Nuclear Medicine — MCQs

Nuclear Medicine Indian Medical PG Practice Questions and MCQs

Question 61: Which one of the following statements concerning Radon is NOT true?

A. Radon decays by alpha emission. (Correct Answer)
B. Radon is a single largest contributor to an individual's background radiation dose.
C. The most stable isotope, 222Rn, has a half-life of 3.8 days.
D. Radon is implicated in lung cancer.

Explanation: ### Explanation **Analysis of the Question** The question asks for the statement that is **NOT true** regarding Radon. Interestingly, in the provided options, Option A is marked as the "correct" answer (the false statement), but scientifically, Radon **does** decay by alpha emission. However, in the context of many standardized medical exams, this question often hinges on the specific technicality of the decay chain or is a "except" type question where the phrasing might be misleading. Let's break down the facts: **1. Why Option A is the "False" Statement (Contextual Analysis)** While Radon-222 decays via alpha emission into Polonium-218, in some examination contexts, it is argued that the primary health hazard isn't the Radon gas itself, but its **short-lived progeny (daughters)** like Polonium, which are solids and deposit in the airway. However, strictly speaking, Radon is an alpha emitter. If this is the keyed answer, it is often due to a technicality in the decay chain description or a specific textbook reference used by the examiners. **2. Analysis of Other Options (True Statements)** * **Option B:** **True.** Radon gas (primarily from soil and building materials) is the **single largest contributor** to the average annual effective dose of natural background radiation (approx. 1.2–2.0 mSv/year). * **Option C:** **True.** The most stable and common isotope, **Radon-222** (derived from the Uranium-238 decay series), has a half-life of **3.82 days**. * **Option D:** **True.** Radon is the **second leading cause of lung cancer** worldwide after tobacco smoking. It is the leading cause among non-smokers. **Clinical Pearls for NEET-PG:** * **Source:** Radon is a colorless, odorless noble gas produced from the decay of **Radium-226**. * **Mechanism of Injury:** Alpha particles from inhaled radon progeny cause double-strand DNA breaks in bronchial epithelium. * **Basement Effect:** Radon levels are highest in basements and poorly ventilated mines because it is heavier than air. * **Synergy:** There is a synergistic (multiplicative) effect between radon exposure and cigarette smoking for lung cancer risk.

Question 62: A 60-year-old woman, bedridden for 2 months, complains of breathlessness and chest pain. Her chest X-ray is normal. What is the next step in management?

A. V/Q scan (Correct Answer)
B. Echocardiography
C. Pulmonary wedge angiography
D. Pulmonary arteriography

Explanation: **Explanation:** The clinical presentation of sudden onset breathlessness and chest pain in a long-term bedridden patient (prolonged immobilization) is highly suggestive of **Pulmonary Embolism (PE)**. **Why V/Q Scan is the correct choice:** In the setting of a suspected PE where the **Chest X-ray is normal**, a Ventilation/Perfusion (V/Q) scan is an excellent diagnostic tool. A "mismatch" (normal ventilation but impaired perfusion) is the hallmark of PE. While CT Pulmonary Angiography (CTPA) is currently the gold standard, among the given options, the V/Q scan is the preferred non-invasive nuclear medicine study, especially when the X-ray is clear, as it carries a high negative predictive value. **Why other options are incorrect:** * **Echocardiography:** Useful for assessing right ventricular strain or ruling out myocardial infarction, but it is not the definitive diagnostic step for PE. * **Pulmonary Arteriography:** This is the "Gold Standard" invasive test. However, due to its invasive nature and risk of complications, it is reserved for cases where non-invasive tests are inconclusive. * **Pulmonary Wedge Angiography:** This is a localized version of arteriography and is not the standard initial diagnostic approach for systemic PE. **Clinical Pearls for NEET-PG:** * **Triad of PE:** Dyspnea, Chest pain, and Hemoptysis (though all three are present in only 20% of cases). * **CXR in PE:** Usually normal (the most common finding). Specific but rare signs include **Hampton’s Hump** (wedge-shaped opacity) and **Westermark sign** (focal oligemia). * **V/Q Scan Interpretation:** A "High Probability" scan in a patient with high clinical suspicion is diagnostic. * **Gold Standard:** CT Pulmonary Angiography (CTPA) is the investigation of choice in modern practice.

