What is the maximum radiation dose (in Gray) that bone tissue can tolerate?
Enhancement in CT contrast is due to -
Normal prevertebral soft tissue thickness in adults at C3 level is
Flask shaped heart is seen in –
NEET-PG 2013 - Radiology NEET-PG Practice Questions and MCQs
Question 41: What is the maximum radiation dose (in Gray) that bone tissue can tolerate?
- A. 50 Gray (Correct Answer)
- B. 30 Gray
- C. 20 Gray
- D. 40 Gray
Explanation: ***Correct Option: 50 Gray*** - The **maximum radiation tolerance dose** for bone tissue is approximately **50-60 Gray (Gy)** based on radiobiology literature and clinical practice. - Among the given options, **50 Gy** represents the most appropriate threshold for bone tolerance. - According to **Emami et al. tolerance doses** and **QUANTEC guidelines**, bone can typically tolerate up to 60 Gy without significant risk of complications. - Doses approaching or exceeding **60 Gy** carry increased risk of **osteoradionecrosis**, particularly in the **mandible and weight-bearing bones**. - **Clinical significance**: In radiation therapy planning, doses of 50-60 Gy to bone are commonly used therapeutically for tumors involving or adjacent to bone. *Incorrect Option: 40 Gray* - 40 Gy is **below the accepted tolerance threshold** for bone tissue. - This dose is generally **well-tolerated** by bone without significant risk of necrosis or fracture. - Commonly used in palliative and definitive radiation protocols without major bone complications. *Incorrect Option: 30 Gray* - 30 Gy is **considerably below** the tolerance limit for bone. - This dose level is **safe for bone tissue** and carries minimal risk of radiation-induced bone damage. - Often used in palliative treatments with excellent bone tolerance. *Incorrect Option: 20 Gray* - 20 Gy is a **low radiation dose** from the perspective of bone tolerance. - This dose is **highly unlikely** to cause any significant bone damage or complications. - Represents a conservative therapeutic dose well within safety margins.
Question 42: Enhancement in CT contrast is due to -
- A. Iodine (Correct Answer)
- B. Gadolinium
- C. Mercury
- D. Silver
Explanation: ***Iodine*** - **Iodine-based contrast agents** are commonly used in CT scans to enhance the visualization of blood vessels, organs, and certain lesions due to their **high atomic number** and ability to absorb X-rays. - The degree of enhancement observed on a CT image is directly proportional to the concentration of **iodine** in the tissue or blood. *Gadolinium* - **Gadolinium-based contrast agents** are predominantly used in **Magnetic Resonance Imaging (MRI)**, not CT scans. - Gadolinium works by altering the **magnetic properties** of water molecules in tissues, thereby improving MRI signal intensity. *Mercury* - **Mercury** is a highly toxic heavy metal and is **not used as a contrast agent** in any imaging modality due to its severe health risks. - While historically used in some medical applications, it has been replaced by safer alternatives. *Silver* - **Silver** is not used as a contrast agent in medical imaging; it has no suitable properties for enhancing images in CT or other common modalities. - It is known for its **antimicrobial properties** and is sometimes used in wound dressings.
Question 43: Normal prevertebral soft tissue thickness in adults at C3 level is
- A. 7 mm (Correct Answer)
- B. 15 mm
- C. 22 mm
- D. 30 mm
Explanation: ***7 mm*** * The normal prevertebral soft tissue thickness in adults at the **C3 level** is approximately **7 mm** (upper limit 5-7 mm) on a lateral cervical spine radiograph. * The **C1-C4 levels** have relatively thin prevertebral soft tissue as they lie above the hypopharynx and esophagus. * Exceeding this measurement can indicate **prevertebral hematoma, edema, abscess**, or other pathologies such as retropharyngeal abscess or cervical spine fracture with soft tissue swelling. *Incorrect: 15 mm* * A measurement of 15 mm at the **C3 level** would be considered **abnormal** and indicates prevertebral soft tissue swelling. * This value exceeds the normal limit by more than double and warrants further investigation. *Incorrect: 22 mm* * A measurement of 22 mm at the **C3 level** is significantly **abnormal** and highly suggestive of serious pathology such as retropharyngeal hematoma, abscess, or cervical spine injury. * Note that 22 mm may be within normal limits at the **C6-C7 level** where the esophagus widens the prevertebral space. *Incorrect: 30 mm* * 30 mm is severely abnormal at the **C3 level**. * While prevertebral soft tissue thickness can be greater at lower cervical levels (C5-C7) due to the esophagus, even at those levels 30 mm would suggest pathology. * Such measurements indicate significant conditions like large hematoma, abscess, or tumor.
Question 44: Flask shaped heart is seen in –
- A. Pericardial effusion (Correct Answer)
- B. TOF
- C. Ebstein anomaly
- D. TAPVC
Explanation: ***Pericardial effusion*** - A **"flask-shaped"** or **"water bottle-shaped" heart** on chest X-ray is a classic finding in significant pericardial effusion. - This appearance results from the accumulation of a large amount of fluid in the **pericardial sac**, which causes the cardiac silhouette to enlarge symmetrically and assume a globular shape. *TOF (Tetralogy of Fallot)* - TOF typically presents with a **"boot-shaped" heart** (coeur en sabot) on chest X-ray due to right ventricular hypertrophy and a concave pulmonary artery segment. - This morphology is distinctly different from the flask-shaped appearance of pericardial effusion. *Ebstein anomaly* - Ebstein anomaly is characterized by apical displacement of the tricuspid valve, leading to **massive right atrial enlargement**. - On chest X-ray, this often results in a **markedly enlarged oval-shaped heart**, which can be quite massive but does not typically have the distinct flask/water bottle shape. *TAPVC (Total Anomalous Pulmonary Venous Connection)* - TAPVC can present with different X-ray findings depending on the type, but a classic finding for the supracardiac type is a **"snowman" or "figure-of-8" heart** in children. - This appearance is due to the dilated superior vena cava and anomalous veins draining to it, not a flask shape.