Improving PET Quantification of Small Animal [Ga-68]DOTA-Labeled PET/CT Studies by Using a CT-Based Positron Range Correction

Abstract

Image quality of positron emission tomography (PET) tracers that emits high-energy positrons, such as Ga-68, Rb-82, or I-124, is significantly affected by positron range (PR) effects. PR effects are especially important in small animal PET studies, since they can limit spatial resolution and quantitative accuracy of the images. Since generators accessibility has made Ga-68 tracers wide available, the aim of this study is to show how the quantitative results of [Ga-68]DOTA-labeled PET/X-ray computed tomography (CT) imaging of neuroendocrine tumors in mice can be improved using positron range correction (PRC). Eighteen scans in 12 mice were evaluated, with three different models of tumors: PC12, AR42J, and meningiomas. In addition, three different [Ga-68]DOTA-labeled radiotracers were used to evaluate the PRC with different tracer distributions: [Ga-68]DOTANOC, [Ga-68]DOTATOC, and [Ga-68]DOTATATE. Two PRC methods were evaluated: a tissue-dependent (TD-PRC) and a tissue-dependent spatially-variant correction (TDSV-PRC). Taking a region in the liver as reference, the tissue-to-liver ratio values for tumor tissue (TLRtumor), lung (TLRlung), and necrotic areas within the tumors (TLRnecrotic) and their respective relative variations (Delta TLR) were evaluated. All TLR values in the PRC images were significantly different (p < 0.05) than the ones from non-PRC images. The relative differences of the tumor TLR values, respect to the case with no PRC, were Delta TLRtumor 87 +/- 41 \% (TD-PRC) and 85 +/- 46 \% (TDSV-PRC). TLRlung decreased when applying PRC, being this effect more remarkable for the TDSV-PRC method, with relative differences respect to no PRC: Delta TLRlung = - 45 +/- 24 (TD-PRC), - 55 +/- 18 (TDSV-PRC). TLRnecrotic values also decreased when using PRC, with more noticeable differences for TD-PRC: Delta TLRnecrotic = - 52 +/- 6 (TD-PRC), - 48 +/- 8 (TDSV-PRC). The PRC methods proposed provide a significant quantitative improvement in [Ga-68]DOTA-labeled PET/CT imaging of mice with neuroendocrine tumors, hence demonstrating that these techniques could also ameliorate the deleterious effect of the positron range in clinical PET imaging.