PET/MR Imaging:

PET/MR imaging is an integrated imaging system that combines the high soft tissue resolution of MR imaging with the quantitative data obtained from PET into a single system. It is a relatively new imaging technique but with potential clinical use in pediatric oncologic and nononcologic processes in additional to its role in research. It is particularly relevant in pediatric patients due to reduced radiation burden compared with PET/CT and the ability to obtain exquisite functional and anatomic imaging in a single imaging session, thereby reducing the number of anesthesia/sedations. This review article focuses on the current as well as future applications of PET/MR imaging in pediatric imaging, including both oncologic and nononcologic indications.

Key points

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PET/MR imaging is particularly relevant in pediatrics due to reduced radiation burden compared with PET/CT.
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PET/MR imaging is a highly promising imaging modality, although it presents many challenges that need to be addressed prior to routine use.
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Current PET/MR imaging use in pediatrics is primarily in oncology.

Introduction

Combined PET/MR imaging is an emerging hardware solution that integrates the advantages of high soft tissue contrast and exquisite anatomic characterization of MR imaging with the high sensitivity of and quantitative physiologic data obtained from PET into a single system. The first combined PET/MR imaging systems were installed in 2010, and since then there have been slowly and steadily increasing technological advancements and clinical adoption. Although the use of PET/MR imaging in the pediatric setting is still in its infancy, it is an attractive imaging technology that could result in radiation dose savings and also negate the need for multiple imaging studies and anesthetic exposures in children. This review article focuses on the current as well as future applications of PET/MR imaging in pediatric imaging, including both oncologic and nononcologic indications.

PET/MR imaging technique

Instrumentation

Broadly speaking, 2 different technical approaches to combined PET/MR imaging have been developed: sequential and concurrent or simultaneous PET/MR imaging.

Sequential PET/MR imaging

In sequential PET/MR imaging, the PET and MR imaging scanners are positioned in-line but separate, with the same patient-transfer tabletop moving between the 2 imaging gantries. The sequential approach is less challenging technically than simultaneous PET/MR imaging, because the distance between the scanners reduces interference of PET photomultiplier tube (PMT) function with magnetic fields from MR imaging and vice versa.

Simultaneous PET/MR imaging

Concurrent imaging with an integrated scanner necessitates addressing the space restrictions inside the MR imaging magnet bore and any possible interference between the 2 modalities. In the first prototypes of integrated scanners, the PMTs needed to be physically separated from the magnetic field of MR imaging using long cables. More recently, the PMTs have been replaced with semiconductor detectors, including avalanche photodiode detectors or silicon photomultipliers that are not sensitive to magnetic fields. Fully integrated simultaneous PET/MR imaging scanning is particularly pertinent to pediatric imaging in decreasing overall scanning time, given limitations of patient compliance and the need for general anesthesia/sedation. The remainder of this review focuses on this type of scanner, which is the preferred and more widely installed scanner type. All new PET/MR imaging systems are now of the simultaneous type.

Workflow, Technical/Protocol Considerations, and Challenges

Workflow considerations

Smooth operation of a PET/MR imaging program requires a great deal of planning, coordination, cross-modality training, and multidisciplinary effort involving pediatric radiologists, nuclear medicine physicians, radiologic technologists, nuclear medicine technologists, nursing staff, and referring physicians. The indications for PET/MR imaging should be established case by case but based on a few preinstituted guidelines and in specific patients and disease groups. Proper patient preparation is key to successful scanning, including choosing the appropriate tracer and determining the need for sedation. PET-related preparation, including fasting prior to injection of F-18 Fluorodeoxyglucose ( ¹⁸ F-FDG) and measures for suppression of physiologic brown adipose tissue uptake, are similar to those for PET/CT. Additionally, patients are screened with an MR safety questionnaire, similar to conventional MR imaging examination screening.