Question 63: What is the physical half-life of radioactive Iodine-131?

A. 8 hours
B. 8 days (Correct Answer)
C. 16 days
D. 60 days

Explanation: **Explanation:** **Iodine-131 (I-131)** is a radioisotope widely used in nuclear medicine for both diagnostic imaging and therapeutic purposes. The **physical half-life** of I-131 is approximately **8.02 days**. This duration is clinically significant as it allows the isotope to remain in the body long enough to deliver a therapeutic dose of radiation to thyroid tissue while being cleared relatively quickly to minimize long-term exposure. **Analysis of Options:** * **8 hours:** This is incorrect. However, **Technetium-99m (Tc-99m)**, the most commonly used diagnostic isotope in radiology, has a half-life of **6 hours**. * **16 days:** This is incorrect and does not correspond to a commonly tested medical isotope. * **60 days:** This is the half-life of **Iodine-125 (I-125)**, which is primarily used in brachytherapy seeds (e.g., for prostate cancer) and RIA (Radioimmunoassay) labs. **High-Yield Clinical Pearls for NEET-PG:** * **Emissions:** I-131 is a **dual emitter**. It emits **Beta particles** (responsible for the therapeutic destruction of thyroid tissue) and **Gamma rays** (allowing for scintigraphy/imaging). * **Mechanism:** It is trapped and organified by the thyroid gland via the Sodium-Iodide Symporter (NIS). * **Clinical Uses:** Treatment of Graves' disease, toxic multinodular goiter, and well-differentiated thyroid cancer (post-thyroidectomy). * **Contraindication:** I-131 is strictly **contraindicated in pregnancy** as it crosses the placenta and can destroy the fetal thyroid gland. * **I-123:** Another isotope of Iodine used for diagnostics; it has a half-life of **13 hours** and emits only gamma radiation, making it safer for simple uptake studies.

Question 64: Which isotope is replacing radium?

A. Cesium
B. Iridium (Correct Answer)
C. Gold
D. Californium

Explanation: **Explanation:** In modern brachytherapy, **Iridium-192** has largely replaced Radium-226. Radium was the first isotope used in radiotherapy, but it fell out of favor due to significant safety concerns: it decays into **Radon gas** (a leakage hazard), has a very long half-life (1,600 years) making disposal difficult, and emits high-energy gamma rays that require heavy shielding. **Why Iridium-192 is preferred:** * **Physical Properties:** It has a high specific activity, allowing for the production of very small "seeds" or wires, which are ideal for interstitial implants. * **Safety:** It does not produce gaseous daughter products. Its lower average photon energy (0.38 MeV) compared to Radium makes radiation protection easier for staff. * **Versatility:** It is the isotope of choice for **High Dose Rate (HDR)** remote afterloading systems. **Analysis of Incorrect Options:** * **Cesium-137:** While it replaced Radium for intracavitary applications (like cervical cancer) due to its longer half-life (30 years) and better safety profile, it has been largely superseded by Iridium-192 in modern HDR units. * **Gold-198:** Used primarily for permanent interstitial implants (e.g., prostate), but its short half-life (2.7 days) makes it unsuitable as a general replacement for Radium. * **Californium-252:** A neutron emitter used experimentally for bulky tumors; it is not a routine replacement for Radium. **High-Yield Clinical Pearls for NEET-PG:** * **Half-life of Ir-192:** ~74 days. * **Half-life of Co-60:** 5.26 years (used in Teletherapy). * **Half-life of Cs-137:** 30 years. * **Brachytherapy Principle:** Follows the **Inverse Square Law**, delivering a high dose to the tumor with a rapid dose fall-off to protect surrounding healthy tissue.

Question 65: Ectopic mucosa of Meckel's diverticulum is diagnosed by which imaging modality?

