ABSNM NMPI flashcards
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32 cards. Read the term and the definition here. The deck at the top of this page is optional.
- Ohm's law
- The relationship V = IR, where V is voltage, I is current, and R is resistance; used to analyze detector circuits.
- Preamplifier function
- Converts the small charge pulse from the detector into a voltage pulse, providing initial amplification and impedance matching.
- Pulse height analyzer
- An electronic circuit that sorts incoming pulses by amplitude, allowing selection of a specific energy window.
- RC time constant
- The product τ = RC; the time for a capacitor to charge to 63% or discharge to 37% of its initial value.
- Paralyzable dead time
- A detector model in which events occurring during dead time extend the dead time; observed rate m = n/(1 + nτ).
- Nonparalyzable dead time
- A detector model in which dead time is fixed after each event; observed rate m = n·e^(−nτ) for random events.
- PMT gain
- The amplification factor G = δ^n, where δ is the dynode multiplication factor and n is the number of dynodes.
- Energy resolution FWHM
- The full width at half maximum of a photopeak divided by its centroid energy, expressed as a percentage; improves with more photoelectrons.
- NaI(Tl) crystal
- A thallium-doped sodium iodide scintillator with high light output and good efficiency for 140 keV gamma rays; resolution ~6–7% at 140 keV.
- Well counter
- A NaI(Tl) detector with a well-shaped crystal providing high geometric efficiency for counting small radioactive samples.
- Dose calibrator
- A reentrant ionization chamber used to measure the activity of radionuclides in becquerels or curies; requires daily constancy checks.
- Geiger–Müller counter
- A gas-filled detector operating in the Geiger region; sensitive to beta and gamma contamination but has low gamma efficiency.
- Photopeak
- The peak in a gamma-ray spectrum corresponding to full energy absorption of the incident photon; its centroid identifies the radionuclide.
- Compton continuum
- The broad distribution of energies below the photopeak caused by Compton scattering of gamma rays in the detector.
- Anger logic
- A positioning method in gamma cameras that uses weighted signals from multiple photomultiplier tubes to determine event location.
- Parallel-hole collimator
- A collimator with parallel holes that produces an image the same size as the object; sensitivity depends on hole diameter and length.
- Pinhole collimator
- A collimator with a single small aperture that magnifies the image; used for imaging small organs such as the thyroid.
- Energy window
- The range of pulse heights accepted by the pulse height analyzer; typically 20% centered on the photopeak.
- Uniformity correction map
- A stored image of a flood source used to correct nonuniform response of the gamma camera during processing.
- Septal penetration
- The passage of photons through collimator septa, which degrades image contrast; depends on septal thickness and photon energy.
- Pixel size
- The field of view divided by the matrix dimension; smaller pixels improve sampling but reduce counts per pixel.
- Integral uniformity
- A measure of the maximum variation in count density across the useful field of view; typically expressed as a percentage.
- Differential uniformity
- A measure of local uniformity calculated over adjacent pixels; more sensitive to small-area nonuniformities than integral uniformity.
- Bar phantom
- A test object with parallel lead bars of varying spacing used to evaluate spatial resolution and linearity of a gamma camera.
- Butterworth filter
- A frequency-domain smoothing filter characterized by cutoff frequency and order; used to reduce noise in nuclear medicine images.
- SPECT reconstruction
- The process of creating tomographic slices from projections, typically using filtered backprojection or iterative methods such as OSEM.
- PET coincidence detection
- Detection of two 511 keV annihilation photons within a timing window (e.g., 4–12 ns) to define a line of response.
- Time-of-flight PET
- PET technique that measures the arrival time difference of annihilation photons to localize the event along the line of response, improving SNR.
- PET randoms
- Coincidence events from unrelated photons; rate ≈ 2τ·R1·R2, corrected by delayed-window or model-based methods.
- Attenuation correction
- Correction for photon attenuation in PET and SPECT, typically using CT or transmission sources, essential for quantitative imaging.
- Daily gamma camera QA
- Daily tests include uniformity flood and energy peaking to ensure stable camera performance before clinical use.
- Dose calibrator constancy
- A daily check using a long-lived source to verify that the dose calibrator response is stable within ±10%.