Metabolic imaging using PET

Carbon-11: Radiochemistry and Target-Based PET Molecular Imaging Applications in Oncology, Cardiology, and Neurology

The positron emission tomography (PET) molecular imaging technique has gained its universal value as a remarkable tool for medical diagnosis and biomedical research. Carbon-11 is one of the promising radiotracers that can report target-specific information related to its pharmacology and physiology to understand the disease status. Currently, many of the available carbon-11 (t_1/2 = 20.4 min) PET radiotracers are heterocyclic derivatives that have been synthesized using carbon-11 inserted different functional groups obtained from primary and secondary carbon-11 precursors. A spectrum of carbon-11 PET radiotracers has been developed against many of the upregulated and emerging targets for the diagnosis, prognosis, prediction, and therapy in the fields of oncology, cardiology, and neurology. This review focuses on the carbon-11 radiochemistry and various target-specific PET molecular imaging agents used in tumor, heart, brain, and neuroinflammatory disease imaging along with its associated pathology.

Physiologic and hypermetabolic breast 18-F FDG uptake on PET/CT during lactation

OBJECTIVE

To investigate the patterns of breast cancer-related and lactation-related ¹⁸F-FDG uptake in breasts of lactating patients with pregnancy-associated breast cancer (PABC) and without breast cancer.

METHODS

¹⁸F-FDG-PET/CT datasets of 16 lactating patients with PABC and 16 non-breast cancer lactating patients (controls) were retrospectively evaluated. Uptake was assessed in the tumor and non-affected lactating tissue of the PABC group, and in healthy lactating breasts of the control group, using maximum and mean standardized uptake values (SUVmax and SUVmean, respectively), and breast-SUVmax/liver-SUVmean ratio. Statistical tests were used to evaluate differences and correlations between the groups.

RESULTS

Physiological uptake in non-breast cancer lactating patients' breasts was characteristically high regardless of active malignancy status other than breast cancer (SUVmax = 5.0 ± 1.7, n = 32 breasts). Uptake correlated highly between the two breasts (r = 0.61, p = 0.01), but was not correlated with age or lactation duration (p = 0.24 and p = 0.61, respectively). Among PABC patients, the tumors demonstrated high ¹⁸F-FDG uptake (SUVmax = 7.8 ± 7.2, n = 16), which was 326-643% higher than the mostly low physiological FDG uptake observed in the non-affected lactating parenchyma of these patients (SUVmax = 2.1 ± 1.1). Overall, ¹⁸F-FDG uptake in lactating breasts of PABC patients was significantly decreased by 59% (p < 0.0001) compared with that of lactating controls without breast cancer.

CONCLUSION

¹⁸F-FDG uptake in lactating tissue of PABC patients is markedly lower compared with the characteristically high physiological uptake among lactating patients without breast cancer. Consequently, breast tumors visualized by ¹⁸F-FDG uptake in PET/CT were comfortably depicted on top of the background ¹⁸F-FDG uptake in lactating tissue of PABC patients.

KEY POINTS

• FDG uptake in the breast is characteristically high among lactating patients regardless of the presence of an active malignancy other than breast cancer. • FDG uptake in non-affected lactating breast tissue is significantly lower among PABC patients compared with that in lactating women who do not have breast cancer. • In pregnancy-associated breast cancer patients, ¹⁸F-FDG uptake is markedly increased in the breast tumor compared with uptake in the non-affected lactating tissue, enabling its prompt visualization on PET/CT.

PET Imaging of Cardiac Hypoxia: Hitting Hypoxia Where It Hurts

Purpose of Review

In this review, we outline the potential for hypoxia imaging as a diagnostic and prognostic tool in cardiology. We describe the lead hypoxia PET radiotracers currently in development and propose a rationale for how they should most appropriately be screened and validated.

Recent Findings

While the majority of hypoxia imaging agents has been developed for oncology, the requirements for hypoxia imaging in cardiology are different. Recent work suggests that the bis(thiosemicarbazone) family of compounds may be capable of detecting the subtle degrees of hypoxia associated with cardiovascular syndromes, and that they have the potential to be “tuned” to provide different tracers for different applications.

Summary

New tracers currently in development show significant promise for imaging evolving cardiovascular disease. Fundamental to their exploitation is their careful, considered validation and characterization so that the information they provide delivers the greatest prognostic insight achievable.

Metabolic Imaging in Humans

Metabolic imaging enhances understanding of disease metabolisms and holds great potential as a measurement tool for evaluating disease prognosis and treatment effectiveness. Advancement of techniques, such as magnetic resonance spectroscopy, positron emission tomography, and mass spectrometry, allows for improved accuracy for quantification of metabolites and present unique possibilities for use in clinic. This article reviews and discusses literature reports of metabolic imaging in humans published since 2010 according to disease type, including cancer, degenerative disorders, psychiatric disorders, and others, as well as the current application of the various related techniques.

Quantitative cardiac positron emission tomography: the time is coming!

In the last 20 years, the use of positron emission tomography (PET) has grown dramatically because of its oncological applications, and PET facilities are now easily accessible. At the same time, various groups have explored the specific advantages of PET in heart disease and demonstrated the major diagnostic and prognostic role of quantitation in cardiac PET. Nowadays, different approaches for the measurement of myocardial blood flow (MBF) have been developed and implemented in user-friendly programs. There is large evidence that MBF at rest and under stress together with the calculation of coronary flow reserve are able to improve the detection and prognostication of coronary artery disease. Moreover, quantitative PET makes possible to assess the presence of microvascular dysfunction, which is involved in various cardiac diseases, including the early stages of coronary atherosclerosis, hypertrophic and dilated cardiomyopathy, and hypertensive heart disease. Therefore, it is probably time to consider the routine use of quantitative cardiac PET and to work for defining its place in the clinical scenario of modern cardiology.

