Radiation Physics and Protection Practice Questions

Q: If the central ray is perpendicular to the film but not to the object, then:

Foreshortening of the image occurs. ***Foreshortening of the image occurs*** - When the **central ray** is perpendicular to the **film** but not to the **object**, the object appears shorter than its actual size because parts of the object closer to the film are projected at a relative angle that compresses the image. - This angular relationship causes the object's dimensions parallel to the central ray to be minimized on the film, leading to **foreshortening**. *Blurring of the image occurs* - **Blurring** typically results from patient movement, insufficient exposure time, or issues with the focal spot size, rather than the angulation between the central ray, object, and film. - While extreme angulation can degrade image quality, a specific blur due to perpendicularity to the film but not the object is less direct than geometric distortion. *Elongation of image occurs* - **Elongation** occurs when the **central ray** is perpendicular to the **object** but not to the **film**, causing parts of the object further from the central point of the beam to be stretched out. - In this scenario, the issue is the central ray being perpendicular to the film but not the object, which creates the opposite effect—foreshortening. *None of the options* - This option is incorrect because **foreshortening** is a distinct and predictable geometric distortion that occurs under the described conditions. - The specific angulation described directly leads to geometric distortion rather than an absence of effect.

Q: In medical radiotherapy, a linear accelerator emits

Electrons and photons. ***Electron and photons*** - Medical linear accelerators (linacs) are designed to produce high-energy **electrons** and **X-rays (photons)** for radiotherapy. - Electrons are accelerated to high speeds and then either used directly for shallow treatments or directed at a heavy metal target to generate X-rays. *Electron and positrons* - While electrons are emitted, **positrons** are generally not produced by standard medical linacs used for radiation therapy, as they are anti-particles of electrons. - Positrons are primarily used in **Positron Emission Tomography (PET)** imaging, not for therapeutic radiation. *Neutrons and positrons* - Standard medical linacs do not emit **neutrons**; neutrons are byproducts of very high-energy photon interactions (above 10-15 MeV) but are not intentionally emitted. - As mentioned, **positrons** are not a primary emission for radiotherapy. *Neutrons only* - **Neutron therapy** utilizes specialized neutron generators or cyclotrons, not typical medical linacs, to produce neutrons for treating certain cancers. - Medical linacs are not designed to solely emit neutrons as their primary therapeutic radiation.

Q: CT numbers of water and bone are respectively:

0, + 1000. ***0, + 1000*** - The **CT number (Hounsfield Unit)** for **water** is defined as **0**, serving as a reference point for all other tissues in CT imaging. - **Bone**, particularly **cortical bone**, has a high density and thus corresponds to a CT number of approximately **+1000 HU**. *100,0* - This option incorrectly assigns a CT number of **100 to water**, which is fundamentally incorrect as water is defined as **0 HU**. - It also assigns **0** to **bone**, which is the CT number for water, not bone. *+1000,-100* - This option correctly identifies **+1000 HU** for dense bone but incorrectly assigns **-100 HU to water**, which is the CT number typically associated with fat, not water. *0,-1000* - While **0 HU** is correct for water, **-1000 HU** is the CT number for **air**, not bone. - Bone has a high positive CT number due to its high density, whereas air has a very low negative CT number.

Q: One gray equals

100 RAD. ***100 RAD*** - The **gray (Gy)** is the SI unit of absorbed radiation dose, defined as **1 joule of energy absorbed per kilogram** of matter - **1 Gy = 100 rad** is the standard conversion factor between SI and traditional units - This conversion is essential in radiation oncology and radioprotection for dose calculations and safety limits - Example: A dose of 2 Gy = 200 rad *1000 RAD* - This is **10 times too high** for the correct conversion - Would result in significant **overestimation** of absorbed dose when converting from grays to rads - Could lead to dangerous errors in radiation therapy planning *10 RAD* - This is **10 times too low** for the correct conversion - Would result in significant **underestimation** of absorbed dose when converting from grays to rads - Could lead to underdosing in radiation therapy or underestimating radiation exposure risks *10000 RAD* - This is **100 times too high** for the correct conversion - Represents a **gross overestimation** of the absorbed dose - Would result in calculation errors of orders of magnitude in radiation dosimetry

