PET/MR in children. Initial clinical experience in paediatric oncology using an integrated PET/MR scanner

How PET/MR Can Add Value For Children With Cancer

PURPOSE

To review how PET/MR technology could add value for pediatric cancer patients.

RECENT FINDINGS

Since many primary tumors in children are evaluated with MRI and metastases are detected with PET/CT, integrated PET/MR can be a time-efficient and convenient solution for pediatric cancer staging. 18F-FDG PET/MR can assess primary tumors and the whole body in one imaging session, avoid repetitive anesthesia and reduce radiation exposure compared to 18F-FDG PET/CT. This article lists 10 action points, which might improve the clinical value of PET/MR for children with cancer. However, even if PET/MR proves valuable, it cannot enter mainstream applications if it is not accessible to the majority of pediatric cancer patients. Therefore, innovations are needed to make PET/MR scanners affordable and increase patient throughput.

SUMMARY

PET/MR offers opportunities for more efficient, accurate and safe diagnoses of pediatric cancer patients. The impact on patient management and outcomes has to be substantiated by large-scale prospective clinical trials.

Pediatric Nuclear Medicine and its Development as a Specialty

Pediatric Nuclear Medicine (PNM) offers to the pediatrician noninvasive procedures, with high clinical impact and low dosimetry. New techniques have been adapted to children, diminishing doses, always looking for less dosimetry, higher sensitivity and higher resolution images. PNM is and will remain a minority subspecialty, but highly complex for general NM physicians due to the different diagnostics in children and due to the higher technical complexity of the examinations. General NM physicians have to be trained and regularly receive CME in this field.

PET/MR Imaging in Musculoskeletal Disorders

There is emerging evidence suggesting that PET/MR imaging will have a role in many aspects of musculoskeletal imaging. The synergistic potential of hybrid PET/MR imaging in terms of acquiring anatomic, molecular, and functional data simultaneously seems advantageous in the diagnostic workup, treatment planning and monitoring, and follow-up of patients with musculoskeletal malignancies, and may also prove helpful in assessment of musculoskeletal infectious and inflammatory disorders. The application of more sophisticated MR imaging sequences and PET radiotracers other than FDG in the diagnostic workup and follow-up of patients with musculoskeletal disorders should be explored.

An update on the role of PET/CT and PET/MRI in ovarian cancer

This review article summarizes the role of PET/CT and PET/MRI in ovarian cancer. With regard to the diagnosis of ovarian cancer, the presence of FDG uptake within the ovary of a postmenopausal woman raises the concern for ovarian cancer. Multiple studies show that FDG PET/CT can detect lymph node and distant metastasis in ovarian cancer with high accuracy and may, therefore, alter the management to obtain better clinical outcomes. Although PET/CT staging is superior for N and M staging of ovarian cancer, its role is limited for T staging. Additionally, FDG PET/CT is of great benefit in evaluating treatment response and has prognostic value in patients with ovarian cancer. FDG PET/CT also has value to detect recurrent disease, particularly in patients with elevated serum CA-125 levels and negative or inconclusive conventional imaging test results. PET/MRI may beneficial for tumor staging because MRI has higher soft tissue contrast and no ionizing radiation exposure compared to CT. Some non-FDG PET radiotracers such as ¹⁸F-fluorothymidine (FLT) or ¹¹C-methionine (MET) have been studied in preclinical and clinical studies as well and may play a role in the evaluation of patients with ovarian cancer.

An overview of PET/MR, focused on clinical applications

Hybrid PET/MR scanners are innovative imaging devices that simultaneously or sequentially acquire and fuse anatomical and functional data from magnetic resonance (MR) with metabolic information from positron emission tomography (PET) (Delso et al. in J Nucl Med 52:1914-1922, 2011; Zaidi et al. in Phys Med Biol 56:3091-3106, 2011). Hybrid PET/MR scanners have the potential to greatly impact not only on medical research but also, and more importantly, on patient management. Although their clinical applications are still under investigation, the increased worldwide availability of PET/MR scanners, and the growing published literature are important determinants in their rising utilization for primarily clinical applications. In this manuscript, we provide a summary of the physical features of PET/MR, including its limitations, which are most relevant to clinical PET/MR implementation and to interpretation. Thereafter, we discuss the most important current and emergent clinical applications of such hybrid technology in the abdomen and pelvis, both in the field of oncologic and non-oncologic imaging, and we provide, when possible, a comparison with clinically consolidated imaging techniques, like for example PET/CT.

Magnetic resonance imaging based functional imaging in paediatric oncology

Imaging is central to management of solid tumours in children. Conventional magnetic resonance imaging (MRI) is the standard imaging modality for tumours of the central nervous system (CNS) and limbs and is increasingly used in the abdomen. It provides excellent structural detail, but imparts limited information about tumour type, aggressiveness, metastatic potential or early treatment response. MRI based functional imaging techniques, such as magnetic resonance spectroscopy, diffusion and perfusion weighted imaging, probe tissue properties to provide clinically important information about metabolites, structure and blood flow. This review describes the role of and evidence behind these functional imaging techniques in paediatric oncology and implications for integrating them into routine clinical practice.

