PT  - JOURNAL ARTICLE
AU  - Anna-Lena Theisen
AU  - Michael Lassmann
AU  - Johannes Tran-Gia
TI  - Towards a patient-specific traceable quantification of SPECT/CT-based radiopharmaceutical distributions.
AID  - 10.2967/jnumed.121.262925
DP  - 2021 Nov 01
TA  - Journal of Nuclear Medicine
PG  - jnumed.121.262925
4099  - http://jnm.snmjournals.org/content/early/2021/11/18/jnumed.121.262925.short
4100  - http://jnm.snmjournals.org/content/early/2021/11/18/jnumed.121.262925.full
AB  - Quantitative SPECT/CT imaging is currently the state-of-the-art for peri-therapeutic monitoring of radiopharmaceutical distributions. Due to poor resolution, however, the verification of SPECT/CT-based activity distributions is of particular importance. Due to the lack of a ground truth in patient measurements, phantoms are commonly used as a substitute for clinical validation of quantitative SPECT/CT. Due to the time-consuming and erroneous preparation of multi-compartment phantoms, e.g. for the kidney, the usually very complex internal activity distributions are typically replaced by one- or two-compartment models. To provide a simplified solution for generating inhomogeneous activity distributions, this work presents a methodology for designing single-compartment phantoms that mimic inhomogeneous spatial activity distributions by internal filling structures of different volume fractions. Methods: A series of phantoms with different filling structures was designed, 3D printed, and measured. After assessing the feasibility of the presented approach in a simple geometry, a set of three patient-specific kidney phantoms was designed based on the contrast-enhanced CT of a patient suffering from metastatic castration-resistant prostate cancer. Internal gyroid structures of different wall thickness were used in renal medulla and cortex to reproduce the inhomogeneous activity distribution observed in a peri-therapeutic SPECT/CT acquisition 24-hours post [177Lu]Lu-PSMA injection (apparent activity concentration ratios of 1:1, 1:3.5, and 1:7.5). After 3D printing, SPECT/CT experiments were performed and the results were compared to the patient data for different reconstruction settings (iterations, subsets, and post-filtering). Results: A good agreement was found between phantom designs and fabricated phantoms (based on high-resolution CT). No internal filling structures were visible in any of the SPECT images, indicating a sufficiently small feature size. While a good visual and quantitative agreement was achieved for certain combinations of filling structure and reconstruction, a histogram analysis indicated an even more complex activity distribution in the patient than represented by the two compartments assumed in our model. Conclusion: The proposed methodology provides patient-specific phantoms mimicking inhomogeneous activity distributions while using a single stock solution, thus simplifying the filling process and reducing uncertainties in the activity determination. This enables an unprecedented possibility for patient-specific evaluation of radiopharmaceutical uptake, reducing uncertainties in internal dosimetry and individualized treatments.