Q: Function of hypo used in fixer is:

Dissolves and removes the unexposed silver halide. ***Dissolves and removes the unexposed silver halide*** - **Sodium thiosulfate**, commonly known as "hypo," is the primary fixing agent, forming soluble complexes with unexposed **silver halide crystals**. - These soluble complexes are then washed away, leaving behind the developed silver image and making the film permanent and light-stable. *Forms a protective layer on the film* - This is not the primary function of hypo; fixers contain **hardeners** (like potassium alum) that stiffen and protect the gelatin emulsion, not the hypo itself. - The protective layer is more related to the hardening component of the fixer, preventing damage during subsequent processing and handling. *Inactivates the carryover developing agent* - The **stop bath** (usually an acidic solution) is responsible for neutralizing any residual developer carried over from the developing stage. - While the acidic nature of some fixers can contribute, it's not the main role of the hypo component. *Prevents damage to gelatin* - The hardening agents in the fixer, such as **potassium alum**, are responsible for **cross-linking gelatin molecules** to prevent swelling and damage. - Hypo's main role is chemical removal of silver halide, not physical protection of the gelatin.

Q: The calculation of Hounsfield units primarily depends on -

Attenuation coefficient of the material. ***Attenuation coefficient of the material*** - Hounsfield units (HU) are **calculated directly from the linear attenuation coefficient (μ)** of the material being scanned. - The formula is: **HU = 1000 × (μ_tissue - μ_water) / (μ_water - μ_air)** - CT scanners measure X-ray attenuation through tissues, and the HU values are derived by comparing the measured attenuation coefficient to that of water (which is assigned 0 HU) and air (assigned -1000 HU). - This is the **primary and direct parameter** used in HU calculation. *Electron density of the material* - While electron density is an **important factor that influences** the attenuation coefficient, it is not directly used in the HU calculation formula. - At CT imaging energies (typically 70-140 keV), Compton scattering predominates, which is dependent on electron density. - However, the relationship is indirect: electron density → affects attenuation coefficient → attenuation coefficient used to calculate HU. *Mass density of the material* - Mass density (ρ) contributes to the attenuation coefficient through the relationship: **μ = ρ × (μ/ρ)**, where (μ/ρ) is the mass attenuation coefficient. - Like electron density, this is an **underlying physical property** that influences attenuation but is not the direct parameter in the HU formula. *Effective atomic number of the material* - Effective atomic number (Z_eff) primarily affects **photoelectric absorption**, which dominates at lower X-ray energies. - At CT energies, photoelectric effect has minimal contribution compared to Compton scattering. - This parameter has **indirect influence** on the attenuation coefficient and therefore on HU values.

Q: Radiation Dose Monitoring in Occupational Workers is done by

TLD Badge. ***TLD Badge (used for monitoring radiation exposure)*** - **Thermoluminescent Dosimeter (TLD) badges** are widely used for monitoring an individual's exposure to ionizing radiation over time. - They work by storing energy from radiation exposure and releasing it as **light when heated**, which is then measured to calculate the accumulated dose. *Collimators (used to shape radiation beams)* - **Collimators** are devices used in radiation therapy and diagnostic imaging to **restrict and shape the radiation beam**, ensuring it only targets the intended area. - They do not measure or monitor the dose received by an individual, but rather **control the spatial distribution** of the radiation. *Grid (used to reduce scatter in imaging)* - An **anti-scatter grid** is placed between the patient and the image receptor in radiography to **absorb scattered radiation**, which degrades image quality. - While essential for image quality, grids do not directly measure or monitor the radiation dose received by an occupational worker. *Linear Accelerator (used for delivering radiation therapy)* - A **linear accelerator (linac)** is a machine used to deliver **external beam radiation treatment** for cancer. - It generates high-energy X-rays or electrons, but it is a **source of radiation** for treatment, not a device for monitoring occupational exposure.