Potential of PET-MRI for imaging of non-oncologic musculoskeletal disease

Early detection of musculoskeletal disease leads to improved therapies and patient outcomes, and would benefit greatly from imaging at the cellular and molecular level. As it becomes clear that assessment of multiple tissues and functional processes are often necessary to study the complex pathogenesis of musculoskeletal disorders, the role of multi-modality molecular imaging becomes increasingly important. New positron emission tomography-magnetic resonance imaging (PET-MRI) systems offer to combine high-resolution MRI with simultaneous molecular information from PET to study the multifaceted processes involved in numerous musculoskeletal disorders. In this article, we aim to outline the potential clinical utility of hybrid PET-MRI to these non-oncologic musculoskeletal diseases. We summarize current applications of PET molecular imaging in osteoarthritis (OA), rheumatoid arthritis (RA), metabolic bone diseases and neuropathic peripheral pain. Advanced MRI approaches that reveal biochemical and functional information offer complementary assessment in soft tissues. Additionally, we discuss technical considerations for hybrid PET-MR imaging including MR attenuation correction, workflow, radiation dose, and quantification.

Comprehensive Oncologic Imaging in Infants and Preschool Children With Substantially Reduced Radiation Exposure Using Combined Simultaneous ¹⁸F-Fluorodeoxyglucose Positron Emission Tomography/Magnetic Resonance Imaging: A Direct Comparison to ¹⁸F-Fluorodeoxyglucose Positron Emission Tomography/Computed Tomography

OBJECTIVE

The aim of this study was to evaluate the clinical applicability and technical feasibility of fluorodeoxyglucose (FDG) positron emission tomography (PET)/magnetic resonance imaging (MRI) compared with FDG PET/computed tomography (CT) in young children focusing on lesion detection, PET quantification, and potential savings in radiation exposure.

METHODS

Twenty examinations (10 PET/CT and 10 PET/MRI examinations) were performed prospectively in 9 patients with solid tumors (3 female, 6 male; mean age, 4.8 [1-6] years). Fluorodeoxyglucose PET/CT and FDG PET/MRI were performed sequentially after a single tracer injection. Lesion detection and analysis were performed independently in PET/CT and PET/MRI. Potential changes in diagnostic or therapeutic patient management were recorded. Positron emission tomography quantification in PET/MRI was evaluated by comparing standardized uptake values resulting from MRI-based and CT-based attenuation correction. Effective radiation doses of PET and CT were estimated.

RESULTS

Twenty-one PET-positive lesions were found congruently in PET/CT and PET/MRI. Magnetic resonance imaging enabled significantly better detection of morphologic PET correlates compared with CT. Eight suspicious PET-negative lesions were identified by MRI, of which one was missed in CT. Sensitivity, specificity, and accuracy for correct lesion classification were not significantly different (90%, 47%, and 62% in PET/CT; 100%, 68%, and 79% in PET/MRI, respectively). In 4 patients, the use of PET/MRI resulted in a potential change in diagnostic management compared with PET/CT, as local and whole-body staging could be performed within 1 single examination. In 1 patient, PET/MRI initiated a change in therapeutic management. Positron emission tomography quantification using MRI-based attenuation correction was accurate compared with CT-based attenuation correction. Higher standardized uptake value deviations of about 18% were observed in the lungs due to misclassification in MRI-based attenuation maps. Potential reduction in radiation dose was 48% in PET/MRI compared with PET/CT (P < 0.05).

CONCLUSIONS

FDG PET/MRI is at least equivalent to FDG PET/CT for oncologic imaging in young children. Specifically, superior soft tissue contrast of MRI results in higher confidence in lesion interpretation. Substantial savings in radiation exposure can be achieved, and the number of necessary imaging examinations can be reduced using PET/MRI compared with PET/CT.

Evaluation of diagnostic performance of whole-body simultaneous PET/MRI in pediatric lymphoma

BACKGROUND

Whole-body (18)F-fluorodeoxyglucose (FDG) positron emission tomography/computed tomography (PET/CT) is the standard of care for lymphoma. Simultaneous PET/MRI (magnetic resonance imaging) is a promising new modality that combines the metabolic information of PET with superior soft-tissue resolution and functional imaging capabilities of MRI while decreasing radiation dose. There is limited information on the clinical performance of PET/MRI in the pediatric setting.

OBJECTIVE

This study evaluated the feasibility, dosimetry, and qualitative and quantitative diagnostic performance of simultaneous whole-body FDG-PET/MRI in children with lymphoma compared to PET/CT.

MATERIALS AND METHODS

Children with lymphoma undergoing standard of care FDG-PET/CT were prospectively recruited for PET/MRI performed immediately after the PET/CT. Images were evaluated for quality, lesion detection and anatomical localization of FDG uptake. Maximum and mean standardized uptake values (SUVmax/mean) of normal organs and SUVmax of the most FDG-avid lesions were measured for PET/MRI and PET/CT. Estimation of radiation exposure was calculated using specific age-related factors.