Technical/protocol considerations

Although numerous (and numerous combinations of) acquisition protocols are currently possible, most pediatric whole-body PET/MR imaging protocols are similar in their fundamental components. A sample protocol with standard MR imaging pulse sequences is shown in Fig. 1 and listed in Tables 1 and 2 . A whole-body (or torso) examination usually includes whole-body (or skull base to thighs) PET and MR acquisitions, with typically MR-based attenuation correction sequences and fat-suppressed T2-weighted and/or inversion recovery sequences acquired at each bed position (usually 3–4 minutes at each position). The most commonly used method for attenuation correction is segmentation based, with a double-echo chemical shift gradient-echo sequence (Dixon technique) that provides separate fat and water images that are then segmented into 4 tissue compartments: soft tissue, fat, lung, and background air. Atlas-based attenuation correction methods may be inaccurate for use in pediatric patients because they are based on adult atlases. Diffusion-weighted imaging (DWI) also can be performed as a part of the whole-body protocol. Additionally, dedicated imaging of a specific body part may be performed based on the indication, including administration of gadolinium-based contrast where needed. For brain imaging, anatomic sequences, such as T2-weighted fluid-attenuated inversion recovery (FLAIR) and/or magnetization-prepared 3-D volumetric T1 gradient-echo sequences, are performed in addition to those for attenuation correction with a typical total acquisition time of 20 minutes ( Table 3 ). The administered activity of ¹⁸ F-FDG for ¹⁸ F-FDG PET/MR imaging is currently the same as for PET/CT and is typically based on North American consensus guidelines for pediatric patients, with a suggested activity of 0.10 mCi/kg to 0.14 mCi/kg (3.7–5.2 MBq/kg) for body PETs and 0.10 mCi/kg (3.7 MBq/kg) for brain PETs. When complete brain or body protocol MR imaging is simultaneously performed, this potentially allows for a further reduction in administered radiotracer dose because of the longer MR imaging acquisition time per bed position compared with PET/CT.

Table 1

Sample MR imaging pulse sequences in pediatric whole-body PET/MR imaging protocol

Sequence (Siemens [Siemens Healthcare, Erlangen, Germany] Biograph mMR)	Plane	Repetition Time (ms)	Echo Time (ms)	Matrix	Comments
T1 in/opposed phase attenuation correction, Dixon volumetric interpolated breath-hold examination	Coronal	3.6	1.2	192 × 121
T2 single-shot (Half-Fourier acquisition single-shot turbo spin-echo)	Axial	1600	95	320 × 260
DWI	Axial	11,000	70	160 × 120	B = 50,400,800
Short tau inversion recovery	Coronal	4000	48	320 × 192	Respiratory triggered for chest and abdomen

Massachusetts General Hospital, Boston, MA.

Table 2

Sample MR imaging pulse sequences in pediatric whole-body PET-MR imaging protocol

Sequence (GE Signa, GE, Chicago, Illinois)	Plane	Repetition Time (ms)	Echo Time (ms)	Matrix	Comments
T1 Dixon attenuation correction	Axial	4	1.6	256 × 128
3-D T1 LAVA-Flex	Axial	4	Minimum	288 × 224	Fusion in 3 planes
DWI	Axial	5500	Minimum	160 × 128	B = 150,400,800
Short tau inversion recovery	Coronal	2000–6500	68	320 × 224	Respiratory navigator for chest and abdomen if good waveform
T2 fast spin-echo, fat suppressed	Axial	8000–12,000	80	320 × 224	Add on at sites of interest

Children’s Hospital of Philadelphia, Philadelphia, PA.