A. Technetium-99m radionuclide scan (Correct Answer)
B. Angiography
C. Computed Tomography (CT)
D. Endoscopy

Explanation: **Explanation:** **Meckel’s diverticulum** is the most common congenital anomaly of the gastrointestinal tract, resulting from the incomplete obliteration of the vitelline duct. Approximately 50% of symptomatic cases contain **ectopic gastric mucosa**, which secretes acid and can lead to painless lower GI bleeding (melena or hematochezia). **Why Technetium-99m is the Correct Answer:** The **Technetium-99m (Tc-99m) Pertechnetate scan**, often referred to as a **"Meckel’s Scan,"** is the gold standard for diagnosis. The pertechnetate ion is actively taken up and secreted by the mucous cells of the **ectopic gastric mucosa**, rather than the intestinal mucosa itself. On a gamma camera, this appears as a "hot spot" (increased uptake) usually in the right lower quadrant, appearing simultaneously with the stomach. **Why Other Options are Incorrect:** * **Angiography:** While it can detect active bleeding (at a rate of >0.5 ml/min), it is invasive and does not specifically identify ectopic mucosa. * **Computed Tomography (CT):** CT is often used to rule out other causes of abdominal pain (like appendicitis) but has low sensitivity for identifying a small Meckel’s diverticulum unless it is inflamed. * **Endoscopy:** Standard upper and lower endoscopies cannot reach the mid-distal ileum where Meckel’s is typically located. **High-Yield Clinical Pearls for NEET-PG:** * **Rule of 2s:** 2% of the population, 2 inches long, 2 feet from the ileocecal valve, 2 types of ectopic tissue (Gastric > Pancreatic), and usually presents before age 2. * **Pharmacological Enhancement:** To increase the sensitivity of the scan, clinicians may use **Pentagastrin** (increases uptake), **H2 blockers (Cimetidine)** (inhibits release from cells), or **Glucagon** (decreases peristalsis). * **False Negatives:** Can occur if there is no ectopic gastric mucosa or if the diverticulum has been "washed out" by brisk bleeding.

Question 66: Technetium-99m is most commonly used in which medical specialty?

A. Cardiology
B. Radiotherapy
C. Radiodiagnosis
D. Nuclear medicine (Correct Answer)

Explanation: **Explanation:** **Technetium-99m (Tc-99m)** is the "workhorse" of **Nuclear Medicine**, used in approximately 80% of all diagnostic nuclear imaging procedures worldwide. It is a metastable nuclear isomer of Technetium-99. **Why Nuclear Medicine is correct:** Nuclear medicine involves the administration of radioactive substances (radiopharmaceuticals) into the body to visualize physiological processes. Tc-99m is the ideal isotope for this because: * **Ideal Half-life:** It has a physical half-life of **6 hours**, which is long enough for metabolic studies but short enough to minimize patient radiation dose. * **Pure Gamma Emitter:** It emits gamma rays at **140 keV**, which is the optimal energy for detection by standard Gamma Cameras. * **Versatility:** It can be tagged to various compounds to image different organs (e.g., Tc-99m MDP for bone scans, Tc-99m Sestamibi for myocardial perfusion). **Why other options are incorrect:** * **Radiotherapy:** This specialty uses high-energy ionizing radiation (like Cobalt-60 or Linear Accelerators) or beta-emitters (like I-131) to *treat* cancer, rather than for diagnostic imaging. * **Radiodiagnosis:** While Nuclear Medicine is a branch of diagnostic imaging, "Radiodiagnosis" typically refers to conventional Radiology (X-rays, CT, MRI) which uses external radiation sources or magnetic fields rather than injected radioisotopes. * **Cardiology:** Although Tc-99m is used in *Nuclear Cardiology* (e.g., SPECT scans), it is a tool provided by the Nuclear Medicine department and is used across many other specialties like Nephrology and Oncology. **High-Yield Clinical Pearls for NEET-PG:** * **Production:** Tc-99m is obtained from a **Molybdenum-99 (Mo-99) generator** (often called a "Moly cow"). * **Common Scans:** * **Tc-99m MDP:** Bone scan (Osteoblastic activity). * **Tc-99m DTPA/MAG3:** Renal imaging. * **Tc-99m Pertechnetate:** Thyroid imaging and Meckel’s diverticulum. * **Tc-99m HMPAO:** Brain perfusion.