RESULTS

Nine PET/MRI scans were performed in eight patients (mean age: 15.3 years). The mean time interval between PET/CT and PET/MRI was 51 ± 10 min. Both the PET/CT and PET/MRI exams had good image quality and alignment with complete (9/9) concordance in response assessment. The SUVs from PET/MRI and PET/CT were highly correlated for normal organs (SUVmean r(2): 0.88, P<0.0001) and very highly for FDG-avid lesions (SUVmax r(2): 0.94, P=0.0002). PET/MRI demonstrated an average percent radiation exposure reduction of 39% ± 13% compared with PET/CT.

CONCLUSION

Simultaneous whole-body PET/MRI is clinically feasible in pediatric lymphoma. PET/MRI performance is comparable to PET/CT for lesion detection and SUV measurements. Replacement of PET/CT with PET/MRI can significantly decrease radiation dose from diagnostic imaging in children.

Hybrid FDG-PET/MR compared to FDG-PET/CT in adult lymphoma patients

PURPOSE

The goal of this study is to evaluate the diagnostic performance of simultaneous FDG-PET/MR including diffusion compared to FDG-PET/CT in patients with lymphoma.

METHODS

Eighteen patients with a confirmed diagnosis of non-Hodgkin's (NHL) or Hodgkin's lymphoma (HL) underwent an IRB-approved, single-injection/dual-imaging protocol consisting of a clinical FDG-PET/CT and subsequent FDG-PET/MR scan. PET images from both modalities were reconstructed iteratively. Attenuation correction was performed using low-dose CT data for PET/CT and Dixon-MR sequences for PET/MR. Diffusion-weighted imaging was performed. SUVmax was measured and compared between modalities and the apparent diffusion coefficient (ADC) using ROI analysis by an experienced radiologist using OsiriX. Strength of correlation between variables was measured using the Pearson correlation coefficient (r p).

RESULTS

Of the 18 patients included in this study, 5 had HL and 13 had NHL. The median age was 51 ± 14.8 years. Sixty-five FDG-avid lesions were identified. All FDG-avid lesions were visible with comparable contrast, and therefore initial and follow-up staging was identical between both examinations. SUVmax from FDG-PET/MR [(mean ± sem) (21.3 ± 2.07)] vs. FDG-PET/CT (mean 23.2 ± 2.8) demonstrated a strongly positive correlation [r s = 0.95 (0.94, 0.99); p < 0.0001]. There was no correlation found between ADCmin and SUVmax from FDG-PET/MR [r = 0.17(-0.07, 0.66); p = 0.09].

CONCLUSION

FDG-PET/MR offers an equivalent whole-body staging examination as compared with PET/CT with an improved radiation safety profile in lymphoma patients. Correlation of ADC to SUVmax was weak, understating their lack of equivalence, but not undermining their potential synergy and differing importance.

A Clinical and Experimental Comparison of Time of Flight PET/MRI and PET/CT Systems

PURPOSE

The purpose of the study was to compare image quality and quantitative accuracy of positron emission tomography/magnetic resonance imaging (PET/MRI) and PET/computed tomography (PET/CT) systems with time of flight PET gantries, using phantom and clinical studies.

PROCEDURES

Identical phantom experiments were performed on both systems. Calibration, uniformity, and standardized uptake value (SUV) recovery were measured. A clinical PET/CT versus PET/MRI comparison was performed using [(18)F]fluoromethylcholine ([(18)F]FCH).

RESULTS

Calibration accuracy and image uniformity were comparable between systems. SUV recovery met EANM/EARL requirements on both scanners. Thirty-four lesions with comparable PET image quality were identified. Lesional SUVmax differences of 4 ± 26% between PET/MRI and PET/CT data were observed (R (2) = 0.79, slope = 1.02). In healthy tissues, PET/MRI-derived SUVs were 16 ± 11% lower than on PET/CT (R (2) = 0.98, slope = 0.86).

CONCLUSION

PET/MRI and PET/CT showed comparable performance with respect to calibration accuracy, image uniformity, and SUV recovery. [(18)F]FCH uptake values for both healthy tissues and lesions corresponded reasonably well between MR- and CT-based systems, but only in regions free of MR-based attenuation artifacts.

Whole-body magnetic resonance imaging in children: technique and clinical applications

Whole-body MR imaging is being increasingly used in children to evaluate the extent of various oncologic and non-oncologic entities. The lack of exposure to ionizing radiation, excellent soft-tissue contrast (even without the use of contrast agents), and functional imaging capabilities make it especially suitable for screening and surveillance in the pediatric population. Technical developments such as moving table platforms, multi-channel/multi-element surface coils, and parallel imaging allow imaging of the entire body with multiple sequences in a reasonable 30- to 40-min time frame, which has facilitated its acceptance in routine clinical practice. The initial investigations in whole-body MR imaging were primarily focused on oncologic applications such as tumor screening and staging. The exquisite sensitivity of fluid-sensitive MR sequences to many different types of pathology has led to new applications of whole-body MR imaging in evaluation of multifocal rheumatologic conditions. Availability of blood pool contrast agents has allowed whole-body MR angiographic imaging of vascular malformations, vasculitides and vasculopathies. Whole-body MRI is being applied for delineating the extent and distribution of systemic and multifocal diseases, establishing diagnoses, assessing treatment response, and surveillance imaging. This article reviews the technique and clinical applications of whole-body MR imaging in children.

