Physicists are now able to capture a snapshot of a single photon, with technique called “compressed ultrafast photography,” which captures up to 1011 frames per second. Charalampos Tsoumpas, PhD, Royal Society industry fellow and a lecturer in medical imaging at the University of Leeds, speculates on the impact this technology could have on medical imaging, in a feature for Physics World:
Today’s best state-of-the-art clinical PET scanners can achieve a resolution of about 3–4 mm in a scan that lasts a couple of minutes and they offer reasonably good (10%) energy resolution, thereby providing vital information about the biochemical behaviour of the radiotracer in the body. Better images are possible using a new variety of machine known as time-of-flight PET (TOF-PET) scanners, which can measure the tiny time difference between when the two annihilation photons are detected. The world’s current fastest TOF-PET commercial scanner, which is built by Siemens, can resolve processes down to just above 200 picoseconds (200 × 10–12 s) (figure 1).
That kind of timing resolution is excellent and, what’s more, the signal-to-noise ratio is 2.5 times higher than with PET scanners that don’t use timing information. So, if – and it’s a big if – detectors could operate at still faster speeds of, say, 10 ps, we’d get even better maps of where the photons originated. Indeed, the signal-to-noise ratio of the images is expected to rise by a factor of 12 according to a formula developed by Maurizio Conti and colleagues from Siemens in 2013 (IEEE Transactions on Nuclear Science 60 87).
Huge improvements of that kind, brought about by high-resolution, ultrafast gamma-ray detectors, could bring many practical benefits in medicine. Clinicians would be able to examine patients faster, reduce the isotope doses they need, and improve the quality, resolution, accuracy and precision of the images that are so vital for medical diagnoses. But such detectors could benefit many other areas too, which is what prompted the European Union (EU) to fund the Fast Advanced Scintillation Timing (FAST) project.
Read more in Physics World.
Featured image: TOF-PET. Our ability to detect gamma rays at ultrahigh speed is of great value in medical imaging, especially in a variant of positron emission tomography (PET) known as time-of-flight PET (TOF-PET). PET involves injecting a patient with a radioactive tracer, which emits positrons that collide with electrons in the patient’s tissue, annihilating each other at a certain point (red flash) to produce two 511 keV gamma-ray photons. The photons travel in opposite directions and are captured by “scintillator” crystals, triggering the release of lower-energy photons that a photodetector converts into a measurable electric signal. TOF-PET, such as Siemens’ device (see top of page), is much more sensitive than ordinary PET thanks to solid-state silicon photomultipliers that reject a lot of background noise, thereby allowing a more precise time of the photon emission to be determined. This information in turn reveals how far the tracer has reached in the body, which helps clinicians to diagnose disease.