Nuclear Medicine Practice Questions

Q: Viability of ischemic myocardium is best diagnosed by?

PET scan. **Explanation:** The assessment of myocardial viability is crucial in patients with ischemic heart disease to determine if revascularization (stenting or bypass) will improve cardiac function. **Why PET Scan is the Correct Answer:** Positron Emission Tomography (PET) using **18F-Fluorodeoxyglucose (FDG)** is considered the **Gold Standard** for diagnosing myocardial viability. The underlying principle is the "mismatch" pattern: ischemic but viable myocardium (hibernating myocardium) switches its metabolism from fatty acids to glucose. A PET scan showing reduced blood perfusion (via N-13 Ammonia) but preserved glucose uptake (via FDG) confirms that the tissue is alive and will likely recover function after revascularization. **Analysis of Incorrect Options:** * **MUGA Scan (Multi-Gated Acquisition):** This is used primarily to calculate the **Left Ventricular Ejection Fraction (LVEF)** and evaluate global/regional wall motion. It does not assess metabolic activity or viability. * **Thallium-201 Scan:** While Thallium-201 (a potassium analog) can assess viability via "redistribution" imaging, it has lower resolution and lower sensitivity compared to PET. It was the traditional choice but has been superseded by PET. * **Coronary Angiogram:** This is an anatomical study that identifies the location and severity of coronary artery stenoses. It cannot determine if the distal muscle tissue is dead (infarcted) or merely hibernating. **NEET-PG High-Yield Pearls:** * **Hibernating Myocardium:** Chronic ischemia leading to reversible LV dysfunction; PET shows Perfusion-Metabolism Mismatch. * **Stunned Myocardium:** Acute ischemia followed by reperfusion; temporary dysfunction despite restored flow. * **MRI (Late Gadolinium Enhancement):** Another highly accurate modality; if transmural enhancement is <50%, the tissue is considered viable. * **Dobutamine Stress Echo:** Assesses "contractile reserve" to predict viability.

Q: Which of the following radioactive isotopes of iodine has the longest half-life?

I-131. **Explanation:** In Nuclear Medicine, understanding the physical half-life of radioisotopes is crucial for both diagnostic imaging and therapeutic applications. The half-life refers to the time required for the radioactivity of an isotope to fall to half its original value. **Why I-131 is Correct:** **Iodine-131 (I-131)** has a physical half-life of approximately **8.02 days**. It is a high-energy isotope that undergoes beta-minus decay, making it highly effective for the **ablation of thyroid tissue** (in Grave’s disease or Thyroid Cancer). Because of its relatively long half-life and high-energy gamma emissions (364 keV), it is used sparingly for diagnosis but is the gold standard for therapy. **Why Other Options are Incorrect:** * **I-123:** This is the preferred isotope for diagnostic thyroid scans because it emits pure gamma radiation (159 keV) with no beta particles, resulting in a lower radiation dose to the patient. It has a much shorter half-life of **13.2 hours**. * **I-132:** This is a very short-lived isotope with a half-life of only **2.3 hours**. It is rarely used in routine clinical practice today. * **Option D:** This is incorrect as each isotope of an element has a unique, fixed physical half-life based on its nuclear stability. **High-Yield Clinical Pearls for NEET-PG:** * **I-123:** Best for diagnostic imaging (Thyroid uptake and scan) due to lower radiation burden. * **I-131:** Best for therapy (Thyrotoxicosis/Malignancy) due to **Beta-particle** emission. * **I-125:** Has a half-life of **60 days** (longest among common medical iodine isotopes), but is primarily used in **Brachytherapy** (e.g., prostate cancer) and RIA (Radioimmunoassay), not routine thyroid imaging. * **Mechanism:** Iodine isotopes are trapped and organified by the thyroid gland via the Sodium-Iodide Symporter (NIS).

Q: Whole body Iodine scan is done in all of the following except?