FDG Whole-Body PET/MRI in Oncology: a Systematic Review

The recent advance in hybrid imaging techniques enables offering simultaneous positron emission tomography (PET)/magnetic resonance imaging (MRI) in various clinical fields. ¹⁸F-fluorodeoxyglucose (FDG) PET has been widely used for diagnosis and evaluation of oncologic patients. The growing evidence from research and clinical experiences demonstrated that PET/MRI with FDG can provide comparable or superior diagnostic performance more than conventional radiological imaging such as computed tomography (CT), MRI or PET/CT in various cancers. Combined analysis using structural information and functional/molecular information of tumors can draw additional diagnostic information based on PET/MRI. Further studies including determination of the diagnostic efficacy, optimizing the examination protocol, and analysis of the hybrid imaging results is necessary for extending the FDG PET/MRI application in clinical oncology.

Speeding up PET/MR for cancer staging of children and young adults

OBJECTIVE

Combining ¹⁸F-FDG PET with whole-body MR for paediatric cancer staging is practically feasible if imaging protocols can be streamlined. We compared ¹⁸F-FDG PET/STIR with accelerated ¹⁸F-FDG PET/FSPGR for whole-body tumour imaging in children and young adults.

METHODS

Thirty-three children and young adults (17.5 ± 5.5 years, range 10-30) with malignant lymphoma or sarcoma underwent a ¹⁸F-FDG PET staging examination, followed by ferumoxytol-enhanced STIR and FSPGR whole-body MR. ¹⁸F-FDG PET scans were fused with MR data and the number and location of tumours on each integrated examination were determined. Histopathology and follow-up imaging served as standard of reference. The agreement of each MR sequence with the reference and whole-body imaging times were compared using Cohen's kappa coefficient and Student's t-test, respectively.

RESULTS

Comparing ¹⁸F-FDG PET/FSPGR to ¹⁸F-FDG PET/STIR, sensitivities were 99.3 % for both, specificities were statistically equivalent, 99.8 versus 99.9 %, and the agreement with the reference based on Cohen's kappa coefficient was also statistically equivalent, 0.989 versus 0.992. However, the total scan-time for accelerated FSPGR of 19.8 ± 5.3 minutes was significantly shorter compared to 29.0 ± 7.6 minutes for STIR (p = 0.001).

CONCLUSION

F-FDG PET/FSPGR demonstrated equivalent sensitivities and specificities for cancer staging compared to ¹⁸F-FDG PET/STIR, but could be acquired with shorter acquisition time.

KEY POINTS

• Breath-hold FSPGR sequences shorten the data acquisition time for whole-body MR and PET/MR. • Ferumoxytol provides long-lasting vascular contrast for whole-body MR and PET/MR. • ¹⁸ F-FDG PET/FSPGR data provided equal sensitivity and specificity for cancer staging compared to ¹⁸ F-FDG PET/STIR.

Simultaneous whole body 18F-fluorodeoxyglucose positron emission tomography magnetic resonance imaging for evaluation of pediatric cancer: Preliminary experience and comparison with 18F-fluorodeoxyglucose positron emission tomography computed tomography

AIM: To describe our preliminary experience with simultaneous whole body ¹⁸F-fluorodeoxyglucose (¹⁸F-FDG) positron emission tomography and magnetic resonance imaging (PET-MRI) in the evaluation of pediatric oncology patients.

METHODS: This prospective, observational, single-center study was Health Insurance Portability and Accountability Act-compliant, and institutional review board approved. To be eligible, a patient was required to: (1) have a known or suspected cancer diagnosis; (2) be under the care of a pediatric hematologist/oncologist; and (3) be scheduled for clinically indicated ¹⁸F-FDG positron emission tomography-computed tomography (PET-CT) examination at our institution. Patients underwent PET-CT followed by PET-MRI on the same day. PET-CT examinations were performed using standard department protocols. PET-MRI studies were acquired with an integrated 3 Tesla PET-MRI scanner using whole body T1 Dixon, T2 HASTE, EPI diffusion-weighted imaging (DWI) and STIR sequences. No additional radiotracer was given for the PET-MRI examination. Both PET-CT and PET-MRI examinations were reviewed by consensus by two study personnel. Test performance characteristics of PET-MRI, for the detection of malignant lesions, including FDG maximum standardized uptake value (SUVmax) and minimum apparent diffusion coefficient (ADCmin), were calculated on a per lesion basis using PET-CT as a reference standard.

RESULTS: A total of 10 whole body PET-MRI exams were performed in 7 pediatric oncology patients. The mean patient age was 16.1 years (range 12-19 years) including 6 males and 1 female. A total of 20 malignant and 21 benign lesions were identified on PET-CT. PET-MRI SUVmax had excellent correlation with PET-CT SUVmax for both benign and malignant lesions (R = 0.93). PET-MRI SUVmax > 2.5 had 100% accuracy for discriminating benign from malignant lesions using PET-CT reference. Whole body DWI was also evaluated: the mean ADCmin of malignant lesions (780.2 + 326.6) was significantly lower than that of benign lesions (1246.2 + 417.3; P = 0.0003; Student’s t test). A range of ADCmin thresholds for malignancy were evaluated, from 0.5-1.5 × 10^-3 mm²/s. The 1.0 × 10^-3 ADCmin threshold performed best compared with PET-CT reference (68.3% accuracy). However, the accuracy of PET-MRI SUVmax was significantly better than ADCmin for detecting malignant lesions compared with PET-CT reference (P < 0.0001; two-tailed McNemar’s test).

