Functional Imaging — MCQs

Q: What is the primary principle behind BOLD (Blood Oxygen Level Dependent) contrast in functional MRI?

Paramagnetic properties of deoxyhemoglobin. ***Paramagnetic properties of deoxyhemoglobin*** - **Deoxyhemoglobin** acts as an endogenous contrast agent because it is **paramagnetic**, which creates local magnetic field inhomogeneities and decreases the **T2* signal**. - During neuronal activity, an oversupply of oxygenated blood (which is **diamagnetic**) flushes out the deoxyhemoglobin, leading to a relative increase in the **MR signal intensity**. *Increased water diffusion in active tissue* - This describes **Diffusion-Weighted Imaging (DWI)**, which measures the random motion of water molecules rather than blood oxygenation levels. - While **diffusion fMRI** is an experimental technique, it is distinct from the standard **BOLD contrast** mechanism which relies on hemodynamic changes. *Accumulation of gadolinium in active neurons* - **Gadolinium** is an exogenous (external) contrast agent used in conventional MRI to enhance visibility of vascularity or blood-brain barrier breakdown. - **BOLD imaging** is a non-invasive technique that does not require the injection of contrast agents like gadolinium to monitor brain activity. *Increased fat content in metabolically active tissue* - Neuronal activity is associated with changes in **glucose metabolism** and **blood flow**, not an increase in local fat or lipid concentration. - **Fat suppression** techniques are actually used in MRI to remove signals from adipose tissue to better visualize underlying structures.

Q: Which of the following is a functional investigation?

PET scan. ### Explanation **Correct Answer: D. PET scan** **Why PET scan is the correct answer:** Positron Emission Tomography (PET) is a **functional (molecular) imaging** modality. Unlike conventional imaging, which visualizes anatomical structures, PET scans detect metabolic and biochemical activity within tissues. It utilizes radiopharmaceuticals, most commonly **18F-Fluorodeoxyglucose (18F-FDG)**, which is a glucose analog. Since malignant cells and inflamed tissues have higher metabolic rates, they take up more FDG, appearing as "hot spots" on the scan. This allows for the detection of disease processes (like malignancy or ischemia) even before structural changes become visible on CT or MRI. **Why other options are incorrect:** * **A, B, and C (CT, MRI, USG):** These are primarily **structural (anatomical) imaging** modalities. They provide detailed information regarding the size, shape, position, and morphology of organs and lesions. While advanced sequences like Functional MRI (fMRI) or Dynamic Contrast-Enhanced CT exist, the standard modalities listed are categorized as anatomical investigations. **High-Yield Clinical Pearls for NEET-PG:** * **Hybrid Imaging:** Modern PET is almost always combined with CT (**PET-CT**) to provide both functional and precise anatomical localization. * **Gold Standard:** PET scan is the gold standard for **staging, restaging, and monitoring treatment response** in various cancers. * **Myocardial Viability:** PET using FDG is the gold standard for assessing myocardial viability in patients with ischemic heart disease. * **Brain Imaging:** It is used to differentiate between radiation necrosis and tumor recurrence, and in the evaluation of dementia (e.g., Alzheimer’s). * **Cyclotron:** The radioisotopes used in PET (like 18F, 11C, 15O) are produced in a cyclotron and have very short half-lives.

Practice by Chapter

Principles of Functional Imaging

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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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