Schindler, Thomas

Valenta, Ines

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EDITORIAL COMMENTARY82Rb PET/CT: entering a new area of myocardial perfusionimaging?Ines Valenta & Thomas Hellmut SchindlerPublished online: 1 June 2012# Springer-Verlag 2012In this issue of the European Journal of Nuclear Medicineand Molecular Imaging, Flotats et al. [1] report a strikingimprovement in image quality of myocardial perfusionstudies with the use of 82Rb PET/CT as compared to con-ventional 99mTc-labelled SPECT/CT. When using PET/CTin concert with the positron-emitting myocardial perfusiontracer 82Rb, the improvement in image quality also mani-fested in a higher interpretative confidence and interreaderagreement of 82Rb perfusion images than observed for99mTc-labelled SPECT/CT. In particular, the current studyis unique in that 82Rb PET/CT and 99mTc-labelled SPECT/CT were performed in the same patient with known orsuspected coronary artery disease (CAD). Previous inves-tigations commonly compared the sensitivity and specificityof cardiac PET versus SPECT perfusion imaging in thedetection of flow-limiting epicardial lesions [2]. For exam-ple, Go et al. [3] showed sensitivity, specificity and accuracyof 95, 82 and 92 % for 82Rb PET and 79, 76 and 78 % for201Tl SPECT for haemodynamically obstructive CADlesions. Further, Stewart et al. [4] reported that overallsensitivity, specificity and accuracy of 82Rb PET for detec-tion of coronary artery lesions ≥50 % diameter stenosis were84, 88 and 85 %, respectively. In comparison to this, theperformance of 201Tl SPECT revealed a sensitivity of 84 %but a low specificity of 53 % and a diagnostic accuracy of79 %. The commonly higher sensitivity in the identificationof flow-limiting epicardial lesions with 82Rb PET ascompared with SPECT imaging, either with 201Tl or99mTc-labelled perfusion tracers, can be related to the higherspatial and contrast resolution of 82Rb PET, while the pho-ton attenuation–free images of PET imaging may accountfor the relative increase in specificity [2].The new investigation conducted by Flotats et al. [1] nowadds further important information by addressing imagequality of myocardial perfusion studies, and the resultingreader confidence, and interreader agreement when applying82Rb PET/CT in direct comparison to 99mTc-labelledSPECT/CT. As it was observed, image quality was estimatedto be excellent or good in 63 % of PET, 33 % of attenuation-corrected (AC) SPECT and 22 % of non-AC SPECTimages. Further, artefact-free images were more frequentlyobserved with PET than with SPECT studies (PET 81 %,AC SPECT 22 % and non-AC SPECT 15 %). In thisdirection, significant liver or bowel uptake (or both) as wellas attenuation artefacts can be assumed to have substantiallyaffected the interpretation of the SPECT/CT images render-ing the interpretative decision-making process more difficultor observer dependent for SPECT/CT than for PET/CTimages. Not surprisingly, the reader certitude to gradestress-rest perfusion imaging into a normal or abnormalfinding was highest for PET (85 %), followed by ACSPECT (30 %) and non-AC SPECT (7 %). This is alsoreflected by the interreader agreement which was superiorfor PET/CT than for those of SPECT/CT perfusion studies.Several methodological aspects may account for theobservations of Flotats et al. [1]. PET cameras identifypaired photons (511 keV of energy each) produced by thepositron annihilation effect. The paired 511 keV travel in theopposite direction at a 180° angle from each other [2]. Thisenables a precise localization of the positron decay applyingthe principle of coincidence detection and without collima-tion as used for SPECT [2, 5]. Without the use of collima-tors, PET cameras have a much higher sensitivity than doI. Valenta : T. H. Schindler (*)Department of Specialties in Medicine, Division of Cardiology,Nuclear Cardiology and Cardiac PET/CT,University Hospitals of Geneva,6th Floor, Rue Gabrielle-Perret-Gentil 4,1211 Geneva, Switzerlande-mail: thomas.schindler@hcuge.chEur J Nucl Med Mol Imaging (2012) 39:1231–1232DOI 10.1007/s00259-012-2154-4SPECT cameras, resulting in a higher spatial resolution inthe range of 4–7 mm. Another aspect is the relatively higherextraction fraction of 82Rb at higher flows than those of99mTc-labelled perfusion tracers [6], which yields an in-crease in contrast resolution. Both the higher spatial andcontrast resolution of 82Rb PET perfusion studies explainthe reported higher sensitivity in the detection of CADlesions as compared with 201Tl or 99mTc-labelled perfusiontracers [2, 6]. Conversely, the robust attenuation correctionof the PET emission data using the transmission source(68Ge rotating rod source or CT) accounts for the reportedincrease in specificity of PET perfusion imaging whencompared to conventional SPECT imaging [2, 7].Another important aspect is that the gamma energy levelsof PET radiotracers with 511 keV are considerably higherthan those for 99mTc-labelled perfusion tracers (140 keV)and 201Tl (70-90 keV) applied in cardiac SPECT imaging.This again renders PET imaging