Table 3

Sample MR imaging pulse sequences in pediatric brain PET-MR imaging protocol

Sequence	Plane	Resolution	Notes
3-D volumetric T1 with inversion preparation	Sagittal (with reformats in other planes)	0.9–1.2 mm ³isotropic	GE: BRAVO Siemens: MPRAGE
3-D volumetric T2 with variable flip angle	Sagittal (with reformats in other planes)	1.0–1.2 mm ³isotropic	GE: CUBE Siemens: SPACE Can be done 2-D in other planes as well
FLAIR	Axial (or coronal for epilepsy)	3–5 mm thickness 256 × 192 in-plane matrix	Can be done 3-D as well
Arterial spin labeling perfusion	Axial (whole brain)	GE: 512 sampling along 8 spiral arms, 3–4 mm slice thickness Siemens: 1.6 m × 1.6 mm × 4 mm	GE: 3-D spiral pseudocontinuous arterial spin labeling Siemens: 3D GRASE pulsed arterial spin labeling Inversion time: 1900–2050 ms
DWI	Axial	3–5 mm thickness 128 × 128 in-plane matrix acquisition	B = 0, 1000—can be fast spin-echo DWI or multishot segmented k-space if needed, albeit with increased scan time—may substitute diffusion tensor imaging acquisition

Children’s Hospital of Philadelphia, Philadelphia, PA.

Technical challenges

Attenuation correction of the PET scan emission data is a prerequisite for accurate quantification of PET and is more challenging with PET/MR imaging due to the lack of bone density information, truncation of MR imaging–based image information along the patient’s arms, and the presence of additional attenuating hardware components, such as radiofrequency coils. There are newer methods of compensating gradient nonlinearities and extending the MR imaging field of view to include the arms and, therefore, decrease PET attenuation correction bias. Imaging of lung parenchyma is difficult with MR imaging due to low proton density, rapid loss of signal because of field inhomogeneity, and respiratory motion. For these reasons, as well as the inferior spatial resolution of MR imaging compared with CT, it is challenging to detect small pulmonary nodules with PET/MR imaging, and chest CT maintains in important role in thoracic staging of pediatric malignancies.

Data processing, display, and storage

Various software packages are offered by the PET/MR imaging system manufacturers and other third-party vendors for hybrid image viewing and evaluation. More work is likely needed to optimize the nuclear medicine workstations to support viewing of PET/MR imaging with various MR imaging acquisitions, including functional data, such as DWI and perfusion-weighted imaging. Despite smaller image matrix size, PET/MR imaging has larger data storage requirements in comparison to PET/CT because more numerous sequences are obtained. PET/MR imaging examinations have typical size requirements ranging from 500 MB to 2 GB.

Radiation Considerations

PET/MR imaging provides significant radiation dose savings compared with PET/CT. In addition to the dose reduction from eliminating CT imaging, the longer acquisition time per bed position associated with PET/MR imaging potentially allows lower administered radiopharmaceutical doses. Depending on the acquisition parameters of the CT scan (diagnostic-quality CT vs localization/attenuation correction CT) accompanying the PET/CT, dose reductions of up to 73% have been reported when performing PET/MR imaging instead of PET/CT.

Imaging evaluation

Image Interpretation Considerations

Reading and interpreting PET/MR imaging data are complex tasks requiring integration of information obtained from multiple MR imaging sequences with the PET data, which necessitates a high level of expertise in both pediatric MR imaging and pediatric nuclear medicine, and is highly demanding on the readers. The images could be interpreted in a joint read-out session between nuclear medicine physicians and pediatric radiologists, or, alternatively, a single pediatric imager who is trained in both nuclear medicine and radiology could interpret them. The former method may be preferred, particularly in situations where complex organ-specific MR acquisition has been performed, and could follow the typical workflow of the department for reading dedicated organ-system MR imaging by subspecialty pediatric radiologists (eg, thoracic radiologists, abdominal radiologists, musculoskeletal radiologists, or neuroradiologists), and separate MR imaging and PET reports may be issued. In contrast, a whole-body PET/MR imaging examination performed without a dedicated diagnostic-quality organ-specific MR imaging is probably most efficiently interpreted by a single hybrid pediatric imager.

