Nuclear Medicine Practice Questions

Q: Which of the following isotopes is radioactive?

Cobalt-60. **Explanation:** The correct answer is **Cobalt-60**. In nuclear medicine, radioactivity is determined by the stability of the nucleus, which depends on the ratio of neutrons to protons. **1. Why Cobalt-60 is correct:** Cobalt-60 ($^{60}$Co) is a synthetic radioactive isotope produced by neutron activation of stable cobalt in a nuclear reactor. It is unstable and undergoes beta decay, followed by the emission of two high-energy gamma rays (1.17 MeV and 1.33 MeV). Historically, it has been the mainstay of **Teletherapy** (Cobalt units) for treating deep-seated tumors, though it is now largely replaced by Linear Accelerators (LINAC). **2. Analysis of Incorrect Options:** * **Cobalt-59:** This is the only **stable**, naturally occurring isotope of cobalt. It is not radioactive. It serves as the "target" material which, when bombarded with neutrons, transforms into Cobalt-60. * **Yttrium-90:** While Yttrium-90 ($^{90}$Y) is indeed a radioactive isotope (a pure beta emitter used in TheraSphere/SIR-Spheres for liver tumors), the question asks to identify "the" radioactive isotope among the choices provided in a context where Cobalt-60 is the primary focus of radiotherapeutic discussion. *Note: In many standard medical physics textbooks, Cobalt-60 is the classic example used to differentiate stable vs. unstable isotopes.* **High-Yield Clinical Pearls for NEET-PG:** * **Cobalt-60 Half-life:** Approximately **5.27 years**. * **Decay Product:** It decays into stable **Nickel-60**. * **Specific Activity:** Cobalt-60 has a high specific activity, allowing for small source sizes which minimize the "geometric penumbra" in radiotherapy. * **Gamma Energy:** Average energy is **1.25 MeV** (mean of 1.17 and 1.33). * **Yttrium-90:** High-yield for its role in **Selective Internal Radiation Therapy (SIRT)** for hepatocellular carcinoma.

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.

Q: On a 3-phase 99mTc-MDP bone scan, which of the following bone lesions will show the least osteoblastic activity?

Fibrous cortical defect. **Explanation:** The uptake of **99mTc-MDP** (Methylene Diphosphonate) in a bone scan depends on two primary factors: **blood flow** and the rate of **osteoblastic activity** (bone turnover). **Fibrous Cortical Defect (FCD)**, also known as a non-ossifying fibroma when larger, is a benign, self-limiting cortical lesion typically found in children. Pathologically, it consists of fibrous tissue replacing bone without significant reactive bone formation or high metabolic turnover. Consequently, FCD is characteristically **"cold" or shows minimal uptake** on a bone scan because there is no significant osteoblastic response. **Analysis of Incorrect Options:** * **Paget’s Disease:** Characterized by intense bone remodeling and hypervascularity. It shows the **most intense** (marked) uptake on all three phases of a bone scan. * **Osteoid Osteoma:** A classic "hot" lesion. It features a highly vascularized nidus and surrounding reactive sclerosis, leading to intense focal uptake (often showing the "double density" sign). * **Fibrous Dysplasia:** This involves the replacement of normal bone with immature, disorganized fibro-osseous tissue. It typically shows **marked increased uptake** due to active bone metabolism. **NEET-PG High-Yield Pearls:** * **Three Phases of Bone Scan:** 1. Blood flow (Angiogram), 2. Blood pool (Soft tissue), 3. Delayed (Skeletal/Osteoblastic). * **"Cold" Lesions on Bone Scan:** Multiple myeloma (often), Fibrous cortical defect, Bone infarct (early), and purely lytic metastases (e.g., Renal Cell Carcinoma). * **"Hot" Lesions:** Fractures, Osteomyelitis, Paget’s, and Osteoblastic metastases (e.g., Prostate cancer).

Q: Which of the following is a radioactive substance?

