Digital PET: What a technologist needs to know

Introduction: Since the invention of combined PET/CT, many advanced have taken place ranging from novel combined modalities to improved reconstruction algorithms and innovative hardware designs that improve patient safety, image quality, and quantitative accuracy. Every year new technologies arrive to meet the demanding clinical need for improved image quality, faster imaging, and smaller lesion detection. For PET-CT imaging, the biggest leap forward over the last two decades has likely been the advent of digital PET and SiPM systems. This work examines the differences between traditional PET and new digital systems, reviews what technologists need to know about translating to these technologies, and how this new technology impacts image quality.

Methods: Hardware components were compared from our traditional state of the art PMT-based PET-CT platform to our newly installed digital PET-CT platform. Multimodal images acquired using multiple tracers were qualitatively compared to observe the difference in image quality and assess any necessary changes to imaging protocols.

Results: One of the biggest hardware improvements in the last few years across all major vendors is the replacement of standard photomultiplier tubes (PMT) with Silicon based photomultipliers (SiPM) as the primary detector technology in state-of-the-art digital systems. With this advancement has come smaller pixel sizes that improve resolution, increased sensitivity that decreases scan times (or reduces dose) and improved electronics that enable more accurate event localization. The table 1 below show the changes between our two systems.

Table 1:

The hardware changes noted improve multiple aspects of image quality with the greatest improvement being faster timing resolution. This increase in TOF not only improves image contrast but also improves lesion detectability leading to reduced patient dose or decreased scanner time. The smaller crystal size also plays a role in better lesion detectability. Combining both the smaller crystals with the faster TOF improves the signal to noise ration and provides sharper images no matter the amount of activity given. This is seen in figures 1 below where the improvements to contrast and resolution are apparent.

Conclusions: Moving from traditional PMT-based PET-CT systems to digital SiPM-based systems will have significant improvements on resolution, sensitivity, and time of flight performance. As a technologist converting protocols from one platform to another there are a few things to consider. Patient scan time or patient dose could be decreased with these improvements or injection and dosing protocols may warrant a review to see if time savings or dose savings can be realized. Improved contrast and resolution can lead to images appearing sharper than from a traditional PMT system so institutional review of image quality may be needed to properly assess what post-processing filters, if any, may need to be applied to meet the needs of the reading radiologists.

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