New knowledge about the bremsstrahlung image of strontium-89 with the scintillation camera.

OBJECTIVE

Strontium-89 ((89)Sr) chloride has been used to treat metastases in bone. A method to visualize the distribution of (89)Sr chloride with a scintillation camera was developed in 1996. Studies using bremsstrahlung imaging have shown that (89)Sr accumulates in bone and that the bremsstrahlung generated from biological tissue surrounding bone does not exceed 30 keV. However, it was not clear how low-energy bremsstrahlung from bone can produce peak energy levels of around 75 keV. We speculate that a different (unidentified) factor is involved.

METHODS

The energy spectrum of an (89)Sr source was acquired with a scintillation camera with or without a low-to-medium-energy general-purpose collimator. The energy window was set at 20-650 keV for 4 windows. A 50-mm thick acrylic block was placed between the scintillation camera and the (89)Sr source to exclude the effects of bremsstrahlung. The energy spectrum of (89)Sr covered with lead was acquired using the scintillation camera without a collimator.

RESULTS

With the collimator the energy spectrum curve was similar to that without the 50 mm of acrylic. The energy spectrum curve showed peaks at about 75, 170, and 520 keV. Without the collimator the energy spectrum showed a similar curve but no peak at 75 keV peak. The curve was similar to that obtained with the scintillation camera and the collimator; however, the curve obtained when the (89)Sr source had been placed in a lead container was similar to that obtained when the source was unshielded, and the collimator was not attached to the scintillation camera.

CONCLUSION

If bremsstrahlung of (89)Sr produces an image, a low-energy spectrum region should decrease when acrylic is placed between the (89)Sr source and the scintillation camera. However, similar curves were obtained both with the acrylic in place and without the acrylic. Therefore, we believe that the radiation detected by the scintillation camera was not bremsstrahlung due to the beta rays of (89)Sr. Most (89)Sr preparations are contaminated by (85)Sr, and most of the gamma ray energy of (85)Sr is 514 keV. The scintillation camera detected the characteristic X-ray energy of about 75 keV from the materials of the collimator (lead and others) through interaction with the gamma rays of (85)Sr.