Image Pitfalls

Certain pitfalls related to MR-based attenuation correction have to be considered to avoid misinterpretation. Quantitative errors are observed in and around skeletal structures (eg, bone metastases and bone marrow) because bone attenuation is often neglected in segmentation-based MR attenuation correction. Additionally, segmentation artifacts also can occur around metal implants due to MR imaging susceptibility artifacts. Even though these artifacts have the potential to affect PET quantification (ie, standardized uptake values [SUVs]) and visual PET interpretation, likely no changes in diagnosis occur, especially when the attenuation maps are checked for artifacts and PET/MR imaging is read in conjunction with the non–attenuation-corrected PET. Truncation artifact that can be seen with PET/CT also may occur with PET/MR due to differences in field of view of PET and MR imaging ( Fig. 2 ). Important pearls and pitfalls in performance and interpretation of pediatric PET/MR imaging and its clinical applications are listed in Box 1 .

Box 1

Pearls, pitfalls, and variants

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The 2 types of PET/MR imaging scanners are sequential and simultaneous, and the latter is preferred and more widely installed.
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In integrated PET/MR imaging only a normal-bore (60-cm) MR imaging system is available.
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PET/MR imaging cannot be performed in patients who have contraindications to MR imaging.
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Fully integrated PET/MR imaging is particularly relevant in children in decreasing overall scanning time, given limited patient compliance and the need for anesthesia/sedation.
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PET/MR imaging protocols should maximize the time of simultaneous acquisition of MR and PET and need to be optimized to achieve the highest diagnostic accuracy.
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PET/MR imaging protocol preferably should not be longer than the equivalent PET/CT plus organ system/region-specific MR imaging for a given indication.
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Dose reductions of up to 73% have been reported when performing PET/MR imaging instead of PET/CT due to lack of the CT component.
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Decreasing the amount of PET tracer administered (due to longer imaging times in PET/MR imaging) could further reduce the radiation dose.
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It is highly recommended to assess the quality of the MR-based attenuation map (μ map) as an initial step in reading PET/MR imaging to assess for potential artifacts.
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Quantitative errors are observed within and around skeletal structures (eg, osseous metastases and bone marrow) because bone attenuation is usually neglected or inaccurately calculated.
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Time-of-flight technology in PET/MR imaging may significantly reduce metal artifacts.
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Segmentation artifacts can occur in MR-based attenuation correction that are mostly observed around metal implants causing MR susceptibility artifacts.
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Because of smaller size of young pediatric patients and resultant relatively larger FOV of the MR imaging portion of the scan, the arms that are by a patient’s side are imaged as a part of the attenuation correction sequences, which avoids attenuation correction errors.
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MR imaging information/data also can be used to improve PET reconstruction and data analysis. For instance, because MR imaging sequences could be obtained during breath-holds or using respiratory gating or, alternatively, used for motion correction. Therefore, PET data could be filtered and reconstructed with motion-free or motion-corrected data, with decrease in motion artifact.
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A majority of the basic system performance and quality-control tests for a clinical PET/MR imaging scanner typically are performed with standard MR imaging and PET phantoms.
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PET/MR imaging examinations have increased data storage size requirements in comparison to PET/CT ranging from 500 MB to 2 GB.
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Advanced functional MR imaging techniques, such as DWI, MR spectroscopy, and perfusion-weighted imaging, in conjunction with PET may further enhance disease detection and characterization.
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For ¹⁸F-FDG examinations, physiologic brown fat uptake needs to be suppressed (by warming or pharmacologic means) to preserve image quality and diagnostic accuracy.
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Small lung nodules are challenging to detect on PET/MR imaging and, therefore, separate CT may need to be acquired in patients likely to have pulmonary metastases.
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For both bone and soft tissue sarcoma, PET/MR imaging does not have incremental value over MR imaging alone for the primary tumor but is beneficial in staging of nodal and distant metastases.
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¹⁸F-FDG PET is useful in the assessment of primary as well as metastatic disease in neuroblastomas that are not mIBG avid or when clinical symptoms and/or conventional imaging modalities, including MR imaging, suggest more disease than the extent of mIBG uptake.
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¹⁸F-FDG uptake by brain tumors correlates approximately with the grade of the tumor and, therefore, is used to image poorly differentiated tumors, such as high-grade gliomas.
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To reduce the effect of post-therapy inflammation, ¹⁸F-FDG PET/MR imaging in assessment of brain tumors should be performed 3 months after end of treatment.
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For brain tumor imaging with ¹⁸F-FDG PET, delayed imaging at 3 hours to 4 hours improves the tumor-to-brain uptake ratio.
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The main neurologic indication for pediatric PET/MR imaging is assisting presurgical localization of epileptogenic focus.
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¹⁸F-FDG PET has a high sensitivity for detecting epileptogenic foci in the temporal lobe with lower sensitivity for extratemporal epilepsy.
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PET/MR imaging may have an important role in the identification of cortical dysplasia by detecting subtle changes on MR images that may initially be reported as normal.
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For accurate interpretation of ¹⁸F-FDG PET for epilepsy, it is beneficial to monitor with electroencephalogram during the ¹⁸F-FDG uptake phase. Seizures that occur during the uptake period may affect tracer distribution and degree of uptake in the epileptogenic foci.
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¹⁸F-FDG is a sensitive agent but nonspecific in evaluation of infection/inflammation. Additionally, assessment for abnormal uptake in regions of normal physiologic distribution is challenging. Simultaneous ¹⁸F-FDG PET/MR imaging can potentially increase the overall accuracy of diagnosis in this clinical setting.

