Nuclear Medicine — MCQs

Nuclear Medicine Indian Medical PG Practice Questions and MCQs

Question 41: All of the following are beta emitting particles except?

A. Strontium 89
B. Technetium 99m (Correct Answer)
C. Phosphorus 32
D. Tin 117m

Explanation: **Explanation:** The core concept tested here is the distinction between **Diagnostic** and **Therapeutic** radionuclides based on their mode of decay. **Why Technetium-99m (Tc-99m) is the correct answer:** Tc-99m is a **pure gamma emitter**. It decays via **isomeric transition**, releasing a 140 keV gamma photon without emitting any particulate radiation (like alpha or beta particles). This makes it ideal for diagnostic imaging (Gamma cameras/SPECT) because gamma rays have high penetrability and low ionization, minimizing the patient's absorbed radiation dose. **Analysis of Incorrect Options (Beta Emitters):** Beta particles ($\beta^-$) are high-energy electrons used primarily for **radionuclide therapy** because they travel short distances in tissue, causing localized cellular destruction. * **Strontium-89:** A pure beta emitter used for the palliative treatment of painful bone metastases (e.g., from prostate cancer). * **Phosphorus-32:** A pure beta emitter historically used for treating Polycythemia Vera and persistent joint effusions (radiation synovectomy). * **Tin-117m:** A unique radionuclide that emits **conversion electrons** (which behave like beta particles) and is used for bone pain palliation. **High-Yield Clinical Pearls for NEET-PG:** * **Ideal Diagnostic Agent:** Tc-99m (Half-life: 6 hours; Energy: 140 keV). * **Pure Beta Emitters (Therapeutic):** Yttrium-90, Strontium-89, Phosphorus-32. * **Mixed Emitters (Beta + Gamma):** Iodine-131 (used for both treating thyroid cancer via beta and imaging via gamma). * **Alpha Emitter:** Radium-223 (used for bone metastases; provides high linear energy transfer over a very short range).

Question 42: Radionuclide scans are useful in all of the following except:

A. Gastrointestinal bleed
B. Cholecystitis
C. Intraabdominal abscess
D. Local staging of tumors (Correct Answer)

Explanation: **Explanation:** Radionuclide scans (Nuclear Medicine) are functional imaging modalities that rely on the physiological activity of tissues rather than detailed anatomical structure. **Why "Local staging of tumors" is the correct answer:** Local staging (T-staging) requires high **spatial resolution** to determine the exact depth of invasion, involvement of adjacent layers, and precise anatomical boundaries. Radionuclide scans (like PET or Bone scans) have poor spatial resolution compared to **MRI or CT**, making them unsuitable for local staging. While PET-CT is excellent for *distant* metastasis (M-staging), it cannot replace MRI/CT for assessing the local extent of a primary tumor. **Why the other options are incorrect:** * **Gastrointestinal bleed:** **99mTc-labeled RBC scans** are highly sensitive for detecting active GI bleeds, capable of detecting rates as low as 0.1 mL/min (more sensitive than angiography). * **Cholecystitis:** **HIDA scan** (Cholescintigraphy) is the **gold standard** for diagnosing acute calculous cholecystitis. Non-visualization of the gallbladder indicates cystic duct obstruction. * **Intraabdominal abscess:** **Gallium-67** or **In-111 labeled Leukocyte (WBC) scans** are specifically used to localize occult infections and abscesses by targeting inflammatory activity. **High-Yield Clinical Pearls for NEET-PG:** * **Meckel’s Diverticulum:** Diagnosed using **99mTc-Pertechnetate** (Meckel’s scan), which targets ectopic gastric mucosa. * **Bone Scan (99mTc-MDP):** Most sensitive for early bone metastasis (except in Multiple Myeloma, where it is often negative). * **Renal Scans:** **DTPA** (Glomerular filtration), **MAG3** (Tubular secretion), and **DMSA** (Renal cortical scarring—the gold standard).

Question 43: What is the test for reversible cardiac ischemia?