CONCLUSION: These results suggest a clinical role for simultaneous whole body PET-MRI in evaluating pediatric cancer patients.

Combined PET/MR: The Real Work Has Just Started. Summary Report of the Third International Workshop on PET/MR Imaging; February 17-21, 2014, Tübingen, Germany

This paper summarises the proceedings and discussions at the third annual workshop held in Tübingen, Germany, dedicated to the advancement of the technical, scientific and clinical applications of combined PET/MRI systems in humans. Two days of basic scientific and technical instructions with "hands-on" tutorials were followed by 3 days of invited presentations from active researchers in this and associated fields augmented by round-table discussions and dialogue boards with specific themes. These included the use of PET/MRI in paediatric oncology and in adult neurology, oncology and cardiology, the development of multi-parametric analyses, and efforts to standardise PET/MRI examinations to allow pooling of data for evaluating the technology. A poll taken on the final day demonstrated that over 50 % of those present felt that while PET/MRI technology underwent an inevitable slump after its much-anticipated initial launch, it was now entering a period of slow, progressive development, with new key applications emerging. In particular, researchers are focusing on exploiting the complementary nature of the physiological (PET) and biochemical (MRI/MRS) data within the morphological framework (MRI) that these devices can provide. Much of the discussion was summed up on the final day when one speaker commented on the state of PET/MRI: "the real work has just started".

Whole-body MRI in paediatric oncology

Imaging plays a crucial role in the diagnosis and follow-up of paediatric malignancies. Until recently, computed tomography (CT) has been the imaging technique of choice in children with cancer, but nowadays there is an increasing interest in the use of functional imaging techniques like positron emission tomography and single-photon emission tomography. These later techniques are often combined with CT allowing for simultaneous acquisition of image data on the biological behaviour of tumour, as well as the anatomical localisation and extent of tumour spread. Because of the small but not negligible risk of radiation induced secondary cancers and the significantly improved overall survival rates of children with cancer, there is an increasing interest in the use of alternative imaging techniques that do not use ionising radiation. Magnetic resonance imaging (MRI) is a radiation-free imaging tool that allows for acquiring images with a high spatial resolution and excellent soft tissue contrast throughout the body. Moreover, recent technological advances have resulted in fast diagnostic sequences for whole-body MR imaging (WB-MRI), including functional techniques such as diffusion weighted imaging. In this review, the current status of the technique and major clinical applications of WB-MRI in children with cancer will be discussed.

Development of a novel depth of interaction PET detector using highly multiplexed G-APD cross-strip encoding

PURPOSE

The aim of this study was to develop a prototype PET detector module for a combined small animal positron emission tomography and magnetic resonance imaging (PET/MRI) system. The most important factor for small animal imaging applications is the detection sensitivity of the PET camera, which can be optimized by utilizing longer scintillation crystals. At the same time, small animal PET systems must yield a high spatial resolution. The measured object is very close to the PET detector because the bore diameter of a high field animal MR scanner is limited. When used in combination with long scintillation crystals, these small-bore PET systems generate parallax errors that ultimately lead to a decreased spatial resolution. Thus, we developed a depth of interaction (DoI) encoding PET detector module that has a uniform spatial resolution across the whole field of view (FOV), high detection sensitivity, compactness, and insensitivity to magnetic fields.

METHODS

The approach was based on Geiger mode avalanche photodiode (G-APD) detectors with cross-strip encoding. The number of readout channels was reduced by a factor of 36 for the chosen block elements. Two 12 × 2 G-APD strip arrays (25 μm cells) were placed perpendicular on each face of a 12 × 12 lutetium oxyorthosilicate crystal block with a crystal size of 1.55 × 1.55 × 20 mm. The strip arrays were multiplexed into two channels and used to calculate the x, y coordinates for each array and the deposited energy. The DoI was measured in step sizes of 1.8 mm by a collimated (18)F source. The coincident resolved time (CRT) was analyzed at all DoI positions by acquiring the waveform for each event and applying a digital leading edge discriminator.

RESULTS

All 144 crystals were well resolved in the crystal flood map. The average full width half maximum (FWHM) energy resolution of the detector was 12.8% ± 1.5% with a FWHM CRT of 1.14 ± 0.02 ns. The average FWHM DoI resolution over 12 crystals was 2.90 ± 0.15 mm.

CONCLUSIONS

The novel DoI PET detector, which is based on strip G-APD arrays, yielded a DoI resolution of 2.9 mm and excellent timing and energy resolution. Its high multiplexing factor reduces the number of electronic channels. Thus, this cross-strip approach enables low-cost, high-performance PET detectors for dedicated small animal PET and PET/MRI and potentially clinical PET/MRI systems.

