PET/CT Principles and Applications — MCQs
Which radiopharmaceutical is commonly used in positron emission tomography (PET) imaging?
Which of the following investigations work on the same principle?
A lady presented with a 4 cm tumor in the left parietal lobe for which she underwent surgery and radiotherapy. After 3 months she presented with headache and vomiting. Which of the following would characterize the lesion in the patient?
Investigation of choice for vascular ring around airway:
Investigation of choice for leptomeningeal carcinomatosis:
Which common tracer in PET is usually administered in the form of a glucose sugar?
Distant bone metastases can be best detected by which of the following imaging techniques?
Statement 1 - A 59-year-old patient presents with flaccid bullae. Histopathology shows a suprabasal acantholytic split. Statement 2 - The row of tombstones appearance is diagnostic of Pemphigus vulgaris.
Which artificial radioisotopes are used in nuclear medicine?
Which radiopharmaceutical is used for a liver scan?
PET/CT Principles and Applications Indian Medical PG Practice Questions and MCQs
Question 1: Which radiopharmaceutical is commonly used in positron emission tomography (PET) imaging?
- A. 18F-FDG (Fluorodeoxyglucose) (Correct Answer)
- B. Oxygen-15 (used in specific PET scans)
- C. Carbon-11 acetate
- D. Nitrogen-13 ammonia
Explanation: ***18F-FDG (Fluorodeoxyglucose)*** - **18F-FDG** is the most widely used radiopharmaceutical in PET imaging, particularly for **oncology**, as it's a glucose analog that accumulates in metabolically active cells. - Its widespread use is due to its favorable physical properties for PET and its ability to reflect **tumor metabolism**. *Carbon-11 acetate* - **Carbon-11 acetate** is used in specific PET applications, primarily for **cardiac imaging** to assess myocardial oxidative metabolism. - It has a very short half-life (around 20 minutes) which limits its availability to centers with on-site cyclotrons. *Oxygen-15 (used in specific PET scans)* - **Oxygen-15** (e.g., O-15 water) is used in highly specialized PET scans for measuring **blood flow** and oxygen metabolism, especially in brain studies. - Its extremely short half-life (approximately 2 minutes) necessitates an on-site cyclotron and immediate use. *Nitrogen-13 ammonia* - **Nitrogen-13 ammonia** is a common radiopharmaceutical for **myocardial perfusion imaging** with PET, reflecting regional blood flow to the heart. - Like other C-11 and O-15 tracers, its short half-life (about 10 minutes) requires proximity to a cyclotron facility.
Question 2: Which of the following investigations work on the same principle?
- A. MRI and PET Scan
- B. CT and MRI
- C. CT and X-ray (Correct Answer)
- D. USG and HIDA Scan
Explanation: ***CT and X-ray*** - Both **Computed Tomography (CT)** and **X-ray** imaging utilize **ionizing radiation** to generate images of the body's internal structures. - They work by passing X-ray beams through the patient, with different tissues absorbing the radiation to varying degrees, which is then detected to create an image. *MRI and PET Scan* - **Magnetic Resonance Imaging (MRI)** uses **strong magnetic fields and radio waves** to create detailed images of soft tissues, based on water content. - **Positron Emission Tomography (PET) scans** use **radioactive tracers** to visualize metabolic activity and blood flow, detecting gamma rays emitted from the patient. *CT and MRI* - **CT scans** use **ionizing radiation** (X-rays) to produce cross-sectional images. - **MRI scans** use **magnetic fields and radio waves** and do not involve ionizing radiation. *USG and HIDA Scan* - **Ultrasound (USG)** uses **high-frequency sound waves** to create real-time images of organs and structures. - **Hepatobiliary Iminodiacetic Acid (HIDA) scans** are a type of nuclear medicine study that uses a **radioactive tracer** to evaluate liver and gallbladder function.
Question 3: A lady presented with a 4 cm tumor in the left parietal lobe for which she underwent surgery and radiotherapy. After 3 months she presented with headache and vomiting. Which of the following would characterize the lesion in the patient?
