State of the Art Hybrid Technology: PET/CT

In the recent years, hybrid positron emission tomography (PET)/computed tomography (CT) scanners have been increasingly utilized in cardiac applications. PET imaging quality has been improved by the use of new scintillators, small detector element size, and fully 3D iterative reconstruction techniques with time-of-flight information and resolution recovery. Further quality enhancements for cardiac imaging can be obtained by tracking and correcting for cardiac and breathing motion with respiratory gating devices and advanced software techniques. The primary tracers used for PET/CT cardiac imaging are Rubidium-82 (⁸²Rb) and Nitrogen-13-ammonia (¹³N-ammonia) and 18-F fluorodeoxyglucose used for myocardial viability imaging. A new F-18 perfusion tracer (F-18 Flurpiridaz) is being evaluated. High-resolution multi-slice CT component of the hybrid scanner allows accurate attenuation correction for PET, measurement of CT calcium, and contrast CT angiography. Hybrid PET/CT protocols have demonstrated increased diagnostic accuracy for the detection of obstructive disease compared with standalone techniques. Radiation dose to the patient is a concern in hybrid imaging because multiple scans are performed in one scanning session. 3D PET acquisition combined with the new low-dose CT protocols can reduce the doses significantly. Hybrid PET/CT scanners have also been utilized for anatomically-guided molecular imaging of plaque biology in the carotid vessels, aorta, and coronary vessels. This review summarizes the state-of-the-art hybrid imaging PET/CT instrumentation and advances in the image quality related to cardiac imaging.     

State of the Art Hybrid Technology: SPECT/CT

Several technological advancements have been observed in the field of cardiovascular imaging in recent years. In particular, the integration of nuclear medicine cameras with multidetector computed tomography scanners provides a unique opportunity to examine cardiac and vascular anatomic abnormalities, on one hand, and their physiological consequences, on the other, during the same examination. Moreover, fusion imaging between computed tomography coronary angiography and single-photon emission computed tomography are merged into a single image, enhancing the clinician’s ability to determine the functional consequences of a coronary stenosis. Finally, the marked increase in the volume of conventional imaging tests in the past few years has contributed to a marked rise in health care costs. In the future, new imaging tests must demonstrate their ability to enhance the quality of care and improve outcomes, while remaining cost-effective, before being approved for widespread use. To this end, validation in sound clinical research outcomes studies will be fundamental.     

一体化PET/MR分析脑默认网络工作机制

目的 利用一体化PET/MR探讨脑默认网络功能连接和葡萄糖代谢的空间相关性。方法 对9名健康人行一体化PET/MR脑成像,获得同步的静息态MR脑功能成像（fMRI）和PET图像。分析静息态脑默认网络脑区连接与葡萄糖摄取分布的相关性。结果 基于静息态fMRI的脑默认网络与PET图像高代谢区域有很好的空间分布相似性。相关性分析显示,右侧后扣带区域内脑功能连接值与相对葡萄糖摄取（rGU）值的相关性最显著（r_s=0.833,P<0.001）。结论 一体化PET/MR可为研究脑默认网络的神经生理机制提供新的手段。     

一体化18F-FDG PET/MR评估缺血性脑血管病

目的 探讨一体化¹⁸F-FDG PET/MR显像对于慢性缺血性脑血管病的应用价值。方法 对10名成年健康志愿者及17例慢性单侧颈内动脉（ICA）或大脑中动脉（MCA）闭塞患者行一体化¹⁸F-FDG PET/MR检查。由2名医师分析图像，定量分析和比较健康志愿者左侧与右侧不同脑区、慢性缺血性脑血管病患者脑梗死患侧与对侧相应区域及脑梗死周围区与对侧相应区域间平均ADC值（ADC_mean）、平均标准化摄取值（SUV_mean）及最大标准化摄取值（SUV_max）的差异。结果 10名健康志愿者MRI均未见异常，¹⁸F-FDG脑代谢图像清晰，各脑区代谢分布对称；左侧与右侧额叶、顶叶、颞叶、枕叶ADC_mean、SUV_mean、SUV_max差异均无统计学意义（P均>0.05）。17例慢性缺血性脑血管病MRI均可见脑梗死灶，¹⁸F-FDG脑代谢图显示患侧均较对侧相应区域ADC_mean、SUV_mean、SUV_max明显减低（P均<0.01）；脑梗死周围区与对侧相应区域比较ADC_mean、SUV_mean、SUV_max亦明显减低（P均<0.01）。结论 利用一体化¹⁸F-FDG PET/MR检查可同时获得脑结构和脑代谢综合信息，全面评价慢性缺血性脑血管病。     

