PET/MRI, Part 2: Technologic Principles

Open-source phantom with dedicated in-house software for image quality assurance in hybrid PET systems

BACKGROUND

Patients' diagnosis, treatment and follow-up increasingly rely on multimodality imaging. One of the main limitations for the optimal implementation of hybrid systems in clinical practice is the time and expertise required for applying standardized protocols for equipment quality assurance (QA). Experimental phantoms are commonly used for this purpose, but they are often limited to a single modality and single quality parameter, lacking automated analysis capabilities. In this study, we developed a multimodal 3D-printed phantom and software for QA in positron emission tomography (PET) hybrid systems, with computed tomography (CT) or magnetic resonance (MR), by assessing signal, spatial resolution, radiomic features, co-registration and geometric distortions.

RESULTS

Phantom models and Python software for the proposed QA are available to download, and a user-friendly plugin compatible with the open-source 3D-Slicer software has been developed. The QA viability was proved by characterizing a Philips-Gemini-TF64-PET/CT in terms of signal response (mean, µ), intrinsic variability for three consecutive measurements (daily variation coefficient, CoVd) and reproducibility over time (variation coefficient across 5 months, CoVm). For this system, averaged recovery coefficient for activity concentration was µ = 0.90 ± 0.08 (CoVd = 0.6%, CoVm = 9%) in volumes ranging from 7 to 42 ml. CT calibration-curve averaged over time was HU = ( 951 ± 12 ) × density - ( 944 ± 15 ) with variability of slope and y-intercept of (CoVd = 0.4%, CoVm = 1.2%) and (CoVd = 0.4%, CoVm = 1.6%), respectively. Radiomics reproducibility resulted in (CoVd = 18%, CoVm = 30%) for PET and (CoVd = 15%, CoVm = 22%) for CT. Co-registration was assessed by Dice-Similarity-Coefficient (DSC) along 37.8 cm in superior-inferior (z) direction (well registered if DSC ≥ 0.91 and Δz ≤ 2 mm), resulting in 3/7 days well co-registered. Applicability to other scanners was additionally proved with Philips-Vereos-PET/CT (V), Siemens-Biograph-Vison-600-PET/CT (S) and GE-SIGNA-PET/MR (G). PET concentration accuracy was (µ = 0.86, CoVd = 0.3%) for V, (µ = 0.87, CoVd = 0.8%) for S, and (µ = 1.10, CoVd = 0.34%) for G. MR(T2) was well co-registered with PET in 3/4 cases, did not show significant distortion within a transaxial diameter of 27.8 cm and along 37 cm in z, and its radiomic variability was CoVd = 13%.

CONCLUSIONS

Open-source QA protocol for PET hybrid systems has been presented and its general applicability has been proved. This package facilitates simultaneously simple and semi-automated evaluation for various imaging modalities, providing a complete and efficient QA solution.

PET/MRI: pictorial review of hepatobiliary and pancreatic applications

PET and MRI both play valuable roles in the management of hepatobiliary and pancreatic (HBP) malignancies. Simultaneous PET/MRI combines the excellent soft-tissue resolution and anatomic details from MRI with functional information from PET in a single comprehensive examination. MRI is the main imaging modality in evaluating HCC, playing important roles in screening, characterization, local extent, and evaluating tumor response, whereas 18F-fluorodeoxyglucose (FDG) PET can help evaluate for lymph node involvement and metastatic disease. In cholangiocarcinoma and pancreatic malignancies, both PET and MRI have excellent utility in initial staging as well as assessing treatment response. In all HBP malignancies, FDG-PET/MRI is a unique problem-solving tool in complex cases and diagnostic challenges, especially after locoregional therapy and when differentiating residual or recurrent viable disease from inflammatory and other benign processes. In this manuscript, we review the role of PET/MRI in the diagnosis, staging, assessing treatment response, and characterizing post-treatment processes. With the introduction of multiple new tracers, the value of PET/MRI has not yet been fully realized, and more studies are needed to demonstrate the utility and efficacy of PET/MRI in improving patient care in hepatobiliary and pancreatic oncology.