Clinical applications

The major applications of PET/MR imaging in pediatrics are discussed in this section. The reader is also referred to Table 4 for a list of major clinical applications, the radiopharmaceuticals used, and pertinent imaging features and evaluation.

Table 4

PET/MR imaging—major clinical applications, radiopharmaceuticals, and pertinent imaging evaluation

	Clinical Applications	Major PET Radiopharmaceuticals	Imaging Assessment
Oncologic	Lymphoma	¹⁸F-FDG	• Hodgkin lymphomas and majority of the different histologic subtypes of non-Hodgkin lymphomas in children are highly ¹⁸F-FDG avid, and may have both nodal and extranodal involvement. ○ For Hodgkin lymphoma, most cases have intrathoracic nodal involvement. ○ For non-Hodgkin lymphomas, mediastinal and hilar involvement is common with lymphoblastic lymphoma and abdominal disease with Burkitt lymphoma. • The MR component (in particular, T2-weighted imaging and DWI) increases the chances of detecting focal or diffuse splenic and hepatic involvement, thus contributing to accurate diagnosis of extranodal disease. • Combination of ¹⁸F-FDG PET and MR imaging effectively assesses extent of bone marrow involvement. • Deauville score is useful in assessment of response to therapy on the interim and end-of-therapy scans, with scores ≤3( ¹⁸F-FDG uptake less than liver) considered negative for disease, with very low likelihood for relapse. • An interim PET scan has a more defined role in predicting the likelihood of treatment failure and relapse in Hodgkin lymphoma than in non-Hodgkin lymphoma.
	Primary bone tumors	¹⁸F-FDG	• Bone sarcomas are typically located in the extremities, but pelvic, rib, and vertebral lesions also can occur, especially in Ewing sarcoma. • For both Ewing sarcoma and osteosarcoma, MR imaging is the preferred modality for diagnosis and local tumor staging. • The utility of ¹⁸F-FDG PET is not completely defined in these malignancies, in particular for osteosarcoma, although greater accuracy in detection of nodal and distant metastases has been reported. • ¹⁸F-FDG PET is more sensitive for skeletal metastases in Ewing sarcoma and osteosarcoma likely related to predominant marrow infiltration in Ewing and osteoblastic metastases in osteosarcoma. • Combined ¹⁸F-FDG PET/MR imaging is likely to increase overall accuracy of staging and response assessment. • A significant disadvantage of PET/MR imaging is the limitation in detecting small pulmonary nodules, which is a common site of metastases.
	Soft tissue sarcomas	¹⁸F-FDG	• These include rhabdomyosarcoma (most common), nonrhabdomyosarcoma soft tissue sarcomas, and desmoid tumors. • The most common locations for rhabdomyosarcomas are head and neck and genitourinary tract. • MR imaging is the preferred modality for diagnosis and local tumor staging. • Soft tissue sarcomas are typically ¹⁸F-FDG avid, and ¹⁸F-FDG is useful in detection of nodal and skeletal metastases. • Combined ¹⁸F-FDG PET/MR imaging, including DWI, is likely to increase overall accuracy of staging and response assessment.
