Tyler Dumouchel

A three-dimensional model-based partial volume correction strategy for gated cardiac mouse PET imaging

Quantification in cardiac mouse positron emission tomography (PET) imaging is limited by the imaging spatial resolution. Spillover of left ventricle (LV) myocardial activity into adjacent organs results in partial volume (PV) losses leading to underestimation of myocardial activity. A PV correction method was developed to restore accuracy of the activity distribution for FDG mouse imaging. The PV correction model was based on convolving an LV image estimate with a 3D point spread function. The LV model was described regionally by a five-parameter profile including myocardial, background and blood activities which were separated into three compartments by the endocardial radius and myocardium wall thickness. The PV correction was tested with digital simulations and a physical 3D mouse LV phantom. In vivo cardiac FDG mouse PET imaging was also performed. Following imaging, the mice were sacrificed and the tracer biodistribution in the LV and liver tissue was measured using a gamma-counter. The PV correction algorithm improved recovery from 50% to within 5% of the truth for the simulated and measured phantom data and image uniformity by 5-13%. The PV correction algorithm improved the mean myocardial LV recovery from 0.56 (0.54) to 1.13 (1.10) without (with) scatter and attenuation corrections. The mean image uniformity was improved from 26% (26%) to 17% (16%) without (with) scatter and attenuation corrections applied. Scatter and attenuation corrections were not observed to significantly impact PV-corrected myocardial recovery or image uniformity. Image-based PV correction algorithm can increase the accuracy of PET image activity and improve the uniformity of the activity distribution in normal mice. The algorithm may be applied using different tracers, in transgenic models that affect myocardial uptake, or in different species provided there is sufficient image quality and similar contrast between the myocardium and surrounding structures.

Uniformity and repeatability of normal resting myocardial blood flow in rats using [13N]-ammonia and small animal PET.

ObjectiveThis study aimed to quantitatively evaluate population variability, regional uniformity and repeatability of myocardial blood flow measurements using [13N]-ammonia and small animal PET. MethodsSerial PET scans were conducted on Sprague–Dawley rats using [13N]-ammonia to study relative perfusion and absolute myocardial blood flow (ml/min/g). FlowQuant automated analysis software was used to produce five-segment polar maps to investigate regional myocardial blood flow differences. The interobserver and intraobserver repeatability was assessed quantitatively using Bland–Altman analysis. ResultsAbsolute myocardial blood flow values were 4.3±1.1 ml/min/g, corresponding to a population variability of 25.5%. There were significant age-related increases in resting myocardial blood flow (r2=0.59, P<0.001). The test–retest differences had a coefficient of repeatability of 24.5% of the mean myocardial blood flow. The operator variability was small, relative to the population variability. ConclusionRepeatable myocardial blood flow values are minimally influenced by operator intervention. However, age-related myocardial blood flow increases must be taken into account when comparing measurements between experimental groups.

Analytical-Based Partial Volume Recovery in Mouse Heart Imaging

Positron emission tomography (PET) is a powerful imaging modality that has the ability to yield quantitative images of tracer activity. Physical phenomena such as photon scatter, photon attenuation, random coincidences and spatial resolution limit quantification potential and must be corrected to preserve the accuracy of reconstructed images. This study focuses on correcting the partial volume effects that arise in mouse heart imaging when resolution is insufficient to resolve the true tracer distribution in the myocardium. The correction algorithm is based on fitting 1D profiles through the myocardium in gated PET images to derive myocardial contours along with blood, background and myocardial activity. This information is interpolated onto a 2D grid and convolved with the tomographs point spread function to derive regional recovery coefficients enabling partial volume correction. The point spread function was measured by placing a line source inside a small animal PET scanner. PET simulations were created based on noise properties measured from a reconstructed PET image and on the digital MOBY phantom. The algorithm can estimate the myocardial activity to within 5% of the truth when different wall thicknesses, backgrounds and noise properties are encountered that are typical of healthy FDG mouse scans. The method also significantly improves partial volume recovery in simulated infarcted tissue. The algorithm offers a practical solution to the partial volume problem without the need for co-registered anatomic images and offers a basis for improved quantitative 3D heart imaging.

