Benefit of time-of-flight in PET: experimental and clinical results

Abstract

Significant improvements have made it possible to add the technology of time-of-flight (TOF) to improve PET, particularly for oncology applications. The goals of this work were to investigate the benefits of TOF in experimental phantoms and to determine how these benefits translate into improved performance for patient imaging.

Methods: In this study we used a fully 3-dimensional scanner with the scintillator lutetium-yttrium oxyorthosilicate and a system timing resolution of approximately 600 ps. The data are acquired in list-mode and reconstructed with a maximum-likelihood expectation maximization algorithm; the system model includes the TOF kernel and corrections for attenuation, detector normalization, randoms, and scatter. The scatter correction is an extension of the model-based single-scatter simulation to include the time domain. Phantom measurements to study the benefit of TOF include 27-cm- and 35-cm-diameter distributions with spheres ranging in size from 10 to 37 mm. To assess the benefit of TOF PET for clinical imaging, patient studies are quantitatively analyzed.

Results: The lesion phantom studies demonstrate the improved contrast of the smallest spheres with TOF compared with non-TOF and also confirm the faster convergence of contrast with TOF. These gains are evident from visual inspection of the images as well as a quantitative evaluation of contrast recovery of the spheres and noise in the background. The gains with TOF are higher for larger objects. These results correlate with patient studies in which lesions are seen more clearly and with higher uptake at comparable noise for TOF than with non-TOF.

Conclusion: TOF leads to a better contrast-versus-noise trade-off than non-TOF but one that is difficult to quantify in terms of a simple sensitivity gain improvement: A single gain factor for TOF improvement does not include the increased rate of convergence with TOF nor does it consider that TOF may converge to a different contrast than non-TOF. The experimental phantom results agree with those of prior simulations and help explain the improved image quality with TOF for patient oncology studies.

Figure 1

Images from the 35-cm diameter phantom measurement with two cold spheres (28, 37 mm) and four hot spheres (10, 13, 17, 22 mm) with 6:1 contrast. (A) non-TOF and (B) TOF images for 1, 2, 5, 10, and 20 iterations (left to right) for 5-min scan time. (C) non-TOF images after 10 iterations and (D) TOF images after 5 iterations for 5, 3, 2, and 1-min scan times (left to right).

Figure 2

CRC vs. noise curves for 13-mm (left) and 17-mm (right) hot spheres with 6:1 contrast in 27- (top) and 35-cm (bottom) cylinders. Scan times on the Gemini TF scanner were 1 ( ), 2 ( ), and 3 ( ) min (27-cm phantom) and 2 ( ), 3 ( ), 4 ( ), and 5 ( ) min (35-cm phantom) with closed symbols for non-TOF and open symbols for TOF reconstruction as a function of number of iterations (1, 2, 5, 10, 15, and 20).

Figure 3

Representative transverse sections of two different patients: low dose CT (left), non-TOF ML-EM (middle), and TOF ML-EM (right). Patient 1 (top) with colon cancer (119 kg, BMI = 46.5) shows a lesion in the abdomen seen in CT much more clearly in the TOF image than in the non-TOF image. Patient 2 (bottom) with abdominal cancer (115 kg, BMI = 38) shows structure in the aorta seen in CT much more clearly in the TOF image than in the non-TOF image.

Figure 4

Patient with non-Hodgkin’s lymphoma (140 kg, BMI = 46). The top row shows representative transverse, sagittal, and coronal images (not triangulated) for non-TOF reconstruction, while bottom row shows the same cross-sectional images for TOF reconstruction. In each of these images the different lesions are seen more clearly in the TOF reconstruction than the non-TOF reconstruction.

Figure 5

Patient with non-Hodgkin’s lymphoma (140 kg, BMI = 46). Anterior projection image after 10 iterations of TOF ML-EM reconstruction is shown (left). The letters (a–g) denote the lesions that were used in the L/B ratio analysis. L/B ratio is plotted vs. noise in the liver ROI for 1–10 iterations for each of the lesions for non-TOF (middle) and TOF (right) reconstructions.

Figure 6

TOF gain as a function of patient mass. The TOF gain for matched noise levels, averaged over the 6–9 lesions (1–2 cm diameter) for each patient, is plotted as a function of patient mass. The error bars reflect the range of TOF gains seen for that patient.