Medullary carcinoma of thyroid. ### Explanation The correct answer is **Medullary Carcinoma of Thyroid (MTC)**. **1. Why Medullary Carcinoma is the correct answer:** The whole-body iodine scan (using I-131 or I-123) relies on the expression of the **Sodium-Iodide Symporter (NIS)**. This symporter is a characteristic of follicular epithelial cells of the thyroid. Medullary carcinoma, however, originates from the **Parafollicular C-cells** (neuroendocrine cells), which do not concentrate iodine. Therefore, MTC is "iodine-cold" and cannot be imaged or treated with radioactive iodine. MTC is instead monitored using serum **Calcitonin** and CEA levels. **2. Why the other options are incorrect:** * **Follicular Carcinoma (FTC):** This is a well-differentiated thyroid cancer (WDTC) derived from follicular cells. It typically retains the ability to trap iodine, making iodine scans essential for detecting distant metastases (especially bone). * **Papillary Carcinoma (PTC):** As the most common WDTC, it also originates from follicular cells. While it may take up iodine less intensely than FTC, it still expresses the NIS protein, allowing for post-operative whole-body scanning to identify residual disease or recurrence. **3. Clinical Pearls for NEET-PG:** * **Differentiated Thyroid Cancers (DTC):** Includes Papillary and Follicular. They are iodine-avid. * **Undifferentiated/Anaplastic Carcinoma:** These lose the ability to trap iodine and thus cannot be scanned with I-131. * **MTC Imaging:** Since MTC doesn't take up iodine, the imaging of choice for recurrence is often **PET-CT** (using 18F-DOPA or 68Ga-DOTATATE) or Ultrasound. * **Pre-requisite for Iodine Scan:** Patients must have high TSH levels (>30 mIU/L) and a low-iodine diet to maximize the sensitivity of the scan.

Q: Which radioisotope is preferred for measuring Glomerular Filtration Rate (GFR)?

Diethylene triamine pentaacetic acid (DTPA). **Explanation:** The measurement of **Glomerular Filtration Rate (GFR)** requires a radiopharmaceutical that is handled exclusively by glomerular filtration, with no significant tubular secretion or reabsorption. **Why DTPA is the correct answer:** **Tc-99m DTPA (Diethylene triamine pentaacetic acid)** is the gold standard radioisotope for GFR estimation. It is a small chelate that is filtered freely by the glomerulus and is not secreted or reabsorbed by the renal tubules. This property allows it to accurately reflect the filtration capacity of the kidneys. It is also used for dynamic renal imaging to evaluate obstructive uropathy. **Why the other options are incorrect:** * **Orthoiodohippuran (OIH):** Labeled with I-131 or I-123, it is primarily used to measure **Effective Renal Plasma Flow (ERPF)** because it is cleared by both filtration (20%) and tubular secretion (80%). * **DMSA (Dimercaptosuccinic acid):** This is a **static cortical imaging agent**. It binds to the proximal convoluted tubules and remains in the renal cortex for a long duration. It is the investigation of choice for detecting **renal scars** and ectopic kidneys, not for GFR. * **MAG 3 (Mercaptoacetyltriglycine):** This is the preferred agent for **dynamic renography**, especially in patients with renal failure. Like OIH, it is primarily cleared by tubular secretion (95%) and is used to measure ERPF, not GFR. **High-Yield Clinical Pearls for NEET-PG:** * **GFR Agent:** Tc-99m DTPA. * **ERPF Agent:** Tc-99m MAG3 (most common) or I-131 OIH. * **Cortical/Scarring Agent:** Tc-99m DMSA. * **Renal Failure:** MAG3 is superior to DTPA because its high extraction fraction provides better images at low filtration rates. * **Captopril Scan:** Used for diagnosing Renovascular Hypertension (Renal Artery Stenosis).

Q: Which of the following investigations is NOT useful in Multiple Myeloma?

BONE SCAN. **Explanation:** The correct answer is **Bone Scan (Technetium-99m MDP)**. **1. Why Bone Scan is NOT useful:** Multiple Myeloma is characterized by **purely lytic lesions** caused by increased osteoclast activity and the suppression of osteoblasts. A traditional Bone Scan (Tc-99m MDP) depends on **osteoblastic activity** (new bone formation) to show "hot spots." Since there is little to no osteoblastic response in myeloma, bone scans often yield **false-negative results**, failing to detect up to 30-50% of lesions. **2. Analysis of other options:** * **MRI (Option B):** This is the **most sensitive** modality for detecting early bone marrow infiltration. It is the gold standard for identifying "smoldering" myeloma and evaluating spinal cord compression. * **CT Scan (Option A):** Low-dose Whole Body CT (WBCT) has replaced the traditional skeletal survey. It is highly effective at detecting small lytic lesions and assessing fracture risks. * **PET Scan (Option D):** FDG-PET/CT is excellent for detecting extramedullary disease and monitoring the response to therapy, as it measures metabolic activity. **Clinical Pearls for NEET-PG:** * **Investigation of Choice (Initial):** Whole Body Low-Dose CT (WBLDCT). * **Most Sensitive for Marrow Infiltration:** MRI. * **Classic Radiographic Sign:** "Punched-out" lytic lesions (especially in the skull). * **The "Cold" Scan:** Remember, Myeloma is a "cold" lesion on a Bone Scan but a "hot" lesion on a PET Scan. * **Exception:** A bone scan may only be positive in Myeloma if there is a healing pathological fracture (which triggers osteoblasts).

Q: In the treatment of papillary carcinoma of the thyroid, radioiodine predominantly destroys neoplastic cells by which mechanism?