PET/MRI evaluation of hepatobiliary tumors

Positron-emission tomography magnetic resonance imaging (PET/MRI) has emerged as a powerful hybrid molecular imaging technique in clinical practice, overcoming initial technical challenges to provide comprehensive anatomic and metabolic information. This advanced modality combines the superior soft tissue contrast of MRI with the metabolic insights of PET, offering advantages in hepatobiliary imaging, including improved detection of small liver metastases and reduced radiation exposure. The evolution of PET/MRI technology has been marked by significant advancements, such as the development of MRI-compatible PET detectors and sophisticated motion compensation techniques. These innovations have enhanced image quality and co-registration accuracy, crucial for hepatobiliary imaging. The integration of time-of-flight capability and silicon photomultipliers has further improved spatial resolution and sensitivity. PET/MRI protocols for liver imaging typically involve a whole-body scan followed by a targeted liver examination, utilizing radiotracers like FDG and DOTATATE. This approach allows for comprehensive staging and detailed liver assessment in a single session, potentially altering management decisions in up to 30% of patients with intrahepatic cholangiocarcinoma. While PET/MRI excels in characterizing various hepatobiliary lesions, including hepatocellular carcinoma and cholangiocarcinoma, challenges remain in differentiating certain benign entities like small hemangiomas from metastases. Ongoing research and clinical experience continue to refine the role of PET/MRI in hepatobiliary imaging, promising improved diagnostic accuracy and patient care.

Imaging Modalities for Head and Neck Cancer: Present and Future

Several imaging modalities are utilized in the diagnosis, treatment, and surveillance of head and neck cancer. First-line imaging remains computed tomography (CT); however, MRI, PET with CT (PET/CT), and ultrasound are often used. In the last decade, several new imaging modalities have been developed that have the potential to improve early detection, modify treatment, decrease treatment morbidity, and augment surveillance. Among these, molecular imaging, lymph node mapping, and adjustments to endoscopic techniques are promising. The present review focuses on existing imaging, novel techniques, and the recent changes to imaging practices within the field.

Radiation Detectors and Sensors in Medical Imaging

Medical imaging instrumentation design and construction is based on radiation sources and radiation detectors/sensors. This review focuses on the detectors and sensors of medical imaging systems. These systems are subdivided into various categories depending on their structure, the type of radiation they capture, how the radiation is measured, how the images are formed, and the medical goals they serve. Related to medical goals, detectors fall into two major areas: (i) anatomical imaging, which mainly concerns the techniques of diagnostic radiology, and (ii) functional-molecular imaging, which mainly concerns nuclear medicine. An important parameter in the evaluation of the detectors is the combination of the quality of the diagnostic result they offer and the burden of the patient with radiation dose. The latter has to be minimized; thus, the input signal (radiation photon flux) must be kept at low levels. For this reason, the detective quantum efficiency (DQE), expressing signal-to-noise ratio transfer through an imaging system, is of primary importance. In diagnostic radiology, image quality is better than in nuclear medicine; however, in most cases, the dose is higher. On the other hand, nuclear medicine focuses on the detection of functional findings and not on the accurate spatial determination of anatomical data. Detectors are integrated into projection or tomographic imaging systems and are based on the use of scintillators with optical sensors, photoconductors, or semiconductors. Analysis and modeling of such systems can be performed employing theoretical models developed in the framework of cascaded linear systems analysis (LCSA), as well as within the signal detection theory (SDT) and information theory.

