Nuclear Medicine Practice Questions

Q: Which radiotracer is commonly used in PET scans?

2-Fluorodeoxyglucose. **Explanation:** **1. Why 2-Fluorodeoxyglucose (18F-FDG) is correct:** PET (Positron Emission Tomography) imaging relies on tracers that emit positrons. **18F-FDG** is a glucose analog and the most widely used radiopharmaceutical in PET imaging. It works on the principle of **increased glycolysis** (Warburg effect) in metabolically active cells, such as malignant tumors, inflammatory sites, and brain tissue. Once injected, FDG is transported into cells by GLUT transporters and phosphorylated by hexokinase into FDG-6-phosphate. Unlike normal glucose, it cannot be further metabolized and becomes "trapped" inside the cell, allowing for the visualization of metabolic activity. **2. Why the other options are incorrect:** * **Technetium (99mTc):** This is the most common tracer used in **Gamma Camera/SPECT** imaging (e.g., bone scans, DTPA scans), not PET. It emits gamma rays, not positrons. * **Chromium (51Cr):** Historically used for labeling red blood cells to estimate **RBC survival time** or sequestering studies; it is not used in PET. * **Cobalt (57Co/58Co):** Previously used in the **Schilling test** to diagnose Vitamin B12 absorption issues; it has no role in PET imaging. **3. High-Yield Clinical Pearls for NEET-PG:** * **Half-life of 18F:** Approximately **110 minutes**. * **Patient Preparation:** Patients must fast for 4–6 hours; blood glucose should ideally be **<150–200 mg/dL** to prevent competition between endogenous glucose and the tracer. * **Physiological Uptake:** Normal "hot spots" include the brain (high glucose demand), heart, kidneys, and urinary bladder (excretion route). * **Brown Fat:** Can cause false positives; prevented by keeping the patient warm or using beta-blockers.

Q: On radionuclide scanning, a toxic adenoma typically appears as which of the following?

Hot nodule. **Explanation:** In radionuclide thyroid scanning (using **Technetium-99m pertechnetate** or **Iodine-123**), the appearance of a nodule depends on its functional activity compared to the surrounding normal thyroid tissue. **Why "Hot Nodule" is correct:** A **toxic adenoma** is a hyperfunctioning, autonomous nodule. It produces thyroid hormones independently of TSH control. This increased metabolic activity leads to an increased uptake of the radionuclide, appearing as an area of intense radioactivity (a **"Hot Nodule"**). Because the high levels of thyroid hormone suppress TSH secretion via negative feedback, the rest of the normal thyroid gland becomes inactive and often does not visualize on the scan (the "suppressed gland" sign). **Analysis of Incorrect Options:** * **Cold Nodule:** These are non-functional areas with less uptake than the surrounding tissue. While 85% of thyroid nodules are cold, they carry a higher risk of malignancy (approx. 15-20%). Toxic adenomas are almost never malignant. * **Warm Nodule:** These have uptake similar to the surrounding normal thyroid tissue. They are usually benign and represent functioning nodules that are not yet autonomous enough to suppress the rest of the gland. * **Neutral:** This is not a standard term used in radionuclide imaging to describe focal thyroid lesions. **High-Yield Clinical Pearls for NEET-PG:** * **Plummer’s Disease:** This refers to a Toxic Multinodular Goiter, which shows multiple "hot" areas on a scan. * **Marine-Lenhart Syndrome:** A rare condition where Graves' disease coexists with functioning (hot) nodules. * **Rule of Thumb:** "Hot" nodules are rarely malignant (<1%), whereas "Cold" nodules require further evaluation via FNAC to rule out carcinoma. * **Drug Interference:** Patients must stop taking antithyroid drugs or exogenous iodine (e.g., amiodarone, contrast) before the scan to avoid false results.

Q: What is the radioisotope of choice for a triple-phase bone scan?

