Nuclear Medicine Practice Questions

Q: What is the scan used to estimate collecting system dilatation?

99mTc-DTPA scan. **Explanation:** The correct answer is **99mTc-DTPA scan**. To estimate collecting system dilatation and differentiate between obstructive and non-obstructive hydronephrosis, a **dynamic renal scintigraphy (Diuretic Renogram)** is performed. **1. Why 99mTc-DTPA is correct:** 99mTc-DTPA (Diethylene Triamine Penta-acetic Acid) is an agent that is handled almost exclusively by **glomerular filtration** (not secreted or reabsorbed). Because it stays within the tubular lumen and follows the flow of urine, it is the gold standard for calculating the **Glomerular Filtration Rate (GFR)** and visualizing the drainage pattern of the collecting system. When combined with a diuretic (Furosemide), it helps determine if a dilated renal pelvis is due to mechanical obstruction or simple stasis. **2. Why other options are incorrect:** * **MAG 3 (Mercaptoacetyltriglycine):** While also used for dynamic renography, it is primarily secreted by the renal tubules. It is the agent of choice in patients with **renal failure** or pediatric cases due to better extraction, but DTPA remains the classic answer for GFR-based drainage studies in standard exams. * **DMSA (Dimercaptosuccinic Acid):** This is a **static** renal scan. It binds to the proximal convoluted tubules and is used to identify **renal scars**, ectopic kidneys, or pyelonephritis. It cannot evaluate the collecting system or drainage. * **Hippuran (I-131 OIH):** An older agent used for Effective Renal Plasma Flow (ERPF). It is rarely used now due to high radiation doses and poor imaging quality. **Clinical Pearls for NEET-PG:** * **Best agent for GFR:** 99mTc-DTPA. * **Best agent for Renal Scarring:** 99mTc-DMSA. * **Best agent for Renal Function in Neonates/Renal Failure:** 99mTc-MAG3. * **Whitaker Test:** The invasive gold standard for differentiating obstructive from non-obstructive dilatation (pressure-flow study).

Q: A hot spot on a bone scan is seen in all the following conditions except:

Fibrous cortical defect. **Explanation:** Bone scintigraphy (Bone Scan) using **99mTc-MDP** depends on two primary factors: **blood flow** and **osteoblastic activity** (bone turnover). A "hot spot" indicates increased tracer uptake due to high bone remodeling. **Why Fibrous Cortical Defect (FCD) is the correct answer:** A Fibrous Cortical Defect (and its larger counterpart, Non-Ossifying Fibroma) is a benign, **non-neoplastic** fibrous lesion typically found in the metaphysis of long bones in children. These lesions are characterized by a lack of active bone turnover or significant osteoblastic reaction. Consequently, they appear **"cold" or "quiet"** on a bone scan. If an FCD appears "hot," it usually indicates a complication like a pathological fracture. **Analysis of Incorrect Options:** * **Osteomyelitis:** Shows intense uptake (hot spot) due to increased vascularity and reactive bone formation in response to infection. * **Fibrous Dysplasia:** This condition involves the replacement of normal bone with fibro-osseous tissue. It characteristically shows **marked, intense tracer uptake** due to high metabolic activity within the lesion. * **Paget’s Disease:** Known for having the **highest tracer uptake** in nuclear medicine. The extreme osteoblastic activity and hypervascularity during the mixed and sclerotic phases result in very "hot" scans. **High-Yield Clinical Pearls for NEET-PG:** * **"Cold" Spots (Photopenia):** Think of Multiple Myeloma (often), early Infarction/Avascular Necrosis, Anaplastic tumors, or Renal Cell Carcinoma metastases. * **Super Scan:** A scan showing intense symmetrical skeletal uptake with faint or absent renal activity; commonly seen in Metastatic Prostate Cancer, Renal Osteodystrophy, and Paget’s Disease. * **Flare Phenomenon:** A temporary increase in uptake seen 3 months after successful chemotherapy, which should not be confused with disease progression.

