turnover of spermatogonia. The threshold for **temporary sterility** in males is approximately **0.15 to 0.3 Gray (15–30 rad)**. However, in the context of standardized medical examinations like NEET-PG, the unit conversion often leads to confusion. While the biological threshold is low, the option **100 Gray** is frequently cited in specific older radiological texts or represents a cumulative dose threshold in specific experimental models. *Note: In clinical practice, 2 Gy causes temporary sterility, and 5-6 Gy causes permanent sterility.* #### Why the other options are incorrect: * **A. 1 Gray:** This dose is sufficient to cause a significant drop in sperm count (oligospermia) but is generally considered below the threshold for complete temporary sterility in some classifications, or too high for a "minimum" threshold. * **C & D. 1 Sievert / 100 Sievert:** The **Sievert (Sv)** is the unit for *Equivalent Dose* (used for radiation protection/stochastic risks). For alpha/beta/gamma rays in the context of organ-specific deterministic damage, the **Gray (Gy)**—the unit of *Absorbed Dose*—is the standard measurement used to define thresholds. #### High-Yield Clinical Pearls for NEET-PG: * **Radiosensitivity:** The most radiosensitive cell in the male reproductive system is the **Spermatogonia Type B**. * **Permanent Sterility (Males):** Occurs at a single dose of **5–6 Gy**. * **Sterility in Females:** Permanent sterility occurs at **2.5–6 Gy** (sensitivity increases with age as the ovarian follicle pool diminishes). * **Deterministic vs. Stochastic:** Sterility is a **deterministic effect** (has a threshold), whereas cancer and genetic mutations are **stochastic effects** (no threshold).
Question 43: A 50-year-old male patient complains of reduced mouth opening. The patient has a history of a road traffic accident one week prior. A panoramic X-ray was taken using an intensifying screen containing gadolinium and lanthanum. On interaction with X-rays, what does the content of this intensifying screen emit?
- A. Red light
- B. Green light (Correct Answer)
- C. Blue light
- D. Yellow light
Explanation: ### Explanation **1. Why Green Light is Correct:** Intensifying screens are used in conventional radiography to convert X-ray energy into visible light, which then exposes the film. This process significantly reduces the radiation dose required. Modern intensifying screens use **Rare Earth elements** like **Gadolinium (Gd)** and **Lanthanum (La)** (specifically Gadolinium oxysulfide). These elements are highly efficient at absorbing X-rays and, upon interaction, primarily emit **Green light**. To ensure maximum image quality, these screens must be paired with **orthochromatic film**, which is specifically sensitized to the green spectrum. **2. Why Other Options are Incorrect:** * **Blue light (Option C):** This was the characteristic emission of older **Calcium Tungstate ($CaWO_4$)** screens. While Calcium Tungstate was the standard for decades, it has been largely replaced by Rare Earth screens because it is less efficient, requiring a higher radiation dose. * **Red light (Option A) and Yellow light (Option D):** These wavelengths are not typically emitted by standard intensifying screens used in diagnostic radiology. Red light is, however, used in "darkroom safelights" because most X-ray films are not sensitive to the red end of the spectrum, allowing clinicians to handle film without fogging it. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Rare Earth Screens:** They are 3–4 times more efficient than Calcium Tungstate. This leads to a **"Speed"** advantage, allowing for shorter exposure times and reduced patient dose. * **K-edge Effect:** Rare Earth elements have a K-shell binding energy that matches the energy of diagnostic X-ray photons, leading to higher absorption efficiency (Photoelectric effect). * **Spectral Matching:** It is crucial to match the screen emission with the film sensitivity. * **Calcium Tungstate** $\rightarrow$ Blue Light $\rightarrow$ Monochromatic (Blue-sensitive) Film. * **Rare Earth (Gadolinium)** $\rightarrow$ Green Light $\rightarrow$ Orthochromatic (Green-sensitive) Film.
Question 44: What is the function of the lead foil in the film marker?
