Update on time-of-flight PET imaging

Abstract

Time-of-flight (TOF) PET was initially introduced in the early days of PET. The TOF PET scanners developed in the 1980s had limited sensitivity and spatial resolution, were operated in 2-dimensional mode with septa, and used analytic image reconstruction methods. The current generation of TOF PET scanners has the highest sensitivity and spatial resolution ever achieved in commercial whole-body PET, is operated in fully-3-dimensional mode, and uses iterative reconstruction with full system modeling. Previously, it was shown that TOF provides a gain in image signal-to-noise ratio that is proportional to the square root of the object size divided by the system timing resolution. With oncologic studies being the primary application of PET, more recent work has shown that in modern TOF PET scanners there is an improved tradeoff between lesion contrast, image noise, and total imaging time, leading to a combination of improved lesion detectability, reduced scan time or injected dose, and more accurate and precise lesion uptake measurement. Because the benefit of TOF PET is also higher for heavier patients, clinical performance is more uniform over all patient sizes.

Keywords: lesion detection; lesion uptake; scan time; time-of-flight PET.

Figure 1

(A) Emission point at a distance d from the center of the scanner within an object of diameter D. The two 511 keV photons are detected in coincidence at times t₁ and t₂. (B) Without precise TOF measurement a uniform probability along the LOR within the object is assumed for each emission point, leading to noise correlations over a portion of image space between the two events as shown here. (C) With TOF information the position of the emission point is localized along the LOR with a precision that is defined by a Gaussian distribution of width Δx. (D) Better localization of the two emission events along their individual LORs leads to reduced (or no, as shown here) noise correlation of the events in image space during image reconstruction.

Figure 2

A. (A) Gain in sensitivity as defined by D/Δx plotted as a function of timing resolution for cylindrical phantoms with three different diameters. (B) TOF gain as a function of activity concentration in a 35 cm diameter by 11.5 cm long uniform cylinder measured on the Super PETT I (SPI) scanner (5). Figure reprinted with permission from (28).

Figure 3

(A) Reconstructed Non-TOF (top row) and TOF (bottom row) images for a 35 cm diameter cylindrical lesion phantom for iteration numbers (left to right) 1, 2, 5, 10, and 20. The phantom has hot spheres (diameters of 22, 17, 13, and 10 mm) with 6:1 uptake relative to background and two cold spheres (37 and 28 mm). (B) Non-TOF (top row) and TOF (bottom row) images for the 35 cm diameter cylindrical lesion phantom for scan times of (left to right) 5, 3, 2, and 1 min. Non-TOF and TOF images are shown for iteration numbers 10 and 5, respectively where the lesion CRC values are at or close to convergence. (C) CRC for the 13 mm diameter sphere plotted as a function of image noise and plotted at iteration numbers 1, 2, 5, 10, 15, and 20. Closed symbols are for Non-TOF and open symbols are for TOF images with scan times of 2 (▲), 3 (◆), 4 (■), and 5 (●) mins. (D) Gain in lesion contrast as measured over several lesions in five different patients. TOF and Non-TOF images were chosen for a fixed number of iterations in order to achieve similar pixel-to-pixel noise in the images. All figures reprinted with permission from (33).

Figure 4

(A) Reconstructed Non-TOF (top row) and TOF (bottom row) images for a 35 cm diameter lesion phantom containing six, 10 mm diameter spheres with 6:1 uptake relative to the background. All images are shown after 20 iterations and are for scan times of (left to right) 1, 2, 3, 4, and 5 mins. (B) NPW SNR for the 10 mm diameter spheres plotted as a function of scan time for Non-TOF (light bars) and TOF (dark bars) images. Figures reprinted with permission from (34).

Figure 5

(A) Reconstructed images for a patient study showing a lesion synthetically inserted in the liver. Arrows indicate the location of the inserted lesion. (B) Results for average ALROC values for liver lesions shown as a function of: BMI (labels of L for BMI < 26 and H for BMI ≥ 26), scan time (labels of 1m or 3m for scan times of 1 min and 3 min, respectively), and image reconstruction (labels of NT for Non-TOF and T for TOF). All figures reprinted with permission from (41).

Figure 6

(A) Average sphere uptake or NUV measured in the lung and liver. (B) Variability of the sphere uptake or NUV measurement in the liver and lung shown as a function of statistical replicates (Repl.), location within the same organ (Loc.), and over different patients (Subj.). Figures reprinted with permission from (43).

Figure 7

(A) Transverse Non-TOF (left) and TOF (right) images of a thorax phantom with a shifted attenuation correction map applied to the data. The arrows in the Non-TOF image show areas of incorrect increased and decreased counts, leading to artifacts in the image. (B) Transverse Non-TOF (left) and TOF (right) images of a thorax phantom with an incorrect normalization applied to the data. The three hot lesions are not all visible in the Non-TOF image which also shows increased artifacts. (C) Transverse Non-TOF (left) and TOF (right) images of a thorax phantom with no scatter correction applied to the data. Figures reprinted with permission from (46).