Nuclear Medicine Practice Questions

Q: Which of the following is selectively filtered into the renal tubules?

DTPA. **Explanation:** The correct answer is **DTPA (Diethylenetriaminepentaacetic acid)**. In renal physiology and nuclear medicine, the handling of radiopharmaceuticals by the nephron depends on their pharmacokinetics. 1. **DTPA (Technetium-99m DTPA):** This agent is handled almost exclusively by **glomerular filtration**. It is not secreted or reabsorbed by the renal tubules. Because its clearance rate is nearly identical to the Glomerular Filtration Rate (GFR), it is the gold standard radiopharmaceutical for measuring GFR and evaluating renal perfusion. **Analysis of Incorrect Options:** * **B. DMSA (Dimercaptosuccinic acid):** This agent binds to the **proximal convoluted tubules** and remains fixed in the renal cortex for a long duration. It is used for **static renal imaging** to detect scars, ectopic kidneys, or pyelonephritis, rather than measuring filtration. * **C. MAG3 (Mercaptoacetyltriglycine):** This is primarily handled by **tubular secretion** (approx. 80%) and only minimally by filtration. It is the agent of choice for dynamic renography in patients with suspected obstruction or renal failure. * **D. EC (Ethylene Dicysteine):** Similar to MAG3, EC is primarily cleared by **tubular secretion**. It offers a better target-to-background ratio but is not a marker for pure filtration. **High-Yield Clinical Pearls for NEET-PG:** * **GFR Marker:** Tc-99m DTPA (Exogenous) and Creatinine/Inulin (Physiological). * **Renal Plasma Flow (RPF) Marker:** PAH (Para-aminohippurate) or Tc-99m MAG3. * **Renal Cortical Imaging:** Tc-99m DMSA. * **Best for Neonates:** MAG3 is preferred over DTPA due to the immature GFR in newborns.

Q: Fluorodeoxyglucose-positron emission tomography (FDG-PET) does not detect which of the following?

Typical carcinoid. **Explanation:** The core concept behind FDG-PET imaging is the **Warburg effect**, where malignant cells exhibit increased glucose metabolism and overexpression of GLUT-1 transporters. The sensitivity of FDG-PET in neuroendocrine tumors (NETs) is directly proportional to the **metabolic rate and proliferation index (Ki-67)** of the tumor. * **Typical Carcinoid (Correct Answer):** These are well-differentiated, low-grade (Grade 1) tumors with a low mitotic rate (<2 mitoses/10mm²) and low metabolic activity. Because they grow slowly and often lack significant GLUT-1 expression, they are frequently **FDG-avid negative**. For these tumors, Somatostatin Receptor Scintigraphy (e.g., **Ga-68 DOTATATE PET/CT**) is the imaging modality of choice. * **Atypical Carcinoid:** These are intermediate-grade (Grade 2) tumors. They have higher mitotic indices and a greater likelihood of being FDG-positive compared to typical carcinoids, as they transition toward more aggressive metabolic pathways. * **Small Cell & Large Cell Neuroendocrine Tumors:** These are high-grade (Grade 3), poorly differentiated carcinomas. They are highly aggressive with rapid cell turnover and high glucose utilization. Consequently, they are consistently **FDG-avid** and are better visualized with FDG-PET than with DOTATATE scans. **Clinical Pearls for NEET-PG:** 1. **The "Flip-Flop" Phenomenon:** Low-grade NETs are DOTATATE positive/FDG negative; High-grade NETs are DOTATATE negative/FDG positive. 2. **Other FDG-PET "Cold" Tumors:** Bronchoalveolar carcinoma (Adenocarcinoma in situ), Renal Cell Carcinoma (clear cell), Prostate cancer, and Mucinous tumors. 3. **Brain & Bladder:** FDG-PET has limited utility in the brain (high baseline glucose use) and urinary tract (excretion of tracer).

Q: MIBG (metaiodobenzylguanidine) is an analogue to which of the following neurotransmitters?

Norepinephrine. **Explanation:** **1. Why Norepinephrine is Correct:** MIBG (Metaiodobenzylguanidine) is a structural analogue of **Norepinephrine** (noradrenaline). It enters neuroendocrine cells via the **Type-1 energy-dependent sodium pump (Uptake-1 mechanism)**, the same transporter used by norepinephrine. Once inside the cell, it is stored in presynaptic storage vesicles. Because it mimics the distribution of sympathetic innervation, it is used to image tumors derived from the neural crest. **2. Analysis of Incorrect Options:** * **Epinephrine:** While chemically related to norepinephrine, MIBG specifically mimics the uptake and storage kinetics of norepinephrine at the sympathetic nerve endings. * **Adenine & Guanine:** These are purine nitrogenous bases involved in the structure of DNA and RNA. They have no structural or functional relationship with the catecholamine pathway or MIBG. **3. Clinical Pearls & High-Yield Facts for NEET-PG:** * **Isotopes Used:** * **I-123 MIBG:** Preferred for diagnostic imaging (better image quality, lower radiation dose). * **I-131 MIBG:** Used for both imaging and high-dose **targeted radiotherapy** (e.g., in malignant pheochromocytoma). * **Clinical Indications:** It is the gold standard for localizing **Pheochromocytoma**, **Neuroblastoma** (staging and response), and Paragangliomas. * **Patient Preparation:** It is crucial to **block the thyroid gland** with Lugol’s iodine or Potassium Iodide before the scan to prevent the uptake of free radioactive iodine. * **Drug Interactions:** Drugs like Tricyclic Antidepressants (TCAs) and Reserpine must be stopped as they interfere with the Uptake-1 mechanism, leading to false-negative results.

