Absolute radiotracer concentration measurement using whole-body solid-state SPECT/CT technology: in vivo/in vitro validation

Assessment of unilateral condylar hyperplasia with quantitative SPECT/CT

Purpose

The current study aimedto assess condylar activityin patients with unilateral condylar hyperplasia (UCH) with quantitative SPECT/CT.

Patients and methods

This retrospective study included patients with UCH who underwent quantitative SPECT/CT. SPECT analysis and quantification of SPECT/CT were performed, and the maximum count per pixel and SUVmax of either side of the condyles were calculated.

Results

39 patients were included in the analysisand classified into three subgroups according to the percentile differential right-left ratio: inactive group, left active (LA) group, and right active (RA) group. Totally, the SUVmax of the affected side is significantly higher than the unaffected side (active:5.93 ± 2.43 vs inactive:3.62 ± 1.76, P < 0.001), SUVmax-based ratios correlated well with the ratios based on maximum count (R = 0.944, P < 0.001). ROC analysis showed poorSUVmaxperformance in differentiation between theactive condyles and the inactive condyles due to the lower area under the curve (AUC) (0.588). In subgroup analysis, the affected side is significantly higher than the unaffected side in active groups with SUVmax, no significant difference was found between the active sides or the inactive sides of active groups. Interestingly, the SUVmax of the left side was statistically higher than that of the right sidein the inactive group (P = 0.01),while the left side of the right active group has significantlylower activitythan that in the inactive group, meanwhile,the right side showed no significant difference. Furthermore, each side showed no significant difference between the left active group and the inactive group.

Conclusions

SUVmax is not an optimal measurement effectively used to evaluate active condyles. However, SUV ratios correlated well with the count ratios, and the left side of condyles showed a peculiar feature in condyle growth status reflected in radioactivity quantified with SPECT/CT, which needs further study to determine the role in the development of the UCH.

Monte Carlo modelling of a compact CZT-based gamma camera with application to 177Lu imaging

Background

Semiconductor gamma-camera systems based on cadmium zinc telluride (CZT) detectors present new challenges due to an energy-response that includes effects of low-energy tailing. In particular, such energy tails produce effects that need to be considered when imaging radionuclides with multiple emissions such as $$^{177}{\mathrm {Lu}}$$ 177 Lu . Monte Carlo simulation can be used to investigate the behaviour of such systems and optimise their use, provided that the detector model closely reflects the real physical detector. The aim of this work is to develop a CZT model applicable for simulation of CZT-based gamma cameras.

Methods

The equations describing the charge transport and signal induction are considered in three dimensions and are solved numerically, and the CZT model is then realised by coupling the detector-response to the photon-transport handled by the SIMIND Monte Carlo program. The CZT model is tuned to reproduce experimentally measured energy spectra of a hand-held gamma camera system for multiple radionuclides ($$^{99\mathrm {m}}{\mathrm {Tc}}$$ 99 m Tc , $$^{123}{\mathrm {I}}$$ 123 I and $$^{177}{\mathrm {Lu}}$$ 177 Lu ) and parallel-hole collimators (MEGP, LEHR) as well as an uncollimated system.

Results

Overall, the model results agree well with measurements across the range of experimental conditions. The applicability of the model is demonstrated by separating energy spectra into components to investigate the interference of high-energy photons on lower energy-windows, where pronounced effects of low-energy tailing for $$^{177}{\mathrm {Lu}}$$ 177 Lu are observed.

Conclusions

The developed model provides understanding of the specifics of the camera response and is expected to be helpful for future optimisation of gamma camera applications.

Quantitative SPECT/CT for dosimetry of peptide receptor radionuclide therapy

Neuroendocrine tumors (NETs) are uncommon malignancies of increasing incidence and prevalence. As these slow growing tumors usually overexpress somatostatin receptors (SSTRs), the use of ⁶⁸Ga-DOTA-peptides (gallium-68 chelated with dodecane tetra-acetic acid to somatostatin), which bind to the SSTRs, allows for PET based imaging and selection of patients for peptide receptor radionuclide therapy (PRRT). PRRT with radiolabeled somatostatin analogues such as ¹⁷⁷Lu-DOTATATE (lutetium-177-[DOTA,Tyr³]-octreotate), is mainly used for the treatment of metastatic or inoperable NETs. However, PRRT is generally administered at a fixed injected activity in order not to exceed dose limits in critical organs, which is suboptimal given the variability in radiopharmaceutical uptake among patients. Advances in SPECT (single photon emission computed tomography) imaging enable the absolute quantitative measure of the true radiopharmaceutical distribution providing for PRRT dosimetry in each patient. Personalized PRRT based on patient-specific dosimetry could improve therapeutic efficacy by optimizing effective tumor absorbed dose while limiting treatment related radiotoxicity.