Beta (β) rays. **Explanation:** The correct answer is **Beta (β) rays**. Radioiodine-131 ($^{131}$I) is the isotope of choice for the treatment of differentiated thyroid cancers (Papillary and Follicular). Its therapeutic efficacy is derived from its decay process, which releases both beta particles and gamma rays. 1. **Why Beta (β) rays are correct:** Beta particles are high-energy electrons with a short tissue penetration range (approximately **0.5 to 2 mm**). This localized range ensures that the radiation dose is concentrated within the thyroid tissue or neoplastic cells that have trapped the iodine, causing DNA damage and cell death while sparing surrounding structures like the parathyroid glands or recurrent laryngeal nerve. 2. **Why other options are incorrect:** * **Gamma (γ) rays:** While $^{131}$I does emit gamma rays, they have high penetration power and exit the body. They are used for **diagnostic imaging** (scintigraphy) to locate metastases but contribute minimally to the actual destruction of the tumor. * **X-rays:** These are electromagnetic waves produced extranuclearly (not via radioactive decay of $^{131}$I) and are used in external beam radiation, not radioiodine therapy. * **Alpha (α) particles:** These have very high linear energy transfer but are not emitted by $^{131}$I. They are used in other therapies (e.g., Radium-223 for bone mets). **High-Yield Clinical Pearls for NEET-PG:** * **Physical Half-life of $^{131}$I:** 8.02 days. * **Mechanism of uptake:** $^{131}$I enters thyroid cells via the **Sodium-Iodide Symporter (NIS)**. * **Pre-requisite:** Patients must have high TSH levels (>30 mIU/L) and be on a low-iodine diet to maximize uptake before therapy. * **Contraindication:** Pregnancy and breastfeeding are absolute contraindications.

Q: What does 'M' stand for in 99Tcm?

Metastable. **Explanation:** In nuclear medicine, the lowercase **'m'** in **Technetium-99m ($^{99m}Tc$)** stands for **Metastable**. **1. Why 'Metastable' is Correct:** A metastable state refers to an excited energy state of an atomic nucleus that has a longer half-life than ordinary excited states (which usually decay in picoseconds). $^{99m}Tc$ is an isomer of $^{99}Tc$. It remains in this high-energy state for a clinically useful period (half-life of **6 hours**) before undergoing **Isomeric Transition**. During this transition, it releases energy in the form of **gamma radiation (140 keV)** to reach a more stable ground state, without emitting alpha or beta particles. This makes it ideal for diagnostic imaging using a Gamma Camera. **2. Why Other Options are Incorrect:** * **Micro/Mega:** These are standard SI prefixes for $10^{-6}$ and $10^6$, respectively. They describe quantity or scale, not the physical state of a radioisotope. * **Metastatic:** This is a clinical term referring to the spread of cancer from a primary site to a distant organ. While $^{99m}Tc$ bone scans are used to detect "metastatic" disease, the 'm' itself describes the nuclear physics of the isotope. **3. High-Yield Clinical Pearls for NEET-PG:** * **Half-life ($T_{1/2}$):** 6 hours (ideal for hospital logistics). * **Energy:** 140 keV (optimal for detection by modern gamma cameras). * **Production:** Derived from a **Molybdenum-99 ($^{99}Mo$) generator** (often called a "Moly cow"). * **Pure Gamma Emitter:** Lack of beta emission minimizes the patient's radiation dose. * **Drug of Choice:** $^{99m}Tc$ is the most widely used radiopharmaceutical in nuclear medicine (used in MDP bone scans, HIDA scans, and Sestamibi scans).

Q: Which of the following conditions cannot be detected by bone scintigraphy?

None of the above. **Explanation:** Bone scintigraphy (Bone Scan), typically performed using **Technetium-99m labeled Methylene Diphosphonate (Tc-99m MDP)**, is a highly sensitive functional imaging modality. It detects changes in bone metabolism and blood flow rather than just structural changes. **Why "None of the above" is correct:** Bone scintigraphy is capable of detecting all three conditions listed. It relies on the principle that any process increasing bone turnover or osteoblastic activity will show increased uptake ("hot spots"), while processes severely compromising blood supply may show decreased uptake ("cold spots"). * **Avascular Necrosis (AVN):** In the early stages, AVN appears as a **"cold spot"** due to the lack of blood supply (infarction). Later, during the repair phase, it may show a "hot rim" due to revascularization and reactive bone formation. * **Fractures:** Bone scans can detect fractures (including stress fractures) within 24–72 hours of injury, often before they become visible on conventional X-rays, due to the immediate increase in osteoblastic repair activity. * **Osteomyelitis:** A **3-phase bone scan** is the gold standard for differentiating osteomyelitis from cellulitis. Osteomyelitis shows increased uptake in all three phases (blood pool, soft tissue, and delayed bone phase). **High-Yield Clinical Pearls for NEET-PG:** * **Sensitivity vs. Specificity:** Bone scans are highly **sensitive** (detects pathology early) but have low **specificity** (many conditions look similar). * **The "Cold" Lesions:** While most pathologies are "hot," remember that **Multiple Myeloma**, early AVN, and some purely lytic metastases (e.g., Renal Cell Carcinoma) can present as "cold" or photopenic areas. * **Flare Phenomenon:** Increased uptake seen after successful chemotherapy for bone metastases, which can be mistaken for disease progression.

