Impact of improved dead time correction on the quantification accuracy of a dedicated BrainPET scanner

Abstract

Objective: Quantitative values derived from PET brain images are of high interest for neuroscientific applications. Insufficient DT correction (DTC) can lead to a systematic bias of the output parameters obtained by a detailed analysis of the time activity curves (TACs). The DTC method currently used for the Siemens 3T MR BrainPET insert is global, i.e., differences in DT losses between detector blocks are not considered, leading to inaccurate DTC and, consequently, to inaccurate measurements masked by a bias. However, following careful evaluation with phantom measurements, a new block-pairwise DTC method has demonstrated a higher degree of accuracy compared to the global DTC method.

Approach: Differences between the global and the block-pairwise DTC method were studied in this work by applying several radioactive tracers. We evaluated the impact on [11C]ABP688, O-(2-[18F]fluoroethyl)-L-tyrosine (FET), and [15O]H2O TACs.

Results: For [11C]ABP688, a relevant bias of between -0.0034 and -0.0053 ml/ (cm3 • min) was found in all studied brain regions for the volume of distribution (VT) when using the current global DTC method. For [18F]FET-PET, differences of up to 10% were observed in the tumor-to-brain ratio (TBRmax), these differences depend on the radial distance of the maximum from the PET isocenter. For [15O]H2O, differences between +4% and -7% were observed in the GM region. Average biases of -4.58%, -3.2%, and -1.2% for the regional cerebral blood flow (CBF (K1)), the rate constant k2, and the volume of distribution VT were observed, respectively. Conversely, in the white matter region, average biases of -4.9%, -7.0%, and 3.8% were observed for CBF (K1), k2, and VT, respectively.

Conclusion: The bias introduced by the global DTC method leads to an overestimation in the studied quantitative parameters for all applications compared to the block-pairwise method.

Significance: The observed differences between the two DTC methods are particularly relevant for research applications in neuroscientific studies as they affect the accuracy of quantitative Brain PET images.

Fig 1. The differences in the DTC factors for both DTC methods for a typical data set.

Data were obtained from a typical [¹¹C]ABP688 volunteer measurement.

Fig 2. The Siemens 3T MR BrainPET insert detector together with the indication of the 19 accepted coincidences for one of the uppermost detector heads.

Within an accepted head pair, coincidences between all 6 blocks of one head with all 6 blocks of the opposed head are allowed.

Fig 3. Statistical distribution of the slope values for V_T and both DTC methods for three exemplary brain regions obtained with the 3T MR-BrainPET insert.

Black line: mean, white line: median, yellow box 25/75% quantile, fences: min/max values.

Fig 4. Statistical distribution of the slope values for BP_ND and both DTC methods for three exemplary brain regions obtained with the 3T MR-BrainPET insert.

Black line: mean, white line: median, yellow box 25/75% quantile, fences: min/max values.

Fig 5

[¹¹C]ABP688 TACs in three relevant brain regions for both DTC methods: (a) ACC, (b) the cerebellum GM, and (c) temporal posterior cortices. (d) shows the relative differences between both DTC methods. A CCF was applied to the reconstruction with the block-pairwise DTC method.

Fig 6. TACs obtained from one [¹⁸F]FET-PET image for three relevant types of VOIs for the global and the block-pairwise DTC methods.

(a) Background VOI, (b) tumor VOI, and (c) max in tumor VOI. (d) shows the relative differences between both DTC methods. For the block-pairwise DTC method, TACs are shown before and after the application of the CCF.

Fig 7. Statistics of differences between the global DTC method and the block-pairwise DTC method for relevant features of the [¹⁸F]FET-PET TBR_max and TBR_mean.

Fig 8. Dependency of the mean relative difference between both DTC methods in the time interval.

(a) Dependency of the mean relative difference between both DTC methods in the time interval 20 to 40 minutes p.i. for TBR_max on the distance of the tumor to the PET FOV isocenter. (b) Dependency of the mean relative difference between both DTC methods in the time interval 20 to 40 minutes p.i. for TBR_max on the tumor size. (c) Dependency of the mean relative difference between both DTC methods in the time interval 20 to 40 minutes p.i. for TBR_mean on the distance of the tumor to the PET FOV isocenter. (d) Dependency of the mean relative difference between both DTC methods in the time interval 20 to 40 minutes p.i. for TBR_mean on the tumor size. Regression lines are shown in red.

Fig 9. TACs and relative differences obtained from one [¹⁵O] water PET study for two relevant types of VOIs for the global and the block-pairwise DTC methods.

For the block-pairwise DTC method, TACs are shown after the application of the CCF.

Fig 10. Statistical distribution of the relative differences for the kinetic parameters CBF (K₁), k₂, and V_T obtained from kinetic modeling of the [¹⁵O]H₂O TACs assuming a one-tissue compartment model.

Black line: mean, white line: median, yellow box 25/75% quantile, fences: min/max values. Left: GM, right: WM.