Absolute Quantification in Diagnostic SPECT/CT: The Phantom Premise

The application of absolute quantification in SPECT/CT has seen increased interest in the context of radionuclide therapies where patient-specific dosimetry is a requirement within the European Union (EU) legislation. However, the translation of this technique to diagnostic nuclear medicine outside this setting is rather slow. Clinical research has, in some examples, already shown an association between imaging metrics and clinical diagnosis, but the applications, in general, lack proper validation because of the absence of a ground truth measurement. Meanwhile, additive manufacturing or 3D printing has seen rapid improvements, increasing its uptake in medical imaging. Three-dimensional printed phantoms have already made a significant impact on quantitative imaging, a trend that is likely to increase in the future. In this review, we summarize the data of recent literature to underpin our premise that the validation of diagnostic applications in nuclear medicine using application-specific phantoms is within reach given the current state-of-the-art in additive manufacturing or 3D printing.

Review on 99mTc radiopharmaceuticals with emphasis on new advancements

Rapid imaging acquisition, high spatial resolution and sensitivity, powered by advancements in solid-state detector technology, are significantly changing the perspective of single photon emission tomography (SPECT). In particular, this evolutionary step is fueling a rediscovery of technetium-99m, a still unique radionuclide within the nuclear medicine scenario because of its ideal nuclear properties and easy preparation of its radiopharmaceuticals that does not require a costly infrastructure and complex procedures. Scope of this review is to show that the arsenal of technetium-99m radiopharmaceuticals is already equipped with imaging agents that may complement and integrate the role played by analogous tracers developed for positron emission tomography (PET). These include, in particular, somatostatin (SST) and prostate-specific membrane antigen (PSMA) receptor targeting agents, and a number of peptide-derived radiopharmaceuticals. Additionally, these recent technological developments, combined with new myocardial perfusion tracers having more favorable biodistribution and pharmacokinetic properties as compared to current commercial agents, may also reinvigorate the prevailing position still hold by technetium-99m radiopharmaceuticals in nuclear cardiology.

Digital Solid-State SPECT/CT Quantitation of Absolute 177Lu Radiotracer Concentration: In Vivo and In Vitro Validation

The accuracy of ¹⁷⁷Lu radiotracer concentration measurements using quantitative clinical software was determined by comparing in vivo results for a digital solid-state cadmium-zinc-telluride SPECT/CT system with in vitro sampling. Methods: First, image acquisition parameters were assessed for an International Electrotechnical Commission body phantom emulating clinical count rates loaded with a lung insert and 6 hot spheres with a 12:1 target-to-background ratio of ¹⁷⁷Lu solution. Then, the data of 28 whole-body SPECT/CT scans of 7 patients who underwent ¹⁷⁷Lu prostate-specific membrane antigen radioligand therapy were retrospectively analyzed. Three users analyzed SPECT/CT images for in vivo urinary bladder radiotracer uptake using quantitative software. In vitro radiopharmaceutical concentrations were calculated using urine sampling obtained immediately after each scan, scaled to SUVs. Any in vivo or in vitro identity relations were determined by linear regression (ideally, slope = 1 and intercept = 0), within a 95% confidence interval. Results: Phantom results demonstrated lower quantitative error for acquisitions using the 113-keV ¹⁷⁷Lu energy peak rather than including the 208-keV peak, given that only low-energy collimation was available in this camera configuration. In the clinical study, 24 in vivo-in vitro pairs were eligible for further analysis, with 4 having been rejected as outliers (via Cook distance calculations). All linear regressions (R ² ≥ 0.82, P < 0.0001) provided identity in vivo-in vitro relations (95% confidence interval), with SUV averages from all users giving a slope of 0.96 ± 0.13, an intercept of -0.07 ± 0.46 g/mL, and an average residual difference of 19.5%. In acquisitions with the lower-energy ¹⁷⁷Lu energy peak, solid-state SPECT/CT imaging provided an accuracy to within approximately 20% of in vivo urinary bladder radiotracer concentrations. Conclusion: This noninvasive in vivo quantitation method can potentially improve diagnosis, patient management, and treatment response assessment and provide data essential to ¹⁷⁷Lu dosimetry.