Whole-body PET/MRI of Pediatric Patients: The Details That Matter

The overall goal of this procedure is to use positron emission tomography/magnetic resonance or PET/MR scans for the staging and restaging of cancer in children. PET/MRI can help diagnose the primary tumor and the degree of metastasis to assign appropriate treatment protocol and to determine the tumor response to the treatment. The main advantage of the PET/MRI scan is that it allows the evaluation of the primary tumor and the whole body in one session with marked and reduced radiation exposure compared to a PET/CT scan.

One to two hours before the scan, infuse ferumoxytol diluted in saline at a one-to-four ratio intravenously into the patient over at least 15 minutes according to FDA protocols. Alternatively, standard gadolinium-based contrast agent can be administered directly before the start of the PET/MRI scan. This means after the localizers and directly before starting the integrated PET/MRI scan.

45 minutes before the scan, check the patient's blood glucose levels. If the patient's blood glucose levels are below 200 milligrams per deciliter, administer radiolabeled 2-deoxy-2-fluoro-D-glucose and one milliliter of saline. If the patient's blood glucose levels are above 200 milligrams per deciliter, consider rescheduling the PET/MRI scan.

At the time of the scan, position the patient in the supine position on the PET/MR table of a three-tesla integrated PET/MRI system bundled with MP24 software. If the patient cannot lay supine, scan the head supine and the whole body in the prone position. For very long patients, acquire the lower legs in a second scanning session.

Close a breathing belt around the patient and position the head coil and two to three anterior coils on the neck, chest, abdomen, pelvis and extremities. Enter the patient accession number for the diagnostic images before initiating the scan. Using the generic localizer protocol, acquire whole body localizers and axial, coronal and sagittal planes selecting the number of stations according to the patient's height.

Prescribe axial PET slabs to three-minute data acquisitions using the maximum field of view. The most critical parts of the protocol are using time-efficient planning and prescribing the PET slabs and MRI sequences with the correct parameters and in the correct consecutive order before initiating the scan. In the setup menu of the prescribed PET task, enable PET reconstruction as one non-attenuation-corrected and one MRI attenuation-corrected PET reconstruction.

Set a matrix size of 192 by 192 and use 28 subsets with two iterations for each PET reconstruction. The reconstructed PET data will be a merged volume. Fill each PET slab with a low-resolution liver acquisition with volume acquisition or LAVA sequence for attenuation correction and a higher resolution LAVA sequence for anatomical co-registration of the PET data.

Under the task menu, select the PET task and click Add Task and Add Post-Processing. Then, select the Auto Bind feature to enable merging of the magnetic resonance attenuation corrections. Click the setup to bring up a new menu and select all the consecutive water attenuation corrections for the PET slabs.

Then, click Accept to enable the software to automatically merge the attenuation corrections. Merge the diffusion-weighted images selecting the first PET slab before clicking on the diffusion-weighted sequence and Add Task. Add a dedicated T2 sequence for evaluation of the lungs and/or mediastinum.

For a local PET/MR scan, place an appropriate coil over the primary tumor and scan the tumor as just demonstrated. Before co-registering the PET and MRI data, select all of the individual MRI datasets of a specific sequence and click AutoBind. The software will construct a merged MRI dataset.

Then, save the merged MRI dataset. The software will automatically name the dataset and add it to the series list. It is critical to merge the MRI data of the same sequence before merging the MRI data with the PET data so that the radiologist can scroll through the scan from head to toe without disruption.

To co-register the merged MRI attenuation-corrected PET data and merged high-resolution LAVA data, select each dataset to be co-registered and click Image QC.The software will show both the individual non-co-registered datasets and the co-registered dataset in two planes in six separate windows. Select the MRI data and window level appropriately since this cannot be adjusted once the co-registered scan is finalized and saved, and select the co-registered PET/MRI dataset and window level the PET data. Select the appropriate size orientation for the co-registered dataset and select batch under the Film/Save menu to save the adjusted co-registered PET/MRI dataset.

Select Loop and scroll all the way to the most cranial slice, then click Set Start and scroll down to the caudal slice and click Set End. Enter an appropriate file name into the description field and click Preview to let the scan scroll from head to toe while checking that the final result is acceptable. Close the preview and select OK to start rendering the PET/MRI scan, then co-register the MRI attenuation-corrected PET data and the dedicated T2 MRI sequence for evaluation of the lungs as just demonstrated.

To generate a PET whole-body maximum intensity projection, select the MRI attenuation-corrected PET in any MRI dataset and click ImageQC. Double-click on the window with PET data only, and hover the cursor in the upper left corner to select view type. Select VR to create a 3D reconstruction of the PET data, then select HD maximum intensity projection to create the PET whole-body maximum intensity projection overview.

To save the overview, click Batch and Rotate. Adjust the number of images to 60 and enter an appropriate file name in the description field. Click preview to confirm that the final result is acceptable, then close the preview and select OK to start rendering the maximum intensity projection overview.

To send the diagnostic images and datasets to picture archiving and communication system, select the appropriate data series and click on the desired network in the Destinations menu. To send source images under a different accession number, click Edit Patient to change the accession number and send the data as just demonstrated. To measure the standardized uptake values, load the co-registered PET/MRI data in an FDA-approved medical imaging viewer to read and analyze the scan.

Select Calculate Standardized Uptake Value Statistics and position the region of interest around the tissue demonstrating the pathological radiolabeled glucose uptake. Hold and drag with the right mouse button to enlarge the region of interest diameter as necessary. The software will then determine both the mean and maximum standardized uptake values.

Here, representative integrated whole-body head-to-toe datasets of MRI, PET and fused PET/MR images of a 10-year-old boy with undifferentiated sarcoma of the 12th rib, status post-resection presenting with pulmonary and cardiac metastases are shown. Data for the two scans was acquired within 40 minutes and the effective dose of the patient during this exam was 3.3 millisievert. The described whole-body PET/MRI protocol can be completed in less than 40 minutes.

When the diffusion-weighted scans are omitted, the scan can be completed in less than 30 minutes. Our PET/MRI protocol has been tailored to the specific needs of children with regards to the slowest reasonably achievable radiation principles, time-efficient data acquisition and a local and whole-body tumor scan in one exam.