Time for a New PET?

Because of budget constraints, I can't even purchase my SPECT/CT, let alone replace my solid but aging PET/CT. Still, it's always exciting to see what's up and coming in the field.

It seems I should have saved my Euros and gone to the European Association of Nuclear Medicine meeting in Munich instead of the SNM in New Orleans earlier this year. Not one, but two major PET/CT upgrades were announced by both GE and Siemens.

GE (GE Healthcare of Chalfont St. Giles, U.K.) brings us the new Discovery PET/CT 600. I've taken the liberty of reproducing the introductory Flash presentation below:

If you peruse the information (and it takes a little work with this particular Flash construction) you will find a number of nice features, including a 70cm bore, and a table that accomodates patients who are gravitationally challenged at up to 500 lbs. We also have Motion Match motion management, an IBM BladeCenter for fast reconstruction, VUE Point HD lesion detectability, and even PET VCAR efficient treatment assessment, which appears to give one the ability to follow lesions on serial scans much like the MIMVista program. But it is the graphic below that I find rather interesting:

I'm not really sure what this means. I've written before about the differences between Bismuth germinate (BGO), which GE has used for a while and cerium-doped lutetium oxyorthosilicate (LSO), the crystal scintillator used by Siemens for the past several years. However, that article is currently off-line because it refers to some equipment that I am currently using, and I would not wish to offend anyone. The only chart I could find concerning the differences is from Siemens:

Basically, this shows that LSO gives a better response to the anhillation radiation coming out of your patient than BGO. The latter might theoretically be more sensitive, stopping more photons if you will, especially if the crystals are thicker, as the green chart seems to indicate. However, the name of the game is counts, and LSO wins hands-down. Daghighian, et. al., writing in IEEE Transactions on Nuclear Science in August, 1993, agree:

A new scintillation crystal, cerium doped lutetium oxyorthosilicate (LSO), is discussed. It has light intensity which is 75% of NaI(Tl), scintillation decay time of 12 ns (30%) and 42 ns (70%), effective Z of 66, and density of 7.4 g/cm3. The fast decay time and scintillation light output of LSO are superior to those of bismuth germanate (BGO) for positron emission tomography (PET) and the stopping power of LSO for 511-keV photons is only slightly lower than BGO. The detection characteristics relevant to PET applications of small crystals of LSO (2×2×10 mm) are compared with those of BGO. The energy resolution at 511 keV is 12% full width half maximum (FWHM) for LSO and two to three times wider for BGO. The coincidence timing of two opposing crystals, using a position sensitive photomultiplier tube (PMT), is 1.4 ns FWHM for LSO and more than three times higher for BGO. Using a fast PMT, coincidence timing for LSO yields 0.46 ns FWHM.

Stopping power seems to be analogous to sensitivity in this setting. Why is GE still using the older and less optimal BGO? Maybe Siemens owns the patents for LSO. Now, there is another scintillator in the wings, LYSO, Cerium doped Lutetium Yttrium Orthosilicate, which is as good or better in some ways than LSO. I know GE had been toying with LYSO, but maybe it hasn't yet panned out.

But back to the green graphic. "...potentially reducing the dose requirement and allowing for faster scans and increased throughput..." Ummmm, well, maybe, but it all depends upon the reference value for comparison, doesn't it? I'm not seeing any claims about 5 minute scans here.

I have no details yet on whether the Discovery PET/CT 600 is 2D/3D, although I will assume that it is. Nothing yet on the CT component, either.

Siemens marketing division is apparently composed of '80's throw-backs, as their new ad campaign reads: "I want my mCT!" (OK, maybe you children are too young to remember the old "I want my MTV" slogan.... Just go ahead and rub it in.) It seems that their new offering is too cool to even be called a PET scanner...welcome to the world of molecular CT with the new Biograph mCT. And what else does the "m" indicate?

maximum-return-on-investment CT.
minimum-dosage CT.
more-patient-comfort CT.
meets-every-need CT.
made-for-the-future CT.

Based on the rather detailed brochure, the mCT appears to be a mating of the latest LSO high-resolution PET with the latest Siemens Definition Adaptive Spiral CT. In fact, it's more of a symbiosis than the older Biographs, as the pieces are much more integrated. This is not just a CT and a PET scanner bolted together. From HealthImaging.com:

“This system looks like a CT, feels like a CT, but it has PET embedded within it,” Markus Lusser, VP Global Marketing and Sales, Siemens Medical Solutions Molecular Imaging, told Health Imaging News. “Just by looking at it, you might think it is just CT—you cannot clearly see it has PET at first glance.”The traditional PET-CT approach is to build the PET scanner next to a CT scanner, Lusser said, with about a 6-foot long tunnel. The Biograph mCT, which has approximately a 3-foot long tunnel that is 78 cm wide, and has a 500-pound-capacity bed, is designed to accommodate a variety of patients.

The mCT can be purchased with different slice-counts, from 40 to 120 slices in CT as well as PET features that range from TrueV extended field-of-view technology to ultraHD-PET. “We are offering 128-slice CT configuration which is unmatched in the PET arena,” Lusser said, adding CT scanners dominating the PET-CT arena are mainly 16 slice.

Notice that the bore is 8cm wider than the GE 600, which might cover that last serving of Hagen Das, but the weight-limit is the same. The CT can resolve down to .24mm, although some of the literature says .33mm instead, still pretty small. And, there is attention paid to dose-limitation:

Taking comfort a step further, Biograph mCT addresses two of the foremost concerns related to scanning: scan time and radiation dose. Biograph mCT’s Adaptive Dose Shield protects the patient from unnecessary pre- and post-spiral radiation, conserving 20% dosage in routine CT exams.With its ultra-low dose capability and super-fast PET acquisition time, Biograph mCT gives you everything you need. So scanning can be less stressful for you and your patients.

By "super-fast" they mean a 5 minute PET/CT exam time. That is pretty darn fast.

I may have to get to RSNA this year after all, just to see these two new additions. Of course, it would be nice to be able to actually buy one....