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

Red blood cell scintigraphy. **Explanation:** The detection of gastrointestinal (GI) bleeding depends on the rate of blood loss. **Red blood cell (RBC) scintigraphy** (typically using Technetium-99m labeled RBCs) is the most sensitive imaging modality because it can detect bleeding rates as low as **0.1 mL/min**. Because the radiopharmaceutical remains in the intravascular compartment for a prolonged period, patients can be scanned repeatedly over 24 hours, making it ideal for detecting intermittent or "obscure" bleeding. **Analysis of Options:** * **Selective Angiography:** While highly specific and capable of therapeutic intervention (embolization), it is less sensitive than scintigraphy. It requires a much higher bleeding rate of **0.5 to 1.0 mL/min** to visualize the extravasation of contrast. * **I-131 Fibrinogen Studies:** This test was historically used to detect deep vein thrombosis (DVT) by incorporating into developing clots; it has no clinical role in the acute detection of GI bleeding. * **Fecal Occult Blood Testing (FOBT):** While this can detect minute amounts of blood in the stool, it is a biochemical screening tool for chronic/microscopic loss (e.g., colorectal cancer screening) and cannot localize the site or manage acute hemorrhage. **NEET-PG High-Yield Pearls:** * **Sensitivity Thresholds:** Scintigraphy (0.1 mL/min) > Angiography (0.5–1.0 mL/min). * **Technetium-99m Sulfur Colloid:** An alternative for GI bleed scans; it is cleared rapidly by the liver/spleen, providing a high target-to-background ratio, but cannot be used for delayed imaging (unlike labeled RBCs). * **Meckel’s Diverticulum:** The "Meckel’s scan" uses **Tc-99m Pertechnetate**, which is taken up by ectopic gastric mucosa, not the blood itself.

Q: What is the typical dose of I-131 given to patients with carcinoma thyroid?

100 mCi. **Explanation:** The management of Differentiated Thyroid Cancer (DTC) often involves **Radioactive Iodine (I-131) Ablation** following a total thyroidectomy. The goal is to destroy any residual thyroid tissue or occult microscopic disease. **Why 100 mCi is correct:** In nuclear medicine, doses are categorized based on their intent: diagnostic or therapeutic. For the treatment of thyroid carcinoma, **therapeutic doses** are required. A dose of **75–150 mCi** (typically 100 mCi) is the standard empirical dose used for remnant ablation. If there is known lymph node metastasis, the dose may be increased to 150 mCi, and for distant metastasis (e.g., lung or bone), it can go up to 200 mCi. **Analysis of Incorrect Options:** * **10 mCi (Option A):** This is a therapeutic dose typically used for **Graves' disease** (Hyperthyroidism), where the goal is to reduce thyroid function rather than eradicate all thyroid tissue. * **10 uCi & 100 uCi (Options C & D):** These are **diagnostic doses** (microcuries). They are used for thyroid uptake studies or diagnostic scans to visualize the gland's morphology without delivering a significant radiation burden. **High-Yield Clinical Pearls for NEET-PG:** * **Prerequisite:** Patients must have a high TSH (>30 mIU/L) before I-131 administration, achieved either by thyroid hormone withdrawal or Recombinant Human TSH (rhTSH). * **Low Iodine Diet:** Patients should follow a low iodine diet for 1–2 weeks prior to the dose to increase the "starvation" and subsequent uptake by thyroid cells. * **Contraindication:** I-131 is strictly contraindicated in **pregnancy** (Category X). * **Mechanism:** I-131 emits **Beta particles** (for therapy) and **Gamma rays** (for imaging). The destructive effect is primarily due to Beta particles.

Q: All of the following are beta emitter particles except?

Technetium 99m. ### Explanation The core concept tested here is the difference between **particulate radiation** (Beta particles) used primarily for therapy and **electromagnetic radiation** (Gamma rays) used for diagnostic imaging. **Why Technetium 99m (Tc-99m) is the correct answer:** Technetium 99m is a **pure gamma emitter**. The "m" stands for metastable, meaning it exists in an excited nuclear state. When it decays to Technetium 99, it releases energy in the form of a **140 keV gamma photon** without emitting any particulate radiation (like alpha or beta particles). This property, combined with its short half-life (6 hours), makes it the "workhorse" of diagnostic nuclear medicine (e.g., Bone scans, MUGA scans) because it provides high-quality images with low radiation dose to the patient. **Analysis of Incorrect Options (Beta Emitters):** * **Strontium 89:** A pure beta emitter used for the **palliative treatment of painful bone metastases** (e.g., from prostate cancer). * **Phosphorus 32:** A pure beta emitter historically used for treating **Polycythemia Vera** and currently used for intracavitary therapy and bone pain palliation. * **Tin 117m:** A unique radioisotope that emits **conversion electrons** (which behave like beta particles) and is used for treating bone pain. **NEET-PG High-Yield Clinical Pearls:** 1. **Pure Beta Emitters:** Remember the mnemonic **"YPS"** — **Y**ttrium-90, **P**hosphorus-32, and **S**trontium-89. These are used for therapy, not imaging. 2. **Iodine-131:** Unlike Tc-99m, I-131 emits **both** beta particles (for treating thyroid cancer/thyrotoxicosis) and gamma rays (allowing for post-therapy imaging). 3. **Alpha Emitters:** Radium-223 is a notable alpha emitter used in metastatic prostate cancer. 4. **Positron Emitters:** Used in PET scans (e.g., Fluorine-18). A positron is essentially a "positive beta particle."