Motion frozen (18)F-FDG cardiac PET

BACKGROUND

PET reconstruction incorporating spatially variant 3D Point Spread Function (PSF) improves contrast and image resolution. "Cardiac Motion Frozen" (CMF) processing eliminates the influence of cardiac motion in static summed images. We have evaluated the combined use of CMF- and PSF-based reconstruction for high-resolution cardiac PET.

METHODS

Static and 16-bin ECG-gated images of 20 patients referred for (18)F-FDG myocardial viability scans were obtained on a Siemens Biograph-64. CMF was applied to the gated images reconstructed with PSF. Myocardium to blood contrast, maximum left ventricle (LV) counts to defect contrast, contrast-to-noise (CNR) and wall thickness with standard reconstruction (2D-AWOSEM), PSF, ED-gated PSF, and CMF-PSF were compared.

RESULTS

The measured wall thickness was 18.9 ± 5.2 mm for 2D-AWOSEM, 16.6 ± 4.5 mm for PSF, and 13.8 ± 3.9 mm for CMF-PSF reconstructed images (all P < .05). The CMF-PSF myocardium to blood and maximum LV counts to defect contrasts (5.7 ± 2.7, 10.0 ± 5.7) were higher than for 2D-AWOSEM (3.5 ± 1.4, 6.5 ± 3.1) and for PSF (3.9 ± 1.7, 7.7 ± 3.7) (CMF vs all other, P < .05). The CNR for CMF-PSF (26.3 ± 17.5) was comparable to PSF (29.1 ± 18.3), but higher than for ED-gated dataset (13.7 ± 8.8, P < .05).

CONCLUSION

Combined CMF-PSF reconstruction increased myocardium to blood contrast, maximum LV counts to defect contrast and maintained equivalent noise when compared to static summed 2D-AWOSEM and PSF reconstruction.

Enhanced definition PET for cardiac imaging

BACKGROUND

We aimed to determine in phantom and cardiac clinical studies the impact of a new high-resolution PET image reconstruction.

METHODS

A phantom with cardiac insert filled with (18)F, 14 (18)F-FDG viability studies and 15 (82)Rb perfusion studies were acquired on a Siemens Biograph-64 (4-ring). The data were reconstructed with 2D- and 3D-attenuation weighted ordered subsets expectation maximization (AWOSEM), and high-definition reconstruction (HD.PET). We calculated wall/cavity contrast, contrast-to-noise ratio (CNR), wall thickness, motion/thickening and ejection fraction.

RESULTS

In the phantom study, we found an increase in defect size (up to 26%), contrast (up to 48%) and CNR (1.9) with HD.PET as compared to standard techniques. The contrast increased on HD.PET images compared to 2D- and 3D-AWOSEM for viability (14.0% +/- 4.8%) and perfusion studies (7.3% +/- 4.3%) (P < .05). Average CNR increased with HD.PET by 79.4% +/- 17.1% and 68.8% +/- 3.0% in viability and perfusion studies respectively (all P < .05). Average wall thickness with HD.PET decreased in the phantom study by 1.3 +/- 0.3 mm and the viability studies by 1.9 +/- 0.7 mm but not in the perfusion studies. The functional measurements were not significantly different for any techniques.

CONCLUSIONS

We demonstrated both in phantom and patient cardiac studies that HD.PET improves image contrast, defect definition, and CNR.

Time course of alterations in myocardial glucose utilization in the Zucker diabetic fatty rat with correlation to gene expression of glucose transporters: a small-animal PET investigation

Diabetic cardiomyopathy is associated with abnormalities in glucose metabolism. We evaluated myocardial glucose metabolism in a rodent model of type 2 diabetes, namely the Zucker diabetic fatty (ZDF) rat, and validated PET measurements of glucose uptake against gene and protein expression of glucose transporters (GLUTs).

METHODS

Six lean and ZDF rats underwent small-animal PET at the age of 14 wk and at the age of 19 wk. The imaging protocol consisted of a 60-min dynamic acquisition with 18F-FDG (18.5-29.6 MBq). Dynamic images were reconstructed using filtered backprojection with a 2.5 zoom on the heart and 40 frames per imaging session. PET measurements of myocardial glucose uptake (MGUp) rate and utilization were determined with an input function derived by the hybrid image-blood-sampling algorithm on recovery-corrected anterolateral myocardial regions of interest. After the PET session at week 19 (W19), hearts were extracted for gene and protein expression analysis of GLUT-1 and GLUT-4. The dependence of MGUp on gene expression of GLUT-1 and GLUT-4 was characterized by multiple-regression analysis.

RESULTS

MGUp in ZDF rats at both week 14 (W14) and W19 (P < 0.006) was significantly lower than MGUp in lean littermate control rats. Moreover, lean rats at W19 displayed significantly higher MGUp than they did at W14 (P = 0.007). Consistent with a diminished MGUp result, gene expression of GLUT-4 was significantly (P = 0.004) lower in ZDF rats. Finally, MGUp significantly (P = 0.0003) correlated with gene expression of GLUT-4.

CONCLUSION

Using small-animal PET, we confirmed alterations in myocardial glucose utilization and validated PET measurement of MGUp against gene and protein expression of GLUTs in the diabetic heart of an animal model of type 2 diabetes.