Is PET-CT the only option?

Abstract

Accurate determination of the anatomic location of normal and pathological sites visualized on PET undoubtedly facilitates optimal interpretation of PET studies. It is now well established that image fusion with structural studies improves the diagnostic accuracy of images generated by PET [1]. PET images can be fused with CT and/or MRI either by using co-registration algorithms for PET and CT scans which have been acquired separately or by using combined PET-CT instruments. However, combined PETCT scanners have certain advantages. Sequential acquisition of PET and CT images (one immediately after the other) minimizes patient and organ motion artifacts since the patient is positioned almost identically for both scans [2]. Furthermore, the CT data can be used for attenuation correction (AC) of the PET emission scan [3, 4]. Traditionally, an external radionuclide source has been employed for this purpose, but the use of CT for AC provides several advantages: First, the transmission scanning time is much shorter for CT than for an external radionuclide source (subminute versus 20–30 min) [4]. Second, transmission scans are acquired by using external radionuclide sources with 511 keV or higher energies (Ge, Cs) which provide images with suboptimal quality for structural assessment of anatomic sites; in contrast, CT images offer high spatial resolution and low noise, facilitating structural assessment of abnormalities visualized [3, 4]. Third, by using X-rays (CT) the cost of replacing external radioactive sources is eliminated for the life of the machine [3]. In addition, combined PET-CT can help to distinguish true lesions from physiologic uptake, particularly in patients whose anatomy is distorted by postoperative or radiation therapy. Many recent studies highlight the importance of the combined units when adopted in the appropriate clinical settings [5–8]. However, PET-CT units as the “only option” may not be realistic for multiple reasons. In a major fraction of patients who undergo PET-CT scanning a diagnostic CT is performed for further assessment of the disease sites, entailing the performance of two CT scans; the radiation dose may consequently reach high levels, especially in the pediatric population. In addition, misalignment between PET and CT images due to voluntary or physiological motion of the lungs, heart, bowel, and other organs may cause artifacts in the fused image [9–12]. CT scans are acquired during breath holding while PET images are taken over 3–4 min, with averaging of data from different fields of view during this extended period. Thus, respiratory artifacts caused by different breathing patterns during the acquisitions of PET and CT data can be significant [9, 10, 13, 14]. Curvilinear cold artifacts paralleling the dome of the diaphragm at the lung/diaphragm interface have been noted on 84% of PET-CT image acquisitions but are not seen on germanium transmission source-corrected images [9]. Another technical limitation of PET-CT scans is the necessity of applying compensatory algorithms to the CT scan data to produce an equivalent degree of attenuation between CT attenuation-corrected PET/CT scans and radionuclide attenuation-corrected PET scans [15]. Therefore, precise quantitative measurement of radiolabeled Eur J Nucl Med Mol Imaging (2007) 34:819–821 DOI 10.1007/s00259-006-0340-y