Molecular Imaging — MCQs

Molecular Imaging Indian Medical PG Practice Questions and MCQs

Question 11: Positron emission tomographic (PET) scanning is useful in the evaluation of which of the following conditions?

A. Mycotic aortic aneurysm (Correct Answer)
B. Solitary pulmonary nodules
C. Staging lung cancer
D. Lymph node involvement by malignancy

Explanation: **Explanation:** The correct answer is **A. Mycotic aortic aneurysm**. While PET scanning is traditionally associated with oncology, its role in detecting **vascular inflammation and infection** is a high-yield concept for NEET-PG. **1. Why Mycotic Aortic Aneurysm is the Correct Answer:** A mycotic aneurysm is an infected arterial wall. PET/CT using **18F-FDG** (Fluorodeoxyglucose) is highly sensitive for this condition because activated inflammatory cells (neutrophils and macrophages) at the site of infection have a significantly high metabolic rate and glucose requirement. FDG-PET can detect these "hot spots" of infection even before structural changes are visible on a conventional CT, making it superior for diagnosing infected prostheses and vascular grafts. **2. Analysis of Other Options:** * **B, C, and D (Solitary pulmonary nodules, Staging lung cancer, Lymph node involvement):** While PET is indeed used for these conditions, they represent the *standard* oncological applications of the technology. In the context of this specific question (often derived from recent clinical updates or Harrison’s/Bailey’s), the focus is on the **emerging or specific utility** in vascular infections where conventional imaging might be equivocal. *Note: If this were a "Multiple Select" question, all would be correct; however, in a single-best-response format, the focus is on the specific inflammatory application.* **Clinical Pearls for NEET-PG:** * **Mechanism:** FDG is a glucose analog; it is taken up by cells via GLUT transporters and phosphorylated but cannot be further metabolized, leading to "metabolic trapping." * **False Positives:** PET can be positive in non-malignant conditions like **Tuberculosis, Sarcoidosis, and Fungal infections** (due to high metabolic activity of granulomas). * **Brain Imaging:** PET is the gold standard for differentiating **Radiation Necrosis** (cold/low uptake) from **Tumor Recurrence** (hot/high uptake). * **Myocardial Viability:** FDG-PET is the "Gold Standard" for identifying hibernating myocardium.

Question 12: Which of the following imaging modalities is used to differentiate tumor recurrence from radiation necrosis?

A. PET scan (Correct Answer)
B. MRI
C. 3D CT
D. USG

Explanation: **Explanation:** The differentiation between **tumor recurrence** and **radiation necrosis** is a classic clinical challenge because both entities often appear identical on conventional structural imaging (like CT or MRI), showing contrast enhancement and perilesional edema. **1. Why PET Scan is the Correct Answer:** The distinction relies on **metabolic activity** rather than anatomy. * **Tumor Recurrence:** Malignant cells are hypermetabolic and demonstrate high uptake of radiopharmaceuticals like **18F-FDG** (Fluorodeoxyglucose) or amino acid tracers (e.g., 11C-Methionine). * **Radiation Necrosis:** This represents dead tissue and vascular injury; it is metabolically inactive (hypometabolic) and shows little to no tracer uptake. Therefore, PET imaging provides the functional data necessary to distinguish "hot" viable tumor from "cold" necrotic tissue. **2. Why Other Options are Incorrect:** * **MRI:** While advanced sequences like MR Spectroscopy (showing high Choline/NAA ratios) can help, standard MRI often fails because both necrosis and recurrence cause blood-brain barrier breakdown and enhancement. * **3D CT:** CT provides excellent structural detail of bone and acute hemorrhage but lacks the metabolic sensitivity to differentiate viable tumor cells from post-radiation changes. * **USG:** Ultrasound has no role in evaluating deep-seated brain tumors or post-radiation changes due to the inability of sound waves to penetrate the adult cranium. **Clinical Pearls for NEET-PG:** * **Gold Standard:** While PET is the preferred functional imaging, **MR Spectroscopy (MRS)** is the most common "advanced MRI" alternative mentioned in exams. * **Tracer of Choice:** In the brain, **18F-FET** or **11C-Methionine** are often superior to FDG because the brain has high baseline glucose uptake, which can mask tumors. * **Hot vs. Cold:** Remember: **Recurrence = Hot (Hypermetabolic)**; **Necrosis = Cold (Hypometabolic).**

Question 13: What isotope is commonly used in PET scans?

