Molecular Imaging Practice Questions

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

PET scan. **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).**

Q: What isotope is commonly used in PET scans?

18-fluoro-deoxyglucose. **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).

Q: What is meant by PET scan?

Positron Emission Tomography. **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.

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

Ultrasound. ***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

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

Contrast Enhanced CT abdomen. **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.

Question 1

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

Accepted Answer

Mycotic aortic aneurysm

Answer

Solitary pulmonary nodules

Answer

Staging lung cancer

Answer

Lymph node involvement by malignancy

Question 2

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

Accepted Answer

PET scan

Answer

MRI

Answer

3D CT

Answer

USG

Question 3

What isotope is commonly used in PET scans?

Accepted Answer

18-fluoro-deoxyglucose

Answer

Phosphorus-32

Answer

Iridium-77

Answer

Radium-226

Question 4

What is meant by PET scan?

Accepted Answer

Positron Emission Tomography

Answer

Positive Emission Tomography

Answer

Positron Energy Tomography

Answer

Positive Energy Tomography

Question 5

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

Accepted Answer

Ultrasound

Answer

CT scan (Contrast-enhanced)

Answer

Plain radiography (X-ray)

Answer

MRI

Question 6

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

Accepted Answer

Contrast Enhanced CT abdomen

Answer

PET scan

Answer

MRI of the abdomen

Answer

Adrenal radionuclide scan

Molecular Imaging — MCQs

Molecular Imaging — MCQs

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