Monte-Carlo simulations of clinically realistic respiratory gated (18)F-FDG PET: application to lesion detectability and volume measurements

In PET/CT thoracic imaging, respiratory motion reduces image quality. A solution consists in performing respiratory gated PET acquisitions. The aim of this study was to generate clinically realistic Monte-Carlo respiratory PET data, obtained using the 4D-NCAT numerical phantom and the GATE simulation tool, to assess the impact of respiratory motion and respiratory-motion compensation in PET on lesion detection and volume measurement. To obtain reconstructed images as close as possible to those obtained in clinical conditions, a particular attention was paid to apply to the simulated data the same correction and reconstruction processes as those applied to real clinical data. The simulations required 140,000h (CPU) generating 1.5 To of data (98 respiratory gated and 49 ungated scans). Calibration phantom and patient reconstructed images from the simulated data were visually and quantitatively very similar to those obtained in clinical studies. The lesion detectability was higher when the better trade-off between lesion movement limitation (compared to ungated acquisitions) and image statistic preservation is considered (respiratory cycle sampling in 3 frames). We then compared the lesion volumes measured on conventional PET acquisitions versus respiratory gated acquisitions, using an automatic segmentation method and a 40%-threshold approach. A time consuming initial manual exclusion of noisy structures needed with the 40%-threshold was not necessary when the automatic method was used. The lesion detectability along with the accuracy of tumor volume estimates was largely improved with the gated compared to ungated PET images.

Influence of PET/CT on radiologists and contrast-enhanced CT on nuclear medicine physicians in patients with lymphoma

AIM

We assessed in this study the influence of contrast-enhanced CT (ceCT) on PET/CT interpretation and PET/CT on ceCT interpretation in patients with lymphoma, before and after chemotherapy.

METHODS

Fifty patients with Hodgkin disease (N.=17) or non-Hodgkin lymphomas (N.=33) were assessed before and after chemotherapy. PET/CT were performed 60 minutes after injection of FDG. Iopamidol was then injected and followed, 50 seconds later, by another CT. PET images were successively reconstructed using non-enhanced CT (PET-) and ceCT (PET+). Four nuclear physicians rated PET- and PET+ in random order. Three radiologists initially rated ceCT alone and then ceCT along with PET+.

RESULTS

Before chemotherapy, global agreement (GA) was 99% (k=0.96) when PET- was compared to PET+. Nine (5%) lesions were discordant, 5 according to PET- and 4 to PET+. After chemotherapy, GA was 99% (k=0.91). Eight (15%) lesions were discordant, 3 according to PET- and 5 to PET+. Before chemotherapy, GA was 97% (k=0.91) when ceCT was compared to ceCT with PET+. Twenty-one (12%) lesions were discordant, 16 when ceCT were analyzed alone and 5 when ceCT was analyzed with PET+. After chemotherapy, GA was 95% (k=0.76). All 30 (35%) discordant lesions were positive according to ceCT alone. A significant difference between the 2 procedures was found in the pelvis and in the groin (P<0.05).

CONCLUSION

PET+ did not differ from PET-, before and after chemotherapy. Fewer abnormalities were observed, when ceCT was analyzed with PET+, particularly after chemotherapy, due to residual masses that are better analyzed with functional imaging.

Long-term follow-up of patients with papillary and follicular thyroid cancer: a prospective study on 715 patients

PURPOSE

This prospective study evaluated the recurrence rate in 715 patients with differentiated thyroid cancer who had no evidence of persistent disease after total thyroidectomy and lymph node dissection in 94% of them followed up by radioiodine ablation (30-100 mCi) and assessed the predictive value of the initial thyroglobulin (Tg) levels for detecting recurrence, both during levothyroxine (LT4) treatment and after TSH stimulation.

PATIENTS AND METHODS

Patients had Tg determinations performed at 3 months on LT4 treatment (Tg1) and at 9-12 months after stimulation by either thyroid hormone withdrawal or recombinant human TSH (Tg2); the Access kit was used (functional sensitivity of 0.11 ng/ml); they had undetectable anti-Tg antibodies. Patients were followed up annually. Predictive values were calculated by comparing Tg levels (Tg1 and Tg2) and the outcome in terms of recurrence.

RESULTS

During the median follow-up of 6.2 yr, 32 patients had a recurrence. Assuming a cutoff level for Tg1 at 0.27 ng/ml, Tg1 sensitivity and specificity reached 72 and 86%, respectively, whereas predictive positive and negative values were 20 and 99%, respectively. With a cutoff level for Tg2 at 1.4 ng/ml, sensitivity and specificity reached 78 and 90%, respectively, whereas positive and negative predictive values were 26 and 99%, respectively.

CONCLUSION

This large prospective cohort of patients presented a low rate of recurrence. Initial Tg measurements allow to predict long-term recurrence with an excellent specificity. Stimulated Tg determination presented a slightly higher sensitivity than Tg determination on LT4. TSH stimulation may be avoided when Tg measured 3 months after ablation is less than 0.27 ng/ml during LT4 treatment.

Development of a generic thresholding algorithm for the delineation of 18FDG-PET-positive tissue: application to the comparison of three thresholding models

An iterative generic algorithm has been developed to compare three thresholding models used to delineate gross tumour volume on (18)F-FDG PET images. 3D volume was extracted and characteristic parameters were measured. Three fitting models using different parameters were studied: model 1 (volume, contrast), model 2 (contrast) and model 3 (SUV). The calibration was performed using a cylindrical phantom filled with hot spheres. To validate the models, two other phantoms were used. The calibration procedure showed a better fitting model for model 1 (R(2) from 0.94 to 1.00) than for model 3 (0.95) and model 2 (0.69). The validation study shows that model 3 yielded large volume measurement errors. Models 1 and 2 gave close results with no significant differences. Model 2 was preferred because it presents less error dispersion and needs fewer characteristic parameters, making it easier to implement. Our results show the importance of developing a generic algorithm to compare the performances of fitting models objectively and to validate results on other phantoms than the ones used during the calibration process to avoid methodological biases.