Joining Forces: PET-CT Adds Essential Information

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The boldface headline on molecular imaging proclaims dramatic market shifts in the types of PET scanners being delivered, according to industry leaders. More than 95 percent of systems sold today are PET-CT hybrid scanners that combine to provide more applications and more imaging information than is possible separately.

Most positron emission tomography exams today are performed for clinical applications in oncology. However, when a high-speed multislice CT module is incorporated, the combined scanner offers opportunities for comprehensive cardiac studies that can reveal the extent of an ischemic area as well as the specific problem within the coronary vasculature that produced the presenting event.

The CT component enables whole-body attenuation correction and exquisite anatomic localization of lesions that yields impressive enhancements to accuracy in a number of applications. For example, when clinicians interpret studies obtained through an individual modality scan, the sensitivity and specificity may reside in the 60 percent and 70 percent range.  But when studies are completed in near simultaneous succession using a PET-CT, the accuracy rate rises to about 90 percent, according to Philips Medical Systems. Clinicians in the field corroborate this assessment. This improved precision affords a level of confidence in staging cancerous conditions, evaluating effectiveness of treatment and guiding biopsies of tumors and in the case of cardiac applications may decrease the necessity for further testing or invasive procedures through the use of CT angiography combined with PET imaging data.

Peter F. Faulhaber, MD, assistant professor of radiology at Case Western Reserve University, director of clinical PET at the University Hospitals in Cleveland and director of nuclear medicine at the Lewis Stokes Veterans Administration Medical Center, explains that when considering molecular imaging, he believes two components are of equal importance -- the radiopharmaceutical used for imaging and the scanner instrument that detects it. Given new tracers that are in development coupled with an "explosion on the instrumentation side," he anticipates major advancements in molecular imaging over the next decade.

Equipment enhancements

Faulhaber uses a Philips Medical Systems Gemini PET-CT scanner at the VA affiliate, where they have a large practice in oncology. "We must keep in mind this is a PET scan, providing crucial information, and the CT makes that scan more precise." In particular, he suggests that with patients who have received treatment, being able to assess which portion of a lesion might still be active is vital information to obtain. The 16-slice CT component of the system assists in decreasing scan times from an hour to 24 minutes for a full body study. In the past, PET scans required both a transmission scan for attenuation correction and then an emission scan for the data gathering portion of the study.

Carter S. Young, DO, FACR, chairman and medical director of medical imaging at Methodist Medical Center of Illinois in Peoria reports that with the latest version of the Philips Gemini GXL, he is able to reduce scan times to 18 minutes, down from 50 minutes with their 3D Allegro machine - and all of this with improvements in image quality.

This medical center is a large community hospital that has been using PET since 1991, and currently performs about 1,200 PET scans per year - with 98 percent of them for oncologic indications. They were the first to install the Gemini GXL, which features new detector technology as well as enhanced reconstruction algorithms.

Reduced scan times improve throughput and improve patient comfort.  "To obtain statistically satisfactory data sets requires one to significantly increase the dose of the radiopharmaceutical," explains Young. "It is not unusual for 2D users to decrease their scan times down to 20 to 25 minutes by using 25 milliCuries (mCi) of FDG." Using the GXL and a typical scan time of 18 minutes, they inject 10 to 11 mCi of the FDG radiotracer while producing what he describes as extremely high-quality images.

The significant reduction of radioactive dose required to obtain these studies holds important implications not only for patients, but also for the technologist and other personnel who perform the scans.    

Jacqueline Brunetti, MD, medical director of radiology at Holy Name Hospital in Teaneck, N.J., is using the Discovery LS PET-CT scanner from GE Healthcare