Q: Which of the following chemicals or isotopes is used in cardiac ventriculography?

Technetium. **Explanation:** **Cardiac Ventriculography** (specifically Radionuclide Ventriculography or MUGA scan) is a nuclear medicine study used to evaluate ventricular function, wall motion, and Ejection Fraction (EF). 1. **Why Technetium is correct:** The procedure utilizes **Technetium-99m ($^{99m}Tc$)**, usually tagged to Red Blood Cells (RBCs) or human serum albumin. Because the radioactive tracer remains within the intravascular compartment, a gamma camera can visualize the blood pool within the cardiac chambers. By synchronizing the image acquisition with an ECG (Gated study), clinicians can calculate the Left Ventricular Ejection Fraction (LVEF), which is the gold standard for monitoring cardiotoxicity (e.g., in patients on Doxorubicin). 2. **Why other options are incorrect:** * **Gallium (Ga-67):** Used primarily for imaging chronic inflammation, infections (like sarcoidosis), or certain tumors (lymphoma). It is not used for functional ventricular imaging. * **Lipodate:** An oral iodinated contrast medium formerly used for cholecystography (gallbladder imaging). It has no role in cardiac imaging. * **Diatrizoate:** A high-osmolar ionic contrast medium used in conventional X-ray angiography. While used in *invasive* catheter-based ventriculography, the term "ventriculography" in the context of isotopes specifically refers to nuclear medicine studies where Technetium is the agent of choice. **High-Yield Pearls for NEET-PG:** * **MUGA Scan:** Stands for Multi-Gated Acquisition scan. * **Best Parameter:** MUGA is considered more accurate and reproducible than Echocardiography for measuring LVEF. * **Radiopharmaceutical:** $^{99m}Tc$-pertechnetate is the most common isotope used in various scans due to its short half-life (6 hours) and optimal energy (140 keV).

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: Which isotope is used for Radioactive Iodine Uptake (RAIU) test?

I-123. The **Radioactive Iodine Uptake (RAIU)** test is a functional study used to measure the metabolic activity of the thyroid gland. It is primarily used to differentiate causes of hyperthyroidism (e.g., Graves' disease vs. Thyroiditis). ### **Explanation of Options** * **A. I-123 (Correct):** I-123 is the isotope of choice for RAIU and thyroid imaging. It is a **pure gamma emitter** with a photon energy of **159 keV**, which is ideal for detection by gamma cameras. Crucially, it has a short half-life of **13.2 hours**, resulting in a significantly lower radiation dose to the patient compared to other isotopes. * **B. I-137 (Incorrect):** This is not a standard isotope used in clinical medicine. It is a fission product with no role in thyroid diagnostics. * **C. I-125 (Incorrect):** While used in laboratory assays (like RIA) and prostate brachytherapy, it is not used for RAIU due to its long half-life (60 days) and low-energy emissions which are difficult to image. ### **Clinical Pearls for NEET-PG** * **I-131 vs. I-123:** While I-123 is preferred for **imaging/uptake** (diagnostic), **I-131** is primarily used for **therapy** (ablation of thyroid cancer or Graves') because it emits **Beta particles**, which cause local tissue destruction. I-131 has a longer half-life of **8 days**. * **Technetium-99m pertechnetate:** Can also be used for thyroid imaging (trapped but not organified), but it is not used for formal "uptake" percentage calculations. * **Contraindication:** Radioactive iodine studies are strictly contraindicated in **pregnancy** and breastfeeding. * **High Uptake:** Seen in Graves' disease and Toxic Multinodular Goiter. * **Low Uptake:** Seen in Subacute Thyroiditis, Factitious Thyrotoxicosis, and Iodine overload (Jod-Basedow effect).

Q: Radioiodine ablation is preferred in which of the following conditions?

