Practical considerations for quantitative clinical SPECT/CT imaging of alpha particle emitting radioisotopes

Abstract

Rationale: Alpha particle emitting radiopharmaceuticals are generating considerable interest for the treatment of disseminated metastatic disease. Molecular imaging of the distribution of these agents is critical to safely and effectively maximize the clinical potential of this emerging drug class. The present studies aim to investigate the feasibility and limitations of quantitative SPECT for 223 Ra, 225 Ac and 227 Th. Methods: Three state-of-the-art SPECT/CT systems were investigated: the GE Discovery NM/CT 670, the GE Optima NM/CT 640, and the Siemens Symbia T6. A series of phantoms, including the NEMA IEC Body phantom, were used to compare and calibrate each camera. Additionally, anthropomorphic physical tumor and vertebrae phantoms were developed and imaged to evaluate the quantitative imaging protocol. Results: This work describes and validates a methodology to calibrate each clinical system. The efficiency of each gamma camera was analyzed and compared. Using the calibration factors obtained with the NEMA phantom, we were able to quantify the activity in 3D-printed tissue phantoms with an error of 2.1%, 3.5% and 11.8% for 223 Ra, 225 Ac, and 227 Th, respectively. Conclusion: The present study validates that quantitative SPECT/CT imaging of 223 Ra, 225 Ac, and 227 Th is achievable but that careful considerations for camera configuration are required. These results will aid in future implementation of SPECT-based patient studies and will help to identify the limiting factors for accurate image-based quantification with alpha particle emitting radionuclides.

Quantitative SPECT/CT imaging of lead-212: a phantom study

“…Similarly to our findings with 212 Pb, they achieved quantitation of 5.6 ml volumes with an error smaller than 19% [43]. The count rate achieved with 212 Pb with the 79 keV energy window was much higher than when imaging 223 Ra [4,6,13,43]. However, a lot of photons contributing to the 79 keV energy peak are characteristic X-rays from the lead collimator and scattered photons from the 2.6 MeV emission of 208 Tl, and hence it would be more relevant to report the number of primary photons detected to compare sensitivity.…”

“…Still, almost 25 times more activity is expected to be administered with 212 Pb compared to 223 Ra therapy, and it is therefore expected that images of reasonable quality can be obtained. Count rates with 227 Th and 225 Ac were lower than with 223 Ra and hence also lower than with 212 Pb [13]. Other studies have attempted to quantify 227 Th and 223 Ra in combination with planar imaging and found that 227 Th can be separated from 223 Ra in the images.…”

“…α-particle therapies have been proposed for a variety of cancers with different targeting agents, and an overview of TATs on the market or in development was provided recently by Sgouros et al [8]. Imaging of several of the α-particle emitting radionuclides has been shown to be feasible; for instance, thorium-227 ( 227 Th) and actinium-225 ( 225 Ac) can be imaged with planar scintigraphy [5,6,[9][10][11][12][13] and SPECT [11][12][13][14]. Planar scintigraphy and SPECT scans of the liver and brain have been performed with bismuth-213 ( 213 Bi) [14][15][16], while terbium-149 ( 149 Tb) can be imaged with positron emission tomography (PET) [17].…”

Experimental evaluation of absolute quantification in 99mTc‐TRODAT‐1 SPECT/CT brain dopamine transporter (DAT) studies

“…On the other hand, a study by Benabdallah et al. showed POP was used to mimic the vertebra region for quantification accuracy using various theranostic radionuclides in SPECT/CT phantom studies 28 . Hence, further experiments with varying POP thicknesses are required.…”

“…27 On the other hand, a study by Benabdallah et al showed POP was used to mimic the vertebra region for quantification accuracy using various theranostic radionuclides in SPECT/CT phantom studies. 28 Hence, further experiments with varying POP thicknesses are required. Our findings suggest that when CT erroneously predicts the skull (made of POP), the activity distribution is underestimated.…”