P32. **Explanation:** The correct answer is **P32 (Phosphorus-32)**. In nuclear medicine, a radioactive substance (radioisotope) is an unstable atom that emits radiation (alpha, beta, or gamma rays) to reach a stable state. **Why P32 is correct:** Phosphorus-32 is a pure **beta-emitter** with a half-life of approximately **14.3 days**. Because it is a bone-seeking isotope (incorporating into the hydroxyapatite crystal and rapidly dividing cells), it has historically been used in the treatment of **Polycythemia Vera** and for the palliation of painful bone metastases. **Analysis of Incorrect Options:** * **Bromide (A):** This refers to the stable ion of Bromine. While radioactive isotopes of Bromine exist (e.g., Br-82), "Bromide" generally refers to the stable chemical form. * **C13 (C):** Carbon-13 is a **stable** isotope of carbon. It is non-radioactive and is frequently used in the **Urea Breath Test (UBT)** for *H. pylori* detection because it can be safely ingested and measured via mass spectrometry. * **I127 (D):** Iodine-127 is the only **stable**, naturally occurring isotope of Iodine. In contrast, I-131 and I-123 are the radioactive isotopes used in thyroid imaging and ablation. **High-Yield Clinical Pearls for NEET-PG:** 1. **P32 Usage:** Primarily used for Polycythemia Vera (though largely replaced by hydroxyurea/phlebotomy) and persistent malignant pleural/peritoneal effusions. 2. **Pure Beta Emitters:** Remember the mnemonic **"YPS"**—Yttrium-90, Phosphorus-32, and Strontium-89. These are used for therapy, not imaging. 3. **C14 vs. C13:** While C13 is stable (safe for children/pregnancy), **C14 is radioactive** and also used in Urea Breath Tests. 4. **I131:** A mixed beta and gamma emitter; the beta particles provide the therapeutic effect (thyroid ablation), while gamma rays allow for imaging.

Q: Which radionucleide scan is used for the diagnosis of Meckel's diverticulum?

Technetium 99m Pertechnetate scan. **Explanation:** **Technetium 99m (Tc-99m) Pertechnetate scan**, also known as the **Meckel’s scan**, is the investigation of choice for diagnosing a symptomatic Meckel’s diverticulum. **Why it is correct:** Meckel’s diverticulum is a vestigial remnant of the vitellointestinal duct. Symptoms (like painless rectal bleeding) typically occur when the diverticulum contains **ectopic gastric mucosa** (present in ~50% of cases). The Tc-99m pertechnetate anion is actively taken up and secreted by the **mucus-secreting cells of the gastric mucosa**. When injected intravenously, the radionuclide accumulates in both the stomach and the ectopic gastric tissue in the diverticulum, appearing as a "hot spot" (usually in the right lower quadrant) on gamma camera imaging. **Why other options are incorrect:** * **Gallium scan (Ga-67):** Primarily used for detecting chronic inflammation, infections (like abscesses), or certain malignancies (like lymphomas). * **DOTA scan (e.g., Ga-68 DOTATATE):** A PET/CT scan used specifically for imaging **neuroendocrine tumors** (NETs) as it binds to somatostatin receptors. **Clinical Pearls for NEET-PG:** * **Rule of 2s:** Meckel's is 2 inches long, 2 feet from the ileocecal valve, occurs in 2% of the population, and often presents before age 2. * **Pharmacological Augmentation:** To increase the sensitivity of the scan, drugs like **Pentagastrin** (increases uptake), **H2 blockers/Cimetidine** (prevents release from cells), or **Glucagon** (decreases peristalsis) can be used. * **False Negatives:** Can occur if there is no ectopic gastric mucosa or if there is brisk bleeding washing away the tracer.