A. Phosphorus-32
B. 18-fluoro-deoxyglucose (Correct Answer)
C. Iridium-77
D. Radium-226

Explanation: **Explanation:** **18-Fluoro-deoxyglucose (18-FDG)** is the most widely used radiopharmaceutical in Positron Emission Tomography (PET). The underlying principle is **metabolic imaging**. 18-FDG is a glucose analog; it is taken up by cells via GLUT transporters and phosphorylated by hexokinase. However, unlike normal glucose, it cannot undergo further glycolysis and becomes "trapped" within the cell (**metabolic trapping**). Since malignant cells have high metabolic rates and increased GLUT-1 expression, they accumulate higher concentrations of 18-FDG, appearing as "hot spots" on the scan. **Analysis of Incorrect Options:** * **Phosphorus-32:** A pure beta-emitter used primarily for therapeutic purposes, such as treating Polycythemia Vera or for intracavitary treatment of malignant effusions. * **Iridium-192 (Note: Iridium-77 is not a standard medical isotope):** Commonly used in **Brachytherapy** (e.g., breast or prostate cancer) as a source for high-dose-rate (HDR) radiation. * **Radium-226:** Historically used in brachytherapy (pioneered by Marie Curie), but largely replaced by safer isotopes like Cesium-137 due to its long half-life and daughter product (Radon gas) risks. **Clinical Pearls for NEET-PG:** * **Physics:** PET relies on **Annihilation Radiation**. A positron emitted by the isotope meets an electron, producing two 511 keV photons traveling in opposite directions (180°). * **Half-life:** The half-life of Fluorine-18 is approximately **110 minutes**. * **Preparation:** Patients must be **fasting (4–6 hours)** and have controlled blood glucose levels (<200 mg/dL) to minimize competition between endogenous glucose and 18-FDG. * **False Positives:** FDG is not cancer-specific; it also accumulates in areas of active **inflammation and infection** (e.g., Tuberculosis).

Question 14: What is meant by PET scan?

A. Positive Emission Tomography
B. Positron Emission Tomography (Correct Answer)
C. Positron Energy Tomography
D. Positive Energy Tomography

Explanation: **Explanation:** **Positron Emission Tomography (PET)** is a functional nuclear medicine imaging technique that visualizes metabolic processes in the body. The correct answer is **Option B** because the technology relies on the detection of pairs of gamma rays emitted indirectly by a **positron-emitting radionuclide** (tracer). **Why the correct answer is right:** When a radiopharmaceutical (like FDG) is injected, the radioisotope undergoes beta-plus decay, emitting a **positron** (an anti-matter particle). This positron travels a short distance before colliding with an electron, resulting in an **annihilation event**. This event produces two 511 keV photons (gamma rays) traveling in opposite directions, which are detected by the PET scanner to create a 3D image of metabolic activity. **Why the incorrect options are wrong:** * **Option A & D:** "Positive" is a common distractor; while positrons have a positive charge, the "P" specifically stands for the particle name (Positron). * **Option C & D:** "Energy" is incorrect as the "E" stands for **Emission**, referring to the release of radiation from within the patient's body (unlike X-rays/CT where radiation is transmitted through the patient). **High-Yield Clinical Pearls for NEET-PG:** * **Most Common Tracer:** **18F-FDG** (Fluorodeoxyglucose), a glucose analog. It is taken up by cells via GLUT transporters and trapped by phosphorylation. * **Clinical Use:** Primarily used for **oncology** (staging and treatment response), **cardiology** (myocardial viability), and **neurology** (epilepsy and Alzheimer’s). * **Key Physics Fact:** The annihilation photons always have an energy of **511 keV**. * **Physiological Uptake:** Normal "hot spots" include the brain, heart, kidneys, and urinary bladder.

