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Lexington VA Medical Center University of Kentucky Medical Center Lexington, KY 40536-0293
I read with great interest the article by Taoka et al. [1] entitled "Factors Influencing Visualization of Vertebral Metastases on MR Imaging Versus Bone Scintigraphy." The authors compared the findings of MR imaging and planar bone scintigraphy in 74 patients with known widely disseminated metastatic disease. Their review showed that findings on bone scans were negative for all intramedullary lesions without cortical involvement shown on MR images; the findings on the bone scans were frequently positive for lesions with cortical involvement.
We had an autopsy-documented case of a patient with bronchogenic carcinoma with three vertebral metastases who had undergone CT, MR imaging, and planar and SPECT (single-photon emission computed tomography) bone scintigraphies. Findings on CT were negative. The planar and SPECT scintigraphy showed one lesion in the L1, whereas MR imaging revealed three lesions: one in the T12 was shown on bone studies; two smaller lesions in the T12 and L1 vertebrae were not seen on bone studies [2]. Thus we concur with the authors' [1] conclusion: Early vertebral metastases tend to be small and located in the intramedullary cavity (bone marrow) without cortical involvement, and, therefore, findings may be positive on MR images but negative on bone scan. Despite this, the authors should correlate SPECT bone scintigraphy with MR imaging, to compare topographic quality images of both diagnostic modalities. SPECT technology is widely available—often acquired by an institution before MR is accessible.
In a planar bone scintigraphy, the posterior projection, as in the four posterior views illustrated by Taoka et al. [1], is the best view for detection of the vertebral lesion. In anterior and lateral projections, the vertebral bone is too far away from the detector of a gamma camera because of the body structures. Because metastatic bone disease is predominantly located in the axial skeleton, hematogenous metastatic tumor cells deposit into the hematopoietic red marrow of the vertebral body, which is located in the anterior portion of the vertebra. Structures behind the vertebral body include the spinal cord, spinous process, facets, and skeletal muscles that would attenuate photon emission from the metastatic lesions from the vertebral body. Thus, even the posterior projection is not ideal and natural for the detection of lesions in the vertebral body by planar vertebral scintigraphy. Therefore, bone SPECT scintigraphy should be performed for vertebral lesions suggestive of disease, especially metastatic lesions.
A gamma camera with SPECT capability is available in almost every community hospital, and vertebral SPECT acquisition can be obtained in 20 to 30 min. The use of vertebral SPECT should be encouraged, because it provides sagittal, coronal, and transverse images topographically for the evaluation of vertebral lesions.
References
University of Oklahoma Health Sciences Center Oklahoma City, OK 73190
We greatly appreciate Dr. Shih's comments with regard to our article, "Factors Influencing Visualization of Vertebral Metastases on MR Imaging Versus Bone Scintigraphy" [1].
As clearly stated in our article, our results were derived purely from the planar bone scintigraphy because SPECT (single-photon emission computed tomography) was not available in all cases. However, it was not the purpose of the article to compare the efficacies of MR and nuclear medicine techniques in detecting bone metastases. We repeatedly stated in the article that nuclear medicine bone scanning remains the standard modality in the evaluation of metastasis to the skeletal system. Nuclear medicine bone scanning provides the most cost-effective way to evaluate the entire skeletal system. Our main purpose was to determine whether a fundamental difference was associated with a positive finding in MR and nuclear medicine bone scans. Specifically, MR imaging is sensitive to bone marrow changes, and therefore is superior in detecting metastasis, particularly in those bones with large bone marrow cavities, but inferior in the evaluation of bones that contain little bone marrow, including small peripheral bones and the ribs. We would therefore agree with Dr. Shih that SPECT should measure the concurrence among modalities in cases with cortical involvement, but additional studies are required to determine whether SPECT increases the detection rate in cases that show marrow involvement only.
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