Q: What is true about Hybrid PET/CT scans?

All of the above. **Explanation:** Hybrid PET/CT imaging is a revolutionary modality that combines the **functional sensitivity** of Positron Emission Tomography (PET) with the **anatomical precision** of Computed Tomography (CT). * **Functional-Anatomical Mapping (Option A & B):** PET utilizes radiopharmaceuticals like **18F-FDG** (Fluorodeoxyglucose) to detect areas of high glucose metabolism (functional data). However, PET alone has poor spatial resolution. By fusing it with CT (anatomical data), clinicians can precisely localize "hot spots" to specific organs or lymph nodes. This synergy allows for accurate staging and biopsy targeting. * **Lesion Differentiation (Option C):** PET/CT is highly sensitive in differentiating benign from malignant lesions. While the spatial resolution limit for PET is typically around **7–10 mm (approx. 1 cm)**, the metabolic signature (Standardized Uptake Value - SUV) helps distinguish malignant growths from stable benign nodules of similar size. **Why "All of the Above" is correct:** The integration of these two technologies overcomes the limitations of each individual modality, providing a comprehensive diagnostic tool that maps metabolic activity directly onto high-resolution anatomical structures. **High-Yield Clinical Pearls for NEET-PG:** * **Radiopharmaceutical:** 18F-FDG is the most common tracer; it is a glucose analog. * **Patient Preparation:** Patients must fast for 4–6 hours, and blood glucose should ideally be ** 2.5 is often suggestive of malignancy (though not definitive).

Q: Which radiotracer is used to visualize the parathyroid glands?

99mTc sestamibi. **Explanation:** **99mTc Sestamibi (Option A)** is the gold standard radiotracer for parathyroid imaging, primarily used to localize parathyroid adenomas. The underlying mechanism relies on the high concentration of **mitochondria** in parathyroid oxyphil cells. Sestamibi is a lipophilic cation that accumulates in these mitochondria. While it is initially taken up by both the thyroid and parathyroid glands, it **washes out** more rapidly from normal thyroid tissue, leaving a persistent "hot spot" in the hyperfunctioning parathyroid gland on delayed images (Dual-phase scintigraphy). **Analysis of Incorrect Options:** * **99mTc Pertechnetate (Option B):** This tracer is trapped by the thyroid gland via the sodium-iodide symporter. It is used for thyroid scintigraphy (e.g., Graves' disease) or to identify ectopic gastric mucosa (Meckel’s diverticulum), but it does not localize parathyroid tissue. * **99mTc Exametazime (HMPAO) (Option C):** This is a lipophilic agent used for **Brain SPECT** imaging to assess cerebral blood flow or for labeling leukocytes (WBC scan) to detect occult infections. * **99mTc MAG3 (Option D):** This is a renal radiopharmaceutical used for **dynamic renal scintigraphy** (Renogram) to assess renal tubular secretion and effective renal plasma flow (ERPF). **High-Yield Clinical Pearls for NEET-PG:** * **Dual-Phase Technique:** Uses Sestamibi alone; images are taken at 15 minutes (early) and 2–3 hours (delayed). * **Dual-Isotope Subtraction:** Uses Sestamibi (parathyroid + thyroid) and Pertechnetate (thyroid only). Subtracting the two leaves only the parathyroid image. * **Ectopic Glands:** Sestamibi is particularly useful for locating ectopic parathyroid glands in the mediastinum. * **Intraoperative Probe:** Sestamibi can be used with a gamma probe during surgery to guide the surgeon to the adenoma.

Q: What is the physical half-life of radioactive Iodine-131?

8 days. **Explanation:** **Iodine-131 (I-131)** is a radioisotope widely used in nuclear medicine for both diagnostic imaging and therapeutic purposes. The **physical half-life** of I-131 is approximately **8.02 days**. This duration is clinically significant as it allows the isotope to remain in the body long enough to deliver a therapeutic dose of radiation to thyroid tissue while being cleared relatively quickly to minimize long-term exposure. **Analysis of Options:** * **8 hours:** This is incorrect. However, **Technetium-99m (Tc-99m)**, the most commonly used diagnostic isotope in radiology, has a half-life of **6 hours**. * **16 days:** This is incorrect and does not correspond to a commonly tested medical isotope. * **60 days:** This is the half-life of **Iodine-125 (I-125)**, which is primarily used in brachytherapy seeds (e.g., for prostate cancer) and RIA (Radioimmunoassay) labs. **High-Yield Clinical Pearls for NEET-PG:** * **Emissions:** I-131 is a **dual emitter**. It emits **Beta particles** (responsible for the therapeutic destruction of thyroid tissue) and **Gamma rays** (allowing for scintigraphy/imaging). * **Mechanism:** It is trapped and organified by the thyroid gland via the Sodium-Iodide Symporter (NIS). * **Clinical Uses:** Treatment of Graves' disease, toxic multinodular goiter, and well-differentiated thyroid cancer (post-thyroidectomy). * **Contraindication:** I-131 is strictly **contraindicated in pregnancy** as it crosses the placenta and can destroy the fetal thyroid gland. * **I-123:** Another isotope of Iodine used for diagnostics; it has a half-life of **13 hours** and emits only gamma radiation, making it safer for simple uptake studies.

