Nuclear Medicine Exam Questions & Answers 2026 (11–20)

Question 1

Which reconstruction algorithm is most commonly used for clinical SPECT and incorporates noise suppression via a filter?

TheoryPractice · Accepted Answer

Filtered back-projection (FBP). 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.

Question 2

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?

TheoryPractice · Accepted Answer

8. 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.

Question 3

Which method is the gold standard for attenuation correction in clinical SPECT/CT?

TheoryPractice · Accepted Answer

CT-based attenuation correction using Hounsfield unit conversion. 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.

Question 4

PET imaging detects pairs of 511 keV photons. What physical process produces these photons?

TheoryPractice · Accepted Answer

Positron-electron annihilation. 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.

Question 5

LSO (lutetium oxyorthosilicate) is preferred over BGO in modern PET scanners primarily because of its:

TheoryPractice · Accepted Answer

Much shorter scintillation decay time enabling time-of-flight capability. 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.

Question 6

Which scanner geometry parameter most increases PET system sensitivity?

TheoryPractice · Accepted Answer

Increasing axial field of view (longer detector rings). 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.

Question 7

In PET, 'random coincidences' arise from two unrelated annihilation events detected within the coincidence time window. How are randoms typically corrected?

TheoryPractice · Accepted Answer

Subtracted using a delayed coincidence channel. 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.

Question 8

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?

TheoryPractice · Accepted Answer

0.04 cps. 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.

Question 9

Daily quality control of a gamma camera should include which test?

TheoryPractice · Accepted Answer

Peaking (energy calibration) and flood uniformity. 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.

Question 10

A gamma camera flood uniformity image shows a 'ring' artifact on SPECT but not on planar images. The most likely cause is:

TheoryPractice · Accepted Answer

Incorrect center-of-rotation (COR) offset. 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.

Nuclear Medicine Exam Questions & Answers 2026 (11–20)

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