Radiology -- eLetters for Strobel et al., 244 (2) 566-574

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Nuclear Medicine:
Klaus Strobel, Reinhard Dummer, Daniela B. Husarik, Marisol Pérez Lago, Thomas F. Hany, and Hans C. Steinert
High-Risk Melanoma: Accuracy of FDG PET/CT with Added CT Morphologic Information for Detection of Metastases
Radiology 2007; 244: 566-574 [Abstract] [Full text] [PDF]

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Electronic letters published:

Non-attenuation-corrected PET images for Detection of Lung Metastases: Guillaume Chuto, MD (5 June 2008)

Non-attenuation-corrected PET images for Detection of Lung Metastases 5 June 2008

Guillaume Chuto, MD,
MD
Nuclear Medicine, Institut Paoli-Calmettes, Marseille, France
Send letter to journal:
Re: Non-attenuation-corrected PET images for Detection of Lung Metastases

chutog{at}marseille.fnclcc.fr Guillaume Chuto, MD

Editor:
I read with great interest the article by Strobel et al that highlights dedicated analysis of coregistered computed tomography (CT) images to improve the accuracy of integrated positron emission tomography (PET)/CT for depiction of metastases in patients with high-risk melanoma (1). In seven out of 124 patients, a metastatic lesion was detected only by using coregistred CT images, including four pulmonary metastases (<1 cm) in four patients. It is known that because emission time is long, breath holding is not possible during PET acquisition and results in motion-related degradation of spatial resolution in PET lung images. However, respiratory motion is also responsible for false attenuation correction of PET images with CT data, especially near the diaphragm (with or without breath holding during CT acquisition) (2). Non-attenuation-corrected (NAC) PET images of the lung are more sensitive than attenuation-corrected (AC) PET images. Surprisingly, the authors did not use NAC PET images for lung analysis in their study. It is possible that some of the four pulmonary lesions could have been detected with NAC PET images before analysis of coregistred CT images.
In our institution, we sequentially analyze NAC PET images of the lung (transaxial and coronal views), AC PET images alone (maximum intensity projection and transaxial), AC PET with coregistred CT (transaxial essentially), and finally coregistred CT alone (no intravenous contrast agent, no breath holding), with reference to AC and NAC PET if needed. The average time of interpretation is less than 20 minutes (range 10–30 minutes).
References
1. Strobel K, Dummer R, Husarik DB, Pirez Lago M, Hany TF, Steinert HC. High-risk melanoma: accuracy of FDG PET/CT with added CT morphologic information for detection of metastases. Radiology 2007;244(2):566-574.
2. Bacharach SL. PET/CT attenuation correction: breathing lessons. J Nucl Med 2007;48(5):677-679.
Dr Strobel and Colleagues Respond:
We read with great interest the comments of Dr Chuto regarding our article. We reviewed the nonattenuated images from our critical patients and found no lung metastases that were positive on the nonattenuated images but negative on the attenuated images. We are curious to know if the hypothesis advanced by Dr Chuto is supported by original literature on lung lesions. On the basis of our experience with thousands of melanoma patients in the last several years, we believe that reviewing CT images with appropriate "lung windows" is essential for finding small lung metastases. The dedicated interpretation of images with lung window settings is much more helpful than reviewing the nonattenuated PET images. We regularly use the nonattenuated images for the evaluation of metallic implants, because we know that metal alters the attenuation correction severely (1). An interesting approach to improving detection of lung metastases is respiratory gating, and the first studies reporting its use have been promising (2,3). Further work is needed to determine if respiratory gating will increase the accuracy of detecting small lung lesions and to evaluate if this time-consuming technique can be included in the daily routine of a busy department.
References
1. Goerres GW, Ziegler SI, Burger C, Berthold T, Von Schulthess GK, Buck A. Artifacts at PET and PET/CT caused by metallic hip prosthetic material. Radiology 2003;226:577-584.
2. Nehmeh SA, Erdi YE, Ling CC, et al. Effect of respiratory gating on quantifying PET images of lung cancer. J Nucl Med 2002; 43:876-881.
3. Nehmeh SA, Erdi YE, Ling CC, et al. Effect of respiratory gating on reducing lung motion artifacts in PET imaging of lung cancer. Med Phys 2002;29:366-371.