Can PET be performed without an attenuation scan?

Journal of Nuclear Cardiology, Sep 2015

Colin Jones BSc, Ran Klein PhD

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Can PET be performed without an attenuation scan?

Received Aug Can PET be performed without an attenuation scan? Colin Jones 0 2 Ran Klein 0 0 Reprint requests: Ran Klein, Department of Nuclear Medicine, The Ottawa Hospital , Ottawa, ON , Canada 1 Department of Nuclear Medicine, The Ottawa Hospital , Ottawa, ON , Canada 2 Department of Systems and Computer Engineering, Carleton University , Ottawa, ON , Canada Positron emission tomography (PET) has emerged as the king of non-invasive molecular imaging, largely due to its ability to quantify tracer concentrations in units of Bq/mL. This ability is heavily reliant on accurate correction of coincidence photon attenuation. In cardiac imaging applications, including myocardial perfusion imaging (MPI), accurate and robust attenuation correction (AC) has been coveted as a means to distinguish between uptake deficits and attenuation artifacts. The notion that PET is able to deliver near perfect AC, along with other advantages, has greatly contributed to the growth of cardiac PET in recent years. From its early days, AC was an integral part of PET. Early instrumentation used weak, rotating transmission sources (e.g., 68Ge/68Ga or 137Cs) to measure attenuation along each line of response. The AC factors were then applied to the measured coincidence data to compensate for attenuation prior to image reconstruction. These transmission scans were lengthy and produced noisy data that propagated into the reconstructed image. With the introduction of hybrid PET-CT, x-ray computed tomography (CT) data could be acquired in a few seconds and manipulated to produce nearly noise-free AC estimates,1 and replaced traditional transmission scans as the de facto AC method in clinical PET. Recently, PET-MR hybrid systems have become available and rely on processing of magnetic resonance images (MR) to derive AC data. MR-based AC is a complex problem requiring further research and development,2 but has been - demonstrated to produce equivalent clinical finding compared to PET-CT.3,4 While the nuclear imaging community welcomed these hybrid technologies due to the shorter scan times and the ability to co-register anatomical and functional information, it quickly became clear that certain challenges lay ahead with regards to AC. Both CT and MR approaches suffer from misalignment problems due to patient motion between acquisition on attenuation and emission data. Rigidbody motion (e.g., of the head) can be effectively corrected using manual or automated registration methods.5 However, compensation for non-rigid patient motion is much more challenging and remains the topic of much research. In the context of nuclear cardiology, respiratory and cardiac motions further compound the challenge. Both CT and MR data are captured split second, resulting in mid-breath and mid-cardiac cycle images that do not fully correspond to PET data that is acquired over many breathing and cardiac cycles, even when emission data are gated (respiratory or cardiac). Breath-hold regiments during CT or MR data acquisition and cardiac triggering are common strategies to minimize motion artifacts within the attenuation image. Nevertheless cardiac imaging always requires verification and often correction to ensure optimal AC registration. Registration is performed with regards to the heart, often resulting in misregistration of other structures in the image (e.g., liver, lung, skeletal muscle and bone). Respiratory or cardiac motion related AC misregistration can be partially accounted for using 4D-CT or -MR techniques; however, 4D techniques typically assume regular periodic motion such as uniform breath cycles and cardiac motion.6 Other, more practical limitations of these technologies have also been acknowledged. CT-AC can be a significant source of radiation to the patient compared to the tracer dose (e.g., *0.5 mSv CT-AC dose vs *1 mSv from a 82Rb scan7) especially if 4D-CT-AC is used to account for respiratory motion.16 PET-MR systems are currently prohibitively expensive, can increase the overall study length, and have significant counter indications (e.g., non-MR compatible implanted devices and claustrophobia). ESTIMATING ATTENUATION FROM THE EMISSION DATA Another approach to correct for attenuation, which was proposed over three decades ago,8 is to determine the attenuation information from the emission scan directly. Nuyts et al. explored this approach in 1999 using two algorithms: maximum likelihood (ML), and maximum a posteriori (MAP) iterative reconstructions using decreasing ordered subsets (OS). The results of both algorithms showed promise in comparison to no AC or standard AC, with MAP outperforming ML.9 Despite promising results, simultaneous estimation of attenuation and activity suffers from cross-talk, propagating errors between both images.10 Time of flight (TOF) information can be used to help localize updates during iterative reconstructions and reduce the cross-talk problem. With increasing prevalence of TOF capabilities in moder (...truncated)


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Colin Jones BSc, Ran Klein PhD. Can PET be performed without an attenuation scan?, Journal of Nuclear Cardiology, 2016, pp. 1098-1101, Volume 23, Issue 5, DOI: 10.1007/s12350-015-0266-5