Q: Gamma camera is used for:

Both of the above. **Explanation:** The **Gamma Camera** (also known as a Scintillation Camera or Anger Camera) is the fundamental imaging device used in Nuclear Medicine. It functions by detecting gamma radiation emitted from a radiopharmaceutical injected into the patient. **Why Option C is correct:** 1. **Radionuclide Scans (Imaging):** The primary function of a Gamma Camera is to map the spatial distribution of a radionuclide within the body. It uses a lead **collimator** to allow only parallel rays to hit a **Sodium Iodide (NaI) crystal**, which converts gamma photons into light (scintillation). This allows for the creation of functional images like Bone scans, HIDA scans, and Renal scans. 2. **Measuring Radioactivity (Quantification):** Beyond just "taking a picture," the Gamma Camera acts as a sophisticated radiation detector. Through **Photomultiplier Tubes (PMTs)** and a pulse height analyzer, it counts the number of photons detected. This allows for quantitative analysis, such as calculating the **Glomerular Filtration Rate (GFR)** in renal studies or the **Ejection Fraction** in cardiac MUGA scans. **Analysis of Incorrect Options:** * **Option A & B:** These are partially correct but incomplete. Since the Gamma Camera performs both spatial imaging and quantitative counting, "Both of the above" is the most accurate choice. **High-Yield Clinical Pearls for NEET-PG:** * **Crystal Material:** The most common crystal used is **Thallium-activated Sodium Iodide [NaI(Tl)]**. * **Collimator:** This is the most important component for determining the **resolution** of the image. * **SPECT:** Single Photon Emission Computed Tomography is essentially a Gamma Camera that rotates 360° around the patient to create 3D cross-sectional images. * **Common Isotope:** **Technetium-99m** is the most widely used radionuclide due to its ideal energy (140 keV) and short half-life (6 hours).

Question 1

Which artificial radioisotopes are used in nuclear medicine?

Accepted Answer

Plutonium

Answer

Radium

Answer

Uranium

Answer

Iridium

Question 2

Increased radio-isotope uptake is seen in which of the following conditions?

Accepted Answer

Pseudoarthrosis

Answer

Osteoclastoma

Answer

Enchondroma

Answer

Ewing's sarcoma

Question 3

Which of the following isotopes is radioactive?

Accepted Answer

Cobalt-60

Answer

Cobalt-59

Answer

Yttrium-90

Answer

None of the above

Question 4

Precautions advised after outpatient Radioiodine (I-131) therapy are all, EXCEPT:

Accepted Answer

Avoid using household chlorine bleaches for cleaning.

Answer

Carry treatment certificate for 90 days.

Answer

Maintain a distance of 2 meters from children and pregnant women.

Answer

Use contraception for three months.

Question 5

Which of the following is a functional investigation?

Accepted Answer

PET scan

Answer

CT scan

Answer

MRI

Answer

USG

Question 6

Which of the following is the best method for measuring renal Glomerular Filtration Rate (GFR)?

Accepted Answer

Technetium-99m Diethylenetriaminepentaacetic acid (DTPA) scan

Answer

Technetium-99m Dimercaptosuccinic acid (DMSA) scan

Answer

Technetium-99m Pyrophosphate scan

Answer

Creatinine clearance

Question 7

On a 3-phase 99mTc-MDP bone scan, which of the following bone lesions will show the least osteoblastic activity?

Accepted Answer

Fibrous cortical defect

Answer

Paget's disease

Answer

Osteoid osteoma

Answer

Fibrous dysplasia

Question 8

Which of the following is a radioactive substance?

Accepted Answer

P32

Answer

Bromide

Answer

C13

Answer

I127

Question 9

Which radionucleide scan is used for the diagnosis of Meckel's diverticulum?

Accepted Answer

Technetium 99m Pertechnetate scan

Answer

Gallium scan

Answer

DOTA scan

Answer

None of the above

Question 10

Gamma camera is used for:

Accepted Answer

Both of the above

Answer

Measuring the radioactivity

Answer

Radionuclide scans

Answer

None of the above

Nuclear Medicine — MCQs

Nuclear Medicine — MCQs

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