Paul Ak

Effects of iterative reconstruction on image contrast and lesion detection in gamma camera coincidence imaging in lung and breast cancers.

To investigate the effects of iterative reconstruction in 18F-fluorodeoxyglucose (FDG) gamma camera coincidence imaging (GCI), image contrast and visual detection obtained by using the iterative ordered-subsets expectation maximization (OSEM) reconstruction, in a phantom and in patients with lung cancer and breast cancer, were compared with those obtained by using the conventional filtered backprojection (FBP) reconstruction. Images of a cylindrical phantom containing hot spheres of various sizes (10-38 mm) were acquired by positron emission tomography (PET) and GCI at various sphere-to-background activity ratios. Forty-one consecutive patients with biopsy-proven cancer of lung (n = 20) and breast (n = 21) underwent PET and GCI on the same day after intravenous injection of 370 MBq of FDG. GCI images reconstructed by the OSEM and the FBP were compared. FDG PET was considered as the standard of reference. In GCI phantom images, OSEM yielded better contrast and signal-to-noise ratio (SNR) than FBP over the range of sphere sizes. Attenuation correction improved both the image measures and sphere detection obtained by the OSEM in GCI. In the study involving patients, FDG PET depicted 41 primary tumours and 25 metastatic lymph nodes. All of the tumours >2 cm in diameter (n = 25), six of the nine tumours 1.5-2.0 cm in diameter (67%), two of seven tumours <1.5 cm in diameter (29%), and 20 metastatic lymph nodes (80%) were detected in attenuation uncorrected GCI reconstructed by the OSEM as well as the FBP. The undetected lesions in GCI were identical between the OSEM and the FBP reconstructions. OSEM yielded significantly greater tumour-to-background (T/B) ratios and lower noise than FBP in GCI (T/B ratios, 4.1±3.2 vs 3.7±2.7, P = 0.02; noise, 0.09±0.04 vs 0.14±0.05, P<0.0001). In conclusion, OSEM yielded better image contrast and less noise than the FBP in GCI, but the lesion detection obtained by the OSEM and the FBP in attenuation uncorrected GCI in patients with lung cancer and breast cancer were similar. Phantom data suggest the potential of OSEM for improving lesion detection in GCI after attenuation correction.

Myocardial viability assessment with gated SPECT TC-99m tetrofosmin % wall thickening: Comparison with F-18 FDG-PET

Object: This study was designed to assess the value of gated SPECT Tc-99m-tetrofosmin (TF) wall thickening (WT) in addition to TF exercise (Ex)/rest myocardial SPECT, in comparison with F-18 fluorodeoxyglucose (FDG)-PET.Methods: The study population consisted of 33 patients with old myocardial infarction (27 men and 6 women; mean age, 62±8 years old). All patients underwent Ex/rest TF SPECT and glucose loading FDG-PET. Polar map images of Ex/rest TF were generated exercise-rest perfusion scintigraphy. LV segments with less than 70% of the maximum TF activity on the exercise image were defined as stress-induced defects. Among these, the segments whose TF activity increased by 10% from exercise to rest images or exceeded 70% of the maximum uptake were defined as reversible (viable) defects. The remaining defects on the rest image were irreversible (non-viable) defect segments, and were considered for viability study on the basis of %WT. %WT was calculated according to the standard method: {(counts ES—counts ED)/counts ED}×100. A viable segment on gated SPECT was defined as a segment whose %WT exceeded the lower limit of the normal value (mean-SD). PET viability was defined as FDG uptake exceeding 50% of the maximum count.Results: Among the 792 segments evaluated in the 33 patients studied, there were 689 PET viable segments. Of the 689 segments analyzed, 198 (29%) were identified as having defects on Ex images. Among these defects, 55 (8%) were reversible or partially reversible, as evidenced by rest images, and 143 (21%) were irreversible. Of the irreversible segments on Ex/rest images, 106 (15%) demonstrated no apparent WT by gated TF SPECT, whereas 37 (6%) segments with irreversible defects did have apparent WT. Overall, the sensitivity of Ex/rest TF perfusion imaging was 79%. Sensitivity was improved from 79% to 85% by combining %WT and perfusion data, but specificity was reduced from 70% to 56%.Conclusion: %WT evaluated from gated TF imaging enhanced myocardial viability assessment in comparison with FDG-PET.