Q: A pregnant woman with head trauma requires a CT scan of the head. What is the most effective radiation protection measure for the fetus?

Reduced mA and kVp. ***Reduced mA and kVp*** - **Optimizing scan parameters** (reducing mA and kVp) is the most effective way to minimize radiation dose during head CT in pregnancy. - Modern CT scanners with **iterative reconstruction** allow significant dose reduction without compromising diagnostic image quality. - The fetal dose from head CT is already negligible (< 0.01 mGy), but dose optimization further reduces any potential risk. - This directly addresses the radiation source rather than attempting to shield scatter radiation. *Lead apron over abdomen* - Lead shielding provides **minimal to no benefit** during head CT as the fetus is far from the primary beam. - Scatter radiation reaching the pelvis from head CT is negligible. - Lead aprons can interfere with **automatic exposure control (AEC)**, potentially increasing rather than decreasing dose. - Modern radiology guidelines (ACR, ICRP) no longer routinely recommend gonadal shielding for most CT examinations. *CT not recommended* - Withholding indicated imaging in trauma is **inappropriate and potentially dangerous**. - The diagnostic benefit of head CT in trauma far outweighs the negligible fetal risk. - **Maternal well-being** is the priority, and missing a critical head injury poses greater risk to both mother and fetus. *Using MRI instead* - While MRI has no ionizing radiation, it is **not appropriate for acute trauma** evaluation. - MRI takes longer to perform, requires patient cooperation, and is less readily available in emergency settings. - CT remains the **gold standard** for acute head trauma assessment.

Question 1

If the central ray is perpendicular to the film but not to the object, then:

Accepted Answer

Foreshortening of the image occurs

Answer

Blurring of the image occurs

Answer

Elongation of image occurs

Answer

None of the options

Question 2

In medical radiotherapy, a linear accelerator emits

Accepted Answer

Electrons and photons

Answer

Electrons and positrons

Answer

Neutrons and positrons

Answer

Neutrons only

Question 3

CT numbers of water and bone are respectively:

Accepted Answer

0, + 1000

Answer

100,0

Answer

+1000,-100

Answer

0,-1000

Question 4

One gray equals

Accepted Answer

100 RAD

Answer

1000 RAD

Answer

10 RAD

Answer

10000 RAD

Question 5

Function of hypo used in fixer is:

Accepted Answer

Dissolves and removes the unexposed silver halide

Answer

Forms a protective layer on the film

Answer

Inactivates the carryover developing agent

Answer

Prevents damage to gelatin

Question 6

The calculation of Hounsfield units primarily depends on -

Accepted Answer

Attenuation coefficient of the material

Answer

Mass density of the material

Answer

Effective atomic number of the material

Answer

Electron density of the material

Question 7

A 55 year old woman diagnosed with ca cervix stage IIb is advised for chemoradiation. Which of the following is the true statement regarding radiation use?

Accepted Answer

Dose/Intensity of radiation is inversely proportional to the square of distance of source

Answer

Rapidly proliferating cells are less affected by radiation

Answer

The small bowel is not significantly affected by radiation

Answer

Small blood vessels are unaffected by radiation

Question 8

Which of the following is a FALSE statement regarding radiation?

Accepted Answer

Small blood vessels are relatively resistant to radiation compared to other tissues

Answer

GI mucosa is one of the most radiosensitive tissues in the body

Answer

Rapidly dividing cells are highly sensitive to Radiation

Answer

The intensity of Radiation is inversely proportional to the square of distance from the source

Question 9

Radiation Dose Monitoring in Occupational Workers is done by

Accepted Answer

TLD Badge

Answer

Collimators

Answer

Grid

Answer

Linear Accelerator

Question 10

A pregnant woman with head trauma requires a CT scan of the head. What is the most effective radiation protection measure for the fetus?

Accepted Answer

Reduced mA and kVp

Answer

Using MRI instead

Answer

Lead apron over abdomen

Answer

Avoid CT, rely on clinical assessment

Radiation Physics and Protection — MCQs

Radiation Physics and Protection — MCQs

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