Hybrid imaging makes headway in cardiac and oncologic imaging, but caveats persist

The combined functional and morphological approach to imaging afforded by PET/CT and SPECT/CT has far-reaching technical, diagnostic, and economic advantages, according to Dr. Gerald Antoch of the department of diagnostic and interventional radiology and neuroradiology at the University Hospital Essen in Germany. He moderated Monday's state-of-the art symposium on the use of PET/CT and SPECT/CT for cardiac and oncologic purposes.

Antoch noted that PET imaging no longer requires transmission sources when attenuation correction can be based on previously acquired CT images. In addition to better PET image quality, the PET examination time can be reduced by 30% to 40%. Patient throughput increases, and examination costs decrease.

From a diagnostic point of view, the combination proves invaluable. Supplementing morphological images with functional data vastly increases the diagnostic yield. By the same token, because a clear anatomic correlation is possible, functional PET data become far more valuable.

The advantages of PET, and now PET/CT, have made it the standard in many institutions for staging and response assessment in different types of lymphoma, according to Dr. Sally F. Barrington, head of the PET Imaging Centre at St. Thomas' Hospital in London.

Accurate staging and response assessment are essential in lymphoma to avoid toxic treatment in patients with good prognosis yet enable dose escalation for poor prognosis patients to increase cure rates. FDG-PET has the ability to detect metabolically active lymphoma independent of nodal size and has better sensitivity for extranodal disease compared with CT. PET staging alters management in up to 30% of adult and pediatric patients, she said.

Data are accumulating to confirm PET/CT's efficacy in assessing early response to treatment. PET performed after two or three cycles of induction chemotherapy is a strong predictor of disease-free and overall survival, independent of clinical stage and other pretreatment prognostic indicators in Hodgkin's disease and non-Hodgkin's lymphoma. PET also has a high negative predictive value in the assessment of residual masses in Hodgkin's disease at completion of chemotherapy and reduces the number of patients with complete response unconfirmed, Barrington said.

Limitations in using PET imaging for lymphoma include false-negative results in some low-grade lymphomas and minimal residual disease and false-positive uptake in infection and inflammation.

Dr. Conor Collins and colleagues at St. Vincent's University Hospital in Dublin cited the four most common cancers imaged with PET/CT:

• lymphoma: staging, treatment evaluation, and assessment of recurrence
• non-small cell lung cancer: preoperative staging and assessment of recurrence
• colorectal cancer: preoperative staging in patients with hepatic metastases and assessment of possible recurrence
• esophageal cancer: preoperative staging, evaluation of treatment response, and assessment of recurrence

Utilization of PET/CT is also increasing in patients with melanoma and cancers of extracranial head and neck, breast, testes, and cervix, Collins said.

He noted that important underlying questions remain about PET/CT, despite its accuracy. What is its impact on patient management? Is it cost-effective? Are there other less expensive imaging alternatives that would provide the same information?

One area where FDG-PET has no role is prostate cancer, he said, though other radiotracers such as fluorine-18 choline and F-18 ethylcholine show promise in the prostate. Research is ongoing into new tracers for specific organs or cellular signatures. These include F-18 ethyltyrosine for brain tumors, F-18 thymidine for cellular proliferation, F-18 DOPA for endocrine tumors, and copper-60 ATSM for cellular hypoxia.

Dr. Philipp A. Kaufmann, director of nuclear cardiology at University Hospital Zurich, discussed PET/CT and SPECT/CT in cardiac imaging. Kaufmann was the senior investigator in a study that produced the Society of Nuclear Medicine's 2006 Image of the Year. The image illustrated perfusion SPECT/CT's importance for assessing low-risk patients with suspected myocardial infarction. The addition of SPECT to CT angiography data increases specificity, mainly the ability to identify the vessel responsible for causing ischemia, he said.

SPECT/CT and PET/CT certainly present the opportunity for a "one-stop shopping approach" for cardiac imaging, Antoch said. But he added that these hybrid imaging modalities are not the last word. While future developments may make cardiac MRI a strong competitor when imaging morphology and function in a single session, research into PET/MR hybrid scanners could round out the playing field.