Synthetic 18F labeled biomolecules that are selective and promising for PET imaging: major advances and applications

The concept of positron emission tomography (PET) based imaging was developed more than 40 years ago. It has been a widely adopted technique for detecting and staging numerous diseases in clinical settings, particularly cancer, neuro- and cardio-diseases. Here, we reviewed the evolution of PET and its advantages over other imaging modalities in clinical settings. Primarily, this review discusses recent advances in the synthesis of 18F radiolabeled biomolecules in light of the widely accepted performance for effective PET. The discussion particularly emphasizes the 18F-labeling chemistry of carbohydrates, lipids, amino acids, oligonucleotides, peptides, and protein molecules, which have shown promise for PET imaging in recent decades. In addition, we have deliberated on how 18F-labeled biomolecules enable the detection of metabolic changes at the cellular level and the selective imaging of gross anatomical localization via PET imaging. In the end, the review discusses the future perspective of PET imaging to control disease in clinical settings. We firmly believe that collaborative multidisciplinary research will further widen the comprehensive applications of PET approaches in the clinical management of cancer and other pathological outcomes.

New Perspectives in Radiological and Radiopharmaceutical Hybrid Imaging in Progressive Supranuclear Palsy: A Systematic Review

Progressive supranuclear palsy (PSP) is a neurodegenerative disease characterized by four-repeat tau deposition in various cell types and anatomical regions, and can manifest as several clinical phenotypes, including the most common phenotype, Richardson's syndrome. The limited availability of biomarkers for PSP relates to the overlap of clinical features with other neurodegenerative disorders, but identification of a growing number of biomarkers from imaging is underway. One way to increase the reliability of imaging biomarkers is to combine different modalities for multimodal imaging. This review aimed to provide an overview of the current state of PSP hybrid imaging by combinations of positron emission tomography (PET) and magnetic resonance imaging (MRI). Specifically, combined PET and MRI studies in PSP highlight the potential of [18F]AV-1451 to detect tau, but also the challenge in differentiating PSP from other neurodegenerative diseases. Studies over the last years showed a reduced synaptic density in [11C]UCB-J PET, linked [11C]PK11195 and [18F]AV-1451 markers to disease progression, and suggested the potential role of [18F]RO948 PET for identifying tau pathology in subcortical regions. The integration of quantitative global and regional gray matter analysis by MRI may further guide the assessment of reduced cortical thickness or volume alterations, and diffusion MRI could provide insight into microstructural changes and structural connectivity in PSP. Challenges in radiopharmaceutical biomarkers and hybrid imaging require further research targeting markers for comprehensive PSP diagnosis.

Multi-Modality Imaging of Atheromatous Plaques in Peripheral Arterial Disease: Integrating Molecular and Imaging Markers

Peripheral artery disease (PAD) is a common and debilitating condition characterized by the narrowing of the limb arteries, primarily due to atherosclerosis. Non-invasive multi-modality imaging approaches using computed tomography (CT), magnetic resonance imaging (MRI), and nuclear imaging have emerged as valuable tools for assessing PAD atheromatous plaques and vessel walls. This review provides an overview of these different imaging techniques, their advantages, limitations, and recent advancements. In addition, this review highlights the importance of molecular markers, including those related to inflammation, endothelial dysfunction, and oxidative stress, in PAD pathophysiology. The potential of integrating molecular and imaging markers for an improved understanding of PAD is also discussed. Despite the promise of this integrative approach, there remain several challenges, including technical limitations in imaging modalities and the need for novel molecular marker discovery and validation. Addressing these challenges and embracing future directions in the field will be essential for maximizing the potential of molecular and imaging markers for improving PAD patient outcomes.

Imaging of lung cancer

Lung cancer is the leading cause of cancer-related mortality globally. Imaging is essential in the screening, diagnosis, staging, response assessment, and surveillance of patients with lung cancer. Subtypes of lung cancer can have distinguishing imaging appearances. The most frequently used imaging modalities include chest radiography, computed tomography, magnetic resonance imaging, and positron emission tomography. Artificial intelligence algorithms and radiomics are emerging technologies with potential applications in lung cancer imaging.