A. Angiography
B. Thallium scan (Correct Answer)
C. MUGA
D. Resting Echocardiography

Explanation: **Explanation:** The correct answer is **Thallium scan (Thallium-201)**. This is a nuclear medicine imaging technique used to assess myocardial perfusion and viability. **Why Thallium scan is correct:** Thallium-201 is a potassium analog that enters viable myocardial cells via the Na+/K+ ATPase pump. In a "Stress-Rest" protocol, the tracer is injected during peak exercise. Areas with **reversible ischemia** show "cold spots" (decreased uptake) during stress because of reduced blood flow, but show "fill-in" (normal uptake) on delayed resting images as the tracer redistributes. If a cold spot persists during rest, it indicates an infarct (non-viable tissue). **Why other options are incorrect:** * **Angiography:** While the "gold standard" for visualizing anatomical coronary artery stenosis, it does not directly assess the functional physiological significance or the viability of the myocardium. * **MUGA (Multi-Gated Acquisition) Scan:** This is used primarily to calculate the **Left Ventricular Ejection Fraction (LVEF)** and evaluate regional wall motion abnormalities, not to differentiate between ischemia and infarction. * **Resting Echocardiography:** This assesses cardiac anatomy and wall motion at a single point in time. It cannot distinguish between a dynamic reversible ischemic state and a permanent scar unless combined with a stressor (e.g., Dobutamine). **High-Yield Clinical Pearls for NEET-PG:** * **Technetium-99m Sestamibi:** Another common perfusion agent; unlike Thallium, it does *not* redistribute, requiring two separate injections (stress and rest). * **PET Scan (FDG):** The "Gold Standard" for detecting **myocardial viability** (hibernating myocardium). * **Hot Spot Imaging:** Technetium-99m Pyrophosphate binds to calcium in damaged cells and is used to image **acute myocardial infarction** (shows as a "hot spot").

Question 44: Following RAIU studies demonstrate which of the following conditions?

A. Graves Disease
B. Toxic Adenoma
C. Multinodular Goiter (MNG)
D. Lingual Thyroid (Correct Answer)

Explanation: ***Lingual Thyroid*** - **RAIU (Radioactive Iodine Uptake)** studies show uptake at the **base of the tongue** with **absence of uptake** in the normal cervical thyroid location. - This pattern is **pathognomonic** for lingual thyroid, where thyroid tissue fails to descend during embryological development and remains at the **tongue base**. *Graves Disease* - Shows **diffusely increased uptake** throughout an **enlarged thyroid gland** in the normal cervical location. - **RAIU values** are typically **elevated (>35%)** with **homogeneous distribution** across both thyroid lobes. *Toxic Adenoma* - Demonstrates a **single hot nodule** with **suppressed uptake** in the surrounding normal thyroid tissue. - The **autonomous nodule** shows intense uptake while the rest of the gland appears **photopenic** due to TSH suppression. *Multinodular Goiter (MNG)* - Exhibits **multiple areas** of increased and decreased uptake creating a **heterogeneous pattern** within an enlarged gland. - Shows **patchy uptake distribution** with both **hot and cold nodules** throughout the cervical thyroid region.

Question 45: What dye is used in SPECT?

A. O-14
B. Tc-99 (Correct Answer)
C. Nitrogen-16
D. Iodine 124

Explanation: **Explanation:** **SPECT (Single Photon Emission Computed Tomography)** utilizes radionuclides that emit **gamma radiation**. The most commonly used radioisotope in clinical practice is **Technetium-99m (Tc-99m)**. 1. **Why Tc-99m is Correct:** Tc-99m is the "workhorse" of nuclear medicine because it has an ideal half-life (6 hours), which is long enough for imaging but short enough to minimize radiation dose. It emits a single photon of 140 keV, which is perfectly suited for detection by gamma cameras used in SPECT. 2. **Why other options are incorrect:** * **O-14, Nitrogen-16, and Iodine-124:** These are primarily **positron emitters** used in **PET (Positron Emission Tomography)** scans, not SPECT. PET requires isotopes that undergo beta-plus decay to produce annihilation photons (511 keV), whereas SPECT detects direct gamma rays. Specifically, O-14 and N-16 have extremely short half-lives (measured in seconds), making them impractical for standard SPECT imaging. **High-Yield Clinical Pearls for NEET-PG:** * **SPECT vs. PET:** SPECT uses single gamma-emitting isotopes (Tc-99m, I-123, Ga-67, Tl-201). PET uses positron-emitting isotopes (F-18, C-11, O-15). * **Tc-99m Production:** It is obtained from a **Molybdenum-99 generator** (the "Moly cow"). * **Common SPECT uses:** Myocardial perfusion imaging (MPI), Bone scans (MDP), and Brain perfusion (HMPAO). * **Half-life Rule:** Tc-99m has a physical half-life of **6 hours**.

Question 46: What is the half-life of Iodine-123?

