Knowledge Hub · Ge-68 Line Source
PET normalization needs a uniform 511 keV source that survives years of routine scanner QA. Eluted Ga-68 decays in hours; a sealed Ge-68 line source — in secular equilibrium with its Ga-68 daughter — delivers PET-energy photons at a predictable rate for 2–3 years. This page unpacks why the half-life choice matters, how NEMA NU-2 frames the normalization measurement, and what ±5 % uniformity buys.
Why this matters
Why Ge-68 / Ga-68 in equilibrium
Ge-68 decays by electron capture (T½ 270.95 d) to Ga-68, which decays by β⁺ (T½ 67.7 min) producing the 511 keV annihilation photons PET scanners see. In secular equilibrium, the source emits PET-relevant photons at a rate controlled by the long-lived parent. The same source remains useful for 2–3 years of routine PET normalization with predictable activity correction.
Based on: IAEA Human Health Series No. 27 — Quality Assurance for PET and PET/CT Systems.
Read source ↗NEMA NU-2 normalization
Modern 3D PET reconstructions require a per-detector-element normalization correction. NEMA NU-2 specifies a line source rotated around the scanner field of view at known activity, sampling the response of every crystal-PMT path. The Ge-68 line source is the standard test object — long-lived enough to survive the scanner's service life, uniform enough that variations in the normalization map come from the detector, not the source.
Based on: NEMA NU-2 Performance Measurements of Positron Emission Tomographs.
Read source ↗±5 % uniformity along length
A normalization measurement assumes the source itself is uniform. Any variation along the length of the line source (hot spot, weld inhomogeneity) shows up in the per-crystal normalization map and becomes a permanent imprint on every quantitative PET scan until the next normalization. ±5 % uniformity, verified at 5 mm intervals along the source before dispatch, is the industry standard that keeps source contribution well below detector contribution.
Based on: Manufacturer QC dossier; NEMA NU-2-2018 uniformity expectations.
Read source ↗Argon-welded sealed integrity
A PET line source carries hundreds of MBq of activity through the scanner gantry on rotation. Source integrity is non-negotiable. Double-wall stainless steel construction with argon-welded ends produces a hermetic seal verified by leak testing at manufacture. The source can be handled, transported and rotated within the scanner with no contamination path — and the routine wipe test at AERB inspection comes back clean.
Based on: AERB Atomic Energy (Radiation Protection) Rules 2004; ISO 2919 sealed source classification.
Read source ↗Dose calibrator cross-check
A nuclear-medicine department dispensing Ga-68 radiopharmaceuticals (DOTATATE, PSMA-11, FAPI) needs a dose calibrator setting for Ga-68 calibrated against a NIST-traceable reference. A Ge-68 point source — in secular equilibrium with its Ga-68 daughter — gives the radiopharmacy a known-activity reference at the 511 keV PET energy for dose-calibrator constancy and geometry-factor verification.
Based on: AAPM TG-181 PET/NM QA guidance; IAEA Human Health Series 27.
Read source ↗Why short-lived sources do not work here
A facility could in principle elute Ga-68 from its Ge-68/Ga-68 radiopharmacy generator and use that for normalization — but the eluted Ga-68 decays in hours, so the normalization would need to be redone weekly or daily. A sealed Ge-68 line source carries the parent activity for years, so the same source survives software upgrades, detector replacements and even scanner relocations.
Based on: IAEA Human Health Series 27.
Read source ↗Specification snapshot
270.95 d
Ge-68 half-life
Parent of Ga-68
511 keV
Annihilation γ
PET imaging energy
±5 %
Uniformity
Verified at 5 mm intervals
2–3 y
Useful service life
From a single source
The IAEA, NEMA, AAPM and AERB documents that frame PET normalization with a sealed Ge-68 line source.
IAEA framework for PET QA programmes including normalization, uniformity and sensitivity expectations.
The standard PET-scanner performance specification covering normalization, uniformity, spatial resolution and sensitivity.
AAPM task-group report framing routine PET / PET-CT QA expectations.
Indian regulatory framework for sealed-source import, handling and decay-store / disposal.