Pediatric Applications of Hybrid PET/MR Imaging




Hybrid PET/MR imaging systems have recently become available for clinical practice. The simultaneous physiologic and anatomic imaging offers the potential to reduce radiation dose and other advantages for pediatric patients. Issues more unique to pediatric imaging, however, must also be addressed, including imaging time and disease sensitivity. Combined with newer tracers and a concerted multidisciplinary effort, the approach has the potential to substantially improve the imaging of a variety of pediatric diseases.


Key points








  • Hybrid PET/MR imaging offers significant ionizing radiation dose reduction, important for the pediatric population.



  • Considerations must be made for pediatric studies, including site selection, scan duration, and sequence selection.



  • New tracers could offer higher diagnostic utility for pediatric populations.




The recent introduction of hybrid PET/MR imaging systems has offered a potentially powerful new tool for diagnostic imaging. The advantages and disadvantages of the hybrid PET/MR imaging systems are most apparent in pediatric patient populations. In general, oncological indications are the primary reasons for pediatric patients requiring PET imaging, although there is also a significant potential role for other applications, such as neurologic diseases. The potential for low-dose molecular and functional imaging is the primary source of interest from pediatric oncologists in the hybrid system. A good example of the combined functional information with soft tissue discrimination is shown Fig. 1 , where the PET/MR images have detected a small metastasis from Ewing sarcoma within the adductor muscle. The MR images offer precise localization of the signal and significantly increased confidence in the involvement of the muscle.




Fig. 1


Coronal ( A C ) and axial ( D F ) PET ( C , F ), MR ( A , D ), and PET/MR imaging fusion ( B , E ) images from a pediatric patient with known Ewing sarcoma. A small metastasis ( blue arrows ) is identified in the left thigh region. The axial images ( D F ) demonstrate that the metastasis is centered in the adductor muscle.

( From Hirsch FW, Sattler B, Sorge I, et al. PET/MR imaging in children. Initial clinical experience in pediatric oncology using an integrated PET/MR imaging scanner. Pediatr Radiol 2013;43(7):873; with permission.)


Despite the potential in the pediatric population, the relative use of PET/MR imaging in the pediatric population and the number of research studies focused on this population are limited. This is likely due to some of the technical complexities of imaging the younger population on the PET/MR imaging scanners and the complexity of poor reimbursement for the pediatric population. These struggles are not specific to pediatric PET/MR imaging and often reflect more global issues in pediatric imaging.




Radiation dose reduction


Even with the potential mortality of the primary pediatric cancers, patients who survive into adulthood may suffer secondary effects from their cancer treatment. Thus, there is a continuing effort to minimize delayed health effects from radiation in this sensitive population. This could reduce the occurrence of secondary cancers, accelerated atherosclerosis, or other health effects. Although somewhat controversial, the potential association of dental radiographs with the later development of meningiomas serves to remind clinicians of the importance of minimizing any x-ray dose.


The CT portion of a PET/CT study is performed for both attenuation correction and anatomic guidance. Typically, CT studies are performed only with intravenous contrast if there is clinical need for higher-quality CT images. Eliminating the CT portion by using MR imaging for attenuation correction and anatomic guidance in the hybrid PET/MR imaging study reduces the total radiation dose by approximately one-half. Furthermore, because many patients are followed longitudinally, the overall dose reduction to a patient can be significant. The lifetime attributable risk cancer from a single abdominal CT has been estimated to be as high as 0.09% for a 5-year-old child, although studies are ongoing to better estimate the actual risk. There are many efforts focused on reducing radiation dose to the most sensitive population. Some studies have called into question the need for PET/CT in the pediatric population, and, currently, follow-up CTs for common pediatric diseases (such as lymphoma), may not be clinically indicated. PET/MR imaging can be a source of dose reduction.




Radiation dose reduction


Even with the potential mortality of the primary pediatric cancers, patients who survive into adulthood may suffer secondary effects from their cancer treatment. Thus, there is a continuing effort to minimize delayed health effects from radiation in this sensitive population. This could reduce the occurrence of secondary cancers, accelerated atherosclerosis, or other health effects. Although somewhat controversial, the potential association of dental radiographs with the later development of meningiomas serves to remind clinicians of the importance of minimizing any x-ray dose.


The CT portion of a PET/CT study is performed for both attenuation correction and anatomic guidance. Typically, CT studies are performed only with intravenous contrast if there is clinical need for higher-quality CT images. Eliminating the CT portion by using MR imaging for attenuation correction and anatomic guidance in the hybrid PET/MR imaging study reduces the total radiation dose by approximately one-half. Furthermore, because many patients are followed longitudinally, the overall dose reduction to a patient can be significant. The lifetime attributable risk cancer from a single abdominal CT has been estimated to be as high as 0.09% for a 5-year-old child, although studies are ongoing to better estimate the actual risk. There are many efforts focused on reducing radiation dose to the most sensitive population. Some studies have called into question the need for PET/CT in the pediatric population, and, currently, follow-up CTs for common pediatric diseases (such as lymphoma), may not be clinically indicated. PET/MR imaging can be a source of dose reduction.




