With the widespread introduction of PET/CT scanners, NaF has reemerged as an ideal agent for bone imaging in combination with advanced software programs

Since the early 1970s radionuclide bone scanning has been a mainstay in the oncologic detection of osseous metastatic disease. Traditionally the bone scan was performed with planar imaging using a radiopharmaceutical such as technetium 99m MDP (methylene diphosphonate.)

After injection into the patient, the radiopharmaceutical incorporates into metabolically active bone, usually at the site of osteoclastic activity. The isotope decays with time, emitting a gamma ray (an x-ray from the nucleus of the atom), which is captured by a detector and displayed on film or on a computer screen. The images are evaluated for “hot spots,” or patterns of abnormal bone metabolism likely to indicate malignancy.

The sensitivity and specificity of planar bone scans varies depending upon tumor type and stage of disease, but generally the sensitivity is between 62 percent and 100 percent and the specificity between 78 percent and 100 percent. The difficulty in interpreting planar bone scans is that many benign entities can cause increased bone metabolism and show hot spots on a bone scan such at degenerative, post traumatic or postoperative change.

With the addition of newer nuclear medicine techniques such as SPECT (single photon emission computed tomography), the sensitivity and specificity for the detection of metastasis does increase over that of planar imaging, but imaging time also dramatically increases and this extra imaging time is often not attainable in ill patients or those who are in pain.

More recently a new pharmaceutical 18 sodium fluoride (18NaF) has come into day-to-day use. This agent is a positron emitting isotope first used in the early 1960s but abandoned because its physical properties (positron emitter) were not suitable for the existing technology of the era, which involved the use of rectilinear and gamma cameras.

With the advent of commercially available Positron Emitting Tomography (PET) scanners in the 1990s, and especially since the widespread introduction of PET/CT scanners, NaF has reemerged as an ideal agent for bone imaging because now the isotope and scanner are optimally matched such that a positron-emitting isotope is captured in a positron scanner. Various software programs are applied to determine the exact site of the radioactive event (spatial localization) and with the addition of a coregisterd low dose CT, the morphology of a lesion can be easily and accurately characterized.

From the patient’s perspective a PET bone scan is more easily tolerated than a planar scan. The PET isotope has a higher energy flux than the planar bone scan agent and imaging time for a PET bone scan is usually on the order of 15-20 minutes versus 45-60 minutes with a planar scan or even longer if SPECT is performed as an adjunct to the planar scan. Because a low dose CT scan accompanies the PET bone scan as a routine part of the examination, the patient usually does not need additional imaging such as plain x rays or MRI to differentiate benign from malignant bone processes.

From the nuclear physician’s perspective, the PET bone scan has a greater sensitivity and specificity than other nuclear medicine techniques. NaF has twice the photon flux as other bone agents (twice the number of radioactive disintegrations per unit time) and for any given lesion is more conspicuous and easier to see. The physical properties of the PET agent such as a shorter half-life and more rapid urinary clearance also contribute to the conspicuity of lesions at PET imaging.

From the oncologist’s perspective a PET bone scan offers increased sensitivity of about 90 percent and increased specificity of 97 percent when compared with planar imaging. The increase is due to the nature of the isotope and scanner (which allow for coincidence detection) and also the presence of a coregistered low dose CT scan allows for accurate anatomic characterization of metabolically active lesions (the hot spots.)

For example PET/CT bone scan can with precision locate a lesion in the bone as intramedullary or not and characterize that lesion as osteolytic or osteosclerotic. Furthermore with the CT images, one can differentiate between benign processes such as degenerative change, subchondral cysts or Paget’s disease versus metastatic disen below. The first example demonstrates abnormal uptake in the left lateral aspect of the L2 vertebral body...on the sagittal reconstructed image from the co registered CT the metabolically active lesion is noted to simply represent degenerative change.

In the second example, a metastatic rib lesion is noted on the CT and corresponds with a hot rib lesion.

Dr. Robbins is a radiologist with Valley Radiologists Inc. in San Jose.

—By Dr. Barry Robbins

Posted on April 23, 2007 11:23 AM