less prone to attenuationartefacts. Conceptually, although the two 511-keV photonsproduced by positron annihilations are less attenuated than asingle 140-keV 99mTc photon, there may be in fact moreattenuation with PET than with SPECT imaging as twophotons must be identified for coincidence detection.According to the principle of coincidence detection, how-ever, the two photons travelling in opposite directions proj-ect along the same ray and they must pass through the sametotal amount of tissue independent of the localization wherethey were emitted [8]. The total measured attenuation inPET imaging therefore can be assumed to be similar any-where along this projection line. This is in contrast toSPECT imaging, where there is no opposing photon in orderto aid in localizing an annihilation event. Because of this,attenuation of photons in SPECT imaging can vary expo-nentially. PET emission data therefore can be preciselycorrected for attenuation by simply multiplying each projec-tion line with the appropriate AC factors determined from atransmission 68Ge line source or CT as compared to morecomplicated ones applied for SPECT [8]. The higher gammaenergy levels of PET radiotracers with 511 keV and verysolid and precise attenuation correction of PET emissiondata do not lead only to an increase in specificity but alsoto an increase in interpretative confidence of PET perfusionimages as observed in the current study [1]. The technolog-ical advantages of PET over SPECT cameras, therefore,have led to a relatively higher sensitivity and specificityof myocardial perfusion PET or PET/CT scanners fordetecting ≥50 or ≥70 % luminal epicardial narrowingaround 92 and 90 %, respectively [2]. On the other hand,we need to bear in mind that reported PET or PET/CTperfusion studies were mostly performed in highly selectedstudy populations, which may have biased, at least in part,the relatively high sensitivity and specificity of PET perfu-sion studies in the detection of CAD lesions. The highspatial and contrast resolution in concert with photonattenuation–free images of PET, however, provide a higherimage quality associated with a higher sensitivity andspecificity of PET/CT perfusion imaging than for conven-tional SPECT or SPECT/CT. This again may explain therelative ease in the interpretation of PET/CT stress-restperfusion images for the detection of flow-limiting epicar-dial lesions [1].How current observations of Flotats et al. [1] with 82RbPET/CT myocardial perfusion imaging compare with itera-tive image reconstruction or the implementation of newalgorithms, so-called proprietary resolution recovery/noisereduction (RRNR) of SPECT/CT systems, or new ultrafastSPECT camera designs [8, 9], remain uncertain, which willcertainly stimulate further comparative investigations andadvancements in the field of scintigraphic myocardialperfusion imaging [10].References1. Flotats A, Bravo PE, Fukushima K, Chaudry M, Merrill J, BengelFM. 82Rb-PET myocardial perfusion imaging is superior to99mTc-labelled agent SPECT in patients with known or suspectedcoronary artery disease. Eur J Nucl Med Mol Imaging 2012.doi:10.1007/s00259-012-2140-x.2. Schindler TH, Schelbert HR, Quercioli A, Dilsizian V. CardiacPET imaging for the detection and monitoring of coronary arterydisease and microvascular health. JACC Cardiovasc Imaging2010;3(6):623–40.3. Go RT, Marwick TH, MacIntyre WJ, Saha GB, Neumann DR,Underwood DA, et al. A prospective comparison of rubidium-82PET and thallium-201 SPECT myocardial perfusion imaging uti-lizing a single dipyridamole stress in the diagnosis of coronaryartery disease. J Nucl Med 1990;31(12):1899–905.4. Stewart RE, Schwaiger M, Molina E, Popma J, Gacioch GM,Kalus M, et al. Comparison of rubidium-82 positron emissiontomography and thallium-201 SPECT imaging for detection ofcoronary artery disease. Am J Cardiol 1991;67(16):1303–10.5. Garcia EV, Faber TL. Advances in nuclear cardiology instrumen-tation: clinical potential of SPECT and PET. Curr CardiovascImaging Rep 2009;2:230–7.6. Bengel FM, Higuchi T, Javadi MS, Lautamäki R. Cardiac positronemission tomography. J Am Coll Cardiol 2009;54(1):1–15.7. Ghosh N, Rimoldi OE, Beanlands RS, Camici PG. Assessment ofmyocardial ischaemia and viability: role of positron emissiontomography. Eur Heart J 2010;31(24):2984–95.8. Garcia EV. Physical attributes, limitations, and future potential forPET and SPECT. J Nucl Cardiol 2012;19(Suppl 1):S19–29.9. Fiechter M, Ghadri JR, Wolfrum M, Kuest SM, Pazhenkottil AP,Nkoulou RN, et al. Downstream resource utilization followinghybrid cardiac imaging with an integrated cadmium-zinc-telluride/64-slice CT device. Eur J Nucl MedMol Imaging 2012;39(3):430–6.10. Kaufmann PA. 82-Rubidium—the dawn of cardiac PET inEurope? Eur J Nucl Med Mol Imaging 2007;34(12):1963–4.1232 Eur J Nucl Med Mol Imaging (2012) 39:1231–1232

82Rb PET/CT: entering a new area of myocardial perfusion imaging?

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82Rb PET/CT: entering a new area of myocardial perfusion imaging?

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