Question 67: Conversion of one nucleus into another was first demonstrated in 1919 by which scientist?

A. Rutherford (Correct Answer)
B. Curie
C. Becquerel
D. Roentgen

Explanation: **Explanation:** The correct answer is **Ernest Rutherford**. In 1919, Rutherford performed the first successful artificial **nuclear transmutation**. He bombarded nitrogen gas with alpha particles, resulting in the production of oxygen isotopes and protons ($^{14}N + \alpha \rightarrow ^{17}O + p$). This experiment proved that the nucleus of an atom could be altered, laying the foundation for modern nuclear medicine and the production of artificial radioisotopes. **Analysis of Incorrect Options:** * **Marie Curie:** Known for her pioneering research on radioactivity and the discovery of **Radium and Polonium**. While she coined the term "radioactivity," she did not demonstrate the first transmutation. * **Henri Becquerel:** Discovered **spontaneous radioactivity** in 1896 while working with uranium salts. His work was observational regarding natural decay rather than intentional conversion. * **Wilhelm Roentgen:** Discovered **X-rays** in 1895. His contribution pertains to electromagnetic radiation rather than nuclear transformation. **High-Yield Clinical Pearls for NEET-PG:** * **Father of Nuclear Physics:** Ernest Rutherford (also discovered the alpha and beta particles and the concept of half-life). * **Artificial Radioactivity:** First discovered by **Irene Joliot-Curie and Frederic Joliot** in 1934 (distinct from Rutherford’s 1919 transmutation). * **Technetium-99m:** The most commonly used radiopharmaceutical in diagnostic nuclear medicine, produced in a Mo-99/Tc-99m generator. * **Unit of Radioactivity:** The SI unit is the **Becquerel (Bq)**; the traditional unit is the **Curie (Ci)**.

Question 68: Selenium methionine scan is used in the diagnosis of which condition?

A. Parathyroid adenoma
B. Pancreatic carcinoma (Correct Answer)
C. Bone metastasis
D. Meckel's diverticulum

Explanation: **Explanation:** **Selenium-75 selenomethionine ($^{75}$Se-methionine)** is a radiopharmaceutical analog of the essential amino acid methionine. The underlying principle of this scan is that cells with high protein synthesis rates will actively uptake this amino acid analog. 1. **Why Pancreatic Carcinoma is Correct:** The pancreas is an organ with high metabolic activity and protein turnover due to the production of digestive enzymes. Pancreatic carcinoma cells exhibit significantly increased protein synthesis compared to normal tissue. Therefore, $^{75}$Se-methionine was historically used to image the pancreas. While it has largely been replaced by modern CT, MRI, and PET-CT (using $^{18}$F-FDG), it remains a classic high-yield fact in nuclear medicine exams. 2. **Analysis of Incorrect Options:** * **Parathyroid Adenoma:** The gold standard nuclear scan is the **Technetium-99m Sestamibi** (MIBI) scan. * **Bone Metastasis:** The investigation of choice is the **Technetium-99m MDP** (Methylene Diphosphonate) bone scan, which detects osteoblastic activity. * **Meckel’s Diverticulum:** This is diagnosed using the **Technetium-99m Pertechnetate** scan (also known as the "Meckel’s scan"), which is taken up by the ectopic gastric mucosa. **High-Yield Clinical Pearls for NEET-PG:** * **Hot vs. Cold:** In pancreatic imaging, a "cold" spot (photopenia) on a Selenium scan typically indicates a space-occupying lesion like a tumor or cyst. * **Other uses of Selenium-75:** It can also be used to image the parathyroid glands, but Sestamibi is far more sensitive and preferred in modern practice. * **Half-life:** $^{75}$Se has a physical half-life of approximately 118 days.

Question 69: Which radioactive element is no longer used in current medical practice?

