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30 free Nuclear Medicine questions
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Instrumentation
Which detector type is most commonly used in a gamma camera?
Correct — B. NaI(Tl) crystals are the standard detector in Anger gamma cameras because of their high light output at 140 keV (the primary energy used in nuclear medicine) and well-established manufacturing. Semiconductor detectors like CZT are used in dedicated cardiac cameras but not the primary gamma camera platform. -
Instrumentation
A parallel-hole collimator is used for a bone scan. What is the primary advantage of this collimator type?
Correct — A. Parallel-hole collimators produce an image that is the same size as the object regardless of distance, making them ideal for whole-body imaging. Converging collimators magnify, diverging collimators minify, and pinhole collimators magnify only small fields of view. -
Instrumentation
A NaI(Tl) gamma camera has a measured photopeak FWHM of 21 keV at 140 keV. What is the percent energy resolution?
Correct — C. Percent energy resolution = (FWHM / photopeak energy) × 100 = (21 / 140) × 100 = 15%. Typical NaI(Tl) cameras achieve 9–12% at 140 keV; 15% indicates some degradation but is still within broad acceptance limits. -
Instrumentation
Intrinsic spatial resolution of a gamma camera is measured WITHOUT the collimator. Which factor most directly degrades intrinsic resolution?
Correct — D. Intrinsic resolution is limited primarily by the statistical spread of scintillation photons within the NaI crystal, which creates uncertainty in locating the interaction. PMT diameter and electronics also contribute, but the photon statistics in the crystal are the dominant factor for intrinsic resolution. -
Instrumentation
For imaging high-energy photons (e.g., 364 keV from I-131), which collimator is required?
Correct — B. I-131 emits 364 keV photons, which require thick septa to prevent penetration. High-energy general purpose (HEGP) collimators have thick lead septa designed for photon energies above ~300 keV. LEHR is for Tc-99m (140 keV) only. -
Instrumentation
What is the primary function of the photomultiplier tube (PMT) in a gamma camera?
Correct — A. PMTs detect the faint light (scintillation photons) produced when gamma rays interact in the NaI(Tl) crystal and amplify that light via successive dynode stages into a measurable electrical pulse. Position determination uses the relative outputs of multiple PMTs (Anger logic). -
Instrumentation
In the Anger position logic circuit, how is the X, Y position of a gamma-ray interaction determined?
Correct — C. Anger logic uses a resistor matrix (or digital equivalent) to compute weighted centroid positions. The X and Y coordinates are calculated from the fraction of light detected by each PMT relative to the total (Z-pulse), giving the centroid of the scintillation event. -
Instrumentation
What is the purpose of the pulse height analyzer (PHA) in a gamma camera?
Correct — D. The PHA discriminates between pulses based on their amplitude (proportional to photon energy). By setting a window around the photopeak (e.g., ±10% of 140 keV), scatter photons with lower energy are rejected, improving image contrast. -
Instrumentation
A gamma camera's observed count rate is 80,000 cps when the true count rate is 100,000 cps. What is the system dead time (τ) assuming a paralyzable model is NOT applicable and using the non-paralyzable formula n = m/(1 − mτ)?
Correct — B. Non-paralyzable: n = m / (1 − mτ), so τ = (1 − m/n) / m = (1 − 80000/100000) / 80000 = 0.20 / 80000 = 2.5 × 10⁻⁶ s = 2.5 µs. -
Instrumentation
SPECT imaging acquires data by rotating the gamma camera detector around the patient. What is the primary benefit over planar scintigraphy?
Correct — A. SPECT reconstructs true 3-D cross-sectional images, dramatically improving contrast-to-noise for small or deep lesions compared to planar projection imaging. Attenuation artifacts still exist in SPECT (corrected separately); sensitivity is not increased by removing the collimator. -
Instrumentation
Which reconstruction algorithm is most commonly used for clinical SPECT and incorporates noise suppression via a filter?
Correct — C. Filtered back-projection applies a ramp filter (removes star artifact) combined with a smoothing window (e.g., Butterworth or Hann) to suppress noise. While OSEM/MLEM are increasingly used, FBP with filtering remains the classic standard reconstruction in SPECT. -
Instrumentation
In OSEM (Ordered Subsets Expectation Maximization) reconstruction, one 'iteration' of OSEM with 8 subsets is equivalent to how many iterations of the standard MLEM algorithm?
Correct — D. Each OSEM iteration processes all subsets sequentially; with S subsets, one OSEM iteration approximates S MLEM iterations. Therefore 1 OSEM iteration with 8 subsets ≈ 8 MLEM iterations, making OSEM ~8× faster to converge. -
Instrumentation
Which method is the gold standard for attenuation correction in clinical SPECT/CT?
Correct — B. CT-based attenuation correction converts Hounsfield units to linear attenuation coefficients at the imaging energy, providing patient-specific, high-resolution maps. This has replaced external transmission sources in modern SPECT/CT systems. -
Instrumentation
PET imaging detects pairs of 511 keV photons. What physical process produces these photons?
