Hybrid SPECT/CT image reveals coronary lesions

David Shenkenberg

Five years ago, cardiac CT scanners had only four detectors. Today that number has jumped to 64. Cardiologists also can fuse CT with single-photon emission computed tomography (SPECT) images. Dr. Oliver Gämperli of University Hospital Zurich in Switzerland believes that that is why the fused SPECT/CT image generated by him and fellow lead researcher, Dr. Philipp A. Kaufmann, became the Society of Nuclear Medicine’s 2006 Image of the Year.

It is part of a study presented at the Society of Nuclear Medicine’s 53rd annual meeting in June. In the study, the researchers used CT and SPECT to detect coronary artery disease, which can lead to heart attack or stroke. Coronary angiography is typically used to detect the disease, but it is invasive and results in a 1 to 2 percent longer stay in the hospital and a 0.1 to 0.2 percent greater chance of mortality than noninvasive procedures such as SPECT/CT. Gämperli said that his research has shown that SPECT/CT can be used to diagnose coronary artery disease, reducing the need for coronary angiography.

This cardiac image is the Society of Nuclear Medicine’s 2006 Image of the Year. The arrows indicate ischemia. © University Hospital Zurich.

SPECT/CT is challenging to perform because of the motion of the heart. Therefore, cardiology has lagged behind other medical disciplines that already combine CT and SPECT imaging.

Both CT and SPECT produce images of a similar resolution and expose the patient to a similar amount of radiation. However, CT displays anatomical features, whereas SPECT shows blood flow. Fusion produces a complete image, as the Image of the Year shows. This allows doctors to detect coronary stenosis and ischemia, both of which are indicators of coronary artery disease.

For SPECT imaging, doctors give patients radioactive tracers that can attenuate as they travel through the heart muscle. By the time the signal travels to the inferior wall, it is much weaker. This problem is increased in obese patients because the tracer must travel through fatty tissue. On the other hand, CT has difficulty evaluating calcified arteries because they produce a strong, opaque signal that is known as the “blooming effect,” obscuring the surrounding coronary tissue.

The Image of the Year comprises six parts. The four on the left are SPECT images. The upper two of these were taken with the heart under stress, and the lower two were taken with the heart at rest. The two images on the right fuse SPECT and CT.

The researchers used a CT scanner and fusion software, both from GE Healthcare in Chalfont St. Giles, UK. They concluded that the combined SPECT/CT is useful for determining whether patients of moderate or low risk require intervention. Further studies must be done to confirm analyses for high- and low-risk patients, Kaufmann said.