Medical Imaging (PET, SPECT, CT)

Modality Requirements at a Glance

Three distinct modalities dominate clinical nuclear imaging, each with a unique scintillator specification profile.

Positron Emission Tomography (PET)

PET detects coincident 511 keV annihilation photons. Speed is the primary constraint. A short decay time reduces random coincidences and allows tighter timing windows, which directly improves image signal-to-noise. High density and high atomic number (Z) are equally critical: they stop 511 keV gammas efficiently in thin detector elements, keeping scanner diameter manageable and sensitivity high. Light yield matters, but decay time and stopping power take priority over raw photon count.

Single-Photon Emission Computed Tomography (SPECT)

SPECT uses collimation rather than coincidence, so it does not impose the same sub-nanosecond timing requirement. Energy resolution becomes more important here, because the system must discriminate the primary photopeak from scattered photons. A modest decay time (hundreds of nanoseconds to a few microseconds) is acceptable. Good energy resolution and high light yield allow finer energy windowing, reducing scatter contamination and improving lesion contrast.

Computed Tomography (CT)

CT operates in current-integration mode with X-ray sources, not radioactive tracers. The demands shift: very low afterglow (no phosphorescence after the X-ray pulse ends), high density for X-ray stopping power, and compatibility with photodiode readout. A scintillator with slow primary decay is acceptable provided afterglow is negligible. Hygroscopicity is a concern in production arrays, so non-hygroscopic materials are strongly preferred.

ScintIQ Materials for Medical Imaging

Five ScintIQ materials are well-established in medical imaging applications. Their key properties are summarized below; full specifications appear on each material's data sheet.

Note: Relative light yield is quoted versus NaI(Tl) = 100, measured with a bialkali PMT. CdWO₄ full datasheet pages are available on request; contact us for material samples and custom sizing.

Understanding the Key Tradeoffs

LYSO(Ce): The PET Standard

LYSO dominates modern PET because it combines the two properties that matter most: density (7.20 g/cm³) and speed (50 ns decay). High density means more 511 keV photon stopping power per unit volume, allowing thin detector elements and compact scanner geometries. The 50 ns decay enables sub-nanosecond coincidence timing windows, substantially reducing random coincidences and improving sensitivity. LYSO is non-hygroscopic and compatible with SiPM readout, which has largely replaced PMTs in newer PET designs. Its relative light yield of 70–80 supports good energy resolution without being the highest available. LYSO is the first material to specify for any new PET detector design.

BGO: Density at Lower Cost

Bismuth germanate (BGO) was the PET workhorse before LYSO became widely available. Its density (7.13 g/cm³) and Z are comparable to LYSO, and it has no afterglow. The limitation is light yield: at 15–20 relative to NaI(Tl), BGO produces far fewer photons per gamma event, which limits energy resolution and makes it less compatible with SiPM readout (SiPMs require more light to overcome their dark noise). BGO also decays slowly (0.3 μs), which limits count-rate performance. It remains relevant in budget-conscious configurations or legacy replacement programs where the original scanner was designed around BGO array geometry.

GAGG(Ce): Flexible Across Modalities

GAGG is a newer oxide scintillator with a useful combination of density (6.60 g/cm³), moderate speed (100 ns), and green emission (520 nm) that suits both standard PMTs and SiPMs. It does not match LYSO's stopping power or BGO's legacy installed base, but it fills a practical role in SPECT detector arrays, small-animal PET, and gamma camera applications where SiPM compatibility and radiation hardness are valued. GAGG is also non-hygroscopic, which simplifies array assembly. Its 35–40 relative light yield is sufficient for energy discrimination at SPECT energies (typically 140 keV for Tc-99m).

CsI(Tl): SPECT and Gamma Cameras

CsI(Tl) is the classic SPECT and gamma camera scintillator. Its 550 nm yellow-green emission is a direct match to photodiodes and silicon sensors, and it delivers 45 relative light yield with only slight hygroscopicity. The slower decay (0.6 / 3.4 μs) is acceptable for SPECT because coincidence timing is not required. CsI(Tl) crystals can be grown in large monolithic slabs or segmented arrays, and they are mechanically rugged. For gamma cameras that use position-sensitive PMTs or SiPM arrays, CsI(Tl) offers a well-proven, cost-effective path.

CdWO₄: CT Arrays and High-Flux X-Ray

Cadmium tungstate has the highest density of the medical imaging group (7.90 g/cm³) and very low afterglow, which is the defining requirement for CT. After each X-ray pulse, the scintillator must return to baseline before the next measurement; afterglow smears slice data and creates artifacts. CdWO₄ is non-hygroscopic and its 540 nm emission is well-matched to silicon photodiodes. The slow primary decay (20 / 5 μs) is not a penalty in current-integration CT readout. CdWO₄ is available in custom-sized bars and arrays; contact Berkeley Nucleonics for dimensional specifications.

Readout Options and System Integration

All ScintIQ materials for medical imaging are available in a range of crystal sizes, surface finishes, and housing configurations. The choice of readout device interacts directly with emission wavelength and light yield.

• SiPM readout: Preferred for LYSO and GAGG in modern PET and compact SPECT. SiPMs operate at low bias voltage, are MRI-compatible, and scale well to pixelated array formats. The ScintIQ SiPM Readout module pairs with LYSO and GAGG crystals for integrated detector assemblies.
• PMT readout: Remains standard for large-area SPECT detectors and gamma cameras using CsI(Tl) or NaI(Tl). Position-sensitive PMTs provide good spatial resolution across large crystal areas. ScintIQ assemblies include crystal-to-PMT optical coupling with light guide options.
• Photodiode readout: The natural choice for CsI(Tl) and CdWO₄ in CT, where green and yellow emission (540–550 nm) matches silicon photodiode quantum efficiency. Photodiodes operate at low voltage without high-voltage supplies, simplifying array electronics.

Detector housing, reflector materials, optical coupling grease, and hermetic encapsulation are all configurable through the ScintIQ custom detector program. Crystal dimensions, pixel pitch, and array formats are specified per project. Contact Berkeley Nucleonics engineering to discuss your detector geometry and count-rate requirements.

ScintIQ Material Data Sheets

Full specifications, energy resolution data, and available configurations for each material are documented in the ScintIQ series data sheets:

CdWO₄ data sheets and sample quantities are available on request. Use the ScintIQ material comparison tool to filter all 20 available scintillators by density, speed, and application.