Progressing Toward a Cohesive Pediatric 18F-FDG PET/MR Protocol: Is Administration of Gadolinium Chelates Necessary?

With the increasing availability of integrated PET/MR scanners, the utility and need for MR contrast agents for combined scans is questioned. The purpose of our study was to evaluate whether administration of gadolinium chelates is necessary for evaluation of pediatric tumors on (18)F-FDG PET/MR images.

METHODS

First, in 119 pediatric patients with primary and secondary tumors, we used 14 diagnostic criteria to compare the accuracy of several MR sequences: unenhanced T2-weighted fast spin-echo imaging; unenhanced diffusion-weighted imaging; and-before and after gadolinium chelate contrast enhancement-T1-weighted 3-dimensional spoiled gradient echo LAVA (liver acquisition with volume acquisition) imaging. Next, in a subset of 36 patients who had undergone (18)F-FDG PET within 3 wk of MRI, we fused the PET images with the unenhanced T2-weighted MR images (unenhanced (18)F-FDG PET/MRI) and the enhanced T1-weighted MR images (enhanced (18)F-FDG PET/MRI). Using the McNemar test, we compared the accuracy of the two types of fused images using the 14 diagnostic criteria. We also evaluated the concordance between (18)F-FDG avidity and gadolinium chelate enhancement. The standard of reference was histopathologic results, surgical notes, and follow-up imaging.

RESULTS

There was no significant difference in diagnostic accuracy between the unenhanced and enhanced MR images. Accordingly, there was no significant difference in diagnostic accuracy between the unenhanced and enhanced (18)F-FDG PET/MR images. (18)F-FDG avidity and gadolinium chelate enhancement were concordant in 30 of the 36 patients and 106 of their 123 tumors.

CONCLUSION

Gadolinium chelate administration is not necessary for accurate diagnostic characterization of most solid pediatric malignancies on (18)F-FDG PET/MR images, with the possible exception of focal liver lesions.

Clinical applications of PET/MRI: current status and future perspectives

Fully integrated positron emission tomography (PET)/magnetic resonance imaging (MRI) scanners have been available for a few years. Since then, the number of scanner installations and published studies have been growing. While feasibility of integrated PET/MRI has been demonstrated for many clinical and preclinical imaging applications, now those applications where PET/MRI provides a clear benefit in comparison to the established reference standards need to be identified. The current data show that those particular applications demanding multiparametric imaging capabilities, high soft tissue contrast and/or lower radiation dose seem to benefit from this novel hybrid modality. Promising results have been obtained in whole-body cancer staging in non-small cell lung cancer and multiparametric tumor imaging. Furthermore, integrated PET/MRI appears to have added value in oncologic applications requiring high soft tissue contrast such as assessment of liver metastases of neuroendocrine tumors or prostate cancer imaging. Potential benefit of integrated PET/MRI has also been demonstrated for cardiac (i.e., myocardial viability, cardiac sarcoidosis) and brain (i.e., glioma grading, Alzheimer's disease) imaging, where MRI is the predominant modality. The lower radiation dose compared to PET/computed tomography will be particularly valuable in the imaging of young patients with potentially curable diseases.However, further clinical studies and technical innovation on scanner hard- and software are needed. Also, agreements on adequate refunding of PET/MRI examinations need to be reached. Finally, the translation of new PET tracers from preclinical evaluation into clinical applications is expected to foster the entire field of hybrid PET imaging, including PET/MRI.

Childhood extracranial neoplasms: the role of imaging in drug development and clinical trials

Cancer is the leading cause of death in children older than 1 year of age and new drugs are necessary to improve outcomes. Imaging is crucial to the drug development process and assessment of therapeutic response. In adults, tumours are often assessed with CT using size criteria. Unfortunately, techniques established in adults are not necessarily applicable in children due to differing pathophysiology, ability to cooperate and increased susceptibility to ionising radiation. MRI, in particular quantitative MRI, has to date not been fully utilised in children with extracranial neoplasms. The specific challenges of imaging in children, the potential for functional imaging techniques to inform upon and their inclusion in clinical trials are discussed.

Beyond whole-body imaging: advanced imaging techniques of PET/MRI

PET/MRI is a hybrid imaging modality that is gaining clinical interest with the first Food and Drug Administration-approved simultaneous imaging system recently added to the clinical armamentarium. Several advanced PET/MRI applications, such as high-resolution anatomic imaging, diffusion-weighted imaging, motion correction, and cardiac imaging, show great potential for clinical use. The purpose of this article is to highlight several advanced PET/MRI applications through case examples and review of the current literature.

Simultaneous whole-body time-of-flight 18F-FDG PET/MRI: a pilot study comparing SUVmax with PET/CT and assessment of MR image quality

PURPOSE

The recent introduction of hybrid PET/MRI scanners in clinical practice has shown promising initial results for several clinical scenarios. However, the first generation of combined PET/MRI lacks time-of-flight (TOF) technology. Here we report the results of the first patients to be scanned on a completely novel fully integrated PET/MRI scanner with TOF.

