Reply: Regarding Dynamic Bone Imaging with ^99mTc-Labeled Diphosphonates and ¹⁸F-NaF: Mechanisms and Applications

REPLY: We thank Dr. Freesmeyer for his interest in our review article on dynamic bone imaging using ^99mTc-labeled diphosphonates and ¹⁸F-NaF in which we postulated that it would technically be possible to perform early soft-tissue phase imaging with ¹⁸F-NaF PET (1), although this technique has not been described in the literature or in recent guidelines (2). Compared with ^99mTc-labeled diphosphonates, ¹⁸F-NaF provides more rapid blood clearance and higher bone-to-background uptake ratios. In combination with dynamic PET acquisition, ¹⁸F-NaF allows for quantitative kinetic modeling of bone blood flow and metabolism for various applications, including investigation of bone viability (3) or diffuse metabolic bone disease (4), although limited to the available field of view. The fast kinetic properties of ¹⁸F-NaF have led to concerns that obtaining a soft-tissue phase would not be feasible with ¹⁸F-NaF PET; instead, ¹⁸F-FDG PET or 3-phase ^99mTc-methyl diphosphonate bone scanning would be required under the assumption that the acquisition of tomographic PET data, even in 3-dimensional mode, may have insufficient temporal resolution to capture the rapid soft-tissue phase of ¹⁸F-NaF (5).

Therefore, we read with great interest the description of a novel technique of 2-phase whole-body ¹⁸F-NaF PET scanning. This technique is similar to performing early whole-body soft-tissue imaging with ^99mTc-labeled diphosphonate bone scanning using a sweep protocol as a screening tool for sites of joint inflammation. The proposed technique is analogous to prior published work on 2-phase or 3-phase ¹⁸F-FDG PET for chronic osteomyelitis (6). ¹⁸F-FDG PET for imaging of osteomyelitis has been found to have excellent sensitivity and specificity for bone infection, with possibly even higher accuracy than the current gold standard radionuclide technique of ^99mTc-hexamethylpropyleneamine oxime– or ¹¹¹In-labeled white blood cell scintigraphy (7,8). Whether the addition of an early phase could augment the ¹⁸F-FDG PET scan and further improve its diagnostic capability is an intriguing question. However, the kinetic behavior of ¹⁸F-FDG and ¹⁸F-NaF clearly differs, with the high net transport of ¹⁸F-NaF into bone expected to provide technical challenges.

Dr. Freesmeyer describes his preliminary experience with early combined angiographic/soft-tissue–phase ¹⁸F-NaF PET within 80 s of injection to acquire a whole-body scan. Using a modern scanner with an extended field of view, he reports that a typical soft-tissue distribution is clearly visually discernible with only slight skeletal uptake noted toward the end of the short acquisition. Similarly, ^99mTc-labeled diphosphonate bone scans often show skeletal uptake on the soft-tissue phase when imaging is delayed to obtain multiple projections. Under the condition that the PET scanner design allows for ultra-short whole-body acquisitions with acceptable image quality, we agree that such a protocol would provide evidence of active inflammation and help distinguish the etiology of observed increased ¹⁸F-NaF osseous uptake. We caution, however, that with the described image protocol, factors such as the injected radiotracer volume and concentration, the duration of radiotracer injection, cardiac output, and renal function are expected to have a significant influence on soft-tissue uptake and, therefore, may interfere with image interpretation.

• © 2013 by the Society of Nuclear Medicine and Molecular Imaging, Inc.