Radiation Physics and Protection Practice Questions

Q: Charge transferred across rows of detector in a 'Bucket brigade' fashion is seen in:

CCD. ***CCD*** - Charged Coupled Devices (CCDs) operate by transferring accumulated charge from one photographic element to the next in a sequential "bucket brigade" fashion to a read-out node. - This method allows for efficient and low-noise conversion of incident light into an electrical signal. *CMOS* - **CMOS (Complementary Metal-Oxide Semiconductor)** sensors read out each pixel individually, rather than transferring charge along a chain. - Each pixel in a CMOS sensor typically has its own **photodetector** and read-out amplifier. *Flat panel detector* - **Flat panel detectors (FPDs)**, commonly used in digital radiography, convert X-rays directly or indirectly into an electrical signal. - They do not use a "bucket brigade" charge transfer mechanism; instead, they have a matrix of **thin-film transistors (TFTs)** that collect and read out the signal from individual detector elements. *PSP* - **Photostimulable phosphor (PSP)** plates store X-ray energy as a latent image, which is then read by scanning with a laser to release light proportional to the absorbed energy. - This is an analog storage and read-out process and does not involve the electronic "bucket brigade" charge transfer found in CCDs.

Q: In MRI, the commonly used field strength is:

1.5 tesla. ***1.5 tesla*** - **1.5 Tesla (T)** is a widely accepted and commonly used field strength for clinical MRI systems, offering an excellent balance of **signal-to-noise ratio (SNR)** and spatial resolution. - This field strength provides high-quality images for a broad range of diagnostic applications, from neuroradiology to musculoskeletal imaging, making it a standard in many hospitals. *100 tesla* - A field strength of **100 Tesla** is far beyond the capabilities of current clinical MRI technology and would be unsafe for human use due to the incredibly powerful magnetic forces. - This extreme field strength is found only in highly specialized research laboratories for material science, not for medical imaging. *0.05 tesla* - **0.05 Tesla** represents a very low field strength, which historically was used in early MRI systems but offers significantly lower **signal-to-noise ratio (SNR)** and image quality compared to modern systems. - Low-field MRI units are sometimes used for specific niche applications like extremity imaging where cost and portability are prioritized over high resolution. *11 tesla* - While experimental MRI scanners at **11 Tesla** exist for advanced research, they are not commonly used in routine clinical practice due to their immense cost, complex maintenance, and potential safety concerns such as specific absorption rate (SAR) limits and patient comfort. - These ultra-high field scanners are primarily employed for neuroimaging research to achieve extremely fine spatial and temporal resolution for functional studies.

Q: Which of the following is not an artificial radio isotope element?

Ra226. ***Ra-226*** * **Radium-226** (Ra-226) is a **naturally occurring radioactive isotope** of radium, found in the uranium decay chain. * It is not artificially produced but rather exists in nature as a product of the **radioactive decay of uranium-238**. *Co-60* * **Cobalt-60** is a **synthetic radioactive isotope** produced by neutron activation of cobalt-59. * It is widely used in **radiation therapy** and industrial applications, making it an artificial radioisotope. *I-125* * **Iodine-125** is an **artificially produced radioisotope** commonly used in brachytherapy and diagnostic imaging. * It is created in a nuclear reactor by bombarding tellurium-124 with neutrons, making it not naturally occurring. *Tc-99m* * **Technetium-99m** is the most widely used **artificial radioisotope** in nuclear medicine for diagnostic imaging. * It is produced artificially from molybdenum-99 generators and does not occur naturally, making it an ideal choice for medical imaging due to its short half-life and gamma emission.

Q: Which of the following is most radiolucent?

Air space. ***Air space*** - **Air** has a very low density, allowing almost all X-rays to pass through it, resulting in a **dark (radiolucent)** appearance on an X-ray image. - In dental radiography, air spaces such as the **maxillary sinus** or nasal cavity appear as distinct dark areas. *Enamel* - **Enamel** is the **most highly mineralized** tissue in the human body, containing approximately 96% inorganic material. - Due to its high mineral content, enamel absorbs a large amount of X-rays, making it appear very **bright (radiopaque)** on an X-ray. *Bone* - **Bone tissue** is highly mineralized, though less so than enamel, and contains significant amounts of calcium and phosphate. - It absorbs a substantial portion of X-rays, appearing **radiopaque (white or light gray)** on a radiograph, indicating its density. *Dentin* - **Dentin** is less mineralized than enamel but more so than pulp, composed of about 70% inorganic material. - It appears **less radiopaque than enamel** but still significantly radiopaque (light gray) compared to soft tissues or air.

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.

Question 1

Charge transferred across rows of detector in a 'Bucket brigade' fashion is seen in:

Accepted Answer

CCD

Answer

CMOS

Answer

Flat panel detector

Answer

PSP

Question 2

In MRI, the commonly used field strength is:

Accepted Answer

1.5 tesla

Answer

100 tesla

Answer

0.05 tesla

Answer

11 tesla

Question 3

Which of the following is not an artificial radio isotope element?

Accepted Answer

Ra226

Answer

CO 60

Answer

1-125

Answer

Tc-99m

Question 4

Which of the following is most radiolucent?

Accepted Answer

Air space

Answer

Enamel

Answer

Bone

Answer

Dentin

Question 5

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 6

In medical radiotherapy, a linear accelerator emits

Accepted Answer

Electrons and photons

Answer

Electrons and positrons

Answer

Neutrons and positrons

Answer

Neutrons only

Question 7

CT numbers of water and bone are respectively:

Accepted Answer

0, + 1000

Answer

100,0

Answer

+1000,-100

Answer

0,-1000

Question 8

One gray equals

Accepted Answer

100 RAD

Answer

1000 RAD

Answer

10 RAD

Answer

10000 RAD

Question 9

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 10

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

Radiation Physics and Protection — MCQs

Radiation Physics and Protection — MCQs

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