MATERIALS AND METHODS

We analyzed data from patients who underwent a clinically indicated F FDG PET/CT, followed by PET/MRI. Maximum standardized uptake values (SUVmax) were measured from F FDG PET/MRI and F FDG PET/CT for lesions, cerebellum, salivary glands, lungs, aortic arch, liver, spleen, skeletal muscle, and fat. Two experienced radiologists independently reviewed the MR data for image quality.

RESULTS

Thirty-six patients (19 men, 17 women, mean [±standard deviation] age of 61 ± 14 years [range: 27-86 years]) with a total of 69 discrete lesions met the inclusion criteria. PET/CT images were acquired at a mean (±standard deviation) of 74 ± 14 minutes (range: 49-100 minutes) after injection of 10 ± 1 mCi (range: 8-12 mCi) of F FDG. PET/MRI scans started at 161 ± 29 minutes (range: 117 - 286 minutes) after the F FDG injection. All lesions identified on PET from PET/CT were also seen on PET from PET/MRI. The mean SUVmax values were higher from PET/MRI than PET/CT for all lesions. No degradation of MR image quality was observed.

CONCLUSION

The data obtained so far using this investigational PET/MR system have shown that the TOF PET system is capable of excellent performance during simultaneous PET/MR with routine pulse sequences. MR imaging was not compromised. Comparison of the PET images from PET/CT and PET/MRI show no loss of image quality for the latter. These results support further investigation of this novel fully integrated TOF PET/MRI instrument.

Whole-Body MRI in Children: Current Imaging Techniques and Clinical Applications

Whole-body magnetic resonance imaging (MRI) is increasingly used in children to evaluate the extent and distribution of various neoplastic and non-neoplastic diseases. Not using ionizing radiation is a major advantage of pediatric whole-body MRI. Coronal and sagittal short tau inversion recovery imaging is most commonly used as the fundamental whole-body MRI protocol. Diffusion-weighted imaging and Dixon-based imaging, which has been recently incorporated into whole-body MRI, are promising pulse sequences, particularly for pediatric oncology. Other pulse sequences may be added to increase diagnostic capability of whole-body MRI. Of importance, the overall whole-body MRI examination time should be less than 30-60 minutes in children, regardless of the imaging protocol. Established and potentially useful clinical applications of pediatric whole-body MRI are described.

Integrated whole body MR/PET: where are we?

Whole body integrated magnetic resonance imaging (MR)/positron emission tomography (PET) imaging systems have recently become available for clinical use and are currently being used to explore whether the combined anatomic and functional capabilities of MR imaging and the metabolic information of PET provide new insight into disease phenotypes and biology, and provide a better assessment of oncologic diseases at a lower radiation dose than a CT. This review provides an overview of the technical background of combined MR/PET systems, a discussion of the potential advantages and technical challenges of hybrid MR/PET instrumentation, as well as collection of possible solutions. Various early clinical applications of integrated MR/PET are also addressed. Finally, the workflow issues of integrated MR/PET, including maximizing diagnostic information while minimizing acquisition time are discussed.

Combined PET/MR: Where are we now? Summary report of the second international workshop on PET/MR imaging April 8-12, 2013, Tubingen, Germany

This workshop was held a year after the initial positron emission tomography/magnetic resonance (PET/MR) workshop in Tübingen, which was recently reported in this journal. The discussions at the 2013 workshop, however, differed substantially from those of the initial workshop, attesting to the progress of combined PET/MR as an innovative imaging modality. Discussions were focused on the search for truly novel, unique clinical and research applications as well as technical issues such as reliable and accurate approaches for attenuation and scatter correction of PET emission data. The workshop provided hands-on experience with PET and MR imaging. In addition, structured and moderated open discussion sessions, including six dialogue boards and two roundtable discussions, provided input from current and future PET/MR imaging users. This summary provides a snapshot of the current achievements and challenges for PET/MR.

MR-PET of the body: Early experience and insights

MR-PET is a novel imaging modality that combines anatomic and metabolic data acquisition, allowing for simultaneous depiction of morphological and functional abnormalities with an excellent soft tissue contrast and good spatial resolution; as well as accurate temporal and spatial image fusion; while substantially reducing radiation dose when compared with PET-CT.

In this review, we will discuss MR-PET basic principles and technical challenges and limitations, explore some practical considerations, and cover the main clinical applications, while shedding some light on some of the future trends regarding this new imaging technique.

Integrated PET/MRI for planning navigated biopsies in pediatric brain tumors

INTRODUCTION

An integrated PET/MRI scanner has been used in selected cases of pediatric brain tumor patients to obtain additional metabolic information about lesions for preoperative biopsy planning and navigation.

PATIENTS AND METHODS

Four patients, age 9-16 years, received PET/MRI scans employing [(11)C]methionine positron emission tomography (PET) and contrast-enhanced 3D-MR sequences for neuronavigation. PET and MR sequences have been matched for neurosurgical guidance. An infrared camera-based neuronavigation system was employed with co-registered MR and PET images fused to hybrid images for preoperative planning, stereotactic biopsy planning, and/or intraoperative guidance.

