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PET/CT Combined Scanners - a 2018 Breakthrough of the Year... and a Personal Story

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Recently, a co-worker in her 20's was diagnosed with a brain tumor!  At times like these, the importance of medical imaging jumps to the fore!

Most people have heard of CT ("CAT") scanners – at least enough to know that they don't actually involve cats – but less well-known are PET scanners (which likewise don't involve pets!), and the synergistic combination of the two.

A Marriage Made in Heaven

What do those scanners do?  And why are they being combined in single devices?

Voted 2018 Breakthrough of the Year by a science magazine, the improved PET/CT combined scanner has been a game changer.
The EXPLORER PET/CT scanner – the world’s first medical imaging system that can capture a 3D image of the entire human body simultaneously – has produced its first human images.

Developed by UC Davis scientists and a multi-institutional consortium, EXPLORER can scan up to 40 times faster, or use up to 40 times less radiation dose, than current PET systems, making it possible to conduct repeated studies in an individual […]
The high-sensitivity scanner can also create movies that track radiolabelled drugs as they move around the body.

Incidentally, I was privileged to work on the development of earlier generations of PET scanners when, fresh out of college in the 1990’s, I held a job at the Lawrence Berkeley Lab as a Research Associate for PET scanner detector development.

Stephen Derenzo, a professor of mine from an Electrical Engineering class at UC Berkeley, was the leader of that group, and he offered me a job.

He recently retired after decades of cutting-edge research.  A PET scanner calibration device got named after him:  “Derenzo phantom (calibration device)”

Back in that research lab, a hot topic was to improve the crystals that catch the gamma rays from the Positron emission (hence the name PET, Positron Emission Tomography).  I remember helping the professor with a number of crystal gamma-ray absorption measurements and computer simulations…

Hindsight

Decades later, it turned out that a lot of the improvements have arisen from ditching the old “photomultiplier tubes” – used to detect the light emitted by the crystals – in favor of Silicon photomultipliers (more info.)  I remember absolutely no talk of that back then...

Another key improvement arose from combining PET scanners with CT scanners, and more recently combining PET with MRI.

I can’t claim any large personal contribution – I was just a Research Associate (basically a student assistant) back then – but it has been satisfying to have lent a hand…  and I’m happy that the improved technology is now better helping patients – including my co-worker, who thankfully had a successful surgery and is recovering.

For more details on the long road of small improvements in PET/CT imaging devices, here's a good 2018 article.

But what are the basic principle behind PET scanners?  That's only half of the modern devices, but the part I worked on and know best – and so I'll focus on that.

How PET (Positron Emission Tomography) Scanners Work, in brief

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Here's a machine in cross section. 
  1. The patient gets injected with a radioactive substance (short-lived and low dose, of course.)
  2. The radioactive emission of gamma rays (similar to X-rays but more energetic), always emitted in pairs, gets picked up by a duo of detectors on opposite chamber walls.
  3. From the pair of signals, the location of the radioactive substance in the body gets determined

A Few More Details

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In the diagram to the left:

(i) The radioactive atoms are isotopes of Fluorine, derived from from Oxygen and Neon by means of nuclear reactions (absorption of, respectively, protons or neutrons by the nuclei of the source atoms)

(ii) The radioactive Fluorine atoms get incorporated in sugar molecules such as FDG (Fludeoxyglucose, aka 2-fluoro 2-deoxy-D-glucose.)

(iii) The radioactive sugar is injected into the patient.  It doesn't stay radioactive for very long, though – sparing the patient from excessive radiation.  The half-life of Fluorine-18 is less than 2 hours.  (Fludeoxyglucose F-18 injections)

(iv) The radioactive Fluorine atoms emit positrons, the anti-matter version of electrons.  In our world dominated by ("regular") matter, it doesn't take long for anti-matter to annihilate itself: the newly-emitted positron runs into an electron, its anti-particle, and boom – the energy released gets carried off by two gamma-ray photons (higher-energy version of X rays) in opposite directions, to be picked up by gamma-ray detectors.

(v) Finally, computation techniques reconstruct the location and time of the original emission, resulting in an image.

"Follow the Money Glucose Trail, and the Bullet Gamma-Ray Marks"

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The counterpart of the "follow the money" of criminal investigations, is "follow the glucose"!

It's no coincidence the the radioactive Fluorine gets attached to glucose (a type of sugar.) Glucose is, of course, a key player in metabolism.  So, area with high metabolic activity will really light up in the image.

Next, it's "ballistic analysis" : if you locate 2 bullet tracks on opposite walls, from the same gun, one can reconstruct where the gun was located in the room.

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Complications of course arise if the bullet ricochets, or if there were multiple shootings....

In loving memory of my sister-in-law Myriam.

She ultimately lost the battle against cancer, but regular PET/CT scans helped monitor it, adjust her treatment, and perhaps contributed to living much longer than originally prognosed.

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