Q: Which radiotracer is commonly used in PET scans?

2-Fluorodeoxyglucose. **Explanation:** **1. Why 2-Fluorodeoxyglucose (18F-FDG) is correct:** PET (Positron Emission Tomography) imaging relies on tracers that emit positrons. **18F-FDG** is a glucose analog and the most widely used radiopharmaceutical in PET imaging. It works on the principle of **increased glycolysis** (Warburg effect) in metabolically active cells, such as malignant tumors, inflammatory sites, and brain tissue. Once injected, FDG is transported into cells by GLUT transporters and phosphorylated by hexokinase into FDG-6-phosphate. Unlike normal glucose, it cannot be further metabolized and becomes "trapped" inside the cell, allowing for the visualization of metabolic activity. **2. Why the other options are incorrect:** * **Technetium (99mTc):** This is the most common tracer used in **Gamma Camera/SPECT** imaging (e.g., bone scans, DTPA scans), not PET. It emits gamma rays, not positrons. * **Chromium (51Cr):** Historically used for labeling red blood cells to estimate **RBC survival time** or sequestering studies; it is not used in PET. * **Cobalt (57Co/58Co):** Previously used in the **Schilling test** to diagnose Vitamin B12 absorption issues; it has no role in PET imaging. **3. High-Yield Clinical Pearls for NEET-PG:** * **Half-life of 18F:** Approximately **110 minutes**. * **Patient Preparation:** Patients must fast for 4–6 hours; blood glucose should ideally be **<150–200 mg/dL** to prevent competition between endogenous glucose and the tracer. * **Physiological Uptake:** Normal "hot spots" include the brain (high glucose demand), heart, kidneys, and urinary bladder (excretion route). * **Brown Fat:** Can cause false positives; prevented by keeping the patient warm or using beta-blockers.

Question 1

What is the best investigation for multiple osteoblastic bone metastases?

Accepted Answer

Bone Scan

Answer

MRI

Answer

CT

Answer

X-ray

Question 2

Viability of ischemic myocardium is best diagnosed by?

Accepted Answer

PET scan

Answer

MUGA scan

Answer

Thallium scan

Answer

Coronary angiogram

Question 3

Which of the following radioactive isotopes of iodine has the longest half-life?

Accepted Answer

I-131

Answer

I-123

Answer

I-132

Answer

All have equal half-life

Question 4

Whole body Iodine scan is done in all of the following except?

Accepted Answer

Medullary carcinoma of thyroid

Answer

Follicular carcinoma of thyroid

Answer

Papillary carcinoma of thyroid

Answer

None of the above

Question 5

Which radioisotope is preferred for measuring Glomerular Filtration Rate (GFR)?

Accepted Answer

Diethylene triamine pentaacetic acid (DTPA)

Answer

Orthoiodohippuran (OIH)

Answer

Dimercaptosuccinic acid (DMSA)

Answer

Mercaptoacetyltriglycine (MAG 3)

Question 6

Which of the following investigations is NOT useful in Multiple Myeloma?

Accepted Answer

BONE SCAN

Answer

CT SCAN

Answer

MRI

Answer

PET

Question 7

In the treatment of papillary carcinoma of the thyroid, radioiodine predominantly destroys neoplastic cells by which mechanism?

Accepted Answer

Beta (β) rays

Answer

X-rays

Answer

Gamma (γ) rays

Answer

Alpha (α) particles

Question 8

What does 'M' stand for in 99Tcm?

Accepted Answer

Metastable

Answer

Micro

Answer

Mega

Answer

Metastatic

Question 9

Which of the following conditions cannot be detected by bone scintigraphy?

Accepted Answer

None of the above

Answer

Avascular necrosis

Answer

Fractures

Answer

Osteomyelitis

Question 10

Which radiotracer is commonly used in PET scans?

Accepted Answer

2-Fluorodeoxyglucose

Answer

Technetium

Answer

Chromium

Answer

Cobalt

Nuclear Medicine — MCQs

Nuclear Medicine — MCQs

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