BGO is a high Z, high-density scintillation material. Due to the high atomic number of bismuth (83) and the material's high density of 7.13 g/cm3, it is a very efficient gamma-ray absorber. Given the high Z value of the material, the photo fraction for gamma-ray absorption is high and as a result, very good peak-to-total ratios are observed.
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It is a relatively hard, rugged, non-hygroscopic crystal which does not cleave. The material does not show any significant self-absorption of the scintillation light.
The scintillation emission maximum is situated at 480nm. The light emission in photons/keV is about 15-20% of NaI(Tl); but, since the emission is partly in the area above 500nm where phototubes are less sensitive, the relative photoelectron yield of a bialkali PMT compared to NaI(Tl) amounts to 10-15%.
Due to the high Z value of the material, the photofraction for γ-ray absorption is high; and BGO scintillation crystals are used in applications where a high photofraction is required (for example, PET scanners) or because of its high detection efficiency (for example, Compton suppression spectrometers). It is a combination of properties that make BGO the material of choice for neutron activation analysis.
The decay time of BGO is about 300ns at room temperature, which is comparable to that of NaI(Tl). As there is no slow component in BGO and the rise time is quite fast (intrinsic scintillator), it is possible to get good timing <2ns with 3” thick crystals.
The scintillation intensity of BGO is a strong function of the temperature. At room temperature, the rate of change with temperature is approximately -1.2%/C. The radioactivity in BGO can make it unacceptable for some applications. We have developed a production process that significantly reduces the natural background, making our BGO well-suited for most applications.
The choice of a PET/CT scanner should not be based solely on the type of crystal used, whether BGO, LSO, or LYSO. This debate has been ongoing since the s, when PET/CT began to establish itself in clinical practice, and to this day, no clear "winner" has emerged.
Manufacturers that opted for BGO have continued using this crystal. Those that chose LSO or LYSO have also stuck with their technologies. If the superiority of one had been decisive, all manufacturers would likely have already shifted to the same model.
Understanding the Differences Between Crystals
BGO crystals have high density and atomic number, which ensures greater intrinsic detection efficiency, in other words, a higher probability of absorbing the photons emitted by the patient.
LSO/LYSO crystals, in turn, have a much shorter light decay time, which enables the use of Time-of-Flight (TOF) technology. Although these crystals generally have lower intrinsic efficiency than BGO, TOF compensates for this limitation by improving image quality.
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It’s important to clarify: TOF does not increase the number of detected photons, but it improves the spatial localization of events by more accurately estimating where the annihilation occurred. The result is an image with a better signal-to-noise ratio, especially useful in whole-body scans or in larger patients.
Comparing TOF and Non-TOF Systems Requires Caution
It is common to use effective sensitivity as a comparative parameter. This is an adjusted form of sensitivity meant to reflect the gain provided by TOF. However, this gain depends on mathematical models specific to each manufacturer, which introduces subjectivity into the comparison. In many cases, the gain may be overestimated.
A More Reliable Parameter: The NEMA Image Quality Test
A more robust alternative for comparing systems is the Image Quality test from NEMA (National Electrical Manufacturers Association).
This test simulates a whole-body scan by inserting spherical lesions ("hot spheres") of different sizes into a homogeneous radioactive background. It allows both a visual assessment and a quantitative analysis of the lesion-to-background contrast.
The recovered contrast is compared to the expected value, revealing how well the system preserves information under different conditions. This is particularly important for small lesions, where the partial volume effect reduces contrast. The better the contrast recovery, the greater the clinical potential of the system.
What Really Matters
In the end, the central question is not whether the system uses BGO or LSO/LYSO, but rather: Does the system deliver high-quality images, with sensitivity, contrast, and robustness suited to the clinical challenges of your practice?
The selection of an imaging system is a complex process that goes far beyond the choice of crystal type. It involves a careful balance of technical performance, clinical requirements, regulatory considerations, and operational constraints. This article focuses on one specific aspect. It is just one part of a much broader evaluation.