Functional Imaging — MCQs
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Which of the following is used to differentiate tumor recurrence and radiation necrosis?
MRI is not better than CT for the detection of which of the following?
What does PET stand for?
Functional Imaging Indian Medical PG Practice Questions and MCQs
Question 11: Which of the following is used to differentiate tumor recurrence and 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 diagnostic challenge because both entities appear remarkably similar on conventional structural imaging (like MRI or CT). **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 glucose uptake. On a **18F-FDG PET scan**, recurrence shows "hot spots" (increased tracer uptake). * **Radiation Necrosis:** This represents dead tissue and inflammatory changes following radiotherapy. It is metabolically inactive or hypometabolic, appearing as "cold spots" (decreased tracer uptake) on PET. * *Note:* Advanced PET tracers like **11C-Methionine** or **18F-FET** (Fluoroethyltyrosine) are even more specific than FDG for brain tumors. **2. Why other options are incorrect:** * **MRI (Conventional):** While MRI is the gold standard for initial diagnosis, both recurrence and necrosis show contrast enhancement and perilesional edema, making them indistinguishable on routine T1/T2 sequences. (Note: MR Spectroscopy/Perfusion can help, but PET remains the classic functional answer). * **3D CT:** CT provides excellent bony detail but lacks the soft tissue resolution and functional data required to assess tissue viability or metabolism. * **USG:** Ultrasound has no role in intracranial imaging in adults due to the skull barrier and lacks the sensitivity for metabolic differentiation. **High-Yield Clinical Pearls for NEET-PG:** * **MR Spectroscopy (MRS):** In tumor recurrence, there is **Increased Choline** (cell membrane turnover) and **Decreased NAA** (neuronal loss). * **Cold vs. Hot:** Radiation necrosis = Cold on PET; Tumor recurrence = Hot on PET. * **Thallium-201 SPECT:** Another functional modality used for this purpose; tumors are Thallium-positive, while necrosis is Thallium-negative.
Question 12: MRI is not better than CT for the detection of which of the following?
- A. Ligament injury
- B. Soft tissue tumors
- C. Meningeal pathology
- D. Calcified lesions (Correct Answer)
Explanation: **Explanation:** The correct answer is **D. Calcified lesions**. **Why CT is superior for Calcification:** Computed Tomography (CT) is based on X-ray attenuation. Calcium has a high atomic number, making it highly radiopaque (hyperdense/white) on CT. In contrast, MRI relies on proton (hydrogen) density. Calcified lesions contain very few mobile protons, resulting in a "signal void" (black appearance). Because many other structures (like cortical bone or rapidly flowing blood) also appear black on MRI, it is difficult to differentiate or quantify calcification using standard MRI sequences. Therefore, CT remains the **gold standard** for detecting intracranial calcifications, renal stones, and osteoid matrix in tumors. **Analysis of Incorrect Options:** * **A. Ligament injury:** MRI is the modality of choice due to its superior **spatial and contrast resolution**, allowing visualization of collagenous structures against fat and fluid. * **B. Soft tissue tumors:** MRI provides excellent **soft tissue characterization**, helping to define tumor margins, neurovascular involvement, and tissue composition (fat, blood, or necrosis) better than CT. * **C. Meningeal pathology:** MRI (especially post-gadolinium T1 sequences) is far more sensitive than CT for detecting leptomeningeal enhancement, dural metastases, or inflammatory changes. **High-Yield Clinical Pearls for NEET-PG:** * **Hyperacute Stroke:** Non-contrast CT is the initial investigation to rule out hemorrhage, but **Diffusion-Weighted Imaging (DWI) MRI** is the most sensitive for early ischemia. * **Posterior Fossa:** MRI is superior to CT because CT is prone to "bone hardening artifacts" in the posterior fossa. * **Susceptibility Weighted Imaging (SWI):** This specific MRI sequence is used to detect "blood or bone" (calcium vs. hemorrhage), but CT is still preferred for primary detection of calcification.
Question 13: What does PET stand for?
- A. Positive electron tomography
- B. Proton-electron therapy
- C. Positron emission tomography (Correct Answer)
- D. Photon emitting tomography
Explanation: **Explanation:** **Positron Emission Tomography (PET)** is a functional nuclear medicine imaging technique that detects gamma rays emitted indirectly by a positron-emitting radionuclide (tracer). The most common tracer used is **18F-Fluorodeoxyglucose (FDG)**, a glucose analog. 1. **Why Option C is Correct:** The process begins when a radioactive tracer undergoes **beta-plus decay**, emitting a **positron** (the antimatter counterpart of an electron). This positron travels a short distance before colliding with an electron in the tissue. This collision results in an **annihilation event**, producing two 511 keV photons (gamma rays) that travel in exactly opposite directions (180°). The PET scanner detects these coincident photons to create a 3D image of metabolic activity. 2. **Why Other Options are Incorrect:** * **Option A:** "Positive electron" is a descriptive term for a positron, but it is not the formal name of the modality. * **Option B:** PET is a diagnostic imaging tool, not a "therapy" (though it guides therapy). * **Option D:** While photons are emitted, the specific mechanism is the emission of a positron leading to photon production; "Photon emitting tomography" is too vague and could technically describe SPECT. **High-Yield Clinical Pearls for NEET-PG:** * **Mechanism:** PET measures **metabolic activity**, whereas CT/MRI measure anatomy. * **Standard Uptake Value (SUV):** A semi-quantitative measure used to differentiate malignant (high SUV) from benign lesions. * **Physiological Uptake:** Normal high FDG uptake is seen in the **brain** (high glucose requirement), **heart**, and **urinary tract** (excretion path). * **Clinical Use:** Primarily used for cancer staging, monitoring treatment response, and identifying myocardial viability.
Practice by Chapter
Principles of Functional Imaging
Practice Questions
Functional MRI
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Diffusion and Perfusion Imaging
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Spectroscopy
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PET and SPECT Functional Imaging
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Functional Imaging in Neurology
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Functional Imaging in Oncology
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Functional Imaging in Cardiology
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Dynamic Contrast-Enhanced Imaging
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Quantitative Imaging Biomarkers
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Integration of Functional and Anatomical Imaging
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Emerging Techniques in Functional Imaging
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