Myocardial Perfusion Imaging with Combined Single-photon Emission Computed Tomography and Multislice Computed Tomography

Myocardial Perfusion Imaging with Combined Single-photon Emission Computed Tomography and Multislice Computed Tomography

Kuikka Jyrki T
European Cardiovascular Disease 2007
Published: June 2007
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Myocardial perfusion imaging (MPI) using combined computed tomography (CT) and single-photon emission CT (SPECT) systems plays an important role in the management of patients with coronary artery disease (CAD).1–4 The method can be used to assess myocardial perfusion and left ventricular function simultaneously. It is an especially valuable tool for assessing short-term risk of CAD, thus effectively guiding decision-making regarding revascularisation.3,4

The system usually consists of a dual-headed, large-detector gamma camera united with a multislice diagnostic CT, and is designed for sequential SPECT and CT imaging. MPI with CT-based attenuation correction (AC) shows a consistent improvement in image quality and in apparent diagnostic accuracy for the identification of CAD compared with non-attenuation corrected (NAC) MPI. However, MPI is susceptible to several complex methodological, biochemical and physiological factors that expose it to several potential artefacts and pitfalls, potentially limiting its diagnostic value (see Figure 1).

CT can be used for enhanced object localisation and tissue AC in SPECT; in addition, by itself it provides assessment of the prognostic significance of coronary calcium score (CCS) and coronary CT angiography (CTA), at least with a 64-slice CT. As an atherosclerosis imaging method, CCS is likely to provide greater long-term risk assessment, and is therefore more useful in determining the need for aggressive medical prevention measures.3 The key advantage of CTA over MPI is that the results are highly unlikely to be normal in patients in whom revascularisation would be warranted; this is in contrast to MPI, where a balanced reduction in perfusion can occasionally result in a normal finding (three-vessel disease) despite the presence of severe and extensive CAD.

This article will briefly review the benefits, artefacts and pitfalls of combined SPECT and multislice CT that may compromise the performance and interpretation of MPI.

Physical Performance of Single-photon Emission Computed Tomography and Computed Tomography
Single-photon Emission Computed Tomography
SPECT images suffer from noise due to low count statistics and poor spatial resolution. New detector technologies and iterative reconstruction packages have been developed to improve SPECT resolution down to 7mm and to decrease noise by simultaneously correcting for attenuation, scatter and collimator response.5,6 These corrections almost halve the acquisition time, and gated MPI examinations can be performed in 15 minutes while maintaining the same signal-to-noise statistics and image quality that previously would have required twice the duration.

Computed Tomography
Combined systems are usually available in six-, 16- or 64-slice CT. A 64-slice system allows the capture of the whole heart in about a six-second breath-hold, while the 16-slice system requires a breath-hold of about 15 seconds in CTA studies. The typical tube voltage is 120–140kV and the tube current ranges from 250 to 700mAm, resulting in a spatial resolution of 0.4mm with relatively low noise of 0.3%. Several excellent reviews provide a better understanding of CCS and CTA.1,8-10

Single-photon Emission Computed Tomography and Computed Tomography Co-registration
CT-based AC is now a recommended technique for improving image quality.7 The attenuation map required for AC-MPI can be acquired using a low-dose CT mode (tube current of 20–30mA).The coregistration of SPECT and the attenuation map need to be verified for every patient, even when a semi-automatic method for detection and correction of SPECT-CT emission–transmission misalignment is used. The misalignment in any direction has to be less than ±3mm.

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