	NF-1	¹⁸F-FDG	• NF-1 patients are at increased risk for developing plexiform neurofibromas that can transform to MPNSTs • ¹⁸F-FDG PET can monitor and detect malignant transformation of plexiform neurofibromas. MPNSTs demonstrate increased ¹⁸F-FDG uptake, and different cutoff values for SUV exist in literature. Sites of focal ¹⁸F-FDG uptake could be used to target biopsy. • MR findings that suggest malignant transformation of neurofibromas include larger size of the mass, peripheral enhancement pattern, and heterogeneity on T1-weighted imaging, perilesional edema, and intratumoral cystic change. • PET/MR imaging may prove useful with complementary information that can further increase the accuracy for assessment of malignant degeneration.
	Neuroendocrine tumors	⁶⁸Ga-DOTATATE ¹⁸F-FDG ¹⁸F-DOPA ¹¹C-hydroxytryptophan	• Neuroendocrine tumors are rare in the pediatric population, with carcinoid of the appendix the most common. • Genetic tumor predisposition is important in the etiology of neuroendocrine tumors in children. • ⁶⁸Ga-DOTATATE is a recently FDA-approved somatostatin receptor PET agent and could be used in diagnosis and staging of these tumors. It is considered equivalent or superior to ¹¹¹In-pentetreotide (OctreoScan) imaging for diagnosis and staging of neuroendocrine tumors. Imaging time and the number of patient visits are decreased for ⁶⁸Ga-DOTATATE. • Head of pancreas, especially the uncinate process, can demonstrate viable physiologic focal or diffuse uptake of ⁶⁸Ga-DOTATATE and is a potential source of misinterpretation because neuroendocrine tumors can occur in the pancreaticoduodenal regions. • An additional source of potential misinterpretation is ⁶⁸Ga-DOTATATE uptake in an accessory splenule. • MR imaging detection rates of these tumors are also high and PET/MR imaging may improve the overall accuracy. Neuroendocrine tumors typically have high signal on T1-weighted fat-suppressed noncontrast image and demonstrate restricted diffusion.
	Neuroblastoma	¹⁸F-FDG PET ⁶⁸Ga-DOTATATE ¹¹C-hydroxyephedrine ¹¹C-epinephrine ¹⁸F-fluorodopamine ¹⁸F-DOPA	• Neuroblastomas most commonly occur in the adrenal gland followed by extra-adrenal retroperitoneum. • Metastases are present in up to 60% cases at diagnosis • ¹⁸F-FDG PET is useful in the assessment of primary as well as metastatic disease in neuroblastomas that are not mIBG avid, or in cases where conventional imaging modalities, including MR imaging, suggest more disease than the extent of mIBG avidity. • ¹⁸F-FDG PET is limited in assessment of bone marrow involvement given normal physiologic uptake in the marrow. • PET/MR imaging with somatostatin receptor agents (eg, ⁶⁸Ga-DOTATATE) may have a larger future role in imaging of neuroblastoma given improved sensitivity of PET, better quantitation of uptake, and shorter uptake times compared with the current standard of mIBG scintigraphy and with the added advantage of having a concurrent MR imaging.
	Primary brain tumors	¹⁸F-FDG Amino acid tracers • ¹¹C-methionine • ¹⁸F-DOPA • O-(2- ¹⁸F-fluoroethyl)- l -tyrosine ¹⁸F-fluorothymidine ¹⁸F-fluoromisonidazole	• There is higher uptake in high-grade tumors compared with low-grade tumors or radiation necrosis/post–therapeutic change with ¹⁸F-FDG PET. • ¹¹C-methionine is more accurate in detection of low-grade and high-grade gliomas and for assessment of the treatment response because of low background uptake in normal brain tissue. • ¹⁸F-fluorothymidine is used for imaging tumor cell proliferation, and its uptake correlates with thymidine kinase-1 activity, an enzyme expressed during the DNA synthesis phase of the cell cycle. Thus, ¹⁸F-fluorothymidine could be used to monitor treatment response and to serve as a prognostic marker in brain tumor imaging. • ¹⁸F-fluoromisonidazole is an agent to image hypoxia, because its metabolites are trapped exclusively in hypoxic cells and could be used to differentiate high-grade and low-grade brain gliomas. Hypoxia is associated with tumor progression and resistance to radiotherapy.