Correct — A. When a positron emitted from a PET radionuclide travels a short distance and annihilates with an electron, the combined mass is converted into two 511 keV photons emitted 180° apart. PET cameras detect these coincident photons to localize the event. -
Instrumentation
LSO (lutetium oxyorthosilicate) is preferred over BGO in modern PET scanners primarily because of its:
Correct — D. LSO has a decay time of ~40 ns vs. BGO's ~300 ns. The fast decay enables time-of-flight (TOF) PET, where the difference in photon arrival times localizes the annihilation event more precisely along the line of response, improving image signal-to-noise ratio. -
Instrumentation
Which scanner geometry parameter most increases PET system sensitivity?
Correct — C. Sensitivity scales with the solid angle subtended by the detector, which increases with axial FOV (longer detector rings). Widening the energy window accepts more scatter; widening the time window increases randoms. Axial FOV extension is the dominant geometry-based sensitivity gain. -
Instrumentation
In PET, 'random coincidences' arise from two unrelated annihilation events detected within the coincidence time window. How are randoms typically corrected?
Correct — B. The delayed coincidence method introduces a time delay (e.g., 128 ns) in one detector channel so that any coincidences measured must be random (true coincidences cannot occur across this delay). These randoms-only counts are subtracted from the prompt channel. -
Instrumentation
For a PET scanner with a singles rate of S = 2 × 10⁶ cps per detector and a coincidence time window of 2τ = 10 ns, what is the approximate random coincidence rate R per detector pair?
Correct — A. R = 2τ × S₁ × S₂ = 10 × 10⁻⁹ × (2 × 10⁶)² = 10 × 10⁻⁹ × 4 × 10¹² = 0.04 cps per detector pair. Random rates increase with the square of singles rate and are minimized by narrow time windows. -
Instrumentation
Daily quality control of a gamma camera should include which test?
Correct — C. NEMA and ACR standards specify daily peaking (verifying the photopeak window is correctly set) and flood-field uniformity (checking detector response homogeneity) as the minimum daily QC for gamma cameras. -
Instrumentation
A gamma camera flood uniformity image shows a 'ring' artifact on SPECT but not on planar images. The most likely cause is:
Correct — D. COR error causes each point source to be back-projected in a small circle rather than a point, producing ring or halo artifacts in reconstructed SPECT images. These rings are not visible in planar images because back-projection is not applied there. -
Instrumentation
Integral uniformity on a gamma camera flood image is calculated as:
Correct — B. NEMA defines integral uniformity as [(Max − Min) / (Max + Min)] × 100%. This normalizes the range to the sum, giving a value between 0–100% that is independent of absolute count level. Acceptable values for clinical cameras are typically <5%. -
Instrumentation
Daily QC for a PET/CT scanner most critically involves:
Correct — A. Daily PET QC includes a blank scan (detects broken crystals/PMTs and provides normalization data) and verification that scanner calibration matches the dose calibrator (cross-calibration). Co-57 floods and collimator checks are gamma camera procedures. -
Instrumentation
What type of detector is used inside a dose calibrator?
Correct — B. Dose calibrators contain a pressurized (argon or nitrogen) ionization chamber that collects the ion current produced by radiation. The high pressure increases sensitivity over air-filled chambers, allowing accurate measurement of clinical radiopharmaceutical activities. -
Instrumentation
NRC regulations require dose calibrator constancy checks to be performed:
Correct — C. 10 CFR 35.60 requires constancy to be measured daily (before the first measurement of patient doses) using a sealed long-lived standard (e.g., Cs-137 or Co-57). Accuracy tests use at least two reference sources annually. -
Instrumentation
A Geiger-Müller (GM) survey meter is most appropriate for:
Correct — A. GM tubes are extremely sensitive to ionizing radiation (useful for detecting low-level contamination) but saturate at high dose rates and cannot measure energy. Ionization chamber-based survey meters (cutie pie) are used for accurate dose-rate measurements. -
Instrumentation
An ionization chamber survey meter (cutie pie) has a scale reading of 4.5 mR/h with a calibration factor of 0.95. What is the corrected dose rate?
Correct — B. Corrected dose rate = Scale reading × Calibration factor = 4.5 × 0.95 = 4.275 mR/h ≈ 4.28 mR/h. Calibration factors adjust for instrument over- or under-response relative to an NIST-traceable standard. -
Instrumentation
A scintillation well counter is used to measure a thyroid uptake standard. If the sample is placed directly at the mouth of the well instead of fully inside it, the measured count rate will:
Correct — D. Well counter sensitivity relies on the sample being surrounded by the crystal (near 4π geometry). Positioning the sample at the mouth reduces solid-angle coverage dramatically, lowering count rate by up to 50% or more. Reproducible, consistent geometry is essential for well counter measurements. -
Instrumentation
What collimator is typically used on a thyroid uptake probe?
