Nuclear Medicine — MCQs

Nuclear Medicine Indian Medical PG Practice Questions and MCQs

Question 51: What is the scan used to estimate collecting system dilatation?

A. 99mTc-DTPA scan (Correct Answer)
B. MAG 3 scan
C. DMSA scan
D. Hippuran scan

Explanation: **Explanation:** The correct answer is **99mTc-DTPA scan**. To estimate collecting system dilatation and differentiate between obstructive and non-obstructive hydronephrosis, a **dynamic renal scintigraphy (Diuretic Renogram)** is performed. **1. Why 99mTc-DTPA is correct:** 99mTc-DTPA (Diethylene Triamine Penta-acetic Acid) is an agent that is handled almost exclusively by **glomerular filtration** (not secreted or reabsorbed). Because it stays within the tubular lumen and follows the flow of urine, it is the gold standard for calculating the **Glomerular Filtration Rate (GFR)** and visualizing the drainage pattern of the collecting system. When combined with a diuretic (Furosemide), it helps determine if a dilated renal pelvis is due to mechanical obstruction or simple stasis. **2. Why other options are incorrect:** * **MAG 3 (Mercaptoacetyltriglycine):** While also used for dynamic renography, it is primarily secreted by the renal tubules. It is the agent of choice in patients with **renal failure** or pediatric cases due to better extraction, but DTPA remains the classic answer for GFR-based drainage studies in standard exams. * **DMSA (Dimercaptosuccinic Acid):** This is a **static** renal scan. It binds to the proximal convoluted tubules and is used to identify **renal scars**, ectopic kidneys, or pyelonephritis. It cannot evaluate the collecting system or drainage. * **Hippuran (I-131 OIH):** An older agent used for Effective Renal Plasma Flow (ERPF). It is rarely used now due to high radiation doses and poor imaging quality. **Clinical Pearls for NEET-PG:** * **Best agent for GFR:** 99mTc-DTPA. * **Best agent for Renal Scarring:** 99mTc-DMSA. * **Best agent for Renal Function in Neonates/Renal Failure:** 99mTc-MAG3. * **Whitaker Test:** The invasive gold standard for differentiating obstructive from non-obstructive dilatation (pressure-flow study).

Question 52: Which isotope of iodine is used in brachytherapy for prostate carcinoma?

A. I-127
B. I-125 (Correct Answer)
C. I-131
D. I-124

Explanation: **Explanation:** **Iodine-125 (I-125)** is the isotope of choice for permanent interstitial brachytherapy in low-risk prostate carcinoma. The underlying medical concept relies on its physical properties: it has a **long half-life (approx. 60 days)** and emits **low-energy gamma radiation (35 keV)**. This allows for a continuous, low-dose rate of radiation to be delivered directly to the tumor over several months, ensuring high local control while minimizing radiation exposure to adjacent critical structures like the rectum and bladder. **Analysis of Incorrect Options:** * **I-127:** This is the only stable, naturally occurring isotope of iodine. It is non-radioactive and therefore has no therapeutic application in oncology. * **I-131:** While used therapeutically, it is primarily used for **thyroid ablation** and treating thyroid cancer. It emits high-energy beta particles and gamma rays with a shorter half-life (8 days), making it unsuitable for the permanent seed implantation required in prostate brachytherapy. * **I-124:** This is a positron emitter used primarily as a radiopharmaceutical for **PET imaging**, not for brachytherapy. **High-Yield Clinical Pearls for NEET-PG:** * **Brachytherapy Isotopes:** Apart from I-125, **Palladium-103 (Pd-103)** is also commonly used for prostate brachytherapy (shorter half-life of 17 days, used for more aggressive tumors). * **I-123:** Used for diagnostic thyroid scans (pure gamma emitter, 13-hour half-life). * **I-131 MIBG:** Used for imaging and treating neuroblastoma and pheochromocytoma. * **Rule of Thumb:** Permanent implants (seeds) use isotopes with low energy and longer half-lives; temporary high-dose-rate (HDR) brachytherapy typically uses **Iridium-192**.