PET/MR imaging technical differences in children


Conventional clinical PET/MR imaging currently uses the Dixon method to estimate the attenuation coefficient in subjects. One small advantage in the pediatric population is that the skeleton may not have fully ossified, which may increase the available signal from the Dixon sequence. This could, in turn, lead to improved attenuation estimation in pediatric patients, though the relative improvement is likely small. The smaller size of pediatric patients also leads to another significant advantage because the patients’ arms are within the field of view (FOV) of the MR imaging portion of the scan. As a result, the arms are imaged as a part of the attenuation correction sequences when the arms are scanned by a patient’s side and accounted for in the attenuation correction. Not accounting for the arms can lead to significant errors in attenuation correction. The arms-down position is also desirable for improved patient comfort. Fig. 2 shows an example of an 8-year-old child with a rhabdomyosarcoma in the right lower extremity and metastatic disease to body of the seventh thoracic vertebrae. The arms are seen in the down position and completely imaged within the FOV of the MR imaging.




Fig. 2


Coronal STIR MR ( A ) and fusion PET/MR ( B ) images from an 8-year-old girl with metastatic rhabdomyosarcoma of the right lower extremity ( dashed arrow ). A metastatic lesion is shown in the thoracic spine ( solid arrow ). The arms of this subject are down and fully imaged on the MR imaging portion of the study.


Early pediatric studies have directly evaluated PET/MR imaging–derived standard uptake values (SUVs) compared with PET/CT-derived SUVs. Lyons and colleagues evaluated the uptake values between the 2 modalities in normal structures and found systematic relative underestimation of the SUVs. Furthermore, the various tissue types had a large range of correlations. Poor correlations were found in organs, such as the lungs and skeletal muscles, whereas much higher correlations were found in solid organs, such as the brain, myocardium, and bone marrow. It is likely that the 3-segment model used in their study severely limits the attenuation correction.


One of the other important concerns raised by many PET/MR imaging studies is the impact of respiration motion during the MR acquisition. The MR attenuation correction images requires significantly longer time compared with conventional CT attenuation images, resulting in respiratory motion within the attenuation correction images. The respiratory motion can cause both misregistration and inaccurate SUV measurements in both the thorax and upper abdominal organs. Although PET acquisitions are also significantly longer than a single breath-hold, the iterative reconstruction algorithms can take into account the cyclic motion if an accurate attenuation map is available. MR imaging confounds the respiratory motion due not only to the long scan time but also the poor visualization of lesions in the lungs due to susceptibility effects. The use of ultrashort echo time sequences allows better delineation of pulmonary nodules compared with conventional dual-echo gradient-echo sequences. Furthermore, capturing motion on MR imaging sequences using navigator echo or other types of approaches can noninvasively measure the respiratory cycle or generate 4-D MR images to assist in PET reconstruction. Combining these approaches for improved attenuation correction with MR imaging gated acquisitions is promising; however, these areas remain under study and their application has not been explored fully in the pediatric population. This is of critical importance for pediatric oncology, because many pediatric cancers can metastasize to the lungs, including osteosarcoma and nephroblastoma.




PET/MR imaging workflow considerations


MR Imaging Sequence Selection


Effective MR imaging sequence selection can substantially enhance the value of the PET/MR imaging study for patients and physicians. This is especially true in the pediatric population where time constraints are more apparent due to patient tolerance and minimizing sedation. Imaging sequences should be selected carefully keeping in mind the exam indication and the potential sites of metastases or treatment related complications.


For whole-body imaging, short-time inversion sequences (STIR) acquired in the coronal plane of particular use for evaluating pathology and lymphadenopathy. At the authors’ institution, the fused whole-body STIR images are regularly used for lesion location. For example, in Fig. 3 , fused coronal fluorodeoxyglucose F 18 (FDG) PET and STIR MR images allow whole-body evaluation of this patient with neurofibromatosis 1. The neurofibromas in this patient can be seen scattered throughout the entire body as bright lesions against the darker background of muscle. Lymph nodes can also be readily identified on STIR images. Other investigators have used the STIR half-Fourier rapid acquisition with relaxation enhancement sequence for whole-body MR imaging of pediatric lymphomas for rapid imaging. One of the significant advantages for PET/MR imaging is the robust developer community working on new faster and higher-resolution MR imaging sequences, most of which may be directly ported to the PET/MR imaging systems. Advances in MR imaging sequence design may be almost immediately adapted for use in MR imaging. Ongoing developments in CT are mostly focused on dose reduction.


Sep 18, 2017 | Posted by in MAGNETIC RESONANCE IMAGING | Comments Off on Pediatric Applications of Hybrid PET/MR Imaging
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