- A. Digital subtraction angiography with dual source CT scan
- B. Gd-enhanced MRI
- C. 99Tc-HMPAO SPECT brain
- D. 18FDG PET Scan (Correct Answer)
Explanation: ***18FDG PET Scan*** - This patient, presenting with new neurological symptoms after **surgery and radiotherapy** for a cerebral tumor, faces a diagnostic dilemma: differentiating between **tumor recurrence** and **radiation necrosis**. - **18FDG PET scans** effectively distinguish between these two conditions because viable tumor cells exhibit high metabolic activity and thus actively take up **fluorodeoxyglucose (FDG)**, while radiation necrosis is metabolically inactive and shows little to no FDG uptake. *Digital subtraction angiography with dual source CT scan* - **Digital subtraction angiography (DSA)** is primarily used to visualize **vascular structures** and is not the modality of choice for differentiating tumor recurrence from radiation necrosis. - A **dual-source CT scan** is useful for rapid imaging and dynamic studies but lacks the metabolic information needed for this specific differentiation. *Gd-enhanced MRI* - While **Gd-enhanced MRI** is excellent for detecting **structural changes** and **blood-brain barrier disruption**, it often cannot definitively differentiate between **tumor recurrence** and **radiation necrosis**. - Both conditions can present with similar **enhancement patterns** on MRI, making differentiation challenging without additional metabolic information. *99Tc-HMPAO SPECT brain* - **99mTc-HMPAO SPECT** measures **regional cerebral blood flow (rCBF)**, which can be altered in both tumors and areas of radiation injury. - However, it does not provide the specific metabolic information (glucose metabolism) needed to reliably distinguish between **viable tumor cells** and **radiation necrosis** as effectively as FDG PET.
Question 4: Investigation of choice for vascular ring around airway:
- A. PET
- B. Catheter directed angiography
- C. MRI
- D. CT (Correct Answer)
Explanation: ***CT*** - **CT angiography (CTA)** is the **investigation of choice** for diagnosing vascular rings due to its ability to provide detailed anatomical visualization of the great vessels and their relationship to the trachea and esophagus. - It offers high spatial resolution, allowing precise identification of the type of vascular anomaly, the degree of **airway and esophageal compression**, and guiding surgical planning. *PET* - **PET scans** are primarily used for assessing **metabolic activity**, particularly in oncology or to evaluate organ function, and do not provide sufficient anatomical detail for vascular rings. - While it can detect metabolically active lesions, it is **not suitable** for visualizing the structural abnormalities of blood vessels and their compressive effects on the airway. *Catheter directed angiography* - **Catheter-directed angiography** is an **invasive procedure** involving radiation and contrast, primarily used for assessing blood flow dynamics, identifying stenosis, or guiding interventions. - While it can visualize vessels, CTA is **less invasive**, provides comparable or superior anatomical detail for vascular rings, and is generally preferred for initial diagnosis. *MRI* - **MRI** can provide good soft tissue contrast and visualize vascular structures without radiation, but it is often **less readily available** and can be more challenging for pediatric patients due to the need for sedation and longer scan times. - For comprehensive anatomical detail including bone and calcifications, and in patients who might struggle with breath-holding, **CT angiography** often offers clearer and more consistent images of complex vascular anatomy.
Question 5: Investigation of choice for leptomeningeal carcinomatosis:
- A. Gd enhanced MRI (Correct Answer)
- B. CT scan
- C. SPECT
- D. PET
Explanation: ***Gd enhanced MRI*** - **Gadolinium-enhanced MRI** is the investigation of choice for **leptomeningeal carcinomatosis** as it can visualize the subtle nodular or linear enhancement along the leptomeninges, indicating tumor dissemination. - It offers superior **soft tissue contrast** and spatial resolution compared to CT, enabling detection of small lesions and accurate mapping of disease extent. *CT scan* - A **CT scan** has limited sensitivity for detecting leptomeningeal involvement due to poor contrast resolution of soft tissues and the dura/arachnoid spaces. - It might show hydrocephalus or large tumor deposits, but subtle leptomeningeal enhancement is often missed. *SPECT* - **Single photon emission computed tomography (SPECT)** is primarily used for functional imaging and is not the investigation of choice for anatomical visualization of leptomeningeal carcinomatosis. - Its resolution is too low to detect the fine structural changes associated with leptomeningeal spread. *PET* - **Positron emission tomography (PET)**, often combined with CT, identifies metabolically active tumor cells and can detect diffuse metastatic disease. - While useful for overall cancer staging and identifying primary lesions, it is less effective than gadolinium-enhanced MRI for directly visualizing the morphology and enhancement patterns of leptomeningeal carcinomatosis due to limited spatial resolution in the CSF spaces.
Question 6: Which common tracer in PET is usually administered in the form of a glucose sugar?
