ABSNM NMPI cheat sheet

Ohm's law: V = IR; power P = VI = I²R = V²/R.

Pulse height is proportional to the charge collected at the preamplifier input: V = Q/C.

Preamplifier converts charge to voltage; amplifier shapes and amplifies pulses; pulse height analyzer sorts by amplitude.

Resistor–capacitor (RC) circuit time constant τ = RC; a pulse decays to 37% after one τ, 50% after 0.693τ.

Paralyzable dead time: observed rate m = n/(1 + nτ); nonparalyzable: m = n·e^(−nτ) for random events.

Photomultiplier tube gain G = δ^n, where δ is dynode multiplication factor and n is number of dynodes.

Energy resolution (FWHM) improves as the square root of the number of photoelectrons collected.

Coincidence resolving time 2τ must be shorter than the time between unrelated events to reject randoms; randoms rate ≈ 2τ·R1·R2.

NaI(Tl) has high light output and good efficiency for 140 keV gamma rays; resolution ~6–7% FWHM at 140 keV.

Gas-filled detectors operate in ionization, proportional, or Geiger–Müller regions depending on applied voltage.

Well counter: NaI(Tl) well crystal with high geometric efficiency for small samples; used for blood/urine counting.

Dose calibrator: reentrant ionization chamber; measures activity in becquerels or curies; daily constancy check required.

Geiger–Müller counter has a thick dead layer and low efficiency for gamma rays; used for contamination surveys.

Energy spectrum: photopeak, Compton continuum, and backscatter peak; photopeak centroid identifies the radionuclide.

Dead-time loss can be corrected using the paralyzable or nonparalyzable model; observed count rate must be corrected to true rate.

Survey meter choice depends on radiation type: GM for beta/gamma contamination, ion chamber for dose rate, scintillation for low-level gamma.

NaI(Tl) crystal thickness typically 3/8 inch (9.5 mm) for 140 keV; thicker crystals improve high-energy efficiency but degrade spatial resolution.

Photomultiplier tubes are optically coupled to the crystal; Anger logic computes event position from weighted PMT signals.

Collimator hole geometry determines sensitivity and resolution: sensitivity ∝ hole diameter² / length²; resolution improves with longer, smaller holes.

Parallel-hole collimator: image size equals object size; pinhole collimator magnifies and is used for small organs (thyroid).

Energy window typically 20% centered on the photopeak (e.g., 126–154 keV for Tc-99m 140 keV).

Uniformity correction maps are acquired with a flood source and applied to subsequent images.

Spatial resolution (FWHM) of a gamma camera is typically 7–10 mm at 10 cm from the collimator.

Septal penetration occurs when photons pass through collimator septa; septal thickness must be sufficient for the photon energy.

Matrix size and pixel size: pixel size = field of view / matrix dimension; smaller pixels improve sampling but reduce counts per pixel.

Photopeak window width affects sensitivity and scatter inclusion; narrower windows reduce scatter but increase noise.

Uniformity QC: integral uniformity and differential uniformity; NEMA protocols specify acceptance limits.

Flood field uniformity correction: acquire 10–20 million counts; calculate integral and differential uniformity.

Spatial resolution and linearity QC: use a bar phantom or line sources; evaluate FWHM and distortion.

Energy resolution QC: measure photopeak FWHM; typical NaI(Tl) value 6–7% at 140 keV.

Count rate performance: test dead time and maximum count rate; paralyzable vs nonparalyzable behavior.

Image filtering: Butterworth and Hanning filters; cutoff frequency and order control smoothing and noise.

SPECT: gamma camera rotates around patient; reconstruction by filtered backprojection or iterative methods (OSEM).

PET: annihilation photons 511 keV emitted at ~180°; coincidence detection with timing window ~4–12 ns.

PET detector: scintillation crystal (e.g., LSO, BGO) coupled to PMT or SiPM; time-of-flight improves SNR.

Randoms in PET: random coincidence rate ≈ 2τ·R1·R2; corrected by delayed-window or model-based methods.

Scatter correction in PET: tail-fitting or Monte Carlo; scatter fraction typically 30–50% in 3D mode.

Attenuation correction: CT-based or transmission-source-based; essential for quantitative PET.

SPECT collimator choice: low-energy high-resolution (LEHR) for Tc-99m; medium-energy for I-131.

PET spatial resolution: ~4–6 mm FWHM; dominated by crystal size and positron range.

Daily QA: dose calibrator constancy, gamma camera uniformity and energy peaking.

Weekly/monthly QA: spatial resolution, linearity, sensitivity, and count rate performance.

Annual QA: full calibration and performance testing per NEMA or AAPM protocols.

Dose calibrator accuracy: ±5% for radionuclides used clinically; constancy within ±10%.

Gamma camera uniformity: integral uniformity typically <5% and differential <3% for useful field of view.

Regulatory: NRC 10 CFR Part 35 and state regulations govern medical use of byproduct material.

Record keeping: QA results must be documented and retained per regulatory requirements.

Acceptance testing: performed before first clinical use; establishes baseline for subsequent QA.