Tc99m-Methylene diphosphonate. **Explanation:** **Tc99m-Methylene Diphosphonate (MDP)** is the radioisotope of choice for a triple-phase bone scan because it is a **phosphonate analogue** that localizes to the bone via **chemisorption**. It binds to the hydroxyapatite crystals in the bone matrix, specifically in areas of high osteoblastic activity and increased vascularity. **Why the other options are incorrect:** * **Tc99m-Sestamibi:** Primarily used for myocardial perfusion imaging (MPI) and parathyroid adenoma localization. It is a lipophilic cation that accumulates in mitochondria. * **Tc99m-Pertechnetate:** Used for thyroid imaging, Meckel’s diverticulum scans, and salivary gland imaging. It behaves similarly to iodine but is not organified. * **Tl201-Thallium:** A potassium analogue used for myocardial viability studies and sometimes in tumor imaging (e.g., differentiating lymphoma from toxoplasmosis in the brain). **Triple-Phase Bone Scan Phases:** 1. **Flow Phase (Blood Flow):** Images taken every 2-3 seconds for 1 minute; assesses arterial supply. 2. **Blood Pool Phase:** Images taken at 5 minutes; assesses tissue vascularity/inflammation. 3. **Delayed Phase (Skeletal):** Images taken at 2-4 hours; assesses osteoblastic activity. **High-Yield Clinical Pearls for NEET-PG:** * **Hot Spots:** Seen in fractures, metastases (osteoblastic), and osteomyelitis. * **Cold Spots:** Seen in early avascular necrosis (AVN), multiple myeloma, and some aggressive lytic metastases (e.g., Renal Cell Carcinoma). * **Osteomyelitis vs. Cellulitis:** In osteomyelitis, all three phases are "positive" (increased uptake). In cellulitis, only the first two phases show increased uptake, while the delayed phase is normal or shows diffuse, non-focal uptake. * **Excretion:** Tc99m-MDP is excreted by the kidneys; hence, the bladder is the critical organ (receives the highest radiation dose).

Q: Which imaging modality is primarily used for the calculation of ejection fraction?

Multiple Gated Acquisition (MUGA) scan. **Explanation:** The **Multiple Gated Acquisition (MUGA) scan**, also known as Equilibrium Radionuclide Angiocardiography (ERNA), is considered the "gold standard" in nuclear medicine for the accurate and reproducible calculation of the **Left Ventricular Ejection Fraction (LVEF)**. **Why MUGA is correct:** The procedure involves labeling the patient's red blood cells with **Technetium-99m (Tc-99m)**. An ECG is used to "gate" the acquisition, synchronizing the gamma camera images with the cardiac cycle. By measuring the radioactivity (counts) at end-diastole and end-systole, the computer calculates the EF based on volume changes rather than geometric assumptions. This makes it highly accurate, especially in patients undergoing cardiotoxic chemotherapy (e.g., Doxorubicin). **Why other options are incorrect:** * **B & D (Thallium-201 and Sestamibi):** These are primarily **Myocardial Perfusion Imaging (MPI)** agents. While modern "Gated SPECT" can estimate EF, their primary clinical utility is to detect ischemia (reversible defects) or infarction (fixed defects). * **C (PET Imaging):** PET (using Rubidium-82 or N-13 Ammonia) is the gold standard for **myocardial viability** and absolute blood flow quantification, but it is not the primary or routine modality for simple EF calculation due to high cost and limited availability. **Clinical Pearls for NEET-PG:** * **Radiopharmaceutical:** Tc-99m labeled RBCs (using the *in vivo*, *in vitro*, or modified method). * **Key Advantage:** MUGA is less operator-dependent than Echocardiography and more accurate in patients with irregular body habitus or lung disease. * **High-Yield Fact:** If a patient’s LVEF drops by >10% during chemotherapy (as measured by MUGA), the oncologist may need to stop or adjust the cardiotoxic drug.

Q: To detect gastrointestinal protein loss, all are used except?

99mTc glycoheptone. **Explanation:** Protein-losing enteropathy (PLE) is characterized by the excessive loss of serum proteins into the gastrointestinal tract. To diagnose this, radiopharmaceuticals must be used that remain within the intravascular space and only appear in the bowel if there is an active leak. **Why 99mTc glycoheptone is the correct answer:** **99mTc glycoheptone** is primarily used in **renal imaging** (specifically for evaluating renal cortical morphology and function). It is rapidly cleared by the kidneys and does not bind to serum proteins in a way that allows for the detection of enteric protein loss. Therefore, it has no role in the diagnosis of PLE. **Analysis of other options:** * **Indium-111 transferrin:** This is a gold-standard radiopharmaceutical for PLE. Transferrin is a large serum protein; when labeled with Indium-111, its presence in the stool or abdominal scans confirms protein leakage. * **α-1 antitrypsin:** While not a radiopharmaceutical, the **fecal α-1 antitrypsin clearance** test is a common biochemical method used to detect and quantify GI protein loss because this protein is resistant to degradation by digestive enzymes. * **99mTc dextran:** This is a macromolecule that mimics the behavior of serum proteins. It is used in scintigraphy to localize the site of protein loss due to its size and intravascular retention. **Clinical Pearls for NEET-PG:** * **99mTc-labeled Human Serum Albumin (HSA)** is another frequently used agent for PLE scintigraphy. * **Chromium-51 (51Cr) chloride** was historically the most common agent but is less used now due to radiation safety and the requirement for 24–96 hour stool collection. * **Scintigraphy Advantage:** Unlike biochemical tests (like α-1 antitrypsin), nuclear medicine scans can **localize** the specific site of the protein leak in the gut.

