Author's personal copy Advances in Attenuation Correction Techniques in PET Habib Zaidi, PhD, PD a, *, Marie-Louise Montandon, PhD a , Abass Alavi, MD b In positron emission tomography (PET), the imaging system records two antiparallel 511 keV photons that are emitted after electron–positron annihilation. In this case, the annihilation photons traverse a total tissue thickness that is equal to the body thickness intersected by the line between the two detectors, also called the line of response (LoR). Since its inception, photon attenuation in biological tissues has been identified as the most important physical degrading factor affecting PET image quality and quantitative accuracy [1]. Photon attenuation usually refers to the combination of photoelectric absorption and Compton scattering. That is, both are components of the general process of photon attenuation [2]. A particularity of PET (in contrast to single- photon emission computed tomography - SPECT) is that the attenuation factor for a given LoR depends on the total distance traveled by both annihilation photons within the attenuating medium, and it is independent of the emission (EM) point along this LoR. This makes attenuation correction (AC) easier, owing to the fact that it only requires a simple premultiplication of the EM data by the resultant POSITRON EMISSION TOMOGRAPHY PET Clin 2 (2007) 191–217 This work was supported by Grant Number SNSF 3100A0-116547 from the Swiss National Foundation. a Division of Nuclear Medicine, Geneva University Hospital, CH-1211 Geneva 4, Switzerland b Division of Nuclear Medicine, Department of Radiology, Hospital of the University of Pennsylvania, 3400 Spruce Street, Philadelphia, PA 19104, USA * Corresponding author. E-mail address: habib.zaidi@hcuge.ch (H. Zaidi). - What is photon attenuation? - Practical and clinical consequences of photon attenuation - Methods for determination of the attenuation map Radionuclide-based transmission scanning X-ray CT-based transmission scanning MRI-guided derivation of the attenuation map Comparison of methods - Attenuation correction strategies in positron emission tomography - Attenuation correction in small animal positron emission tomography - Pitfalls of CT-based attenuation correction in positron emission tomography and potential solutions Artifacts resulting from polychromaticity of x-ray photons and beam hardening Artifacts resulting from misregistration between emission and transmission data Artifacts arising from the use of contrast-enhanced CT Truncation artifacts Artifacts arising from metallic objects Artifacts arising from x-ray scatter in CT images - Summary - References 191 1556-8598/07/$ – see front matter ª 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.cpet.2007.12.002 pet.theclinics.com