Parameterizing Image Quality of TOF versus Non-TOF PET as a Function of Body Size
Positron emission tomography (PET) is a nuclear medicine diagnostic imaging exam of metabolic processes in the body. Radiotracers, which consist of positron emitting radioisotopes and a molecular probe, are introduced into the body, emitted radiation is detected, and tomographic images are reconstructed. The primary clinical PET application is in oncology using a glucose analogue radiotracer, which is avidly taken up by some cancers.
It is well known that PET performance and image quality degrade as body size increases, and epidemiological studies over the past two decades show that the adult US population's body size has increased dramatically and continues to increase. Larger patients have more attenuating material that increases the number of emitted photons that are scattered or absorbed within the body. Thus, for a fixed amount of injected radioactivity and acquisition duration, the number of measured true coincidence events will decrease, and the background fractions will increase. Another size-related factor, independent of attenuation, is the volume throughout which the measured coincidence counts are distributed: for a fixed acquisition duration, as the body size increases, the counts are distributed over a larger area. This is true for both a fixed amount of radioactivity, where the concentration decreases as size increases, and a fixed concentration, where the amount radioactivity increases with size.
Time-of-flight (TOF) PET is a recently commercialized technology that allows the localization, with a certain degree of error, of a positron annihilation using timing differences in the detection of coincidence photons. Both heuristic and analytical evaluations predict that TOF PET will have improved performance and image quality compared to non-TOF PET, and this improvement increases as body size increases. The goal of this dissertation is to parameterize the image quality improvement of TOF PET compared to non-TOF PET as a function of body size. Currently, no standard for comparison exists.
Previous evaluations of TOF PET's improvement have been made with either computer-simulated data or acquired data using a few discrete phantom sizes. A phantom that represents a range of attenuating dimensions, that can have a varying radioactivity distribution, and that can have radioactive inserts positioned throughout its volume would facilitate characterizing PET system performance and image quality as a function of body size. A fillable, tapered phantom, was designed, simulated, and constructed. The phantom has an oval cross-section ranging from 38.5 × 49.5 cm to 6.8 × 17.8 cm, a length of 51.1 cm, a mass of 6 kg (empty), a mass of 42 kg (water filled), and 1.25-cm acrylic walls.
For this dissertation research, PET image quality was measured using multiple, small spheres with diameters near the spatial resolution of clinical whole-body PET systems. Measurements made on a small sphere, which typically include a small number of image voxels, are susceptible to fluctuations over the few voxels, so using multiple spheres improves the statistical power of the measurements that, in turn, reduces the influence of these fluctuations. These spheres were arranged in an array and mounted throughout the tapered phantom's volume to objectively measure image quality as a function of body size. Image quality is measured by placing regions of interest on images and calculating contrast recovery, background variability, and signal to noise ratio.
Image quality as a function of body size was parameterized for TOF compared to non-TOF PET using 46 1.0-cm spheres positioned in six different body sizes in a fillable, tapered phantom. When the TOF and non-TOF PET images were reconstructed for matched contrast, the square of the ratio of the images' signal-to-noise ratios for TOF to non-TOF PET was plotted as a function, <italic>f</italic>(<italic>D</italic>), of the radioactivity distribution size, <italic>D</italic>, in cm. A linear regression was fit to the data: <italic>f</italic>(<italic>D</italic>) = 0.108<italic>D</italic> - 1.36. This was compared to the ratio of <italic>D</italic> and the localization error, <italic>σd</italic>, based on the system timing resolution, which is approximately 650 ps for the TOF PET system used for this research. With the image quality metrics used in this work, the ratio of TOF to non-TOF PET fits well to a linear relationship and is parallel to <italic>D/σd</italic>. For <italic>D</italic> < 20 cm, there is no image quality improvement, but for radioactivity distributions <italic>D</italic> > 20 cm, TOF PET improves image quality over non-TOF PET. PET imaging's clinical use has increased over the past decade, and TOF PET's image quality improvement for large patients makes TOF an important new technology because the occurrence of obesity in the US adult population continues to increase.
Positron emission tomography
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