Quantitative [¹⁸F]-Naf-PET-MRI Analysis for the Evaluation of Dynamic Bone Turnover in a Patient with Facetogenic Low Back Pain

With this technique, we use PET-MRI to identify physiologic biomarkers of bone turnover. We believe these physiologic biomarkers may more accurately reflect facet joint pathology than CT and MRI static images. We believe this technique may help us identify painful and symptomatic joints in the lumbar spine.

We also hope that this technique can be applied to other regions of the spine, such as the cervical and thoracic spine for identification of symptomatic facet joints, and even degenerative disc disease. For simultaneous PET and magnetic resonance image or MRI acquisition, using a posterior array central molecular imaging array coil and a 3.0 PET and MR imager, center the field of view in both imaging modalities to cover the lower spine region from T-12 to S-3. Set the appropriate parameters for a PET acquisition and for a clinical MRI sequence for a lumbar spine protocol.

Immediately before acquiring the MRI sequences, intravenously inject a 0.1 millimeters per kilogram of gadobutrol contrast into the patient’s antecubital fossa and a 2.96 megabecquerel per kilogram radioactive dose of sodium fluoride F-18. Next, using three separate temporal phases centered over the lower spine, perform a 60-minute dynamic PET scan acquiring the first phase of the scan with 12 frames of 10 seconds each, the second phase of four frames of 30 seconds each, and the last phase of 14 frames of four minutes each. Then set a 60 centimeter field of view with a three millimeter filter cutoff, a standard Z-axis filter, no cardiac 3D filter, 28 subsets, and a Time-of-Flight ordered subsets expectation maximization with four iterations to reconstruct the PET data on the console.

For analysis of the acquired PET and MR images, transfer the images to a dedicated work station equipped to analyze dynamic PET data. Locate the bilateral facet joints of the lumbar spine from L-1 to L-2 and L-5 to S-1. Visually triangulate with the sagittal and axial plane T-2 MR images and record the the slice number of the approximate center to identify the center point of each lumbar facet joint.

With the patient data open in the view tab, click the volume of interest button and select sphere object. Within the predefined popup window, enter 7.5 millimeters as the radius and click create new volume of interest. Left click to place a 7.5 millimeter spherical volume of interest in the center of each facet joint, adjusting the sphere by left clicking and dragging until the volume is visually centered on the facet.

When a volume of interest has been placed onto each facet, place a five millimeter spherical volume of interest in the right iliac crest in the central marrow cavity to exclude the cortex involvement as a reference region. Then position the volume of interest so the edges are within the marrow entirely. Right click on the axial image and select data inspection to measure the diameter of the abdominal aorta proximal to its bifurcation.

To calculate the partial volume correction coefficient, left click on the right side of the aortic wall and move the cursor to the left side of the aortic wall to record the distance of the aortic wall diameter in the data inspector window. Next, left click the volume of interest button and select circle region of interest to create a circular region of interest with a radius of half of the aortic diameter. Click create new volume of interest and left click in the center of the aorta, repositioning the volume as necessary to ensure the circle approximates the aortic wall position.

Then descend one slice in the axial plane and create a second circular region of interest to make a cylindrical region of interest from the two overlapping circular regions of interest. To create a partial volume corrected arterial input, use the recovery coefficients derived from the PET computed tomography phantom to apply the recovery coefficients to the image-based measurement over the descending aorta. Then substitute this partial volume-corrected arterial input into the image analysis software for use in kinetic modeling and the accurate quantification of tracer kinetics.

Here, represented of sodium fluoride F-18 PET, axial T-2 fat suppressed, and axial T-1 post-contrast fat suppressed MR images through the level of the L-3 to L-4 facet joints can be seen. In this table, the Patlak kinetic model, standardized uptake value mean and maximum, and MRI facet arthropathy grade for each of the 10 sampled facet joints in a representative patient have been summarized. When the patient Patlak kinetic model influx rates were plotted against the standardized uptake value mean and the MRI-based facet arthropathy grades, the facet joint with the highest MRI grade of degenerative facet arthropathy had the highest Patlak kinetic model and standardized uptake value mean values.

We are in the midst of a clinical trial to validate our technique in patients with lumbar facet joint pain. We look forward to publishing this data soon. There’s a steep learning curve for understanding how to do the analysis with the PET MR software.

Additionally, it can be difficult to locate the facet joints for those people unfamiliar with spinal anatomy. As a result, we believe that a visual depiction of the image processing steps will greatly enhance understanding of our process.