¹³¹I activity quantification of gamma camera planar images

Area of Science:

• Nuclear Medicine
• Medical Imaging Physics
• Radiopharmaceutical Therapy

Background:

• Accurate quantification of radiopharmaceutical activity in target tissues is crucial for effective therapeutic administration.
• Thyroid conditions like hyperthyroidism and differentiated thyroid cancer are treated with radioactive iodine (I-131).
• Estimating activity in patients using gamma cameras (GC) requires precise sensitivity calibration.

Purpose of the Study:

• To develop and validate a procedure for estimating I-131 activity in thyroid tissues during treatment.
• To investigate the impact of lesion characteristics (size, distance) on gamma camera sensitivity.
• To reduce uncertainty in measuring thyroid uptake activity for individual patients.

Main Methods:

• Utilized a high-energy (HE) collimator gamma camera (GC) to acquire planar images of I-131.
• Calculated mean fluence (G) using MCNPX code and intrinsic efficiency (η₀) from calibration measurements.
• Estimated sensitivity (S) as G × η₀, validated with SIMIND code and experimental measurements.

Main Results:

• Sensitivity values for Siemens e-cam ranged from 7 to 35 cps MBq⁻¹, and for Philips Skylight from 6 to 29 cps MBq⁻¹.
• Sensitivity was found to be strongly dependent on lesion size (r) and distance (d) from the GC.
• Scatter and penetration photons significantly influenced sensitivity, unlike direct photons.

Conclusions:

• A procedure to estimate patient-specific acquisition sensitivity in thyroidal gamma camera planar imaging was established.
• Individual sensitivity values can significantly reduce uncertainty in measuring thyroid uptake activity.
• This method enhances the accuracy of therapeutic I-131 administration for thyroid conditions.