Improvements on a patient-specific dose estimation system in nuclear medicine examination

Internal radiation dose estimation using multiple D-shuttle dosimeters for positron emission tomography (PET): A validation study using NEMA body phantom

PURPOSE

Internal radiation dosimetry plays an important role in ensuring the safe use of positron emission tomography (PET) technology and is a legal requirement in most countries. We propose a new technique to estimate the internal radiation dose in PET studies by means of multiple D-shuttle dosimeters attached on the body surface of the patient.

METHODS

Radioactivity in a source organ was estimated iteratively using measurements from multiple D-shuttle dosimeters with a maximum-likelihood expectation-maximization (MLEM) algorithm with dose response from a source to a D-shuttle dosimeter computed by Monte Carlo simulation. To validate our technique, we performed a phantom study using a National Electrical Manufacturers Association (NEMA) body phantom. The fillable compartments (torso cavity and six spheres) of the phantom were filled with ¹⁸ F-FDG mixed with pure water using an 800:1 sphere-to-background radioactivity concentration ratio. The radioactivity concentrations present in the torso cavity and six spheres were 0.00165 MBq/mL and 1.32 MBq/mL, respectively. The initial radioactivities of the torso cavity and six spheres (treated as source organs) were 15.9 MBq (torso cavity), 34.7 MBq (37 mm sphere), 15.1 MBq (28 mm sphere), 7.27 MBq (22 mm sphere), 3.26 MBq (17 mm sphere), 1.54 MBq (13 mm sphere), and 0.697 MBq (10 mm sphere). Eleven D-shuttle dosimeters were attached to the NEMA body phantom surface to obtain information on body surface dose and a mathematical NEMA body phantom has been modeled in the Heavy Ion Transport Code System (PHITS) Monte Carlo simulation code.

RESULTS

Radioactivity was estimated in 2 min intervals over a 110-min total dose time using our proposed technique. A significant correlation (R² = 0.992) was found between actual radioactivity and estimated radioactivity at every 2 min interval for each source organ. The estimated initial radioactivity (mean with standard deviation) was 16.5 ± 0.311 MBq (torso cavity), 33.0 ± 0.624 MBq (37 mm sphere), 15.7 ± 0.189 MBq (28 mm sphere), 7.11 ± 0.738 MBq (22 mm sphere), 4.17 ± 0.083 MBq (17 mm sphere), 1.48 ± 0.469 MBq (13 mm sphere), and 0.865 ± 0.313 MBq (10 mm sphere), which were very close to the actual initial radioactivity measurements for each source organ.

CONCLUSIONS

The phantom study showed that our technique worked successfully. This technique could be used to estimate internal radiation dosimetry in a clinical PET study.

Noninvasive measurement of radiopharmaceutical time-activity data using external thermoluminescent dosimeters (TLDs)

In this study, we present a new method for estimating the time-activity data using serial timely measurements of thermoluminescent dosimeters (TLDs). The approach is based on the combination of the measurement of surface dose using TLD and Monte Carlo (MC) simulation to estimate the radiopharmaceutical time-activity data. It involves four steps: (1) identify the source organs and outline their contours in computed tomography images; (2) compute the S values on the body surface for each source organ using a MC code; (3) obtain a serial measurement of the dose with numerous TLDs placed on the body surface; (4) solve the dose-activity equation to generate organ cumulative activity for each period of measurement. The activity of each organ at the time of measurement is simply the cumulative activity divided by the timespan between measurements. The usefulness of this method was studied using a MC simulation based on an Oak Ridge National Laboratory mathematical phantom with ¹⁸F-FDG filled in six source organs. Numerous TLDs were placed on different locations of the surface and were repeatedly read and replaced. The time-activity curves (TACs) of all organs were successfully reconstructed. Experiments on a physical phantom were also performed. Preliminary results indicate that it is an effective, robust, and simple method for assessing the TAC. The proposed method holds great potential for a range of applications in areas such as targeted radionuclide therapy, pharmaceutical research, and patient-specific dose estimation.