Detectability of small objects in PET/computed tomography phantom images with Bayesian penalised likelihood reconstruction.

OBJECTIVES: This study investigated the subcentimetre lesion detection gains of a Bayesian penalised likelihood reconstruction (BPLR) (Q.Clear, GE Healthcare, Milwaukee, USA) in PET/computed tomography (CT) phantom images and compares observer performance with ordered subset expectation maximisation (OSEM) reconstruction images (VUE Point HD, GE Healthcare). METHODS: Images were presented to three blinded experienced observers to identify lesions and assign confidence ratings. Responses were analysed using jackknife alternative free receiver operator characteristic (JAFROC) software. Phantom lesions (active and nonactive) were constructed using putty. Seventy nonactive and 93 (F) active lesions, with diameters of 3, 5 or 7 mm were suspended in active backgrounds at varying contrast ratios (2:1-32:1) within an National Electrical Manufacturers Association 2012 phantom. PET/CT images were acquired with a GE Discovery 710 and reconstructed using both BPLR (penalisation coefficient 400) and high-definition attenuation corrected (HDAC) OSEM (2 iterations, 24 subsets). RESULTS: Small but significant (P = 0.009) visual detection gains were seen for active lesions with BPLR [weighted JAFROC figure of merit (wJAFROC FOM) = 0.77] over OSEM (FOM = 0.74). When split by subset, these improvements were significant for 5 mm and lesion to background ratio of 8:1. No significant (P = 0.514) differences were seen for the identification of nonactive lesions of any size (BPLR FOM = 0.74 and OSEM FOM = 0.73). CONCLUSIONS: Significant detection gains were demonstrated for small active lesions with BPLR over OSEM. Coupled with the significant increase in contrast-to-noise ratio, these results support the use of BPLR in the imaging of small active (≤7 mm) lesions but show no improvement with BPLR in the identification of true negative lesions.

Prevalence of incidental thyroid malignancy on routine 18F-fluorodeoxyglucose PET-CT in a large teaching hospital.

PURPOSE: To quantify incidental thyroid pathology including malignancy on routine 18F-FDG PET-CT scans To compare standardised uptake values (SUVmax) in thyroid malignancy subtypes METHODS AND MATERIALS: This is a retrospective study of all 18F-FDG PET-CT scans (n = 6179) performed in a teaching hospital between June 2010 and May 2019. RIS database search of reports for the word "thyroid" was performed. Studies with evidence of thyroid uptake were included. Patient age and gender, primary indication for PET scan (malignant or non-malignant), thyroid result on PET (diffuse or focal tracer uptake, SUVmax), ultrasound and FNAC results were recorded. RESULTS: Incidental abnormal thyroid tracer uptake as a proportion of all 18F-FDG PET-CT scans was 4.37% (n = 270). Out of region patients (n = 87) whose records could not be obtained were excluded leaving a study group of n = 183. Ninety-four in this group had focal uptake, and 89 had diffuse uptake. Fifty-five patients in the focal group had undergone further investigations. Of these, 30 were thought to be benign on USS alone, and 25 patients underwent USS/FNAC. Thirteen (24%) malignancies were identified (5 papillary, 6 follicular, 1 poorly differentiated thyroid cancer, 1 metastatic malignancy). Mean SUVmax for papillary carcinoma was noted to be 8.2 g/ml, and follicular carcinoma was 12.6 g/ml. CONCLUSION: Incidental abnormal thyroid 18F-FDG PET-CT uptake in PET-CT scans of 4.37% is in keeping with the known limited literature. Rather similar number of patients was noted in the focal and diffuse tracer uptake categories in the final study group. Around quarter of the focal lesions were identified to be malignant, implying focal lesions should always be further investigated.

Same day cerebral perfusion and dopamine transporter imaging for differential diagnosis of cerebral impairment.

When cognitive impairment is first evident it can be difficult to distinguish between different conditions such as idiopathic Parkinson's disease and Lewy body dementia. Imaging both cerebral perfusion and dopamine transporter function has been shown to provide accurate differentiation between the most common conditions. At present cerebral perfusion and dopamine transporter imaging is conducted on separate days. Carrying out both scans on the same day has the potential to benefit the patient through the social convenience of one visit to hospital and the earlier availability of results. This work considered whether it was possible to obtain diagnostic quality images from Ioflupane (123I) single positron emission tomography (SPECT) acquired at the same time as or four hours after a Exametazime (99mTc) SPECT on the same day. Possible changes to the Ioflupane (123I) SPECT acquisition and processing protocols such as a new energy window and use of a resolution recovery algorithm were explored using phantom studies. Initial phantom results show that when a four hour delay between acquisitions is used comparable contrast to noise ratios can be achieved (4.23 vs. 4.63) with an insignificant loss in resolution (11.51 mm vs. 11.35 mm full width at half maximum (FWHM)). An offset energy window (159 keV -5.5% & +15.5%) was found to provide the highest contrast to noise ratio. This work provides a proof of concept for same day imaging.