New PET scanners promise drastically reduced scan times

August 1, 2006
Greg Freiherr
Greg Freiherr

Siemens extends field-of-view, Philips offers time-of-flight, introducing speedy exams to whole-body imaging

A major weakness of positron imaging has always been speed or, more accurately, the lack of it. Piggybacking these devices on CT only exacerbated the problem. Now two vendors are doing something about it.

Philips Medical Systems and Siemens Medical Solutions have come up with two different ways to speed exams. Their focus is on standard body lengths from the neck to the knees as well as small areas of the body such as the head.

Philips' Gemini TF (TrueFlight) is the first commercial time-of-flight PET/CT. It records each positron annihilation (the event that causes two gamma ray photons to fire away from each other at almost exactly 180 degrees ) with such precision that the location of the radiotracer can be pinpointed exactly. Siemens' Biograph TruePoint PET/ CT uses an extended field-of-view to capture more axial volume for each step or bed position during the exam.

"So now, if we do the imaging from neck to knee, we can cover the same axial field-of-view as before in four bed positions in eight minutes," said David Townsend, Ph.D., a professor of medicine and radiology at the University of Tennessee Medical Center in Knoxville, the first U.S. site to install the Biograph TruePoint. "This reduces scan time by a factor of two."

Full-body exams on the Biograph TruePoint take 10 minutes or less, he said. Philips executives make the same claim for the Gemini TF, even for obese patients, who typically need more time, said Jim Cavanaugh, director of global marketing for Philips Nuclear Medicine PET/CT.

"We are marketing this system as delivering less than 10 minutes whole-body FDG imaging for everybody," he said. "There is no differentiation for large and small patients, and that is one of the key features of this product."

Because time-of-flight calculations narrow the area where the radiotracer might be located, patient size does not matter. Early clinical experience supports this contention.

"We haven't systematically compared head-to-head obese to thin patients, but [the Gemini TF] certainly improves our ability to assess obese patients," said Dr. M. Donald Blaufox, chair of nuclear medicine at Montefiore Medical Center in the Bronx, where more than 400 patients have been scanned on the new Philips device since its arrival this spring.

Blaufox and colleagues are trying to nail down the optimal protocol for using the Gemini TF, experimenting with one- and two-minute images at each bed position, which is substantially faster than could be accomplished on the standard Gemini PET scanner.

Faster exam times translate into increased productivity as well as patient comfort and decreased patient movement. That can lead to better images, particularly when patients have difficulty holding still. But, like all PET cameras, the enhanced versions of Gemini and Biograph allow users to trade time for improved image quality.

Longer exam times mean more photon counts and better images. Consequently, users can zip through exams faster than before yet achieve comparable images. Or they can complete their exams in the same amount of time as they now spend but produce higher quality images.

"Those who don't care about time can choose to spend the usual length of a procedure and get a lot better image quality than they can today," said Piotr Maniawski, senior marketing manager for PET/CT development at Philips.

Alternatively, these two variables-exam time and photon count-might be optimized to reduce the dose of radiotracer. Michael Reiterman, president of Siemens Molecular Imaging, says striking the right balance can cut radiation exposure to patients by as much as 50%.

"We can reduce the dose, increase throughput, and create and produce great images," Reiterman said.

DIFFERENT MEANS

Although the gains are similar, Siemens and Philips achieve them by markedly different means. Siemens produces an extended field-of-view by adding 33% more detector material. The end result is a 78% jump in photon detection, according to the company. To enhance image quality further, the Biograph TruePoint PET/ CT automatically adjusts reconstruction and scatter correction algorithms to match patient habitus. The system, like other TruePoints now rolling off Siemens' production line, features a new gantry and detector mounting design. The detector's four rings extend 21.6 cm, producing 32,448 individual pixels.

Gemini TF harnesses a new crystal material, extraordinarily precise electronics, and advanced computing hardware and software to deliver time-of-flight information. In place of the GSO (gadolinium oxyorthosilicate) crystal built into the Gemini GXL detector, Philips has applied a hybridized crystal constructed from lutetium and yttrium oxyorthosilicate, which the company refers to as LYSO.

The critical consideration in time-of-flight PET is the time needed to generate a flash of light in response to the crystal being hit by a gamma ray. This is called the rise time. LYSO has a rise time of 650 picoseconds. Along with an array of advanced electronics that sample the detector once every 25 psec, the fast rise time allows calculations that narrow down the location of the radiotracer.

This is achieved by recording each of the two gamma rays emitted when a positron collides with-and is annihilated by-an electron. The two photons fire at a 180 degrees angle from each other, following the annihilation of a positron by an electron. This pair of photons has a specific position associated with it in the field-of-view. Therefore, when the position of the photons as a distinct pair is recorded, the location of the annihilation event can be determined within a relatively short distance.

The rapid sampling that homes in on the event generates a lot of data, which are handled using a computing platform that is twice as fast as the one Philips builds into its 64-slice CTs. Even so, reconstruction takes about 20 minutes. Philips minimizes the impact of this computing by beginning the calculations immediately after the first positron annihilation is recorded. As a result, previous images are available about a minute after the scan is done. Fully reconstructed images take another 10 minutes.

"We have a very scalable computing architecture, which has allowed us to distribute the processing across additional circuit boards," said Ian Farmer, senior vice president and general manager for Philips nuclear medicine.

GREATER EXPENSE

Biograph TruePoint and Gemini TF cost more to purchase than other PET/CT devices. Exactly how much more depends on the configuration. The added price, however, comes with unprecedented horsepower, the kind that has the potential to boost productivity and a department's bottom line.

PET/CT pioneer Townsend has scanned more than 360 patients since the Biograph TruePoint was installed in late February at the University of Tennessee.

The improvement in time is dramatic. A breast cancer patient, initially scanned with an older generation system, required 28 minutes, according to Townsend. The exam was performed using seven bed positions, each lasting four minutes. With Biograph TruePoint, the same quality scan was conducted in 15 minutes with five bed positions lasting three minutes each.

The Biograph TruePoint performs well even with the most challenging patients. Townsend used the system successfully to assess a 267-pound patient who was imaged in 12 minutes utilizing six bed positions lasting two minutes each. The short scan was particularly important in this case, as the patient suffered from claustrophobia.

"It produced a diagnostically satisfactory study that would not have been possible with an older system," he said.

When scans last as long as traditional studies, the detail is striking. Townsend described a lung cancer patient examined for 20 minutes using four bed positions, each lasting five minutes. The structure of the kidneys was clearly evident. Even shorter scans, such as a 10-minute study-two minutes per five bed positions-generated extraordinary delineation of the spine.

"The scan quality we are getting is unparalleled," he said.

Blaufox is similarly upbeat about the Gemini TF, describing it as a "significant advance" over the old version. His opinion represents a change of heart.

"When (Philips) first presented the machine to me that way, I didn't believe it," he said. "But I am convinced now that it is a major improvement in the technology and the ability to produce an image."