A. 6 hours
B. 8 days
C. 13 hours (Correct Answer)
D. 14 days

Explanation: **Explanation:** **Iodine-123 (I-123)** is a cyclotron-produced radioisotope widely used in diagnostic nuclear medicine, particularly for thyroid imaging and uptake studies. The correct answer is **13 hours**, which represents its physical half-life. This duration is ideal for clinical practice as it allows sufficient time for thyroid localization while ensuring the radiation dose to the patient remains low. **Analysis of Options:** * **A. 6 hours:** This is the half-life of **Technetium-99m (Tc-99m)**, the most commonly used radioisotope in nuclear medicine. * **B. 8 days:** This is the half-life of **Iodine-131 (I-131)**. Due to its longer half-life and high-energy beta emission, I-131 is primarily used for the treatment of thyrotoxicosis and thyroid cancer, rather than routine diagnostic imaging. * **C. 13 hours (Correct):** The specific physical half-life of I-123. * **D. 14 days:** This is the approximate half-life of **Phosphorus-32 (P-32)**, used in the treatment of polycythemia vera. **High-Yield Clinical Pearls for NEET-PG:** * **Mode of Decay:** I-123 decays via **electron capture** and emits **gamma rays** (159 keV), which is the optimal energy for gamma camera detection. * **Clinical Use:** It is the preferred isotope for thyroid scans (especially in children) and MIBG scans (for neuroblastoma/pheochromocytoma) because it lacks the damaging beta-particle emission found in I-131. * **Comparison:** Unlike Tc-99m (which is only trapped by the thyroid), I-123 is both **trapped and organified**, providing a more accurate assessment of thyroid function.

Question 47: What is the best imaging modality for neuroendocrine tumors?

A. PET scan
B. CECT scan
C. Radionuclide scan (Correct Answer)
D. MRI with gadolinium scan

Explanation: **Explanation:** Neuroendocrine tumors (NETs) are unique because they overexpress **Somatostatin Receptors (SSTR)** on their cell surfaces. The **Radionuclide scan** (specifically Somatostatin Receptor Scintigraphy or SRS) is considered the best imaging modality because it provides functional information based on this molecular expression, allowing for the detection of both the primary tumor and occult metastases that might be missed on structural imaging. * **Radionuclide scan (Correct):** Traditionally, the **Octreotide scan** (using Indium-111 pentetreotide) was the gold standard. However, in modern practice, **68Ga-DOTANOC/DOTATATE PET-CT** (a specialized radionuclide scan) is the investigation of choice due to its superior sensitivity and resolution. * **PET scan (Incorrect):** While a specialized PET (68Ga-DOTANOC) is ideal, a standard **18F-FDG PET scan** is often negative in well-differentiated NETs because these tumors are slow-growing and have low glucose metabolism. * **CECT and MRI (Incorrect):** These are structural imaging modalities. While excellent for anatomical localization and surgical planning, they cannot confirm the neuroendocrine nature of a lesion or detect small functional metastases as effectively as radionuclide imaging. **High-Yield Clinical Pearls for NEET-PG:** * **Investigation of Choice (IOC) for NETs:** 68Ga-DOTANOC PET-CT (a type of radionuclide scan). * **Most sensitive marker for NETs:** Chromogranin A (CgA). * **Theranostics:** The same SSTR receptors targeted for imaging can be used for treatment with **177Lu-DOTATATE** (Peptide Receptor Radionuclide Therapy - PRRT). * **Exception:** Insulinomas often lack SSTR; therefore, Endoscopic Ultrasound (EUS) or intraoperative USG is often preferred over radionuclide scans for them.

Question 48: A HIDA scan is useful in which of the following conditions?

A. Acute cholecystitis (Correct Answer)
B. Meckel's diverticulum
C. Colonic angiodysplasia
D. Diverticulitis

Explanation: **Explanation:** **HIDA Scan (Hepatobiliary Iminodiacetic Acid Scan)**, also known as cholescintigraphy, is a nuclear imaging study used to evaluate the function of the gallbladder and the biliary system. 1. **Why Acute Cholecystitis is Correct:** The underlying principle is the **obstruction of the cystic duct**, which is the primary cause of acute cholecystitis. In a normal scan, the radiotracer (Technetium-99m labeled HIDA) is taken up by the liver and excreted into the bile ducts, eventually filling the gallbladder. In acute cholecystitis, the cystic duct is blocked (usually by a stone); therefore, while the tracer enters the common bile duct and the duodenum, it **fails to visualize the gallbladder** even after 4 hours or following morphine administration. This "non-visualization" is the hallmark diagnostic finding. 2. **Why Other Options are Incorrect:** * **Meckel’s Diverticulum:** This is diagnosed using a **Technetium-99m Pertechnetate scan** (Meckel’s scan), which targets ectopic gastric mucosa. * **Colonic Angiodysplasia:** This is typically evaluated via colonoscopy or **Technetium-99m labeled RBC scan** (for active bleeding). * **Diverticulitis:** This is a clinical diagnosis usually confirmed by a **Contrast-Enhanced CT (CECT) scan** of the abdomen. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard:** While Ultrasonography is the initial investigation of choice for gallstones, **HIDA scan is the most sensitive/Gold Standard** for diagnosing acute cholecystitis. * **Morphine Augmentation:** If the gallbladder is not seen by 60 minutes, low-dose morphine is given to constrict the Sphincter of Oddi, increasing biliary pressure to force the tracer into the gallbladder. * **Biliary Atresia:** HIDA scan is also used to differentiate biliary atresia (no tracer in the gut) from neonatal hepatitis (tracer reaches the gut).