Cardiac PET/MRI

Medical imaging plays an important role in clinical management of patients with cardiac diseases and in preclinical and clinical research. The recent availability of hybrid PET/MRI devices that combine positron emission tomography (PET) with magnetic resonance imaging (MRI) opens up new opportunities. Technical advancements have been necessary to make the two systems with different underlying principles work well together. Growing evidence points to a significant value of this novel modality for imaging of the myocardium and the coronary arteries in order to gain broad insight into the morphological, functional, molecular and cellular aspects of cardiac pathophysiology. PET/MRI can deliver the combined information of stand-alone PET and MRI with improved spatial and temporal co-registration; it can additionally be used to improve PET image quality and quantification accuracy by addressing factors such as motion and partial volume effects, making PET/MRI more than its parts.     

Whole-body MR/PET: applications in abdominal imaging

Over the last few decades it has been shown that novel technologies and technological progress rapidly change the working environment of radiologists and nuclear medicine physicians. Thus, new possibilities, e.g., in tumor staging and therapy monitoring, but also new challenges arise. Recently, it could be shown that the integration of magnetic resonance imaging (MRI) and positron emission tomography (PET) is technically possible. The evolvement of new dedicated hybrid MR/PET systems for whole-body imaging in humans offers new potential in multimodal imaging. Especially simultaneous measurement of PET and MRI datasets allows for insights in metabolic and functional processes, particularly in oncologic demands, but also in cardiovascular and cerebral imaging. In this work-in-progress review article, a technical summary including the method-inherent challenges are given. Furthermore, possible clinical applications and research interests are addressed.     

Colorectal cancer staging: comparison of whole-body PET/CT and PET/MR

Purpose

Correct staging is imperative for colorectal cancer (CRC) since it influences both prognosis and management. Several imaging methods are used for this purpose, with variable performance. Positron emission tomography–magnetic resonance (PET/MR) is an innovative imaging technique recently employed for clinical application. The present study was undertaken to compare the staging accuracy of whole-body positron emission tomography–computed tomography (PET/CT) with whole-body PET/MR in patients with both newly diagnosed and treated colorectal cancer.

Methods

Twenty-six patients, who underwent same day whole-body (WB) PET/CT and WB-PET/MR, were evaluated. PET/CT and PET/MR studies were interpreted by consensus by a radiologist and a nuclear medicine physician. Correlations with prior imaging and follow-up studies were used as the reference standard. Correct staging was compared between methods using McNemar’s Chi square test.

Results

The two methods were in agreement and correct for 18/26 (69%) patients, and in agreement and incorrect for one patient (3.8%). PET/MR and PET/CT stages for the remaining 7/26 patients (27%) were discordant, with PET/MR staging being correct in all seven cases. PET/MR significantly outperformed PET/CT overall for accurate staging (P = 0.02).

Conclusion

PET/MR outperformed PET/CT in CRC staging. PET/MR might allow accurate local and distant staging of CRC patients during both at the time of diagnosis and during follow-up.

     

SimPET: a Preclinical PET Insert for Simultaneous PET/MR Imaging

Purpose

SimPET/M7 system is a small-animal dedicated simultaneous positron emission tomography and magnetic resonance imaging (PET/MRI) scanner. The SimPET insert has been upgraded from its prototype with a focus on count rate performance and sensitivity. The M7 scanner is a 1-T permanent magnet-based compact MRI system without any cryogens. Here, we present performance evaluation results of SimPET along with the results of mutual interference evaluation and simultaneously acquired PET/MR imaging.