Nononcologic	Epilepsy	¹⁸F-FDG	• Interictal ¹⁸F-FDG PET demonstrates focal hypometabolism in the epileptogenic region and is more sensitive in temporal than extratemporal epilepsy for detection of the seizure focus. • MR imaging is the modality of choice for assessment of an underlying structural lesion in patients with epilepsy, such as mesial temporal sclerosis, cortical malformations, cortical dysplasias, brain tumors, and ischemic brain injury, among others. • Functional MR imaging can be used to noninvasively map motor and language functions and may be used as part of surgical planning to predict and limit postoperative neurologic deficits. • Combined PET/MR imaging can have incremental value over PET or MR imaging alone in the assessment of epileptogenic focus, with an added benefit of a single imaging session.
	Inflammatory/infectious processes • FUO • Neuroinflammation • Vasculitis • Spondylodiscitis • Arthritis • IBD	¹⁸F-FDG	• ¹⁸F-FDG PET may be useful in the setting of FUO when conventional imaging modalities do not yield a diagnosis. • FUO could be related to underlying infectious, inflammatory or oncologic processes, and ¹⁸F-FDG PET demonstrates uptake at sites of disease involvement. • ¹⁸F-FDG uptake within malignancies or inflammatory processes in or adjacent to sites of high physiologic activity (eg, marrow and genitourinary and gastrointestinal tracts) may be obscured, and simultaneous PET/MR imaging is likely to add benefit in such situations. • Both ¹⁸F-FDG PET and MR imaging (including DWI) have been used in assessment of the extent and degree of bowel involvement with active IBD demonstrating increased ¹⁸F-FDG uptake and lower apparent diffusion coefficient values, respectively.
	Cardiac applications Evaluation of myocardial viability and perfusion in children with congenital or acquired coronary abnormalities	Metabolic imaging ¹⁸F-FDG Perfusion imaging ⁸³Ru ¹³N-ammonia ¹⁵O-water	• MR imaging is the imaging standard for assessment of myocardial viability, with scar tissue demonstrating delayed gadolinium enhancement. • Myocardial perfusion PET provides an accurate qualitative assessment of myocardial perfusion as well as quantification of regional myocardial blood flow. • ¹⁸F-FDG PET in conjunction with perfusion PET also could be used for evaluation of myocardial viability. Hibernating myocardium demonstrates a perfusion-metabolism mismatch, whereas scar tissue demonstrates no perfusion or metabolism. • Combined PET/MR imaging potentially is advantageous by providing simultaneous morphologic and functional information simultaneously. • Anatomic extent and ¹⁸F-FDG avidity of inflammatory tissue in myocarditis or sarcoidosis and cardiac tumors also could be assessed after suppressing the normal myocardial uptake after preparation with a special high-fat, low-carbohydrate diet.