Q: Selenium methionine scan is used in the diagnosis of which condition?

Pancreatic carcinoma. **Explanation:** **Selenium-75 selenomethionine ($^{75}$Se-methionine)** is a radiopharmaceutical analog of the essential amino acid methionine. The underlying principle of this scan is that cells with high protein synthesis rates will actively uptake this amino acid analog. 1. **Why Pancreatic Carcinoma is Correct:** The pancreas is an organ with high metabolic activity and protein turnover due to the production of digestive enzymes. Pancreatic carcinoma cells exhibit significantly increased protein synthesis compared to normal tissue. Therefore, $^{75}$Se-methionine was historically used to image the pancreas. While it has largely been replaced by modern CT, MRI, and PET-CT (using $^{18}$F-FDG), it remains a classic high-yield fact in nuclear medicine exams. 2. **Analysis of Incorrect Options:** * **Parathyroid Adenoma:** The gold standard nuclear scan is the **Technetium-99m Sestamibi** (MIBI) scan. * **Bone Metastasis:** The investigation of choice is the **Technetium-99m MDP** (Methylene Diphosphonate) bone scan, which detects osteoblastic activity. * **Meckel’s Diverticulum:** This is diagnosed using the **Technetium-99m Pertechnetate** scan (also known as the "Meckel’s scan"), which is taken up by the ectopic gastric mucosa. **High-Yield Clinical Pearls for NEET-PG:** * **Hot vs. Cold:** In pancreatic imaging, a "cold" spot (photopenia) on a Selenium scan typically indicates a space-occupying lesion like a tumor or cyst. * **Other uses of Selenium-75:** It can also be used to image the parathyroid glands, but Sestamibi is far more sensitive and preferred in modern practice. * **Half-life:** $^{75}$Se has a physical half-life of approximately 118 days.

Question 1

What is the scan used to estimate collecting system dilatation?

Accepted Answer

99mTc-DTPA scan

Answer

MAG 3 scan

Answer

DMSA scan

Answer

Hippuran scan

Question 2

A hot spot on a bone scan is seen in all the following conditions except:

Accepted Answer

Fibrous cortical defect

Answer

Osteomyelitis

Answer

Fibrous dysplasia

Answer

Paget's disease

Question 3

Which radioisotope is NOT the choice for a Triple phase Bone scan?

Accepted Answer

Tc99m-Methyl diphosphonate

Answer

Tc99m-Sestamibi

Answer

Tc99m-Peectinate

Answer

Tc99m-Thallium201 subtraction

Question 4

What is the most sensitive test to detect gastrointestinal bleeding?

Accepted Answer

Red blood cell scintigraphy

Answer

Selective angiography

Answer

I 131 fibrinogen studies

Answer

Fecal occult blood testing

Question 5

What is the typical dose of I-131 given to patients with carcinoma thyroid?

Accepted Answer

100 mCi

Answer

10 mCi

Answer

10 uCi

Answer

100 uCi

Question 6

All of the following are beta emitter particles except?

Accepted Answer

Technetium 99m

Answer

Strontium 89

Answer

Phosphorus 32

Answer

Tin 117m

Question 7

What is true about Hybrid PET/CT scans?

Accepted Answer

All of the above

Answer

Functional-anatomical mapping

Answer

Provide images that help to pinpoint abnormal metabolic activity to respective anatomical structures

Answer

Can differentiate benign from malignant lesions as small as 1 cm

Question 8

Which radiotracer is used to visualize the parathyroid glands?

Accepted Answer

99mTc sestamibi

Answer

99mTc pertechnetate

Answer

99mTc exametazime

Answer

99mTc MAG3

Question 9

What is the physical half-life of radioactive Iodine-131?

Accepted Answer

8 days

Answer

8 hours

Answer

16 days

Answer

60 days

Question 10

Selenium methionine scan is used in the diagnosis of which condition?

Accepted Answer

Pancreatic carcinoma

Answer

Parathyroid adenoma

Answer

Bone metastasis

Answer

Meckel's diverticulum

Nuclear Medicine — MCQs

Nuclear Medicine — MCQs

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