Q: Which of the following isotopes is predominantly a beta emitter?

32 Phosphorus. **Explanation:** **32 Phosphorus ($^{32}$P)** is a pure **beta ($\beta^-$) emitter**. In nuclear medicine, beta emitters are primarily used for **therapeutic purposes** because beta particles have high linear energy transfer (LET) and short tissue penetration (a few millimeters), allowing for localized tissue destruction without significant damage to distant organs. **Analysis of Options:** * **32 Phosphorus (Correct):** It emits high-energy beta particles. Clinically, it was historically used for treating Polycythemia Vera and is still used for intracavitary treatment of malignant effusions and certain bone metastases. * **125 Iodine:** This isotope decays via **electron capture** and emits low-energy **gamma rays** and Auger electrons. It is commonly used in prostate brachytherapy and radioimmunoassays. * **51 Chromium:** It decays by **electron capture** and emits **gamma rays**. Its primary clinical use is the labeling of Red Blood Cells (RBCs) to determine RBC volume or survival time. * **99m Technetium:** This is a **pure gamma emitter** (isomeric transition). Due to its ideal half-life (6 hours) and energy (140 keV), it is the most widely used radiopharmaceutical for **diagnostic imaging** (scintigraphy). **High-Yield Clinical Pearls for NEET-PG:** * **Pure Beta Emitters:** Remember the mnemonic **"YPS"** — **Y**ttrium-90, **P**hosphorus-32, and **S**trontium-89. These are used for therapy, not imaging. * **Diagnostic vs. Therapeutic:** Diagnostic isotopes are usually Gamma emitters (for detection by Gamma cameras), while Therapeutic isotopes are usually Beta or Alpha emitters (for cell killing). * **131 Iodine:** Unlike $^{125}$I, $^{131}$I is a **mixed beta and gamma emitter**, making it useful for both treating thyroid cancer (beta) and imaging (gamma).

Q: What is the recommended dose of I-131 for carcinoma of the thyroid?

Both A and B. **Explanation:** The management of differentiated thyroid carcinoma (DTC) often involves **Radioactive Iodine (RAI) therapy** using I-131 following a total thyroidectomy. The dosage is stratified based on the patient's risk of recurrence and the presence of metastases. 1. **Low-Risk Patients (Option A):** For patients with small, localized tumors and no evidence of extrathyroidal extension or lymph node involvement, the goal is **remnant ablation**. Lower doses ranging from **30 to 100 mCi** are sufficient to destroy any remaining normal thyroid tissue, facilitating future monitoring with Thyroglobulin levels. 2. **High-Risk Patients (Option B):** For patients with gross extrathyroidal extension, incomplete tumor resection, or distant metastases (e.g., lung or bone), higher doses are required for **adjuvant therapy or treatment**. Doses typically range from **100 to 200 mCi** to ensure the destruction of microscopic or macroscopic residual disease. **Why Option C is correct:** Since both A and B represent the standard clinical protocols for different risk categories, "Both A and B" is the most accurate answer. **Why Option D is incorrect:** While 50 mCi falls within the low-risk range, it is too specific and fails to account for the higher doses required in advanced or metastatic cases. **High-Yield Clinical Pearls for NEET-PG:** * **Preparation:** Patients must have a high TSH (>30 mIU/L) before therapy, achieved either by thyroid hormone withdrawal (4 weeks) or recombinant human TSH (rhTSH). * **Diet:** A low-iodine diet is recommended for 1–2 weeks prior to treatment to increase I-131 uptake. * **Contraindication:** Pregnancy and breastfeeding are absolute contraindications for I-131 therapy. * **Side Effects:** Acute sialadenitis (salivary gland swelling) and bone marrow suppression (at very high doses) are important complications to remember.