Question 15: Which of the following imaging modalities is most appropriate for initial evaluation of suspected acute appendicitis in a young adult patient?

A. Ultrasound (Correct Answer)
B. CT scan (Contrast-enhanced)
C. Plain radiography (X-ray)
D. MRI

Explanation: ***Ultrasound*** - **First-line imaging modality** for suspected acute appendicitis in young adults, especially in children, pregnant women, and young females - **Advantages:** No ionizing radiation, readily available, cost-effective, can be performed at bedside - **High specificity** (>90%) when positive findings are present - **Graded compression technique** helps visualize the appendix and assess for periappendiceal inflammation - **Limitations:** Operator-dependent, may be difficult in obese patients or with overlying bowel gas *CT scan (Contrast-enhanced)* - **Most sensitive imaging modality** (sensitivity >95%) for acute appendicitis - Considered when ultrasound is inconclusive or technically difficult - **Gold standard** in adults, especially in obese patients - Provides excellent visualization of the appendix and complications (perforation, abscess) - However, involves **ionizing radiation**, making it less ideal as first-line in young patients *MRI* - **Preferred in pregnant women** when ultrasound is inconclusive - No ionizing radiation exposure - High accuracy but **limited availability**, longer scan time, and higher cost - Not typically used as first-line imaging in non-pregnant young adults *Plain radiography (X-ray)* - **Limited role** in diagnosing acute appendicitis - Non-specific findings; may show fecalith, loss of psoas shadow, or signs of perforation - Cannot reliably visualize the appendix - **Not recommended** as initial imaging for suspected appendicitis

Question 16: The most appropriate first-line imaging modality to detect adrenal metastasis due to bronchogenic carcinoma is:

A. PET scan
B. MRI of the abdomen
C. Adrenal radionuclide scan
D. Contrast Enhanced CT abdomen (Correct Answer)

Explanation: **Contrast Enhanced CT abdomen** - **Contrast-enhanced CT abdomen** is generally considered the most sensitive and cost-effective imaging modality for detecting **adrenal metastases**. - It allows for detailed visualization of adrenal gland morphology, including size, shape, and enhancement patterns, which can help differentiate benign from malignant lesions. *PET scan* - While **PET (Positron Emission Tomography) scans** are highly sensitive for detecting metabolically active metastatic disease, they are often used as a secondary imaging modality to characterize indeterminate lesions found on CT or MRI. - **PET scans** can have false positives in benign adrenal tumors (e.g., adenomas rich in fat) and are less readily available or higher in cost for initial screening compared to CT. *MRI of the abdomen* - **MRI of the abdomen** can be very useful for further characterization of adrenal masses, especially for distinguishing between lipid-rich adenomas and metastases. - However, for initial detection, especially in the context of screening for distant metastases from bronchogenic carcinoma, **CT is generally preferred due to its wider availability, speed, and lower cost**. *Adrenal radionuclide scan* - **Adrenal radionuclide scans** (e.g., using MIBG or iodocholesterol) are primarily used for functional imaging of adrenal glands, typically to detect specific types of tumors like pheochromocytomas or aldosteronomas. - These scans are **not sensitive for detecting adrenal metastases** from bronchogenic carcinoma, as the metastatic lesions do not typically exhibit the specific uptake patterns targeted by these radiotracers.

Practice by Chapter

Principles of Molecular Imaging

Practice Questions

Radiopharmaceuticals for Molecular Imaging

Practice Questions

PET Radiochemistry

Practice Questions

Molecular Imaging in Oncology

Practice Questions

Molecular Imaging in Cardiology

Practice Questions

Molecular Imaging in Neurology

Practice Questions

Molecular Imaging in Inflammation and Infection

Practice Questions

Reporter Gene Imaging

Practice Questions

Cell Tracking and Imaging

Practice Questions

Nanoparticles in Molecular Imaging

Practice Questions

Theranostics

Practice Questions

Future Directions in Molecular Imaging

Practice Questions

Want unlimited practice?

Get full access to all questions, explanations, and performance tracking.

Start For Free