Left ventricular mass index measured by quantitative gated myocardial SPECT with99mTc-tetrofosmin: a comparison with echocardiography

Objective: Left ventricular mass is an important determinant of diagnosis and prognosis in patients with heart disease. The aim of the present study was to validate measurement of the left ventricular mass index (LVMI) by quantitative gated myocardial SPECT (QGS) with99mTc-tetrofosmin by comparing it with echocardiography.Methods: QGS and M-mode echocardiography (Echo) were performed within one month of each other in 179 patients. M-mode echocardiography was carried out according to Devereuxs method. QGS images were acquired one hour after injection of99mTc-tetrofosmin at rest. Myocardial volume was defined as the volume between the endocardial and epicardial surface in the end-diastolic phase. LVMI (g/m2) was defined as myocardial volume divided by myocardial specific density and corrected for body surface area. QGS LVMI measurements were performed twice by the same observer and independently by two different observers. Regional hypoperfusion in the group of patients with old myocardial infarction (n=26) was evaluated semiquantitatively on the basis of the total defect score on the resting99mTc-tetrofosmin SPECT images.Results: Among the patients as a whole QGS LVMI was significantly correlated with Echo LVMI (r=0.96, p<0.001). Intra-observer and inter-observer analyses showed significant reproducibility (r=0.99 and r=0.98, respectively, p<0.001). In the patients with old myocardial infarction, but QGS LVMI was significantly lower than Echo LVMI (p<0.001), and the magnitude of the underestimation was closely related to the severity of the perfusion defect on the resting SPECT images.Conclusions: Measurements of LVMI by99mTc-tetrofosmin QGS are reproducible and consistent with echocardiograpic estimates. Underestimation in patients with severe perfusion defects must be taken into consideration.

Clinical value of lung uptake of iodine-123 metaiodobenzylguanidine (MIBG), a myocardial sympathetic nerve imaging agent, in patients with chronic heart failure

This study investigated the clinical value of I-123 MIBG pulmonary accumulation and washout in patients with chronic heart failure (CHF). Nineteen patients with CHF and 15 normal volunteers (NL) were included. The uptake ratio of heart to mediastinum (H/M), that of lung fields to mediastinum (L/M), and washout rate (WR) of the heart and lung fields were calculated in anterior planar images and compared with results of echocardiography and cardiac catheterization. In the CHF group, the lung uptake in delayed images increased and lung WR was decreased suggesting pulmonary endothelial lesions. Furthermore, there was a negative correlation between right and left lung WR and pulmonary arterial diastolic pressure (PA(d)) and pulmonary arterial systolic pressure (PA(s)) in the CHF group. Since the WR of MIBG reflected PA, it may be used as an index of severity of cardiac dysfunction.

Gamma camera coincidence imaging in oncology

Abstract Imaging positron-emitters, such as 18 F-fluorodeoxyglucose (FDG), using a gamma camera in coincidence mode is an exciting development in recent years. The proven clinical utility of FDG positron emission tomography (PET) in oncology and high cost of a PET scanner led to the development of this cheaper alternative. Gamma camera coincidence imaging (GCI) is capable of both positron and single-photon imaging. There are substantial differences in system design and performances between GCI and PET. GCI yields a spatial resolution comparable to that of PET, but the system sensitivity and lesion-detection capability, particularly for smaller lesions, are inferior. Despite these limitations, the clinical experiences with GCI in various oncological patients are so far encouraging. Recent advances in detectors, attenuation correction, and iterative reconstruction algorithms look promising to develop coincidence imaging as an appealing alternative to PET for clinical positron imaging.