PET/MRI system design

Introduction  

The combination of clinical MRI and PET systems has received increased attention in recent years. In contrast to currently used PET/CT systems, PET/MRI offers not only improved soft-tissue contrast and reduced levels of ionizing radiation, but also a wealth of MRI-specific information such as functional, spectroscopic and diffusion tensor imaging. Combining PET and MRI, however, has proven to be very challenging, due to the detrimental cross-talk effects between the two systems.     

Detection of dopaminergic neurotransmitter activity using pharmacologic MRI: Correlation with PET,microdialysis, and behavioral data

The metabolic activation resulting from direct dopaminergic stimulation can be detected using auto-radiography, positron emission tomography (PET) or, potentially, fMRI techniques. To establish the validity of the latter possibility, we have performed a number of experiments. We measured the regional selectivity of two different dopaminergic ligands: the dopamine release compound D-amphetamine and the dopamine transporter antagonist 2β-carbomethoxy-3β-(4-fluoropheny) tropane (CFT). Both compounds led to increased signal intensity in gradient echo images in regions of the brain with high dopamine receptor density (frontal cortex, striatum, cingulate cortex ? parietal cortex). Lesioning the animals with unilaterally administered 6-hydroxydopamine (6-OHDA) led to ablation of the phMRI response on the ipsilateral side; control measurements of rCBV and rCBF using bolus injections of Gd,-DTPA showed that the baseline rCBV and rCBF values weire intact on the lesioned side. The time course of the BOLD signal changes paralleled the changes observed by microdialyisis measurements of dopamine release in the striatum for both amphetamine and CFT; peaking at 20–40 min after injection and returning to baseline at about 70–90 min. Signal changes were not correlated with either heart rate, blood pressure or pCO₂. Measurement of PET binding in the same animals showed an excellent correlation with the phMRI data when compared by either measurements of the number of pixels activated or percent signal change in a given region. The time course for the behavioral measurements of rotation in the 6-OHDA lesioned animals correlated with the phMRI. These experiments demonstrate that phMRI will become a valuable, noninvasive tool for investigation of neurotransmitter activity in vivo. Key words: fMRI; dopamine; amphetamine; CFT.     

PET measurements of dopaminergic pathways in the brain.

Positron emission tomography (PET) measurements of dopaminergic pathways have revealed several new insights into the role of dopamine in the pathophysiology and pharmacology of brain diseases such as Parkinson's disease (PD), dystonia and schizophrenia. PET studies of regional blood flow or metabolism identifies sites of regional pathology. Drug-induced changes in flow or metabolism indicate the function of dopamine-mediated pathways. Measurements of radioligand binding in vivo with PET reveals abnormalities associated with specific diseases and the actions of various drugs that affect the dopaminergic system. Finally, PET measurements of the uptake of analogues of levodopa provide clues to the function of dopamine pathways potentially important for diagnosis and treatment of disease like PD.     

Integrated PET/CT in the preoperative staging of lung cancer: A prospective comparison of CT,PET and integrated PET/CT

Purpose

The purpose of this study was to evaluate the role of integrated PET/CT in the staging of lung cancer compared with CT alone or PET alone.

Materials and methods

Thirty-three patients underwent integrated PET/CT for the staging of lung cancer. The tumor, node and metastasis (TNM) stages were assessed with CT, PET and integrated PET–CT and compared with the surgical and pathological staging.

Results

CT correctly evaluated the (T) status in (64%) of the patients, PET in (59%) and PET/CT in (86%). CT correctly evaluated the (N) status in (73%) of the patients, PET in (76%), and PET/CT (88%) with accuracy, sensitivity, specificity, PPV and NPV were 73%, 78%, 71%, 50% and 94% for CT, 76%, 67%, 79%, 55% and 95% for PET and 88%, 89%, 88%, 73% and 100% for PET/CT respectively, and for (M) status were 91%, 86%, 92%, 75% and 96% for CT, 88%, 71%, 92%, 71% and 92% for PET and 97%, 100%, 96%, 88% and 100% for PET/CT respectively. Regarding the overall TNM staging CT correctly staged 24 patients. PET correctly staged 23 cases while PET/CT correctly staged 30 cases. A significant difference in the accuracy of overall tumor staging between PET/CT and CT (P = 0.0412) or PET (P = 0.0233).

