Sometimes in life strange coincidences happen. For example, a few weeks ago I wrote an article on the science behind computed tomography scanners that are ubiquitous in modern hospitals. Then today there is an article in the local newspaper reporting that the hospital I was at in April has just purchased and installed a PET/CT medical imaging device. And so rather than accept this as a random coincidence, I will take it as a sign that my readers would like to know more about this imaging method.

This machine is known as a hybrid imaging technology, because it uses two very different forms of medical imaging. The first of these methods in the traditional computed tomography or CT scan. The patient lies down on a table, and is moved into a large ring. Inside the ring is an x-ray emitter, just like the ones used by doctors for the past century to look for broken bones or other medical issues. Except where those x-ray machines take a single two-dimension image, the CT scanner takes approximately one hundred images covering anywhere from 180o to 220o around the patient. These images are loaded into a computer, which then uses the reconstruction techniques that I discussed in the previous article to produce a cross-section of the patient's insides. (In some more modern CT scanners, an actual 3D model of the patients internal organs can be produced and used for virtual reality surgery or for radiologists and gastroenterologists to "fly through" the patient's colon!)

The CT scan is easy to access, affordable for the patient and the hospital, and produces high resolution images with a relatively small dose of radiation. But it is also limited in what can be imaged, and is useless for observing and testing a number of physiological functions.

And so we come to the second half of the hybrid, the positron emission tomography (PET) scan. This is a newer and more specialized imaging modality than the CT scan, but it has advantages when measuring biological processes.

Depending on the nature of the medical examination, the patient will be either ingest or be injected with a radioactive substances (often this is fluorodeoxyglucose, containing the isotope 18F, which is not relevant to the discussion here but fun to use in Scrabble). The body absorbs this substance, and processes it the same way that glucose is processed. It gets absorbed into those organs or tissues that are active, including various tumors. Once the patient's body has processed the FDG, they are placed inside a PET scanner.

The fluorine isotope has a very interesting and useful property. Once formed, it rapidly decays away into lighter nuclei, but in doing so it produces a number of antimatter particles known as positrons. Each positron travels a few millimeters through the body before annihilating with an electron. Because the positrons have a relatively low energy, this annihilation produces two high energy photons with energy of exactly 511keV and which travel in opposite directions. If both photons are detected by the machine, then a line can be drawn between the two detection sites and we know that some of the FDG has been absorbed somewhere in the body along that line. By detecting multiple electron-positron annihilations, we can calculate where all of these lines intersect and know that something in the body at that location is absorbing our glucose substitute. 

The end result is that the radiologist receives an image or 3D model of where in the patient's body the glucose has been deposited. I am not a medical doctor, and so I won't try to explain the complex biological processes involved here, but this provides the specialists with a map of tumor sites or other sites of interest in the patient's diagnosis and treatment.

The problem with a PET scan is that the photons being used are an order of magnitude more energetic than x-rays, and are potentially much more damaging to the healthy tissues in the human body. As such, lower dosages are needed to make the process safe, and that contributes to making the PET scan a low resolution medical imaging technology. A PET scan cannot produce a high resolution image without creating other medical issues for the patient.

And that is why the PET/CT scanner is such an interesting piece of equipment. The patient can receive a low resolution, low dosage PET scan to map out the diagnostically interesting regions. At the same time - or close to it - the patient will also receive a higher resolution CT scan that maps out and images their internal organs in great detail. If the machine is properly calibrated, the two images can be superimposed upon each other to give the doctor a very precise map.

It is the best of both worlds!