New PET scanners improve Dx in problem patients

From time-of-flight to fusion imaging, innovations promise faster scanning of obese patients

PET/CT scanners optimized to counter the challenges of obesity, cancer, cardiovascular disease, and Alzheimer's disease will be focal points at the 2006 RSNA meeting. Vendors will roll out a range of enhancements that include better methods for scanning obese patients, faster whole-body coverage for cancer staging, and lower radiation exposure.

TruFlight, a time-of-flight imaging technique introduced on the Philips Gemini TF this year, will attract attendees interested in learning about the technical innovations that propelled this coincidence detection method into clinical practice. Early adopters of Gemini TF have found that imaging obese patients is one winning application of TruFlight. Previously hard-to-detect thoracic cancers in overweight patients light up with fluorine-18 FDG imaging like never before with conventional PET/CT imaging.

TruePoint, featured on Siemens' Biograph family of PET/CT scanners, boosts the photon counts 78% by increasing the axial coverage of its detectors one-third. Higher count rates translate into faster scanning times for better overall lesion detection and whole-body scans in less than 10 minutes, according to Markus B. Lusser, vice president of worldwide molecular imaging sales and marketing.

GE Healthcare will emphasize cardiovascular applications with the rollout of the 64-slice version of its Discovery VCT PET/CT scanner. Visitors will see 64-slice coronary angiograms fused with cardiac perfusion images obtained with rubidium-82 or nitrogen-13 ammonia.

Earlier in 2006, coronary artery imaging applications were a memorable part of the Society of Nuclear Medicine meeting because of clinical trials demonstrating the high sensitivity of fused coronary artery CT and perfusion SPECT for coronary artery disease, noted Karthik Kuppusamy, general manager of nuclear medicine for GE. The company plans to use its Chicago exhibit to demonstrate the promise of fusion PET/CT in the same clinical arena.

TIME-OF-FLIGHT PROMISE

The commercial introduction of TOF is the culmination of a 20-year research effort to apply the technique to everyday diagnosis. Researchers have known since the invention of PET that true coincidence detection is theoretically possible. PET radioisotopes, such as fluorine-18, emit positrons as they decay. After traveling a few millimeters, they strike electrons and annihilate, emitting high-energy photons traveling at the speed of light in nearly opposite directions.

TOF-PET uses timing information from the geometry of photons traveling at a known speed and angle, but the technique has been difficult to implement because of the incredible speeds involved and small distances covered. PET detectors must achieve a time resolution of a few hundred trillionths of a second (picoseconds) to localize the origin of paired photons to within 5 mm of their actual point of origin.

The following innovations contribute to the 650-picosecond timing resolution that makes TruFlight possible, according to Scott Smith, Gemini TF product manager:

- lutetium-yttrium oxyorthosilicate (LYSO) scintillating crystals that maximize counts;

- small crystal arrays to boost spatial resolution and pinpoint the trajectory of photons;

- photo multiplier tubes that perform 10 times faster than conventional technology;

- timing electronics that measures very small differences in time with extreme accuracy over months of use without recalibration; and

- a powerful computer and new reconstruction algorithms to address TOF's demanding computation requirements.

Early studies performed on a Gemini TF at the University of Pennsylvania beta site confirmed the advantages of TOF for large patients, said Simon Cherry, Ph.D, a professor of biomedical engineering at the University of California, Davis, speaking at the SNM meeting in June. Specifically, several colon cancer metastases were detected with TOF in a man with a 15.75-inch-diameter waist; the cancers were overlooked on images from the same scanner without TOF.

"The gains are real and actually demonstrated," Cherry said at the meeting.

The greater sensitivity of TOF can also be used to increase image acquisition speed in patients of average weight and height. Improved acquisition leads to increased throughput or can be used to reduce the injected dose of radiopharmaceuticals to decrease radiation exposure. One Philips customer routinely images 18 patients in an eight-hour day, Smith said. Whole-body PET scans can be acquired in less than 10 minutes.

With this clinical experience as a backdrop, Philips will unveil the next step in the technology at the RSNA meeting, a 64-slice version of the Gemini TF. The addition of Philips' Brilliance 64-detector CT to the platform will aid PET/CT cardiac imaging and allow users to perform diagnostic CT alone, Smith said.

Philips rolled out its 16-slice version of Gemini TF at the European Congress of Radiology in February and in June at the SNM meeting. Beta testing was performed at the University of Pennsylvania Medical Center, Montefiore Medical Park in New York City, and University Hospital in Cleveland.

RAISING THE COUNT RATE

Siemens claims multiple benefits from the one-third increase of axial volume coverage from its TruePoint series of PET/CT scanners. More axial coverage translates into a wider field-of-view, meaning fewer bed positions for whole-body imaging completed in less than 10 minutes or a brain scan performed in a single bed position, Lusser said. More axial coverage

raises the count rate 78%, enabling the system to detect lesions as small as 5 mm and allowing the user to reduce patient radiation exposure by half.

For cardiovascular imaging, Siemens will showcase Circulation, a CT evaluation tool designed to streamline workflow while it automates calcium scoring, vessel analysis, and myocardial viability evaluations.

The firm will also introduce Version 2.0 of True-D software, featuring full respiratory gating, allowing clinical comparisons of images at multiple time points during a single exam or among several series studies. Slices appropriate for cross-correlation are reconstructed and presented automatically to simplify interpretation and improve accuracy of response-to-therapy studies. Lesion diameter, volume, and FDG metabolic activity data can be captured and compared.

GE is moving in a similar direction with PET Volume Computer-Assisted Reading. VCAR generates automated reports covering changes in tumor diameter, volume, total standard uptake unit volumes, and peak standardized uptake value. Results from up to eight studies can be simultaneously tracked and charted to plot treatment progress, Kuppusamy said.

PET ALZHEIMER'S PACKAGE

Siemens and GE will also unveil new neurology software to raise diagnostic confidence in Alzheimer's disease with PET/CT imaging. Siemens' Scenium Version 2.0 is an automated brain evaluation tool that sorts through on-board case files for images to help diagnose and stage dementias.

GE has secured FDA 510(k) clearance for its PET Alzheimer's package. The software calls up comparison studies showing characteristic FDG uptake patterns according to patient age and disease severity. FDG uptake rates are quantified. A version of the software has been applied to Phase II and III clinical trials of carbon-11 PIB, a promising Alzheimer's PET/CT imaging agent.

The 64-slice Discovery VCT will be the hub of the GE PET/CT exhibit, according to Kuppusamy. The company will also introduce IQ-Fusion, software designed for combining 3D PET and CT data sets for hybrid image presentation.

Discovery VCT will be promoted as a one-stop cardiac imaging tool for performing complete functional and anatomic PET/CT exams in 35 minutes. Workstation displays will underscore new cardiac workflow improvements. GE will also stress best practices to help radiologists optimize the quality of CT coronary artery images and PET perfusion studies performed with the Discovery VCT.

Looking back at his 16 years of experience at PET system design, Cherry remembers few breakthroughs. Yet the steady adoption of incremental improvements, like those to be featured at this year's RSNA meeting, will continue to have major influence on PET and PET/CT performance, he said.