Correct — C. Thyroid uptake probes use a flat-field (or open-face) collimator with a single bore designed to view the thyroid from ~30 cm distance. This geometry accepts photons from the entire gland without the complex hole patterns of imaging collimators. -
Instrumentation
A 24-hour thyroid uptake calculation is: (Neck CPM − Background CPM) / (Standard CPM − Background CPM) × 100%. The neck counts 12,500 cpm, thigh (background) 500 cpm, standard 50,000 cpm, standard background 500 cpm. What is the uptake?
Correct — A. Uptake = (12500 − 500) / (50000 − 500) × 100 = 12000 / 49500 × 100 = 24.24% ≈ 24.5% (rounding to 1 decimal). The thigh measurement estimates tissue background contribution not from thyroid uptake. -
Instrumentation
As source-to-collimator distance increases for a parallel-hole collimator, spatial resolution:
Correct — B. Geometric resolution for a parallel-hole collimator = d(L + b)/L, where d = hole diameter, L = hole length, b = source-to-face distance. As b increases, geometric resolution degrades. Total system resolution combines intrinsic and geometric components in quadrature.
Nuclear Medicine sample questions
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Instrumentation Before performing SPECT acquisitions, the center-of-rotation (COR) offset is verified to be within acceptable limits. What clinical consequence occurs if COR is significantly off?
A. Ring or halo artifacts in reconstructed transaxial images ✓
B. Incorrect photopeak causing high scatter fraction
C. Loss of planar uniformity on flood images
D. Incorrect SUV calculations on co-registered CT
Correct — A. A COR offset means the mechanical rotation axis does not align with the assumed electronic center. Back-projection of each projection arc creates ring/donut artifacts in transaxial slices. This is a SPECT-specific artifact not seen in planar imaging.
Patient Care What is the minimum number of patient identifiers required before administering a radiopharmaceutical?
A. One (name only)
B. Two (e.g., name and date of birth) ✓
C. Three (name, DOB, and MRN)
D. Four (name, DOB, MRN, and address)
Correct — B. The Joint Commission and SNMMI standards require at least two unique patient identifiers (such as full name and date of birth) before any procedure to prevent wrong-patient errors.
Radiation Physics The nucleus of an atom contains which two types of particles?
A. Electrons and neutrons
B. Protons and neutrons ✓
C. Protons and electrons
D. Electrons and positrons
Correct — B. The nucleus is composed of protons (positively charged) and neutrons (no charge). Electrons orbit the nucleus in shells outside the nucleus.
Imaging Procedures What is the most commonly used radiopharmaceutical for bone scintigraphy?
A. Tc-99m MDP ✓
B. Tc-99m HMPAO
C. Tc-99m MAA
D. Tc-99m DTPA
Correct — A. Tc-99m MDP (methylene diphosphonate) is the standard radiopharmaceutical for bone scintigraphy because it binds to hydroxyapatite in bone matrix proportional to osteoblastic activity and blood flow. The others are used for brain perfusion, lung perfusion, and renal/GFR studies, respectively.
Quality Control How often must a dose calibrator's accuracy be checked according to NRC regulations?
A. At installation, following repair, and at least annually ✓
B. Daily before each use
C. Weekly on a rotating schedule
D. Every 6 months only
Correct — A. NRC 10 CFR 35.60 requires dose calibrator accuracy checks at installation, following repair or adjustment, and at least annually using at least two different reference standard sources.
Nuclear Medicine Before using a Geiger-Mueller (GM) survey meter in the clinic, what is the first quality control step a technologist should perform?
A. Check the battery and perform the response check with the built-in source ✓
B. Wipe the detector probe with 70% isopropyl alcohol
C. Zero the analog needle against a lead shield
D. Perform a linearity test with a certified reference standard
Correct — A. Daily pre-use QC for GM meters includes verifying adequate battery status and confirming instrument response using the built-in check source, ensuring the meter is functional before conducting area surveys.
Radiation Safety Which SI unit is used to express absorbed radiation dose?
A. Gray (Gy) ✓
B. Sievert (Sv)
C. Becquerel (Bq)
D. Curie (Ci)
Correct — A. The Gray (Gy) is the SI unit of absorbed dose, equal to 1 joule of energy deposited per kilogram of tissue. The Sievert measures effective dose (biological effect), while Becquerel and Curie measure radioactivity.
Radiopharmacy Which radionuclide is produced by neutron activation in a nuclear reactor and commonly used in bone imaging?
A. Molybdenum-99 ✓
B. Technetium-99m
C. Fluorine-18
D. Gallium-67
Correct — A. Molybdenum-99 is produced by neutron bombardment of U-235 in a nuclear reactor via fission, or by neutron activation of Mo-98. It is the parent radionuclide for Tc-99m generators used in bone and other nuclear medicine studies.
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Safe and effective care environment
Health promotion and maintenance
Psychosocial and physiological integrity
Pharmacology, infection control and patient safety
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