Conclusion

The integrated PET/CT is superior to either CT or PET in the staging of lung cancer which has an important impact on selection of the appropriate treatment regimen.     

Modulation of dopaminergic and glutamatergic brain function: PET studies on parkinsonian rats.

Degeneration of dopaminergic neurons of the substantia nigra pars compacta is a cardinal feature of Parkinson's disease (PD). Although uncertain, the pathology has been suggested to derive from a malfunction of the complex interaction between dopaminergic and metabotropic glutamate receptors (mGluRs). To further address this issue, we investigated the imaging profile and expression of dopamine D(2) receptors and mGluRs in a classic parkinsonian rodent model induced by the toxin 6-hydroxydopamine. METHODS: Adult male Sprague-Dawley rats (250-300 g) received a stereotaxic injection of 8 mug/2 muL of 6-hydroxydopamine (n = 6) or saline solution (n = 4) in the right medial forebrain bundle. Small-animal PET was performed on all rats 4 wk after the surgical procedure to assess dopamine transporter (DAT) status using (11)C-2beta-carbomethoxy-3beta-(4-fluorophenyl)-tropane (CFT), as well as dopamine D(2) receptor and mGluR(5) modulation using (11)C-raclopride and 2-(11)C-methyl-6-(2-phenylethynyl)-pyridine ((11)C-MPEP), respectively. Behavioral studies were also conducted 6 wk after lesioning by d-amphetamine challenge. Immunohistochemistry and Western blotting were carried out at 8 wk after lesioning to confirm dopamine fiber, neuronal loss, and level of striatal mGluR(5) expression. RESULTS: PET images showed decreased (11)C-CFT binding on the lesioned side, including the structures of the striatum, hippocampus, and cortex, compared with the contralateral intact side. Interestingly, dopamine D(2) receptors and mGluR(5) upregulation were observed in the right striatum, hippocampus, and cortex, using (11)C-raclopride and (11)C-MPEP, respectively. A negative correlation was also found between the percentage change in mGluR(5) expression and DAT function. Finally, tyrosine hydroxylase immunoreactivity confirmed both dopamine fiber loss (t test, P < 0.01) and neuronal loss (t test, P < 0.01) on the lesioned side. These changes were accompanied by a strongly enhanced mGluR(5) expression in the right striatum of the lesioned side analyzed by Western plot. CONCLUSION: These findings support the existence of compensatory mechanisms in nigrostriatal dopamine degeneration and provide new insights that help further dissect some of the pathways underlying neurodegeneration. In addition, these results reconfirm that PET is a valuable tool for multilevel receptor studies, significantly contributing to the understanding of pathogenic mechanisms and ultimately opening new avenues in the study of neuroprotective approaches toward PD.     

PET and PET/CT in pediatric oncology

18F-fluorodeoxyglucose positron emission tomography (FDG-PET) and FDG-PET/computed tomography (CT) are becoming increasingly important imaging tools in the noninvasive evaluation and monitoring of children with known or suspected malignant diseases. In this review, we discuss the preparation of children undergoing PET studies and review radiation dosimetry and its implications for family and caregivers. We review the normal distribution of 18F-fluorodeoxyglucose (FDG) in children, common variations of the normal distribution, and various artifacts that may arise. We show that most tumors in children accumulate and retain FDG, allowing high-quality images of their distribution and pathophysiology. We explore the use of FDG-PET in the study of children with the more common malignancies, such as brain neoplasms and lymphomas, and the less-common tumors, including neuroblastomas, bone and soft-tissue sarcomas, Wilms' tumors, and hepatoblastomas. For comparison, other PET tracers are included because they have been applied in pediatric oncology. Multiple multicenter trials are underway that use FDG-PET in the management of children with neoplastic disease; these studies should give us greater insight into the impact FDG-PET can make in their care. PET is emerging as an important diagnostic imaging tool in the evaluation of pediatric cancers. The recent advent of dual-modality PET-computed tomography (PET/CT) imaging systems has added unprecedented diagnostic capability by revealing the precise anatomical localization of metabolic information and metabolic characterization of normal and abnormal structures. The use of CT transmission scanning for attenuation correction has shortened the total acquisition time, which is an especially desirable attribute in pediatric imaging. Moreover, expansion of the regional distribution of the most common PET radiotracer, FDG, and the introduction of mobile PET units have greatly increased access to this powerful diagnostic imaging technology. Here, we review the clinical applications of PET and PET/CT in pediatric oncology. General considerations in patient preparation and radiation dosimetry will be discussed.     