PET-MRI for evaluation of response to radiochemotherapy in patients with locally advanced cervical cancer

OBJECTIVE

We aimed to analyze the diagnostic test accuracy of positron emission tomography and a magnetic resonance imaging scan (PET-MRI) fusion in evaluating tumor response after radiochemotherapy in patients with locally advanced cervical cancer.

METHODS

Patients treated at two institutes between January 2008 and December 2016 were studied retrospectively. Re-evaluation by positron emission tomography (PET) and magnetic resonance imaging (MRI) was performed in a non-concurrent way 4-8 weeks after treatment. A nuclear medicine doctor and a radiologist (subsequently referred as "radiologists"), both experts in gynecological oncology, re-examined the post-treatment MRI and positron emission tomography-computed tomography (PET-CT) separately, and then performed a fusion of these examinations. In this study we describe this "a posteriori fusion methodology", with two levels, enabling limitation of anatomical shifts. The gold standard was anatomical pathology analysis of the surgical specimen, since all patients underwent surgery following this radiological re-evaluation. The radiologists' degree of certainty in their diagnoses, and the impact of fusion on their diagnostic confidence were assessed by the radiologists, using two Likert judgment scales. They also adjudicated on possible changes of interpretation after the fusion.

RESULTS

Thirty-one patients were included. The PET-MRI fusion has a sensitivity of 79% and a specificity of 90%. The positive predictive value (PPV) was 94%, and the negative predictive value (NPV) was 69%. In 45% of cases (n=13), radiologists reported an improvement in their degree of certainty in their diagnosis using a Likert judgment scale, due to inspecting the PET and MRI fused. A change in interpretation of tumor response was observed using a Likert judgment scale in 31% of cases.

CONCLUSION

PET-MRI fusion improves the radiologist's own diagnostic confidence in assessing response to concurrent radiochemotherapy in locally advanced cervical cancer. More studies using a latest generation hybrid system will be necessary to further compare to MRI and PET-CT.

PET/MR Part 4: Clinical Applications of PET/MRI

Position emission tomography (PET) and magnetic resonance imaging (MRI) as a hybrid modality provides novel imaging opportunities. While there are a very broad array of pathologies that could benefit from PET/MRI, there is only a narrow range of applications where benefit over standard care justifies the higher resource utilization and, in particular, offers a net positive trade-off over PET/CT. This benefit is generally associated with the omission of CT and the associated radiation dose from the patient workup. This manuscript provides a summary of the generally accepted clinical applications of PET/MRI in both adult and pediatric populations. While there are a number of potential applications and certainly exciting research that may expand applications in the future, the purpose of this paper was to focus on current, mainstream applications. This is the final manuscript in a four-part integrated series sponsored by the SNMMI-TS PET/MR Task Force in conjunction with the SNMMI-TS Publication Committee.

PET/MR Part 3: PET/MRI Protocols and Procedures

The emergence of position emission tomography (PET) and magnetic resonance imaging (MRI) as a hybrid modality has demanded new approaches to protocol and procedure. While protocols for MRI and PET individually lend themselves to synergistic and simultaneous approaches, there are a number if unique challenges and patient preparations that require consideration. This manuscript provides an insight into the protocols, procedures and challenges associated with simultaneous PET/MRI in both adult and pediatric populations. While protocols may be specific to applications or pathologies of interest, a richer discussion of the clinical applications of PET/MRI is beyond the scope of this manuscript and will be detailed in part 4 of the series. The foundations of PET/MRI protocols is an understanding of the various MRI sequences which are outlined succinctly. The principles outlined for protocols and procedures are general in nature and specific application will vary among departments. Given the procedures of PET is well established amongst the readership of this journal, the manuscript provides an emphasis on MR factors unless specific variations in standard PET protocol or procedure are driven by the simultaneous MRI. This manuscript is the third in a four-part integrated series sponsored by the SNMMI-TS PET/MR Task Force in conjunction with the SNMMI-TS Publication Committee.