Procedures

Following NEMA NU 4-2008 standard, we evaluated the performance of the SimPET system. The M7 MRI compatibility of SimPET was also assessed by analyzing MRI images of a uniform phantom under different PET conditions and PET count rates with different MRI pulse sequences. Mouse imaging was performed including a whole-body ¹⁸F-NaF PET scan and a simultaneous PET/MRI scan with ⁶⁴Cu-NOTA-ironoxide.

Results

The spatial resolution at center based on 3D OSEM without and with warm background was 0.7 mm and 1.45 mm, respectively. Peak sensitivity was 4.21 % (energy window?=?250–750 keV). The peak noise equivalent count rate with the same energy window was 151 kcps at 38.4 MBq. The uniformity was 4.42 %, and the spillover ratios in water- and air-filled chambers were 14.6 % and 12.7 %, respectively. In the hot rod phantom image, 0.75-mm-diameter rods were distinguishable. There were no remarkable differences in the SNR and uniformity of MRI images and PET count rates with different PET conditions and MRI pulse sequences. In the whole-body ¹⁸F-NaF PET images, fine skeletal structures were well resolved. In the simultaneous PET/MRI study with ⁶⁴Cu-NOTA-ironoxide, both PET and MRI signals changed before and after injection of the dual-modal imaging probe, which was evident with the exact spatiotemporal correlation.

Conclusions

We demonstrated that the SimPET scanner has a high count rate performance and excellent spatial resolution. The combined SimPET/M7 enabled simultaneous PET/MR imaging studies with no remarkable mutual interference between the two imaging modalities.

     

Application of MR/PET in Oncologic Imaging

The present review aims to depict the possibilities offered by hybrid imaging with magnetic resonance positron emission tomography (MR/PET). Recently, new whole-body MR/PET scanners were introduced allowing for the combination of both modalities outside the brain. This is a challenge for both modalities: For MRI, it is essential to provide anatomical images with high resolution. Additionally, diffusion-weighted imaging (DWI), proton spectroscopy, but also dynamic contrast-enhanced imaging plays an important role. With regard to PET, the technical challenge mainly consists of obtaining an appropriate MR-based attenuation correction for the PET data. Using MR/PET, it is possible to acquire morphological and functional data in one examination. In particular, children and young adults will benefit from this new hybrid technique, especially in oncologic imaging with multiple follow-up examinations. However, it is expected that PET/CT will not be replaced completely by MR/PET because PET/CT is less cost-intensive and more widely available. Moreover, in lung imaging, MRI limitations still have to be accepted. Concerning research, simultaneous MR/PET offers a variety of new possibilities, for example cardiac imaging, functional brain studies or the evaluation of new tracers in correlation with specific MR techniques.     

An integrated MR/PET system: prospective applications

Radiology is strongly depending on medical imaging technology and consequently directing technological progress. A novel technology can only be established, however, if improved diagnostic accuracy influence on therapeutic management and/or overall reduced cost can be evidenced. It has been demonstrated recently that Magnetic Resonance Imaging (MRI) and Positron Emission Tomography (PET) can technologically be integrated into one single hybrid system. Some scientific arguments on the benefits are obvious, e.g., that simultaneous imaging of morphological and functional information will improve tissue characterization. However, crossfire of questions still remains: What unmet radiological needs are addressed by the novel system? What level of hardware integration is reasonable, or would software-based image co-registration be sufficient? Will MR/PET achieve higher diagnostic accuracy compared to separate imaging? What is the added value compared to other hybrid imaging modalities like PET/CT? And finally, is the system economically reasonable and has the potential to reduce overall costs for therapy planning and monitoring? This article tries to highlight some perspectives of applying an integrated MR/PET system for simultaneous morphologic and functional imaging.