Question 53: A hot spot on a bone scan is seen in all the following conditions except:

A. Osteomyelitis
B. Fibrous dysplasia
C. Paget's disease
D. Fibrous cortical defect (Correct Answer)

Explanation: **Explanation:** Bone scintigraphy (Bone Scan) using **99mTc-MDP** depends on two primary factors: **blood flow** and **osteoblastic activity** (bone turnover). A "hot spot" indicates increased tracer uptake due to high bone remodeling. **Why Fibrous Cortical Defect (FCD) is the correct answer:** A Fibrous Cortical Defect (and its larger counterpart, Non-Ossifying Fibroma) is a benign, **non-neoplastic** fibrous lesion typically found in the metaphysis of long bones in children. These lesions are characterized by a lack of active bone turnover or significant osteoblastic reaction. Consequently, they appear **"cold" or "quiet"** on a bone scan. If an FCD appears "hot," it usually indicates a complication like a pathological fracture. **Analysis of Incorrect Options:** * **Osteomyelitis:** Shows intense uptake (hot spot) due to increased vascularity and reactive bone formation in response to infection. * **Fibrous Dysplasia:** This condition involves the replacement of normal bone with fibro-osseous tissue. It characteristically shows **marked, intense tracer uptake** due to high metabolic activity within the lesion. * **Paget’s Disease:** Known for having the **highest tracer uptake** in nuclear medicine. The extreme osteoblastic activity and hypervascularity during the mixed and sclerotic phases result in very "hot" scans. **High-Yield Clinical Pearls for NEET-PG:** * **"Cold" Spots (Photopenia):** Think of Multiple Myeloma (often), early Infarction/Avascular Necrosis, Anaplastic tumors, or Renal Cell Carcinoma metastases. * **Super Scan:** A scan showing intense symmetrical skeletal uptake with faint or absent renal activity; commonly seen in Metastatic Prostate Cancer, Renal Osteodystrophy, and Paget’s Disease. * **Flare Phenomenon:** A temporary increase in uptake seen 3 months after successful chemotherapy, which should not be confused with disease progression.

Question 54: Which radioisotope is NOT the choice for a Triple phase Bone scan?

A. Tc99m-Sestamibi
B. Tc99m-Peectinate
C. Tc99m-Methyl diphosphonate (Correct Answer)
D. Tc99m-Thallium201 subtraction

Explanation: **Explanation** The **Triple Phase Bone Scan (TPBS)** is a nuclear imaging study primarily used to differentiate between osteomyelitis and cellulitis. It utilizes **Technetium-99m labeled phosphonates**, most commonly **Tc-99m Methyl Diphosphonate (MDP)**. **1. Why Tc-99m MDP is the Correct Answer (as the choice FOR the scan):** The question asks which isotope is NOT the choice; however, in standard medical practice and NEET-PG contexts, **Tc-99m MDP** is the **gold standard** and the primary isotope used for bone scans. It localizes to the bone via **chemisorption** onto the hydroxyapatite crystal surface. The three phases include: * **Phase 1 (Flow):** Assesses perfusion (0–60 seconds). * **Phase 2 (Blood Pool):** Assesses soft tissue vascularity (1–5 minutes). * **Phase 3 (Delayed/Skeletal):** Assesses bone turnover (2–4 hours). **2. Analysis of Other Options:** * **Tc-99m Sestamibi:** Primarily used for myocardial perfusion imaging and parathyroid localization; it does not localize to bone hydroxyapatite. * **Tc-99m Pertechnetate:** Used for thyroid, Meckel’s diverticulum, and salivary gland imaging. * **Thallium-201:** A potassium analog used for myocardial imaging and tumor viability, not for standard triple-phase bone evaluation. *(Note: There appears to be a pedagogical discrepancy in the provided key. In standard radiology, Tc-99m MDP **is** the isotope of choice for a Bone Scan. If the question asks which is NOT the choice, the answer should logically be A, B, or D. However, if the key marks MDP as the "Correct Answer" to the question "Which is NOT the choice," it may imply a distractor or a specific clinical contraindication, though MDP remains the universal standard.)* **High-Yield Clinical Pearls:** * **Osteomyelitis:** Shows "Hot" uptake in all three phases. * **Cellulitis:** Shows "Hot" uptake in Phase 1 and 2, but is "Cold" or normal in Phase 3. * **Fourth Phase:** A 24-hour delayed scan is sometimes added in patients with peripheral vascular disease or diabetes to improve specificity.