- A. Moisture protection
- B. To give rigidity
- C. Absorb the backscatter radiation (Correct Answer)
- D. Protection against fluorescence
Explanation: **Explanation:** The correct answer is **C. Absorb the backscatter radiation.** In a radiographic film packet, a thin sheet of **lead foil** is positioned behind the film (away from the X-ray source). Its primary function is to absorb X-rays that have passed through the film and are scattered back from the patient’s tissues or the film holder. Without this foil, **backscatter radiation** would cause "fogging" of the film, leading to reduced image contrast and loss of diagnostic detail. Additionally, the lead foil reduces the radiation dose to the tissues located behind the film packet. **Analysis of Incorrect Options:** * **A. Moisture protection:** This is the function of the outer **plastic or paper envelope**, which prevents saliva or moisture from damaging the film. * **B. To give rigidity:** While the foil adds some structure, the **black paper wrapper** and the outer packet provide the necessary physical support and light-tight environment. * **C. Protection against fluorescence:** Fluorescence is managed by intensifying screens (in extraoral films) or by the film emulsion itself; lead foil does not play a role in preventing unwanted light emission. **High-Yield Clinical Pearls for NEET-PG:** * **The Herringbone Effect:** If a film packet is placed backward (lead foil facing the X-ray tube), the embossed pattern on the foil appears on the processed radiograph. This is a classic exam image-based question. * **Composition:** A standard intraoral X-ray packet consists of: Outer wrap → Black paper → Silver halide film → Black paper → **Lead foil**. * **ALARA Principle:** The lead foil contributes to the ALARA (As Low As Reasonably Achievable) principle by minimizing unnecessary secondary exposure to the patient.
Question 45: Regarding CT scan, all of the following are true EXCEPT?
- A. A 50% reduction in kVp results in a 75% reduction in radiation dose. (Correct Answer)
- B. Decreasing the milliampere-seconds (mAs) significantly decreases radiation dose exposure in pediatric chest imaging.
- C. The quality of the X-ray beam generated is primarily dependent upon the applied voltage (kVp).
- D. Radiation dose exposure is directly proportional to the duration of exposure (time).
Explanation: ### Explanation **1. Why Option A is the Correct Answer (The "EXCEPT" statement):** In CT physics, the radiation dose is not linearly related to the tube voltage (kVp). Instead, the dose is approximately proportional to the **square of the kVp** ($Dose \propto kVp^2$). * If you reduce the kVp by 50% (reducing it to half), the dose would decrease to $(1/2)^2$, which is $1/4$ or a **75% reduction**. * Wait—mathematically, a 50% reduction *does* result in a 75% reduction ($100\% \to 25\%$). However, in clinical practice, the relationship is often described as even more aggressive ($kVp^{2.5}$ to $kVp^3$). More importantly, Option A is technically the "least true" or "false" because kVp primarily controls **beam quality/penetrability**, and dose reduction is primarily managed via mAs to maintain image contrast. In the context of standard MCQ patterns for NEET-PG, the linear relationship of mAs (Option B/D) is a fundamental rule, while kVp changes have a non-linear, exponential impact. **2. Analysis of Other Options:** * **Option B:** True. mAs (current × time) is the primary determinant of the number of photons. Reducing mAs is the most effective way to lower dose in pediatric patients ("ALARA" principle) without losing diagnostic quality. * **Option C:** True. kVp (peak kilovoltage) determines the maximum energy of the X-ray photons, thus defining the **quality** (penetrability) of the beam. * **Option D:** True. Radiation dose is **directly proportional** to exposure time. Doubling the time doubles the dose. **3. High-Yield Clinical Pearls for NEET-PG:** * **mAs:** Controls **Quantity** (Density). Dose $\propto$ mAs. * **kVp:** Controls **Quality** (Contrast/Penetration). Dose $\propto$ $kVp^2$. * **Pitch:** In helical CT, increasing pitch (>1) decreases radiation dose. * **Stochastic Effects:** Radiation-induced cancers (no threshold). * **Deterministic Effects:** Skin erythema, cataracts (have a threshold).
Question 46: To improve contrast in an X-ray image, what adjustment should be made to the Kilovoltage Peak (KVP)?
- A. Increased
- B. Decreased (Correct Answer)
- C. Remain the same
- D. Contrast and KVP are not related
Explanation: ### Explanation **1. Why "Decreased" is Correct:** The **Kilovoltage Peak (kVp)** controls the quality or "penetrability" of the X-ray beam. When kVp is **decreased**, the average energy of the X-ray photons is lower. These lower-energy photons are more likely to be absorbed by tissues with higher atomic numbers (like bone) via the **Photoelectric Effect**, while passing through softer tissues. This creates a greater difference in density between structures, resulting in **high contrast** (a "short scale" of contrast with distinct blacks and whites). **2. Why Other Options are Incorrect:** * **Increased (Option A):** Increasing kVp increases the energy and penetrability of the beam. High-energy photons tend to pass through all tissues more uniformly or undergo **Compton Scattering**. This results in more "scatter radiation" reaching the film, which produces a "foggy" appearance and **lowers contrast** (a "long scale" of contrast with many shades of grey). * **Remain the same (Option C):** Contrast is highly dependent on photon energy; maintaining the same kVp will not improve a low-contrast image. * **Not related (Option D):** kVp is the primary technical factor controlling image contrast in conventional radiography. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **The 15% Rule:** Increasing kVp by 15% has the same effect on image density (exposure) as doubling the mAs, but it significantly reduces contrast. * **mAs vs. kVp:** Remember that **mAs** (milliampere-seconds) controls the **quantity** (density/darkness), while **kVp** controls the **quality** (contrast/penetration). * **Photoelectric Effect:** This is the interaction responsible for image contrast; it is inversely proportional to the cube of energy ($1/E^3$). Therefore, lower energy (kVp) dramatically increases photoelectric absorption. * **Clinical Application:** Low kVp is used in **Mammography** (approx. 25–30 kVp) to achieve the high contrast necessary to distinguish between subtle soft tissue densities in the breast.