Question 49: In the diagnosis of acute myocardial infarction, a hot spot is seen with which imaging modality?

A. Thallium-201 scan
B. Technetium-99m scan (Correct Answer)
C. Strontium-90 scan
D. Iodine-127 scan

Explanation: **Explanation:** The correct answer is **Technetium-99m scan** (specifically **Tc-99m Pyrophosphate**). **1. Why Technetium-99m is correct:** In the setting of an acute myocardial infarction (AMI), irreversible cell damage leads to an influx of calcium into the necrotic myocytes. **Tc-99m Pyrophosphate (PYP)** has a high affinity for these calcium deposits. When injected, the radiopharmaceutical accumulates in the infarcted area, creating a **"Hot Spot"** (increased uptake) on imaging. This scan is typically positive between 24 and 72 hours after the onset of an infarct. **2. Why other options are incorrect:** * **Thallium-201 scan:** Thallium is a potassium analog that is taken up by *viable* myocardium. In an MI, the dead tissue cannot take up the tracer, resulting in a **"Cold Spot."** It is used for viability and stress testing, not for "hot spot" imaging of an acute infarct. * **Strontium-90:** This is a pure beta-emitter used primarily in radiotherapy (e.g., for pterygium or bone pain palliation), not for diagnostic cardiac imaging. * **Iodine-127:** This is the stable, non-radioactive isotope of Iodine. Radioactive isotopes like I-123 or I-131 are used for thyroid imaging, but not for acute MI diagnosis. **High-Yield Clinical Pearls for NEET-PG:** * **Hot Spot Imaging:** Tc-99m Pyrophosphate (detects necrosis). * **Cold Spot Imaging:** Thallium-201 or Tc-99m Sestamibi (detects ischemia/infarction as a perfusion defect). * **PET Scan:** The "Gold Standard" for myocardial viability is **FDG-PET**, which identifies "hibernating myocardium." * **Rule of Thumb:** "Hot" means the tracer goes to the pathology; "Cold" means the tracer goes to healthy tissue and avoids the pathology.

Question 50: Which iodine isotope is used for the treatment of carcinoma of the thyroid?

A. Iodine 131 (Correct Answer)
B. Iodine 123
C. Iodine 213
D. All of the above

Explanation: **Explanation:** The correct answer is **Iodine-131 (I-131)**. The fundamental principle behind using I-131 for thyroid carcinoma is its ability to emit **Beta ($\beta$) particles**. While I-131 emits both gamma rays (used for imaging) and beta particles, it is the high-energy beta radiation that travels a short distance (approx. 1–2 mm) within the tissue, causing localized ionization and destruction of follicular thyroid cells. This makes it the gold standard for **ablating thyroid remnants** post-surgery and treating **metastatic differentiated thyroid cancer** (Papillary and Follicular). **Analysis of Options:** * **Iodine-123 (B):** This is a pure **gamma emitter** with a shorter half-life (13 hours). It is ideal for **diagnostic imaging** (scintigraphy) and calculating thyroid uptake because it provides high-quality images with a much lower radiation dose to the patient. It lacks the beta emission required for therapy. * **Iodine-213 (C):** This is not a standard isotope used in clinical practice for thyroid disorders. * **All of the above (D):** Incorrect, as only I-131 possesses the therapeutic properties necessary for tissue destruction. **High-Yield Clinical Pearls for NEET-PG:** * **Physical Half-life:** I-131 has a half-life of **8 days**. * **Mechanism of Uptake:** Iodine enters thyroid cells via the **Sodium-Iodide Symporter (NIS)**. * **Contraindication:** Radioactive iodine is strictly **contraindicated in pregnancy** (crosses the placenta and can destroy the fetal thyroid) and breastfeeding. * **Preparation:** Patients must have high TSH levels (>30 mIU/L) and follow a low-iodine diet to maximize the uptake of the isotope by cancerous cells.

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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