Question 55: Which radionuclide is most commonly used for infarct scanning?

A. Tc-99m pyrophosphate (Correct Answer)
B. Tc-99m Sestamibi
C. Tc-99m Sulfur Colloid
D. Tc-99m Red blood cells

Explanation: **Explanation:** **Tc-99m Pyrophosphate (PYP)** is the radionuclide of choice for **infarct-avid imaging** (hot spot scanning). The underlying mechanism involves the accumulation of calcium within irreversibly damaged myocardial cells following an acute myocardial infarction (AMI). Tc-99m PYP binds to these calcium hydroxyapatite crystals and denatured proteins in the necrotic tissue. * **Timing:** It becomes positive 12–24 hours after an infarct, peaks at 48–72 hours, and usually returns to baseline after 1–2 weeks ("fades"). **Analysis of Incorrect Options:** * **Tc-99m Sestamibi:** Used for **myocardial perfusion imaging** (cold spot scanning). It distributes in the myocardium proportional to blood flow and is used to assess ischemia and viability, not specifically to label necrotic tissue. * **Tc-99m Sulfur Colloid:** Primarily used for **reticuloendothelial system (RES) imaging**, such as liver-spleen scans or bone marrow imaging. * **Tc-99m Red Blood Cells (RBCs):** Used for **MUGA scans** (to assess ventricular ejection fraction) or for detecting gastrointestinal bleeds and hemangiomas. **High-Yield Clinical Pearls for NEET-PG:** * **"Hot Spot" vs. "Cold Spot":** Tc-99m PYP is a "Hot Spot" agent (labels the dead tissue); Thallium-201 and Sestamibi are "Cold Spot" agents (label healthy tissue). * **Sensitivity:** PYP scans are most sensitive for transmural (Q-wave) infarcts. * **The "Doughnut Sign":** Large anterior wall infarcts may show central photopenia (no uptake in the center due to lack of blood flow to deliver the tracer), creating a doughnut appearance. * **Persistent Positive Scan:** If a PYP scan remains positive months after an MI, it suggests a poor prognosis or ventricular aneurysm.

Question 56: What is the most sensitive test to detect gastrointestinal bleeding?

A. Selective angiography
B. Red blood cell scintigraphy (Correct Answer)
C. I 131 fibrinogen studies
D. Fecal occult blood testing

Explanation: **Explanation:** The detection of gastrointestinal (GI) bleeding depends on the rate of blood loss. **Red blood cell (RBC) scintigraphy** (typically using Technetium-99m labeled RBCs) is the most sensitive imaging modality because it can detect bleeding rates as low as **0.1 mL/min**. Because the radiopharmaceutical remains in the intravascular compartment for a prolonged period, patients can be scanned repeatedly over 24 hours, making it ideal for detecting intermittent or "obscure" bleeding. **Analysis of Options:** * **Selective Angiography:** While highly specific and capable of therapeutic intervention (embolization), it is less sensitive than scintigraphy. It requires a much higher bleeding rate of **0.5 to 1.0 mL/min** to visualize the extravasation of contrast. * **I-131 Fibrinogen Studies:** This test was historically used to detect deep vein thrombosis (DVT) by incorporating into developing clots; it has no clinical role in the acute detection of GI bleeding. * **Fecal Occult Blood Testing (FOBT):** While this can detect minute amounts of blood in the stool, it is a biochemical screening tool for chronic/microscopic loss (e.g., colorectal cancer screening) and cannot localize the site or manage acute hemorrhage. **NEET-PG High-Yield Pearls:** * **Sensitivity Thresholds:** Scintigraphy (0.1 mL/min) > Angiography (0.5–1.0 mL/min). * **Technetium-99m Sulfur Colloid:** An alternative for GI bleed scans; it is cleared rapidly by the liver/spleen, providing a high target-to-background ratio, but cannot be used for delayed imaging (unlike labeled RBCs). * **Meckel’s Diverticulum:** The "Meckel’s scan" uses **Tc-99m Pertechnetate**, which is taken up by ectopic gastric mucosa, not the blood itself.