MLEM Reconstructed Image Resolution from the LabPET Animal Scanner

Background: PET image resolution is a function of scanner intrinsic resolution and reconstruction method. The purpose of this study was to measure reconstructed image resolution vs. MLEM iterations on the new LabPET 3.6 animal scanner. Methods: A Micro Deluxetrade hot rods phantom filled with an 18F solution was scanned for 60 min, and images were reconstructed using 10 to 1000 MLEM iterations. To estimate the image resolution, peak activity values were measured for each rod and compared to the theoretical values of partial-volume recovery obtained by convolving a 2D-Gaussian model with circles of the known rod diameters. Results were confirmed visually by convolving the estimated Gaussian model with a high resolution CT image. Results: FWHM image resolution improved from 2.1 to 1.3 mm with 10 to 1000 MLEM iterations. CT image convolution with this Gaussian model faithfully reproduced the measured resolution in images reconstructed with 200 MLEM iterations. Conclusion: Initial measurement of the LabPET transverse image resolution is consistent with that expected from a system with individual detector readout.

Recovery of partial volume losses in cardiac mouse PET imaging using a combined 1D/2D and a combined 1D/3D model

Micro-PET image resolution is on the order of the left ventricle (LV) wall thickness in a mouse heart. Mouse LV images are thus subject to partial volume (PV) losses, impeding the ability to quantify tracer activity in cardiac muscle. In this study, 2D and 3D PV correction (PVC) models are proposed for mouse imaging. ECG gated PET images are acquired and a 1D model is used to extract the LV wall contours and adjacent activity. This information is used to build either 2D or 3D images to derive the regional recovery coefficients in 2D or 3D. A 2D mouse heart phantom was created physically and digitally to test the 2D algorithm. The phantom was designed with 8 cardiac gates and variable wall thicknesses. The physical phantom was imaged with the Inveon small animal PET scanner and an effective Gaussian resolution of 1.3 mm FWHM was derived from the image. A 3D simulation was created based on the MOBY phantom assuming isotropic resolution. The 3D PVC model was applied to the simulation. Finally, ECG gated FDG mouse images were obtained with the Inveon and the 2D PVC algorithm was applied in the basal slices of the heart, with resolution also estimated directly from the image. The 2D PVC algorithm was found to reduce bias in the 2D measured and simulated phantom activity from 40% to 5% while also restoring image uniformity. The 3D PVC performed on the MOBY simulation reduced bias from 40% to 15% while increasing image homogeneity across all planes throughout the heart. A similar pattern was observed in the FDG mouse hearts using an estimated transverse resolution of 1.35 mm. With current technology, PVC is mandatory to restore quantitative accuracy in small animal cardiac PET imaging. This study indicates that the proposed methodology has the ability to partially restore the expected activity distribution. While mouse imaging was the focus of the present study, this algorithm could be used for LV imaging in other species where LV thickness is on the order of the system resolution.

Reconstructed Image Resolution of the LabPET Small Animal Scanner Measured Using Simulated Partial Volume Effects

PET image resolution is a function of scanner intrinsic resolution and reconstruction method. The purpose of this study was to measure the reconstructed image resolution of the LabPETtrade small animal scanner. A Micro Deluxetrade hot rods phantom was filled with an 18F solution and scanned. The 3D emission data were rebinned using single slice rebinning with various ring differences. Image reconstruction was performed using FBP and variable MLEM iterations in normal and high resolution modes. To estimate the image resolution, the relative peak recovery values were recorded for each hot rod and compared to simulated values of partial volume recovery coefficients obtained by convolving a 2D-Gaussian model with circles of the known rod diameters. FWHM image resolution was shown to improve from 2.3 to 1.1 mm with 20 to 500 MLEM iterations while FBP had a resolution between 1.7-1.8 mm. Initial measurement of the LabPETtrade transverse image resolution is consistent with that expected from a system with individual detector readout. This methodology is applicable to other PET systems and may be particularly useful when using iterative reconstruction.