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

Summary

Imaging techniques that reflect dynamic bone turnover may aid in characterizing a wide range of bone pathologies. We present detailed methodologies for performing and analyzing dynamic [¹⁸F]-NaF-PET-MRI data in a patient with facetogenic low back pain using the lumbar facet joints as a prototypical region of interest.

Abstract

Imaging techniques that reflect dynamic bone turnover may aid in characterizing a wide range of bone pathologies. Bone is a dynamic tissue undergoing continuous remodeling with the competing activity of osteoblasts, which produce the new bone matrix, and osteoclasts, whose function is to eliminate mineralized bone. [¹⁸F]-NaF is a positron emission tomography (PET) radiotracer that enables visualization of bone metabolism. [¹⁸F]-NaF is chemically absorbed into hydroxyapatite in the bone matrix by osteoblasts and can thus noninvasively detect osteoblastic activity, which is occult to conventional imaging techniques. Kinetic modeling of dynamic [¹⁸F]-NaF-PET data provides detailed quantitative measures of bone metabolism. Conventional semi-quantitative PET data, which utilizes standardized uptake values (SUVs) as a measure of radiotracer activity, is referred to as a static technique due to its snapshot of tracer uptake in time.  Kinetic modeling, however, utilizes dynamic image data where tracer levels are continuously acquired providing tracer uptake temporal resolution. From the kinetic modeling of dynamic data, quantitative values like blood flow and metabolic rate (i.e., potentially informative metrics of tracer dynamics) can be extracted, all with respect to the measured activity in the image data. When combined with dual modality PET-MRI, region-specific kinetic data can be correlated with anatomically registered high-resolution structural and pathologic information afforded by MRI. The goal of this methodological manuscript is to outline detailed techniques for performing and analyzing dynamic [¹⁸F]-NaF-PET-MRI data. The lumbar facet joint is a common site of degenerative arthritis disease and a common cause for axial low back pain.  Recent studies suggest [¹⁸F]-NaF-PET may serve as a useful biomarker of painful facetogenic disease.  The human lumbar facet joint will, therefore, be used as a prototypical region of interest for dynamic [¹⁸F]-NaF-PET-MRI analysis in this manuscript.

Introduction

Standard clinical imaging techniques of bone pathology are primarily limited to characterizing structural changes, which can be nonspecific. For example, asymptomatic morphologic abnormalities related to the normal aging may be indistinguishable from degenerative alterations which are responsible for severe pain and disability¹. Bone is a dynamic tissue undergoing continuous remodeling with the competing activity of osteoblasts, which produce the new bone matrix, and osteoclasts, whose function is to eliminate mineralized bone². [¹⁸F]-NaF is a positron emission tomography (PET) radiotracer that enables visuali....

Protocol

This prospective feasibility study recruited patients after obtaining Human Study IRB approval and complying with HIPAA regulations.

1. Phantom

1. Fill a hollow cylindrical phantom with an insert that has hollow cylinders with a range of diameters (5 - 38 mm) with 185 MBq of [¹⁸F]-NaF.
2. Generate an attenuation map of the phantom using CT or a template that was generated previously for this phantom.
3. Place the phantom into the center of PET/MR and acquire PET data for 5-10 min recording the resultant image using the imaging console.
4. Reconstruct using the imaging console with the CT-based attenu....

Results

¹⁸NaF-PET uptake values are measured in the bilateral facet joints at the L1-L2 through L5-S1 vertebral levels for a total of 10 ROIs in a single representative patient with axial low back pain. Representative [¹⁸F]-NaF-PET, axial T2 fat suppressed, and axial T1 post-contrast fat-suppressed MR images through the level of the L3-L4 facet joints are shown in Figure 2.  The K_{i_Patlak}, SUV_mean, SUV_max, and MRI facet arthropathy grade for eac.......

Discussion

In this methodological manuscript, we have provided background on the potential utility of dynamic [¹⁸F]-NaF-PET-MRI for evaluating a wide range of bone pathologies and have outlined the techniques for dynamic [¹⁸F]-NaF-PET-MRI image acquisition and analysis using the human lumbar facet joints as prototypical regions of interest. Dual modality PET-MRI allows for acquisition of dynamic PET data over a time period similar to that required for MR data acquisition alone, thus maximizing the overlap of s.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Gadolinium Contrast agent (Gadovist)	Bayer	na	1.0mmol/ml solution for IV injection.
[18F]-NaF Radiotracer	na	na	2.96 MBq/kg
GE Signa PET-MRI Scanner	General Electric	na	3.0Tesla 60cm Bore PET-MRI scanner
PMOD Kinetic Modeling Software	PMOD Technologies, LLC	na	Version 3.8