PET Scans in the Future

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One of the most considerable current areas of advancement in PET technology is the development of PET-MRI hybrid scans. Click here for more information. 

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Aside from this, there have been many recent developments in PET technology. These include dual time point imaging and time-of-flight (TOF) PET. As technology progresses, these will become better integrated into clinical technology. 

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Dual Time Point Imaging 

Most cancer cells have a higher metabolic activity and accumulate more FDG during the PET scan uptake period (the period after radiotracer injection and before scanning) than regular cells. However, some infections/inflammations also lead to an increased glucose metabolism rate, mimicking malignant tumours and making it hard to establish if certain areas of increased radioactive activity are indeed cancerous. Dual time point PET is a technique where a scan is performed after a fixed (standard) uptake phase and then repeated after a delay. [1]

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Some tumours have an increase in their uptake of FDG with time, compared to inflammation/infection which has a decrease with time. Where there is uncertainty as to whether an area is malignant, repeating the scan as late as reasonably possible increases the tumour-to-background contrast and therefore increases the sensitivity of the technique. [2]

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Time-of-Flight PET

Time-of-flight (TOF) PET was initially developed in the 1980s but had very limited sensitivity and spatial resolution. Current generation TOF PET scanners have the highest resolutions and sensitivities of any clinical PET scanner. In modern TOF PET scanners there is an improved compromise between contrast, image noise, and total imaging time. This reduces scan times and radiation doses for patients, and makes it easier to detect malignancies using the equipment. [3]

 

​Upon detection of an annihilation photon pair in opposite detectors, the distance, d, of the annihilation event along the line of response is given by 

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(1)

 

where c is the speed of light, and the arrival times of the two photons are t1 and t2. [3]

 

Conventionally, in PET scanners the difference in arrival time (t2−t1) was not known precisely enough to determine the emission point. The emission event was taken to have an equal probability of occurring anywhere on the line of response. 

 

In TOF PET scanners, because the difference in arrival times of the two photons is precisely measured, the scanners can accurately localise the annihilation. The uncertainty in this localisation is determined by the timing resolution, Δt (this is taken as the full-width-half-maximum of the histogram of TOF measurements from a point source). Spatial uncertainty, Δx, is then 

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(2)

 

This spatial uncertainty from the TOF element is around an order of magnitude worse than the detector spatial resolution (around 5mm in modern scanner) so image reconstruction (similar to that in conventional PET scanners) is still required to produce tomographic images. However, during this image reconstruction, signal noise is projected over a fewer number of steps, leading to a less noisy image (and improved signal to noise ratio). [4] 

 

TOF PET is now included in many commercial PET-CT scanners, and its use in PET-MRI is growing rapidly. [4]

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References:

[1] Basu, S. et al. (2011). Fundamentals of PET and PET-CT scanning, New York: New York Academy of Sciences. pp. 11-13

[2] Chan, W. L. et al. (2011). Dual-time-point (18)F-FDG-PET/CT imaging in the assessment of suspected malignancy.J Med Imaging. 55(4). pp. 379-90.

[3] Surti, S. (2015). Update on time-of-flight PET imaging J Nucl Med. 56(1). Pp. 98–105.

[4] Vandenberghe, S. et al. (2016) Recent developments in time-of-flight PET JNMMI Physics 3(3). pp. 1-6

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