Question 57: What is the typical dose of I-131 given to patients with carcinoma thyroid?

A. 10 mCi
B. 100 mCi (Correct Answer)
C. 10 uCi
D. 100 uCi

Explanation: **Explanation:** The management of Differentiated Thyroid Cancer (DTC) often involves **Radioactive Iodine (I-131) Ablation** following a total thyroidectomy. The goal is to destroy any residual thyroid tissue or occult microscopic disease. **Why 100 mCi is correct:** In nuclear medicine, doses are categorized based on their intent: diagnostic or therapeutic. For the treatment of thyroid carcinoma, **therapeutic doses** are required. A dose of **75–150 mCi** (typically 100 mCi) is the standard empirical dose used for remnant ablation. If there is known lymph node metastasis, the dose may be increased to 150 mCi, and for distant metastasis (e.g., lung or bone), it can go up to 200 mCi. **Analysis of Incorrect Options:** * **10 mCi (Option A):** This is a therapeutic dose typically used for **Graves' disease** (Hyperthyroidism), where the goal is to reduce thyroid function rather than eradicate all thyroid tissue. * **10 uCi & 100 uCi (Options C & D):** These are **diagnostic doses** (microcuries). They are used for thyroid uptake studies or diagnostic scans to visualize the gland's morphology without delivering a significant radiation burden. **High-Yield Clinical Pearls for NEET-PG:** * **Prerequisite:** Patients must have a high TSH (>30 mIU/L) before I-131 administration, achieved either by thyroid hormone withdrawal or Recombinant Human TSH (rhTSH). * **Low Iodine Diet:** Patients should follow a low iodine diet for 1–2 weeks prior to the dose to increase the "starvation" and subsequent uptake by thyroid cells. * **Contraindication:** I-131 is strictly contraindicated in **pregnancy** (Category X). * **Mechanism:** I-131 emits **Beta particles** (for therapy) and **Gamma rays** (for imaging). The destructive effect is primarily due to Beta particles.

Question 58: Parathyroid adenoma is best diagnosed by which imaging modality?

A. Ultrasound (USG)
B. CT scan
C. I-131 scan
D. Tc99m-Sestamibi scan (Correct Answer)

Explanation: **Explanation:** **Tc99m-Sestamibi (MIBI) scan** is the gold standard and most sensitive imaging modality for localizing a parathyroid adenoma. The underlying principle is based on **differential washout kinetics**. Sestamibi is taken up by both the thyroid and parathyroid glands (via mitochondria-rich cells). However, the tracer washes out rapidly from normal thyroid tissue but is **retained much longer** in hyperfunctioning parathyroid tissue (adenomas). Delayed imaging (at 2–3 hours) typically shows a persistent "hot spot" in the parathyroid while the thyroid signal has faded. **Analysis of Incorrect Options:** * **Ultrasound (USG):** While often the first-line investigation due to its low cost and lack of radiation, it is operator-dependent and frequently fails to detect retro-esophageal or mediastinal (ectopic) adenomas. * **CT Scan:** Useful for anatomical mapping of large or ectopic glands, but lacks the functional specificity of nuclear medicine. It is not the primary diagnostic tool. * **I-131 Scan:** This is used for imaging **thyroid** tissue (specifically for thyroid cancer or hyperthyroidism) and has no role in parathyroid imaging as parathyroid cells do not trap iodine. **High-Yield Clinical Pearls for NEET-PG:** * **Dual-Phase Technique:** This refers to the MIBI scan described above (Early vs. Delayed imaging). * **Dual-Isotope Subtraction:** Another method using **Tc99m-Pertechnetate** (thyroid only) and **Tl-201** (both); subtracting the two images leaves only the parathyroid. * **Ectopic Adenomas:** MIBI is superior for detecting adenomas in the mediastinum. * **SPECT/CT:** Combining MIBI with CT (SPECT/CT) provides the highest sensitivity and precise anatomical localization for surgery.