- A. Aluminum - 12
- B. Fluorine 18 (Correct Answer)
- C. Carbon 11
- D. Oxygen 15
Explanation: ***Fluorine 18*** - **18F-FDG** (Fluorodeoxyglucose) is the most common PET tracer, utilizing **Fluorine-18** as its radioactive component. - FDG is a glucose analog, meaning it mimics glucose and is taken up by metabolically active cells, allowing for imaging of **glucose metabolism**. *Aluminum - 12* - **Aluminum-12** is not a common radionuclide used in PET imaging. - The most common tracers in PET are **positron emitters** like Fluorine-18, Carbon-11, Nitrogen-13, and Oxygen-15. *Carbon 11* - **Carbon-11** can be used in PET tracers (e.g., 11C-methionine), but it is **less common** than 18F-FDG due to its shorter half-life. - Its short half-life (20 minutes) requires an **on-site cyclotron** for production, limiting its widespread use. *Oxygen 15* - **Oxygen-15** is employed in PET tracers (e.g., 15O-water for cerebral blood flow), but it has an **even shorter half-life** (2 minutes) than Carbon-11. - Its extremely short half-life makes it **impractical** for routine clinical use in the form of a glucose sugar.
Question 7: Distant bone metastases can be best detected by which of the following imaging techniques?
- A. Bone scan (Correct Answer)
- B. CT
- C. Intravenous venogram
- D. PET scan
Explanation: ***Bone scan*** - A **bone scan** is highly sensitive for detecting **osteoblastic activity**, which is characteristic of most bone metastases. - It involves injecting a **radioactive tracer** (usually technetium-99m methylene diphosphonate) that accumulates in areas of increased bone turnover, making it excellent for surveying the entire skeletal system. *PET scan* - While a **PET scan** (Positron Emission Tomography) can detect bone metastases, especially with **FDG-PET**, it is generally more expensive and may not be as sensitive for purely **osteoblastic lesions** as a bone scan. - Its primary role is often in assessing metabolic activity of the primary tumor and other distant soft tissue metastases. *CT* - **CT scans** (Computed Tomography) are excellent for assessing bone anatomy, cortical destruction, and soft tissue involvement, but they are generally less sensitive for detecting early or widespread **osseous metastatic disease** compared to a bone scan. - CT provides detailed anatomical information but may miss early **marrow involvement** that alters bone metabolism. *Intravenous venogram* - An **intravenous venogram** is an imaging technique used to visualize veins, primarily for detecting **thrombosis** or venous insufficiency. - It has no role in the detection of **bone metastases**, as it provides no information about bone structure or metabolic activity.
Question 8: Statement 1 - A 59-year-old patient presents with flaccid bullae. Histopathology shows a suprabasal acantholytic split. Statement 2 - The row of tombstones appearance is diagnostic of Pemphigus vulgaris.
- A. Statements 1 & 2 are correct, 2 is not explaining 1 (Correct Answer)
- B. Statements 1 and 2 are correct and 2 is the correct explanation for 1
- C. Statements 1 and 2 are incorrect
- D. Statement 1 is incorrect
Explanation: ***Correct: Statements 1 & 2 are correct, 2 is not explaining 1*** **Analysis of Statement 1:** - A 59-year-old patient with **flaccid bullae** and **suprabasal acantholytic split** on histopathology is the classic presentation of **Pemphigus vulgaris** - The flaccid (easily ruptured) nature of bullae distinguishes it from tense bullae seen in bullous pemphigoid - The suprabasal location of the split (just above the basal layer) with acantholysis (loss of cell-to-cell adhesion) is pathognomonic - **Statement 1 is CORRECT** ✓ **Analysis of Statement 2:** - The **"row of tombstones" or "tombstone appearance"** is indeed a diagnostic histopathological feature of Pemphigus vulgaris - This appearance results from basal keratinocytes remaining attached to the basement membrane while suprabasal cells separate due to acantholysis - The intact basal cells standing upright resemble a row of tombstones - **Statement 2 is CORRECT** ✓ **Does Statement 2 explain Statement 1?** - Statement 2 describes a **histopathological appearance** (tombstone pattern) that is a **consequence** of the suprabasal split - However, it does NOT explain the **underlying cause** of the flaccid bullae or the suprabasal split - The true explanation involves **IgG autoantibodies against desmoglein 3 (and desmoglein 1)**, which attack intercellular adhesion structures (desmosomes), causing **acantholysis** - Therefore, **Statement 2 does NOT explain Statement 1** ✗ *Incorrect: Statement 2 is the correct explanation for Statement 1* - While both statements describe features of Pemphigus vulgaris, the tombstone appearance is a descriptive finding, not an explanatory mechanism *Incorrect: Statements 1 and 2 are incorrect* - Both statements are medically accurate descriptions of Pemphigus vulgaris features *Incorrect: Statement 1 is incorrect* - Statement 1 correctly describes the cardinal clinical and histopathological features of Pemphigus vulgaris
Question 9: Which artificial radioisotopes are used in nuclear medicine?