PET/MRI of the hepatobiliary system: Review of techniques and applications

Simultaneous positron emission tomography and MRI (PET/MRI) is an emerging technology that offers the benefits of MRI, including excellent soft tissue contrast, lack of ionizing radiation, and functional MRI techniques, with the physiologic information provided by PET. Although most PET/MRI systems are currently installed in tertiary care centers, PET/MRI technology is becoming increasingly widespread. The usefulness of PET/MRI varies by tumor type and organ system and has been shown to have utility in evaluation of primary and secondary hepatic neoplasms. Understanding the appropriate applications, techniques and relevant imaging findings is important for practicing radiologists considering or currently utilizing PET/MR for the evaluation of primary liver neoplasms, including hepatocellular carcinoma (HCC), as well as staging of biliary neoplasms including cholangiocarcinoma and gallbladder cancer, identification of liver metastases, and staging of neuroendocrine tumor.     

A look ahead: PET/MR versus PET/CT

Introduction  

Integration of positron emission tomography (PET) and magnetic resonance (MR) has become a topic of increasing interest to the imaging community over the past two years.     

PET and PET/CT in endocrine tumours

Functional information provided by PET tracers together with the superior image quality and the better data quantification by PET technology had a changing effect on the significance of nuclear medicine in medical issues. Recently introduced hybrid PET/CT systems together with the introduction of novel PET radiopharmaceuticals have contributed to the fact that nuclear medicine has become a growing diagnostic impact on endocrinology. In this review imaging strategies, different radiopharmaceuticals including the basic mechanism of their cell uptake, and the diagnostic value of PET and PET/CT in endocrine tumours except differentiated thyroid carcinomas will be discussed.     

PET/CT in lymphoma: prospective study of enhanced full-dose PET/CT versus unenhanced low-dose PET/CT.

PET/CT combines functional and morphologic data and increases diagnostic accuracy in a variety of malignancies. This study prospectively compares the agreement between contrast-enhanced full-dose PET/CT and unenhanced low-dose PET/CT in lesion detection and initial staging of Hodgkin's disease and non-Hodgkin's lymphoma. METHODS: Forty-seven biopsy-proven lymphoma patients underwent a 18F-FDG PET/CT study that included unenhanced low-dose CT and enhanced full-dose CT for initial staging. Patients who had undergone previous diagnostic CT for initial staging were excluded. For every patient, each modality of PET/CT images was evaluated by either of 2 pairs of readers, with each pair comprising 1 experienced radiologist and 1 experienced nuclear physician. While evaluating one of the 2 types of PET/CT, the readers were unaware of the results of the other type. Lesion detection, number of sites affected in each anatomic region, and disease stage were assessed. Agreement between techniques was determined by the kappa-statistic, and discordances were studied by the McNemar test. Clinical, analytic, histopathologic, diagnostic CT, and PET data; data from other imaging techniques; and follow-up data constituted the reference standard. RESULTS: For region-based analysis, no significant differences were found between unenhanced low-dose PET/CT and contrast-enhanced full-dose PET/CT, although full-dose PET/CT showed fewer indeterminate findings and a higher number of extranodal sites affected than did low-dose PET/CT. Agreement between the 2 types of PET/CT was almost perfect for disease stage (kappa = 0.92; P < 0.001). CONCLUSION: Our study showed a good correlation between unenhanced low-dose PET/CT and contrast-enhanced full-dose PET/CT for lymph node and extranodal disease in lymphomas, suggesting that unenhanced low-dose PET/CT might suffice in most patients as the only imaging technique for the initial staging of lymphomas, reserving diagnostic CT for selected cases.