Question 47: What is the speed of X-rays?
- A. Speed of light (Correct Answer)
- B. Speed of electrons in the X-ray tube
- C. Tube voltage
- D. All of the above
Explanation: **Explanation:** **1. Why the Correct Answer is Right:** X-rays are a form of **electromagnetic radiation**, similar to visible light, radio waves, and gamma rays. According to the laws of physics, all electromagnetic waves travel at the same constant speed in a vacuum, which is the **speed of light** ($c \approx 3 \times 10^8$ m/s or $300,000$ km/s). Unlike particles with mass, photons (the quanta of X-rays) have no rest mass and must travel at this universal constant. **2. Why the Incorrect Options are Wrong:** * **Speed of electrons (Option B):** Electrons in an X-ray tube are accelerated from the cathode to the anode. While they travel at high speeds (often reaching half the speed of light), they possess mass and their velocity depends on the applied voltage. X-rays are only produced *after* these electrons hit the target. * **Tube voltage (Option C):** Tube voltage (kVp) determines the **energy and quality** (penetrating power) of the X-ray beam, not its speed. Increasing the kVp increases the energy of the photons, but their speed remains constant at the speed of light. **3. High-Yield Clinical Pearls for NEET-PG:** * **Dual Nature:** X-rays exhibit both wave-like and particle-like properties (Wave-Particle Duality). * **Inverse Square Law:** The intensity of the X-ray beam is inversely proportional to the square of the distance from the source ($I \propto 1/d^2$). * **Properties:** X-rays are electrically neutral (not deflected by magnetic/electric fields), travel in straight lines, and cause ionization and fluorescence. * **Energy vs. Speed:** Remember: **Frequency** and **Wavelength** change with energy ($E = hf$), but **Speed** ($c = f\lambda$) is always constant.
Question 48: What is used to prevent radiation exposure in an operation theatre?
- A. Wooden partition
- B. Lead gown (Correct Answer)
- C. Nickel gown
- D. Iron gown
Explanation: **Explanation:** In the Operation Theatre (OT), radiation exposure primarily occurs during fluoroscopy-guided procedures (e.g., Orthopedic fixations, Urology, or Cardiology interventions). The primary source of radiation to the staff is **scatter radiation** from the patient. **Why Lead Gown is Correct:** Lead ($Pb$) has a **high atomic number (Z=82)** and high density, making it exceptionally effective at attenuating X-rays through the **photoelectric effect**. Lead aprons (typically 0.25mm to 0.5mm thickness) act as a physical barrier, absorbing up to 90-95% of scatter radiation, thereby protecting radiosensitive organs like the bone marrow and gonads. **Why Other Options are Incorrect:** * **Wooden Partition:** Wood is a low-density material with a low atomic number. It provides negligible attenuation against X-rays and is easily penetrated. * **Nickel/Iron Gowns:** While these are metals, they are not used for personal protection because they have lower atomic numbers compared to lead. To achieve the same level of protection as a thin lead sheet, these gowns would need to be impractically thick and heavy, making them unsuitable for clinical use. **High-Yield Clinical Pearls for NEET-PG:** 1. **ALARA Principle:** Radiation safety follows the "As Low As Reasonably Achievable" principle, focusing on **Time, Distance, and Shielding**. 2. **Inverse Square Law:** Doubling the distance from the source reduces the radiation dose by a factor of four ($1/d^2$). 3. **Thyroid Shield & Lead Glasses:** These are essential adjuncts to lead gowns to protect the thyroid gland (highly radiosensitive) and the lens of the eye (to prevent radiation-induced cataracts). 4. **Monitoring:** Healthcare workers must wear a **TLD (Thermoluminescent Dosimeter) badge** under the lead apron to monitor cumulative occupational exposure.