- A. Radium
- B. Uranium
- C. Plutonium (Correct Answer)
- D. Iridium
Explanation: ### Explanation **Correct Answer: C. Plutonium** In nuclear medicine, radioisotopes are categorized as either **natural** (found in nature) or **artificial** (man-made via nuclear reactors or cyclotrons). **Plutonium (specifically Pu-238)** is an artificial radioisotope produced in nuclear reactors. While not used as a diagnostic tracer or therapeutic agent for internal administration, it has a significant historical and niche clinical application as a power source for **Radioisotope Thermoelectric Generators (RTGs)** in long-lived **cardiac pacemakers**. Its high energy density and long half-life made it ideal for devices requiring decades of operation without battery replacement. **Analysis of Incorrect Options:** * **A. Radium:** This is a **naturally occurring** radioactive metal found in uranium ores. While Radium-223 is used in treating bone metastases (Xofigo), the element itself is classified as natural. * **B. Uranium:** This is a **naturally occurring** heavy metal. It is the raw material used to produce artificial isotopes but is not used directly in clinical nuclear medicine. * **C. Iridium:** While Iridium-192 is used in Brachytherapy, it is generally classified as a transition metal used in "sealed sources" for radiotherapy rather than being the classic example of an "artificial radioisotope" in the context of general nuclear medicine tracers (like Technetium-99m). However, in the context of this specific question, Plutonium is the most distinct "artificial/man-made" element. **High-Yield Clinical Pearls for NEET-PG:** * **Technetium-99m (Tc-99m):** The most commonly used artificial radioisotope in diagnostic nuclear medicine (produced in a Mo-99/Tc-99m generator). * **Cyclotron-produced isotopes:** Include F-18 (used in PET scans), I-123, and Thallium-201. * **Reactor-produced isotopes:** Include I-131, Mo-99, and Xenon-133. * **Therapeutic Alpha Emitter:** Radium-223 is the first alpha-emitting radiopharmaceutical approved to improve survival in castration-resistant prostate cancer with bone metastases.
Question 10: Which radiopharmaceutical is used for a liver scan?
- A. Tc-99m sulphur colloid (Correct Answer)
- B. Tc-99m mebrofenin
- C. Tc-99m MIBI
- D. Tc-99m DTPA
Explanation: **Explanation:** The correct answer is **Tc-99m sulphur colloid**. The underlying principle for a liver-spleen scan is the **phagocytic activity of the Reticuloendothelial System (RES)**. When Tc-99m sulphur colloid is injected intravenously, the particles (sized 0.1–1.0 μm) are cleared from the blood by Kupffer cells in the liver (80–90%), splenic macrophages (5–10%), and bone marrow. This scan is primarily used to evaluate functional liver anatomy and detect "cold nodules" (e.g., abscesses or tumors) or "hot spots" (e.g., Focal Nodular Hyperplasia). **Analysis of Incorrect Options:** * **Tc-99m Mebrofenin:** This is an IDA (Iminodiacetic acid) derivative used for **HIDA scans**. It evaluates the **hepatobiliary system** (hepatocyte uptake and biliary excretion) and is the gold standard for diagnosing Acute Cholecystitis. * **Tc-99m MIBI:** Primarily used for **Myocardial Perfusion Imaging** and Parathyroid imaging. It is taken up by mitochondria. * **Tc-99m DTPA:** A chelating agent cleared by glomerular filtration, used for **Renal Dynamic Scans** to assess GFR and obstructive uropathy. **High-Yield Clinical Pearls for NEET-PG:** 1. **Colloid Shift:** In portal hypertension or cirrhosis, there is decreased liver uptake and increased uptake in the spleen and bone marrow. 2. **Focal Nodular Hyperplasia (FNH):** This is the only liver lesion that typically appears "hot" or "isointense" on a sulphur colloid scan due to the presence of Kupffer cells. 3. **Hot Spot on Liver Scan:** Classically seen in **Superior Vena Cava (SVC) Obstruction** (due to collateral flow via the vein of Sappey).
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