Q: What is the most sensitive test to detect gastrointestinal bleeding?

Red blood cell (RBC) scintigraphy. **Explanation:** The detection of gastrointestinal (GI) bleeding depends on the rate of blood loss. **Red blood cell (RBC) scintigraphy (Technetium-99m labeled RBC scan)** is the most sensitive test because it can detect bleeding at rates as low as **0.1 mL/min**. 1. **Why RBC Scintigraphy is Correct:** This nuclear medicine study involves labeling the patient's own RBCs with Tc-99m and re-injecting them. Because the tracer remains in the intravascular compartment for a prolonged period (up to 24 hours), it allows for repeated imaging. This makes it highly sensitive for detecting **intermittent or slow bleeding** that other modalities might miss. 2. **Why Other Options are Incorrect:** * **Selective Angiography:** While highly specific and capable of therapeutic intervention (embolization), it is less sensitive than a tagged RBC scan. It requires a much higher bleeding rate of **0.5–1.0 mL/min** to visualize the extravasation of contrast. * **Fibrinogen Studies:** These are used to assess coagulation status or fibrinogen levels (e.g., in DIC) but have no role in localizing or detecting the site of active GI bleeding. * **Stool for Occult Blood (Guaiac/FIT):** While these can detect microscopic blood, they are screening tools for chronic blood loss (e.g., colorectal cancer) and are not used to localize or quantify active, acute GI hemorrhage. **High-Yield Clinical Pearls for NEET-PG:** * **Most sensitive test:** RBC Scintigraphy (0.1 mL/min). * **Minimum rate for Angiography:** 0.5 mL/min. * **Gold standard for localization/initial management:** Endoscopy (Upper GI/Colonoscopy). * **Provocative Angiography:** May be used if the RBC scan is positive but standard angiography is negative.

Question 1

Which radiotracer is commonly used in PET scans?

Accepted Answer

2-Fluorodeoxyglucose

Answer

Technetium

Answer

Chromium

Answer

Cobalt

Question 2

On radionuclide scanning, a toxic adenoma typically appears as which of the following?

Accepted Answer

Hot nodule

Answer

Cold nodule

Answer

Warm nodule

Answer

Neutral

Question 3

What is the radioisotope of choice for a triple-phase bone scan?

Accepted Answer

Tc99m-Methylene diphosphonate

Answer

Tc99m-Sestamibi

Answer

Tc99m-Pertechnetate

Answer

Tc201-Thallium subtraction

Question 4

Which imaging modality is primarily used for the calculation of ejection fraction?

Accepted Answer

Multiple Gated Acquisition (MUGA) scan

Answer

Single-Photon Emission Computed Tomography (SPECT) using thallium 201

Answer

Positron Emission Tomography (PET) myocardial perfusion imaging

Answer

Sestamibi scan with pharmacological stress

Question 5

A young female with a history of renal calculi complains of bone pain and abdominal cramps. Investigations reveal multiple fractures and elevated serum calcium and parathyroid hormone (PTH) levels. Which of the following is the best investigation to arrive at a definitive diagnosis?

Accepted Answer

Sestamibi scan

Answer

CECT neck

Answer

Radioiodine scan

Answer

Ultrasound neck

Question 6

To detect gastrointestinal protein loss, all are used except?

Accepted Answer

99mTc glycoheptone

Answer

Indium-111 transferrin

Answer

α-1 antitrypsin

Answer

99mTc dextran

Question 7

Which of the following isotopes is predominantly a beta emitter?

Accepted Answer

32 Phosphorus

Answer

125 Iodine

Answer

51 Chromium

Answer

99m Technetium

Question 8

What is the recommended dose of I-131 for carcinoma of the thyroid?

Accepted Answer

Both A and B

Answer

30-100 mCi in low-risk patients

Answer

100-200 mCi in high-risk patients

Answer

50 mCi

Question 9

Which among the following is/are pure beta emitter(s)?

Accepted Answer

Phosphorus-32

Answer

I-131

Answer

Samarium

Answer

Radium-226

Question 10

What is the most sensitive test to detect gastrointestinal bleeding?

Accepted Answer

Red blood cell (RBC) scintigraphy

Answer

Selective angiography

Answer

Fibrinogen studies

Answer

Stool for occult blood

Nuclear Medicine — MCQs

Nuclear Medicine — MCQs

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