Nuclear Medicine Practice Questions

Q: Which radiotracer is most commonly used for PET imaging in oncology?

18F-Fluorodeoxyglucose (18F-FDG). ***18F-Fluorodeoxyglucose (18F-FDG)*** - **18F-FDG** is a glucose analog that accumulates in **metabolically active** cells, making it the standard for detecting malignant tumors which exhibit high **glycolysis**. - It has an ideal half-life of approximately **110 minutes**, allowing for efficient diagnostic imaging and clinical distribution. *Technetium-99m* - This is the most common tracer for **SPECT** imaging and bone scans, not PET imaging, due to its **gamma emission** properties. - It does not measure **glucose metabolism**, which is the primary mechanism required for standard oncological PET scans. *Gallium-67* - Primarily used in the past for detecting **inflammation**, infections, or certain lymphomas via **SPECT** imaging. - It has a long half-life of 78 hours and relies on **transferrin binding** rather than the metabolic pathways used in PET/CT. *Iodine-131* - This isotope is specifically used for the diagnosis and treatment of **thyroid carcinoma** and hyperthyroidism. - It emits both **beta and gamma radiation**, making it more suitable for **radionuclide therapy** than routine PET diagnostic imaging.

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: 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 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

Why does 18F-FDG accumulation occur in inflammatory lesions despite PET being primarily used for cancer detection?

Accepted Answer

Activated macrophages and neutrophils have increased glucose metabolism

Answer

Inflammatory cells have no glucose metabolism

Answer

FDG is specific only for malignant cells

Answer

Inflammation causes decreased blood flow reducing FDG delivery

Question 2

Which radiotracer is most commonly used for PET imaging in oncology?

Accepted Answer

18F-Fluorodeoxyglucose (18F-FDG)

Answer

Technetium-99m

Answer

Gallium-67

Answer

Iodine-131

Question 3

A "cold spot" on bone scintigraphy is seen in all the following conditions except?

Accepted Answer

Eosinophilic granuloma

Answer

Fibrous cortical defect

Answer

Pseudarthrosis

Answer

Multiple myeloma

Question 4

Non-visualization of the gallbladder is a feature seen in acute cholecystitis in which of the following investigations?

Accepted Answer

HIDA scan

Answer

USG abdomen

Answer

CECT abdomen

Answer

MRCP

Question 5

Which of the following is the best method for measuring renal Glomerular Filtration Rate (GFR)?

Accepted Answer

Technetium-99m Diethylenetriaminepentaacetic acid (DTPA) scan

Answer

Technetium-99m Dimercaptosuccinic acid (DMSA) scan

Answer

Technetium-99m Pyrophosphate scan

Answer

Creatinine clearance

Question 6

What is the best investigation for multiple osteoblastic bone metastases?

Accepted Answer

Bone Scan

Answer

MRI

Answer

CT

Answer

X-ray

Question 7

Viability of ischemic myocardium is best diagnosed by?

Accepted Answer

PET scan

Answer

MUGA scan

Answer

Thallium scan

Answer

Coronary angiogram

Question 8

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 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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