RESULTS

All patients showed hot spots of increased amino acid transport in PET and contrast-enhancing lesions in MRI. In three of the four patients, PET hot spots were congruent with contrast-enhancing areas in MRI. In two patients, frame-based stereotactic biopsies were taken from thalamo-mesencephalic lesions. One patient underwent second-look surgery for the suspicion of recurrent malignant glioma of the posterior fossa. One incidental frontal mass lesion was subtotally resected. No complications occurred. Hybrid imaging was helpful during the procedures to obtain representative histopathologic specimens and for surgical guidance during resection. Co-registered images did match with intraoperative landmarks, tumor borders, and histopathologic specimens.

CONCLUSION

The integrated PET/MRI scanner offers co-registered multimodal, high-resolution data for neuronavigation with reduced radiation exposure compared to PET/CT scans. One examination session provides all necessary data for neuronavigation and preoperative planning, avoiding additional anesthesia in the small patients. Hybrid multimodality imaging may improve safety and yield additional information when obtaining representative histopathologic specimens of brain tumors.

Update on pediatric leukemia and lymphoma imaging

Together, leukemia and lymphoma account for half of all childhood malignancies. Leukemia and lymphoma arise from similar cell lines and can have overlapping imaging features; however, the clinical presentation, imaging strategies, and treatment protocols can vary substantially based on the specific subtype. Although imaging does not play a central role in staging or monitoring disease in childhood leukemia, findings on imaging may be the first indication of the diagnosis. Advanced imaging, especially positron emission tomography/computed tomography, has moved to the forefront of staging and treatment response evaluation in Hodgkin's disease and non-Hodgkin's lymphoma. Imaging also plays a key role in evaluating the myriad of treatment complications that are commonly seen with chemotherapy and associated neutropenia. Future efforts will be largely focused on decreasing radiation exposure to these children, utilizing reduced or radiation-free modalities, such as positron emission tomography/magnetic resonance and diffusion-weighted whole-body imaging with background suppression, as well as refining surveillance imaging strategies. The purpose of this article is to briefly review the classification of pediatric leukemia and lymphoma, illustrate common imaging findings at presentation throughout the body, describe staging and therapeutic response evaluation, and show a spectrum of commonly encountered complications of treatment.

Potential Pediatric Applications of PET/MR

Medical imaging with multimodality and whole-body technologies has continuously improved in recent years. The advent of combined modalities such as PET/CT and PET/MR offers new tools with an exact fusion of molecular imaging and high-resolution anatomic imaging. For noninvasive pediatric diagnostics, molecular imaging and whole-body MR have become important, especially in pediatric oncology. Because it has a lower radiation exposure than PET/CT, combined PET/MR is expected to be of special use in pediatric diagnostics. This review focuses on possible pediatric applications of PET/MR hybrid imaging, particularly pediatric oncology and neurology but also the diagnosis of infectious or inflammatory diseases.

Hybrid PET/MR imaging in two sarcoma patients - clinical benefits and implications for future trials

PET/MRI is an evolving hybrid imaging modality which combines the inherent strengths of MRIs soft-tissue and contrast resolution and PETs functional metabolic capabilities. Bone and soft-tissue sarcoma are a relatively rare tumor entity, relying on MRI for local staging and often on PET/CT for lymph node involvement and metastatic spread evaluation. The purpose of this article is to demonstrate the successful use of PET/MRI in two sarcoma patients. We also use these patients as a starting point to discuss how PET/MRI might be of value in sarcoma. Among its potential benefits are: superior TNM staging than either modality alone, decreased radiation dose, more sensitive and specific follow-up and better assessment of treatment response. These potentials need to be investigated in future PET/MRI soft-tissue sarcoma trials.

Clinical and research applications of simultaneous positron emission tomography and MRI

Evaluation of the molecular processes responsible for disease pathogenesis and progression represents the new frontier of clinical radiology. Multimodality imaging lies at the cutting edge, combining the power of MRI for tissue characterization, microstructural appraisal and functional assessment together with new positron emission tomography (PET) tracers designed to target specific metabolic processes. The recent commercial availability of an integrated clinical whole-body PET-MRI provides a hybrid platform for exploring and exploiting the synergies of multimodal imaging. First experiences on the clinical and research application of hybrid PET-MRI are emerging. This article reviews the rapidly evolving field and speculates on the potential future direction.

Initial experience of MR/PET in a clinical cancer center

Magentic Resonance/positron emission tomography (PET) has been introduced recently for imaging of clinical patients. This hybrid imaging technology combines the inherent strengths of MRI with its high soft-tissue contrast and biological sequences with the inherent strengths of PET, enabling imaging of metabolism with a high sensitivity. In this article, we describe the initial experience of MR/PET in a clinical cancer center along with a review of the literature. For establishing MR/PET in a clinical setting, technical challenges, such as attenuation correction and organizational challenges, such as workflow and reimbursement, have to be overcome. The most promising initial results of MR/PET have been achieved in anatomical areas where high soft-tissue and contrast resolution is of benefit. Head and neck cancer and pelvic imaging are potential applications of this hybrid imaging technology. In the pediatric population, MR/PET can decrease the lifetime radiation dose. MR/PET protocols tailored to different types of malignancies need to be developed. After the initial exploration phase, large multicenter trials are warranted to determine clinical indications for this exciting hybrid imaging technology and thereby opening new horizons in molecular imaging.