Question 59: What is true about Hybrid PET/CT scans?

A. Functional-anatomical mapping
B. Provide images that help to pinpoint abnormal metabolic activity to respective anatomical structures
C. Can differentiate benign from malignant lesions as small as 1 cm
D. All of the above (Correct Answer)

Explanation: **Explanation:** Hybrid PET/CT imaging is a revolutionary modality that combines the **functional sensitivity** of Positron Emission Tomography (PET) with the **anatomical precision** of Computed Tomography (CT). * **Functional-Anatomical Mapping (Option A & B):** PET utilizes radiopharmaceuticals like **18F-FDG** (Fluorodeoxyglucose) to detect areas of high glucose metabolism (functional data). However, PET alone has poor spatial resolution. By fusing it with CT (anatomical data), clinicians can precisely localize "hot spots" to specific organs or lymph nodes. This synergy allows for accurate staging and biopsy targeting. * **Lesion Differentiation (Option C):** PET/CT is highly sensitive in differentiating benign from malignant lesions. While the spatial resolution limit for PET is typically around **7–10 mm (approx. 1 cm)**, the metabolic signature (Standardized Uptake Value - SUV) helps distinguish malignant growths from stable benign nodules of similar size. **Why "All of the Above" is correct:** The integration of these two technologies overcomes the limitations of each individual modality, providing a comprehensive diagnostic tool that maps metabolic activity directly onto high-resolution anatomical structures. **High-Yield Clinical Pearls for NEET-PG:** * **Radiopharmaceutical:** 18F-FDG is the most common tracer; it is a glucose analog. * **Patient Preparation:** Patients must fast for 4–6 hours, and blood glucose should ideally be **<150–200 mg/dL** to prevent competition between endogenous glucose and FDG. * **Brown Fat:** Can cause false positives; beta-blockers or keeping the patient warm can minimize this. * **Standardized Uptake Value (SUV):** A semi-quantitative measure of FDG avidity; SUV >2.5 is often suggestive of malignancy (though not definitive).

Question 60: Which radiotracer is used to visualize the parathyroid glands?

A. 99mTc sestamibi (Correct Answer)
B. 99mTc pertechnetate
C. 99mTc exametazime
D. 99mTc MAG3

Explanation: **Explanation:** **99mTc Sestamibi (Option A)** is the gold standard radiotracer for parathyroid imaging, primarily used to localize parathyroid adenomas. The underlying mechanism relies on the high concentration of **mitochondria** in parathyroid oxyphil cells. Sestamibi is a lipophilic cation that accumulates in these mitochondria. While it is initially taken up by both the thyroid and parathyroid glands, it **washes out** more rapidly from normal thyroid tissue, leaving a persistent "hot spot" in the hyperfunctioning parathyroid gland on delayed images (Dual-phase scintigraphy). **Analysis of Incorrect Options:** * **99mTc Pertechnetate (Option B):** This tracer is trapped by the thyroid gland via the sodium-iodide symporter. It is used for thyroid scintigraphy (e.g., Graves' disease) or to identify ectopic gastric mucosa (Meckel’s diverticulum), but it does not localize parathyroid tissue. * **99mTc Exametazime (HMPAO) (Option C):** This is a lipophilic agent used for **Brain SPECT** imaging to assess cerebral blood flow or for labeling leukocytes (WBC scan) to detect occult infections. * **99mTc MAG3 (Option D):** This is a renal radiopharmaceutical used for **dynamic renal scintigraphy** (Renogram) to assess renal tubular secretion and effective renal plasma flow (ERPF). **High-Yield Clinical Pearls for NEET-PG:** * **Dual-Phase Technique:** Uses Sestamibi alone; images are taken at 15 minutes (early) and 2–3 hours (delayed). * **Dual-Isotope Subtraction:** Uses Sestamibi (parathyroid + thyroid) and Pertechnetate (thyroid only). Subtracting the two leaves only the parathyroid image. * **Ectopic Glands:** Sestamibi is particularly useful for locating ectopic parathyroid glands in the mediastinum. * **Intraoperative Probe:** Sestamibi can be used with a gamma probe during surgery to guide the surgeon to the adenoma.

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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