JNM: CT attenuation correction boosts SPECT MPI prognostic value

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SPECT myocardial perfusion imaging with technetium-99m (Tc99m) shows normal findings. Source: Daniel S. Berman, MD, director of cardiac imaging and nuclear cardiology at Cedars-Sinai Heart Institute

CT attenuation correction improved risk stratification among patients undergoing SPECT myocardial perfusion imaging (MPI), according to a study published in the February issue of the Journal of Nuclear Medicine.

Although SPECT MPI is widely employed in the evaluation of patients with coronary artery disease, studies are plagued by attenuation artifacts that can decrease specificity and reduce diagnostic accuracy, which, in turn, reduces the cost-effectiveness of SPECT MPI.

CT attenuation correction offers a possible method to improve image quality and diagnostic accuracy, and its availability is increasing with deployment of multi-detector CT systems, offered Aju P. Pazhenkottil, MD, from the cardiac imaging service at University Hospital Zurich in Switzerland, and colleagues. In addition, low-dose CT images can be used for coronary calcium scoring.

Pazhenkottil and colleagues undertook the study to assess the impact of CT attenuation correction on the prognostic value of SPECT MPI, enrolling 876 consecutive patients who underwent a 1-d adenosine stress-rest 99mTc—tetrofosmin SPECT MPI for known or suspected coronary artery disease between May 2005 and June 2008 at the cardiac imaging service at University Hospital Zurich.

The researchers derived a summed stress score (SSS) by adding percentage uptake scores according to a 0 to 4 scale for 20 segments and used established filtered back projection cutoff values: less than 4, normal;  4 to 8, mildly abnormal; 9 to 13, moderately abnormal; and more than 13, severely abnormal. Cutoff values had not been established for CT attenuation correction. Cardiac death and nonfatal myocardial infarction provided study endpoints.

During a mean follow-up of approximately 2.3 years, a total of 184 adverse events occurred in 145 patients, including 35 major cardiac adverse events (MACE), with 16 cardiac deaths and 19 nonfatal myocardial infarctions, reported the researchers.

“With attenuation correction, prognostically relevant summed stress score cutoff is shifted toward lower values,” continued the researchers. Specifically, they noted the sharpest increase from low to intermediate MACE rate between a summed stress score of 0 and 1 to 3 and determined optimal cutoff attenuation correction values of 0, 1 to 8 and more than 8 to describe low, intermediate and high risk for MACE and all adverse events. Patients with a CT attenuation correction score of zero did maintain a low event rate over four years, implying a warranty period of more than four years for this group, they added.

“The present results indicate that CT attenuation correction is helpful for risk stratification of SPECT-MPI, adding incremental prognostic value,” summed Pazhenkottil and colleagues. They continued, “The discrimination between true and false defects has been one of the major challenges in nuclear cardiology. Our results show that attenuation correction, especially with CT, successfully reduces the number of false-positive results.”

Pazhenkottil and colleagues suggested additional studies in larger populations to further determine the prognostic value of CT attenuation in different subgroups to assess the impact of sex and obesity.