PET-CT Scanner

Overview

A PET-CT scanner combines two imaging methods in one gantry. Positron emission tomography (PET) maps where a positron-emitting tracer has concentrated, showing function such as glucose metabolism, while X-ray computed tomography (CT) provides the surrounding anatomy at high spatial resolution. Acquired back to back without moving the patient, the two datasets are fused so a metabolically active spot, for example an FDG-avid tumour, is pinned to an exact anatomical location. Hybrid PET-CT, introduced clinically around 2001, quickly replaced standalone PET because the CT also supplies the attenuation map PET reconstruction needs.

The system pairs a stationary PET Detector Ring with a rotating CT Subsystem in a common Gantry, a precise Patient Table, Coincidence Electronics, and a Reconstruction Computer.

How PET detection works

The patient receives a tracer such as fluorine-18 FDG, a glucose analogue. Each F-18 nucleus emits a positron, which travels a short distance and then annihilates with an electron, converting their mass into two 511 keV gamma photons flying out in almost exactly opposite directions. The PET Detector Ring surrounds the patient with hundreds of Detector Blocks, each a LYSO Crystal Array array read by a SiPM Array. When a gamma is absorbed the crystal scintillates and the silicon photomultiplier records the event's energy and arrival time. LYSO is chosen because it is dense, stopping 511 keV gammas efficiently, and fast, giving the sub-nanosecond timing that time-of-flight imaging requires.

Detection alone is not enough; PET needs to know that two gammas came from the same annihilation. The Coincidence Electronics handle this: the Coincidence Processor accepts two events only if they arrive within a roughly 4 ns window, which means they almost certainly came from one annihilation somewhere along the line joining the two detectors, the line of response. Modern scanners add time-of-flight: the Time-of-Flight Timing Board measures the tiny difference in arrival times, around 200–400 picoseconds, and from it estimates where along that line the annihilation occurred, sharpening the image and cutting noise.

How the CT works

The CT Subsystem is a conventional CT mounted just in front of the PET ring. Its X-ray Tube and opposed CT Detector Arc spin together on the CT Rotor several times per second, the High-Voltage Generator supplying 80–140 kV. As they rotate, the CT Detector Arc records how much each X-ray path is attenuated, and these projections reconstruct into cross-sectional slices. Power and the high data rate cross the spinning joint through the CT Slip Ring. Besides its diagnostic images, the CT provides a map of tissue density that PET reconstruction uses to correct for the gammas absorbed inside the body before they reached the ring, without this attenuation correction, deep structures would falsely appear cold.

Reconstruction and fusion

Event data from every block is gathered by the Data Concentrator and sent to the Reconstruction Computer, where the Reconstruction Server and its GPU Boards run iterative algorithms that build the PET volume from millions of lines of response, applying CT-based attenuation and scatter correction. The PET and CT volumes are inherently co-registered because the patient did not move between them, and the Operator Console displays the fused result.

Mechanics and support systems

The Patient Table must advance the patient through both fields of view with very little sag, so its carbon-fibre Carbon-Fibre Table Top includes Deflection Compensation to keep PET and CT slices aligned. Detector performance depends on temperature: the Cooling System holds the LYSO and SiPM at a stable temperature with the Detector Temperature Sensor feedback, because both scintillation light yield and SiPM gain drift with temperature, while the Water Chiller also removes the considerable heat of the X-ray tube. The Power Distribution cabinet conditions three-phase mains for the high-power CT generator and the sensitive PET electronics alike, and the Covers and Bore Liner and smooth Bore Liner hide the spinning CT behind a clean 70–80 cm patient tunnel.