Post-surgery papillary carcinoma. **Explanation:** The correct answer is **D. Post-surgery papillary carcinoma.** Radioiodine (I-131) ablation is a standard adjuvant therapy following total thyroidectomy in patients with differentiated thyroid cancers (DTC), such as Papillary and Follicular carcinoma. The primary goal is to destroy any residual microscopic thyroid tissue or occult metastatic foci. This procedure facilitates the use of Serum Thyroglobulin as a highly sensitive tumor marker for follow-up and improves the sensitivity of future diagnostic whole-body iodine scans. **Why other options are incorrect:** * **A. Pregnancy:** This is an **absolute contraindication**. I-131 crosses the placenta and can destroy the fetal thyroid gland, leading to irreversible cretinism and potential teratogenicity. * **B. Graves’ Disease:** While I-131 is a treatment modality for Graves’, it is generally reserved for adults who fail medical therapy (Antithyroid drugs). It is not the "preferred" first-line treatment in all demographics, especially when compared to the definitive role it plays in post-surgical cancer management. * **C. Young Patients:** In children and adolescents, physicians generally prefer medical management or surgery over radioiodine due to the theoretical long-term risk of secondary malignancies and the potential impact on gonadal function. **High-Yield Clinical Pearls for NEET-PG:** * **Isotope of Choice:** I-131 is used for therapy (Beta emitter), while I-123 or I-131 (in lower doses) is used for imaging. * **Preparation:** Patients must have high TSH levels (>30 mIU/L) to ensure maximum uptake, achieved either by thyroid hormone withdrawal or recombinant human TSH (rhTSH). * **Safety:** Patients must avoid pregnancy for at least 6–12 months following I-131 therapy. * **Contraindications:** Pregnancy, breastfeeding, and severe uncontrolled thyrotoxicosis.

Q: A bone scan in multiple myeloma typically shows which of the following?

Cold spot. **Explanation:** In **Multiple Myeloma (MM)**, the characteristic bone lesions are purely **osteolytic**. The pathophysiology involves the activation of osteoclasts by myeloma cells (via RANKL expression) and the simultaneous inhibition of osteoblasts. A standard **Technetium-99m MDP Bone Scan** relies on osteoblastic activity (bone formation) to show "hot spots." Because MM lacks a reactive osteoblastic response, the lesions do not take up the tracer. Instead, they appear as **"Cold spots"** (areas of decreased or absent uptake) or may even appear normal, leading to a high false-negative rate (approx. 30-50%). **Analysis of Options:** * **A. Hot spot:** This is the typical finding for osteoblastic or mixed lesions (e.g., Prostate cancer metastases, Paget’s disease, or healing fractures). It is rarely seen in MM unless a pathological fracture is actively healing. * **C & D. Diffuse uptake patterns:** "Diffusely increased uptake" (Super scan) is characteristic of metabolic bone diseases (Hyperparathyroidism) or widespread osteoblastic metastases. MM does not present with diffuse metabolic alterations of the entire skeleton in this manner. **High-Yield Clinical Pearls for NEET-PG:** * **Gold Standard for Imaging MM:** Whole-body Low-Dose CT (WBLDCT) or MRI is now preferred over the traditional skeletal survey. * **Bone Scan vs. Skeletal Survey:** A skeletal survey (X-ray) is more sensitive than a bone scan for MM, but even X-rays require 30-50% bone mineral loss to be visible. * **Punch-out Lesions:** The classic radiographic description of MM on X-ray. * **Exception:** If a bone scan *is* positive (hot) in a suspected MM patient, always rule out a pathological fracture.

Question 1

Which of the following is selectively filtered into the renal tubules?

Accepted Answer

DTPA

Answer

DMSA

Answer

MAG3

Answer

EC

Question 2

Fluorodeoxyglucose-positron emission tomography (FDG-PET) does not detect which of the following?

Accepted Answer

Typical carcinoid

Answer

Atypical carcinoid

Answer

Small cell neuroendocrine tumor

Answer

Large cell neuroendocrine tumor

Question 3

MIBG (metaiodobenzylguanidine) is an analogue to which of the following neurotransmitters?

Accepted Answer

Norepinephrine

Answer

Epinephrine

Answer

Adenine

Answer

Guanine

Question 4

Which of the following chemicals or isotopes is used in cardiac ventriculography?

Accepted Answer

Technetium

Answer

Gallium

Answer

Lipodate

Answer

Diatrizoate

Question 5

What isotope is commonly used in PET scans?

Accepted Answer

18-fluoro-deoxyglucose

Answer

Phosphorus-32

Answer

Iridium-77

Answer

Radium-226

Question 6

What is the most suitable radioisotope of iodine for treating hyperthyroidism?

Accepted Answer

I131

Answer

I123

Answer

I125

Answer

I132

Question 7

Which isotope is used for Radioactive Iodine Uptake (RAIU) test?

Accepted Answer

I-123

Answer

I-137

Answer

I-125

Answer

None of the above

Question 8

Radioiodine ablation is preferred in which of the following conditions?

Accepted Answer

Post-surgery papillary carcinoma

Answer

Pregnancy

Answer

Graves disease

Answer

Young patients

Question 9

A bone scan in multiple myeloma typically shows which of the following?

Accepted Answer

Cold spot

Answer

Hot spot

Answer

Diffusely increased uptake

Answer

Diffusely decreased uptake

Question 10

Distant metastasis of bone can be best detected by?

Accepted Answer

Bone scan

Answer

NMI

Answer

CT

Answer

Intravenous venogram

Nuclear Medicine — MCQs

Nuclear Medicine — MCQs

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