A. Radium 226 (Correct Answer)
B. Cobalt 60
C. Cesium 137
D. Iridium 192

Explanation: **Explanation:** **Radium-226 (Option A)** is the correct answer because it has been entirely phased out of modern clinical practice. Historically, Radium-226 was the first isotope used in brachytherapy (discovered by Marie Curie). However, it is no longer used due to significant safety concerns: it has an extremely long half-life (1,600 years), decays into a hazardous daughter product (**Radon-222 gas**), and is a "bone-seeker," posing a high risk of bone marrow toxicity and osteosarcoma if accidental internal exposure occurs. **Why the other options are incorrect:** * **Cobalt-60 (Option B):** Still used in external beam radiotherapy (Telecobalt units) and Gamma Knife surgery, particularly in developing countries, due to its high-energy gamma emission. * **Cesium-137 (Option C):** While its use in brachytherapy has decreased, it is still utilized in blood irradiators and some manual afterloading systems. It replaced Radium because it is safer to handle. * **Iridium-192 (Option D):** This is currently the **most commonly used** isotope for High-Dose-Rate (HDR) brachytherapy due to its high specific activity and small source size. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard for HDR Brachytherapy:** Iridium-192 (Half-life: 74 days). * **Permanent Implants (Prostate Cancer):** Iodine-125 or Palladium-103 are preferred. * **Historical Context:** Radium-226 was replaced by Cesium-137, which has since been largely replaced by Iridium-192. * **Safety:** The "Radon hazard" is unique to Radium-226, requiring hermetically sealed containers that are no longer standard in modern oncology.

Question 70: Which of the following statements regarding the use of nitrates in myocardial perfusion studies is true?

A. Nitrates act as a stress agent.
B. Nitrates cause decreased myocardial flow in areas of ischemia.
C. Nitrates enhance the detection of myocardial viability when using Thallium-201 as a radiopharmaceutical. (Correct Answer)
D. Nitrates result in reduced collateral flow into the ischemic zone.

Explanation: **Explanation:** The use of nitrates in myocardial perfusion imaging (MPI) is a high-yield concept focused on distinguishing between **hibernating myocardium** (viable but under-perfused) and **infarcted tissue** (non-viable scar). **1. Why Option C is Correct:** Nitrates (like Nitroglycerin) are potent venodilators and coronary vasodilators. When administered before radiopharmaceutical injection (especially **Thallium-201** or **Tc-99m Sestamibi**), they reduce preload and improve collateral blood flow to chronically ischemic areas. This "recruits" flow to hibernating segments, allowing the tracer to reach viable cells that would otherwise appear as "cold spots" on a standard stress test. Therefore, nitrate-augmented MPI increases the sensitivity for detecting **myocardial viability**. **2. Why the Other Options are Incorrect:** * **Option A:** Nitrates are not stress agents. Stress agents include vasodilators (Adenosine, Regadenoson) or inotropes (Dobutamine) used to induce ischemia. Nitrates are used in **rest studies** to improve perfusion. * **Option B:** Nitrates **increase** myocardial flow to ischemic areas by dilating collateral vessels and reducing left ventricular end-diastolic pressure (LVEDP). * **Option D:** Nitrates **enhance** collateral flow, which is the primary mechanism by which they help identify viable tissue in the "watershed" ischemic zones. **Clinical Pearls for NEET-PG:** * **Gold Standard for Viability:** While Nitrate-augmented MPI is excellent, **FDG-PET** remains the gold standard for assessing myocardial viability. * **Thallium-201:** Known for its "redistribution" property; it acts as a potassium analog. * **Hibernating Myocardium:** Chronic ischemia leading to reversible LV dysfunction; it improves after revascularization. * **Stunned Myocardium:** Acute ischemia followed by reperfusion; the dysfunction is temporary and resolves spontaneously.

Practice by Chapter

Physics of Nuclear Medicine

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Radiopharmaceuticals

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Radiation Detection in Nuclear Medicine

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Thyroid Scintigraphy

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Bone Scintigraphy

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Renal Nuclear Medicine

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Cardiac Nuclear Medicine

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Pulmonary Nuclear Medicine

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Neurological Nuclear Medicine

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PET/CT Principles and Applications

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Radionuclide Therapy

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Radiation Safety in Nuclear Medicine

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