Technical Note: Characterization of technology to detect residual injection site radioactivity

Each year in the United States, approximately 18.5 million nuclear medicine procedures are performed. Various quality control measures are implemented to reduce image errors and improve quantification of radiotracer distribution. However, there is currently no routine or timely feedback about the quality of the radiotracer injection. One potential solution to evaluate the injection quality is to place a topical scintillation sensor near the injection site to record the presence of residual activity. This work investigates a sensor design for identification of injections where the prescribed radioactive activity is not fully delivered into the patient's circulation (an infiltration).

The sensor consists of a single unshielded bismuth germanate (BGO) crystal (3 mm × 3 mm × 3 mm). Using radioactive sources with gamma energies that span the range commonly used in nuclear medicine, we quantified energy resolution and linearity. Additionally, we computed sensitivity by comparing the calculated incident activity to the activity measured by the sensor. Sensor output linearity was calculated by comparing measured data against the radioactive decay of a source over multiple half-lives. The sensor incorporates internal temperature feedback used to compensate for ambient temperature fluctuations. We investigated the performance of this compensation over the range of 15°C-35°C.

Energy spectra from four sensors were used to calculate the energy resolution: 67% for ^99m Tc (141 keV), 67% for ¹³³ Ba (344 keV), 42% for ¹⁸ F (511 keV), and 32% for ¹³⁷ Cs (662 keV). Note that the energy used for ¹³³ Ba is a weighted average of the three photon emissions nearest to the most abundant (356 keV). Sensor energy response was linear with a difference of 1%-2% between measured and predicted values. Energy-dependent detector sensitivity, defined as the ratio of measured photons to incident photons for a given isotope, decreased with increasing photon energy from 55.4% for ^99m Tc (141 keV) to 3.3% for ¹³⁷ Cs (662 keV). Without compensation, error due to temperature change was as high as 53%. Temperature compensation reduced the error to less than 1.4%. Sensor output linearity was tested to as high as 210 kcps and the maximum magnitude error was 4%.

The performance of the sensor was adequate for identification of excessive residual activity at an injection site. Its ability to provide feedback may be useful as a quality control measure for nuclear medicine injections.