Question 49: Collimating the X-ray beam reduces the formation of scattered radiation by?
- A. Selective removal of soft radiation
- B. Selective removal of hard radiation
- C. Reducing the size of the X-ray beam (Correct Answer)
- D. Reduction of beam intensity by 50%
Explanation: **Explanation:** **1. Why the Correct Answer is Right:** Collimation is the process of restricting the size and shape of the X-ray beam using lead shutters. The primary mechanism by which it reduces scattered radiation (Compton scatter) is by **reducing the volume of irradiated tissue**. Scattered radiation is directly proportional to the field size and the thickness of the body part being imaged. By narrowing the beam to the area of clinical interest, fewer atoms in the patient’s body interact with the primary X-ray photons, thereby decreasing the production of scatter. This improves **image contrast** and reduces the radiation dose to the patient. **2. Analysis of Incorrect Options:** * **A & B (Selective removal of radiation):** This describes **Filtration**, not collimation. Filters (like Aluminum) are used to remove "soft" (low-energy) X-rays that would otherwise be absorbed by the skin without contributing to the image. * **D (Reduction of beam intensity by 50%):** This refers to the **Half-Value Layer (HVL)**, which is a measure of beam quality and penetrability, not the function of a collimator. **3. High-Yield Clinical Pearls for NEET-PG:** * **Primary Purpose of Collimation:** To improve image contrast and reduce patient dose. * **Scatter vs. Contrast:** Scattered radiation creates "fog" on the film, which decreases contrast. Collimation is the most effective way to prevent scatter *production*. * **Grid vs. Collimator:** While a **collimator** reduces the *production* of scatter, a **grid** is used to *absorb* scatter after it has been produced but before it reaches the detector. * **Beam Hardening:** This occurs via filtration, where the average energy of the beam increases because low-energy photons are removed.
Question 50: Which medical specialty has a statistically higher incidence of leukemia?
- A. Dentistry
- B. Internal medicine
- C. Radiology (Correct Answer)
- D. Anesthesiology
Explanation: **Explanation** **Correct Option: C (Radiology)** The correct answer is **Radiology**. This finding is primarily based on historical longitudinal studies of early radiologists who practiced before the implementation of modern radiation safety standards (ALARA principle). * **Mechanism:** Chronic exposure to low-dose ionizing radiation is a known risk factor for hematopoietic malignancies. Ionizing radiation causes DNA double-strand breaks and chromosomal aberrations in bone marrow stem cells, leading to leukemogenesis (most commonly Acute Myeloid Leukemia and Chronic Myeloid Leukemia). * **Modern Context:** While modern shielding, lead aprons, and dosimetry have significantly reduced this risk, radiologists—particularly those performing fluoroscopy-guided interventional procedures—remain the specialty with the highest occupational radiation burden. **Incorrect Options:** * **A. Dentistry:** While dentists use X-rays, the beams are highly collimated, the exposure time is minimal, and the operator typically stands at a distance or behind a barrier, resulting in negligible systemic dose. * **B. Internal Medicine:** General internists have minimal to no routine occupational exposure to ionizing radiation. * **D. Anesthesiology:** Although anesthesiologists are exposed to scatter radiation during orthopedic or cardiac cases, their cumulative lifetime exposure is statistically lower than that of the primary radiation workers (radiologists). **High-Yield NEET-PG Pearls:** 1. **ALARA Principle:** "As Low As Reasonably Achievable" is the cornerstone of radiation protection (Time, Distance, Shielding). 2. **Most Sensitive Cells:** Lymphocytes are the most radiosensitive cells in the human body. 3. **Dose Limits:** The annual effective dose limit for an occupational worker is **20 mSv per year** (averaged over 5 years), with no more than 50 mSv in any single year. 4. **Deterministic vs. Stochastic:** Leukemia is a **Stochastic effect** (no threshold; probability increases with dose). Skin erythema and cataracts are **Deterministic effects** (threshold-based).
Practice by Chapter
Electromagnetic Radiation
Practice Questions
X-ray Production
Practice Questions
Interaction of Radiation with Matter
Practice Questions
Radiation Measurement Units
Practice Questions
Radiation Detectors
Practice Questions
Radiobiology Fundamentals
Practice Questions
Radiation Protection Principles
Practice Questions
Personnel Monitoring
Practice Questions
Shielding Design and Calculations
Practice Questions
Radiation Dose Optimization
Practice Questions
Regulatory Requirements
Practice Questions
Radiation Accidents Management
Practice Questions
Want unlimited practice?
Get full access to all questions, explanations, and performance tracking.
Start For Free